<%BANNER%>

Controlled Self-Assembly of Metal-Organic Networks

Permanent Link: http://ufdc.ufl.edu/UFE0024355/00001

Material Information

Title: Controlled Self-Assembly of Metal-Organic Networks Tuning the Magnetic Behavior of Prussian Blue Analogs and the Coordination Topology of Langmuir-Blodgett Films
Physical Description: 1 online resource (177 p.)
Language: english
Creator: Gardner, Justin
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: analogs, anisotropy, blue, ctist, films, langmuir, magnetic, magnetism, nanoparticles, photomagentism, prussian, thermal, thin
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In order to modify their magnetic behavior and prepare Prussian blue analog materials suitable for future application, the transition metal composition and dimensionality of lattices were controlled, using nanoparticle and thin film synthetic procedures, as well as, transition metal ion doping. The cobalt iron Prussian blue analog, AjCokFe(CN)6l.nH2O, is known to exhibit photoinduced and thermal charge transfer induced spin transitions (CTIST). The sign and magnitude of photoinduced change in magnetization in microcrystalline samples of the ternary transition metal Prussian blue analog Na(alpha)Ni(1-x)CoxFe(CN)6b.nH2O were controlled by tuning the transition metal composition and introducing competing superexchange energies. Additionally, inclusion of NiII ions into the lattice initiates dilution of the spin-crossover material, reducing the magnitude and hysteresis of the thermal CTIST. The mechanism of the CTIST is the same as that present in the parent cobalt iron Prussian blue analog, denoted by the generation of new moments for compositions with x > 0. These results are qualitatively explained by simple mean field models. Prussian blue analog thin films were generated using a sequential adsorption method, tuning their transition metal composition and thickness by exchanging aqueous metal reactants and number of deposition cycles. The Prussian blue analog thin films displayed unique magnetic behavior, namely directional anisotropy, not observed in bulk powders. The magnitude of magnetic anisotropy was controlled by film thickness and transition metal composition. Scaling with the spin values of incorporated metal ions, the behavior can be qualitatively explained by considering the contributions of g value and single ion anisotropies. CTIST active Co-Fe PBA nanoparticles were synthesized using oil-water microemulsion procedures to restrict particulate growth. By tuning the concentration of interstitial cations and particle size, we demonstrated the ability to tune the temperature at which the thermal CTIST event takes place and the width of thermal hysteresis of the transition. Supramolecular assembly of metal-organic networks was investigated by reacting Langmuir monolayers of 4,4 ,4 -tricarboxytriphenylmethyl octadecyl ether with transition metal ion subphases. The precipitation and coordination topology of these networks was shown to be dependent upon the symmetry of the amphiphile, coordination geometry and charge of the subphase ions, and subphase acidity.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Justin Gardner.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Talham, Daniel R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024355:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024355/00001

Material Information

Title: Controlled Self-Assembly of Metal-Organic Networks Tuning the Magnetic Behavior of Prussian Blue Analogs and the Coordination Topology of Langmuir-Blodgett Films
Physical Description: 1 online resource (177 p.)
Language: english
Creator: Gardner, Justin
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: analogs, anisotropy, blue, ctist, films, langmuir, magnetic, magnetism, nanoparticles, photomagentism, prussian, thermal, thin
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: In order to modify their magnetic behavior and prepare Prussian blue analog materials suitable for future application, the transition metal composition and dimensionality of lattices were controlled, using nanoparticle and thin film synthetic procedures, as well as, transition metal ion doping. The cobalt iron Prussian blue analog, AjCokFe(CN)6l.nH2O, is known to exhibit photoinduced and thermal charge transfer induced spin transitions (CTIST). The sign and magnitude of photoinduced change in magnetization in microcrystalline samples of the ternary transition metal Prussian blue analog Na(alpha)Ni(1-x)CoxFe(CN)6b.nH2O were controlled by tuning the transition metal composition and introducing competing superexchange energies. Additionally, inclusion of NiII ions into the lattice initiates dilution of the spin-crossover material, reducing the magnitude and hysteresis of the thermal CTIST. The mechanism of the CTIST is the same as that present in the parent cobalt iron Prussian blue analog, denoted by the generation of new moments for compositions with x > 0. These results are qualitatively explained by simple mean field models. Prussian blue analog thin films were generated using a sequential adsorption method, tuning their transition metal composition and thickness by exchanging aqueous metal reactants and number of deposition cycles. The Prussian blue analog thin films displayed unique magnetic behavior, namely directional anisotropy, not observed in bulk powders. The magnitude of magnetic anisotropy was controlled by film thickness and transition metal composition. Scaling with the spin values of incorporated metal ions, the behavior can be qualitatively explained by considering the contributions of g value and single ion anisotropies. CTIST active Co-Fe PBA nanoparticles were synthesized using oil-water microemulsion procedures to restrict particulate growth. By tuning the concentration of interstitial cations and particle size, we demonstrated the ability to tune the temperature at which the thermal CTIST event takes place and the width of thermal hysteresis of the transition. Supramolecular assembly of metal-organic networks was investigated by reacting Langmuir monolayers of 4,4 ,4 -tricarboxytriphenylmethyl octadecyl ether with transition metal ion subphases. The precipitation and coordination topology of these networks was shown to be dependent upon the symmetry of the amphiphile, coordination geometry and charge of the subphase ions, and subphase acidity.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Justin Gardner.
Thesis: Thesis (Ph.D.)--University of Florida, 2009.
Local: Adviser: Talham, Daniel R.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024355:00001


This item has the following downloads:


Full Text

PAGE 1

1 CONTROLLED SELF-ASSEMBLY OF METAL-ORGANIC NETWORKS: TUNING THE MAGNETIC BEH AVIOR OF PRUSSIAN BLUE ANALOGS AND THE COORDINATION TOPOLOGY OF LANGMUIR-BLODGETT FILMS By JUSTIN EDWARD GARDNER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2009

PAGE 2

2 2009 Justin Edward Gardner

PAGE 3

3 To Edward P. Kirven: A great teacher, a great man, and above all else, a great friend

PAGE 4

4 ACKNOWLEDGMENTS My pursuit of a graduate degree in chem istry has been a long and arduous journey, but one well worth the wait. Though I have spent countless hours in classr ooms, weary afternoons in the laboratory, and sleepless nights in fr ont of the computer screen, all my efforts would have been futile if not for the numerous people, who have aided me along this course. One of the first people to inspire in me a desire to learn chemistry was Dr. Frank Swicker, a man who I had the honor of learning from for two years at Chris tian Brothers high school. This journey then continued at the University of the South, where I met and studi ed under a group of awe-inspiring professors, in particular Dr. Edward Kirven a nd Dr. Robert Bachman, who helped develop in me a desire to both learn and help others to learn chemistry. The influence of those around me did not subsid e, but rather intensif ied as my continued education led me to the University of Florida. After listening to his orientation talk given on coordination complexes and metallomesogens, I chose to work in the laboratory of Dr. Daniel R. Talham. Throughout my tenure, he has given me invaluable a dvice and information, which has aided in my development into a more knowledge able chemist. But beyond helping with my chemistry education, Dr. Talham, with his co ntinuous enthusiasm and refusal to allow complacency, helped me become a more self-motivated chemist and productive thinker. I also want to take the time to thank the others from the Talham research group and the rest of the chemistry department, who have been instrumental in my success over the year s. First, I want to thank Franz Frye, Ian Rummel, and Matthieu Du mont for our many conversations, which have both aided the progression of resear ch projects and helped keep en tertainment and sanity thriving in CLB 413. I also want to thank Monique W illiams, Candace Zieleniuk, Eric Libra, Jonathon Sommer, and Roxy Lowry who have helped the da ys, months, and even years pass more easily

PAGE 5

5 by being those who would give you their friendship, listen and understand the hardships that we all go through, and make the weekends more enjoyable. Much of the research that I have conducted through the past five years has required the help of others outside of the department. Much of this assistance was provided by Dr. Mark W. Meisel and Daniel Pajerowski, a graduate student in his research group. All of the magnetic data herein was obtained by this research group. I also want to thank both Dr. Meisel and Dan for their help in the interpretation of this data and th e theory behind the result s. I would also like to thank those at the Major Analytical Instrument ation Center for all their help in analyzing countless cyanometallate samples, especially Ben Pletcher, who has gone out of his way to help me with the TEM analysis of numerous nanopa rticle samples and explanations of related instrumental methods. Special thanks also need to be given to the group of gentlemen who helped guide the completion and future progression of this work with their candid opinions and scientific insight. Due to the help which they offered and the grea t deal of respect which I hold for them, I now want to take time to thank my supervisory committee: Daniel Talham, Mark Meisel, Ronald Castellano, George Christou, and Mike Scott. Finally, I would like to thank those who mean the most in my life, those who always believe in me, who are always there with suppor t: my girlfriend Jessica Orlowski, my extended family, and especially my parents Doug and Mart ha Gardner. Thanks go out to you all for the nightly prayers, supportive letters and phone calls and their forgiving acceptance of infrequent contact and visits and prolonged times of separation. For all these reasons and many more, I give all of my thanks and love to you all.

PAGE 6

6 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ........10 LIST OF FIGURES.......................................................................................................................11 LIST OF ABBREVIATIONS........................................................................................................ 18 ABSTRACT...................................................................................................................................21 CHAP TER 1 INTRODUCTION TO PRUSSIAN BLUE ANALOGS........................................................ 23 Prussian Blue and Prussian Blue Analogs.............................................................................. 23 Photoinduced and Thermal CT IST in Cobalt Iron PBAs ....................................................... 25 Prussian Blue Analog Thin Film and Nanoparticle Fabrication .............................................29 Magnetic Anisotropy in Prussian Blue Analog Thin Film s.................................................... 32 Scope of Research...................................................................................................................34 2 PHOTOMAGNETISM AND THERMAL CTIST IN TERNARY TRANSITION METAL PRUSSIAN BLUE ANALOGS ............................................................................... 36 Introduction................................................................................................................... ..........36 Experimental Section........................................................................................................... ...40 Materials..........................................................................................................................40 Synthesis..........................................................................................................................40 Na1.3 Ni4.0[Fe(CN)6]3.0 20.0 H2O (xsynthesis = 0, x = 0.00)....................................... 40 Na1.7Co0.8Ni3.2[Fe(CN)6]3.0 17.7 H2O (xsynthesis = 0.2, x = 0.22)............................. 41 Na1.7Co1.8Ni2.2[Fe(CN)6]3.1 19.7 H2O (xsynthesis = 0.4, x = 0.45)............................. 41 Na1.8Co2.6Ni1.4[Fe(CN)6]3.0 18.5 H2O (xsynthesis = 0.6, x = 0.66)............................. 41 Na1.8Co3.5Ni0.5[Fe(CN)6]2.9 15.3 H2O (xsynthesis = 0.8, x = 0.87)............................. 41 Na1.9Co4.0[Fe(CN)6]3.0 17.4 H2O (xsynthesis = 1.0, x = 1.00).................................... 41 Instrumentation and Characterization.............................................................................. 41 Chemical and structur al characterization ................................................................. 41 Magnetic measurements........................................................................................... 42 Results Section........................................................................................................................43 Chemical and Physical Characterization......................................................................... 43 Low Temperature Magnetization....................................................................................50 High Temperature Magnetization.................................................................................... 56 Discussion Section............................................................................................................. .....58 Low Temperature Magnetization....................................................................................58 Photoinduced decrease in magnetization................................................................. 58

PAGE 7

7 Evolution of magnetic properties............................................................................. 61 High Temperature Magnetization.................................................................................... 63 Conclusions.............................................................................................................................66 3 MAGNETIC ANISOTROPY IN PRUSSI AN BLUE ANALOG THIN-FILMS .................. 67 Introduction................................................................................................................... ..........67 Experimental Section........................................................................................................... ...68 Materials..........................................................................................................................68 Film Preparation..............................................................................................................69 Rb0.61Co4.0[Cr(CN)6]2.87nH2O fast thin film (1a-f)..................................................69 Rb0.88Ni4.0[Cr(CN)6]3.00nH2O fast thin film (2a-g).................................................. 69 RbjNik[Cr(CN)6]lnH2O powder spin-cast thin film (2h)......................................... 69 Rb0.70Cu4.0[Cr(CN)6]2.90nH2O fast thin film (3a-f)..................................................70 Rb0.27Zn4.0[Cr(CN)6]2.77nH2O fast thin film (4a-f)..................................................70 Rb0.74Co4.0[Fe(CN)6]2.84nH2O fast thin film (5a-f)..................................................70 Rb0.96Ni4.0[Fe(CN)6]2.79nH2O fast thin film (6a-f)................................................... 70 Rb0.54Cu4.0[Fe(CN)6]2.72nH2O fast thin film (7a-f)..................................................71 Rb0.54Zn4.0[Fe(CN)6]2.83nH2O fast thin film (8a-f)..................................................71 Instrumentation................................................................................................................ 71 Analysis Preparation and Procedures..............................................................................72 Results Section........................................................................................................................73 Thin Film Generation and Characterization.................................................................... 73 Prussian blue analog thin film precipitation............................................................. 73 PBA thin film structural characterization........................................................................ 74 PBA thin film chemical characterization................................................................. 79 Thin Film Magnetic Characterization............................................................................. 80 Low-temperature magnetization.............................................................................. 80 Thin film anisotropy: transition metal ion dependence............................................ 84 Thin film anisotropy: thickness dependence............................................................ 86 Discussion...............................................................................................................................88 Thin Film Precipitation.................................................................................................... 88 Magnetic Anisotropy in PBA Thin Films....................................................................... 88 Conclusions.............................................................................................................................91 4 SIZE DEPENDENT THERMAL CTIST IN COB ALT IRON PRUSSIAN BLUE ANALOGS........................................................................................................................ .....93 Introduction................................................................................................................... ..........93 Experimental Section........................................................................................................... ...96 Materials..........................................................................................................................96 Synthesis..........................................................................................................................96 NajCok[Fe(CN)6]lnH2O bulk powder precipitation (1-4)........................................96 NajCok[Fe(CN)6]lnH2O small bulk powder precipitation (2a)................................ 97 NajCok[Fe(CN)6]lnH2O nanoparticle precipitation (3a and 3b).............................. 97 Instrumentation and Characterization.............................................................................. 97 Chemical and structur al characterization ................................................................. 97

PAGE 8

8 Magnetic measurements........................................................................................... 98 Results.....................................................................................................................................98 Chemical and Physical Characterization......................................................................... 98 Chemical composition determination....................................................................... 98 Structural characterization......................................................................................101 Thermal CTIST Behavior..............................................................................................107 Variable temperature FT-IR................................................................................... 107 High temperature magnetism................................................................................. 110 Discussion Section............................................................................................................. ...113 Nanoparticle Precipitation.............................................................................................113 Thermal CTIST............................................................................................................. 114 Increasing TCTIST....................................................................................................114 Evolution of CTIST slope and hysteresis............................................................... 115 Conclusions...........................................................................................................................116 5 INTRODUCTION TO SUPRAMOLECULAR ASSEMBLY WITH LANGM UIR MONLAYERS............................................................................................... 118 Supramolecular Chemistr y and Self-assem bly..................................................................... 118 Langmuir Monolayers and Self-assembly at Interfaces....................................................... 120 Monolayer and Thin Film Characterization......................................................................... 124 Brewster Angle Microscopy..........................................................................................124 FT-IR Spectroscopy and Atomic Force Micr oscopy of Langmuir-Blodgett Film s...... 125 Scope of Research.................................................................................................................127 6 TOPOLOGICAL CONTROL OF TWODIME NSIONAL METAL-ORGANIC NETWORKS........................................................................................................................128 Introduction................................................................................................................... ........128 Experimental Section........................................................................................................... .131 Synthesis........................................................................................................................131 Materials.................................................................................................................131 Instrumentation....................................................................................................... 131 4,4,4-tricyanotriphenylmethanol (1)................................................................... 132 4,4,4-tricyanotribenzylmethyl dodecyl ether (2)................................................. 132 4,4,4-tricyanotriphenylmethyl octadecyl ether (3) .............................................. 133 4,4,4-tricarboxytriphenylmethyl octadecyl ether (4).......................................... 134 Monolayers and LB Films............................................................................................. 134 Materials.................................................................................................................134 Substrate preparation.............................................................................................. 134 Instrumentation....................................................................................................... 135 Monolayer and LB film preparation....................................................................... 136 Results Section......................................................................................................................136 Langmuir Monolayers................................................................................................... 136 Amphiphile design and LB film fabrication.......................................................... 136 Tris-cyano amphiphiles..........................................................................................138 Tris-carboxy amphiphiles: pH depe ndence of network form ation......................... 142

PAGE 9

9 Tris-carboxy amphiphiles: charge de pendence of network form ation................... 145 Langmuir-Blodgett Films.............................................................................................. 149 Tris-cyano amphiphiles..........................................................................................149 Tris-carboxy amphiphile FT-IR spectroscopy....................................................... 151 Tris-carboxy amphiphile AFM............................................................................... 153 Discussion Section............................................................................................................. ...156 Tris-cyano Amphiphiles................................................................................................ 156 Tris-carboxy Amphiphiles............................................................................................. 158 Conclusions...........................................................................................................................160 APPENDIX A TRANSMISSION ELECTRON MICROGRAPH S OF HE TEROSTRUCTURED THIN FILMS...................................................................................................................................161 B TRANSMISSION FT-IR SPECTRA OF PBA THIN FILMS .............................................162 C NMR SPECTRA OF AMPHIPHILIC MOLECULES......................................................... 164 LIST OF REFERENCES.............................................................................................................167 BIOGRAPHICAL SKETCH.......................................................................................................177

PAGE 10

10 LIST OF TABLES Table page 2-1 Molecular formulas for synthesized NaNi1-xCox[Fe(CN)6]nH2O compounds.............. 46 2-2 Unit cell parameters and particulate size of NaNi1-xCox[Fe(CN)6]nH2O compounds as a function of x................................................................................................................46 2-3 Magnetic ordering temperatures (Tc) and coercive fields (Hc) as a function of x for NaNi1-xCox[Fe(CN)6]nH2O compounds........................................................................54 2-4 Width of thermal hysteresis and % of CTIST active material in NaNi1-xCox[Fe(CN)6]nH2O as a function of x...............................................................56 3-1 AFM measured Prussian blue analog thin film thickness and roughness data.................. 74 3-2 Molecular composition and characteristic CN stretches for Prussian blue analog RbjMk[M(CN)6]lnH2O thin films synthesized with the sequential adsorption method... 80 3-3 Magnetic anisotropy of RbjMk[M(CN)6]lnH2O thin films as a function of divalent transition metal ion at 2 K.................................................................................................. 85 4-1 Transition metal elemental analyses and proposed m olecular formulas for NajCok[Fe(CN)6]lnH2O compounds..................................................................................99 4-2 Equilibrium particulate size of NajCok[Fe(CN)6]lnH2O compounds as a function of surfactant and transi tion metal reagen t concentration..................................................... 102 4-3 Variable temperature FT-IR results for NajCok[Fe(CN)6]lnH2O bulk powder 3 and nanoparticles 3a and 3b ...................................................................................................110 4-4 Size dependence of thermal CTIST in NajCok[Fe(CN)6]lnH2O. TCTIST is defined as the temperature upon which the thermal hysteresis is centered (Tc = (T1/2up-T1/2down). 112 6-1 List of the alkyl tail peak stretching frequencies a nd half-m aximum widths for amphiphiles 2 and 3 over aqueous subphases of NaCl, AgNO3, CuCl2, and Ni(NO3)2..150 6-2 List of the alkyl tail peak stretching frequencies a nd half-m aximum widths for amphiphile 4 over aqueous subphases of NaCl(aq), Fe(SO4)2(aq), and La(NO3)3(aq)..........152 6-3 Roughness and film thickness study results for monolayers of amphiphile 4 over aqueous subphases of Fe(SO4)2(aq), and La(NO3)3(aq), transferred onto hydrophilic silicon substrates (RMS roughness of 0.062 nm)............................................................ 155

PAGE 11

11 LIST OF FIGURES Figure page 1-1 Idealized unit cell of Prussian blue, FeIII 4[FeII(CN)6]3 14H2O........................................23 1-2 Representative unit cell of a Prussian blue analog, AjM k[M(CN)6]l nH2O (where A is an alkali metal ion and M,M are transition metal ions ); these networks are isostructural to Prussian blue............................................................................................. 24 1-3 Representative unit cell of the low spin, low tem perature phase Co-Fe PBA, A4CoIII 4[FeII(CN)6]4 nH2O, where A is an alkali cation..................................................26 1-4 Representative unit cell of the high s pin, high temperature phase Co-Fe PBA, CoIII 4[FeII(CN)6]2.66 nH2O................................................................................................27 1-5 Thermal CTIST in Co-Fe PBAs........................................................................................ 28 1-6 Photoinduced CTIST in Co-Fe PBAs................................................................................29 1-7 Schematic depicting the sequential absorp tion of cyanom etallate networks onto the polymer substrate Melinex................................................................................................. 31 1-8 Nanoparticle synthesis of Prussian blue analogs using the m icroemulsion technique......32 1-9 Temperature dependence of the magnetization of the Ni-Fe PBA after field cooling in 20 G with the sam ple aligned parallel and perpendicular to the magnetic field............ 33 1-10 The time dependence of the photoind uced m agnetization of a film of RbjCok[Fe(CN)6]l nH2O, prepared using sequential adsorption onto a two-dimensional, cyanometallate template la yer, when the external field is parallel and perpendicular to th e plane of the film......................................................................... 34 2-1 Field cooled (FC) magnetic susceptibility vs. temperature data, for heterostructured sandwich f ilm (NiCr/CoFe/NiC r) described above, in He = 100 G are shown for the parallel and perpendicular orie ntations of the film with re spect to the applied field........ 37 2-2 Change in magnetism ( M) vs. tim e of irradiation fo r the parallel and perpendicular orientations of the heterost ructured sandwich film (NiCr/ CoFe/NiCr) in an external applied field of 100 G........................................................................................................ 38 2-3 Cross-sectional schematic of the heterostructured sa ndwich film (NiCr/CoFe/NiCr) ......38 2-4 Relative fraction of RbjCok[Fe(CN)6]lnH2O (black) as a function of thin film depth in ~180 nm and ~800 nm hetero structured sandwich films...............................................39 2-5 Measured atomic composition of nick el, cobalt, and iron for com pounds of the general molecular formula NaNi1-xCox[Fe(CN)6]nH2O as a function of xsynthesis..........44

PAGE 12

12 2-6 Color variation of NaNi1-xCox[Fe(CN)6]nH2O as a function of x................................. 44 2-7 FT-IR spectra and respective fi tting param eters for synthesized NaNi1-xCox[Fe(CN)6]nH2O compounds as a function of x............................................45 2-8 TEM micrographs for NaNi1-xCox[Fe(CN)6]nH2O compounds for different values of x.....................................................................................................................................47 2-9 Room temperature XRD reflection of NaNi1-xCox[Fe(CN)6]nH2O compounds, showing a continuous evolution of unit cell parameters with x......................................... 48 2-10 TEM image of a post-synthesis, manually mixed NaNi1-xCox[Fe(CN)6]nH2O compound with x = 0.6...................................................................................................... 49 2-11 Photoinduced CTIST in NaNi1-xCox[Fe(CN)6]nH2O compounds.................................51 2-12 Mean-field calculations of molar ma gnetic susceptibilities as a function of tem perature at 10 G in th e dark and light states.................................................................52 2-13 Magnetization as a function of applied m agnetic field in both the dark and light states, measured at 2 K....................................................................................................... 53 2-14 Magnetic behaviour of post synthesis m ixed NaNi1-xCox[Fe(CN)6]nH2O, x = 0.66.....55 2-15 Molar magnetic susceptibility as a fu nction of temperature f or 12 nm and 30 nm particles of Na1.3Ni4.0[Fe(CN)6]3.0 20.0 H2O in the dark FC state measured with He 10 G....................................................................................................................................55 2-16 Temperature dependent magnetizat ion studies as a function of CoII concentration, x, for NaNi1-xCox[Fe(CN)6]nH2O......................................................................................57 2-17 Temperature dependent magnetization studi es, with an applied filed of 5 kG, for 73 nm particles of NaNi1-xCox[Fe(CN)6]nH2O, with x = 1.00...................................... 58 2-18 Field dependent, photomagentism of NaNi1-xCox[Fe(CN)6]nH2O, x = 0.66.................60 2-19 Dependence of the Curie-Weiss temperature, C-W, coercive field, HC, and maximum in the ZFC susceptibility, Tpeak, before and after photoexciation for NaNi1-xCox[Fe(CN)6]nH2O, as a function of x..............................................................62 2-20 Dependence of thermal hysteresis, nu m ber of spin-crossover active nearest neighbors, zSCO, percentage of CTIST active materi al, and unit cell lattice parameters as a function of x................................................................................................................64 3-1 Prussian blue analog thin film thickness as a function of deposition cycles using a sequential absorption m ethod............................................................................................ 74

PAGE 13

13 3-2 AFM images of 30 cycle RbjMk[Cr(CN)6]lnH2O thin films synthesized using the sequential deposition method............................................................................................. 75 3-3 AFM images of 30 cycle RbjMk[Fe(CN)6]lnH2O thin films synthesized using the sequential deposition method............................................................................................. 76 3-4 AFM image of a 30 cycle Rb0.54Zn4.0[Fe(CN)6]2.83nH2O thin film synthesized using the sequential deposition method....................................................................................... 76 3-5 AFM images of 53, 118, 179, and 330 nm Rb0.61Co4.0[Cr(CN)6]2.87nH2O thin films synthesized using the seque ntial deposition method......................................................... 77 3-6 AFM images of 67, 132, 191, and 325 nm Rb0.74Co4.0[Fe(CN)6]2.84nH2O thin films synthesized using the seque ntial deposition method......................................................... 77 3-7 SEM images of 200 cycle RbjMk[Cr(CN)6]lnH2O thin films........................................... 77 3-8 SEM images of 200 cycle RbjMk[Cr(CN)6]lnH2O thin films........................................... 78 3-9 SEM images of 200 cycle RbjMk[Fe(CN)6]lnH2O thin films........................................... 78 3-10 SEM images of 200 cycle RbjMk[Fe(CN)6]lnH2O thin films........................................... 78 3-11 AFM image of a spin cast thin film of RbjNik[Cr(CN)6]lnH2O microcrystalline, bulk powders..............................................................................................................................79 3-12 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjMk[Cr(CN)6]lnH2O thin films, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 82 3-13 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjMk[Fe(CN)6]lnH2O thin films, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 82 3-14 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjNik[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 83 3-15 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjMk[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 85 3-16 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjMk[Fe(CN)6]lnH2O compounds, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 85 3-17 Magnetic anisotropy of RbjNik[Cr(CN)6]lnH2O thin films as a function of film thickness, displaying a gradual decrease in anisotropy with increas ed film thickness...... 86

PAGE 14

14 3-18 FC and ZFC magnetic susceptibi lity vs. tem perature data, for RbjNik[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel and perpe ndicular film orientations with respect to the applied field................................................................................................. 87 3-19 Schematic of the easy axis in th e Prussian blue analog thin film s..................................... 89 3-20 Schematic of the tetragonal distortion of a Cr3+ ion..........................................................91 4-1 MT versus T plots for NajCok[Fe(CN)6]lnH2O Prussian blue anal ogs as a function of interstitial cation concentration and transi tion metal ion ratio, dur ing the cooling and warming process, at H = 5000 G....................................................................................... 95 4-2 FT-IR spectra and respective fi tting param eters for synthesized NajCok[Fe(CN)6]lnH2O compounds as a function of cation concentration and particle size.................................................................................................................. ....100 4-3 TEM images of microcrystalline and nanoparticle NajCok[Fe(CN)6]lnH2O compounds from Table 4-1.............................................................................................. 101 4-4 Particle distributions of the four bul k powder cobalt iron Prussian blue analogs, NajCok[Fe(CN)6]lnH2O, as a function of alka li cation concentration............................. 102 4-5 Particle distribution of 2 and 2a. Upon increasing the reactant concentration and decreasing the reaction tim e, the particle size decreased from 237 40.1 nm to 72.8 14.9 nm.........................................................................................................................103 4-6 Particle distribution of 3, 3a, and 3b Upon increasing the surfactant concentration, the particle size decreased from 557.1 77.2 nm to 14.4 1.1 nm and 12.7 1.1nm.... 103 4-7 Absorbance IR spectra of cobalt iron Prussian blue analogs, 1 4 .................................105 4-8 Size dependent behavior of the absorb ance FT-IR spectra of cobalt iron Prussian blue analog nanoparticles, 2a, 3a, and 3b ........................................................................106 4-9 Size dependence of thermal CTIST in NajCok[Fe(CN)6]lnH2O nanoparticles, 3b and 3a......................................................................................................................................108 4-10 Size dependent behavior of thermal CTIS T exhibited by variable tem perature FT-IR spectra of NajCok[Fe(CN)6]lnH2O nanoparticles 3a and 3b ...........................................109 4-11 Temperature dependent magnetization studi es as a function of partic le size for NajCok[Fe(CN)6]lnH2O. T vs T as measured in a SQUID magnetometer with an applied field of 5 kG........................................................................................................ 111 5-1 The assembly of a metal organic fram ework (MOF) with diamonoid coordination topology from the copolymerization of metal ions with organic linkers......................... 119

PAGE 15

15 5-2 Synthetic strategy for the self-assembly of twodim ensional coordinate covalent networks at the air-water interface...................................................................................121 5-3 A schematic representation of charac terization of Langm uir monolayers throughout the films compression..................................................................................................... 122 5-4 A schematic representation of the prin ciple behind Brewster angle m icroscopy (BAM)..............................................................................................................................125 5-5 A schematic depiction of the deposition of a monolayer and multi-layer Langm uir-Blodgett film onto a hydrophilic solid support...............................................126 6-1 Excised layers of a Prussian Blue an alogue and silver cont aining diam ondoid type network, yielding a two-dimensional square grid lattice and honeycomb networks....... 129 6-2 Schematic of two-dimensional (6,3) honeycom b metal-organic networks formed from the coordination of amphiphillic molecules to transition metal ions in the aqueous subphase.............................................................................................................131 6-3 Target amphiphiles with three-fold symmetry and three coordination nodes ................. 137 6-4 Synthesis procedure for amphiphiles 2, 3, and 4 .............................................................138 6-5 MMA versus time isotherms for 2 and 3 over a 2 m M aqueous subphases of NaCl.......139 6-6 Pressure versus area isotherms for 2 and 3 over 2 m M aqueous subphases of NaCl, AgNO3, CuCl2, and Ni(NO3)2..........................................................................................140 6-7 BAM images of Langmuir monolayers of 2 and 3 over aqueous subphases of NaCl at pressures of 0 m N/m and 5 mN/m................................................................................... 140 6-8 BAM images of Langmuir monolayers of 2 over aqueous subphases of AgNO3, CuCl2, and Ni(NO3)2 at pressures of 0 and 5 mN/m........................................................ 141 6-9 BAM images of Langmuir monolayers of 3 over aqueous subphases of AgNO3, CuCl2, and Ni(NO3)2 at pressures of 0 and 5 mN/m........................................................ 141 6-10 Pressure versus area isotherms for 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at pHs of 4.9, 5.7, and 3.5; 4.0, 4.5, a nd 3.0; and 3.0, 4.0, and 2.5 compared to 4 over a NaCl subphase............................................................................................... 143 6-11 BAM images of Langmuir monolayers of 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at surface pressures of 0, 5, and 10 mN/m and low pHs............... 144 6-12 BAM images of Langmuir monolayers of 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at surface pressures of 0, 1, and 10 mN/m at higher pHs.............. 145

PAGE 16

16 6-13 Pressure versus area isotherms for amphiphile 4 over aqueous subphases of Fe(SO4)2(aq) at pHs of 3.0 and 2.5 and NaCl(aq), La(NO3)3(aq) at pHs of 4.0 and 3.0 and NaCl(aq), and Fe(SO4)2(aq) at a pH of 3.0, La(NO3)3(aq) at a pH of 4.0, and NaCl(aq)........146 6-14 BAM images of 4,4,4triscarboxybenzyl m ethyloctode cyl ether over subphases of Fe(SO4)2(aq) (pH = 2.50) and La(NO 3 ) 3(aq) (pH = 3.0) at surface pressures of 0 and 10 mN/m..........................................................................................................................147 6-15 BAM images of 4,4,4triscarboxybenzyl m ethyloctode cyl ether over subphases of Fe(SO4)2(aq) (pH > 2.50) and La(NO 3 ) 3(aq) (pH > 3.0) at surface pressures of 0 and 10 mN/m..........................................................................................................................147 6-16 Pressure versus area isotherms for amphiphile 4 over NaCl(aq) at pHs of 2.5 and 5.3..... 148 6-17 BAM images of 4,4,4triscarboxybenzyl m ethyloctode cyl ether over subphases of NaCl (aq) at a high pH 5.3 and a low pH of 2.5 at surface pressures of 0 and 10 mN/m...............................................................................................................................149 6-18 ATR-FTIR of LB films of 2 and 3 spread over aqueous subphases of NaCl(aq), AgNO3(aq), CuCl2(aq) and Ni(NO3)2(aq) transferred onto a silicon ATR crystal, with the use of Y-type Langmuir-Blodgett transfers............................................................... 150 6-19 ATR-FTIR of LB films of 4 spread over aqueous subphases of NaCl(aq), CuCl2(aq), MnBr2(aq), and Fe(SO4)2(aq)...............................................................................................151 6-20 ATR-FTIR of LB films of 4 spread over aqueous subphases of NaCl(aq), Fe(SO4)2(aq), and La(NO3)3(aq). The spectra presented display the regions containing stretching peaks of the amphiphiles alkyl ta ils and the aromatic carbonyls................................... 153 6-21 Two-dimensional AFM images of bare silicon and transfer red m onolayers of amphiphile 4 spread over subphases of Fe(SO4)2(aq) and La(NO3)3(aq) at a surface pressure of 5 mN/m..........................................................................................................154 6-22 Two-dimensional AFM images of bare silicon and transfer red m onolayers of amphiphile 4 spread over subphases of Fe(SO4)2(aq) and La(NO3)3(aq)............................155 6-23 Reaction scheme of tricarboxy amphiphiles with divalent and trivalent transition m etal subphase ions during two-dimensional network precipitation............................... 158 6-24 Perpendicular schematic view of two-dim ensional honeycomb metal-organic networks formed from the coordi nation of a Langmuir monolayer of 4,4,4-tricarboxytriphenylmethyl octadecyl either to divale nt and trivalent transition metal ions from an aqueous subphase.............................................................................. 159 A-1 High resolution TEM images of ~180 nm and ~800 nm Prussian blue analog heterostructured sandwich films, comprised of a layer of RbjCok[Fe(CN)6]lnH2O deposited between two layers of RbjNik[Cr(CN)6]lnH2O...............................................161

PAGE 17

17 A-2 High resolution TEM images of Prussian blue analog heterostructured sandwich film s, comprised of a layer of RbjCok[Fe(CN)6]lnH2O deposited between two layers of RbjNik[Cr(CN)6]lnH2O................................................................................................161 B-1 FT-IR spectra and respective fi tting param eters for synthesized RbjMj[Cr(CN)6]lnH2O fast thin film compounds as a function of M............................. 162 B-2 FT-IR spectra and respective fi tting param eters for synthesized RbjMj[Fe(CN)6]lnH2O fast thin film compounds as a function of M............................. 163 C-1 Proton (1H) NMR of 4,4,4-tricyanotri benzylmethyl dodecyl ether ( 2 ) in CDCl3.........164 C-2 Carbon (13C) NMR of 4,4,4-tricyanotribenzylmethyl dodecyl ether ( 2 ) in CDCl3......164 C-3 Proton (1H) NMR of 4,4,4-tri cyanotribenzylmethyl octadecyl ether ( 3 ) in CDCl3......165 C-4 Carbon (13C) NMR of 4,4,4-tricyanotribe nzylmethyl octadecyl ether ( 3) in CDCl3....165 C-5 Proton (1H) NMR of 4,4,4-tri carboxytribenzylmethyl octadecyl ether ( 4) in CD3OD.............................................................................................................................166 C-6 Carbon (13C) NMR of 4,4,4-tricarboxytribe nzylmethyl octadecyl ether ( 4) in CD3OD.............................................................................................................................166

PAGE 18

18 LIST OF ABBREVIATIONS A Alkali cation AFM Atomic force microscopy AQ Aqueous solution ATR Attenuated total reflectance BAM Brewster angle microscopy C Condensed CCD Charge coupled device CHN Carbon hydrogen nitrogen CN cyanide CTIST Charge transfer induced spin transition DI Deionized EDS Energy dispersive spectroscopy emu Electromagnetic unit EPR Electron paramagnetic resonance FC Field cooled FCC Face centered cubic FT-IR Fourier transform infrared G Gas G Gauss GIXD Grazing incidence x-ray diffraction HC Coercive field HD Dipolar field HE External applied magnetic field HS High spin

PAGE 19

19 HT High temperature K Kelvin LB Langmuir-Blodgett LC Liquid condensed LE Liquid expanded LS Low spin LT Low temperature M Transition metal M Transition metal MMA Mean molecular area MOF Motel-organic frameworks NMR Nuclear magnetic resonance OD Outer diameter PBA Prussian blue analog PVP Polyvinylpyrrolidone RMS Route mean squared S Spin value SBU Secondary building units SCO Spin crossover SEM Scanning electron microscopy SQUID Superconducting quantum interference device TC Magnetic ordering temperature TCTIST Temperature around which the th ermal spin crossover centers, (T1/2up+T1/2down)/2 Tf Freezing temperature associated with spin-glass materials

PAGE 20

20 T1/2up Temperature at which half of the spin crossover material has transitioned in the heating cycle T1/2down Temperature at which half of the spin crossover material has transitioned in the cooling cycle TRZ Triazole TEM Transmission electron microscopy XRD X-ray diffraction ZFC Zero field cooled Magnetic susceptibility

PAGE 21

21 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CONTROLLED SELF-ASSEMBLY OF METAL-ORGANIC NETWORKS: TUNING THE MAGENTIC BEHAVIOR OF PRUSSIAN BLUE ANALOGS AND THE COORDINATION TOPOLOGY OF LANGMUIR-BLODGETT FILMS By Justin Edward Gardner May 2009 Chair: Daniel R. Talham Major: Chemistry In order to modify their ma gnetic behavior and prepare Pr ussian blue analog materials suitable for future application, the transition me tal composition and dimensionality of lattices were controlled, using nanoparticle and thin f ilm synthetic procedures, as well as, transition metal ion doping. The cobalt ir on Prussian blue analog, AjCok[Fe(CN)6]l .nH2O, is known to exhibit photoinduced and thermal char ge transfer induced spin transitions (CTIST). The sign and magnitude of photoinduced change in magne tization in microcryst alline samples of the ternary transition metal Prussian blue analog NaaNi1-xCox[Fe(CN)6]b .nH2O were controlled by tuning the transition metal composition and intr oducing competing superexchange energies. Additionally, inclusion of NiII ions into the lattice initiate s dilution of the spin-crossover material, reducing the magnitude and hysteresis of the thermal CTIST. The mechanism of the CTIST is the same as that pres ent in the parent c obalt iron Prussian blue analog, denoted by the generation of new moments for compositions with x > 0. These results are qualitatively explained by simple mean field models. Prussian blue analog thin films were ge nerated using a sequential adsorption method, tuning their transition metal composition and th ickness by exchanging aqueous metal reactants

PAGE 22

22 and number of deposition cycles. The Prussian blue analog thin films displayed unique magnetic behavior, namely directional anisotropy, not obs erved in bulk powders. The magnitude of magnetic anisotropy was controlled by film thickness and transition metal composition. Scaling with the spin values of incorporated metal i ons, the behavior can be qualitatively explained by considering the contributions of g value and single ion anisotropies. CTIST active Co-Fe PBA nanoparticles were sy nthesized using oil-water microemulsion procedures to restrict particulate growth. By t uning the concentration of interstitial cations and particle size, we demonstrated the ability to tune the temperature at which the thermal CTIST event takes place and the width of ther mal hysteresis of the transition. Supramolecular assembly of metal-organi c networks was investigated by reacting Langmuir monolayers of 4,4,4-tr icarboxytriphenylmethyl octadecyl ether with transition metal ion subphases. The precipitation and coordinatio n topology of these networks was shown to be dependent upon the symmetry of the amphiphile, coordination geometry and charge of the subphase ions, and subphase acidity.

PAGE 23

23 CHAPTER 1 INTRODUCTION TO PRUSSIAN BLUE ANALOGS Prussian Blue and Prussian Blue Analogs Prussian blue is a m olecular based compound, with the formula FeIII 4[FeII(CN)6]3 14H2O, comprised of a face-centered cubic (FCC) lattice of divalent and trivalent iron ions bound together by linear cyanide bridging ligands, Figure 1-1. Long used as a colorful blue pigment, this coordinate covalent compound garnered considerable interest in th e scientific community when it was found to exhibit magnetic or dering at low temperatures in 1928.1 In was not until 1968 that the magnetic behavior of Prussian blue was determined to be long range ferromagnetic ordering at low temp eratures (< 5.6 K).2 The crystal structure was subsequently solved through x-ray crystallography and further supported with neutron scattering experiments ten years later.3,4 Further studies on this compound have led to the iden tification of an extensiv e list of interesting properties like electrochromism,5,6 heavy metal filtration,7 peroxide sensing,8,9 hydrogen storage,10 and catalytic ability.11 FeIIFeIICNFeIIFeIICNFigure 1-1. Idealized unit cell of Prussian blue, FeIII 4[FeII(CN)6]3 14H2O. Prussian blue is comprised of a FCC lattice of FeII and FeIII ions bridged by cyanide ligands. Notice that the formal charge of the idealized unit cell possesses a charge of 4-.

PAGE 24

24 Prussian blue analogs (PBAs) are a class of mate rials that are isostructu ral to Prussian blue. These compounds, with the general formula AjM k[M(CN)6]l nH2O (where A is an alkali metal ion and M,M are transition metal ions), are achieve d by replacing one or both of the iron ion sites of Prussian blue with another transition metal ion, Figure 12. In order to achieve unit cell charge balance and precipitate extended coordi nate covalent networks Prussian blue and Prussian blue analogs incorporate cyanometallate vacancies and interstitial cations to decrease negative charge and increase positive charge within the compound. MIIIMIICNcyanometallate vacancy alkali cation MIIIMIICNcyanometallate vacancy alkali cation Figure 1-2. Representative unit ce ll of a Prussian blue analog, AjM k[M(CN)6]l nH2O (where A is an alkali metal ion and M,M are transition metal ions ); these networks are isostructural to Prussian blue. Interstitial alkali cations and cyanometallate vacancies are incorporated to obtain charge balan ce in the network. Incomplete coordination spheres, caused by cyanometallate vacan cies, are filled with coordinating water molecules. The extensive list of aqueous soluble transition metal and alkali metal salts increases the ease with which the elemental composition, and therefor e the properties, of these materials can be tuned. Recent findings have shown these co mpounds to exhibit high temperature magnetic

PAGE 25

25 ordering,12-16 photoinduced magnetism,17-35 thermal charge transfer induced spin transition(CTIST),28,30,36 and magnetic anisotropy,37-41 making them the focus of numerous research groups investigating molecular magnetism With binary PBAs having been extensively studied, ternary transition metal Prussian blue analogs, with the general molecular formula AM (1-x)M x[M(CN)6] nH2O, have started to draw attent ion due to additional properties observed in these compounds, like magnetic pole inversions42,43 and dilution of spin-crossover behavior.44 Photoinduced and Thermal CTIST in Cobalt Iron PBAs Though several cyanometallates and coordina tion compounds exhibit thermally induced spin-crossover and photoinduced magnetism, one of the most extensively st udied systems is the CTIST active cobalt iron Prussian blue analog (AjCoII k[FeIII(CN)6]l nH2O). Photo-induced magnetism was first reported in cobalt iron (Co-Fe) Prussian blue analogs by Hashimoto and coworkers in 199617, and these compounds have subsequent ly been extensively investigated by several groups.18-35,37-41 The magnetic behavior of these compounds has been found to be dependent upon alkali cation concentration, tran sition metal ion ratio, network dimensionality, and external stimuli. As previously mentioned, Prussian blue analogs incorporate al kali cations and/or cyanometallate vacancies into th eir lattices to achieve charge balance upon network formation. The manner in which this charge balance is achie ved has a predominant effect on the ligand field of the cobalt ions in the structure, and therefor e the magnetic properties. If charge balance is achieved solely by inclusion of positive alkali cations, the resultant rigid lattice will have no cyanometallate vacancies and four intersti tial cations per unit cel l; each cobalt atoms coordination sphere is now comprised of six n itrogen atoms yielding a strong ligand field. In this case, this system exists as CoIII LS-FeII LS transition metal pairs, each metal having an electron

PAGE 26

26 configuration of d6 (t2g 6) and spin value, S = 0.20 On the other hand, if charge balance is achieved by the incorporation of cy anometallate vacancies, the resu ltant lattice will have one and a half cyanometallate vacancies and zero interstiti al cations per unit cell. Each cobalt atoms coordination sphere is now comprised of four nitrogen and two oxygen atoms, arising from coordinating water molecules, yielding a weak ligand field; now, this system exists as CoII HS-FeIII HS transition metal pairs. The electron c onfigurations of the cobalt and iron metal centers are now d7 (t2g 5eg 2) and d5 (t2g 5) with spin states of S = 3/ 2 and 1/2 respectively. These metal centers are antiferromagnetically coupled, yi elding an overall parama gnetic (S = 1) state.20 This population of the anti-bonding eg orbitals also leads to an expansion in the analogs high spin lattice structure, as compared to that of th e low spin state lattice. The representative Co-Fe Prussian blue analog unit cells, accompanied by their respective electron configurations and the over all spin states of their respective cobalt and iron ions, are shown in Figures 1-3 and 1-4, below. FeII LSCoIII LSCNalkali cation FeII LSCoIII LSCNalkali cation oCoIII LSFeII LSegt2gt2g 6(S = 0)t2g 6(S = 0) oCoIII LSFeII LS o o oCoIII LSFeII LSegt2gt2g 6(S = 0)t2g 6(S = 0) Figure 1-3. Representative unit cell of the low spin, low temperature phase Co-Fe PBA, A4CoIII 4[FeII(CN)6]4 nH2O, where A is an alkali cation. Charge balance is achieved in the lattice by inclusion of four intersti tial cations. The electron occupancy of the CoIII LS and FeII LS ions leads to an overall spin value of zero for the Prussian blue analog material.

PAGE 27

27 cyanometallate vacancy CoII HSFeIII LSCNcyanometallate vacancy CoII HSFeIII LSCNoCoII HSFeIII LSegt2gt2g 5eg 2(S = 3/2)t2g 5(S = 1/2) o oCoII HSFeIII LSegt2gt2g 5eg 2(S = 3/2)t2g 5(S = 1/2) Figure 1-4. Representative unit cell of the high spin, high temperature phase Co-Fe PBA, CoIII 4[FeII(CN)6]2.66 nH2O. Charge balance is achieved by inclusion of cyanometallate vacancies. Antiferromagnetically coupled CoII HS and FeIII LS ions lead to an overall spin value of S = 1. When the lattice of the Co-Fe Prussian blue analog incorporates both interstitial cations and cyanometallate vacancies to achieve charge balance, as shown in Fi gure 1-2, the cobalt ions can possess a coordination sphere consisting of approximately five nitrogen and one oxygen atom and experience an intermediate ligand field. Due to the materials intermediate octahedral splitting ( o), the thermal energy at room temperature allows for an electron transfer from the FeII sites to the CoIII ions and the population of the CoIII eg orbitals, increasing the magnetization of the compound. The resultant high spin (HS) electronic configuration and magnetic behavior would resemble that of the latt ice with no interstitial cations Upon cooling this compound to temperatures of ~50 K, the population of the eg orbitals is no longer fa vorable and the reverse electron transfer occurs, decreasing the magnetization. At this point, the low spin (LS) electronic configuration and magnetic behavior mimics that of the lattice with no cyanometallate vacancies. This change in the electronic configuration, and therefore magnetism, of the compound is known as charge transfer induced spin transition (CTIST).20,22,30 While heating to induce this thermal

PAGE 28

28 process, the low temperatur e, low spin, diamagnetic CoIII LS-FeII LS pairs undergo a charge transfer from the FeII ions to the CoIII ions to form the excited state of CoII LS-FeIII LS. This system then undergoes a spin transition, rela xing into the paramagnetic, longlived metastable state of CoII HSFeIII LS. The nature of this CTIST behavior, temperature and rate of spin transition and width of hysteresis, has been shown to be heavily dependent upon the type of alkali cation and its concentration in the lattice. The thermal CTIST is further explained with a potential energy well diagram, along with a sample spin transition curve in Figure 1-5. LS HSrM-N & T G HS LS 1.00.5 0.0T T1/2downT1/2upHS/ T A B LS HSrM-N & T G HS LS LS HSrM-N & T G HS LS LS HSrM-N & T G HS LS 1.00.5 0.0T T1/2downT1/2upHS/ T A B Figure 1-5. Thermal CTIST in Co-Fe PBAs. A) Potential energy diagrams of the Co-Fe PBA at high and low temperatures. At low temper atures, the Gibbs free energy of the low spin phase is lower, making it the ground st ate and forcing electron occupancy of the t2g orbitals. At higher temper atures, the Gibbs free energy of the high spin phase is lower, facilitating a charge transfer indu ced spin transition, marked by the electron occupancy of the eg orbitals. B) Representative spin transition curve of thermal CTIST. Hysteresis, a sign of lattice coope rativity, is marked by a difference in T1/2up and T1/2down, the temperatures at which half of the spin crossover material has transitioned in the heating (up arrow) and cooling (down arrow) cycles.20 The CTIST effect found in the Co-Fe Prussian blue analog can also be induced by photoirradiation. At temperatures below 20 K, diamagnetic CoIII LS-FeII LS pairs can be photo-excited, using 600 nm light, into a long-lived metastable state of ferrimagnetic CoII HS-FeIII LS; this process

PAGE 29

29 can be reversed by either irradiation with 450 nm light or thermal cycling above 100 K. This photomagnetic behavior is depicted in Figure 1-6.17,18 Internucleardistance EnergyFeII-CN-CoIII(LS) t2g 6 t2g 6FeIII-CN-CoII(LS) t2g 5 t2g 6eg 1FeIII-CN-CoII(HS) t2g 5 t2g 5eg 2Charge Transfer Relaxation Diamagnetic Ferrimagnetic TemperatureAB Internucleardistance EnergyFeII-CN-CoIII(LS) t2g 6 t2g 6FeIII-CN-CoII(LS) t2g 5 t2g 6eg 1FeIII-CN-CoII(HS) t2g 5 t2g 5eg 2Charge Transfer Relaxation Diamagnetic Ferrimagnetic Temperature Internucleardistance EnergyFeII-CN-CoIII(LS) t2g 6 t2g 6FeIII-CN-CoII(LS) t2g 5 t2g 6eg 1FeIII-CN-CoII(HS) t2g 5 t2g 5eg 2Charge Transfer Relaxation Diamagnetic Ferrimagnetic Temperature TemperatureAB Figure 1-6. Photoinduced CTIST in Co-Fe PBAs A) Potential energy diagrams of the photoinduced CTIST present in the Co-Fe PBA. At low temperatures, the ground state, diamagnetic CoIII LS-FeII LS species can be photoexcit ed into the high energy CoII LS-FeIII LS pairs, through a charge transfer from the iron to the cobalt ions. This species then relaxes through a spin tran sition into the ferr imagnetic, long-lived metastable state CoII HS-FeIII LS. B) Representative magnetization ( ) vs. temperature plots of the photoinduced CTIST of Co-Fe Prussian blue analog. Upon photoexcitation, an increase in magnetization is observed from the dark (black) to light (yellow) state.20 Prussian Blue Analog Thin Film and Nanoparticle Fabrication Recent developments in thin film and nanopartic le fabrication have been extended to this class of materials in an attempt to both tune their magnetic behavior and further the progression toward future application. Prussian blue analogs microcrystalline structure and their inability to wet most surfaces make the fabrication of hom ogenous thin films rather difficult. Extensive research into the preparation of such structures has led to the development of a multitude of synthetic techniques for the precipitation of Pr ussian blue and PBA thin films, such as electrochemical deposition,45-49 spin-casting and dip-coati ng of colloidal dispersions, 45,50

PAGE 30

30 adsorption onto Langmuir monolayers,51-55 Langmuir-Blodgett film templating,56 and sequential absorption techniques.40,41,57-60 Though they allow for the deposi tion of PBA thin films, ma ny of these techniques possess drawbacks, which will hinder the subsequent analysis and future application of these materials. While electrochemical deposition allows for the cont rolled fabrication of uniform thin films, this technique requires absorption to take place on a smooth, conducting surface; this places strict limitations on analytical techniques and the possible substrates used with the technique. Spin-casting and dip-coating posse ss the benefits of being able to easily tune thin film composition and utilize a wide range of applicable substrates for thin-film deposition, but these techniques lack the ability to finely control film thickness or form films with homogenous morphology. Adsorption of Prussian blue analog particles onto Langmuir monolayers and the use of Langmuir-Blodgett template films allow for the use of a multitude of solid supports, fine control of film thickness, a nd the manipulation of transiti on metal composition, but often produce films containing considerable amounts of organic material and large separations between Prussian blue analog material. The se quential absorption techni que has been shown to construct films of uniform coverage, have th e ability to easily control the composition and thickness of these films, and th e use of multiple deposition subs trates, Figure 1-7. This method of cyanometallate thin film fabrication has b een adapted to tune their ion transport ability61, magnetic anisotropy, and photomagnetism.37-41Another technique currently employed by many researchers to manipulate the microscopic scale a nd properties of a variety of materials (metallic, semiconductor, coordinate covalent, etc.) is nanop article fabrication. T hough Prussian blue and several of its analogs naturally form colloidal suspensions of nanoparticles from aqueous aggregation require added syntheti c solution, precipitation reactions, fine control of particulate

PAGE 31

31 Deposition Cycle x Melinex substrate MII MII MII MII MIII MII MIII MII MII MIII MII MIII MIII AjMII k[MIII(CN)6]lnH2O MIIMIII(CN)6 3-& A+ Deposition Cycle x Melinex substrate Melinex substrate MII MII MII MII MII MII MII MII MII MII MIII MII MIII MII MII MIII MII MIII MIII MII MIII MII MII MIII MII MIII MII MII MIII MII MII MII MIII MII MII MIII MII MIII MII MII MIII MIII AjMII k[MIII(CN)6]lnH2O AjMII k[MIII(CN)6]lnH2O MIIMIII(CN)6 3-& A+ Figure 1-7. Schematic depicting the sequential absorption of cya nometallate networks onto the polymer substrate Melinex. Linear increas es in film thickness are observed with multiple deposition cycles. size and the prevention of particle procedures.62-69 Recent advancements in the synthesis of monodisperse, size-controlled nanoparticles have utilized several techniques for nanoparticle preparation and isolation incl udingencapsulating polymers, microemulsions of non-miscible solvents and surfactants, and reaction solutions of high reactant concentrati on and ionic strength. Encapsulating polymers, such as polyvinylpyrroli done (PVP), control equilibrium particle size by encapsulating the precipitated pa rticle and restricting further growth64,65, while microemulsions of non-miscible solvent mixtur es (i.e. cyclohexane/water) and amphiphillic surfactants (i.e. sodium dioctyl sulfosuccinate, Igepal CO 520, a nd octadecylamine) restrict the quantity of available reactants and the size of the micro-reactor droplets within the reaction system.66,67 These approaches also utilize their coor dinating polymers and surfactants to both

PAGE 32

32 stabilize the nanoparticles and prev ent particulate aggreg ation. The research reported herein will utilize a microemulsion nanoparticle synthesis depicted in Figure 1-8. Na3Fe(CN)6(aq)& NaCl (aq)CoCl2 (aq) & NaCl (aq) Combine Solutions Stir Mixture 3 hrs Cyclohexane & Igepal CO 520 Na3Fe(CN)6(aq)& NaCl (aq)CoCl2 (aq) & NaCl (aq) Combine Solutions Stir Mixture 3 hrs Cyclohexane & Igepal CO 520 Figure 1-8. Nanoparticle s ynthesis of Prussian blue analogs using the microemulsion technique. The microemulsions are formed by treating cyclohexane solutions of the surfactant Igepal CO 520 with aqueous precursor solu tions. After nanoparticle precipitation, often marked by a solution color change, nanocrystalline products are isolated by solvent induced flocculation. Magnetic Anisotropy in Prussian Blue Analog Thin Films The abundance of thin film synthetic tech niques and wide array of electrochemical, sensory, and magnetic phenomena exhibited by cyanometallates has drawn considerable attention to Prussian blue analog thin films in the past few decades. Due to possible application in the development of magneto-o ptical storage media thin film materials, which exhibit strong, directional magnetic anisotropy, ar e of particular interest to those investigating molecular magnetism.70,71 While the magnetism and magnetic anisot ropy of thin films of transition metals, alloys, and metal complexes have been extens ively investigated, the presence of magnetic anisotropy in Prussian blue analog thin films a nd its origins have not been studied to same extent.37-41

PAGE 33

33 While investigating two-dimensional netw orks and nano-scale thin films of the self-assembled Prussian blue analogs, our lab wa s the first to discover anisotropy in the low temperature magnetization and photoindu ced magnetism of these materials.37,72 When thin films of the Ni-Fe Prussian blue analog were oriented parallel to an app lied magnetic field, an increased magnetism, as compared to that when the film oriented perpen dicular to the applied field, was observed, Figure 1-9. Similar behavi or was observed in other PBA thin films. Figure 1-9. Temperature dependenc e of the magnetization of the Ni-Fe PBA after field cooling in 20 G with the sample aligned parallel (filled circles) and perpendicular (open ciclres), to the magnetic field. The beak in the field cooled magnetization data at Tc = 8 is indicative of the magnetic an isotropy. The measuring field was 20 G.72,73 Photomagnetic anisotropy was later observed in thin films of the c obalt iron Prussian blue analogs.56 When these films are oriented parallel to the applied magnetic field, an increase in photomagnetism is detected, while there is a decrease in magnetization upon photoexcitation when the film is oriented perpendicular to the a pplied field, Figure 1-10. Subsequent studies of Co-Fe PBA thin films lead to the discovery of a photomagnetic thickness dependence and films with an opposite sign of photomagnetic anisotropy.38,40 The magnetic anisotropy observed in

PAGE 34

34 these studies have been attributed to structural anisotropy directed from the interface between the substrate and precipitated thin films and an easy magnetic axis within the samples. Figure 1-10. The time dependence of the phot oinduced magnetiza tion of a film of RbjCok[Fe(CN)6]l nH2O, prepared using sequential adsorption onto a two-dimensional, cyanometallate template la yer, when the external field is parallel and perpendicular to th e plane of the film.56,73 In order to further understa nd the origins of the anisotr opy found in the photoinduced and temperature dependent magnetization of Prussian blue analog thin films, our investigations of sequentially absorbed thin films is being expanded to include othe r divalent transition metal ions (NiII, CuII, and ZnII) and hexacyanometallates (Cr(CN)6 3-). Without a clear understanding of this phenomenon, the utilization of a variety of transiti on metal ions with different spin values will elucidate other contributing f actors to thin film anisotropy, such as single ion and g value anisotropy. Scope of Research In the following research investigations, we re po rt the intelligent de sign of Prussian blue analog systems, tuning their photomagnetic, th ermal CTIST, and magnetically anisotropic

PAGE 35

35 behavior by controlling the materials dimensions and transition metal composition. In Chapter 2, we report the first photoinduced decrease in magnetization in th ese materials resulting from the creation of new transition metal ion spins, by utilizing co mpeting magnetic superexchange energies in ternary Prussian blue analog lattices,. In Chapter 3, we also demonstrate the ability to tune the magnitude of magnetic anisotropy in PBA thin films by controlling film thickness and transition metal composition, and we present new ideas about the origins of this previously seen phenomenon. Finally in Chapter 4, by controlli ng the size and composition of CTIST active CoFe PBA nanoparticles, we demonstrate the ability to tune the temperature at which the thermal CTIST event takes place and the width of thermal hysteresis in the transition.

PAGE 36

36 CHAPTER 2 PHOTOMAGNETISM AND THERMAL CTIST IN TERNARY TRANSITION METAL PRUSSI AN BLUE ANALOGS Introduction Bi metallic Prussian blue analogs (PBAs), a class of molecular based compounds, with the general molecular formula AjM k[M(CN)6]lnH2O (where A is an alkali metal ion and M M are transition metal ions), have garnered considerab le attention in the field of molecular magnetism due to the ease with which their composition can be tuned and the diverse range of magnetic properties they exhibit like room temperature magnetic ordering12-16, photoinduced magnetism,17-35 thermal CTIST,28,30,36 and magnetic anisotropy.37-41 Photo-induced magnetism was first reported in the coba lt-iron Prussian blue analog, K0.2Co1.4[Fe(CN)6].9H2O, by Hashimoto and co-workers17 and has since been extensivel y investigated by several groups.17-35 At low temperatures, diamagnetic CoIIILS-FeIILS (S = 0) pairs can be photo-excited into a longlived, metastable state of ferrimagnetic CoIIHS-FeIIILS (S = 1) pairs by way of a charge transfer induced spin transition (CTIST), incr easing the magnetization of the compound. While investigating the phot oinduced magnetism in cobalt iron Prussian blue analog (AjCok[Fe(CN)6]lnH2O) thin films, our lab discovered th at these films exhibit anisotropic photoinduced magnetism, a behavior not s een in the bulk powder materials.37 When the films are oriented parallel to an exte rnal applied magnetic field (H e ) during photoirradiation, the magnetism increases, while a decrease in magnetism is observed when the films are oriented perpendicular to H e during photo-excitation. Though the ph otomagnetic anisotropy is not found in microcrystalline samples, the mechanism of the CTIST phenomenon was found to be the same as that found of the bulk materials. To increa se the temperature at which the photomagnetism could be observed, we then extended our studi es of thin film magnetism to multilayered

PAGE 37

37 heterostructures. Sandwiching a layer of the photomagnetic RbjCok[Fe(CN)6]lnH2O between two layers of the high Tc, ferromagnetically ordering RbjNik[Cr(CN)6]lnH2O led to new behavior, not seen in eith er of the pure materials.74,75 The magnetic ordering temperature of the heterostructured thin film increased to 70 K, and photoirradiation causes a decrease in magnetization for all film orientations, Figures 2-1 and 2-2. These obser vations are in sharp contrast to the low temperature magnetic orde ring, <18 K, and photom agnetic anisotropy found in the pure cobalt iron Prussian blue analog thin films. 020406080100 0 2 4 6 8 10 12 14 ( 10 -6 emu/cm2)Temperature (K) Figure 2-1. Field cooled (FC) ma gnetic susceptibility vs. temperature data, for heterostructured sandwich film (NiCr/CoFe/NiC r) described above, in He = 100 G are shown for the parallel (blue) and perpendi cular (red) orientations of the film with respect to the applied field. The increase in magneti zation near 70 K indi cates the onset of magnetic ordering within the film. This novel photoeffect is attributed to heterogeneously distributed transition metal ions at the interfaces between th e two parent compounds, leading to competitive ferromagnetic and antiferromagnetic superexchange mechanisms in the material, Figure 2-3 and 2-4. High resolution TEM micrographs of microtomed cross sections of ~200 nm and ~800 nm

PAGE 38

38 heterostructured thin films can been seen in Appendix A. -1012 12.65 12.70 12.75 12.80 (10-6 emu/cm2)Time (Hours)A-1012 4.14 4.16 4.18 4.20 (10-6 emu/cm2)Time (hours)B Figure 2-2. Change in magnetism ( M) vs. time of irradiation for the A) parallel and B) perpendicular orientations of the heterost ructured sandwich film (NiCr/CoFe/NiCr) in an external applied field of 100 G. A decrease in magnetism is observed for both orientations. Figure 2-3. Cross-sectional sche matic of the heterostructured sandwich film (NiCr/CoFe/NiCr). The expanded representative lattice on th e right indicates thorough mixing of the transition metal ions at the interface of the pure CoFe and NiCr layers. Interfacial mixing is denoted by the brown lattice, wh ile the pure analogs, NiCr and CoFe, are displayed in purple and grey. Magnetic exch ange energies of the possible transition metal pairs at the interface are JNiCr ~ 5.6 cm-1, JCoCr ~ 1.4 cm-1, JNiFe ~ 3.9 cm-1, and JCoFe ~ -1.7 cm-1. To test this hypothesis, trimeta llic Prussian blue analogs, in which ions of the pure parent Co-Fe PBA system can be exchanged for another ion, introducing a ferromagnetic component, were considered. Interest in th ese ternary transition metal PBAs, AM''(1-x)M'x[M(CN)6]nH2O (where M'' and M' occupy equivalent lattice si tes) arises from the additional properties not

PAGE 39

39 observed in the binary systems, like photoinduced magnetic pole inversion42,43 and spin-crossover dilution.44 To investigate the origin of the photoinduced decrease in 050100150200 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of CoFe PBAFilm Depth (nm)A0200400600800 0.0 0.2 0.4 0.6 0.8 1.0 Fraction of CoFe PBAFilm Depth (nm)B Figure 2-4. Relative fraction of RbjCok[Fe(CN)6]lnH2O (black) as a function of thin film depth in A) ~180 nm and B) ~800 nm heterostruct ured sandwich films. The relative analog concentrations, obtained from EDS line s cans, have been normalized to a total transition metal concentration of one. magnetization seen in the heterost ructured thin films, ternary Prussian blue analogs, with the general molecular formula NaNi1-xCox[Fe(CN)6]nH2O, were first synthesized. The structure of the Prussian blue analog is comprised of a c ubic iron sublattice, which is interpenetrated by a separate cubic sublattice with a statistical mixture of cobalt and nickel ions Sodium is utilized as the interstitial cation to produ ce a wide thermal hysteresis, and nickel ions within the lattice give rise to ferromagnetic supe rexchange pathways between NiII and FeIII.30,76 This ternary transition metal Prussian bl ue analog displays either a photoinduced increase or decrease in magnetism depending upon the materi als relative cobalt fraction, x = [Co]/([Co] + [Ni]), the applied magnetic field, and the temperature. Though the second example of a photoinduced decrease in magnetization in Prussian bl ue analogs, this is the first reported compound where the si gn of the photoinduced change in magnetization can be controlled by chemical composition. A dependence upon the extent of CoII and NiII ion mixing

PAGE 40

40 was observed for the ordering temp erature, coercive field, the am ount of photoactive material, and the magnitude and hysteresis width of the ther mal CTIST. These results can be qualitatively understood with simple molecular field theories. Experimental Section Materials Na3Fe(CN)6(aq) was synthesized by oxidizing Na4Fe(CN)6(aq) with Cl2(g) and used in situ .30 Deionized water, used in synthetic proce dures, was obtained from a Barnstead NANOpure system with a resistiv ity of at least 17.8 M All other reagents were purchased from SigmaAldrich or Fisher-Acros and used without further purification. Synthesis Ternary tran sition metal Prussian blue analogs with an approximate molecular formula NaNi1-xCox[Fe(CN)6]nH2O were precipitated out of wa ter by treating 20 mM aqueous solutions of Na3Fe(CN)6(aq) with equivolume solutions comprised of a mixture of NiCl2 and CoCl2 (20 mM in transition metal ion concentration) and NaCl (2 M). The concentration of CoII (x) in the precipitated Prussian blue analogs was controlled by varyi ng the volume ratio of CoII (aq) and NiII (aq) in the aqueous reaction solutions from 0 to 1 (xsynthesis = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0), while keeping the reaction mixtures tota l transition metal ion concentration 20 mM. The reaction mixtures were stirred for three hour s in the open atmosphere, and the precipitated powders were then isolated by centrifugation. Th e precipitated Prussian blue analogs were rinsed three times with water and dried under a stream of nitrogen gas, yielding solids of NaNi1-xCox[Fe(CN)6]nH2O. Na1.3 Ni4.0[Fe(CN)6]3.0 20.0 H2O (xsynthesis = 0, x = 0.00) Anal. Calcd for C18H40N18O20Na1.3Co0.0Ni4.0Fe3.0: C, 17.15; H, 3.20; N, 20.00; Co, 0.00; Ni, 18.62; Fe, 13.29. Found: C, 17.16; H, 2.87; N, 19.33; Co, 0.00; Ni, 18.62; Fe, 13.29.

PAGE 41

41 Na1.7Co0.8Ni3.2[Fe(CN)6]3.0 17.7 H2O (xsynthesis = 0.2, x = 0.22) Anal. Calcd for C18.0H35.4N18.0O17.7Na1.7Co0.9Ni3.1Fe3.0: C, 17.59; H, 2.90; N, 20.52; Co, 4.32; Ni, 14.81; Fe, 13.64. Found: C, 17.75; H, 2.84; N, 19.91; Co, 4.05; Ni, 14.93; Fe, 13.76. Na1.7Co1.8Ni2.2[Fe(CN)6]3.1 19.7 H2O (xsynthesis = 0.4, x = 0.45) Anal. Calcd for C18.6H39.4N18.6O19.7Na1.7Co1.8Ni2.2Fe3.1: C, 17.37; H, 3.03; N, 20.26; Co, 8.25; Ni, 10.04; Fe, 13.46. Found: C, 17.23; H, 2.95; N, 19.55; Co, 8.32; Ni, 10.15; Fe, 13.37. Na1.8Co2.6Ni1.4[Fe(CN)6]3.0 18.5 H2O (xsynthesis = 0.6, x = 0.66) Anal. Calcd for C18.0H37.0N18.0O18.5Na1.8Co2.6Ni1.4Fe3.0: C, 17.71; H, 2.96; N, 20.66; Co, 12.15; Ni, 6.51; Fe, 13.28. Found: C, 17.37; H, 2.88; N, 19.62; Co, 12.42; Ni, 6.52; Fe, 13.48. Na1.8Co3.5Ni0.5[Fe(CN)6]2.9 15.3 H2O (xsynthesis = 0.8, x = 0.87) Anal. Calcd for C17.4H30.6N17.4O15.3Na1.8Co3.5Ni0.5Fe2.9: C, 17.90; H, 2.64; N, 20.89; Co, 17.67; Ni, 2.51; Fe, 13.88. Found: C, 17.90; H, 2.69; N, 20.25; Co, 17.57; Ni, 2.61; Fe, 13.87. Na1.9Co4.0[Fe(CN)6]3.0 17.4 H2O (xsynthesis = 1.0, x = 1.00) Anal. Calcd for C18.0H34.8N18.0O17.4Na1.9Co4.0Ni0.0Fe3.0: C, 17.60; H, 2.86; N, 20.53; Co, 19.19; Fe, 13.64. Found: C, 17.67; H, 2.54; N, 20.07; Co, 19.08; Fe, 13.70. Instrumentation and Characterization Chemical and structural characterization Fourier tran sform infrared (FT-IR) spectra, ta ken of samples prepared as KBr pellets or mounted between NaCl plates, were record ed using a Thermo Scientific Nicolet 6700 spectrometer. A Joel 2010F spectrometer wa s used to perform energy dispersive x-ray spectroscopy (EDS) and transmission electron mi croscopy (TEM) to determine transition metal composition and particle size, by analyzing 50 part icle counts with ImageJ imaging software.77 Samples were deposited as methanol dispersi ons onto 400 mesh copper grids with holey carbon support films, purchased from Ted Pella Inc. Carbon, hydrogen, and nitrogen (CHN)

PAGE 42

42 percentages were found with co mbustion analysis performed by the University of Florida Spectroscopic Services laboratory. A Philips APD 3720 powder diffractometer with a Cu K source was used to perform room temperature x-ray diffraction (XRD) experiments, in order to investigate the compounds crystal structures and characteristic lattice constants.78 Powder samples were mounted on glass slides and pressed onto double-sided cellophane tape. Room temperature powder XRD diffractograms were utilized to model the products structure by a Rietveld refinement using EXPGUI79 interface for GSAS80. A single-phase model with Fm 3m space group symmetry was used to approximate the structure of the Prussian blue analogs. Th e cobalt and nickel atoms were forced to occupy equivalent sites in the network. Atomic occupancies were determined by the previously described chemical formulas, allowing the oxygen at oms to vary. The same site symmetries as are present in Prussian blue were used, re placing iron vacancies by six legating oxygen atoms from water molecules.3 Placement of the oxygen atoms of in terstitial water molecules at the 32 f Wyckoff and 192 l positions was passable.31 Magnetic measurements Magnetic measurements were performed by the University of Florida, Department of Physics using a Quantum Design MPMS XL superconducting quantum interference device (SQUID) magnetometer.81 A room temperature halogen ligh t source (1-2 mW) with a bundle of ten optical fibers, ~270 m O.D. (Ocean Optics Model 200) was used to introduce light into the sample chamber of the SQUID for photomagnetic measurements. Powder samples were mounted on pieces of cellophane tape, rapped aro und a plastic straw, to increase the optical cross-section. High temperature data (T > 100K) were taken using gelcaps as sample holders. Backgrounds were subtracted from the magnetic data by using the measur ed mass susceptibility

PAGE 43

43 of similar sample holders. Prior to recordi ng magnetic data, the SQUID was demagnetized and subsequently allowed to relax for at least two hours. The same demagnetization protocol, during which the magnetic field is oscillated to zero by successive ramps starting at 20kG, was used for all low field measurements in 10 G. Results Section Chemical and Physical Characterization Prussian blue analogs with the general form ula NaCoxNi1-x[Fe(CN)6] 4.5H2O were precipitated from aqueous media with xsynthesis values of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, where xsynthesis is defined as the re lative cobalt fraction, [CoII (aq)]/([CoII (aq)] + [NiII (aq)]), present during the synthetic procedure. The chemical formulas were determined consider ing the results of EDS, FT-IR, and CHN analyses. The Co, Ni, and Fe ratio s were taken directly from the EDS results, because these elements displayed signals, wh ich had a high signal to noise ration and were reproducible. The elemental per centages of C, H, and N in the compounds were taken from combustion analyses. Using the results of comb ustion analyses, the elemental percentage of oxygen in the precipitated solids was calculate d by assuming all hydrogen and oxygen atoms in the compounds are in H2O molecules. The amount of NiII and CoII present in the precipitated solid closely mimics the ratios used in the synthetic procedures, with a slight te ndency for the compounds to favorably incorporate more CoII into their lattices, as is seen in Figure 2-5 and Table 2-1, below. The gradual change in composition of the precipitated solids is also accompanied by a change in color from purple to yellow, as the concentration of NiII increased in the Prussian bl ue analogs; this observation can be seen in Figure 2-6.

PAGE 44

44 0.00.20.40.60.81.0 0 5 10 15 20 Atomic Mass %x synthesis Figure 2-5. Measured atomic composition of nickel ( ), cobalt ( ), and iron ( ) for compounds of the general molecular formula NaNi1-xCox[Fe(CN)6]nH2O as a function of xsynthesis. Figure 2-6. Color variation of NaNi1-xCox[Fe(CN)6]nH2O as a function of x. The relative ratios of FeII and FeIII in the isolated compounds were determined by fitting and integrating the peaks assigned to the cyanide stretches in the FT-IR spectra of each species, assuming all extinction coefficients to be equi valent. The FT-IR spectrum of the pure cobalt hexacyanoferrate (x = 1.0) displays cyanide stre tching peaks, which have been assigned to the CoII HS-FeIII LS, CoIII LS-FeII LS, CoII HS-FeII LS, and the linkage isomerized CoII-FeIII phases, at 2160, 2117, 2098, and 2060 cm-1, respectively.30,82 The spectrum belonging to the pure nickel

PAGE 45

45 hexacyanoferrate (x = 0.0) displays two cyan ide stretching peaks, assigned to the cyanide bridged NiII-FeIII and NiII-FeII metal pairs, at 2164 and 2123 cm-1.83 2200 2100 2000 x = 1.00 Wavenumber (cm-1) x = 0.87 x = 0.66 x = 0.45Absorbance units x = 0.22 x = 0.00 Compound 0 (cm-1) W (cm1 ) A (I cm1 ) 2163.2 24.7 30.7 2122.9 22.3 5.0 2043.0 26.9 1.4 x = 0.00 2078.6 40.5 2.5 2163.3 24.6 9.2 2120.1 20.6 1.0 2095.9 42.1 0.9 x = 0.22 2044.8 38.1 1.2 2161.5 22.1 8.9 2119.6 26.6 1.3 2069.5 42.8 1.6 x = 0.45 2041.0 25.2 0.7 2160.7 23.7 6.3 2118.6 29.0 1.1 2070.3 48.1 1.9 x = 0.66 2038.2 24.4 0.6 2161.2 31.8 5.7 2116.9 15.2 0.4 2094.8 38.3 2.3 x = 0.87 2040.5 35.1 1.5 2159.9 20.8 23.7 2114.5 12.3 0.9 2094.0 27.9 3.6 x = 1.00 2056.8 41.6 5.5 Figure 2-7. FT-IR spectra (black) and resp ective fitting parameters for synthesized NaNi1-xCox[Fe(CN)6]nH2O compounds as a function of x. All fits (green) were performed using four Lorentzian lines (red). is defined as the peak stretching energy, W is defined as the peak half-maxim um width, and A is defined as the area under the curve. 4 1 22 ,04 2i ii i iW WA I

PAGE 46

46 As the concentration of NiII in the lattice is increased at the expense of CoII ions, the intensities of the three peaks near 2120, 2090, and 2060 cm-1 decrease, while that of the peak near 2160 cm-1 remains relatively unchanged and a peak near 2125 cm-1 emerges. These changes in the intensities of the peak s indicate both the reduction in the number of cobalt-iron pairs and the subseque nt formation of nickel-iron pair s with the gradua l inclusion of NiII ions into the structure. All FT-IR spectra an d their tabulated results are shown in Figure 2-7. Finally, the amount of sodium ions in the chem ical formulas was assigned according to charge balance of the network in conjunction with CH N and EDS analyses, with the known constraints of detecting light atoms with the latter technique.84 The resultant expanded chemical formulas are listed in Table 2-1. Table 2-1. Molecular form ulas for synthesized NaNi1-xCox[Fe(CN)6]nH2O compounds xsynthesis x Chemical Formula 0.0 0.0 Na1.3NiII 4.0[FeIII(CN)6]2.7[FeII(CN)6]0.3 20.0 H2O 0.2 0.22 Na1.7CoII 0.8NiII 3.2[FeIII(CN)6]2.5FeII(CN)6]0.5 17.7 H2O 0.4 0.45 Na1.7CoII 1.8NiII 2.2[FeIII(CN)6]2.5FeII(CN)6]0.6 19.7 H2O 0.6 0.66 Na1.8CoII 2.6NiII 1.4[FeIII(CN)6]2.2FeII(CN)6]0.8 18.5 H2O 0.8 0.87 Na1.8CoII 3.5NiII 0.5[FeIII(CN)6]1.8FeII(CN)6]1.1 15.3 H2O 1.0 1.00 Na1.9CoII 4.0[FeIII(CN)6]2.2FeII(CN)6]0.8 17.4 H2O Table 2-2. Unit cell parameters and particulate size of NaNi1-xCox[Fe(CN)6]nH2O compounds as a function of x x Unit cell length () Edge length (nm) 0.0 10.2396(2) 15.59 3.40 0.22 10.2491(7) 26.50 5.25 0.45 10.2558(3) 28.71 6.92 0.66 10.2680(2) 38.74 7.65 0.87 10.2893(7) 117.21 22.68 1.0 10.3072(6) 237.79 40.12

PAGE 47

47 x = 0.00 x = 0.22 x = 0.45 x = 1.00 x = 0.87 x = 0.66 x = 0.00 x = 0.22 x = 0.45 x = 1.00 x = 0.87 x = 0.66 Figure 2-8. TEM micrographs for NaNi1-xCox[Fe(CN)6]nH2O compounds for different values of x. All scale bars shown are 100 nm. The structure of the compounds was then inve stigated with TEM and powder XRD. For identical synthesis protocols, excepting the ratio of CoII (aq) to NiII (aq), the average size of the particles evolves continuousl y, becoming larger as more CoII ions are introduced into the lattice, Table 2-2 and Figure 2-8. The TEM images of the trimetallic Prussian bl ue analogs also indica te that the defined edges and cubic shape of the part icles are lost as the amount of atomic mixing of the divalent transition metal site in the lattice is increas ed, Figure 2-8. Similarly, the unit cell constants evolve continuously when the relative concentration of CoII ions in the lattice is changed from x = 0.00 to x = 1.00, Table 2-2 and Figure 2-9. This c ontinuous evolution of particle size and the absence of the secondary precip itates of the parent compounds, coupled with evolution of the

PAGE 48

48 lattice constants while keepi ng the same single-phase space group, indicate a homogeneous mixing of the NiII and CoII ions within the lattice. 16.016.517.017.518.0 x = 1.00 x = 0.87 x = 0.45x = 0.66x = 0.22 Intensity (arb. units)2 (degrees)x = 0.00 Figure 2-9. Room temperature XRD reflection of NaNi1-xCox[Fe(CN)6]nH2O compounds, showing a continuous evolution of unit ce ll parameters with x. Backgrounds have been subtracted and the inte nsities have been normalized. To verify that the evolution of both the pr ecipitates color and ma gnetic behavior is a consequence of intimate atomic mixing of CoII and NiII ions in the divalent metal sites, and not a result of macroscopic mixing of the cobalt iron and nickel iron Prussi an blue analogs, a post synthesis, homogenous mixture of the parent PBA compounds, with transition metal composition equivalent to the xsynthesis = 0.6 compound, was synthesized as a control. Though the powder sample appears brown in color, with a hue si milar to the corresponding ternary Prussian blue analog, TEM micrographs reveal that the sample is comprised of a bimodal dispersion of particles, with average particle edge lengt hs closely matching thos e of the two parent

PAGE 49

49 compounds, Figure 2-10. The additive nature of this samples magnetic behavior will be demonstrated in the following sections. 100 nm a) b) 100 nm 100 nm B A Figure 2-10. TEM image of a post-synthesis, manually mixed NaNi1-xCox[Fe(CN)6]nH2O compound with x = 0.6. TEM micrographs of the manually mixed sample indicates the presence of A) CoFe and B) NiFe PB As, with equivalent edge lengths of the previously reported x = 0.0 and 1.0 sample s. The scale bars shown is 100 nm. Finally, control over particle size for a give n x value was shown possible by varying the concentration and the amount of tim e that the particles are in solu tion prior isolation. Increasing the transition metal ion concentrations of th e reaction solutions from 2 mM to 40 mM and decreasing the reaction time from 3 hours to 30 minutes lead to a decrease in average particle edge length of the x = 1 compound from 238 nm to 73 nm. Allowing x = 0.0 precipitated solids to remain in their reaction solutions for 3 days after the initial reaction period enable to further Oswald ripening to form particles with a larger average edge length (30 nm). As will be shown later, no appreciable differences were seen in the low temperature magnetization of these compounds, as compared to the x = 0.0 and 1.0 reported above.85,86

PAGE 50

50 Low Temperature Magnetization The tem perature and time dependences of the dc magnetic susceptibilities, (T) and = M/H, of the six ternary transition metal Prussian blue analogs are shown in Figure 2-11. The temperature dependences of the dc magnetic susceptibilities were r ecorded for samples in a field of 10 G between 2 K and 30 K, while the time dependences of the compounds photomagnetism were recorded at 2 K in a field of 10 G. A clear bifurcation of the field-cooled (FC) and zero-field-cooled (ZFC) curves at a lower temper ature than the Prussian blue analogs magnetic ordering temperature (Tc), with a peak in the ZFC vs. T plots, is observed for all samples, indicating the spin-glass nature of the materials. A gradual scaling of the magnetic ordering temperature was also seen, the Tc increasing as the concentration of NiII ions in the lattice is increased, Table 2-3. An increase in the magnetic ordering temperature was also observed after photoillumination, for thos e compounds containing CoII ions, indicating an increase in the magnetic coherence within the samples. All compounds, which have cobalt ions incorporated into the lattice, display a ch ange in their magnetization when exposed to white light. Though present in all the mixed samples, the most striking effect is observed in the x = 0.66 sample. At 5 K and in 10 G, the compound displays an obvious decrease in magnetization when photoexcited. This same photoinduced decrease in magnetism was also seen in the x = 0.45 sample, although to a lesser extent. These experimental results observed in the SQUID magnetometry data agree qualitatively with si mple mean field calculations, Figure 2-11 and 2-12.

PAGE 51

51 no effectAx = 0.00 505 510 515 x = 0.22 344 346 348 x = 0.45 167 168 169 170 171 x = 0.66 (emu/mol) 100 200 300 x = 0.87 010203040 400 450 500 x = 1.00 time (hours) 0 100 200 300 400 x = 0.00B 0 100 200 300 400 500 x = 0.22 0 100 200 300 x = 0.45 (emu/mol)0 50 100 150 x = 0.66 0 100 200 300 400 x = 0.87 051015202530 0 100 200 300 400 500 x = 1.00 T (K) Figure 2-11. Photoinduced CTIST in NaNi1-xCox[Fe(CN)6]nH2O compounds. A) Molar magnetic susceptibility as a function of time of sample irradiation. Experiments were preformed at 5 K and 10 G. B) Molar magnetic susceptibility as a function of temperature in the dark FC ( ), ZFC ( ) and photoexcited states ( ), measured with He 10 G. All magnetic measurements are represented per mole of sample; x corresponds to molecular formulas listed in Table 2-1.

PAGE 52

52 0 100 200 300 400 500 600 x = 0.00 0 100 200 300 400 x = 0.22 0 50 100 150 x = 0.45 (emu/mol)-150 -100 -50 0 x = 0.66 0 100 200 300 400 x = 0.87 051015202530 0 100 200 300 400 500 x = 1.00 T (K) Figure 2-12. Mean-field calcu lations of molar magnetic susc eptibilities as a function of temperature at 10 G in the dark (solid line) and light states (dashed line). All magnetic calculations are represented per mole of sample; x corresponds to molecular formulas listed in Table 2-1.81

PAGE 53

53 -4 -2 0 2 4 6 x = 0.00 (b) -6 -4 -2 0 2 4 6 x = 0.22 -4 -2 0 2 4 6 x = 0.45 M (emu G / mol)-4 -2 0 2 4 x = 0.66 -4 -2 0 2 4 6 x = 0.87 -20020406080 -2 -1 0 1 2 3 x = 1.00 H (kG) Figure 2-13. Magnetization as a function of applied magnetic field in both the dark ( ), and light states ( ), measured at 2 K. High field magnetization always increases after photoirradiation, even for those samples showing a photodecrease at low fields.

PAGE 54

54 Field dependent magnetization studies were then performed on the series of compounds at 2 K and in applied magnetic fields up to 70 kG, and this data is represented in Figure 2-13 and Table 2-3. All samples show an increase in ma gnetization at high, applie d magnetic fields; those materials exhibiting photomagnetism also showed an increase in magnetization from the dark to the light state. A dependence upon photoirradiation and x was also seen in the magnitude of the compounds coercive fields, increasing after illumination and decreasing as the degree of divalent transition metal mixing was increased. Table 2-3. Magnetic or dering temperatures (Tc) and coercive fields (Hc) as a function of x for NaNi1-xCox[Fe(CN)6]nH2O compounds x Magnetic Ordering Temperature (Tc) (K) Coercive Fields (Hc) Light (G) Dark (G) 0.00 27 3470 3470 0.22 23 1980 1985 0.45 20 1374 1368 0.66 17 930 910 0.87 13 1380 1060 1.00 17 3036 2671 To confirm that the observed behavior is due to an intimate mixing of the CoII and NiII ions with the lattice, a post-synthesis, manually mixed sample of separately precipitated nickel hexacyanoferrate (x = 0) and cobalt hexacyanof errate (x = 1) powders was prepared, and its magnetic behavior was subsequently investigat ed. Though the chemical composition of this sample is the same as that of the x = 0.66 material, a magnetic increase is observed upon photoirradiation, as opposed to the previ ously described phot odecrease. The FC (T) plots clearly show the magnetic orderings of both bi nary species, x = 0.00 and 1.00, as well as the previously noted photoincrease in magnetization, Figure 2-14. High and low field magnetization increases, as well as increased coercive fields, are observed after photoirradiation.

PAGE 55

55 05101520253035 0.20 0.22 0.24 0.26 (emu/gram)t (hours)A051015202530 0 10 20 30 40 (10 -2 emu/gram)T (K)B-2-101234567 -40 -30 -20 -10 0 10 20 30 40 H C,Dark ~ 2710 G H C,Light ~ 3140 GM (emuG/gram)H (Tesla)C Figure 2-14. Magnetic behaviour of post synthesis mixed NaNi1-xCox[Fe(CN)6]nH2O, x = 0.66. A) Molar magnetic susceptibilit y as a function of time of sample irradiation. Experiments were preforme d at 5 K and 10 G. B) Molar magnetic susceptibility as a function of temperature in the dark FC ( ) and photoexcited states ( ), measured with He 10 G. C) Magnetization as a function of applied magnetic field in both the dark ( ), and light states ( ), measured at 2 K. To verify that the materials evolution in low temperature magnetism is independent of particle size, larger nickel hexacyanoferrate particles, with an equivalent chemical formula, were grown (~30 nm). The FC (T) plot indicates magnetic ordering at 27 K, the same Tc seen in the smaller 12 nm particles, Figure 2-15. 0102030 0 100 200 300 400 x (emu/mol)Temperature (K)A0102030 0 100 200 300 400 (emu/mol)T (K)B Figure 2-15. Molar magnetic susceptibility as a function of temperature for A) 12 nm and B) 30 nm particles of Na1.3Ni4.0[Fe(CN)6]3.0 20.0 H2O in the dark FC ( ) state measured with He 10 G.

PAGE 56

56 High Temperature Magnetization The therm al CTIST effect was then studied in the NaNi1-xCox[Fe(CN)6]nH2O compounds by recording the temperature depend ences of the dc magnetic susceptibility temperature product ( T) in 5 kG and in the temperature range of 100 K to 300 K for various values of x. A thermal sweep rate of < 0.5 K/min was utilized to insure equilibrium was maintained throughout the spin-crossover event. The results of these st udies are displayed in Figure 2-16, below. For comparison, combined Bethe-Peierls-Weiss sp in-crossover and Weiss mean field magnetization calculations fo r these compounds are also presented. All NaNi1-xCox[Fe(CN)6]nH2O samples with x > 0 demonstrate a thermally induced CTIST, which appears to be able to be cycled wi th temperature. This behavior is indicated by the sudden decrease of the magnetic susceptibility upon cooling the mate rials below ~170 K and the subsequent increase in magnetization after warming. These CTIST events also exhibit thermal hysteresis, which is characteristic of the cooperativity of the transition. The width of this thermal hysteresis and the magnitude of the CTIS T event, and therefore the amount of material that undergoes the CTIST, increase as the concentra tion of Co in the lattice increases, Table 2-4. Table 2-4. Width of thermal hysteresi s and % of CTIST active material in NaNi1-xCox[Fe(CN)6]nH2O as a function of x x Thermal Hysteresis (T1/2upT1/2down) (K) CTIST active material (%) 0.00 no effect 0 0.22 2 3 0.45 13 3 0.66 21 3 0.87 13 13 1.00 38 69 Finally, a decrease in the ferromagnetic slope in T, characteristic of the NajNik[Fe(CN)6]lnH2O (x = 0.00) compound, can be seen as more Co is in troduced into the lattice. These experimental observations closely mimic the predicted behavior of the mean field calculations, Figure 2-16.

PAGE 57

57 7 8 9 10 Ax = 0.00 10.0 10.5 11.0 11.5 x = 0.22 12.2 12.4 12.6 12.8 x = 0.45 12.4 12.6 12.8 x = 0.66T (emu K / mol) 12 13 14 x = 0.87 100150200250300 5 10 15 x = 1.00 T (K) 7 8 9 10 Bx = 0.00 9.0 9.5 10.0 x = 0.22 10.7 10.8 10.9 x = 0.45 T (emu K / mol)12.2 12.4 12.6 x = 0.66 12 13 14 x = 0.87 100150200250300 5 10 15 x = 1.00 T (K) Figure 2-16. Temperature dependent ma gnetization studies as a function of CoII concentration, x, for NaNi1-xCox[Fe(CN)6]nH2O. A) T vs T as measured in a SQUID magnetometer with an applied field of 5 kG. B) The results of mean field calculations as shown for T vs T. The magnetic signals are expressed per mole of compound, using the chemical formulas reported above.81

PAGE 58

58 To validate the claim that the evolution in the thermal hysteresis and fraction of CTIST active material is dependent upon the composition of the ternary Prussian blue analog, and not the equilibrium size of the part icles, a smaller cobalt hexacya noferrate (x = 1.00) particulate sample was precipitated (73 nm) and its magnetic behavior was investigated between 100 and 300 K, Figure 2-17. Similar to the prior repor ted x = 1.00 complex, the temperature dependent magnetization plots of T vs. T demonstrate wide thermal hyste resis of 24.1 K, with 77 % of the expected amount of material tran sitioning during the CTIST event. 100150200250300 5 10 15 T(emuK/mol)T (K) Figure 2-17. Temperature dependent magnetization studies, with an applie d filed of 5 kG, for 73 nm particles of NaNi1-xCox[Fe(CN)6]nH2O, with x = 1.00. Discussion Section Low Temperature Magnetization Photoinduced decrease in magnetization The observation of a photoindu ced d ecrease in magnetizat ion in heterostructured, sandwich films of the photoactive, ferrimagnetic RbjCok[Fe(CN)6]lnH2O deposited between two

PAGE 59

59 layers of ferromagnetic RbjNik[Cr(CN)6]lnH2O and the extensive mixing of the Prussian blue analogs, found in these films, led us to believe th at these photoeffects are a result of the interplay between ferromagnetic and antiferromagnetic exchange, Figure 2-2, 2-3, and 2-4. To test this conjecture, a se ries of ternary transition meta l Prussian blue analogs were synthesized (NaNi1-xCox[Fe(CN)6]nH2O, with x ranging from 0.00 to 1.00) and their magnetic behavior compared to that predicted by numerical calcu lations and observed in the heterostructured thin films. The results of mean field calculations predict a decrease in magnetization upon photoirradiation for the NaNi1-xCox[Fe(CN)6]nH2O samples, which have enough NiII ions in the lattice ferromagnetically coup led to iron ions. Though seen in both of the x = 0.45 and 0.66 samples, this behavior is most cl early seen in the latter of the tw o. This sample has an adequate amount of ferromagnetically coupled Ni-Fe pairs to drive the Fe subl attice parallel to the applied field, while possessing enough photoactive Co-Fe pairs to exhibit an appreci able CTIST effect. In a low field (10 G), the major spins of photoexcited, ferrimagnetic Co-Fe pairs align antiparallel to the applied fiel d because of antiferromagnetic s uperexchange between the Co and Fe ions within the lattice. Sin ce the iron sublattice has previously aligned parallel to the applied field due to the presence of fe rromagneticly coupled Ni ions, a photodecrease in magnetization is observed, Figure 2-11. In the low applied field of 10 G, a decrease in the m easured susceptibility is observed below approximately 12 K, while an in crease is observed above this temperature, indicating a temperature dependence of this phe nomenon. This is due to the thermal population of system excited states, where the Co spins alig n parallel to those of the Fe and Ni ions and hence the applied field, a result of thermal energy overcoming the exchange energies. Also, if a sufficient external magnetic fiel d is applied to the sample, a photoincrease in magnetization is

PAGE 60

60 observed, due to the reduction in energy of the system by aligning the Co-Fe pairs with the applied field, as apposed to with th e superexchange, Figures 2-13 and 2-18. This increase in magnetization at high fields is observed in all the measured samples, regardless of x, and proves that the increase in magnetization is due to additional spins being generated during photoirradiation. These results indicate that the mechanism for this photoinduced magnetization is the sa me CTIST seen in the pure NajCok[Fe(CN)6]lnH2O (x = 1.00) material. The photoeffect seen in these compounds is reproducible and can be reversed with thermal cycling above 150 K. The strong correlation between the experimental results observed in the ternary Pr ussian blue analogs and the pr edictions established from the mean field calculations, as well as the sim ilarities in magnetic behavior seen in the heterostructured thin films and x = 0.66 comp ound, support the claim that the photoinduced decreases in magnetization is due to the interp lay between ferromagnetic and antiferromagnetic superexchange interactions within the lattice. Figure 2-18. Field depe ndent, photomagentism of NaNi1-xCox[Fe(CN)6]nH2O, x = 0.66. The sample was measured at T = 5 K at low fi eld (H = 10 G) and high field (H = 1 kG).

PAGE 61

61 It is also important to note that there is a time dependence of the photoinduced magnetic effect in these materials. When the samples we re remeasured after one month, the photodecrease was found to be stronger by a few percent. This change in the photomagnetic properties may be a result increased atomic mixing within the sa mples, arising from solid state diffusion of interstitial cations within the lattice. Evolution of magnetic properties The NaNi1-xCox[Fe(CN)6]nH2O compounds reported above exhibit spin glass magnetic behavior, as evidenced by a bifurcation of th e FC and ZFC magnetization curves, Figure 2-11. When the samples are cooled without the presence of an external applied field, ferrimagnetic domains within the solids are oriented randoml y. After cooling below the samples ordering temperature, an increase in magnetization is obser ved due to short range magnetic interactions. Further cooling the sample below its freezing temperature, Tf decreases the magnetization, and the material resembles an antiferromagnet. If a sufficient, external field is then applied to the sample, the randomly oriented domains align, and the compounds magnetization will increase, as a result of long range, magnetic correlations. Upon warming the sample, the once aligned magnetic domains begin to randomize, and the magnetization of the sample decreases. With the addition of sufficient thermal energy, the FC magnetic data will resemble that of the ZFC. There is an inherit complexity to the magnetic behavi or in these compounds as a result of the spin glass-like nature present in both pa rent compounds.25,87-91 Because of this, the lack of monotonic scaling found among the magnetic properties is not surp rising. This reflecte d in the evolution of the Curie-Weiss temperature, the coercive fi eld, and the peak in the zero-field-cooled susceptibility, Figure 2-19. A gradual scaling in the magnetic propertie s of the two binary Prussian blue analogs is observed with minima present near x ~ 0.8.

PAGE 62

62 15 20 25 A1 2 3 HC (kG) B0.00.20.40.60.81.0 10 15 20 25 Tpeak (K)C-W (K)x C Figure 2-19. Dependence of the Curie-Weiss temperature, C-W, coercive field, HC, and maximum in the ZFC susceptibility, Tpeak, before ( ) and after () photoexciation for NaNi1-xCox[Fe(CN)6]nH2O, as a function of x. Coerci ve fields were obtained at T = 2 K after sweeping to 70 kG. The Curie-Weiss temperatures were determined by fitting data, from 10 K above the ordering te mperature to 50 K, with the functional form ~ 1/(TC-W). These results can be compared with previous investigations of ternary transition metal Prussian blue analogs.92-97 Similar non-monotonic scaling of magnetic behavior is observed in a series of NixMn1-x[Cr(CN)6]nH2O ternary Prussian blue, where an increase in the coercive field and decrease in the magnetic ordering temper ature is observed on the background of a linear evolution between the values of the parent compounds as a function of x.92,93 In contrast to these two systems, monotonic scaling of the transiti on temperatures and magnetic behavior as a function of x was observed in several ternary Prussian blue analogs, Cu[CoxFe1-x(CN)6], Ni[CoxFe1-x(CN)6], Fe[CoxFe1-x(CN)6], Ni[CrxFe1-x(CN)6] and Fe[CrxFe1-x(CN)6].94-97

PAGE 63

63 This non-monotonic scaling of the magnetic pr operties appears to be related to the presence of competing ferromagnetic and antiferr omagnetic exchange energies, the extent of transition metal mixing the divalent sites, a nd divalent versus trivalent transition metal substitution. These factors can lead to slight stru ctural changes, an increase in network disorder, and therefore, a loss of structur al and magnetic cooperativity. High Temperature Magnetization The width of the hysteresis of th e therm al CTIST, defined as T1/2up-T1/2down, decreases when NiII ions are included into the pure cobalt hex acyanoferrate material. This dilution of the spin crossover event is strongly associated wi th a decrease in the number of thermal CTIST active nearest neighbors, zSCO, present with the la ttice, Figure 2-20. zSCO is determined by considering the number of cyanometallate vacancies, the relative amount of FeII and FeIII in the lattice, and x, using the previously reported molecular formulas, Table 2-1. While the dilution of CTIST active species is th e dominant contributor to the decrease in thermal hysteresis, changes in the local environm ents of the active specie s are also present and play an active role. The diluti on of spin-crossover species has pr eviously been investigated in [FexZn1-x(2-pic)3]Cl2EtOH, where the reduction in the ther mal hysteresis was attributed to the elastic interactions innate to these magneto-structural transitions.98,99 In recent studies of the CTIST behavior in PBAs, the CTIST diluted Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86.05H2O was compared to its parent compound Rb0.81Mn[Fe(CN)6]0.95.24H2O.100 In this case, no appreciable change in the width of th e hysteresis loop was observed. As previously mentioned, the number of CT IST active nearest neighbors, and hence the amount of CTIST active material, qui ckly reduce upon the introduction of NiII ions within the PBA lattice, Figure 2-20. More specifically, the fraction of sp in transitioning material, as

PAGE 64

64 compared to the amount expected from the chemi cal formula, decreases from 69% to 13% to 3% when looking at the x = 1.00, 0.87, and 0.66 samples respectively. 0.00.20.40.60.81.0 0 20 40 60 80 100 10.24 10.26 10.28 10.30 0 1 2 3 4 0 10 20 30 40 % CTIST activex Unit cell length() z sco Thermal hysteresis (K) Figure 2-20. Dependence of thermal hysteresi s, number of spin-crossover active nearest neighbors, zSCO, percentage of CTIST active materi al, and unit cell lattice parameters as a function of x. Thermal hysteresis (T1/2up-T1/2down) is defined as the difference in the temperature, at which half of the spin-crossover active material has transitioned to the HS state during the warming cycle and th e temperature at whic h half of the spincrossover active material has transitioned to the LS state during the cooling cycle. The percentages of CTIST active material were established from the chemical formula of the compounds, the room temperature FT-IR, and the ch ange in magnetic susceptibility from the

PAGE 65

65 thermal CTIST. We believe this decrease in th e CTIST active material to be related to the stabilization of CoHS-FeLS pairs arising from the inclusion of NiII ions into the lattice and subsequent variations of the compounds unit cells, Figure 2-20 and Table 2-2. The CoLSFeLS phase with sodium counter ions has a lattice constant of 9.9721 while the CoHSFeLS phase has a lattice constant of 10.3033 .31 This latter case is similar to the x = 1.00 (10.3072 ). On the other hand, the Ni-Fe Prussian bl ue analog (x = 0.0) has a latti ce constant of 10.2396 which is comparable to previously reported value of 10.229 .76 The lattice constants of the reported NaNi1-xCox[Fe(CN)6]nH2O appear to scale monotonically as a function of x, Figure 2-20. This is consistent with unit cell changes seen in other ternary metal Prussian blue analogs.92-97 As a result of Ni-Fe pair formation, it is no l onger energetically favorable for the Co-Fe metals pairs in NaNi1-xCox[Fe(CN)6]nH2O to exist in the LS state, due to the added strain that would be placed on the lattice. Since a portion of the Co-Fe pairs are now forced into the HS state, the amount of PBA material which can transition in the LS state, and therefore undergo CTIST, is reduced. The FT-IR spectra of the compounds also prov ide evidence supporting th e stabilization of the lattice with the inclusion of NiII ions. As the amount of NiII ions in the divalent sites of the networks is increased, the st retching frequency of cyanid e stretching peak near 2160 cm-1 increases from 2160 cm-1 in the pure cobalt hexacyanoferrate to 2163 cm-1 in the case of the pure nickel hexcaynoferrate. Modeling the cyanide br idged metal centers after two masses connected by a spring, this change in the peak stretching fr equency indicates a stab ilization of the bond and an increased rigidity of the lattice, when nickel is included into the network. A similar reduction of CTIST active material was observed by Cafun et al. in an investigation of the dilution of cobalt hexacyanoferrate. In the study, the CTIST dilution was

PAGE 66

66 achieved by placing ZnII ions in the divalent metal site of the lattice or by placing hexacyanocobaltate in the cyanometallate site. Similarly, a significant sensitivity of the CTIST effect was observed, with a larger than expect ed reduction in the magnitude of the behavior.44 Conclusions In our current study, we demonstrated the tune able synthesis of a series of trimetallic Prussian blue analogs with the general molecular formula NaNi1-xCox[Fe(CN)6]nH2O. The NaNi1-xCox[Fe(CN)6]nH2O system is the first example of a compound in which competing superexchange energies dictat e the sign of the photoinduced CTIST magnetization. These compounds exhibit either a photoincrease or photo induced decrease in magnetization for certain values of x, temperature, and applied magnetic field. The ease with which the molecular composition of these ternary transition metal Prussi an blue analogs can be tuned allows for the intelligent control of both the magnitude and sign of the photoeffect through stoichiometry modification. In addition, dilution of the spin crossover active species of the ternary Prussian blue analog decreases the width of the thermal hysteresis of the CT IST. The experimental results and the origins of the magnetic behaviour are qualitatively explained using mean field calculations.

PAGE 67

67 CHAPTER 3 MAGNETIC ANISOTROPY IN PRUSSIAN BLUE ANALOG THIN-FILMS Introduction Research of thin film s of coordination compounds continues to provide materials with novel magnetic properties.37-41,56,59,72,101-108 One such series of c oordination compounds, which has drawn considerable attention, are Prussian blue analogs, AjM k[M(CN)6]lnH2O, where A is an alkali ion and M and M are transition metal ions. On e Prussian blue analog with immense potential in the development of magneto-optical storage media is the photomagnetic cobalt iron PBA, K0.2Co1.4[Fe(CN)6].9H2O. The photomagnetism of this compound was first described by Hashimoto and co-workers.17 The cobalt iron Prussian blue analogs have since been extensively studied in order to modify the magnitude of this transition and temp erature at which the photoinduced CTIST phenomenon takes place. In order to achieve these goals, investigations have focused on the manipulation of molecula r composition and the synthesis of nanoscale materials using nanoparticle and thin film fabrication techniques.17-41,56,59,72,107,108 While investigating nano-scale thin films of the cobalt iron Prussian blue analogs (AjCok[Fe(CN)6]lnH2O), our lab was the first to discove r anisotropy in the low temperature magnetization of these materials.37 When the thin films were oriented parallel to an applied field, an increased temperature dependent magnetis m is observed, as compared to that when the film oriented perpendicular to the applied fi eld. Similar magnetic anisotropy has also been observed in nanometer-scaled AjNik[Cr(CN)6]lnH2O and heterostructured Prussian blue analog thin films.56,74,75,101,102 While studying the photoinduced magne tism in cobalt iron Prussian blue analog thin films, our lab also discovered th at these films exhibit anisotropic photo-induced magnetism, a behavior not seen in the bulk powder materials.37,56 When the films are oriented parallel to an external applied magnetic field (H e ) during photoirradiation, the magnetism

PAGE 68

68 increases, while a decrease in magnetism is observed when the films are oriented perpendicular to H e during photo-excitation. The magnetic anisot ropy observed in these studies has been attributed to structural anis otropy directed from the inte rface between the substrate and precipitated thin films producing an easy magnetic axis within the samples. Our investigations of magnetic anisotropy in Pr ussian blue analog thin films is now being expanded to include a variety of di valent transition metal ions (CoII, NiII, CuII, and ZnII) and hexacyanometallates (Fe(CN)6 3and Cr(CN)6 3-). The fabrication of binary PBA thin films with tunable thickness and molecular composition was demonstrated using sequential adsorption and spin casting methods. All of the Prussian blue analog thin films displayed magnetic anisotropy, which could be tuned with the exchange of tran sition metal ions within the lattice of the PBA thin films. The thickness dependence of the magnetic anisotropy was investigated in the RbjNik[Cr(CN)6]lnH2O thin films; minimal thickness dependence upon magnetic anisotropy was identified, decreasing slightly w ith increased thin film thickness. This magnetic anisotropy in the PBA thin films is explained by single-ion anisotropy and a magnetic easy axis within the material. These may be caused by Jahn-Teller la ttice distortions induced by magnetic ordering at low temperatures and lattice strains initiated by the structure-directi ng interface between the Melinex and the PBAs. Experimental Section Materials K3Cr(CN)6 was synthesized by treating aqueous so lutions of potassium cyanide with CrCl3H2O and was used after recrys tallization from methanol.109 Deionized water, used in synthetic procedures, was obtained from a Barnst ead NANOpure system with a resistivity of at least 17.8 M Melinex, a polyethylene terephtha late polymer 535/380 gauge, was obtained from DuPont Teijin films and was used as the solid support for all of the thin films reported

PAGE 69

69 herein. Prior to use, Melinex substrates were washed with methanol and water to clean the surface and dried under a stream of nitrogen gas. All other r eagents were purchased from Sigma-Aldrich or Fisher-Acros and us ed without further purification. Film Preparation Rb0.61Co4.0[Cr(CN)6]2.87nH2O fast thin film (1a-f) A Melinex solid support was immersed 5 times in 10 mM Co(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 10 mM K3[Cr(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen.73 Rb0.88Ni4.0[Cr(CN)6]3.00nH2O fast thin film (2a-g) A Melinex solid support was immersed 5 times in 10 mM Ni(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 10 mM K3[Cr(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, 200, and 400 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. RbjNik[Cr(CN)6]lnH2O powder spin-cast thin film (2h) A solution of 10 mM K3[Cr(CN)6](aq) and 12.5 mM RbNO3(aq) was treated with 10 mM Ni(NO3)2H2O(aq) precipitating a microcrystalline powder product. The solid precipitate was isolated by centrifugation, was rinsed three times with DI water, and once with methanol. The isolated precipitate was then dried under a stream of nitrogen, yielding a light blue powder. The Prussian blue analog precipitat e was then dispersed in ether at a concentration of 2.5 mg/mL.73

PAGE 70

70 Rb0.70Cu4.0[Cr(CN)6]2.90nH2O fast thin film (3a-f) A Melinex solid support was immersed 5 times in 10 mM Cu(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 10 mM K3[Cr(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. Rb0.27Zn4.0[Cr(CN)6]2.77nH2O fast thin film (4a-f) A Melinex solid support was immersed 5 times in 10 mM Zn(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 10 mM K3[Cr(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. Rb0.74Co4.0[Fe(CN)6]2.84nH2O fast thin film (5a-f) A Melinex solid support was immersed 5 times in 5 mM Co(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 20 mM K3[Fe(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen.73 Rb0.96Ni4.0[Fe(CN)6]2.79nH2O fast thin film (6a-f) A Melinex solid support was immersed 5 times in 10 mM Ni(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 20 mM

PAGE 71

71 K3[Fe(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. Rb0.54Cu4.0[Fe(CN)6]2.72nH2O fast thin film (7a-f) A Melinex solid support was immersed 5 times in 10 mM Cu(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 20 mM K3[Fe(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. Rb0.54Zn4.0[Fe(CN)6]2.83nH2O fast thin film (8a-f) A Melinex solid support was immersed 5 times in 10 mM Zn(NO3)2H2O(aq) and rinsed twice with DI water. Next, the Melinex substrate was then dipped 5 times in 20 mM K3[Fe(CN)6](aq) and 12.5 mM RbNO3(aq) and then rinsed with DI water. This deposition cycle was repeated 5, 10, 20, 30, 60, and 200 times in order to manufacture thin films of varying thickness. After Prussian blue analog deposition, the films were rinsed with DI water and methanol and dried under a stream of nitrogen. Instrumentation FT-IR spectra of the Prussian blue analog th in films were recorded using a Thermo Scientific Nicolet 6700 spectrometer. Tappi ng mode AFM experiments were performed on nitrogen-dried samples using a Multimode AF M with a Nanoscope IIIa controller (Digital Instruments, Santabarbara, CA) and commer cially available silicon cantilever probes (Nanosensors, Phoenix, AZ). Scanning elec tron microscopy (SEM) images and energy-

PAGE 72

72 dispersive x-ray spectroscopy (EDS) spectra were obtained using a Hitachi S-4000 FE-SEM. Magnetic measurements were made using a Quantum design MPMS XL superconducting quantum interference device (SQUID) magneto meter. A bundle of 10 optical fibers, 270 m OD (Ocean Optics Model 200), was used to introduc e a room temperature halogen light source of 1-2 mW power into the SQUID magnetometer. Analysis Preparation and Procedures Melin ex substrates, used for transmission FT -IR experiments, were first cleaned with methanol and water and later coated by a 200 cycl e sequential deposition of the Prussian blue analogs to be analyzed. FT-IR spectra, of these samples, were obtained with a 32 transient scan at a resolution of 1 cm-1, with the Melinex supports were orie nted at a 45 degree angle to the incident beam, to maximize signa l to noise. Backgrounds spectr a of the solid supports were subtracted out of each thin film spectrum. Solid supports were cut into squares with sizes of 1 cm2 and mounted to magnetic disks with double sided tape for AFM analysis. Thickness, roughness, and surface coverage and morphology data were obtained by averaging the measurements of 5 different scans covering areas of 0.01 mm2. Thickness was determined by inducing a phase boundary in the Prussian blue analog thin films, preventing PBA deposition with a clear acrylic, and measuring the difference in height between the solid support and the surface of the film. The root mean square of film height deviation was used to represent the roughness of the films. Melinex supports, used for sequential absorption of 200 cycl e thin films, were cut into squares with sizes of 1 cm2, mounted onto a metal puck, and coated with a thin film of carbon for SEM measurements. For SQUID measurements, f ilms were cut into ~10.5 mm2 squares and stacked in order to increase the signal to noise fo r the measurements. Backgrounds for the solid supports were subtracted by assuming all of the weight of a film was from Melinex and subsequently

PAGE 73

73 subtracting that contribution to the magnetization by using prev iously measured Melinex mass susceptibility.81 Results Section Thin Film Generation and Characterization Prussian blue analog thin film precipitation The Prussian blue analog thin film s, with the general molecular formula of RbjM k[M(CN)6]lnH2O, were synthesized by the spin casting of precipitated powders onto Melinex sheets and the sequential absorption of divalent transition metal cations (Co2+, Ni2+, Cu2+, and Zn2+), interstitial Rb+ cations, and tri-anionic cyanometallates Cr(CN)6 3and Fe(CN)6 3from aqueous solutions. The sequential adsorp tion procedure was adapted from previously reported techniques by several research groups, Fi gures 1-7. Continuous PBA films of tunable thickness were deposited by varying the number of de position cycles during thin film fabrication. Though Melinex was the primary substrate used in this investigation, this method can also be applied to other substrates. With a nitrogen-containing, adhesion-promoti ng coating, Melinex directs the sequential absorption of PBAs by first binding divalent transition metal ions from aqueous solutions to the surface of the polymer support. Subsequent emersion into aqueous, cyanometallate solutions enables the formation of M-CN-M bonds and initiates the precipitation of extended coordinate covalent networks. The sequential deposition met hod lends itself to fine control of thin film thickness, which can be linearly increased by re peating deposition cycles Previously used by our group to study the photomagnetic behavior of c obalt iron and heterostru ctured Prussian blue analog thin films, we are now extending this met hod to the fabrication of an extensive list of other binary transition metal PBAs.

PAGE 74

74 PBA thin film structural characterization The evolutio n of thin film thickness, r oughness, and surface topology as a function of deposition cycles was investigated with atomic force microscopy. The thickness and roughness of the PBA thin films increase with additional deposition cycles, Figure 3-1 and Table 3-1. 02 04 06 0 0 100 200 300 400 Film thickness (nm)Deposition cyclesMCr(CN)6 Films02 04 06 0 0 100 200 300 400 MFe(CN)6 Films Film thickness (nm)Deposition cycles Figure 3-1. Prussian blue analog thin film thickness as a function of deposition cycles using a sequential absorption method. M = Co (squa re), Ni (triangle), and Cu (star). Uncertainty bars represent the standard deviation in the measurement of thin film thickness. Table 3-1. AFM measured Prussi an blue analog thin film thickness and roughness data. RMS is defined as the average of the height deviations taken from the mean thickness plain. Errors in film thickness are from the standa rd deviations of thickness measurements. PBA Number of cycles Average film thickness (nm) RMS roughness (nm) Unit cells per deposition cycle CoCr 10 53 6 25 5.5 20 118 10 76 30 179 14 180 60 330 28 97 NiCr 10 55 9 33 4.8 20 107 8 14 30 150 8 60 60 295 11 79 CuCr 10 27 4 30 2.8 20 49 4 24 30 83 10 33 60 165 12 92 CoFe 10 67 3 14 5.1 20 132 6 85 30 191 5 48

PAGE 75

75 Table 3-1. Continued 60 325 25 223 NiFe 10 30 3 29 3.4 20 58 5 33 30 106 3 69 60 196 8 65 CuFe 10 29 1 14 2.4 20 52 2 18 30 81 3 40 60 150 6 39 Though the thickness of each Prussian blue analog follows this general trend in thickness evolution, the rate at which the thickness of th e thin films increase is highly dependent upon the divalent transition metal ion, decreasing with atom ic number, Table 3-1 and Figures 3-2 and 3-3. It is important to note that similar thin film roughness is observed for each PBA at similar film thickness. This evolution in precipitation rate closely mimics the Irving-William stability series, an observation resultant of tr ansition metal ion charge dens ity, lability, and Jahn-Teller distortion. In no other systems was this more evid ent than in the ZnCr and ZnFe PBA thin films, as will be seen in later AFM images, Figure 3-4. Though uniform substrate coverage is achieved for the majority of the PBA thin films, only the fo rmation of isolated crysta llites is observed in the ZnCr and ZnFe thin films. This depositi on behavior prevents the measurement of MCr and MFe (M = Co, Ni, and Cu) film thickness until 10 deposition cycles have been completed. Film thickness for all ZnCr and ZnFe Prussian blue analog thin films could not be measured. 1500 nm 750 nm 0 nm CoCr N iC r CuCr 1500 nm 750 nm 0 nm CoCr N iC r CuCr Figure 3-2. AFM images of 30 cycle RbjMk[Cr(CN)6]lnH2O thin films synthesized using the sequential deposition method. Uniform coverage and similar surface morphology is observed for CoCr and NiCr. Decreased film thickness (179, 150, and 83 nm) is observed after changing the di valent transition metal site from Co to Ni to Cu.

PAGE 76

76 750 nm 375 nm 0 nm CoFe NiFe CuFe 750 nm 375 nm 0 nm CoFe NiFe CuFe Figure 3-3. AFM images of 30 cycle RbjMk[Fe(CN)6]lnH2O thin films synthesized using the sequential deposition method. Uniform surface coverage and similar surface morphology is observed for each film. Decreased film thickness (191, 106, and 81 nm) is observed after changing the divalent tr ansition metal site from Co to Ni to Cu. ZnFe 750 nm 375 nm 0 nm ZnFe 750 nm 375 nm 0 nm Figure 3-4. AFM image of a 30 cycle Rb0.54Zn4.0[Fe(CN)6]2.83nH2O thin film synthesized using the sequential deposition method. More deposi tion cycles did not result in uniform substrate coverage, only increased size of isolated PBA particulates. The degree of substrate coverage and surface mo rphology of the Prussian blue analog thin films was then analyzed with atomic force microscopy and further supported using scanning electron microscopy. Upon the completion of five deposition cycles, unifor m substrate coverage with small particulates is achieved for all PBA thin films, except thos e utilizing Zn as the divalent transition metal ion, Fi gure 3-4. After ten deposition cy cles, smooth uniform thin films are observed for the MCr and MFe (M = Co, Ni, and Cu) PBA thin films, as evidenced by the AFM images of CoCr and CoFe Prussian blue analog thin films, Figur es 3-5 and 3-6. The smooth nature of these films evolves into a rough er surface coverage with added cycles. This rough surface morphology is retained in the Prussian blue analog thin films while developing

PAGE 77

77 even thicker films, Figures 3-5 and 3-6. As seen in the prior AFM image of a 30 cycle ZnFe thin film, repeated deposition cycles do not induce the formation of a uniform film, but only increase the size of isolated particulates. SEM images of 200 cycle Prussian blue analog thin films, used for EDS analysis, confirm the observations made about surface morphology w ith the prior AFM investigations, Figures 3-7 through 3-10. 1500 nm 750 nm 0 nm CoCr 1500 nm 750 nm 0 nm 1500 nm 750 nm 0 nm 1500 nm 750 nm 0 nmCoC r 5 cycle 10 cycle 20 cycle 30 cycle Figure 3-5. AFM images of 53, 118, 179, and 330 nm Rb0.61Co4.0[Cr(CN)6]2.87nH2O thin films synthesized using the sequential depositi on method. Surface coverage is achieved after 5 cycles. Surface morphology is maintained throughout film preparation with roughness increasing with increased film thickness. 750 nm 375 nm 0 nm CoFe 750 nm 375 nm 0 nm 750 nm 375 nm 0 nm 750 nm 375 nm 0 nmCoFe 5 cycle 10 cycle 20 cycle 30 cycle Figure 3-6. AFM images of 67, 132, 191, and 325 nm Rb0.74Co4.0[Fe(CN)6]2.84nH2O thin films synthesized using the sequential depositi on method. Surface coverage is achieved after 5 cycles. Surface morphology is maintained throughout film preparation with roughness increasing with increased film thickness. CoCr N iC r CuCr ZnCr CoCr N iC r CuCr ZnCr Figure 3-7. SEM images of 200 cycle RbjMk[Cr(CN)6]lnH2O thin films. Uniform substrate coverage and rough surface morphology is indicated for the prior three films, while large isolated crystals are seen in the ZnCr analog. The magnification is 2000x and scale bars are 10 m.

PAGE 78

78 ZnCr CuCr N iC r CoCr ZnCr CuCr N iC r CoCr Figure 3-8. SEM images of 200 cycle RbjMk[Cr(CN)6]lnH2O thin films. The magnification is 10,000x and scale bars are 1 m. N iFe CuFe ZnFe CoFe N iFe CuFeZnFe CoFe Figure 3-9. SEM images of 200 cycle RbjMk[Fe(CN)6]lnH2O thin films. Uniform substrate coverage and rough surface morphology is indicated for the prior three films, while large isolated crystals are seen in the ZnFe analog. The magnification is 2000x and scale bars are 10 m. N iFe CuFe ZnFe CoFe N iFe CuFeZnFe CoFe Figure 3-10. SEM images of 200 cycle RbjMk[Fe(CN)6]lnH2O thin films. The magnification is 10,000x and scale bars are 1 m. Low magnification SEM images of RbjMk[Cr(CN)6]lnH2O and RbjMk[Fe(CN)6]lnH2O (M = Co, Ni, and Cu) revealed the continuous nature of the thin films and indicate the presence of large, bulk-like powder precipitates on the surface. Upon increasing the magnification five-fold, new observations came to light. While they previous ly appeared quite rough, one can now see that the films appear to be uniform and smooth, when looking past the large crystallites lying on the surface. One can also see what appear to be individual cubic particles embedded into the thin film. The presence of cubic structures within the films, the continued growth of discrete islands in the Zn containing films, and the rough, powder like appearance of the films surface indicate

PAGE 79

79 that the sequential depos ition method allows for the dissolvi ng and recrystallization of already precipitated Prussian blue analog films and the in clusion of previously precipitated solids from the aqueous solutions. While this may cause the in clusion of defects into the PBA lattice, it may also allow for structural anneali ng of the thin films and enable structural coherence to permeate through the film to a greater exte nt than in related powders. To investigate the magnetic anisotropy of Prussian blue analog thin films fabricated by other synthetic methods, spin cast films of RbjNik[Cr(CN)6]lnH2O microcrystalline powders upon Melinex substrates were made. Though uni form coverage of the solid support was achieved, the morphology observed in the sequentia l deposition thin films was not retained, and the surface displays a substantia l degree of roughness (58.6 nm), Figure 3-11. The thickness of the film could not be accurately measured due to the lack of a deposition boundary and large degree of roughness. 500 nm 0 nm 500 nm 0 nm 500 nm 0 nm Figure 3-11. AFM image of a spin cast thin film of RbjNik[Cr(CN)6]lnH2O microcrystalline, bulk powders. Surface coverage is achi eved with a RMS roughness of 58.6 nm. PBA thin film chemic al characterization The chemical formulas of the thin films were determined considering the results of EDS and FT-IR analyses. The alkali and transition metal ratios were taken directly from the EDS results, because these elements had a high signa l to noise ratio and we re reproducible. The

PAGE 80

80 relative ratios of FeII and FeIII in the precipitated thin films were determined by fitting and subsequently integrating the peaks assigned to th e cyanide stretches in th e FT-IR spectra of each species, assuming all extinction coefficients to be equivalent. Alka li and transition metal composition, characteristic cyanide peak stretching frequencies, and the resultant thin film molecular formulas are summarized in the tables Table 3-2. Strong agreement between cyanide peak stretching frequencies in th e FT-IR spectra of the thin films and those of the representative bulk powders confirms that the targeted materials were deposited upon the solid supports.30,82,106,110-113 The transmission FT-IR spectra and a summary of fitting parameters of the PBA thin films are supplied in Appendix B, Figures A-1 and A-2. Table 3-2. Molecular compositi on and characteristic CN stretc hes for Prussian blue analog RbjMk[M(CN)6]lnH2O thin films synthesized with the sequential adsorption method PBA Chemical composition % Rb % Cr % Fe % Co % Ni % Cu % Zn CN stretches (cm-1) CoCr Rb0.61Co4.0[Cr(CN)6]2.87nH2O 7.94 36.35 55.72 2171, 2137 NiCr Rb0.88Ni4.0[Cr(CN)6]3.00nH2O 10.94 36.54 52.54 2175, 2135 CuCr Rb0.70Cu4.0[Cr(CN)6]2.90nH2O 8.21 38.71 53.32 2187, 2125 ZnCr Rb0.27Zn4.0[Cr(CN)6]2.77nH2O 3.66 37.98 58.37 2191, 2140 CoFe Rb0.74Co4.0[FeIII(CN)6]2.62[FeII(CN)6]0.22nH2O 9.81 37.39 52.81 2163, 2118, 2074 NiFe Rb0.96Ni4.0[FeIII(CN)6]2.22[FeII(CN)6]0.57nH2O 11.81 34.33 53.86 2167, 2116 CuFe Rb0.54Cu4.0[FeIII(CN)6]2.37[FeII(CN)6]0.35nH2O 6.60 33.61 59.8 2174, 2138, 2102 ZnFe Rb0.54Zn4.0[FeIII(CN)6]2.78[FeII(CN)6]0.05nH2O 7.62 36.00 56.37 2169, 2099 Thin Film Magnetic Characterization Low-temperature magnetization The temperature dependences of the dc magnetic susceptibilities, M, of the Prussian blue analog RbjMk[M(CN)6]lnH2O thin films were investigated as a function of transition metal

PAGE 81

81 composition and film thickness and are shown in Fi gures 3-12, 3-13, and 3-14. The temperature dependences of the dc magnetic su sceptibilities were r ecorded for samples (200 cycles) oriented parallel and perpendicular to an applied field of 100 G between 2 K and 150 K. While oriented parallel to the applied field, these thin films exhibit magnetic behavior similar to the known bulk powders. Ferromagnetic and ferrimagnetic ordering is observed in the Prussian blue analog thin films, marked by an abrupt increase in the dc magnetic susceptibilities at low temperatures. A gradual evolution of the magne tic ordering temperature, Tc, is observed with a change in transition metal composition, increasing w ith higher spin states and increased J values. These experimental values of Tc agree with those previously reported for the bulk powders.82,110,114-116 A clear bifurcation of the field-cooled (FC) and zero-field-cooled (ZFC) curves, below the Prussian blue analogs magnetic ordering temperatures, with a peak in the ZFC vs. T plots, is observed for all samples, indicating the sp in-glass nature of the materials. To investigate the thickness de pendence of the thin film magn etic behavior (anisotropy, ordering temperature, spin glass nature) of the Prussian blue analogs, sequentially adsorbed and spin cast thins films of Rb0.88Ni4.0[Cr(CN)6]3.00nH2O with thickness ranging from 55 nm to 2.2 m were fabricated on Melinex solid suppor ts, and their dc magnetic susceptibilities were monitored as a function of temperature. Thes e data were then compared to that of the representative bulk powders. All films exhibited a magnetic ordering temperature between 70 and 80 K and glassy behavior upon the onset of fe rromagnetic ordering, which is consistent with the representative bulk powder. Surprisingly, the 55 nm thick sequentially adsorbed and spin cast films have their freezing temperatures Tf below 20 K, similar to the bulk powder, while the bifurcation in the FC and ZFC magnetic succeptibili ty data is above 20 K for the thicker films. This indicates that the thicker films ar e exhibiting less glassy behavior.

PAGE 82

82 0 1 2 3 4 5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 020406080100 0.0 0.4 0.8 1.2 1.6 CuCr Tc = 70 K NiCr Tc = 70 K M (10 -2 emu/sample)CoCr Tc = 30 K M (10 -5 emu/sample) M (10 -6 emu/sample)T (K) Figure 3-12. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjMk[Cr(CN)6]lnH2O thin films, in He = 100 G for the parallel (blue) and perpendicular (red) film orientat ions with respect to the applied field. 01 02 03 0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 M (10-3 emu/sample)T (K)CuFe Tc = 23 K M (10-2 emu/sample)NiFe Tc = 28 K Figure 3-13. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjMk[Fe(CN)6]lnH2O thin films, in He = 100 G for the parallel (blue) and perpendicular (red) film orientat ions with respect to the applied field.

PAGE 83

83 020406080100 0.0 0.7 1.4 2.1 2.8 3.5 0.0 0.3 0.6 0.9 1.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0 4 8 12 16 20 M (10-4 emuG/sample) T (K) M (10-1 emuG/sample) M (10-1 emuG/sample) M (10-2 emuG/sample) spin cast 2.2 m 1.1 m 55 nm M (emu/gram)bulk powder Figure 3-14. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjNik[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel (blue) and perpendicu lar (red) film orientations with respect to the applied field.

PAGE 84

84 Thin film anisotropy: tran sition metal ion dependence The dependence of magnetic susceptibility upon sample orient ation was then investigated for the Prussian blue analog RbjMk[M(CN)6]lnH2O thin films as a function of transition metal composition and film thickness and is shown in Fi gures 3-15, 3-16, and 3-17. The temperature dependences of the dc magnetic susceptibilities were first recorded for samples (200 cycles) oriented parallel and perpendicular to an appl ied field of 100 G between 2 K and 150 K. The magnetizations of the thin films were normalized to a value of one in order to simplify further magnetic analysis and sample comparisons. For all samples, except the CuFe PBA, magnetic anisotropy is observed at low temperatures. When the Prussian blue analog thin films were oriented parallel to the applied magnetic field, the onset of magnetic order is more rapid, and the magnitude of sample magnetization is greater, as opposed to th e perpendicular orie ntation. Though the Rb0.54Cu4.0[Fe(CN)6]2.72nH2O thin film displays a small amount of magnetic anisotropy, the magnetization is greater in the perp endicular orientation. This is the opposite effect as seen in the other PBA thin films. Though the phenomenon appears to be present in all the PBA thin films, exchanging the divalent transition metal io n or hexacyanometallate in the Prussian blue analog lattice enables the magnitude of the ma gnetic anisotropy to be controlled at low temperatures. For example, by exchanging the NiII with CuII in the RbjMk[Cr(CN)6]lnH2O Prussian blue analog thin film, the magnitude of the magnetic anisotropy (Mparallel / Mperpendicular) is decreased (Ni-Cr Mparallel / Mperpendicular = 4.74, Cu-Cr Mparallel / Mperpendicular = 2.02), Table 3-3. Similar effects are seen when th e hexacyanochromate ion of the RbjNik[Cr(CN)6]lnH2O Prussian blue analog thin film is exchange d for hexacyanoferrate (Ni-Cr Mparallel / Mperpendicular = 4.74, NiFe Mparallel / Mperpendicular = 2.06)

PAGE 85

85 010203040 0.0 0.2 0.4 0.6 0.8 1.0 020406080100 020406080100 CuCr NiCr Magnetization (arb. units)T (K)CoCr T (K) T (K) Figure 3-15. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjMk[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel (blue) and perpendicu lar (red) film orientations with respect to the applied field. Magnetic succeptibilities have been normalized to one for comparison simplification. 0102030 0102030 0.0 0.2 0.4 0.6 0.8 1.0 NiFeMagnetization (arb. units) T (K)CuFe T (K) Figure 3-16. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjMk[Fe(CN)6]lnH2O compounds, in He = 100 G for the parallel (blue) and perpendicu lar (red) film orientations with respect to the applied field. Magnetic succeptibilities have been normalized to one for comparison simplification. Table 3-3. Magneti c anisotropy of RbjMk[M(CN)6]lnH2O thin films as a function of divalent transition metal ion at 2 K Prussian blue analog Magnetic anisotropy (Mparallel/Mperpendicular) NiFe 2.06 CuFe 0.89 CoCr 2.50 NiCr 4.74 CuCr 2.02

PAGE 86

86 Thin film anisotropy: thickness dependence The thickness dependence of the magnetic anis otropy of sequentially adsorbed and spin cast thins films of Rb0.88Ni4.0[Cr(CN)6]3.00nH2O with thickness ranging from 55 nm to 2.2 m were fabricated on Melinex solid supports was then investigated. All of the thin film samples displayed magnetic anisotropy, though it generall y decreased with film thickness, Figures 3-17 and 3-18. Like the thicker films, less anisotropy is observed in the spin cast films as well. These observations indicate that the th icker films and spin cast films exhibit magnetic behavior more reminiscent of representative bulk powders. This observation is in contrast to the evolution in the glassy nature of the thin films, as previously discussed. 10 cycle200 cycle400 cyclespin cast 1 2 3 4 5 Mparallel(T=2K)/Mperpendicular(T=2K)Film thickness Figure 3-17. Magneti c anisotropy of RbjNik[Cr(CN)6]lnH2O thin films as a function of film thickness, displaying a gradual decrease in anisotropy with increased film thickness. Little dependence upon thin film thickness is observed in the magnetic anisotropy of the dc magnetic susceptibili ty of the thin films.

PAGE 87

87 020406080100 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Magnetization (arb. units)T (K) spin cast 2.2m 1.1m 55 nm bulk powder Figure 3-18. FC (closed squares) and ZFC (open squares) magnetic susceptibility vs. temperature data, for RbjNik[Cr(CN)6]lnH2O compounds, in He = 100 G for the parallel (blue) and perpendicu lar (red) film orientations with respect to the applied field. Magnetic succeptibilities have been normalized to one for comparison simplification.

PAGE 88

88 Discussion Thin Film Precipitation Binary transition metal Prussian blue analog thin films, with the general molecular formula of RbjMk[M(CN)6]lnH2O, were fabricated with sequential absorption and spin cast techniques. These two synthetic procedures lend themselves to the preparation of continuous, coordination compound films of tunable thickness and compositi on onto a variety of substrates. Although the sequential deposition allows for fine control of film thickness, the PBA lattices precipitate onto the solid support at a rate of several unit cells per deposition cycle. Though the thickness of each Prussian blue analog thin film increases with re peated deposition cycles, the rate of deposition is highly dependent upon the nature of the divalent transition metal ion a nd hexacyanometallate, decreasing from left to right across the first ro w transition metal series. This evolution in precipitation rate is a result of increased ion labi lity of the divalent transition metal ions and closely mimics their behavior as predicted by the Irving-William stability series. The lability of the transition metal ions and the use of the sequential deposition method allows for the dissolving and re crystallization of the already pr ecipitated Prussian blue analog films. This facilitates structural annealing of the thin films and enables structural coherence to permeate through the film to a greater extent than in related powders. Th is equilibrium process would possibly allow for the formation of PBA lattices with different structures or more lattice defects and cyanometallate vacancies. This helps explain the decreased interstitial cation concentration of the Cu and Zn containing PBA thin films, as well as the precipitation of isolated crystallites in the ZnCr and ZnFe Prussian blue analog thin films. Magnetic Anisotropy in PBA Thin Films The temperature dependences of the dc magnetic susceptibilities, M, of the Prussian blue analog RbjMk[M(CN)6]lnH2O thin films were investigated as a function of transition metal

PAGE 89

89 composition and film thickness. Ferromagnetic a nd ferrimagnetic ordering were observed in the Prussian blue analog thin films, accompanied by spin glass behavior. The magnetic ordering temperatures and spin glass behavior of the Prussian blue analog thin f ilms are consistent with the previously reported bulk powders. 25,83-87,110,114-116 The decrease in glassy behavior with increased film thickness in the RbjNik[Cr(CN)6]lnH2O thin films may be due to increased structural coherence in the films as a result of structural annea ling during the additional deposition cycles. For all Prussian blue analog thin film sa mples, except the CuFe PBA, the magnetic susceptibility when oriented parallel to the applied magnetic field was greater that when perpendicular to the applied magne tic film, suggesting that the th in films contain a magnetic easy axis in the plane of the film, Figure 3-19. Applied Magnetic Field Easy AxisEasy Axis Ferromagnetic domains PBA thin film Applied Magnetic Field Easy AxisEasy Axis Ferromagnetic domains PBA thin film Easy AxisEasy Axis Ferromagnetic domains PBA thin film Figure 3-19. Schematic of the easy axis in the Prussian blue analog thin films. When the sample is orientated parallel to the appl ied magnetic field the magnetic domains preferentially align with the easy axis and applied field leading to a greater magnetization, than observed in the perpendicular orientation.

PAGE 90

90 A magnetic easy axis is a direction in which the magnetic material has a greater susceptibility to an applied magnetic field. This is due to a pr eferred, lower energy orientation of the magnetic spin moments. With the easy axis in the plane of the substrate and thin film, Hparallel aligns the magnetic moments with the applied fi eld and hence each other, while Hperpendicular allows for the spins to be randomized. This additive effect in the parallel orientation cau ses an increased thin film magnetization parallel to the applied field, as opposed to the perpendicular orientation. The magnetic anisotropy observed in the CuFe Prussian blue analog thin films, though small, had the opposite effect. Prussian blue analog compounds are composed of face-centered cubic lattices and should not exhibit any stru ctural or magnetic anisotropy. The presence of magnetic anisotropy in the thin films, a property not found in the bulk powders, indi cates that the interface between the solid support and the Prussian blue an alog thin film plays a ro le in the formation of the easy axis by directing the precipitation of the film and its structure.41,117,118 Another contribution to the magnetic anisotropy observed in these thin film systems is single-ion anisotropy. During th e deposition process, large Pru ssian blue analog domains are grown, which may be strained by the coordinatin g amine functional groups of the Melinex thin film. This stress placed upon the network during thin film fabrication could in turn be reduced through coherent, Jahn-Teller distortions upon magne tic ordering at low temperatures. This distortion of the lattice would induce structural anisotropy in the thin films and lift the degeneracy of the multiple spin states of the transition metal ions, Figure 3-20. This would in turn cause the preferential al ignment of the spin-orbit inte raction along a given axis or orientation. Because the magnetic ordering temperatures are not affected by film orientation, it is likely that the magnetic ordering of the PBA th in films at low temperatures is inducing the distortion from the perfect octahedral symmetry of these compounds.

PAGE 91

91 Tetragonal distortion Octahedral Cr3+S = 1/2, 3/2 Distorted octahedral Cr3+/2 /2 D Tetragonal distortion Tetragonal distortion Octahedral Cr3+S = 1/2, 3/2 Distorted octahedral Cr3+/2 /2 D D Figure 3-20. Schematic of th e tetragonal distortion of a Cr3+ ion. The break in symmetry and added zero field splitting energies (D) lifts th e degeneracy of the possible spin states. Also observed in these Prussian blue analog thin films is an ion dependency on the magnitude of magnetic anisotropy. The observation and magnitude(Mparallel / Mperpendicular) of the anisotropy in the dc magnetic susceptibility of the thin films depend on the overall spin of the magnetically coupled transition metal ions in the lattice and the possible spin states in which each ion can exist. Conclusions Prussian blue analog thin films with a vari ety of divalent transition metal ions (CoII, NiII, CuII, and ZnII) and hexacyanometallates (Fe(CN)6 3and Cr(CN)6 3-) were fabricated with sequential adsorption and spin cas ting methods. All of the Pru ssian blue analog thin films displayed magnetic anisotropy, which could be tune d with the exchange of transition metal ions within the lattice of the PBA thin films. The expe rimental anisotropies observed in the thin films displays a strong correlation with those predic ted by a spin-only approx imation of single ion anisotropy. The magnetic anisotropy in the dc magnetization data of RbjNik[Cr(CN)6]lnH2O thin films showed very little thickness depende nce, indicating the permeation of structural

PAGE 92

92 coherence throughout the film. We have attributed the magnetic anisotropy observed in these systems to Jahn-Teller lattice distortions init iated by magnetic ordering at low temperatures. Low temperature EPR and UV-VIS spectral analyses may give some structural insight, which will aid in further validating this claim. Furthe r understanding of the orig ins of this phenomenon may lead to the development of new classes of magneto-optical storage devices.

PAGE 93

93 CHAPTER 4 SIZE DEPENDENT THERMAL CTIST IN CO B ALT IRON PRUSSIAN BLUE ANALOGS Introduction A variety of transition metal coordination polymers and cyan ometallate networks exhibit low-temperature photoinduced magnetism, 17-35 thermally induced CTIST,28,30,36 and spin crossover phenomenon near room temperature.36,119-125 Spin crossover and CTIST-active coordination compounds displaying magnetic bistabi lity show evidence of po ssible application in the development of new magnetic and memory storage devices. Major challenges in the fabrication of new magn etic devices include the control of the size and composition of the SCO material, as well as the temperature and width of thermal hysteresis associated with the spin crossover event. One synthetic approach capab le of tuning the composition and size of spin crossover compounds is the us e water-in-oil microemulsion and encapsulating polymers to restrict growth and prevent aggregation during na noparticle precipitation. Such nano-crystalline materials often demonstrate physical properties and magnetic behavior different from their macro-sized counterparts.63-69,81,126-129 Nanoparticles of FeII containing spin crossover compounds have recently been synthesized by modifying procedures reported by Mann and by Ma llah and coworkers. Investigations of the [Fe(Htrz)2(trz)](BF4) coordination polymer performed by Pardo-Ibez and coworkers demonstrated the synthesis of SCO nanoparticle s with a wide thermal hysteresis centered around ~360 K and a steep onset of magnetism.126 Subsequent investigati ons by Ltard and coworkers on [Fe(NH2trz)3](Br)2 presented nanoparticle fabricati on of related compounds, which were accompanied by a decrease in thermal hysteresis and the temperature at which the materials magnetism increases.127 Later studies conducted by Mallah and coworkers, illustrated the precipitation of Hoffman clathrate {Fe(pz)[Pt(CN)4]} nanoparticles.128 These nano-sized

PAGE 94

94 networks displayed more gradual thermal spin crossover, decreased thermal hysteresis, and reductions in the fraction of spin transitioning material. The synthesis of Prussian blue analog nanopartic les has also garnered considerable interest in the field of molecular magnetism and the deve lopment of magnetic devi ces; of no exception is the Co-Fe Prussian blue analog.63-69,81 Though the ability to control the equilibrium particle size, photomagnetism, low temperature magnetic behavior, and critical size of this Prussian blue analog have been investigated, the evolution of the thermal CTIST event with particle size has not yet been reported. Exhibiting a th ermally induced CTIST transition from Co IIILS-Fe IILS (S = 0) pairs to the metastable state of ferrimagnetic Co IIHS-Fe IIILS (S = 1) at a variety of temperatures, one viable candidate for such a study is the NajCok[Fe(CN)6]lnH2O Prussian blue.30 Hashimoto and coworkers investigated bulk powde rs of this material and demonstrated the ease with which the behavior of the CTIST phenomenon (TCTIST, rate of magnetic increase, and thermal hysteresis width) can be tuned with alka li cation concentration and transition metal ion ratios. They observed that the CTIST temperature (TCTIST) could be shifted from 200 K to 280 K, simply by varying the Co:Fe ratio and incr easing the average number of interstitial sodium ions per unit cell from 1.08 to 1.90, Figure 4-1.30 Though the temperature of the CTIST transition was changed, the thermal hysteresis rema ined ~40 K. They also found that when the sodium ion concentration was lowered beyond this range, the CoFe PBA exis ted solely in the HS state between 50 and 300 K, while it resided in the LS state when the con centration was higher. Unlike previous investigations of the photomagentic behavior of the CoFe Prussian blue analogs, size dependence studies of th e thermally induced CTIST phenomenon have not yet been extended to nanoparticles.

PAGE 95

95 Na.07Co1.5Fe1.0Na.37Co1.37Fe1.0Na.53Co1.32Fe1.0Na.60Co1.26Fe1.0Na.94Co1.15Fe1.0 Na.07Co1.5Fe1.0Na.37Co1.37Fe1.0Na.53Co1.32Fe1.0Na.60Co1.26Fe1.0Na.94Co1.15Fe1.0 Figure 4-1. MT versus T plots for NajCok[Fe(CN)6]lnH2O Prussian blue analogs as a function of interstitial cation concentration and transi tion metal ion ratio, during the cooling (i) and warming (ii) process, at H = 5000 G.30,73 The thermal CTIST phenomenon of NajCok[Fe(CN)6]lnH2O nanoparticles, synthesized by microemulsion and aqueous synthetic methods, are reported herein. Thes e nanoparticles display

PAGE 96

96 thermal CTIST consistent with their respective microcrystalline powders. Though the nanoparticles exhibit the same CTIST phenom enon found in the bulk powders, the onset temperature of the transition was increased, by as much as ~60 degrees, to temperatures as high as 320 K. This change in the thermal CTIST was also accompanied by a decrease in thermal hysteresis width and magnitude of CIST transi tion with decreasing particle size. Though diminished thermal hysteresis and CTIST magnitude are qualitatively consistent with previously reported Monte Carlo simulations on spin crossover materials130-131 and experimental magnetic data from investigations of spin crossover nanopa rticles, the increase in transition temperature is an intriguing new development. Experimental Section Materials Na3Fe(CN)6(aq) was synthesized by oxidizing Na4Fe(CN)6(aq) with Cl2(g) and used in situ.30 Deionized water, used in synthetic procedures, was obtained from a Barnstead NANOpure system with a resistivity of at least 17.8 M All other reagents were purchased from SigmaAldrich or Fisher-Acros and used without further purification. Synthesis NajCok[Fe(CN)6]lnH2O bulk powder precipitation (1-4) Four Co-Fe Prussian blue analogs with differe nt transition metal rati os and alkali cation concentrations were prepared using proce dures previously described by Hashimoto and coworkers.30 Microcrystalline powders were synthesi zed by treating aqueous solutions of 2 mM Na3Fe(CN)6(aq) and NaCl(aq) (0-5 M) with equivolume so lutions comprised of 2 mM CoCl2(aq) and NaCl(aq) (0-5 M). The concentration of NaCl(aq) and temperature of the four reaction systems are the following: 1 (0 M, r.t), 2 (1 M, r.t), 3 (3 M, r.t), and 4 (5 M, 75C). The reaction mixtures were stirred for three hours in the open atmosphere, and the precipitated powders were then

PAGE 97

97 isolated by centrifugation. The precipitated Prussian blue analogs were rinsed three times with water and dried under a stream of nitrogen gas. NajCok[Fe(CN)6]lnH2O small bulk powder precipitation (2a) A smaller microcrystalline powder of 2 was synthesized by modifying the synthetic approach described above. Aqueous solutions of 40 mM Na3Fe(CN)6(aq) were mixed with equivolume solutions comprised of 40 mM CoCl2(aq) and 2 M NaCl(aq). The reaction mixture was stirred for 30 minutes in the open atmosphere, and the precipitated powder was then isolated by centrifugation. The precipitated Prussian blue analog was rinsed three times with water and dried under a stream of nitrogen gas. NajCok[Fe(CN)6]lnH2O nanoparticle precipitation (3a and 3b) Two sets of Co-Fe Prussian blue analog nanopa rticles were synthesized by modifying the procedures previously repor ted by Yamada and coworkers.67 Two separate microemulsions were formed by the drop-wise addition of 350 L aqueous solutions of 20 mM CoCl2(aq) with 1 M NaCl(aq) and 20 mM Na3Fe(CN)6(aq) with 1 M NaCl(aq) to 10 mL cyclohexane solutions of Igepal CO 520. The size of the aqueous droplets of th e microemulsions was controlled by varying the concentration of Igepal CO 520 in the cyclohexane solutions: 3a (1.5 g / 10 mL ) and 3b (1.75 g / 10 mL). After thirty minutes of stirring, allowing for microemulsion equilibrium, the two microemulsion solutions were combined and allowe d to react for three ho urs. The nanoparticles were then flocculated by the addition of ethanol and isolated by centrifugation. The precipitated particles were washed three times with ethanol and dried under a stream of nitrogen gas. Instrumentation and Characterization Chemical and structural characterization Fourier transform infrared (FT-IR) spectra, ta ken of samples prepared as KBr pellets or mounted between NaCl plates, were record ed using a Thermo Scientific Nicolet 6700

PAGE 98

98 spectrometer. Variable temperature FT-IR experiments were conducted using a OMEGA CN76000 thermocouple device. A Joel 2010F sp ectrometer was used to perform energy dispersive X-ray spectroscopy (EDS) and transm ission electron microsco py (TEM) to determine transition metal composition and particle size, by analyzing 15-50 particle counts with ImageJ imaging software.77 Samples were deposited as metha nol dispersions onto 400 mesh copper grids with holey carbon support films, purchased from Ted Pella Inc. Magnetic measurements Magnetic measurements were performed by th e University of Florida Department of Physics using a Quantum Design MPMS XL superconducting quantum interference device (SQUID) magnetometer.81 High temperature data (T > 100K) were taken in a field of 5000 G with temperature sweep rates of dT/dt < 1K/min, using gelcaps as sample holders. Backgrounds were subtracted from the magnetic data by usi ng the measured mass susceptibility of similar sample holders. Prior to recording magnetic data, the SQUID was demagnetized, during which the magnetic field is oscillated to zero by succes sive ramps starting at 20kG, and subsequently allowed to relax for at least two hours. Results Chemical and Physical Characterization Chemical composition determination Microcrystalline powders and nanoparticles of the Co-Fe Prussian blue analog with the general formula NajCok[Fe(CN)6]lnH2O were precipitated from aqueous media and water-in-oil microemulsions, using the surfact ant Igepal CO 520. To initia te particulate precipitation, solutions containing sodium chloride and sodium ferricyanide were treated with solutions of cobalt chloride and sodium chloride. The chemi cal formulas were determined by considering the results of EDS, combustion analyses, and FT-IR spectroscopy. The Co and Fe ratios were taken

PAGE 99

99 directly from the EDS results, because these el ements displayed signals with a high signal to noise ratio and were reproducible. The elem ental percentages of C, H, and N in the microcrystalline compounds were taken from co mbustion analyses. Us ing the results of combustion analyses, the quantity of molecular wa ter in the precipitated solids was calculated by assuming all hydrogen and oxygen atoms in the compounds are in H2O molecules. The relative ratios of FeII and FeIII in the isolated compounds were determined by fitting and integrating the peaks assigned to the cyan ide stretches in the r oom temperature FT-IR spectra of each species, assumi ng all extinction coefficients to be equivalent. The FT-IR spectrum of the pure cobalt hex acyanoferrate displays cyanid e stretching peaks at 2160, 2117, 2098, and 2060 cm-1, which have been assigned to the CoII HS-FeIII LS, CoIII LS-FeII LS, CoII HS-FeII LS, and the linkage isomerized CoII-FeIII phases.30,81 The individual FT-IR spectra and their respective fitting parameters can be seen in Figure 4-2. Finally, the sodium ion content in the chemical formulas was assigned according to char ge balance of the network in conjunction with EDS analyses. The resultant expanded chemical formulas are listed in Table 4-1. Table 4-1. Transition metal elemental anal yses and proposed molecular formulas for NajCok[Fe(CN)6]lnH2O compounds PBA Elemental composition e xperimental (calculated) % Co % Fe % C % H % N 1 Na0.15CoII 4.0[FeIII(CN)6]2.53FeII(CN)6]0.14 19.8 H2O 20.28(20.29) 12.84(12.83) 17.49( 16.56) 1.71(1.72) 20.41(19.32) 2 Na1.91CoII 4.0[FeIII(CN)6]2.21FeII(CN)6]0.82 17.4 H2O 19.08(19.08) 13.70(13.69) 17.67( 17.69) 2.54(2.84) 20.07(20.63) 2a Na1.79CoII 4.0[FeIII(CN)6]1.97FeII(CN)6]0.97 17.9 H2O 19.29(19.28) 13.38(13.43) 18.90(17. 33) 2.95 (2.93) 22.05(20.21) 3 Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O 18.60(18.61) 14.04(14.05) 18.11( 18.13) 2.64(2.66) 21.11(21.13) 3a Na2.26CoII 4.0[FeIII(CN)6]1.42FeII(CN)6]1.50 n H2O 57.80(59.10) 42.20(40.10) 3b Na2.11CoII 4.0[FeIII(CN)6]1.57FeII(CN)6]1.35 n H2O 57.90(59.10) 42.10(40.10) 4 Na3.81CoII 4.0[FeIII(CN)6]3.93FeII(CN)6]0.02 n H2O 50.33(51.66) 49.67(48.34)

PAGE 100

100 22002150210020502000 0.0 0.5 1.0 Wavenumbers (cm-1) 0.0 0.5 1.0 Absorbance (arbitrary units)0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 0.0 0.5 1.0 3b 2a 4 3 2 3a 1PBA 0 (cm-1) W (cm1 ) A (I cm1 ) 2160.3 17.8 19.0 2113.5 10.0 0.6 1 2099.9 13.9 1.1 2158.9 22.2 30.6 2112.1 16.6 2.4 2 2091.4 20.8 9.1 2158.9 18.3 7.9 2120.5 38.1 16.0 3 2093.3 23.8 3.4 2156.7 17.4 7.8 2122.6 42.8 82.3 4 2090.0 54.7 0.4 2159.3 18.5 21.2 2115.1 18.5 4.9 2a 2093.5 25.4 15.1 2149.4 16.3 0.8 2116.4 39.9 62.5 3a 2094.6 47.6 67.1 3b 2116.0 44.1 37.5 2095.0 39.1 32.2 Figure 4-2. FT-IR spectra (black) and respective fitting parameters for synthesized NajCok[Fe(CN)6]lnH2O compounds as a function of cati on concentration and particle size. All fits (red) were perf ormed using either three or tw o Lorentzian lines (green). is defined as the peak stretching energy, W is defined as the peak half-maximum width, and A is defined as the area under the curve. 3 1 22 ,04 2i i i i iW WA I (1)

PAGE 101

101 Structural characterization The structure of the compounds was then inve stigated with TEM and room temperature FT-IR spectroscopy. The TEM images of the bimetallic Prussian blue analogs indicate that the defined edges and cubic shape of the particle s are maintained by both the microcrystalline powders and nanoparticle samples, Figure 4-3. Th e slight tendency of the particles to exhibit more rounded edges with lower sodium ion conc entrations appears to be a result of the incorporation of more cyanometa llate vacancies and increase in the number of defects within the lattice. There is no clear evolution in the equi librium particle edge length of the bulk powders with varying concentrations of interstitial cations. This is indicated by the gradual increase and subsequent decrease in equilibr ium particle edge length with in creased sodium content, Figures 4-3 and 4-4. On the other hand, the average si ze of the particles decr eases significantly with increased reactant and surfactant concentration and decreased reaction times, Table 4-2. Figure 4-3. TEM images of mi crocrystalline and nanoparticle NajCok[Fe(CN)6]lnH2O compounds from Table 4-1.

PAGE 102

102 Table 4-2. Equilibrium particulate size of NajCok[Fe(CN)6]lnH2O compounds as a function of surfactant and transition metal reagent concentration PBA sample Surfactant concentration (g / 10 mL cyclohexane) Co2+ and Fe(CN)6 3(mM) Particle edge length (nm) 1 0 2 120 38 2 0 2 238 40 2a 0 40 73 15 3 0 2 557 77 3a 1.5 20 14.4 1.1 3b 1.75 20 12.7 1.1 4 0 2 319 41 200400600 0 5 10 Particle edge length (nm)0 1 2 Normalized particle count0 2 4 0 9 18 Na3.81(4) Na2.38(3) Na1.91(2) Na0.15(1) Figure 4-4. Particle distribu tions of the four bulk powder c obalt iron Prussian blue analogs, NajCok[Fe(CN)6]lnH2O, as a function of alkali cation concentration. First, by increasing the concentration of the aqueous reactants twenty fold and decreasing the reaction times, the average particle edge length of Na1.91CoII 4.0[FeIII(CN)6]2.21FeII(CN)6]0.82 17.4 H2O (2) was decreased from 237 40.1 nm to 72. 8 14.9 nm, Figure 4-5. The changes in the synthetic method prevent Oswald ripening and initiate an increa sed number of crystallization sites, decreasing the quantity of aqueous reactants available for the formation of each particle.

PAGE 103

103 100200300400 0 2 4 Particle edge length (nm) 0 11 22 Na1.9+/-0.1(2) Normalized particle countNa1.9+/-0.1(2a) Figure 4-5. Partic le distribution of 2 and 2a. Upon increasing the re actant concentration and decreasing the reaction time, the particle size decreased from 237 40.1 nm to 72.8 14.9 nm. 0200400600800 0 1 2 Particle edge length (nm) Normalized particle count510152025 0 6 12 0 6 12 Na2.3+/-0.1(3) Na2.3+/-0.1(3a) Na2.3+/-0.1(3b) Figure 4-6. Partic le distribution of 3, 3a, and 3b. Upon increasing the su rfactant concentration, the particle size decreased from 557.1 77. 2 nm to 14.4 1.1 nm and 12.7 1.1nm.

PAGE 104

104 Control over the average particle size was fu rther demonstrated by using surfactants and varied concentrations of Igepal CO 520. The concentrations of the aqueous reactants were also changed to induce reduction in particle size. The average particle edge length of Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O (3) was decreased from 557.1 77.2 nm to 14.4 1.1 nm and 12.7 1.1 nm in nanoparticle samples 3a and 3b, Figure 4-6. Also, the standard deviation in equilibrium particle si ze decreased with the use of the microemulsion synthetic method. The use of water-in-oil mi croemulsions limits the particle growth by decreasing the amount of available aqueous reacta nts, and the encapsulati ng surfactant stabilizes the outside of the particles, limiting further growth by Oswald ripening. Room temperature FT-IR spectroscopy was then us ed to verify the oxidation states of the iron ions within the lattices and determine the relative amounts of CoII HS-FeIII LS (HS), CoIII LS-FeII LS (LS), CoII HS-FeII LS (reduced) pairs within the material s. The intensities of the peaks in the FT-IR spectra corresponding to these phase s, and therefore the relative amounts of the different spin state pairs within the cobalt iron Prussian blue analogs, is affected by both the concentration of interstitial sodium ions and aver age particle size. FT-IR spectra of the seven compounds are seen in Figures 4-7 and 4-8. The sodium ion concentration dependent behavi or of the Prussian blue analog was first investigated, in order to make comparisons to smaller nanocrystalline powders and nanoparticle samples of equivalent chem ical formulas, Figure 4-7.30 The FT-IR spectra of Na0.15CoII 4.0[FeIII(CN)6]2.53FeII(CN)6]0.14 19.8 H2O (1) displays a major peak at 2160 cm-1 and two minor peaks at 2116 and 2102 cm-1. This indicates that the compound exists primarily in the HS state at room temperature, while a small fr action of the Co-Fe pairs remains in the LS and reduced, paramagnetic state. As the concentratio n of interstitial sodium ions is increased in

PAGE 105

105 compounds Na1.91CoII 4.0[FeIII(CN)6]2.21FeII(CN)6]0.82 17.4 H2O (2) and Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O (3), the relative fractio n of HS Co-Fe pairs decreases, while the amount of LS material increases. This is denoted by the decrease in relative intensity of the peaks near 2160 cm-1 and the increase in those near 2120 and 2090 cm-1. The change in relative intensities of these peaks is accompanied by a shift to lower wave numbers. This change in behavior a result of the increased ligand field and d-orbital splitting and the subsequent stabilization of the LS state as the ground state. After the sodium ion concentration is further increased, reducing the number of cy anometallate vacancies, the FT-IR spectrum of Na3.81CoII 4.0[FeIII(CN)6]3.93FeII(CN)6]0.02 n H2O (4) now displays a prominent peak at 2121 cm-1 and two shoulders at 2156 and 2088 cm-1. This shows that the majority of the Co-Fe pairs are now in the LS state, while a small fraction st ill remains in the HS and reduced states. 2300225022002150210020502000 0.00 0.25 0.50 0.75 1.00 Na3.81(4) Na2.38(3) Na1.91(2) Absorbance unitsWavenumber (cm-1)Na0.15(1) Figure 4-7. Absorbance IR spectra of cobalt iron Prussian blue analogs, 1 4. Changes in the FT-IR spectra and spin states of the PB As are displayed as a function of cation concentration.

PAGE 106

106 The effect of particle size upon the Prussian blue analog behavior wa s then monitored with FT-IR. Nanoparticles of the Co-Fe Prussian blue analog, 2a, 3a, and 3b, were synthesized, and their FT-IR spectra were compared to those of their resp ective bulk powders, 2 and 3, Figure 4-8. 2200215021002050 0.0 0.2 0.4 0.6 0.8 1.0 238 nm 73 nm Absorbance unitsWavenumber (cm-1)Na1.9+/-0.1(2) 2200215021002050 0.0 0.2 0.4 0.6 0.8 1.0 Na2.3+/-0.1(3)13 nm 14 nm Absorbance unitsWavenumber (cm-1)560 nm Figure 4-8. Size dependent behavior of the ab sorbance FT-IR spectra of cobalt iron Prussian blue analog nanoparticles, 2a, 3a, and 3b. For comparison, the FT-IR of the respective bulk powders, 2 and 3, have been included. Changes in the FT-IR spectra and spin states of the PBAs are disp layed as a function of particle size. The bulk powders FT-IR spectra display thr ee peaks corresponding to the presence of HS, LS, and reduced states. As the particle size of 2 was decreased from ~238 to ~ 73 nm, the relative amount of HS Co-Fe pairs decreased, and the number of LS Co-Fe pairs increased, denoted by the change in the peaks intensities. The increase in LS Co-Fe pairs within the lattice leads these smaller particles to exhibit structur al and magnetic properties more similar to those Co-Fe Prussian blue analogs with a higher conc entration of interstitial cations. This same general behavior was seen in the series of compounds, 3, 3a, and 3b. As the particle size was decreased from ~560 to ~14 and ~13 nm, almost all of the HS Co-Fe pairs are eliminated, leading to a dominating presence of LS and reduced material in the nanoparticles. These data lead to the observation that the smaller Prussi an blue analog particles favor the formation of CoIII LS-FeII LS and CoII HS-FeII LS pairs within the lattice.

PAGE 107

107 Thermal CTIST Behavior Variable temperature FT-IR The size dependence of the thermally induced CTIST of NajCok[Fe(CN)6]lnH2O Prussian blue analogs was first monitored using variable temperature FT-IR. In their previous report, Hashimoto and coworkers demonstrated that the thermal CTIST effect observed in NajCok[Fe(CN)6]lnH2O bulk powders can be quantitative ly investigated by monitoring the relative intensities and peak energies of the cyanide stretching modes.30 In order to determine the size dependency of this phenomenon, FT-IR spectra of nanocrystalline Co-Fe PBAs (3a and 3b) were recorded at different temperatures, Figure 4-9, and compared to that of their respective bulk material Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O (3), Figure 4-7. FT-IR spectra were taken for nanoparticle samples 3a and 3b at three different temperatures: room temperature, 340 a nd 335 K, and 350 and 355 K. For compounds 3a and 3b, single, prominent peaks at 2110 and 2106 cm-1 indicate the stabilization of the CoIII LS-FeII LS state in the material. The shift in peak energy from 2120 cm-1 designates the presence of CoII LS-FeII LS pairs within the lattice. U pon warming the samples to ~340 K, a small shoulder at 2156 cm-1 emerges in the FT-IR spectra and the peak energies of the cyanide stretching modes shift to lower wave numbers of 2096 and 2110 cm-1. These changes in the FT-IR spectra suggest the formation of CoII HS-FeIII LS pairs from CoIII LS-FeII LS metal pairs through a thermal CTSIST event and the possible formation of more CoII HS-FeII LS pairs through thermal decomposition. Thermal decomposition is supported by the non-reversible natu re of the transition as seen in the magnetic data; the magnitude of T decreases slightly with repeated cycling. The fraction of the nanoparticle Co-Fe pairs transitioning during the CTIST seems to be very small, illustrated by slight decreases in th e intensity and peak energy corres ponding to the LS state and the emergence of only a small shoulder at 2156 cm-1.

PAGE 108

108 2200215021002050 0.0 0.2 0.4 0.6 0.8 1.0 Na2.38 (13 nm)355 K 335 K Absorbance unitsWavenumber (cm-1)295 K 2200215021002050 0.0 0.2 0.4 0.6 0.8 1.0 Na2.38 (14 nm) 350 K 340 K Absorbance unitsWavenumber (cm-1)295 K Figure 4-9. Size dependence of thermal CTIST in NajCok[Fe(CN)6]lnH2O nanoparticles, 3b and 3a. Variable temperature FT-IR spec tra indicate the formation of CoII HS-FeIII LS and CoII HS-FeII LS pairs through thermal CTIST and decomposition. In order to confirm these qualitative obs ervations, the FT-IR spectra at the three temperatures were fitted using two and three Lo rentzian lines. The area under these lines was subsequently integrated, yiel ding the relative amounts of CoII HS-FeIII LS, CoIII LS-FeII LS and CoII HS-FeII LS in the particles. The results of this analysis can be seen in Figure 4-10 and Table 4-3. At room temperature, 295 K, 17.9 % of the Co-Fe pairs in Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O (3) exist in the HS state as opposed to 21.8 % in the reduced state. Heating the microcrystallin e powder did not lead to a change in the FT-IR spectra, indicating that all CTIST active material ha s already transitioned into the HS state. At the same temperature, 295 K, a much smaller pe rcentage (0.6 and 0.0 %) of Co-Fe pairs in 3a and 3b exist in the HS state, while a higher percen tage (51.5 and 46.2 %) of the Co-Fe metal pairs exist in the non-CTIST ac tive, reduced state. Upon h eating the two nanocrystalline samples to ~340 and finally to ~350 K, the fraction of HS and reduced material in each sample increases at the expense of the CoIII LS-FeII LS pairs. As the particle size decreases, a smaller percentage of the Co-Fe pairs transition into the HS state upon warming. For compounds 3a and 3b, 13.2 and 2.6 % of the Co-Fe pairs in the material have transitioned to the HS state.

PAGE 109

109 22002150210020502000 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 Wavenumber (cm-1) Absorbance units350 K 340 K 295 K Na2.38 (14 nm)2200215021002050 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Wavenumber (cm-1) Absorbance units355 K 335 K 295 K Na2.38 (13 nm) 3a 0 (cm-1) W (cm1 ) A (I cm1 ) 2149.4 16.3 0.8 2116.4 39.9 62.5 295 K 2094.6 47.6 67.1 2158.5 38.3 10.0 2111.1 35.3 33.9 340 K 2091.2 38.1 64.4 2158.1 54.4 15.0 2110.3 33.2 30.8 350 K 2090.6 37.4 67.9 3b 0 (cm-1) W (cm1 ) A (I cm1 ) 2116.0 44.1 37.5 295 K 2095.0 39.1 32.2 2152.4 7.2 0.2 2111.4 31.0 22.7 335 K 2091.8 29.1 24.8 2156.0 15.0 1.2 2120.0 28.9 9.0 355 K 2095.0 38.9 48.9 Figure 4-10. Size dependent behavior of ther mal CTIST exhibited by variable temperature FT-IR spectra (black) of NajCok[Fe(CN)6]lnH2O nanoparticles 3a and 3b. All fits (red) were performed using either two or three Lorentzian lines (green).

PAGE 110

110 Table 4-3. Variable temp erature FT-IR results for NajCok[Fe(CN)6]lnH2O bulk powder 3 and nanoparticles 3a and 3b PBA sample Temperature (K) Relative spin-state fraction % HS % LS % Reduced 3 295 17.9 60.3 21.8 3a 295 340 350 0.6 47.9 51.5 9.3 31.3 59.4 13.2 27.1 59.7 3b 295 335 355 0.0 53.8 46.2 0.4 50.5 49.4 2.6 16.7 80.7 This reduction in the CTIST effect appears to be a result of both the reduction in particle size and an increase in the fraction of non-CTIST active Co-Fe pairs. Though the magnitude of the CTIST phenomena decreased from sample 3 to 3a, the percentage of LS material which undergoes CTIST into the HS states ([%HSHT-%HSLT]/[%HSLT+%LSLT]) marginally increases from 23 to 26%. Upon further h eating of the nanoparticles, no additional CTIST behavior is observed, and the samples decompose. Though the transition in the nanoparticles is small in magnitude, it is significant that the temperature at which the phenomena occurs has changed. The phase transition is complete in 3 at 295 K, but it is not until ~350 K that CTIST effect has reached completion in the nanoparticle samples 3a and 3b. High temperature magnetism The size dependence of the thermal CTIS T effect was then studied in the NajCok[Fe(CN)6]lnH2O compounds by recording the temperature dependences of the dc magnetic susceptibility temperature product (T) in 5 kG and in the temperature range of ~100 K to ~375 K for particles of varying sizes. A thermal sweep rate of < 1 K/min was utilized to insure equilibrium was maintained throughout the spin-crossover event. The results of these studies are displayed in Figure 4-11 and summarized in Table 4-4.

PAGE 111

111 150200250 4 8 12 16 4 8 12 16 T (K)238 nm (2) 73 nm (2a) Na1.91 T (emuK/mol) 200250300350 6.5 7.0 7.5 8.0 1.8 2.1 2.4 2.7 3 6 9 12 T (K) T (10 -3 emuK/g) T (10 -3 emuK/g) T (emuK/mol)557 nm (3) 13 nm (3b) 14 nm (3a) Na2.38 Figure 4-11. Temperature dependent magnetizati on studies as a function of particle size for NajCok[Fe(CN)6]lnH2O. T vs T as measured in a SQUID magnetometer with an applied field of 5 kG. For powder sample s, the magnetic signals are expressed per mole of compound, using the chemical formulas reported above. Data for nanoparticle samples is reported per gram of compound measured. All NajCok[Fe(CN)6]lnH2O samples demonstrate a thermally induced CTIST, which appears to be able to be cycled with temperature. This behavior is indicated by the sudden decrease of the magnetic susceptibility upon cool ing the materials to temperatures of ~ 150 K and the subsequent increase in magnetization af ter warming to ~350 K. These CTIST events also exhibit thermal hysteresis, which is character istic of the cooperativity of the transition. The lack of agreement of the experimental T values with the spin-only a pproximations is due in part

PAGE 112

112 to the orbital angular momentum contributions of the CoII ions. Remnant magnetism at lower temperatures is a result of incomplete transiti on from the HS to the LS state upon cooling. The temperature at which the CTIST event occurs (Tc = [T1/2up + T1/2down]/2), the width of this thermal hysteresis, and the magnitude of the CTIST event depend heavily on both the composition and the equilibrium particle size of the Prussian blue analog particulates, Table 4-4. The slight tail in the low temperature magnetic data of the nanoparticle samples (3a and 3b) is likely due to an artifact of the measuremen t caused by high temperatures and poor background subtraction. Table 4-4. Size dependence of thermal CTIST in NajCok[Fe(CN)6]lnH2O. TCTIST is defined as the temperature upon which the thermal hysteresis is centered (Tc = (T1/2up-T1/2down). PBA sample TCTIST and thermal hysteresis (K) T1/2up T1/2down TCTIIST Thysteresis CTIST active material % 2 215 178 197 37 69 2a 225 200 213 25 78 3 275 246 261 29 74 3a 328 311 320 17 3b 328 298 313 30 Upon decreasing the equilibrium particle size of 2 and 3, the temperature at which the thermal CTIST phenomenon takes place for these compounds increased 16 and ~60 K, respectively. It is noteworthy to mention that the transition te mperature for the latter compound, 3, has now been shifted to above room temperature, 320 K. For each NajCok[Fe(CN)6]lnH2O PBA stoichiometry, a decrease in thermal hyste resis (~ 12 K) was observed with decreasing particle size, excepting sample 3b. The charge transfer spin transition also becomes more gradual, loosing the abrupt nature seen in the bulk powders. This change in the CTIST effect is associated with both a loss of cooperativity in the lattice and an increase in surface volume fraction.127,128,130 Surrounded by fewer unit cells on each side this region of the particulates is more accommodating to the lattice distortions a ssociated with the thermal CTIST phenomenon,

PAGE 113

113 enabling it to transition from the more compact LS lattice to an expanded HS network more easily.130 Discussion Section Nanoparticle Precipitation Microcrystalline and nan oparticle samples of NajCok[Fe(CN)6]lnH2O were synthesized using aqueous and microemulsion synthetic methods. We demonstrated the ability to control the equilibrium particle size and transition metal stoi chiometry of the precipitated solids by tuning the reactant and surfactant concentrations. Increasing the concentration of the aqueous precursors and Igepal CO 520 and decreasing th e length of the reaction times lead to the precipitation of nanocrystalline solids smaller than their bulk counterparts with decreased standard deviations in particle edge length. These changes in the phys ical structure of the precipitated solids are due in part to the prevention of Oswald ripening and the increase in crystallization sites within th e reaction mixtures. When studyi ng the physical structure of the bulk powder samples, the equilibrium particle size increased and the cubic shape became better defined with increasing interstitial cation concentr ation in the solids. These changes are a result of a decrease in lattice defects and cy anometallate vacancies in the compounds. Along with its ability to control the equilibrium particle size of the Prussian blue analog particulates, the use of Igepal CO 520 water-in-oil microemulsions a ppears to also have an effect on the metal oxidation and spin states within the na noparticle lattices. An increase in surfactant concentration not only leads to a decrease in par ticle size and subsequent increase in the relative fraction of surface material in the precipitated so lids, but also an increase in the relative amount of CoIII LS-FeII LS and CoII HS-FeII LS pairs, Figure 4-9 and Table 4-3. The formation of a higher fraction of CoIII LS-FeII LS pairs may be attributed to retarded PBA crystallization as a result of hindered reactant diffusion within the microemulsi on reaction systems. The slow reaction times

PAGE 114

114 can lead to the precipitation of particulate cores with a lower abundance of defects and cyanometallate vacancies. The lattices cobalt ions would experience an increased ligand field, making the CoIII LS-FeII LS phase the ground state at higher temp eratures, decreasing the magnitude of the CTIST transition and possible shifting it to higher temperatures, Figure 4-11. With precipitation rates increasing with particle size, the outside of the particle would inherently incorporate more defects and cyan ometallate vacancies and fewer interstitial sodium ions. This material would in turn be comprised of primarily CoII HS-FeII LS and CoIII HS-FeII LS pairs, adding to the remnant magnetization at lower temperatur es, Figure 4-11. Though the particles may be comprised of a sodium rich and sodium defici ent regions, the average sodium ion composition would be of an intermediate level. Thermal CTIST Increasing TCTIST An evolution in the transition metal stoichio metry and equilibrium particle size lead to changes in the temperature at which the thermally induced CTIST occurs in NajCok[Fe(CN)6]lnH2O compounds. An increase in the inte rstitial sodium concentration from 1.91 to 2.38 sodium ions per unit cell caused a shift in the CTIST onset temperature from 197 K for Na1.91CoII 4.0[FeIII(CN)6]2.21FeII(CN)6]0.82 17.4 H2O (2) to 261 K for Na2.38CoII 4.0[FeIII(CN)6]2.30FeII(CN)6]0.87 16.6 H2O (3). This evolution in CTIST behavior is due to the stabilization of the LS state at higher temperatures.30 With an increase in the interstitial cation concentration, the averag e ligand field strength around the c obalt ions increases, causing a large separation between the eg and t2g d-orbitals and a stabilization of the LS spin state. At higher temperatures, the energy se paration between the LS and HS states decreases, allowing for the population of the HS state and a charge transfer induced spin transition.

PAGE 115

115 A reduction in the equilibrium particle size, accompanied by an increase in the relative fraction of low spin and reduced Co-Fe pairs, also leads to an increase in temperature at which the CTIST phenomenon occurs. The CTIST trans ition temperatures shifted from 197 and 261 K for 2 and 3 to 213, 320, and 313 K for 2a, 3a, and 3b, respectively. It is important to note that this effect is opposite of previously reported shifts in the transition temperatures of SCO compounds.126-128 The incorporation of the non-CTIST active CoII HS-FeII LS pairs and the use of a coordinating surfactant may inhibit any thermal e xpansion of the Prussian blue analogs. By restricting the lat tice expansion associated with the CT IST phenomenon, the more compact, LS lattice of the PBA material would be stabilized at high temperatures, a nd the energy barrier of the CTIST transition would be increased. This w ould in turn force the phenomenon to take place at higher temperatures in the nanoparticles, as co mpared to their bulk solids. Another possible influence of the shift in transition temperature would be the inclusion of a higher fraction of CoIII LS-FeII LS pairs in the CTIST transitioning particle core, effectively changing the composition of the spin transitioning CoFe PBA unit cells. As described above, increased interstitial ion concentration and ligand field of the surrounding c obalt ions stabilizes the LS state and forces the transition to higher temperatures. Evolution of CTIST slope and hysteresis The slope of the charge transfer induced sp in transition and the width of the thermal hysteresis, defined as T1/2up-T1/2down, decrease as the size of the PB A particulate decreases and as the relative fraction of reduced Co-Fe pairs in the material is increased. The decrease in equilibrium particle size and the surfactants ability to isolate individual particles lead a loss of cooperativity in NajCok[Fe(CN)6]lnH2O nanoparticles.126-128 With a decrease in the number of Co-Fe pairs transitioning at once, a decrease in the thermal hysteresis associated with the spin transition is observed. This is associate with diminished magnetic interaction between particles

PAGE 116

116 enabling smoother spin transitions with smaller ac tivation barriers. Anot her contributing factor to decreased hysteresis width is an increase in surface volume fraction. With fewer neighboring unit cells, the surface of the par ticles may undergo lattice distorti ons associated with the CTIST effect more easily. This allows for smoother tr ansitions between the LS and HS states, marked by a decrease in the slope of the transition and a lower hysteresis width. A decrease in the size distribution, as seen in the nanocrystalline samples, has also been attributed to a loss in thermal hysteresis in related compounds.128 Finally, the incorporation of CoII HS-FeII LS pairs leads to the dilution of CTIST active material within the lattice. Th is leads to further loss of coope rativity within the precipitated solids and diminishes the thermal hysteresis width. As mentioned previously, the dilution of the CTIST has been investigated in Prussian blue analogs, by doping the latt ices with diamagnetic species. The CTIST diluted Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.862.05H2O was compared to its parent compound Rb0.81Mn[Fe(CN)6]0.951.24H2O, but in this case, no appr eciable change in the width of the hysteresis loop was observed.96 Other studies of CTIST dilution in ternary transition metal Prussian blue analogs have also attributed a re duction in the magnitude of the CTIST event to incorporation of diamagnetic species44. We cannot, as off yet, conclude that this is the origin of the decreased magnitude of the CTIST effect in our systems. Conclusions In our current study, we demonstrat ed the tuneable synthesis of NajCok[Fe(CN)6]lnH2O Prussian blue analog nanoparticles using a microemulsion synthesis procedure. These nanoparticles displayed thermal CTIST consistent with their respec tive microcrystalline powders. Upon decreasing the size of the Prussian blue analog particulates, the transition from the LS to HS phase became more gradual and th e thermal hysteresis was diminished. There was also a change in the temperat ure at which the CTIST phenome non was observed; compared to

PAGE 117

117 their respective microcrystalline bulk powders, the TCTIST of the nanoparticles shifted to higher temperatures. Nanoparticle samples 3a and 3b exhibited transition temp eratures above room temperature (320 and 313 K, respectively). To ou r knowledge, this is th e first Co-Fe Prussian blue analog nanoparticle displaying thermal CTSIT above room temperature, as well as, the first study displaying the size dependence of this effect in these materials. Fu rther investigations on the systematic orientation of such particles may lead to the devel opment of magnetically switchable devices, applicable at ambient temperatures.

PAGE 118

118 CHAPTER 5 INTRODUCTION TO SUPRAMOLECULAR ASSEMBLY WITH LANGM UIR MONLAYERS Supramolecular Chemistry and Self-assembly The technology industrys constant pursuit to minimize component and device size and scientists desire to mimic biological systems and intelligently tune compounds composition and properties have led to extensive research in the design and synthesis of nano-scale materials. To circumvent the fast approaching size limitati ons afforded to us by current physical and lithographical techniques, chemists are ardently searching for new synt hetic pathways, which will allow for the manufacture of such systems.132 One such approach, extensively utilized by synthetic chemists, is supramolecular assembly, th e synthesis of large, complex structures from smaller, simple, molecular building blocks.133 This approach to synthetic chemistry has lent itself to the development of novel materials with a diverse set of properties and applications in such fields as molecular magnetism, catalysis, molecular recognition, optics, and gas or ion absorption. In order to coordinate these molecular building blocks and form thermodynamically stable extended networks, supramolecular chemistry employs a variety of interactions from the weak Van der Waals forces, pi-pi stacking, and hydr ogen bonding to stronger coordinate covalent bonds. Utilizing bonding types of intermediate strength allows for the formation of robust networks, while keeping a certain level of structural lability to enable the ma terials to anneal into more stable structures with extended lattices.134,135 Due to its predictable nature, capacity to direct coordination geometry, and flexibility in constituent exchange, a modular approach to reticular supramolecular chemistry has been ex tensively studied. Using this approach, the desired coordination geometry a nd properties of the supramolecular network has to first be determined. After choosing atomic and mol ecular building blocks with the appropriate

PAGE 119

119 composition, structure and binding geometry, these components can then be synthesized into secondary building units (SBUs) w ith high levels of symmetry and rigidity. In turn, these SBUs can be polymerized, through structure annealing, into three-dimensional, extended metal-organic frameworks (MOFs). Figure 5-1. The assembly of a metal organi c framework (MOF) with diamonoid coordination topology from the copolymerization of metal ions with organic linkers.136 Though an endless library of coordinating, organic ligands and the many coordination geometries of transition metal ions may lead to an almost infinite number of synthetic combinations of SBUs with va rious coordination geometries, onl y a short list of synthesized MOF geometries exists. The incorporation of transition metal ions and conjugated organic ligands can lead to the formation of assemb lies with increased network rigidity, vibrant coloration and the observation of such properties as molecular magnetism, catalytic ability, electrical conductance, gas absorption, and charge transfer phenomena.136-142 The application of many of th ese supramolecular structures and coordinate covalent networks may require the attachment of these st ructures to devices with the use of nanoscale

PAGE 120

120 wiring and circuitry; this entails depositing them onto a substrate or positioning them at an interface. In order to eliminate the possible problems associated with the synthesis and subsequent placement of these networks at an interface, one can synthesize these supramolecular networks at the interface itself. One such boundary, used extensively by our research group, is the air-water interface, making it an appropriate system with which to carry out our study of self-assembly at an interface. Utilizing La ngmuir monolayers at the air-water interface, amphiphillic ligands can be used to direct the coordination geometry and synthesis of extended networks, utilizing its polar head groups predetermined orientat ion and coordination geometry. Langmuir Monolayers and Se lf-assembly at Interfaces Our studies of supramolecular assembly at in terfaces focus on the ai r/water interface and the use of Langmuir monolayers. This air/li quid interface affords us many benefits and eliminates common obstacle that other interfaces namely the liquid/liquid, liquid/solid, and solid/solid interfaces, do not. First, utiliz ing Langmuir monolayers spread over aqueous subphases eliminates the problem of diffusion re striction of reactants, which would otherwise limit the reactivity of our system. The air/wate r interface also guarantees us a smooth, uniform surface and can act as a structure director, a ssuring that our networks will be limited to two-dimensional coordination. Using Langmuir monolayers to study network self-assembly also allows us to use several analyt ical techniques, like attenuating-total-reflectance (ATR) fourier transform infrared (FT-IR) spectroscopy, atom ic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD), and Brewster angle microscopy (BAM). In addition, Langmuir monolayers and the extended networ ks, which they are used to fo rm, are easily transferred to solid supports, with the use of Langmuir-Blodgett (LB) film met hods, for future application and further characterization of their stru ctural and physical properties.

PAGE 121

121 In order to precipitate extended networks at the air-water interface, an amphiphillic ligand is first spread over the water surface, forming a uniform, molecular monolayer. The subphase often contains transition metal ions or metal complexes. Bonding of the amphiphillic molecules to the subphase components, using coordinating functiona lities like cyanide and carboxylic acid functional groups, results in the formation of a co ordination polymer at the air-water interface. The desired coordination topology an d two-dimensional nature of th e precipitated networks is controlled by the symmetry of the amphiphile s and subphase components, charge of the reactants, and the dimensional limitations of the interface itself, Figure 5-2. Figure 5-2. Synthetic strategy for the self-asse mbly of twodimensional coordinate covalent networks at the air-water in terface. The coordination topology of the networks is controlled by the geometry of the amphiphile s (black) and subphase reactants (red). The three possible two-dimensional networ k topologies shown on the right are the (6,3) honeycomb, (4,4) face-centrered cubic, and linear chain topologies.143,144 To construct a Langmuir monolayer, one first dissolves the desired amphiphillic molecule into a volatile organic solvent, which will not di ssolve into the subphase over which it will be spread. Next, the dissolved amphiphile is spr ead upon the subphase between a set of movable barriers, which skim the surface. Once the volatil e solvent evaporates, th e molecules will spread over the surface, orienting themselves with pol ar heads below the water surface and aliphatic carbon chains above the interface. To study the natu re of the monolayer and monitor the order of

PAGE 122

122 the amphiphillic molecules, the surface pressure of the monolayer is often monitored, by a Wilhelmy balance, during monolayer compression a nd displayed in a pressure vs. area isotherm, as can be seen in Figure 5-3. As the barrier s of the trough are moved toward each other and the surface pressure increases, the monolayer may undergo several phase transitions. The observation of these phases is dependent upon te mperature and surface area available for each amphiphile, and may not be observed for all species and in all conditions. Liquid ExpandedL iquid Condensed Gas Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Hydrophobic Tail Hydrophilic Head Group Liquid ExpandedL iquid Condensed Gas Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Mean Molecular Area (2)Surface Pressure (mN/m) Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Hydrophobic Tail Hydrophilic Head Group(G) (LE) (LC)Liquid ExpandedL iquid Condensed Gas Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Hydrophobic Tail Hydrophilic Head Group Liquid ExpandedL iquid Condensed Gas Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Mean Molecular Area (2)Surface Pressure (mN/m) Mean Molecular Area (2)Surface Pressure (mN/m)LE & G LC & LE LC LE Hydrophobic Tail Hydrophilic Head Group(G) (LE) (LC) Figure 5-3. A schematic representation of ch aracterization of Langm uir monolayers throughout the films compression. As the monolaye r is compressed, the proximity of the amphiphiles is increased leading to increased organization and phase transitions. The change in surface tension of monolayer s through the compression sequence is monitored to produce surface pressure vs. mean molecular area isotherms. At ambient temperatures and with the barriers spread wide allowing a large area per molecule, the monolayer may exist in a two-dimens ional gas phase; in this state, the molecules have little or no interaction between each other. As the barriers compress the monolayer, increasing the amphiphiles proximity and interacti on, the surface pressure rises; this evolution in monolayer behavior is often accompanied by a pha se transition. The monolayer can then exist in a liquid-expanded (LE) phase, which is analo gous to a three-dimensional liquid. As the

PAGE 123

123 barriers are moved even closer t ogether and the monolayer is fu rther compressed, the film can then go through another phase tran sition into the two-dimensiona l liquid condensed (LC) phase, similar to a three-dimensional so lid, though still lacking long range order. As the monolayer is compressed and goes through these phase transiti ons, the phases of the monolayer will often coexist in a state of equilibrium Plateaus in the isotherm mark the times at which the phases exist together in equilibrium. Though observed in many monolayer systems, all of these phases may not be observed for all amphiphiles at room temperature. The behavior of the isotherms over the reactant subphases can give informati on about the formation of the two-dimensional coordination polymers formed at th e air-water interface. By extr apolating the steepest portion of the pressure versus area isotherm, one can acqui re the minimum mean molecular area of the amphiphillic molecule at zero applied pressure.145 Differences in the mean molecular area, slope, and collapse pressure of the isotherms over pure water and transiti on metal ion containing subphases give evidence of interaction betw een the amphiphiles and subphase components. Numerous studies have shown the propensity of Langmuir monolayers to form extended two-dimensional networks and multi-component systems through inter-am phiphile interactions and the selective coordination of subphase ions, small molecules, and extended molecular networks. In order to develop novel systems w ith applications in molecular electronics and optical devices, Palacin and coworkers have con ducted investigations into the fabrication of Langmuir-Blodgett films comprised of amphiphilli c metal-containing heterocyclic, thiophene, and porphyrin compounds.146-149 The self-assembly of these networks primarily utilized electrostatic, van der Waals, and pi stacking interactions between adjacent molecular species. Similar self-assembly at interfaces was c onducted by Drain and coworkers, during their investigations of porphyrin complexes.150,151 Lending themselves as a medium capable of

PAGE 124

124 directing the self-assembly of molecular complexes from aqueous subphases, Langmuir monolayers were used by Yakhmi and coworkers to control the crystalliza tion and orientation of Prussian blue analogs and allow for their transfer to solid supports.104,152 The fabrication of extended two-dimensional, metal-organic la ttices, based on layered metal phosphonates and cyanometallate networks have been extensively i nvestigated in our own re search group. Metalphosphonate Langmuir-Blodgett films were inves tigated by Byrd, Petruska, Fanucci, and coworkers to development novel two-dimensi onal, magnetic networks and metal-organic surfaces which could be used as solid supports for three-dimensional network templating, biological sensing, and catalysis studies.153-157 To control the magne tic properties of twodimensional, metal-organic networks through pr edetermined network topology, Culp utilized pentacyano ferrate and terpyrid ine based amphiphiles to fabricate cyanometallate LangmuirBlodgett films.56,59,72,101,102,143,158 Using these amphiphiles and th e strong coordinate covalent bonds formed between cyanide ligands and transi tion metal ions, he was able to precipitate square grid and linear chain networks based on Prussian blue analogs. These two-dimensional PBA systems were subsequently shown to have th e ability to form multi-layered solids, template extended coordinate covalent networks and possess molecular magnetism. Monolayer and Thin Film Characterization Brewster Angle Microscopy Though the nature of the isotherm gives a cons iderable amount of information about the films behavior on the air-water interface, the amphiphiles propensity to form ordered monolayers can also be studied with the use of Brewster angle microscopy (BAM).159 This technique is now widely used to image Langmuir monolayers in situ. The basis of the technique is Brewster angle refracti on and the Brewster angle, B, which is given by B = tan-1 (n1/n2), where n1and n2 are the refractive indices of two joined media. Linearly polarized light, at the

PAGE 125

125 Brewster angle, aimed at the interface of two media with different refractive indices, will transmit entirely from one media to the other. The Brewster angle of th e air-water interface is ~53. When running a BAM experiment at the air-wat er interface, a polarized laser is directed towards the surface of the water, a nd the angle of incidence is set to the Brewster angle. This causes all incident light to be internally refrac ted. When this happens, the camera of the BAM does not detect any light. Wh en a monolayer is spread upon the water surface, the beam encounters a new refractive index, and its reflec tivity changes, Figure 5-4. The change in reflectivity allows for the reflected light to be detected by a high-resolution CCD camera; the detected light is then used to form images of the monolayer and study its phase behavior.160 water water film air air N o reflection Reflection water water film air air N o reflection Reflection Figure 5-4. A schematic representation of th e principle behind Brewster angle microscopy (BAM). The introduction of a film at th e air-water interface changes the refractive index of the surface. Internally refracted light is not reflected away from the surface and can now be detected by a camera. This allows for real time imaging of Langmuir monolayers. FT-IR Spectroscopy and Atomic Force Microscopy of Langmuir-Blodgett Films Further characterization of this reaction syst em requires the deposition of these films onto a solid support. The Langmuir-Blodgett (LB) t echnique, seen in Figure 5-5, allows for the transfer of monolayers from the water surface onto both hydrophobic and hydrophilic surfaces, to form monolayer and multi-layered thin films.161,162 When using this technique, the film is first compressed to the desired surface pressure, and the substrate is submerged into the aqueous

PAGE 126

126 subphase. When transferring onto a hydrophillic surface, the polar head-groups and precipitated networks of monolayers interact with the substrate. Upon lif ting the solid support from the subphase, a single layer of the monol ayer is transferred onto the su rface. The substrate can again be submerged into the subphase, through the monolaye r, and lifted back out, depositing a bilayer of the amphiphillic network and producing a multi-layered Langmuir-Blodgett film. This process can be repeated in order to pr oduce thin films of varied thickness. Figure 5-5. A schematic depiction of the deposition of a monolayer and multi-layer Langmuir-Blodgett film onto a hydrophilic solid support. Repeating the deposition process allows for the fine control in th e preparation of thin films of varying thickness. FT-IR spectroscopy studies of the transferred thin films enable one to monitor the coordination behavior of the am phiphiles by evaluating the differen t spectral peaks associated with the amphiphile. Attenuated total internal re flectance (ATR) crystals are used to give the method monolayer sensitivity; the geom etry of the crystal is such that the incident IR radiation is internally reflected, resulting in multiple passes of the beam through the sample. The vibrational

PAGE 127

127 modes of the C-H, C-N, and C-O bond stretches ar e readily observed in the monolayers FT-IR spectra of multi-layered LB films, and will be used to study the behavior of the amphiphiles studied herein. The peak multiplicity and peak stretching frequencies of the C-H stretches gives information about the packing density and orie ntation of the alkyl tails of the amphiphiles,163 while those of the C-N and C-O stretches give information about the coordination of subphase components by the polar head groups of the amphiphiles. The surface morphology and thickness of the tran sferred films can be investigated using atomic force microscopy, AFM. With the approp riate choice of tips and slow scanning rates, AFM can give detailed surface images and roughness analyses, with nanometer resolution, as well as accurate depth measurements with a reso lution on the order of angstroms. These studies can be performed over sample areas as small as tens of nanometers, or as large as several microns. While the structure of the coordination polymer networ k of the thin films are often masked by thick layers of alkyl chains from th e amphiphiles, informati on about the coordination topology of the networks can be elucidat ed from the thickness of the films. Scope of Research The work presented herein is aimed at understanding the self-assembly of extended coordinate covalent networks at an interface. In Chapter 6, we demonstrate the control of network precipitation and coordination topology with amphiphile symmetry and coordination geometry, along with the acidity and transition me tal composition of the aqueous subphase using data obtained from surface pressure isotherms, BAM, ATR FT-IR and AFM.

PAGE 128

128 CHAPTER 6 TOPOLOGICAL CONTROL OF TWO-DIMENSIONAL METAL-ORGANIC NETWORKS Introduction Because of the approaching size limitations of conventional synthetic and lithographic techniques, supramolecular assembly has gather ed considerable attention in the design and synthesis of many recent nanoscale materials.133 This approach of constructing large, extended networks from smaller molecular building blocks utilizes both weak attractive forces, such as electrostatic, van der Waals, and pi stacking in teractions, and strong intr amolecular forces, such as coordinate covalent bonding.134,135 Future analysis and applic ation of these materials and envisioned devices, such as cellular mimicking monolayers, molecular circuitry, and nanoscale and thin film magnetic materials, will most likely require them to be transferred to a solid support or placed on a surface. In order to av oid the possible problems associated with the synthesis and subsequent placement of these materials onto a substrate, one can use the alternative method of synthesizing these materials at the desired interface. With an extensive library of common synthetic routes and a growing understanding of how this knowledge can be applied to supramolecular chemistry, there is now an increasing desire in the scientific community to understand how both reactants and th e interface itself can be used to direct and modify the synthesis, dimensionality, and topology of self-assembled networks. Using the air-water interface as a medium to study the self-assembly at interfaces provides us an inexpensive and readily available system with which to work. This air/liquid interface eliminates the diffusion restriction of reactants and provides a smooth, uniform surface, which can act as a structure director, assuring that our networks will be limited to two-dimensions. The use of Langmuir monolayers to st udy network self-assembly afford s us the use of a plethora of analytical techniques to monitor network formation and enable the easy transfer of these films to

PAGE 129

129 solid supports by the Langmuir-Blo dgett technique for further applic ation and characterization of their physical and struct ural properties. Past research studies have shown Langmuir mo nolayers have the ability to form extended two-dimensional molecular solids56,59,153-157 and selectively bind ions, small molecules,146-151 and extended molecular networks104,152 in order to assemble multi-co mponent extended systems. Though there are a variety of different geometries and connectivity possible in three-dimensional supramolecular structures, there are primarily two coordination geometries found in these twodimensional grids: a square grid, 44, network and a honeycomb shaped, 63, grid (Schlafli symbol defining the number of nodes in the shortest circuit associated with thos e nodes and the number of angles at each node).164 These two coordination topologies can be pictured by excising a single layer from three-dimensional structures of a Prussian Blue analogue and the diamondoid network formed by coordination of 4,4,4-tricyanotriphenylmethanol and silver cations respectively, Figure 6-1. Figure 6-1. Excised layers of a Prussian Blue analogue (A) and silver containing diamondoid type network (B), yielding a two-dimens ional square grid lattice and honeycomb networks.73,152,165 A B A B

PAGE 130

130 In previous investigations, our research group has demonstrated the use of Langmuir monolayers to coordinate subphase ions to form continuous metal-orga nic lattices, based on layered metal phosphonates156,157 and cyanometallate networks.72,101,102 Utilizing long-chain pentacyano ferrate amphiphiles to coordinate cationic transition metal species from the subphase, Culp demonstrated the precipitation 44 square grid networks of Prussian blue analogs. The coordination topology of the networks was predetermined by the four-fold symmetry of the cyanometallate amphiphiles and the octahedral coordination sphere of the aqueous transition metal reagents. These systems were subsequently shown to have the ability to form multi-layered solids,56 template extended coordinate covalent solids,59 and possess molecular magnetism.56,59,72,101,102 After demonstrating the ability to pr ecipitate square grid networks using Langmuir monolayers, our interests have now turn ed to the synthesis of Langmuir-Blodgett films with a hexagonal coordination topology. There is an extensive list of solid-state honeycomb and diamonoid networks composed of transition metal ions bridged by organic molecules, possessing a variety of coordinating functional groups, such as nitriles, amines, phosphonates, and carboxylic acids.166-172 In an attempt to predetermine the coordination t opology of these networks, researchers used octahedrally and tetrahedrally hybridized transition metal ions and coordinating organic ligands with multiple coordination nodes and three-fold sy mmetry. In this present research study of supramolecular assembly, we will demonstrate th e use of the air/water interface and Langmuir monolayers to direct the synthe sis of two-dimensional, meta l-cyanide and metal-carboxylate networks, with honeycomb topology, using similar coordination strategies to those shown in Figure 6-2. Here in, we repor t that the reaction of the C3 symmetric amphiphile, 4,4,4-triccarboxytriphenyl methyl octadecyl ether with cationic transitio n metal ion subphases

PAGE 131

131 resulting in the formation of two-dimensional metal-organic frameworks; the evidence offered indicates that the self-assembled ne tworks possess hexagonal coordination. Figure 6-2. Schematic of two-dimensional (6 ,3) honeycomb metal-organic networks formed from the coordination of amphiphillic molecules (black) to transition metal ions (red) in the aqueous subphase. Coordination nodes are denoted by the sides of triangles and the terminal ends of thick lines. Thr ee-fold symmetry is de noted by triangles and y-shapes, while thick lines denote two-fold symmetry. Network formation and topology was controlled by the three-fold, coordination symmetry of the amphiphile and subphase ions, the charge of the subphase transition metal cations, acidity of the aqueous subphases, and the dimensional restricti on of the air/water interf ace. The observations were supported by surface pressure vs. mean molecular area isotherms, Brewster angle microscopy, ATR FT-IR experiments and atomic force microscopy. Experimental Section Synthesis Materials All reagents were purchased from Sigma-Aldric h or Fisher-Acros and used without further purification. Dry solvents we re obtained from a SECA Solvent Dispensing System designed by J. C. Meyer. All reactions were performed under an inert atmosphere. Instrumentation All nuclear magnetic resona nce (NMR) spectra were obtained on a Gemini 300 MHz spectrometer, Appendix C. The characteristic so lvent peaks were used as reference values. Fourier transform infrared (FT-IR) spectra as KBr pellets were recorded with a Bruker Vector 22 A B C

PAGE 132

132 IR. Melting points were obtained on a M EL-TEMP capillary melting point apparatus. Combustion analyses and mass spectrometry were performed by the University of Florida Spectroscopic Services laboratory, in order to obtain elemental percentages of carbon, hydrogen, and nitrogen and molecular ion masses. 4,4,4-tricyanotriphenylmethanol (1)173 This synthetic procedur e was modified from that previously reported.173 A solution of 4-bromobenzonitrile (4.0559 g, 22.5 mmol) in TH F (100mL) was stirred at -78C under dry argon and treated drop wise with a solution of n-butyl lithium (8.2mL, 2.5M in hexane, 20.5 mmol) over an hour. The resulting mixture was kept at -78C for one hour, and was then treated slowly with a solution of methyl-4 -cyanobenzoate (1.6920 g, 10.5 mmol) in THF (100mL). The resulting solution was then kept at -78C for two hours and was then slowly warmed to room temperature. The reaction was quenched with ice water. Ether (~100mL) was added to the mixture and the mixture was then washed twice with a NaHCO3/NaHSO4 solution (100mL), washed twice with water (100mL), and once with brine (50mL), keeping the organic phase. The organic phase was then dried over MgSO4 and filtered. Volatiles were removed under reduced pressure and the re sulting residue was put to vac uum over night. The residue was purified by column chromatography174 (70% Hexane, 30% Ethyl A cetate) and recrystallized from THF as a white solid (1.5 g, 20 %): m.p. 228C (225 C). IR (KBr) (cm-1): 3450, 3100, 2900, 2250, and 1610. 1H NMR (MHz, CDCl3) : 3.32 (s, 1 H, OH), 7.40 (d, 6 H, Har), 7.66 (d, 6 H, Har). 13C NMR (MHz, CDCl3) : 81.2, 112.3, 118.1, 128.4, 132.3, and 149.4. 4,4,4-tricyanotribenzylmeth yl dodecyl ether (2)175 A solution of 1 (0.1536 g, 0.4580 mmol) and sodium hydride (0.0421 g, 1.75 mmol) in THF (100mL) was stirred at reflux under dry ar gon for two hours and was then treated slowly with a solution of 1-bromododecane (0.15mL) in THF (50mL). The reaction mixture was then

PAGE 133

133 kept at reflux for four days. The reaction mi xture was then quenched with water. Ether (~100mL) was added to the mixture and the mixture was then washed twice with a NaHCO3/NaHSO4 solution (100mL), washed twice with water (100mL), and once with brine (50mL), keeping the organic phase. The organic phase was then dried over MgSO4 and filtered. Volatiles were removed by evaporation under redu ced pressure and the resulting residue was put to vacuum over night. The residue was purif ied by column chromatography (70% Hexane, 30% Ethyl Acetate) and (100% Hexane) and isolated as an oil (0.018 g, 8%). Elemental Analysis calculated in %: C: 81.1; H: 7. 4; N: 8.3; found: C: 80.5; H: 9.0; N: 7.0. IR (KBr) (cm-1): 3100, 2900, 2800, 2250, and 1620, 1490. 1H NMR (MHz, CDCl3) : 0.86 (t, 3H), 1.26 (m, 18H), 1.64 (p, 2H), 2.98 (t, 2H), 7.49 (d, 6 H, Har), 7.65 (d, 6 H, Har). 13C NMR (MHz, CDCl3) : 14.0, 22.6, 26.1, 29.3, 29.7, 29.9, 31.9, 35.0, 64.6, 85.6, 112.0, 118.1, 128.9, 132.2, and 147.2. 4,4,4-tricyanotriphenylmethyl octadecyl ether (3) A solution of 1 (0.1660 g, 0.50 mmol) and sodium hydride (0.04 g, 1.5 mmol) in THF (100mL) was stirred at reflux unde r dry argon for two hours and was then treated slowly with a solution of 1-iodooctodecane (0.23 g, 0.60 mmol ) in THF (50mL). The reaction mixture was then kept at reflux for four da ys. The reaction mixture was then quenched with water. Ether (~100mL) was added to the mixture and the mixture was then washed twice with a NaHCO3/NaHSO4 solution (100mL), twice with water (100mL), and once with brine (50mL), keeping the organic phase. The or ganic phase was then dried over MgSO4 and filtered. Volatiles were removed by evaporation under redu ced pressure and the resulting residue was put to vacuum over night. The residue was purif ied by column chromatography (70% Hexane, 30% Ethyl Acetate) and (100% Hexane) and isolated as an oil and recrystallized from hexanes (0.026 g, 9%). Elemental Analysis calculated in %: C: 81.7; H: 8.4; N: 7.2; found: C: 81.5; H: 8.9; N: 6.7. IR (KBr) (cm-1): 3100, 2900, 2250, and 1610. 1H NMR (MHz, CDCl3) : 0.88 (t,

PAGE 134

134 3H), 1.26 (m, 30H), 1.63 (p, 2H), 2.99 (t, 2H), 7.50 (d, 6 H, Har), 7.65 (d, 6 H, Har). 13C NMR (MHz, CDCl3) : 14.1, 22.6, 26.1, 28.1, 28.7, 29.3, 29.4, 29.5, 29.6, 31.9, 32.8, 33.9, 64.5, 85.5, 111.9, 118.0, 128.9, 132.1, and 147.2. 4,4,4-tricarboxytriphenylmeth yl octadecyl ether (4)176 A mixture of 3 (1.200 g, 2.042 mmol) and potassium hydroxide (1.711 g, 30.4 mmol) in 20 mL ethylene glycol and 5mL water was heated at reflux for 96 hours. The solution was cooled to r.t. and acidified to pH 1 by additi on of aqueous HCl (1 M). The resulting solid precipitate was separate d by filtration, washed with water, and dried under vacuum. The solid was purified by column chromatography (100 % He xane) and (100% Metha nol) and was isolated as a flaky tan solid (0.2939 g, 22%), after ro tary evaporation. Elemental Analysis for C40H54O8: calculated in %: C: 72.8; H: 8.25; found: C: 72.9; H: 8.38. IR (KBr) (cm-1): 3400, 3075, 2925, 2854, 1693, 1607, 1419, 1184, and 1079. 1H NMR (MHz, CD3OD) : 0.87 (t, 3H), 1.26 (m, 30H), 1.63 (p, 2H), 3.08 (t, 2H), 7.54 (d, 6 H, Har), 7.98 (d, 6 H, Har) and 12.94 (s, 3H, HOH). 13C NMR (MHz, CDCl3) : 14.7, 23.8, 27.5, 29.1, 30.8, 33.1, 65.1, 87.4, 129.7, 130.5, 149.7, and 169.3. MS (643.36 (M-H-) Monolayers and LB Films Materials All reagents were used as r eceived or synthesized above. Subphases were prepared with water, having a resistivity of 17.8 to 18.1 M from a Barnstead NANOpure system. Substrate preparation Single-crystal (100) silicon wafers, purchas ed from Semiconductor Processing Company (Boston, MA), were used as deposition substrat es for atomic force microscopy (AFM). These substrates were cleaned using the RCA cleaning pr ocedure and dried under a stream of nitrogen. Samples for attenuated-total-reflectance (ATR )-FTIR experiments were prepared on Silicon

PAGE 135

135 ATR parallelograms (45, 50 mm x 10 mm x 3 mm) The silicon ATR crystals were cleaned with piranha etch (75% Hydrogen peroxide, 25% Sulf uric Acid) before use to remove any debris. Hydrophobic substrates are prepared by modifying substrates with a deposition of a monolayer of OTS. Instrumentation The Langmuir-Blodgett experiments we re performed using KSV 2000 and 5000 Instruments modified to operate with hydr ophobic double barriers on homemade Teflon troughs with surface areas of 717 cm2 (15.0 cm x 57.8 cm) and 953.4 cm2 (13.0 cm x 71.8 cm), respectively. The surface pressure was measured with a filter paper Wilhemy plate suspended from a KSV microbalance. Brewster angle mi croscopy experiments were performed using a Nanofilm Technology GmbH BAM2plus with the LB trough mentioned above. A polarized Nd:YAG laser (532 nm, 50 mW) was used with a CCD camera (572 x 768 pixels). The instrument is equipped with a scanner that al lows an objective of no minal magnification of 10x and 20x to be moved along the optical axis. For the 10x objective, a laser power of 50%, maximum gain, and a shutter time of 1/50 s are used. The incident beam is set at the Brewster angle in order to obtain a minimum signal before the monolayer is spread upon the trough. A piece of black glass is placed at the bottom of the trough in order to absorb the internally refracted beam to avoid stray light The polarizer and analyzer are set at 0 for all experiments. The laser and camera are mounted on an x-y stag e that allows for the examination of the monolayer at different regions. FT-IR spectra of transferred LB films were recorded using a Thermo Scientific Nicolet 6700 spectrometer A Harrick TMP stage was used for ATR experiments. ATR-FTIR spectral runs consist of 60 minutes of scans at a resolution of 4 cm-1. Tapping mode AFM experiments were performed on nitrogen-dried samples using a Multimode

PAGE 136

136 AFM with a Nanoscope IIIa controller (Dig ital Instruments, Santabarbara, CA) and commercially available silicon cantilever probes (Nanosensors, Phoenix, AZ). Monolayer and LB film preparation The amphiphillic molecules 2 4 were spread over aqueous transition metal subphases using chloroform solutions with conc entrations ~ 4 mg/ 10mL. Amphiphiles 2 and 3 were spread over 2mM aqueous subphases of NaCl, CuCl2, Ni(NO3)2, and AgNO3. Amphiphile 4 was spread over 4mM aqueous solutions of NaCl, CuCl2, MnBr2, FeSO4, and La(NO3)3; the subphase pHs were adjusted with dilute aqueous solutions of NaOH(s) and HCl(aq). All monolayers were allowed to equilibrate for 30 minutes upon spread ing and were then compressed at a rate of 1mN/min with a maximum speed 5 mN/min. The stability of the spread monolayers and aqueous solubility of the amphiphiles were anal yzed with the use of a creep test, where monolayers of amphiphiles 2 and 3 were compressed to and held at a constant pressure of 1mN/m, after which a kinetic study of the m ean molecular area as a function of time was performed. Monolayer and multilaye r thin films were transferred as Y-type films at a rate of 2mm/ min onto hydrophilic substrates at a surface pressure held c onstant at 5, 10, and 20 mN/m at ambient temperatures, in order to utilize ATR FT-IR spectrometry and AFM in the study of the coordination behavior of the amphiphiles with the alkali and transition metal aqueous subphases. Results Section Langmuir Monolayers Amphiphile design and LB film fabrication In order to achieve the desired coordinati on geometry, amphiphiles with three-fold symmetry and three coordination no des and tetrahedrally and octa hedrally hybridized transition metal ions in the aqueous subphase were employed. Because of its propensity to form two and

PAGE 137

137 three dimensional honeycomb networks with transition metal ions, tr i(4-cyanophenyl)methanol was chosen as the primary building block to construct the possible targ et amphiphiles shown in Figure 6-3.165 These systems exhibit several attributes, which make them suitable for this study. O X X X O X X X O N X X X N Figure 6-3. Target amphiphiles with three-fold symmetry and three coordination nodes. X is defined as a coordination node comprised of either a cyano or carboxylic acid functional group. First, carbon substitution chemistry has been extensively studied, whic h enables one to easily vary substituents on the central carbon atom. The tetrahedral methyl center supplies a rigid, three-fold symmetry for the amphiphiles, and the para-substituted aromatic rings provide three nitrile functional groups, which are easily oxid ized to carboxylic acid functionalities, as coordination nodes. Carboxylic acid and cyano co ordination nodes are known to spontaneously form strong, coordinate covalent bonds with transition metal ions and can supply electronegativity to the amphipi hles polar head group. Fina lly, these molecules also lend themselves to alkylation, giving th e amphiphiles some hydrophobic nature. The ease with which the amphiphile could be sy nthesized and our ability to tune both the length of the hydrophobic, alkyl tail and type of coordinating func tionality present on the polar head group led us to choose the center amphiphile for this study. The synthetic reaction scheme for the amphiphiles reported above is shown in Figure 6-4. Though each step of the synthesis

PAGE 138

138 exhibited low product yields, th is approach afforded the su ccessful fabrication of three amphiphiles with varied alkyl tail lengths (C12 and C18) and two different coordinating functionalities (CN and CO2H). Br CN + NC O O nBuLi,THF -78oC OH CN NC CN OH CN NC CN NaH,RX THF,reflux OR CN NC CN 1)KOH,ethyleneglycol reflux 2)HCl OR CO2H HO2C CO2H Figure 6-4. Synthesis procedure for amphiphiles 2, 3, and 4. Coordination reactions between these three am phiphiles and transition metal ions from the subphase can lead to the preci pitation of two-dimensional, hexagonal networks with two different coordination topologies, Fi gure 6-2 (A and C). If the am phiphiles coordinate metal ions from the subphase which bind in a linear or bent fashion, a hexagonal network with a large mean molecular area (~120 2/ molecule) will be formed, Figure 6-2 (A). On the other hand, if the amphiphiles bind to transition metal ions which coordinate in a facial manner, a honeycomb network with a smaller mean molecular area (~80 2/ molecule) will be precipitated, Figure 6-2 (C). This observati on allows for further control ov er network coordination topology with the variation of transition metal ion reactants. Tris-cyano amphiphiles The behaviors of amphiphiles 2 and 3 at the air/water interface were studied for aqueous subphases of 2mM MII (M = Cu and Ni) and MI (M = Na and Ag) and are represented with mean

PAGE 139

139 molecular area vs. time creep tests, pressu re vs. area isotherms, and BAM images. The stability of the monolayer can be studied from the slope of the creep test isotherms, and the effect of transition metal coordination to th e tris-cyanobenzyl methyl head groups of the amphiphiles can be inferred from the shapes of the individual isotherms and the contrast and appearance of the BAM images of the monolayers. For both amphiphiles, 2 and 3, creep test plots with sma ll negative slopes indicate the amphiphiles propensity to form monolayers wi th low aqueous solubility, Figure 6-5. 01020304050 60 70 80 MMA ( 2 /molecule)Time (min) Figure 6-5. MMA versus time isotherms for 2 (black) and 3 (red) over a 2 mM aqueous subphases of NaCl. Static slopes of near zero indicate low amphiph ile solubility into the subphase. For each amphiphile studied, the compression isotherms contained two distinct regions corresponding to 1) a mixture of two-dimensional gas analogous phase and liquid-expanded (LE) phase and 2) a LE phase. Comparing the co mpression isotherms of the two amphiphiles, 2 and 3, over the transition metal ion subphases to those over NaCl(aq), one observes little change in the collapse pressure or take-off point of the isotherms, 2 mN/m and 5 2/molecule respectively. The mean molecular areas (MMAs) of amphiphiles 2 and 3 shifted from 45 and 50 2/molecule to ~48 2/molecule, and the collapse pressures de crease from 11 and 13 mN/m to 10 and 11 mN/m, upon monolayer compression and interacti on with the transition metal subphases. One

PAGE 140

140 also sees little or no change in the slope or shape of these isotherms. This behavior is depicted in the isotherms of Figure 6-6. Figure 6-6. Pressure ve rsus area isotherms for 2 and 3 over 2 mM aqueous subphases of NaCl (black), AgNO3 (red), CuCl2 (green), and Ni(NO3)2 (blue). The behaviors of both amphiphiles were then studied over the four subphases with the use of Brewster angle microscopy by analyzing BAM images of the monolayers throughout monolayer compression. BAM images of monolayers of 2 and 3, held at surface pressures of 0 and 5 mN/m, over the four aqueous subphases ar e shown in Figures 6-7, 6-8, and 6-9. Figure 6-7. BAM images of Langmuir monolayers of 2 and 3 over aqueous subphases of NaCl at pressures of 0 mN/m and 5 mN/m. 40 80 120 0 4 8 12 Surface Pressure (mN/m)Mean Molecular Area (2) 40 80 120160 0 4 8 12 Surface Pressure (mN/m)Mean Molecular Area (2)2 3 2 3 5 mN/m5 mN/m 0 mN/m0 mN/m

PAGE 141

141 Figure 6-8. BAM images of Langmuir monolayers of 2 over aqueous subphases of AgNO3, CuCl2, and Ni(NO3)2 at pressures of 0 and 5 mN/m. Figure 6-9. BAM images of Langmuir monolayers of 3 over aqueous subphases of AgNO3, CuCl2, and Ni(NO3)2 at pressures of 0 and 5 mN/m. At lower MMAs and higher surface pressures, the monolayers exist in the liquid expanded phase, denoted by the presence of un iform, fluid regions with lighter contrast. This behavior is observed while the amphiphiles are spread over all three transition meta l ion aqueous subphases and strongly resembles that of the amphiphile when spread over the non-coordinating aqueous Ag+ Cu2+Ni2+ 0 mN/m 0 mN/m0 mN/m Ag+ Cu2+Ni2+ 0 mN/m 0 mN/m0 mN/m 5 mN/m 5 mN/m 5 mN/m 5 mN/m 5 mN/m 5 mN/m

PAGE 142

142 subphase of sodium chloride. These observations lend evidence that the two-dimensional coordinate covalent networks are not forming. Tris-carboxy amphiphiles: pH de pendence of network formation The behavior of amphiphile 4 at the air/ water interface was then studied for 4mM M3+ (M = La), M2+ (M = Cu, Mn, and Fe) and M+ (M = Na) subphases and is represented in the following pressure vs. area isotherms and BAM images. The ability of the amphiphiles tricarboxybenzyl methyl head group to coordina te transition metal ions from the aqueous subphases can be deduced from the shapes of th ese isotherms and the appearance of the BAM images of the monolayers. Amphiphile 4 was first studied for its ability to co ordinate divalent subphase ions with varying levels of Lewis acidi ty. For each metal ion (Cu2+, Mn2+, and Fe2+) system studied, the compression isotherms of amphiphile 4 contained three distinct regions corresponding to 1) a mixture of two-dimensional gas analogous phase and liquid-expanded (LE) phase, 2) a LE phase, and 3) a liquid condensed (LC) phase, when spread over subphases with pHs 3.25, 4.00, and 2.50 respectively. This behavior strongly resembled that of 4 over a NaCl(aq) subphase. As the pH of these subphases was increased, the sepa ration between the phases of the monolayers became less distinguishable, the slope of the is otherms decreased, the take off points of the isotherms shifted from ~80 2/molecule to ~120 2/molecule, and the collapse pressures of the monolayers decreased, as compared to 4 over NaCl(aq) subphase. This behavior is seen in the amphiphiles isotherms over the re spective subphases, Figure 6-10. The changes seen in the isotherms support the notion that the amphiphiles ar e coordinating the transition metal ions from the subphase and forming rigid, two-dimensional networks.

PAGE 143

143 Figure 6-10. Pressure versus area isotherms for 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at pHs of 4.9, 5.7, and 3.5 (blue); 4.0, 4.5, and 3.0 (green); and 3.0, 4.0, and 2.5 (red) compared to 4 over a NaCl subphase (black). This behavior was then studied over the three transition metal subphases by analyzing BAM images of the monolayers. Below are BAM images of the monolayers over aqueous subphases of M2+ (M = Cu, Mn, and Fe) at low pHs (3.25, 4.00, and 2.50) throughout their compression, Figure 6-11. For amphiphile 4 fluid regions of dark and light contrast are present at high MMAs and low surface pres sures over each subphase, indicating the presence of the gas analogous and liquid expanded phase of the monola yer. Upon compression, the more condensed areas of the monolayers are forced together and the films undergo phase transitions into a liquid expanded phase. Upon further compression, at low MMAs and higher su rface pressures, the 4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )Cu2+ Mn2+ Fe2+

PAGE 144

144 monolayers appear to undergo another phase transition and may now exist in the liquid condensed phase, denoted by the presence of unifo rm, fluid regions with even lighter contrast. Figure 6-11. BAM images of Langmuir monolayers of 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at surface pressures of 0, 5, and 10 mN/m and low pHs. The pHs of these subphases were subsequently ra ised to pHs of 4, 5.7, and 4 respectively and were again studied with Brewster angle microscopy duri ng a compression sequence, Figure 6-12. At high MMAs and low surface pressures, la rge rigid regions of bright contrast existed throughout the viewing area of the BAM, indi cating that the monolay ers existed in the condensed phase. Upon compression, these c ondensed areas of the monolayers are forced together, eliminating areas free of amphiphile Upon further compression, these areas of condensed monolayer are further squeezed toge ther, forming continuous sheets of condensed phase, until they are forced upon one another, initi ating monolayer collapse, indicated by bright Cu2+ Mn2+ Fe2+ 5 mN/m 0 mN/m 10 mN/m 5 mN/m 10 mN/m 0 mN/m 5 mN/m 10 mN/m 0 mN/m

PAGE 145

145 lines between adjacent regions. The differences seen in the BAM images of the monolayers over the subphases at different levels of acidity indicate that the amphiphiles are able to coordinate the transition metal ions from the subphase to form extended networks. The network formation also seems to be pH dependent, with network formation only present at higher pHs. Figure 6-12. BAM images of Langmuir monolayers of 4 over aqueous subphases of CuCl2, MnBr2, and Fe(SO4)2 at surface pressures of 0, 1, and 10 mN/m at higher pHs. Tris-carboxy amphiphiles: charge dependence of network formation The dependence of network coordination t opology upon the charge of the subphase transition metal ions was then inve stigated. For both metal ion (Fe2+ and La3+) system studied, the compression isotherms of the amphiphile, over subphases with pHs 2.5and 3.0 respectively, contained three di stinct regions corresponding to 1) a mixture of two-dimensional gas analogous phase and liquid-e xpanded (LE) phase, 2) a LE phase, and 3) a liquid condensed Cu2+ Mn2+ Fe2+ 5 mN/m 5 mN/m 10 mN/m 10 mN/m 0 mN/m 0 mN/m 5 mN/m 10 mN/m 0 mN/m

PAGE 146

146 (LC) phase and strongly resembled that of the amphiphile over a NaCl(aq) subphase. As the pH of these subphases was increased, the separati on between the phases of the isotherms became less distinguishable, the slope of the isotherms decreased, th e take off points of the isotherms shifted to a different mean molecular area (75 and 120 2/molecule respectively), and the collapse pressures of the monolayers decreased, as compared to amphiphile over NaCl(aq) subphase (80 2/molecule). This behavior is displaye d in the isotherms below, Figure 6-13. Figure 6-13. Pressure versus area isotherms for amphiphile 4 over aqueous subphases of Fe(SO4)2(aq) at pHs of 3.0 (green ) and 2.5 (red) and NaCl(aq) (black), La(NO3)3(aq) at pHs of 4.0 (green) and 3.0 (red) and NaCl(aq) (black), and Fe(SO4)2(aq) at a pH of 3.0 (red), La(NO3)3(aq) at a pH of 4.0 (green), and NaCl(aq) (black). At low pHs (pH = 2.5 and 3.0 for M2+/3+ = Fe and La respectively) fluid regions of dark and light contrast are present at high MMAs and low surface pressures over each subphase, indicating the presence of the expanded and liquid expanded phase of the monolayer. After sufficient compression, the monolayer exists in the liquid expanded phase, denoted by the 4080120160 0 10 20 30 40 50 60 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 60 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 )Fe2+La3+ Fe2+ vs La3+

PAGE 147

147 presence of uniform, fluid regions with lighter contrast; an example of this behavior is shown in the BAM images of Figure 6-14. Figure 6-14. BAM images of 4,4,4-triscarboxybenzyl met hyloctodecyl ether over subphases of Fe(SO4)2(aq) (pH = 2.50) and La(NO3)3(aq) (pH = 3.0) at surface pressures of 0 and 10 mN/m. Figure 6-15. BAM images of 4,4,4-triscarboxybenzyl met hyloctodecyl ether over subphases of Fe(SO4)2(aq) (pH > 2.50) and La(NO3)3(aq) (pH > 3.0) at surface pressures of 0 and 10 mN/m. Fe2+ La3+0 mN/m 0 mN/m Fe2+ La3+0 mN/m 0 mN/m 10 mN/m 10 mN/m 10 mN/m 10 mN/m

PAGE 148

148 After raising the pH of the subphases, the beha vior of amphiphile cha nged drastically. At high MMAs, rigid regions of light er contrast are present in th e BAM images, indicative of the existence of a condensed phase, Figure 6-15. As compression of the monolayers proceeds, these smaller regions of the condensed phase are forced together. During this pr ocess the formation of larger, uniform regions of the condensed phase is observed in the view ing area Brewster angle microscope. The behavior observed in the isothe rm plots and BAM images above give evidence that at higher pHs two discrete extended networ ks with different mean molecular areas form upon the amphiphillic coordination of Fe2+ and La3+ ions from the subphase. While studying the coordinating ability of amphiphile 4 for subphase ions and the pH dependence of this ability, it is important to note the behavior of the amphiphile over a subphase of sodium ions at varying levels of acidi ty. It is essential to point out that 4s behavior over a NaCl(aq) subphase is independent of subphase acidity. These obser vations are displayed by the compression isotherms and BAM images in Figures 6-16 and 6-17. 4080120160 0 10 20 30 40 50 Surface Pressure (mN/m)Mean Molecular Area ( 2 ) Figure 6-16. Pressure versus area isotherms for amphiphile 4 over NaCl(aq) at pHs of 2.5 (red) and 5.3 (black).

PAGE 149

149 Figure 6-17. BAM images of 4,4,4-triscarboxybenzyl met hyloctodecyl ether over subphases of NaCl(aq) at A) and B) a high pH 5.3 and B) and D) a low pH of 2.5 at surface pressures of 0 and 10 mN/m. Langmuir-Blodgett Films Tris-cyano amphiphiles Langmuir-Blodgett (LB) films of 2 and 3, over the four subphases, were formed by Y-type deposition onto a hydrophilic silicon ATR crystal. Analysis of the alkyl ta il and aromatic nitrile vibrational modes was performe d with ATR-FTIR, comparing these functionalaties peak frequencies, along with the peak multiplicity a nd half-maximum widths, over the three transition metal subphases to that over an aqueous sodium ch loride subphase. The fi rst region of the FT-IR spectra analyzed was that containing the peak corre sponding to the stretch of the aromatic nitriles of the amphiphile. With respect to both amphiphiles 2 and 3, the stretching frequency of the aromatic nitrile did not shift fr om its original value of 2230 cm-1,when the amphiphiles were spread over the three transition metal subphases, as is shown in the FT-IR spectra and table below. The peaks corresponding to the alkyl ch ains were then analy zed, studying their peak stretching frequencies, peak multiplicities, and peak half-maximum widths. ATR FT-IR spectra pH = 5.3 pH = 2.5 0 mN/m 0 mN/m 10 mN/m 10 mN/m

PAGE 150

150 of amphiphile 2 and 3, spread over the four aqueous subpha ses, display two alkyl stretching peaks centered at 2924 and 2854 cm-1 and 2920 and 2850 cm-1, respectively. When amphiphile 2 was spread over NaCl(aq), these peaks half-maximum widths were 27 and 19 cm-1, while amphiphile 3s spectra contained peaks with half-m aximum widths of ~19 and ~12 cm-1, respectively. As seen in the FT-IR spectra on Figure 6-18 and Table 6-1, the peak stretching frequencies, multiplicity, and half-maximum widths do not change appreciably for amphiphile 2 and slightly increase for amphiphile 3, when spread over the three transition metal ion subphases, as compared to being spread over the non-coordinating sodium ion subphase. 30002800260024002200 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm-1) 30002800260024002200 0.00 0.04 0.08 0.12 Absorbance unitsWavenumber (cm-1)2 3 Na+Ag+Cu2+Ni2+Na+Ag+Cu2+Ni2+ 30002800260024002200 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm-1) 30002800260024002200 0.00 0.04 0.08 0.12 Absorbance unitsWavenumber (cm-1) 30002800260024002200 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm-1) 30002800260024002200 0.00 0.04 0.08 0.12 Absorbance unitsWavenumber (cm-1)2 3 Na+Ag+Cu2+Ni2+Na+Ag+Cu2+Ni2+ Figure 6-18. ATR-FTIR of LB films of 2 and 3 spread over aqueous subphases of NaCl(aq), AgNO3(aq), CuCl2(aq) and Ni(NO3)2(aq) transferred onto a silicon ATR crystal, with the use of Y-type Langmuir-Blodgett transfers. Table 6-1. List of the alkyl tail peak stretching frequencies and half-maximum widths for amphiphiles 2 and 3 over aqueous subphases of NaCl, AgNO3, CuCl2, and Ni(NO3)2 Aqueous Subphase Peak Half-Maximum Widths (2) 2924 cm-1 2854 cm-1 (cm-1) Peak Half-Maximum Widths (3) 2920 cm-1 2850 cm-1 (cm-1) NaCl 27 19 17 10 AgNO3 27 19 19 12 CuCl2 27 19 19 14 Ni(NO3)2 27 19 21 14

PAGE 151

151 With no change in the FT-IR spectra, the behavior of amphiphiles 2 and 3 does not appear to be changing when spread over the f our different aqueous subphases. This leads us to believe that the amphiphiles are not coor dinating transition metal ions from the subphase.177-179 Tris-carboxy amphiphile FT-IR spectroscopy To continue the study of amphiphile 4s ability to coordinate di valent transition metal ions from the aqueous subphase, LB films of 4, after being spread over the three subphases, were formed by Y-type deposition ont o silicon ATR crystal for FT-IR analysis. Analyses of the aromatic carbonyl stretching modes was performe d, comparing the peak frequencies of the carbonyl groups over the three transition metal subphases to that over an aqueous sodium chloride subphase. In each case, the relative intensity of the car bonyl stretch at 1693 cm-1 diminished, while those of the CO2 stretches (1605 and 1412 cm-1) increased, Figure 6-19. 180017001600150014001300 50 75 100 TransmittanceWavenumber (cm -1 )180017001600150014001300 2.0 2.2 2.4 2.6 2.8 3.0 TransmittanceWavenumber (cm -1 )180017001600150014001300 1.45 1.50 1.55 1.60 TansmittanceWavenumber (cm -1 )180017001600150014001300 0.44 0.48 0.52 0.56 TransmittanceWavenumber (cm -1 )NaCl Cu2+Fe2+Mn2+ Figure 6-19. ATR-FTIR of LB films of 4 spread over aqueous subphases of NaCl(aq), CuCl2(aq), MnBr2(aq), and Fe(SO4)2(aq).

PAGE 152

152 These changes in the FT-IR spectra lend evid ence to the deprotonation of the carboxylic acid functional groups and the subseque nt coordination of transition metal ions from the subphase.177 Similar ATR FT-IR experiments were performed in order to continue the study on the charge dependence of the monolayer coordina tion topology. The half-maximum widths and multiplicity of the peaks, corresponding to the CH2 stretches of the hydrocarbon tails of the amphiphillic molecules upon coordinating the Fe(SO4)2aq and La(NO3)3aq subphase ions, were compared to that of the amphiphile over the NaCl(aq) subphase. With respect to the FT-IR spectrum of the amphiphile spread over the NaCl(aq) subphase, the half-widths of the two peaks at 2942 and 2852 cm-1 increase by 5 and 2 cm-1 respectively when the amphiphile is spread over the Fe(SO4)2(aq) subphase, while remaining relatively unchanged when the amphiphile was spread over the La(NO3)3(aq) subphase. It was also seen that the aromatic carbonyl stretch at 1693 cm-1 fades, while the peaks at 1412 and 1605 cm-1 grow in relative intensity, when spreading the amphiphile over the La(NO3)3 and Fe(SO4)2 subphases, as compared to the FT-IR spectrum of 4 over the non-coordinating NaCl(aq) subphase. These observations indicate interactions between the amphiphiles coordina ting functional groups an d the transition metal ions of two subphases. They also support the idea that the amphiphiles spread over the La(NO3)3 subphase are in closer proximity with their alky l tails arranged in a more orderly fashion than those over the Fe(SO4)2 subphase.177-180 These results are seen in Figure 6-20 and summarized in the Table 6-2. Table 6-2. List of the alkyl tail peak stretching frequencies and half-maximum widths for amphiphile 4 over aqueous subphases of NaCl(aq), Fe(SO4)2(aq), and La(NO3)3(aq) Aqueous Subphase Peak Half-Maximum Widths 2924 cm-1 2852 cm-1 (cm-1) NaCl(aq) 20 12 Fe(SO4)2(aq) 25 14 La(NO3)3(aq) 22 12

PAGE 153

153 3000 2900 2800 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 180017001600150014001300 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 300029002800 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 180017001600150014001300 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 )AB Na+Fe2+La3+Na+Fe2+La3+3000 2900 2800 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 180017001600150014001300 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 300029002800 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 ) 180017001600150014001300 0.00 0.02 0.04 0.06 0.08 0.10 Absorbance unitsWavenumber (cm -1 )AB Na+Fe2+La3+Na+Fe2+La3+ Figure 6-20. ATR-FTIR of LB films of 4 spread over aqueous subphases of NaCl(aq), Fe(SO4)2(aq), and La(NO3)3(aq). The spectra presented display the regions containing stretching peaks of the A) amphiphiles alkyl tails and the B) aromatic carbonyls. Tris-carboxy amphiphile AFM In order to probe the propensity of the am phiphiles to form uniform monolayers, through coordination of subphase metal ions the films ability to coat a surface, and to further delineate between the two distinct monolayers formed over the two aqueous subphases, AFM studies were performed on transferred LB-films of 4 over monolayers of Fe(SO4)2(aq) and La(NO3)3(aq) at a variety of surface pressures. The results were compared to those from AFM studies of bare slides of the silicon substrat es, upon which the monolay ers were transferred. The first aspect of the coordina ted monolayers studied was their ability to be transferred to a solid support and coat a surf ace. Langmuir-Blodgett films of 4 over Fe(SO4)2(aq) and La(NO3)3(aq) subphases were transferred to silicon slid es at a surface pressu res of 5 and 20 mN/m and studied with atomic force microscopy, comparing their roughness, uniformity and surface coverage to that of bare sili con. Bare silicon surface is a uniformly smooth surface, having a root mean square roughness of 0.062 nm, with no defects or height ch anges, aside from dust particles. One sees a different view when studying the AFM study result s of the transferred

PAGE 154

154 Langmuir-Blodgett films. The LB films of 4 transferred at a surface pressure of 5 mN /m, over the Fe(SO4)2(aq) and La(NO3)3(aq) subphases, display the ability to coat the surface of the hydrophilic silicon, although defects are seen the transferred monolayers, as in seen in Figure 6-21. Though the coated surfaces appear rough due to the many defects in the transferred monolayers, the LB films, spread over the Fe(SO4)2(aq) and La(NO3)3(aq) subphases, are relatively smooth with a RMS roughness of .195 and .123 nm respectively. Notice that the roughness, measured at areas of uniform coverage, of the thin film formed by the coordination of the amphiphile and the trivalent transition metal subphase is smoother than that of the amphiphile spread over the divalent aqueous subphase. Figure 6-21. Two-dimensional AFM images of bare silicon and transferred monolayers of amphiphile 4 spread over subphases of Fe(SO4)2(aq) and La(NO3)3(aq) at a surface pressure of 5 mN/m. Langmuir-Blodgett films of 4 over Fe(SO4)2(aq) and La(NO3)3(aq) subphases were then transferred to silicon slides at a surface pressure of 20 mN/m and studied with atomic force microscopy. Looking at these two thin films, one can see uniform coverage of the silicon slides in both cases, with little or no defects in the LB film, Figure 6-22. Looking at the AFM data listed in the Table 6-3, one can again see that the transferred LB films, originally spread over the Fe(SO4)2(aq) and La(NO3)3(aq) subphases, are smooth, with a RMS roughness of 0.119 and .160 nm respectively. When comparing these results to those of the monolayers transferred at the Silicon Fe2+ La3+5 mN/m 5 mN/m 5 mN/m

PAGE 155

155 lower surface pressure described above, one obs erves a more continuous thin film with fewer defects and higher percentage of surface coverage, along with slight changes in monolayer roughness. AFM measurements of the transfe rred LB films at a pressure of 5 mN/m were then used to determine thin film thickness a nd roughness of the distinct monolayers to differentiate between the two network coordination topol ogies; the results are summarized in Table 6-3. As described in the paragraphs above, the LB film of the monolayer spread over a La(NO3)3(aq) subphase was smoother than that spread over a Fe(SO4)2(aq) subphase, indicating closer packed and more organized alkyl tails in the prior film. Meas uring the height difference between the silicon surface and the surface of the thin film, the th ickness of the two LB films, spread over Fe(SO4)2(aq) and La(NO3)3(aq) subphases, were found to be 1.148 and 1.589 nm respectively, indicating a closer packing and more upright orientation of the alkyl tales in the latter monolayer. Figure 6-22. Two-dimensional AFM images of bare silicon and transferred monolayers of amphiphile 4 spread over subphases of Fe(SO4)2(aq) and La(NO3)3(aq). Table 6-3. Roughness and film thickness st udy results for monolayers of amphiphile 4 over aqueous subphases of Fe(SO4)2(aq), and La(NO3)3(aq), transferred onto hydrophilic silicon substrates (RMS roughness of 0.062 nm) Aqueous Subphase Transfer Surface Pressure (mN/m) RMS Roughness (nm) Film Thickness (nm) Fe(SO4)2(aq) 5 20 0.20 0.04 0.12 0.03 1.15 0.12 La(NO3)3(aq) 5 20 0.12 0.02 0.16 0.04 1.59 0.18 Silicon Fe2+La3+20 mN/m 20 mN/m 20 mN/m

PAGE 156

156 Discussion Section Tris-cyano Amphiphiles The amphiphillic behavior of compounds 2 and 3 and their ability to assemble into extended coordinate covalent networks by binding subphase ions have been extensively studied over aqueous transition metal subphases (AgNO3, CuCl2, and Ni(NO3)2); these observations were then compared to the amphiphiles behavior over a aqueous subphase of NaCl(aq). In their mean molecular area vs. time isotherms, minor negative slopes indicate compounds 2 and 3 monolayer stability and low water solubility. Well-defined surface pressure vs. mean molecular area isotherms possessing a phase transition from a mixture of liquid expanded and two-dimensional gas phases to homogenous mo nolayers of liquid expanded phase are accompanied by Brewster angle microscope images throughout monolayer compression mimicking this same behavior. These observa tions demonstrate these compounds viability as amphiphiles for this study. Surface pressure vs. mean mo lecular area isotherms and BA M images of monolayers of compounds 2 and 3 over the transition metal ion subphases were then compared with those over a sodium chloride subphase. In the case of both amphiphiles, the shape, slope, collapse point, and take-off point of the isotherms did not change an appreciable amount when the subphase was changed from the NaCl(aq) subphase to the transition metal io n subphases. BAM images of the monolayers compression over all the subphases indi cate that each monolayer studied contained a coexistence of dynamic regions of liquid expanded and gas-anal ogous phases. It was not until a phase transition, result ant of monolayer compression, that th e formation of a uniform film of liquid expanded phase occurred. One last impor tant observation from these isotherms is that both amphiphiles mean molecular area, ~45 2 (2) and ~50 2 (3), appear to be considerably less than the optimized cross-sectional ar ea of the polar tris-cyano head group (~75

PAGE 157

157 2/molecule). The fluid nature of the monolayer less than optimized m ean molecular area, and the lack of behavior change seen in the amphiphiles isotherms lead us to conclude that there is minimal amphiphile/subphase interaction and th e transition to more condensed phases during monolayer compression is due to the maximization of van der Waals and pi stacking interactions. In order to determine the extent of the intera ction between the transition metal ions of the subphase and amphiphiles 2 and 3, Langmuir-Blodgett films of th e monolayers were deposited on ATR crystal for FT-IR analysis in order to co mpare the spectra of the transferred monolayers over the transition metal ion s ubphases to those over a NaCl(aq) subphase. Comparing the multiplicity, half-maximum widths, and stretching frequencies of the peaks corresponding to the alkyl tails and aromatic nitriles of 2 and 3, no significant differences were seen in the spectra over all four aqueous subphases. These observati ons indicate that there was no change in the packing alignment or density of the alkyl tails of the amphiphiles or interaction between the coordination nodes of the amphiphiles and the subphase ions. The inability of the amphiphiles to coordinate transition metal ions from the subphase to form extended networks is due to their failure to make charge balanced networks with those ions. In order to address this pr oblem, the nitrile functional gr oups on the polar head group of the amphiphiles were oxidized to carbonyl groups, yielding a tri-acid species. Upon spreading the resultant molecules over an aqueous subphase, the deprotonation of the amphiphiles will yield tri-anionic species; this will allow for the formation of charge-balanced covalent networks with cationic transition metal ions from the subpha se. If the acidity of the aqueous subphase is kept low, the equilibrium of the reaction will li e to the right, enabling coordination between the amphiphiles and subphase ions, Figure 6-23.

PAGE 158

158 2R(CO2H)3+ 3MIIR(CO2H)3+ MIII2R(CO2)3MII 3+ 6H+R(CO2)MIII+ 3H+ 2R(CO2H)3+ 3MIIR(CO2H)3+ MIII2R(CO2)3MII 3+ 6H+R(CO2)MIII+ 3H+3 2R(CO2H)3+ 3MIIR(CO2H)3+ MIII2R(CO2)3MII 3+ 6H+R(CO2)MIII+ 3H+ 2R(CO2H)3+ 3MIIR(CO2H)3+ MIII2R(CO2)3MII 3+ 6H+R(CO2)MIII+ 3H+3 Figure 6-23. Reaction scheme of tricarboxy amphi philes with divalent (A) and trivalent (B) transition metal subphase ions during twodimensional network precipitation. Tris-carboxy Amphiphiles To achieve charge balance within the precipita ted networks, we chose to use the tri-anionic amphiphile 4,4,4-triccarboxytri phenylmethyl octadecyl ether to continue our study of the self-assembly of coordinate c ovalent networks. Amphiphile 4s ability to assemble into extended coordinate covalent networks by coordi nating subphase ions, and the pH dependence of these reactions, have was studied over aqueous transition metal subphases (MnBr2, CuCl2, Fe(SO4)2, and La(NO3)3); these observations were then comp ared to the amphiphiles behavior over an aqueous subphase of NaCl(aq). Changes in the monolayers behavior after being spread over transition metal subphases at higher pHs (increases in MMA, decrease in collapse pressure, the existence of rigid islands of condensed phase and the diminished intensities of the FT-IR spectaras carbonyl peaks) observed with Brewst er angle microscopy, surface pressure vs. MMA isotherms, and FT-IR spectroscopy, indicated ne twork formation through th e interaction between the coordinating functional groups of the amphiph iles and the subphase re actants. Upon raising the acidity of the subphase solutions, the separa tion in the behavior ove r all four subphases becomes indistinguishable, providing evidence to the pH dependence of the network formation. The dependence of network coordination topo logy upon the charge of the subphase ions was then investigated by reac tion of Langmuir monolayers of 4 over aqueous subphase of Fe(SO4)2, and La(NO3)3. Two possible extended networks formed from subphase ion

PAGE 159

159 coordination by amphiphile 4 are shown in the schematic below, Figure 6-24. The schematic illustrates the dependence of the networks c oordination topology and the amphiphiles mean molecular area upon subphase cation charge. O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O M3+M 3 + M3+M3+M3+ M3+ O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O M 2 + M 2 + M 2 + M 2 + M 2 + M 2+ A B Figure 6-24. Perpendicular schematic view of two-dimensional honeycomb metal-organic networks formed from the coordinati on of a Langmuir monolayer of 4,4,4tricarboxytriphenylmethyl octad ecyl either to A) divalent and B) trivalent transition metal ions from an aqueous subphase. The respective mean molecular areas of the amphiphiles in the two netw orks are ~120 and ~80 2. As in the monlayer systems discussed above, similar evidence supporting two-dimensional network formation was provi ded by BAM, ATR FT-IR, and surface pressure vs. MMA isotherms. Of greater importance are the differences in behavior observed for these two monolayer systems. When spread over Fe(SO4)2(aq) and La(NO3)3(aq) subphases, the mean molecular area of amphiphile 4 was observed to be 120 and 80 2/molecule, respectively. These experimental mean molecular area s accurately correspond to the id ealized networks pictured in Figure 6-24. Further analyses, FT-IR and AF M, of Langmuir-Blodgett films of the two monolayers indicated closer, more crystalline packing of the alkyl tails and thicker and smoother

PAGE 160

160 LB films when spread over the trivalent metal subphase, as opposed to the divalent transition metal subphase. These observations support th e idea that the amphiphiles, when over the La(NO3)3(aq) subphase as compared to a Fe(SO4)2(aq) subphase, are in closer proximity to each other, lending evidence that two discrete, hexago nal networks are forming, each pictured above. Each honeycomb network has its own coordina tion topology and amphiphile mean molecular area, and these topologies are dependent upon the charge of the subphase ions and coordination geometry of the reactants. Conclusions Reaction of the amphiphiles 4,4,4-tricyano tribenzylmethyl dodecyl ether and 4,4,4tricyanotribenzylmethyl octodecyl ether with monovalent and divalent transition metal ion subphase did not result in the formation of two-di mensional metal-organic networks, due to their inability to form charge balance st ructures. Spreading monolayers of 4,4,4-tricarboxytribenzyl methyl octodecyl ether over trivalen t and divalent transition metal ion subphases resulted in the self-assemb ly of two-dimensional networks By varying the pH of the metal ion subphases, the pH dependence of the network formation can also observed. The network coordination topology was controlled both by subphase cation charge and amphiphile coordination symmetry. These finding are reinforced with the aid of pressure vs. area isotherms, BAM images, ATR FT-IR, and AFM measurements a nd give us a better understanding of the topological control and self-asse mbly of two-dimensional networks. Such systems may lend themselves to the templating and fabrication of thin films of analog porous materials, which may be used for chemical sensing, filtration, and abso rption. To explicitly confirm the structure and topology of the individual two-dimensional networks GIXD experiments are needed.

PAGE 161

161 APPENDIX A TRANSMISSION ELECTRON MICROGRAPHS OF HETEROSTRUCTURED THIN FILMS 100 nm 100 nm 100 nm 100 nm Figure A-1. High resolution TEM images of (lef t) ~180 nm and (right) ~800 nm Prussian blue analog heterostructured sandwich fi lms, comprised of a layer of RbjCok[Fe(CN)6]lnH2O deposited between two layers of RbjNik[Cr(CN)6]lnH2O. The prior film was synthesized with alte rnating deposition cycles of 10 absorption cycles of each aforementioned analog, wh ile the latter had repeat cycles of 40 deposition cycles. The sequential adsorp tion technique used in the synthetic procedure is analogous to that used in the preparation of the binary transition metal Prussian blue analog thin films. A lack of contrast difference in the prior film indicates extensive mixing of the two Pr ussian blue analogs throughout the film, while the latter appears to have three disc rete layers with interfacial mixing at the interfaces of each in dividual layer. Figure A-2. High resolution TEM images of Pr ussian blue analog hete rostructured sandwich films, comprised of a layer of RbjCok[Fe(CN)6]lnH2O deposited between two layers of RbjNik[Cr(CN)6]lnH2O. The observed fracturing of the thin films is common in the microtomed samples due to the stress of section preparation and the interaction between the electron beam and sample TEM grids. Scale bars are 500 nm.

PAGE 162

162 APPENDIX B TRANSMISSION FT-IR SPECTRA OF PBA THIN FILMS 220021502100 Wavenumber (cm -1 ) Absorbance (arb. units) ZnCr CuCr NiCr CoCrPBA 0 (cm-1) W (cm1 ) A (I cm1 ) 2172.6 28.7 5.6 CoCr 2134.3 12.1 0.2 2175.0 22.1 3.6 NiCr 2135.0 42.6 1.3 2186.2 25.8 2.3 CuCr 2123.2 29.8 2.3 2186.6 31.6 2.4 ZnCr 2145.6 58.6 1.6 Figure B-1. FT-IR spectra (black) and respective fitting parameters for synthesized RbjMj[Cr(CN)6]lnH2O fast thin film compounds as a function of M. All fits (green) were performed using two Lorentzian lines (red). is defined as the peak stretching energy, W is defined as the peak half-max imum width, and A is defined as the area under the curve.

PAGE 163

163 220021502100 Absorbance (arb. units)Wavenumber (cm -1 ) ZnFe CuFe NiFe CoFePBA 0 (cm-1) W (cm1 ) A (I cm1 ) 2162.5 18.7 2.9 2124.6 31.0 3.44 CoFe 2108.0 26.4 2.4 2165.2 19.2 13.1 NiFe 2120.3 42.6 3.3 2173.9 19.8 2.7 CuFe 2100.7 11.2 0.3 2170.3 29.7 29.7 ZnFe 2097.0 10.9 0.5 Figure B-2. FT-IR spectra (black) and respective fitting parameters for synthesized RbjMj[Fe(CN)6]lnH2O fast thin film compounds as a function of M. All fits (green) were performed using two and th ree Lorentzian lines (red). is defined as the peak stretching energy, W is defined as the peak half-maximum width, and A is defined as the area under the curve.

PAGE 164

164 APPENDIX C NMR SPECTRA OF AMPHIPHILIC MOLECULES 12CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O 12CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 12CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O N N N O Figure C-1. Proton (1H) NMR of 4,4,4-tricyanotr ibenzylmethyl dodecyl ether (2) in CDCl3. 12CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O 12CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 12CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O Figure C-2. Carbon (13C) NMR of 4,4,4-tricyanotri benzylmethyl dodecyl ether (2) in CDCl3.

PAGE 165

165 18CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O 18CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 18CCNH 11 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O Figure C-3. Proton (1H) NMR of 4,4,4-tricyanotrib enzylmethyl octadecyl ether (3) in CDCl3. 18CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O 18CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 18CCNcarbon 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity N N N O Figure C-4. Carbon (13C) NMR of 4,4,4-tricyanotribe nzylmethyl octadecyl ether (3) in CDCl3.

PAGE 166

166 18CO2Hhydrogen 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity C O 2 H C O 2 H C O 2 H O 18CO2Hhydrogen 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 18CO2Hhydrogen 10 9 8 7 6 5 4 3 2 1 0 Chemical Shift (ppm) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity C O 2 H C O 2 H C O 2 H O Figure C-5. Proton (1H) NMR of 4,4,4-tricarboxytri benzylmethyl octadecyl ether (4) in CD3OD. 18cco2Hcarbon 220 200 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity C O 2 H C O 2 H C O 2 H O 18cco2Hcarbon 220 200 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity 18cco2Hcarbon 220 200 180 160 140 120 100 80 60 40 20 0 Chemical Shift (ppm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Normalized Intensity C O 2 H C O 2 H C O 2 H O Figure C-6. Carbon (13C) NMR of 4,4,4-tricarboxytri benzylmethyl octadecyl ether (4) in CD3OD.

PAGE 167

167 LIST OF REFERENCES (1) Davidson, D.; Welo, L.A.; J. Phys. Chem. 1928, 32, 1191. (2) Ito, A.; Suenaga, M.; Ono, K. J. Chem. Phys. 1968, 48, 3597. (3) BuserH. J.; Schwarzenbach D.; Petter, W.; Ludi. A. Inorg.Chem. 1977, 16, 2704. (4) Day, P.; Herren, F.; Ludi, A.; Gudel, H. U.; Hulliger, F.; Givord, D. Helv. Chim. Acta 1980, 63, 148. (5) Rosseinsky, .D. R.; Glidle, A. J. Electrochem. Soc. 2003, 150, 9. (6) Roger, M.J.; Reynolds, J. R. J. Mat. Chem. 2005, 15, 22. (7) Nigrovic V. Phys. Med. and bio. 1965, 10, 81. (8) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. Anal. Chem. 2000, 72, 1720. (9) Dawei, P.; Jinhua, C.; Lihua, N. ; Wenyan, T.; Shouzhuo, Y. Anal. Biochem. 2004, 324, 115. (10) Kaye, S. S.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 6506. (11) Itaya, K.; Shoji, N.; Uchida, I. J. Am. Chem. Soc. 1984, 106, 3423. (12) Gadet, V.; Mallah, T.; Castro, I.; Verdaguer, M.; Veillet, P. J. Am. Chem. Soc. 1992, 114, 9213. (13) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554. (14) Entley, W.; Girolami, G. Science 1995, 268, 397. (15) Ferlay, S.; Mallah, T.; Ouahs, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (16) Hatlevik, O.; Buschmann, W.; Zhang, J.; Manson, J.; Miller. J. Adv. Mater. 1999, 11, 914. (17) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (18) Sato, O.; Einaga, Y.; Iyoda, T.; Fu jishima, A.; Hashimoto, K. J. Phys. Chem. B 1997, 101, 3903. (19) Yoshizawa, K.; Mohri, F.; Nuspl G.; Yamabe, T. J. Phys. Chem. B 1998, 102, 5432. (20) Sato, O.; Einaga, Y.; Fujishima, A.; Hashimoto, K. Inorg. Chem. 1999, 38, 4405.

PAGE 168

168 (21) Yokoyama, T.; Kiguchi, M.;Ohta, T.; Sato, O.; Einaga, Y.; Hashimoto, K. Phys. Rev. B 1999, 60, 9340. (22) Bleuzen, A.; Lomenech, C.; Escax, V.; Villain, F.; Varret, F.; Cartier dit Moulin, C.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, 6648. (23) Cartier dit Moulin, C.; Villain, F.; Bleuzen, A. ; Arrio, M.-A.; Sainctavit, P.; Lomenech, C.; Escax, V.; Baudelet, F.; Dartyge, E.; Gallet, J.-J.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, 6653. (24) Goujon, A.; Roubeau, O.; Varret, F.; Dolbecq, A.; Bleuzen, A.; Verdaguer, M. Eur. Phys. J. B 2000, 14, 115. (25) Pejakovi D.; Manson, J.; Miller, J.; Epstein, A. Phys. Rev. Lett. 2000, 85, 1994. (26) Champion, G.; Escax, V.; Cartier dit Moulin, C.; Bluezen, A.; Villian, F.; Baudelet, F.; Dartyge, E.; Verdaguer, M. J. Am. Chem. Soc. 2001, 123, 12544. (27) Goujon, A.; Varret, F.; Escax, V.; Bleuzen, A.; Verdaguer, M. Polyhedron 2001, 20, 1347. (28) Escax, V.; Bleuzen, A.; Cartier dit Moulin, C.; Villain, F.; Goujon, A.; Varret, F.; Verdaguer, M.; J. Am. Chem. Soc. 2001, 123, 12536. (29) Kawamoto, T.; Asai, Y.; Abe, S.; Phys. Rev. Lett. 2001, 86, 348. (30) Shimamoto, N.; Ohkoshi, S.; Sato, O.; Hashimoto, K. Inorg. Chem. 2002, 41, 678. Reprinted with permission from Inorganic Chemistry, 2002. (31) Hanawa, M.; Moritomo, Y.; Kuriki, A.; Tateishi, J.; Kato, K.; Takata, M.; Sakata, M. J. Phys. Soc. Jpn. 2003, 72, 987. (32) Bleuzen, A.; Escax, V.; Iti, J.-P.; Mnsch, P.; Verdaguer, M. C. R. Chimie 2003, 6, 343. (33) Bleuzen, A.; Excax, V.; Ferrier, A.; Villain, F. ; Verdaguer, M.; Munsch, P.; Itie, J.-P. Angew. Chem. Int. Ed., 2004, 43, 3728. (34) Park, J.-H.; Frye, F. A.; Anderson, N. E.; Pajerowski, D. M.; Huh, Y.-D.; Talham, D. R.; Meisel, M. W. J. Mag. Magn. Mater. 2007, 310, 1458. (35) Li, D.; Clrac, R.; Roubeau, O.; Hart, E.; Mathonire, C.; Le Bris, R.; Holmes, S. M. J. Am. Chem. Soc. 2008, 130, 252. (36) Kosaka, W.; Nomura, K.; Hashimoto, K.; Ohkoshi, S.-i. J. Am. Chem. Soc. 2005, 127, 8590. (37) Park, J.-H.; imr, E.; Meisel, M. W.; Huh, Y. D.; Frye, F.; Lane, S.; Talham, D. R. App. Phys. Lett. 2004, 85, 3797.

PAGE 169

169 (38) Park, J.-H.; Frye, F.; Lane, S.; imr, E.; Huh, Y. D.; Talham, D. R.; Meisel, M. W. Polyhedron 2005, 24, 2355. (39) Yamamoto T.; Umemura Y.; Sato O.; Einaga, Y. J. Am. Chem. Soc. 2005, 127, 16065. (40) Frye, F. A.; Pajerowski, D. M.; Lane, S. M.; Anderson, N. E.; Park, J.-H.; Meisel, M. W.; Talham, D. R. Polyhedron 2007, 26, 2281. (41) Frye, F. A.; Pajerowski, D. M.; Park, J.-H.; Meisel, M. W.; Talham, D. R. Chem. Mater. 2008, 20, 5706. (42) Ohkoshi, S.-i.; Yorozu, S.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. App. Phys. Lett. 1997, 70, 1040. (43) Ohkoshi, S.-i.; Hashimoto, K. J. Am. Chem. Soc. 1999, 121, 10591. (44) Cafun, J.-D.; Londinire, L.; Rivire, E.; Bleuzen, A. Inorg. Chim. Acta 2008, 361, 3555. (45) Neff, V. J. Electrochem. Soc. 1978, 125, 886. (46) Itaya, K.; Ataka, T.; Toshima, S. J. Am. Chem. Soc, 1982, 104, 4767. (47) Buschmann, W.; Paulson, S.; Wynn, C.; Girt u, M.; Epstien, A.; White, A.; Miller, J. Chem. Mater. 1988, 10, 1386. (48) Ohkoshi, S.; Einaga, Y.; Fujishima, A.; Hashimoto, K. J. Electroanal. Chem. 1999, 473, 245. (49) Tacconi, N.; Rajeshwar, K. Chem. Mater. 2003, 15, 3046. (50) Toshima, N.; Liu, K.; Kaneko, M. Chem. Lett. 1990, 485. (51) Ravaine, S.; Lafuente, C.; Mingotaud, C. Langmuir 1998, 14, 6347. (52) Mingotaud, C.; Lafuente, C.; Amiell, J.; Delhaes, P. Langmuir 1999, 15, 289. (53) Romualdo-Torres, G.; Agricole, B.; Mingot aud, C.; Ravaine, S.; Delhaes, P. Langmuir 2003, 19, 4688. (54) T. Yamamoto, Y. Umemura, Sato, O.; Einaga, Y. Chem. Lett. 2004, 33, 500. (55) T. Yamamoto, Y. Umemura, Sato, O.; Einaga, Y. J. Am. Chem. Soc. 2005, 127, 16065. (56) Culp, J.; Park, J-H,; Frye, F.; Huh, Y-D. Meisel, M.; Talham, D. Coordination Chem. Rev. 2005, 249, 2642. Reprinted with permission fr om Coordination Chemistry Reviews, 2005. (57) Millward, R.; Madden, C.; Sutherl, I.; Mortimer, R.; Fletcher, S.; Marken, F. Chem. Commun. 2001, 1994.

PAGE 170

170 (58) Pyrasch, M.; Tieke, B.; Langmuir 117 2001, 117, 7706. (59) Culp, J.; Park, J.-H.; I. Benitez, Huh, Y.-D.; Meisel, M.; Talham, D. Chem. Matter. 2003, 15, 3431. (60) Park, J.-H.; Huh, Y.-D.; imr, E.; Gamble, S.; Talham, D.; Meisel, M.; J. Magn. Magn. Mater. 2004, 272-276, 1116. (61) Jin, W.; Toutianoush, A.; Pyrasch, M.; Schnepf, J.; Gottschalk, H.; Rammensee, W.; Tieke, B. J. Phys. Chem. B 2003, 107, 12062. (62) Vaucher, S,; Li, M.; Mann, S. Angew. Chem. Int. Ed. Engl. 2000, 39, 1793. (63) Ng, C. W.; Ding, J.; Chow, P. Y.; Gan, L. M.; Quek, C. H. J. Appl. Phys. 2000, 87, 6049. (64) Uemura, T.; Kitagawa, S. J. Am.Chem. Soc. 2003, 125, 7814. (65) Uemura, T.; Ohba, M.; Kitagawa, S. Inorg. Chem. 2004, 43, 7339 (66) Vaucher, S.; Fielden, J.; Li, M.; Dujardin, E.; Mann, S., Nano. Lett. 2002, 2, 225. (67) Yamada, M.; Arai, M.; Kurihara, M.; Sakamoto, M.; Miyake, M., J. Am. Chem. Soc. 2004, 126, 9482. (68) Moore, J. G.; Lochner, E. J.; Ramsey, C.; Dalal, N. S.; Stiegman, A. E. Angew. Chem. 2003, 115, 2847. (69) Catala, L.; Mathonire. C.; Gloter, A.; Stephan, O.; Gacoin, T.; Boilot, J.-P.; Mallah, T. Chem. Commun. 2005, 746. (70) Kahn, O. Molecular Magnetism. VCH Publishers: New York, 1993. (71) Coronado, E.; Delhaes, P.; Gatteschi, D.; Miller, J. Molecular Magnetism: From Molecular Assemblies to the DevicesI. Kluwer Academic Publis hers: Dordrecht, 1996. (72) Culp, J; Park, J.-H.; Stratakis, D.; Meisel, M.; Talham, D. J. Am. Chem. Soc. 2002, 124, 10083. Reprinted with permission from Journa l of the American Chemical Society, 2002 (73) Permission for the reuse of this figure was obtained by the publishing journal. (74) Park, J.-H. Ph. D. Dissertation, University of Florida, Gainesville, 2006. (75) Frye, F. A. Ph. D. Dissertation, University of Florida, Gainesville, 2007. (76) Juszczyk, S.; Johansson, C.; Hanson, M.; Ratuszna, A; Ma ecki, G. J. Phys: Condens. Matter 1994, 6, 5697. (77) Rasband, W. S. ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http://rsb.info.nih.gov/ij/, 1997-2008.

PAGE 171

171 (78) Room temperature x-ray diffraction experime nts were performed and subsequent data was analyzed by Daniel Pajerowski, Department of Physics, University of Florida. (79) Toby, B. H. J. Appl. Cryst. 2001, 34, 210-213. (80) Larson, A. C.; Von Dreele, R. B. Los Alamos National Laboratory Report LAUR 2000, 86-748. (81) Magnetic measurements and mean field calculat ions were performed and subsequent data was analyzed by Daniel Pajerowski, Department of Physics, University of Florida. (82) Ng, C. W.; Ding, J.; Gan, L. M. J. Solid State Chem. 2001, 156, 400. (83) Sato, O. J. Solid State Electrochem. 2007, 11, 773. (84) Due to the known problems of detecting light atoms, the EDS data were not exclusively used to determine the sodium content. (85) Pajerowski, D. M.; Frye, F. A.; Talham, D. R.; Meisel, M. W. New J. Phys. 2007, 9, 222. (86) Frye, F. A.; Pajerowski, D. M.; Anderson, N. E.; Long, J.; Park, J.-H.; Meisel, M. W., Talham, D. R. Polyhedron 2007, 26, 2273. (87) Pejakovi D. A.; Manson, J. L.; Miller, J. S.; Epstein, A. J. J. Appl Phys. 2000, 87, 6028. (88) Pejakovi D. A.; Manson, J. L.; Miller, J. S.; Epstein, A. J. Synth. Met. 2001, 122, 529. (89) Pejakovi D. A.; Manson, J. L.; Kitamura, C. ; Miller, J. S.; Epstein, A. J. Polyhedron 2001, 20, 1435. (90) Pejakovi D. A.; Kitamura, C.; Miller, J. S.; Epstein, A. J. Mol. Cryst. Liq. Cryst. 2002, 374, 289. (91) Phu, P. K.; Giang, T. N.; Minh, N. V. Communications in Physics, 2008, 18, 43. (92) Ohkoshi, S.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Phys. Rev. B 1997, 56, 11642. (93) Ohkoshi, S.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Inorg. Chem. 1997, 36, 268. (94) Widmann, A.; Kahlert, H.; Petrovic-Prelevic, I. ; Wulff, H.;, Yakhmi, J. V.; Bagkar, N.;, Scholz, F. Inorg. Chem. 2002, 41, 5706. (95) Bagkar, N.; Widmann, A.; Kahlert, H.; Ravikum ar, G.; Yusuf, S. M.; Scholz, F.; Yakhmi, J. V. Philos. Mag. 2005, 85, 3659. (96) Widmann, A.; Kahlert, H.; Wulff, H.; Scholz, F. J. Solid State Electrochem. 2005, 9, 380. (97) Schwudke, D.; Stsser, R.; Scholz, F. Electrochem. Comm. 2000, 2, 301.

PAGE 172

172 (98) Sorai, M.; Ensling, J.; G tlich, P. Chem. Phys. 1976, 18, 199. (99) Spiering H.; Meissner E.; Kppen H.; M ller E. W.; G tlich P. Chem. Phys. 1982, 68, 65. (100) Lummen, T. T. A.; Gengler, R. Y. N.; Rudolf, P., Lusitani, F.; Vertelman, E. J. M.; van Koningsbruggen, P. J.; Knupfer, M.; Molodt sova, O.; Pireaux, J.-J.; van Loosdrecht, P. H. M. J. Phys. Chem. C 2008, 112, 14158. (101) Culp, J.; Park, J.-H.; Meisel, M.; Talham, D. Inorg. Chem. 2003, 42, 2842. (102) Culp, J.; Park, J.-H.; Benitez, I.; Meisel, M.; Talham, D. Polyhedron. 2003, 22, 2125. (103) Agusti, G.; Cobo, S.; Gaspar, G.; Moussa, N.; Szilagyi, P; Palfi, V; Vieu, C; Munoz, C.; Real, J.; Bousseksou, A. Chem. Matter. 2008, 20, 6721. (104) Bagkar, N.; Choudhury, S.; Kim, K.-H.; C howdhury, P.; Lee, S.-I.; Yakhmi, J. Thin Film Solids. 2006, 513, 325. (105) Sato, O.; Hayami, S.; Einaga, Y.; Gu, Z.-Z. Bull. Chem. Soc. Jap. 2003, 76, 443. (106) Torres, G.; Agricole, B.; Delhaes, P.; Mingotaud, C. Chem. Matter. 2002, 14, 4012. (107) Yamamoto, T.; Umemura, Y. ; Sato, O.; Einaga, Y. Chem. Matter. 2004, 16, 1195. (108) Yamamoto, T.; Umemura, Y. ; Sato, O.; Einaga, Y. Chem. Letters.. 2004, 23, 500. (109) Gigelow, J. Inorg. Syn. 1946, 11, 203. (110) Yun, H.; Yao-Dong, D.; Hong-Bo, H.; Jun, L.; Yaun-Fu, H. Chinese Physics. 2004, 13, 746. (111) Rodriguez-Hernandez, J.; Reguera, E.; Lima, E.; Balmaseda, J.; Martinez-Garcia, R.; Yee-Madeira, H. J. Phys.Chem. Solids. 2007, 68, 1630. (112) Kosaka, W.; Tozawa, M.; Hashimoto, K.; Ohkoshi, S.-i. Inorg. Chem. Comm. 2006, 9, 920. (113) Paraschiv, C.; Andruh, M.; Sutter, J.-P. Inorg. Chem. Acta. 2003, 351, 385. (114) Zentkova, M.; Mihalik, M.; Kovac, J.; Ze ntka, A.; Mitroova, Z; Lukacova, M.; Kavecansky, V.; Kiss, L. Phys. Stat. Sol. 2006, 243, 272. (115) Ohkoshi, S.-i.; Hashimoto, K. Chem. Phys. Lett. 1999, 314, 210. (116) Nishino, M.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 1998, 297, 51. (117) Gersten, J.; Smith, F. The Physics and Chemistry of Materials, John Wiley & Sons, Inc.: New York, 2001.

PAGE 173

173 (118) Kahn, O. Molecular Magentism, VCH Publishers Inc.: New York, 1993. (119) O. Kahn, E. Codjovi, Y. Garcia, P. J. van Koningsbruggen, R. Lapouyade and L. Sommier, ACS Symp. Series, 1996, 20, 298. (120) Y. Garcia, P. J. van Koningsbruggen, R. La pouyade, L. Fourns, L. Rabardel, O. Kahn, V. Ksenofontov, G. Levchenko and P. Gtlich, Chem. Mater., 1998, 10, 2426. (121) Gutlich, P.; Garcia, Y.; Goodwin, H. Chem. Soc. Rev. 2009, 29, 419. (122) Niel, V.; Martinez-Agudo, J.; M unoz, M.; Gaspar, A.; Real, J. Inorg. Chem. 2001, 40, 3838. (123) Halcrow, M. Chem. Soc. Rev. 2008, 37, 278. (124) Ohkoshi, S.-i.; Ikeda, S.; Hozumi, T.; Kashiwagi, T.; Hashimoto, K. J. Am. Chem. Soc. 2006, 128, 5320. (125) Vertelman, E.; Maccallini, E.; Dimitris, G.; Rudolf, P.; Bakas, T.; Luzon, J.; Broer, R.; Pugzlys, A.; Lummen, T.; Loosdr echt, P.; Koningsbruggen, P. Chem. Matter. 2006, 18, 1951. (126) Coronado, E.; Galan-Mascaros, J.; Monrrabal-Capilla, M; Garcia-Martinez, J; PardoIbanez, P. Adv. Mater. 2007, 19, 1359. (127) Forestier, T.; Mornet, S.; Daro, N.; Nishihara, T.; Mouri, S.-i.; Tanaka, K.; Fouche, O.; Freysz, E.; Letard, J.-F. Chem. Commun. 2008, 4327. (128) Volatron, F.; Catala, L.; Riviere, E.; Gloter, A.; Stephan, O.; Mallah, T. Inorg. Chem. 2008, 47, 6584. (129) Fornasieri, G.; Bleuzen, A. Angew. Chem. Int. Ed. 2008, 47, 7750. (130) Kawamoto, T.; Abe, S. Chem. Commun. 2005, 3933. (131) Kawamoto, T.; Abe, S. J. of Phys: Conference Series. 2005, 21, 3933. (132) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, (7), 18231848. (133) Holliday, B. J.; Mirkin, C. A. Angew. Chem.-Int. Ed. 2001, 40, (11), 2022-2043. (134) Lehn, J.; Atwood, J.; Davies, J.; McNicol, D.; Vogtle, V. Comprehensive Supramolecular Chemistry, Pergamon Press: Oxford, 1996. (135) Phip, D.; Stoddart, J. Angew. Chem.-Int. Edit. Engl. 1996, 35, 1155.

PAGE 174

174 (136) Eddaoudi, M.; Moler, D. B.; Li, H. L.; Chen, B. L.; Reineke, T. M.; O'Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, (4), 319-330. Adapted in part from Accounts of Chemical Research, 2001. (137) Jensen, P.; Price, D. J.; Batten, S. R.; Moubaraki, B.; Murray, K. S. Chem.-A Eur. J. 2000, 6, (17), 3186-3195. (138) Lang, J. P.; Xu, Q. T.; Yuan, R. X.; Abrahams, B. F. Angew. Chem.-Int. Ed. 2004, 43, (36), 4741-4745. (139) Yaghi, O. M.; Li, H. L.; Davis, C.; Richardson, D.; Groy, T. L. Acc. Chem. Res. 1998, 31, (8), 474-484. (140) Kim, J.; Chen, B. L.; Reineke, T. M.; Li, H. L.; Eddaoudi, M.; Moler, D. B.; O'Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2001, 123, (34), 8239-8247. (141) Ockwig, N. W.; Delgado-Friedrichs, O.; O'Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2005, 38, (3), 176-182. (142) Rosi, N. L.; Eddaoudi, M.; Kim, J.; O'Keeffe, M.; Yaghi, O. M. Crystengcomm 2002, 401-404. (143) Culp, J. T. Ph. D. Dissertation, University of Florida, Gainesville, 2002. (144) Hill, R. J.; Long, D. L.; Champness, N. R.; Hubberstey, P.; Schroder, M. Acc. Chem. Res. 2005, 38, (4), 335-348. (145) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848-1906. (146) Isz, S.; Weissbuch, I.; Kjaer, K.; Bouwan, W.; Als-Nielsen, J. Palacin, S.; RuaudelTeixier, A.; Leiserowitz, L.; Lahav, M. Chem. Eur. J. 1997, 3, 930. (147) Porteu, F.; Palacin, S. Nanostruct. Based Mol. Mater. 1992, 209. (148) Carniato, S.; Roulet, H.; Dufour, G.; Palaci n, S.; Barraud, A.; Millie, P.; Nenner, I. J. Phys. Chem. 1992, 96, 7072. (149) Tran-Thi, T.; Lipskier, J.; Simoes, M.; Palacin, S. Thin Solid Films. 1992, 210, 150. (150) Milic, T.; Garno, J.; Batteas, J.; Smeureanu, G.; Drain, C. Langmuir, 2004, 20, 3974. (151) Milic, T.; Chi, N.; Yablon, D.; Flynn, G.; Batteas, J.; Drain, C. Angew. Chem. Int. Ed. 2002, 41, 2117. (152) Sipra, C.; Bagkar, N.; Dey, G.; Subramanian, H.; Yahkmi, J. Langmuir. 2002, 18, 7409. Reprinted with permission from Langmuir, 2002 (153) Byrd, H.; Whipps, S.; Pike, J.; Jingfei, M.; Nagler, S.; Talham, D. J. Am. Chem. Soc. 1994, 116, 295.

PAGE 175

175 (154) Byrd, H.; Pike, J.; Talham, D. J. Am. Chem. Soc. 1994, 116, 7903. (155) Petruska, M.; Fanucci, G.; Talham, D. Chem. Mater. 1998, 10, 177. (156) Fanuci, G.; Talham, D. Langmuir, 1999, 15, 3289. (157) Fanucci, G.; Backov, R.; Fu, R.; Talham, D. Langmuir, 2001, 17, 1660. (158) Culp, J.; Morgan, A.; Meisel, M.; Talham, D. Mol. Cryst. Liq. Cryst. 2002, 376, 383. (159) Talham, D. R. Chem. Rev. 2004, 104, (11), 5479-5501. (160) Meunier, J. Colloids And Surfaces A-Physicoche mical And Engineering Aspects 2000, 171, (1-3), 33-40. (161) Ulman, A. An Introduction to Ultrathin Organic F ilms: From Langmuir-Blodgett to SelfAssembly, Academic Press: Boston, 1991. (162) Roberts, C. G. Langmuir-Blodgett Films, Plenum Press: New York, 1990. (163) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Film Sol. 1983, 76, 67. (164) Hill, R.; Long, D.; Champness, N.; Hubberstey, P.; Chroder, M. Acc. Chem. Res. 2005, 38, 335. (165) Ferlay, S.; Koenig, S.; Hosseini, M.; Pansanel, J.; Cian, A.; Kyritsakas, N. Chem. Commun. 2002, 218. Reprinted with permission fr om Chemical Communications and the Royal Chemical Society, 2002. DOI: 10.1039/b109444k (166) Fan, J.; Sui, B.; Okamura, T.; Sun, W. Y.; Tang, W. X.; Ueyama, N. J. Chem. Soc.-Dalt. Trans. 2002, (20), 3868-3873. (167) Wan, S. Y.; Li, Y. Z.; Okamura, T.; Fan, J. A.; Sun, W. Y.; Ueyama, N. Eur.J. Inorg. Chem. 2003, (20), 3783-3789. (168) Zhao, W.; Fan, J.; Okamura, T.; Sun, W. Y.; Ueyama, N. N. J. Chem. 2004, 28, (9), 1142-1150. (169) Zhao, W.; Fan, H.; Okamura, T.; Sun, W. Y.; Ueyama, N. J. Sol. St. Chem. 2004, 177, (7), 2358-2365. (170) Choi, H. J.; Suh, M. P. J. Am. Chem. Soc. 1998, 120, (41), 10622-10628. (171) Mikami, S.; Sugiura, K.; Miller, J. S.; Sakata, Y. Chem. Let. 1999, (5), 413-414. (172) Georgiev, I. B., C. L.; Bosch, E. J. Supra. Chem. 2001, (1), 153-155. (173) Dunnebacke, D.; Neumann, W. P.; Penenory, A.; Stewen, U. Chemische Berichte 1989, 122, (3), 533-535.

PAGE 176

176 (174) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, (14), 2923-2925. (175) March, J.; Smith, M., March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 5th ed.; John Wiley &Sons Inc. : New York, 2001; 'Vol.' p 477. (176) Laliberte, D.; Maris, T.; Wuest, J. D. J. Org. Chem. 2004, 69, (6), 1776-1787. (177) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds. John Wiley & Sons, Inc.: NewYork, 1963. (178) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (179) Wood, K. A.; Snyder, R. G.; Strauss, H. L. J. Chem. Phys. 1989, 91, 5255. (180) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.

PAGE 177

177 BIOGRAPHICAL SKETCH Justin Edward Gardner had the honor of bei ng born into the arm s of Doug and Martha Gardner on a cold December day in Memphis, Tenne ssee. Raised by his parents in the southeast, the influence of his family, namely his grandpa rents from the Mississipp i delta and small towns of the mid-west, was never far from reach. His education started at Crum p elementary and Kirby middle schools and further progressed, under the watc hful eye of the brothe rs of the Catholic church, at Christian Brothers High School. He later pursued a liberal arts education at the picturesque University of the South in Sewan ee, TN, a quiet community in the great Smokey Mountains. During four foggy years on The Mount ain, his interests in chemistry drew him into countless coffee-break convers ations with Dr. Edward Kirven and led him to investigate metallomesogens in the research group of Dr. Robert Bachman. After graduating from Sewanee, his educational journey, in both chemistry and li fe, led him further south to The Swamp. For the past half decade, he conducted research on the behavior of two-dimensional self-assembled networks and the synthetic modification of the magnetic properties of Prussian blue analogs and received his Ph. D. in Inorganic Chemistry. Though he left the University of Florida with a rich knowledge of chemistry and greater understanding of lifes cha llenges, his continuous growth and education will now proceed in the Pacific Northw est, while launching a career with Intel Co.