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Growth and Photomagnetic Properties of Nano and Mesoscale Coordination Polymer Heterostructures

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Title:
Growth and Photomagnetic Properties of Nano and Mesoscale Coordination Polymer Heterostructures
Creator:
Gros, Corey R
Place of Publication:
[Gainesville, Fla.]
Florida
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University of Florida
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english
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1 online resource (205 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
TALHAM,DANIEL R
Committee Co-Chair:
CHRISTOU,GEORGE
Committee Members:
VEIGE,ADAM S
WEI,WEI
MEISEL,MARK W
Graduation Date:
12/18/2015

Subjects

Subjects / Keywords:
Coordination polymers ( jstor )
Eggshells ( jstor )
Ions ( jstor )
Irradiation ( jstor )
Ligands ( jstor )
Magnetic fields ( jstor )
Magnetism ( jstor )
Magnetization ( jstor )
Magnets ( jstor )
Platelets ( jstor )
Chemistry -- Dissertations, Academic -- UF
heterostructures -- photomagnetism
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bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Chemistry thesis, Ph.D.

Notes

Abstract:
Photomagnetic coordination polymer materials provide an intriguing route toward spin control and manipulation in potential light-assisted magnetic recording and spintronic applications, however challenges such as short lifetimes and low temperature restrictions limit the viability of such materials in devices. Recent work explores a new mechanism of light-switchable magnetism by developing heterostructures of Prussian blue analogue (PBA) coordination polymers, resulting in synergistic photomagnetic behavior beyond the sum of each component's light-induced response. Specifically, irradiation and resulting structural changes of a light-sensitive component induces strain on a mechanically-coupled magnetic component, thus altering its magnetization. The work herein demonstrates the general nature of this light-induced strain mechanism by exploring non-PBA photoactuators which differ both in structure and morphology to the magnetic PBA components used previously. This work focuses on the coupling of two- and three-dimensional photoactive Fe(II) spin crossover networks with the ferromagnetic nickel hexacyanochromate (NiCr) PBA in layered thin film and particle architectures. Controlled fabrication of the film components using solution deposition methods is explored to achieve targeted rough and smooth surface topographies. Nano- and mesoscale thin film heterostructures with varied surface topographies are then developed to evaluate the role of film thickness and interfacial roughness on the photomagnetic behavior of the film. Two-dimensional light-sensitive networks are synthesized as nano- and microscale platelets for photoactive components of heterostructure particles. Coupling the two-dimensional light-sensitive components and three-dimensional cubic PBA network exploits large structural differences to gain insight into the extent of structural coupling with greatly reduced epitaxial growth at the interface between them. The observance of structural and photomagnetic activity despite minimal epitaxy demonstrates how this general light-induced strain mechanism may be utilized in a variety of heterostructures by coupling strain-sensitive and light-sensitive materials. ( en )
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In the series University of Florida Digital Collections.
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Includes vita.
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Includes bibliographical references.
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Description based on online resource; title from PDF title page.
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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.
Thesis:
Thesis (Ph.D.)--University of Florida, 2015.
Local:
Adviser: TALHAM,DANIEL R.
Local:
Co-adviser: CHRISTOU,GEORGE.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2016-12-31
Statement of Responsibility:
by Corey R Gros.

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Applicable rights reserved.
Embargo Date:
12/31/2016
Classification:
LD1780 2015 ( lcc )

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GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROSTRUCTURES By COREY RACHELLE GROS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2015

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© 2015 Corey Rachelle Gros

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To safety, to tea time, to quarters, and to betting Volkswagens

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4 ACKNOWLEDGMENTS It was o nce said that the mind is not a vessel to be filled but a fire to be kindled. My journey through graduate school has done nothing short of this , fuelling a passion for exploration and the pursuit of knowledge. It is with the help and guidance of many peopl e that I complete this fulfilling experience, and to them I am incredibly grateful. First and forem ost I thank my advisor Dr. Daniel Talham, whose guidance has been instrumental in my development as a scientist and as a person. With his mentorship I was ab le to flouris h; allowing me the freedom to make mistakes and the stubbornness to fix them. His patience, wisdom, and clarity of thought have been an inspiration; qualities I aspire to as I take my first steps into the professional world. I am also grateful to Eric Lambers Science department , with whom many enlightening discussions have developed my knowledge beyond the bench top. My graduate school career would not have been nearly as enjoyable or successful without the insight , camaraderi e , and support of the Talham group members. In particular I thank Emi ly Pollard, Matt Andrus, Olivia Risset, Carissa Li, Yichen Li, Hao Liu, Divya Rajan, Akhil Ahir, Caue Ferreira, Alli Garnsey, Ashley Felts , Carolyn Averback , James Sternberg, Steven LoCic ero, and John Cain who in many way s contributed to my work or my sanity. I sincerely thank the entire group wh o contributed to cultivating an atmosphere which was both educational and fun. T here are no other folks with whom have an impromptu dan ce party or survive a lab rainstorm . Witho ut the help of my collaborators this work would not have been complete, and for that I thank the physicists who have given their time to help my work become successful. I thank Dr. Mark Meisel for his enthusiasm and support throughout the project, as well as his students Marcus Peprah , Pedro Quintero , and post doctoral scientist Dr. Tatiana Brinzari for the

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5 magnetometry measurements and setup of the variable temperature FTIR system, in addition to contributions from John Cain and Ashley Felts from the Talham group . I thank Dr. Mariana Sendova and Dr. Brian Hosterman at the New College of Florida for the opportunity to become familiar with new techniques outside of UF. I thank Dr. Khalil Abboud, his teaching assistants Matt Andrus, Ashley Felts, and Annaliese Thuijs , and the X ray Crystallography lab for assistance with variable temperature powder X ray diffraction measurements. Additionally I thank members of the chemistry department specifically A ndrew Mowson, Annaliese Thuijs, Adeline Fournet, Kylie Mitchell, and Gianna DiFrancesco whose insight ful discussions continue to aid my understanding of science. I thank Dr. Daniel Talham, Dr. Mark Meisel, Dr. George Christou, Dr. Adam Veige, and Dr. Da vid Wei for generously d onating their time to serve on my committee . Lastly, I thank my family and friends for their unwavering support as I complete my time in Florida. I am indebted to my family in Canada who have supported me from across the continent a s I spend the years who have kept me sane , and to those in Gainesville both inside and outside of the chemistry department who have made this a ride worth taking.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ............... 4 LIST OF TABLES ................................ ................................ ................................ ........................... 9 LIST OF FIGURES ................................ ................................ ................................ ....................... 10 LIST OF ABBREVIATIONS ................................ ................................ ................................ ........ 16 ABSTRACT ................................ ................................ ................................ ................................ ... 20 CHAPTER 1 OVERVIEW ................................ ................................ ................................ ........................... 22 2 LIGHT INDUCED MAGNETIZATION CHANGES IN A COORDINATION POLYMER HETE ROSTRUCTURE OF A PRUSSIAN BLUE ANALOGUE AND A HOFMANN LIKE Fe(II) SPIN CROSSOVER COMPOUND ................................ ............. 38 Introduction ................................ ................................ ................................ ............................. 38 Experimental Method s ................................ ................................ ................................ ............ 40 Film Deposition ................................ ................................ ................................ ............... 40 Characterization ................................ ................................ ................................ ............... 4 1 Heterostructure Deve lopment ................................ ................................ ................................ . 43 Photomagnetic Behavior of Single Phase and Heterostructure Films ................................ .... 43 Conclusion ................................ ................................ ................................ .............................. 47 3 NICKEL HEXACYANOCHROMATE FILM DEPOSITION STUDIES: PROBING THE INFLUENCE OF INTERFACE ROUGHNESS ON THE PHOTOMAGNETIC PROPERTIES OF NiCr PBA BASED HETEROSTRUCTURES ................................ ........ 54 Introduction ................................ ................................ ................................ ............................. 54 Experimental Methods ................................ ................................ ................................ ............ 56 Film Deposition ................................ ................................ ................................ ............... 56 C haracterization ................................ ................................ ................................ ............... 59 Development of NiCr PBA Thin Films ................................ ................................ .................. 61 ................................ ................................ .............................. 61 ........ 63 Substrate functionalization ................................ ................................ ....................... 64 Effect of deposition solvent on film growth ................................ ............................. 65 Effect of precursor concentration on film growth ................................ .................... 66 Characterization and Photomagnetic Properties of Fe(azpy)[Pt(CN) 4 ]/NiCr PBA Heterostructure Films ................................ ................................ ................................ .......... 67 ................................ ... 68

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7 PBA deposition ................................ ................................ ................................ ..................... 69 PBA deposition ............... 71 PBA deposition ................................ ................................ ................................ ..................... 72 Conclusion ................................ ................................ ................................ .............................. 74 4 SYNERGISTIC PHOTOMAGNETIC EFFECTS IN A HETEROSTRUCTURE OF HOFMANN LIKE [Fe(4 PHENYLPYRIDINE) 2 [Ni(CN) 4 ]·0.5H 2 O] AND K 0.4 Ni[Cr(CN) 6 ] 0.8 · n H 2 O COORDINATION POLYMERS ................................ .................. 98 Introduction ................................ ................................ ................................ ............................. 98 Experimental Methods ................................ ................................ ................................ .......... 100 Synthesis ................................ ................................ ................................ ........................ 100 Fe(ph py) 2 [Ni(CN) 4 ] seed particle synthesis (4 1) ................................ .................. 100 Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure particle synthesis (4 2) ............... 100 Charact erization ................................ ................................ ................................ ............. 101 Results ................................ ................................ ................................ ................................ ... 104 Synthesis and Characterization of Single Phase and Heterostructure Particles ............ 104 Hofmann Phase Spin Transition in 4 1 and 4 2 ................................ ............................ 106 Magnetic Measurements ................................ ................................ ................................ 108 Discussion ................................ ................................ ................................ ............................. 109 Photoeffects in Single Phase and Heterostructure Particles ................................ .......... 109 Structural and Chemical Changes in the Heterostructure ................................ .............. 112 Conclusion ................................ ................................ ................................ ............................ 114 5 EXPANDING THE TWO DIMENSIONAL HOFMANN/NiCr PBA HETEROSTRUCTURE FAMILY: INVESTIGATING NiCr PBA GROWTH ON MESO AN D MICROSCALE Fe(X Py) 2 [Ni(CN) 4 ] {X = H, 3 Cl, phpy} PLATELETS ................................ ................................ ................................ ........................ 128 Introduction ................................ ................................ ................................ ........................... 128 Experimental Methods ................................ ................................ ................................ .......... 130 Synthesis ................................ ................................ ................................ ........................ 130 Fe(X py) 2 [Ni(CN) 4 ] particles {X = H, 3 Clpy} (5 1a 5 6a) ............................... 130 Fe( phpy) 2 [Ni(CN) 4 ] particle synthesis (5 7a) ................................ ........................ 131 Fe(X py) 2 [Ni(CN) 4 ]/NiCr PBA {X = H, 3 Clpy, phpy} heterostructures (5 1b 5 7b) ................................ ................................ ................................ ................. 132 Characterization ................................ ................................ ................................ ............. 132 Particle Synthesis and Morphology ................................ ................................ ...................... 134 Fe(X py) 2 [Ni(CN) 4 ] {X = H, 3 Cl} particles ................................ ................................ 134 Fe(phpy) 2 [Ni(CN) 4 ] particles ................................ ................................ ........................ 136 Fe(X py) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure particles ................................ ............. 138 Thermal and Light Induced Spin Transitions ................................ ................................ ....... 139 Conclusion ................................ ................................ ................................ ............................ 140 6 CONCLUDING REMARKS ................................ ................................ ................................ 158

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8 AP PENDIX A DEPOSITION AND CHARACTERIZATION OF CrCr PBA FILMS AND CrCr PBA/Fe(azpy)[Pt(CN) 4 ] HETEROSTRUCTURES ................................ .................... 161 B EFFECT OF LIGAND CONC ENTRATION IN SYNTHESIS OF HOFMANN LIKE Fe(II) SPIN CROSSOVER NETWORKS ................................ ................................ ............ 168 C FTIR CHARACTERIZATION METHODS OF NANO SIZED SPIN CROSSOVER PARTICLES ................................ ................................ ................................ ......................... 174 D SUBSTRATE COMMENTS REGARDING GROWTH AND CHARACTERIZATION OF COORDINATION POLYMER FILMS ................................ ................................ ......... 179 E PERMISSION TO REPRODUCE COPYRIGHTED MATERIAL ................................ ..... 185 LIST OF REFERENCES ................................ ................................ ................................ ............. 196 BIOGRAPHICAL SKETCH ................................ ................................ ................................ ....... 205

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9 LIST OF TABLES Table page 2 1 Warming and cooling rates for magnetometry studies. ................................ ..................... 42 3 1 ................................ .............. 57 3 2 ................................ ............ 58 3 3 Select sample characteristics of Hofmann/NiCr PBA heterostructure samples. ............... 58 3 4 Warming and cooling rates for magnetometry studies. ................................ ..................... 60 4 1 Cooling and warming rates used in magnetometry measurements. ................................ . 104 4 2 Metal ratio analysis for 4 1 and 4 2 obtained by bulk Energy Dispersive X ray Spectroscopy (EDS) ................................ ................................ ................................ ......... 115 4 3 Room temperature FTIR fitting parameters for s ingle phase and heterostructure . ......... 118 5 1 Synthetic parameters and co nditions for single phase particles . ................................ ..... 131 5 2 Synthetic paramet ers for heterostructure syntheses. ................................ ........................ 132 B 1 Select precursor concentrations and volumes used in syntheses of Hofmann particles. . 170 D 1 Atomic percentages of M454 and M535 surface coatings determined from XPS survey spectra. ................................ ................................ ................................ .................. 183

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10 LIST OF FIGURES Figure page 1 1 Scheme of the PBA cubic structure. ................................ ................................ .................. 32 1 2 CoFe NiCr PBA core shell hetero structures. ................................ ................................ .... 33 1 3 Photomagnetism in NiCr CoFe NiCr PBA multilayer heteros tructu re film. .................... 33 1 4 PXRD peak broadening observed in shell PBA of photomagnetic PBA heterostructure . ................................ ................................ ................................ ................... 34 1 5 Schematic energy dia gram illustrating a general LIESST event from the LS ground state to the HS metastable stat e. ................................ ................................ ......................... 35 1 6 Examples of 2D and 3D Hofma nn like structures. ................................ ........................... 35 1 7 Structural fragment of the 3D Hofmann like Fe(azpy)[Pt(CN) 4 ]. ................................ .... 36 1 8 Scheme depicting the sequential adsorption deposition of PBA thin films. ...................... 36 1 9 Scheme of the proposed structure of Fe(4 phpy) 2 [Ni(CN) 4 ]. ................................ ........... 37 2 1 Cross section scheme illustrating the layered architecture of the Fe(azpy)[Pt(CN) 4 ]/NiCr PBA heterostructure. ................................ ................................ .. 48 2 2 SEM images of cross sections of th e heterostructure film. . ................................ .............. 48 2 3 ATR FTIR spectra showing the growth of CN stretches as the heterostructure film layers develop ................................ ................................ ................................ .................... 49 2 4 The thermal SCO behavior of the Fe(azpy)[Pt(CN) 4 ] single phase material as measured by Raman spectroscopy. . ................................ ................................ ................... 49 2 5 XPS spectra after NiCr PBA deposition and Fe(azpy)[Pt(CN) 4 ] deposition. .................... 50 2 6 XPS data of the heterostructure after various Fe(azpy)[ Pt(CN) 4 ] deposition cyc les . ........ 51 2 7 Photomagnetic response of single phase Hofma nn and heterostructure films . ................. 52 2 8 Normalized dif ference plots (lig ht dark) for the magnetic response of single phase Hofmann and heterostructure fi lms. . ................................ ................................ ................. 52 2 9 Magnetization ver sus time of irradiation normalized to the dark state, for Fe(a zpy)[Pt(CN) 4 ] and NiCr/Fe(azpy)[Pt(CN) 4 ] heterostr ucture. . ................................ .... 53 2 10 Magnetization versus temperature plot of a NiCr PBA film before irradiati on and after irradiation. . ................................ ................................ ................................ ................. 53

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11 3 1 cycle. ................................ ................................ ................................ ................................ .. 76 3 2 r PBA films as deposition cycles are increased . . ................................ ................................ ................................ ..................... 77 3 3 SEM and AFM images of NiCr PBA films deposited using various precursor chnique. . ................................ ................................ ............ 78 3 4 deposit ion technique. . ................................ ................................ ................................ ........ 79 3 5 Comparison of average roughness as a fu nction of average thickness for various ......................... 80 3 6 XPS survey spectrum of a CPTES functionalized glass slide after the begi nning stages of NiCr PBA deposition (5 cycles).. ................................ ............................ 80 3 7 Solvent influence on NiCr determined via XPS.. ................................ ................................ ................................ ......... 81 3 8 2 O solvent ratio for 3 13 (50% MeOH) and 3 14 (25% MeOH) . ................................ ................................ 82 3 9 SEM data of NiCr s deposited using varying precursor concentration . ................................ ................................ ................................ ..................... 83 3 10 Average roughness as a function of average thickness for various samples comparing various NiCr and ......................... 84 3 11 AFM characterization of NiCr ............................... 85 3 12 Representative XPS survey ch layer of the heterostructure . ................................ ................................ ............................... 86 3 13 AFM and SEM images depicting various control of Hofmann layer deposition for heterostructure sampl es.. ................................ ................................ ................................ .... 87 3 14 Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA deposition samples . ....... 88 3 15 Magnetization vs. temperature difference plots (light dark) for heterostructures presented in Figure 3 PBA films. ................ 89 3 16 Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA layers with varying thickness. ............... 90

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12 3 17 Ma gnetization vs. temperature difference plots (light dark) for heterostructures presented in Figure 3 PBA layers with varying thickness. ................................ ................................ ................................ ............................ 91 3 18 SEM images of the heterostructure surface after deposition of the NiCr PBA and Hofmann layers. ................................ ................................ ................................ ................. 92 3 19 AFM characterization of NiCr ............................. 9 3 3 20 SEM and schemes depicting film cross sections of smooth and rough NiCr PBA ................................ ................................ ............................. 94 3 21 Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA depositions.. ................................ ......... 95 3 22 Magneti zation vs. temperature difference plots (light dark) for heterostructures presented in Figure 3 PBA films. ................................ ...... 96 3 23 Magnetization vs temperature (left) and magneti zation vs. time (right) plots for a Cr PBA layer with a thickness >400 nm and average roughness >100 nm.. ................................ ................................ ...................... 97 3 24 Magnetization vs. temperature differ ence plots (light dark) for the heterostructure presented in Figure 3 PBA layer with a thickness >400 nm and average roughness >100 nm. ................................ ................................ ....... 97 4 1 TGA analysis of F e(phpy) 2 [Ni(CN) 4 ] . ................................ ................................ ............. 115 4 2 TEM images of the Hofm ann like seed particles Fe(phpy) 2 [Ni(CN) 4 ] and the product o f subsequent NiCr PBA addition. ................................ ................................ ................... 115 4 3 AFM particle thickness data of single phase Fe(phpy) 2 [Ni(CN) 4 ] . ................................ . 116 4 4 EDS 2D map of a heterostructure particle Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA .. .................. 116 4 5 EDS linescan of heterostructure particles Fe(phpy) 2 [Ni(CN) 4 ] /NiCr PBA . .................... 116 4 6 TEM images depicting homogeneous precipitation of cubic NiCr PBA particles during a failed heterostructure synthesis.. ................................ ................................ ........ 117 4 7 Room temperature FTIR data of Fe(phpy) 2 [Ni(CN) 4 ] and Fe(phpy) 2 [Ni(CN) 4 ] /NiCr PBA. . ................................ ................................ ................................ ................................ 117 4 8 Room temperature FTIR data of for Fe(phpy) 2 [Ni(CN) 4 ] and Fe(phpy)2[Ni(CN)4] /NiCr PBA . . ................................ ................................ ................... 118 4 9 Room tempera ture PXRD patterns of the single phase Hofmann compound , single phase NiCr PBA reference, and heterost ructure .. ................................ ............................ 119

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13 4 10 Structural model of compound Fe(phpy) 2 [Ni(CN) 4 ] .. ................................ ..................... 120 4 11 Variable temperature FTIR data illustrating the Hofm ann spin transition in single phase Fe(phpy) 2 [Ni(CN) 4 ] and Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure compounds.. ................................ ................................ ................................ ..................... 121 4 12 Variable temperature PXRD data of select reflections in Fe(phpy) 2 [Ni( CN) 4 ] and Fe(phpy)2[Ni(CN)4]/NiCr PBA ................................ ................................ ...................... 122 4 13 Field cooled SQUID magnetometry data of Fe(phpy)2[Ni(CN)4] and Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA before and after irradiation.. ................................ ......... 123 4 14 SQUID difference plot of single phase Fe(phpy) 2 [Ni(CN) 4 ] and Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure. ................................ ............................. 123 4 15 Low temperature (5 K) zero fie ld cooled minor hysteresis loop measurements of the Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure before and after irradiation. ................. 124 4 16 SQUID magnetometry data of Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA bef ore and after irradiation normalized to the magnetization value at T=26 K r eplotted from Figure 4 13(b) . ................................ ................................ ................................ ................................ 125 4 17 the Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure collected imm ediately after synthesis and sev eral days after synthesis . ................................ ................................ ...................... 125 4 18 Comparison of peak widths in select normalized Hofmann and heterostructure ligand modes. ................................ ................................ ................................ .............................. 126 4 19 Magnetization vs. time data illustrating the photoirradiation of the heterostr ucture sample. ................................ ................................ ................................ ............................. 127 5 1 SEM images of bulk Fe(py) 2 [Ni(CN) 4 ] platelets. . ................................ ........................... 142 5 2 SEM images and particle size analysis of bulk Fe(3 Clpy) 2 [Ni(CN) 4 ] p latelets. . ........... 142 5 3 FTIR spectra of bulk Fe(py) 2 [Ni(CN) 4 ] and Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets . ................ 143 5 4 PXRD patterns of bulk Fe(py) 2 [Ni(CN) 4 ] and Fe(3 Clpy) 2 [Ni(CN) 4 ] plate lets . ............. 143 5 5 SEM images of Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets illustrating the varying particle size and polydispersity as a function of precursor solution concentrations used in synthesis. . ................................ ................................ ................................ ........................ 144 5 6 AFM image and cross section analysis of dropcasted Fe(3 Clpy) 2 [Ni(CN) 4 ] particles. . ................................ ................................ ................................ .......................... 145

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14 5 7 Particle size distributions of Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets illustrating the varying particle size and polydispersity as a function of precursor solution concentrations used in synthesis. ................................ ................................ ................................ ............. 145 5 8 FTIR spectra of Fe(3 Clpy) 2 [Ni(CN) 4 ] samples produced from syntheses targeting particle size control by varying precursor concentrations. ................................ .............. 146 5 9 SEM images and particle size distrib utions illustrating the effect of synthesis temperature on Fe(3 Clpy)[Ni(CN) 4 ] platelet size. ................................ ......................... 147 5 10 SEM images and particle size distributions illustrating the effect of precursor addition m ethod/drop rate on Fe(3 Clpy)[Ni(CN) 4 ] platelet size. . ................................ .. 148 5 11 FTIR and PXRD data depicting the compositional and structural changes that occur upon ligand exchange from H 2 O phpy in the 2D Ho fmann like networks Fe(H 2 O) 2 [Ni(CN) 4 ] and Fe(phpy) 2 [Ni(CN) 4 ]. . ................................ ................................ 149 5 12 SEM images and particle size distributi on of Hofmann platelets before and after H 2 O phpy ligand exchange. ................................ ................................ .......................... 150 5 13 SEM images of bulk Fe(py) 2 [Ni(CN) 4 ] platelets and Fe(py) 2 [Ni(CN) 4 ]/NiCr PBA heterostructures. ................................ ................................ ................................ ............... 151 5 14 SEM images of seed Fe(3 Clpy) 2 [N i(CN) 4 ] particles and the respective heterostructures. ................................ ................................ ................................ ............... 152 5 15 FTIR spectra of Fe(X py) 2 [Ni(CN) 4 ]/NiCr PBA heterostructures {X = ph, py, 3 Clpy}. . ................................ ................................ ................................ .......................... 153 5 16 SEM images of he terostructures synthesized with Fe(3 Clpy) 2 [Ni(CN) 4 ] seed particles using two different NiC r PBA precursor concentrations. . ................................ 154 5 17 SEM image s comparing NiCr PBA size and morphol ogy on the surface of py and 3 Clpy Hofmann analogues to the NiCr PBA growth on the surfac e of the phpy Hofmann analogue . ................................ ................................ ................................ .......... 155 5 18 Field cooled magn etiz ation vs. temperature and magnetization vs time plots of a single phase Fe(3 Clpy) 2 [Ni(CN) 4 ] and Fe(3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure before and after irradiation.. ................................ ................................ .... 156 5 19 SQUID diff erence plots (light dark) of MT vs T Fe(3 Clpy) 2 [Ni(CN) 4 ] and M vs T Fe(3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA of field cooled ir radiation measurements. ................. 157 A 1 ATR FTIR data illustrating the increase in intensity of the CrCr number of deposition cycles increases. ................................ ................................ ............ 163 A 2 AFM images of CrCr PBA films after 20 and 30 deposition cycles. . ............................. 164

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15 A 3 SEM images of CrCr PBA films after 20 and 30 deposition cycles. . .............................. 165 A 4 SEM images of 60 cycle CrCr PBA base layer and heterostructure after Hofmann deposition. ................................ ................................ ................................ ........................ 166 A 5 SQUID magnetometry experiments of heterostructur e CrCr PBA/Hofmann sample. .... 167 B 1 TEM images of NiCr PBA seed particles an d attempted NiCr PBA/Fe(pz)[Pt(CN) 4 syntheses. ................................ ................................ ................................ ......................... 172 B 2 FTIR spectra of NiCr PBA, B 1, and B 2 . ................................ ................................ ...... 172 B 3 FTIR data of samples B 3, B 4, B 5, and B 6 illustrating differences in ligand and water content with varying Hofmann particle synthesis. ................................ ................. 173 C 1 Optical images and TEM/SEM images of various spin crossover compounds synt hesized, contrasting the appearance of the crystalline flakes and powder produced from drying the compoun d . ................................ ................................ .............. 177 C 2 FTIR spectra of the compounds described in Figure C 1 ground into KBr pellets , correlating the crystalline or powder nature of the dry sample with the extent of scattering anomalies observed in the IR data. ................................ ................................ .. 177 C 3 FTIR spectra of nanoscale Fe(phpy) 2 [Ni(CN) 4 ] crystallin e flakes collected via ATR and transmission experiments using a pressed KBr pellet. ................................ .............. 178 D 1 Optical images of various rough microscale PB, PBA, and PBA/Hofmann heterostructure films deposited v ia aqueous SA onto Melinex 454 substrates. . .............. 183 D 2 XPS survey spectra and high resolution dat a of M454 and M535 surfaces. ................... 183 D 3 Magnetization vs. temperature data of an ODT substr ate. . ................................ .............. 184

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16 LIST OF ABBREVIATIONS A Alkali ion a Lattice parameter AFM Atomic force microscopy ATR FTIR A ttenuated total reflectance F ourier transform infrared spectros copy azpy A zopyridine cm 1 Inverse centimeter CN (or NC) Cyanide CoCr Cobalt hexacyanochromate CoFe Cobalt hexacyanoferrate CP Coordination polymer CPTES (3 cyanopropyl)triethoxysilane CrCr Chromium hexacyanochromate CTIST Charge transfer induced spin transition CuFe Copper hexacyanoferrate EDS Energy dispersive spectroscopy emu Electromagnetic unit EtOH Ethanol eV Electron volt EXAFS Extended X ray absorption fine structure FC Field cooled FeCr Iron hexacyanochromate FTIR Fourier transfo rm infrared spectroscopy G Gauss H Field

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17 h Hours HS High spin hv Irradiation K Kelvin KNiCr Potassium nickel hexacyanochromate LIESST Light induced excited spin state trapping LS Low spin M Magnetization MeOH Methanol mg Milligram min Minute mL Millilitre mM Millimolar mm Millimetre mmol Millimole mT Millitesla mW Milliwatt Nd:YAG Neodymium doped Yttrium Aluminum Garnet NiCr Nickel hexacyanochromate NiFe Nickel Hexacyanoferrate nm Nanometer PBA Prussian blue analogue Ph Phenyl phpy Phenylpyridine PXRD Powder X ray diffraction

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18 py Pyridine pz Pyrazine R a Mean roughness RbCoFe Rubidium cobalt hexacyanoferrate S Spin value SA Sequential adsorption SCO Spin crossover SEM Scanning electron microscopy SQUID Superconducting quantu m interference device T Temperature or Tesla T 1/2 Temperature at which high spin and low spin states are equally populated while heating through spin crossover T 1/2 Temperature at which high spin and low spin states are equally p opulated while cooling through spin crossover T C Magnetic ordering temperature TBA Tetrabutylammonium TEM Transmission electron microscopy TGA Thermogravimetric analysis V Volt VT FTIR Variable temperature Fourier transform infrared spectroscopy VT PXRD Variable temperature powder X ray diffraction W Watt XPS X ray photoelectron spectroscopy 1D One dimensional

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19 2D Two dimensional 3D Three dimensional Å Angstrom ° C Degree Celsius Change in Out of plane bending frequency In plane bending frequency Bragg angle Wavelength Micrometer Stretching frequency Magnetic susceptibility

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20 Abstract of Dissertation Presented to the Graduate S chool of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROSTRUCTURES By Corey Rachelle Gros Dec ember 2015 Chair: Daniel Talham Major: Chemistry P hotomagnetic coordination polymer materials provide an intriguing route toward spin control and manipulation in potential light assisted magnetic recording and spintronic applications, however chall enges such as short lifetimes and low temperature restrictions limit the viability of such materials in devices. Recent work explores a new mechanism of light switchable magnetism by developing heterostructures of Prussian blue analogue (PBA) coordination polyme rs, resulting in synergistic photomagnetic behavior beyond the sum of each induced response. Specifically, the irradiation and resulting structural changes of a light sensitive component induces strain on a mechanically coupled magnetic c omponent, thus altering its magnetization. The work herein demonstrates the general nat ure of this light induced magnetomechanical mechanism by exploring non PBA photoactuators which differ both in structure and morphology to the magnetic PBA components us ed previously. This work focuses on coupling two and three dimensional (2D and 3D) photoactive Hofmann like Fe(II) spin crossover networks , specifically Fe(phpy) 2 [Ni(CN) 4 ] {phpy = 4 phenylpyridine} and Fe(azpy)[Pt(CN) 4 azopyridine} respectiv ely, with the ferromagnetic nickel hexacyanochromate (NiCr) PBA to create new photoactive coordination polymer heterostructures in layered thin film and particle architecture s. In both cases, inducing

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21 the light induced excited spin state trapping ( LIESST ) effect in the photoactive component of the heterostructure results in a decrease in magnetization, despite photo increase observed in the single phase photoactuators. Controlled fabrication of NiCr PBA films using solution d eposition methods is demonstrate d to achieve targeted rough and smooth surface topographies for heterostructure interface engineering . Nano and mesoscale thin film heterostructures with varied surface roughnesses are then developed to correlate the role of film thickn ess and interfacial roughness with extent of ph otomagnetic behavior . The t wo dimensional light sensitive Fe(phpy) 2 [Ni(CN) 4 ] network is synthesized as nano scale platelets for photoactive components of heterostructure particl es. Coupling the 2D phenylpyridine based photoactu ator and 3D cubic PBA network exploits large structural differences to gain insight into the extent of light induced synergy despite significant lattice mismatch . Additionally, the change in photoresponse of the Hofmann like compound is amplified by incorp oration into the heterostructure. Observing s tructural and photomagnetic activity despite coupling structurally different networks demonstrates how this light induced magnetomechanical effect may be generally used as an effective approach to creating photo active heterostructures .

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22 CHAPTER 1 OVERVIEW Molecule based materials have been of interest in recent decades in an effort to explore alternatives to conventional inorganic solid state materials for use in technological applications. 1 3 The versatility of targeted structure and bonding motifs available by employing a properties. Furthermore, the rational design of molecular materials often affords tunable structures, physical properties, and reactivity via synthetic and post synthetic routes not available with traditional solids. Coordination polymers (CPs) are a clas s of molecule based materials which have garnered attention for a variety of potential applications such as gas storage and separations, ion storage, catalysis, biomedical imaging and drug delivery, negative thermal expansion materials, magnetism, and phot omagnetism. 4 11 C oordination polym er s are one , two , or three dimensional arrays of transition metal ions bridged by ligands which allow for structural versatility and control over morphology, size, and architecture of designed structures and composites. Reasonable control of CP size and morphology of particles and films invites the exploration and optimization of siz e dependent properties, where the influence of size reduction on material properties is a common consideration from a technological standpoint. 12, 13 Surfactant based techniques utilizing microemulsions 14, 15 and surfactant free techniques which utilize charge stabilization 16 are often employed in the synthesis of nanoparticles. C oordination polymer thin films have been fabricated using a variety of techniques ranging from particle deposition onto a solid substrate via spin coating, drop casting , and Langmuir Blodgett methods to layer by layer approaches such as sequential adsorption (SA) of CP components onto a solid support. 17 21

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23 Prussian blue analogues (PBAs) are a well studied family of CPs, largely due to the simplicity of structure leading to easily tailored physical properties and modest control of both particle size and film thick ness using mild synthetic conditions. The PBA structure consists of transition metal ions bridged by cyanide in an octahedral fashion, resulting in a robust three dimensional cubic network (Figure 1 1). Necessarily, charge balance requires the addition of positive charge or the removal of excess negative charge in the form of hexacyanometallate vacancies with water molecules filling the empty coordination sites. Judicious se lection of transition metal ions as well as controlling the concentration of interstitial cations and vacancies within the structure yields versatility in physical properties. Furthermore, multifunctional materials, in some cases achieving synergistic prop erties , can be achieved by adopting a mixed material or heterostructure architecture of two or more PBAs. Examples of Prussian blue (PB) and PBA heterostructures have featured gradient Gd PB particles, which combine the photothermal capability of the PB co mponent with the MRI contrast enhancement from Gd 3+ ions near the particle surface 22 and PBA cathode materials for Li ion storage involving a high charge capacity copper hexacyanoferrate PBA core , which is structurally unstable surrounded by a structurally robust nickel hexacyanoferrate PBA shell 23 . Regarding photomagnetism, examples of light sensitive magnetic PBAs as both core shell particles and multi layered thin films hav e demonstrated that a photo inert magnetic PBA can be coupled to a photoactive cobalt hexacyanoferrate (CoFe) PBA resulting in light induced synergy between structural and magnetic properties. Additionally, recent studies have focused on the growth of heter ostructure PBA materials, specifically the influence of interfacial epitaxy on the growth mechanisms observed in various PBA heterostructures. 24

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24 Photomagnetic PBA heterostructures typically involve CoFe PBA and pressure sensitive hexacyanochromate based PBAs. This heterostructure was first demonstrated using ferromagnetic nickel hexacyanochromate (NiCr) PBA in NiCr CoFe NiCr PBA multi layered films 25 and CoFe NiCr PBA core shell particles 26 (Figure 1 2). Structural changes associated with the photo induced charge transfer induced spin transition (CTIST) of the CoFe PBA perturbs the magnetic response of the coupled NiCr PBA. Similar photo induced changes have since been observed in heterostructures with CoCr PBA 27 and CrCr PBA 28 magnetic components, the former probes the photoeffects exhibited by cross like and traditional core shell particle morphologies whereas the latter demonstrates that the photoswitching can occur at higher temperatures. The basis of the photoinduced changes in magnetization is believed to be mechanical in natur e, resulting from structural changes occurring in the CoFe PBA during the CTIST event. Specifically, the CoFe PBA is known to adopt multiple local electronic configurations within metal ion pairs namely the entropically favorable high spin (HS) Co 2+ NC F e 3+ and enthalpically favorable low spin (LS) Co 3+ NC Fe 2+ states which differ considerably in lattice parameter (a). While retaining its cubic Fm 3 ¯ m structure, a change in the lattice parameter of the unit cell occurs due to population/depopul antibonding e g of external stimuli, most of ten via thermal cycling in which the ground state electronic configuration switches between HS and L S, or irradiation at low temperature (typically below 100 K) where the compound transitions from an energetically favourable LS state to a metastable HS state, which is structurally and electronically similar to the high temperature HS state. Incorporating the CoFe PBA into a PBA heterostructure by coupling to a magnetic PBA

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25 via an interface provides a CTIST induced mechanism of structurally distorting the coupled PBA lattice, thus altering its magnetization (Figure 1 3). 25 The mechanism behind the light induced magnetic changes observed in PBA heterostructures has been probed recently using powder X ray diffraction (PXRD) in core shell particles. 27, 29 The photomagnetic response is described as a magnetomechanical effect 30, 31 resulting from the mechanical coupling of the two components across an interface. Upon cooling, the CoFe cores in the PBA heterostructures undergo a lattice contraction consequently straining the magnetic PBA shell lattice, observed as a significant broadening in shell reflections (Figure 1 4). PXRD data indicate that the shell magnetically orders at low tem perature while strained, and that irradiating the strained heterostructure relieves the strain in the magnetic shell by repopulating the CoFe HS stat e. The structural evidence supports that upon relieving strain in the magnetic shell, the preferred magneti c anisotropy axis shifts. Consequently, magnetic domains initially aligned with the magnetic field upon cooling to reorient away from the applied field, thus decreasing the overall magnetic response of the material. Recent work modeling this penetration de pth has been shown specifically in CoFe NiCr PBA and CoFe CoCr PBA core shell systems. 27 The PBA heterostructure family represents a model system for studying the intricacies of strain mediated light switchable magnetic processes due to the simplistic PBA cubic structure and si milarity of lattice constants. These strain mediated alterations of physical properties can be explored beyond PBA h eterostructures, however, utilizing structurally different composites res ulting in multifunctional materials exhibiting synergistically responsive properties. To date, PBAs have been coupled with various materials resulting in multifunctional n anocomposites, examples including Au NiFe NiCr PBA core multi shell structures exhibi ting both plasmonic

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26 and magnetic properties, and metal oxide PB nanocomposites behaving as thermal ablation agents responsive to magnetic field and light. 32, 33 Herein, we aim to venture beyond PBA only heterostructures by coupling non PBA CP photoactuators with magnetic NiCr PBA in pursuit of new photomagnetic heterostructures consisting of structurally different components. By expanding upon the PBA heterostructure family to incl ude various other photoactive CPs, we demonstrate that structurally different materials may be integrated into multifunc tional heterostructures where desired properties may be produced or amplified via mechanical coupling of the components. By expanding be yond PBA based photoactuators, we aim to illustrate the strain induced photoswitching mechanism is general in hexacyanochromate PBA based heterostructures. In search of alternative photoactuators for CP heterostructures, Fe(II) spin crossover (SCO) materia ls are attractive candidates for strain mediated magnetization switching. Fe(II) SCO compounds are a well studied class of CPs including molecular compounds, one dimensional, 34 two dimensional, 35 and three dimensional 15, 36 networks in which Fe(II) centres are ligated by mono or multi dentate N dono r organic molecules . Similar to CoFe PBA, Fe(II) SCO materials undergo spin transitions, adopting both S=2 HS and S=0 LS d 6 electron configurations. The materials are known to exhibit large changes in Fe N bond lengths of approximately 0.2 Ã… upon spin transition. Based on the nature of the compounds and/or guest population within the networks, Fe(II) SCO compounds may undergo complete or partial spin transitions in one or multiple steps, in many cases exhibiting thermal hysteresis due to cooperativity of the SCO event. 37 The ability to stabilize multiple ground states allows reversible switching via thermal cycling in addition to other physical and chemical stimuli such as application of light, pressure, or the presence of guest molecules which may be intercalated i nto

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27 the structure. 18, 38 51 Given the strikingly different electron configurations in the Fe d o rbitals, spin states are commonly monitored via optical, vibrational, magnetic, and M ssba u er techniques. In some cases, Fe(II) SCO compounds may undergo a LS HS transition upon irradiation at low temperature, first observed by McGarvey and Lawthers in the solution phase and Decurtins et al . in the solid state (Figure 1 5 ). 44, 52 Subsequently named the light induced excited spin state trapping (LIESST) effect, a metastable HS state is typically achieved via ave been utilized. Early work from Haus er et. al. systematically probed the timescale of HS trapping at low temperature of a series of dilute [Fe(Ptz) 3 ](BF 4 ) 2 based compounds, introducing the inverse energy gap law, which relates the relative energy gap be tween LS and HS states to the timescale of metastable HS population and subsequent relaxation to the LS state. 53 Similarly, Létard utilized a theoretical relative stability of HS and LS states in a comprehensive s tudy of published Fe(II) SCO materials to derive an expression relating the temperature regime of thermal SCO to the relaxation temperature from the metastable HS LS states referred to as T (LIESST). 48 Selection of photoactive Fe(II) compounds appropriate for integration into CP heterostructures considers the above relations, specifically targeting compounds with SC O behavior at relatively low temperatures (below 150 K) with the aim of utilizing LIESST active compounds with metastable HS states beyond 50 K. In the present work, Hofmann like two dimensional and three dimensional networks are employed as photoactuators in CP heterostructures. Named after the 2D cyanide bridged clathrate [Ni(CN) 2 · NH 3 ·C 6 H 6 ] originally reported by Hofmann and K ü spert in 1897, 54 the Hofman n like networks are composed of octahedrally coordinated metal centers bridged

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28 equatorially by tetracyanometallate anions in a square planar geometry to form 2D bimetallic planes. The dimensionality of the overall network is dictated by the denticity of organic ligands coordinated to the axial Fe sites. Specifically, 3D structures are achieved by utilizing bismonodentate organic ligands capable of coordinating to Fe centres of adjacent bimetallic planes, whereas 2D structures typically consist of pyridine like monodentate organic ligands coordinated axially that are c stacking with ligand s of adjacent layers (Figure 1 6 ). The Hofmann like family of CPs are of particular interest due to the ease in manipulating the structure toward desired spin transition and structural properties. Typical research into this class of compounds has focused largely on particle/film feature size control, 12, 55 57 gas storage and sensing via pore expansi on through ligand selection, 18, 19, 58, 59 exploring and tuning the SCO properties of each compound, 36, 43, 45, 47, 60 68 and achieving or enhancing bistability of SCO properties toward potential room temperature applications utilizing thermal and photo control. 41, 42, 49, 69 72 The 3D Hofmann like network Fe(azpy)[Pt(CN) 4 ] {az azopyridine} reported in 2008 by Agustí et al. 60 (Figure 1 7 ) is , to date, the only published 3D Hofmann like network which alludes to exhibiting a LIESST effect in thin films and particles demonstrated vi a Raman spectroscopy. Synthesised as either a nanocrystalline powder or thin film via SA, the report indicates a drastic dependence of SCO behavior on hydration level, where the presence of H 2 O within the lattice stabilizes the LS state of the material, th us shifting the T 1/2 to considerably higher temperatures. The dehydrated form of the compound is reported to exhibit HS trapping below 100 K. The 3D nature of the network and photoactive properties invite the use of such a compound as a photoactuator in mu ltilayer films via SA techniques.

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29 In Chapter 2, the Fe(azpy)[Pt(CN) 4 ] compound described above is utilized as a photoactuator for light sensitive magnetic heterostructures when coupled to the NiCr PBA in a mesoscale bilayer film architecture. 73 The SA technique is used to deposit both layers, notably creating a relatively rough interface between the two films due to the PBA deposition method. The resulting heterostructure is the first known example using a non PBA p hotoactuator to induce changes in magnetization when coupled to a magnetic PBA. Inducing the LIESST effect of the photoactuator results in a decrease in magnetization of the heterostructure that is attributed to strain mediated processes in the NiCr PBA co mponent. Additionally, the LIESST effect is observed magnetically for the first time in the single phase Hofmann like film, which is then correlated to the magnetic behavior of the heterostructure. Chapter 3 involves exploring and optimizing NiCr PBA SA d eposition parameters toward minimizing interfacial roughness in Fe(azpy)[Pt(CN) 4 ]/NiCr PBA heterostructure films. The SA of PBA films typically involves alternative submersion of a solid support into solutions of the component ions, thus developing a film with repeate d deposition cycles (Figure 1 8 ). Previous single and multi layer PBA films from our group deposited via SA have a characteristically rough surface topography upon deposition of each layer as a result of the aqueous deposition method used. 25, 73 75 This strategy relies on nucleation of PBA crystallites formed upon sequential mixing of reactant solutions which de posit at the surface of the solid support. Alternative SA methods exist to develop smooth and uniform films, involving functionalized substrates and modified deposition conditions to promote ion adhesion while removing physisorbed ions more effectively tha n our previous work. Although such deposition methods have been demonstrated to more closely follow an ideal layer by layer growth of PBA films, 76, 77 these efforts typically require signific ant time periods (up to 30 minutes per deposition cycle) and usually result in only

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30 the initial steps of film growth and nucleation on a surface unreasonable for mesoscale PBA deposition. Herein we demonstrate SA growth via controlled nucleation and a la yer by layer approach, contrasting the control of film thickness and roughness as a result of using each technique. Furthermore, SA deposition parameters are explored with the aim of producing nano and mesoscale NiCr PBA films with greatly reduced roughne ss from previous mesoscale films from our group. Finally, initial photomagnetic measurements are reported which probe the effect of film thickness and interfacial roughness on the extent of light induced magnetic changes in the Fe(azpy)[Pt(CN) 4 ] heterostru cture. Another route toward structural variation in NiCr PBA based heterostructure components employs 2D Hofmann like networks, where further diversity is seen in the lack of common lattice parameters, structure, and particle morphology. Such issues are so mewhat circumvented with the 3D Fe(azpy)[Pt(CN) 4 ] based films where the Fe Pt Fe distance of the bimetallic plane matches reasonably to that of the NiCr PBA unit cell, and the SA deposition method facilitates interfacing the two materials in a relatively s traight forward manner. Several 2D Hofmann structures further deviate from the PBA structure, where common features involve a ligand terminated particle surface, a reduction in lattice symmetry resulting in strikingly different unit cell parameters, and a corrugated bimetallic plane which is expected to drastically limit epitaxial growth of any PBA lattice at the Hofmann particle surface. Chapter 4 couples the 2D photomagnetic Fe(phpy) 2 [Ni(CN) 4 ] {phpy = 4 phenylpyridine} Hofmann like platelets first reporte d by Seredyuk et. al. (Figure 1 9 ) with a NiCr PBA network to explore the extent of synergistic photomagnetic changes exhibited by structurally and morphologically different CPs. 78 Although the crystal stru cture is n ot known , the compound is reported to have structural features si milar to the 2D Hofmann family probed via bulk vibrational

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31 spectroscopy as well as the local Fe environment via extended X ray absorption fine structure (EXAFS) analysis. 78 The structural and magnetic behavior of the single phase photoactuator and heterostructure materials are highlighted, as well as observed interfacial magnetic signatures not previously seen in PBA heterostructures resulting from the Ho fmann platelet morphology. Chapter 5 utilizes the 2D Hofmann like systems Fe(X py) 2 [Ni(CN) 4 ] {X = H, 3 Cl , ph } in an attempt to address syn thetic limitations of the above phenylpyridine based compound. Specifically, the nanoscale particle size inhibits our ability to observe NiCr PBA growth at the Hofmann particle surface, consequently limiting our understanding of the heterostructure interface. The chloropyridine based analogue exhibits a partial SCO upon cooling as observed in li terature, as well as a LIESST effect firs t reported in this chapter. Furthermore, a new approach to synthesizing mesoscale Fe(phpy) 2 [Ni(CN) 4 ] platelets via ligand substitution is explored. These Hofmann compound s are synthesiz ed as nano , meso , and micros cale platelets as platforms for NiCr PBA growth to provide insig ht into the nature of the interface formed from coupling the two CP lattices. Preliminary results explorin g the photomagnetic behavior of a single phase Fe(3 Clpy) 2 [Ni(CN) 4 ] and Fe(3 Clpy) 2 [Ni (CN) 4 ]/NiCr PBA heterostructure are presented. Chapter 6 consists of concluding remarks regarding t he CP heterostructures presented in previous chapters. The conclusions will reflect on the nature of coupling of structurally different coordination polymer s at an interface of the CP heterostructures as well as the influence of interfacial coupling on photoinduced magnetic responses of each material. Additionally, potential future directions will be discussed. Complementary experiments are presented in the a ppendices following the conclusion.

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32 Figure 1 1. Scheme of the PBA cubic structure. C N M H 2 O A

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33 Figure 1 2. CoFe NiCr PBA core shell heterostructures. A) TEM cross section and scheme of a multilayer heterostructure film. Scale bar is 100 nm. B) TEM image of a CoFe NiCr PBA core shell particle. Reprinted with permission from Pajerowski et al. C opyright (2010) American Chemical Society, and Dumont et al. Copyright (2011) American Chemical Society. 25, 26 Figure 1 3. Photomagnetism in NiCr CoFe NiCr PBA multilayer heterostructure film. Representative of NiCr film and particle samples, a decrease in magnetization is es are measured with the light off the former data collected pre irradiation and the latter collected post irradiation. Reprinted with permission from Pajerowski et al. Copyright (2010) American Chemical Society. 25

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34 Figure 1 4. PXRD peak broadening observed in shell PBA of photomagnetic PBA heterostructure. PXRD patterns depicting the 200 reflection of a RbCoFe KCoCr PBA core shell heterostructure. Upon cooling, the contraction of the CoFe PBA componen t strains the mechanically coupled KCoCr PBA shell, observed as broadening in the KCoCr reflection. Irradiating the heterostructure at low temperature results in reversal of the KCoCr reflection broadening, suggesting a relief of the initial CTIST induced strain. Reprinted with permission from Risset et al. Copyright (2014) American Chemical Society. 27

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35 Figure 1 5 . Schematic energy diagram illustrating a general LIESST event from the LS ground state to the HS metastable state. The switching between states may occur by Reproduced with permission from G ü tlich et al. Copyright (2001) Elsevier. 79 Figure 1 6 . Examples of 2D and 3D Hofmann like structures. A) 2D Fe(py) 2 [Ni(CN) 4 ] {M = Ni}. B) 3D Fe(pz)[Pt(CN) 4 ] {M = Pt}. A) Adapted from a structure published by Kitazawa et al. 65 B) Reprinted with permission from Molnár et al. Copyright (2007) WILEY VGH Verlag GmbH & Co. KGaA, Weinheim. 56 a) b )

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36 Figure 1 7 . Structural fragment of the 3D Hofmann like Fe(azpy)[Pt(CN) 4 ]. Reprinted with permissi on from Agustí et al. Copyright (2008) American Chemical Society. 60 Figure 1 8. Scheme depicting the sequential adsorption deposition of PBA thin films. Reprinted from Justin Gardner Ph.D. thesis. 80

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37 Figure 1 9 . Scheme of the proposed structure of Fe(4 phpy) 2 [Ni(CN) 4 ]. For clarity, the structure of the phpy ligands is shown only for the central iron atom, all others are represented as L. Reprinted with permission from G aspar et al. Copyright (2014) WILEY VGH Verlag GmbH & Co. KGaA, Weinheim. 64

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38 CHAPTER 2 LIGHT INDUCED MAGNETIZATION CHANGES IN A COORDINATION POLYMER HETEROSTRUCTURE OF A PRUSSIAN BLUE ANALOGUE AND A HOFMANN LIKE Fe (II) SPIN CROSSOVER COMPOUND Introduction Light controllable magnetic materials are promising candidates for optically controlled or energy assisted magnetic recording routes leading to orders of magnitude increases in information storage density. 81 83 Furthermore, photo generation and control of spins can increase speeds in spintronics and spin photonics based processing. 84 Despi te these potentially high impact payoffs, inducing and manipulating spins with light remains a considerable challenge that is currently mate rials limited. Efforts to control magnetism with light go back nearly 50 years , 85 and m uch recent focus has cent ere d on light induced magnetization in II VI and III V dilute magnetic semiconductors, although the effects only persist for short times. 86, 87 On the other han d, photoinduced magnetization resulting from localized charge transfer can persist for years in some coordination polymer systems, but the effects are restricted to low temperature. 5, 88 Recen tly, other work from our group described a new mechanism for switching magnetism with light involving coordination polymer heterostructures , in which a light sensitive component elastically couples across an interface to a non photoactive magnetic componen t altering its magnetization. 25, 26, 74 Members of the family of Prussian blue analogues, A j Co k [Fe(CN) 6 ] l n H 2 O (A is generally a monovalent alkali cation) are known to undergo a charge transfer induced spin transition (CTIST) . 5, 88, 89 When coupled to a magnetic analogue that is not photoactive, such as Rb 0. 8 Ni 4.0 [Cr(CN) 6 ] 2.9 n H 2 O (NiCr PBA) , the NiCr PBA Reprinted with permission from Gros, C.R.; Peprah, M.K.; Hosterman, B.D.; Brinzari, T.V.; Quintero, P.A.; Sendova, M.; Meisel, M.W.; Talham, D.R. J. Am. Chem. Soc. 2014 , 136 , 9846. Copyright 2014 American Chemical Society.

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39 magnetization can be significantly altered by photo triggering the CTIST in t he CoFe PBA lattice. The effect has been observed in both thin film and p article heterostructures and is most pronounce d in heterostructures with dimensions on the order of 100 500 nm. 25, 26, 74 This new approach offers the opportunity to rethink light controllable magn etism by separating the photo event from the magnetic spins. We report here a new photomagnetic heterostructure that for the first time uses something other than a CoFe PBA as the photoactive component, thereby illustrating that the mechanism can be genera l. In previous examples, lattice changes associated with the CTIST elastically couple across the interface to realign magnetic domains in the magnetic component. When s earching for alternative photoactive networks to couple to the magnetic PBA, coordinat ion polymer spin crossover compounds involving Fe 2+ become promising candidates, as alterations in metal ligand bond lengths upon high spin to low spin transition lead to significant changes in unit cell dimensions. 48, 79 If the low spin to high spin transition is induced via the LIESST effect (light induc ed excited state spin trapping) the structural change can be used to elastically couple to the magnetic PBA to alter its magnetis m. To demonstrat e this idea, a thin film of the Hofmann like SCO network {Fe(azpy)[Pt(CN) 4 x H 2 O} (azpy = 4,4 azopyridi ne), recently reported by Agustí et. al., 60 was coupled to a thin film of NiCr PBA (Ni[Cr(CN) 6 ] 0.7 n H 2 O) (Figure 2 1). Below the magnetic ordering temperature of the NiCr PBA (T c = 70 K) , a light induced chang e in magneti zation is observed. The sign and magnitude of the heterostructure magnetization change cannot be accounted for by the LIESST effe ct alone, but the temperature profile of the magnetization clearly shows that the changes in the NiCr PBA network are correlated to the spin state of the Hofmann like SCO network .

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40 Experimental Methods Film Deposition Materials: All reagents were purchased from Sigma Aldrich and used as received unless otherw ise specified. Synthesis of 4,4 azopyridine (azpy), K 3 Cr(CN) 6 , and (TBA) 2 Pt(CN) 4 (TBA= tetrabutylammonium) follows literature methods . 25, 90, 91 Bev l Ledge microscope slides (1.0 mm) , microscope cover glass slides (0.2 mm), and Melinex 454 (obtained from DuPont Teijin films) were used as film substr ates after cleaning with piranha solution (H 2 SO 4 :H 2 O 2 , 2:1) or methano l. (Warning: Piranha solution reacts rapidly with organic material and should be handled with extreme caution). Deposition of NiCr/Fe(azpy)[Pt(CN) 4 ] films : Heterostructure films were developed on glass substrates with both the NiCr PBA and Fe(azpy)[Pt(CN) 4 ] deposited using a sequential adsorption method . Initial NiCr PBA dep osition was carried out at room temperature following literature methods. 25 The su bstrate was soaked in a 10 mM aqueous solution of NiCl 2 ·6H 2 O for 5 seconds, followed by submerging in a solution containing 1 0 mM K 3 Cr(CN) 6 for 5 seconds to complete one cycle. Following each cycle, the substrate was rinsed with deionised wat er. The process was repeated for 10 cycles to develop the NiCr film. After PBA deposition, the film was rinsed with acetone and dried under air flow. The deposition of the Fe(azpy )[Pt(CN) 4 ] layer was performed at low temperature by keeping precursor solutions at 78 °C . The NiCr coated substrate was soaked alternatively in 10 mM ethanolic solutions of Fe(BF 4 ) 2 ·6H 2 O, (TBA) 2 Pt(CN) 4 , and azpy fo r 60 s econds , with rinsing in pure EtOH in between each step. This process was repeated for 40 cycles to produce a continuous spin crossover film . All depositions were performed in lab atmosphere. The single phase Fe(azpy )[Pt(CN) 4 ] films were grown on Meli nex substrates and carried out as described above after initially treating the substrates by

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41 soaking them in a 30 mM ethanolic azopyridine solution overnight. To develop thicker films for single phase characterization, 30 mM precursor solutions were used i n the deposition. Ni[Cr(CN) 6 ] 0.67 n H 2 O (NiCr PBA): XPS metal ratio analysis, Ni:Cr 3:2 (Figure 2 5). ATR FTIR (Figure 2 3) (CN)=2175 cm 1 . Fe(azpy)[ Pt(CN) 4 ] x H 2 O (Hofmann phase): XPS metal ratio analysis, with Fe:Pt 1:1 (Figure 2 5). ATR FTIR (Figure 2 3) (CN)=2169 cm 1 calculated via s ubtraction of initial NiCr signal from heterostructure, consistent with single phase Hofmann (CN) frequency (2170 cm 1 ). All analyses are consistent with previously published data on these compounds. 25, 60, 92 Characterization Fourier transform infrared spectroscopy (FTIR) was performed with a Nicolet 6700 Thermo Scientific spectrometer. Films were grown on a multi reflection silicon ATR crystal and measured afte r each layer was deposited using a Harrick ATR accessory. To increase signal to noise, IR spectra are an average of 500 scans with a resolution of 1 cm 1 . Variable temperature Raman spectra were acquired using a Kaiser Optical RXN 1 Raman microscope system, with an excitation wavelength of 785 nm and power of 25 mW. Temperature control was achieved with a Linkam THMS600 liquid nitrogen cryo stage. SEM images were collected with an FEI XL 40 FEG SEM. SEM film samples were fractured and mounted on the side to create cross sections of the film. X ray photoelectron spectroscopy (XPS) measurements were per formed with a Physical Electronics PHI 50 00 VersaProbe II XPS system ray source . XPS data were co llected with a pass energy of 93.9 eV at 0.8 eV/step (survey spectra) or 23.5 eV at 0.1 eV /step (Ni 2p 3/2 , Cr 2p 3/2 , Fe 3p, Fe 2p 3/2 , Pt4f, and Pt 4d 5/2 regions) averaging 10 40 scans over a spot size of 0.2 X 0.2 mm. XPS data were fit and metal ratios calculated using PHI Multipak XPS software.

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42 Magnetic data were collected on a commercial Quantum Design MPMS XL 7 using similar protocols for both configurations. The samples were held in a drinking stra w and oriented perpendicular to the applied magnetic field. Both dark state and light state data were taken with a custom made optic sample rod (OSR 1.0) . 93 More specifically, the sample probe consists of eight str ands of optical fibers (Ocean Optics, Model 200 UV/VIS) going through a stainless steel rod Fiber Lite® High Intensity Illuminator (Series 180 by Dolan Jenner Industries Inc.) fitted with a Quartzline lamp (General Electric EKE 21 V, 150 W). The power reaching the sample position is nominally 1.0 mW. The heterostructure and single phase Fe(azpy )[Pt(CN) 4 ] x H 2 O were field cooled from room temperature to 5 K a nd measured in a 10 mT and 1 T applied fields, respectively. The application of 1 T for the single phase Hofmann like film was necessary to distinguish its magnetic signal from the signal of the supporting substrate. The heterostructure was irradiated at 5 K for 3 hours whereas the single phase Hofmann film was irradiated at the same temperature for 3.5 hours. The light state data were acquired with the light off to show the persistence of the metastable HS state in both configurations. Table 2 1 provides d etails about the warming and cooling rates employed. Finally, in both dark and light states, the data were collected while warming from 5 K to 90 K for the heterostructure and 5 K to 150 K for the single phase Hofmann film. Table 2 1. Warming and cooling r ates for magnetometry studies. Sample Cooling Rates Warming Rates a Single phase Hofmann like sample 2 K/min for 300 K to 100 K 5 K /min for 100 K to 5 K 1 K/min for 5 K to 100 K 5 K/min for 100 K to 150 K Heterostructure 10 K/min for 300 K to 10 K 5 K/m in for 10 K to 5 K 5 K/min for 5 K to 85 K 10 K/min for 85 K to 90 K a When the data are being acquired, the sample is stable at a given temperature. The maximum rates for changing the temperature between set points are listed.

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43 Heterostructure Developmen t Each component of the thin film heterostructure was deposited using sequential adsorption methods that have frequently been used for both PBA films 25, 74, 94 and Hofmann like thin films 18, 55, 95 . The N iCr PBA was deposited from aqueous precursor solutions at room temperature yielding a polycryst alline film ~200 nm thick . T he deposition is terminated with the addition of hexacyanochromate ions that provide a nitrogen functionalized surface to promote the coordination of iron ions and begin nucleation of the Hofmann like network. Subsequent Hofmann ne twork deposition was performed in ethanolic precursor sol utions at low temperature ( 78 ° C ) to reduce ligand desorption , 95 thus allowing for the controlled growth of a 3D network with an average thickness of approx imately 50 nm. The successful deposition of the film on the substrate is seen via SEM (Figure 2 2). ATR FTIR data support the deposition of each layer of the film by monitoring growth of peaks in the (CN) region corresponding to bridging networks of the respective layers (Figure 2 3). Using XPS (Fig ure 2 6) the layered structure of the film is confirmed upon deposition of each component, in which the emergence of the surface Fe(azpy)[Pt(CN) 4 ] signals and a decrease of underlying NiCr PBA sig nals is observed. Photomagnetic Behavior of Single Phase and Heterostructure Films The field cooled magnetic response of the heterostructure before irradiation shows the typical features of a NiCr PBA film, with fe rromagnetic ordering below 70 K ( Figure 2 7 ). 25 Since the thin Fe(azpy)[Pt(CN) 4 ] layer is paramagnetic it does not contribu te significantly to the magnetic signal of the heterostructure sample ( vide infra ) i n a small field (10 mT ). Upon photoirradiation with white light at low temperature, a decrease in the overall magnetization of the heterostructure is observed . After t urning the light off, this reduced magnetization is maintained and upon warming persists up to 60 K, slightly below the ordering temperature of the NiCr PBA phase. The NiCr PBA material is known to not be photoactive, but the Hofmann like

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44 material is capable of undergoing a LIESST effect. 60 Consequently , photoirradiation of the Hofmann layer influences the magnetic response of the NiCr lay er, similar to the persistent photoinduced magnetization changes seen previously in mixed PBA heterostructures containing photoactive components. 25, 26, 96 Although the photoactivity o f the heterostructure correlate s with the LIESST effect of the Fe(azpy)[Pt(CN) 4 ] layer, the sign of the change contradicts the mag netization increase normally associated with a LS to HS transition (Figure 2 7). Magnetization vs temperature before and after irradiation for a Fe(azpy)[Pt(CN) 4 ] film alone shows the increase in magnetization upon irradiation that is expected as th e light generates HS iron center s in the spin transition c ompound. Typical LIESST behavio r 48 is observed as the increased magnetization persists after irradiation with a T (LIESST) ~53 K. As the Hofmann like compoun d is not magneti cally ordered, the signal is significantly weaker than that for the het erostructure, so the data of the single phase Fe(azpy)[Pt(CN) 4 ] are for a thicker film (> to detect the magnetizatio n above the weak background signal of the supporting substrate. By comparing the difference plots before and after irradiation (Figure 2 8 ) for both the single phase Fe(azpy)[Pt(CN) 4 ] and the NiCr PBA/Fe(azpy)[Pt(CN) 4 ] heterostructure, s triking similaritie s between the temper ature profiles are revealed. The temperature regime for which the heterostructure experiences a light induced decrease in magnetization mimics the Hofmann like network HS trapping and relaxation behavior, implying a correlation between the two events in the heterostructure. Furthermore, the magnetization decrease in the heterostructure suggests that the photoinduced change is more than simply an additive effect of the Hofmann material independent of the NiCr PBA, but rather results from the coupling of the two materials via th e interface to perturb the NiCr PBA magnetic response .

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45 As a possible cause of the photoinduced magnetization change in the heterostructure, we might consider that the transition from the LS to the HS state of iron c enter s in the Fe(azpy)[Pt(CN) 4 ] network produces spins at the interface that are able to align antiferromagnetically through cyanide bridges to the Cr 3+ ions at the PBA surface. If this mechanism was the source of the magnetic decrease, however, the disapp earance of the effect would be evident at temperatures that reflect the strength of the Fe 2+ NC Cr 3+ coupling. Typically iron hexacyanochromate PBA networks magnetically order below 25 K . 97, 98 Thus the presence of a significant magnetization decrease up to 60 K in the heterostructure is not sufficiently explained by interfacial antiferromagnetic interactions. Furthermore, the magnitude of the decrease for a 200 nm NiCr PBA layer i mplies that the magnetic perturbation likely penetrates beyond the surface layer of the NiCr PBA, involving depths of several unit cells . O ur current understanding is the structural change associated with the spin state transition of the Fe(azpy)[Pt(CN) 4 ] c ouples to the NiCr PBA, resulting in a magnetomechanical effect 30, 31 in a portion of the PBA net work that experiences domain distortion/realignment . 96 Spin transitions in Fe(II) complexes are well known to undergo relatively large structural changes within the [FeN 6 ] coordination sphere as a consequence of the promotion or removal of electrons to or from antibonding Fe e g orbitals, resulting in Fe N bond length changes of approximately 0.2 Ã… . 99, 100 Linkages at the film interface couple the networks, so structural changes in the LIESST active Fe(azpy)[Pt(CN) 4 ] layer influence the NiCr PBA. Upon cooling, the Fe(azpy)[Pt(CN) 4 ] thermally transitions to the LS state (Figure 2 4 ), undergoing a lattice contraction that alters the underlying NiCr PBA near the interface (these elastic coupling effects have been seen in mixed PBA heter ostructures, where they are easier to quantify). 29 More specifically, this alteration distorts domains in the interface strained region from highly

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46 anisotropic to less anisotropic forms. With further cooling belo w 70 PBA domains magnetically order with their moments more aligned with the magnetic field. At base temperature, the application of light leads to the HS state of the Hofmann like layer via the LIESST effect, and the associated struc tural changes relax the interface strained domains, which reassume their original level of anisotropy with their moments less aligned with the applied magnetic field. Even slight structural changes can be enough to reorient magnetic anisotropy axes, i n thi s case causing them to reorient away from the applied field direction, leading to a net decrease in magnetization. Upon warming, the trapped HS state of the Hofmann like lattice relaxes at T (LIESST) bringing the structures of the networks, and therefore t he magnetization, back to the con ditions similar to those established when the NiCr PBA was first cooled below its ordering temperature. Further support for this mechanism comes by comparing the temperature profile of the light induced effects of the PBA H ofmann heterostructure to that of the mixed PBA heterostructure reported previously. 25 In the mixed PBA heterostructure, the optical CTIST of the CoFe PBA relaxes in the vicinity of 140 K, well above the NiC r PBA ordering temperature, so the light induced state alter s the magnetization of the NiCr PBA up to its T c near 70 K. On the other hand, the Hofmann T (LIESST) of ~53 K is below T c of the NiCr PBA. As a consequence, the photoeffects of the heterostructur e diminish before the ordering temperature is reached . The m agnitude of the change is associated with the ex tent of strain induced at the interface and the depth to which it has an effect, as well as by the size of magnetic domain s. The measured photoinduc ed magnetization change s sug gest the perturbed region extends beyond just the ions at the inter face and involves reorienting do mains several unit cells deep. 29, 96 Compared to othe r magnetic PBA lattices, the NiCr PBA appears to be more susceptible to interface effects

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47 and may be related to the significant pressure dependence of the NiCr PBA magnetic response. 101, 102 Conclusion In summary, a Hofmann like spin crossover compound was coupled with a nickel hexacyanochromate Prussian blue analogue to develop a new type of coordination polymer heterostructure in which a light induced change in one network induces a magnet ization change in the other. For the first time, the interface mediated photoinduced magnetization change in coordination polymer heterostructures is observed with structurally different materials, demonstrating that this mechanism for switching magnetism with light can be general .

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48 Figure 2 1. Cross section scheme illustrating the layered architecture of the Fe(azpy)[Pt(CN) 4 ]/NiCr PBA heterostructure. Figure 2 2. SEM images of cross sections of the heterostructure films. Images illustrate the continuous film deposited on the substrate surface. Black arrows beside image highlight the interface between the glass substrate and the film.

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49 Figure 2 3 . ATR FTIR spectra showing the growth of CN stretches as th e film devel ops. Data were collected aft er the deposition of the NiCr PBA layer ( ( CN ) = 2175 cm 1 ) and the Hofmann like layer (heterostructure, black t race) . The Fe(azpy)[Pt(CN) 4 ] trace (blue ) was calculated by subtraction, yielding a peak position 1 ) that is consistent with single phase Fe(azpy)[Pt(CN) 4 ] films and as seen for similar Fe NC Pt bridging modes. 60, 92, 103 Fi gure 2 4. The thermal SCO behavior of the Fe(azpy)[Pt(CN) 4 ] single phase material as measured by Raman spectroscopy. Raman spectra recorded at 303 K and 123 K (left) exhibit thermally induced changes including peak shifts and the disappearance of modes ass ociated with the HS state (i.e. 1497cm 1 ) as temperature is reduced. The HS fraction is monitored as a function of temperature (right) from the normalized intensity of the HS mode at 1497cm 1 relative to a mode which is unaffected by the spin transition (1 160 cm 1 ). The resulting HS fraction vs. temperature curve reveals an incomplete spin transition with ca. 40% HS material at low temperature with a T 1/2 210 K, which is consistent with previously reported results.

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50 Figure 2 5. XPS spectra showing binding energy regions for Ni 2p 3/2 and Cr 2p 3/2 afte r NiCr deposition (top), and the Fe 2p and Pt 4f regions after Fe(azpy)[Pt(CN) 4 ] deposition (bottom). Metal ratios for each layer (Ni:Cr = 3:2, Fe:Pt = 1:1) are consistent with the respective target materials.

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51 Figure 2 6 . XPS data of the heterostructure after various Fe(azpy)[Pt(CN) 4 ] deposition cycles illustrating the growth of the Hofmann network as deposition cycles are increased. A) Ni 2p 3/2 , Cr 2p 3/2 , Fe 3p, and Pt 4d 5/2 spectral regions highlighting t he appearance of Fe and Pt signals as well as the disappearance of Ni and Cr signals , thus demonstrating the layered arch itecture of the heterostructure with the Hofm ann film growth on the surface of the NiCr PBA film. B) The peak areas of spectra for each region in (a) normalize d to the initial peak intensity (Ni and Cr) or final peak intensity (Fe and Pt) are plotted as a function of deposition cycle. a) b)

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52 Figure 2 7. Photomagnetic response of single phase Hofmann and heterostructure films. A) MT vs temperature plot normalized to M T at T=70 K, illustrating a photoinduced increase in magnetization. The single phase fi lm was both FC and data collected in a field of 1 T. The inset shows the T (LIESST) of the single phase film, defined as the minimum of the d(MT/MT 70K )/dT plot. B) M vs T data in which the heterostructure film exhibits a photoinduced decrease in magnetizat ion. The heterostructure film was FC and data collected in a field of 100 G. The inset highlights the region below 50 K in which the decrease in magnetization is the largest. Figure 2 8. Normalized differences (light dark) in magnetic responses of the single phase Hofmann and heterostructure films. The difference plots highlight the temperature regions in which population and relaxation of the photoinduced HS state occur in the single phase SCO material and the associated ligh t induced effect on the heterostructure. a) b)

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53 Figure 2 9 . M agnetization versus time of irradiation (at 5 K) normalized to the dark state, for Fe(azpy)[Pt(CN) 4 ] in 1 T ( top ) and NiCr/Fe(azpy)[Pt(CN) 4 ] heterostructure ( bottom ). The r esults show a gradual increase in magnetization during illumination of the Fe(azpy)[Pt(CN) 4 ] for 3.5 hours. On the other hand , the NiCr/Fe(azpy)[Pt(CN) 4 ] heterostructure shows an abrupt decrease in magnetization upon irradiation. The heterostructure was i lluminated for 3.0 hours. At t = 0 hrs, the light in switched on for both the heterostructure and single phase Ho fmann. Figure 2 10 . Magnetization versus temperature plot of a NiCr PBA film before irradiation (dark) and after i rradiation with the light turned off (light), illustrating that the NiCr film alone does not exhibit photoinduced changes in magnetization. The film was cooled in a field of 100 G and collected while warming in the same field.

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54 CHAPTER 3 NICKEL HEXACYANOC HROMATE FILM DEPOSITION STUDIES: PROBING THE INFLUENCE OF INTERFACE ROUGHNESS ON THE PHOTOMAGNETIC PROPERTIES OF NiCr PBA BASED HETEROSTRUCTURES Introduction Coordination polymer thin films are of interest toward various applications which require tunable material properties at the nanoscale. 104 106 Furthermore, nanopatterning and highly controlled nanoscale growth have been effective approaches to exploring si ze reduction effects on the physical properties of materials. 55, 107 One potential application is in magnetic devices, specifically toward light assisted magnetic recording, as seen in photoswitchable coordination polymer materials which exhibit multiple stable physical states controlled by an external stimulus. 79 Examples of magneto optical properties, specifically observed in photoma gnetic materials, have been explored as PBA heterostructures in which a photoactive component induces a structural change in a coupled magnetic component, thus altering the magnetic response of the heterostructure. Such examples typically include the CoFe PBA as a photoactuator coupled to a magnetic hexacyanochromate based PBA; however as seen in chapter 2, the CoFe PBA photoactuator may be replaced by other compounds namely the Hofmann like Fe(azpy)[Pt(CN) 4 ] spin crossover compound which exhibit contro lled spin transitions resulting in structural changes. 25 28, 73, 74 A photoinduced change in the NiCr PBA magnetization was observed upon irradiation of the thin film heterostructure which couples the structurally different Hofmann and PBA components. As described previously, the light induced alteration in magnetization is attributed to a magnetomechanical effect, 30, 31 in which the NiCr PBA component experiences strain mediated magnetic changes due to the thermal and photoinduced spin transitions of the coupled photoactuators.

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55 Our investigation of the photomagnetic Hofmann/NiCr PBA heterostructure films invites further exploration of the relationship between structurally different coordination polymers and the extent of photoactivity. In the current chapter, we f ocus on the role of the interface, specifically the NiCr PBA surface features that contribute to the heterostructure interface. The NiCr PBA film growth is performed using the sequential adsorption (SA) technique to understand growth mechanisms and optimiz e deposition parameters for film growth with targeted thickness and roughness. The sequential adsorption (SA) technique, first developed by Decher and coworkers, 108 has been utilized as a facile template assisted asse mbly method to develop single or multiphase thin films without special chemical modifications. 17, 18, 56, 76, 95, 109 111 In addition to the inexpensive and often environmentally friendly nature of the technique, SA provides facile development of multi layer films with complex architectures /functionality and property engineering via controlling surface interactions of the film with ionic or macromolecular building blocks. Herein, SA is used in two ways to produce NiCr PBA films as used in chapter 2 and previous literatur e from our group 25, 74, 94 mimicking a layer by layer approach, often seen in literature to monitor the nucleation of PBA materi als on a substrate. The NiCr PBA film thickness, roughness, and morphology are Furthermore, the photomagnetic properties of Hofmann/PBA heterostructures are explored, in which the influence of thickness and roughness of the NiCr PBA component developed by different SA techniques is probed. The various photomagnetic responses are then addressed in the context of previous photoactive PBA heterostructures to better understand the

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56 role of the interfacial topo graphy in mediating mechanical strain in the Hofmann/PBA heterostructures. Experimental Methods Film Deposition Materials: All reagents were purchased from Sigma Aldrich and used as received unless otherw ise specified. Syntheses of 4,4 azopyridine (azpy), K 3 Cr(CN) 6 , and (TBA) 2 Pt(CN) 4 (TBA= tetrabutylammonium) w ere modified from literature methods. 25, 90, 91 Bev l Ledge microscope slides (1.0 mm) and Labcraf t microscope cover glass slides (0.2 mm) were used as film substr ates after cleaning with piranha solution (H 2 SO 4 :H 2 O 2 , 2:1). (Warning: Piranha solution reacts rapidly with organic material and should be handled with extreme caution). Substr ate functionali zation : In select film deposition experiments, glass substrates were functionalized with (3 cyanopropyl)triethoxysilane (CPTES) in lab atmosphere using a method modified from common silanization techniques. 112 Initially, glass slides cleaned with Piranha solution (see above) were immersed in a 3% CPTES solution (EtOH/H 2 O/CPTES 95:2:3) for 60 minutes, followed by soak rinsing the slides in EtOH three times for 5 minutes each to remove exces s CPTES. The sl ides are then dried under a stream of air and subsequently annealed in the oven at 150 ° C for at least 12h. After annealing, the slides are allowed to cool to room temperature and are thoroughly rinsed with MeOH. Functionalized substrates are stored in a m ethanolic 2 mM NiCl 2 · 6H 2 O solution until use. PBA film depositions: The piranha cleaned glass su bstrate was soaked in a n aqueous solution of NiCl 2 ·6H 2 O for 5 seconds, followed by submerging in an aqueous solution containing K 3 Cr(CN) 6 for 5 seco nds to complete one cycle. Solution concentrations were typically 10 mM, 6 mM, and 4 mM. Following each cycle, the s ubstrate was rinsed with deioniz ed wat er. The process was repeated for 2 20 cycles to develop the NiCr film. After PBA

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57 deposition, the film was rinsed with acetone and dried under air flow. Select deposition 1. Ni[Cr(CN) 6 ] 0.67 n H 2 O metal ratio analysis : 60:40 (Ni:Cr). FTIR: (CN) = 2175 cm 1 . Table 3 Sample Solution Concentrations Deposition Cycles 3 1 10 mM 40 3 2 10 mM 20 3 3 10 mM 10 3 4 10 mM 6 3 5 10 mM 5 3 6 10 mM 2 3 7 6 mM 15 3 8 6 mM 10 3 9 6 mM 5 3 10 4 mM 30 3 1 1 4 mM 20 3 1 2 4 mM 10 PBA film depositions: cleaned or CPTES functionalized glass slides. Microscope cover glass slide s (0.2 mm thick) are used as substrates for depositions involving SQUID characterization in order to minimize glass background, whereas experiments not involving magnetometry are grown on standard microscope slides (1 mm thick). Glass slides are typically submerged in each precursor solution for 30 seconds while rinsing in between precursor solutions. Agitated rinsing in two consecutive solvent baths for a total of 6 8 seconds is performed to ensure thorough removal of excess precursor ions from the substra te surface. Solvents used for both precursor and rinse solutions are typically H 2 O, MeOH, or a mixture of the two. After deposition, samples are sprayed with acetone and dried in air . Table 3 2 highlights select deposition parameters for samples deposited K 0.46 Ni [Cr(CN) 6 ] 0. 82 n H 2 O metal ratio analysis : 55:45 (Ni:Cr). FTIR (CN)=2175 cm 1 .

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58 Table 3 Sample Concentration Cycles Deposition Solvents NiCl 2 Sol. K 3 Cr(CN) 6 Sol. Rinse 3 1 3 10 mM 80 H 2 O H 2 O H 2 O 3 1 4 10 mM 40 MeOH MeOH/H 2 O (1:1) MeOH 3 1 5 5 mM 60 MeOH/H 2 O (1:1) MeOH/H 2 O (1:1) MeOH/H 2 O (1:1) 3 1 6 5 mM 60 MeOH/H 2 O (1:3 ) MeOH/H 2 O (1:3) MeOH/H 2 O (1:3) 3 1 7 5 mM 80 MeOH MeOH/H 2 O (1:1) MeOH 3 1 8 10 mM 40 MeOH MeOH/H 2 O (1:1) MeOH 3 1 9 10 mM 80 MeOH MeOH/H 2 O (1:1) MeOH 3 20 10 mM 180 MeOH MeOH/H 2 O (1:1) MeOH Heterostructures: Fe(azpy)[Pt(CN) 4 ] growth: In the Hofmann/NiCr PBA heterostructures, the Hofmann like Fe(azpy)[Pt(CN) 4 ] layer is deposited on top of se lect NiCr carried out using the same protocol as in Chapter 2. The Fe(azpy )[Pt(CN) 4 ] layer was deposited at low temperature by keeping precursor and rinse solutions at 78 °C . The NiCr coated substrate was soaked alternatively in 10 mM ethanolic solutions of Fe(BF 4 ) 2 ·6H 2 O, (TBA) 2 Pt(CN) 4 , and azpy for 60 s econds , with rinsing in pure EtOH in between each step. This process was repeated for 25 or 40 cycles to produce a continuous spin crossover film . All depositions were performed in lab atmosphere. Table 3 3 summarizes the heterostructure samples discussed in this chapter. XPS metal ratio analysis for the Fe(azpy)[Pt(CN) 4 ] layer (for NiCr PBA characterization, see above .): 50:50 (Fe:Pt). ATR FTIR: (CN)= 2175 cm 1 (NiCr) 2160 cm 1 (Hofmann). Table 3 3. Select sample characteristics of Hofmann/NiCr PBA heterostructure samples. Heterostructure Sample Hofmann cycles NiCr Base Layer NiCr Deposition type 3 21 40 Sample 3 3 Fast 3 22 40 Sample 3 4 Fast 3 23 20 Sample 3 12 Fast 3 24 25 Sample 3 3 Fast 3 25 30 (30 mM) Sample 3 3 Fast 3 26 25 Sample 3 18 Slow 3 27 40 Sample 3 19 Slow 3 28 40 Sample 3 20 Slow 3 29 40 Sample 3 2 Fast

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59 Characterization SEM images were collected with an FEI XL 40 FEG SE M at a beam energy of 15 kV. For cross section imaging, film samples were fractured to 1 2 mm in height and mounted so that the fractured side is oriented toward the electron gun to image cross sections of the film. Atomic force microscopy (AFM) measuremen ts were performed using a Digital Instruments Multimode scanning probe microscope. Data were collected in tapping mode using a standard uncoated silicon tip. Thickness measurements were performed by removing select areas of the film sample with tweezers to expose the substrate surface, thus creating a step for thickness analysis. Images are processed using Nanoscope 5.31r1 software from Digital Instruments. X ray photoelectron spectroscopy (XPS) measurements were per formed with a Physical Electronics PHI 50 00 VersaProbe II XPS system ray source . XPS data were co llected with a pass energy of 93.9 eV at 0.8 eV/step (survey spectra) or 23.5 eV at 0.1 eV /step (Ni 2p 3/2 and Cr 2p 3/2 regions) averaging 10 40 scans over a spot size of 0.2 X 0.2 mm 2 . XPS data were fit and metal ratios calculated using PHI Multipak XPS software. Magnetic data were collected on a commercial Quantum Design MPMS XL 7 using similar protocols for both configurations. The samples were held in a drinking straw and or iented perpendicular to the applied magnetic field. Both dark state and light state data were collected custom made optic sample rod s OSR 1.0 93 and OSR 2.0 . 96 Sche matics and detailed descriptions of the sample rods can be f ound in the respective references. Briefly , the OSR 1.0 sample probe consists of eight strands of optical fibers (Ocean Optics, Model 200 UV/VIS) going through a stainless steel rod (Outer Diamete was achieved using a Fiber Lite® High Intensity Illuminator (Series 180 by Dolan Jenner

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60 Industries Inc.) fitted with a Quartzline lamp (General Electric EKE 21 V, 150 W). The power reaching the 1.0 mW. Samples measured using OSR 1.0 PBA layers. OSR 2.0 is a design modified from OSR 1.0, utilizing quartz rods in place of the fiber optics resulting in better irrad iation efficiency. The heterostructure samples were field cooled from room temperature to 5 K or 10 K and measured in a 10 mT applied field. The heterostructures were irradiated at 5 K or 10 K for 0.5 3 hours depending on the sample. The light state data w ere acquired with the light off to show the persistence of the metastable HS state in both configurations. Table 3 4 provides details about the warming and cooling rates employed. Finally, in both dark and light states, the data were collected while warmin g from 5 K to 90 K. Table 3 4 . Warming and cooling rates for magnetometry studies. Sample Cooling Rates Warming Rates a 3 21 10 K/min from 300 K to 10 K 5 K/min from 10 K to 5 K 5 K/min from 5 K to 85 K 10 K/min from 85 K to 90 K 3 22 10 K/min from 300 K to 10 K 5 K/min from 10 K to 5 K 5 K/min from 5 K to 38 K 10 K/min from 40 K to 64 K 5 K/min from 65 K to 85 K 10 K/min from 85 K to 90 K 3 23 10 K/min from 300 K to 5 K 5 K/min from 5 K to 38 K 10 K/min from 40 K to 64 K 5 K/min from 65 K to 85 K 10 K/m in from 85 K to 90 K 3 24 10 K/min from 300 K to 5 K 5 K/min from 5 K to 38 K 10 K/min from 40 K to 64 K 5 K/min from 65 K to 85 K 10 K/min from 85 K to 90 K 3 25 10 K/min from 300 K to 10 K 5 K/min from 10 K to 5 K 5 K/min from 5 K to 38 K 10 K/min fro m 40 K to 64 K 5 K/min from 65 K to 85 K 10 K/min from 85 K to 90 K

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61 Table 3 4. Continued Sample Cooling Rates Warming Rates a 3 26 2.5 K/min from 300K to 10 K 10 K/min from 10 K to 14 K 3 K/min from 14 K to 16 K 1 K/min from 16 K to 82 K 3 27 2.5 K/min from 300K to 5 K 1 K/min from 5 K to 100 K 3 28 2.5 K/min from 300K to 5 K 1 K/min from 5 K to 100 K 3 29 10 K/min from 300 K to 5 K 5 K/min from 5 K to 38 K 10 K/min from 40 K to 64 K 5 K/min from 65 K to 85 K 10 K/min from 85 K to 90 K a When the dat a are being acquired, the sample is stable at a given temperature. The maximum rates for changing the temperature between set points are listed. Development of NiCr PBA Thin Films The development of NiCr PBA thin films described herein is based on the seq uential submersion of a film substrate alternately into NiCl 2 and K 3 Cr(CN) 6 precursor solutions. Deposition parameters such as precursor concentration, deposition solvent, and deposition based on rinsing e fficiency of the substrate (Figure 3 1) are explored in an effort to minimize surface roughness and to better achieve films with targeted thickness and morphology. sferring the substrate from one precursor solution to the next without removing excess precursor ions. Consequently, film deposition relies on mixing of precursor solutions at the substrate surface to produce a polycrystalline film. With this method of dep osition, growth is most easily controlled by ion concentration in solution and number of deposition cycles, taking advantage of NiCr PBA insolubility in aqueous media. Although moderate control of film thickness may be achieved on the microscale, the techn ique is based on simultaneous mixing of precursor solutions near the

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62 substrate surface which inherently produces rough films. The rough topography is due to spontaneous nucleation of NiCr PBA nanoparticles at the substrate surface. In this case, the substr ate identity has some influence in directing film growth, however this is largely limited to the solvent/substrate compatibility which allows mixing of precursor solutions and resulting crystallite formation across the surface. On hydrophobic surfaces, the NiCr PBA nucleation occurs almost exclusively at substrate edges as seen in the case of hydrophobic substrates with aqueous precursor solutions . Development of polycrystalline NiCr technique, is demonstra ted via SEM as a function of deposition cycle (Figure 3 3). Considering 10 mM NiCl 2 and K 3 Cr(CN) 6 aqueous precursor solutions ( 3 1 , 3 2 , 3 3 , and 3 4 ), an increase in both film thickness and surface roughness is observed as the number of deposition cycles increases. The increase in both film roughness and thickness is consistent with the nature of PBA nucleation on the surface, specifically a crystallite deposition rather than a layer by layer mechanism. Varying precursor solution concentrations presents a facile way to control film 4, SEM and AFM images reveal the varying size and morphology of crystallites on the substrate surface in addition to film thickness as a function of solution co ncentration. With increasing precursor concentration, an increase in both thickness and roughness is observed. This trend implies that the number of nucleation sites and overall ionic concentration near the substrate surface during each deposition cycle increasing with larger solution concentrations dictates the overall rate at which film growth occurs (Figure 3 5). Of notable interest is the change in roughness that occurs upon varying concentration. When comparing films of similar thickness, we observ e an increase in

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63 film roughness with increasing precursor solution concentration (Figure 3 6). Relatively uniform film thickness can be achieved with lower concentrations of precursor solutions; however at extremely low concentrations, typically below 3 mM , and shorter submersion times, film growth is limited. Given the particle based nucleation nature of the film, sufficiently low concentrations likely prevents nucleation due to increasing surface free energy, where below a critical crystallite size nanopa rticle dissolution is more favorable than growth. 113 by layer deposition of pr ecursor ions rinsing steps between substrate submersions in precursor solutions, thus removing excess ions from the film surface and restricting film growth to c oordination of targeted framework components in each step. The films produced using the layer by layer approach tend to exhibit a step wise growth at the nanoscale, resulting in much higher control of film thickness and reduced surface roughness. Here it i s important to note that thoroughly removing the excess precursor ions is essential to controlling the layer by layer approach. Insufficient removal of excess ions results in uncontrolled nucleation at the film surface consequently increasing film roughnes s. Alternatively, uncontrolled NiCr PBA growth may occur due to a sufficiently high concentration of precursor ions in the rinse solutions. Therefore, frequent monitoring and refreshing the rinse solvent is necessary to prevent uncontrolled PBA formation. La yer by layer dep osition of NiCr PBA films can be attempted in a variety of ways including using substrate functionalization, optimizing precursor concentrations, and optimizing the solvent systems used in the precursor and rinse solutions to promote cont rolled growth. Appropriate selection of these parameters is important for controlling both the density of

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64 nucleation sites on the surface and adsorption/desorption of component ions, which overall affects the growth rate and surface topography of the NiCr PBA films. Substrate functionalization Substrate functionalization is common to research in materials and biomedical applications, often to promote film growth or to alter the surface chemistry for immobilization of targeted molecules/proteins. 114 116 Although NiCr PBA films have been deposited on glass (and other oxide) surfaces without functionalization, typically via surface hydroxylation using a piranha solution trea tment, surface functionalization allows better control of nucleation site density and in some cases patterning of nucleation sites to direct patterned film growth. In this PBA films provides regular nucl eation sites for Ni 2+ coordination, seeding the NiCr PBA film growth. Alternatively, depositions utilizing piranha treated surfaces are capable of yielding films comparable to those performed with CPTES functionalized surfaces; however depositions with pir anha treated surfaces are typically less reproducible. The CPTES functionalization of glass is performed in a manner similar to the commonly used silane functionalization protocols, 112 utilizing silanol f ormation and subsequent adhesion to the substrate in an ethanolic solution. Annealing the substrate at high temperature (~150 °C) drives off H 2 O resulting in covalent binding of the silane to the glass surface, thus depositing nitrile functional groups on the surface of the substrate. Evidence of successful CPTES functionalization of the glass substrate is observed using XPS (Figure 3 6) , in which strong C 1s and N 1s signals are observed, attributed to good coverage of the cyanopropyl groups on the glass s urface.

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65 Effect of deposition solvent on film growth The deposition solvent plays a vital role in tuning the solubility of the precursor ions and NiCr PBA product in solution, thus affecting the film development. Ideally only H 2 O would be used, since both due to the high solubility of precursor ions in aqueous solutions and the relative insolubility of the NiCr PBA product. However, as seen with 3 13 , NiCr deposition using 10 mM precursor solutions does not occur from H 2 O solvent only. Successful deposition requires reducing the solubility of the NiCr component ions, therefore a similar deposition using almost exclusively MeOH (a 1:1 MeOH/H 2 O mixture for K 3 Cr(CN) 6 is used due to insolubility in pure MeOH) is employed. Using 10 mM p recursor solutions and MeOH for precursor and rinse solvents, full NiCr PBA coverage of the glass substrate is achieved ( 3 14 ). The results of the depositions with H 2 O ( 3 13 ) and MeOH ( 3 14 ) are deter mined by XPS analysis (Figure 3 7). After 80 deposition cycles in H 2 O, only the substrate signals are observed in the XPS survey spectrum, specifically the O1s at ~531 eV and Si 2s and 2p below 200 eV. Only 40 cycles in MeOH reveals a survey spectrum ty pical of NiCr PBA networks featuring signals from Ni, Cr, C, N, and O, as well as a significant scattering background. Furthermore, the lack of Si 2s and 2p signals indicates that a continuous film is generated. AFM imaging supports XPS results for 3 12 (F igure 3 16), revealing a continuous 20 nm thick film deposited using MeOH. AFM imaging was attempted on 3 13 , however no film was observed on the substrate from the H 2 O deposition, which confirms the XPS results. To further explore how tuning solubility af fects film growth and surface topography, mixtures of MeOH and H 2 O in various ratios were used in both precursor and rinse solutions. Film samples were deposited using MeOH/H 2 O ratios of 1:1 ( 3 15 ) and 1:3 ( 3 16 ) with all other deposition parameters kept c onstant (5 mM precursor concentrations). AFM and SEM analysis

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66 (Figure 3 8) reveals that the solvent mixture with higher MeOH percentage ( 3 15 ) yields a continuous film with small surface features and a low average roughness (R a percentage o f MeOH is reduced ( 3 16 ) in the solvent mixture from 50% to 25%, the surface features appear much larger, yielding a densely packed island like topography rather than a continuous film, indicated by regions of bare substrate visible in both SEM and AFM ima ges. Additionally, as highlighted by AFM 3D cross sections, surface features are much larger as H 2 O is increased in the solvent mixture, contributing significantly to increased surface roughness in 3 16 (R a 9 n m). The difference in feature size and cont inuity of the NiCr PBA films resulting from density of nucleation sites on the surface is consistent with increasing precursor solubility as the H 2 O fraction of the solvent mixture is increased. Effect of precursor concentration on film growth When att empting to understand the effect of deposition parameters on targeted film growth, precursor concentration must be considered. Typically, low precursor concentrations ( 1 mM) have been used in literature requiring long submersion times (up to 15 minutes) and EtOH based solvent systems to monitor the nucleation of PBA films on a substrate. 76, 76, 77, 117, 118 While satisf actory for controlling the initial stages of PBA film growth, long submersion times and low precursor concentrations are inefficient for growing thicker films (20 100 nm), therefore, we explore the role of precursor concentration in film deposition to unde rstand optimum concentrations for generating NiCr PBA films of reasonable thickness for probing magnetic properties. As mentioned above, low precursor concentrations require long soaking times for full precursor binding, which is not necessarily feasible f or performing many deposition cycles, and often results in island like film growth. Higher concentrations, however, often result in uncontrolled nucleation on the surface or excessive solvent waste produced to effectively remove physisorbed species. Note t hat ideal precursor concentration likely varies

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67 with the solvent mixture used, therefore ideal concentrations for one solvent mixture may not correlate with another. To probe the effect of precursor concentration on film topography, films were grown with 5 mM ( 3 17 ) and 10 mM ( 3 18 ) precursor solution concentrations in a predominately methanolic solution/rinse solvent system (MeOH for NiCl 2 and rinses, MeOH/H 2 O (1:1) for K 3 Cr(CN) 6 solution), with all other deposition parameters kept constant. SEM imaging (F igure 3 9) illustrates that the lower concentration solutions yield larger island like nucleation on the substrate, whereas the higher concentration solutions yield smaller features which form a continuous film on the substrate. Given a relatively short su bmersion time (30 seconds per solution) and considering ion diffusion time, higher concentration solutions are more likely to produce more complete coverage and nucleation sites on the surface, thus resulting in smaller features and continuous films. Cha racterization and Photomagnetic Properties of Fe(azpy)[Pt(CN) 4 ]/ NiCr PBA Heterostructure Films Heterostructure development is carried out as described previously in this chapter, PBA deposition techniques in order to d erive correlations dictated by the NiCr PBA surface topography NiCr PBA deposition technique produced a rough NiCr PBA layer with an average thickness of PBA films of varying thickness and roughness to illustrate photomagnetic properties. Additionally, deposition of the Hofmann layer a t various thicknesses and nucleation control is compared to illustrate the relative insensitivity of Hofmann deposition control to its function as a photoactuator. It is important to note that very limited control of the Hofmann layer is utilized in

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68 the fo llowing depositions, largely due to difficulty efficiently rinsing the substrate resulting in uncontrolled nucleation. Given the low temperature ( 78 °C) of the deposition and resulting viscosity of the EtOH solvent in addition to the relatively high conce ntration of the precursor solutions, efficient removal of excess precursor components from the film surface is difficult to execute, therefore uncontrolled nucleation to some extent is likely. ion PBA based heterostructures is shown in Figure 3 11. The films include NiCr PBA layers with average thicknesses of approximately 40 nm ( 3 2 3 ), 96 nm ( 3 22 ), and 200 nm ( 3 2 1 , 3 24 , and 3 25 ), with average roughnesses of approximately 11 nm , 33 nm, and 70 nm, respectively. The average thickness of the Hofmann layer deposited on top of the NiCr PBA is kept relatively consistent from ~30 50 nm ( 3 21 , 3 22 , and 3 23 ), however 3 24 utilized fewer deposition cyc les (25 vs 40 cycles) and 3 25 utilized precursor solutions with much higher concentrations (30 vs 10 mM) on a consistent 200 nm NiCr PBA base to produced Hofmann layers which vary in both thickness and film quality. Specifically, 3 24 is grown with fewer cycles to determine whether the heterostructure may be photoactive with a thinner Hofmann layer, and 3 25 with a Hofmann layer approximately 1 10 m thick (deposited in a fashion involving high concentration precursor solutions which result in significant uncontrolled nucleation of Hofmann compound) is described to determine whether the Hofmann thickness and the extent of control over Hofmann nuclea tion has a significant impact on the photomagnetic properties of the heterostructure. The contrasting heterostructure thickness as a result of typical deposition control ( 3 21 ) and uncontrolled nucleation ( 3 25 ) is highlighted in Figure 3 13, depicting mes oscale and microscale heterostructure thicknesses respectively. Additionally, representative XPS survey spectra are

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69 shown for the respective layers as additional characterization in Figure 3 12, illustrating the successful deposition of each layer (as seen in Ni, Cr, C, N, and O signals for the NiCr PBA layer, and the additional Fe and Pt signals of the Hofmann compound). The layered architecture of the heterostructure is implied by the spectra, where the disappearance of the NiCr PBA signals (specifically from Ni and Cr) and appearance of Fe and Pt signals is evident after the Hofmann layer is deposited. PBA deposition Introduced in Chapter 2, 3 21 composed of a ~200 nm thick NiCr PBA lay er and a Hofmann film with ~50 nm thick base layer exhibits magnetic behavior in the dark state typical of NiCr PBA films and particles. Upon irradiation at low temperature, a photoinduced decrease in magnetization is observed up to the T (LIESST) of the Ho fmann compound. The M vs. T plots before and after irradiation are shown in Figure 3 14. Notably, the magnitude of the photoinduced decrease in magnetization for all rough NiCr PBA samples, while significant , appears to be smaller than that of previous CoF e/NiCr PBA heterostructures likely due to weaker structural coupling of components at the interface. Despite varying the Hofmann layer thickness ( 3 24 and 3 25 ), including significant uncontrolled nucleation in the thicker Hofmann film, the heterostructu res exhibit photomagnetic decreases of similar magnitude, suggesting that thickness and roughness of the Hofmann layers used here (>30 nm thick). Upon vary ing the NiCr PBA deposition and keeping the Hofmann layer relatively consistent, the heterostructure exhibits varied photo magnetic properties (Figure 3 16 ). When considering the thicker and rougher films 3 21 and 3 2 2 with NiCr PBA thicknesses of 96 nm a nd 200 nm and roughnesses of 33 nm and 70 nm respectively a significant photoinduced decrease is observed, whereas the thinner and smoother NiCr PBA layer 3 23 with a NiCr -

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70 PBA thickness of 40 nm and roughness of 11 nm exhibits a negligible change in magnetization upon irradiation, despite demonstrating the magnetic profile and ordering of a typical thin NiCr PBA film. The diminished photomagnetic behavior of the heterostructure incorporating the thin PBA layer invites the question of why no change in magnetization is observed upon irradiation in the nanoscale film, yet the mesoscale films are photoactive. It is common to observe differences in material properties due to size reduction effects below a threshold length scale, however even th e thin film retains bulk magnetic properties, as seen in its magnetic ordering behavior. Therefore, film thickness alone is unlikely the cause of the atypical photomagnetic response. Furthermore, PBA core shell particle systems exhibit photoinduced magneto mechanical behavior in PBA magnetic shells as thin as ~11 nm. 27 An alternative suggestion is that the rigid solid support may hinder NiCr PBA lattice deformation to some lengthscale; as a result a threshold film thickness must be achieved in order to observe magnetomechanical effects. This issue is addressed using a functionalized substrate and controlled deposition methods , , and will be discussed later in the chapter. Our attention is now drawn to the NiCr PBA surface topography as a possible explanation for varying photomagnetic responses, which creates the structure of the interface through which the photoinduced stress is applied. When comparing photoactive heterostructures presented th us far, the NiCr PBA surfaces all consist of relatively large features creating a rough interface. In contrast, the photo inactive sample consists of much smaller NiCr surface crystallites and a relatively smooth surface. As the interface likely has a sign ificant influence

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71 over the photomagnetic properties, the difference in surface roughness may contribute to the light induced response of the heterostructure. PBA deposition To further explore the influence of interfacial roughness on the photomagnetic properties exhibited by the Hofmann/NiCr used to generate thin, smooth NiCr PBA films as base layers in the heterostructures. Specifically , NiCr PBA films with average thicknesses ranging from approximately 20 80 nm are generated with average surface roughnesses an order of magnitude smaller (~2 5 nm) than in the terostructure layer ( Figure 3 18 ) demonstrate the deposition of smooth NiCr PBA layers in the thinnest ( 3 26 ), intermediate ( 3 27 ), and thickest ( 3 28 ) heterostructure samples, as well as the increased size of surface features upon deposition of the Hofmann layer. AFM ima ges and cross section analysis (Figure 3 19) reveal NiCr PBA thicknesses of 20 ± 1 nm (thin), 35 ± 2 nm (intermediate) and 83 ± 4 nm (thick), and average roughnesses of 2.8 ± 0.3 nm (thin), 2.9 ± 0.2 nm (intermediate), and 4.4 ± 0.5 nm (thick). Fig ure 3 20 illustrates the significant difference in surface roughness 3 20 ) and rough ( 3 3 ) surface topographies. This NiCr PBA film thickness range was targeted for mu ltiple reasons: firstly the thin films approximately 20 nm and 35 nm thick are of similar thickness to analogous shells of PBA core shell systems. Reducing interfacial roughness to a comparable magnitude as the core shell PBA particles may allow us to estimate the penetration depth of interfacial strain given the greater thickness uniformity of smooth films. Additionally, evaluating the photomagnetic behavior of a film system similar to the heterostructure particle systems may provide insight into the c apability of the structural changes in the Hofmann like framework to elastically couple to

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72 the structurally different NiCr PBA network. Secondly, the length scales of the NiCr PBA thicknesses span a range of magnetic behaviors exhibited by NiCr PBA materia ls. The thinner film, at approximately 20 nm in average thickness, approaches the limit at which size reduction effects may occur in the magnetic response of NiCr PBA, whereas the intermediate thickness (35 nm) is both within the bulk range of NiCr PBA th ickness and analogous to that of the photo inactive rough NiCr heterostructure ( 3 23 ). The thicker smooth film (83 nm thick) is of similar thickness to the rough NiCr PBA layer (96 nm) in the photoactive heterostructure, making it a reasonable sample for c omparison of photomagnetic behavior. Additionally, comparing the thickest smooth film with a photoactive heterostructure provides insight into the ability of a rigid substrate to influence structural perturbations in the NiCr PBA framework. It is important to note, however, that the cyanopropyl groups on the substrate surface are expected to dampen any structural influence that the rigid substrate may impart. Given similar thicknesses of the NiCr PBA layers, cture suggests that the film thickness exceeds the penetration depth of any potential substrate influence on the NiCr PBA lattice. PBA deposition Photomagnetic experiments were carried out below the ordering temperature of the NiCr samples remained consistent to directly compare film samples. Notably, none of the heterostructures with smooth NiCr PBA surfaces exhibit significant photomagnetic changes upon irr adiation, as seen in Figure 3 21 . All heterostructure samples exhibit ferro magnetic ordering of the NiCr PBA layer below 60 K, however the thinnest sample undergoes a much more gradual thermal relaxation of magnetization upon heating the sample, likely attributed to size reduction effects. Upon irradiation, the difference in magn etization as

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73 negligible change in magnetization relative to photoactive heterostructures with rough interfaces. Hypotheses for the lack of photoactivity in the smooth samples include possible structural coherence limitations due to the potentially small NiCr PBA grain size observed in AFM images, or alternatively the photoeffects rely on some threshold surface roughness to apply the necessary stress to the NiCr P BA surface. When considered alongside the photo inactive 3 23 ) with an average roughness of ~11 nm compared to photoactive samples with average roughnesses >30 nm, the greater surface roughness appears to correlate with the extent o f photoactivity in the heterostructure. The interfacial roughness of such heterostructures alone, however, does not definitively explain photoactive behavior, especially when considering the interfaces of photoactive PBA core shell particles. In these part icle systems, the average interfacial roughness is likely within 1 PBA unit cell (~1 nm), yet a significant photoinduced magnetic decrease is observed (typically 10 30% of the total shell magnetization). 26 On e unique feature of the core shell morphology is the encapsulation of the photoactive component by the magnetic component, which is reproduced to varying extents in the film systems. When considering NiCr PBA surface topography, the smooth film provides a relatively flat surface for the growth of the photoactive Hofmann layer, whereas the rough film essentially provides troughs or pockets in which the photoactive material may nuc leate, emphasized in Figure 3 20 etween NiCr PBA crystallites in rough films may be required in order to sufficiently mimic the encapsulation demonstrated with the PBA core shell particles, thus resulting in photomagnetic PBA behavior. Support for this is exhibited by the reverse NiCr CoF e PBA core shell heterostructure particles, in which the

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74 magnetic component is the core and the photoactive component is the shell. In this architecture, the heter ostructure exhibits a substantially smaller photomagnetic response. Film roughness, however, is likely not the only factor as seen by reduced photoinduced decreases which are observed in heterostructures with thicker and rougher NiCr PBA layers (>400 nm average thickness, 3 29 ), where the bulk character of the NiCr PBA dominates the magnetic respo nse, dwarfing the pho tomagnetic behavior (Figure 3 23 ). The magnitude of the photomagnetic changes in 3 29 is comparable to thinner mesoscale NiCr layers ( 3 21 , 3 22 ), suggesting that beyond a threshold film thickness or surface roughness, further increasi ng these parameters will likely have a negligible influence on the photomagnetic properties of the heterostructure. The extent of photomagnetic behavior is likely a combination of both structural factors (complicated by the partial spin transition of the H ofmann compound) and the surface roughness. Conclusion In the development of NiCr PBA/Hofmann heterostructures, the NiCr PBA film deposition has been studied in a variety of ways to understand both the deposition factors responsible for film development a nd the resulting influence of each deposition technique on photomagnetic behavior upon inclusion into heterostructure films. Control of the NiCr PBA modeling a la yer by layer deposition approach, resulting in varied extents of control over film concentrations to tune film thickness and roughness by controlling crystallite size whereas the the precursor adsorption/desorption and surface coverage of the film substrate in a layer by layer growth mechanism.

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75 Unlike core shell parti cle systems, the rough heterostructure interfaces tend to correlate with photomagnetic behavior, whereas the heterostructures with smoother interfaces are relatively photo inactive. The photomagnetic experiments of these bilayer films suggest that surface roughness likely promotes the light induced magnetomechanical effects observed in the heterostructures.

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76 Figure 3 cycle. NiCl 2 K 3 Cr(CN) 6 Rinse Rinse (x2) Rinse (x2) K 3 Cr(CN) 6 NiCl 2 NiCl 2

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77 Figure 3 PBA films as deposition cycles are increased. All precursor concentrations used in deposition are 10 mM, and all scale bars are 1 m. A) 5 cycles ( 3 5 ). B) 10 cycles ( 3 3 ). C) 20 cycles ( 3 2 ). D) 40 cycles ( 3 1 ). a) b) c) d)

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78 Figure 3 3. SEM and AFM images of NiCr PBA films deposited using various precursor ique. SEM images of films with a) 10 mM, b) 6 mM, and c) 4 mM precursor concentrations illustrating the increasing size of surface features as the deposition concentration increases. AFM images an d section height analysis of 10 cycle films deposited with d ) 10 mM, e) 6 mM, and f) 4 mM precursor concentrations, revealing the increasing thickness and roughness of films with increasing deposition concentration. a) b) c) d) e) f) Vertical distance = 112.9 nm Vertical distance = 40.8 nm Vertical distance = 9 5.1 nm

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79 Figure 3 4. Variation in film thickness with increasing deposition cy deposition technique. 10 mM (black), 6 mM (red), and 4 mM (blue) precursor solutions illustrate that decreasing concentrations result in thinner and more uniform films.

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80 Figure 3 5. Comparison of averag e roughness as a function of average thickness for 10 mM (black), 6 mM (red), and 4 mM (blue) precursor solution concentrations using the roughness is observed upon increasing p recursor concentration. Figure 3 6. XPS survey spectrum of a CPTES functionalized glass slide after the beginning stages of NiCr PBA deposition (5 cycles) . Strong C1s and N1s signals are observed, attributed to good cov erage of the cyanopro pyl groups on the glass surface, as well as very weak Ni 2p doublet.

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81 Figure 3 7. Solvent influence on NiCr determined via XP S. A) Analysis of sample 3 11, deposited using H 2 O only, depicts prominent Si 2p/2s and O1s signals from the glass substrate in the survey spectrum, indicating that NiCr PBA is not present on the substrate surface. B) In the survey spectrum for 3 12, depos ited using predominantly MeOH, signals from Ni, Cr, C, N, K, and O are observed, as well as a strong scattering background typically observed with NiCr PBA samples. The absence of Si signals indicates that full coverage is achieved rather than islands depo sited on the surface. a) b)

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82 Figure 3 8. 2 O solvent ratio for 3 13 (50% MeOH) and 3 14 (25% MeOH). Data sets of 3 13 and 3 14 illustrate the continuity and smaller feature si ze produced from the 50% MeOH solutions relative to the island like growth observed from the 25% MeOH solutions. A) SEM image of 3 13 (scale bar = 500 nm). B) SEM image of 3 14 (scale bar = 500 nm). C) AFM height image of 3 13. C) AFM height image of 3 14 . E) Cross section height analysis of 3 13. F) Cross sectio n height analysis of 3 14. G) 3D cross section of 3 13. H) 3D cross section of 3 14. a) b) c) d) e) Vertical distance = 30.2 nm Vertical distance = 32.2 nm f) g) h)

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83 Figure 3 9. SEM data of NiCr concen tration. A) 5 mM. B) 10 mM. Lower precursor concentrations favor island like deposition whereas higher precursor concentrations favour more complete coverage producing a continuous film. Scale bars = 500 nm in both images. a) b)

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84 Figu re 3 10. A verage roughness as a function of average thickness for various samples comparing various NiCr PBA films (replotted from Figure 3 5) techniqu e is highlighted when compared to the rate of roughness increase observed in exhibits a marg inal increase in surface roughness as the film develops, consistent with a true layer by layer deposition of PBA components.

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85 Figure 3 1 1 . AFM characterization of NiCr NiCr layer in 3 23, with an a verage thickness of 40 nm and R a = 11 nm. B) NiCr layer in 3 22, with an average thickness of 96 nm and R a = 33 nm. C) NiCr layer in 3 21, with an average thickness of 194 nm and R a = 71 nm. Vertical distance = 38.5 nm Vertical distance = 95 .1 nm Vertical distance = 192.6 nm a) b ) c )

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86 Figure 3 12. Representative layer of the heterostructure. S ignals associated with the NiCr PBA material (blue) are observed after the deposition of the first layer, and these signals diminish after depositing the Ho fmann layer as Fe and Pt signals appear (black) illustrating the layered architecture of the heterostructure. Ni 2p Cr 2p Pt 4f Fe 2p

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87 Figure 3 13. AFM and SEM images depicting various control of Hofmann layer deposition for heterostructure samples. A) A FM data for 3 21 , with an average heterostructure thickness of ~250 nm and a Hofmann layer thickness of ~50 nm. B) SEM secondary electron (left) and backscatter (right) images of 3 25 cross sections, illustrating the thickness and roughness of the Hofmann layer deposited with high precursor concentrations resulting in uncontrolled nucleation. Average Hofmann thickness is estimated to be 1 10 m. Scale bars are 50 m and 5 m for left and right images respectively. Arrows beside image highlight the interface between the heterostructure film and the substrate edge. Vertical distance = 239.7 nm a) b )

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88 Figure 3 14. Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA deposition sample s measured before and after irradiation with the light off. All samples were field cooled and data collected in a field of 100 G. All samples exhibit decreases in magnetization upon irradiation, despite varying Hofmann thickness. A) 3 24, with a Hofmann th ickness of approximately 20 nm. B) 3 21, with a Hofmann thickness of approximately 50 nm. C) 3 25, with a Hofmann thickness a) b) c)

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89 Figure 3 15.Magnetization vs. temperature di fference plots (light dark) for heterostructures presented in Figure 3 PBA films.

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90 Figure 3 16 .Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA layers with varying thickness. Samples measured before and after irradiation with the light off. All samples were field cooled and data collected in a field of 100 G. Heterostructures in (a) and (b) wit h NiCr PBA layers 96 nm (3 22) and 200 nm (3 21) thick exhibit decreases in magnetization upon irradiation. C) 3 23, with a relatively thin and smooth NiCr PBA layer (40 nm thick and 11.0 nm average roughness), exhibits a negligible change in magnetization. a) b) c)

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91 Figure 3 17.Magnetization vs. temperature difference plots (light dark) for heterostr uctures presented in Figure 3 16 PBA layers with varying thickness.

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92 Figure 3 18 . SEM images of the heterostructure surface after deposition of the NiCr PBA and Hofmann la NiCr PBA deposition, and the increased surface roughness resulting from subsequent Hofmann deposition. A) NiCr layer of 3 26 . B) Hofmann layer of 3 26 . C) NiCr layer of 3 27 . D) Hofmann l ayer of 3 27 . E) NiCr layer of 3 28 . F) Hofmann layer of 3 28 . a) c ) e ) b ) d ) f )

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93 Figure 3 19 . AFM characterization of NiCr PBA layer in All height scales are 200 nm, illustrating that the NiCr surface remains consistently smooth as NiCr thickness is increased. A) NiCr layer in 3 26 , with an average thickness of 21 nm and R a = 2.8 nm. B) NiCr layer in 3 27 , with an average thickness of 35 nm and R a = 2.9 nm. C) NiCr layer in 3 28 , w ith an average thickness of 84 nm and R a = 4.4 nm. Vertical distance = 19.0 nm Vertical distance = 33.7 nm Vertical distance = 83.9 nm a) b ) c )

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94 Figure 3 20 . SEM and schemes depicting film cross sections of smooth and rough NiCr PBA ars (1 m) and magnifications are equal in the images. A) SEM image of 3 20 , ill ustrating the relatively smooth surface (R a = 4.4 nm) of the ~83 nm film. B) SEM image of 3 3 , illust rating the relatively rough surface (R a = 71 nm) of the ~200 nm fil m . C) Cross section scheme of the SEM image in (a). D) Cross section scheme of the SEM image in (b). a) b) c) d)

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95 Figure 3 21 . Magnetization vs temperature (left) and magnetization vs. time (right) plots for PBA depositions. M vs T data are collected before and after irradiation with the light off. All samples were field cooled and data collected in a field of 100 G. All smooth NiCr based heterostructures show negligible changes in magnetization upon irradiation. Heterostructures in (a), (b), and (c) include smooth NiCr PBA layers 21 nm (3 26 ) and 35 nm (3 27), and 8 3 nm (3 28) thick. Note that in (c) , the slight translation of the light magnetization is likely an artifact of the measurement rather than a decrease in magnetization. a) b) c)

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96 Figu re 3 22.Magnetization vs. temperature difference plots (light dark) for heterostr uctures presented in Figure 3 21 ( 3 26 , 3 27 , 3 28 ) PBA films. Note that for sample 3 28 , the difference observed below 60 K is likely an artifact of the measurement rather than a real photoinduced decrease in magnetization.

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97 Figure 3 23 . Magnetization vs temperature (left) and magnetization vs. time (right) plots for 3 29, a heterostructur PBA layer with a thickness >400 nm and average roughness >100 nm. The film is measured before and after irradiation with the light off. The sample was field cooled and data collected in a field of 100 G. The photomagnetic decrease observed upon irradiation of 3 29 is of similar PBA layers, despite the much thicker and rougher NiCr PBA layer shown here. The comparable magnitudes of the photomagnetic decreases for t surface roughness on the photomagnetic properties is limited in these heterostructures for mesoscale NiCr layer thicknesses. Figure 3 24.Magnetization vs. temperature differenc e plots (light dark) for the heterostructure presented in Figure 3 23 PBA layer with a thickness >400 nm and average roughness >100 nm.

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98 CHAPTER 4 SYNERGISTIC PHOTOMAGNETIC EFFECTS IN A HETEROSTRUCTURE OF HOFMANN LIKE [Fe(4 P HENYLPYRIDINE) 2 [Ni(CN) 4 ]·0.5H 2 O] AND K 0.4 Ni[Cr(CN) 6 ] 0.8 · n H 2 O COORDINATION POLYMERS Introduction Interest in coordination polymer materials is recently expanding due to their variety of physical properties which can be exploited for applications such as gas storage and separations, ion storage, catalysis, informa tion storage, and sensors . 8, 100, 119 126 The ability to control size and morphology of such materials lends itself to possible technological applications, where size reduction effects and the control and optimization of physical properties at the nanoscale are commonly pursued. 72, 100, 127, 128 In particul ar, the areas of magnetism and photomagnetism in nanoscale coordination polymers are of recent interest in a field typically dominated by traditional inorganic solids. 15, 79, 129 The ability to tune the magnetic properties of robust coordination polymer materials, however, provides an attractive class of materials for fundamental studies toward potential applications. In addition, coupling coordination polym ers in heterostructure architectures allows the increase in functionality of materials, including the possibility of synergy between material properties. 23, 104 , 130 Both the rational design of such materials and understanding the fundamental pr ocesses responsible for photomagnetic phenomena are important for possible utility. Previous work from our group has coupled different Prussian blue analogue (PBA) coordi nation polymers in nanoparticle 26 28 and thin film 25, 74 heterostructures t o explore a new mechanism for switching magnetism with light. By forming an interface between separate photoswitchable and magnetic components within an appropriate size regime, a light induced structural change in a photoactive component influences the ma gnetic response of a photo inactive magnetic component. A common photoactuator in the PBA heterostructures is cobalt

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99 hexacyanoferrate ( CoFe PBA ) , which undergoes a charge transfer induced spin transition (CTIST) that can be triggered either thermally or a t low temperatures with light resulting in significant structural changes. B y coupling the photoactive component with PBAs with relatively high ferro or ferrimagnetic ordering temperatures, such as nickel hexacyanochromate ( NiCr PBA) and chromium h exacyanochromate (CrCr PBA), 131, 132 the magnetic response of the magnetic component can be altered upon irradiation. The individual magnetic components themselves are not photoactive and the photoi nduced magnetic response is the result of magnetomechanical coupling across the interface. The synergistic photomagnetic effects are not limited to heterostructures of Prussian blue analogues with similar lattice parameters and structures , as was recently demonstrated when a Hofmann like iron spin crossover compound, Fe(azpy)[Pt(CN) 4 ] (azpy = 4,4' azopyridine), was used as a photoactuator coupled to NiCr PBA in a heterostructured thin film. 73 Similar to the CoFe PBA , Fe(II) spin crossover compounds are known to undergo significant Fe N bond length changes of approximately 0.2 Ã… upon spin transition . By using the light induced excited spin state trapping (LIESST) effect in the Hofmann like compound, magnetization chan ges are observed in the NiCr PBA component, a result of elastic coupling across the interface with the transition of the structurally different Hofmann compound. To further demonstrate that the mechanism is general, the present study develops a nanopartic le heterostructure based on a Hofmann like network, here a two dimensional network , Fe(phpy) 2 [Ni(CN) 4 ] (phpy = 4 phenylpyridine), first reported by Seredyuk et. al. 78 Although the crystal structure of the single phase Hofmann like compound is not known, powder diffraction and XAS along with other spectroscopic techniques have been used to determine both the local environment of the Fe centers and the overall Hofmann like framework. 78 Additionally, the

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100 compound was shown to exhibit both a thermal spin crossover and a LIESST effect. Herein, the structural changes associated with the spin crossover and LIESST effect in the two dimensional Hofmann like material are utilized to manipula t e the magnetic response of NiCr PB A component, resulting in a new photomagnetic heterostructure. Experimental Methods Synthesis All reagents were purchased from Sigma Aldrich and used as received unless otherwise specified. Synthesis of K 3 Cr(CN) 6 , and (TB A) 2 Ni(CN) 4 (TBA= tetrabutylammonium) followed literature methods. 25, 90 Fe(phpy) 2 [Ni(CN) 4 ] seed particle synthesis (4 1) Synthesis of seed particles was adapted from literature methods . 78 A 20 mL methanolic solution of 4 phenylpyridine (100 mM) was added dropwise to a 20 mL methanolic solution of Fe(BF 4 ) 2 ·6H 2 O (50 mM) while stirring. Following the addition of 4 phenylpyridine, a 20 mL methanolic so lution of (TBA) 2 Ni(CN) 4 (50 mM) was added dropwise and allowed to stir overnight. Upon addition of (TBA) 2 Ni(CN) 4 , immediate precipitation of a yellow product occurs. The target compound was isolated by centrifugation and washed twice with methanol before s toring in 20 mL MeOH for subsequent syntheses. A 0.5 mL aliquot of the MeOH suspension was removed and MeOH evaporated for further characterization of the dry compound. Fe(phpy) 2 [Ni(CN) 4 ]·0. 5H 2 O EDS metal ratio analysis: 52.9:47.1 (Fe:Ni). FTIR: (CN) = 21 52 cm 1 (s), 2157 cm 1 (s). Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure particle synthesis (4 2) Heterostructure particles are synthesised by a method similar to that used previously in our group. 24, 26 28 An aliquot of the stock seed suspension ( 4 1 redispersed in 500 mL H 2 O while stirring. Aqueous 50 mL solutions of NiCl 2 ·6H 2 O (2 mM) and

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101 K 3 Cr(CN) 6 (2.2 mM) were added dropwise simultaneously to the stirring suspension of seed particles via peristaltic pump over a period of 4 h ours and allowed to stir for 24 hours. The particles were isolated by centrifugation and washed twice with a H 2 O/acetone mixture (5:1). After washing, a small aliquot of the suspension was removed for TEM sample preparation, with the rest dried in air for analysis. Fe(phpy) 2 [Ni(CN) 4 ]·0.5H 2 O/K 0.4 Ni[Cr(CN) 6 ]0.8·nH 2 O EDS metal ratio analysis: 25.1:54.3:20.6 (Fe:Ni:Cr). FTIR: (CN) Hofmann = 2152 cm 1 (s), 2157 cm 1 (s); (CN) NiCr PBA =2132 cm 1 (w ), 2171 cm 1 (m). Characterization Fourier transform infrared spectroscopy (FT IR) data were collected on a Thermo Scientific Nicolet 6700 spectrometer. A pproximately 2 mg of sample was di spersed in a KBr matrix and pressed at approximately 3000 PSI into a pellet for analysis. Room temperature FTIR data (300K) were collected at a resolution of 1 cm 1 averaging 16 scans, measured over a range of 4000 400 cm 1 . Variable temperature FTIR measu rements were carried out by mounting the KBr sample into a copper sample holder coupled to a Displex cryo system in a sealed sample chamber equipped with BaF 2 windows. Temperature control was achieved with a Lakeshore 325 temperature controller coupled to a resistive heater. Samples were cooled from 300 K to 20 K at a rate of approximately 5 K/min, allowing the temperature to stabilize at each setpoint. FTIR samples were irradiated at 20 K using a halogen lamp with a power output of 50 W placed approximatel y 3 cm from the sample . Data were collected at a resolution of 0.5 cm 1 over an energy range of 4000 750 cm 1 . Transmission electron microscopy (TEM) was performed on a JEOL 2010F high resolution transmission electron microscope with a beam energy of 200 k V. TEM samples were ( 4 1 ) or H 2 O ( 4 2 ) and dispersing with sonication. Approximately 0.1 mL of each diluted

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102 suspension was then dropped onto a grid and air dried for analysis (carbon film on a holey carbon support, 400 mes h copper grid, Ted Pella Inc.). Platelet size for 4 1 was measured using ImageJ imaging software. Platelet face length s in this report are measured as the distance between parallel edges of platel ets where discrete edges are observed. Energy dispersive X ray spectroscopy (EDS) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the high resolution TE M. Atomic percentages for Fe, Ni , and Cr were determined by averaging EDS scans in 3 4 regions of each sample grid. Water content of the single phase Fe(phpy) 2 [Ni(CN) 4 ] sample was measured by thermogravimetric analysis (TGA) using a TGA Q5000 instruments from TA Instruments. Data were collected from 300 600 K with a N 2 purg e flow rate of 10 mL/min and a ramp rate of 10.00 K/min and reported in Figure 4 1 . AFM measurements were collected using a Digital Instruments Multimode SPM in tapping mode. The AFM sample ( 4 1 ) was prepared by diluting a suspension with 4 mL toluene followed by sonication to thoroughly disperse the particles. The sample was then dropped onto a methanol cleaned 1 cm 2 glass slide (cut from standard 1.0 mm Bev l Ledge microscope slides) and so lvent allowed to evaporate before measurement. Synchrotron powder X ray diffraction (PXRD) data were collected on beamline 11 BM at the Advanced Photon Source (APS, Argonne National Laboratory, Argonne, IL) using a wavelength of 0.414169 Å. Samples were l oaded into a Kapton capillary tube (inner diameter of range of 0.5 30° with a step size of 0.001° and a scan rate of 0.01°/sec. Data were integrated using GSAS(II) 133 and background subtractions were performed using the FullProf Suite.

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103 Structural modeling was performed using Diamond 3 software to index PXRD data for the single phase Hofmann compound at 295 K. Structural refinemen t was not attempted due to the complexity of the diffraction pattern attributed to a low symmetry l attice and nano sized particles . To index the diffraction pattern, a rough structure was developed initially from extending the unit cell of a published crys tal structure for Fe(py) 2 [Ni(CN) 4 ]. 65 Subsequent alterations of the unit cell parameters were performed to reasonably match the calculated reflection positions to experimental PXRD data. Additionally, atom posi tions within the unit cell were adjusted slightly to more accurately reflect intensities observed in experimental data, as well as to maintain reasonable bond lengths and angles initially distorted by indexing efforts. Magnetic measurements were performed using a commercial Quantum Design MPMS SL 7. Samples were prepared by spreading powder samples between two pieces of transparent tape and mounted in a homemade optic sample rod 96 (OSR 2.0) connected to a h alogen lamp (400 2200 nm). More specifically, the sample probe consists of eight strands of optical fibers r Lite® High Intensity Illuminator (Series 180 by Dolan Jenner Industries Inc.) fitted with a Quartzline lamp (General Electric EKE 21 V, 150 W). The power reaching the sample position is nominally 1.0 mW. Samples 4 1 and 4 2 were field cooled and data co llected at fields of 0.1 T and 100 G respectively. For compound 4 1 , dark data were collected while cooling from 300 5 K before irradiating for 6 hours. After he light off while warming from 5 300 K. Data for 4 2 pre irradiation (dark) and post irradiation with the light off (light) were collected while warming from 5 100 K. Cooling and warming rates used for 4 1 and 4 2 are listed in Table 4 1.

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104 Table 4 1. Cooling and warming rate s used in magnetometry measurements. Sample Cooling Rates Warming Rates a 4 1 2 K/min for 300 K to 100 K 5 K /min for 100 K to 5 K 1 K/min for 5 K to 90 K 2 K/min for 90 K to 300 K 4 2 2 K/min for 300 K to 10 0 K 4 K/min for 10 0 K to 5 K 1 K/min for 5 K to 90 K 2 K/min for 90 K to 300 K a When the data are being acquired, the sample is stable at a given temperature. The maximum rates for changing the temperature between set points are listed. Results Synthesis and Characterization of Single Phase and Hete rostructure Particles The synthesis of Fe(phpy) 2 [Ni(CN) 4 ] seed particles was adapted from the procedure used by Seredyuk et al., 78 utilizing high concentration solutions to yield nanoscale par ticles in a platelet morp hology common to the family of Hofmann type materials. 41, 66 Although synthesized previously, we report the particle size and morphology of the seed compound for the first time. The heteros tructures are prepared with technique s commonly used to form PBA core shell heterostructure particles, in which resuspending the seed compound in water followed by addition of the NiCr PBA precursors allows for heterogeneous precipitation of the PBA, likel y involving exchange of surface phenylpyridine ligands with [Cr(CN) 6 ] 3 to form Cr CN Fe linkages. TEM images reveal the particle size and morphology of seed ( 4 1 ) and heterostructure ( 4 2 ) nanoparticles (Figure 4 2). Compound 4 1 forms rectangular plate lets with average face sizes of 24.8 ± 8.1 nm . Particle aggregation is observed, as they do not inherently possess surface charge or sterics to avoid sintering. Complimentary AFM imaging of individual platelets (Figure 4 3) indicates an average platelet thickness of 4.3 ± 0.8 nm , with face lengths agreeing reasonably with TEM data. The average Fe:Ni ratio determined by energy dispersive X ray analysis (EDS) is approximately 1:1 as expected for the Hofmann like material (Table 4 2).

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105 Additionally, a small amount of H 2 O resides in the framework, indicated by the 1.6% mass loss with TGA analysis (Figure 4 1). The h eterostructure sample ( 4 2 ) shows little change in size, morphology, and aggregation from the seed compound , as seen in Figure 4 2 , yet EDS 2D map ping (Figure 4 4) and 1D linescans (Figure 4 5) of individual particles and small clusters indicate metals from both components (Fe, Ni, Cr) are present in individual particles. The sample appears homogenous and there is no evidence of much larger cubic Ni Cr PBA particles that typically form under these synthetic conditions (Figure 4 6), indicating the NiCr PBA precipitates on the surface of the Hofmann like seed particles rather than separately. The presence of both the Hofmann like and Prussian blue analo gue components is observed using infrared spectroscopy (Figure 4 7). Bands dimensional Fe Ni network are observed for compound 4 1 at 2158cm 1 and 2153cm 1 , consistent with many reported examples for this fa mily of compounds. 66, 92, 134 Additionally, the presence of the phenylpyridine ligand is evident in the lower energy region of the spectrum. For the heter ostructure ( 4 2 ), addition al CN) modes indicate NiCr PBA in the sample, and a broad O H signal from H 2 O intrinsic to the PBA network also appears . Deconvolution of the CN) bands for the heterostructure provides qualitative insight into its bulk composition (Figure 4 8, Table 4 3 ). Notably, a pair of relatively narrow peaks at 2158 cm 1 and 2153 cm 1 , attributed to the Hofmann component, and broad peaks at 2171 cm 1 and 21 3 1 cm 1 , attributed to bridging and terminal NiCr PBA modes, indicate the presence of each extended networ k within the heterostructure. Additionally, a broad at 2160 cm 1 attributed to interfacial Fe Cr is also observed, 135 a result of coordination of the NiCr PBA to the surface Fe atoms of the Hofmann particles through the hexacyanochromate

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106 ions, observable because of the relatively large surface to volume ratio of the Hofmann platelets. Evidence for these linkages also appears in the magnetic response, described later in this report. Powder X ray diffraction data were collected at 295 K t o confirm the presence of each component in the heterostructure sample (Figure 4 9). To better understand the structure of single phase Hofmann ( 4 1 ) structure in the context of particle surface/heterostructure interface and the trends in structural change s upon spin transition, the Hofmann diffraction pattern is roughly indexed to a structural model consistent with the family of 2D Hofmann frameworks (see Figure 4 10 for predicted structure and unit cell parameters). The model indicates the general structu re is similar to that of the Fe(py) 2 [Ni(CN) 4 ] {py = pyridine} 65 , consisting of a corrugated 2D Fe NC Ni CN Fe plane with an extended interplanar distance due to the additional length of the 4 phenylpyridine lig and. Interdigitated phenylpyridine ligands reside stacking interactions typically observed in this family of materials. The diffractio n pattern of 4 2 contain s reflections consistent w ith the FCC NiCr PBA lattice (a estimated from 200 and 400 peak center s) and the seed Hofmann compound, confirming the presence of both bulk structures in the heterostructure. The absence of diffraction peaks consistent with other PBA materials ( such as FeCr PBA) implies that there is no significant mixing of material s, homogeneous precipitation of FeCr PBA or formation of PBA core shell materials during synthesis. Hofmann Phase Spin Transition in 4 1 and 4 2 Variable temperature FTIR is used to m onitor the thermal and photoinduced spin transitions associated with the Hofmann like component in both 4 1 and 4 2 (Figure 4 11). As observed previously in similar networks, 136 indicat e structural changes within the seed c ompound as it undergoes thermal and photoinduced spin

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107 transitions. Behavior of the seed particles and the heterostructure are si milar. Upon cooling from room temperature to 130 K, there is little change in the peak position or intensity of the HS mode at 828 cm 1 . Further cooling below 130 K reveals a decrease in intensity of the original HS mode at 828 cm 1 and the appearance of a new mode at 830 cm 1 transition. No significant change in the spectrum oc curs below 70 K . Once cooled to 20 K, irradiation with white light reverses the peak changes observed thermally, evidence of the light induced excited spin state trapping ( LIESST) effect. Specifically, a decrease in intensity of the mode at 830 cm 1 and an increase in original mode at 828 cm 1 are exhibited by both the single phase compound and the heterostructure. The spin transition characteristic of the Hofmann phase is r etained when the seed particles are coated with the NiCr PBA shell. Structural changes associated with the spin transition can be followed with powder X ray diffraction. 78 The diffraction pattern of 4 1 illustrates st r uctural changes exhibited by the single phase Hofmann compound upon cooling. A thermal contraction is observed upon cooling from transition is induced by cooling fr om 16 0 12), evidenced by reflections shifting to a much greater extent. Note, however, that not all reflections shift, suggesting that the structural change here is likely anisotropic. Specifically, no significant changes in the positions of h00 reflections (correlating with the interplanar direction) are observed , as seen with th e 600 reflection in Figure 4 12. This lack of change in peak position indicates that spin crossover may be accompanied by a rearrangement of ligand packing between the bimetallic planes, reducing the impact of the spin transition on the lattice parameter in that direction. Upon cooling, similar structural changes are observed in 4 2 , further demonstrating that the Hofmann like phase spin transition is retained when part of the heterostructure. The

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108 reflections associated with the Hofmann like component undergo changes consistent with the pure material, whereas the NiCr reflections undergo only slight shifts associated with thermal contraction of the lattice. Magnetic Measurements The magnetic response of 4 1 and 4 2 as a function of temperature and the influence of irradiation with white light was recorded us ing SQUID magnetometry (Figure 4 13 ). Field cooled data for the paramagnetic 4 1 were collected at 1000 G, wher eas data for the heterostructure were obtained with a smaller measuring field, 100 G , due to the ferromagnetic NiCr PBA component of 4 2 4 1 is typical for compounds containing Fe(II) in the S=2 high spin state. Upon thermal cycling, compound 4 1 exhibits a partial spin transition with T 1/2 = 125 K, T 1/2 = 130 K, and a hysteresis width of 5 K. This incomplete spin transition was observed previously in the Hofmann like network, residual HS fracti on more prevalent in networks with an appreciable water content. 137 Below 50 K, further decrease in magnetizat ion is attributed to zero field splitting of the residual HS iron centers. At 5 K, the LIESST effect is indu ced via irradiation with white light, populating a metastable HS state, resulting in an increase in magnetization. Thermal relaxation of the of approximately 65 K (Figure 4 13 a inset). The magnetic response of 4 2 is dominated by the ferromagnetically ordered NiCr PBA component below T c = 65 K. In contrast to 4 1 , upon population of the Hofmann like network Fe(II) HS state after irradiation , the magnetization decreases in the heterostructure. The photomagnetic dec rease persists after the light is turned off until thermally relaxed above 50 K. This light induced change is attri buted to a decrease in the NiCr PBA magnetization. The

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109 magnetization change in the paramagnetic Hofmann like component is much smaller and n ot seen in the pre sence of the ferromagnetic NiCr PBA. Discussion Photoeffects in Single Phase and Heterostructure Particles A new photomagnetic heterostructure has been developed by coupling the photoactive Hofmann like Fe(phpy) 2 [Ni(CN) 4 ] with the ferroma gnetic NiCr PBA, resulting in a nanoparticle system in which the magnetization of the NiCr PBA component can be altered with light. Earlier examples of coupling a light switchable network with a magnetic one in coordination polymer heterostructures to ach ieve photo induced magnetization changes focused on pairing isostructural Prussian blue analogues in either thin films 25, 73, 74 or nanometer scale part icles. 26 28, 74, 138 More recently it was shown that similar photoeffects can be achieved by coupling a n on PBA photoactuator with a NiCr PBA network in heterostructure thin films. 73 The present study, for the first time, achieves light switchable magnetism by coupling dissimilar networks in a nanoparticle heterostruc ture. The magnetization change in the heterostructure correlates with the LIESST effect in the Hofmann like compound, but is associ ated with the NiCr PBA network. The sign of the magnetization change is negative despite spin generation in the Hofmann net work upon undergoing LIESST. Furthermore, the light induced changes in the heterostructure diminish upon warming to T (LIESST), where the Hofmann LS state is repopulated, providing further evidence of synergism between the two heterostructure components. No tably, the magnetization change in the heterostructure is approximately 50 times larger than the change associated with the seed particles of compound 1 dark) plots normalized to the amount of photoactive material (Figure 4 14). Given that Fe(phpy) 2 [Ni(CN) 4 ] is paramagnetic and does not magnetically order, the absolute magnetization change upon spin -

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110 crossover i n 4 1 is small relative to the ferromagnetic moment of the NiCr PBA network. When coupled to the NiCr PBA in the heterostructure, the magnetization change due to the LIESST effect is amplified. The photomagnetic behavior of the heterostructure is attribu ted to magnetomechanical effect s 30, 31 resulting from structural changes transmitted across the interface between the elastically coupled Hofmann and PBA networks, as seen in previous PBA hetero structures. 25 28, 96 Specifically, the structural contraction exhibited by the Hofmann compo und upon cooling applies a stress through the interface to the NiCr PBA, consequently the NiCr PBA component magnetically orders in a strained state. Light induced structural expansion of the Hofmann component relieves strain in the NiCr PBA lattice, alter ing the anisotropy of the magnetically ordered interfacial domains. Reorienting the NiCr magnetic domains away from the applied magnetic field results in a decreased magnetization upon irradiation of the heterostructure. The photoeffect persists after irra diation until warming through the Hofmann T (LIESST), in which thermal relaxation of the metastable HS state returns the NiCr PBA lattice to a strained state. The magnitude of the photo response observed in the current Hofmann/NiCr PBA heterostructure is co nsiderably smaller than previous photoactive PBA heterostructures, suggesting that the penetration depth of the strain experienced by the NiCr PBA lattice is greatly reduced with the Hofmann photoactuator. Magnetomechanical perturbations in this case are e xpected to be smaller in the Hofmann/NiCr PBA system, primarily due to limited epitaxial growth restricted by dissimilar lattice structures. The light induced changes in NiCr PBA anisotropy are further demonstrated using minor hysteresis loop studies perfo rmed on the heterostructure sample ( 4 2 ), described in Figure 4 15. A detailed discussion of the experiment, designed and conducted by Marcus Peprah, can be

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111 found in reference 139 . The minor loops before irradiation are symmetric about the origin of the M vs. H plot, indicating that the strained NiCr PBA allows the magnetic domains to more easily align with the applied field. Irradiating the heterostructure relieves strain in the NiCr PBA, consequently changes the local anisotropy of NiCr magnetic domains a nd thus shifting the minor loops away from the origin below 300 G. At sufficiently high applied fields the photoinduced effects diminish as a result of overcoming the NiCr magnetocrystalline anisotropy, as seen with fields of 500 G and greater. A slight in crease in magnetization of the heterostructure in both the dark and light states below 20 K is not seen in the seed particles or in pure NiCr PBA. 131 The temperature of this feature is consistent with the strength of in teraction between Fe 2+ and Cr 3+ metal centers in analogous PBA compounds, 97, 98 and likely reflects the interfacial covalent coupling of the two components in the heterostructure. The observation of magnetic exchange through the interfacial linkages is unique relative to related core shell systems. Unlike previous mesoscale cubic PBA particles used as cores, the thin nanoscale platelets of Fe(phpy) 2 [Ni(CN) 4 ] afford much larger sur face area, by an order of magnitude, resulting in a higher contribution from the heterostructure interface, in this case the cyanide bridged Fe NC Cr linkages. This magnetic feature increases in the light state, consistent with Fe(II) LS to HS conversion u pon irradia tion (Figure 4 16 ). Evidence of Fe NC incorporates a significant contribution from Fe NC Cr vibrational modes. Over time, an additional vibrational mode emerges at lower frequencies likely from a linkage isomeris ed Fe CN Cr (Figure 4 17), consistent with the behavior observed in iron hexacyanochromate PBA particles. 98, 135 The lack of separate reflections in PXRD data associated with the FeC r PBA phase, however, support the conclusion that the Fe NC Cr linkages are located primarily at the

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112 heterostructure interface rather than a result of homogeneous nucleation, which would result in detectable coherent diffraction. Structural and Chemical C hanges in the Heterostructure The core shell particles combine s tructurally different networks. Specifically, t he NiCr PBA forms a three dimensional cubic network of cyanide bridged transition metal ions , whereas the Hofmann like material consists of two d imensional layers of cyanide bridged transition metal ions he stacking interactions of interlayer phenylpyridine ligands , coor dinated axially to the Fe center s, as commonly seen the family of two dimensional Hofmann compounds. A typical structural feature of the 2D Hofmann family utilizing pyridi ne 65 and halogenated pyridine 66 ligands is the corrugated structure of the bimetallic cyanide bridged plane, which is supported by EXAFS experiments for the phenylpyri dine analogue. 65, 66, 78 The corrugated nature of the plane is further supported by the indexed powder diffraction pattern of the single phase Hofmann compound pres ented in this report. As a result, the growth of the NiCr PBA on the Hofmann platelet surface is unlikely to be epitaxial, rather island like nucleation is expected at the interface. Although the size regime and morphology of the heterostructure component s do not allow for visually observing interfacial coupling via TEM imaging, the EDS mapping indicates the presence of Cr as well as Ni and Fe distributed across the face of each particle cluster. Additionally, the different particle sizes for each material typically observed when grown under the synthetic protocols used in this report are not seen. The NiCr PBA generally forms as 80 100 nm cubic particles while the Hofmann like particles are sub 40 nm platelets. Supported by the lack of homogeneous precipitation of NiCr PBA particles and the uniformity of the nanoscale platelet clusters from single phase to heterostructure, the element maps indicate that a heterostructure is formed upon addition of the NiCr PBA components rather than side

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113 nucleation. The structural and vibrational data confirm that the Hofmann and NiCr PBA frameworks remain intact, rather than significant material mixing upon heterostructure synthesis. Lattice coupling is further suggested by room temperature FTIR experiments, where a symmetric broadening of the Hofmann ligand modes is observed upon heterostructure formation (Figure 4 18 ). Disparity in peak broadening of various ligand modes indicates that some modes may be more strongl y coupled to changes in the Fe 2+ coordinatio n environment than o thers, exemplified by the extent of broadening in pyridine based and phenyl based modes. Such disparity in affected ligand modes upon perturbation is common to Hofmann like frameworks, typically reported as changes in the peak position of ligand modes upon spin transition. 92 upon growth of NiCr PBA is evident in VT PXRD and VT FTIR experiments. PXRD directly s upports significant structural changes in the Hofmann compound through the thermal spin transition, additionally revealing the anisotropic nature of the changes in the Hofmann framework. The VT FTIR experiments indicate that the spin transition can be indu ced in the heterostructure with light at low temperature, evidenced by the re emergence of HS modes at low temperature upon irradiation. It is important to n ote that although the LIESST effect in the Hofmann compound can be monitored directly via SQUID mag netometry for the si ngle phase component, cannot be directly observed via magnetometry in the heterostruc ture material given the weak paramagnetic behavior of the Hofmann component. Thus the spectral changes associated with the LIESST effect in the low temperature FTIR measurements are a key component to relating the alteration in NiCr magnetiz ation to the Hofmann LIESST event.

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114 Conclusion A new heterostructure system has been developed with two structurally different coordination polyme r networks that exhibit synergistic photomagnetic effects at low temperature. A light induced alteration of the NiCr PBA magnetization is observed by utilizing the LIESST effect in the paramagnetic Hofmann like compound, which is attributed to a magnetomec hanical effect at the interface. Furthermore, the inclusion of the LIESST active Hofmann compound into a magnetic heterostructure in essence enhances the magnitude of the neral magnetomechanical mechanism by expanding the library of photoactive materials in various morphologies capable of imparting strain to manipulate the physical properties of the heterostructure.

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115 Figure 4 1. TGA analysis of compound 4 1 . The TGA analysis. The initial weight loss of approximately 1.6% attributed to guest water molecules within the Hofmann lattice, suggesting an overall chemical formula of Fe(phpy) 2 [Ni(CN) 4 ]·0.5H 2 O . Figure 4 2 . TEM images of t he Hofman n like seed particles ( 4 1 ) and the product of subsequ ent NiCr PBA addition ( 4 2 ). A) Sample 1 , illustrating nanoscale clusters of Hofmann platelets. B) Sample 4 2 , upon addition of NiCr PBA component, no significant change to platelet size or mor phology is observed. Table 4 2 . Me tal ratio analysis for 4 1 and 4 2 obtained by bulk Energy Dispersive X ray Spectroscopy (EDS) Sample Fe (%) Ni (%) Cr (%) 4 1 47.02 ± 1.76 52.83 ± 1.65 n/a 4 2 25.19 ± 0.59 54.25 ± 0.26 20.56 ± 0.46 a ) b)

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116 Figure 4 3. AFM particle thickness data of single phase Fe(phpy) 2 [Ni(CN) 4 ]. A) AFM image of seed Hofmann particles ( 4 1 ) dispersed on a glass surface after drop casting from a toluene suspension. B) Trace analysis of AFM image illustrating particle height. An average particle thickness of 4.3 ± 0.8 nm is determined by the mean of the vertical distances of over 100 particles. Figure 4 4 . EDS 2D map of a heterostructure particle. The EDS map illustrates the presence of metals from both components in the s ame region of the heterostructure particle. Figure 4 5 . EDS linescan of heterostructure particles ( 4 2 ) illustrating the presence of metals from both compounds in the heterostructure particle, supporting the heterogeneous growth of the NiCr PBA mate rial on the Hofmann like seed compound. The data shown above are consistent with multiple scans on different areas of the sample. a ) ) b )

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117 Figure 4 6 . TEM images depicting homogeneous precipitation of cubic NiCr PBA particles during a failed heterostructure syn thesis. The above images contrast the size and morphology of Hofmann and NiCr PBA particles, highlighting the ease in detecting a physical mixture of the particles rather than heterogeneous growth of NiCr PBA on the Hofmann particles during heterostructure synthesis. Figure 4 7. Room temperature FTIR data of 4 1 and 4 2 . The FTIR data exhibit vibrational modes associated with the seed Hofmann like compound ( 4 1 ) and each component of the heterostructure material ( 4 2 ).

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118 Figure 4 8 . Room temperature FTIR data of 4 1 and 4 2 depicting the deconvolution of the , as well as a table summarizing modes attributed to the bridging (b) and terminal (t) modes of each compound. Table 4 3 . Room temperature FTIR fitting parameters for 4 1 and 4 2 . 4 1 4 2 PBA reference Peak Center (cm 1 ) FWH M (cm 1 ) Peak Center (cm 1 ) FWHM (cm 1 ) Peak Center (cm 1 ) FWHM (cm 1 ) NiCr PBA (t) n/a n/a 2132 16.9 2130 a ----Hofmann (b) 2139 9.4 2143 10.5 n/a n/a Hofmann (b) 2152 5.5 2152 8.1 n/a n/a Hofmann (b) 2157 6.9 2157 9.8 n/a n/a FeCr inter face (b) n/a n/a 2160 18.6 2160 b ----NiCr PBA (b) n/a n/a 2171 24.7 2174 a 21.2 a a Data reported in reference 25. b Data reported in reference 135 . a) b)

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119 Figure 4 9 . Room temperature (295 K) PXRD patterns of the single phase Hofma nn compound ( 4 1 ), single phase NiCr PBA reference, and heterostructure ( 4 2 ). Data are normalized to the Hofmann reflection at 2.03°. Reflections from both the single phase Hofmann compound and NiCr PBA are observed in the heterostructure sample.

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120 Unit Cell Parameters a 24.46 Å 90° b 7.48 Å 107.2° c 6.80 Å 90° Figure 4 10. Structural model of compound 4 1 . A ) Predicted structural model and unit cell parameters of the unit cell and extended structure of the single phase Hofmann compound ( 4 1 ) derived from PXRD pattern indexed to a monoclinic C2/m (no. 12) space group. B) Experimental diffraction pattern and calculated pattern from selected unit cell parameters. a) b)

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121 Figure 4 11 . Var iable temperature FTIR data illustrating the Hofmann spin transition in single phase and heterostructure compounds. VT FTIR data monitor the changes in a phpy 4 1 (a,b) and heterostructure 4 2 (c,d) as t he Hofmann component undergoes thermal and photoinduced spin transitions. A partial thermal spin transi tion from HS (828 cm 1 ) to LS (830 cm 1 ) is observed upon cooling each sample, which is reversed upon irradiation at 20 K.

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122 Figure 4 12 . Variable temperature PXRD data of select reflections in 4 1 and 4 2 . Diffraction patterns are collected at 295 K (black ), 160 K (red), and 90 K (blue), illustrating the similarities of Hofmann reflection shifts due to spin transition of the Hofmann material in each compound. 111 600 020

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123 Figure 4 13 . Field cooled SQUID magnetometry data of 4 1 and 4 2 before and after irradiation. A) Single phase Hofmann compound exhibits a thermal spin crossover a t T 1/2 = 125 K and a LIESST effect below T = 70 K. The sample was cooled and data collected in an applied field of 1000 G. A T (LIESST) = 65 K was determined, defined photoinduced HS state to the LS state. B) Heterostructure irradiation reveals a decrease in magnetic response of the heterostr ucture as a result of inducing the LIESST effect in the Hofmann component. The sample was cooled and data collected in an applied field of 100 G. Figure 4 14 . SQUID difference plot (light dark) of 4 1 and 4 2 . Data are normal ized to the amount of photoactive Hofmann component illustrating the temperature profiles of the photoinduced changes and relaxation processes. Comparison of the two data sets highlights the increased magnitude of the photoeffect in the heterostructure rel ative to the single phase component alone.

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124 Figure 4 15. Low temperature (5 K) zero field cooled minor hysteresis loop measurements before and aft er irradiation. Magnetization vs. field data illustrating minor hysteresis loop field sweeps of heterostructure before (a and b) and after (c and d) irradiation measured with the light off. The low field regions of interest (b) and (d) are highlighted from the full field sweeps provided in (a) and (c) respectively. The dark data show symmetric loops. Up to the 300 G sweep, a shift in the hysteresis loops is observed after irradiation resulting in asym metric loops, indicating a change in anisotropy of the Ni Cr magnetic domains upon strain relief. The 500 G and 1000 G loops are symmetric about the origin, indicating that the applied field is strong enough to overcome the anisotropic nature of the NiCr PBA domains; consequently the heterostructure does not exhi bit light induced changes in anisotropy. Data are reproduced from the PhD. Dissertation of Marcus K. Peprah. 139

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125 Figure 4 16. SQUID magnetometry data of 4 2 before and after irradia tion normalized to the magnetization value at T=26 K. These data are replotted from Figure 4 13(b), expanding on the temperature region from 5 30 K. These data illustrate the relative magnitudes of the increase in magnetization below 20 K due to Fe Cr inte ractions at the interface of the heterostructure particles. Upon irradiation, the LIESST event in the Hofmann component increases the number of HS Fe 2+ centers, resulting in additional Fe Cr interactions depicted by an increased magnetization from the dark state. Figure 4 17. 4 2 collected immediately after synthesis (black) and several days after synthesis (red). Over time, a mode below 2100 cm 1 appears, attributed to linkage isomerism of bridging CN groups at th e heterostructure interface. The rearrangement from Fe NC Cr to Fe CN Cr bridging modes results in much lower vibrational frequencies (~2075 cm 1 ).

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126 Figure 4 18 . C omp arison o f peak widths in select normalized Hofmann and heterostructure ligand modes . Select modes associated with the pyridine (py) and phenyl (ph) rings of the phenylpyridine ligand exhibit broadening to varying extents, indicating that some modes are more strong ly influenced by heterostructure formation than others. Vibrational modes were assigned from literature calculations of the phenylpyridine molecule. 136

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127 Figure 4 19. Magnetization vs. t ime data illustrating the photoirradiation of the heterostructure sample ( 4 2 ). A decrease in magnetization is observed, which persists after the light is turned off.

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128 CHAPTER 5 EXPANDING THE TWO DIMENSIONAL HOFMANN/NiCr PBA HETEROSTRUCTURE FAMILY: INVES TIGATING NiCr PBA GROWTH ON MESO AND MICROSCALE Fe(X Py) 2 [Ni(CN) 4 ] {X = H, 3 Cl, phpy} PLATELETS Introduction Magnetomechanical coupling between Hofmann like networks and ferromagnetic NiCr PBA in nano and mesoscale heterostructures were investigated in previous chapters of this work. Fabricated as both bilayer films and heterostructure nanoparticle architectures, various combinations of CPs have yielded photoactive nanostructures. In these systems, thermal and light induced structural changes in the photoactuator attributed to the LIESST effect were observed in both the single phase photoactuator and heterostructure. Furthermore, photoinduced structural changes in the Hofmann component correlate with photomagnetic behavior of the heterostructure. Alt hough various techniques are utilized to observe the magnetomechanical property of the heterostructure, a sufficient understanding of local lattice coupling at the interface is elusive due to the size regime and morphology of the CP nanostructures and limi tations of the instrumentation used. Unlike many PBA heterostructure particles in which image contrast and EDS mapping allow differentiation of heterostructure components and component morphologies with electron microscopy, 23, 26, 27, 140 these techniques are ineffective in the previous Hofmann/PBA systems. Furthermore, shell growth modes in PBA heterostructures such as island forma tion and continuous epitaxial growth can be determined and understood based on structural parameters of the heterostructure components, 24 leading to a more thorough understanding of observed magnetomechanical ef fects in the context of interfacial coupling and lattice misfit. To expand on the 2D Hofmann/NiCr PBA system presented in Chapter 4, describing the structural and photomagnetic properties of the nanoscale Fe(phpy) 2 [Ni(CN) 4 ]/NiCr PBA

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129 heterostructure, prel iminary work utilizing alternative 2D Hofmann networks is presented to address questions regarding the nature of NiCr PBA growth on the Hofmann platelet surface. Furthermore, the synthesis and resulting morphology and size of Hofmann platelets is studied t o develop heterostructure particles of varying photoactuator thickness. Proposed studies involve probing the influence of Hofmann thickness on the ability to transmit structural changes via the interface. To this aim, three different 2D Hofmann compounds a re synthesized as meso and microscale models of the nanoscale photoactive platelets presented previously. Initial heterostructure efforts include Fe(X py) 2 [Ni(CN) 4 ] {X = H, 3 Clpy} Hofmann networks, 65, 141, 142 synthesized here as bulk particles similar to the synthesis of bulk Fe(3 Fpy) 2 [Ni(CN) 4 ] platelets reported in literature. 66 The pyridine based analogue, while it does not exhibit a LIESST effect, has a known crystal structure with metal cyanide planes (creating the heterostructure interface) similar to those expected in the Fe(phpy) 2 [Ni(CN) 4 ] analogue, as proposed in Chapter 4. Microscale seed particles allow facile ima ging of both seed particle morphology and heterostructure morphology via electron microscopy. The 3 chloropyridine analogue is used as a Hofmann network for a Hofmann/PBA heterostructure, which exhibits a cooperative partial spin crossover as demonstrated in literature. 142 The LIESST properties of the single phase 3 Clpy based compound and preliminary photomagnetic studies of a Fe(3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure are presented here. Additionally, seed part icle size studies are probed to target photoactive platelets of a desired thickness, and NiCr PBA growth is carried out to observe the heterostructure morphology directed by the Hofmann network. Finally, the synthesis of mesoscale Fe(phpy) 2 [Ni(CN) 4 ] is dem onstrated via ligand substitution of Fe(H 2 O) 2 [Ni(CN) 4 ] to create seed particles ideal for imaging via electron

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130 microscopy, thus providing a means to determine the nature of NiCr PBA growth on the surface of the phenylpyridine analogue materials. Such infor mation provides a better understanding of the CP interfacial coupling in the heterostructure, which may be applied to the nanoscale photoactive heterostructure system described in Chapter 4. Experimental Methods Synthesis All reagents were purchased from Sigma Aldrich or Acros Organics and used as received unless otherw ise specified. Synthesis of K 3 Cr(CN) 6 , and (TBA) 2 Ni (CN) 4 was modified from literature methods . Fe( X py) 2 [Ni(CN) 4 ] particles {X = H, 3 Clpy} (5 1a 5 6a) Synthesis of Hofmann particles was a dapted from literature methods. 66 Generally, a methanolic solution of Fe(BF 4 ) 2 · 6H 2 O (or FeSO 4 · 7H 2 O for 5 2a , 5 2a* ) and X py in stoichiometric amounts was added dropwise to a stirring aqueous solution of K 2 Ni(CN) 4 and allowed to stir for several hours (or 45 minutes for sample 5 2a* ) before isolating and washing the product. Table 5 1 below lists synthetic parameters and conditions used in each synthesis. The precursor solution addition was performed using a separa tory funnel with an addition rate slower than 10 mL/hr, with the exception of sample 5 3a* which was added via peristaltic pump at a controlled rate of ~10 mL/hr. Samples are synthesized at room temperature, with the exception of sample 5 2a* which was syn thesized in an ice bath at 4 °C. After stirring, the light yellow precipitate is filtered and washed with 1:1 MeOH/H 2 O. The precipitate is stored in MeOH for subsequent use, with a small aliquot dried in air for analysis. EDS metal ratio analysis (Fe:Ni:Cl ): 42.59:57.41:0 ( 5 1a ), 26.49:23.65:52.98 ( 5 2a ), 26.52:21.88:51.39 ( 5 2a* ),

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131 23.73:23.42:52.85 ( 5 3a ), 23.11:23.34:53.55 ( 5 4a ) 25.31:25.92:48.77 ( 5 5a ), 24.79:24.97:50.24 ( 5 6a ). FTIR : (CN) py Hofmann = 2159 cm 1 (s) ; (CN) 3 Clp y Hofmann 2156 cm 1 (s). Table 5 1. Synthetic parameters and conditions for samples 5 1a 5 6a. Sample X py Precursor quantity (Fe 2+ /Ni(CN) 4 2 , X py ) Solution Volumes Addition Method Temperature 5 1a py 0.4, 0.8 mmol 20 mL Separatory Funnel Room Temp. 5 2a 3 Clpy 0.25, 0.5 mmol 30 mL Separatory Funnel Room Temp. 5 2a* 3 Clpy 0.25, 0.5 mmol 30 mL Separatory Funnel 4 ° C 5 3a 3 Clpy 0.25, 0.5 mmol 30 mL Separatory Funnel Room Temp. 5 3a* 3 Clpy 0.25, 0.5 mmol 3 0 mL Pump Room Temp. 5 4a 3 Clpy 0.25, 0.5 mmol 25 mL Separatory Funnel Room Temp. 5 5a 3 Clpy 0.25, 0.5 mmol 20 mL Separatory Funnel Room Temp. 5 6a 3 Clpy 0.25, 0.5 mmol 10 mL Separatory Funnel Room Temp. Fe( phpy) 2 [Ni(CN) 4 ] particle synthesis (5 7a) Mesoscale Fe(phpy) 2 [Ni(CN) 4 ] platelets (5 7a) were synthesized using ligand substitution of Fe(H 2 O) 2 [Ni(CN) 4 ] · xH 2 O particles (5 7a*) . Initially, Fe(H 2 O) 2 [Ni(CN) 4 ] · xH 2 O platelets are synthesized by adding a 10 mM aqueous Fe(BF 4 ) 2 · 6H 2 O (100 mL) solution to a 10 mM aqueous K 2 Ni(CN) 4 ( 10 0 mL) solution via separatory funnel, using an addition rate slower than 2 0 mL/hr. After stirring several hours, the suspension is filtered and washed with H 2 O before resuspending in MeOH for storage. To perform ligand substit ution, a concentrated methanolic suspension of Fe(H 2 O) 2 [Ni(CN) 4 ] · xH 2 O particles (~190 mg in 4 mL) is added quickly to a stirring MeOH/H 2 O (1:1) solution of phpy (200 mM, 10 mL). The resulting yellow suspension is stirred for several hours before filtering and washing with MeOH/H 2 O (1:1) mixture. The particles are stored in MeOH until further use. EDS metal ratio analysis (Fe:Ni): 47.54:52.46 ( 5 7a* ), 44.58:55.42 ( 5 7a ). FTIR : (CN) H 2 O Hofmann = 2155 cm 1 (s) ; (CN) phpy Hofmann 2154 cm 1 (s).

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132 Fe( X py) 2 [Ni(CN) 4 ]/NiCr PBA {X = H, 3 Clpy, phpy} heterostructures (5 1b 5 7b) Heterostructure samples are synthesised by a method similar to Chapter 4 and as used previously in our group. 24, 26 28, 140 An aliquot of the stock seed ) was redispersed in 500 mL H 2 O while stirring. Aqueous 50 mL solutions of NiCl 2 ·6H 2 O and K 3 Cr(CN) 6 were added dropwise simultaneously to the stirring suspension of seed particles via peristaltic pump over a period of 5 hours . Solution concentrations for each deposition are listed in Table 5 2. After addition, the suspension is allowed to stir for up to 24 hours . The particles are then isolated by filtration and rinsing with H 2 O, before storing in MeOH. EDS metal ratio analysis (Fe:Ni:Cr:Cl): 32.06:57.23:10.70:0 ( 5 1b ), 23.90:26.4 4:1.12:48.55 ( 5 4b ), 21.73:28.12:4.55:45.61 ( 5 4c ), 17.58:32.33:9.67:40.41 ( 5 5b ), 14.07:38.34:18.01:29.58 ( 5 6b ), 33.02:56.53:10.45:0 ( 5 7b ). FTIR: (CN) 3 Clpy Hofmann = 2156 cm 1 (s); (CN) py Hofmann = 2159 cm 1 (s); (CN) phpy Hofmann = 2155 cm 1 (s); (CN) NiCr PBA = 2172 2176 cm 1 ( w m). Table 5 2. Synthetic parameters for heterostructure syntheses. Sample Seed Compound NiCl 2 · 6H 2 O K 3 Cr(CN) 6 Volumes 5 1b 5 1a 0.1 mmol 0.11 mmol 50 mL 5 4b 5 4a 0.04 mmol 0.042 mmol 50 mL 5 4c 5 4a 0.1 mmol 0.11 m mol 50 mL 5 5b 5 5a 0.2 mmol 0.22 mmol 50 mL 5 6b 5 6a 0.2 mmol 0.22 mmol 50 mL 5 7b 5 7a 0.1 mmol 0.11 mmol 50 mL Characterization Fourier transform infrared spectroscopy (FT IR) data were collected on a Thermo Scientif ic Nicolet 6700 spectrometer. A pproximately 2 mg of dried sample was dispersed in a KBr matrix and pressed (a~3000 PSI) into a pellet for analysis. Room temperature FTIR data (300K) were collected at a resolution of 1 cm 1 averaging 16 scans, measured over a range of 4000 400 cm 1 . SEM images were collected with an FEI XL 40 FEG SEM with a beam energy

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133 of 15 kV. Samples were drop casted onto a glass slide which was then mounted with carbon tape onto a standard mount. Transmission electron microscopy (TEM) was performed on a JEOL 2010F high resolution transmission electron microscope with a beam energy of 200 kV. TEM samples were suspension with approximately 0.25 mL of acetone . Approximately 0.1 mL of each diluted suspension was then dr opped onto a grid and air dried for analysis (carbon film on a holey carbon support, 400 mesh copper grid, Ted Pella Inc.). Energy dispersive X ray spectroscopy (EDS) was performed with an Oxford Instruments EDS X ray Microanalysis System coupled to the hi gh resolution TE M. Atomic percentages for Fe, Ni , and Cr were determined by averaging EDS scans in 3 4 regions of each sample grid. Powder X ray diffraction (PXRD) measurements were collected on a Panalytical XPert Powder diffractometer with a Cu K source ( = 1.54 Å ). Powder samples were measured over a 2 range of 5 50 ° with a step size of 0.008 ° /step. Magnetic measurements were performed using a commercial Quantum Design MPMS SL 7. Samples were prepared by spreading powder samples between two pieces of transparent tape and mounted in a homemade optic sample rod 96 (OSR 2.0) connected to a halogen lamp (400 2200 nm). More specifically, the sample probe consists of eight strands of optical fibers ( Lite® High Intensity Illuminator (Series 180 by Dolan Jenner Industries Inc.) fitted with a Quartzline lam p (General Electric EKE 21 V, 150 W). The power reaching the sample position is nominally 1.0 mW. Samples 5 5a and 5 5b were field cooled and data collected at fields of 0.1 T and 100 G respectively. After ta were then collected with t he light off while

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134 warming from 5 300 K. Data were collected pre irradiation (dark) and post irradiation with the light off (light) while warming from 5 100 K. Table 4 1. Cooling and warming rates used in magnetometry measurem ents. Sample Cooling Rates Warming Rates a 5 5a 2 K/min for 300 K to 5 K 1 K/min for 5 K to 100 K 2 K/min for 100 K to 300 K 5 5b 2 K/min for 300 K to 5 K 1 K/min for 5 K to 100 K 2 K/min for 100 K to 300 K a When the data are being acquired, the sample is stable at a given temperature. The maximum rates for changing the temperature between set points are listed. Particle Synthesis and Morphology Fe( X py) 2 [Ni(CN) 4 ] {X = H, 3 Cl} particles Bulk pyridine and chloropyridine based Hofmann particles are synt hesized using a typical protocol for bulk microcrystalline 2D Hofmann compounds. 142 Bulk synthesis of the pyridine analogue ( 5 1a ) resulted in poorly defined platelets with face lengths ranging from ~100 nm to seve ral m (Figure 5 1). Optimization of the synthesis was not pursued beyond attempting a heterostructure synthesis using the py based particles. The bulk chloropyridine analogue ( 5 2a ) synthesis yields platelet particles with average face length >1 m and a larg e size dispersion (where 1 is roughly 21% and 29% of total face length and thickness respectively), similar to the particle size reported for the 3 fluoropyridine analogue. 66 SEM images and particle size distribu tion of the bulk chloropyridine based ( 5 2a ) particles illustrating the size and morphology of the bulk product is presented in Figure 5 2. FTIR and PXRD characterization of the bulk py and 3 Clpy based Hofmann compounds. Chloropyridine based Hofmann plat elets of various sizes, with average face lengths and thicknesses ranging from ~100 1000 nm and ~6 130 nm respectively, were synthesized by varying the concentration of Hofmann precursor solutions from 16 mM to 50 mM (Fe 2+

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135 concentrations). As seen in the b ulk Fe(3 Clpy) 2 [Ni(CN) 4 ] particles ( 5 2a ), size dispersion of both platelet face lengths and thicknesses are relatively large, with the standard deviation approximately 20 30% of the respective total sizes. SEM images of 5 3a , 5 4a , 5 5a , and 5 6a as well as AFM characterization of 5 6a is shown in Figure 5 5 and Figure 5 6, with the measured particle size distributions in Figure 5 7. The FTIR spectra of these samples are presented in Figure 5 8, illustrating the vibrational modes associated with the bridgi ng Fe NC Ni network and chloropyridine ligands. Slight differences in spectra are typical with small differences in H 2 O content, seen in both the contribution in O H stretching region and the asymmetry in the CN stretch. Although the surfactant free techni que used here results in polydisperse samples, an active platelet surface is retained to facilitate heterostructure growth. Regarding heterostructure attempts with similar surfactant based seed particle syntheses (nanoscale phpy analogue synthesized with N aAOT), sample color change and preliminary characterization suggest that surfactant coated seed particles may be a viable alternative for heterostructure syntheses due to the weak interactions expected between particle surface and surfactant; however resul ts from these preliminary experiments will not be discussed here. Precursor addition rate and synthesis temperature were varied in an attempt to produce more monodisperse 3 Clpy based Hofmann compounds. The effect of temperature was shown previously to re duce platelet size dispersion in 3 Fpy surfactant based syntheses. 66 Bulk Fe(3 Clpy) 2 [Ni(CN) 4 ] syntheses performed at room temperature ( 5 2a ) and at 4 ° C ( 5 2a* ) result in particles of similar size and morphology, as seen in Figure 5 9. A small decrease in both platelet face length and thickness is observed in the sample synthesized at 4 ° C, however the relative standard deviations of each sample are comparable (20 30%).

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136 The addition rate consistency using a sep aratory funnel was proposed as a possible source of polydispersity in platelet sizes, due to the drop rate consistently slowing drastically (in some cases stopping completely). In syntheses using separatory funnels, drop rate is initially set around 10 mL/ hr and adjusted periodically to maintain the addition rate, however the average rate is considered to be <10 mL/hr. To explore the effect of inconsistent addition rates, an equivalent synthesis performed using a peristaltic pump with a consistent addition rate of 10 mL/hr was performed. The particle size comparison of samples synthesized using a separatory funnel ( 5 3a ) and peristaltic pump ( 5 3a* ) is shown in Figure 5 10. Both platelet face lengths and thicknesses similar in both lengthscale and size dispe rsion, with minor differences attributed to a few large platelets in the 5 3a* sample shifting the averages to larger values. The particle influence on particle s ize or morphology. Fe( phpy) 2 [Ni(CN) 4 ] particles The pyridine and chloropyridine based Hofmann networks were explored as a microscale model of the nano sized Fe(phpy) 2 [Ni(CN) 4 ] platelets described in Chapter 4 in order to better understand the interfacial Hofmann/NiCr PBA coupling, however slight structural differences or ligand influences may alter the NiCr PBA growth mechanism on the Hofmann surface. Due to phpy lability and solubility limitations, meso and microscale Fe(phpy) 2 [Ni(CN) 4 ] platelets do not form using reasonable precursor concentrations and the synthetic techniques described thus far. In order to produce phpy based platelets large enough to image via SEM, an alternative synthetic route is presented involving ligand exchange of a pre formed Ho fmann platelets. Fe(H 2 O) 2 [Ni(CN) 4 ] platelets ( 5 7a* ) are good candidates as sacrificial particles for ligand substitution, owing mainly to the lability of the H 2 O ligand and microscale product. Ligand

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137 substitution is performed by adding a suspension of 5 7 a* platelets to a concentrated phpy solution in a MeOH/H 2 O solvent system. The H 2 O phpy substitution is immediately observed (a few seconds to minutes depending on the relative phenylpyridine and particle concentrations) as a color change from a pale white yellow compound to a bright yellow color consistent with the nanoscale Fe(phpy) 2 [Ni(CN) 4 ] product. A similar experiment performed entirely in MeOH results in substitution; however the rate of color change is significantly slower (up to an hour or longer). Differences in reaction rates can be attributed to the insolubility of phenylpyridine in H 2 O, driving ligand substitution in MeOH/H 2 O mixtures. FTIR and PXRD characterization of 5 7a* and 5 7a is presented in Figure 5 11, illustrating the addition of ph py vibrational modes in the 5 7a FTIR spectrum and the 5 7a diffraction pattern matching that of the nanoscale Fe(phpy) 2 [Ni(CN) 4 ] reference reasonably well. Differences in reflection intensity between the two phpy based materials are expected due to differ ences in preferred particle orientations of the two samples (See Figure C 1 in Appendix C for details on sample orientation issues). SEM images and particle size analysis (Figure 5 12) illustrate effect of ligand substitution on the 2D platelet morphology. Unsurprisingly, a large reduction in platelet face length is 5 7a micrographs in which many platelet prism shapes appear to have a smaller face to thickness aspect ratio. Ad in the 5 7a histogram can be understood by the observed platelet fracture upon ligand substitution. Notably, the edge surface features of the phpy based platelets appear rippled, unlike the s mooth edges observed in the H 2 O based particles (and particles from previous py and 3 Clpy based Hofmann analogues). These edge features suggest that the relative position of adjacent 2D bimetallic planes likely must shift to accommodate coordination an d subsequent

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138 rearrangement of ligand packing of the phenylpyridine molecules between the planes. Alternatively, the visible layers within the particle cross sections may result from particles fracturing. The visible particle surface features are likely a c ombination of both events, as ligand diffusion and simultaneous large scale structural rearrangement likely facilitate particle fracturing. Fe( X py) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure particles Heterostructure synthesis efforts for all Hofmann seed platele ts described previously results in a drastic change in platelet surface features, as seen via SEM imaging in Figures 5 13 and 5 14. In all cases, the NiCr PBA appears to form islands on both the faces and edges of the Hofmann platelets, although the featur es are more easily observed on larger platelets. Island formation is reasonable when considering the structure of the bimetallic planes in the network and that the surface of the Hofmann like platelet faces is terminated in the pyridine like ligands. The c orrugated nature of the 2D planes would limit local epitaxial growth of the NiCr PBA network. Additionally, the lability of the surface ligands has not been quantified or probed in depth, however once some degree of ligand exchange occurs at the surface, p referential coordination of subsequent NiCr PBA components to pre seeded hexacyanochromate moieties may direct NiCr growth. Consequently, it is possible that only a portion of surface ligands are replaced. Bulk EDS measurements (reported in the experiment al section) suggest that Fe, Ni, and Cr are present in all samples, and vibrational modes associated with the NiCr PBA components are observed in most cases using FTIR (Figure 5 15). In samples such as 5 4b and 5 4c , IR evidence of NiCr PBA is barely visib le due to the small portion of NiCr in each sample. Notably, varying the concentration of NiCr PBA precursors does not result in larger surface NiCr

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139 seed particle. SEM and TEM images of samples 5 4b and 5 4c (Figure 5 16) reveal an increase in island density on the platelet surface as the precursor concentration is increased from 0.8 mM to 2.0 mM, however the islands are similar in size and morphology. This similar size in features supports the partial ligand exchange and preferential growth at NiCr nucleation sites proposed here. When contrasting the NiCr PBA feature size on the py based ( 5 1b ) or 3 Clpy based ( 5 4c ) platelets with the phpy based ( 5 7b ) platelets (Figure 5 17), the surface features on the phpy based platelets appear to be considerably smaller than those of the py and 3 Clpy based platelets. Furthermore, the images suggest that the NiCr PBA material forms a pseudo of NiCr PBA nucleation sites, rather than the more discreet NiCr PBA islands observed with 5 1b and 5 4c . These differences in morphology may be explained by possible differences in lability of the phenylpyridine ligand, in which the surface phpy ligands may be more easily replaced than the py or 3 Clpy ligands. Thermal and Light Induced Spin Transitions To assess potential photoactive behavior of the heterostructures described in this chapter, preliminary magnetic data were collected on a single phase F e(3 Clpy) 2 [Ni(CN) 4 ] sample ( 5 5a ) and the associated Hofmann/NiCr PBA heterostructure ( 5 5b ). The thermal spin transition properties of single phase bulk Hofmann framework have been reported in literature, 142 which exhibits a partial cooperative spin crossover centered near T = 115 K. Although the potential LIESST behavior was not addressed, the relatively low T 1/2 suggests that a metastable HS state may be achieved with this framework. The thermal and photomagnetic properties of 5 5a and 5 5b are presented in Figure 5 18. The magnetic response of the field cooled single phase and heterostructure samples were recorded before (dark) and after (light) irradiation while warming from low temperature in applied fields of 1000 G and 100 G respectively. The single phase Hofmann compound exhibits

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140 a LIESST effect as seen by the increase in magnetization both in the M vs T and M vs time plots. The M vs T inset depicts the derivative of the light state data, indicating the T (LIE SST) = 63 K and approximate SCO temperature (T 1/2 of the sample. The heterostructure ( 5 5b ), as seen with previous Hofmann/PBA heterostructure samples, exhibits a photoinduced decrease in magnetization upon irradiation, inducing the LIESST effec t in the Hofmann component. The photoinduced changes in magnetization observed are consistent with description of magnetomechanical behavior in previous CP heterostructure samples. The M vs time data demonstrate that the photoinduced changes in magnetiz ation are persistent at low temperature after the light is turned off. The relaxation behavior of the Hofmann LIESST behavior and decrease in heterostructure magnetization is illustrated in Figure 5 19, illustrating the similar temperature regimes of the c orrelated events. Conclusion The synthesis and morphology of various Hofmann and Hofmann/NiCr PBA heterostructure particles were presented herein. A facile strategy toward size control of Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets while conserving an active p latelet surface was demonstrated, revealing particle sizes ranging from ~100 1000 nm. Platelet size control strategies via methods that do not passivate the seed platelet surface are preferred to perform subsequent heterostructure growth, allowing potentia l semi quantitative studies correlating the spin transition properties of the Hofmann component and photomagnetic properties of the heterostructure as a function of seed platelet size. Furthermore, synthesis of mesoscale Fe(phpy) 2 [Ni(CN) 4 ] platelets via li gand substitution provide potential interface comparisons to the nanoscale heterostructure reported in Chapter 4. Furthermore, qualitative comparisons regarding NiCr PBA growth on the various Hofmann platelet surfaces were discussed, providing potential in sight into NiCr PBA growth on

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141 the Hofmann surface which may aid in future synthesis of targeted systems as well as current magnetic behavior of similar systems. The magnetic behavior of the presented single phase and heterostructure system illustrate a pr omising system for future photomagnetic experiments. The single phase Fe(3 Clpy) 2 [Ni(CN) 4 ] compound and Fe(3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure sample exhibit photoinduced magnetic behavior consistent with previous photoacti ve Hofmann/PBA and PBA/PBA heterostructure systems, suggesting that magnetomechanical behavior is further expanded to this class of heterostructures. With the success of complementary microscopic techniques to better understand the coupling of the two comp onents across the interface, the current system appears promising in developing new mechanically coupled photoactive CP heterostructures.

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142 Figure 5 1. SEM images of bulk Fe(py) 2 [Ni(CN) 4 ] platelets ( 5 1a ). Scale bars = 500 nm. Figure 5 2. SEM images and particle size analysis of bulk Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets ( 5 2a ).

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143 Figure 5 3. FTIR spectra of bulk F e(py) 2 [Ni(CN) 4 ] ( 5 1a ) and Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets ( 5 2a ). Figure 5 4. PXRD patterns of bulk Fe(py) 2 [Ni(CN) 4 ] ( 5 1a ) and Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets ( 5 2a ).

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144 a) b) c) d) Figure 5 5. SEM images of Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets illustrating the varying particle size and polydispersity as a function of precursor solution concentrations used in synthesis . A) 5 3a 5 4a . Scale bars 5 5a (right). D) 5 6a

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145 Figure 5 6. AFM image and cross sect ion analysis of dropcasted 5 6a particles used to determine the average platelet thickness of the sample. Figure 5 7. Particle size distributions of Fe(3 Clpy) 2 [Ni(CN) 4 ] platelets illustrating the va rying particle size and polydispersity as a function of precursor solution concentrations used in synthesis. Platelet face lengths and thicknesses are measured from SEM images of samples, with the exception of 5 6a thickness which was measured via AFM.

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146 Figure 5 8. FTIR spectra of Fe(3 Clpy) 2 [Ni(CN) 4 ] samples produced from syntheses targeting particle size control by varying precursor concentrations.

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147 Figure 5 9. SEM images and particle size distributions illustrating the effect of synthesis temperature on Fe(3 Clpy)[Ni(CN) 4 ] platelet size. Syntheses were carried out at room temperature ( 5 2a ) and at 4 ° C ( 5 2a * ). 5 2a 5 2a*

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148 Figure 5 10. SEM images and particle size distributions illustrating the effect of precursor addition method/drop rate on Fe(3 Clpy)[Ni(CN) 4 ] platelet size. Precursor solutions were added using a separatory funnel with an inconsistent addition rate slower than 10 mL/hr ( 5 3a ) and a peristaltic pump with a consistent addition rate of ~10 mL/hr ( 5 3 a * ). 5 3a 5 3a*

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149 Figure 5 11. FTIR (a) and PXRD (b) data depicting the compositional and structural changes that occur upon ligand exchange from H 2 O phpy in the 2D Hofmann like networks Fe(H 2 O) 2 [Ni(CN) 4 ] ( 5 7a* ) and Fe(phpy) 2 [Ni(CN) 4 ] ( 5 7a ). In (b), a nanoscale Fe(phpy) 2 [Ni(CN) 4 ] sample i s used as a reference for the phenylpyridine based analogue. a) b )

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150 a) b) Figure 5 12. SEM images and particle size distribution of Hofmann platelets before ( 5 7a* ) and after ( 5 7a ) H 2 O phpy ligand exchange. A) Fe(H 2 O) 2 [Ni(CN) 4 ] ( 5 7a*) . B) Fe(phpy) 2 [Ni(CN) 4 ] ( 5 7a ) .

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151 a) b) Figure 5 13. SEM images of bulk Fe(py) 2 [Ni(CN) 4 ] platelets ( 5 1a ) and Fe(py) 2 [Ni(CN) 4 ]/NiCr PBA heterostructures ( 5 1b ). Heterostructure images highlight t he island like growth of NiCr PBA on the surface of Hofmann platelets. A) 5 1a . B) 5 1b

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152 a) b) c ) Figure 5 14. SEM images of seed Fe(3 Clpy) 2 [Ni(CN) 4 ] particles of various sizes described in Figures 5 5 and 5 7 (left) and the respectiv e Fe(3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructures (right) . A) 5 4a (left), 5 4b (right) . B) 5 5a (left), 5 5b (right) . C) 5 6a (left), 5 6b (right) .

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153 Figure 5 15. FTIR spectra of Fe(X py) 2 [Ni(CN ) 4 ]/NiCr PBA heterostructures {X = ph, py, 3 Clpy}. Vibrational modes of from both the Hofmann and NiCr PBA components are observed to varying extents. The 5 4b and 5 4c NiCr PBA contribution in the relatively low NiCr PBA present in the sample (as illustrated in Figure 5 16).

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15 4 a) b ) Figure 5 16. SEM images of heterostructur es synthesized with 5 4a Fe(3 Clpy) 2 [Ni(CN) 4 ] seed particles using two different NiCr PBA precursor concentrations. The samples were synthesized with 0.04/0.42 mM (top) and 0.1/0.11 mM (bottom) NiCr precursor solutions. The images suggest that the NiCr PBA material is deposited as island like growth, and that increasing NiCr PBA precursor concentrations results in an increased density of NiCr PBA islands. A) 5 4b (0.04/0.042 mM precursor solutions) . B) 5 4c (0.1/0.11 mM precursor solutions) .

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155 a) b) Figure 5 17. SEM images comparing NiCr PBA size and morphology on the surface of py ( 5 1b ) and 3 Clpy ( 5 4c ) Hofmann analogues to the NiCr PBA growth on the surface of the phpy Hofmann analogue ( 5 7b ). For repeated syntheses using various pl atelet seed compounds and the same precursor solution concentrations, NiCr PBA surface features appear consistently smaller on the phpy based platelets than on the py based and 3 Clpy based platelets A) 5 1b (left) and 5 4c (right). B) 5 7b .

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156 Figure 5 18. Field cooled magnetization vs. temperature (left) and magnetization vs time (right) plots of a single phase Fe(3 Clpy) 2 [Ni(CN) 4 ] ( 5 5a ) and Fe( 3 Clpy) 2 [Ni(CN) 4 ]/NiCr PBA heterostructure ( 5 5b ) before and after irradiation. A) Single phase Hofmann compound ( 5 5a ) exhibits a LIESST effect below T = 65 K and T 1/2 above T = 105 K. The sample was cooled and data collected in an applied field of 1000 G. The dM/dT vs T data are shown in the inset, emphasizing the temperatures at which thermal and photoinduced spin transitions occur. B) Heterostructure i rradiation reveals a decrease in magnetic response of the heterostructure as a result of inducing the LIESST effect in the Hofmann component. The sample was cooled and data collected in an applied field of 100 G. a) b )

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157 Figure 5 19 . SQUID difference plots (light dark) of MT vs T ( 5 5a ) and M vs T ( 5 5b ) of field cooled irradiation measurements, highlighting the temperature regions of the photoeffect in the single phase and heterostructure samples.

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158 CHAPTER 6 CONCLU DING REMARKS T his dissertation has highlighted several examples of coordination polymer heterostructures with the common theme of coupling structurally different materials to produce synergistic properties. From a fundamental standpoint, the preceding studies expand upo n previous photomagnetic PBA heterostructure systems exhibiting magnetomechanical synergy , thus emphasizing the general nature of the interface mediated photoeffects in the coordination polymer heterostructures . By u tilizing photoactuators other than CoFe PBA in NiCr based heterostructures , the work herein demonstrate s the photoinduced manipulation of physical properties in heterostructures with non ideal lattice coupling. A variety of coordination polymer morphologies and architectures have been explored throughout the document, providing insight into aspects of the photomagnetic effects not seen previously in PBA heterostructures. The series of thin films coupling the nickel hexacyanochromate PBA K x Ni[Cr(CN) 6 ] y · n H 2 O with the Hofmann like network Fe(azpy)[ Pt(CN) 4 ] demonstrates that one coordination polymer may be deposited onto another to create synergy between the material properties despite their dissimilar structures. As thin film deposition is widely used in the creation of functional materials toward m any applications, the optimization and tuning of physical properties as well as understanding fundamental aspects of heterostructure development is important. Because the structurally induced phenomena are mediated by the interface between the two coordin ation polymer components, NiCr PBA growth in these systems is pursued, and controlling thickness and s urface roughness is achieved to a greater extent than seen previously in photoactive PBA heterostructure films. Exploring parameters such as precursor con centration and solvents used in the sequential adsorption film depositi ons provides insight into effectively

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159 promoting precursor adhesion and limiting ion desorption, resulting in a layer by layer growth mechanism. Furthermore, the nature of light induced magnetic per turbat ion is explored by contrasting the photomagnetic response of heterostructures with rough NiCr surface topographies to the photoinactivity of heterostructures incorporating smooth NiCr surface topographies. As a result, varying NiCr roughn ess highlights the correlation between interface topography and the light induced magnetization changes observed in the Hofmann/PBA films. Beyond demonstrating the photoactive behavior of heterostructures incorporating structurally different components , an understanding of the lattice coupling at the interface is necessary to gain insight into the mechanism of the photoinduced magnetization changes. The studies presented here have evaluated the capability of nanoscale ph otoactive platelets to impart strain on a coupled NiCr PBA. The particle morphology of the 2D Fe(phpy) 2 [Ni(CN) 4 ] H ofmann system allows a unique advantage of detecting interfacial interactions due to the high surface to volume ratio inherent to nanoscale platelets. Fe Cr linkages are observed magnetically and spectroscopically, supporting the interfacial coupling of the coordination polymers. Additionally, modeling the structure of the Hofmann component provides an understanding of the Hofmann/PBA interface as well as the anisotropy of the Hofm ann spin transition. Structural changes are induced in the Hofmann component via thermal cycling and irradiation at low temperature , supporting the structural origin of the photomagnetic behavior exhibited by the heterostructure. Despite the limited lattic e coupling and differences in particle morphology between the Hofmann and NiCr PBA components, the heterostructure exhibit s photoactive behavior. Additionally, coupling the paramagnetic photoactuator to the ferromagnetic NiCr PBA results in a n increased ph otoresponse by an order of magnitude, essentially amplifying the light switchable property exhibited by the Hofmann component.

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160 New insight into interface mediated alteration of physical properties opens routes for further exploration of structure property relationships between coordination polymer materials which are coupled at an interface. Future work expanding the family of Hofmann like photoactuators utilized in Hofmann/NiCr PBA systems aims to shed light on the nature of the coupling between Hofmann an d PBA systems in a variety of morphologies and size regimes, thus probing the extent of light induced influence which can be achieved by coupling structurally different materials.

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161 APPENDIX A DEPOSITION AND CHARACTERIZATION OF CrCr PBA FILMS AND CrCr PBA/Fe(azpy)[Pt(CN) 4 ] HETEROSTRUCTURES Deposition of CrCr described herein, as well as subsequent integration into a bilayer heterostructure with the Hofmann like Fe(azpy)[ Pt(CN) 4 ] similar to those described in the NiCr PBA/Fe(azpy)[Pt(CN) 4 ] system described in Chapters 2 and 3. The development of the CrCr PBA film and the heterostructure is described via microscopy and FTIR, and the photomagnetic behavior is monitored using SQUID magnetometry. Film deposition description CrCr PBA : Glass slides (1.0 mm thick for microscopy, 0.2 mm thick for SQUID) or silicon ATR crystals (ATR FTIR) were submerged in piranha solution for ~0.5 hours to both clean the substrates and create a hyd rophilic surface. The substrates were then rinsed with H 2 O before beginning CrCr PBA deposition. The substrates were first submerged in a 10 mM aqueous solution of CrCl 2 for 3 seconds, followed by submersion in a 10 mM aqueous solution of K 3 Cr(CN) 6 . In the case of the CrCl 2 precursor solution, a piece of Zn metal and trace amounts of HCl (~0.1 mL) were added to reduce the Cr precursor which had partially oxidized. The substrate is then rinsed by submersion with agitation in H 2 O for ~5 seconds to complete on e deposition cycle. In the films analyzed below, deposition cycles were repeated from 20 130 times. After deposition, the samples are rinsed with H 2 O and then acetone to dry. CrCr PBA/Fe(azpy)[Pt(CN) 4 ] heterostructures: A 60 cycle CrCr PBA film deposited a s described above was used as the base layer in a PBA/Hofmann heterostructure film. After deposition of the CrCr PBA layer, the Hofmann layer was deposited as described in chapters 2 and 3. Briefly, the substrate was submerged in 10 mM ethanolic solutions of Fe(BF 4 ) 2 · H 2 O,

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162 (TBA) 2 Pt(CN) 4 , and azpy, for 1 minute each, rinsing with EtOH in between each precursor solution. The deposition is carried out in an acetone/dry ice bath at approximately 75 78 °C. The deposition cycle is repeated 40 times to generate th e Hofmann film. CrCr PBA film development/topography ATR FTIR is used to monitor the growth of the CrCr PBA film on the surface of a silicon ATR crystal, and an increase of intensity of the CrCr PBA (CN) is observed at approximately 2187 cm 1 (Figure A 1 ) as the number of deposition cycles increases from 50 130. AFM (Figure A 2) and SEM (Figure A 3) images were collected on films with 20, 30, and 40 deposition cycles to analyze the CrCr film topography as the films develop. Island formation of CrCr PBA m aterial is observed in all samples, with increasing density of CrCr islands as deposition cycles are increased from 20 to 30 cycles. CrCr PBA/Fe(azpy)[Pt(CN) 4 ] heterostructure SEM images reveal the film topography after deposition of the 60 cycle CrCr PBA base layer and subsequent Hofmann layer (Figure A 4). Neither deposited layer forms a continuous film, rather the CrCr PBA forms a densely packed island like layer and the Hofmann layer shows a discontinuous film with smooth regions of Hofmann material on the surface of the CrCr PBA islands. A SQUID magnetometry experiment was performed to evaluate the magnetic and photomagnetic properties of the heterostructure film. Field cooled and measured in an applied field of 100 G, the sample exhibits magnetic order ing of the CrCr PBA material below 230 K in both the dark and light data of magnetization vs temperature plots (Figure A 5a). No photoinduced change in magnetization is observed, as seen in the magnetization vs time data (Figure A 5b). A vertical translati on of the data set after irradiation is observed in the

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163 magnetization vs. temperature curve, and is attributed to an artifact of the measurement rather Figure A 1. ATR FTIR data illustrating the increase in intensity of the CrCr number of deposition cycles increases.

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164 Figure A 2. AFM images of CrCr PBA films after 20 and 30 deposition cycles. A/20 cycles. B/30 cycles. Vertical distance = 90 .5 nm Vertical distance = 105.4 nm a) b )

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165 Figure A 3. SEM images of CrCr PBA films after 20 and 30 deposition cycles. A/20 cycles. B/30 cycles. a) b)

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166 Figure A 4. SEM images of (a) 60 cycle CrCr PBA base layer, and (b) heterostructure after Hofmann deposition. a) b)

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167 Figure A 5. SQUID magnetometry experiments of heterostructure CrCr PBA/Hofmann sample. A) Field cooled magnetization vs. temperature data collected before and after irradiation. B) Magneti zation vs time data, illustrating no change in magnetization in the sample after irradiation.

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168 APPENDIX B EFFECT OF LIGAND CONCENTRATION IN SYNTHESIS OF HOFMANN LIKE Fe(II) SPIN CROSSOVER NETWORKS Synthesis of Hofmann like frameworks involves the assembl y of divalent iron ions, tetracyanometallate ions, and organic ligand components into desired 2D and 3D networks in solution. Typically the ionic components form two dimensional bimetallic plane via coordination of tetracyanometallate moieties bridging Fe centers in the equatorial plane. Fe centers are intended to coordinate neutral monodentate or bismonodentate organic ligands in the axial positions, however solvent molecules (often H 2 O) may compete for these coordination sites under favorable conditions. Literature examples of Hofmann like syntheses often include an excess of ligand or high concentration (low volume) solutions to favor the coordination of the organic ligand over . 38, 41, 78, 143 Common synthetic protocols used by our group for both shell syntheses (in core shell systems) and large particle sizes involve relatively dilute suspensions/solutions in order to achieve targeted m aterials. Attempted syntheses of 2D Fe(phpy) 2 [Ni(CN) 4 ] and 3D Fe(pz)[Pt(CN) 4 ] Hofmann like networks, to increase particle size and as a shell material for core shell particle architectures respectively, resulted in various levels of ligand coordination as described below. Attempted synthesis of NiCr PBA/Fe(pz)[Pt(CN) 4 ] core shell particles NiCr PBA seed particles were synthesized by adding 200 mL aqueous NiCl 2 · 6H 2 O (15 mM) and K 3 Cr(CN) 6 (16 mM) solutions dropwise simultaneously to 200 mL of H 2 O while s tirring. NiCr PBA precipitate was isolated by centrifugation and rinsed with H 2 O and stored in H 2 O for future use as seed particles.

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169 Hofmann heterostructure synthesis was carried out by resuspending 80 100 mg of seed NiCr PBA particles in 50 mL MeOH. Not e that H 2 O was also attempted as solvent for shell synthesis, however no Hofmann material precipitated at the precursor solution concentrations used (2 mM). 50 mL methanolic solutions containing 2 mM ( B 1 ) or 4 mM ( B 2 ) Fe(BF 4 ) 2 · 6H 2 O and 4 mM ( B 1 ) or 8 mM ( B 2 ) pz and 50 mL MeOH/H 2 O (1:1) solutions containing 2 mM ( B 1 ) or 4 mM ( B 2 ) K 2 Pt(CN) 4 were added dropwise via peristaltic pump (rate ~ 8 mL/hr) to the stirring MeOH suspension of NiCr PBA particles and allowed to stir several hours. Particles were isolated by centrifugation and rinsed with MeOH/H 2 O (9:1). Note that overall pz concentration in the NiCr PBA suspension does not exceed 1.34 mM in B 1 synthesis and 2.67 mM in B 2 synthesis. TEM analysis of the B 1 and B 2 indicate that a mixture of parti cles is produced rather than a heterostructure (Figure B 1). Furthermore, the large Hofmann particles resemble platelets typical of thin sheets formed by 2D sheets rather than thicker platelets observed with many 3D network syntheses. Transmission FTIR dat a (Figure B 1 and 2169 cm 1 from NiCr PBA and bridging Fe NC Pt networks respective, however pz ligand modes are not present. With the low pz concentrations used in both syntheses, the main Hofmann product formed is likely the 2D Fe(H 2 O) 2 [Pt(CN) 4 ] · x H 2 O network. Attempted s ynthesis of Fe(phpy ) 2 [Ni (CN) 4 ] platelets >50 nm The literature synthesis protocol for bulk Fe(phpy) 2 [Ni(CN) 4 ] utilizes small solution volumes (~2 mL) and relatively high precursor concentrations to yield th e desired product, 78 resulting in nano sized platelets (<40 nm). Early attempts to grow larger platelets focused on dilute precursor solutions and slow addition via peristaltic pump, reducing nucleation sites and thu s promoting growth of larger particles.

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170 Bulk (nano sized) Hofmann particles have been successfully synthesized by the dropwise addition of a 20 40 mL MeOH Fe 2+ /pz precursor solution to a stirring 10 20 mL MeOH solution of (TBA) 2 Ni(CN) 4 . In these cases, st oichiometric amounts of precursors are used, with concentration of Fe 2+ generally exceeding 40 mM. An alternative synthetic method aimed toward larger volume syntheses involves simultaneously adding MeOH Fe(BF 4 ) 2 · H 2 O and (TBA) 2 Ni(CN) 4 solutions slowly drop wise to a stirring MeOH solution, which has shown varied levels of success in coordinating the desired phpy ligand. Experimental volumes/concentrations attempted for three examples of dilution experiments using this alternative synthetic method are reporte d below in Table B 1. Table B 1. Select precursor concentrations and volumes used in syntheses of Hofmann particles. Sample Fe(BF 4 ) 2 · 6H 2 O solution (TBA) 2 Ni(CN) 4 solution phpy solution Volume Concentration Volume Concentration Volume Concentration B 3 30 mL 16.7 mM 30 mL 16.7 mM 500 mL 2 mM B 4 30 mL 16.7 mM 30 mL 16.7 mM 80 mL 12.5 mM B 5 20 mL 12.5 mM 20 mL 12.5 mM 100 mL 50 mM B 6 a 15 mL 40 mM 15 mL 40 mM 15 mL 80 mM a Note that B 6 is a reference nano sized platelet sample synthesized using the bul k technique described above. FTIR data (normalized to the CN stretch) illustrate the result of syntheses B 3 , B 4 , B 5 , and B 6 described above (Figure B 3). Note that syntheses utilizing higher precursor volumes and lower phpy concentrations result in a larger intensity in the O H stretching region, suggesting H 2 O is incorporated into the framework in place of phpy when phpy concentration is not sufficiently high in the reaction flask. The two extremes, B 3 and B 6 representing low and high phpy concentra tions respectively, depict easily detected differences in framework composition. Firstly, ligand modes are not observed in the B 3 spectrum, however strong evidence of H 2 O is seen both in the large O H stretching and bending modes and the single bridging F e NC Ni mode at 2160 cm 1 . Sample B 6 , however, features an low intensity feature

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171 in the O H stretching region and two partially 1 and 2152 cm 1 . It is important to note here that the two cyanide modes can be resolved due to the insignificant contribution of the 2160 cm 1 mode attributed to bridging CN modes from coordinated water in place of phpy. In cases with higher water content, the separate Hofmann CN modes may not be resolved due to the additional relatively broad CN c ontribution at 2160 cm 1 . The optimal resolution for data collection is 1 cm 1 for these samples, in which narrow Hofmann modes may be resolved without observing vibration rotation features from atmospheric gases.

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172 Figure B 1. TEM i mages of NiCr PBA seed particles and attempted NiCr PBA/Fe(pz)[Pt(CN) 4 syntheses. A) NiCr PBA seed particles. Scale bar = 100 nm. B) B 1 . Scale bar = 200 nm. C) B 2 . Scale bar = 200 nm. Figure B 2. FT IR spectra of NiCr PBA, B 1 , and B 2 . a) b) c)

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173 Figure B 3. FTIR data of samples B 3 , B 4 , B 5 , and B 6 illustrating differences in ligand and water content with varying Hofmann particle synthesis.

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174 APPENDIX C FTIR CHARACTERIZATION METHODS OF NANO SIZED SPIN CROSSOVER PARTICLES FTIR in this work has been used extensively to detect vibrational modes suggesting the presence of CP components in powder samples, and monitoring structural changes in various SCO net works as a function of temperature. The technique used most often used here is grinding a small amount of dry powder samples (~1 3 mg) into 200 300 mg of a KBr matrix and applying pressure (~3000 PSI) for approximately 1 minute to create a KBr pellet. Alte rnative methods used include dropcasting suspensions of powder samples in a volatile solvent (i.e. MeOH or acetone) onto a pressed KBr pellet or ATR multi reflection crystal. These various sample preparation techniques provide both unique challenges and ad vantages when collecting data of CP samples. Standard powder samples are isolated and washed after synthesis, before storing in a solvent typically MeOH until further use. For FTIR analysis where powders are diluted in the KBr matrix, an aliquot of th e stored suspension is first dropped onto a watch glass and the solvent is allowed to evaporate to produce dry powder for analysis. With many of the nano sized products, such as the 2D Fe(phpy) 2 [Ni(CN) 4 ] platelets, this method of powder isolation results i n millimeter scale crystalline flakes (Figure C 1). With respect to FTIR analysis, two large difficulties arise from the nature of the crystalline flake form of the powder. Firstly, attempts to resuspend the powder once dried are largely unsuccessful, desp ite varying solvents and excessive ultrasonic agitation. For this reason, only small aliquots of suspension are dried, leaving the majority of any compound produced stored in solvent. The size regime of the crystalline flakes also proves challenging for sa mple preparation when compared to powder samples. Powder samples often result from drying compounds which are composed of meso or microscale particles which likely are not highly oriented as the solvent

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175 dries. Nano sized platelets and particles as describ ed in this work often result in crystalline flakes, possibly attributed to nanoparticles with surface functionality that may direct oriented stacking. Notably, larger particles of similar compositions (i.e. Fe(phpy) 2 [Ni(CN) 4 ]) to the nanoparticles dry as p owders rather than crystalline flakes. The crystalline nature of the flakes is problematic when attempting to redisperse the product evenly into the KBr matrix. Given the difficulty in redispersing and grinding the crystalline product into a powder, sample flakes remain on the same length scale (or greater) than the wavelength of the incident mid IR energy. Consequently, anomalous dispersion of refractive indices, resulting in the Christiansen effect 144 , is commonly observed when attempting to prepare ground KBr samples with the crystalline flakes. The impacts of this scattering event are illustrated in Figure C 2, as seen by comparing the baseline and peak shapes in data collected on nano sized samples (par ticles produced from fast addition of Fe(azpy)[Pt(CN) 4 ] film deposition waste solutions and bulk Fe(phpy) 2 [Pt(CN) 4 ] synthesis) to mesoscale Fe(phpy) 2 [Pt(CN) 4 ] powder samples. Specifically, distortion in the vicinity of strongly absorbing bands is observed as a drastic decrease in absorbance on the higher wavenumber side of bands and an increase in absorbance on the lower wavenumber side of the band. 144 One method to circumvent spectral anomalies due to scat tering is to analyze the sample using an ATR technique rather than a transmission experiment. Dropcasting the suspended sample onto an ATR crystal may be carried out on samples which are difficult to grind to eliminate artifacts. The ATR technique has limi tations as well, however, which should be understood when trying to analyze spectra, and make comparisons to data collected via the KBr pellet method. Firstly, the multi reflection ATR crystals used in our group (with faces approximately 1 x 5 cm) require much more sample to coat both sides relative to the ~1 3 mg

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176 used in KBr pellets. Insufficient coverage typically results in low signal. Secondly, unlike KBr, the ATR crystals are not transparent to IR radiation, therefore selection of the crystal material is important for analysis of specific vibrational modes. For instance, the Si ATR crystal may be used for analysis of the (CN) stretching region, however it will absorb energy in the same region of Hofmann ligand modes. Typically a Ge ATR crystal is used in samples with organic components, where the crystal does not absorb greatly above 700 cm 1 . It is important to note tha t inherent differences ATR and transmission experiments limit the ability to directly compare data using the two techniques. With the ATR technique, penetration depth of the evanescent wave varies with the wavelength of electromagnetic radiation, therefore relative intensities of absorption bands will appear distorted compared to transmission experiments. In particular, intensities in the low wavenumber region will appear larger and the high wavenumber region will appear lower due to differences in the effe ctive sampling volume from varying penetration depth. This effect is illustrated in Figure C 3, where FTIR spectra are collected on a bulk Fe(phpy) 2 [Ni(CN) 4 ] sample.

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177 Figure C 1. Optical images and TEM/SEM images of various spin crosso ver compounds synthesized, contrasting the appearance of the crystalline flakes (a and b) and powder (c) produced from drying the compound. A) Nano sized Fe(azpy)[Pt(CN) 4 ] particles. TEM scale bar = 200 nm. B) Nano sized Fe(phpy) 2 [Ni(CN) 4 ] platelets. TEM s cale bar = 200 nm. C) Larger Fe(phpy) 2 [Ni(CN) 4 Figure C 2. FTIR spectra of the compounds described in Figure C 1 ground into KBr pellets, correlating the crystalline or powder nature of the dry sample with the extent of scattering anomalies observed in the IR data. a) b) c)

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178 Figure C 3. FTIR spectra of nanoscale Fe(phpy) 2 [Ni(CN) 4 ] crystalline flakes collected via ATR and tran smission experiments using a pressed KBr pellet.

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179 APPENDIX D SUBSTRATE COMMENTS REGARDING GROWTH AND CHARACTERIZATION OF COORDINATION POLYMER FILMS Thin films in this work were grown on several substrates to optimize sample signal based on the technique. The substrates attempted are described below, with comments on the utility of the substrate. Microscope slide (MS)/ Microscope cover glass slide (MCG) Glass slides were the most commonly used substrate in this work, as it is a suitable material for many o f the characterization techniques and could be functionalized for controlled film growth. The MS and MCG were practical for different uses, owing to their thicknesses of 1.0 mm and 0.2 mm respectively. Both types of slides are piranha washed before use to clean contaminants from the surface, including the hydrophobic coating from the MCG slides. Removal of hydrophobic contaminants on the substrate surface is necessary for aqueous solution deposition. The MS slides are primarily useful for imaging techniques (AFM and SEM). The glass surface is resistant to deformation upon light scratching with tweezers, making this an ideal candidate for step height (film thickness) measurements using AFM. Due to the thickness of the MS slide (1.0 mm), SQUID film measurement s are not performed due to the large glass background. Additionally, cutting the thick glass slides after film deposition for analysis inevitably causes damage around the edges of the resulting film pieces, thus the substrate is not ideal for analysis such as imaging film cross sections via SEM. The MCG slides, 0.2 mm thick, were selected to overcome some of the challenges encountered with MS substrates while using a surface of similar composition and structure. The 0.2 mm thick glass greatly reduces the ba ckground in SQUID measurements, therefore this was the main substrate utilized in magnetometry measurements of materials containing ferromagnetic

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180 NiCr PBA. The thin glass can be difficult to cut into desired sizes; however when done carefully the MCG can b e fractured in a relatively controlled fashion with minimal damage to the CP film on the glass surface. In this case, a razor blade or a diamond tip pen was used to fracture the glass. Alternative methods, although not utilized extensively, include submerg ing the thin glass substrate in liquid N 2 , allowing glass fracture to occur much more easily when frozen with minimal contact between film and cutting device. Melinex 454 Melinex 454 (M454) was attempted in much of the early PBA and PBA Hofmann thin film depositions. This substrate is a polyethylene terephthalate (PET) sheet with an unknown hydrophobic surface coating on both sides. The hydrophobic nature of the surface does not allow for nano or mesoscale controlled film growth, however M454 may be used to deposit rough films several micrometers thick via aqueous sequential adsorption (Figure D 1). M454 is a more suitable substrate for deposition of the single phase Fe(azpy)[Pt(CN) 4 ] films, which involves EtOH solutions rather than H 2 O, resulting in favor able solvent substrate interactions required for film deposition. Film depositions performed by previous members of our group utilized a similar substrate, Melinex 535 (M535), comprising the same PET base and an unknown hydrophilic surface. Although not id eal for deposition due to the unknown coating, the substrate allows deposition due to favorable interaction with H 2 O. XPS spectra of M454 and M535 are presented in Figure D 2, and atomic percentages of C, N, and O determined from XPS survey spectra are rep orted in Table D 1. The immediate difference observed between the two samples is the large difference in nitrogen composition between the hydrophobic and hydrophilic samples. Furthermore, high resolution data in the N 1s region reveal two inequivalent nitr ogen environments in the M535 sample, specifically a second nitrogen signal shifted to higher eV

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181 (~407 eV) indicating that the nitrogen is likely coordinated to one or more electronegative oxygen atoms. The large N content and possible N O functionality likely contributes to the hydrophilicity of the M535 surface. The Melinex substrates are advantageous for SQUID analysis for two reasons. Firstly, the plastic substrate is easily cut with scissors, resulting in predictable sample size and simple sample preparation and loading compared to glass. Secondly, the Melinex has a relatively small diamagnetic background, therefore multiple slides may be stacked in the SQUID probe for an increase in magnetic signal. Although not used in this work, the plastic subs trate would be preferred for microtoming the sample to create cross sections for TEM analysis, a technique in which a hard glass substrate is not suitable. Melinex substrates are not well suited for AFM step height measurements, due to easier deformation o f the polymer substrate upon scratching with tweezers. Unlike glass, the Melinex coating is often damaged when removing the deposited PBA films with tweezers, thus creating an artificially large step height (film thickness). The flexible nature of the Meli nex substrate makes orienting the sample for SEM cross section imaging difficult, therefore is not recommended over a glass surface. Office Depot transparency In search of a hydrophilic substrate with similar SQUID advantages to Melinex 454, a polyester transparency ( ODT, Office Depot Color Inkjet transparencies. Product#: 753641 ) was attempted as a substrate for NiCr PBA growth. The coating on one side of the substrate was removed by sonication several times in EtOH. The surface contact with H 2 O after c leaning with EtOH was sufficient for aqueous SA deposition, and NiCr PBA films were successfully

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182 The limitations of the ODT substrate are similar to those of Melinex, where AF M film thickness measurements and SEM cross section measurements are not reasonable with ODT. The ease of sample preparation for SQUID measurements is similar to that of Melinex as well. We do note, however, that while low temperature measurements (<100 K) of NiCr PBA films deposited on ODT were successful with this substrate, anomalous behavior at higher temperatures (>300 K) were observed (Figure D 3). Although the origin of the high temperature features are not understood, we attribute the high temp erature behavior to the ODT background. For this reason among other characterization challenges, the use of ODT as a film substrate was not continued.

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183 Figure D 1. Optical images of various rough microscale PB, PBA, and PBA/Hofmann heterostructu re films deposited via aqueous SA onto Melinex 454 substrates. A) NiCr PBA film. B) PB film. C) CrCr PBA/Fe(azpy)[Ni(CN) 4 ] film. Figure D 2. XPS survey spectra and high resolution data of M4 54 and M535 surfaces. A) Survey spectra illustrating C 1s, N 1s, and O 1s signals from each substrate coating. B) High resolution spectra of the N 1s region for each substrate coating. Table D 1. Atomic percentages of M454 and M535 surface coatings determ ined from XPS survey spectra. Substrate C N O Melinex 454 69.7 % 2.3 % 28.0 % Melinex 535 65.6 % 13.0 % 21.4 % a) b) c)

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184 Figure D 3. Magnetization vs. temperature data of an ODT substrate. Data were collec ted at 5000 G.

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185 APPENDIX E PERMISSION TO REPRODUCE COPYRIGHTED MATERIAL Title: Persistent Photoinduced Magnetism in Heterostructures of Prussian Blue Analogues Author: Daniel M. Pajerowski, Matthew J. Andrus, Justin E. Gardner, et al Publ ication: Journal of the American Chemical Society Publisher: American Chemical Society Date: Mar 1, 2010 Copyright © 2010, American Chemical Society Logged in as: Corey Gros PERMISSION/L ICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE This type of permission/license, instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order. Please note the following: Permission is granted for your request in both print and electronic formats, and translations. If figures and/or tables were requested, they may be adapted or used in part. Please print this page for your records and send a copy of it to your publisher/graduate school. Appropriate credit for th e requested material should be given as follows: "Reprinted (adapted) with permission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words. One time permission is granted only for the use specified in your request. No additional uses are granted (such as derivative works or other editions). For any other uses, please submit a new request. If credit is given to another source for the material you requested, permiss ion must be obtained from that source. Copyright © 2015 Copyright Clearance Center, Inc. All Rights Reserved. Privacy statement . Terms and Conditions . Comments? We would like to hear from you. E mail us at customercare@copyright.com

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186 Title: Photoinduced Magnetism in Core/Shell Prussian Blue Analogue Heterostructures of KjNik[Cr(CN)6]l·nH2O with RbaCob[Fe(CN)6]c·mH2O Author: Matthieu F. Dumont, Elisabeth S. Knowles, Amandine Guiet, et al Publication: Inorganic Chemistry Publisher: American Chemical Society Date: May 1, 2011 Copyright © 2011, American Chemical Society If you're a copyright.com user, you can login to RightsLink using your copyright.com credentials. Already a RightsLink user or want to learn more? PERMISSION/LICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE This type of permission/license, instead of the standard Terms & Conditions, is sent to you because n o fee is being charged for your order. Please note the following: Permission is granted for your request in both print and electronic formats, and translations. If figures and/or tables were requested, they may be adapted or used in part. Please print th is page for your records and send a copy of it to your publisher/graduate school. Appropriate credit for the requested material should be given as follows: "Reprinted (adapted) with permission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words. One time permission is granted only for the use specified in your request. No additional uses are granted (such as derivative works or other editions). For any other uses , please submit a new request. If credit is given to another source for the material you requested, permission must be obtained from that source. Copyright © 2015 Copyright Cle arance Center, Inc. All Rights Reserved. Privacy statement . Terms and Conditions . Comments? We would like to hear from you. E mail us at customercare@copyright.com

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187 Title: Light Induced Changes in Magnetism in a Coordination Polymer Heterostructure, Rb0.24Co[Fe(CN)6]0.74@K0.10Co[Cr(CN)6]0. 70·nH2O a nd the Role of the Shell Thickness on the Properties of Both Core and Shell Author: Olivia N. Risset, Pedro A. Quintero, Tatiana V. Brinzari, et al Publicatio n: Journal of the American Chemical Society Publisher: American Chemical Society Date: Nov 1, 2014 Copyright © 2014, American Chemical Society If you're a copyright.com user, you can login to RightsLink using your copyright.com credentials. Already a RightsLink user or want to learn more? PERMISSION/LICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE This type of permission/license, instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order. Please note th e following: Permission is granted for your request in both print and electronic formats, and translations. If figures and/or tables were requested, they may be adapted or used in part. Please print this page for your records and send a copy of it to you r publisher/graduate school. Appropriate credit for the requested material should be given as follows: "Repr inted (adapted) with permission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words. One time permission is granted only for the use specified in your request. N o additional uses are granted (such as derivative works or other editions). For any other uses, please submit a new request. If credit is given to another source for the material you requested, permission must be obtained from that source.

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188 Title: Thermal and Light Induced Spin Crossover Phenomena in New 3D Hofmann Like Microporous Metalorganic Frameworks Produced As Bulk Materials and Nanopatterned Thin Films Author: Gloria Agustí, Saioa Cobo, Ana B. Gaspar, et al Publication: Chemi stry of Materials Publisher: American Chemical Society Date: Nov 1, 2008 Copyright © 2008, American Chemical Society If you're a copyright.com user, you can login to RightsLink using your copyright.com credentials. Already a RightsLink user o r want to learn more? PERMISSION/LICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE This type of permission/license, instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order. Please note the following: Permission is granted for your request in both print and electronic formats, and translations. If figures and/or tables were requested, they may be adapted or used in part. Please print this page for your records and send a copy of it to your publisher/graduate school. Appropriate credit for the requested material should be given as follows: "Reprinted (adapted) with permission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words. One time permission is granted only for the use specified in your request. No additional uses are granted (such as derivative works or other editions). F or any other uses, please submit a new request. If credit is given to another source for the material you requested, permission must be obtained from that source.

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189 Title: The Effect of Pressure on the Cooperative Spin Transition in th e 2D Coordination Polymer {Fe(phpy)2[Ni(CN)4]} Author: Ana B. Gaspar,Georgiy Levchenko,Sergey Terekhov,Gennadiy Bukin,Javier Valverde Muñoz,Francisco J. Muñoz Lara,Maksym Seredyuk,José A. Real Publication: European Journal of Inorganic Chemistry Publish er: John Wiley and Sons Date: Dec 20, 2013 Copyright © 2014 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim Logged in as: Corey Gros Account #: 3000969130 This Agreement be tween Corey Gros ("You") and John Wiley and Sons ("John Wiley and Sons") consists of your license details and the terms and conditions provided by John Wiley and Sons and Copyright Clearance Center. License Number 3736841018973 License date Oct 26, 2 015 Licensed Content Publisher John Wiley and Sons Licensed Content Publication European Journal of Inorganic Chemistry Licensed Content Title The Effect of Pressure on the Cooperative Spin Transition in the 2D Coordination Polymer {Fe(phpy) 2[Ni(CN)4]} Licensed Content Author Ana B. Gaspar,Georgiy Levchenko,Sergey Terekhov,Gennadiy Bukin,Javier Valverde Muñoz,Francisco J. Muñoz Lara,Maksym Seredyuk,José A. Real Licensed Content Date Dec 20, 2013 Licensed Content Pages 5 Ty pe of use Dissertation/Thesis Requestor type University/Academic Format Print and electronic Portion Figure/table Number of figures/tables 1 Original Wiley figure/table number(s) Figure 1 Will you be translating? No Title of your thesis / dissertation GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROSTRUCTURES

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191 Title: Photoswitchable coordinatio n compounds Author: Philipp Gütlich,Yann Garcia,Theo Woike Publication: Coordination Chemistry Reviews Publisher: Elsevier Date: October 2001 Copyright © 2001 Elsevier Science B.V. All rights reserved. Logged in as: Corey Gros Account #: 3000969130 This is a License Agreement between Corey Gros ("You") and Elsevier ("Elsevier"). The license consists of your order details, the terms and conditions provided by Elsevier , and the payment terms and conditions . License Number 3736850993349 License date Oct 26, 2015 Licensed content publisher Elsevier Licensed content publication Coordination Chemistry Reviews Licensed co ntent title Photoswitchable coordination compounds Licensed content author Philipp Gütlich,Yann Garcia,Theo Woike Licensed content date October 2001 Licensed content volume number 219 Licensed content issue number n/a Number of pag es 41 Type of Use reuse in a thesis/dissertation Portion figures/tables/illustrations Number of figures/tables/illustrations 1 Format both print and electronic Are you the author of this Elsevier article? No Will you be transl ating? No Original figure numbers Figure 1 Title of your thesis/dissertation GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROSTRUCTURES Expected completion date Dec 2015 Estimated size (number of pa ges) 150 Elsevier VAT number GB 494 6272 12 Permissions price 0.00 USD VAT/Local Sales Tax 0.00 USD / 0.00 GBP Total 0.00 USD

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192 Title: Light Induced Magnetization Changes in a Coordination Polymer Heterostructure of a Prussian Blue Analogue and a Hofmann like Fe(II) Spin Crossover Compound Author: Corey R. Gros, Marcus K. Peprah, Brian D. Hosterman, et al Publication: Journal of the American Chemical Society Publisher: American Chemical Society Date: Jul 1, 2014 Copyright © 2014, American Chemical Society Logged in as: Corey Gros Account #: 3000969130 PERMISSION/LICENSE IS GRANTED FOR YOUR ORDER AT NO CHARGE This type of permission/license , instead of the standard Terms & Conditions, is sent to you because no fee is being charged for your order. Please note the following: Permission is granted for your request in both print and electronic formats, and translations. If figures and/or tables were requested, they may be adapted or used in part. Please print this page for your records and send a copy of it to your publisher/graduate school. Appropriate credit for the requested material should be given as follows: "Reprinted (adapted) with per mission from (COMPLETE REFERENCE CITATION). Copyright (YEAR) American Chemical Society." Insert appropriate information in place of the capitalized words. One time permission is granted only for the use specified in your request. No additional uses are gr anted (such as derivative works or other editions). For any other uses, please submit a new request.

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193 Title: A Combined Top Down/Bottom Up Approach for the Nanoscale Patterning of Spin Crossover Coordination Polymers Author: G. Molnár ,S. Cobo,J. A. Real,F. Carcenac,E. Daran,C. Vieu,A. Boussekso u Publication: Advanced Materials Publisher: John Wiley and Sons Date: Jul 17, 2007 Copyright © 2007 WILEY VCH Verlag GmbH & Co. KGaA, Weinheim Logged in as: Corey Gros Accou nt #: 3000969130 This Agreement between Corey Gros ("You") and John Wiley and Sons ("John Wiley and Sons") consists of your license details and the terms and conditions provided by John Wi ley and Sons and Copyright Clearance Center. License Number 3738061148042 License date Oct 29, 2015 Licensed Content Publisher John Wiley and Sons Licensed Content Publication Advanced Materials Licensed Content Title A Combined Top Do wn/Bottom Up Approach for the Nanoscale Patterning of Spin Crossover Coordination Polymers Licensed Content Author G. Molnár,S. Cobo,J. A. Real,F. Carcenac,E. Daran,C. Vieu,A. Boussekso u Licensed Content Date Jul 17, 2007 Licensed Content Pages 5 Type of use Dissertation/Thesis Requestor type University/Academic Format Print and electronic Portion Figure/tabl e Number of figures/tables 1 Original Wiley figure/table number(s) Scheme 1 Will you be translating? No Title of your thesis / dissertation GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROSTRUCTURES Expected completion date Dec 2015 Expected size (number of pages) 150 Requestor Location Corey Gros 214 Leigh Hall GAINESVILLE, FL 32611

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195 Title: Spin crossover behaviour of the coordination polymer FeII(C5H5N)2NiII(CN)4 Author: Takafumi Kitazawa,Yuji Gomi,Masashi Takahashi,Masuo Takeda,Miki En omoto,Akira Miyazaki,Toshiaki Enoki Publication: Journal of Materials Chemistry Publisher: Royal Society of Chemistry Date: Dec 31, 1969 Copyright © 1969, Royal Society of Chemistry Logged in as: Corey Gros Account #: 3000969130 Thank you very much for your order. This is a License Agreement between Corey Gros ("You") and Royal Society of Chemistry. The license consists of your order details, the terms and conditions provided by Royal Society of Chemistry, and the payment terms and conditions . License Number 3738070097543 License date Oct 29, 2015 Licensed content publisher Royal Society of Chemistry Licensed co ntent publication Journal of Materials Chemistry Licensed content title Spin crossover behaviour of the coordination polymer FeII(C5H5N)2NiII(CN)4 Licensed content author Takafumi Kitazawa,Yuji Gomi,Masashi Takahashi,Masuo Takeda,Miki Enomoto,Aki ra Miyazaki,Toshiaki Enoki Licensed content date Dec 31, 1969 Volume number 6 Issue number 1 Type of Use Thesis/Dissertation Requestor type academic/educational Portion figures/tables/images Number of figures/tables/image s 1 Distribution quantity 1 Format print and electronic Will you be translating? no Order reference number None Title of the thesis/dissertation GROWTH AND PHOTOMAGNETIC PROPERTIES OF NANO AND MESOSCALE COORDINATION POLYMER HETEROS TRUCTURES Expected completion date Dec 2015 Estimated size 150 Total 0.00 USD

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205 BIOGRAPHICAL SKETCH Corey Gros was born in Winnipeg, Manitoba and raised in Victoria, British Columbia, Canada. A proud Canuck, s he migrated to Alabama where received her Bachelor of Science in chemistry in 2009 from the University of M ontevallo while competing for the Lady Falcons socc er program from 2005 2008 . In August, 2010 she began her graduate career at the University of Florida to pursue her Ph.D . under the supervision of Professor Dan iel Talham. She successfully defende d her dissertation in the fall of 2015 .