Incorporating two-dimensional, inorganic, extended-lattice structures and magnetic properties into ultrathin organic films

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Incorporating two-dimensional, inorganic, extended-lattice structures and magnetic properties into ultrathin organic films
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Seip, Candace Tricia, 1971-
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Table of Contents
    Title Page
        Page i
    Dedication
        Page ii
    Acknowledgement
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
    List of Tables
        Page vii
    List of Figures
        Page viii
        Page ix
        Page x
    Abstract
        Page xi
        Page xii
    Chapter 1. Investigations of two-dimensional inorganic materials and introduction to the langmuir-blodgett method
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    Chapter 2. Langmuir-blodgett films of known layered solids: Preparation and structural properties of octadecylphosphonate bilayers with divalent metals
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    Chapter 3. An electron paramagnetic resonance study of a langmuir-blodgett film of manganese octadecylphosphonate and comparison of the magnetic properties to sold-state manganese alkylphosphonates
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    Chapter 4. A magnetic langmuir-blodgett film
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    Chapter 5. Magnetic characterization of a diluted antiferromagnetic langmuir-blodgett film
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    References
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    Biographical sketch
        Page 128
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Full Text










INCORPORATING TWO-DIMENSIONAL, INORGANIC,
EXTENDED-LATTICE STRUCTURES AND MAGNETIC PROPERTIES
INTO ULTRATHIN ORGANIC FILMS












By

CANDACE TRICIA SEIP


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


1997




























To Mom and Dad

&

My Grandparents.













ACKNOWLEDGMENTS
I would like to thank my advisor Dr. Daniel Talham for letting me work with him

during my time in Florida. He has taught me many skills that I am sure to take with me

wherever I go. I am also grateful to Dr. Mark Meisel for performing all the static magnetic

measurements on each manganese-containing LB film. I would also like to thank him for

all his useful discussions pertaining to magnetism. I also thank the University of Florida

Major Analytical Instrumentation Center for the use of their X-ray photoelectron

spectrometer and electron microscope, and especially Eric Lambers for many helpful XPS

discussions and Augusto Morrone for providing instruction on the operation of the electron

microscope. Special thanks go to Andy Boeckl for performing X-ray diffraction

measurements on all the LB films. I also would like to thank my Ph.D. committee

members Dr. Russell Drago, Dr. Dave Richardson, Dr. John Reynolds, and Dr. Elizabeth

Seiberling, for their dedication in attending my seminars and oral examination and their

insightful comments and questions, which have proven to be very useful in helping me

attain a greater knowledge and understanding of my research.

I am indebted to all my undergraduate chemistry professors, especially Dr. Lynn

Mihichuk and Dr. Keith Johnson. They provided me with a solid understanding of

chemistry, introduced me to research, and encouraged me to further my education in

chemistry.

I would like to thank my parents, Ken and Gayle Seip, for all their love, constant

encouragement and support. They provided me with the confidence to believe in myself

and made everything I've accomplished, possible. Special thanks go to my Uncle David

and Aunt Julie for lending me their laptop computer, which helped immensely in the

preparation of this dissertation.








Finally, I would like to acknowledge some of my friends for their companionship

and support. I thank Angela for being an especially good friend and a person I can always

count on. I thank her for being a great shopping buddy and for occasionally reminding me

that I can't spend all my time in the lab! I also thank her for initiating me into the Energizer

skating club, which provided me with hours, and miles, of endless fun with only minor

injuries. I thank "Bry Guy" for being a special friend who was always there whenever I

needed him. I hope you have fun while you're away, and hope you don't get sea sick! I

would also like to thank Steve Joerg for his special friendship, chemistry discussions, and

spiritual support throughout the years. I also thank my most recent friends, Mary and BJ,

for making my last two semesters here a lot of fun. It's too bad you didn't get here sooner!

Lastly, I thank Cheryl, Richard, and Treena for being my dear friends for many years. I

look forward to the friendship we'll share for many more years to come.













TABLE OF CONTENTS

Page


ACKNOWLEDGMENTS.......................................................................... iii

LIST OF TABLES.................................................................................. vii

LIST OF FIGURES ............................................................................... viii

ABSTRACT......................................................................................... xi

CHAPTERS

1 INVESTIGATIONS OF TWO-DIMENSIONAL INORGANIC
MATERIALS AND INTRODUCTION TO THE LANGMUIR-
BLODGETT METHOD.................................................................... 1

Low Dimensional Inorganic Solids....................................................... 1
Organic/Inorganic Layered Compounds.................................................. 5
Ultrathin Organic Films.................................................................... 11
Organic Films formed Using Self-Assembly Techniques......................... 12
The Langmuir-Blodgett Method......................................................14
Theory of Low-Dimensional Magnetism................................................ 19
Scope of Dissertation...................................................................... 23

2 LANGMUIR-BLODGETT FILMS OF KNOWN LAYERED SOLIDS:
PREPARATION AND STRUCTURAL PROPERTIES OF
OCTADECYLPHOSPHONATE BILAYERS WITH DIVALENT
METALS .................................................................................... 30

Introduction................................................................................. 30
Experimental Section....................................................................... 37
Materials ................................................................................ 37
Substrate Preparation and Deposition Procedure................................... 37
Instrumentation......................................................................... 38
Results and Discussion.................................................................... 39
Film Deposition........................................................................ 39
X-ray Photoelectron Spectroscopy .................................................. 41
Ellipsometry and X-ray Diffraction.................................................. 47
FTIR Spectroscopy.................................................................... 50
Extended Lattice Structure............................................................ 57
Summary.................................................................................... 59












3 AN ELECTRON PARAMAGNETIC RESONANCE STUDY OF A
LANGMUIR-BLODGETr FILM OF MANGANESE
OCTADECYLPHOSPHONATE AND COMPARISON OF THE
MAGNETIC PROPERTIES TO SOLID-STATE MANGANESE
ALKYLPHOSPHONATES............................................................... 61

Introduction................................................................................. 61
Experimental Section....................................................................... 65
Materials ................................................................................ 65
Instrumentation......................................................................... 66
Procedure.......................................................................... 66
Results and Discussion.................................................................... 67
Summary.................................................................................... 76

4 A MAGNETIC LANGMUIR-BLODGETT FILM..................................... 78

Introduction................................................................................. 78
Experimental Section.......................................................................82
Materials ................................................................................ 82
Instrumentation......................................................................... 82
Procedure...............................................................................82
Results and Discussion.................................................................... 83
Structure and High Temperature Magnetic Behavior of Manganese
Octadecylphosphonate Langmuir-Blodgett Films............................. 83
Magnetic Behavior of the Manganese LB Film at Low Temperatures
(5K-25K)........................................................................... 83

5 MAGNETIC CHARACTERIZATION OF A DILUTED
ANTIFERROMAGNETIC LANGMUIR-BLODGETT FILM....................... 91

Introduction................................................................................. 91
Experimental Section....................................................................... 93
Materials ................................................................................ 93
Instrumentation......................................................................... 94
Substrate Preparation.................................................................. 94
Preparation of Alternating Manganese/Cadmium Octadecylphosphonate
LB Films ........................................................................... 95
Preparation of Mixed Manganese/Cadmium Octadecylphosphonate LB
Films................... ........................................................ 97
Results and Discussion.................................................................... 98
Summary .................................................................................. 116

REFERENCES ............................................................................... 120

BIOGRAPHICAL SKETCH................................................................ 128













LIST OF TABLES


Tableag
2-1 Deposition Conditions for the Preparation of Metal Octadecylphosphonate
LB film s..................................................................................... 38

2-2 Relative Intensities of the Metal and Phosphorus XPS Signals for Single
Bilayers of the Divalent Metal Octadecylphosphonate LB Films ..................... 45

2-3 Interlayer Spacings, Indices of Refraction, and Alkyl Chain Tilt Angles
for the Divalent Metal Octadecylphosphonate LB Films............................... 48
2-4 Comparison of the Infrared v(P032-) Frequencies of Powders and LB
films of Divalent Metal alkylphosphonates.............................................. 55

3-1 Structural and Magnetic Parameters for Mn(O3PR).H20 Solids..................... 64













LIST OF FIGURES


Fig= pag
1-1 The crystal structure of KCP, K2Pt(CN)4Co10.0323H20................................ 3

1-2 Illustration of the unit cell of the K2NiF4 structure...................................... 4

1-3 Atomic positions of the atoms in alkylammonium metal tetrahalides.................. 6

1-4 Crystal structure of (C10oH21NH3)2CdCl4............................................... 7

1-5 Crystal structure of Ca(O3PCH3).H20.................................................. 10

1-6 Self-Assembly of octadecyltrichlorosilane on a hydroxylated surface............... 14

1-7 Formation of a Langmuir Monolayer at the air/water interface........................ 15

1-8 A Pressure-Area Isotherm................................................................. 16

1-9 Deposition of a Langmuir Monolayer onto a Hydrophobic Substrate................ 17

1-10 A Langmuir-BlodgettBilayer............................................................. 17

1-11 Effects of Space Dimension and Magnetic Spin Interaction on Magnetic
Transitions. Theory predicts a transition to long range magetic order at
finite temperatures for only specific combinations of space and spin
dim ensions.................................................................................. 22

2-1 Illustration of a Metal Organophosphonate Compound................................ 31

2-2 X-ray crystal structure of cadmium methylphosphonate, Cd(O3PCH3)-H20
(A) Packing diagram, and (B) Cadmium coordination................................. 34
2-3 X-ray crystal structure of calcium hexylphosphonate, Ca(H03PC6H13)2 .........36

2-4 Deposition of a divalent metal alkylphosphonate Langmuir-Blodgett film..........40

2-5 (A) XPS survey spectrum from one bilayer of a magnesium
octadecylphosphonate LB film. (B) XPS multiplex spectrum of a single
bilayer of magnesium octadecylphosphonate........................................... 43
2-6 Illustration of the XPS Experiment and the Factors that Influence
Quanification of the XPS Spectrum...................................................... 44








2-7 Determination of Overlayer Thicknesses in LB Films of the Manganese
Octadecylphosphonates.................................................................... 46

2-8 X-ray diffraction from 15 bilayers of a cadmium octadecylphosphonate LB
film.......................... ... ....... ........ ...... ........ ..........48

2-9 Ellipsometric data for a cobalt octadecylphosphonate LB film........................ 49

2-10 FTIR spectra from 10 bilayer LB films of magnesium, calcium, cobalt,
cadmium, and manganese octadecylphosphonate...................................... 51

2-11 Intensity of the Va(P032-) and HOH bend as a function of multilayers for
the deposition of cadmium octadecylphosphonate LB film............................ 53
2-12 FTIR comparison of cadmium ethylphosphonate powder and cadmium
octadecylphosphonate LB film............................................................ 54

2-13 FTIR comparison of solid-state calcium ethylphosphonate,
Ca(03PC2H5)-H20........................................................................ 56
3-1 Structure of manganese phenylphosphonate............................................ 63

3-2 Orientations with respect to the magnetic field. Ho, of the LB film stacked in
a conventional EPR tube................................................................... 67
3-3 EPR spectra of the manganese octadecylphosphonate LB film at 275 K, 62
K and 20 K ................................................................................. 69
3-4 EPR linewidth as a function of sample orientation at room temperature............. 71

3-5 Temperature dependence of the inverse of the area of the EPR signal from
the manganese octadecylphosphonate LB film.......................................... 72

3-6 Temperature dependence of the integrated area of the EPR signal from the
manganese octadecylphosphonate LB film............................................ 72

3-7 EPR linewidth as a function of temperature for a powder sample of
manganese propylphosphonate and for the manganese
octadecylphosphonate LB film............................................................ 74

4-1 Structure of Mn(O3PC6H5).H20 viewed down the a axis............................ 81

4-2 Magnetization vs. temperature for an 81 bilayer film with the measuring
field applied parallel to the plane of the film............................................. 86

4-3 Magnetization vs. applied field at 2K, normalized to the value at 5T, with
the applied field directed perpendicular and parallel to the plane of the film.........87

4-4 Illustration of the orientation of the manganese spins in manganese
organophosphonates....................................................................... 88
4-5. Magnetization at 2K in the vicinity of zero field showing hysteresis during
cycling between 5T....................................................................... 90








5-1 Illustration of Two Types of Alternating Manganese/Cadmium
Octadecylphosphonate LB films.......................................................... 96

5-2 Integrated areas of the manganese, cadmium, and phosphorus XPS signals
of two alternating manganese/cadmium phosphonate LB films..................... 100

5-3 Magnetization vs. temperature for a type I Langmuir-Blodgett film of
manganese octadecylphosphonate with each manganese layer separated by
one bilayer of cadmium octadecylphosphonate....................................... 102

5-4 Magnetization vs. applied field measured perpendicular to the plane of the
film at 2K for the type I manganese LB .............................................. 105

5-5 Magnetization vs. temperature for a type I manganese
octadecylphosphonate LB film with the measuring field applied
perpendicular to the plane of the film ................................................. 107

5-6 Magnetization vs. applied field at 2K for the type II manganese LB film......... 109

5-7. Illustration of the orientation of manganese moments in manganese
octadecylphosphonate LB films........................................................ 111

5-8 XPS and magnetization experiments on mixed manganese/cadmium
octadecylphosphonate LB films containing 34% cadmium.......................... 115

5-9 XPS and magnetization behavior of a mixed manganese/cadmium
octadecylphosphonate LB films containing 12% cadmium.......................... 118













Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

INCORPORATING TWO-DIMENSIONAL, INORGANIC,
EXTENDED-LATITICE STRUCTURES AND MAGNETIC PROPERTIES
INTO ULTRATHIN ORGANIC FILMS

By

Candace Tricia Seip

May, 1997



Chairman: Daniel R. Talham
Major Department: Chemistry

This dissertation presents experimental results from the fabrication, structural

determination, and magnetic characterizations of a series of single layer and multilayer,

extended-lattice, divalent metal alkylphosphonate Langmuir-Blodgett (LB) films. LB films

of M(03PC18H37).H20, M = Mn, Cd, Co, Mg and Ca(HO3PC1gH37)2 were prepared and

structurally characterized. The results demonstrate that inorganic extended-lattice networks

can be incorporated into ultrathin organic films using LB methodology. Structural

identification of single layer and multilayered metal octadecylphosphonate LB films indicate

that the organic groups form close-packed, organized films with the metal ions providing

extra rigidity and structural order to the films. Metal cations and phosphonate anions

comprising the inorganic portions of the LB films occur in ratios equivalent to those

observed in analogous metal alkylphosphonate solid-state layered compounds.

Magnetic investigations involving LB films of manganese octadecylphosphonate,
Mn(03PC18H37).H20 reveal that the manganese ions undergo two-dimensional








antiferromagnetic Heisenberg exchange at high temperatures. Below 13 K, the LB films

undergo a transition to spontaneous long range magnetic order. The manganese spins of

the coupled nearest neighbor moments do not exactly cancel due to low site symmetry and

weak ferromagnetism is observed below the ordering temperature. In the ordered state, the
LB films also exhibit magnetic hysteresis, a signature of magnetic memory. The results

demonstrate the first example of a magnetic Langmuir-Blodgett film.

Magnetic susceptibility measurements performed on diluted manganese

octadecylphosphonate LB films, MnxCdl.-x(O3PC18gH37)-H20 exhibit spontaneous

magnetization at lower temperatures than those observed in the pure manganese films. The

results have been fit to low-dimensional magnetic models. The compiled data presented in

this dissertation indicate that magnetic properties usually observed exclusively in inorganic

solid-state materials may be incorporated into organic Langmuir-Blodgett films.













CHAPTER 1
INVESTIGATIONS OF TWO-DIMENSIONAL INORGANIC MATERIALS AND
INTRODUCTION TO THE LANGMUIR-BLODGETT METHOD


Low Dimensional Inoganic Solids

Low dimensional materials are defined as infinite in one or two spatial directions.1

Isolated single chains, layers, fibers and thin films of varying but finite thickness all

comprise this category. A vast amount of physical phenomena are inherent in systems

having restricted dimensionality and are frequently studied to enhance understanding of

fundamental theories. As a consequence of the realized properties present in low-

dimensional solids, many of these materials have proven to be useful in many technological

applications. The investigation of low-dimensional solids and their properties has lead to

the discovery of 1-d and 2-d conductors and superconductors, 2-d semiconductor

interfaces and 1-d and 2-d molecular crystals and liquid crystals to mention just a few.1-3

Many of the physical properties mentioned above are found in inorganic materials,

although recently many examples of low-dimensional organic materials having magnetic

and electrical properties are emerging.3 A classic example of a one-dimensional linear
chain compound is K2Pt(CN)4C10.33H20, commonly referred to as KCP. The crystal

structure of KCP, shown in Figure 1-1,2 consists of chains of mixed valence platinum ions
contained in square planar Pt(CN)4 stacks. Potassium ions fill the channels between the

stacks and separate successive platinum chains. The extra chloride ions, also contained in

the channels cause the partial oxidation and therefore short bond lengths of the platinum
ions. Interest in KCP is mainly attributed to the conductivity that exists along the platinum

chains and its overall behavior as a metal.2 Many other one-dimensional chain





















Figure 1-1. The crystal structure of KCP, K2Pt(CN)4Co10.33H20. KCP consists of closely spaced square planar Pt(CN)4
groups. Along the platinum chains, KCP is an electrical conductor and behaves as a metal, however, ordinary ionic properties
are observed perpendicular to the chains. Potassium cations (dotted circles) and chloride anions (filled circles) are contained in
the channels between the platinum stacks and separate the chains of Pt(CN)4 anions. Adapted from reference 2.











0
0


00


0P


0


CP
CP
O


O
0




%
*Q
QD
OJ
0


0
0


9)


0 8








compounds,4 both organic and inorganic, exist but will not be discussed further in this
dissertation. Rather, the focus of this work will concentrate on the two-dimensional
materials and their inherent properties, especially those related to magnetism.
After theories of low-dimensional magnetism were established, thorough magnetic
investigations of two-dimensional materials began. Initial studies focused mainly on purely
inorganic compounds crystallizing in the K2NiF4 structure. The K2NiF4 structure, as
illustrated in Figure 1-2, contains magnetic NiF2 layers separated by two sheets of
nonmagnetic KF.5 Nearest neighbor magnetic ions within a NiF2 layer undergo large

antiferromagnetic exchange. Interlayer interactions between layers also occur but the order
of coupling is much smaller than the intralayer coupling. It was soon realized that any









= Ni

Q=F














Figure 1-2. Illustration of the unit cell of the K2NiF4 structure. Substantial experimental
work has been performed on compounds having the K2NiF4 structure due to the two-
dimensional framework of the metal ions.








deviation from the ideal isotropic two-dimensional system, such as the presence of

interlayer coupling, single-ion anisotropy, anisotropy in the exchange mechanism, and

crystal imperfections may introduce long-range order at finite temperatures.6 With this

discovery came the need for solid-state materials with more two-dimensional structures.

Organic/Inorganic Layered Compounds

Layered compounds consisting of two-dimensional arrays of inorganic material

separated by non-functional organic material are excellent models of two-dimensional

systems and are a better approach to reaching two-dimensionality over the classical purely

inorganic materials. In these solid-state materials, interactions in the inorganic layers are

better confined within each layer since the organic groups force the inorganic layers farther

apart. Communication between succesive inorganic layers is dependent upon the size of

the organic spacer. As the organic groups become larger, inorganic interlayer interactions

diminish. Two classes of solid-state compounds synonymous with two-dimensional

models are the alkylammonium metal tetrahalide perovskites and the metal

organophosphonates.
In the perovskite family, (CnHI2n+NH3)2MX4, where M = a divalent metal ion; X

= a halide, metal ions are linked into two-dimensional sheets by sharing equatorial halides
with neighboring MX6 octahedra, as illustrated schematically in Figure 1-37. NH-3+

groups occupy octahedral holes and are electrostatically held into place by hydrogen bonds

between the hydrogens and the halides. Hydrocarbon chains on the ammonium groups are

directed away from the metal sheets and therefore separate successive metal layers. The

layered nature of the perovskites is clearly shown in the crystal structure of

(Co10H21NH3)2CdC14,7 Figure 1-4. In this solid-state material, cadmium ions form two-

dimensional layers suspended between layers of alkylammonium groups. Van der Waals

interactions between the terminal methyls of the alkylammonium groups of adjacent layers

are responsible for keeping consecutive layers together.7'8





























S. =Halide
= CnH2n+INH3






Figure 1-3. Atomic positions of the atoms in alkylammonium metal tetrahalides. The
metal ions are linked into a two-dimensional sheet by edge sharing MX6 octahedra.
Nitrogen atoms fit into octahedral holes and the hydrocarbon groups are directed away
from the metal ion plane.


Historically, the perovskites have been investigated for three main reasons.9 The
first reason was due to the temperature dependent structural phase transitions that many of
the layer perovskite halide salts undergo. With increases in temperature the NH3+ groups

have free rotation, and the hydrogen bonding that exists between the hydrogens on the
NH3+ groups and the halides is found to vary.9 Changes in the color observed in copper





















































Figure 1-4. Crystal structure of (Co10H21NH3)2CdCl4.7 These layer type perovskites
have proven to be useful in the study of solid-state two-dimensional compounds.








perovskites as a function of temperature were attributed to these structural transitions.7

Changes in the structures of the perovskites were also related to interesting lattice vibrations

and dielectric properties.7 Perovskites have also received attention due to the possibility of

using the inorganic layers as templates for organic reactions such as polymerization.9 In

metal perovskites possessing organic groups with chemical functionality, the inorganic

metal ions direct the organic groups and hold them in place such that they could react with

each other stereospecifically to form ordered polymers. In addition to this concept,

research on molecular composities has been initiated. The perovskites may be modified in

such a way that the properties of the organic groups and inorganic ions complement each

other in order to produce a wide variety of new properties. The third major reason that the

perovskite halide salts have been of interest involves their interesting magnetic properties.

Since these solids model two-dimensional materials, perovskites bearing paramagnetic

metal ions have been the focus of low-dimensional magnetic studies in order to probe the

theoretical predictions of 2-d magnetism. It is reported that perovskites,
(CnH2n+INH3)2MC14, with M = Cu and Cr are ferromagnetic10'11 and those containing
Mn are antiferromagnetic.12 The transition to a magnetically ordered state was in most

cases attributed to a crossover from two-dimensionality to three dimensionality since the

perovskites are part of a three-dimensional framework.

The transition metal phosphonates, represented by the molecular formula,
M(O3PCnH2n+1).H20 where M is a divalent metal ion, are also examples of mixed

organic/inorganic layered solids and have been used as low-dimensional model

compounds.9 In these materials, metal ions are bridged by phosphonate groups forming

two-dimensional extended-lattice sheets of metal ions separated by the organic substituents

on the phosphonate ligands.13-18 The structure of calcium methylphosphonate is shown in

Figure 1-5.13 Many other divalent metal ions also form complexes with a variety of

organophosphonate groups and are structurally similar to calcium

methylphosphonate.14,15,19 In cadmium, manganese, cobalt, zinc, nickel, chromium,






















Figure 1-5. Crystal structure of Ca(O3PCH3)-H20. Structural parameters were taken from reference 13. In the metal
alkylphosphonates metal ions form two-dimensional sheets suspended between alkylphosphonate groups. Adjacent layers are
held together by weak van der Waals interactions between terminal methyl groups on the alkylphosphonates. (Key: Ca cross-
hatched circles; 0 small open circles; P dotted circles; C shaded circles; H small shaded circles).





10








magnesium and some calcium alkylphosphonates, the metal ions are octahedrally

coordinated, bonded to six oxygen atoms. Five oxygens are from phosphonate groups and

the sixth coordination site is filled by a water molecule. Layers are held together by van der

Waals interactions between terminal methyl groups.13-15'18'20 Divalent metal

phosphonates have also shown promise in practical applications in the area of separations,

catalysis, chemical sensors, and biological recognition.21-23 In the solid-state,

phosphonate chemistry is rich and continues to be pursued.

Ultrathin Organic Films

The quasi-two-dimensional nature of the previously mentioned layered solids has

been useful in investigating phenomena in two-dimensions. However, thin films of these

layered materials are possibly more useful in application devices and synthetically, offer

more structural control. Two approaches used to achieve organized assemblies of organic

molecules at solid surfaces are Langmuir-Blodgett film methods and organic self-assembly

methods. Self-assembly procedures have been developed to form single layers and

multilayers of tetravalent Zr24"29 and HfO and divalent Zn and Cu23 organophosphonates

for potential applications ranging from non-linear optics28'29 and separations23 to chemical

sensors23 and interfaces.24 Problems related to most self-assembled films is their fragility

and lack of long range structural order.30 Researchers also recognize the potential of

Langmuir-Blodgett films in a variety of industrial applications such as molecular

electronics, microelectronics, optics, biosensors, and catalysis,31-33 to more academic

applications such as cell membrane models and two-dimensional model compounds.34-5

Thin films having a layered nature are excellent models for probing chemistry and physics

restricted to two-dimensions. Organic/Inorganic alternating layered Langmuir-Blodgett

films are especially useful in this area. They may readily be engineered to maximize the

distance between the inorganic lattices by systematically increasing the size of the organic

groups between the inorganic layers.









This dissertation focuses on research based on Langmuir-Blodgett films of a variety

of divalent metal alkylphosphonates. The reason for choosing phosphonate molecules in

conjunction with the LB method of synthesis is dual. First, the LB technique offers much

control over the interlayer spacings between inorganic ions. Secondly, phosphonic acid

containing a long hydrocarbon chain was the chosen amphiphile since in the solid-state,

single crystals of the metal phosphonates containing small organic groups are difficult to

synthesize and are almost impossible to obtain in crystalline form when the phosphonates

contain alkyl groups consisting of more than six carbon atoms. To date, no single crystal

X-ray diffraction data have been reported on any divalent metal alkylphosphonate

containing more than six carbon atoms.

Organic Films formed Using Self-Assembly Techniques

The formation of monolayer and multilayer films using the self-assembly method

relies on the affinity of molecules for each other and the affinity of the molecules for

surfaces. The spontaneous chemisorption of thiols on gold, organosilicon molecules on

hydroxylated surfaces, and carboxylic acids on silver are all examples of self-assembly

reactions. From an energetic standpoint, the self-assembling molecule can be divided into

three parts the headgroup, which forms a bond with the substrate; the alkyl chain, which

provides van der Waals interactions between molecules and affects the order of the film;

and the terminal group which are normally disordered at room temperatures.33 Clearly the

attraction of the molecular headgroup for the surface is the driving force behind molecular

self-assembly. The disadvantage of applying self-assembled films to the investigation of

two-dimensional properties results from the strong interactions that exist between the

monolayer and the surface. Generally, these interactions are so strong that the properties of

the self-assembled monolayer become coupled to the substrate and the monolayer becomes

part of a three-dimensional structure. For example, it was demonstrated by Whitesides et

al.36 that self-assembled monolayers of docosanethiol on various gold surfaces form with








structures reflective of the respective surface to which they adhere.37 When docosanethiol

is self-assembled on Au(1 11) surfaces, the symmetry of sulfur atoms in the monolayer is

hexagonal indicating that sulfur binds at certain hollow sites on the Au(l 11) surface.37

However, when docosanethiol is self-assembled on Au(100) surfaces, the symmetry of the

alkanethiol assembly is base-centered square. Whitesides determined that on the Au(100l)

surfaces, sulfur binds chemisorbs on both on-top and threefold hollow sites in an

alternating fashion.37 It was found that the resulting monolayers of the docosanethiols on

the two different surfaces also have differences in the tilt angle of the alkyl chains.37 This

clearly indicates that the structure and properties of self-assembled monolayers are

contingent upon the structure of the substrate to which they chemisorb.

As in the case of the assembly of thiols on gold, numerous investigations involving

the structure of self-assembled monolayers and multilayer of organosilanes on silicon or

glass have been reported.33'38"40 The self-assembly reaction of octadecyltrichlorosilane on

a hydroxylated surface is dipicted schematically in Figure 1-6. In this reaction,

octadecyltrichlorosilane is dissolved in an organic solvent containing a substrate such as

glass or silicon. Si-Cl bonds react with the OH groups present on the surface of the

substrate and with neighboring Si-O groups to form a network of Si-O-Si Bonds.33 van

der Waals interactions between the long chain alkyl groups also occur and increase the

order of the monolayer. As a result of the covalent bonds between the head group and

substrate in addition to the interactions between molecules, self-assembled monolayers of

alkylsilanes on glass or silicon form close-packed, ordered films on the surface of the

substrate. The monolayers terminate with methyl groups which chemically modify wetting

properties of the substrate.33 The research presented in this dissertation makes use of this

self-assembly reaction to obtain close-packed, highly ordered hydrophobic substrates onto

which monolayers are then deposited. When monolayers of different amphiphilic molecules

are deposited, via the Langmuir-Blodgett technique, onto these surface modified substrates

interactions between the amphiphilic molecules and the surface are eliminated.








-OH 0-Si %AANAAAA/ CH3
< S I
p

0

OH + CH3(CH2)17SiCl3___ 0- Si'CAtA tVQ3
OH 0- SitA/ASAt CH3
I | O|


P
OH 0-Si A/VAAAAA CH3
- CH3(CH2)17SiC13 -->



P
-OH 0- Si NA.AAAAAA CH3




Figure 1-6. Self-Assembly of octadecyltrichiorosilane on a hydroxylated surface.



The Langmuir-Blodgett Method

Axnphiphilic molecules may be organized at an air-water interface using a
Langmuir-Blodgett trough. The formation of a Langmuir monolayer, developed by Irving
Langmuir~l, is depicted in Figure 1-7. The procedure consists of dropping a volatile
solution of an amphiphilic molecule onto a water surface contained inside a teflon 'trough'
having movable teflon barriers. The solvent evaporates and the polar end of the amphiphile
wants to dissolve into the water subphase; the hydrophobic end is directed away fromn the
aqueous surface. At this stage, the molecules are separated from each other by large
distances and are labeled as a two-dimensional "gas"~ as shown in part A of Figure 1-~7.33
The organization of the molecules on the surface of the subphase is monitored by the
change in surface tension of pure water and the surface tension of water covered with a
monolayer. This difference in surface tension is equivalent to the surface pressure and is
"-OH ~- O-Si,.VNA IAAAAAA CH3
< S I

M- OH M- O-- Si A A/A ^CH r3

Figure 1-6. Self-Assembly of octadecyltrichlorosilane on a hydroxylated surface.



The Langmuir-Blodgett Method

Amphiphilic molecules may be organized at an air-water interface using a
l~angmuir-Blodgett trough. The formation of a Langmuir monolayer, developed by Irving
Langmuir41, is depicted in Figure 1-7. The procedure consists of dropping a volatile
solution of an amphiphilic molecule onto a water surface contained inside a teflon 'trough'
having movable teflon barriers. The solvent evaporates and the polar end of the amphiphile
wants to dissolve into the water subphase; the hydrophobic end is directed away from the
aqueous surface. At this stage, the molecules are separated from each other by large
distances and are labeled as a two-dimensional "gas" as shown in part A of Figure 1-7.33
The organization of the molecules on the surface of the subphase is monitored by the
change in surface tension of pure water and the surface tension of water covered with a
monolayer. This difference in surface tension is equivalent to the surface pressure and is











Wilely treading Solution
A Plate
Barrier

SAnalogous
Iil
>,_______________________^ hn

B
"Liquid"
Analogous
Film
.______________^Rhn

C
C"Solrt"
Analogous
Film


Figure 1-7. Formation of a Langmuir Monolayer at the air/water interface. A-C depict
the three stages in the organization of the monolayer as monitored by the measurement of
the surface pressure of the aqueous subphase.


measured using a platinum plate, more commonly known as a Wilhelmy plate, suspended

from a microbalance.42 At this stage, the barriers move and compress the molecules at the

air/water interface. The molecules become closer together and undergo a phase transition

from a "liquid" state, Figure 1-7B, to eventually a "solid" state where the molecules make

up a closely packed and uniformly oriented monolayer42 as illustrated in Figure 1-7C.

During the formation of the Langmuir monolayer, the surface pressure is recorded as a

function of the area per molecule since the total number of molecules and the area that the

monolayer occupies is known. A typical plot of a pressure-area isotherm is shown in

Figure 1-8. In the gas region of the pressure-area isotherm, interactions between molecules

are negligible and surface pressure is low. As the molecules are compressed, interactions

between molecules increase and surface pressure slowly rises. As the molecules are

compressed further, surface pressure increases rapidly as the head groups and hydrophobic












"Solid"
I

I"Liquid"

S"Gas"




Mma(A2)

Figure 1-8. A Pressure-Area Isotherm. Typically three distinct regions are observed in
the formation of a Langmuir Monolayer signified by changes in slope of the line.


portions of the molecules form a close-packed, organized monolayer.
The next stage in the Langmuir-Blodgett experiment involves the transfer of the

Langmuir monolayer onto the surface of a solid support. The vertical deposition method,
illustrated in Figure 1-9, is the most conventional method of transferring monolayers onto

solid supports and was developed by Blodgett and Langmuir in 1937.43 The procedure

involves moving a substrate through a Langmuir monolayer which is held at a surface

pressure chosen in such a manner that the monolayer will maintain its close-packed,
uniformly organized structure. When the substrate is hydrophobic, as shown in Figure 1-

9, the monolayer will be transferred on the down-stroke with the hydrophobic alkyl chains

of the monolayer interacting with the surface. Upon withdrawal of the substrate from the
subphase, hydrophilic interactions of the head-groups will occur and result in the
deposition of a second monolayer onto the substrate. It is common to call the transfer of

two complete monolayers onto a substrate, deposition of a Langmuir-Blodgett bilayer.
Therefore, in this dissertation, a bilayer is defined as consisting of two organic portions

and one inorganic portion as shown in Figure 1-10.






























Figure 1-9. Deposition of a Langmuir Monolayer onto a Hydrophobic Substrate.
Transfer of a monolayer results from the hydrophobic interactions between the substrate
and the hydrophobic part of the monolyaer as the substrate is lowered through the
monolayer. When the substrate is withdrawn through the air/water interface, interactions
between the polar portions of the monolayer result in the transfer of a complete bilayer of a
LB film.


Figure 1-10. A Langmuir-Blodgett Bilayer. The sticks represent the hydrophobic alkyl
groups in the bilayer and the balls represent the hydrophilic portion of the bilayer.








Saturated fatty acid molecules were the pioneer molecules used by Langmuir and

Blodgett over 60 years ago44 and have continued to maintain their popularity in this field.

The literature is flooded with studies of long chain carboxylic acids and their relation to

Langmuir monolayers and LB films. One of the most challenging aspects of ultrathin film

research lies in the structural characterization of such minute amounts of materials. A great

advancement in the area of Langmuir-Blodgett films came from the modification of surface

sensitive anayltical techniques to probe the molecular structure of ultrathin films.

Zasadzinski, Dutta, and Blasie were just a few of the many researchers to make significant

contributions to this advancement by providing insight to the in-plane structure and

morphology of LB films of fatty acid salts using high-resolution X-ray diffraction methods

and scanning probe microscopy.32'45-49 From their structural reports on Langmuir

monolayers and Langmuir-Blodgett films, one may better understand how to modify

variables in the LB experiment to fabricate thin films with significant control at a molecular

level.

The in-plane structure of LB films of barium and lead stearate was determined using

X-ray diffraction methods.48,'49 By comparison of the diffraction patterns obtained from

multilayers of the films with the diffraction patterns obtained from powdered samples of

lead stearate, assignment of a unit cell for the films was possible.48 In another more direct

series of investigations, Zasadzinski also assigned unit cells for LB films of many fatty acid

salts using AFM. Zasadzinski's discoveries revealed that the structure of transferred LB

films of fatty acid salts did not always maintain the same structure as the corresponding

floating Langmuir monolayer. A number of factors can influence the structure of the

transferred film. AFM studies on arachidate monolayers and Langmuir-Blodgett

monolayer and multilayer films with and without divalent metal ions, exhibit disordered

arrangements of the alkyl groups for those films lacking the metal ions, to organized

crystalline arrangements of the alkyl groups in those films containing divalent metal ions.45

It was also realized that the interaction of the metal ions with the carboxylate headgroups








attributed mechanically stable films regardless of substrate, deposition conditions, number

of layers, and AFM imaging conditions. It was postulated that the strong interactions

between the metal ions and the oxygens of the carboxylate groups provide the energetically

driving force for organization.45 In another strikingly remarkable AFM study, experiments

revealed that with the substitution of Ba2+ for Cd2+ in fatty acid LB films the structural

parameters of the molecular organization changed dramatically.50 The authors speculate

that the molecular areas of the fatty acid films are dictated by the size and bonding

requirements of the metal ion and the alkane packing is forced to adopt the best

configuration compatible with the molecular area.50 Further AFM studies on lead and

manganese carboxylate LB films revealed different molecular areas and alkyl chain

packing than those of the corresponding carboxylate films containing cadmium and

barium. This difference may be attributed to the stronger intralayer interactions present in

the lead films, and to a smaller extent the manganese films.3235 A conclusion may be

drawn from these thorough results: the most durable and closely packed LB film may be

constructed by maximizing intralayer interactions of the divalent metals, and the alkyl

chains will tilt to maximize the van der Waals interactions between them.


Theory of Low-Dimensional Magnetism

Understanding magnetic phenomena in two-dimensions continues to be a goal of

materials research. The transition from magnetic exchange between two paramagnetic ions

to spontaneous magnetization at some finite temperature depends on two factors: lattice

dimensionality and spin dimensionality. Predictions of when to expect spontaneous

magnetism in low-dimensional materials dates back to 1930 when Bloch concluded from

his theory of spin waves with Heisenberg exchange, that ferromagnetism was impossible

in two-dimensional systems.51 This theory initiated much research into the magnetic

behavior of systems with low lattice dimensionality. Onsager provided a proof that if

magnetic exchange was anisotropic, or Ising type, spontaneous magnetization could occur








at finite temperatures in two-dimensions.52 This was further validated by Mermin and

Wagner who published a proof that ferromagnetism and antiferromagnetism cannot exist at

finite temperatures for Heisenberg and XY spin interactions, but no conclusive argument

could be made for Ising type exchange.53 The theoretical predictions of when to expect a

transition from short-range magnetic exchange to spontaneous long range order as a

function of spin and space dimensionality are summarized in Figure 1-11.

The physical predictions were tested by many scientists investigating the magnetic

behavior of many quasi two-dimensional materials. Numerous magnetic investigations

have involved materials crystallizing with the K2NiF4 structure. For example, temperature

dependent magnetic studies display two-dimensional antiferromagnetic exchange at high

temperatures in K2MnF4, La2CuO4, and K2NiF4.11,54-56 In each complex, a transition

from short range order to long range order occurs. Although the observation of an ordered

state in these quasi two-dimensional compounds seemed to oppose theoretical predictions,

the transition was justified by a crossover in lattice dimensionality or spin dimensionality.

This crossover property in two-dimensional layered materials is dependent upon the

distance between magnetic layers and the magnetic correlation length within a single

magnetic layer. A lattice dimensionality crossover will occur when the interactions between

magnetic layers is larger than the correlation length within a layer. A dimensionality

crossover from 2d Heisenberg to 3d Heisenberg is the driving force for an ordered state in
complexes of La2CuO4.55 On the other hand, when the correlation length or the size of the

magnetically correlated spins is larger than the magnetic coupling between layers, a

crossover in spin dimensionality is sited as the explanation for an observed transition to
long range order. This is the case in K2MnF4 and K2NiF4. As the temperature is

decreased, the correlation lengths increase and a transition from 2d Heisenberg to 2d Ising

is predominant.11'54'56

The layered perovskites were also a popular choice for magnetization experiments

on two-dimensional systems. Many alkylammonium metal tetrahalides were synthesized






















Figure 1-11. Effects of Space Dimension and Magnetic Spin Interaction on Magnetic Transitions. Theory predicts a transition
to long range magetic order at finite temperatures for only specific combinations of space and spin dimensions.








Lattice Dimensionality


--t-- ---.-" -.--_T

1D 2D 3D


SNo Transition Long Range Long Range
| atT>OK Orderat T > 0 K Orderat T > 0 K
Ising
Jxy = 0_____ ____________



No Transition No Transition Long Range
at T > 0 K at T > 0 K Orderat T > 0 K

XY or Planar
Jz=0




5 No Transition No Transition Long Range
-at T > 0 K at T > 0 K Orderat T > 0 K

Heisenberg
Jxy = Jz ___________ ________________ _______


N

N



+








II
II








and variations with respect to the length of the alkyl group and metal were made. Although

these compounds are anisotropic, they remain part of a three-dimensional network and at

low temperatures spontaneous magnetization occurs. The explanation for the observation

of the transition in most cases, is attributed to the dimensionality crossover from two to

three-dimensions and no theories have been disproven.

The magnetization measurements of a series of manganese alkylphosphosphonates

were performed by Day et al.57 Static susceptibility measurements on manganese methyl,

ethyl, propyl, and butylphosphonate exhibit a transition to long range order at about 15K.

Manganese is a Heisenberg nucleus with S = 5/2, and the observation of spontaneous

magnetization is attributed to a dimensionality crossover from two-dimensions to three-

dimensions.58 Since these solids are part of a three-dimensional crystal, a better two-

dimensional system is a true monolayer where the magnetic coupling is restricted to two-

dimensions.

In 1975, Pomerantz began a study of Langmuir-Blodgett films of manganese

alkylcarboxylates in order to investigate magnetism in two dimensions.59 He constructed

Langmuir-Blodgett films containing spin 5/2 manganese ions which are Heisenberg in

nature. The magnetic behavior of the two-dimensional film was probed with EPR

spectroscopy. For the EPR experiments, sample preparation involved transferring

monolayers of manganese stearate onto several quartz plates and stacking together as many

plates as possible using bits of mylar as spacers between the plates.60 Extensive electron

paramagnetic resonance experiments were performed on the films and observation of

antiferromagnetic exchange was reported. Evidence of a transition from short range order

to long range order was cited, but no direct observation of an ordered state was achieved.60


Scoe of Dissertation

There is currently high interest in engineering mixed organic/inorganic materials

where features of the organic and inorganic components complement each other leading to








new solid-state structures and materials with composite or even new properties. Some

examples of recent interest are layered inorganic solids with organic intercalates,61'62

organic/inorganic low-dimensional solids,9'21'6364 zeolites and other open framework host

materials with organic guests,65'66 metal oxide mesostructures formed with the aid of

organic surfactants,67-69 and thin film heterostructures built-up from alternating layers of

organic polyelectrolytes and colloidal inorganic polyions.70'71 In some cases, a stable

inorganic lattice facilitates spatial and orientational control of organic molecules. On the

other hand, new inorganic lattice structures are sometimes formed resulting from

cooperative interactions between the organic and inorganic components. In all cases, there

is the promise of developing new materials with properties not seen in purely organic or

purely inorganic solids.

The Langmuir-Blodgett technique allows for the organization of molecular

assemblies at air/water interfaces. The assemblies may be transferred onto solid substrates

and in favorable cases, well ordered ultrathin films are formed. Langmuir monolayers are

constructed from amphiphilic molecules containing long organic groups and polar

headgroups, with long chain carboxylic acids historically being the most popular choice of

amphiphile. It is long been realized that metal ions added to the subphase under monolayer

films of fatty acids can be incorporated into Langmuir-Blodgett monolayer and multilayer

films.44 Divalent metal ions crosslink the carboxylate groups forming a two-dimensional

inorganic lattice and enhance the stability of the transferred films.33'42'72 The addition of

an inorganic extended lattice into LB films opens the possibility of introducing physical

properties that are typical of the inorganic solid-state. Through recent structural studies on
LB films of fatty acid salts,32'46'73-75 it also became clear that the ionic interactions

between the polar carboxylate headgroup and the metal ion often determine the destiny of

the transferred film. Historically however little attention has been placed on using these

favorable interactions to gain more control over the deposited film. It seems evident to

utilize these favorable polar headgroup interactions found in carboxylic acid films of








amphiphilic molecules with metal ions to purposefully control and dictate the structures of

LB films consisting of different amphiphilic molecules. Once it is demonstrated that

crystalline inorganic extended-lattice structures may be incorporated into ultrathin films and

the inorganic lattice retains its anticipated behavior, the nature of the organic portion of the

films may be given functionality and the possibility exists to fabricate LB films with hybrid

organic/inorganic properties.

To begin these investigations, hybrid organicfinorganic LB films in which the

organic portions bear no other purpose than to isolate the inorganic layers were fabricated

to determine if inorganic extended-lattice structures could be incorporated into organic

films. Chapter two presents a thorough structural study of LB films of a series of divalent

metal octadecylphosphonates. The Langmuir-Blodgett technique is used in conjunction

with the favorable lattice energy found to be present in solid-state metal phosphonate

compounds to purposefully control molecular structure and properties to form high quality,

durable ultrathin organic films. It will be demonstrated that extended-lattice monolayers

and multilayers of some of the metal phosphonates can be prepared using the Langmuir-

Blodgett (LB) method.76,77 The lattice energy between the metal ions and the oxygens of

the phosphonate head groups, determines the structure of transferred LB films of divalent

metal alkylphosphonates. The alkyl chains vary their packing, tilt angle, and tilt direction

to achieve close-packing in the organic/inorganic thin films. Single layer and multilayer LB

films of manganese, cadmium, cobalt, magnesium, and calcium octadecylphosphonate

were prepared and the structures of the films are compared to the structures of the solid-

state structures to verify that the films are isostructural with the solid-state metal

organophosphonates. It will be demonstrated using the especially compelling example of

the calcium octadecylphosphonate LB film that the structure of each octadecylphosphonate

LB film depends on the identity of the metal ion. The strong metal/phosphonate binding

interaction determines the structure of the LB films just as in the solid-state phosphonates.








To provide further evidence that organized inorganic extended-lattice structures may

be incorporated into LB films, a detailed electron paramagnetic resonance (EPR)

investigation was performed on LB films of manganese octadecylphosphonate and

presented in chapter three. The magnetic properties of the manganese LB film are

compared to the structurally analogous manganese alkylphosphonate powdered samples.

The magnetic interactions between the paramagnetic manganese ions in powered

samples of a series of manganese alkylphosphonates with varying alkyl chain lengths were

probed using static magnetic susceptibility measurements.57'58 It was found that the

manganese ions undergo two-dimensional Heisenberg antiferromagnetic exchange at high

temperatures. At approximately 15 K a transition from short range antiferromagnetic order

to one of long range order occurs and a weak ferromagnetic moment is observed.57'58 The

exhibited magnetic properties of these solids require that the inorganic layer be organized

and crystalline.

Chapter three summarizes the results of an investigation using variable temperature

EPR spectroscopy on a 50 bilayer LB film of manganese octadecylphosphonate. As it will

be pointed out, the magnetic exchange in the manganese LB film is similar to the magnetic

exchange observed in the powered manganese alkylphosphonates. From the EPR studies,

the manganese LB film can best be described as a two-dimensional Heisenberg

antiferromagnet. The g values of the LB film are characteristic of Mn2+ in a nearly cubic

field and are essentially isotropic. The dependence of the EPR linewidth on sample

orientation in the magnetic field is consistent with the behavior predicted for a two-

dimensional lattice with antiferromagnetic Heisenberg exchange. The temperature

dependence of the integrated area of the EPR signal, which is proportional to spin

susceptibility is presented. A value for the antiferromagnetic exchange constant was

extrapolated from the results and the value agrees closely with the exchange constants of

the analogous solid-state manganese phosphonates. EPR linewidths of the manganese

octadecylphosphonate LB film increase rapidly as the temperature is lowered below 30 K.








This is characteristic of a system approaching a magnetic ordering transition. Using EPR

spectroscopy, an ordering transition cannot be observed since the linewidth becomes too

broad and the signal is too weak to measure below 17 K

With evidence from the EPR study of the manganese octadecylphosphonate LB film

pointing to the likely occurance of a transition from short range two-dimensional

Heisenberg antiferromagnetic exchange to long range order, there was desire to probe the

magnetic behavior of the LB film at lower temperatures. In chapter four, static magnetic

susceptibility experiments were performed on multilayer samples of manganese

octadecylphosphonate LB films. In agreement with the magnetization studies of the

powered manganese alkylphosphonate samples, the LB film also undergoes a transition to

long range order at 13.5 K Below the ordering temperature the manganese LB film

exhibits a weak ferromagnetic moment The weak moment in the ordered state is due to

antiferromagnetic ordering where coupled nearest neighbor moments do not exactly cancel

due to spin canting. These results are the first demonstration of cooperative ordering in an

LB film. The LB film also displays magnetic memory below the ordering temperature.

The effect is small but nevertheless this is the first observation of a magnetic LB film. The

magnetic exchange is due to superexchange via the phosphonate ligand and the extended-

lattice structure of the film provides sufficient structural order for magnetic ordering to take

place. As a result of these magnetic investigations, it is established conclusively that

extended-lattice layered structures can be incorporated into LB films and properties

normally associated with solid-state inorganic structures can be incorporated into ultrathin

organic films.

It is well established1 that any magnetic interlayer interactions that occur in quasi

two-dimensional solids act to increase the ordering temperature and any nonmagnetic
impurities cause a suppression in the ordering temperature.55 In chapter five the

manganese octadecylphosphonate LB films are manipulated in such a way as to investigate

the importance of the interlayer magnetic exchange. In the pure manganese LB films,








where ordering occurs at 13.5 K, the separation between successive manganese layers is

about 50A and it is predicted that any exchange that occurs between layers is minimal. In

order to accurately asses this prediction, LB films of alternating layers of manganese and

cadmium octadecylphosphonate were prepared.

Since cadmium is diamagnetic, the layers of cadmium octadecylphosphonate act as
nonfunctional spacers in these new expanded manganese films. The separation between

adjacent manganese layers is now increased from 50A to 100A. Magnetic susceptibility

measurements performed on the expanded films resulted in an ordering temperature of 10.5

K, about 3 K lower than the ordering temperature of the pure manganese LB films.

In addition, Langmuir-Blodgett films of manganese octadecylphosphonate with

manganese layers separated by two layers of cadmium octadecylphosphonate were also

prepared. In these films, the distance between manganese layers is now tripled relative to

the pure manganese LB film. Again, magnetic measurements illustrate an ordering

temperature at 10.5 K. It seems unlikely that the suppression in the transition temperature

in the two expanded manganese LB films relative to the pure manganese LB film is due to
interlayer coupling since the interlayer distance in the pure manganese film is so large.

Instead a more likely explanation of the temperature suppression is due to small nonmagnetic

impurities in the manganese layers of the alternating manganese/cadmium films. These

impurities due to cadmium ions in the manganese layers, act to break up the domains of

manganese spins. With the decrease in the correlation length of the manganese spins, a

reduction in the ordering temperature is expected. To provide further evidence of this

explanation magnetic measurements were performed on two additional LB samples. The

first sample contained 13% Cd mixed into the manganese layers. With the large amount of

nonmagnetic impurities, the correlation lengths of the manganese spins was so small no

transition to long range order occurred. Instead, the manganese spins acted independently

and the results were comparable to a normal paramagnet. In a second sample, 5% Cd was
mixed into the manganese layers. Magnetization measurements revealed a suppression of





29

the ordering temperature. The results of chapter five illustrate that the magnetic behavior of

the LB films obeys the predictions set by basic theories of magnetism, but most importantly

it demonstrates that inorganic properties observed in solid-state materials may be

incorporated into Langmuir-Blodgett films.













CHAPTER 2
LANGMUIR-BLODGETT FILMS OF KNOWN LAYERED SOLIDS: PREPARATION
AND STRUCTURAL PROPERTIES OF OCTADECYLPHOSPHONATE BILAYERS
WITH DIVALENT METALS

Introduction

The Langmuir-Blodgett technique33'42'43'72'78 is one of the oldest and most elegant

approaches known that allows researchers to purposefully arrange molecules into organized

assemblies. Molecules, usually amphiphiles, are first compressed to a close-packed

monolayer at a water surface followed by transfer of the assembly as a monolayer to a solid

support Multilayer films are formed through repeated deposition cycles. While the LB

method is normally considered a technique for organizing organic molecules, inorganic

ions are often incorporated into transferred films.33'43'72 The metal ions crosslink charged

molecules and enhance the films' stability and processibility, but they are generally thought

of as passive elements in the otherwise organic assemblies. While there are recent studies

that show how the identity of the inorganic ions can influence the LB film structure,32 there

has been relatively little effort by researchers to control the structure and function of the

inorganic component of LB films.33'72 If the inorganic network can be purposefully

developed, then the alternating hydrophobic and hydrophilic layered structure of LB films,

coupled with the ability to control layer-by-layer deposition, should provide a unique

opportunity to explore the fabrication of mixed organic/inorganic layered solids and thin

films.

The approach that is used to investigate control of the inorganic component of LB

films is to base the films on known inorganic layered structures and incorporate inorganic

extended lattice structures into the hydrophilic portion of the LB assemblies.76'77'79 There

are numerous examples of inorganic and mixed organic/inorganic layered solids,9'80 and








the objective is to use LB methods to prepare examples of these extended lattice inorganic
layers. If the inorganic lattice favors a layered structure it might complement the layered

nature of organic LB film assemblies. These concepts were previously
demonstrated76'77'79 in the characterization of LB films of zirconium octadecylphosphonate

and manganese octadecylphosphonate which are each analogs of known metal phosphonate
layered solids.15'17'21'81 Although the tetravalent and divalent metal phosphonate solid-

state structures are slightly different,15,17'21'81 these solids are all characterized by sheets
of metal ions that are bonded above and below the metal ion plane by layers of the
organophosphonates as illustrated by the cartoon shown in Figure 2-1. The phosphonate

ligands bridge metal ions forming the extended lattice layers, and adjacent layers are
separated by van der Waals interactions between the organic groups. The zirconium

phosphonate and manganese phosphonate LB films have been shown to have the same

inorganic lattice structure as the analogous solids.76'77'79




Organic Molecular
5 Solid Network


PO3PP3P3PO03
M M M M Inorganic Extended
M M M M T -
/ \ / \ / Lattice Network
P03P03P03P03








Figure 2-1. Illustration of a Metal Organophosphonate Compound. In the solid-state,
these materials consist of two dimensional arrays of organic molecular networks separated
by inorganic extended lattice networks.








This chapter describes LB films modeled after divalent metal organophosphonates.

Thorough structural studies by Mallouk and co-workers13-15'21'22 and Cao and

coworkers16 have identified two series of solid-state divalent metal organophosphonates,
M(O3PR).H20 (M = Mg, Mn, Zn, Ca, Co, Cd; R=n-alkyl, aryl group) and M(HO3PR)2

(M = Ca). Each forms a layered structure and interlayer distances vary to accommodate the

different R groups. For M=Mg, Mn, Co, and Zn in the first series, the layered

phosphonates are isostructural, crystallizing in an orthorhombic space group.14'15 Each

phosphonate group bridges four metal ions and the metal ions are coordinated by five

oxygens from four different phosphonate groups. The distorted octahedral coordination of

the metal ion is completed by a water of hydration as seen in Figure 2-2 for the crystal

structure of cadmium methylphosphonate.16 For M=--Ca or Cd in the same series, a

structure of slightly lower symmetry is adopted.13,14'16 In the second series, calcium

forms 1:2 salts with alkylphosphonates having alkyl groups containing five carbon atoms

or greater as illustrated in Figure 2-3.13.14
This chapter will illustrate how the LB approach for depositing metal phosphonate

films is quite general and offers a compelling example of how the choice of metal ion can

be used to control the structure of the deposited LB films. The detailed description of the

preparation and characterization of octadecylphosphonate LB films with a series of divalent

metals including Mn2+, Mg2+, Cd2+, Co2+ and Ca2+ will be presented. While different

structures are observed, each LB film will be shown to adopt the same metal-phosphonate

binding as the known solid-state analogs. The Mn2+, Cd2+, Co2+ and Mg2+

octadecylphosphonate LB films form M(O3PR)'H20 structures, and the calcium
octadecylphosphonate LB film forms with a 1:2 metal to phosphonate stoichiometry and

formula Ca(HO3PR)2. The different structures demonstrate the important role that the
inorganic extended lattice plays in organizing the LB films and that the metal phosphonate

LB film structure can be controlled by choice of metal ion.






















Figure 2-2. X-ray crystal structure of cadmium methylphosphonate, Cd(O3PCH3)-H20. (A) Packing diagram, and (B)
Cadmium coordination. Structural coordinates taken from reference 16.








34







0-





















Figure 2-3. X-ray crystal structure of calcium hexylphosphonate, Ca(H03PC6H13)2. (A) Packing diagram, and (B) calcium
coordination. Structural coordinates taken from reference 13.





36








0















All chemicals were purchased and used without further purification.
Octadecylphosphonic Acid, CH3(CH2)17P(O)(OH)2, 98% was purchased from Alfa

Aesar (Ward Hill, MA). MnCl2-4H20 (99.6%), CoCl2-6H20 (99%), and CaC12
(97.8%) were purchased from Fisher Scientific (Fair Lawn, NJ). CdCl2.2.5H20 was
obtained from Aldrich (Milwaukee, Wisconsin). Mg(NO3)2.6H20 was obtained from
Mallinckrodt, Inc. (Paris, Kentucky). Octadecyltrichlorosilane (OTS, C1I8H37SiC13,
95%) was purchased from Aldrich and stored under N2. A Barnstead Nanopure (Boston,

MA) purification system produced water with an average resistivity of 18 MD2-cm for all

experiments.

Substrate Prepration and Deposition Procedure

Single crystal (100) silicon wafers, purchased from Semiconductor Processing

Company (Boston, MA), were used as deposition substrates for XPS and ellipsometry
measurements. Glass slides, (Buehler, Lake Bluff, IL) were the substrates in the XRD
experiments. The silicon and glass substrates were cleaned using the RCA cleaning
procedure82 then dried under N2. Germanium attenuated-total-reflectance (ATR) crystals,
(45 50mm x 10mm x 3mm) purchased from Wilmad Glass (Buena, NJ), were used as

substrates for the infrared experiments. Octadecyltrichlorosilane (OTS) was self-
assembled39'83 onto the substrates to make them hydrophobic. The clean substrates were
placed in a 2% solution of OTS in hexadecane for 30 minutes, rinsed with chloroform to
remove any excess hexadecane, and dried under a stream of nitrogen.
Divalent metal octadecylphosphonate Langmuir-Blodgett films were prepared by

spreading octadecylphosphonic acid onto an aqueous subphase containing a salt of the








appropriate metal at a concentration of 5 x 10-4 M. The pH of the subphase was adjusted

appropriately using HC1 or KOH. Target pressures, barrier speeds, and subphase pH

varied depending on the metal used as recorded in Table 2-1.

Table 2-1. Deposition conditions for the preparation of metal octadecylphosphonate LB
films.

LB Filma Subphase pH Target Pressure (mN/m)

Mn(O3PC18H37)'H20 5.2 5.6 20

Cd(O3PC18H37),H20 4.2- 5.0 17

Mg(O3PC18H37)-H20 7.4- 7.6 17

Ca(HO3PC18H37)2 7.0-8.10 17

Co(HO3PC18H37) 5.5-5.8 17
a) Barrier speeds = 5mm/min on the upstroke; 8mm/min on the downstroke.

Instrumentation

The LB films were prepared using a KSV (Stratford, CT) 3000 trough modified to

operate with double barriers. The surface pressure was measured with a platinum

Wilhelmy plate suspended from a KSV microbalance.

Infrared spectra were recorded with a Mattson Instruments (Madison, WI) Research

Series-1 FTIR spectrometer using a narrow-band mercury cadmium telluride detector. LB

films were deposited on OTS-covered Ge ATR crystals and a Harrick (Ossining, NY) TMP

stage was used for the ATR experiments. Polarized FTIR-ATR spectra were taken with s-

and p- polarized light. All ATR spectra consisted of 1000 scans at 2.0 cm-1 resolution and

were referenced to the OTS-covered Ge ATR crystal or the appropriate s- or p- polarized

background.

X-ray photoelectron spectra were obtained using a Perkin-Elmer (Eden Prairie,
MN) PHI 5000 Series spectrometer. All spectra were taken using the Mg Ka line source at

1253.6 eV. The spectrometer has a typical resolution of 2.0 eV, with anode voltage and








power settings of 15 kV and 300 W, respectively. Typical operating pressure was 5 x 10-9

atm. Survey scans were performed at a 45" take-off angle with a pass energy of 89.45 eV.

Multiplex scans, 140 scans over each peak, were run over a 20-30 eV range with a pass

energy of 35.75 eV. The observed relative intensities were determined from experimental

peak areas normalized with atomic and instrument sensitivity factors.84'85

X-ray diffraction was performed with a Philips APD 3720 X-Ray Powder
Diffractometer using the Cu Ka line, X = 1.54 A, as the X-ray source.

Ellipsometry measurements were performed on a Rudolph Instruments (Fairfield,

NJ) Series 431A Universal Ellipsometer using a 70" angle of incidence with a helium-neon
laser, X = 632.8 nm, as the source.

Results and Discussion


Film Deposition

The divalent metal octadecylphosphonate LB films are transferred onto solid

surfaces by compressing the monolayer to an optimum pressure, Table 2-1, then lowering

a hydrophobic substrate through the film at 8mm/min, transferring the film in a tail-to-tail

fashion on the down-stroke. The substrate is then raised from the subphase through the

monolayer at a speed of 5mm/min, forming a head-to-head bilayer as demonstrated in

Figure 2-4. All of the films discussed in this paper have been transferred onto solid

supports made hydrophobic with a self-assembled monolayer of OTS. Slow deposition

speeds for the up-stroke allow draining of the water from the film and aid in the

crystallization of the inorganic lattice. To prepare crystalline LB films of the phosphonates

on solid surfaces, the optimum pH of the subphase depends on the identity of the metal ion;

Table 2-1. The metal phosphonates are soluble in acid solutions so if the subphase pH is

too low, metal ions are not incorporated into the Langmuir monolayer, but if the subphase

pH is too high phosphonate groups cross-link the M+2 ions at the air/water interface












Meniscus
Region


03H2 P03H2 P03H2 P03H2 P03H2
M(aq)


Meniscus Region






Solid Support
Solid Support


Monolayer at
Air/Water
interface


Subphase


Figure 2-4. Deposition of a divalent metal alkylphosphonate Langmuir-Blodgett film.
Interactions between the hydrophobic substrate and the hydrophobic part of the Langmuir
monolayer result in the transfer of one layer of the phosphonic acid. Bonding of the metal
ions in the subphase to the oxygens of the phosphonate groups results in the incorporation
of metal ions in the LB film and upon withdrawal of the substrate from the subphase, a
second layer of alkylphosphonate is deposited onto the substrate.


Pull


O-P03H2 1
-PO3H2
'"PO03H2
S--P,03H2








making the Langmuir monolayer too rigid to transfer. Therefore, the optimum subphase

pH is the highest possible pH at which the monolayer is not too rigid to transfer.

Multilayers of the metal phosphonate films are not formed by continuous deposition of the

films because the Langmuir monolayer becomes too rigid with time. Instead, one bilayer is

transferred to a substrate, then the monolayer is removed from the water surface and a new

octadecylphosphonic acid monolayer is formed. For each of the metal phosphonate systems

studied, this deposition technique produces mulitlayer films with each successive transfer

having a transfer ratio of 1.00.1.

X-ray photoelectron SDectroscooy

In the XPS experiment, photons of sufficiently high energy strike a sample and

ionize atoms in the sample. The kinetic energy, KE, of the ejected electrons depends on the
energy of the incoming photons, hv, and the binding energy, BE, of the atom from which

the electron originated according to the Einstein photoelectric law: KE = hv-BE.86 Since

elements have characteristic binding energies, the position of the observed peaks in the

XPS spectrum may be used to identify the elements in the sample. Figure 2-5 shows the

XPS survey and multiplex spectra from one bilayer of a magnesium octadecylphosphonate

LB film. In all LB films, elemental analyses determined by XPS, reveals that the only

atoms present in each are carbon, oxygen, phosphorus, and the appropriate metal

Obtaining quantitative information from the XPS experiment, however, is not as

straightforward. Many instrumental and experimental factors play a role in determining

the intensity of each peak in the XPS spectrum.87 As illustrated in Figure 2-6, incoming

X-rays having intensity Io enter the surface of the sample and penetrate to some depth, dm,

below the surface where there exist Ni atoms of element i. Electrons from these elements

will be removed from each occupied orbital. The probability for photoejection from each
orbital, the photoionization cross section, y, is different for each orbital and different for a

given orbital in different elements. Tables of cross section values have been compiled from






















Figure 2-5. (A) XPS survey spectrum from one bilayer of a magnesium octadecylphosphonate LB film. (B) XPS multiplex
spectrum of a single bilayer of magnesium octadecylphosphonate. The magnesium 2p and phosphorus 2p signals are shown.
Integrated intensities are consistent with a 1:1 Mg to P ratio after accounting for the film geometry, photoelectron energies, and
the appropriate elemental and instrumental sensitivity factors.











8

6


z 4


2


0


10


8


S6


4


2


0


100 80
Binding Energy (eV)


60 40


1000 800 600 400 200











Detector


X-rays


LB Film
Surface


dm1
00oooo oooo roooooooeeeooooooo Ni

e e
e
Figure 2-6. Illustration of the XPS Experiment and the Factors that Influence
Quanification of the XPS Spectrum.


experimental measurements of relative peak areas on materials of known compositionn.4

Electrons from atomic energy levels will be emitted in all directions as shown in Figure 2-

6, but only those aimed at the detector will be counted. The fraction aimed at the detector
will be A/4t2, where A is the area of the sample. In order to be detected, the electrons

must travel through the sample and escape. The inelastic mean free path of the electrons is

defined as the distance over which about 60% of the electrons can travel before losing

energy in collisions with other electrons.87 The amount of electrons reaching the
detector,ki, depends exponentially on the inelastic mean free path of the electrons, e,

where ki = edm/Aecos0. Since the detector reports only a fraction of the photoelectrons

that arrive, D, the integrated area of the XPS signal also depends on this quantity. The total

intensity, I, for element i, is equal to the product of all the above factors: Ii =
IoedNiO(A/4it2)iD.87 The factors related to instrument geometry, X-ray flux, and

photoelectron production remain constant and are often incorporated into manufacturers







tables of atomic sensitivity factors. The peak areas in the octadecylphosphonate LB films
have been integrated and corrected with atomic sensitivity factors84'85 to yield the observed
relative intensities for each element, as summarized in Table 2-2.

Table 2-2. Relative Intensitiesa of the Metal and Phosphorus XPS Signals for Single
Bilayers of the Divalent Metal Octadecylphosphonate LB Films.
LB Film XPS Peak Observed Relative Calculated Relative
Intensity (3%) Intensityb (%)
Mn(03PC18H37)-H20 Mn2p 37 40
P2p 63 60
Cd(03PC18H37)'H20 Cd3d 44 47
P2p 56 53
Mg(03PC18H37)'H20 Mg2p 47 51
P2p 53 49
Co(03PC18H37)H20 Co2p 38 38
P2p 62 62
Ca(H03PC18H37)2 Ca2p 29 32
P2p 71 68
a) Intensities are reported as the percentage of the sum of the integrated areas of the metal
and phosphorus peaks after correcting for elemental sensitivity factors.
b) Assuming a layered structure, using a model described in reference.88


Atomic sensitivity factors,I'*, will have different values for different peaks of
different elements. Peaks measured for various elements having considerably different
energies will have significantly different inelastic mean free paths. As a consequence, the
relative intensity of the peaks will not reflect the appropriate concentrations of the elements
in the sample since the sampled volume of those electrons with a larger mean free path will
be larger. A model was derived to accurately predict the expected relative XPS signal
intensities for the elements based on the layered film geometry.88 Seah and Dench
developed an equation to estimate the inelastic mean free path of a photoelectron in an
organic film, ) = 10[49/(KE2) + 0.11(KE)0.5] (A).89 The intensity of an element, A, is

given by an attenuation equation:87







00 ^ 1
A A= A Xesin I
where is IA the atomic sensitivity factor and dm is the overlayer thickness. The relative
intensity for element A is given by:87
I- exp -dmd

IA ,Y exp -dm 1o00x -d 1+..
A ,'- Lz,A(sin0). I -exp Ee,B(SinO) "

The overlayer thicknesses depend on the position of the atoms in the sample and may be
estimated from the interatomic distances as illustrated in Figure 2-7.


Surface





24.3 A
/?\ /


/ 22.5 A

\ -


0 0 0 0 0 0
Mn Mn Mn
0 0 0 0 0 0 0 0 0





Substrate
\lI/ \ I/ \lI/





Figure 2-7. Determination of Overlayer Thicknesses in LB Films of the Manganese
Octadecylphosphonates.


For the Mn2+, Cd2+, Co2+ and Mg2+ octadecylphosphonate LB films, the calculated ratios
assume the 1:1 metal to phosphorus stoichiometry present in the M(O3PR)-H20 solid-state
phases. Table 2-2 compares the calculated metal phosphorus ratios with the experimentally








observed ratios. Within experimental uncertainty (3%), the observed ratios for the Mn2+,
Cd2+, Co2+ and Mg2+ films are consistent with the bulk metal phosphonate stoichiometry.

Elemental analysis of calcium octadecylphosphonate LB films via XPS reveals a calcium to

phosphorus ratio of 1:2. In the solid-state, calcium alkylphosphonates form two layered

phases with differing stoichiometries that depend on the length of the alkyl chain.13'14
Salts with a 1:1 calcium phosphonate stoichiometry, Ca(O3PCnH2n+I)'H20, form when

n = 1-5; and 1:2 calcium phoshonates, Ca(HO3PCnH2n+I)2, form for n > 5.13,14 The

XPS findings for the calcium phosphonate LB films are consistent with the stiochiometry

observed in the solid-state calcium phosphonate compounds containing long chain

hydrocarbons.

Ellipsometry and X-ray Diffraction

X-ray diffraction illustrates the layered nature of the LB films. The X-ray

diffraction pattern obtained from a 15 bilayer cadmium octadecylphosphonate LB film is

shown in Figure 2-8 where several orders of 001 reflections are observed. Interlayer

spacings derived from Figure 2-8 and similar diffraction patterns for the manganese,

magnesium, cobalt and calcium octadecylphosphonate LB films are summarized in Table 2-

3. Interlayer thicknesses range from 46.7 48.5 A which are reasonable for

octadecylphosphonate bilayers.77

The deposition of multilayered films was followed by ellipsometry to ensure that a

consistent amount of material is transferred with each bilayer. For all of the LB films, the

thickness increases linearly with the number of layers, indicating that the same amount of

material is being transferred to the OTS covered silicon substrate after each deposition

cycle. A plot of LB film thickness as a function of the number of layers for cobalt

octadecylphosphonate is shown in Figure 2-9 where the solid line is a linear fit to the data.

The cobalt behavior is representative of all the films. Ellipsometry relates the thickness and
refractive index of a thin homogeneous film to measurable parameters, A and T,33 where








1.00


0.54


0.36


0.16


0.04 da, A- 10

0.00 ....
0 5 10 15 20
20 (degrees)

Figure 2-8. X-ray diffraction from 15 bilayers of a cadmium octadecylphosphonate LB
film.

Table 2-3. Interlayer Spacings,a Indices of Refraction,b and Alkyl Chain Tilt Anglesc
for the Divalent Metal Octadecylphosphonate LB Films.
LB Film Interlayer Spacing (A) Refractive Tilt Angle
(+0.2A) Index (degrees)
Mn(03PC18H37).H20 48.5 1.60 32
Cd(03PC18H37).H20 48.2 1.55 36
Mg(03PC18H37).H20 47.6 1.62 40
Co(03PC18H37)'H20 47.2 1.58 40
Ca(H03PC18H37)2 46.7 1.59 42
a) Determined by X-ray diffraction from films of 15 Bilayers.
b) Determined by ellipsometry.
c) Tilt angle of the alkyl chain with respect to the film normal, determined by measuring
the Va(CH2) band in two polarizations using ATR-FTIR.











800





S400



O0 I
*S 200-



0 4 8 12 16
Bilayers


Figure 2-9. Ellipsometric data for a cobalt octadecylphosphonate LB film. The
thickness of the OTS layer and oxide layer on the silicon wafer has not been accounted
for,giving rise to the non-zero thickness intercept.


the angles A and T give the change in phase and change in amplitude of plane-polarized

light respectively, of light reflected off the film. Data obtained from ellipsometry were used

in conjunction with data from the X-ray studies to calculate refractive indices of the organic

films. Since interlayer distances obtained from X-ray diffraction are independent of optical

constants, the value of the refractive index of the film was varied until the ellipsometric

thicknesses agreed with interlayer thicknesses determined from X-ray diffraction.

Refractive indices determined for the films range from 1.55 to 1.62, and the results are

summarized in Table 2-3. Refractive indices for organic monolayers are often estimated to

be 1.5,90 based on the assumption that the monolayer is crystalline and its refractive index

should be similar to polyethylene 1.49-1.55.33 The introduction of metal ions should

increase the refractive index of an organic film, thus substantiating the higher refractive

indices obtained for our LB films relative to those of pure polyethylene films. The values








determined for the LB films are comparable to the refractive index of 1.54 previously

measured for solid-state hafnium-l,10-decanediylbis(phosphonate).30

FTIR Sp&ctroscopX

Figure 2-10 shows the FTIR spectra from 1000-4000 cm-1 of each of the divalent

metal octadecylphosphonate films. Peaks at 2955, 2918, and 2850 cm"1 are assigned to
the asymmetric methyl (va(CH3)), asymmetric methylene (va(CH2)), and symmetric

methylene (vs(CH2)) stretches, respectively of the octadecyl chain. It has been shown that

the position and shape of the va(CH2) and vs(CH2) absorption bands reflect the

conformational order and packing of the aliphatic chains in monolayers.39'91'92 For long-
chain hydrocarbons such as n-alkylthiols or polyethylene the energy of the va(CH2) band

ranges from 2918-2920 cnm-1 when the aliphatic chain is in an all-trans conformation, to

2924-2928 cmr1 when a "liquid-like" alkane contains a large percentage of gauche
bonds.39,91 The observed position of the Va(CH2) IR band at 2918 cm-1 in each divalent

metal octadecylphosphonate LB film implies the alkyl chains are fully extended with all-
trans conformation. The vs(CH2) band is also an indicator of the state of the hydrocarbon

chains. This band is particularly sensitive to the average local environment of an individual

chain within the monolayer indicating the density of packing of the monolayer.91"92 The
peak position of the vs(CH2) band for crystalline hydrocarbons lies at 2850 cm-1 and shifts

to higher energies, 2856 cm- 1, as the hydrocarbon chains become less close-packed.91
The appearance of the vs(CH2) band at 2850 cm-1 for each ocadecylphosphonate LB

system is consistent with high density, crystalline-like phases in the monolayers. The full
width at half maximum, FWHM, of the Va(CH2) absorption band is another measure of the

conformational order of the alkyl chains in the films,39'91 where an organized close-packed

monolayer gives a FWHM of 17 cm' 1 and a randomly oriented film can result in a FWHM
of greater than 35 cm-1.77,93 A FWHM of 17 cm-1 of the va(CH2) bands in each of the

divalent metal phosphonate LB films is consistent with the peak-position analyses and




















































3500 3000 2500 2000 1500 1000

Wavenumbers (cm"1)



Figure 2-10. FTIR spectra from 10 bilayer LB films of magnesium, calcium, cobalt,
cadmium, and manganese octadecylphosphonate.








indicates crystal-like organization of the all-trans alkyl chains. The position and shape of

these hydrocarbon absorption stretching modes do not change with the deposition of

additional bilayers indicating that the films maintain their structure with each succesive

transfer.

The appearance of two unresolved bands at approximately 3400 cm-1 and a band at

1608 cm- 1 in the IR spectra of all but the calcium octadecylphosphonate LB films indicates

the presence of lattice water in the films. Lattice water absorbs at 3550-3200 cm1l and at

1630-1600 cm'1 due to OH stretching and HOH bending modes, respectively.94 In bulk

manganese, cadmium, magnesium, and cobalt alkylphosphontes, each metal ion is bound

by five oxygen atoms from the phosphonate anions and oxygen from water fills the sixth

coordination site.2,3,5 For the LB films formed with these metal ions, the intensity of the

water bending mode was followed as successive bilayers were transferred to a Ge ATR

crystal, Figure 2-11. A linear increase in area of the water bending mode suggests that

water is stoichiometrically incorporated into the lattice and is consistent with coordination

of water to the metal ions. In addition, the relative intensities of the water bending modes
versus the P032- stretching modes in the LB films is similar to the relative intensities seen

in the powdered solids. This further suggests that the coordinated water is included in the

metal-phosphonate lattice of the LB film, as it is in the analogous solid-state metal

phosphonates.

The absense of the water modes in the calcium film is further evidence that the

calcium octadecylphosphonate LB film forms the same structure as the 1:2 calcium

phosphorus solid-state phase. The main differences in the structure between solid-state

calcium organophosphonates with a 1:1 calcium phosphonate stoichiometry and those with

a 2:1 calcium phosphorus ratio lie in the coordination environment of the calcium ion.

Calcium ions in the 1:1 salts have an approximately pentagonal-bipyramidal coordination

bound by 7 oxygens, one of which is a water of hydration.13 In the 1:2 calcium salts,
calcium atoms are bound by eight oxygens from RPO3H- groups resulting in a distorted












S3.0 HOH Bend
"U V,(PO3)
3
| 2.0

I
4 1.0 -
o U

00 *0 0* *"
0.0
0 2 4 6 8 10 12
Bilayers



Figure 2-11. Intensity of the va(P032) and HOH bend as a function of multilayers for
the deposition of cadmium octadecylphosphonate LB film. The linear increase in the bands
indicates the composition of the film is maintained throughout the deposition process. A
linear increase in the intensity of the water bending mode is evidence for the stoichiometric
incorporation of water into the film.


environment. In these compounds, the hydroxyl group of RPO3H" is not coordinated to
calcium and no water is bound to the calcium ions.13 The absence of the v(OH) and HOH

bend absorptions in the LB film of calcium octadecylphosphonate suggests that the film

structure is more like the 1:2 salt. To make additional structural comparisons, the IR
spectra of the phosphonate LB films were compared to the IR spectra of solid-state metal
alkylphosphonates. To illustrate, the 900-1800 cm-1 region of the infrared spectra for a

powder sample of cadmium ethylphosphonate and cadmium octadecylphosphonate LB film
are compared in Figure 2-12. Progressions of IR peaks in the 1175-1400 cm"1 region are
assigned to CH2 rocking and wagging modes of all-trans alkyl chains95 for both the LB

film of cadmium octadecylphosphonate and solid-state cadmium ethylphosphonate. Also
common to both solid-state alkylphosphonates and the analogous LB films are small



























1800 1600 1400 1200 1000
Wavenumbers (cm"1)

Figure 2-12. FTIR comparison of cadmium ethylphosphonate powder and cadmium
octadecylphosphonate LB film. The position of the phosphonate stretching bands in the
two materials is an indicator of the structural similarities.

methylene scissor deformation bands observed at 1467 and 1410 cm-1. The latter band
corresponds to the CH2 group adjacent to the phosphonate group.96'97 The absence of a

strong P=O stretch96'98 in the 1350-1250 cm-1 region or the 1250-1110 cm-1 region for

free and hydrogen bonded modes, respectively, indicates that all of the phosphonate groups

are ionized in the LB film as they are in the solid-state sample. On the other hand, the
P032- stretching modes are strong and well resolved in both the LB films and powdered

solids. In the IR spectrum of the LB film shown in figure 2-12, the band at 1089 cm-1 is
assigned to the asymmetric P032- stretch and bands at 992 cm-1 and 960 cml1are the
P032- symmetric stretches, split as a result of the lower than C3v local symmetry of the

phosphonate groups. The analogous bands appear at 1089 cm-1, 984 cm"1, and 957 cm-1
in the cadmium ethylphosphonate powder. The P032- stretching modes are very sensitive

to local symmetry,13 and the close agreement between the frequencies observed for the LB








film and those seen for the powdered solids indicates that the LB films have the same
phosphorus-oxygen-metal-water extended lattice network as the solid compounds. The
frequencies of the P032- stretching modes for each of the LB films are listed in Table 2-4

along with the frequencies observed in analogous solid-state samples. Like the cadmium
case, the phosphonate binding in the manganese, cobalt, and magnesium phosphonate
films is similar to the corresponding solid-state M(O3PR)-H20 phase.


Table 2-4. Comparison of the Infrared v(P032-) Frequencies of Powders and LB films
of Divalent Metal alkylphosphonates.

Compound Va(P032") (cm- 1) Vs(P032-) (cm-1)
Mn(O3PC2H5)-H20a 1087 1017,988,964
Mn(03PC18H37)'H20b 1087 1003,977
Cd(03PC2H5)-H20a 1089 984,957
Cd(03PC18H37)'H20b 1089 992,960
Mg(O3PC18H37)H20b 1102 1024,999
Ca(03PC2H5)-H20a 1093 1026,993
Ca(H03PCO10H21)2a 1146 1072,1045
Ca(H03PC18H37)2b 1146 1072, 1045
a) Powder sample, measured as a KBr pellet.
b) LB film, measured on Ge ATR crystal.


The IR spectrum of the calcium octadecylphosphonate LB film is compared to the

IR spectra of calcium decylphosphonate and calcium ethylphosphonate in Figure 2-13.
There are differences in the P-0 stretching region of the two solid-state calcium
phosphonate compounds arising from the different structures of the two phases. The P-0
stretching region of the calcium phosphonate LB film is remarkably similar to that of
calcium decylphosphonate. This result, in agreement with XPS data, is conclusive
evidence that the structure of the calcium octadecylphosphonate film fabricated on the LB
trough is similar to the calcium alkylphosphonate solid-state compounds of molecular
formula, Ca(HO3PCnH2n+I)2.




























1800 1600 1400 1200 1000
Wavenumbers (cm-1)

Figure 2-13. FTIR comparison of solid-state calcium ethylphosphonate,
Ca(03PC2H5)-H20; (B) calcium decylphosphonate Ca(H03PCo10H21)2, powder, and (C)
an LB film of calcium octadecylphosphonate. The similarity of the spectra of the solid-state
calcium decylphosphonate and the calcium phosphonate LB film suggests the LB film
structure is the same as those solid-state calcium alkylphosphonates containing long-chain
alkyl groups.


Polarized ATR-FTIR was used to establish the tilt angles of the octadecylphosphonate

molecules in the divalent metal phosphonate LB films. The tilt angle of IR-active

vibrational modes within layered organic films can be determined from the ratio of IR

absorbances measured in two polarization directions.33'99,'100 The absorbance of the

Va(CH2) band was recorded with s- and p-polarized light for each film, and the resulting

tilts of the alkyl chain with respect to the normal of the metal ion plane are given in Table 2-

3. Since the LB films are each comprised of octadecylphosphonate bilayers, their layer

thickness should be similar, with differences arising from variations in the tilting of the

alkyl chains. The less the alkyl chains tilt from the normal, the larger the layer thickness.








The alkyl chain tilt angles, determined by polarized FTIR, are similar for each of the films

and are consistent with the layer thicknesses measured by X-ray diffraction.

Extended Lattice Su'ucture

Typically, inorganic ions in LB films such as metal carboxylate films serve to add

stability to the transferred film by crosslinking and holding together the organic portion of
the film. Although the ionic interactions between the polar headgroup and the metal ion in

metal carboxylate LB films determine the structure of the transferred film, little attention has

been devoted to using ionic headgroup interactions to purposefully control monolayer
structure. Zasadzinski performed AFM studies on a series of divalent metal carboxylate LB

films which revealed that the structures of the organic films were dependent upon the nature
of the metal ion/headgroup interaction.32 In these materials, the alkyl chains vary their
packing, tilt angle, and tilt direction to achieve close-packing in the film, but it is the metal

ion/headgroup lattice energy that dictates the molecular area.
Many investigations have been carried out to determine the structure of solid-state

divalent metal organophosphonates. Syntheses of divalent metal phosphonates by varying
the metal ions as well as the organic groups has lead to the conclusion that the structure of
these hybrid organic/inorganic materials is directed by the choice of metal ion. In 1979,
Cunningham et al. deduced that the metal ions in the series M(03PC6H5)-H20 (M = Mg,

Mn, Co, Cu, Ni) have an octahedral coordination but were unable to carry out X-ray
structure determinations.19 Several years later, crystals of Mn(03PC6H5)*H20 were

grown and an X-ray structure refined in the orthorhombic space group Pmn2l was

reported.15 In 1990, a thorough structural study on two series of divalent metal
phosphonates, M(O3PR)*H20 (M = Mg, Mn, Zn, Ca, Co, Cd; R=n-alkyl, aryl group) and
M(HO3PR)2 (M = Ca) was completed.13 For M=Mg, Mn, Co, Zn in the first series, the
phosphonates crystallize in the orthorhombic layered structure of Mn(03PC6H5).H20,

with interlayer distances to accommodate the different R groups. For M=Ca or Cd in the








same series, a structure of lower symmetry is adopted. For calcium, the larger ionic radius

is expected to be the reason for the difference.13 In the second series, calcium forms 1:2

salts with alkylphosphonates having alkyl groups containing five carbon atoms or greater.

Cao et al.16 determined that cadmium forms 1:1 salts with phosphonic acids crystallizing in
the orthorhombic space group, Pna2li with cadmium ions octahedrally coordinated by five

phosphonate oxygen atoms and a water molecule. The only difference in structure between
the cadmium, manganese and magnesium phosphonates is that the MO6 octahedron in

manganese and magnesium methylphosphonates possesses a mirror symmetry whereas the
CdO6 octahedron in cadmium alkylphosphonates does not16 The stacking of the layers

along the a axis in Cd(O3PCH3)-H20, for example, are different from the stacking of the

layers in manganese and magnesium organophosphonates.16 In the latter, the layers are
translationally related along the b axis whereas in Cd(O3PCH3).H20 the layers are related

by an a-glide plane perpendicular to the b axis. The a-glide causes the layers to repeat in

every other layer along the a axis, resulting in an a-axis dimension twice of the interlayer

distance.16 In all solid-state transition metal phosphonates it is, therefore, the inorganic

lattice energy that dictates the observed layered structures regardless of the organic group.

The differences between the cadmium, manganese, cobalt, magnesium and calcium

structures is attributed to more ionic bonding within the inorganic lattice in the calcium

solids and more covalent bonding in the metal lattices in the other solids.

We have taken advantage of these ionic headgroup interactions to purposefully

control monolayer structure and have shown that the inorganic lattice in the LB films can be

used to dictate the structure of the LB film. The extended lattice structures are formed

during the deposition process. The metal phosphonate "precipitates" upon draining of

water from the film as the substrate is withdrawn from the subphase. Since the source of

organophosphonic acid is restricted to the pre-arranged Langmuir monolayers, the metal

phosphonate "crystallization" is controlled by the deposition procedure. The deposition is

limited to a single layer at a time, and the metal phosphonate layers grow exclusively








parallel to the substrate. XPS data indicate that the M(O3PR)-H20 films are all fully

ionized. In the FTIR, the P032- stretching modes are very sensitive to the mode of

phosphonate binding,13'101 and in the LB films they are as well resolved as they are in the

solids, indicating that the metal phosphonate binding is uniform throughout the film. There

is no evidence for large areas of amorphous metal-phosphonate structure.

Summary

When comparing the structural characterization of the different LB films, it can be

concluded that each divalent metal phosphonate LB film forms with an extended lattice

metal phosphonate network. The metal phosphonate bonding in each case is understood by

comparing the LB films to known metal phosphonate solids, and each film adopts a known

solid-state layered structure. The in-plane layered structure is determined by optimizing the

metal-phosphonate binding. Although the organic groups are the same in each case, even

the metal to phosphonate ratios can be changed by choice of divalent metal. This is

dramatically illustrated by the different structures seen for the Cd2+ and Ca2+ films where

the ionic radii are expected to be similar.13'16 Such differences are well known in solid-

state chemistry, but are not as widely considered when evaluating LB film structure,

although the importance of the metal/headgroup interaction has recently been pointed-out in

structural studies of a series of metal carboxylate LB films.32 We have fabricated, layer-

by-layer, high quality single-layer analogs of known solid-state structures via a wet

chemical method. Previously the preparation of single layers of an inorganic solid was

only achieved by dry processes including chemical vapor deposition, physical vapor

deposition, ion beam deposition or electrochemical methods.102-104 These methods are

limited to substrates whose surface structure is suitably matched to the depositing layer,

and controlling molecular orientations is very difficult103 As a consequence of the

extended lattice structures, properties normally associated soley with inorganic compounds





60


can now be incorporated with ease into organic films using the Langmuir-Blodgett

technique.














CHAPTER 3
AN ELECTRON PARAMAGNETIC RESONANCE STUDY OF A LANGMUIR-
BLODGETT FILM OF MANGANESE OCTADECYLPHOSPHONATE AND
COMPARISON OF THE MAGNETIC PROPERTIES TO SOLID-STATE MANGANESE
ALKYLPHOSPHONATES


Introduction


The layered nature of many solid-state metal phosphonates has attracted attention

from researchers with a variety of interests. Solid-state structural studies have revealed

several new layered structures for different combinations of metal ion and

organophosphonate group.13'14'16'18'105-108 The large internal surface areas in the solids,

that result from the layered structures, suggest potential applications as catalyst supports

and sorbents, and in sensing and separation applications.16'20'22'109 The metal-

phosphonate binding interaction has also been used in procedures to deposit monolayer and

multilayer thin films.21'26-30'110-114 The process deposits layers of functionalized organic
molecules by alternately exposing a surface to solutions of metal ions and o,0-

disubstituted organophosphonic acids, and has been used to prepare thin films for

applications that require oriented assemblies of organic molecules.28',29,'110'111 In yet

another area of interest, some of the transition metal phosphonates have been the subject of

magnetic studies where the layered structures give rise to unusual magnetic properties and

serve as useful models for investigating magnetic behavior in two-dimensional

systems.57'58'115

Manganese organophosphonates crystallize in the orthorhombic space group

Pmn2li. The crystal structure of Mn(03PC6H5)-H20 reported by Cao et al.15 nicely shows

the layered nature of the solids and a view of the manganese phosphonate plane is

reproduced in Figure 3-1.15 The structure consists of layers of manganese ions, which are






















Figure 3-1. Structure of manganese phenylphosphonate, Mn(03PC6H5).H20 viewed down the b (stacking) axis. Phenyl
groups were omitted for clarity. The powdered manganese alkylphosphonates are isostructural with the phenylphosphonate
shown here. Structural data were taken from reference 15. (Cross-hatched circles Mn; large dotted circles P; small open
circles 0).











roughly co-plana, octahedrally coordinated by five phosphonate oxygens and one water of
hydration. The phenyl groups are pointed away from the inorganic layer, approximately
perpendicular to the manganese ion plane, and make van der Waals contacts between
layers. The manganese alkyl phosphonates, Mn(O3PCnH2n+)'H20O, have the same in-
plane structure as the phenyl analog and the interlayer spacing varies as the alkyl group is
changed.15'57 Unit cell parameters for some manganese alkylphosphonates are given in
Table 3-1.



Table 3-1. Structural and Magnetic Parameters for Mn(O3PR).H20 Solids(a)

Compound a, A b, A c, A -J/k, K
Mn(O3PCH3)-H20 5.82 8.82 4.90 2.70
Mn(O3PC2H5)-H20 5.83 10.24 4.87 2.78
Mn(03PC3H7)-H20 5.84 11.71 4.91 2.48
Mn(03PC4H9)-H20 5.84 14.72 4.91 2.48
(a) Reference 2357




Carling et al.57 have performed static susceptibility measurements on powder
samples of a series of manganese alkylphosphonates, Mn(O3PCnH2n+l)-H20 (n = 1-4).
The susceptibility as a function of temperature for each sample is characteristic of
antiferromagnetic exchange in a low-dimensional lattice, and values of the nearest neighbor
exchange, J, determined by Carling et al. from fitting the data to a model for a two-
dimensional Heisenberg antiferromagnet, are listed in Table 3-1.57 The exchange
constant, J, is defined according to the Hamiltonian, H = -J-SiSj, and a value of J < 0
indicates antiferromagnetic exchange. Each of the materials orders antiferromagnetically
with ordering temperatures in the range 14.8-15.1 K.57 Below the Ndel temperature, TN,








the presence of a weak magnetic moment indicates that these materials are best described as

canted antiferromagnets, which are also called "weak ferromagnets."57

In the last chapter, it was demonstrated that inorganic extended-lattice monolayers

of a variety of the metal phosphonates could be prepared using Langmuir-Blodgett film

deposition methods.76,77 In the case of manganese octadecylphosphonate, it was shown

using X-ray diffraction, XPS and FrIR that the LB film has the same structure that is

observed in several of the solid-state manganese alkylphosphonates.76 In this chapter, a

detailed EPR study of the manganese octadecylphosphonate LB film is presented, and the

magnetic behavior of the LB film is compared to published magnetism studies on the series

of solid-state manganese alkylphosphonates.57 It will be demonstrated that properties such

as magnetic exchange inherent to the inorganic lattice can be incorporated into organic

Langmuir-Blodgett films.

EPR experiments on low-dimensional systems provide detailed information about

spin dynamics in magnetically exchange-coupled systems. 116'117 In this chapter it will be

demonstrated from the EPR results, that the LB film of manganese octadecylphosphonate is

well described as a two-dimensional manganese lattice with nearest-neighbor Heisenberg

antiferromagnetic exchange, and the magnitude of the exchange is the same as observed in

the solids. The EPR behavior confirms that the manganese phosphonate LB film contains

an inorganic extended-lattice structure.


Experimental Section


Materials

Octadecylphosphonic acid, C18H37P03H2, was used as purchased from Alfa

chemicals (Ward Hill, MA). Manganese chloride tetrahydrate, MnCl2-4H20, was used as

purchased from Fisher Scientific (Orlando, FL).











Langmuir-Blodgett films were prepared using a KSV (Stratford, CT) 3000

Langmuir-Blodgett trough modified to operate with double barriers. Purified water having

a resistivity of 18 MO-cm was used. EPR spectra were recorded on a Bruker (Billerica,

MA) ER 200D spectrometer modified with a digital signal channel and a digital field

controller. Data were collected using a U.S. EPR (Clarksville, MD) SPEC300 data

acquisition program and analyzed using a U.S. EPR EPRDAP data analysis program.

Temperature was controlled using an Oxford Instruments (Witney, England) ITC 503

Temperature Controller and ESR 900 cryostat

Procedure

Manganese octadecylphosphonate LB films were prepared by spreading

octadecylphosphonic acid onto an aqueous subphase containing 5 x 10-4 M MnCl2.4H20

held in a pH range of 5.2-5.6. The monolayer was compressed to a pressure of 17 mN/m

and bilayers were transferred onto a 625 mm2 calcium arachidate covered mylar substrate at

speeds of 8 mm/min on the downstroke and 5 mm/min on the upstroke. Multilayers cannot

be formed by continuous deposition of the film due to cross-linking of the phosphonate

groups by the manganese ions at the air/water interface. After deposition of a bilayer, the

monolayer was removed from the surface of the trough and a new octadecylphosphonic

acid monolayer was formed. Films containing 50 bilayers were prepared and transfer

ratios were in the range 0.98-1.08.

Samples for EPR studies were deposited onto mylar sheets. After film deposition,

the mylar was cut into thin strips that were stacked and placed in a conventional EPR tube.

The oriented sample could then be rotated with respect to the magnetic field, as shown in

Figure 3-2. The LB film sample has a common interlayer or b axis, all the layers are

parallel to the substrate, but because the film is composed of ordered domains the in-plane















Ho rI





0 0
[)=90 =0


Figure 3-2. Orientations with respect to the magnetic field, Ho, of the LB film stacked in
a conventional EPR tube.


orientation (ac plane) is circularly averaged. The angle 0 is defined as the angle between
the film normal and the direction of the static field, as illustrated in Figure 3-2.

Results and Discussion

Representative EPR signals of the LB film are shown in Figure 3-3. At high
temperatures the shape of the EPR line is mainly dominated by two factors, dipolar
interactions and exchange interactions.116 Dipolar interactions between paramagnetic
centers tend to broaden the EPR lines, while exchange interactions tend to narrow lines.116
In low-dimensional materials the exchange interaction is less effective than in three-
dimensional materials because the number of spins which are exchange coupled to a given
spin is less than in the three-dimensional case.116 The signals for both the LB film and the
solid-state samples are dipolar broadened as expected for a lattice of manganese ions, and
no Mn2+ hyperfine splitting is observed. The g-values are characteristic of Mn2+ in a





















Figure 3-3. EPR spectra of the manganese octadecylphosphonate LB film at 250 K, 50 K, and 20 K. The sample was
oriented with ) = 0* (Figure 3-2). The EPR signal broadens substantially as the temperature is lowered below 30 K.











































5000


2000 3000 4000
Magnetic Field (Gauss)








nearly cubic field and are consistent with the g-value of 1.99 observed for a powder sample

of Mn(O3PC3H7)-H20. In contrast to the powdered solid-state samples, the LB films have

a common b-axis orientation and the g values vary continuously as the sample is rotated

with respect to the external field, although the values are nearly isotropic ranging from 1.99

to 2.00.

The room temperature EPR linewidth of the LB film is plotted as a function of

sample orientation in Figure 3-4. Spectra were taken as the sample was rotated every five

degrees with respect to the magnetic field. The shape of the plot is consistent with the

behavior predicted for a two-dimensional lattice with antiferromagnetic Heisenberg

exchange.60,'116'118-120 In contrast to a three-dimensional material where the linewidth
decreases according to (cos24 1) as the sample is rotated from 0" to 90", in low-

dimensional materials, the contribution to the linewidth originating from spin diffusion has
a (3cos2 1)n dependence where n = 4/3 for a one-dimensional lattice and n = 2 for a two-

dimensional lattice.116'118-120 The expected orientational dependence of the linewidth for a

two-dimensional Heisenberg antiferromagnet is, therefore,
AH = A + B(3cos2o 1)2 (3-1)

where A and B encompass exchange and other dipolar interaction terms.116'119'120 A fit of

the data with A = 218 G and B = 20 G is superimposed on the data in Figure 3-4. The
linewidth has a minimum of 218 G at the magic angle = 54.7" where the secular

contribution of spin diffusion vanishes.116

The integrated area of the EPR signal is proportional to the spin susceptibility, and a

plot of I/area vs. temperature is shown in Figure 3-5. An extrapolation of the high

temperature data intercepts the temperature axis at -34 K indicating nearest neighbor

antiferromagnetic exchange. The data are also expressed in Figure 3-6 as area vs.

temperature with the solid line being a fit to the data. The area of the EPR signal gradually

increases as the temperature is lowered and reaches a maximum near 25 K. The area then

decreases rapidly until the EPR signal is lost around 17 K. The shape of the plot is






71








350





300



250-





200
250





200-





150 ---- I --- I ---- I --- I ---- l --- I ---- l
0 50 100 150

Orientation (Degrees)








Figure 3-4. EPR linewidth as a function of sample orientation at room temperature. The
sample orientation is defined in Figure 3-2. The solid line is a fit to AH = A + B(3cos2o -
1)2 with A = 218 G and B = 20 G. The behavior is characteristic of a two-dimensional
lattice with antiferromagnetic Heisenberg exchange.
























-50 0 50 100 150 200 250
Temperature (K)

Figure 3-5. Temperature dependence of the inverse of the area (arbitrary units) of the
EPR signal from the manganese octadecylphosphonate LB film. The solid line is a linear
fit to the data above 80 K. An extrapolation of the high temperature data intercepts the
temperature axis at -34 K, indicating antiferromagnetic exchange.


0 75 150 225


Temperature (K)


Figure 3-6. Temperature dependence of the integrated area (arbitrary units) of the EPR
signal from the manganese octadecylphosphonate LB film. The solid line is a fit to the data
using equation 2 for a two-dimensional lattice with Heisenberg antiferromagnetic exchange
with exchange constant J/k = -2.8 K. Since the EPR intensity is plotted as arbitrary units,
the fit has been fixed to the EPR intensity at 110 K.








characteristic of antiferromagnetic exchange in a low-dimensional solid,60,llS,121,122 and

the behavior is nearly identical to the temperature dependent static susceptibility of the

powdered solid-state manganese alkylphosphonates.57
Although there is no exact solution for the magnetic susceptibility X, of a quadratic-

layer Heisenberg antiferromagnet, the temperature dependence can be described by a

numerical series expansion122
Ng2B2/X = 30 + y,(Cn/0n'-1) (3-2)
where 0 = kT/JS(S+1), g is the Lande factor, N is the number of spins, tB is the Bohr

magneton, and J is the nearest neighbor exchange constant. The coefficients, Cn, for S =
5/2 have been calculated up to n = 6 by Lines.121 The value of the exchange constant can

be estimated by the temperature of maximum susceptibility, Tmax, according to kTmax/J =

2.06S(S+1).123 Taking Tmax as 25 K yields J/k = -2.8 K and a fit to the data according to

equation 2 is plotted as the solid line in Figure 3-6. Because the EPR intensity is plotted as

arbitrary units, the fit has been fixed to the EPR intensity at 110 K. The value of the

exchange constant, J/k = -2.8 K, agrees closely with the exchange constants for the bulk

metal alkylphosphonates obtained by Carling et al.57 and shown in Table 3-1. The fit of

the EPR intensity to the numerical expression in equation 3-2 is further evidence that the

LB film is a two-dimensional antiferromagnetic exchange-coupled lattice. The magnitude

of the exchange is nearly identical to the solid-state analogs and suggests that the in-plane
Mn-O-Mn interactions in the film are similar to those seen in the solid state-structures.

The peak-to-peak widths of the EPR signals of the manganese

octadecylphosphonate LB film and a powder sample of manganese propylphosphonate are
plotted as a function of temperature in Figure 3-7. In both cases the EPR linewidth remains
approximately constant as the temperature is lowered below room temperature. As the
temperature approaches 30 K, the linewidths broaden rapidly until the signals become so

broad they are lost below 17 K. The powder sample undergoes a magnetic ordering
transition at TN = 14.90 K to a canted antiferromagnetic state.57 The increase in linewidth











2000


S1600 Powder Sample
LB Film

1200


800


4001

0 -------- -------
0 100 200 300
Temperature (K)
Figure 3-7. EPR linewidth as a function of temperature for a powder sample of
manganese propylphosphonate and for the manganese octadecylphosphonate LB film. The
rapid increase in linewidths seen below 30 K is characteristic of systems approaching a
magnetic ordering transition.


is characteristic of a system approaching a magnetic ordering transition and is caused by

antiferromagnetic fluctuations. Large variations in the local field, caused by regions of

short range order fluctuating to achieve long range order, result in the large linewidths.

The LB film also experiences antiferromagnetic fluctuations, although by EPR we cannot

observe whether or not an ordering transition occurs because the signal becomes too broad

and vanishes at the temperature of the anticipated transition.

Magnetic ordering is not predicted to occur in a truly two-dimensional Heisenberg

lattice.15253,124 When ordering is observed in layered systems it occurs as a result of either

anisotropy in the exchange or a dimensionality crossover in a temperature regime where the

interlayer exchange, Jl, becomes important.1,58118 It is interesting to note that in the

series Mn(O3PCnH2n+l)-H20, where ordering does occur, dimensionality crossover has

been observed to occur at values of the reduced temperature, (TN T)/TN, of 0.085, 0.015,








and 0.010 for n = 2, 3,4, respectively.58 For the LB films, interlayer exchange is not

expected as the interlayer spacing is 48.5 A. Also, as a result of the depositon process,

layers are not in registry. The LB films should behave as isolated monolayers and

dimensionality crossover should not occur.
The EPR behavior of the manganese phosphonate LB films can be compared to

previous studies of magnetic exchange between metal ions in LB films. The most

extensive studies are those of Pomerantz et al.34'60'125'126 on LB films of manganese

stearate. Based on linewidth analyses and temperature dependent EPR intensity data,

antiferromagnetic exchange was also observed in the manganese stearate LB film and the

value of the nearest-neighbor exchange was estimated as J/k = -1.0 + 0.4 K.125 This value

is smaller than the -2.8 K value for the manganese phosphonates and is related to

differences in the in-plane structure of the two films. In the metal phosphonate film, in-
plane bonding is well described by the structures of the known solid-state analogs. The

manganese ions are nearly co-planar and each site is coordinated by five phosphonate

oxygens and one water molecule. Adjacent manganese ions are bridged by a single
phosphonate oxygen which mediates magnetic superexchange. The structure of the

manganese stearate films is less clear.34 The Mn2+ ions are certainly bridged by the

carboxylate headgroups, but the mode of binding is not known. TED and AFM studies

suggest a rectangular arrangement of Mn2+ ions.32

The observation of large shifts in resonance field at low temperatures was cited as
evidence for spontaneous magnetization in the manganese stearate films.127 Since the

exchange is predominantly antiferromagnetic, the magnetic moment was attributed to
canting of the antiferromagnetically ordered spins. However, direct observation of the

ordered state has not been observed. An alternative explanation offered by Pomerantz127

for the increased magnetization is the possibility of an antiferromagnetic lattice containing

manganese ion vacancies, which cannot be discounted considering the LB film deposition

method. The only evidence for ordering in the manganese phosphonate films is the large








increase in linewidth at low temperature. As was discussed above, this is a precursor effect

and is not itself evidence for ordering, although, the similarities in the behavior of the LB

film and the powdered solids where ordering does occur are striking. It will be informative

to study the magnetization of these films to lower temperature, but because of the limited

sample size, the use of standard methods for measuring the static susceptibility have not yet

been possible. In the next chapter however, a method of characterization was developed so

magnetic susceptibility of the LB films at at low temperatures, down to 5 K, could be

measured. The results of the magnetic investigations will be discussed in detail in the next

chapter.

Summary


EPR studies of manganese octadecylphoshonate Langmuir-Blodgett films reveal

that the magnetic exchange in the films is identical to that exhibited in solid-state manganese

alkylphosphonates. The high temperature magnetic behavior, above TN of the powders,

for both the film and the solid-state materials is characteristic of antiferromagnetic

Heisenberg exchange in a two-dimensional lattice. At room temperature, the angular

dependence of the EPR linewidth is characteristic of a two-dimensional lattice of

manganese spins. Temperature dependent behavior of the manganese LB film indicates the

manganese Heisenberg spins undergo nearest neighbor antiferromagnetic exchange in a

two-dimensional lattice. The value of the antiferromagnetic exchange constant of the

manganese octadecylphosphonate LB film is comparable to the values of the

antiferromagnetic exchange constants for the structurally analogous manganese

alkylphosphonate powders. While the solid-state manganese alkylphosphonates order to

form "weak ferromagnets", direct observation of a transition to long-range order is not

possible in the LB films using EPR. However, the EPR linewidth at low temperature is

characteristic of a system approaching a magnetic ordering transition. Magnetization








measurements are needed below 17 K to explore the ground state of the two-dimensional

LB film.

The magnetic studies help confirm that the LB film has the same extended lattice

structure as the solid-state alkylphosphonates. These results demonstrate that single layers

of known solid-state layered structures can be prepared using LB methods. Since the

inorganic lattice energy is the dominant interaction in the metal phosphonate layered

structures, the possibility exists of using these structures to organize LB films of

organophosphonates containing organic groups other than alkyl chains, in analogy to the

known solid-state metal phosphonates based on functional organic groups.













CHAPTER 4
A MAGNETIC LANGMUIR-BLODGETT FILM


Introduction

Known for most of this century, the Langmuir-Blodgett (LB) method is perhaps the

earliest technique to afford molecular level control over the dimensions of supramolecular

assemblies.43 The LB technique organizes amphiphilic molecules into a close-packed

monolayer, at a water surface, which is then transferred to a solid support that is pushed or

pulled through the film at the air/water interface.33'42'72 Many fundamental areas of

research have made use of this ability to arrange molecules and control the chemistry of

interfaces, including studies of membrane dynamics,72 biomineralization at organic

interfaces,128 and electron and energy transfer processes in controlled geometries,78 while

some practical applications of LB films include use in organic-based electronic devices,129

organic non-linear optical devices,130'131 and chemical and biochemical sensors.72

However, many potential applications have not been realized because of the metastable

nature of the layered organic assemblies, and in particular, the demonstration of physical

properties that require long-range structural order, such as superconductivity or magnetic

order, has been elusive. In this dissertation thus far the approach to developing long-range

two-dimensional structural order in LB films is to utilize the inorganic lattice energy of

known solid-state layered structures and incorporate inorganic extended lattice structures

into the hydrophilic portion of Langmuir-Blodgett assemblies.76'77 In addition to adding

structural order to the film, this approach provides the opportunity to introduce physical

properties normally associated with inorganic extended lattice structures. As a

demonstration of this concept, the characterization of a magnetic LB film will be presented

in this chapter. Manganese octadecylphosphonate LB films undergo a transition to long








range magnetic order and below the ordering temperature exhibit a spontaneous

magnetization characteristic of a "weak ferromagnet" These results are the first

demonstration of cooperative ordering phenomena in LB films.

There are several examples of mixed organic/inorganic layered compounds where

polar ionic networks are separated by nonpolar organic networks,9 and the LB films

studied here are modeled after one such class of materials, the family of solid-state layered

transition metal phosphonates.17,21,64 Figure 4-1 shows the crystal packing diagram for

manganese phenylphosphonate using the crystal coordinates given by Cao et al.15 In

manganese phenylphosphonate and other manganese phosphonates containing straight

chain alkyl groups, metal ions are crosslinked by the phosphonate groups which bind both

above and below the metal ion plane.15 The metal ions are coordinated by five oxygens

from four different phosphonate groups and the distorted octahedral coordination is

completed by a water of hydration. Layers are held together by van der Waals interactions

between the organic groups in adjacent layers. The distance between manganese planes in

succesive layers depends on the sizes of the organic groups.

The magnetic properties of a series of manganese alkylphosphonates

Mn(O3PCnH2n+l)-H20, n = 1-4, have been investigated by Carling et al,57'58 and these

authors have shown that at high temperatures the manganese phosphonates behave as two-

dimensional Heisenberg antiferromagnets and that at lower temperatures (14.8 15.1 K)

each member of the series undergoes a magnetic ordering transition. The ordered state has

a spontaneous magnetization due to incomplete cancellation of the antiferromagnetically

coupled moments. Such systems are known as canted antiferromagnets or "weak

ferromagnets". The manganese octadecylphosphonate films described in this chapter are

LB analogs of the solid-state manganese phosphonates, possessing structural and magnetic

properties that are comparable to the solids.






















Figure 4-1. Structure of Mn(O3PC6H5)-H20 viewed down the a axis. Crystallographic data taken from reference 15. (Cross-
hatched circles Mn; large dotted circles P; small open circles 0).





81










Experimental Section


Materials

Octadecylphosphonic Acid, CH3(CH2)17P(O)(OH)2,98% was purchased from

Alfa Aesar chemicals (Ward Hill, MA). Manganese chloride tetrahydrate and calcium

chloride were used as purchased from Fisher Scientific (Fair Lawn, NJ). Arachidic Acid,

C19H39COOH, was used with no further purification as purchased from Aldrich

Chemicals. Mylar sheets, purchased from Dupont, were sonicated in ethanol for 20

minutes prior to use.

Instrumentation

Langmuir-Blodgett films were prepared using a KSV (Stratford, CT) 3000

Langmuir-Blodgett trough modified to operate with double barriers. Purified water having
a resistivity of 18 MO-cm was used. Magnetization experiments were performed using a

Quantum Design MPMS SQUID magnetometer. A gelcap and plastic straw were used as a

sample holder during the measurements. The background signals arising from the gelcap

and straw were measured independently and subtracted from the raw data.

Procedure

Manganese octadecylphosphonate bilayers were prepared using conventional LB

deposition techniques.76 Octadecylphosphonic acid was spread on an aqueous subphase

containing 5 x 10-4 M MnCl2-4H20 and held in a pH range of 5.2-5.6. The monolayer

was compressed to a surface pressure of 17 mN/m and bilayers, one layer on the

downstroke and one layer on the upstroke, were deposited onto hydrophobic substrates.

For the magnetic measurements, an 81 bilayer film was deposited onto both sides of a

calcium arachidate covered mylar sheet having a total area of 5.5 cm2 and resulting in a








sample of 6 x 10-7 mol which was then cut up into small pieces, 3mm x 3mm. The mylar

pieces were stacked and oriented in a gelcap which was held in the magnetometer with a

plastic straw.


Results and Discussion


Structure and High Temperature Magnetic Behavior of Manganese Octadecylphosphonate
Langmuir-Blodgett Films

As discussed in the two previous chapters and as a brief review, the deposited

manganese films are layered and several orders of the (001) Bragg peak are observed in X-

ray diffraction, corresponding to an interlayer separation of 48.5A. A 1:1 manganese

phosphorus ratio in the films was confirmed by XPS. In the ATR-FTIR spectra, the

shapes and energy of the C-H stretching bands are consistent with an organized array of

all-trans alkyl chains, and the P-O stretching modes confirm that the metal-phosphonate

binding interactions in the LB film are the same as those in the solids. Magnetic exchange

in the LB film was demonstrated as a result of the EPR study79 showed that the magnitude

of the nearest-neighbor exchange, J/kB = -2.8K, is nearly identical to the values found by
Carling et al.57'58 for the solid-state manganese phosphonates. At temperatures below

50K, the EPR line width begins to increase and becomes too broad to measure below 17K.

This behavior is consistent with antiferromagnetic fluctuations, a precursor to magnetic

ordering, but no direct observation of the ordered state is seen in the EPR.

Magnetic Behavior of the Manganese LB Film at Low Temperatures (5 K 25 K)

An ordering transition was observed in the static magnetization measurements of

manganese octadecylphosphonate LB films. For the magnetic measurements, the

background contribution of the gelcap and straw was measured independently and

subtracted from the data. The temperature dependent magnetization of the LB film,

measured upon warming the film between 5K and 25K and recorded with the measuring








field of 0.01T parallel to the plane of the film, is shown in Figure 4-2A for the cases where

the sample was cooled from room temperature in zero applied field (ZFC) and where the

sample was cooled in a magnetic field of 0.1 T (FC). The ZFC data show the signature of

an ordering transition at 13.5 K. The FC data also show the ordering transition, and the

increased magnetization below the ordering temperature (TN) is evidence for spontaneous

magnetization of the film. The spontaneous magnetization is shown more clearly in Figure

4-2B, where the difference between the FC and ZFC magnetization (AM) is plotted. While

the mass of the sample is small compared to the sample holder and mylar substrate, the

difference plot subtracts the signal due to the sample support and allows quantification of

the film magnetization. Below TN, the magnetization increases and begins to level off as

the temperature approaches 5K. The magnitude of the magnetization is calculated from the

magnetic data of the film measured in the parallel orientation. Using the equation, M =
N(gfenfo/R)eB132 where N = Avogadro's number, gB = the Bohr magneton, and taking M

= 44.5 emu/mol from the data, the moment at 5K corresponds to a ferromagnetic moment
of gfefnopB = 8 x 10-3. The weak moment in the ordered state is consistent with

antiferromagnetic ordering of the lattice where coupled nearest neighbor moments do not

exactly cancel due to low site symmetry giving rise to a "weak ferromagnetic" state. The

magnitude of the ferromagnetic moment is similar to the moments observed for the

manganese alkylphosphonate solid-state analogs.57 A canting angle of the spins in the

manganese LB film is also determined from the magnetic data. For an isotropic (g=2)
S=5/2 spin, the effective ferromagnetic moment, pff = g[S(S + 1)]1V2B = 5.92.132 A
canting angle a, is calculated using a = sin-l(;fero/eff). In the manganese LB films, a

canting angle of 0.1" is observed. This angle is similar to the angles observed in the

powdered manganese alkylphosphonate samples.57
Field dependent magnetization data obtained at 2K at applied fields up to 5T are

shown in Figure 4-3 for two orientations of the film. When a system is in the


























Figure. 4-2. Magnetization vs. temperature for an 81 bilayer film with the measuring
field applied parallel to the plane of the film. (A) Comparison of the data taken upon
warming the film after cooling in zero applied field (ZFC) and cooling in a field of 0. IT
(FC). In both cases the measuring field is 0.01T. The ordering transition (TN) is seen
as the discontinuity in the ZFC plot at 13.5K. (B) The difference of the two plots in
(A) showing the spontaneous magnetization below TN.














. . I * I U U I


A









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T(K)






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1.0


T=2K 0
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00 0
0
0
0 0
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0.6 0



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0.4 0

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H (T)





Figure. 4-3. Magnetization vs. applied field at 2K, normalized to the value at 5T, with
the applied field directed perpendicular (filled diamonds) and parallel (open circles) to the
plane of the film. The spin flop transition is seen in the perpendicular orientation at 2.5T.
A small inflection is seen in the parallel orientation due to imperfect alignment of the
individual mylar sheets with respect to the field.








antiferromagnetic state, T < TN, and the field is applied parallel to the easy axis, a phase

transition known as a spin flop transition can occur.132 At the critical field value, known

as the spin flop field, the moments flop perpendicular to the field.132 At this point, a

discontinuity in the magnetization is observed. For the case where the applied field is

oriented perpendicular to the plane of the film, shown as the filled diamonds in Figure 4-3,

there is evidence for a spin-flop transition at Happ = 2.5T. The spin-flop transition in this

orientation indicates that the axis of antiferromagnetic alignment is perpendicular to the

plane of the film and therefore perpendicular to the manganese phosphonate layers. This

type of arrangement is shown schematically in Figure 4-4. While the magnetic structures

of the solid-state organophosphonates have not yet been determined, the magnetic structure

of a purely inorganic isomorph, KMnPO4-H20, is known from magnetic scattering in its

neutron diffraction profile133 The structure shows that the manganese moments are

antiferromagnetically coupled within the manganese phosphate planes and aligned

perpendicular to the planes, which is consistent with the behavior observed for the LB film.


















Figure 4-4. Illustration of the orientation of the manganese spins in manganese
organophosphonates. Each manganese moment is canted with respect to the two-
dimensional plane. Antiferromagnetic coupling between nearest neighbor moments results
in a net magnetic moment along the two-dimensional plane of manganese ions.