THE NATURE OF ELECTRONIC STATES IN CONDUCTING POLYMER NANO-NETWORKS

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the

Graduate School of The Ohio State University

By

Oludurotimi O. Adetunji, B.S.

* * * * *

The Ohio State University 2008

Dissertation Committee: Approved by Professor Arthur J. Epstein, Adviser

Professor Micheal Poirier

Professor Jay Gupta Adviser Graduate Program in Physics Professor Ciriyam Jayaprakash

ABSTRACT

The nature of the electronic states of charge carriers and the origin of metal to insulating (MI) transition, in highly interconnected conducting polymer nanostructure network, notably Polyaniline nanofibers (PAN-N), were determined via temperature dependent DC conductivity, optical reflectance (300 cm-1 – 50,000 cm-1), electron paramagnetic resonance (EPR) and X-ray diffraction probes.

The nanostructured network has a room temperature (RT) conductivity of ~ 1

S/cm, similar to that of conventional disordered Polyaniline in the Emeraldine Salt (ES) form, but this value is small when compared to the Mott minimum for metallic conductivity (~ 100 S/cm). Therefore, the signature of the temperature dependent DC

conductivity (s dc (T ) ) of PAN-N should be dominated by phonon activation, with very little or no metallic contribution. In fact, the signature of the charge transport of PAN-N films shows a large “metallic” contribution from RT to ~ 235 K, with the change in

s dc (T ) from RT to the peak of the maximum conductivity ~ 200%. We attribute this large metallic contribution to a “mechanically-induced” crossover from metallic to insulating behavior, due to the “fragile” nature of the conductance at interfiber interfaces.

By mechanically modifying the nanostructure morphology via applied pressure, the

ii signature of the charge transport resembles that of conventional disordered Polyaniline, having a broad MI peak and moderate change in conductivity( < 15% ) from RT to the

peak conductivity. The reduced energy activation W[º d lns dc (T )/ d lnT ] of PAN-N has a negative slope at low temperatures, which suggest that the charge carriers are localized by disorder.

Similarly, the dielectric functions for all measured temperature reveal that the charge carriers within the network are localized. EPR measurements show a temperature dependent Pauli susceptibility between 300 K and 130 K, and below 130 K, we see the onset of a Curie-like contribution to the magnetic susceptibility. The nanostructured network has a low magnetic susceptibility, dominated by a weak Curie component, with the density of Curie spins of ~ 1 spin per 200 (2-ring) repeat unit. This suggests that a significant fraction of the spins are paired up as bipolarons, implying that most of the charge carriers are localized.

Structural studies indicate that the nanostructure films are ~ 50 % crystalline with a coherence length of ~ 2 nm. This coherence length is similar to the values reported earlier for conventional disordered Polyaniline with higher conductivity. This suggests that the nature of conductance within the interfiber interfaces affects the effective conductivity of the network.

The data of other charge dynamics including optical, magnetic, and structural probes suggest that the role of interfiber contacts within the network contributes largely

iii to the “metallic” signature of thes dc ()T . We conclude that the MI behavior is due to the

“fragile” nature of the conductance at the nanostructure interface, while disorder and localization dominate the charge dynamics.

iv

Dedicated to my wife and son, Olajumoke and OgoOluwa;

and my parents, James and Oladunni

v ACKNOWLEDGMENTS

I would like to express my sincere gratitude to my advisor, Professor Arthur J.

Epstein for his advice, guidance, constant encouragement and invaluable discussions during the course of my research. My appreciation also goes to Dr. Ronald J. Tonucci of the Naval Research Laboratory (NRL) for welcoming me into his lab as an intern and for the great insights I learned from performing research in his laboratory. I also would like to thank Dr. Nan-Rong Chiou for providing high quality nanostructured Polyaniline samples and without whom none of this work would have been possible. I would also like to thank Professor Jay Gupta, Professor Micheal Poirier and Professor Nandini Trivedi for sharing their valuable time as committee members. I gratefully acknowledge the discussions with Dr. V.N. Prigodin, whose theoretical insight was used in the analysis of this study. I am greatly indebted to Dr. Youngmin Kim, Dr. Jung Woo Yoo and Dr. Raju

Nandyala for helping with the early stage of my research. My thanks also to the past and current members of Epstein’s laboratory- Drs. Fang Chi Hsu, June-Hyoung Park, Jeremy

D. Bergeson, Dr. Derek Lincoln, Mr. Travis S. Steinke, Mr. Yurri Bataiev, and Mr. Louis

Nemzer, Mr. Austin Carter, Ms. Chia-Yi Chen, Ms. K. Deniz Duman, Mr. Jesse Martin and Mr. Mark Murphey. I would also like to thank Ms. Jenny Finnell for helping with administrative issues.

vi The support of the Department of Physics during my time at the Ohio State

University has been greatly appreciated along with the financial support of the National

Science Foundation (NSF) through the IGERT program and the NSEC program.

Finally, I would like to extend a special thanks to my wife, Olajumoke (Queen), our son Ogo-Oluwa, my parents and parents-in-law, and siblings for their loving support and encouragement with which this research can be accomplished. I also thank my friends Emmanuel Olawale and Oluwayomi Fasalojo and their families for their love and prayers throughout my graduate career.

vii

VITA

August 14 1977……...... Born Ilorin, Nigeria

2002……………………………………...... B.Sc. Fisk University, Nashville, Tennessee.

2002-2004……………………………………. Graduate Teaching Associate Department of Physics The Ohio State University, Columbus, Ohio.

June-December, 2006………………………….Physical Scientist II Naval Research Laboratory, Washington, District of Columbia.

2004-2008 ……………………………………. NSF-IGERT Fellow NSF-NSEC-Polymer Biomed Fellow The Ohio State University

PUBLICATIONS

Research Publications

O. O. Adetunji, U. N. Roy, Y. Cui, J. –O. Ndap, and A. Burger, “Growth of Cr and Co doped CdSe crystals from high temperature selenium solutions,” Journal of Electronic Materials 31 7 (2002).

U. N. Roy, B Mekonen, O. O. Adetunji, K. Chattopadhyay, F. Kochari, Y. Cui, A. Burger, and J. T. Goldstein, “ Compositional variations and phase stability during horizonatal Bridgman growth of AgGaTe2 crystals,” Journal of Crystal Growth 241 135 (2002). viii

J.-O. Ndap, C. I. Rablau, K. Morrow, O. O Adetunji, V.A. Johnson, K. Chattopadhyay, R. H. Page, and A. Burger, “ Infrared spectroscopy of chromium-doped cadmium selenide,” Journal of Electronic Materials 31 802 (2002).

O. O. Adetunji, U. N. Roy, A. Burger, “Study of Comparison between Chromium and Cobalt as a doping efficiency in CdSe”, published in National Conference of Undergraduate Research Proceedings, NCUR proceedings, July 2001.

J.-O. Ndap, K. Chattopadhyay, O. O. Adetunji , D. E. Zelmon, and A. Burger “Study of thermal diffusion of Cr2+ in bulk ZnSe,” Journal of Crystal Growth 240 176 (2002).

J-O. Ndap, O. O. Adetunji, K. Chattopadhyay, C. I. Rablau, S. U. Egarievwe, X. Ma, S. Morgan and A. Burger, “High-temperature solution growth of Cr 2+: CdSe for tunable mid-IR laser application” Journal of crystal growth 211 290 (2000).

S. U. Egarievwe, H. Chen, K. Chattopadhay, J.-O. Ndap, X. Ma, O. O. Adetunji, T. McMillan, A. Burger, and R. B. James, "Study of Au/CdZnTe/CdS m-i-n Detectors Fabricated by Sputtering Technique," Proceedings of SPIE 3768 3768 (1999).

FIELD OF STUDY

Major Field: Physics

· Experimental Condensed Matter Physics · Optical Studies of Nanostructure Conducting Polymer · Electrical and Transport Studies on Nanostructured Conducting Polymers · Electron Paramagnetic Resonance

ix TABLE OF CONTENTS Page Abstract...... ii Dedication...... v Acknowledgments ...... vi Vita...... viii List of Figures...... xiii List of Tables ...... xviii

Chapters:

1 Introduction...... 1

2. Theoretical Background and Methodology ...... 7

2.1. Response Functions ...... 7 2.1.1 Drude Model...... 8 2.1.2 The Lorentz Oscillator Model...... 12 2.1.3 Propagation in a medium ...... 15 2.1.4 Propagation through an interface...... 18 2.1.5 Kramer-Kronig Dispersion Relations ...... 20 2.1.6 The Sum Rule ...... 23

3. Electronic State and Transport Processes in Conducting Polymers ...... 25

3.1. Polyacetylene : The Su-Schrieffer-Heeger (SSH) Model ...... 26 3.2. Soliton, Polaron and Bipolaron...... 32 3.3. Polyaniline ...... 36 3.4. Metal to Insulator transition (IMT) ...... 39 3.4.1. Role of dopant...... 39 3.4.2. Role of temperature ...... 40 x 3.4.3. Role of structural order...... 44 3.5. Anderson Localization...... 45 3.6. Mott Variable Range Hopping Model(VRH) ...... 47 3.7. Review of Band and Dielectric Transport ...... 50 3.8.Resonance Quantum Tunneling...... 51

4. Experimental Apparatus and Techniques ...... 54

4.1. Reagent for the preparation of nanostructure polyaniline ...... 54 4.2. Dilute Polymerization...... 54 4.3. Experimental Probes...... 55 4.3.1 Direct Current Conductivity ...... 55 4.4. Reflectance Probe ...... 58 4.4.1. UV-VIS Reflectance...... 60 4.4.2. Absolute Specular Reflectance Accesory (ASRA) ...... 63 4.4.3. Fourier Transform Infrared(FTIR) Spectroscopy...... 64 4.4.4. Bruker IFS 66v/S ...... 68 4.4.5. Kramers-Kronig Calculations...... 70 4.5. X-ray Diffraction ...... 74 4.6. Electron Paramagnetic Resonance(EPR ...... 75 4.6.1. Cyogenics...... 77 4.6.2. Magnetic Suceptibilty...... 78 4.7. Mechanical Probe ...... 81 4.7.1. Pressure Cell Design...... 81

5. Experimental Results ...... 83

5.1. Morphology of Polyaniline Nanostructured Network ...... 83 5.2. DC Conductivity: Proximity to the Quasi IMT ...... 83 5.3. Reflectance...... 92 5.3.1. Optical Conductivity...... 96 5.3.2. Dielectric Constant ...... 99 5.4. Electron Paramagnetic Resonance...... 100 5.5. X-ray Studies of Nanostructured Polyaniline ...... 108 5.6. Mechanical Probe ...... 111

xi 6. Work at Naval Research Laboratory ...... 115

6.1. Introduction...... 116 6.2. Experiment...... 117 6.3 Materials and Methods...... 119 6.3.1 Magnetic Bacteria ...... 119 6.3.2 Channel Glass ...... 120 6.3.3 Cuvettes ...... 121 6.3.4 Bacteria Separation Assembly...... 122 6.3.5 Light Scattering...... 123 6.4 Results and Discussion ...... 124 6.5 Conclusion ...... 125 6.6 Acknowledgment...... 125

7. Discussion...... 130

7.1. Summary and Conclusion...... 131 7.2. Epilogue: Direction for Future Study ...... 132

Bibliography ...... 134

xii

LIST OF FIGURES

Figure ...... Page

1 Examples of pristine conjugated polymers. A common feature of all conducting polymers is a conjugated pathway with alternating single and double bonds (from[6] ...... 6

2. Characteristic frequency dependence of the Drude model. The curves 2 2 2 are calculated for 4πNe /m = w p = 30 eV and h / t= 0.02 eV, [ 5]...... 11

3 Frequency dependence of the Lorentzian dielectric function...... 14

4 Perfect dimerized trans-polyacetylene with dimerization coordinate un. (a) A phase (b) B phase[82], (c) π-band structure of a perfect dimerized chain[81] ...... 30

5 The total energy per CH group site as a function of u[82] ...... 31

6 One electron density of states for A- or B- phase trans- polyacetylene [73] ..31

7 Complex conjugated defect in polyacetylene [67] ...... 34

8 The energy levels and occupation for soliton, polaron and bipolaron state in the gap[66]...... 35

9 Absorbance spectra of leucoemeraldine base, emeraldine base and pernigraniline base in N-methyl pyrrolidinone(NMP)[69-71] ...... 37

10 The schematic of protoniic acid doping of emeraldine base (EB) form of polyaniline(from[68]) ...... 38

11 a) s DC (T ) for PPy-PF6 and PAN-CSA from RT to 20mk for selected samples(from, [32,19]), (b) Log-log plot of W= dln(s ( T )) / d ln T vs temperature for PAN-CSA in the metallic, critical and insulating regimes [116] and T-dependent conductivity of polyaniline nanostructure[103]...... 43

xiii Figure ...... Page

12. a)The Anderson transition and, b) the form of the wavefunction in an Anderson metal-insulator transition, c) The Fermi gas state where the Fermi level lies in the region of localized states(from, [38])...... 46

13 a) X-ray from diffracting intensity vs 2q microdensitometer reading from samples A-E scattering of HCl doped XPAN-ES and PAN-ES, b) ) lns 1/ 2 vs T for HCl doped XPAN-ES and PAN-ES, c) Is the slope T0 of the T-dependent dc conductivity of the various XPAN-ES and PAN-ES samples (from,[43]) ...... 49

14 Schematic view on the structure of polymers. The lines represent poymer chains and the dashed squares marl the region where the polymer chains demonstrate the crystalline order[from([31]) ...... 52

15. Electronic conduction between metallic grains (well-packed and well over -lapping chain regions) embedded in amorphous media (a) the localization radius of electronic states in metallic grain is of the grain size and is of scale of the polymer unit in the amorphous media; (b) intergrain charge transfer effectively is provided by tunneling through resonance states in the amorphous regions. (from, [31,26])...... 53

16 Four-probe confriguration for measurement of room temperature and temperature dependent DC conductivity ...... 57

17 Schematic of reflectance spectroscopy accessory used for reflectance measurements on the Varian 5000 spectrometer [109 ] ...... 62

18 Schematic of the setup in the Absolute Reflectance Accessory [109] ...... 63

19 Schematic diagram of Michelson interferometer (after [51) ...... 65

20 Schematic diagram of Bruker IFS 66v/S FTIR spectrometer. A) light sources(two sources can be mounted at the same time). B) aperture, C) beamsplitter, D) fixed mirror, E) moving mirror, F) sample/reference position G) Si photoconductive bolometer, H) MCT (Mercury Cadmium Telluride) photoconductive detector, I) DTGS(DeuteratedTriGlycine Sulfate) thermal detector. It should be noted the drawing is not to the exact scale (after [52])...... 69 xiv Figure ...... Page

21 Comparism of (a)s (w)and (b) e (w) for PAN-N using different high frequency extrapolations...... 72

22 Schematic view of a typical X-ray Diffraction experiment. ki is the incident vector, kf is the scattered wave vector and q is the scattering vector as q = kf – ki (after [53] )...... 74

23 Variation of the spin state energies as a function of the applied magnetic field, after Bruker Instruments, Inc,[54]...... 76

24 Block diagram of a microwave bridge of an EPR spectrometer from Bruker Instruments, Inc) ...... 80

25 Block diagram of a microwave bridge of an EPR spectrometer (from Bruker Instruments, Inc)...... 80

26 Upper: The pressure cell with the 4-probe configuration showing the gold electrode and the drop cast film in 2 cm rectangular glass substrate. Lower: a) Upper and Lower platforms, b) Acrylic contact and c) Contact close up...... 82

27 (a) The scanning electron micrograph (SEM) of polyaniline nanostructures obtained in 1M HClO4 (aq) dilute polymerization, (b) The corresponding SEM image after the application of ~27 MPa pressure to the nanostructure ..87

28 a) (s dc (T )) polyaniline nanostructure and pressed polyaniline nanostructure as a function of temperature B) reveal the sharp and broad peak of nanostructured polyaniline film and pressed nanostructure respectively,

lower insert is the normalized conductivitys dc (T )/s dc (300K) vs T for polyaniline nanostructure film ...... 88

29 The reduced activation energy(W) as a function of temperature for nanostructure film...... 89

-1/2 30 ln sdc vs T of dc conductivity of HClO4 doped nanostructure , Polyaniline the filled square is the for nanostructure film while the filled circle is for the nanostructure pressed film (pellet). b) shows the low temperature range where the samples obey the quasi 1-D VRH model ...... 90

31 The normalized conductivitys dc (T )/s dc (300K) vs T for polyaniline nanostructure film, polyaniline nanostructure pellet and conventional polyaniline pellet...... 91

xv Figure ...... Page

32 (a) RT Reflectance for nanostructure polyaniline/HClO4 film b) Reflectance as a function of wavenumbers for PAN-N/HClO4 film. The insert is the reflectance ploted in semilog scales to reveal the mid-IR vibrations ...... 93-94

33 Temperature dependence of infrared reflectance for nanostructure polyanilne/HClO4 film...... 95

34 Temperature dependence of optical conductivities as a function of wavenumbers for nanostructure polyanilne/HClO4 film ...... 97

35 Optical conductivity as a function of wavenumber for (a) PAN-N film showing the higher energies, (b) showing the extrapolation to small wavenumbers ...... 98

36 The temperature dependent dielectric constant of PAN-N/HClO4. The insert is the dielectric constant for less conducting samples [29]...... 99

37 Magnetic susceptibility as a function of temperature for nanostructure polyanilne/HClO4 film ...... 104

38 The cT vsT for nanostructure polyanilne/HClO4 film...... 105

39 Linewidth peak to peak and linewidth full width at half maximum as a function of temperature for nanostructure polyanilne/HClO4 film ...... 106

' 40 Peak to peak amplitude ym plotted as a function of the square root of the microwave power P. The dotted line is to show linear dependence at low powers for nanostructured polyanilne/HClO4 film...... 107

2 41 The plot of 1/s as a function of microwave power H1 ...... 108

42 The RT X-ray for small angle and large angle (b) large angle ...... 110

43 Scanning electron microscopy (SEM) image showing (a) PAN-N with no applied weight, (b) with small applied weight, (c) no applied weight at higher magnification, (d) large and small applied weights at the same magnification as (c) ...... 111

xvi Figure ...... Page

44 Resistance as a function of applied pressure ...... 113

45 Resistance as a function of applied pressure and Time ...... 114

46 a) Experimental setuip for filling the cuveettes b) Bacterial probing setup ..118

47 SEM image of the 6 micron channel glass(membrane)...... 121

48 The picture of the assembled filtration device structure with the 0.3 Telsa Neodymium iron boron magnet used to align the field parallel to the channels in the channel (the membrane)...... 122

49 Forward Scattered Light intensity as a function of time as magnetic bacteria migrate from cuvette-A through the membrane to cuvette-B ...... 126

50 Normalized Absorbance to a standard cuvette pathlength...... 127

51 Absorbance as a function of concentration of the cells derived fom serial dilution...... 128

52 Concentration of magnetic bacteria as a function of time from fit . parameters and the number of magnetic bacterial as function of time from fit parameters ...... 129

xvii LIST OF TABLES

1. Configurations of optical components for different frequency range measurements on Bruker IFS 66v/S FTIR spectrometer[84] ...... 69

2. Parameter calculated from quasi 1-D VRH model and magnetic data...... 86

3. Parameter extracted from X-ray diffraction Data...... 109

xviii

CHAPTER 1

INTRODUCTION

Conventional polymers are known to have good dielectric properties and are used for many applications, such as electrical insulation, replacement for wood, ceramics, in shoes and clothes. However, in 1977 a new form of polymer was discovered that conducts electricity (often referred to as a “conducting polymer”) [1,2]. Unlike conventional or saturated polymers that have no electronic interest because all of their carbon valence electrons are paired, conducting or conjugated polymers have one unpaired electron (p-electron) per carbon atom. They contain a p-conjugated system in which the carbon pz orbital overlaps with the orbital of the consecutive carbon atoms leading to electron delocalization along the backbone of the polymer chain. The classic picture is polyacetylene, in which three of the four electrons in its outer most shell are sp2 hybridized. These three sp2 electrons form s bonds between two neighboring carbons and the hydrogen. However, since carbon has four electrons in its outermost shell, there will be one unpaired p electron left to occupy the carbon pz orbital. The overlap of p electrons from various carbon atoms result in the formation of a p band. Each of the delocalized orbitals has only one unpaired electron with a partially filled band, typical of a metal that has finite density of states at the Fermi level. 1 However, because to bond length dimerization due to the Peirels instability [3], there is an opening of a gap at the Fermi surface, therefore the p band split into p- and p*- bands typical of a semiconductor or insulator having no partially filled bands, resulting in the lowering of electronic energy of the occupied states.

When the p-conjugated systems are doped, charge carriers are introduced into the electronic structure. In highly doped systems, the charge carriers are mobile along the backbone of the polymer chains. This results in a “metallic” state [2]. The charge carriers are stored in novel states such as solitons for systems with degenerate ground state and as polaron and bipolaron for systems with non-degenerate ground states.

There are many research interests associated with p-conjugated systems because of the “metallic” properties they have when they are highly doped. Their transport properties, along with their metallic and non-metallic behaviors, are highly influenced by doping level and structural disorder. When p-conjugated systems are doped the conductivity of the pristine conjugated organic polymer can be tuned from an insulating to metallic regime with its room temperature conductivity comparable to a typical metal.

For example, the electrical conductivity of polyacetylene is as high as 105 S/cm, while that of lead is 4.8x104 S/cm. These types of polymers are also known as “electronic polymers.” The doping processes in electronic polymers are not typical of substitutional doping process in inorganic semiconductors. In electronic polymers, the dopants are positioned interstitially between the polymer chains to donate or accept charges from the polymer backbone. An increase in the electrical conductivity of conjugated polymers resulting from an increase in dopant concentration was first reported in 1977 for polyacetylene doped with controlled amounts of halogens (chlorine, bromine, iodine) and 2 with arsenic pentafluoride[1]. Following this first report, similar trends have been seen in other types of conjugated polymers [7,9,11].

Several doping methods have evolved, like redox doping, through which conducting polymers and their derivatives can be doped either by chemical or electrochemical processes [6,12]. Another process, involving no dopant ions, is possible in which the conjugated organic polymer is exposed to radiation of energy larger than its band gap, causing electrons to be excited across the band resulting in “photo-doping” of the polymer. Doping can also be caused by a non-redox method in which, unlike the redox method, the number of electrons in the polymer backbone does not change during the doping process. The first example of such doping was seen with the emeraldine base form of polyaniline. When the emeraldine base was treated with aqueous protonic acids, an increase of about ten orders of magnitude was noticed in its conductivity [13-15].

Over the decade, many of improvements in the processing of these conducting polymers have led to greater values for their conductivity and both n-type (electron donating) and p-type (electron accepting) dopants have been used to induce an insulating- metal transition in conducting polymers [4,5,11]. More recently, a new method for preparing polyaniline called “self-stablized dispersion polymerization” (SSDP) was reported by Lee et al [37], which showed “true metallic” behavior. However, their dielectric constant did not show the “true metallic” behavior down to very low energies

(<2meV) that was reported a decade earlier by Kohlman et al [32]. However, the SSDP free standing films of the polyaniline doped with camphorsulphonic acid (CSA) cast from a meta-cresol solution revealed significant improvement in some of the transport properties of polyaniline compared to previously studied samples. The SSDP samples

3 showed behavior similar to that of a Drude metal, with resistivity decreasing as temperature is lowered to 5 K, and signatures of their optical conductivity and frequency dependent dielectric constant were similar to that of a Drude metal up to 0.04 eV.

Overall, with enhancement in the room temperature conductivity, many traditional signatures of a metal, such a negative dielectric constant, temperature independent Pauli susceptibility, and a linear dependence of the thermoelectric power on temperature have characterized the conducting polymers [19,32,102,106].

Over the past five years, there has been a growing interest in nanostructured conducting polymers over their conventional counterpart, for potential applications in sensors, biosensors and electronics. Although the chemical properties of conventional conducting polymers films and their nanostructured network films are the same, the morphologies and the nature of disorder are important as they affect the nature of charge carriers within the nanostructured network. Different synthesis techniques have been reported for making conducting polymers nanostructures (eg. nanorod, nanofibers, nanowires) [8,10], however, the techniques can be cumbersome and expensive. This led to the development of dilute polymerization a [58-59] novel, simple and scalable techniques used for synthesizing the nanostructured conducting polymers, notably polyaniline nanostructures used in this thesis. Nanostructured polyaniline doped with perchloric acid were probed with temperature dependent direct current transport measurements (which provide insight into the metallic or insulating nature of electrons at the Fermi level), temperature dependent electron paramagnetic resonance (which gave information on whether the charge occupies spinless bipolaron or polaron states), temperature dependent optical reflectance (which helps in the determination of the role of

4 disorder within the nanoscale network. As the material becomes strongly disordered, the wavefunctions of the charge carriers become localized and this can be confirmed by the signature of the dielectric constant derived from the optical data), and mechanical probes

(which gives insight to the “elastic,” “compressive,” and “relaxation” states of the nanostructure within the nanoscale network and interfiber contacts).

In a recent charge transport study of a polyaniline nanostructured network, it was suggested that the sharp maximum in conductivity at a certain temperature (Tmax) is the signature of a metal-semiconductor transition [103]. However, the study failed to provide other concomitant charge carrier results to support this claim. In this thesis, it is claimed that this assertion is incorrect because the RT conductivity is much less that the Mott minimum metallic conductivity. By employing detailed temperature dependent probes of structure and charge dynamics, such as temperature dependent X-ray diffraction, conductivity, electron paramagnetic resonance, and optical reflectance, it was determined that the origin of the ‘metallic’ signature of the nanostructure polyaniline was related to the nature of the interfiber and intrafiber contacts within the network.

5

Figure 1: Examples of pristine conjugated polymers. A common feature of all conducting polymers is a conjugated pathway with alternating single and double bonds [85].

6 CHAPTER 2

THEORETICAL BACKGROUND AND METHODOLOGY

2.1 Response Functions

There are several response functions that discribe the response of a material to incident electromagnetic radiation. From the Kramers-Kronig relations and Fresnel equations, response functions and optical constants such as the complex index of refraction (n), dielectric constant (ε), optical conductivity (σ) and absorption coefficient

(α) of the material can be determined. The signature of the complex dielectric function gives information about localization or delocalization of charge carriers within a given material. The localization of charge carriers can be explained by the Lorentz model where the electrons of an atom are assumed to be bound to the nucleus by a spring-like restoring force. Therefore, the Lorentz model is used for insulators, having a quantum equivalent restoring force that includes all direct interband transistions. While the delocalized, or free, electrons in metals could be explained by the Drude model having a quantum equivalent that includes all intraband transitions. In the next few sections optical constants, Fresnel reflection coefficients, the Kramer-Kronig relations, absorption models, dispersion models, the Drude model, the Lorentz model and the complex index of refraction are briefly reviewed. 7 2.1.1 Drude Model

Drude model is widely used to explain the free electron behavior in materials, notably within a metal. In this model, the valence electrons are assumed bound while the conduction electrons are delocalized around the ion cores.

The motion of the free electron is governed by the equation 2.1, where e is the charge of the electron, m is the mass of electron, t is mean free time between collisions or the scattering time. Equation 2.1 represents the relationship between the damping force

m dr from scattering ( ) and the electric driving force (eE) . t dt

d2rm d r m+ = - eE , (2.1) dt2 t dt

Here E(t)=Re(E(w) (e-iwt)), therefore we seek a steady state solution of the form r(t)=

Re(r(w) e-iwt)). If we substitute the complex r and E into in equation 2.1 we will obtain the solution for equation 2.1

eE / m r = . (2.2) w2 +i w/ t

From the relationship between electric displacement D and electric polarization P and electric field E, we know that

D=eOE+P (2.3) where eO is the permittivity of free space. Polarization can be written in term of the dipole moment p=-er. Therefore we can rewrite equation 2.3 in terms of dipole moments as

D=eOE+ å Ni pi (2.4) i

8 where N is the average number of such molecules per unit volume at r. From the above relations, we can derive the Drude dielectric function as [45,47]

2 % 4pNe 1 eDrude =1 - , (2.5) mw2 + i w/ t

where N is the number of electrons per unit volume, assuming that the material is nonmagnetic. Therefore the real and imaginary parts can be written as,

w2 t 2 e( w ) = 1 - p (2.6) 1 1+ w2 t 2

w2 t e() w = p , (2.7) 2 w(1 + w2 t 2 )

4pN e2 where w2 = , is the plasma frequency and w is the external frequency. The p m plasma frequency serves as the boundary region for collective oscillations between the

low frequency region where e1(w) is negative and light does not propagate, and the high

frequency region where light propagate and e1(w) is positive. The frequency

dependence of the Drude metal, Figure 1, shows e1(w) increasing monotonically as

frequency increases. At low frequency, e1(w) can have large negative values. This large negative dielectric constant due to the electron falling out of phase with the electric field because of its inertia [30]. Therefore, in the low frequency limit (wt << 1) , we can rewrite the Drude relation as follows.

9 2 2 e1() w = -w p t (2.8)

2 e2()/ w = w p t w. (2.9)

10

Figure 2: Characteristic frequency dependence of the Drude model. The curves are

2 2 2 calculated for 4πNe /m = w p = 30 eV and h / t= 0.02 eV, [ 45].

11

If we take into consideration such interactions as electron-phonon, electron-electron, electron screening, immobile ion and charge effects the appropriate expression for the dielectric constant for a metal becomes

w2 t 2 e » e - p with e >1. (2.10) 1 ¥ 1+ w2 t 2 ¥

This includes the high frequency term. However, at low frequency equation 2.10 reduces back to equation 2.8.

2.1.2 The Lorentz Oscillator Model

A classical model for explaining the behavior of insulators was introduced by

Lorentz. In this picture, the electrons of an atom are assumed to be localized to the nucleus by a spring like force. We can describe the motion of the electron via equation

2.11.

d2r d r m+ m G + m w2r = - e E . (2.11) dt2 dt 0

dr where the term mG provides an energy loss mechanism due to various scattering dt

2 mechanisms in the solid and mw0r represents a Hooke’s law restoring force. Within this model, it is assumed that the mass of the nucleus is infinite, or else the reduced mass should be used. It is important to note that a small magnetic force (–ev x B/c) arising from the interaction of electron with the light’s magnetic field component, is neglected

12 because the velocity of light is much larger than that of the electron (i.e the term v/c is small). We can then derive the dielectric function following a similar procedure for the

Drude dielectric function derivation by considering a field that is varying in time as e-iwt:

2 % 4pe N eLorentz =1 + 2 2 . (2.12) m()w0 - w - i Gw

Therefore, the real and imaginary part are, respectively

2 2 2 4pNe w0 - w e1( w ) = 1 + 2 2 2 2 2 (2.13) m ()w0 - w + G w

4pNe2 Gw e2 () w = 2 2 2 2 2 . (2.14) m ()w0 - w + G w

Here we have assumed that the material is nonmagnetic, therefore we set m = 1.

For a classical atom having more than one electron per atom, we can generalize the above result to

2 N % 4pe j eLorentz =1 + å 2 2 , (2.15) mj ()wj - w - i G j w where the atomic density NN= å j . j

A corresponding quantum mechanical derivation of interband transition gives

2 4pe Nf j e~ = 1+ (2.16) Lorentz å 2 2 m j (w j - w ) - iGj w

Here hw j is the energy of transition between two atomic states and f j is the oscillator strength, which is the relative probability of a quantum mechanical transition. For free atoms, it satisfies the sum rule

13 å f j = 1 (2.17) j

Figure 3: The frequency dependence of dielectric function [45]

Figure 3 shows the frequency dependence of the Lorentzian dielectric function. As the frequency increases, ε1(ω) increases, except in the region of width≈ Γ centered around the transition energy ω0. There is also a region of anomalous dispersion near ω0, where an increase in frequency leads to a decrease in ε1(ω). Figure 3 also reveals that at low frequencies, Lorentz insulators are always positive. However, it is possible to have a

14 negative ε1(ω), depending on the background dielectric constant at high energies even in an insulator.

2.1.3 Propagation in a Medium

Four quantities describe the electromagnetic state of materials. These are the electric polarization P, the volume density of the magnetic dipole, magnetization M, the electric charge density r, and the current density J. Considering a medium where there is no external charge and currents, these quantities are related to the macroscopically averaged electromagnetic fields E and H via the Maxwell equations [27,30].

m ¶H Ñ´E = - (2.18) c¶ t

Ñ×E = 0 (2.19)

4ps e ¶E Ñ´HE = + (2.20) c c¶ t

Ñ ×H = 0 (2.21)

Here we assumed that the material is isotropic so that e is not a function of spatial coordinates. Then we can assume the linear relations for the fields:

DEPE=e0 + = e (2.22)

BHMH=m0 () + = m (2.23)

JE= s . (2.24)

Equation 2.24 is usually written as a function of frequency (w ) and is known as the frequency-dependent conductivity or optical conductivity, since it involves absorption

15 due to photons. At zero frequency, optical conductivity becomes the DC electrical conductivity for an isotropic material.

From equations (2.18)-(2.21), we look for solution with the time dependence e-iwt.

Since:

¶2E Ñ´() Ñ´E = (2.25) ¶t 2 the wave equation for the electric field can be derived [30,47]

em¶2EE4 psm ¶ Ñ2E = + . (2.26) c2¶ t 2 c 2 ¶ t

Since Ñ×E = 0 , the solutions are restricted to plane waves form

E= E0expi ( q × x -w t ) = ( E 1 e 1 + E 2 e 2 )exp i ( q × x - w t ) , (2.27)

where E0 is perpendicular to wavevector q, which is assumed to be complex and E1 and

E2 are complex amplitude numbers. Therefore from (2.24) and (2.25), we can derive

w2 4 ps q2 =m( e + i ). (2.28) c2 w

So, the complex dielectric function (e% ) is defined as

4p e% º e +i e = e + i s , (2.29) 1 2 1 w

It is important to note that by rewriting the equation of motion; equation 2.1 in term of momentum as

dP P + = -eE (2.30) dt t

16 and employing E(t)=Re(E(w) (e-iwt)), we will obtain solution for P in term of current density J as

neP(w) (ne2 / m)E(w) s E(w) J(w) = - = = DC (2.31) m 1/t - iw 1- iwt

From equation 2.22, the coefficient of proportionality between J(w) and E(w) is the AC conductivity s (w)

s s (w ) = DC 1 - iwt (2.32)

Therefore, the dielectric constant is related to the frequency dependent conductivity using

2.28, and, from the dispersion relation q= (/)w c n% , the complex index of refraction is

n% =em% º n + ik . (2.33)

In general, for anisotropic absorbing media, e% , n% , m are complex second-order tensors.

However, in the case of anisotropic, non-magnetic (µ = 1) materials, the relations between optical constants can be derived from equations (2.27), (2.28) and (2.32)

2 2 e1 =()n - k (2.34)

e2 =4 ps / w = 2nk , (2.35) and from the definition for absorption coefficient (α) and penetration depth (δ)

1dI 2w k a º - = (2.36) I dx c

c c d = » . (2.37) (2psmw )1/ 2 wk

17

2.1.4 Propagation through an Interface

Considering the boundary between two linear media, we could have both reflection and transmission at normal incidence. The fundamental laws of geometrical

optics reveals that angle of incidence qi and reflection q r are equal. It also shows that

Sinq n n from Snell’s law, t = i , where i is the ratio of the index of incidence to the Sinqi nt nt index of refraction (transmission). By applying boundary conditions to Maxwell’s equations in which the normal component of D and B are continuous and the tangential components of E and H are continuous, the transverse electric (TE) mode (also called s or s polarization, where the electric field vector of the incident wave is perpendicular to the plane of incidence,) satisfies:

EEEi+ r = t (2.38)

-qi E icosq0 + q r E r cos q 0 = - q t E t cos q 1 , (2.39)

where Ei , Er and Et are amplitudes of incident, reflected, and transmitted electric field vectors, and q’s are for wave vectors. In the transverse magnetic (TM) mode (also called p or p polarization), where the magnetic field vector of the incident wave is perpendicular to the plane of incidence, the boundary conditions for E are given as

qi E i- q r E r = q t E t (2.40)

EEEicosq0+ r cos q 0 = t cos q 1 . (2.41)

From Snell’s law and the equations above, we can derive the Fresnel’s equations for the

coefficients of reflection ( rs and rp ) and transmission (ts and t p ). 18

% % æEr ö n0cosq0-- n 1 cos q 1 sin( q 0 q 1 ) rs =ç ÷ = = - (2.42) E % % sin(q+ q ) èi øTE n0cosq0+ n 1 cos q 1 0 1

% æEt ö 2 n0 cosq0 2cos q 0 sin q 1 ts =ç ÷ = = (2.43) E % % sin(q+ q ) èi øTE n0cosq0+ n 1 cos q 1 0 1

% % æEr ö n0cosq1-- n 1 cos q 0 tan( q 0 q 1 ) rp =ç ÷ = = - (2.44) E % % tan(q+ q ) èi øTM n0cosq1+ n 1 cos q 0 0 1

% æEt ö 2 n0 cosq0 2cos q 0 sin q 1 t p =ç ÷ = = (2.45) E % % sin(q+ q )cos( q - q ) èi øTM n1cosq0+ n 0 cos q 1 0 1 0 1

For normal incidence (q = 0) , we get

% % n0- n 1 r= r = r = (2.46) p s 01 % % n0+ n 1

% 2n0 t= t = t = . (2.47) p s 01 % % n0+ n 1

* % % % % 2 2 én0- n 1 ù é n 0 - n 1 ù (1 - n ) + k Rº r r* = = (2.48) 01 01 ê% % ú ê% % ú 2 2 ën0+ n 1 û ë n 0 + n 1 û (1+n ) + k

% % When light is incident from vacuum, n0=1, n 1 = n + ik . For thin films we will need to consider multiple internal reflections and interference between light paths for the reflectance and the transmittance.

19 2.1.5 Kramer-Kronig Dispersion Relations

Kramer-Kronig (K-K) relations serves as the mathematical tool used to calculate imaginary part of response functions since only the real part of response function can be measured. The measured component has to be over a broad spectral range in order to be able to apply K-K analysis. K-K relations are also integral formulas that relate a dispersive process to an absorption process. All dispersion relations are a direct result of causality i.e there has to be a cause before an effect. For example, until light waves reaches the sample there cannot be any reflected waves off the sample. Application of

K-K analysis to the measured real part of reflectivity is widely used to obtain its imaginary part. The combined data containing both the real and imaginary components of reflectivity can be used to acquire several optical constants. From the obtained optical constants, physical interpretation are given to the nature of charge carriers in a given material.

If we probe a system with an external field, the system will produce a response that can be described by a response function. Let the induced response as denoted as X (x, t) and the corresponding external stimulus as f(x, t), we can express both the induced response and the external stimulus as

¥ X(x , t)= ò G( x , x ',t,t')f( x ',t')d x 'dt' , (2.49) -¥ where G(x, x´, t, t´) is a linear response function. The optical conductivity, index of refraction, and dielectric function are examples of response functions. If we assume that

20 the wavelength of the variable field is long, we can ignore the spatial dependence of the functions[45]. Therefore, the Fourier transform of equation 2.49 is

X(w )= G()() w f w . (2.50)

From Cauchy’s theorem, it is assumed that G(ω) decreases quickly as of the semicircle goes to infinity, so we get

1¥ G' (w ) G()w= Pò d w ' (2.51) ip-¥ w ' - w where P corresponds to the principle value of the integral and ω is on the real axis. The real and imaginary parts of the equation (2.51) can be expressed as

2¥ w'G' Im ( w ) ReG (w )= P d w ' (2.52) ò 2 2 p0 () w' - w

2w¥ ReG' ( w ) ImG (w ) = - P d w ' , (2.53) ò 2 2 p0 () w' - w

This shows that the real and imaginary parts are not independent.. Therefore, we now have a general expression for all dispersion relations. If we apply the condition G(-ω) =

G*(ω), we can write similar expression for any physical system. For example, in the dielectric functions, (e% -1 = 4 p Na% ), where a% is the complex polarizability, e% -1 must also be causal [45]. Therefore, dielectric constants can be related as

2 ¥ w'' e() w e( w )- 1 = P2 d w ' (2.54) 1 ò 2 2 p0 () w' - w

2w ¥ [e ( w' )- 1] e() w= - P1 d w ' . (2.55) 2 ò 2 2 p0 () w' - w

21 For the coefficients of reflectivity, from the equation (2.48)

iq (w) r01(w) = R(w)e , ln r01(w) = ln R(w) + iq (w) (2.56)

Therefore, the Kramers-Kronig relation between ln R(w) and θ(ω) can be written as

w¥ lnR' ( w ) q() w= - P d w ' . (2.57) ò 2 p0 () w' - w2

Therefore, as discussed earlier, from known measured reflectance R(ω) of semi-infinite materials, we can calculate the imaginary part of reflectance (or the phase shift, θ(ω)) from equation 2.56. Optical constant can then be derived using the appropriate equations including 2.34-2.35, 2.46, and 2.56. For accurate calculation, the integration should be done over the whole spectral range from zero to infinity. There are suitable extrapolations at low and high energies range that are employed to meet this requirement.

Similarly, for thin samples we can obtain the coefficients of transmittivity from equation

2.41 and using

% n2 Tº t t** = t t , (2.58) % 1 1 1 1 n0

% % assuming n2= n 0 , for identical incident and exit media. We can express t1(w) and

lnt1(w) as

if (w) t1(w) = T(w)e , lnt1(w) = ln T(w) + if(w) . (2.59)

This leads to a similar K-K relation for the coefficients of transmission:

w¥ lnT' ( w ) f() w-2 pw d = - P d w ' . (2.60) 1 ò 2 p0 () w' - w2

22 Here the thickness of the thin sample is included and the calculation and the derivation of optical constants is quite complicated.

2.1.6 The Sum Rule

The Sum rule arises from causality and the dynamic laws of motion. It is derived from the asymptotic nature of optical functions and the K-K dispersion relations. At high frequencies (ω> ωc ) where there is no more absorption, the dielectric function has the asymptotic form

) 2 lim e (w) = 1+ (w p w) + ...... (2.61) w ® ¥

The real part of equation 2.53 in the limit that w ® ¥ is

2 e1 (w) = 1+ (w p w) (2.62)

and the imaginary part ε2 → 0 since in the limit w ® ¥ , ε2 falls off faster than w -2

Therefore, we write equation 2.60 in terms of the K-K dispersion relation for the real part, following equations 2.51 or 2.53 at sufficiently high frequency ωc

w 2 2wcw'' e() w 2 w c e( w )= 1 -p = 1 -P2 d w ' » 1 - w ' e ( w ' ) d w ' (2.63) 12ò 2 2 2 ò 2 w p0() w' - w pw 0

23 -2 Then, by letting ωc → ¥ , and equating the powers of w we can get

¥ p w 'e (w ' )dw ' = w 2 (2.64) ò 2 p 0 2

2 2 where wp =4 pN e / m . We can rewrite equation 2.63

¥ p p4 p Ne2 2p 2e 2 N w' e()() w ' d w ' = w 2 = = cell (2.65) ò 2 p 0 2 2 m mVcell

where Vcell is the unit cell volume and Ncell is number of charges in the unit cell, since

Ncell = N Vcell.

24 CHAPTER 3

ELECTRONIC STATE AND TRANSPORT PROCESSES IN CONDUCTING POLYMERS

According to Bloch Theory, the electron wave functions are delocalized in a perfect crystal with periodic potential. The conduction band is partially filled, having one electron per atom in the periodic lattice. Similarly, a periodic polymer backbone (one dimensional conducting polymer systems) with one unbonded pz orbital per atom would form a “Metallic Bloch state” having a partially filled band. However, due to lattice distortion there is pairing of unbonded pz orbitals of successive sites along the polymer chain called dimerization. This opens up an energy gap at the Fermi surface thereby lowering the electronic energy of filled states. Overall, this distortion caused by Peierls instability changes the periodicity of the lattice, leading the first Brillouin zone to be half as large ( Dk =1/ 2 a ), and the unit cell to be twice the original length [3], resulting in splitting of metallic band into filled valence and empty conduction bands making undoped conducting polymers either a semiconductor or an insulator. In this section, the electronic structure of undoped conducting polymers is explained using a quasi 1-D model developed for polyacetylene, an example of conducting polymers

25 3.1 Polyacetylene : The Su-Schrieffer-Heeger Model

The cis- and trans- isomers of polyacetylene are shown in Figure 1. They are composed of repeat units of carbon and hydrogen. The Su-Schrieffer-Heeger (SSH) model has been used to explain one-dimensional electronic structure in polyacetylene

[48-50]. This model assumes weak interchain coupling and shows that the existence of alternating single and double bonds allow lowering of total energy. Equation 3.1 describes the Hamiltonian of the SSH model for trans-polyacetylene

H SSH = H p + H p - ph + H ph, (3.1) where

+ + H p = -t0 å(an+1,s an,s + an,s an+1,s ) (3.2) n,s t0 is the hopping integral between nearest neighbor π electrons for uniform chain

+ (undimerized chain), the a n,s and an,s stand for creation and annihilation operator for π electrons of s = ±1/2 on the nth (CH) group.

+ + H e- ph = a å(un+1 - un )(an+1,s an,s + an,s an+1,s ). (3.3) n,s

th α is the electron-phonon constant and un is the displacement of the n CH unit from a state where all the carbons are separated equally.

2 P K 2 H = å n + å(u - u ) . (3.4) ph 2M 2 n+1 n

The phonon Hamiltonian contains the mass M of each (CH) repeat, the spring constant K, and momentum P, and displacement un. We can now rewrite equation (3.1) as

26 H SSH = Hp + Hp - ph + H ph 2 + + K 2 Pn = -å(t0 -a(un+1 - un ))(an+1,s an,s + an,s an+1,s ) + å(un+1 - un ) + å n,s 2 2M 2 + + K 2 Pn = - åtn+1,n (an+1,s an,s + an,s an+1,s ) + å(un+1 - un ) + å , (3.5) n,s 2 2M

where tn+1,n is the hopping integral. In a perfect chain, these hopping integrals are

t = t - t single bond n+1,n 0 1 (3.6) = t0 + t1 double bond.

If the displacements un are zero so that the polymer chain is undimerized, the

Hamiltonian becomes

c + c v+ v H = å Ek (ak ,s ak ,s - ak,s ak,s ) (3.7) k ,s

Therefore, there will be one π electron per CH unit, suggesting a metallic chain with a partially filled valence band as shown in Figure 4b. However due to Peierls instability, the one dimensional chain is unstable with respect to a lattice distortion. Therefore, a gap will open at wavevector 2kF, which is at the Fermi level, lowering the total energy of the system. The resulting electron band structure now resembles that of an insulator. The configuration coordinates for a perfectly dimerized chain are given as

n un = (-1) u, (3.8) where u is a fixed value. We can now rewrite equation (3.5) in terms of u, after neglecting the kinetic energy term as

27 + + K 2 H SSH (u) = - åtn+1,n (an+1,s an,s + an,s an+1,s ) + å(un+1 - un ) n,s 2 (3.9) n + + 2 = - å[t0 + (-1) 2au](an+1,s an,s + an,s an+1,s ) + 2NKu , n,s where N is number of (CH) groups in a chain. By diagonalizing the Hamiltonian of

Equation (3.9), the Hamiltonian can be rewritten as

c + c v+ v 2 H = å Ek (ck ,s ck,s - ck,s ck ,s ) + 2NKu , (3.10) k,s

2 2 1/ 2 where Ek = (e k + D k ) with εk = 2t0 cos (ka) is the gap parameter defined by ∆k = 4αu

c+ c v+ v sin(ka) and ck ,s ck,s ( or c k,s ck,s ) is the number operator which counts the number of quasiparticles with wavevector k and spin s in the conduction (c) or valence (v) band.

The ground state energy per (CH) can be calculated from equation 3.9 by integrating over the first Brillouin zone (-π / (2a) < k < π / (2a))

E (u) 4t 0 = - 0 E(1- z 2 ) + 2Ku 2 , (3.10) N p

2 where E(1-z ) is the elliptic integral and z = t1 / t0 = 2αu / t0 . For small z, we have

E(1-z2) = 1+ ½ (ln4/|z| - ½ ) z2 +…. ……………….. Therefore we can substitute this into equation (3.10) which gives

E (u) 4t 1 4 1 4t 2t 4 1 0 = - 0 [1+ (ln - )z 2 ] + 2Ku 2 = - 0 - 0 (ln - )z 2 + 2Ku 2 . (3.11) N p 2 z 2 p p z 2

The plot of E0 ( u ) / N versus u in Figure 5, shows that there are two energy minima due to spontaneous symmetry breaking. This twofold degeneracy of the ground state is due to Pierels instability. The SSH model gives the minimum-energy for the distortion u0

28 2 ~ 0.04 Ǻ for α = 4.1 eV/Ǻ, K = 21 ev/Ǻ , and t0 = 2.5 eV. This value is consistent with values obtained from structural and NMR studies[ 64-65].

E 1 - C = [E (u ) - E (0)] = -0.015. (3.12) N N 0 0 0

The density of states per site can be obtained from the first derivative of Ek : L N 1 N 0 (E) = = | E | , D £| E |£ 2t0 , 2p | dE / dk | p 2 2 2 2 k (4t0 - E )(E - D ) (3.13) = 0, otherwise,

where ∆ = 4αu0 = 2t1 and the graphical relation is plotted in Figure 6.

29

Figure 4: Perfect dimerized trans- polyacetylene with dimerization coordinate un. (a) A phase (b) B phase [50], (c) Peierls distortion and gap opening in polyacetylene. Without Peierls splitting, the single pz orbital on each carbon atom will distribute equally in both directions forming a 1-d metal. However, the Pierels distortion lowers the total energy with a p bond forming along every other s bond as well as a small decrease of this bond length and an increase in the other s bond length(from [66]) and (d) (c) π-band structure of a perfect dimerized chain [49].

30

Figure 5: The total energy per (CH) group site (electronic plus lattice distortion) as a function of u. Note that the double minimum associated with the spontaneous symmetry breaking and the twofold-degenerate ground state [50].

N0 (E)

Figure 6: One-electron density of states for A- or B- phase trans- polyacetylene [67].

31 3.2 Soliton, Polaron and Bipolaron

In conducting polymers, charges are stored in novel states called solitons, polarons and bipolarons depending on the nature of the system’s ground state. For degenerate ground states, charges injected into the polymer backbone via doping, thermal excitation, or photoexcitation are stored in soliton or polaron states. For non-degenerate ground states, the preferred states are either bipolaron or polaron.

Doping of conducting polymers is unlike the doping in conventional semiconductors, where charges are added to the system, which can potentially lead to the formation of electron or hole excitations. However, if we consider the classic conducting polymer, polyacetylene, the charges added to the polymer chain take up sites along the chain and the charges lead to local distortion of bonding forms A or B phase. The lowest energy excitation in the degenerate ground state is a domain wall separating the A-phase from B-phase called a soliton. A soliton has a unique spin-charge relation in which the neutral soliton has no charge but spin ½, which the charged soliton has a ± e charge and no spin. A solition on a chain leads to an energy level exactly in the middle of the gap which can be occupied by 0,1, or 2 electrons.

A polaron state is formed in a non-degenerate ground state when a charge soliton and a neutral soliton are bound. A bipolaron state is formed when two charge solitons are bound or when two charged polarons are bound, having zero spin and charge ±2e.

Polarons and bipolarons both introduce two energy levels within the gap, but the occupations of the two levels are different. In the case where bipolaron are formed by two bound charged polarons, the interaction of the polaron level results into the splitting of the polaron energy levels. The uppermost level merges with the conduction band 32 while the lowest level merges with the valence band, so the electron filling results in a spinless state. Figure 7 shows the schematic structure of soliton, polaron and bipolaron in conjugated polyacetylene, while Figure 8 shows the energy levels with in the gap.

33

Figure 7: Complex conjugated defect in polyacetylene [67]

34

S0 S+ S–

conduction band (LUMO)

(HOMO) valence

2+ 2– 0 + + – BP BP S P S P c.b.

v.b.

Figure 8: The energy levels and occupation for soliton, polaron and bipolaron states in the gap[66].

35 3.3 Polyaniline

There are three different oxidation states of polyaniline. The completely reduced lecoemeraldine base (LEB) form, the half oxidized emeraldine base (EB) form, and the fully oxidized Pernigraline base (PNB) form, all of which are insulating. Their repeat units are comprised of two benezene rings for the LEB, three benezene rings and one quinoid ring for the EB, and a benezene ring and one quinoid ring for the PNB. Through protonation of the imine nitrogen site (-N =) , the conductivity of the emeraldine base form may be enhanced by a factor of 1010 by increasing the electrical conductivity for the base sample from ~ 10-10 to ~ 1 S/cm for the fully protonated emeraldine salt form [74-

83] without changing the electron concentration.

There are two different nitrogen sites in the EB form of polyaniline, resulting in two bonding configuration for the nitrogen sites. These are the amine nitrogen (-NH -) and the imine nitrogen (-N =) sites. The amine nitrogen site has two nonbonding electrons in a pz orbital perpendicular to the plane of nitrogen and the two adjacent carbon atoms. The imine nitrogen site has an electron in the pz orbital, forming a p bond with a carbon atom on the benezene ring, and a lone electron pair in a s oribital in the nitrogen plane.

EB can be doped with a protonic acid (H +A -). The proton ( H + ) forms a s bond with the imine nitrogen site via their lone electron pair, while the counterion (A - ) remains close to the H + in order to maintain charge neutrality in the system. Following the association of the proton with the imine site, the quinoid ring undergoes a redox reaction and thereby changes to a benzenoid ring. This results to the formation of a

36 bipolaron lattice. However, from the measurement of a growing Pauli-like spin concentration with increased doping [72-73], it was suggested that the bipolaron split to form two polarons, thus forming polaron lattice.

Figure 9: Absorption spectra of leucoemeraldine base, emeraldine base and pernigraniline base in N-methyl pyrrolidinone (NMP) [69-71].

37

Figure 10: The schematic of protonic acid doping of emeraldine base (EB) form of polyaniline (from [68]) 38 3.4 Metal to Insulator transition (IMT)

3.4.1 Role of dopant

There are many research interests in p-conjugated systems because of the

“metallic” properties they obtain when highly doped. Their transport properties, both metallic and non-metallic behaviors, are highly influenced by doping level and structural disorder. When p-conjugated systems are doped, the conductivity of the pristine conjugated organic polymer can be tuned from the insulating to metallic regime with room temperature values comparable to that of a typical metal (for example, the electrical conductivity of doped polyacetylene can be as high as 105 S/cm, while that of lead is

4.8x104 S/cm). These types of polymers are also known as “electronic polymers.” The doping processes in electronic polymers are not typical of the substitutional doping processes in inorganic semiconductors. In electronic polymers, the dopants are positioned interstitially between the polymer chains to donate or accept charges from the polymer backbone. An increase in the electrical conductivity of conjugated polymers resulting from an increase in dopant concentration was first reported in 1977 for polyacetylene doped with controlled amounts of halogens (chlorine, bromine, iodine) and with arsenic pentafluoride [1]. Following this first report, similar trends have been seen in other types of conjugated polymers [7-11].

Several doping methods have evolved, like redox doping, in which all conducting polymers and their derivatives can be doped either by chemical or electrochemical processes [6,12]. Another doping involves no dopant ions, instead, the conjugated organic polymer is exposed to radiation of energy larger than its band gap, causing electrons to be excited across the band, resulting in “photo-doping” of the polymer. 39 Doping can also be caused by a non-redox method in which, unlike the redox method, the number of electrons in the polymer backbone does not change during the doping process.

The first example of such doping was seen in the emeraldine base form of polyaniline.

When the emeraldine base was treated with aqueous protonic acids, an increase of about ten orders of magnitude was noticed in its conductivity [13-15].

Over the decade, many of improvements in the processing of these conducting polymers have led to greater values for their conductivity and both n-type(electron donating) and p-type(electron accepting) dopants have been used to induce an insulating- metal transition in conducting polymers [4,5,11].

3.4.2 Role of temperature

Although the room temperature conductivities of highly doped electronic polymers are comparable to that of conventional metals, they generally decrease as the temperature is reduced [16-18]. They have an unusual signature which is different from conventional metals. An experimental definition of a true metal requires that the DC conductivity remain finite as temperature approaches zero (Tà0). However, when the system is near the insulator-metal boundary, the plot of conductivity versus temperature is not enough to determine if the system is in the IMT regime. For example, in many electronic polymers, including polyaniline, polyacetylene, polypyrrole and poly(p- pheneylene vinylene) that have shown IMT over a wide temperature range, more analysis on the s(T) versus T has been done [6,19]. For all of these electronic polymers, insulating, critical and metallic regimes have been identified using Zabrodski plots of the reduced activation energy (W) against temperature (T) [20]. 40 W = -T{d ln r(T) dT} = - d(ln r) d(lnT) = d (lns ) d(lnT) (3.14)

From the plot of W versus T, one can better evaluate the transport properties of the electronic polymers as follows

I) In metallic regime, resistivity (r) is finite as Tà0

II) In critical regime, resisitivity is not activated, however it follows a power law s(T) = aT-b.

III) In insulating regime, the system is activated and follows a Mott variable range hopping mode

1/(n+1) (VRH): s=s0exp(-T0/T) , where n is the dimensionality of the charge transfer [21].

For the metallic regime, the slope of W(T) plot is positive and there is a finite conductivity at zero-temperature. Therefore s (T ) can be expressed in the form

s (T ) = s O + f (T) (3.15) where f (T ) is determined by the interaction and localization contributions to the conductivity [21]. In the critical regime, W is temperature independent for a wide range of temperature and the slope of W(T) plot is zero. In the insulating regime, the slope of

W(T) plot is negative and the resistivity follows the variable range hopping model for

x which ln r a (To /T ) , the reduced activation energy W is

W[º d lns dc (T )/ d lnT ] (3.16)

in the insulating regime, W can be written as

log10W (T ) =A - x log10 T (3.17)

41 where A = x log10 T0 + log10 x . From plot of log10 W against log10 T in equation (3.17), the slope x could be determined, and from x, the dimensionality of the sample could be obtained.

It has been observed that even though the room temperature conductivity of some

2 2 polymers exceeds the Mott minimum metallic conductivity (s min = 0.026e / ha)~10 S/cm in three dimensions, where a is the interatomic spacing, the negative temperature coefficient W implies that the polymer is in the insulating regime. Although there has

been debate about the existence of Mott’s minimum conductivity [25,87], s min still provides a good indicator to the closeness of a systems to the metal insulator transition.

The product of the Fermi wavevector (kF) and the mean free path ( l ) is another indicator

of the measure of disorder in a material. When k Fl ~ 1,s ~ s min ,when k Fl << 1 all

states at Fermi energy are localized, for k Fl >> 1 the system is in the metallic regime.

Crossover from insulating regime to metallic regime has been seen in many conducting polymers (such as polyaniline, polyacetylene, and polyppyrole) when a magnetic field is applied [21,32]. Plots of conductivity against temperature and activation energy W against temperature T with field and without field, is given in Figure 11.

Recent charge transport of nanostructure polyaniline is characterized by a sharp maximum and >200 % increase in the DC conductivity from RT to the maximum of the conductivity ({See Figure 11 d})

42 b

c

T( K)

Figure 11: a) s DC (T ) for PPy-PF6 and PAN-CSA from RT to 20mk for selected samples(from, [32,19]), (b) Log-log plot of W= dln(s ( T )) / d ln T vs temperature for PAN-CSA in the metallic, critical and insulating regimes[116] and T-dependent conductivity of polyaniline nanostructure[103]

43 3.4.3 Role of structural order

Unlike the band transport in ordered or crystalline materials, transport in disordered systems is by hopping among localized states. The temperature-dependent behavior in disordered metals is by thermal activation, which is not typical of band transport. The conductivity of highly doped conducting polymers increases as temperature is increased, and can follow either an activation type or a weak logarithmic behavior [11,17]. Therefore, highly doped conjugated polymers are considered disordered metals. The temperature dependency of conductivity in doped polyacetylene, polypyrrole and polyaniline is characteristic of disordered metals close to the boundary of an insulator-metal transition [19,22]. The transport properties of conducting polymers are dominated by disorder for which wave functions of the charge carriers may become localized.

In a conventional disordered material, IMT has been explained based on the

Anderson disordered-driven localization-delocalization transition [23-25]. However, the

Anderson model is based on homogenous disorder and does not fulfill the dielectric response behavior of highly doped conducting polymers or metallic polymers. It is also noted that the samples (highly doped polyaniline and polypyrole) with the highest room temperature conductivity show insulating behavior, while a less conductive sample has a metallic-like positive W slope at low T. This cannot be explained with the homogeneous disorder model. Recently, a new model was introduced called the resonance quantum tunneling model for the metallic inhomogenous polymer systems which agrees with the frequency dependent experimental results of polyaniline and polyppyrole. [26].

44 3.5 Anderson Localization

This model describes localization that occurs in a homogenous disordered material for which the material is electrically the same in all direction (isotropic).

Assuming a perfect crystal having a periodic potential, the electron wave functions form

Bloch waves delocalized through out the solid [27].

Anderson showed that the wave function in a random potential may be changed if the randomness is very large. Anderson demonstrated that electronic wave functions may become localized if the random component of disorder potential (W) is very strong compared to the electronic bandwidth (B). (See Figure 12). The localized wave function has the form of

y (r) µ exp(-r /x )Re(y 0 ) (3.18) where ξ is the localization length of the state. Mott later demonstrated that states at the center of the band are delocalized, while states at the band tail are more easily localized, since these states are formed from localized orbitals [24]. The critical energy at which states changes from localized to extended is called the mobility edge (Ec) [24]. Ec is a transition between a metal and an insulator. The mobility edge (Ec) separates localized states in the band tail from extended states in the center of the band (see Figure 12c). The electronic properties of materials then depend on the position of the Fermi level (EF ) relative to the Ec. If EF lies in the range of localized states due to strong potential disorder, the material is nonmetallic even though there is a finite density of states at the

Fermi level. However, if EF lies in the range of extended states, it can have a finite value of σdc (T) as Tà0 and the material shows metallic behavior.

45

Figure 12: (a) The Anderson transition and (b) the form of wave function in an Anderson metal-insulator transition. (c) The Fermi gas state where the Fermi level lies in the region of localized states (from, [38]).

46 3.6 Mott Variable Range Hopping Model (VRH)

Electrons conduct electricity in disordered systems via phonon-activated hopping.

Electrons hops from occupied states to nearby empty states. Hopping from one site (n) below the Fermi level to site (m) above the Fermi level is determined by two factors, the

DE -2aR Boltzmann factor, exp[ - ] , and the Tunneling factor, e . R here is the hopping k BT

-1 distance, α = x is the localization length and DE= Em-En, is the energy difference for an electron to overcome, neglecting the electron-electron interaction. The Boltzmann factor will choose a site with the smallest possible energy. Therefore, an optimum hop for an electron among sites leads to a competition between R and DE, given as

DE Pmn µ exp(-2aR - ) (3.19) k BT

As discussed earlier, the Mott variable range model is used in systems with strong disorder (for which the disorder potential energy is greater than the energy bandwidth)

[24]. When electron correlation is neglected, the temperature-dependent conductivity of

Mott’s model [28] has the form

é 1 ù d +1 ê æ T0 ö ú s = s 0 exp - ç ÷ (3.20) ê è T ø ú ë û where s0 is a temperature independent or weakly temperature dependent prefactor. T0 is the effective energy barrier between localized states and is also the measure of the degree

47 of disorder in the disordered region and d is the dimensionality. The localization (ξ) length is connected to T0 via equation 3.21.

c T0 = d (3.21) k B N(EF )x where kB the Boltzmann constant, c is the proportionality constant and N(EF) density of states at the Fermi level. When the dimension is known, the plot of ln(s) versus T 1/ d +1 gives information about the nature of the crystallinity and coherence length. As the coherence length increases, To, the effective energy separation between localized states, decreases. Figure 13, shows x-ray scattering experiment data and their corresponding To data extracted from the plot of ln(s) versus T 1/ 2 . Efros and Shklovski showed that by including Coulomb interaction between localized electrons and holes, there is a change in the hopping transport, especially at low temperatures, so the temperature dependence of conductivity will be

é 1 ù ' 2 æ T 0 ö ê ç ÷ ú s = s 0 exp - ç ÷ (3.22) ê è T ø ú ëê ûú

' 2 T 0 = e eL where e is the electron charge, ande is the dielectric constant[86].

48 a)

b)

c)

d)

Figure 13: a) X-ray from differacting intensity vs 2q microdensitometer reading from samples A-E scattering of HCl doped XPAN-ES and PAN-ES b) ln(s) vs T 1/ 2 for HCl doped XPAN-ES and PAN-ES c) Is the slope T0 of the temperature-dependent dc conductivity of the various XPAN-ES and PAN-ES samples (from,[43]).d) SEM image of conventional polyaniline[133].

49 3.7 Review of Band and Dielectric Transport

There are generally two kinds of charge transport mechanisms: band [30] and hopping [2]. Band transport follows the Drude theory for free electrons, in which the electrons are accelerated by the applied electric field and scattered by impurities and phonons. Therefore, as phonon scattering increases, the conductivity is reduced. This kind of transport occurs in metals with little disorder.

In hopping transport, phonon scattering aids electron transport. As the phonon scattering increases, conductivity is increased. At finite temperature, electrons can hop from one localized state to another by either releasing or absorbing phonons. The electronic transitions are inelastic and the motion of the electrons could be described as incoherent diffusion. This kind of transport is associated with semiconductors, amorphous semiconductors, and “dirty” metals.

Due to high disorder and localization of electrons even in single chains of polymers, hopping should be the ideal mechanism for transport. For polymers in the insulating states, with conductivity less than the Mott minimum, it does follow the hopping transport model. However, for the highly doped polymers with conductivity above the Mott minimum value, neither band nor hopping transport can completely describe their signatures. For example, in highly doped polyaniline, polypyrrole and polyacetylene the conductivity increases with increasing temperature with a finite value of conductivity at low-kelvin temperature smaller than the value at room temperature

[29,32,39,40]. This is not a typical signature of conventional band transport however; although this may be possible if one considers the effect of localization due to disorder.

Nevertheless, the frequency dependent conductivity and dielectric constant for the highly 50 doped polymers cannot be explained by the two transport mechanisms discussed earlier.

A new model was then developed called the quantum resonance tunneling to explain the unusual frequency dependent signatures seen in metallic polymers.

3.8 Resonance Quantum Tunneling

Highly doped conducting polymers (Polyaniline and Polypyrole) exhibit unusual optical constant signatures (frequency dependent dielectric and conductivity) at low frequencies, which originates from the small fraction of the total number of conducting electrons with long scattering times and small plasma frequencies [29]. Based on the unusual signatures of the highly doped conducting polymer, they can be viewed as metallic islands with delocalized electrons embedded in an amorphous media of poor chain order [26].

The mechanism of transport in these metallic polymers does not fit either the band or hopping charge transport. A charged transport mechanism was developed by Prigodin and Epstein (PE) [26,31] to explain the anomalies seen in highly doped polymers. PE showed that a network of highly conducting polymer grains connected by resonance tunneling through the strongly localized states in amorphous media is responsible for the anomalous behavior in metallic polymers [26]. PE claimed that direct tunneling between chains is exponentially suppressed because the metallic grains are always spatially separated by amorphous regions. They used the granular model [41] to explain the quantum resonance tunneling and demonstrated that there is a critical coupling between grains which allow the medium to undergo transition from insulating state to metallic state. 51

Figure 14, Schematic view on the structure of polymers. The lines represent polymer chains and the dashed squares mark the region where the polymer chains demonstrate the crystalline order [from [31].

In this granular model for polymers, the mean energy level spacing (∆E) of the metallic grain is estimated to be [33]

1 DE = , (3.22) Ν(EF )N ^ N||

where N(EF) is the density of states per unit cell and N┴ chains are densely packed over the length N|| in cell units. The level broadening, δE, can be estimated as

dE = 2N^ gDE , (3.23)

where g is the transmission coefficient [34] or dimensionless conductance (in units of

2e2/ћ) of the chain-links between the grains. The IMT in electronic disordered systems occurs as given by Thouless [35] when the level broadening δE is of the order of level spacing ∆E (see Figure 15b). Thouless’ condition reduced equation (3.23) to the critical 52 coupling terms in the form of transmission coefficient gC as 2N ^ gC =1. For PANI-

-2 CSA, gC »10 [34,36]. The value of g c relative to g determines the material property.

When g < gC , the material is a dielectric but it is metallic if g > g c . Thus, the electrical phase of a material depends on the intergrain coupling. Therefore, the system is an insulator in the limit of weak intergrain coupling.

Figure 15:, Electronic conduction between metallic grains (well-packed and well overlapping chain regions) embedded in amorphous media. (a) the localization radius of electronic states in metallic grain is of the grain size and is of scale of the polymer unit in the amorphous media; (b) intergrain charge transfer effectively is provided by tunneling through resonance states in the amorphous regions. (from, [26,31]).

53 CHAPTER 4

EXPERIMENTAL APPARATUS AND TECHNIQUES

4.1 Reagents for the preparation of Nanostructure Polyaniline

Aniline (Aldrich) was distilled under vacuum before use. Deionized water (OSU

Chemical Reagent Store), Ammonium peroxydisulfate (APS; Aldrich), and dopant acids were used directly as received without further purification. Perchloric acid (HClO4) was used as the dopant acid for work in this thesis. The result of elemental sample analysis done by Atlantic Microlab, Inc. located in Norcross Georgia gave the ratio of perchlorate to Nitrogen as 1: 3.66. The ph of the solution tested by ph paper was ~ 1.

4.2 Dilute Polymerization

The synthesis of the nanostructure polyaniline was done by Dr. Nan-Rong Chiou according to his procedure [58-59]. In the preparation of the nanostructure polyaniline,

0.1 M of aniline was dissolved in a small amount of 1M dopant acid (HClO4) solution and then was transferred to the ammonium peroxydisulfate (APS) solution also dissolved in 1M dopant acid (HClO4). The solution was mixed thoroughly at room temperature until the color of the solution changed. The solution was then kept at room temperature

54 with no disturbance to the apparatus. After 1 hour, the dark-green precipitate was collected via a Buchner funnel and then purified portion wise with deionized water. The purification with deionized water was continuous until the filtrate becomes colorless and the pH of the suspension reached ~5. Throughout the process, the molar ratio aniline to

APS was kept at 1.5.

4.3 Experimental Probes

Several probes were utilized during this study. Here is a brief review of the charge transport direct current (dc), temperature dependent dc conductivity, temperature dependent reflectance measurement, temperature dependent electron paramagnetic measurement, and X-Ray diffraction measurement data.

4.3.1 Direct Current Conductivity

Standard 4-probe measurements were used to determine the room temperature conductivity of the nanostructured network. Four gold electrodes were evaporated onto a

2 x 2 cm glass slide after the glass substrates had undergone appropriate photolithographic patterning. The basic step of photolithograph are cleaning of the substrate, applying photoresist to the substrate, softbake of the substrate, placing the substrate on top of the mask, the exposure of the substrate to UV light, and then developing of the substrate. Cleaning was done by scrubbing the substrate was a swab containing actetone and then cleaning the surface with Isopropanol. Photoresist was spincoated on to the surface of the substrate. The substrate was then softbaked at about

100 oC. Then the substrate was place on to the mask while UV light was illuminated on to 55 the mask to the substrate containing resist. Then the substrate was developed using AZ

400 K Developer obtained from AZ Electronic Material. PAN-N was drop-cast onto the substrate with an active area of 0.2 cm by 2 cm. Current was applied through the outer electrodes while the voltage drop was measured across the inner two electrodes. The distance between the two inner electrodes was ~ 2 mm, as controlled by the photolithographic patterning. A TLA-Tencor Alpha-step 500 surface profiler was used to determine the thickness of the films, a typical thickness for a film was in the range 2-5 mm. A Kiethley 2400 source meter was used to provide the input current and simultaneously to measure the voltage drop across the inner electrodes. To reduce the effect of finite contact voltage between the electrode and the active area, the current

polarity was changed (Ià -I) and the given resistance was calculated as Rdc = (V23 ) / I14 as shown in figure 16. The conductivity of the material can then be derived based on equation 4.1

1 l s dc = (4.1) Rdc tw

Where l,t and w are the sample length, thickness and width respectively. For each measurement, the sample ohmic contacts were checked by varying the input current to this see the direct proportionally characteristic of the voltage drop. The current across the two outer electrodes was then adjusted such that the dissipated power is always less than

10-7 watts, thereby reducing any heating effects that could potentially damage the sample.

56 To measure the s dc (T ) , the sample was placed in a Quantum Design Physical Properties

Measurement System (PPMS). The PPMS is equipped with a cryostat so that temperature may be varied with the range 2 K -350 K. The PPMS is computer controlled and could be programmed to run long sequences, provided that the sample is within its power limitation. Samples are wired to a puck (with 12 electrical leads) which also provides thermal contact to the sample chamber.

A

2.5 mm

1 2 3 4

V

Figure 16: Four probe configuration for measurement of room temperature and temperature dependent dc conductivity

Wiring of sample to the puck is accomplished by attaching thin copper wire to all the sample leads and then to the puck. Indium pressed was used to attach the thin copper wire to all four sample leads and the puck. The PPMS was utilized for better control of applied current. A Keithley 2400 Sourcemeter was operated via a Labview program to set

57 the environment for the PPMS sample chamber and for the acquisition of the charge transport data.

4.4 Reflectance probe

Two spectrometers were used for the measurement of reflectance on the PAN-N

-1 /HClO4 films over a broad energy from 300-50000 cm . The two spectrometers used were a Varian Cary 5000 for the UV-VIS-NIR (4000-50000 cm-1) range, equipped with both an integrating sphere and a near normal specular reflectance accessory, and a Bruker

66v/S interferometer for the MIR-FIR range (4000-250 cm-1). Each of the spectrometers was equipped with different light sources and detectors unique for each region of energy.

Data was obtained for three different energy regimes. These regions were the UV-VIS-

NIR, MIR and the FIR regime, and they were merged together to obtain the whole energy range.

PAN-N/HClO4 was drop-cast on glass substrate with thickness of the film typically ~ 10mm. Special consideration was taken regarding the thickness of films and the penetration depth of the radiation. Generally, to avoid transmittance of light, the thickness of the film has to be greater than the penetration depth of the radiation. Due to the roughness of the PAN-N surface, the effect of scattering losses was minimal and corrected by using a slightly gold coated (8 nm) PAN-N sample as the reference “mirror” during the MIR-FIR measurements. This over coating technique has been widely used to determine the specula reflectance of several materials [107-108]. One of the assumptions of this technique is that the over coating is thin enough that it does not charge the

58 nanostructure of the sample. All reflectance data were measured at near-normal incidence.

Therefore, the ratio of the sample reflectance with respect to the sample reference

“mirror” when multiplied by the known reflectance of gold yields a good approximation of the actual specular reflectance of PAN-N /HClO4 sample. Due to the limitation of the thickness of the film and the conductivity of the film, we could not obtain reflectance to very low energies (below 0.034eV). However, we used constant extrapolation to extend the data to low energies. The samples were films of PAN-N with voids and nanowires.

The thickness of the film was typically ~10 mm. The thickness of the films are compared to the penetration depth (d ), since the sample will transmit if penetration depth of the radiation is larger than the sample thickness. For a dissipative medium,

c d = (4.2) 2pmws

where c is the speed of light and s is the conductivity at frequencyw , from equation 4.2, d increases as s decreases. The samples were tested at low energies to check if they transmit light.

59 4.4.1 UV/Vis/NIR Reflectance

Varian Cary 5000 is equipped with accessories for determining both the absolute specular reflectance (using the “W configuration”) and the diffuse reflectance. The diffuse reflectance accessory consists of an integrating sphere from which specular component of reflectance can be extracted. The absolute reflectance accessory is used to obtain the ‘mirror-like’ reflectance off the surface of a “smooth” sample. The diffuse reflectance accessory however, is used to obtain reflectance light in several directions of a sample and it is widely used to obtain reflectance off a rough surface. The integrating sphere accessory is used to determine both the total and the diffuse reflectance. Specular reflectance is then determined by subtracting the diffuse reflectance component from the total reflectance.

Varian Cary 5000 is a dispersive double beam spectrophotometer. It is equipped with a deuterium lamp (190-340 nm) and a tungsten lamp (300-2500 nm) covering the

UV-VIS-NIR (55,000 cm-1 -4000 cm-1) regions. The layout of the diffuse reflectance accessory (DRA) is shown in figure 17, for which the integrating sphere acts as both sample port and reference port. The ports are positioned so that the incident beam is at an angle of 8 degrees, close to normal incidence used for normal incidence reflectance equations. The DRA integrating sphere is equipped with a photomultiplying tube (190 -

900 nm) and PbS (820 - 2500 nm) photodetectors with preamplifier. The DRA is a double-beam ratio spectrometer, which measures the reflected energy of the sample beam relative to the reflected energy of the reference beam. The first measurement recorded is

the background correction ( BC ) which is the ratio between the sample and the reference beams using pressed spectralon at both sample and reference ports 60 E Bc = S , (4.3) ER

Here ES (ER) is the energy of the sample (reference) beam. In order to determine the sample reflectance, the sample is placed in the sample port and the spectralon is placed at the reference port. The spectrometer then measures the following energy ratio

ES RS RS FR = = Bc (4.4) ER RR RR where RR (RS) is the reflectance of the material at the reference(sample) port. The final data recorded and stored by the spectrometer is

F R R = S (4.5) BC RR

The actual reflectance of the sample is then determined by multiplying the stored data by the reflectance of the spectralon (reference material)

F R = R R (4.6) S Bc R

The Spectralon reference material was calibrated at Lab Spheres (located in North Sutton,

NH) in accordance to the National Institute of Standards and Technology (NIST).

61

Figure 17. Schematic of reflectance spectroscopy accessory used for reflectance measurements on the Varian 5000 spectrometer [109].

62

4.4.2 Absolute Specular Reflectance Accessory (ASRA)

This accessory consists of a dual ‘VW’ configuration, with two toroidal mirrors

(T1 and T2) per beam and one spherical mirror (S1). S1 is an adjustable mirror that allows them to be used for both calibration and as sample measurement. Hence, the same optical components are in the light path during both calibration and measurement of the mounted sample. As measurement is taken, reflectance of light off the surface of the mounted sample is recorded. This corresponds to the absolute reflectance of the sample.

Figure 18: Schematic of the setup in the Absolute Reflectance Accessory [109 ]

63

4. 4.3 Fourier Transform Infrared (FTIR) Spectroscopy

A Fourier Transform Infrared (FTIR) spectrometer manufactured by Bruker

(Bruker IFS 66v/S) was used to determine the reflectance of the samples in the range of

200-8,000 cm-1. The basic components of an FTIR spectrometer are a light source,

Michelson interferometer and a detector. The Michelson interferometer is a device that divides a light beam into two paths via a beam splitter and it also consists of a fixed and a movable mirror. The beam of light source is partially reflected of the beamsplitter to the fixed mirror and partially transmitted through the beam splitter to the moving mirror.

The two light beams travelling in different paths reflect of the two mirrors and then recombine at the beamsplitter. At this point, both constructive and destructive interference occurs on the position of the fixed mirror in respect to the moving mirror.

The beam then passes through the sample and the reflective property of the sample then modifies the beam and it is then measured by the detector. The high throughput and the ability to signal average in a short time leading to an enhanced sign to noise ratio gives

FTIR system a great advantage over the grating systems for IR measurements.

64

Figure 19: Schematic diagram of Michelson interferometer (after [51]).

65

As shown in figure 19, an interferometer consists of two mutually perpendicular plane mirrors, one of which can move along the axis perpendicular to its plane. The moving mirror is either moving with a constant velocity or is held at equally spaced points for fixed short periods of time and rapidly stepped between these stops. Let us now describe the intensity of light at the detector using mathematical equations starting from the electric field form of the source given below.

i (q.r -wt ) E (r, t) = E 0 e (4.7)

Where q is the wave vector, r is a position vector, ω is the angular frequency, t is the time and E0 is the amplitude of the electric field. From figure 19, the light travels a distance S to the beam splitter with both transmission and reflection coefficient of tb and rb respectively. The reflected beam travels a distance L to the fixed mirror and the transmitted beam goes a variable distance of L + x/2 to a moving mirror. The reflected beam and the transmitted beam have a reflection coefficient ry and phase φy , and a transmission coefficient rx and phase φx. If we consider the superposition of electric fields of a single frequency arriving from the two mirror at the position of the sample

( assign t=0). We can describe the intensity of the light at the detector as

iqS iqL ij y iqL iq ( L + x / 2 ) ij x iq ( L + x / 2 ) iqD E D ( x ) = E O e [rb e ry e e t b + t b e rx e e rb ]e (4.8)

q = 2πn , n is the wavenumber of the light. If we assume that the reflection coefficient amplitudes for the mirrors are similar such that we have, ry ≈ rx ≈ 1 and defining φ(q) =

φy - φx and Φ = q(D + S + 2L) + φy, we can rewrite ED () x as

iF i(2pn x + j ( n )) ED( x )= E0 r b t b e [1 + e ] (4.9) 66 .

Thus, the light intensity amplitude at the detector is

c 2 S = E E * (4.10) D 8pwm D D

* SD() xº E D () x E D () x = 2 S0 (2pn )RT[1cos(2 b b + pn x + j ())] n (4.11) where Rb and Tb are reflectance and transmittance of the beam splitter and

* S0 (x) = E0 (x)E 0 (x) is the intensity of the source at frequency w before entering the

Michelson interferometers. An expression that characterize the total intensity at the detector which include all frequencies at once is given as

¥1 ¥ I() xº S (,) xn d n = S ()()[1cos(2 n e n + pn x + j ())] n d n (4.12) Dò0 D2 ò 0 0 b

where eb( n )º 4RT b b , is the efficiency of a beam splitter. A scan of the total intensity as a function of the path length is given as the interferogram and the “interferogram” is given as

1 ¥ g()xº I () x - I () ¥ = S ()cos(2 n pn x + j ()) n d n (4.13) DD 2 ò0

where SS()()()nº D n e b n and, γ(x) is the cosine Fourier transform of S (n ), and

1 ¥ I()()()¥ = Sn e n d n (4.14) D2 ò0 0 b and

¥ I(0)= S (n ) e ( n ) d n = 2 I ( ¥ ) (4.15) Dò0 0 b D

Using exponential Fourier transforms as we can rewrite γ(x) as

67 1 ¥ g()())x= S n ei(2pn x+ j ( n ) d n (4.16) 4 ò-¥

S()n contains the spectral information about the intensity of the light emitted at each wavenumber from the source. The specular reflected light from a sample emerging from the FTIR spectrometer would be

SSRs()()n= n s (4.17) where Rs is the reflectance of the sample. The first reflectance is measured relative to the standard reference mirror with the light reflected off the standard mirror. The second reflectance is made of the sample. If we assume that S()n is constant for both measurements, where Rr is the reflectance of the reference, the final reflectance data would be

SSRR()()n n R =s = s = s (4.18) SSRr()()n n r Rr

4.4.4 Bruker IFS 66v/S

Bruker IFS 66v/S was used for all the low energy measurements. It is equipped with a Michelson interferometer and can be evacuated down to a pressure of 2 mbar.

Measurement can be made from room temperature down to 20 Kelvin. The APD cryogenics Heli-Tran LT-3-110 continuous liquid He flow cryostat is used and the temperature is controlled by Oxford ITC 503 temperature controller. The spectrometer is

68 controlled via a personal computer and is equipped with a home-made modified reflectance accessory with an 11o incident angle. Reflectance measurement could be made in the range of 15 cm-1 -10,000 cm-1. Figure 20 shows the schematic diagram of the spectrometer.

Range(cm-1) Beamsplitter Source Detector Window 150 - 650 3.5µ Mylar Hg lamp Si Bolometer Polyethylene 40 - 650 6µ Mylar + Ge Hg lamp Si Bolometer Polyethylene 20 – 100 50µ Mylar Hg lamp Si Bolometer Polyethylene 600 - 8000 KBr Glowbar DTGS KBr

Table 4.1. Configurations of optical components for different frequency range measurements on Bruker IFS 66v/S FTIR spectrometer[84].

Figure 20: Schematic diagram of Bruker IFS 66v/S FTIR spectrometer. A) light sources (two sources can be mounted at the same time). B) aperture, C) beam splitter, D) fixed mirror, E) moving mirror, F) sample/reference position, G) Si photoconductive bolometer, H) MCT (Mercury Cadmium Telluride) photoconductive detector, I) DTGS (Deuterated TriGlycine Sulfate) thermal detector. It should be noted the drawing is not to the exact scale (after [52]).

69 4.4.5 Kramers-Kronig Calculations

Optical constants are determined from a Kramers-Kronig analysis of reflectance data. The KK analysis requires that integration to be performed form 0 to ¥ . However, data acquired are limited to the lowest as well as the highest measured wavenumbers. To make up for this limitation, reasonable extrapolations at low and high frequencies of the reflectance data are made. Reflectance of material with high conductivity due to free electrons are represented at low frequencies (wt << 1) by the Hagen-Rubens (H-R) relations[45 ]

2w . R(w) = 1- (4.19) ps

(2w) This equation requires that <<1 , which means that for a sample with ps conductivitys ~ 100 S / cm , the H-R relation is reasonable for w << 750cm -1 , while for samples with s ~ 10 S / cm , the H-R relations is reasonable for w << 75cm -1 [85]. RT conductivity of PAN-N/HClO4 is ~ 1 S/cm and reflectance was determined up to

300 cm -1. RT conductivity and the upper limit of measured reflectance are too small for

H-R to be used appropriately. Therefore, a constant reflectance low frequency approximation was used for the low frequencies extrapolation. However, the result of

H-R and constant approximations were determined and compared in Figure 21. The data showed very little change in their calculated optical constants at low energies. However, the calculated optical conductivity from the constant extrapolation reflectance data at

-1 10 cm is very close to the measured DC conductivity of PAN-N/HClO4 film.

70 The high frequency approximation is used where both bound and free electrons

response to electric field just like free electrons. At very high frequencies ( w > w free ), the reflectance is extrapolated to infinite frequencies and the expression for the reflectance of free electrons is given as

w R(w) = R ( free ) 4 (4.20) free w

Where Rfree is the reflectance of the sample at the frequencies w free . In this thesis, the range of reflectance measured did not cover all the interband transitions. The highest

measured frequency corresponds towlast . The frequency range between wlast and w free in which there are contribution from interband transition to the reflectance is account for by extrapolating reflectance as

w R(w) = R ( last ) s (4.21) last w

Where Rlast is the reflectance of the highest measure frequency (wlast ). For this study

-1 w free = 100,000cm and s=2 were used for data analysis. These parameters gave similar results for the calculated optical absorption and the measured optical absorption.

71

(a) Constant Hagens-Rubens 100

10 ) w (

s

1

10 100 1000 Wavenumber(cm-1)

35 (b) Constant Hagens-Rubens 30

25

20 ) w

( 15 e

10

5

0

-5 10 100 1000 10000 Wavenumber(cm-1)

Figure 21: Comparison of (a) s (w) and (b) e (w) for PAN-N/HClO4 using different high frequency extrapolations.

72

4.5 X-ray Diffraction

Structural XRD experiments were performed using a conventional X-ray scanning diffractometer (Rigaku X-ray). The X-ray radiation source was Cu-Ka radiation

0 l = 1.542 A and the scattered radiation diffractograms were collected over the range

2q » 4 - 60O .

Structural studies were performed using the Rigaku Geigerflex X-ray

0 diffractometer equipped with a Cu tube (a line with l = 1.5406A ) as the source and a scintillator was used as a detector. The diffractogram was recorded as a function of the number of counts at the scintillator versus the scattering angle 2θ. The PAN-N/HClO4 solution sample were dropped cast on a glass substrate. During the measurements, care was taken such that only the polymer was exposed to the X-ray radiation. The basic equation of X-ray-diffractometry is the Bragg formula for d-spacing

l= 2d sin q , (4.22) where λ is the x-ray wavelength, or alternatively

4p sin q q = , (4.23) l where q is the scattering vector, as shown in figure 22. From the X-ray diffractogram, the crystalline domain size or the structural coherence length ξ could be estimated using

Debye-Scherrer's formula [53],

0.9l x = , (4.24) D(2q )cos q

73 where D(2q ) is a full width at half maximum of diffraction peak. The percentage crystallinity can be estimated as the area of diffractogram under the crystalline peaks divided by the total area of the diffractogram and is given as

S %(crystallity)= cryst 100% (4.25) Stotal

Figure 22. Schematic view of a typical X-ray Diffraction experiment. ki is the incident wave vector, kf is the scattered wave vector and q is the scattering vector as q = kf – ki (after [53]).

74 4.6 Electron Paramagnetic Resonance (EPR)

The fundamental principle of electron paramagnetic (or spin) resonance is based on the fact that electrons display fine structure in the presence of static magnetic field.

This results into the splitting of the energy levels (Zeeman splitting) of the degenerate electrons having spin up (ms=1/2) and spin down (ms=-1/2). The Hamiltonian describing a material in the presence of a magnetic field under an undamped condition is given as

H = -µ · H = ge m B H 0 S Z (4.26)

Where g is the g-factor (2.0023 for a free electron), µB is the Bohr magneton

eh -21 (= = 9.274096´10 erg/Gauss), H0 is the magnetic field (in Gauss) at resonance 2mec and SZ is z component of the spin quantum number. When the energy of the microwave

radiation is the same as the energy of the splitting of the ± ms = 1/ 2 energy levels in the static field there will be a resonance which is governed by equation (4.26)

DE = hn = hw = g e m B H 0 (4.27)

here DE is the energy difference between the spin up and spin down energy levels of the electrons, h is Planck’s constant, ν is the frequency of the incident microwave energy.

Following the above equation, one can experimentally vary either the magnetic field or the microwave frequency. However, in our EPR measurements we always sweep the field instead of the microwave due to the difficulty in getting the cavity to resonate at variable 75 microwave frequencies. As the field is swept, it crosses a resonance regime and the microwave signal (reflected or unabsorbed) from the cavity is compared with the incident microwaves. The raw data is first derivative of absorption. By analyzing the raw data we derive the spectrum intensity, lineshape, linewidth, the magnetic paramagnetic susceptibility and get further insight to the spin motion with our sample. By analyzing the raw data we derive the spectrum intensity, lineshape, linewidth, the magnetic paramagnetic susceptibility and get further insight to the spin motion with our sample.

Figure 23: The change of the microwave resonance with and without the sample in the resonant cavity [38].

Measurements were carried out using a Bruker ELEXSYS E500 spectrometer. The sample was evacuated at a pressure of ~ 10 5 torr and the samples were sealed inside a quartz tube. During an experiment, the sample is first placed inside the cavity. The 76 turning of the microwave source is performed until an on resonance and correct phase is achieved. At critical coupling, the cavity absorbs the energy incident upon it rather than reflecting it. The samples are position along the axis of the cavity to minimize loss which could affect the Q of the microwave cavity. The sensitivity is directly proportional to the value of the Q. The Q factor is defined as

2p (energy stored) n Q = = res (4.28) energy dissipated per cycle Dn

Where the energy dissipated per cycle is the amount of energy lost during one

microwave period, n res is the resonant frequency of the cavity and Dn is the width at the half height of the resonance.

4.6.1. Cryogenics

The Bruker ELEXSYS E500 spectrometer was equipped with an ESR 900 Cryostat and a GFS(Gas Flow Shielded) 650 Trasnfer tube for automated regulation of He flow using a step motor for the needle valve in the transfer line [55,56]. Temperature was regulated by the ITC (Intelligent Temperature Controller) 503 using PID optimization

[57]. The ESR cryostat operates using a continuous flow of helium gas over the sample- containing quartz tube place within it. Liquid helium is drawn through the entire apparatus by suction provided by the GF3 pump and the flow is controlled with a needle valve located in the GFS650 transfer tube. Helium from the GFS650 goes in the ESR900 cryostat and is warmed by a copper (Cu) heater to the required temperature, then flows

77 over to where EPR quartz tube is housed. By proper regulation of the whole apparatus, well stabilized temperature down to 5 K can be achieved by continuous flow of helium gas.

4.6.2. Magnetic Susceptibility

Magnetic susceptibility relates the degree of magnetization (M) of a material in response to an applied magnetic field (H). A linear relationship between M and H occurs in a host of materials. For an isotropic material, the relation is given as

M = cH, (4.29) where c is called the magnetic susceptibility. Negative c correspond to a class of material called diamagnetic while a positive c is for paramagnetic materials. The fundamental models have been used to explain paramagnetic behaviors in materials are the Curie and Pauli spin susceptibility. Curie susceptibility originates from the localized charges, which can be described by Boltzman statistics and the spin of which follow the

Curie law [30]

2 2 N A ge m B S(S +1) c C = , (4.30) 3k BT

23 where cC is the Curie susceptibility, NA is Avogadro’s number (6.02 X10 particles ), T

-16 is the temperature in Kelvin , kB is the Boltzmann constant (1.38 X 10 erg/K), and S is the value of spin quantum number. Curie paramagnetic susceptibility is temperature dependent as shown in equation 4.30.

78 On the other hand, Pauli susceptibility originates from the delocalized carriers which obey Fermi-Dirac statistics as shown in the Pauli law [27]:

c P N(EF ) = 2 , (4.31) 2m B where cp is Pauli paramagnetic susceptibility and N(EF) is the density of states. Pauli susceptibility is temperature independent.

Generally, charge carriers in a material can exhibit both Curie and Pauli components.

In this case, the obtained susceptibility can be written as

c = c C + c P (4.32)

C => = + c (4.33) T P

=> cT = C + c P , (4.34) where C is the Curie constant. From the plot of measured susceptibility vs. 1/T, both

Curie and Pauli components can be extracted.

79

Figure 24: Block diagram of a microwave bridge of an EPR spectrometer (from Bruker

Instruments, Inc).

Hall probe Iris Screw

Figure 25: The general layout of an EPR spectrometer (From Bruker Instruments, Inc).

80

4.7 Mechanical probe

4.7.1 Pressure Cell Design

In order to probe the gradual change in the nature of charge carriers in

PAN-N/HClO4 due to applied pressure, we designed a pressure cell. The cell was fabricated by the physics machine shop in accordance to the design. The pressure cell was capable of isolating all of the weight placed upon it to the active area on our glass substrates. The cell consists of a lower platform fitted with four rods and an upper platform to carry weight. The lower platform has a slot carved in it so that our substrates are held tightly in place. There is also a central groove carved into the lower platform so that the active area on the substrate can be placed in such a way so that the upper platform can make contact with it. The upper platform is fitted with an acrylic surface that makes contact with the drop-cast polyaniline. We chose acrylic because it is an insulator, it is inert with respect to the HClO4-doped polyaniline, and it is hard. The upper platform also has four holes drilled in it so that it fits snugly over the four rods of the lower platform. This design ensures that the acrylic surface makes even contact with surface of the PAN-N/HClO4. Figure 26, shows the image of the fabricated pressure cell.

To apply pressure to the PAN-N/HClO4, we gradually added weight to the sample in increments of one-pound via the pressure cell. We estimated the applied pressure as the ratio of the applied force divided by the area of our sample.

81

Figure 26: Upper: The pressure cell with the 4-probe configuration showing the gold electrode and the drop cast film in 2cm by 2cm rectangular glass substrate. Lower: a) Upper and Lower platforms, b) Acrylic contact and c) Contact close- up

82 CHAPTER 5

EXPERIMENTAL RESULTS

5.1 Morphology of Polyaniline Nanostructured Network

Except otherwise stated, Perchloric acid (HClO4) was used as the dopant acid for the nanostructure polyaniline (PAN-N) discussed in this dissertation. Therefore, PAN-N actually implies PAN-N/HClO4. Figure 27 shows the Scanning Electron Microscopy

(SEM) of nanostructured polyaniline (PAN-N) and pressed polyaniline nanostructure (P-

PAN-N). The morphology of PAN-N shows highly interconnected nanofibers, but P-

PAN-N shows a more compact morphology. At high temperatures (>230 K), their T- dependent DC conductivity transport signatures are different.

5.2. DC Conductivity: Proximity to “Quasi IMT”

Earlier studies of conducting polymers focused on the signature of the DC and microwaves transport measurements as the condition for a MI transition [17,106].

However, the condition for MI transition is not limited to DC conductivity, since other

signatures such as the sign of the slope of W[º d lns dc (T )/ d lnT] vs T [20,29,43,102],

83 thermoelectric power [43], and the signature of the dielectric function (e (w)) obtained from the Kramer-Kronig analysis of optical reflectance are necessary to have a complete picture of the MI transition [16,19,29,32,102]. In conventional highly conducting polyaniline free standing films the signature of the temperature dependent conductivity at high temperature looks metallic because the electrons are scattered by phonons. The temperature dependent DC conductivity of PAN-N prepared by dilute polymerization is characterized by a sharp maximum at ~ 235 K (Tmax). This is consistent with the sharp maximum at ~230 K seen in nanostructured polyaniline doped with HCl prepared by interfacial polymerization [103]. However, this is a different signature from that of conventional polyaniline films and pellets, which have a broad maximum in their

T-dependent conductivity, signifying [29,43,104] the crossover from metallic to insulating behavior. The change in the conductivity of the highly conducting conventional polyaniline from Tmax to room temperature (RT), is typically < 15%

[29,43,104] with RT conductivity greater or close to the Mott minimum metallic conductivity (MMMC) ( 0.03e 2 / ha ~100 S/cm) [24,64,105]. Conventional polyaniline samples with RT conductivity below the MMMC show dielectric behavior with conductivity decreasing monotonically with decreasing temperature below RT. The RT

DC conductivitys dc (RT ) of the nanostructured (PAN-N/HClO4) ~ 1 S/cm, which is very small compared to the Mott minimum metallic conductivity of ~ 100 S/cm. Therefore, we expect the charge transport of the nanostructure to be dominated by phonon activation

84 with very little or no metallic contribution. In fact, the charge transport of polyaniline

nanostructured films shows a large “metallic” contribution (the change in s dc (T ) from

RT to the peak of the maximum conductivity is > 200 %) and an unusual sharp MI transition peak. However, by applying pressure of ~ 27 MPa using a pellet press to the nanostructure, the “nano-morphology” was modified, while the RT DC conductivity was enhanced. The signature of the charge transport in the pressed polyaniline nanostructure

(P-PAN-N) resembled that of conventional doped polyaniline films, having a broad MI

peak and moderate change in conductivity from s dc (RT ) to the peak conductivity (see

Figure 28). P-PAN-N also shows the absence of this usual large metal-like behavior seen in the PAN-N. Figure 28 shows the temperature dependent DC conductivity for the

PAN-N film and P-PAN-N. For the PAN-N film, s increases as the temperature is decreased from RT until ~ 235 K. Below that, there was a decrease in s as the temperature was reduced to ~ 15 K, and the reduced activation energy

W º d lns dc (T )/ d lnT vs T was plotted (see Figure. 29). The increase of W with decreasing T implies that the system was in the insulating regime, which further suggests that the metallic signature seen in the nanostructure is not due to scattering by phonons.

The charge transport dynamics for the PAN-N film and P-PAN-N both follow a quasi 1-

T D variable range-hopping (VRH) model [43,106], wheres µ exp[(- o )1/ 2 ], and T can dc T o be interpreted as an effective energy barrier between localized states, determined by the degree of disorder. The onset of the 1-D VRH for the PAN-N film is just below ~235 K, while for the P-PAN-N, the model holds for temperature below 100 K. From the fit of ln

-1/2 3 sdc vs T to a 1-D VRH model (See Figure 30) , we estimate To as ~ 2.5 x 10 K for 85 the PAN-N film and ~ 1.39 x 10 3 K for the P-PAN-N film. These values suggest that the

PAN-N film has a greater degree of disorder in the disordered regions compared to the P-

PAN-N/HClO4. Normalized conductivity of P-PAN-N, PAN-N, and pellets of conventional polyaniline/HClO4 films are given in Figure 31. The signature of conventional polyaniline is dominated by hopping at all temperature, the signature of the

P-PAN-N film is affected by scattering by phonons because of disorder at high temperature and at low temperature (<250 K) charge transport is dominated by hopping.

The signature of PAN-N film is dominated by hopping below ~235 K. In order to understand the origin of the unusual transition in the PAN-N T-dependent DC conductivity, other extensive intrinsic bulk electronic properties studies of PAN-N were made. From equation 3.21 and magnetic data given in this chapter estimate of localization lengthy and the density of states for PAN-N were determined. The parameter determined form from transport data and magnetic data is given in Table 2.

Parameter PAN-N-HClO4 P-PANN-HClO4 PANI –HCl sample E[43]

s (RT) (S/cm) ~0.4 ~2 ~10

3 3 3 To (K) 2.5 x 10 1.39 x 10 3.8 x 10

-6 -5 X p ** 1.67 x 10 Not measured 1.2 x 10

Table 2: Parameter calculated from quasi 1-D VRH model and magnetic data

** Unit is in emu/mole * 2rings , » emu/mole. 2rings

86

A

B

Figure. 27: (a) The scanning electron micrograph (SEM) of polyaniline nanostructures obtained in 1M HClO4 (aq) dilute polymerization, (b) The corresponding SEM image after the application of ~27 MPa pressure to the nanostructure.

87

PAN-N P-PAN-N 2

(S/cm) 1 dc s

0 0 50 100 150 200 250 300 Temperature(K)

PAN-N P-PAN-N

1 (S/cm) dc s

150 200 250 300 Temperature(K)

Figure 28) A) s dc ()T for nanostructured polyaniline and pressed nanostructured polyaniline as a function of temperature. B) Note the sharp and broad peaks of nanostructured polyaniline film and pressed nanostructure respectively. The lower insert is the normalized conductivitys dc (T )/s dc (300K) vs T for the nanostructured polyaniline film. 88

10 PAN-N

1

(T)/dlnT s W=dln

0.1 10 100 T(K)

Figure 29: The reduced activation energies (W) as a function of temperature for a nanostructured film.

89

PAN-N 1 P-PAN-N

0.1

(S/cm) s

0.01

0.05 0.10 0.15 T-1/2(K-1/2)

1 PAN-N P-PAN-N

0.1

(S/cm) s

0.01

0.10 0.15 T-1/2(K-1/2)

-1/2 Figure 30: a) ln sdc vs T of dc conductivity of HClO4 doped nanostructure polyaniline, the filled square is the for nanostructure film while the filled circle is for the nanostructure pressed film (pellet). b) shows the low temperature range where the samples obey the quasi 1-D VRH model

90

1

PAN-N

(300 K) (300 0.1 P-PAN-N dc s Pan-Pellet (T)/ dc s 0.01

0 50 100 150 200 250 300 Temperature(K)

Figure 31: The normalized conductivity s dc (T)/s dc (300K) vs T for polyaniline nanostructured film, pressed polyaniline nanostructured and conventional polyaniline pellet.

91 5.3 Reflectance

In order to explore the role of charge dynamics, PAN-N was probed using optical reflectance in the range of 300 cm-1 – 50,000 cm-1. Due to the thickness of the film (~10 mm), the material becomes transparent at lower energies, where the penetration depth is greater than the thickness of the sample, so accurate reflectance cannot be obtained. The reflectance was extrapolated as a constant at energies below 300 cm-1 as discussed in section 4.5. To reduce the effect of scattering losses due to roughness of the PAN-N, the reflectance was determined using the gold surface coating technique [107-108]. The reflectance was first measured, then the sample was coated with a thin layer of gold

(~100 Å) the reflectance was measured again, and the first reflectance was normalized with the reflectance after the evaporation of gold. At 300 cm-1, the reflectance is ~ 0.5, at higher energies the wavenumber dependence is weaker up to 10,000 cm-1. The oscillations in the mid IR (500-2,500 cm-1) are due to phonons, and there is no significant change in the T-dependence at mid IR. However, at Far-IR (below 600 cm-1) the reflectance starts to deviate slightly. Figure 31 shows the RT reflectance of PAN-N with the constant extrapolation at low energy. The low T-reflectance probe could not be used due to the lack of an integrating sphere equipped with a cryostat in the UV/Vis/NIR range.

Therefore, the room temperature reflectance data was used for extrapolation to low temperature and for the Kramers-Kronig calculation. The reflectance of metals typical approaches 1 at low energies and remains high up to the plasma frequency of the material. Similarly, for highly conducting conventional polyaniline, the reflectance demonstrates Drude-like behavior approaching 1 at low energies. The reflectance of

PAN-N resembles that of extensively disordered conventional polyaniline [85]. The low 92 absolute reflectance at low energies suggests that charge carriers are localized within the nanostructured network. Figure 32 shows the T-dependent reflectance for PAN-N. There is no indication of a plasma edge at high energy for the spectrum, suggesting that the charge carriers are localized within the PAN-N network.

0.8 300 K PAN-N/HClO 4 0.7

0.6

0.5

0.4

0.3 Reflectance

0.2

0.1

0.0 100 1000 10000 Wavenumber(cm-1)

Figure 32 a) The RT reflectance of PAN-N. Below the red arrow (300 cm -1), low frequency extrapolation was used.

93

0.6 0.6 300 K 250 K 200 K 0.5 0.5 100 K 20 K 0.4 0.4 0.3

Reflectance 0.2 0.3 Reflectance 0.1 0.2 0.0

1000 10000 0.1 Wavenumber(cm-1)

0.0 10000 20000 30000 40000 50000 Wavenumber(cm-1)

Figure 32 b) Reflectance as a function of wavenumbers for PAN-N/HClO4 film. The insert is the reflectance ploted in semilog scales to reveal the mid-IR vibrations

94

0.54 300 K 250 K 200 K 0.52 100 K 20 K 0.50

0.48

Reflectance 0.46

0.44

0.42

0.40 300 350 400 450 500 550 600 650 Wavenumber(cm-1)

Figure 33: Temperature dependence of infrared reflectance for PAN-N/HClO4 films.

95

5.31 Optical Conductivity

The optical conductivity (s(w)) was obtained through a Kramers-Kronig analysis of the reflectance data. Figure 34 shows the temperature dependent s(w) for the PAN-N film. The optical conductivity of the PAN-N increases with decreasing wavenumbers from ~11,000 cm-1, showing a signature expected of free electrons, until it reaches a

-1 maximum optical conductivity around ~1150 cm in the mid IR. At low energies below the maximum s(w), there is a deviation of s(w) from Drude conductivity. s(w) at low energies scales like the measured RT DC conductivity of PAN-N. The s(w) oscillations in the mid-IR range correspond to the vibrational modes in polyaniline.

The signature of PAN-N is similar to that of highly disordered polyaniline films on the insulating side of the MI transition [85]. The temperature dependent far-IR s(w) for

PAN-N follows a similar trend as the RT data. s(w) first increases, then is suppressed at lower temperatures. However, this does not follow the trend seen in the T-dependent DC conductivity, having a significant increase in conductivity as temperature is lowered from

RT to ~235 K. Here, the changes in the optical conductivities at different temperatures are small.

96

150 300 K 250 K 200 K 100 K 100 20 K )(S/cm) w ( s 50

0 10 100 1000 Wavenumber(cm-1)

Figure 34: T-dependent optical conductivity as a function of wavenumber for PAN-N films

97

150 (a) 300 K 250 K 200 K 100 K 100 20 K )(S/cm) w ( s 50

0 10 100 1000 10000 Wavenumber(cm-1)

10 (b)

300 K 8 250 K 200 K 6 100 K 20 K )(s/cm)

w (

s 4

2

0 10 100 Wavenumber(cm-1)

Figure 35: Optical conductivity as a function of wavenumber for (a) PAN-N film showing the higher energies, (b) showing the extrapolation to small wavenumbers. 98 5.3.2. Dielectric Constant

The dielectric constant determined via Kramers-Kronig analysis gives information about the nature of charge carriers with in the nanostructured network. The charge carriers are either localized or delocalized within the network. The signature of the dielectric constant does not cross zero, meaning that the polarization and the electric field is in phase. This also suggests that most of the conducting electrons are localized. Apart

-1 from the weak dispersion of localized electrons at ~ 1200 cm , charge carriers in PAN-N network are localized.

40 0.01 0.1 1 35 30 25 300 K Energy(eV)

) 20 250 K w

( 200 K e 15 100 K 10 20 K 5 0

10 100 1000 10000 Wavenumber(cm-1)

Figure 36: The temperature dependent dielectric constant of PAN-N/HClO4. The insert is the dielectric constant for less conducting samples [29].

99 This also suggests that the crossover from metal to insulator behavior seen in the T- dependent DC conductivity for PAN-N is not due to delocalization of charge carriers.

The signature of the dielectric constant suggests that localized carriers dominate the charge dynamics. The absolute value of the dielectric constant for PAN-N/HClO4 is similar to values report previously for less conducting polyaniline doped with camphor sulfluric acid (PAN/CSA) [29].

5.4 Electron Paramagnetic Resonance

The spin susceptibility provides further insight into the nature of the charge carriers and the effect of disorder. The magnetic susceptibility of the PAN-N films have been estimated from EPR-integrated intensities calibrated against a 2,2-Diphenyl-1-

Picrylhydrazyl (DPPH) standard, and is plotted in Figure 37 as magnetic susceptibility(c) vs T and in Figure 38 as cT vs T. The magnetic susceptibility of PAN-N at high temperature (>150 K) is temperature-independent and attributed to delocalized Pauli spins. At lower temperatures, especially below 50 K, there is a dramatic increase in the magnetic susceptibility attributed to localized Curie spins. The total magnetic susceptibility is composed of both Pauli and Curie component, and the fit to cT (red solid line in figure 38) is over the temperature range for which the data is linear (30 K- 300 K).

The solid line is the fit to cT = c PT + C where c P is the temperature independent Pauli susceptibility and C is the Curie constant. The Pauli and Curie susceptibilities are

-5 -1 -3 -1 estimated as cp= 1.67 x 10 emu mol ·2rings and C=1.75 x 10 emu K mol .2rings

-1 The positive slope in Figure 38 indicates a finite N(EF ) » 0.61 states (eV per 2 rings) ,

100 and the density of curie spins (C) is 1 spin per 200 2-rings repeat units. The magnetic

susceptibility shows that the network has a weak RT Curie susceptibility (cC ) (~5.94 x

10 -6 emu/mol 2 rings). The value of the density of Curie spins and the Pauli components are small compared to conventional polyaniline doped with HCl [43]. This suggests that a significant fraction of the spins are paired in bipolarons, since unpaired spins are strongly localized.

The EPR linewidths are generally determined by spin-spin interactions, narrowing mechanics (spin diffusion, and rotation) and spin-lattice relaxation. For a non- complicated system with homogeneous broadening and possessing Lorentzian lines, the effects of narrowing mechanisms are governed by the effective spin-spin relaxation time

T2. For an EPR spectrum with a Lorentzian line shape, the peak to peak linewidth is written as

2 1 1 DH P-P = ( + ) (5.1) 3g T2 2T1

Where g is the gyromagnetic ratio, T2 is the effective spin-spin relaxation time, and T1 is the spin-lattice relaxation time [88]. The linewidth is inversely related to the spin motion and therefore an increase in conductivity will increase the effectiveness of spin diffusion.

T1 was estimated by using the saturation method. The first derivation of absorption signal is given as [88],

101 2 0' 1 (4 / 3) (H - H 0 )ym Y (H ) = 2 2 (5.2) 1/ 2DH P-P {1+ (1/ 3)[H - H 0 ) /(1/ 2)DH P-P ] }

and the magnetic resonance absorption Y (H ) is given as [88],

0 H1 ym Y (H ) = 2 2 2 2 2 (5.3) 1+ (H - H 0 ) g T2 + (1/ 4)H1 g T1T2

and the saturation factor is defined is given as [88],

1 s = 2 2 (5.4) 1+ (1/ 4)H1 g T1T2

0' 2 2 ym is the maximum amplitude below saturation for which (1/ 4)H1 g T1T2 <<1, H 0 is the

2 magnetic field at the maximum of the absorption signal, and H1 is the power of the

1 microwave. At H - H = ± DH the peak to peak amplitude of the first derivative 0 2 P-P

' ' curve Y = ym has the form [88]

' ym 0' 3'2 ( ) = [ym ]s (5.5) H1

The relations between 1/ s and T1 is expressed as

2 / 3 élim(y ' / H ) ù ê m 1 ú ê H ú 1 2 2 1/ s = 1®0 = 1+ H g T T (5.6) ê (y ' / H ) ú 4 1 1 2 ê m 1 ú ëê ûú

102

Figure 40 shows the saturation characteristic with corresponding peak to peak values for various squares of microwave power. The plot of 1/s as a function of microwave power

2 2 -7 H1 is given in Figure 41. T1 was estimated from the slope of 1/s vs H1 as 3.23 x 10 sec which is in the order of values reported previously for conventional polyaniline[132].

103

3.0x10-4 PAN-N

2.0x10-4

1.0x10-4 (emu/mole.2rings) c

0.0 0 50 100 150 200 250 300 T(K)

Figure 37: Magnetic susceptibility as function of temperature for nanostructured polyanilne/HClO4 film.

104

PAN-N Fit to c T + C (30 K - 300 K) p 6.0x10-3

4.0x10-3

-3 T(emu K/mol. 2 rings) 2 K/mol. T(emu 2.0x10 c

0 50 100 150 200 250 300 T(K)

Figure 38: The cT vsT for nanostructured polyanilne/HClO4 film

105

2.0 PAN-N 1.9

1.8

1.7

1.6

FWHM 1.5

1.4

1.3

1.2 50 100 150 200 250 300 Temperature(K)

Figure 39: Linewidth peak to peak and linewidth full width at half maximum as a function of temperature for nanostructured polyanilne/HClO4 films.

106

4000

3000

2000

Peak to peak amplitude peak to Peak 1000

0 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 P1/2( W1/2)

' Figure 40: Peak to peak amplitude ym plotted as a function of the square root of the microwave power P. The dotted line is to show linear dependence at low powers for nanostructured polyaniline/HClO4.

107

0.022 Relaxation time parameter for PANN/HClO 0.021 4 _linear fit 0.020 Slope =4.534x 10-1 0.019 =1/4gT T 1 2 -7 0.018 T =3.23x10 sec 1 1/S 0.017 T =7.26x10-8sec 2 0.016

0.015

0.004 0.008 0.012 0.016 H 2 1

2 Figure 41: The plot of 1/s as a function of microwave power H1

5.5 X-ray Studies of Nanostructure Polyaniline

Structural studies indicate that the nanostructured films are ~ 50 % crystalline with coherence length of ~ 2 nm. The coherence length is similar to the values reported earlier for conventional polyaniline with less disorder and higher conductivity [101].

However, the effective RT conductivity in the nanostructured network is small. We suggest that since the average diameter of the nanostructured fiber is ~ 40 nm, which is a factor of 20 more than the coherence length, the X-ray diffraction (XRD) probe sees a partially crystalline sample similar to bulk conventional polyaniline. Therefore, the XRD measurement sees a ~50 % crystalline sample. However, due to mechanical contact and

108 the increase in the “fragile” nature of interfiber contacts, the effective conductivity of the network is significantly reduced. From the analysis of the RT X-ray data, the following parameters given in table 3 were determined

2q d, Å L, Å FWHM

Coherence Length ( D (2q ))o

~19o ~4.67 20.1 ~0.078

~25 o ~3.56 12.5 ~0.113

~42 o ~2.14 6.8 ~0.192

Table 3: Parameter extracted from X-ray diffraction data

The coherence length was calculated using Scherrer’s Formula (equation 4.24) and the

Bragg Formula (equation 4.22). These are similar to coherence lengths reported earlier for conventional polyaniline doped with HCl [ 101 ].

109

Figure 42: The RT X-ray for (a) small and big angle (b ) large angle

110 5.6. Mechanical probe

The role of interfiber contacts in the determination of electrical resistance (R) of

PAN-N is presented. There are changes in the PAN-N electrical resistance as a function of applied pressure. As different known pressures were applied to the nanostructure, the electrical resistance change. At low applied force, there is no obvious change in the morphology of PAN-N. However, at higher applied force there is significant change in its morphology. These changes modify the interfiber contact resistance of the nanostructure, inducing changes in the electrical resistance of the PAN-N. .

Figure 43: Scanning electron microscopy images showing, (a) PAN-N with no applied weight, (b) with small applied weight, (c) no applied weight at higher magnification, (d) large and small applied weights at the same magnification as (c).

111 The morphology of PAN-N does not show any noticeable change for low applied pressure (P) ( £~ 13MPa) (see Figure 43). However, we noticed a significant change in the morphology for applied pressure of ³ 27MPa (see figure 27) and an enhancement in the conductivity as the network become less resistive.

RT four-probe DC measurements of resistance R vs pressure P up to ~ 13.4 MPa were performed via the pressure cell setup shown in Figure 26. The weight was applied gradually to the PAN-N at a rate of one pound per half minute. The one pound corresponds to a pressure of ~ 0.22 MPa. We ran this cycle twice: gradually applying weight and then removing the weight at the same rate. The first application of pressure caused the PAN-N to become more resistive. Completely removing the pressure resulted in the sample becoming more conductive. The data shown in Figure 44 shows that though there is some resistivity recovered, the system is not totally reversible after such a large pressure has been applied. Applying pressure to this sample again, however, resulted in a decrease in resistivity. We found that removing the weight again had a negligible effect on the resistivity due to saturation. Figure 45 shows the behavior of

PAN-N as applied pressure is gradually released from PAN-N. From the results in this section, it was determined that the nature of charge carriers in the polyaniline nanostructure depends on “elasticity”, “compression”, and “relaxation” of the nanofibers and the interfiber contacts within the nanoscale network.

112

3.4x104 PANN/HClO 4 3.2x104

3.0x104

2.8x104

4 2.6x10

2.4x104 Intial application Intial removal 4 Saturated application Resistance(Ohms) 2.2x10 Saturated removal 2.0x104

4000 8000 12000 Pressure(KPa)

PANN/HClO 4

28000 Initial application Initial removal Second application 27500 Second removal Third application Third removal

27000 Resistance(Ohms) 26500

0 1000 2000 3000 4000 5000 6000 7000 Pressure (KPa)

Figure 44: Resistance as a function of applied pressure

113

) 6.45 4 6.05 6.40 6.10

6.15 6.35 6.20 6.30

6.25 6.25 6.30 6.20 6.35 6.40 6.15 4.0 3.5 6.45 6.10 3.0 Resistance( Ohms X 10 X Ohms Resistance( 6.05 2.5 4 2.0 3 1.5 2 1.0 Pressure(MPa) 1 Time(mins)

Figure 45: “Compression,” “Relaxation” and “Stretching” of Interfiber contacts as gradual pressure is released from the polymer network.

114

CHAPTER 6

OPTICAL DETECTION AND MAGNETIC SEPARATION OF MAGNETOTATIC BACTERIA, MAGNETOSPIRILUM SP. AMB-1, THROUGH CHANNEL GLASS FILTERS

In partial fulfillment of the NSF Integrative Graduate Education and Research

Traineeship (IGERT) fellowship requirement, I was an intern for 6 months at the Naval

Research Laboratory (NRL) in Washington, DC. This chapter summarizes the work I did and its potential application.

Magnetotatic bacteria are being explored for potential applications based on their intracellular nano-sized magnetite crystals called magnetosomes. Within

Magnetospirilum sp. AMB-1, the nano-sized magnetite crystals reside in a linear array, which act like small magnetic, compass needles, allowing the bacteria to orient and swim along the direction of an externally applied magnetic field. Here, we report a simple but novel method of separating, detecting and extracting live motile magnetic bacteria, from a culture containing both dead and live bacteria, and toxins using the strain

Magnetospirilumsp.AMB-1.

115 6.1 Introduction

Research on magnetotatic bacteria began following their accidental discovery over three decades ago by Richard P. Blakemore [110]. Magnetotatic bacteria are prokaryotes containing intracellular nano-sized magnetite (Fe304) or greigite(Fe3S4) crystals called magnetosomes enclosed in their membrane. The magnetosomes form nearly perfect nano-crystals of uniform size and are therefore of particular interest in nanotechnology and biotechnology. Recent research efforts on magnetic bacteria have focused on understanding the cell biology and the mechanism of the formation of magnetosomes in magnetic bacteria [111-114]. Others have quantified their magnetic moments, cellular magnetism, their growth conditions and their potential usage in the removal of pollutants from the environment [116-120]. These findings could provide better insight on how to biologically synthesize high quality magnetic nanocrystals in large amounts for potential application in medical research. For instance, such magnetic nanocrystals could be used in magnetic resonance imaging as a probe to improve the image contrast for early detection of tumor [124-125]. Magnetotatic bacteria swim and orient in direction of an external magnetic field. These unique characteristics are being exploited for potential application of magnetic bacteria to biotechnology. For instance it has been reported that there is a reduction in their speed as their exposure to concentration of toxic compound increases [122]. These characteristics are also affected depending on how they are grown. For almost all strains of magnetotatic bacteria when grown in an aerobic environment, they grow but with limited or no magnetosome production. This results into random orientation and slow motility of the bacteria when magnetic fields are applied [122,123]. 116 In this chapter, we report for the first time the separation of motile live magnetic bacteria from a culture containing both dead and live magnetic bacteria and toxins using glass structures with highly uniform micron sized channels as a membrane for filtration and an external applied magnetic field.

6.2. EXPERIMENT

Live magnetic bacteria orient and swim along magnetic field lines while dead magnetic bacteria orient but do not swim, remaining stationary. This property of the magnetic bacteria was exploited in developing the separation technique described below.

The separation device for the magnetic bacteria consists of a thin glass membrane containing a high density of uniform micron sized channels placed between two chambers. The first chamber contains the motile magnetic bacteria to be separated while the second chamber is filled with sterile growth media. A 0.3 Telsa neodymium iron boron magnet is used to align and guide the magnetic bacteria through the glass membrane when the applied magnetic field is applied parallel to the direction of the channels in the glass. Separated live magnetic bacteria were optically detected using light scattering technique in the mie regime. The forward scattered light was collected as time elapsed

117 Experimental design

Maintaining a positive 1) 2) pressure at the Cuvette growth media side Growth Media Channel Glass Caps a) AMB -1

Filling done in a nitrogen environment 3) 4) 5)

Want zero ~ ~ mass transfer ~ ~

b)

Figure 46: a) Experimental setup for filling the cuvettes, b) Bacterial probing setup

118 6.3 Materials and Methods

6.3.1 Magnetic Bacteria

Magnetospirillum magneticum AMB-1, also known as wildtype magnetic bacteria was used to demonstrate the separation mechanism. Magnetospirillum sp AMB-1

(ATCC 700264) was purchased from the American Type Culture Collection (A.T.C.C).

The AMB-1 cells were cultivated using the modified magnetospirillum growth medium

(MSGM)[12]. The modified MSGM contained one liter of distilled water, a 10ml

Wolfe’s vitamin solution, 5ml Wolfe’s mineral solution, 0.12g sodium nitrate, 0.68 g potassium phosphate, 0.035g ascorbic acid, 0.37g tartaric acid, 0.37g succinic acid, 0.05g sodium acetate, and 2ml of ferric quinate solution. The solution was adjusted to a pH of

6.75 with NaOH and then autoclaved at 1210C for at least 2 hours making the medium sterile. In order to minimize the oxygen content of the growth media, it was kept in a continuous flow nitrogen gas glove bag. The AMB-1 cells were initially cultured anaerobically, and then inoculated into 15ml Falcon tubes containing the growth medium.

The tube were then sealed with a minimal atmospheric content and cultured inside an incubator at 28oc. The inoculation was done in a nitrogen glove box to reduce the introduction of oxygen into the system. They were harvested after 3 days when on the average greater than 95% of the cells were alive at a cell density of ~9.8 x107cells/ml.

On examination of the bacteria under a Nikon elipsce TE2000-S Transmission microscope a significant number (>95%) of them align and swim in the direction of an applied field.

119 6.3.2 Channel Glass

The channel glass materials are non- magnetic and are composed of glass rods containing an etchable glass and inert glass fused together. A detailed procedure for the fabrication of Channel glass was reported earlier [115]. They can be fabricated from the micron range down to the nanometer range. In the fabrication of our filtration device we used a channel glass cut perpendicular to the length of the glass rod, with a thickness of

370 mm and a uniform channel diameter of 6 mm, a 10 mm center to center channel spacing and a high density of holes available for bacterial transfer. The channel glass was polished on both sides using a coarse polished paper and polishing cloth both from

Buehler together with diamond powder of fineness ranging from 12 mm -0.25 mm. Both sides of the channel glass were parallel to each other. The surfaces are flat and smooth, minimizing any turnaround of the bacteria due to any bumpy surface of the channel glass.

The average turn around distance of bacteria is less than the thickness of glass.

The channel glass was etched at the rate of 1m/min in a glass bottle containing 2% acetic with a trace of nitric acid. The nitric acid helped to prevent polymerization of the solution during etching. The glass bottle was mounted on a rotating stage at 10 rpm.

The rotating stage enables exposing both surfaces of the channel glass equally to the etchant. After more than 4 hours, both sides of the channel glass was completely etched.

It was rinsed thoroughly and allowed to air dry for 2-4hours before it was used in the device structure.

120

Figure 47: SEM image of the 6micron channel glass (membrane)

6.3.3 Cuvettes

VersaFluor Microcuvette was purchased from Bio-rad and modified into component of our separation device structure. The 3x3mm, 4-sided optically clear polystyrene cuvettes were used through out the experiment. For each of the cuvette used, a 2mm diameter hole was drilled on 3 sides of the 4-sided cuvette. The hole centers were drilled about 1mm from the base of the cuvette. Two of the holes were on the opposite side of each other and the third hole is on the adjacent side of the other two holes. The holes on the opposite sides were covered with a highly transparent window for optical measurements. Highly transparent and optically clear windows were used to remove potential light scattering from the surface of the cuvette.

121

Figure. 48: The picture of the assembled filtration device structure with the 0.3 Telsa Neodymium iron boron magnet used to align the field parallel to the channels in the channel glass (the membrance).

6.3.4 Bacteria Separation Assembly

Our goal is to separate live magnetic AMB-1 from a culture containing live and dead magnetic AMB-1 and toxins. The device for the separation was made by assembling two of the drilled polystyrene cuvettes together. A piece of channel glass was sandwiched between the third holes of two microcuvettes. The structure was then mounted on a horizontally lying cuvette then on a microscope slide to maintain adequate structural support for the device. The applied external magnetic field to the device was provided by a 2x2x0.5 inches Neodymium iron boron magnet placed 1.2cm below the device structure. This magnet generates magnetic field between 0.1-0.15 Telsa depending on where you are probing in the highly transparent optically clear window in the device structure. This field is much greater than the minimum 0.5 Gauss needed for 122 the bacteria to orient and swim following the magnetic field lines. The cuvettes were labeled cuvette-A and cuvette-B. For cell transfer and separation we used cells dispersed in growth medium at a cell density of 9.8x107ml-1. Cuvette-A was filled first with about

60ml of growth media and half of the growth media was allowed to migrate to cuvette-B.

We then removed the growth media from cuvette-A and quickly filled it with 27ml of

AMB-1 (volume of the probing area of our device) this helped to create a positive pressure from cuvette-B on cuvette- A. Therefore, minimizing any back flow magnetic bacteria into cuvette-B. Since we need to maintain no mass flow at the start of the transfer experiment, we allowed the cuvette system to reach a no mass flow state by applying magnetic field perpendicular to the channel direction in the membrane. In this orientation, when the magnetic field is perpendicular to the channel direction magnetic bacteria are not observed to transfer across the membrane for several hours.

6.3.5 Light scattering

Optical characterization of the migrated live cells in cuvette-B was done using light scattering. The magnetic bacteria AMB-1 are helically shaped with a diameter of about 0.5mm and about 2-20mm long. These dimensions are in the Mie scattering regime since we are probing the system with a monochromatic laser light of 635nm wavelength.

The block diagram for detecting the scattering center as bacteria migrate to cuvette-B is shown in Figure 46. The monochromatic laser light was sent into a beam shaping aperture of about 0.3cm in diameter. The light was then sent through the two optically clear transparent windows in cuvette-B to mirror-1 and then to mirror-2. The light from mirror-2 was then sent through a second aperture which helped in blocking any unwanted 123 scattered light from the Laser. At this point the forward scattered light was collected by a

Thorlabs DET110 High-Speed Silicon Photo Detector with a spectral response 350-

1100nm. The photodiode was connected to a variable resistor used to increase the output voltage. The variable resistor was then connected to Data Acquisition (DAQ) SC-2345 signal conditioning connector block with an analog to digit converter. The output of the

DAQ Sc- 2345 was connected to the computer that read out the digital output through a

National Instruments photodiode voltage measurement labview program.

6.4. RESULTS AND DISCUSSION

As we probe cuvette-B with the monochromatic laser light and concomitantly applied external magnetic field parallel or perpendicular to the direction of channels in the channel glass in our device, the magnetic cells began to orient and swim along the direction of the applied field lines. When the field is perpendicular to the channels no transfer was noticed. When the field lines are in the same direction as the channels in our device we immediately noticed the presence of magnetic bacteria as scattering centers being introduced into cuvette-B that initially contained only the growth media. As time elapsed, these scattering centers were increased leading to a decrease in the transmission signal (following an exponential decay). The forward scattered light was measured and a reduction in the transmission signal was noticed as more live magnetic cells are separated from the culture in cuvette-A (containing magnetic live, dead and other toxins) to cuvette-B. The raw data in Figure 49 was converted to Absorbance unit and normalized to the path length of a standard 1cm2 cuvette. In order to estimate the transfer rate of the magnetic bacteria we measured the absorbance of the magnetic bacteria as a function of 124 known concentration of the magnetic cells (Figure 51). From the fit parameters of Figure

50 and Figure 51, we derived Figure 51 (concentration of magnetic bacteria/ml as a function of time) by dividing the slope of Figure 51 by the slope of Figure 51. The slope of Figure 52 multiplied by the volume of the cuvette is the transfer rate of AMB-1 through the membrane. We estimated a transfer rate for AMB-1 passing through a membrane of 1cm2 area to be 8.09 x 10 6 amb-1/mins.

6.5. CONCLUSION

Using the combined properties of magnetic bacteria and channel glass we have successfully separated magnetic live AMB-1 from a culture containing magnetic live and dead AMB-1 and other toxins. This novel method could be used to separate other live magnetic bacteria from a culture containing dead and live magnetic bacteria.

6.6. ACKNOWLEDGEMENT

The authors will like to thank Naval Research Laboratory, Washington, DC,

DARPA BioMagnetics Program and Office of Naval Research. This work is supported in part by the NSF-IGERT Grant No dge-0221678. Special thanks to Carol Ibe for growing the bacteria used in this research.

125 AMB-1 Transfer 4100 y = m1*exp(-m0/m2)+m3 Value Error m1 270.27 3.173 m2 68.528 1.2597 4050 m3 3826.4 3.3549 Chisq 84352 NA R 0.99363 NA

4000

3950

Forward Scattered Light Intensity 3900 0 10 20 30 40 50 60 Time(mins)

Figure 49: Forward Scattered Light intensity as function of time as magnetic bacteria migrate from cuvette-A through the membrance to cuvette-B.

126 AMB-1 0.03

0.025

0.02

0.015

0.01 Modified Absorbance Modified

0.005

0 0 5 10 15 20 25 30 35 40

Time(mins)

Fig.ure 50:. Normalized Absorbance to a standard cuvette pathlength

127 OD at 635nm AMB-1 0.08 y = 1.2112e-09x R= 0.9924

0.07

0.06

0.05

0.04 OD at 635nm at OD

0.03

0.02 2 107 3 107 4 107 5 107 6 107 7 107 Concentration(cells/ml)

Figure. 51. Absorbance as a function of concentration of the cell derived from serial dilution

128 Number of amb-1 Number of AMB-1 5 108

8.09x10 6 amb/mins 4 108

3 108

2 108 Number of amb-1 1 108

0 0 10 20 30 40 50 60 Time(mins)

Concentration/area/min AMB-1 8 109 1.414x108/amb-1/area/min 7 109

6 109

5 109

4 109

3 109

2 109

1 109 Concentration(amb-1/area/min) 0 0 10 20 30 40 50 60 Time(mins)

Figure 52: Concentration of magnetic bacteria as a function of time from the fit parameters. Figure 8b. The number of magnetic bacterial as a function of time from the fit parameters.

129 CHAPTER 7

DISCUSSION

The results of the bulk electronic properties probe of PAN-N, such as the activation energy (W) derived from DC conductivity, optical conductivity and dielectric constant shows that the charge carriers are localized within the PAN-N network. Structural study also reveals parameters typical of conventional polyaniline as well as no significant change in the structure of PAN-N. Magnetic data support dominance of Bipolaron formation in PAN-N. Since all the charge dynamic probes suggest that the charge carriers are localized, and that the signature of P-PAN-N does not show a large metal-like contribution, the origin of the metal-like signature seen in the T-dependent DC conductivity of PAN-N should be related to the nature of conductance of the interfiber interface. The SEM image of PAN-N and P-PAN-N also suggest that interface contribution affects the signature of T-dependent DC conductivity. Mechanical probes also show that as the network is under an applied pressure, there are changes in its electrical resistance. This change is as a result of the nature of conductance within the interfiber network.

130

7.1 Summary and Conclusion

In summary, the T-dependent DC conductivity transport data of nanostructured polyaniline exhibits an unusual “metallic” signature, while the T-dependent pressed- nanostructure exhibits “metallic” charge transport signature typical of disordered polyaniline. The reduced activation energy (W) determined via the charge transport data of the nanostructure showed that the charge carriers are in the localized limit of MI transition. The dielectric constant further revealed that the charge carriers within the nanostructured network are localized. Pressure probe suggests that charge transport in the PAN-N network is related to the state of the interfiber contacts within the nanofiber network. As pressure is being applied to the nanofiber network, the network is either in an “elastic state,” “compressive state,” or “relaxation state.” When the PAN-N is in the elastic state, the system is more resistive. In the compressive state, the system is less resistive as the interfiber contact is enhanced, while in the relaxation state the system is effectively less resistive. XRD data show partially crystalline nanostructures having a localization length of ~2nm, typical of polyaniline. However, the effective conductivity of the network is less than the Mott minimum conductivity (~ 100 S/cm), and none of the charge dyanimics probes reveal metallic signatures. Therefore, we propose that the unusual “metallic” signature seen in the T-dependent charge transport is due to the

‘fragile” nature of conductance within the nanostructure network, hence it is

“mechanically-induced.” This implies that the charge transport dynamics within the nanostructure is controlled by the nature of the conductance at the nanostructure

131 (interfiber) interfaces and interaction with phonons. At lower temperatures, the interfiber contacts are strongly coupled, therefore the effect of “fragile” interfiber contact is minimized. As the network gains thermal energy (intermediate temperatures), the interfiber contacts are weakly coupled, so the effects of phonons dominate the charge carrier. At higher temperatures (>235 K), the effects of interfiber contacts dominate due to the “fragile” nature of conductance at the interfiber interfaces, while the effect of phonons beyond the mechanically-induced MI transition is reduced. Therefore, as the temperature is increased above the mechanically-indueced MI transition, the network is more resistive, hence the signature of charge transport is “quasi-metallic.” As the morphology of the nanostructure is significantly modified by applying pressure on the nanostructure, the signature of the charge transport resembles that of a conventional film with disorder.

7.2 Epilogue: Directions for Future Study

Future study should focus on using atomic force microscopy to study the conductance of single fibers, thereby eliminating the effect of interfiber contacts. New synthesis routes should be developed so that thicker free standing films (> 50 mm) of the nanostructure could be made with higher conductivities. This will allow studies of very far infrared reflectance to be made, thereby minimizing any errors due to extrapolation to low wavenumbers.

Also, the processing of the nanostructures should be improved and the disorder should be controlled so that the roles of disorder and effect of delocalization and

132 localization can be studied. Transport measurement could be extended to mK range in order to understand what controls the nature of charge carriers at such low temperature.

The studies could also be extended to other conducting polymers nanostructured networks doped with HClO4 or other acids in order to understand the nature of carriers in such systems.

Finally, this work could be extended to creating mechanical sensor based on nanostructured films.

Future work with the bacterial should be focused on coating the separation membrane with superhydrophobic and superhydrophilic conducting polymer. Based on the surface treatment optical monitoring of rate of cells transfer could be determined.

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