Abstract

A nanocrystalline and porous p-Polyaniline/n-WO3 dissimilar heterojunction at ambient temperature is reported. The high-quality and well-reproducible conjugated polymer composite films have been fabricated by oxidative polymerization of anilinium ion on predeposited WO3 thin film by chemical bath deposition (CBD). X-ray diffraction analysis of the deposited WO3 films revealed that as-deposited film was amorphous; however, an amorphous-to-crystalline phase transition was observed by reason of thermal annealing at 573K for 1h. A pseudo-orthorhombic structure for Polyaniline with crystallite size of 4.79nm was observed while hybrid Polyaniline/WO3 film was amorphous but retained monoclinic phase of tungsten oxide particles dispersed in Polyaniline matrix. Atomic force microscopy (AFM) analysis revealed a homogenous but irregular cluster of faceted spherically-shaped grains with pores. Again the thicknesses measured are 104, 72 and 158nm for WO3, Polyaniline and composite film. Scanning electron microscopy (SEM) analysis corroborates the AFM results. The optical absorption analysis of WO3, Polyaniline and Polyaniline/WO3 hybrid films showed that direct optical transition exist in the photon energy range 2.50 – 3.20eV, 2.75 – 3.30eV and 3.50 – 4.00eV with bandgaps of 2.80, 3.00 and 3.70eV respectively. The refractive index has peak at 315nm, 460nm and 445nm in the dispersion region 300 – 1100nm respectively. The high frequency dielectric constant ∞ ε , and the carrier concentration to effective mass ratio, N/m* were found to be 1.37 and 1.45x1039cm-3 for WO3 film, 1.66 and 8.48x1038cm-3 for Polyaniline film, and 1.58 and 1.10x1039cm-3 for Polyaniline/WO3 hybrid film. The temperature dependence of the electrical resistivity of the deposited films follows the semiconductor behaviour. The activation energy ΔE calculated was 2.0meV, 6.0meV and 14.0meV for WO3, Polyaniline and Polyaniline/WO3 film respectively. C-V characteristics (Mott-Schottky plots) of nWO3 and p-Polyaniline coated on pure Cu electrodes showed that the flat band potential was -791meV/SCE and 830meV/SCE while the values estimated for the donor concentration are 6.17x107 cm-3 and 6.20x1010 cm-3 respectively.

TABLE OF CONTENT

Title page - - - - - - - - - - i

Certification - - - - - - - - - - ii

Dedication - - - - - - - - - - iii

Acknowledgement - - - - - - - - - iv

Table of Contents - - - - - - - - - vi

List of Tables - - - - - - - - - - ix

List of Figures - - - - - - - - - - x

Abstract - - - - - - - - - - xiii

CHAPTER ONE: INTRODUCTION

1.0 Introduction - - - - - - - - - 1

1.1 Objective of the Study - - - - - - - 2

1.2 Research Methodology - - - - - - - 3

1.3 Chemical Deposition Techniques - - - - - - 4

1.4 Concept of Solubility and Ionic Product - - - - - 6

1.5 Particles - - - - - - - - - 7

1.5.1 Nucleation and Growth - - - - - - - 7

1.5.2 Dispersion and Agglomeration - - - - - - 8

CHAPTER TWO: LITERATURE REVIEW

2.0 Review of Tungsten Oxide (WO3) and Polyaniline Thin Films - - 14

2.1 Generality abut Tungsten Oxide (WO3) Thin Films - - - - 14

2.1.1 The Different Lattice Structures of WO3 - - - - - 16

2.1.2 Electronic and Surface Structure of WO3 - - - - - 19

2.2 Generality about Polyaniline Thin Films - - - - - 21

2.3 Theoretical Background of Heterojunction - - - - - 25

2.3.1 Introduction to Heterojunction - - - - - - 25

2.3.2 Theoretical Consideration of Energy Band Diagram - - - - 27

2.4 Heterojunction Models - - - - - - - 29

2.4.1 Current-Voltage (I-V) studies - - - - - - - 31

2.4.2 Capacitance – Voltage (C-V) Studies - - - - - - 32

2.5 Characterization Techniques - - - - - - - 34

2.5.1 X-Ray Diffraction (XRD) - - - - - - - 34

2.5.2 Surface Morphology (Scanning Electron Microscope; SEM) - - - 35

2.5.3 Optical Properties (UV-VIS-NIR Spectrophotometry) - - - - 37

2.5.4 Electrical Resistivity Measurement - - - - - - 39

2.5.5 Fourier Transform Infrared Spectroscopy (FTIR) - - - - 40

2.5.6 Thermo-emf Measurement - - - - - - - 41

CHAPTER THREE: EXPERIMENTAL DETAILS

3.0 Experimental Details - - - - - - - 42

3.1 Experimental Set-up for Deposition of Polyaniline and WO3 Thin Films - - - - - - - - 42

3.2 Substrate Purification/Cleansing - - - - - - 43

3.3 Deposition of Polyaniline and WO3 Thin Films - - - - 43

3.4 p-Polyaniline/n-WO3 Heterojunction Studies - - - - 45

3.5 Characterization of Samples - - - - - - - 46

CHAPTER FOUR: RESULTS AND DISCUSSION

4.1 Chemical Consideration - - - - - - - 47

4.1.1 Reaction Mechanism of WO3 - - - - - - - 47

4.1.2 Reaction Mechanism of Polyaniline - - - - - - 47

4.2 Structural Analysis - - - - - - - - 48

4.3 Atomic Force Microscopy (AFM) Results - - - - - 52

4.4 Scanning Electron Microscopy (SEM) Results - - - - 53

4.5 Energy Dispersive X-ray (EDX) Results - - - - - 55

4.6 Optical Absorption Studies (UV-Vis-NIR Spectrophotometry) - - 55

4.7 Resistivity (Electrical) Measurement - - - - - - 62

4.8 Capacitance - Voltage (C-V) Characteristics - - - - - 64

4.8.1 Construction of Energy Band Diagram - - - - - 67

4.9 Hall Effect and Diffusion Coefficient Determination - - - - 69

CHAPTER FIVE: CONCLUSION

5.0 Conclusion - - - - - - - - - 71

REFERENCES - - - - - - - - - - 73