Fabrication and characterisation of multilayer thin film using self assembly of colloidal gold and silica nanoparticles


Doctoral Thesis / Dissertation, 2011
108 Pages

Excerpt

Table of Contents

Acknowledgements

Abstract

List of Figures

List ofTables

List of Abbreviations

List of chemicals used

1.Introduction
1.1 Background
1.2 Statement of the problem
1.3 Objective of research work
1.4 Hypothesis
1.5 Scope and limitations of research work
1.6 Organization of the research thesis

2.Literature Review
2.1’Top-down’ versus ’bottom-up’ assembly
2.2 Colloidal nanoparticles
2.2.1 Colloidal gold nanoparticles
2.2.2 Colloidal silica nanoparticles
2.3 Polyelectrolytes
2.3.1 Chitosan
2.3.2 PDDA poly(diallyldimethylammoniumchloride)
2.4 Surface passivation of colloidal nanoparticles
2.5 Stabilisation ofnanoparticles
2.5.1 Electrostatic stabilization
2.5.2 Steric stabilisation
2.6 Surfaceplasmonresonance
2.7 Layer-by-layerselfassembly
2.8 Theforces guiding colloidalassembly
2.9 TheroleofpHbuffers
2.10 Theformationandstructureofthinfilms
2.11 Wettability of thin films
2.12 Electricalpropertiesofmetal-insulatorfilms
2.13 Quantumtunneling
2.14 Fowler-Nordheim(F-N)tunneling
2.15 Thermalpropertiesofthinfilms
2.16 Optical properties of thin multilayered structures
2.17 Effectivemediumtheory
2.18 Maxwell-Garnettmodel
2.19 Thinfilmopticalfilter

3 Methodology
3.1 Synthesisofcolloidalnanoparticles
3.2 Surface passivation of colloidal nanoparticles using different polymers
3.3 Synthesis and surface modification of gold nanoparticles
3.4 Synthesis and surface modification of silica nanoparticles
3.5 Thinfilmarchitectures
3.6 Layer-by-layer self assembly using dip coating system
3.7 Thinfilmfabricationusing layer-by-layerselfassembly
3.8 Types of substrates
3.9 Synthesis of pH buffers for colloidal solutions of gold & silica nanoparticles
3.10 Fabrication of gold-silica thin films arrays
3.11 Measurement ofWCA (Water Contact Angle) over glass substrates
3.12 Optical characterisation of thin multilayer films
3.13 Optical absorbance of gold silica thin film through air and water medium
3.14 Study ofthelightinterferencethrough gold-silica thinfilm
3.15 Electrical characterisation of gold-silica multilayer thin films
3.16 Measurement of electrical rectification for gold-silica thin film
3.17 DC-capacitance measurements for gold-silica thin films
3.18 Current-Voltage measurements with change in temperature
3.19 Current measurement with variable temperature and fixed voltage
3.20 ApplicationofMaxwell-Garnetttheory tometal-dielectric multilayer films
3.21 TransmissionElectronMicroscopy
3.22 Scanning Electron Microscopy
3.23 Simulationof opticalspectra gold-silicathinfilms

4 Results and discussions
4.1 Synthesis of colloidal nanoparticles
4.2 Particle Size Distribution of gold and silica nanoparticles
4.3 Glass substrate functionalisation
4.4 Measurement ofWCA (Water Contact Angle)
4.5 Growth of gold-silica multilayers
4.6 Effect of pH of buffers for fabrication of multilayers thin films
4.7 Fabrication of gold-silica multilayer thin films
4.8 Optical absorbance of chitosan capped gold-silica thin films
4.9 Optical absorbance ofPDDA capped gold-silica thin films
4.10 Thin film fabrication using different volume ratio of gold nanoparticles
4.11 Optical filter application
4.12 Deconvolution of optical absorbance graphs for gold-silica multilayers
4.13 Effect of the film thickness on filter transmittance
4.14 Application of Maxwell-Garnett theory to gold-silica multilayer films
4.15 Optical absorbance of gold silica thin film through air and water medium
4.16 Study of the light interference through gold-silica thin film
4.17 Optical transmission through gold-silica thin film
4.18 Electrical characterisation of gold-silica multilayer thin films
4.19 DC-capacitance measurements for gold-silica thin films
4.20 Measurement of Rectification for gold-silica thin
4.21 Current-voltage measurements with change in temperature
4.22 Current measurement with variable temperature and fixed voltage
4.23 Fowler-Nordheim (F-N) plot for gold-silica multilayer films
4.24 Multilayer thickness measurements

5 Conclusions and future recommendations

Appendix I

Appendix II

Appendix III

ACKNOWLEDGEMENTS

First and foremost, I owe my deepest gratitude to my esteemed Ph.D. supervisor Prof. Joydeep Dutta, not only was he readily available for me, as he so is generously for all of his students, but he always read and responded to the drafts. His oral and written comments are always extremely helpful, perceptive and appropriate, for the completion of research study. I feel very fortunate having him as my advisor for the research study and also grateful to him for all the wonderful time we shared in the past four years.

I would like to pass my special thanks to Prof.G. L. Hornyak, Dr. Waleed S. Mohammed and Assoc. Prof. Erik L. J. Bohez, for availability for discussions, providing current information, words of advice and incentive for conducting this study.

I gladly express my gratitude to Dr.Rachana Kumar for her valuable ideas and discussions that provided much help. Being my work partner, I always enjoyed her continual encouragement and support.

My graduate studies would not have been possible without the academic and social challenges and diversions provided by all my colleagues. Special dedications to all my group mates of AIT nano group, who helped me a lot to collect the necessary data needed during the course of my work and also provided all the necessary information required for the compilation of this research study report.

I wish to acknowledge the Higher Education Commission (HEC), Government of Pakistan for its generous financial support throughout my Ph.D. studies at Asian Institute of Technology (AIT).

At the last, I am grateful to my parents and family members who supported me morally and spiritually during my stay at AIT.

ABSTRACT

A novel approach of multilayered thin film based on layer-by-layer deposition using colloidal nanoparticles was carried out in this work. The films were made by the self-assembly of oppositely charged metal and dielectric nanoparticles, alternately capped with polymers. Synthesized colloidal suspensions of gold nanoparticles (~20nm) and silica nanoparticles (~30nm) were used as the building blocks for the self-organisation of the films. Capping with PDDA and chitosan was used effectively to control the optical absorption of the surface plasmon resonance peaks of the gold nanoparticles. Using different combinations of layer formation, absorption characteristics in the near-ultraviolet (NUV), green and blue region were controlled through capping and varying the thickness of the film. Capping with chitosan or PDDA reduced the absorption peak of the coated silica nanoparticles in a similar fashion. Peak absorption in the UV range was achieved by assembling bare silica nanoparticles layers onto layers of gold nanoparticles. Transmission color was controlled (less than 1% color distance per added bi-layer) by changing the film thickness.

Optical modeling of multilayer thin films constructed with oppositely charged nanoparticles helped us to understand phenomenon such as surface plasmon resonance, absorbance, transmittance and reflectance. Maxwell-Garnett effective medium theory in this case is applied in quasi-static limit to multilayer composite consisting of host material silica and inclusion material gold nanoparticles. Maxwell Garnett optical simulations is correlated with experimental spectra obtained for the thin film composites. The thickness of layers, size and spacing of metal inclusion is varied to alter the optical properties for the required device applications.

The multilayered thin film of gold and silica resembles a structure consisting of large charge sheets of metal separated by a dielectric layer. When the applied electric potential reaches a threshold value, it drives the electrons to tunnel through the charge sheets producing a rectification effect. Therefore current-voltage measurements of the multilayer thin films were performed to calculate the threshold voltages. The electrical capacitance in these multilayer devices was modified with the change in thickness of the dielectric layers between two conducting layers and calculated by capacitance-voltage measurements of multilayer stack. I-V measurements were also measured with a change in temperature of the multilayer films at any fixed voltage. Fowler-Nordheim plots applied to current-voltage characteristics were helpful in determination of the tunneling behavior of thin Au-SiO2 films. Electron micrographs provided the particle size distribution for the colloidal nanoparticles and determination of the thickness and particle arrangement in the multilayered films.

List of Figures

Figure 1.1: Schematic of organic chemical addition method of nanocomposite films

Figure 1.2: Self assembly of oppositely charged layers on substrates

Figure 2.1: The Illustration of -top down” and -Bottom up” self assembly technique

Figure 2.2: Nano-Gold colloids exhibit different colours at different sizes

Figure 2.3: Synthesis of silica nano particles

Figure 2.4: Chemical structure of a chitosan polymer

Figure 2.5: The molecular structure ofPDDA

Figure 2.6: Idealised morphologies of different types of nanocomposite particles

Figure 2.7: Electrostatic stabilization due to columbic repulsion

Figure 2.8: Steric stabilisation by the overlapping ofhighly concentrated polymer

Figure 2.9: Schematic representation of interaction of nanoparticle with light

Figure 2.10: The layer by layer self assembly scheme

Figure 2.11: Different degrees of wetting

Figure 2.12: Current-voltage characteristics of a device with lbl deposition

Figure 2.13: The Metal insulator insulator diode

Figure 2.14: Fowler-Nordheim tunneling at a Metal-Insulator interface

Figure 2.15: Electrical resistivity dependence as a function of temperature for metals

Figure 2.16a: Dielectric constant of gold as a function of wavelength

Figure 2.17: The inhomogeneous system is replaced by an effective medium

Figure 2.18: The geometry of a Maxwell Garnett composite material

Figure 2.19: The optical absorption spectra for the mesoporous silica

Figure 3.1a: TSC (trisodium citrate) capped gold nanoparticles

Figure 3.2a: Chitosan capped gold as first layer

Figure 3.3: (a) Dip coating station with rotating stage

Figure 3.4: Schematic of the layer-by-layer growth ofPDDA capped silica nanoparticles

Figure 3.5: Experimental setup for the electropolishing process

Figure 3.6: Electronically controlled contact angle measurement setup

Figure 3.7: Spectrometer test setup for measurements of optical parameters

Figure 3.8: Setup for measurement of optical absorbance of gold-silica thin film

Figure 3.9 Test setup for study oflight interference through gold-silica thin film

Figure 3.10. Schematic of the 4-point probe method

Figure 3.11. (a) Schematic diagram for measurement of rectification of multilayers

Figure 3.12. Schematic of Capacitance-voltage measurement setup for MIM thin films

Figure 3.13. Test setup for current-voltage measurements with change in temperature

Figure 3.14. (a) 100 bilayer thin film ofPDDA silica and TSC gold with sputtered gold

Figure 3.15. TEM at National Metal and Materials Technology Center (MTEC)

Figure 3.16. SEM at National Metal and Materials Technology Center (MTEC)

Figure 4.1: Optical absorbance of gold nanoparticles

Figure 4.2: Optical absorbance ofPDDA capped silica nanoparticles

Figure 4.3: TEM ofPDDA capped silica nanoparticles

Figure 4.4: TEM of gold nanoparticles showing particle size

Figure 4.5: TEM of silica nanoparticles showing particle size

Figure 4.6: Surface charge distribution over the glass substrate

Figure 4.7: Optical image of the DI water

Figure 4.8: Layer-by-layer deposition consists of sequential dipping

Figure 4.9: 30 multilayer of chitosan capped silica & gold

Figure 4.10: PAA-Au & PDDA, 50 layers, PDDA buffer of pH 4

Figure 4.11: Optimised process time for layer-by-layer deposition

Figure 4.12: Multilayer thin films of gold and silica nanoparticles

Figure 4.13: An array of (a) 3 circular holes, (b) 4 circular holes, (c) 12 circular holes

Figure 4.14 (a) Aluminium tube without coating, (b) Aluminium tube after deposition

Figure 4.15: Optical spectra of chitosan capped gold and silica nanoparticles film

Figure 4.16: Optical spectra ofPDDA capped gold and silicananoparticles film

Figure 4.17 Optical absorbance spectra of 50 bilayers of gold nanoparticles

Figure 4.18 Optical absorbance spectra of75 bilayers of gold nanoparticles

Figure 4.19: Optical spectra of chitosan capped (gold and silica) nanoparticles film

Figure 4.20: Dotted black line represents deconvolution

Figure 4.21: Deconvolution of the optical spectra

Figure 4.22: Transmittance of chitosan-capped silica and gold NPs

Figure 4.23: Change of red (650 nm), green (510 nm) and blue (475 nm)

Figure 4.24: MG simulated spectra of 20 nm gold nanoparticles in water

Figure 4.25: MG simulated spectra of20 nm gold nanoparticles with increase in layers

Figure 4.26: Optical spectra ofPDDA-silica-gold film with and without water medium

Figure 4.27: (a) Visible light through an uncoated glass slide

Figure 4.28: Transmission spectra of the chitosan-gold and silica thin film

Figure 4.29: Current-voltage measurements for thin film of chitosan coated silica

Figure 4.30: DC capacitance measurements for thin film of chitosan coated silica

Figure 4.31: Current-voltage measurements through the crossection of a 50 bilayer

Figure 4.32: Current-voltage measurements through the crossection of a 100 bilayer

Figure 4.33: Graph of current-voltage I-V measurements ofPDDA-silica/gold thin film

Figure 4.34: Current-voltage measurements for 100 bilayer thin film ofPDDA silica

Figure 4.35: F-N plot for 100 bi-layer thin film ofPDDA-silica/gold

Figure 4.36: Crossectional SEM of (a) 10bi-layers ofPDDA capped silica

Figure 4.37: Scanning electron micrograph of (a) 50 bi-layers ofPDDA capped silica

List ofTables

Table 2.1: Some examples of weak and strong polyelectrolytes

Table 2.2: Comparison between different deposition methods on glass

Table 4.1: Comparison of the measured values of peak absorbance

Table 4.2: Comparison of the measured values of peak absorbance

Table 4.3: The process variables for the dip coating fabrication method

Table 4.4: Comparison of the peak absorbance and peak wavelength shown in fig.4.17

Table 4.5: Comparison of the peak absorbance and peak wavelength shown in fig.4.18

Table 4.6: Measured values ofFWHM for absorbance spectra

List of Abbreviations

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List of chemicals used

Ammonia

APTS (3-Aminopropyl trimethoxysilane) Chitosan

Gold(III) chloride hydrate Methanol

PDDA poly(diallyldimethylammonium chloride) TEOS (Tetraethyl-orthosilicate)

Trisodium citrate

Chapter 1 INTRODUCTION

1.1 Background

Colloidal metal nanoparticles with mid-nanometer (20-100 nm) sizes are of particular interest due to their intense surface plasmon responses 1-2 and broader cross sections for light scattering 3-4. The encapsulation or coating of nanoparticles has been of special appeal for the controlled drug release 5, genes effect 6, and other bioactive agents 7-8. Traditionally nanoparticles are encapsulated using dry and wet based techniques. The dry methods comprise of: (a) chemical vapor deposition9 (b) physical vapor deposition 10, (c) plasma treatment 11, and (d) precipitation of nanoparticles using pyrolysis of polymeric organic materials 12. Wet methods for capping nanoparticles are: (a) emulsification and solvent evaporation techniques 13 (b) sol-gel process using self assembly 14. The combination of organic chemistry with capping of polymers on nanoparticle materials has brought the developments in the form of nanocomposite surfaces, where the optical properties of the inorganic particles can be tuned by the choice of polyelectrolytes and their inherent functionality 15-16. Composite nanoparticles such as surface modified nanoparticles or core/shell nanoparticles can be termed as special class of nanocomposites. When these nanocomposites have an inorganic core with an organic shell coating, they can be termed as hybrid nanoparticles. The inorganic core may comprise of metal nanoparticle or a metal oxide, and the organic coating either a surfactant, polymerized monomer, or some organic molecule 17. The choice of synthetic techniques and the investigation of the physical properties of the polymeric nanocomposites, organic-inorganic layered structures can be fabricated for interesting thin film applications 18-19-20. Figure 1.1 shows the organic chemical addition and phase-transfer methods to prepare lead zirconate titanate (PZT) composite films embed with large numbers of gold nanoparticles.

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Figure 1.1 Schematic of organic chemical addition method of nanocomposite films 21

Gold nanoparticles are well known to produce strong absorption resulting from the collective oscillation of the conduction band electrons in the metals 22-23. Extensive studies has been carried out to understand the phenomena of plasmonic coupling occur in the individual metal nanoparticles however their collective behavior is still less explored and presently a subject of academic and practical interest.

Since the isostatic point of silica is about pH 2-3 in water 24, the silica particles in sols have large negative charge at neutral and basic conditions 25-26. The films prepared by the electrophoretic sol-gel deposition are basically composed of monodispersed spherical particles, and have a lot of open spaces among these particles. If the open spaces are filled with some organic polymers, new types of inorganic-organic composite films with unique characteristics are expected to be obtained.

The developments in nanotechnology provide a way to engineer nanoparticles using thin films for the real world applications 27-28-29. Nanoparticulate thin films are fabricated using thin layers of composite materials with a thickness of few nanometers 30-31. The ionic self assembly technique, introduced by Decher 32-33 provides an efficient method to control the composition of the multilayer thin film assemblies and the thickness of each layer on the nanometer scale. Most of the charged materials, including almost all kinds of polyions, organic/inorganic particles, dyes and other biosystems have been integrated into multilayer assemblies using this technique. These multilayer thin film have potential applications in nonlinear optical materials and conductive films 34-35. Figure 1.2 demonstrates the formation of multilayers of oppositely charged nanoparticles over the substrates.

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Figure 1.2 Self assembly of oppositely charged layers on substrates 36

The layer-by-layer (LBL) self assembly method with a precise control of film thickness and choice of materials has been applied to the fabrication of the 3-D superstructure 37-38. The driving forces in preparation of such composite structures involve ion-ligand and ligand- metal bridges 39, covalent bonding 40, and electrostatic attraction among oppositely charged nanoparticles 41. This method has been applied to develop composite films consisting of noble metal nanoparticles or binary inorganic colloids.

Jafri et al. developed a pressure sensing unit using novel physical concepts for an ultra­sensitive and reliable pressure sensor that can work up to 1.5 M Pascal (15 Bar) pressure ranges which can be integrated into standard Tsunami early warning detection systems. Different combinations of manganese doped zinc sulfide nanoparticles and chitosan with negatively charged gold nanoparticles and polyacrylic acid (PAA) were used to build multilayers. It was argued that when external pressure was applied, the polymers got compressed and thus reducing the gap between layers of nano particles that resulted in changing current voltage characteristics. Sujira et al. demonstrated the self organization of thin films composed of chitosan capped zinc sulphide nanoparticles and electrostatically stabilized gold nanoparticles by the layer-by-layer process. The current-voltage characteristics of these multilayers films showed a current blockade through the device until the applied voltage level reached a critical onset value. After which the current increased linearly in both the forward and reversed biased conditions.

1.2 Statement of the problem

Encapsulation or coating of the nanoparticles is of great concern because of their finitely smaller dimensions with high surface area high surface energy of the nanoparticles. Most of the inorganic sols such as gold and silica nanoparticle are not uniformly dispersed and tend to agglomerate 42-43. Polyelectrolytes can stabilize them against agglomeration by coating them that can provide electrosteric stabilization and surface charges as well. In order to get the appropriate charge on the colloidal nanoparticles, modification of the surface of nanoparticles is necessary or pH value of the colloidal solution can be adjusted, since the degree of ionization of the capping polyelectrolytes also vary with the pH.

It is necessary to understand the optical properties of colloids and its subsequent effects in the multilayers that gives a direct correlation to the manner; the nanoparticles have been assembled in the layers. The electromagnetic field excites the metal nanoparticles producing a strong optical absorption contributed due to the collective oscillation of electrons on the particle surface, known as surface plasmon resonance (SPR). This resonant frequency is controlled by the particle shape, size, material, and surrounding environment. By altering these parameters, the frequency can be tuned over a broad range of wavelengths that produces the layered coatings being very attractive for diverse applications.

Typically, multilayer structures are built up from dilute solution using alternate layers of oppositely charged polymers. Most of the self assembled thin film have focused on polyelectrolytes in their fully charged state, such as the strong polyelectrolytes. The combination of both weak and strong polyelectrolytes for capping the inorganic colloidal nanoparticles will help to understand the pH dependence on the thickness behavior of the deposited films.

In many applications using high nanoparticle fillings to assemble thin multilayered films suffered due to the aggregation of the nanoparticles resulting in irregular nanostructure features inside the multilayered thin film. The electrical and mechanical properties of the multilayered structures can be improved with uniformly shaped and controlled sizes of the nano dispersions with narrow particle size distribution of the colloids.

Devices constructed with layers of conducting nanoparticles separated by dielectric nanoparticle layers normally show capacitive effect having an onset voltage in the multilayer structure 44. This conduction onset voltage relies on the total number of individual particle layers used for formation of the multilayered devices 45. Conduction phenomena in this kind of devices is still not fully understood, but it is assumed that due to the presence of polymer and dielectric layer in between stacks of conducting gold layers, tunneling of electrons occur.

1.3 Objectives of the research work

The key objectives of this research work are listed below,

1.Synthesis of colloidal gold and silica nanoparticles using wet chemical methods.
2. Surface passivation of colloidal nanoparticles with oppositely charged polyelectrolytes.
3. Self-assembly of colloidal nanoparticles over various substrates to form multilayers with different number of layers and architectures using layer-by-layer dip coating method.
4. Optical modeling of multilayer thin films based on Maxwell-Garnett effective medium theory using different volume concentration of colloidal nanoparticles.
5. Measurement of electrical parameters such as voltage, current, resistance and capacitance of the multilayer thin films with different number of layers.
6. Application of gold-silica multilayer thin films for specific electrical and optical devices.

The colloidal growth of inorganic nanoparticles coated with organic polyelectrolytes enabled us to produce innovative nanostructures. During the coarse of this research study, various strategies are employed to build multilayer thin films using colloidal nanoparticles of metal and dielectric materials, alternately capped with organic polyelectrolytes thus to improve the electrostatic stability in formation ofheterogeneous structures.

Gold has the advantage of being the least susceptible of all metals to oxidation and a good electron conductor. Citrate ions provide electrostatic stabilisation to the gold nanoparticles in a colloid formed from the reduction of gold chloride by trisodium citrate 46.Colloidal silica was first synthesized by stöber process that showed remarkable colloidal stability 47. Silica nanoparticles were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol and followed by the catalysis using ammonia. Polyelectrolytes such as chitosan and PDDA Poly(diallyldimethylammonium chloride, are highly polar compounds and capable of forming hydrogen bonds thus capping the colloidal gold and silica nanoparticles and providing electrosteric stabilisation to the nanoparticles in the colloid. Polyelectrolyte capped colloidal nanoparticles were self assembled into multilayered thin films using layer-by-layer deposition process.

Current-voltage measurements of the multilayered devices helped to build electronics devices that behaved as resistor with a variable capacitance.

Optical modeling of multilayer thin films consisting of metal nanoparticles was carried out using Maxwell-Garnett Effective Medium theory using quasi-static limit to understand the assembly of these multilayer films.

1.4 Hypothesis

The addition of inorganic spherical nanoparticles to organic polymers allowed the modification of the chemical and physical properties of the nanoparticles. The extent of binding of colloidal nanoparticles by polymer depend on the concentration of nanoparticles in solutions, polyelectrolyte concentration, pH and other factors. The phenomena of electronic tunneling was anticipated through these metal-insulator multilayered devices. It was expected that by changing the number and thickness of layers, size and spacing of metal inclusion, the electrical and optical properties can be tuned to match the innovative device applications.

1.5 Scope and limitations of the research work

Layer-by-layer process for the fabrication of multilayer thin films was carried out using an automated dip coating machine. Most of the process parameters dip time, dry time, wash time and pH of the colloidal solutions were controlled but still any variations in the environmental conditions such as temperature and moisture can cause the non uniformity in multilayers for different set of experiments. Provision of a dry nitrogen system assembled with the dip coating machine would help to dry the moisture absorbed by the film during the fabrication process. Silica nanoparticles and the polyelectrolyte (chitosan) are well known materials to absorb moisture, therefore the gold-silica multilayer films had to keep in humidifier to avoid any errors in electrical and optical measurements. The software _Scout‘ used for the simulations of gold- silica composite films has some assumptions, such as no light scattering for the nanoparticles less than 20nm and considers uniform diameter of the colloidal nanoparticles, these conditions does not perfectly match with the experimental results as the colloidal nanoparticles do not have a perfect particle size rather a range of sizes with an average value for the highest count of all particle sizes. Moreover, the electrical conduction mechanism and the optical properties of the gold-silica thin films has been less explored so interpretations of the results were sought carefully. The electronic microscopes such as Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) are not available in-house that caused a time delay for the characterisation of the colloidal nanoparticles and the multilayer films. Availability of the electronic microscopes at the lab would help to perform more set of experiments and broaden the scope of research work.

1.6 Organization of the research thesis

This research thesis has been organized into five chapters. Introduction of the colloidal nanoparticles and thin film fabrication is given in chapter 1. The detailed literature review covering the topics about synthesis of the colloidal gold and silica nanoparticles and their coating using the polyelectrolytes is given in chapter 2. The layer-by-layer dip coating method and the electrostatic forces guiding the self assembly is also explained in this part. A brief review about the potential application of these multilayer structures is also included in this part. The methodology used during the research study is given in chapter 3 that includes the electron microscopy and electrical test bench. The discussion about the results for most of the experimentation carried out during the research work is detailed in chapter 4. Finally a brief statement concluding the research study is given in chapter 5 along with some recommendations for the future work.

Chapter 2 LITERATURE REVIEW

2.1 'Top-down' versus 'bottom-up' assembly

Nanoscale devices and materials can be fabricated using either -top-down” or -bottom- up” fabrication methods 48-49-50. A variety of -top-down” techniques involving deposition or lithographic etching have been developed to make such materials 51-52-53. Top-down techniques takes a bulk material, do necessary machining and modification and finally shape it into a desired product. However, one thing they all have in common is the use of very big, energy intensive machinery in order to make very small scale things 54-55. The limits of lithographic techniques and optical microscopy are determined by the wavelength of light 56. The smallest distance between two lines resolved by an optical instrument (microscope or a lithographic system) is explained by Abbe‘s principle(Jmin = % λ/η sinO ~ λ/2) where Θ is the angle of the incident light 57. For a wavelength, λ=400-700 nm for the visible light, the maximum resolving power thus is half the wavelength (~ 300 nm). In order to reduce wavelength, sources become more expensive and the optics are hard to construct. The choice of using either approach of thin film fabrication depends on the average size of the nanoparticles as shown in figure 2.1.

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Figure 2.1 The Illustration of "top down" and " Bottom up" self assembly technique with the size scale forcolloids 58

In the "bottom-up" approach, molecular components are chemically self-assembled into functional materials and devices by the use of intermolecular forces 59. There are two steps involved in the fabrication of thin films with nanoparticles using _bottcm up‘ approach. In the first step, the synthesis of the colloidal nanoparticles is carried out using wet chemical methods and the second step is followed by self-assembly of these nanoparticles on suitable substrates to build thin films. The colloidal way of synthesis for nanoparticles has many advantages such as uniform dispersion of the particles within the colloid results in a narrow particle size distribution. Moreover, the colloidal route of synthesis allows the surface-modification of these nanoparticles by suitable surfactants using solution-based self-assembly.

A colloidal system can be considered a replica of bottom-up approaches. Generally speaking, a colloid or colloidal suspension is a dual step system of substance forming a kind of mixture in which the primary material is converted into smaller particles (colloid particles) which are then uniformly dispersed into the second material 60. The mixture can be termed as colloidal system, colloidal suspension or colloidal solution. The disperse or particle phase can be virtually any type of small liquid droplets, bubbles, amorphous or crystalline solids, or macromolecules (or collections of small molecules, e.g. micelles) to name a few. Likewise, the continuous phase can be polymers, solids, liquids, gases etc.

The size-dependent characteristics of the colloidal nanoparticles has attracted a growing interest to use them as building blocks for the fabrication of thin films 61. Metallic gold nanoparticles are better electron conductor making them a good candidate for electron transport applications 62. It also has a surface chemistry that suits to attachment of sulphur-based organic molecules in self assembly of multilayered thin films. The colloidal silica nanoparticles are good electrical insulator and with a low refractive-index are suited for electrical and optical applications 63.

2.2.1 Colloidal gold nanoparticle

In the year 1856-7, Michael Faraday was the first to synthesize colloidal gold using reduction of an aqueous solution of gold chloride with phosphorous resulting in a ruby-colored liquid 64. In this work, gold is used for multilayered film because of its high refractive index and wide wavelength range that can effect optical properties of thin films such as transmittance and reflectance 65-66. The apparent color of the gold colloid can be blue, red, or violet as shown in figure 2.2, depends on the shape and size of the nanoparticles as well as their agglomeration in the solution. Generally, the appearance of the red color for a gold colloid attributes to the surface plasmon effect and indicates a chemically stable solution with particle diameter around 20 nm 67. The agglomeration is indicated by the color changes to blue or violet 68. Most of preparation methods for gold colloids are based on precursors solutions that contain gold complexes such as (AuCl4) with tetrachloroauric acid HAuCl4 69.

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Figure 2.2 Nano-Gold colloids exhibit different colors at different sizes and concentrations 70

2.2.2.Colloidal silica nanoparticle

Silicon compounds are denser and less soluble and more readily form three-dimensional structures. Silicon compounds tend to form crystals that may incorporate or exclude water molecules. In the inorganic world of silicon, water is incorporated into the silicate crystalline structure. For example, when water is incorporated into the crystal lattice of silicates, nanoscale dislocations are formed. The sol-gel technique is commonly used for synthesis of silica nanoparticles. The chemical reaction is a combination of hydrolysis and condensation of the metal alkoxide 71 as illustrated in figure 2.3.

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Figure 2.3 Synthesis of silica nanoparticles 72

SiO2 nanoparticles are used to make electronic substrates, thin film substrates, electrical insulators, humidity sensors and thermal insulators where the particle size and size distribution plays an important role 73. Therefore, there is a need to control the particle diameter and particle size distribution for the colloidal silica nanoparticles suspension.

For optical applications, the width of the photonic band gap directly depends on the index contrast. However, a high refractive-index contrast is limited by the availability of materials with low refractive index. Gases are the materials with lowest refractive index (air @ 1.0). Nano porous SiO2 thin film materials from sol-gel process have refractive indices lower than that of dense SiO2, proper mechanical strength, and low light scattering 74.

2.3 Polyelectrolytes (PEL)

Polymers offer a number of advantages for microsystems: low material cost for high­volume fabrication; polymers are flexible and transparent75; wide ranges of material properties and surface chemistries are available; and polymers are chemically and biologically compatible. PEL uses the combined effect of both the electrostatic and steric stabilization forces that results in a stabilized suspension of colloidal nanoparticles 76. Polycations have shown a strong interaction with the metal precursor, due to the formation of ion-pair with the negatively charged tetrachloroauric acid anions. Moreover the columbic forces are also responsible for the stabilization of negatively charged gold colloids by adding cationic polyelectrolytes 77. The hydrophobic side-groups in the water-soluble homopolymers copolymers attaches with the metal surface whereas the hydrophilic part combines with the dispersion medium78.

Table 2.1: Some examples of weak and strong polyelectrolytes

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Some examples of the commonly used polyelectrolytes are given in table 2.1 shows that they can be catergorised as ’weak’ and ’strong’ types. A ’strong’ polyelectrolyte dissociates completely for the specific pH values of the solution. A ’weak’ polyelectrolyte is partially dissociated in the solution depending upon its dissociation constant. Therefore weak polyelectrolytes don‘t get totally charged in solution, their degree of dissociation can be modified by changing the pH of the solution 79-80. The chemical properties of polyelectrolytes are also affected by the degree of charge. The dissociation of polyelectrolyte releases ions that affects the ionic strength of the solution which in turn changes the colloidal stability of the solution 81.

2.3.1 Chitosan

Chitosan is a biocompatible polymer with polycationic properties in slightly acidic conditions 82-83. Extracted from chitin, which is the second most abundant natural organic polymer. Chitin forms the structural backbone of the exoskeleton of shrimps, crabs, lobsters etc. Chitosan is extracted by alkaline deacetylation of chitin wherein the amide groups in the chitin monomer is deacetylized to amine (NH2) groups. The presence of free amines in chitosan makes it insoluble in water at neutral pH and in strong acids 84-85. In dilute acids, the amines get protonated to form NH3+ making it possible for chitosan to be soluble in dilute acids. For example, chitosan easily dissolves in 1% acetic acid showing net positive charge due to the presence of the amine group 86. The chemical structure of chitosan is given in figure 2.4.

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Figure 2.4 Chemical structure of a chitosan polymer [86]

2.3.2 PDDA

Poly(diallyl dimethylammonium) chloride (PDDA), is a cationic polyelectrolyte that is soluble in water. PDDA has the ability to reduce and stabilize the colloidal gold nanoparticles (AuNPs) 87-88. An interesting fact about PDDA is that it is a quaternary ammonium polyelectrolyte 89. Therefore besides reducing and stabilizing the amine-coated metal nanoparticles, it can passivate the surface using quaternary ammonium or salts. Figure 2.5 shows the chemical structure of PDDA.

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Figure 2.5 The molecular structure of PDDA [89]

Generally metal colloids are unstable due to aggregation that causes precipitation 90-91-92. These nanoparticles are capped with a layer of ligand to stabilize them against aggregation as well as corrosive chemical reactions 93-94. When the interparticle distance is short i.e. in nanometers, the van der Waals forces among the nanoparticles overcome the competing repulsive forces resulting in interparticle attraction. However the repulsion among the tiny solid particles provides the colloidal stability in a liquid medium 95.

Generally, the metallic nanoparticles are synthesized either by the controlled segregation at the metal-organic solvent interface or by the reduction of metal salts 96-97-98:

A rapid nucleation is needed to produce highly monodisperse particles, that keeps the solution below saturation level thus enabling the slow consumption of all the precursors controlled growth. The rapid nucleation can be achieved by decomposition of metallic precursors followed by the rapid addition of the reducing agent. During the synthesis process, the colloidal solution needs to be stabilised to limit the agglomeration and a sustained growth rate. The particle aggregation is controlled by using the capping agents covering the particle surface that also helps in functionalization of the colloid 99.

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Figure 2.6 Idealized morphologies of different types of nanocomposite particles. Core shell (1,8), occluded (2), raspberrylike (3, 6), hairy (4), shell crosslinked (5), multilayered (7), and hybrid interpenetrated organic/inorganic networks (9) 100

Nano composites are formed by capping inorganic nanoparticles, clusters, clays, oxides or metals, in combination with a variety of sol-gel derived polymers, organic polymers or biological enzymes as illustrated by the various morphologies given in figure 2.6. Using different synthesis methods and thin film strategies, hybrid structure are formed and then analysed using their catalytic properties and magnetic, optical, and electrical arising from the enhanced mechanical and thermal stability originating from the polymeric matrix. Different combination of size and composition (semiconductor, metal and oxide) of these nanomaterials, have been used for the formation of nanocomposite thin films 101-102.

Dendrimers interact weakly with the gold nanoparticles due to the presence of alcohol and amine functionalities and preserve the electronic properties 103. While the thiols are proved to have a strong interaction with the metal surface producing an electrical charged atmosphere. The surrounding layers of these stabilizers prevent the interaction of the nanoparticles with the incoming ligands 104.

2.5 Stabilisation of colloidal nanoparticles

The attachment of a stabilizing agent to nanoparticles lowers the surface free energy and, therefore, the reactivity of the particles is reduced. A physical barrier around the nanoparticles is provided by these stabilizers that prevents a direct interaction between the metal surfaces. A change in their surface charge occurs that enhances its stability toward agglomeration.

2.5.1 Electrostatic stabilization

The basis of electrostatic stabilization are the repulsive forces that exist between same- charged nanoparticles in the colloid 105. The charge produces because of unsaturated surface atoms. But this electrostatic charge gives stabilization to the particles. As the gold nanoparticles are sythesized by reduction with citrate, that results in the adsorption of citrate ions over the surface of the gold nanoparticles and as a result, a negatively charged surface surrounds the particle as demonstrated in figure 2.7. An electrical double-layer covers the surface of gold nanoparticles due to the presence of these cations in solution forms [ 106.

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Figure 2.7 Electrostatic stabilization due to columbic repulsion between the adsorbed ions on the particle surfaces

2.5.2 Steric stabilization

Steric stabilization is achieved due to the steric repulsion between the large hydrophobic chains of stabilizing agents [ 107. The nanoparticles are driven away by the osmotic pressure due to increased volume fraction of polymers in the surrounding region. Generally, the steric mechanisms provides the stabilization for metal colloids in nonpolar solvents. Long-chain polymers, thiols, or amines generally play a significant role in stabilization of nanoparticles in solvents 108. Figure 2.8 shows a sterically stabilised system due to osmotic pressure caused by the long chained polymers surrounding the surface of the nanoparticles.

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Figure 2.8 Steric stabilisation is caused by the overlapping of highly concentrated polymer layers surrounding the nanoparticle

2.6 Surface plasmon resonance

Metallic nanoparticles exhibits unique electronic, chemical, optical, and magnetic properties that are quite different from those of both individual atoms and bulk matter 109-110. The interaction of electromagnetic fields with discrete systems is characterised by their polarizabilities and is strongly dependent on both dielectric and geometric properties of nanoparticles and surrounding medium 111-112. The surface plasmon resonance, described as the oscillation of the surface plasma electrons induced by the electro-magnetic field is used to characterize most of the metal clusters 113-114-115; consequently, optical spectroscopy (UV- Vis spectroscopy) is used to investigate their microstructure. The results of this absorption (intensity, position, shape, etc.) are directly related to the topology, nature and structure of the cluster. In real, the absorption frequency is a property of the specific metal where aggregation phenomena is reflected by the eventual peak shifting 116, the peak intensity is related to the

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Figure 2.9 Schematic representation of the interaction of a small metal nanoparticle with light, (b) particle dipolar radiation, (c) Quadrupole radiation of larger particles [120]

particle diameter 117, the absorption wavelength is related to the particle shape 118, the shift of the absorption with rise of temperature indicate growth of a cluster 119, and so on. Furthermore, information about the composition of the inner structure (inter-metallic or core/shell) for bimetallic particles, can be examined from the absorption frequency 120. A Schematic representation of the interaction of a small metal nanoparticle with light is shown in figure 2.9.

2.7 Layer-by-layer self assembly

In most of the applications of layers using deposition by adsorption from solution, the application by spraying was initiated by Schlenoff 121 and the use of spin-coaters was introduced by Wang and Hong 122-123 . The advantage of these deposition techniques is that that only small quantity of solutions are required to coat relatively larger surfaces. The concept of electrostatic interactions of charged colloids and multilayer films composed began with Iler's, who exploited the sequential deposition of positive alumina fibrils and negative silica colloids in the year 1966 124. However this novel process was not widely used until the early 1990s, until Decher and coworkers, at Gutenberg University in Germany, succeeded in development of multilayer films that were optically transparent through the use of different polyelectrolytes 125. This concept of self assembly demonstrated the use of size-independent substrates to build periodic nanostructures.

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Figure 2.10 The layer-by-layer self assembly scheme 126

The basic idea behind the electrostatic self assembly is simple as shown in figure 2.10. A single layer of film is assembled through the use of ionic bond formation on the oppositely charged substrate. After that the loosely adsorbed molecules are removed by rinsing in ultrapure water three or four times and the substrate is then immersed in the relative colloidal solution to form a single anionic/cationic monolayer. The process of rinsing is repeated in ultrapure water. In this way, a single bilayer of anionic/cationic material is fabricated. Repeating this self­assembly process produces a multilayered thin film structure.

2.8 The forces guiding colloidal assembly

Colloidal assembly offers two primary advantages. First, it typically does not require energy intensive, high technology deposition processes (like vacuum chambers, electron beams, ion beams) 127. Secondly, the forces which govern colloidal assembly (described above) are all naturally occurring physical phenomena; that is we need only to supply the directions and Mother Nature can be made to work for us 128.

Electrostatics and van der Waalforces

Two of the more prominent aspects of interparticle interactions are electrostatic repulsion and van der Waals (VDW) attraction. VDW attraction between materials, which can be estimated using their frequency dependent dielectric constants and refractive indexes, is the primary cause for particle aggregation and/or adhesion to substrate surfaces 129. Electrostatic repulsion arises between any two similar surfaces within an aqueous environment that develop a specific surface charge, or zeta potential. This surface charge leads to the development of counter-ionic layers in the adjacent solvent surrounding each particle which become mutually repulsive as they overlap (upon particle approach) 130. With few exceptions, electrostatic repulsion serves to stabilize colloidal dispersions, counteracting the VDW attraction between particles.

Depletionforces

In mixtures containing either particles with multimodal size distributions or mixtures of particles and polymers in solution, forces can arise due to local differences in species concentrations. Geometric exclusion of the smaller species from the interstitial spaces between larger particles causes an osmotic pressure that results in an attractive force between the larger particles which scales as ~ kT per polymer molecule (referring to the thermal energy, k being Boltzman‘s constant and T being the absolute temperature) 131.

Capillaryforces

When particles are protruding through either a liquid-liquid interface or a vapor-liquid interface, the curvature of the meniscus between the particles can induce strongly attractive or repulsive forces as the liquids involved minimize their surface free energy 132.

Hydrogen bonding

The term -H-bond” comes from that fact that these interactions only involve hydrogen atoms, which by their tendency to be positively polarized will orient themselves between their covalently bonded atom and electronegative atoms in close proximity.

Hydrogen bonds are not specific to water, and can occur between electronegative atoms (such as Cl, F, N, O, etc.) and H atoms that are attached to other similar electronegative atoms through covalent bonds. The H-bond interaction strength is approximately ~20 kJ/mole, which is less than ionic or covalent interactions (~ 500 kJ/mole), but sufficiently greater than kT to mediate intra- and intermolecular assemblies.

2.9 The role of pH buffers

Buffers are composed of a weak acid and its salt or a weak base and its salt. An acid or a base, if mixed with water will produce OH' or H3O+ ions that increase or decrease the total pH. A pH buffer solution resists pH change due to the fact that addition of a base or an acid is neutralized by the existing A- / HA.

The acid added to the solution becomes neutral by the conjugate base (A-) that is transformed to the acid (HA). Similarly a base becomes neutral by acid (HA), and then is changed to the conjugate base (A-). Hence addition ofbases or acids changes the A- /HA ratio.

If the mmol values of both A- and HA are determined then the volume of 1M solution of NaOH can be calculated. The amounts of the conjugated base and the conjugated acid can be calculated through the use ofHenderson-Hasselbalch equation 133 as given in equation (2.1).

pH = pKa + log (A-) / (HA) (2.1)

2.10 The formation and structure of thin films

There are some important differences that give thin films the unique characteristics.

The size effects

One of the properties of the physically small dimensions is the size effects of the material that includes quantum mechanical tunneling and surface scattering of charge carriers.

Method of filmpreparation

The optical and electrical properties of metal and insulator films depends largely on the deposition conditions, The synthesis of colloidal solutions, structural and electronic defect concentrations, chemical composition and stoichiometry, dislocation densities, density, void or porosity content, grain morphology, electron trap densities, eventual contact reactions, etc. However metals are less affected by these conditions yet the insulators (e.g., oxides, nitrides) are particularly prone to these effects.

The deposition of organic and inorganic materials employing the techniques such as dip­coating, spray-coating, spin-coating and roller-coating provides a simple alternative to the more time consumed processes of vacuum evaporation. Moreover these deposition methods also provide flexibility to be used on a variety of substrates (glass, plastics, metal electrodes and inorganic wafers) to deposit polymers and nanoparticles. A Comparison between different deposition methods is given in table 2.2.

High chemical reactivity

The thin films are susceptible to time aging because of atmospheric oxidation, corrosion, sulfidation, absorption of water vapor, and low-temperature solid-state reactions.

Degree of film continuity

The improper deposition conditions can give rise to discontinuous films with island structure and the conduction mechanism is different as in case of continuous films.

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Table 2.2 : Comparison between different deposition methods on glass 134

2.11 Wettability of thin films

The surface wettability plays an important role in the fabrication of thin films. A good wettability (contact angle less than 90o) is required for absorption of colloids over substrate surfaces and poor wettability (contact angle greater than 90o) reduces the attachment of colloidal particles as can be seen from figure 2.11. An increased surface roughness also helps to improve the wettability of film. Moreover, an enhanced surface coverage is also necessary to deposit monolayers on the structured surfaces.

The Young‘s equation details the classical contact angle Θ of a liquid drop placed over a flat homogeneous surface as given in equation (2.2) 135.

Ylg CosΘ = Ysg -Ysl (2.2)

Where Ylg, Ysv and Ysl are the surface tensions or surface energies of the liquid-vapor, solid- vapor and solid-liquid phase interfaces.

The hydrophilic property also depends on surface treatment of the glass substrate by silanes such as APTS(3-aminopropyl-trimethoxysilane) and the pH of the starting solution in the dip coating process. The presence of excessive H+ ions on the substrate surface can be a reason that acid solutions turns a surface more hydrophilic.

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Figure 2.11 Different degrees of wetting, (a) High wetting with smaller contact angle, (b) Low wetting with larger contact angle

2.12 Electrical properties of metal-insulator films

Electrical impedance can be measured as a function of several parameters like temperature, applied voltage, atmosphere surrounding the sample, etc. Each type of measurement gives complementary information about the sample, providing that the electrical contacts are well defined. Current-Voltage (I-V) characteristics are mainly used to characterize the electrical interactions between the electrical contacts and the sample to be measured. Metal-insulator contacts are typically schottky barriers. If the barrier is high, the contact impedance is high when the contact is reverse polarized. If the barrier is sufficiently low, the contact behaves like a linear resistor for both polarizations. Information on the barrier height can be obtained from I-V curves. When the insulator is highly resistive, non-linear, power-law I-V curves are often observed, which indicate various forms of carrier injection. Current-Voltage characteristics of a device with LbL deposition of gold and zinc sulfide nanoparticles is shown in figure 2.12.

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Figure 2.12 Current-voltage characteristics of a device with LbL deposition of gold and zinc sulfide nanoparticles having 150 layers a) Current vs. Supply voltage b) Current vs. Device voltage [135]

2.13 Quantum tunneling

The quantum tunneling is the mechanical process which permits the electrons to penetrate through a tiny potential barrier in case of nano structures. In 1958, Esaki's was the first to observe the electron tunneling effects in semiconductors. After that, Giaever also demonstrated the tunneling of electrons through a very thin layer of oxide placed in between metal electrodes of a superconductor either in a normal or superconducting state 136.The results helped in the understanding of important charging effects such as Columbic oscillations and Coulomb blockade.

A thin film metal insulator metal (MIM) diode can be termed as a quantum tunneling device as shown in figure 2.13. The condition for electron tunneling is that the insulator layer must be thin compared to the de Broglie electron wavelength. For the device to work as a diode, there tunneling direction is always preferred, that results in steep energy level in the direction of the voltage bias.

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Figure 2.13 Metal insulator insulator metal (MIIM) diode: Cross section of diode. Energy levels for no bias, forward bias, and reverse bias 137

2.14 Fowler-Nordheim (F-N) tunneling

Fowler-Nordheim (F-N) tunneling is the quantum tunneling of carriers across a triangular potential barrier in the presence of an external electric field. The barrier of the insulator is lowered due to the field E so that the electron tunneling occurs from the metal Fermi level into the oxide conduction band. After the carriers tunneled into the insulator they can move freely within the valence/conduction band of the insulator. To understand this current conduction mechanism, experimental current-voltage (I-V) characteristics are plotted as ln(JFN/E2ox) - 1/Eox, known as Fowler-Nordheim plot. The tunneling depends on the applied electric field (E) and insulator thickness (di). Fowler-Nordheim tunneling occur when the interfacial potential barrier (Ob) satisfies the following equation,

illustration not visible in this excerpt138

The experimental data can be plotted to fit into straight lines the slope of which can determine the barrier height under the applied E field as illustrated in figure 2.14.

2.15 Thermal properties of thin films

The evolution of the conductivity of the thin film as a function of temperature gives information on the conduction mechanisms. In the case of a metal, the electrical conductivity decreases when the temperature increases. Initially proposed for the bulk materials, the Matthiessen‘s rule explains that the electron scattering processes are also valid for the thin metal films and contributes towards the total resistivity (pT) of the metal across various layers of the film through the relation given in equation (2.4)

Pt = PTh + Pi + Pd (2.4)

Where pTh = thermal resistivity, pI = Impurity resistivity, pD = Defect resistivity

The collusion of electrons with the oscillating atoms (phonons) that have changed their equilibrium state inside the crystal lattice are the primary source of the thermal pTh or phonon activity which increases linear with temperature. This causes a positive temperature coefficient of the resistivity. Figure 2.15 shows the dependence of electrical resistivity as a function of temperature for metals.

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Figure 2.15 Electrical resistivity dependence as a function of temperature for metals 139

2.16 Optical properties of thin multilayered structures

Classical electrodynamics precisely explains the scattering and absorption of the electromagnetic waves with the particles diameters in nanometer. James Clerk Maxwell formulated the equations which explain the macroscopic electromagnetic fields are termed as Maxwelľs Equations. These laws of explains the dynamics of the magnetic and electric parts of an electromagnetic field, as well as their mutual relationship. Maxwelľs equations for an oscillating electromagnetic field are given in equations (2.5),(2.6),(2.7) and (2.8) 140.

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Where Η denotes the magnetic field, Ε denotes the electric field, ω is the angular frequency, μ denotes the permeability, and ε is termed as the permittivity (dielectric constant).Therefore Maxwelľs Equations are helpful in calculations for the dielectric constant and optical properties for sphere of arbitrary radius.

The optical parameters of spherical metallic nanoparticles can be adjusted by tuning their physical dimensions. As the diameter of nanoparticles is increased, the plasmonic peaks get wider and shifted towards the longer wavelengths. The dielectric properties of the material also determine the intensity and position of the plasmonic resonances. Another important parameter is particle extinction measurement. Extinction is the combination of scattering and absorption processes. The applied electromagnetic field accelerates the charged nanoparticles that results in scattering and eradiation.

[...]


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Title
Fabrication and characterisation of multilayer thin film using self assembly of colloidal gold and silica nanoparticles
College
Asian Insitute of Technology
Author
Year
2011
Pages
108
Catalog Number
V334799
ISBN (eBook)
9783668243040
ISBN (Book)
9783668243057
File size
4050 KB
Language
English
Tags
fabrication
Quote paper
Dr Zaheer Abbas Khan (Author), 2011, Fabrication and characterisation of multilayer thin film using self assembly of colloidal gold and silica nanoparticles, Munich, GRIN Verlag, https://www.grin.com/document/334799

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