Transparent conducting oxides (TCOs) have attracted extensive attention for decades due to their remarkable applications in optoelectronic devices. The development of functional nanostructured TCOs with unique properties, and an expansion of their functionalities are therefore research directions of significant current interest. Among TCOs, In2O3 is widely applied because of its high charge carrier concentration and mobility, as well as the ease with which it can be deposited as a thin film. The important role of surfaces in tuning properties in materials shows the importance of studying nanostructured materials with high surface areas. In this thesis I examined the synthesis of phase-controlled In2O3 nanocrystals (NCs) and showed the effect of doping and composition on the materials properties. Owing to the relevance of size, structure, and composition for manipulating properties of nanomaterials, synthesis of well-defined nanocrystals of pure and doped In2O3 has been of considerable interest for fundamental studies as well as for technological applications. Phase controlled synthesis of colloidal In2O3 NCs was achieved via a size-structure correlation. The study of the morphological and phase transformations of In2O3 NCs during their growth in solution implies that corundum (rh-In2O3) is a transient structure in the formation of cubic bixbyite (bcc-In2O3) phase. The formation of NCs smaller than 5 nm leads to the spontaneous stabilization of metastable phases owing to the surface energy and/or surface stress contributions, both of which are dependent on size. The growth beyond the critical size lowers the potential energy barrier height and causes the nanocrystal phase transformation. In addition, phase transformation of colloidal In2O3 NCs in the temperature range of 210-260 ˚C during their synthesis in solution was studied using a combination of structural and spectroscopic methods, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and analyzed data using Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) and interface nucleation models. The phase transformation occurs via nucleation of bcc-In2O3 phase at the interface between contacting rh-In2O3 NCs, and propagates rapidly throughout the NC volume. In situ high temperature XRD patterns collected during nonisothermal treatment of In2O3 NCs reveal that phase transformation of smaller NCs occurs at a faster rate and lower temperature, which is associated with the higher packing density and contact formation probability of smaller nanoparticles. Owing to the fact that NC surfaces and interfaces play a key role in phase transformation, their control through the synthesis conditions and reaction kinetics is an effective route to manipulating NC structure and properties. Although, doping semiconductor NCs is crucial for enhancing and manipulating their functional properties, the doping mechanism and the effects of dopants on the nanocrystal growth and structure are not well understood. We show that dopant adsorption to the surfaces of colloidal In2O3 NCs during incorporation inhibit NC growth and leads to the formation of metastable rh-In2O3 for nanocrystals smaller than ca. 5 nm. Direct comparison between Cr3+ and Mn3+ dopants indicates that the nanocrystal structure directly determines the dopant incorporation limits and the dopant electronic structure, and can be predicted and controlled. These results enable a new approach to designing multifunctional nanostructures and understanding the early stages of crystal growth in the presence of impurities. Nanocrystalline films fabricated from colloidal Cr3+- and Mn3+- doped In2O3 nanocrystals exhibit strong ferromagnetic ordering up to room temperature. The absence of ferromagnetism in the free standing transition metal (TM)-doped In2O3 NCs and appearance of ferromagnetism only in TM:In2O3 films prepared from colloidal NCs, are attributed to the formation of extended structural defects, proposed to be oxygen vacancies at the NC interfaces. In fact, in TM:In2O3 NCs with high surface to volume ratios, more oxygen vacancies are present at the surface of NCs and networking of NCs in the prepared film causes an increase in grain-boundary defects at the interfaces. A comparative study of magnetic circular dichroism (MCD) spectra of Cr3+-doped bcc-In2O3 and Cr3+-doped rh-In2O3 revealed that Cr3+ ions distinctly occupy different symmetry sites in corundum and bixbyite crystal structure of In2O3. In fact, a change in the crystal structure of In2O3 from bixbyite to corundum changes the electronic configuration of Cr3+. By manipulating the NC composition and structure in solution we applied a one-step synthesis of ternary gallium indium oxide (GIO) nanocrystals with variable crystal structures. The structures and sizes of GIO NCs can be simultaneously controlled, owing to the difference in the growth kinetics of In2O3 and Ga2O3 NCs, and the polymorphic nature of both materials. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, were used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor -acceptor pair (DAP) recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In3+ occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga2O3 nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition.

Transparent conducting oxides (TCOs) have attracted extensive attention for decades due to their remarkable applications in optoelectronic devices. The development of functional nanostructured TCOs with unique properties, and an expansion of their functionalities are therefore research directions of significant current interest. Among TCOs, In2O3 is widely applied because of its high charge carrier concentration and mobility, as well as the ease with which it can be deposited as a thin film. The important role of surfaces in tuning properties in materials shows the importance of studying nanostructured materials with high surface areas. In this thesis I examined the synthesis of phase-controlled In2O3 nanocrystals (NCs) and showed the effect of doping and composition on the materials properties. Owing to the relevance of size, structure, and composition for manipulating properties of nanomaterials, synthesis of well-defined nanocrystals of pure and doped In2O3 has been of considerable interest for fundamental studies as well as for technological applications. Phase controlled synthesis of colloidal In2O3 NCs was achieved via a size-structure correlation. The study of the morphological and phase transformations of In2O3 NCs during their growth in solution implies that corundum (rh-In2O3) is a transient structure in the formation of cubic bixbyite (bcc-In2O3) phase. The formation of NCs smaller than 5 nm leads to the spontaneous stabilization of metastable phases owing to the surface energy and/or surface stress contributions, both of which are dependent on size. The growth beyond the critical size lowers the potential energy barrier height and causes the nanocrystal phase transformation. In addition, phase transformation of colloidal In2O3 NCs in the temperature range of 210-260 ˚C during their synthesis in solution was studied using a combination of structural and spectroscopic methods, including X-ray diffraction (XRD), transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy, and analyzed data using Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) and interface nucleation models. The phase transformation occurs via nucleation of bcc-In2O3 phase at the interface between contacting rh-In2O3 NCs, and propagates rapidly throughout the NC volume. In situ high temperature XRD patterns collected during nonisothermal treatment of In2O3 NCs reveal that phase transformation of smaller NCs occurs at a faster rate and lower temperature, which is associated with the higher packing density and contact formation probability of smaller nanoparticles. Owing to the fact that NC surfaces and interfaces play a key role in phase transformation, their control through the synthesis conditions and reaction kinetics is an effective route to manipulating NC structure and properties. Although, doping semiconductor NCs is crucial for enhancing and manipulating their functional properties, the doping mechanism and the effects of dopants on the nanocrystal growth and structure are not well understood. We show that dopant adsorption to the surfaces of colloidal In2O3 NCs during incorporation inhibit NC growth and leads to the formation of metastable rh-In2O3 for nanocrystals smaller than ca. 5 nm. Direct comparison between Cr3+ and Mn3+ dopants indicates that the nanocrystal structure directly determines the dopant incorporation limits and the dopant electronic structure, and can be predicted and controlled. These results enable a new approach to designing multifunctional nanostructures and understanding the early stages of crystal growth in the presence of impurities. Nanocrystalline films fabricated from colloidal Cr3+- and Mn3+- doped In2O3 nanocrystals exhibit strong ferromagnetic ordering up to room temperature. The absence of ferromagnetism in the free standing transition metal (TM)-doped In2O3 NCs and appearance of ferromagnetism only in TM:In2O3 films prepared from colloidal NCs, are attributed to the formation of extended structural defects, proposed to be oxygen vacancies at the NC interfaces. In fact, in TM:In2O3 NCs with high surface to volume ratios, more oxygen vacancies are present at the surface of NCs and networking of NCs in the prepared film causes an increase in grain-boundary defects at the interfaces. A comparative study of magnetic circular dichroism (MCD) spectra of Cr3+-doped bcc-In2O3 and Cr3+-doped rh-In2O3 revealed that Cr3+ ions distinctly occupy different symmetry sites in corundum and bixbyite crystal structure of In2O3. In fact, a change in the crystal structure of In2O3 from bixbyite to corundum changes the electronic configuration of Cr3+. By manipulating the NC composition and structure in solution we applied a one-step synthesis of ternary gallium indium oxide (GIO) nanocrystals with variable crystal structures. The structures and sizes of GIO NCs can be simultaneously controlled, owing to the difference in the growth kinetics of In2O3 and Ga2O3 NCs, and the polymorphic nature of both materials. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, were used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor -acceptor pair (DAP) recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In3+ occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga2O3 nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition.

Transparent conducting oxides (TCOs) usually have high conductivity and transparency in the visible range and have been widely used in daily life. Recently, TCOs have attracted great interest due to their potential applications in various new optical and electrical devices (flat-panel displays, energy efficient windows, etc.). Nanostructured TCOs can induce new size related properties, for example, when sizes of TCOs are controlled at the nanometer scale, various defects can introduce different defect-related optical emissions. These new nanostructured TCOs combining traditional and new size dependent properties may be used for construction of next generation optical devices. To investigate the optical properties of TCOs at nanoscale, in this thesis, several new kinds of colloidal nanocrystals (NCs) of TCOs have been synthesized and their optical emission and transparency have been explored. The first part of my work focuses on ITO (indium tin oxide) NCs demonstrates phase and size dependence of surface plasmon absorption in the near infrared region. The second part of the thesis describes colloidal synthesis of γ-Ga2O3 with size tunable photoluminescence, further study reveals that the photoluminescence is defect related and can be tuned by changing the defect concentration. In the last part of my study, I develop a methodology for lanthanide doped γ-phase Ga2O3 NCs and reveal tunable chromaticity of the lanthanide doped NCs.

Transparent conducting oxides (TCOs) usually have high conductivity and transparency in the visible range and have been widely used in daily life. Recently, TCOs have attracted great interest due to their potential applications in various new optical and electrical devices (flat-panel displays, energy efficient windows, etc.). Nanostructured TCOs can induce new size related properties, for example, when sizes of TCOs are controlled at the nanometer scale, various defects can introduce different defect-related optical emissions. These new nanostructured TCOs combining traditional and new size dependent properties may be used for construction of next generation optical devices. To investigate the optical properties of TCOs at nanoscale, in this thesis, several new kinds of colloidal nanocrystals (NCs) of TCOs have been synthesized and their optical emission and transparency have been explored. The first part of my work focuses on ITO (indium tin oxide) NCs demonstrates phase and size dependence of surface plasmon absorption in the near infrared region. The second part of the thesis describes colloidal synthesis of γ-Ga2O3 with size tunable photoluminescence, further study reveals that the photoluminescence is defect related and can be tuned by changing the defect concentration. In the last part of my study, I develop a methodology for lanthanide doped γ-phase Ga2O3 NCs and reveal tunable chromaticity of the lanthanide doped NCs.

Metal oxides are a very important class of materials with a wide range of photovoltaic applications. Transparent conducting oxides (TCOs) are the primary front contact materials used in thin film solar cells. Identification of methods for reducing the resistivity of these materials would have significant benefits. Development of p-type TCOs would provide alternative back contact materials and could enable further development of technologies such as bifacial, window and multijunction cells. A series of studies into these areas is presented in this work. Aluminium doped zinc oxide (AZO) is a well-known n-type TCO consisting entirely of Earth-abundant materials. Targets were manufactured from AZO powder, which was synthesised using a patented emulsion detonation process developed by Innovnano S.A. All films showed good optical transmission. Resistivity was found to decrease with both increasing time and temperature up to 300 degree C. Temperatures above 300 degree C were found to be detrimental to film formation, with increasing amounts of damage to the crystal structure and consequent increases in the resistivity. The effect of alloying molybdenum oxide with molybdenum nitride through reactive sputtering in a mixed oxygen-nitrogen atmosphere was investigated. All alloys were found to show p-type behaviour. Resistivity was found to improve with increased nitrogen content, in contrast to optical transmission, which reduced. A selection of compositions were deposited onto CdTe cells as back contacts. These cells showed an increase in efficiency with increasing nitrogen content. Work function was found to increase with increasing oxygen content, but all work functions were low. Resistivity was shown to correlate strongly with efficiency, caused by a corresponding increase in cell voltage. This implies that to form an ohmic contact on CdTe with p-type materials, work function may be less important than resistivity. The copper oxides are p-type, but uses are limited by the narrow band gaps. Cupric oxide was chosen for investigation and for alloying with other oxides with the aim of increasing the band gap. It was found that temperature and deposition environment have significant impacts on sputtered cupric oxide (CuO) films, with low temperatures and high oxygen environments producing the lowest resistivities. Extrinsic sodium doping was found to reduce the resistivity by up to four orders of magnitude. High oxygen content sodium-doped films were found to have carrier concentrations two orders of magnitude higher than that of indium tin oxide.

There are a number of “smart” coatings that can be applied to glass. These include self-cleaning coatings based on titanium dioxide, and low-E coatings based on fluorine-doped tin oxide. Products are often more desirable with colour options such as Pilkington Activ BlueTM. There are currently no alternatives to body tinting glass to achieve colour, which is a time-consuming and expensive procedure. The work in this project details a number of coloured coatings via the AACVD or combustion processing of metal nitrate/urea precursors.

Epsilon-near-zero materials are an emerging class of nanophotonic materials which engender electromagnetic field enhancement and small phase variation due to their approximate zero permittivity. These quasi-static fields facilitate a number of unique optical properties such as supercoupling, subwavelength confinement, and enhanced light-matter interactions, which has made epsilon-near-zero media a rapidly expanding field of optical physics. Contemporary methods of realizing a system with zero permittivity rely on microwave cavities/waveguides or complex metal-dielectric metamaterials; however, both techniques require advanced fabrication and their operational wavelength is fixed relative to their geometric and optical parameters. It remains an open and substantial challenge to realize an epsilon-near-zero material at pertinent wavelengths, particularly near- and mid-infrared, with tunable/dynamic properties. The focus of this thesis is the exploration of transparent conducting oxides for the development of epsilon-near-zero nanophotonic phenomena and applications. Transparent conducting oxides have an inherent low permittivity, in addition to simple fabrication and tunable optical properties, making them exceptionally promising. Application of transparent conducting oxide films for highly confined modes, nonlinear/ultrafast optics, and strongly coupled systems are discussed.

In this study, the characteristics of LPD-NiO, and lithium-doped LPD-NiO filmson glass substrate were investigated. In our experiment, we do some measurement about physical, chemical, electrical and optical properties for LPD-NiO and lithium-doped LPD-NiO films and discussed with them. The NiO film thickness was characterized by field emission scanning electron microscopy (FE-SEM), structure was characterized by X-ray diffraction (XRD), chemical properties were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). Electrical properties were characterized by four-point probe, and optical properties were characterized by a reflecting spectrograph. The thermal annealing was used to improve the characteristics of LPD-NiO and lithium-doped LPD-NiO films in nitrogen, air and nitrous oxide ambient. For lithium doping, the lithium chloride was used as the doping solution and the electrical characteristics were enhanced. After thermal annealing in air at 400 oC, the resistivity of NiO films is 7.5 ¡Ñ 10-1 ohm-cm and can be lowed to 7.2 ¡Ñ 10-3 ohm-cm with lithium doping.

abstract: The object of this body of work is to study the properties and suitability of zinc oxide thin films with a view to engineering them for optoelectronics applications, making them a cheap and effective alternative to indium tin oxide (ITO), the most used transparent conducting oxides in the industry. Initially, a study was undertaken to examine the behavior of silver contacts to ZnO and ITO during thermal processing, a step frequently used in materials processing in optoelectronics. The second study involved an attempt to improve the conductivity of ZnO films by inserting a thin copper layer between two ZnO layers. The Hall resistivity of the films was as low as 6.9×10-5 -cm with a carrier concentration of 1.2×1022 cm-3 at the optimum copper layer thickness. The physics of conduction in the films has been examined. In order to improve the average visible transmittance, we replaced the copper layer with gold. The films were then found to undergo a seven orders of magnitude drop in effective resistivity from 200 -cm to 5.2×10-5 -cm The films have an average transmittance between 75% and 85% depending upon the gold thickness, and a peak transmittance of up to 93%. The best Haacke figure of merit was 15.1×10-3 . Finally, to test the multilayer transparent electrodes on a device, ZnO/Au/ZnO (ZAZ) electrodes were evaluated as transparent electrodes for organic light-emitting devices (OLEDs). The electrodes exhibited substantially enhanced conductivity (about 8×10-5 -cm) over conventional indium tin oxide (ITO) electrodes (about 3.2×10-5 -cm). OLEDs fabricated with the ZAZ electrodes showed reduced leakage compared to control OLEDs on ITO and reduced ohmic losses at high current densities. At a luminance of 25000 cd/m2, the lum/W efficiency of the ZAZ electrode based device improved by 5% compared to the device on ITO. A normalized intensity graph of the colour output from the green OLEDs shows that ZAZ electrodes allow for a broader spectral output in the green wavelength region of peak photopic sensitivity compared to ITO. The results have implications for electrode choice in display technology. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2010

Transparent conducting oxides (TCOs) have become an integral part of modern life through their essential role in touchscreen technology. The growing demand for cheap and superior transparent conducting layers, primarily driven by the smart phone market, has led to renewed efforts to develop novel TCOs. Currently, the most widely used material for transparent conducting applications is Sn-doped indium oxide (ITO), which has outstanding optical and electrical properties. This material is expensive though, due to the extensive use of In, and efforts to develop new low-cost transparent conducting oxides (TCO) have become increasingly important. Similarly attempts to reduce the cost of the fabrication and post-sintering steps used in making doped metal oxide thin films through innovative technologies have gained a lot of attention. With these points in mind, this research project has focused on the development of a novel low-cost aerosol assisted physical deposition method for TCO thin film fabrication and the development of new highly conducting materials to replace the expensive ITO for TCO applications. In this study, a new and simple aerosol assisted vapour deposition technique (i.e AACT) is developed to fabricate TCO films using TCO nanoparticle suspensions. Firstly, to test the validity of the method, ITO thin films are fabricated on float glass substrates from a nanoparticle suspension. The influence of the deposition parameters on the structural and opto-electronic properties of the thin films are investigated to understand the intricacies of the process. In order to investigate the fabrication of replacement materials for ITO, a range of doped zinc oxide powders are synthesised and processed using microwave radiation. Nominally, Al doped ZnO (AZO), Ga doped ZnO (GZO), Si doped ZnO (SZO), Cu doped ZnO (CZO) and Mn doped ZnO (MZO) singly doped ZnO powders are all investigated to determine the best metal dopants for transparent conducting ZnO. AZO and GZO pellets are found to present the best electrical conductivity for the singly doped microwave fabricated powders with values of 4.4 x 10-3 and 4.3 x 10-3 Ω.cm achieved reproducibly. In an effort to further improve the properties of ZnO, co-doping experiments, utilising the two best dopants from the previous work (i.e. Al and Ga) is investigated. ZnO structures that are co-doped with Al and Ga (AGZO) are found to exhibit significantly enhanced electrical properties than the singly doped powders. Typically, electrical conductivity value of 5.6 x 10-4 Ω.cm is obtained for AGZO pellets, which is an order of magnitude better than the previously fabricated materials. Finally, the best AZO, GZO and AGZO materials are utilised to fabricate thin films using the previously verified AACT technique. Further investigations into the opto-electrical properties of the resulting thin films is presented prior to the utilisation of the best films in a practical application. Transparent heaters are fabricated using the best AGZO thin films, which are capable of reaching a mean temperature of 132.3 °C after applying a voltage of 18 V for 10 min. This work highlights the potential for using highly conducting AGZO, particularly fabricated by the microwave synthesis route, as a potential alternative for ITO in a wide variety of applications. The research also highlights the advantages of using microwaves in the thermal processing of TCO materials which significantly reduces the energy impact of the production process.