ZnO nanowire arrays (NWAs) and Al-doped ZnO (AZO) cathodes were applied in hybrid organic / inorganic solar cells for lower-cost solar energy. Parameters for the electrodeposition of ZnO NWAs and the fabrication of NWA-free baseline devices were systematically optimized using ITO as the cathodes. High efficiencies of up to 5.4% were achieved. Incorporation of the ZnO NWAs into the baseline devices significantly reduced their efficiencies due to possible shorting in the active layer. Devices fabricated using AZO cathodes were characterized. The AZO-based devices achieved efficiencies of up to ~4.8%, showing promising results for the application of AZO as an ITO alternative. Formation of numerous large nanoplatelets was observed during the electrodeposition of ZnO NWAs on AZO cathodes. The NWAs grown on AZO cathodes were also non-uniform. Future studies were proposed to address the issues with incorporation of ZnO NWAs in hybrid solar cells and their combination with AZO cathodes.

ZnO nanowire arrays (NWAs) and Al-doped ZnO (AZO) cathodes were applied in hybrid organic / inorganic solar cells for lower-cost solar energy. Parameters for the electrodeposition of ZnO NWAs and the fabrication of NWA-free baseline devices were systematically optimized using ITO as the cathodes. High efficiencies of up to 5.4% were achieved. Incorporation of the ZnO NWAs into the baseline devices significantly reduced their efficiencies due to possible shorting in the active layer. Devices fabricated using AZO cathodes were characterized. The AZO-based devices achieved efficiencies of up to ~4.8%, showing promising results for the application of AZO as an ITO alternative. Formation of numerous large nanoplatelets was observed during the electrodeposition of ZnO NWAs on AZO cathodes. The NWAs grown on AZO cathodes were also non-uniform. Future studies were proposed to address the issues with incorporation of ZnO NWAs in hybrid solar cells and their combination with AZO cathodes.

A wide variety of tunable physio-chemical properties make ZnO nanowires a promising candidate for functional device applications. Although bottom-up grown nanowires are producible in volume, their high-throughput device integration requires control over dimensions and, more importantly, of precise placement. Thus development of top-down fabrication routes with accurate device positioning is imperative and hence pursued in this thesis. ZnO thin film transistors (TFT) were fabricated using solution based precursor zinc neodecanoate. A range of ZnO thin films were prepared by varying process parameters, such as precursor concentrations and annealing temperatures, and then analysed for their optical and electrical characteristics. ZnO TFTs prepared from a 15 % precursor concentration and annealing at 700 $^\circ$C exhibited best device performance with a saturation mobility of 0.1 cm$^2$/V.s and an on/off ratio of 10$^7$. Trap limited conduction (TLC) transport was found to be dominant in these devices. A direct-write electron beam lithography (EBL) process was developed using zinc naphthenate and zinc neodecanoate precursors for the top-down synthesis of ZnO nanowires. Nanoscale ZnO patterns with a resolution of 50 nm and lengths up to 25 $\mu$m were fabricated. A linear mobility of 0.5 cm$^2$/V.s and an on/off ratio of $\sim$10$^5$ was achieved in the micro-FETs with 50 $\mu$m channel width. Interestingly, on scaling down the ZnO channel width down to 100 nm, almost two orders of magnitude enhancement in the linear mobility was observed, which reached $\sim$33.75 cm$^2$/V.s. Such increment in the device performance was attributed to the formation of larger grains and thus reduction in the grain-boundary scattering. Six volatile organic compounds (VOCs) were sensed at room temperature using the direct-write EBL fabricated ZnO devices under UV sensitisation. As the surface-to-volume ratio increases with the decreasing channel width (from 50 $\mu$m to 100 nm), sensing response of the ZnO devices becomes more significant. Ppm level detection of various VOCs was observed; with a 25 ppm level Anisole detection being the lowest concentration. Additionally, using 100 nm device, detection of 10 ppm NO$_2$ was achieved at room temperature. The sensing response towards NO$_2$ was found to be increased with UV illumination and sensor temperature. This led to exhibit $\sim$171 % sensing response for a 2.5 ppm level of NO$_2$.

No description available.

Piezoelectric materials are widely used for sensors, actuators and trasducers. Traditionally, piezoelectric applications are dominated by multicomponent oxide ferroelectrics such as lead zirconate titanate (PZT), which have the advantage of high piezoelectric coefficients. Recently, one-dimensional piezoelectric nanostructures such as nanowires of zinc oxide (ZnO) and gallium nitride (GaN) has gained a lot of attention due to their combined piezoelectric and semiconducting properties. The focus of this thesis is to study the processing and electric properties of such piezoelectric thin films and nanostructures for various applications. There is an increasing interest to form thin films of multicomponent ferroelectric oxides such as PZT on three-dimensional structures for charge storage and MEMS applications. Traditional vapor phase deposition techniques of PZT offer poor conformality over threedimensional surfaces due to their reactant transport mechanisms. As an alternative, solgel synthesis may provide new process possibilities to overcome this hurdle but the film quality is usually inferior, and the yield data was usually reported for small device areas. The first part of this study is dedicated to the characterization of the electric properties and yield of PZT thin film derived from the sol-gel process. PZT thin films with good electric property and high yield over a large area have been fabricated. La doping was found to double the breakdown field due to donor doping effect. LaNiO3 thin films that can be coated on a three-dimensional surface have been synthesized by an all-nitrate based sol-gel route, and the feasibility to form a conformal coating over a three-dimensional surface by solution coating techniques has been demonstrated. ZnO and GaN micro/nanowires are promising piezoelectric materials for energy harvesting and piezotronic device applications. The second part of this study is focused on the growth of ZnO and GaN micro/nanowires by physical vapor deposition techniques. The morphology and chemical compositions are revealed by electron microscopy. Utilizing the as-grown ZnO nanowires, single nanowire based photocell has been fabricated, and its performance was studied in terms of its response time, repeatability, excitation position and polarization dependence upon He-Cd UV-laser illumination. The excitation position dependence was attributed to the competition of two opposite photo- and thermoelectric currents originated from the two junctions. The excitation polarization dependence was attributed to the difference in optical properties due to crystallographic anisotropy. Employing the as-grown GaN nanowires, single nanowire based strain sensor is demonstrated, and its behavior is discussed in terms of the effect of strain-induced piezopotential on the Schottky barrier height.

No description available.

One-dimensional (1D) nanostructures have gained tremendous attention over the last decade due to their wide range of potential applications. Particularly, ZnO 1D nanostructures have been investigated with great interest due to their versatility in synthesis with potential applications in electronics, optics, optoelectronics, sensors, photocatalysts and nanogenerators. The thesis deals with the challenges and the answer to grow ZnO 1D nanostructure by vapor phase transport (VPT) continuously without any length limitation. The conventional VPT technique has been modified for the non-catalytic growth of ultralong ZnO 1D nanostructures and branched structures in large area with controllable aspect ratio. It has been shown that the aspect ratio can be controlled both by thermodynamically (temperature) and kinetically (vapour flux). The thesis also deals with the fabrication of carbon nanotube (CNT) -ZnO based multifunctional devices and the field emission performance of ZnO nanowires by employing various strategies. The entire thesis has been organised as follows: Chapter 1 deals with Introduction. In this chapter, importance of ultralong nanowires and significance of ultralong ZnO nanowires has been discussed. Various efforts to grow ultralong ZnO nanowire with their advantages and disadvantages have been summarised. Lastly the significance of forming ZnO nanowires based nano hybrid structures and importance of doping in ZnO nanowires and has also been discussed. Chapter 2 deals with experimental procedure and characterization. In this chapter, a single step VPT method for the growth of ultralong ZnO nanowires that incorporates local oxidation barrier for the source has been described. The synthesized nanowires were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman & photoluminescence. Chapter 3 deals with growth of ZnO nanowires, controlling the aspect ratio of ZnO nanowires, and role of other experimental aspects. In this chapter, a way to grow nanowires continuously without any apparent length limitation, a way to control the diameter of the nanowires kinetically without catalyst particle or seed layer and obtaining smaller diameter of the nanowires by non-catalytic growth as compared to that set by the thermodynamic limit has been discussed. Furthermore, the significance and importance of local oxidation barrier on source for protecting them from degradation, ensuring the continuous supply of vapour and enabling the thermodynamically and kinetically controlled growth of nanowires has been discussed. Lastly, the scheme for large area deposition and a method to use same source material for several depositions has been presented. Chapter 4 deals with multifunctional device based on CNT -ZnO Nanowire Hybrid Architectures same device can be used as a rectifier, a transistor and a photodetector. In this chapter, the fabrication of CNT arrays-ZnO nanowires based hybrid architectures that exhibit excellent high current Schottky like behavior with p-type conductivity of ZnO has been discussed. CNT-ZnO hybrid structures that can be used as high current p-type field effect transistors (FETs) and deliver currents of the order of milliamperes has been presented. Furthermore, the p-type nature of ZnO and possible mechanism for the rectifying characteristics of CNT-ZnO has been discussed. Lastly, the use of hybrid structures as ultraviolet detectors where the current on-off ratio and the response time can be controlled by the gate voltage has been presented and also an explanation for photoresponse behaviour has been provided. Chapter 5 deals with the substrate-assisted doping of ZnO nanowires grown by this technique. In this chapter, the non-catalytic growth of ZnO nanowires on multiwalled carbon nanotubes (MWCNTs) and soda lime glass (SLG) with controlled aspect ratio has been presented. The elemental mapping to confirm the presence and distribution of carbon and sodium in ZnO nanowires and the transport studies on both carbon and sodium doped ZnO has also been presented. Furthermore the stability of carbon doped ZnO has also been presented. Lastly, the advantage of growing ZnO nanowires on MWCNTs and overall advantage associated with this technique has been discussed. Chapter 6 deals with formation of ZnO nanowire branched structures. In this chapter, a possibility to grow ZnO nanowires on already grown ZnO nanowires has been demonstrated. The formation of branched structure during multiple growth of ZnO nanowire on ZnO nanowire has been presented and evolution of aspect ratio in these branched structures has been discussed. Furthermore, the advantage of using ZnO branched structures and also the ZnO nanoneedles on MWCNT mat for field emission has been presented. Chapter 7 summarizes all the findings of the thesis.

Die vorliegende Arbeit beschäftigt sich mit der elektrischen Charakterisierung von Zinkoxid-Nanodrähten, die mittels gepulster Laserablation (PLD) hergestellt wurden. Ausgehend von den so generierten ZnO-Nanodraht-Ensembles werden Methoden zu deren elektrischer Untersuchung diskutiert und auf praktische Anwendbarkeit hin verglichen. Die entwickelten Methoden werden auf Ensembles von auf n-leitenden ZnO- und ZnO:Ga-Dünnschichten aufgewachsenen Phosphor-dotierten ZnO-Nanodrähten angewendet. Deren reproduzierbares, in Strom–Spannungs- (I–U-) Kennlinien beobachtetes diodenartiges Verhalten wird genauer beleuchtet. Im Zusammenhang mit der elektrischen Charakterisierung einzelner ZnO-Nano-drähte werden experimentelle Methoden zur Vereinzelung und zur Kontaktierung der vereinzelten ZnO-Nanodrähte diskutiert. Dabei werden sowohl etablierte Methoden wie Elektronenstrahllithographie (EBL) als auch neue Techniken wie elektronen- und ionenstrahlinduzierte Deposition (EBID/IBID) und Strom–Spannungs-Rastersondenmikroskopie (I-AFM) behandelt und ihre Eignung für eingehende elektrische Untersuchungen und reproduzierbare Messungen analysiert. Die geeignetsten Methoden werden schließlich eingesetzt, um spezifischen Widerstand sowie Ladungsträgermobilität und -dichte sowohl in nominell undotierten als auch in Aluminium-dotierten ZnO-Nanodrähten zu untersuchen und zu vergleichen. In der Ableitung der physikalischen Materialparameter aus den Messdaten wird dabei besonderes Augenmerk auf die Einbeziehung der geometrischen Besonderheiten der Nanodrähte gegenüber Volumenmaterial- und Dünnschichtproben gelegt. Im Zuge dessen wird unter anderem ein Modell für den elektrischen Widerstand in Nanodrähten mit ihrer Länge nach veränderlichem Querschnitt abgeleitet.