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CHAPTER ONE

INTRODUCTION

1.1 Background of the Research

Nanoscience evolution and the advent of nanowire fabrication marked a new epoch in optoelectronics ¹. Characteristic investigation for achieving efficient light absorption, charge separation transport and collection had culminated in the synthesis of both organic and inorganic semiconductor nanowires ^2-3. The d-block transition elements of the periodic table are all metals of economic importance. Zinc, which is a group II element, finds numerous potential applications, such as smart windows, solar thermal absorber, optical memories and photoelectrocatalysis ^4-5.

Nowadays, the products of semiconductor industry are spread all over the world and deeply penetrate into the daily life of humans. The starting point of semiconductor industry was the invention of the first semiconductor transistor in 1947.³ Since then, the semiconductor industry has kept growing enormously. In the 1949’s, the information age of humans was started on the basis of the stepwise appearance of quartz optical fiber, group III-V compound semiconductors and gallium arsenide (GaAs) lasers. During the development of the information age, silicon (Si) keeps the dominant place on the commercial market, which is used to fabricate the discrete devices and integrated circuits for computing, data storage and communication. Since Si has an indirect band-gap which is not suitable for optoelectronic devices such as light emitting diodes (LEDs) and laser diodes, GaAs with direct band-gap stands out and fills the blank for this application. As the development of information technologies continued, the requirement of ultraviolet (UV)/blue light emitter applications became stronger and stronger which is beyond the limits of GaAs. Therefore, the wide band-gap semiconductors such as gallium nitride (GaN) and zinc oxide (ZnO), i.e. the third generation semiconductors, come forth and turn into the research focus in the field of semiconductor.

ZnO is a typical II-VI semiconductor material with a wide band-gap of 3.37 eV at room temperature. Although its band-gap value is closer to GaN (3.44eV), its exciton binding energy is as high as 39eV, which is much higher than that of GaN (25eV). Therefore, theoretically, we can harvest high efficient UV exciton emission and laser at room temperature, which will strongly prompt the applications of UV laser in the fields of benthal detection, communication and optical memory with magnitude enhancement in the performance. Moreover, the melting point of ZnO is 1954⁰C, which determines its high thermal and chemical stability. Again, ZnO owns a huge potentially commercial value due to its cheaper price, abundant resources in nature, environmentally friendly, simple fabrication processes and so on. Therefore, ZnO has turned into a new hot focus in the field of short-wavelength laser and optoelectronic devices in succession to GaN in the past decade.

It is believed by many researchers that ZnO is a more prospective candidate for the next generation of light emitters for solid state lighting applications than GaN, even though the GaN-based LEDs have been commercialised and currently dominated the light emission applications in UV/blue wavelength range. This is because ZnO has several advantages compared to GaN. The two outstanding factors are;

1. The exciton binding energy of ~39eV at room temperature is much higher than that of GaN (~25eV), which can enhance the luminescence efficiency of ZnO based light emission devices at room temperature, and lower the threshold for lasing by optical pumping. ^6-7
2. The growth of high quality single crystal substrates is easier and of lower cost than GaN.^6-7

Increasingly interesting properties and potential applications of ZnO have been discovered. One of the most attractive aspects is that it is relatively simple for ZnO to form various nanostructures including highly ordered nanowire arrays, tower-like structures, nanorods, nanobelts, nanosprings and nanorings ⁸. Due to the special physical and chemical properties derived from the nanostructures, ZnO has been found to be promising in many other applications, such as  sensing ^9-10, catalysis ^11-12, photovoltaics ¹³ and nano-generators ^14-16, just to mention but a few.

In order to utilize the applications of nanostructure materials, it usually requires that the crystalline morphology, orientation and surface architecture of nanostructures can be well controlled during the preparation processes. For ZnO nanostructures, although different fabrication methods such as vapor-phase transport ¹⁷, pulsed laser deposition ¹⁸, chemical vapor deposition and electrochemical deposition,¹⁹ have been widely used to prepare ZnO nanostructures, the complex processes, sophisticated equipments and high temperature requirement make them very hard for large-scale production for commercial application. On the contrary, aqueous chemical method is of great advantage due to much easier operation and very low growth temperature (95⁰C) ²⁰. ZnO nanostructures grown by this method show poor orientation and different crystalline structures due to the fact that, the optimum conditions required for the growth of these nanostructures is still grossly understudied. Hence, it is still a significant challenge to obtain controllable growth of ZnO nanostructures. It is therefore imperative to investigate the various conditions necessary for the growth of well align ZnO nanostructures.

1.2. NANOWIRES

A nanowire is a nanostructure, with the diameter of the order of a nanometer (10⁻⁹ meters). Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length ²¹. At these scales, quantum mechanical effects are important — which coined the term “quantum wires”. Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au), semiconducting (e.g., Si, InP, GaN, ZnO, etc.), and insulating (e.g., SiO₂, TiO₂).

Typical nanowires exhibit ratios (length-to-width ratio) of 1000 or more. As such they are often referred to as one-dimensional (1-D) materials. Nanowires have many interesting properties that are not seen in bulk or 3-D materials. This is because electrons in nanowires are quantum confined laterally and thus occupy energy levels that are different from the traditional continuum of energy levels or bands found in bulk materials. Peculiar features of this quantum confinement exhibited by certain nanowires manifest themselves in discrete values of the electrical conductance. Such discrete values arise from a quantum mechanical restraint on the number of electrons that can travel through the wire at the nanometer scale ²¹.

Nanowires also show other peculiar electrical properties due to their size. Unlike carbon nanotubes, whose motion of electrons can fall under the regime of ballistic transport (meaning the electrons can travel freely from one electrode to the other), nanowire conductivity is strongly influenced by edge effects. The edge effects come from atoms that lay at the nanowire surface and are not fully bonded to neighboring atoms like the atoms within the bulk of the nanowire. The unbonded atoms are often a source of defects within the nanowire, and may cause the nanowire to conduct electricity more poorly than the bulk material. As a nanowire shrinks in size, the surface atoms become more numerous compared to the atoms within the nanowire, and edge effects become more important.

Furthermore, the conductivity can undergo a quantization in energy: i.e. the energy of the electrons going through a nanowire can assume only discrete values, multiple of the Von Klitzing constant (G) = 2e²/h (where e is the charge of the electron and h is the Planck’s constant). The conductivity is hence described as the sum of the transport by separate channels of different quantized energy levels. The thinner the wire is, the smaller the number of channels available to the transport of electrons.

The quantized conductivity is more pronounced in semiconductors like Si or GaAs than in metals, due to lower electron density and lower effective mass. Quantized conductance can be observed in 25 nm wide silicon fins, resulting in increased threshold voltage. ²¹

• Applications of Nanowire