ABSTRACT
Zinc Oxide (ZnO) nanowires with hexagonal structure were successfully synthesized by chemical bath deposition technique. The obtained nanowires were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and spectrophotometer. The SEM micrographs revealed the morphology of ZnO nanowires with diameter between 170.3 and 481nm and showed that the pH of the bath solution, 8.1 is the optimized value to form ZnO nanowires with hexagonal shape. The XRD pattern of the samples revealed that ZnO nanowire has a hexagonal crystallite structure and further showed that the crystallite size supported by Scherrer’s equation increase with increasing annealing temperature(0.536 nm, 0.541nm, 0.557 nm at 1000C,  1500C and 2000C) respectively. The EDX analysis revealed the elemental compositions of samples and confirmed the presence of Zn and O. The results of the optical analysis showed that ZnO nanowire has high absorbance in the ultraviolet and infrared regions with high transmittance in the visible region. The results further revealed that the absorbance of the nanowire increase with increasing annealing temperature. Its high absorbance in the ultraviolet region suggest that it can be use as solar harvester for trapping solar energy for photovoltaic panel which is capable of converting sunlight radiation directly to electricity for commercial or industrial purpose.
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
1.2.1. Applications of Nanowire
In addition to variations in size and material, nanowire research also spans a wide range of applications.
1.2.1.2. Gas Sensor
Nanowire sensors have been demonstrated to detect the presence of many gases, including oxygen 22, hydrogen, NO2, and ammonia 23, at very low concentrations. Nanowires also have been used to detect ultraviolet light 24, changes in pH 25, and the presence of low-density lipoprotein cholesterol 26. These sensors generally function by measuring changes in the electrical or physical properties of the nanowire in the presence of the target analyte. The sensing
capabilities of nanowires may be enabled by selective doping or by surface modifications that enhance their affinities for certain substances.
1.2.1.3. Medical Applications
Nanowires have been used to coat titanium implants. 27 Doctors have discovered that muscle tissues sometimes do not adhere well to titanium, but when coated with the nanowires, the tissue can anchor itself to the implant, reducing the risk of implant failure. Nanowires are used for the production of nanoscale sensors for various purposes, one of which is in the detection of potential biomarkers of cancer. For example, nano-sized sensing wires that are laid across a microfluid channel can pick up molecular signatures of the particles and can relay the information instantaneously and these sensors can detect altered genes that are associated with cancer and ultimately have the potential to provide information on the location of the malignant
genes 28. Nanowires also play important roles in nano-size devices like nanorobots. Doctors
could use the nanorobots to treat diseases like cancer 29.
1.2.1.4. Magnetic Storage Medium
The most attractive potential applications of nanowires lie in the magnetic information storage medium. Studies have shown that periodic arrays of magnetic nanowire arrays possess the capability of storing forty gigabytes of information per square centimeter of area (40G/cm2) 30. The small diameter, single domain nanowires of Ni, Co fabricated into the pores of porous anodic alumina has been found to be most suitable for the above purpose. The high aspect ratio of the nanowires results in enhanced coercivity and suppresses the onset of the
‘superparamagnetic limit’, which is considered to be very important for preventing the loss of magnetically recorded information between the nanowires. Suitable separation between the nanowires is maintained to avoid the inter-wire interaction and magnetic dipolar coupling. It has been found that nanowires can be used to fabricate stable magnetic medium with packing density
> 1011 wires/cm2 31.
1.2.1.5. Electronic Applications
Some nanowires are very good conductors or semiconductors, and their miniscule size means that manufacturers could fit millions more transistors on a single microprocessor. As a result, computer speed would increase dramatically. Nanowires possess the potential for use in numerous electronic applications. Junctions of semiconductor nanowires such as GaAs and GaP
have shown good rectifying characteristics 32. Several semiconductor devices such as junction
diodes , memory cells and switches 33, transistors, field-effect transistors (FETs), light emitting diodes (LEDs) and inverter 34 etc have already been fabricated using nanowire junctions. The field effect transistors made of nanowires exhibit remarkably modified conductance behaviour 35 and they are very attractive because of their morphological advantages. The operational speed of FETs made of nanowires is much faster compared to that of bulk FETs.
1.2.1.6. Optical Applications
Uniform morphology and interesting optical properties of nanowires have raised their potential for various optical applications. The n–p junction of nanowires has been found to be capable of light emission, by virtue of their photoluminescence (PL) or electroluminescence (EL) properties. The use of p–n junction nanowires has been contemplated for laser applications. It
has been established that ZnO nanowires of wire diameter smaller than the wavelength of emitted light exhibits lasing actions at lower threshold energy compared to their bulk counterpart. 36 This has been attributed to the exciton confinement effect in the laser action, which decreases the threshold lasing energy in nanowires. This effect has been observed in small diameter ZnO (385 nm diameter) and GaN nanowires. The huge surface area and the high conductivity along the length of nanowires are suitable for inorganic–organic solar cell 37.
It is to be noted that when the intensity of the incident photons are increased the electron density of the sub-band edges also increases, due to the above fact, these quantum wires develop strong nonlinearity. Therefore, nanowires may be used to develop optical switches. These optical switches will be able to operate at lower energy and with enhanced switching speed compared to the known switches.
Nanowire can be used in field emission devices such as flat panel displays because of the significant drop in the work function of the surface electrons in those small diameter and high curvature tips of the nanowires 37.
1.3. STATEMENT OF PROBLEM
Literature have shown that, man depend majorly on non-renewable energy source (coal, fossil fuel and natural gas) as the primary energy source. However, the energy generated from these sources is limited and their waste products non-environmentally friendly, costly, and limited in quantity. Because nature does not produce them at the same rate that they are being consumed they are bound to expire one day. Based on this, it is imperative to search for a more abundant, environmentally friendly, clean, cheap and sustainable energy source as an alternative source.
In view of these, the optical property of ZnO nanowire is investigated to find its potency as solar harvester as renewable energy for industrial and commercial purpose.
1.4. AIM AND OBJECTIVES OF THE STUDY
This research work is aimed at synthesizing and characterizing ZnO nanowire and the specific objectives of the study were as follows:
(a) Fabrication of ZnO nanowires using chemical bath deposition method.
(b) Microstructure characterization of deposited ZnO nanowires using SEM, XRD, UV, EDX.
(c) Investigation of the effect of post annealing temperature on the optical property of the grown ZnO nanowires.
(d) Investigation of the effect of pH on the structure and morphology of the synthesized ZnO nanowires.
(e) Determination of the crystallite size of the synthesized ZnO nanowires.
1.5. JUSTIFICATION OF THE STUDY
The growth of ZnO nanowire with hexagonal structure was successfully achieved. These
nanowires are useful in varied areas of life to man. Due to its high absorbance in the ultraviolet region of light spectrum, it can be used as solar harvester, smart windows, solar thermal absorber and optical memories. In electronics, nanowires can be use as devices such as junction diodes, memory cells and switches, transistors, FETs, LEDs and inverter. Nanowire can also be used as sensors that can detect the presence of many gases, including oxygen, hydrogen, NO2, and ammonia, at very low concentrations. They can also be used to detect ultraviolet light, changes in pH, and the presence of low-density lipoprotein cholesterol.
This material content is developed to serve as a GUIDE for students to conduct academic research
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