2.1 Structural properties of ZnO

Reduction of an object size results in large surface to volume ratio hence the surface turn out to more vital and that large surface to volume ratio greatly affected the chemical, electrical and optical properties of nanomaterials. Quantum effects owing to size confinement in nanostructures occurs, when the typical size of the object is equivalent to the crucial length (range 1–10 nm) of the equivalent physical properties'screening length, then the mean free path of electrons; 0-D quantum dots, 1-D quantum dots, and 2-D quantum well are the characteristic structure

Low power consumptions, best crystallinity, and high integration density 1-D with high aspect ratio are shown by the 1-D nanostructures. The nanostructure materials show high sensitivity to surface chemical reactions, with increased surface-to-volume ratio and a Debye length matching with small size. Tunable band

Smart and functional materials are based on metal oxides [10]. Synthesis and fabrication of devices based on metal oxide semiconductor have become more important recently, because the tuning of physical properties of these metal oxides is so easy. Among these MOS, ZnO is a material that has strong piezoelectric and optical properties on the bases of its wide band gap, stability at high temperature, large surface-to-volume ratio, and high excitonic binding energy. They are used in solar cells, photocatalysis, and antibacterial active material. Therefore research work has been carried out on ZnO nanostructures. Metal oxide materials possess electrical, chemical, and physical properties that are highly sensitive to the changes in a chemical environment, through a variety of detection principles based on ionic, conducting, photoconducting, piezoelectronic, pyroelectronic, and luminescence

Doping is another technique utilized to improve ultraviolet (UV) sensing properties of metal oxides, where the dopant atoms are believed to act as activators for surface reactions. In MOS, the electrical, optical, and chemical properties can be changed by adding the doping materials or by creating oxygen defects which results in large concentration of carriers, mobility, and electrical resistivity. So doping

Up to now, various metal oxides' 1-D nanostructures (SnO2 nanowhiskers, In2O3 nanowires, ZnO nanorods, WO3 nanowires, TiO2 nanowires etc.) have been fabricated into film-type nanosensors by means of thermal evaporation or vapor transport method. The most widely studied substances are SnO2 and ZnO nanowires [13]. In this research work, 1-D n-ZnO nanostructures (nanowires, nanorods, nanobelts with needle-like ends, and typical nanobelts) were grown by using vapor transport method using VLS mechanism on n-type Au-coated silicon substrate Si (100). The electrical and optical properties of ZnO nanostructures were

As work done in this chapter mainly deals with ZnO semiconductor, structural

offers another avenue for expanding their sensing capability [12].

investigated using different characterization techniques [14–37].

2. Important properties of metal oxide semiconductors

properties of ZnO material are presented below.

gap is enabled by size confinement [2]. In the recent past, various synthesis methods, such as vapor phase method, electrochemical method, liquid phase methods, and solution-gel methods, were used. Out of these growth techniques, vapor transport method, using vapor-liquid-solid (VLS) growth mechanism or VS growth, is one of the finest growth techniques used for the growth of metal oxide semiconductor nanostructures. It is a cost-effective easy method used to create

many single-crystalline 1-D nanostructures [3–11].

forms.

Gas Sensors

properties [12–21].

102

ZnO is a key technological and prominent material. One of the important properties of ZnO is that it has a wide band gap that makes it suitable for optoelectronic applications of short wavelength. ZnO has high excitonic binding energy (60 meV) at room temperature by ensuring efficient excitonic emission. It has been noted that disordered nanoparticles and thin films at room temperature have ultraviolet (UV) luminescence. In addition, due to the unavailability of centrosymmetry in wurtzite structures that combines with large electromechanical coupling which result in strong piezoelectric and pyroelectric properties and make ZnO a prominent material in the use of mechanical actuators and piezoelectric sensors. As a versatile functional material, ZnO has a different group of growth morphologies, such as nanocombs, nanowires, nanobelts, nanosprings, etc. These ZnO nanostructures are easily obtained, even on cheap substrates such as glass. As work done in this thesis mainly deals with ZnO semiconductor, structural properties of this material are presented below.
