Nawaz, Muhammad Asif
Islamia University of Bahawalpur
Nanotechnology is a technology of resizing and the applications of nanosized materials. Zinc oxide (ZnO) nanostructures became a potential candidate in various applications due to their direct wide bandgap, high exciton binding energy, good chemical stability, radiation hardness, notable optoelectrical, pyroelectric and piezoelectric properties. By doping with different metals (T = Co, Mg, Sr, Sb and P) into ZnO matrix, various properties of ZnO nanostructure could be tuned. The aim of this dissertation was to develop a simple approach to synthesis as-grown and T-doped ZnO by using hydrothermal technique because of its low cost, simple solution process, environment friendly and easy to handle. The grown samples were characterized to study the enhancement in different functional properties for applied applications. The first part of dissertation deals with synthesis of as-grown ZnO nanostructure via hydrothermal method. Microstructural and morphological properties of as-grown and annealed ZnO can be calculated by X-ray diffraction (XRD), Williamson-Hall (W-H) analysis via three different models of uniform deformation model (UDM), uniform deformation stress model (UDSM) and uniform deformation energy density model (UDEDM) and transmission electron microscopy (TEM) respectively. These models were used to calculate other physical parameters such as stress, strain and energy density values. The results showed that the estimated value of crystallite size (D) from W-H analysis by using UDM is highly inter-correlated with the particle size estimated from TEM and do not show much deviation. The second part of the dissertation demonstrates hydrothermal synthesis of T-doped ZnO and study the effect of T dopant levels on the morphological, microstructural, dielectric, magnetic and optical properties of ZnO nanostructure. Dielectric constant of cobalt (Co) doped ZnO samples have been found to be decreased by increasing the dopant levels. Loss tangent results of doped samples reveals the decrease in hopping frequency of charge carriers between metals ions with dopant ion. Considerable increases of optical bandgap while maintaining good hexagonal structure with no other impurity by increasing the Co dopant concentration have been observed. The antimony (Sb) doping shows stronger effect on the length of nanowires (NWs) which is ~ 11 times longer and slight red-shift is observed in photoluminescence (PL) peak as compared to as-grown ZnO. It is argued that the observed red-shift and broadening in PL response are associated with Sb-doping level and/or from Sb defect states partially. For strontium (Sr) doped ZnO films, the surface morphological, microstructural properties, optical binding energy and band-to-band emission have been examined. It is observed that the crystalline quality improved by increasing the Sr dopant level while optical bandgap and band-to-band emission is decreased due to lattice distortion and generation of active defects in ZnO crystal; it may cause bandgap tailing. Magnesium (Mg) doped ZnO nanorods (NR‘s) samples were annealed at 450 ºC, 550 ºC and 650 ºC under the argon (Ar) environment. The XRD spectrum shows that the Mg-doped ZnO NR‘s are crystalline in nature and wurtzite hexagonal phase. The particle size of 550 ºC and 650 ºC Mg-doped ZnO NR‘s estimated from TEM is ~ 126 and 154 nm respectively. We have studied the progression of crystallite size (D) through the use of X-ray peak profile analysis (XPPA). Scherrer formula and three different models of W-H analysis are used which show the increase in grain size and decrease in microstrain by increasing the annealing temperature of Mg-doped ZnO NR‘s. Phosphorus (P) doped ZnO samples have been sintered in air in the range 500-1000 ºC with a step of 100 ºC for one hour in a programmable diffusion furnace. It is observed that forward shift in 2 theta value of the XRD pattern of ZnO sintered at 1000 ºC occurs. This forward shift may correlate with the involvement of P ions with vacancy at Zn-site. Theoretical calculations on a 32-atom super cell of P-doped ZnO further supported the argument and suggests the phosphorous dopant are shallow acceptor PZn-2VZn complex on Zn-site. Typical PL spectra displayed band-to-band emission peak and an additional donor-acceptor peak at ~ 3.14 eV in all P-doped samples.