الفهرس 
AbstractOnly 14 pages are availabe for public view
 |  | from | 79 |  |  |
| | | from | 79 | | |
Abstract
Zinc sulfide (ZnS) belongs to the category of II-VI semiconductors and has been extensively investigated, ever since electro-luminescence (EL) was observed from it by application of alternating current (AC) electric fields. It has been widely used as a semiconductor material for different types of display applications which include light emitting diodes, cathode ray tubes, liquid crystal display (LCD), backlight displays for cellular phones, personal digital assistant (PDA) and palmtop computers. ZnS exists in two crystallographic forms namely the cubic zinc blend structure and hexagonal wurtzite structure with corresponding band gaps of 3.7 and 3.8 eV respectively at room temperature. Earlier reports suggest that, cubic form of ZnS is the most stable crystallographic form at room temperature and it gets converted to hexagonal form upon heat treatment at temperatures above 1020°C. Optical properties of both forms of ZnS can be modified by doping them with different transition metal ions. For example, cubic form of bulk ZnS samples doped with Cu and/or Cl ions gives blue and green emission depending upon doping concentration. Multicolor emission from ZnS can also be achieved by suitably modifying size of ZnS particles as well as dopant ions. For example, number of ZnS based nano-materials have been developed, which can give emission in different wavelength region, by doping ions such as Mn2+, Ag+, Cu2+ or Cu+, etc. There are also many reports on ZnS nanoparticles doped with different ions and their incorporation in polymer matrices for developing polymer based luminescent devices. Among the large number of investigations on doped ZnS nanoparticles/nano-crystals, majority of them deals with the copper doped samples. In copper doped ZnS samples, upon UV excitation, emission peaks around 450 nm (blue emission) and around 525 nm (green emission) are observed. It is widely accepted that green emission or the G-band is arising from electron-hole recombination involving conduction band (levels -close to conduction band) and Cu Zn acceptor levels. The origin of blue emission is still under investigation and it is thought to be arising from the donor-acceptor recombination involving Cu Zn acceptors and Cu donors (copper ions at the interstitial site). It is worth mentioning here that most of the above-mentioned studies are on cubic form of ZnS lattice. As ZnS exists in both cubic and hexagonal forms, it will be interesting to study how this structural transition from cubic to hexagonal form (wurtzite structure) is affecting luminescence from samples. Luminescence properties namely emission spectra and excited state lifetime are expected to change when ZnS is stabilized into cubic phase by reducing particle size to nanometers. In such cases copper doping can change significantly the luminescent properties. Evaluation of electro luminescent properties of ZnS:Cu nanoparticles and its comparison with respect to that of bulk is an important aspect to be considered while exploring the possibility of developing improved ZnS based devices. It is always desirable to have powder electroluminescence from Cu doped ZnS nanoparticles as it will be helpful for the development of flexible, bright display devices. However, getting powder electro luminescence from nanoparticles appear to be more difficult than the corresponding bulk. Keeping the above aspects in mind, in the present study, the importance of DMS in interdisciplinary materials science and future spintronic applications, the present thesis is focused on the synthesis and physical properties of pure and doped with this motivation, Cu doped ZnS nano clusters were prepared by a non-vacuum approach, and their structural and optical properties have been studied. To shed light on this topic, ZnS:Cu nanocrystals were characterized with their structure using UV-Vis and photoluminescence (PL) spectroscopy, HRTEM, and XRD. Additionally, the optical, structural, dielectric and magnetic properties depend on dopant concentration.