QUANTUM DOTS AND CORE SHELL NANOPARTICLES OF ZnS/ZnO – SYNTHESIS AND OPTICAL PROPERTIES

Abstract

Quantum Dots (QD) of semiconductors could provide confinement of charge carriers in all the three dimensions and their electronic and optical properties are size dependent. Due to high surface to volume ratio, surface charges on QDs play a detrimental role in luminescence properties. With the formation of different type of core shell structure such as lower band gap core/higher band gap shell (e.g., ZnO core/ZnS shell) or inverted core/shell (e.g., ZnS core/ZnO shell) different possibilities of band structure modification and charge carrier confinement can be tailored. These structures exhibits different properties depending upon the band alignment of the core and shell semiconductor particles and hence contribute differently towards optical properties. Doping also plays a crucial role for introducing definite physical properties in quantum dots and core shell nanoparticles. This dissertation reports the synthesis of ZnS Quantum Dots, ZnS/ZnO Inverted core shell structure and also ZnO/ZnS core shell structure. XRD and High resolution electron microscopy images unequivocally establish the formation of core shell structure. Quantum confinement effect has been observed in optical absorption and photoluminescence properties. Enhancement of ZnO excitonic emission in core shell structure and fast decay due to excitonic recombination has been observed. Effect of doping core QDs and/or shell material with magnetic impurity Chromium has been studied at room temperature and below. Doping changes the magnetic property of both ZnO and ZnS from diamagnetic to paramagnetic with enhancement of magnetization at lower temperature.