Investigation of confinement effects in ZnO quantum dots.

We report a simple method for the synthesis of Na(+) doped and stable zinc oxide quantum dots, using the quantum confinement atom method. An intense broad green photoluminescence (PL) was observed with a maximum located at approximately 535 nm when excited by UV radiation of 332 nm. The PL peak intensity is found to be highly dependent on the size of the quantum dots (QDs). Electron microscopy observation revealed that the radius of the QD was approximately 1 nm, which clearly indicated that the QDs are in the strong quantum confinement region (exciton Bohr radius, r(B), for bulk ZnO is 1.8 nm). Phase purity of ZnO and the presence of Na(+) was confirmed by x-ray diffraction (XRD) and atomic absorption spectroscopy (AAS), respectively. The results are well incremented by x-ray photoelectron spectroscopy (XPS) studies. Intentional ageing of QDs for several days under controlled experimental conditions such as temperature, relative humidity and pH etc, facilitated the formation of various nanostructures with a slight red shift in the PL peak position. Time resolved emission spectroscopy measurements indicated that PL decay time changes from 35 ns for QDs to 1660 micros for nanocrystals. The observed high-intensity and stable green PL emissions have been analyzed and thoroughly discussed.