Effect of Variation of pH and Shell Thickness upon the Optical and Structural Characteristics of PVA Capped CdSe and CdSe/ZnO Nanoparticles

Abstract

Poly-Vinyl Alcohol (PVA) capped Cadmium Selenide (CdSe) and Cadmium Selenide/Zinc Oxide (CdSe/ZnO) core/shell semiconductor/semiconductor nanoparticles have been synthesized using wet chemical precipitation method. The pH of each solution was varied ranging from 9.5 to 11.5. Particle size of the CdSe nanoparticles was estimated using Brus Equation. The results of UV-Visible spectroscopy of core CdSe samples show blue shifting of absorption edges in a range of 342-377 nm compared to that of the bulk CdSe 712 nm and red shifting of the absorption edges of the core /shell samples in a range (346-357) nm in comparison to the core sample, over which the shell is deposited. For CdSe core nanoparticles, the band gap values were found to be in the range of 3.70–3.90 eV, which is larger than the bulk CdSe of 1.74 eV. Also the band gap values for the core/shell nanoparticles were in the range of 3.75-3.80 eV. The positions of excitonic emission peak obtained from photoluminescence spectra for the core is around 323 nm and for the core/shell samples is around 324nm. The average crystallite size of the core/shell CdSe/ZnO sample was obtained from XRD spectra in the range of 62-69 nm and for the core CdSe sample it was of 11-31 nm. The core and the core/shell samples were more or less spherical as obtained from the SEM analysis. Some of the core nanoparticles were 3-8 nm in size whereas the core/shell nanoparticles were 20-50 nm in size as obtained from HRTEM analysis.