Effect of annealing temperature and ambient on the structural and optical properties of Eu3+-doped ZnS nanocrystals

Abstract

This work explores the influence of post-synthesis annealing temperature and ambient on the structural and optical properties of Eu3+ ion doped in ZnS nanocrystals prepared via chemical precipitation route. Annealing was perform in nitrogen, sulfurizing and reducing ambient at three different temperatures (200 °C, 300 °C, and 400 °C) each. All the nanocrystals show predominantly the cubic phase of ZnS crystal structure without oxidation. Slight variations in lattice constant due to the substitution of Eu3+ at the Zn2+ site was observed especially in samples treated in reducing ambient. Energy transfer between ZnS and Eu3+ was observed. Photoluminescence emission intensity of doped nanocrystals treated at 200 °C was dominated by the dopant ion emission lines, while the defect bands dominated after annealing at 300 °C in reducing and sulfurizing the ambient. The strength of the emission intensities were in the increasing order of treatment ambient; sulfurizing > nitrogen > reducing at 200 °C and 300 °C and the reverse order at 400 °C. The combination of the defect related emission bands and Eu3+ emission lines gave white light spectrum which was confirmed using the CIE chromaticity coordinate. White light emission was obtained only when the samples were annealed at 300 °C in the three ambient. Excitation wavelength for white light emission is 395 nm, while the same samples produced red emission when excited at 465 nm. Both wavelengths are ideal for application with nUV and blue InGaN-based LEDs.