Highly Formed Luminescent Oxygen Trap States in Thermochemically Prepared CdS Nanocrystals and Improvement of the Luminescence Property

Abstract

CdS nanocrystals were prepared through a thermally activated (thermochemical) method. The synthesis was performed in two different oxygen-saturated and Ar purged atmospheres. Na2S2O3 was used as a heat-sensitive source of S in the experiments. Thioglycerol was also applied as the capping agent to restrain the growth. The growth was activated by heating and controlled by solution pH. The results demonstrated a considerably higher luminescence intensity for the synthesis in oxygen-saturated atmosphere. This could show the role of luminescent oxygen trap states created inside the CdS nanocrystals. The improved luminescence was observed for the synthesis at different solution pHs. Nevertheless, the growth rate was slower and luminescence intensity was higher for the synthesis at higher pHs. This trend was followed for the solution pHs in the range of 4.0–7.0. For the synthesis at solution pH of 8.0 the growth rate was unexpectedly increased. This could be attributed to the epitaxial growth of CdS nanocrystals in this pH. The best photoluminescence intensity was achieved for the CdS nanocrystals prepared in oxygen-saturated atmosphere at solution pH of 8.0. The crystalline structure of these NCs was composed of 70% of the cubic and 30% of the hexagonal crystalline phases of the CdS material. This was calculated by simulation of X-ray diffraction pattern of the CdS nanocrystals.