Effect of ZnS shell on optical properties of CdSe-ZnS core-shell quantum dots

摘要

A modified microemulsion technique was used to synthesize CdSe-ZnS core-shell quantum dots (QPs) of ～5 nm diameter. Their nonlinear optical absorption was observed with nanosecond laser radiation of 532-nm wavelength. Linear optical studies show confinement effects. There is a new emission band around 745 nm under excitation of 532 nm which is an indication of the generation of deep trap states in CdSe QPs. As the ZnS shell thickness increases, there could be an increased electron-hole pair generation leading to large free-carrier concentration which in turn results in enhancement of nonlinear optical absorption. In addition, it was found that the optical limiting threshold gets decreased as the ZnS shell thickness increases.