Controlling Microscopic Surface Structure, Crystalline Size and Crystallinity of CdS and ZnS Nanocrystallites

Abstract

Size controlled CdS and ZnS nanocrystallites with well-defined crystallinity were simply prepared by reacting M²⁺ (M=Cd²⁺ or Zn²⁺) and S^2- (H₂S or Na₂S) in N,N-dimethylformamide (DMF) without adding any surface stabilizers. The mean diameter of CdS nanocrystallites can be rigorously controlled over a range from 1.8 to 4.2 nm maintaining the hexagonal crystalline structure by varying the sulfur source and by regulating the preparation temperature. The band-gap dependence estimated from the absorption peak wavelength on the size of the hexagonal CdS nanocrystallites was in good agreement with the pseudopotential calculation using lattice parameters of hexagonal CdS. In the system of ZnS nanocrystallites, crystallinity was controlled by changing the surface modifying reagent. Reactive organic molecules, which can bind onto the surface of ZnS nanocrystallites, induce phase transition of ZnS nanocrystallites from the hexagonal to the cubic phase, maintaining a crystalline size of 3 nm at ambient temperature and pressure.