Transfer from trap emission to band-edge one in water-soluble CdS nanocrystals

A simple and rapid route to water-soluble CdS nanocrystals stabilized by citrate was reported, and the transfer of citrate-stabilized CdS NCs from trap emission to band-edge one was studied systematically for the first time. It was found that heating in air, alkaline activation and illumination, all efficiently manipulated surface states of CdS NCs and controlled the emission states, leading to transferring CdS NCs from a broad trap emission (FWHM ~125 nm) to their strong, narrow band-gap emission (FWHM ~25 nm), comparable to that of CdS NCs synthesized by organic routes. Lifetime decay kinetic studies demonstrated that the average lifetimes for CdS NCs before and after transferred were 131.1 and 32.7 ns, respectively. The freshly-synthesized NCs were predominated by trap emission (~94%), while the transferred CdS NCs with well cubic structure dominated by band-edge emission (up to 91%). The tunable emissions of CdS NCs from violet to green could be achieved by controlling emission states of CdS NCs with different Cd/S molar ratios. The transfer mechanisms of CdS NCs from trap to band-edge emission were proposed to be epitaxial growth of a Cd(OH)(2) shell on CdS NCs core. The transition probability of energy states before and after transferred was further investigated.