Surface-functionalized cadmium chalcogenide nanocrystals: A spectroscopic investigation of growth and photoluminescence

The growth mechanism of CdTe nanocrystals (NCs) vis-à-vis CdS with stabilizing shell of cysteine has been examined. The nucleation and growth were found to be well separated for CdS NCs whereas there is partial overlap of the two events for CdTe NCs. The initial growth rate (dr/dt) of CdS NCs was higher than that of CdTe NCs. As the reflux time was increased, the growth rate showed a linear decline with increasing particle size. Three distinct stages of growth were identified with respect to particle size distribution: focusing, defocusing and equilibrium. The extent of focusing for CdTe NCs (49%) was much larger than that of CdS NCs (23%). With increasing time of reflux, photoluminescence quantum efficiency (PLQE) rose initially and started decreasing after reaching a peak value. A maximum PLQE of 0.13 and 0.16 was observed for CdS and CdTe NCs, respectively, for the same size of 3.4 nm. The particle size increased with increasing concentration of the capping ligand, cysteine and maximum PLQE was obtained at an optimum molar ratio of Cd2+:cysteine. Interestingly, on increasing the ionic strength of the initial synthetic mixture, photoluminescence (PL) intensity of CdS NCs was substantially enhanced while the particle size remained unaffected.