The role of intrinsic and surface states on the emission properties of colloidal CdSe and CdSe/ZnS quantum dots

Time Resolved Photoluminescence (TRPL) measurements on the picosecond time scale (temporal resolution of 17 ps) on colloidal CdSe and CdSe/ZnS Quantum Dots (QDs) were performed. Transient PL spectra reveal three emission peaks with different lifetimes (60 ps, 460 ps and 9-10 ns, from the bluest to the reddest peak). By considering the characteristic decay times and by comparing the energetic separations among the states with those theoretically expected, we attribute the two higher energy peaks to +/- 1(U) and +/- 1(L) bright states of the fine structure picture of spherical CdSe QDs, and the third one to surface states emission. We show that the contribution of surface emission to the PL results to be different for the two samples studied (67% in the CdSe QDs and 32% in CdSe/ZnS QDs), confirming the decisive role of the ZnS shell in the improvement of the surface passivation.