Picosecond photoluminescence decay time in colloidal nanocrystals: The role of intrinsic and surface states

Picosecond time-resolved photoluminescence measurements were performed on CdSe core and CdSe/ZnS core/shell colloidal quantum dots (QDs). Photoluminescence (PL) emission is observed to originate from intrinsic +/- 1(U) and +/- 1(L) bright states with lifetimes of 60 and 450 ps, respectively, and from a long living component with nanosecond lifetimes. The latter is attribuited to the emission from surface states (ss) approximately 16 and 13 meV below the +/- 1(L) state for core and core/shell QDs, respectively. We show that in the temperature range between 15 and 70 K the three recombination processes compete and they are thermally populated through different pathways (+/- 1(L) -> +/- 1(U) and ss -> +/- 1(L)).