Radiative lifetimes and orbital symmetry of electronic energy levels of CdS nanocrystals: Size dependence

Using time-resolved photoluminescence spectroscopy, we studied the electronic levels of semiconductor nanocrystals (NCs) with small spin-orbit coupling such as CdS. Low temperature radiative lifetimes indicate that the lowest-energy transition changes from orbital allowed to orbital forbidden with decreasing NC size. Our results are well explained by a size-dependent hierarchy of s- and p-orbital hole levels that is in agreement with theoretical predictions. Around the critical NC radius of similar to 2 nm we observe an anticrossing of s- and p-orbital hole levels and large changes in transition rates.