Size-dependent orbital symmetry of hole ground states in CdS nanocrystals

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 rates 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 ~2 nm, we observe an anti-crossing of s- and p-orbital hole levels and large changes in transition rates.