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

Abstract

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 $\ensuremath{\sim}2\text{ }\text{nm}$ we observe an anticrossing of $s$- and $p$-orbital hole levels and large changes in transition rates.