Effect of ZnS shell formation on the confined energy levels of ZnSe quantum dots

Abstract

Photoluminescence excitation spectroscopy was employed to investigate the electronic structure of ZnSe/ZnS core/shell quantum dots. Four excited states viz. $1{S}^{e}\ensuremath{-}1{S}_{3/2}^{h}$, $1{S}^{e}\ensuremath{-}2{S}_{3/2}^{h}$, $1{P}^{e}\ensuremath{-}1{P}_{3/2}^{h}$, and $1{S}^{e}\ensuremath{-}1{S}^{SO}$ are observed in ZnSe and ZnSe/ZnS core/shell quantum dots. The experimentally observed excited states for ZnSe/ZnS quantum dots are analyzed on the basis of reported ``effective mass approximation'' calculations. The photoluminescence quantum efficiency increased from 2% for ZnSe quantum dots to 42% for ZnSe/ZnS quantum dots. X-ray photoelectron spectroscopic and transmission electron microscopic investigations suggest formation of uniform ZnS shell on ZnSe. The electron energy levels of ZnSe/ZnS core/shell quantum dots are investigated as a function of core diameter and ZnS shell thickness, and are compared with bare ZnSe quantum dots. Seven different sizes (ranging between 20 to $52\text{ }\text{\AA{}}$) are probed using size-selective photoluminescence excitation technique. Upon building a shell of ZnS on ZnSe quantum dots, the transition from three hole states ($1{S}_{3/2}^{h}$, $2{S}_{3/2}^{h}$, $1{S}^{SO}$) to $1{S}^{e}$ remain well defined and have negligible relative shift, suggesting that the valence-band offset is larger than the energy of these states. With increasing ZnS shell thickness, an observed increase in the transition probability of $1{S}^{e}\ensuremath{-}2{S}_{3/2}^{h}$ state is due to modification of hole states caused by ZnS shell. The relative shift of the $P$ exciton peak $(1{P}^{e}\ensuremath{-}1{P}_{3/2}^{h})$ with increase in shell thickness is due to a loss of confinement energy of $P$ electron state. The energy of $1{P}^{e}\ensuremath{-}1{P}_{3/2}^{h}$ is found to be remarkably independent as a function of core diameter.