Photoluminescence excitation spectroscopy was employed to investigate the electronic structure of ZnSe/ZnS core/shell quantum dots. Four excited states viz. $1S^e-1S_{3/2}^h$, $1S^e-2S_{3/2}^h$, $1P^e-1P_{3/2}^h$, and $1S^e-1S^{SO}$ 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 $52\text{Å}$) are probed using size-selective photoluminescence excitation technique. Upon building a shell of ZnS on ZnSe quantum dots, the transition from three hole states ($1S_{3/2}^h$, $2S_{3/2}^h$, $1S^{SO}$) to $1S^e$ 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 $1S^e-2S_{3/2}^h$ state is due to modification of hole states caused by ZnS shell. The relative shift of the $P$ exciton peak $\left(1P^e-1P_{3/2}^h\right)$ with increase in shell thickness is due to a loss of confinement energy of $P$ electron state. The energy of $1P^e-1P_{3/2}^h$ is found to be remarkably independent as a function of core diameter.

• Received 28 April 2008

DOI:https://doi.org/10.1103/PhysRevB.78.125421