Controlling Charge Carrier Overlap in Type-II ZnSe/ZnS/CdS Core–Barrier–Shell Quantum Dots

Abstract

We describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals, which exhibit a type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth. The results show that a single monolayer of ZnS mitigates the overlap significantly, while four and more monolayers effectively suppress band edge absorption and emission. Transient absorption spectra reveal a broad distribution of excitons with mixed S and P symmetry, which become allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. We present a model of the core/shell interface based on cation diffusion, which allows one to estimate the extent of the diffusion layer from optical spectra.

Switchable dissociation of excitons bound at strained CdTe/CdS interfaces

“…This effect can in principle be countered by introduction of a graded interface to mitigate point defects. 1,2,66 However, a fast, non-radiative channel is required to define the off state when using the model system for switching. Interestingly, the surface treatment also suppressed holeextraction by 4-MPH.…”

An Insight into the Interface through Excited‐State Carrier Dynamics for Promising Enhancement of Power Conversion Efficiency in a Mn‐Doped CdZnSSe Gradient Alloy

“…Recently, Boldt et al. summarized the role of the interface in alloyed structures for efficient charge separation . The low crystal mismatch and miscible structure of CdSe and CdS NCs allows the formation of CdS x Se 1− x alloy NCs that have been studied extensively .…”

Graded Shells in Semiconductor Nanocrystals

“…A lower limit of 270 °C has been shown before for the occurrence of phase mixing to form Cd x Zn 1 − x Se from CdSe/ZnSe core/shell nanocrystals. This temperature regime was observed as well for the combinations CdS/ZnS and ZnSe/CdS [26,28,29]. The degree of alloying strongly depends on temperature and could be observed as a blue-shift of the band-edge absorption and emission lines when going from CdSe/CdS to CdSe/CdS/ZnS or from ZnSe/CdS to ZnSe/CdS/ ZnS nanocrystals.…”

“…These structures have a type-II configuration with electrons localised in the CdS shell and holes confined to the ZnSe core. Without a ZnS barrier layer the electron ground state is partially delocalised into the core, approaching quasi-type-II behaviour, but overlap is mitigated when a ZnS barrier layer is added [29]. Transient absorption (TA) spectra reveal two bleaches, which correspond to the spatially indirect, band edge state 1S 3/2 -1S e (λ = 465 nm) and the excited state 2S 3/2 -1S e (λ = 530 nm), in which the hole extends to the CdS shell (see Figure 3).…”