Excitons in core-only, core-shell and core-crown CdSe nanoplatelets: interplay between in-plane electron-hole correlation, spatial and dielectric confinement

TL;DR

This study models excitonic properties in CdSe nanoplatelets, revealing how lateral size, confinement, and heterostructure type influence electron-hole correlation, energy, and lifetime, with implications for optoelectronic applications.

Contribution

It provides a semi-analytical framework to understand exciton behavior in various CdSe NPL configurations, highlighting the effects of size, confinement, and heterostructure type.

Findings

Exciton lifetimes are less than 1 ps in large NPLs due to superradiance.

Intermediate confinement occurs in NPLs smaller than 20 nm, altering exciton behavior.

Type-II heterostructures can achieve exciton lifetimes up to microseconds.

Abstract

Using semi-analytical models we calculate the energy, effective Bohr radius and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size $L \gtrsim 20$ nm the exciton behavior is essentially that in a quantum well, with superradiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for $L < 20$ nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, different response is observed for core/shell and core/crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g. in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching $\mu s$.