Efficient Diffusive Transport of Hot and Cold Excitons in Colloidal Type II CdSe/CdTe Core/Crown Nanoplatelet Heterostructures

Abstract

Cadmium chalcogenide colloidal quantum wells or nanoplatelets (NPLs), a class of new materials with atomically precise thickness and quantum confinement energy, have shown great potential in optoelectronic applications. Short exciton lifetimes in two-dimensional (2D) NPLs can be improved by the formation of type II heterostructures, whose properties depend critically on the mechanism of exciton transport. Herein, we report a study of room-temperature exciton in-plane transport mechanisms in type-II CdSe/CdTe core/crown (CC) colloidal NPL heterostructures with the same CdSe core and different CdTe crown sizes. Photoluminescence excitation measurements show unity quantum efficiency for transporting excitons created at the crown to the CdSe/CdTe interface (to form lower-energy charge-transfer excitons). At near band edge excitation, the crown-to-core transport time increases with crown size (from 2.7 to 5.6 ps), and this size-dependent transport can be modeled well by 2D diffusion of thermalized excitons in the crown with a diffusion constant of 2.5 ± 0.3 cm2/s (about a factor of 1.6 times smaller than the bulk value). With excitation energy above the band edge, there is an increased contribution of hot exciton transport (up to 7% of the total excitons at 400 nm excitation with diffusion constant that is over twice that of cold excitons). The percentage of hot exciton transport decreases with increasing NPL sizes and decreasing excess excitation photon energy. The observed ultrafast and efficient hot and cold exciton crown-to-core transport suggests their potential applications as light-harvesting and light-emitting materials.