Peculiarities of exciton interaction and relaxation in colloidal CdSe/CdS nanoplatelets

Semiconductor nanocrystals have been actively studied due to their unique physical and chemical properties and are actively being implemented in nanophotonics devices. Nanostructures created by the colloidal synthesis with design shape, size and crystal structure are widely used. Recently, colloidal semiconductor quantum wells (nanoplatelets) have been created. Colloidal semiconductor nanoplatelets (NPLs) are atomically flat nanocrystals which demonstrate a zinc blend crystal structure with a [001] axis. Strong quantum confinement of NPLs and high exciton binding energy are provided by anisotropic of nanocrystals with several nanometers thick and tens of nanometers in lateral dimensions which can be used to tune the optical absorption and photoluminescence spectra. In this paper we present the peculiarities of excitation, interaction and relaxation of excitons in colloidal CdSe nanoplates depend on type and thickness of shell in the case of one-photon exciton excitation by laser pulses (λ=540 nm, τ=10 ns). The linear and nonlinear absorption spectra of colloidal CdSe NPLs were studied. The linear absorption spectrum of the NPLs demonstrate three well-resolved absorption bands that correspond to heavy hole, light hole and spin-orbital exciton transitions at room temperatures due to the almost complete absence size dispersion of nanocrystals. The differential transmission spectra allowed us to reveal experimentally the lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing for the first time. In addition, the features of exciton-exciton interaction, exciton-phonon interaction, as well as the process of energy transfer between light and heavy excitons to exciton relaxation were investigated. The rate equations describing the exciton-exciton and exciton-electron interactions was applied for analyzing the recombination and interaction of excitons in the colloidal NPLs under high excitation densities. This work was partially supported by the Russian Foundation for Basic Research №20-32-70001