Area- and Thickness-Dependent Biexciton Auger Recombination in Colloidal CdSe Nanoplatelets: Breaking the "Universal...

Colloidal nanoplatelets (NPLs) have shown great potentials for lasing applications due to their sharp absorption and emission peaks, large absorption cross sections, large radiative decay rates, and long multiexciton lifetimes. How multiexciton lifetimes depend on material dimensions remains unknown in two-dimensional (2D) materials, despite being a key parameter affecting optical gain threshold and many other properties. Herein, we report a study of room-temperature biexciton Auger recombination time of CdSe NPLs as a function of thickness and lateral area. Comparison of all NPLs shows that the biexciton lifetime does not increase linearly with volume, unlike previously reported "universal volume scaling law" for quantum dots. For NPLs of the same thickness (∼1.8 nm), the biexciton lifetime increase linearly with their lateral area (from 143.7 ± 12.6 to 320.1 ± 17.1 ps when the area increases from 90.5 ± 21.4 to 234.2 ± 41.9 nm²). The biexciton lifetime depends linearly on (1/E_k(e))^7/2 (E_k(e) is the electron confinement energy) or nearly linearly on d⁷ (d is NPL thickness). The observed dependence is consistent with a model in which biexciton Auger recombination rate scales with the product of exciton binary collision frequency and Auger recombination probability in biexciton complexes. The linear increase of Auger lifetimes with NPL lateral areas reflects a 1/area dependence of the binary collision frequency for 2D excitons and the thickness-dependent biexciton Auger recombination time is attributed to its strong dependence on the degree of quantum confinement. This model may be generally applicable to exciton Auger recombination in quantum confined 1D and 2D nanomaterials.