Auger Recombination in CdSe and CsPbBr3 2D Colloidal Nanoplatelets
Tim Lian, Emory University
Two-dimensional (2D) colloidal nanoplatelets (NPLs) are an emerging class of quantum well materials that exhibit many unique properties, including uniform quantum confinement, narrow thickness distribution, large exciton binding energy, giant oscillator strength effect, long Auger lifetime, and high photoluminescence quantum yield. These properties have led to great potentials in optoelectrical applications, such as lasing materials with a low threshold and large gain coefficient. Many of these properties are determined by the structure and dynamics of band-edge excitons in these 2D materials. Motivated by both fundamental understanding and potential applications, the properties of 2D excitons have received intense recent interests. We have carried out a series of studies on fundamental exciton properties in 2D NPLs, including lateral size the 2D exciton (i.e. exciton center-of-mass coherent area); exciton in-plane transport mechanism; size and thickness dependence of bi-exciton Auger recombination rate, and optical gain mechanism and threshold. In this talk I will focus on the size, thickness and material dependence of bi-exciton Auger recombination rates. We show that In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linearly on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness. In CdPbBr3 1D nanorods and 2D NPLs, the biexciton Auger lifetimes increase linearly with the rod length and NPL lateral areas, respectively, and the lifetimes are much shorter than CdSe nanocrystals with similar volume. These observations can be explained by a model in which the Auger recombination rate for 1D nanorods (NRs ) and 2D NPLs is a product of binary collision frequency in the non-quantum confined dimension, and Auger probability per collision. The former gives rise to the linear dependence on the lateral areas in 2D NPLs and rod length in 1D NRs. The Auger recombination proability per collision depends on material property and the degree of quantum confinement, which gives rise to nonlinear dependence on the thickness of NPLs and diameter of NRs, as well as material dependence of Auger lifetimes. Thus, the Auger lifetimes of 2D NPLs and 1D nanorods deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We believe that his model is generally applicable to all 1D and 2D materials.