Auger Recombination in Colloidal, Two-Dimensional Semiconductor Nanocrystals

Auger recombination (AR), a nonradiative recombination process in which an electron and a hole recombine and transfer its excess energy to a third carrier, negatively impacts the efficiencies of devices made from colloidal semiconductor nanocrystals (NCs) because of fast AR rates. Colloidal, two-dimensional semiconductor nanoplatelets (NPLs) offer a solution to the problem of fast AR rates in colloidal NCs while preserving other benefits of NCs such as band gap tunability and high photoluminescence efficiency. In NPLs, AR rates are reduced owing to quantum confinement in only one dimension with bulk-like lateral dimensions. However, the mix of quantum-confined and bulk-like dimensions in NPLs along the thickness and lateral axes, respectively, makes it difficult to predict their photophysical behaviors. I will discuss our investigations into two aspects of AR in two-dimensional NPLs: (1) the scaling of AR rates with NPL area and volume, and (2) the temperature dependence of AR rates.<br/><br/>1: Both zero- and one-dimensional NCs have been shown to follow a “universal volume scaling” relationship for AR rates as a function of volume. However, both experimental and theoretical efforts have yet to agree on the relationship between AR rate and NPL area and volume. We combine experiment with a new theoretical model to ultimately convey that AR rates scale with NPL area but not volume.<br/><br/>2: Conceptually, AR is expected to transition from temperature-dependent behavior in bulk semiconductors to temperature-independent behavior in QDs as a result of flattened band structure in the latter that facilitates satisfaction of linear momentum conservation. In NPLs, the expected behavior is unknown, and AR as a function of temperature has never been studied. We investigate the temperature dependence of biexciton lifetime and fluence-dependent emission in colloidal CdSe NPLs and compare the behavior to that observed with QDs. For NPLs, upon temperature reduction, biexciton lifetime surprisingly decreases and emission intensity increases nearly linearly with fluence rather than saturating, consistent with dominant radiative recombination rather than nonradiative AR. Notably, this suggests that NPLs differ fundamentally from isotropic NCs, and that AR cannot be assumed to be the dominant recombination mechanism for all NCs across all temperatures.