Interfacial Electronic Structure in Graded Shell Nanocrystals Dictates Their Performance for Optical Gain

The interface of semiconductor nanocrystals is a critical factor for determining their performance in light emissive applications. Traditional nanocrystals have an abrupt termination of the core/shell interface. Recent synthetic work has focused upon developing graded core/shell interfaces via alloying. Here, we employ femtosecond state-resolved pump/probe spectroscopy, temperature-dependent photoluminescence spectroscopy, and a microscopic theory of interfacial charge trapping to reveal the manner in which a graded interface controls the main optical gain metrics: threshold, bandwidth, and lifetime in the CdSe/Cd,Zn,S core/shell system. Photoluminescence spectroscopy in conjunction with semiclassical electron transfer theory reveals the absence of an interfacial electronic state. This absence of a surface/interfacial state is unique to these nanocrystals with a graded shell structure, enabling trap free performance. Excitonic state-resolved pump/probe spectroscopy reveals that the higher excitons do not have the same symmetries as spherical CdSe nanocrystals, thereby enabling increased bandwidth. These pump/probe experiments further reveal the unique electronic structure of the band-edge biexciton which enables single exciton gain in these nanocrystal systems. Finally, the long gain lifetimes are discussed in light of the absence of a surface/interfacial electronic state. These experiments provide the first direct view of how interfacial electronic structure can be probed and understood so as to optimize their performance for light emission and optical gain for the metrics of threshold, bandwidth, and lifetime.