Enhanced and robust room temperature excitonic structures in Li-doped CsPbBr3 nanocrystals

Abstract

We investigated the nonlinear photoluminescence (PL) response of CsPbBr₃ nanocrystals (NCs) embedded in a Cs₄PbBr₆ matrix under sub-bandgap 800 nm laser excitations at room temperature (RT). Adding Cs₄PbBr₆ matrix allowed us to stabilize these materials, making them very robust under high-fluence laser excitation. Furthermore, doping our structures with Li up to 40% leads to a substantial increase in the PL. This fact indicates that Li acts to passivate surface/interface states and also can form complexes with trap states such as the positively charged bromine vacancies, leading to their saturation and the suppression of nonradiative recombination. One of the significant aspects of our observation is that even at RT, the PL shows rich and well-resolved excitonic structures, which are typically observed at low temperatures. Our optical excitations was performed at two different fluence regimes. At low fluences, log–log fits of peak PL intensity versus fluence give slopes near 2. This fact is consistent with two-photon absorption as the dominant excitation pathway. At higher fluences, for one transition, we observe spectral narrowing and a faster increase in intensity, while other transitions demonstrate saturation. This observation indicates the possibility of optical gain or amplified spontaneous emission in our structures, even in the presence of Li doping.