Stimulated Emission through an Electron-Hole Plasma in Colloidal CdSe Quantum Rings

Abstract

Colloidal CdSe quantum rings (QRs) are a new class of nanomaterials synthetized via thermo-chemical edge reconfiguration of thinner CdSe nanoplatelets [1],[2]. In the latter, the photo-physics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, that can merge to form excitonic molecules (biexcitons), giving rise to net stimulated emission along the molecule-to-exciton recombination pathway.[3] On the other hand, little is known on the nature of elementary excitations in thicker CdSe QRs - whether they are excitons or free electron-hole pairs- and their behavior at high density regime. Here, we show that charge carriers in QRs condense into a hot uncorrelated electron-plasma at high density opposed to the stable exciton gas found in thinner nanoplatelets. Through strong band gap renormalization, this plasma state is able to produce sizable optical gain with a broadband spectrum. Next, we show that the typical signatures of excitonic transitions are indeed absent in QRs. The gain is limited by a second order radiative recombination process and the buildup is counteracted by a typical charge cooling bottleneck. Overall, our results show that weakly confined QRs are a unique system to study uncorrelated electron-hole dynamics in nanoscale materials. [1] Fedin, I.; Talapin, D. V. Colloidal CdSe Quantum Rings. J. Am. Chem. Soc. 2016, 138, 9771-9774. [2] Salzmann, B. B. V.; Vliem, J. F.; Maaskant, D. N.; Post, L. C.; Li, C.; Bals, S.; Vanmaekelbergh, D. From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfi guration. Chem. Mater 2021. [3] Geiregat, P.; Tomar, R.; Chen, K.; Singh, S.; Hodgkiss, J. M.; Hens, Z. Thermodynamic Equilibrium between Excitons and Excitonic Molecules Dictates Optical Gain in Colloidal CdSe Quantum Wells. J. Phys. Chem. Lett 2019, 10, 3637-3644.