Superior optical gain in zinc selenide colloidal nanocrystals induced by Coulomb-correlated electron–hole plasma

Colloidal semiconductor nanocrystals (NCs) have been recognized as promising solution-processable gain media; however, the lasers with state-of-the-art performance exclusively originate from the cadmium- and lead-based NCs. Herein, we for the first time unravel that high-quality heavy-metal-free ZnSe/ZnS NCs show superior optical gain and lasing performance when the sizes exceed the quantum confinement regime. Corroborated by comprehensive transient spectroscopy, we reveal that the optical gain in large ZnSe/ZnS NCs originates from the novel Coulomb-correlated electron–hole plasma (C-EHP) instead of high-order multi-exciton recombination. Thanks to the formation of a four-level system and the suppression of Auger recombination, the C-EHP renders low gain threshold (9.4 μJ/cm2), high gain coefficient (>6500 cm−1), and long gain lifetime (∼4 ns). Such desirable gain properties compete well with those of classic CdSe NCs and enable the construction of a high-performance laser device. This work represents significant progress toward the development of solution-processable non-heavy-metal nanocrystal lasers.