Broadband Tunable Optical Gain from Ecofriendly Semiconductor Quantum Dots with Near-Half-Exciton Threshold.

Optical gain in solution-processable quantum dots (QDs) has attracted intense interest toward next-generation optoelectronics; however, the development of optical gain in heavy-metal-free QDs remains challenging. Herein, we reveal that the ZnSe 1- x Te x -based QDs show excellent optical gain covering the violet to near-red regime. A new gain mechanism is established in the alloy QDs, which promotes a theoretically threshold-less optical gain thanks to the ultrafast carrier localization and suppression of ground-state absorption by the Te-derived isoelectronic state. Further, we disclose that the hot-carrier trapping represents the main culprit to exacerbate the gain performance. With the increase of Te-to-Se ratio, a sub-band-gap photoinduced absorption (PA) appears and extinguishes the optical gain. To overcome this issue, we modulate the inner ZnSe shell thickness, and the gain is recovered by reducing the overlap between the gain and PA regions in the Te-rich QDs. Our finding represents a significant step toward sustainable QD-based optoelectronics.