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Sub-single exciton optical gain threshold in colloidal semiconductor quantum wells with gradient alloy shelling

Colloidal semiconductor quantum wells have emerged as a promising material platform for use in solution-processable light-generation including colloidal lasers. However, application relying on their optical gain suffer from a fundamental complication due to multi-excitonic nature of light amplification in common II-VI semiconductor nanocrystals. This undesirably increases the optical gain threshold and shortens the net gain lifetime because of fast nonradiative Auger decay. Here, we demonstrate sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient alloyed shell colloidal quantum wells. This sub-single exciton ensemble-averaged gain threshold of Ng = 0.80 (per particle) resulting from impeded Auger recombination along with a large absorption cross-section of quantum wells enables us to observe the amplified spontaneous emission starting at a low pump fluence of 800 nJ cm-2, at least three-folds better than the previously best reported values among all colloidal semiconductor nanocrystals. Moreover, long optical gain lifetimes of 800 ps accompanied with modal gain coefficients of 2,000 cm-1 are achieved. Finally, using these gradient shelled quantum wells, we show a vertical cavity surface-emitting colloidal laser operating at an ultralow lasing threshold of 7.5 micro-joule cm-2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers.