Giant Alloyed Hot Injection Shells Enable Ultra-Low Optical Gain Threshold in Colloidal Quantum Wells.

As attractive materials system for high-performance optoelectronics, colloidal nanoplatelets (NPLs) benefit from atomic-level precision in thickness, minimizing emission inhomogeneous broadening. Much progress has been made to enhance their photoluminescence quantum yield (PLQY) and photostability. However, layer-by-layer growth of shells at room temperature has to date resulted in defects that limit PLQY and thus curtail the performance of NPLs as an optical gain medium. Here we introduce a hot-injection method growing giant alloyed shells using an approach that reduces core/shell lattice mismatch and suppresses Auger recombination. Near-unity PLQY is achieved with a narrow full-width-at-half-maximum (20 nm), accompanied by emission tunability (from 610 to 650 nm). The biexciton lifetime exceeds one nanosecond, an order of magnitude longer than in conventional colloidal quantum dots (CQDs). Reduced Auger recombination enables record-low amplified spontaneous emission threshold of 2.4 µJ cm-2 under one-photon pumping. This is lower, by a factor of 2.5× than the best previously reported value in nanocrystals (6 µJ cm-2 for CdSe/CdS NPLs). Here we also report single-mode lasing operation with 0.55 mJ cm-2 threshold under two-photoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm-2 from CdSe/CdS CQDs in Fabry-Pérot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultra-high performance NPLs.