Green CdSe/CdSeS Core/Alloyed‐Crown Nanoplatelets Achieve Unity Photoluminescence Quantum Yield over a Broad Emission Range

Abstract Cadmium‐based nanoplatelets as optical display and lasing materials are widely explored and exhibit great advantages, owing to their narrow emission linewidths, anisotropic transition‐dipole distributions, and low lasing thresholds. However, in the green range, the photoluminescence quantum yield (PLQY) and emission tunability of nanoplatelets are still inferior to that of quantum dots. In this work, a new synthesis protocol is developed, enabling core/crown nanoplatelets to grow continuously from elementary precursors to their final form. A new heterostructure of CdSe/CdSeS core/alloyed‐crown nanoplatelets is produced that realizes 100% PLQY, the continuous tunability of emission peaks in between 502 and 550 nm, and low full‐width‐at‐half‐maximum (FWHM) of less than 15 nm. Achieving these excellent properties in all three aspects at the same time is unprecedented. In addition, the time‐resolved photoluminescence (TRPL) spectra of these nanoplatelets show a mono‐exponential decay characteristic, and the nanoplatelet film can also show 100% PLQY and a mono‐exponential decay characteristic, indicating the suppression of trap states. The high‐quality nanoplatelets achieved in this work provide a solid foundation for developing nanoplatelet‐based light sources, like light‐emitting diodes and lasers, with much higher efficiency, color purity, and lower working thresholds.