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.

Colloidal semiconductor CdSe nanoplatelets (NPLs) feature ultranarrow and anisotropic emissions. However, the optical performance of blue‐emitting NPLs is deteriorated by trap states, currently exhibiting tainted emissions and inferior photoluminescence quantum yields (PLQYs). Here, near trap‐free blue‐emitting NPLs are achieved by the controlled growth of the core/crown. Deep trap states in NPLs can be partially suppressed with the asymmetrical crown growth and are further suppressed with the growth of the small core and the alloyed symmetrical crown, yielding NPLs with pure blue emissions and near‐unity PLQYs. Exciton dynamic research based on these NPLs indicates that the trap emission stems from surface traps. Besides, light‐emitting diodes exhibiting ultranarrow emission centered around 461 nm with full‐width‐at‐half‐maximums down to 11 nm are fabricated using these NPLs.

Colloidal II–VI group nanoplatelets (NPLs) possess ultranarrow emission line widths, for which they have great promise in achieving the purest display color in solution-processed light-emitting diodes (LEDs). Red NPL-LEDs have shown extremely saturated red color with high efficiency, while the green and blue ones lag far behind. Herein, we report green NPL-LEDs with the purest color in accordance with the Rec. 2020 standard and the peak external quantum efficiency (EQE) of 9.78%. By carefully controlling the aspect ratio, capping ligands, and purifications of CdSe/CdSeS core/alloyed-crown NPLs, NPL films with excellent flatness and unity photoluminescence quantum yields (PLQYs) are realized, laying a solid foundation for improving LED performance. Furthermore, via tuning the carrier injection balance, the record-high EQE for green NPL-LEDs is achieved. The electroluminescence (EL) exhibits an extremely saturated green color with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.163 0.786), which demonstrates their great potential in applications of ultrahigh-definition display technology. Our findings would help to further improve the performance of all NPL-LEDs.

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Colloidal nanoplatelets (NPLs), a class of semiconductor nanocrystals, have attracted considerable attention as a promising gain material for their ultralow amplified spontaneous emission (ASE) and lasing thresholds. However, there exist spectral gaps, especially in the green-color range, that NPLs cannot fully cover. The recently developed CdSe/CdSeS core/ alloyed-crown NPLs with excellent tunability across the greencolor range offer the possibility to remedy this deficiency. Here, the ASE and lasing characteristics of this new type of NPL are investigated. A remarkably low ASE threshold of 16 μJ/cm 2 at 522 nm is measured, the lowest among core/crown NPLs. Microlasers are fabricated by spin-coating them on second-order distributed feedback (DFB) cavities developed in silicon nitride (SiN) substrates. The microlasers exhibit an ultralow lasing threshold of 9 μJ/cm 2 at 522 nm. Moreover, they can cover a spectral range of 505−535 nm with all clean single-mode emissions. Picosecond time-and spectral-resolved photoluminescence (PL) spectroscopy reveals that the gain band is determined by the biexciton emission bandwidth. The vigorous development of NPLs with low lasing thresholds in a broad spectral range will greatly facilitate the realization of nanocrystal-based lasers.