Giant Alloyed Hot Injection Shells Enable Ultralow Optical Gain Threshold in Colloidal Quantum Wells

Abstract

As an attractive materials system for highperformance 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, to date, layer-by-layer growth of shells at room temperature has 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 1 ns, 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 a 0.55 mJ cm −2 threshold under twophotoexcitation, which is also the best among nanocrystals (compared to 0.76 mJ cm −2 from CdSe/CdS CQDs in the Fabry−Peŕot cavity). These findings indicate that hot-injection growth of thick alloyed shells makes ultrahigh performance NPLs.

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…With the further increase of shell thickness, the averaged stimulated emission threshold is derived to be as low as ≈4.4 µJ cm −2 for the thick‐shell CdSe/CdS NPLs with 6 MLs. This ASE threshold is one of the lowest values reported for II–VI core/crown (4.15 µJ cm −2 ) and core/shell (≈2.4 and 6 µJ cm −2 ) NPL heterostructures, which is lower than the best previous reported values for phase‐pure CdSe/CdS QDs (16 µJ cm −2 ) and CdSe‐based core/alloy‐layer/shell QDs (≈6 µJ cm −2 ), enables them to be highly promising candidates for low‐threshold colloidal semiconductor lasing. However, an increased trend of ASE threshold is observed for CdSe/CdS NPLs with increasing CdS shells larger than 6 MLs.…”

Compositional Grading for Efficient and Narrowband Emission in CdSe-Based Core/Shell Nanoplatelets

“…30 This was recently implemented by synthesizing highly luminescent CdSe/ZnS NPLs. 24,31 However, the efficient photoluminescence in CdSe/ZnS NPLs came with broadening of the emission linewidth and an irregular shell growth. Our first goal was to study these problems and see if they could be avoided to obtain high-quality ZnS shells.…”

Record High External Quantum Efficiency of 19.2% Achieved in Light‐Emitting Diodes of Colloidal Quantum Wells Enabled by Hot‐Injection Shell Growth

“…The HIS approach has been utilized for the synthesis of 0D CQDs successfully since early 90s, providing the uniform size distribution and high QY . Recently, we have shown the optimized synthesis of shell growth using HIS approach, which results in highly photostable and near‐unity PLQY emitting core/shell CQWs . The wide‐bandgap shell (e.g., ZnS) provides high photostability and reduced Auger recombination for exciton harvesting applications.…”