Highly Stable, Near‐Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero‐Nanoplatelets Enabled by ZnS‐Shell Hot‐Injection Growth

Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c‐ALD) provides the ability to produce their core/shell heterostructures. However, as c‐ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near‐unity efficiency CdSe/ZnS NPLs are shown using hot‐injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI‐shell hetero‐NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c‐ALD shell‐coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI‐shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm −2 . Despite being annealed at 500 K, these ZnS‐HI‐shell NPLs possess low gain thresholds as small as 25 µJ cm −2 . These findings indicate that the proposed 573 K HI‐shell‐grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.