Expanding the Family of Heterostructured CdSe/CdS Core/Crown Nanoplatelets: Six- and Seven-Monolayers-Thick Species.

The growth of atomically flat CdSe nanoplatelets (NPLs) thicker than 5 monolayers (ML) remains a major challenge in colloidal semiconductor synthesis, particularly for core/crown heterostructures. Here we report the successful synthesis of zinc-blende CdSe NPLs with unprecedented thicknesses of 6 and 7 ML, exhibiting sharp photoluminescence at 579 and 596 nm, respectively. We demonstrate that these thick NPLs can serve as cores for CdSe/CdS core/crown heterostructures, confirmed by lateral size expansion and the emergence of characteristic CdS absorption features. Through density-functional theory calculations, we uncover a critical relationship between NPL shape and crown growth: fluoride species bind three times stronger to {100} facets of rectangular NPLs compared to {110} facets of square NPLs, effectively poisoning crown growth on rectangular species. This mechanistic insight explains the shape-dependent success of crown formation and provides a framework for controlling two-dimensional (2D) semiconductor growth. Our optimized synthesis of thick core NPLs and their crown-enhanced derivatives significantly expands the spectral range of CdSe-based NPLs, advancing their development as narrow-line width red emitters.