Direct Synthesis of Six-Monolayer (1.9 nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 nm

Here, quasi-two-dimensional semiconductor nanoplatelets (NPLs) have garnered widespread interest because of their uniquely narrow emission spectra and favorable characteristics for optical gain and lasing.(1–5) Strongly quantum confined along thickness, NPLs exhibit the electronic structure of quantum wells determined by their thickness, which can be controlled with atomic precision.(6–10) Among all semiconductor NPLs, CdSe NPLs are probably the most studied. So far, four kinds of CdSe NPLs, emitting at around 396 nm, 463 nm, 513m, and 553 nm, have been intensely investigated for almost a decade.(11–18) Despite some early confusions in identifying actual thicknesses of those NPLs, they have been successfully assigned to be 2-monolayer (2 ML) to 5-monolayer (5 ML) thickness by high-resolution TEM.(19) Here, nML thickness corresponds to n chalcogen layers sandwiched between n + 1 metal layers. Another kind of NPLs with 6MLs of CdSe, once briefly mentioned in 2013,(20) however, was not explored, likely because its synthesis proved hard to reproduce. The 6 ML CdSe NPLs can be prepared by the colloidal atomic layer deposition (c-ALD) method,(21) but there have been no reports on direct synthesis of 6 ML CdSe NPLs, and their direct synthesis has been in doubt as being difficult by traditional approaches.(22,23)