Since their discovery cadmium chalcogenide nanoplatelets (NPLs) attracted immense attention due to the fact that they exhibit optoelectronic properties, which are superior to the properties of counterpart nanocrystals of other dimensionalities with the same composition. In addition, due to their shape, they can be arranged into oriented assemblies further extending control over the optical and electronic features. One of the particular features of such NPLs is atomic smoothness of their surface and strong one-dimensional quantum confinement in the thickness direction, which can be controlled down to one monolayer. On the other hand, the ability to change thickness only in integer steps becomes a severe disadvantage preventing precise tuning of spectral position of absorption and photoluminescence bands.

To some extent, spectral tunability was achieved by varying the lateral size of NPLs, and their surface modification via ligand exchange or shell growth. However, these approaches are considerably limited and still not nearly as flexible as the simple size-tuning of quantum dots. In addition, they generally result in a red-shift of optical bands of NPLs, thus highlighting another longstanding challenge for NPLs – the lack of bright emitters in the UV-blue region.

Another more powerful approach consists in the modification of nanocrystal composition through the synthesis of alloyed nanoparticles. Although some advances in this direction have been already achieved by modifying existing protocols for the synthesis of CdSe and CdS NPLs, the synthesized nanoparticles either suffered from poor photoluminescence quantum yields of < 5 % or a limited range of available compositions.

In this work, we demonstrate that both of these issues can be overcome by employing highly reactive stearoychalcogenides, which rapidly react with cadmium carboxylate yielding small polydisperse nanocrystals. At later reaction stages, these nanocrystals serve as a monomer source for the expansion of NPLs in the lateral direction in a slow Ostwald-like ripening process guided by acetate ions. The separation of these stages essentially decouples the precursor conversion step and lateral growth of NPLs thus affording much better control over the lateral dimensions of NPLs, which in turn is a significant factor for achieving brightly emitting nanocrystals. Conveniently, unlike in previously reported procedures, the Se/(Se+S) ratio in the synthesized NPLs is essentially the same as in the starting reaction mixture which enables straightforward control over the nanocrystal composition. Moreover, we demonstrate that by using essentially the same procedure and changing the reaction temperature it is possible to control the thickness of NPLs thus further expanding the spectral range of the synthesized nanocrystals.

Optical spectroscopy measurements show that the increase of selenium content in the alloy results in a steady red shift of absorption and photoluminescence bands consistent with the shrinkage of the material bandgap. Due to their relatively small lateral size enabled by controlling the lateral growth, obtained alloyed CdSe_xS_1-x NPLs exhibit bright band-edge emission covering the blue-green region from 380 to 520 nm with quantum yields of around 30–50 %, which are ca. 10 and 2 times higher than previously reported for respectively 3.5 and 4.5-monolayer thick NPLs with similar composition.