The influence of external dielectric environments is well understood for 2D semiconductor materials but overlooked for colloidally grown II–VI nanoplatelets (NPLs). In this work, we synthesize MX (M = Cd, Hg; X = Se, Te) NPLs of varying thicknesses and apply the Elliott model to extract exciton binding energiesreporting values in good agreement with prior methods and extending to less studied cadmium telluride and mercury chalcogenide NPLs. We find that the exciton binding energy is modulated both by the relative effect of internal vs external dielectric and by the thickness of the semiconductor material. An analytical model shows dielectric screening increases the exciton binding energy relative to the bulk by distorting the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate NPLs are colloidal analogues of van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

We demonstrate the preparation of mesoscale semiconductor (II–VI) nanoplatelets (NPLs) for the first time using colloidal seeded growth. These nanoplatelets are quantum-confined and atomically precise but grown to a length scale compatible with conventional optical imaging and microscopic manipulation (even reaching >1 μm2) offering an opportunity to bridge the application space between nanocrystals and two-dimensional (2D) materials. Using CdTe as a model system, we develop a seeded growth procedure, show the parameters that control extension, and apply them to a variety of thicknesses and compositions. In situ spectroscopy demonstrates that addition onto the nanoplatelet seeds is not continuous and likely occurs through ripening. Finally, we use correlative optical and electron microscopy to demonstrate that at large sizes, photoluminescence (PL) mapping of the entire structure can be resolved including spatial inhomogeneities. Overall, these results show that nanoplatelets can be compared to 2D semiconductors while maintaining the advantages of scalable colloidal synthesis, thickness tunability, and solution processability.

Colloidally grown (II-VI) semiconductor nanoplatelets display significantly smaller lateral dimensions than their mechanically exfoliated 2D van der Waal (vdW) semiconductor counterparts. Here, we show that a seeded growth procedure allows us to significantly extend the lateral area of atomically precise nanoplatelets to the mesoscale (>1 μm^2). Using CdTe nanoplatelets as a model system, we optimize reaction parameters to expand a variety of nanoplatelets with different thickness and compositions. In situ spectroscopy results demonstrate that large NPLs grow through a mechanism of lateral ripening of seeds. Correlative optical spectroscopy and electron microscopy measurements show that the photoluminescence displays resolvable spatial inhomogeneities, similar to 2D semiconductors. Overall, these mesoscale nanoplatelets can be analogized to vdW semiconductors, with the added advantages of scalable colloidal synthesis, thickness tunability and solution processability.

The influence of external dielectric environments is well understood for 2D semiconductor materials but is overlooked for colloidally-grown II-VI nanoplatelets (NPLs). In this work, we synthesize MX (M=Cd, Hg; X= Se, Te) NPLs of varying thicknesses, and apply a modified Elliott model to fit excitonic absorption features and report exciton binding energies for cadmium telluride and mercury chalcogenides for the first time. Our observations indicate that the exciton binding energy is modulated by the dielectric screening of semiconductor material by the external ligand environment. Furthermore, NPL binding energies show a dependence on the number of monolayers consistent with relative effect of internal vs. external dielectric. To describe this, we derive an analytical electrostatic model, reinforcing the hypothesis that the external environment increases the exciton binding energy relative to the bulk—due to the distortion of the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate that NPLs are colloidal analogues of Van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

The influence of external dielectric environments is well understood for 2D semiconductor materials but is overlooked for colloidally-grown II-VI nanoplatelets (NPLs). In this work, we synthesize MX (M=Cd, Hg; X= Se, Te) NPLs of varying thicknesses, and apply a modified Elliott model to fit excitonic absorption features and report exciton binding energies for cadmium telluride and mercury chalcogenides for the first time. Our observations indicate that the exciton binding energy is modulated by the dielectric screening of semiconductor material by the external ligand environment. Furthermore, NPL binding energies show a dependence on the number of monolayers consistent with relative effect of internal vs. external dielectric. To describe this, we derive an analytical electrostatic model, reinforcing the hypothesis that the external environment increases the exciton binding energy relative to the bulk—due to the distortion of the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate that NPLs are colloidal analogues of Van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.