The effect of external dielectric environments on excitonic properties 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 for cadmium telluride and mercury chalcogenides for the first time. The exciton binding energy is tuned both by the relative effect of internal vs. external dielectric and the thickness of the semiconductor material. A semi-empirical 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 mixing in polarizable solvents. Our results suggest NPLs are colloidal analogues of Van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

Dielectric screening modulates semiconductor nanoplatelet excitons

ACS Fall 2021

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