Density functional theory examination of surface defects, substitution, and passivation on HgTe (111) surface for applications in colloidal quantum dots

Abstract

Colloidal quantum dots (CQDs) and especially those of HgTe offer a potential pathway to highly efficient and economical infrared photodetectors. Herein, we utilized first principles density functional theory to examine the chemical and optoelectronic properties of these materials. We demonstrate that an abundant source of trap states on HgTe (111) surfaces are unpassivated mercury atoms at the surface of the nanocrystal. Furthermore, we show that mercury vacancies, which contribute under-coordinated tellurium sites on the surface, do not appear to have an outsized impact on mid-gap states, unlike other II-VI CQD systems. Critical to device engineering, we present a theoretical method for the universal control of the conduction type in HgTe CQDs, regardless of the synthesis employed. Specifically, the substitution of indium into mercury sites at the surface of the nanocrystal induces n-type doping while p-type doping can be obtained through the adsorption of silver on FCC sites on mercury rich surfaces. During this investigation, we also confirm the observation of a ligand dipole dependent Fermi level. While further experimentation is warranted, this could enable higher performing devices with shorter ligands and precisely engineered band alignments.