First Principles Simulations of Hg_xCd_1-x(S,Se) Optical Properties

Mercury cadmium chalcogenide alloy quantum dots (QDs) are a promising material to meet the rapidly growing need for materials with emission in the biological near-infrared (NIR) window (700-2,000 nm) where tissue transparency is high and autofluorescence noise is low. Cation exchange-mediated alloying is an effective way to tune the QD emission wavelength continuously across the entire visible and NIR spectra without altering the QD’s physical dimensions. Here we describe the optical properties of zinc blende Hg_xCd_1-xS and Hg_xCd_1-xSe alloys from first principles simulations across the compositional space. We use density functional theory to compute electronic band structures of the binary mercury and cadmium chalcogenide systems and their intermediate ternary alloys. We then compute the corresponding optical spectra and describe the extent of the effect of spin-orbit coupling on the calculated photophysical properties. Comparison to measured absorption spectra of mercury cadmium chalcogenide alloy QDs provides insights into how photophysical parameters can be achieved with specific compositions and nanocrystal sizes.