First Principles Simulations of Hg_xCd_1-xS and Hg_xCd_1-xSe Optical Properties

Mercury cadmium chalcogenide quantum dots (QDs) are promising fluorescent materials for bio-imaging and device applications due to facile tuning of their emission wavelength from the visible to short wave infrared (SWIR; 900 – 2000 nm). High-quality nanocrystals are achieved via cation exchange-mediated synthesis as de novo syntheses of alloy nanocrystals are hard to control. While the relative degree of exchange of cadmium to mercury is used to control the emission wavelength, precise control of the final emission wavelength remains elusive. A first-principles prediction of the relationship between the mercury-to-cadmium ratio in the alloyed QDs and their optical properties can inform the design and synthesis of these materials.<br/><br/>This talk will describe calculated optical spectra of both bulk mercury cadmium chalcogenide materials and nanocrystals composed of HgS, HgSe, CdS, and CdSe. We used density functional theory to calculate the band gap energies and the optical properties via complex dielectric functions and absorption spectra of zinc blende Hg_xCd_1-xS and Hg_xCd_1-xSe alloys as a function of alloy composition. We then applied the generalized quasi-chemical approximation to describe the relationship between band gap energies and optical features with respect to the material composition under different synthesis conditions, including thermodynamic equilibrium. We found that the optical absorption features at the band edge are highly sensitive to even a small mercury fraction (x&lt;0.2) in the material. We considered the effects of spin-orbit coupling and found that while it did alter the band gap energies by up to 0.5 eV, the complex dielectric function features were similar and only shift in energy. Our bulk alloy calculations are consistent with trends observed experimentally and our nanocrystal calculations can be used to adjust the energy range to account for the effect of quantum confinement. Together these results can be used to identify target compositions of mercury cadmium alloy QDs needed to achieve desired optical properties.