Optical and Electronic Properties of Defective Copper Selenide Predicted from Density Functional Theory

Plasmonic nanomaterials have been shown to have exciting applications in catalysis and medicine. Most plasmonic materials are precious metals such as Au and Ag. There is interest in developing non-precious metal plasmonic substrates due to lower material costs. Copper chalcogenide nanoparticles are very promising materials because their optical properties can be tuned. We investigate the optical and electronic properties of copper selenide, Cu2-xSe, through density functional theory (DFT). Cu2-xSe is only plasmonically active in the oxidized state, i.e., for x> 0. Moreover, it is known to be superionic due to the mobility of the Cu atoms. We have used both generalized gradient approximation (GGA) and meta-GGA functionals to compute structural, electronic, and optical properties of cubic Cu2-xSe, for x= 0, 0.25, 0.5, and 0.75. We performed ab-initio molecular dynamics simulations to simulate disorder within the lattice, taking 50 snapshots sampled from each simulation and relaxing them to their local ground states. We used the HSE06+U meta-GGA functional to calculate electronic structures, including optical band gaps. We found that the average optical gap increases with increasing oxidation, while electronic band gaps were only observed for the x = 0 structure. We observed that ionic disorder induces unpredictable changes in the optical gaps of Cu2-xSe for each fixed value of x.