Theoretical Study of the Impact of Vacancies and Disorder on the Electronic Properties of Cu2–xSe

Copper selenide (Cu2–xSe) is a promising material for plasmonic nanoparticle applications. Cu2–xSe becomes plasmonically active in the oxidized state (x > 0), where free carrier density increases with increasing oxidation. It also has considerable cation (Cu) disorder. To date, there has not been a theoretical study of the impact of the degree of oxidation and cation disorder on the electronic and optical properties of Cu2–xSe. We used density functional theory (DFT) to investigate the effects of the concentration of Cu vacancies and disorder on the properties of Cu2–xSe. We used both generalized gradient approximation (GGA) and hybrid-GGA functionals to compute the structural, electronic, and optical properties of the cubic phase of Cu2–xSe for x = 0, 0.25, 0.5, and 0.75. We performed ab initio molecular dynamics simulations at 300 K to simulate disorder, taking snapshots sampled from simulations of periodic supercells with different levels and arrangements of defects. The HSE06+U hybrid-GGA functional was used to calculate the electronic properties, including the optical band gaps, for a total of 400 different configurations. We found that the average optical band gap increases with increasing oxidation. We also found that only the stoichiometric, x = 0, materials are semiconductors, and the electronic band gap generally increases, increasing disorder.