Electronic Structure Insights into the Tunable Luminescence of CuAlₓFe₁–ₓS₂/ZnS Nanocrystals

We describe the electronic structure and spectroscopic properties of CuAlₓFe₁–ₓS₂ nanocrystals and their core/shell structures. The as-synthesized CuAlₓFe₁–ₓS₂ core exhibits a tetragonal chalcopyrite structure. The core material exhibits tunable band gap that spans the entire visible to near-infrared spectrum, from 3.48 to 0.53 eV. This tunability is achieved by varying the mole fraction of aluminum and iron from 1:0 to 0:1. The band gap variation with composition deviates from Vegard’s law and corresponds to a bowing coefficient of 1.59 eV. Our observations are interpreted using density functional theoretical calculations. In particular, we find that the significant bowing is well accounted for through significant localization of the Fe electronic states. Most significantly, CuAlₓFe₁–ₓS₂ shows photoluminescence upon making a shell of zinc sulfide, which is tunable from 400 to 1400 nm (3.1 to 0.89 eV). CuAlₓFe₁–ₓS₂/ZnS are until date the only visible-infrared tunable nanocrystal fluorophore composed entirely of earth abundant elements with atomic numbers 30 and lower.