Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of Pyrite CuSe₂ Films on SrTiO₃

We perform molecular beam epitaxy growth and scanning tunneling microscopy study of copper diselenide (CuSe₂) films on SrTiO₃(001). Using a Se-rich condition, the single-phase pyrite CuSe₂ grows in the Stranski–Krastanov (layer-plus-island) mode with a preferential orientation of (111). Our careful inspection of both the as-grown and post-annealed CuSe₂ films at various temperatures invariably shows a Cu-terminated surface, which, depending on the annealing temperature, reconstructs into two distinct structures 2×√3 and √3×√3-R30°. The Cu termination is supported by the depressed density of states near the Fermi level, measured by in-situ low temperature scanning tunneling spectroscopy. Our study helps understand the preparation and surface chemistry of transition metal pyrite dichalcogenides thin films.