Formation of Two-Dimensional Copper Selenide on Cu(111) at Very Low Selenium Coverage

Using scanning tunneling microscopy (STM), we observed that adsorption of Se on Cu(111) produced islands with a (root 3 x root 3)R30 degrees structure at Se coverages far below the structure's ideal coverage of 1/3 monolayer. On the basis of density functional theory (DFT), these islands cannot form due to attractive interactions between chemisorbed Se atoms. DFT showed that incorporating Cu atoms into the root 3-Se lattice stabilizes the structure, which provided a plausible explanation for the experimental observations. STM revealed three types of root 3 textures. We assigned two of these as two-dimensional layers of strained CuSe, analogous to dense planes of bulk klockmannite (CuSe). Klockmannite has a bulk lattice constant that is 11% shorter than root 3 times the surface lattice constant of Cu(111). This offers a rationale for the differences observed between these textures, for which strain limits the island size or distorts the root 3 lattice. STM showed that existing step edges adsorb Se and facet toward < 1 (2) over bar1 >, which is consistent with DFT.