Superstructures of carbon on V(1 0 0)

Carbon adsorption on V(1 0 0) was studied by both experimental methods and density functional theory. At low carbon coverages of ΘC=0.18 ML and oxygen below the experimental detection limit, measured scanning tunneling microscopy (STM) images show both areas of local c(2×2) structure and 〈0 1 0〉 oriented rows of C atoms. At higher coverages of ΘC=0.41 ML, mainly 〈0 1 0〉 oriented C rows with some local p(1×2) patterns are formed. The observed c(2×2) pattern is attributed to the presence of oxygen, since a mixture of carbon and oxygen favors the c(2×2) superstructure according to both the STM and the ab initio results. The calculations show that for ΘC=0.50 ML the p(1×2) structure is more stable than c(2×2) by 0.13 eV per adsorbed atom. From the ab initio results it is predicted that p(1×2) changes into c(2×2) at a mixed coverage of about ΘC≈0.37 ML and ΘO≈0.13 ML. The geometry of the c(2×2) structure was determined using quantitative low energy electron diffraction showing good agreement with the ab initio data. Also the simulated STM images agree well with the experimental STM data.