Abstract
The morphological, crystallographical and electronic structure of a well-ordered pseudomorphic sulphur monolayer (S ML) bonded at the (001) surface of a niobium substrate was investigated experimentally and theoretically. The system exhibits strong sulphur-niobium bonds with fourfold symmetry which do not exist in bulk niobium chalcogenides. These bonds enhance the localization of occupied bands near the Fermi energy in two Nb MLs under the S ML and, due to changed symmetry and stoichiometry, push the p-states of the S ML to deeper binding energies as compared to Nb chalcogenides. The S ML shows strong iono-covalent bonding to Nb(001) with bonding states mainly due to S-3p and antibonding states due to Nb-4d orbitals.