Surface core-level shift and electronic structure on transition-metal diboride (0001) surfaces

Core levels of NbB2(0001) and ZrB2(0001) surfaces were measured by x-ray photoelectron spectroscopy (XPS). Large surface core-level shift (SCLS) to 1.6-eV shallower binding energy is observed in the B 1s core of the boron terminated NbB2(0001). By an ab initio density functional theory calculation, this SCLS is well reproduced. The analysis shows that the SCLS reflects not the total charge of the atom but the local charge density in the vicinity of the nucleus. The conventional XPS interpretation is not available in such a system with large intra-atomic charge redistribution. The electronic structure which leads the charge redistribution is discussed.