Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(110)-(2 × 1) O

Abstract

We obtained scanning tunneling microscopy (STM) light emission spectra of a Ni(110)-(2 × 1) O surface. A mosaic of nanoscale bright and dark domains was observed in the STM image. When the tip was fixed over the dark domain, stepwise structures were observed at 100 and 150 meV below the quantum cutoff in the STM light emission spectra. These energies were consistent with the reported vibrational energies of oxygen atoms adsorbed on the Ni(110)-(2 × 1) O surface, and indicated the expected isotope shifts between 16 O and 18O. Therefore, these stepwise structures are induced by the vibrations of adsorbed oxygen atoms. The 100-meV mode corresponded to the vibration polarized parallel to the surface. A theory describing the coupling between the STM light emission and vibration polarized parallel to the surface is proposed. We conclude that the oxygen atoms cover the dark domain and bare Ni atoms correspond to the bright domain. When the exposure level of oxygen was approximately 1/10 of that for Ni(110)-(2 × 1) O, bright-imaged nanostructures were observed in the terrace where bare Ni atoms are exposed. The step structure caused by the vibrations of adsorbed oxygen was observed in the STM light emission spectra of the bright-imaged nanostructures, showing that these structures are covered by oxygen atoms. In other words, the isolated nanostructures covered by oxygen atoms are imaged not as dark, but as bright, unlike in the case of Ni(110)-(2 × 1) O observed by STM.