Surface oxidation of a Nb(100) single crystal by scanning tunneling microscopy

The surface structures of a Nb(100) single crystal in the initial stages of oxidation have been investigated by ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). A ladder-like (3×10) structure was observed on the surface after numerous cycles of Ar-ion sputtering and flash annealing at 1973 K. The ladder-like structure could be identified as a NbO2 overlayer on a Nb(100) surface due to the segregation of oxygen on the surface. The ladder-like structure gradually changed to a disordered structure upon exposure to oxygen at room temperature and was restored by annealing at above 853 K in UHV. The oxidation process on the Nb(100) surface is discussed.