| Summary: | 碩士 === 國立清華大學 === 物理學系 === 94 === We have performed a high resolution photoemission study on ultrathin NiO films on Ag(100) surfaces. The ultrathin NiO films were prepared under UHV conditions on Ag(100) substrate by evaporation of metallic Ni in an O2 atmosphere. The Ni deposition rate was measured by a quartz crystal thickness monitor (QCTM) and the thickness of NiO films on Ag(100) was determined by reflection high energy electron diffraction (RHEED). We found that the substrate temperature and the O2 pressure affect greatly the composition and structure of NiO films on Ag(100) for monolayer (ML) coverage. For the high temperature (~ 180℃) growth, the incorporation rate of Ni into the silver subsurface region is faster than the reaction rate of Ni with oxygen and PES data show substantial metallic Ni for submonolayer coverages. For the room temperature (RT) growth, the low energy electron diffraction shows a (2×1) pattern (in two orthogonal domains) for 1.2 ML NiO films on Ag(100) at O2 pressures in the 5×10-7 ~ 8×10-7 Torr range; while the (2×1) pattern evolves to a (1×1) pattern for O2 pressure above 8×10-7 Torr or with the increase of film thickness. The best (2×1) pattern was observed for 0.9 ML NiO films on Ag(100) at O2 pressure of 7.5×10-7 Torr with post-annealing at 180℃ in an O2 atmosphere. The (2×1) LEED pattern is transformed into a (1×1) pattern upon annealing at 350℃ for 15 min.
PES results indicate that the ultrathin NiO films on Ag(100) becomes an insulator at coverage above ~ 3.2 ML. However, there are finite density of states at Fermi level for 0.9 ML (2×1) NiO/Ag(100) and 2 ML (1×1) NiO(100)/Ag(100). This means that they have metallic character, and the metallic character of 0.9 ML (2×1) NiO/Ag(100) is stronger than that of 2 ML (1×1) NiO(100)/Ag(100). Angle-resolved photoemission spectroscopy has been used to study the band dispersion of 0.9 ML (2×1) NiO/Ag(100), 2 ML (1×1) NiO(100)/Ag(100) as well as 10 ML NiO(100)/Ag(100). The band dispersion of 10 ML NiO(100)/Ag(100) was similar to that of bulk NiO(100) crystal. Weak dispersion of valence bands was observed on 0.9 ML (2×1) NiO/Ag(100), which is very different from that on 2 ML (1×1) NiO(100)/Ag(100). The details of the above observation are discussed in this thesis.
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