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Vanadium dioxide (VO2) exhibits a metal-insulator phase transition at approximately 70 °C. Two different crystal structures, M1 and M2 phases, in the insulating state have been reported. The M2 phase is expected to be a Mott insulator, whereas the M1 phase is expected to act as a band insulator. It is important to clarify the origin and transition properties of the M2 phase. In this study, we fabricated VO2 nanostructures via sputtering deposition and subsequent lamp annealing at various O2 pressures. Temperature-dependent XRD measurements revealed that the nanostructured VO2 exhibits the M2 phase in temperatures just before the phase transition from M1 to R (metal state) during the heating process. The ratio of V5+/(V4+ + V5+) in nanostructured VO2 was estimated from XPS results; an increase in this ratio resulted in the broadening of the temperature range of the M2 phase. Furthermore, in the cooling process, direct phase transitions from R to M1 were observed. Structural changes are likely to occur in different pathways during heating and cooling. Our findings should contribute to the identification of the phase transition mechanism and to the development of a Mott field-effect transistor that utilizes nanostructured VO2 in the M2 phase.

We investigated the reduction of a CeO2(111) surface after the deposition of size-selected Pt8 clusters on it via x-ray photoelectron spectroscopy (XPS). Electron transfer from Pt to CeO2(111) results in the reduction of Ce4+ to Ce3+ and the subsequent formation of Ptδ+ atoms at the interface. We quantitatively estimated the reduction from the Ce3+/(Ce3+ + Ce4+) ratio determined from the Ce3d spectra. Approximately 30% of Ce4+ under or in the perimeter of the Pt8 cluster was reduced to Ce3+. The XPS spectra showed an increase in the ratio of Ce3+/(Ce3+ + Ce4+) with an increase in the amount of Pt8 clusters deposited on the CeO2(111) surface. The reduction of 3.7 Ce4+ to Ce3+ per Pt8 clusters, followed by the formation of 3.7 Ptδ+ atoms per Pt8 clusters was observed at 0.006 ML (mono layer) Pt coverage. The actual Ce3+ formation influenced by Pt deposition was approximately 2.5%. We estimate that approximately 30% of Ce4+ under or in the perimeter of a Pt8 cluster was reduced to Ce3+. At a higher coverage of 0.015 ML, the reduction of 2.2 Ce4+ to Ce3+ per Pt8 cluster, followed by the formation of 3.2 Ptδ+ atoms per Pt8 cluster was observed. This implies the aggregation of Pt clusters occurred at a higher Pt coverage. Atomically precise quantification of the electron transfer across the Pt–CeO2(111) interface can enable an improved understanding of the metal-support interaction.