W. Yamaguchi

Energy-dependent deposition processes of size-selected Ag nanoclusters on highly-oriented pyrolytic graphite

Singly charged cations of silver atoms and clusters (Agn+, n=1,3,5,7,9) were deposited on highly oriented pyrolytic graphite substrates at well-controlled, various collision energies. The total amount of Ag atoms remaining on the substrates after collision was quantified by measuring the Ag 3d5/2 photoelectron intensities. The collision energy dependence of the amount of deposited Ag atoms revealed that, for all the species, there are three distinct energy regions, for which soft landing, rebounding, or implantation is a dominant process, and that the energy ranges for the processes strongly depend on the cluster size. The deposition efficiency vs collision energy curve for each cluster is well fitted to that for Ag1, by considering the difference between the contact area of Ag1 with the surface and that of the cluster, reflecting the compactness of the clusters. Boundaries between the different deposition regimes for the clusters were less distinct than those for Ag1. Considering anisotropy in shape of a...

Fragmentation and ion-scattering in the low-energy collisions of small silver cluster ions (Agn+: n=1−4) with a highly oriented pyrolytic graphite surface

Fragmentation and ion-scattering processes in the low-energy (0–200 eV) collisions of Agn+ (n=1–4) with a highly oriented pyrolytic graphite (HOPG) surface have been investigated by employing a tandem time-of-flight mass spectrometer. It was found that the fragmentation of scattered cluster ions is due to unimolecular dissociation in the energy range studied. A marked difference between the fragmentation pattern of incident Ag3+ and that of Ag4+ has been found: The intact scattered cluster ion was observed for Ag3+ while only fragment ions for Ag4+. From the incident energy dependence of fragment ion intensities, it was deduced that internal energies of the scattered parent ions have upper and lower limits. These limitations are probably due to the adsorption and the implantation of the projectile clusters on the surface. Ion-scattering yield was found to increase with cluster size. Both the incident energy and cluster size dependencies of ion-scattering yields were reproduced by a model calculation in wh...

Influence of CDW stacking disorder on metal-insulator transition in 1T-TaS2

Abstract We have systematically studied the metal–insulator (MI) transition in 1T-Ta1−xS2 single crystals grown by the iodine vapor transport method under various sulfur pressures. From the chemical analyses and X-ray diffraction measurements, it is concluded that the obtained crystals contain Ta vacancies up to x∼5%. The MI transition temperature TMI is found to decrease with decreasing x. However, this can hardly be interpreted by the carrier doping effect on the Mott transition, in which TMI should be very sensitive to band filling, suggesting that the formation of Ta disorders in the nearly stoichiometric compounds may play an important role. STM images of a nearly stoichiometric crystal which shows no MI transition reveal characteristic mesh-like microstructures and the MI transition induced by the electric field of the STM tip. This also indicates that CDW stacking faults along the c-axis, possibly induced by Ta disorders, is important in determining the transition temperature.

Non-destructive deposition and diffusion–aggregation of size-selected silver nanoclusters on glassy carbon substrates as probed by real-time X-ray photoelectron spectroscopy

Abstract Silver nanoclusters, generated and size-selected in the gas phase and soft landed on low-temperature glassy carbon substrates, were characterized by real-time X-ray photoelectron spectroscopy (XPS). It has been found that the 3d core–electron binding energies of the deposited clusters gradually decrease with time in size-specific manners. The observation clearly showed that the clusters are deposited intact on the surface and diffuse and aggregate, leading to larger clusters. This demonstrates that the real-time XPS can provide key information on states and behaviors of size-selected nanoclusters deposited on surfaces.

Anomalous domain structure in 1T-TaS2-xSex observed using scanning tunneling microscopy

On using scanning tunneling microscopy we observed charge-density-wave (CDW) structures in layered compounds 1T-TaS2−xSex (x=0, 0.3, 0.5) at room temperature, and found a new domain structure for the x=0.3 compound. The new domain structure has a mesh-like pattern, in which the domain boundaries are imaged with brighter contrast only at specific sample voltages, implying that the domain structure has an electronic origin. Moreover, the CDW arrangement is irregularly distorted, while its amplitude is spatially unchanged. These features are in clear contrast to those of the hexagonal domains in the x=0 compound, suggesting that the structural transformation with respect to x is not a well-defined phase transition with abrupt disappearance of the hexagonal domains, but a continuous restructuring of CDW. We argue for the mesh-like structure to be a kind of moire pattern, originating from the local variation of CDW stacking along the c-axis.

Microtip-assisted metal–insulator transition in a layered chalcogenide

The layered compound 1T–TaS1.7Se0.3 forms a nanoscale domain structure, separated by mesh-like domain walls, above its bulk metal–insulator transition temperature TMI of ∼180 K. Scanning tunneling microscopy and spectroscopy of the compound demonstrated that each metallic domain can be converted to insulating one by successive scans of the probe tip just above TMI. This tip-assisted phenomenon is consistently explained by assuming that the domain structure arises from irregular distortion of charge density waves, and that the stacking pattern of charge density waves plays an essential role in the metal–insulator transition.

Metal-insulator transition in 1T-TaS2-xSex

Abstract The transition metal dichalcogenide, 1T-TaS2, shows metal–insulator transition (MIT) accompanying the nearly commensurate (NC)–commensurate (C) charge density wave (CDW) phase transition at 180 K. It has been proposed that the low-temperature C-CDW phase is a Mott insulator. However, our specific heat and magnetic susceptibility measurements in the 1T-TaS2−xSex system revealed no indications of effective mass enhancement or intrinsic localized spins in the insulating phase. So we propose a new model for the MIT in which the interlayer coupling of the CDW superstructure plays an important role.