Kazuhiro Takami

Development of a scanning tunneling microscope for in situ experiments with a synchrotron radiation hard-X-ray microbeam.

A scanning tunneling microscope dedicated to in situ experiments under the irradiation of highly brilliant hard-X-rays of synchrotron radiation has been developed. In situ scanning tunneling microscopy (STM) observation was enabled by developing an accurate alignment system in ultrahigh vacuum. Despite the noisy conditions of the synchrotron radiation facility and the radiation load around the probe tip, STM images were successfully obtained at atomic resolution. Tip-current spectra were obtained for Ge nano-islands on a clean Si(111) surface by changing the incident photon energy across the Ge absorption edge. A current modification was detected at the absorption edge with a spatial resolution of the order of 10 nm.

Development and Application of Multiple‐Probe Scanning Probe Microscopes

In the research of advanced materials based on nanoscience and nanotechnology, it is often desirable to measure nanoscale local electrical conductivity at a designated position of a given sample. For this purpose, multiple-probe scanning probe microscopes (MP-SPMs), in which two, three or four scanning tunneling microscope (STM) or atomic force microscope (AFM) probes are operated independently, have been developed. Each probe in an MP-SPM is used not only for observing high-resolution STM or AFM images but also for forming an electrical contact enabling nanoscale local electrical conductivity measurement. The worlds first double-probe STM (DP-STM) developed by the authors, which was subsequently modified to a triple-probe STM (TP-STM), has been used to measure the conductivities of one-dimensional metal nanowires and carbon nanotubes and also two-dimensional molecular films. A quadruple-probe STM (QP-STM) has also been developed and used to measure the conductivity of two-dimensional molecular films without the ambiguity of contact resistance between the probe and sample. Moreover, a quadruple-probe AFM (QP-AFM) with four conductive tuning-fork-type self-detection force sensing probes has been developed to measure the conductivity of a nanostructure on an insulating substrate. A general-purpose computer software to control four probes at the same time has also been developed and used in the operation of the QP-AFM. These developments and applications of MP-SPMs are reviewed in this paper.

Scanning Tunneling Microscopy Combined with Hard X-ray Microbeam of High Brilliance from Synchrotron Radiation Source

In situ scanning tunneling microscopy (STM) with highly brilliant hard X-ray irradiation was enabled at SPring-8. To obtain a good signal-to-noise ratio for elemental analysis, an X-ray beam with a limited size of 10 µm was aligned to a specially designed STM stage in ultrahigh vacuum. Despite various types of noises and a large radiation load around the STM probe, STM images were successfully observed with atomic resolution. The use of a new system for elemental analysis was also attempted, which was based on the modulation of tunneling signals rather than emitted electrons. Among tunneling signals, tunneling current was found to be better than tip height as a signal to be recorded, because the former reduces markedly the error of measurement. On a Ge nanoisland on a clean Si(111) surface, the modulation of tunneling current was achieved by changing the incident photon energy across the Ge absorption edge.

Surface reconstruction of TiC(001) and its chemical activity for oxygen

The TiC(001) surface has been observed by scanning tunneling microscopy with atomic resolution in ultrahigh vacuum. After the sample was cleaned by repeated heating at ∼1300 °C, we observed the 1×1 structure. On the other hand, the √×√ structure due to the ordered carbon vacancies formed when the sample was annealed at ∼1150 °C. We demonstrated the chemical activity of carbon vacancies of both structures for oxygen. The TiC(001)-1×1 surface with intentionally increased carbon vacancies and the TiC(001)-√×√ surface changed to a defect-free 1×1 structure with a small amount of oxygen, owing to the preferential adsorption of oxygen on the carbon vacancies.

Electrical conduction of organic ultrathin films evaluated by an independently driven double-tip scanning tunneling microscope.

The electrical transport properties of organic thin films within the micrometer scale have been evaluated by a laboratory-built independently driven double-tip scanning tunneling microscope, operating under ambient conditions. The two tips were used as point contact electrodes, and current in the range from 0.1 pA to 100 nA flowing between the two tips through the material can be detected. We demonstrated two-dimensional contour mapping of the electrical resistance on a poly(3-octylthiophene) thin films as shown below. The obtained contour map clearly provided an image of two-dimensional electrical conductance between two point electrodes on the poly(3-octylthiophene) thin film. The conductivity of the thin film was estimated to be (1-8) × 10(-6) S cm(-1). Future prospects and the desired development of multiprobe STMs are also discussed.

Structural study of initial growth of nickel on yttria-stabilized zirconia by coaxial impact-collision ion scattering spectroscopy

The atomic structure of yttria-stabilized ZrO2 (YSZ) surfaces was investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS). After a simple treatment to clean the surface, the outermost plane of the clean YSZ (001) surface was found to be a Zr layer. Compared with the clean surface, the initial stage of Ni/YSZ interface formation was studied after the vapor deposition of nickel at room temperature in ultrahigh vacuum (UHV). It was revealed that Ni atoms grow in order even at a thickness of 2 monolayers (MLs) in UHV. The Ni growth conserved the 4-fold symmetry of the YSZ (001) substrate structure, suggesting a certain interaction between the Ni and Zr atoms. However, after air exposure, the ordering structure disappeared and changed into a random phase.