Rei Hobara

Electron–Hole Recombination Time at TiO2 Single-Crystal Surfaces: Influence of Surface Band Bending

Photocatalytic activity is determined by the transport property of photoexcited carriers from the interior to the surface of photocatalysts. Because the carrier dynamics is influenced by a space charge layer (SCL) in the subsurface region, an understanding of the effect of the potential barrier of the SCL on the carrier behavior is essential. Here we have investigated the relaxation time of the photoexcited carriers on single-crystal anatase and rutile TiO2 surfaces by time-resolved photoelectron spectroscopy and found that carrier recombination, taking a nanosecond time scale at room temperature, is strongly influenced by the barrier height of the SCL. Under the flat-band condition, which is realized in nanometer-sized photocatalysts, the carriers have a longer lifetime on the anatase surface than the rutile one, naturally explaining the higher photocatalytic activity for anatase than rutile.

In situ resistance measurements of epitaxial cobalt silicide nanowires on Si(110)

We have performed in situ resistance measurements for individual epitaxial CoSi2 nanowires (NWs) (approximately 60 nm wide and 5μm long) formed on a Si(110) surface. Two- and four-point probe measurements were done with a multitip scanning tunneling microscope at room temperature. The NWs were well isolated from the substrate by a Schottky barrier with zero-bias resistance of 107Ω. The resistivity of the NWs was 30μΩcm, which is similar to that for high-quality epitaxial films. The NW resistance was essentially unchanged after exposure to air.

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Electronic Transport in Multiwalled Carbon Nanotubes Contacted with Patterned Electrodes

The electrical conductance of 0:8 � 5-mm-long multiwalled carbon nanotubes (MWCNT) was measured at room temperature in a multiprobe scanning tunneling microscope (STM)-scanning electron microscope (SEM) system and a conventional prober system, by bringing the MWCNTs into contact with patterned metal electrodes. The contact resistance between the CNTs and metal electrodes was sufficiently small. The conductance was proportional to A=L (and also to B=L, within our experimental error), where A, B, and L are the cross section, circumference, and length of CNTs. This indicates the occurrence of diffusive transport. A nonlinear current-voltage characteristic was obtained; the conductance increased steeply with current. A multiprobe STM-SEM system was very useful for measuring individual CNTs. [DOI: 10.1143/JJAP.43.L1081]

Transport at surface nanostructures measured by four-tip STM ☆

Abstract For in situ measurements of local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One is a ‘four-tip STM prober’, in which independently driven four tips of scanning tunneling microscope (STM) are used for four-point probe conductivity measurements. The probe spacing can be changed from 500 nm to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several μm. These probes were installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions were in situ observed. The probes can be positioned precisely on aimed areas on the sample with aid of piezo-actuators. With these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and influence of atomic steps upon conductivity could be directly measured. The STM prober is mainly described here.

Two-Dimensional Superconductor with a Giant Rashba Effect: One-Atom-Layer Tl-Pb Compound on Si(111).

A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having √3×√3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.

Electron-hole recombination on ZnO(0001) single-crystal surface studied by time- resolved soft X-ray photoelectron spectroscopy

Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained numerically using PES data, showing a depletion layer which extends to 1 μm. Temporal evolution of the photovoltage effect is well explained by a recombination process of a thermionic model, giving the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This lifetime agrees with a temporal range reported by the previous time-resolved optical experiments.

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.

Selective Growth of Straight Carbon Nanotubes by Low-Pressure Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) were grown by low-pressure thermal chemical vapor deposition using pure ethylene. It was found that straight CNTs, which were composed of bundled single- or double-wall CNTs and multiwall CNTs, were preferentially bridged between Fe nanoparticles under a low pressure of 100 Pa. Moreover, utilizing this method, we attempted to grow CNT bridges between patterned Ta electrodes. By nitriding the surface of the Ta electrodes, Fe nanoparticles with a moderate size were effectively formed, resulting in bridging CNTs between the electrodes.

Electrical Characterization of Metal-Coated Carbon Nanotube Tips

Electrical characteristics of bare and metal-coated carbon nanotube (CNT) tips were investigated with an independently driven four-tip scanning tunneling microscope (STM). The CNT was glued on a W tip apex and wholly coated ex situ by metal thin layers. The resistance between the CNT-tip end and the W supporting tip scattered very widely from ca. 50 kΩ to infinity for the bare tips, while coating the tip with a 6-nm-thick PtIr film stably reduced the resistance to less than approximately 10 kΩ. The W coating was also effective for stabilizing the resistance, although they showed slightly larger resistance (ca. 50 kΩ). The metal-coated tips kept their low resistance and flexibility even after 100 repeated contacts to an object for conductivity measurements. They are expected to be useful for nanometer-scale transport measurements with multiprobe STM as well as for conventional single-tip STM.