Toru Hirahara

Redox Control and High Conductivity of Nickel Bis(dithiolene) Complex π‑Nanosheet: A Potential Organic Two-Dimensional Topological Insulator

A bulk material comprising stacked nanosheets of nickel bis(dithiolene) complexes is investigated. The average oxidation number is -3/4 for each complex unit in the as-prepared sample; oxidation or reduction respectively can change this to 0 or -1. Refined electrical conductivity measurement, involving a single microflake sample being subjected to the van der Pauw method under scanning electron microscopy control, reveals a conductivity of 1.6 × 10(2) S cm(-1), which is remarkably high for a coordination polymeric material. Conductivity is also noted to modulate with the change of oxidation state. Theoretical calculation and photoelectron emission spectroscopy reveal the stacked nanosheets to have a metallic nature. This work provides a foothold for the development of the first organic-based two-dimensional topological insulator, which will require the precise control of the oxidation state in the single-layer nickel bisdithiolene complex nanosheet (cf. Liu, F. et al. Nano Lett. 2013, 13, 2842).

Evidence of Dirac fermions in multilayer silicene

Multilayer silicene, the silicon analogue of multilayer graphene, grown on silver (111) surfaces, possesses a honeycomb (√3 × √3)R30° reconstruction, observed by scanning tunnelling microscopy at room temperature, past the initial formation of the dominant, 3×3 reconstructed, silicene monolayer. For a few layers silicene film we measure by synchrotron radiation photoelectron spectroscopy, a cone-like dispersion at the Brillouin zone centre due to band folding. π* and π states meet at ∼0.25 eV below the Fermi level, providing clear evidence of the presence of gapless Dirac fermions.

Large surface-state conductivity in ultrathin Bi films

In situ microscopic-four-point probe conductivity measurements were performed for ultrathin Bi films on Si(111)-7×7. From the extrapolation of thickness-dependent conductivity and decrease in conductivity through surface oxidization, we found clear evidence of large surface-state conductivity (σSS∼1.5×10−3Ω−1∕◻ at room temperature) in Bi(001) films. For the thinnest films (∼25A), the transport properties are dominated by the highly inert surface states that are Rashba spin-split, and this suggests the possibility of using these Bi surface states for spintronics device application.

Direct detection of grain boundary scattering in damascene Cu wires by nanoscale four-point probe resistance measurements

Four-terminal conductivity measurements of damascene copper (Cu) wires with various widths have been performed using platinum-coated carbon nanotube (CNT) tips in a four-tip scanning tunneling microscope. Using CNT tips enabled the probe spacing to be reduced to 70 nm, which is the shortest probe spacing in interconnect wire measurements achieved so far. The measured resistivity of Cu increased as the line width decreased and direct evidence of individual grain boundary scattering was observed when the probe spacing was varied on a scale comparable to the grain size of the Cu wires (∼200 nm).

Origin of the surface-state band-splitting in ultrathin Bi films: from a Rashba effect to a parity effect

Following our previous work (Hirahara T et al 2007 Phys. Rev. B 76 153305), we have performed spin- and angle-resolved photoemission measurements at 120K on ultrathin Bi films grown on a Si substrate, focusing on the split surface-state bands near the Fermi level. We found clear experimental evidence that these states show Rashba-type spin-split behavior near ¯ 0, but the splitting is lost near ¯ M where they overlap with the bulk band projection. This can be explained as a change in the origin of the splitting from a Rashba effect to a parity effect as revealed by ab initio calculations.

Fermi-Level Tuning of Topological Insulator Thin Films

Topological insulators are insulating materials but have metallic edge states with peculiar properties. They are considered to be promising for the development of future low energy consumption nano-electronic devices. However, there is a major problem: Naturally grown materials are not truly insulating owing to defects in their crystal structure. In the present study, we have examined the electronic structure and transport properties of topological insulator ultrathin Bi2Te3 films by angle-resolved photoemission spectroscopy and in situ transport measurements. To realize a truly bulk insulating film, we tried to tune the Fermi-level position using two methods. The first of these, i.e., changing the Si substrate temperature during film growth (350?450 K) to reduce the defects in the grown films, had some effect in reducing the bulk residual carriers, but we could not fabricate a film that showed only the surface states crossing the Fermi level. The second method we employed was to incorporate Pb atoms during film growth since Pb has one less electron than Bi. When the films were grown at around 350 K, we observed a systematic shift in the Fermi level and obtained a bulk insulating film, although it was not possible to move the Dirac point just at the Fermi level. The change in the measured film conductivity was consistent with the shift in the Fermi level and suggested the detection of the surface-state conductivity. For films grown at a higher substrate temperature (450 K), the Fermi level could be tuned only slightly and a bulk n-type film was obtained. Pb incorporation changes the shape of the Dirac cone, suggesting the formation of a stoichiometric ternary alloy of Bi, Pb, and Te, which is another topological insulator.

Electrical conduction of Ge nanodot arrays formed on an oxidized Si surface

Carrier transport mechanism on Ge nanodot arrays formed on SiO2 monolayer covering over the Si surface is investigated by microscopic four-point-probe measurements combined with core-level photoemission spectroscopy and scanning tunneling microscopy. Different conduction natures are found depending on whether or not the nanodots and the substrate are directly connected by subnanometer-sized voids penetrating the SiO2 layer. In the presence of the voids, conductivity is regulated by the dot-size through quantum-size effect.

Direct observation of a gap opening in topological interface states of MnSe/Bi2Se3 heterostructure

High-quality MnSe(111) film was bilayer-by-bilayer grown epitaxially onto the Bi2Se3(111) surface using molecular beam epitaxy. Reversal scenario with quintuple layer-by-layer growth of Bi2Se3 onto the MnSe film was also realized. Angle-resolved photoemission spectroscopy measurements of Bi2Se3 capped with two bi-layers of MnSe revealed that an energy gap of about 90 meV appears at the Dirac point of the original Bi2Se3 surface, possibly due to breaking the time-reversal symmetry on the Bi2Se3 surface by magnetic proximity effect from MnSe.

Quantum-Size Effect in Uniform Ge–Sn Alloy Nanodots Observed by Photoemission Spectroscopy

Nanodots of super-saturated Ge–Sn alloy formed on a Si substrate covered with a SiO2 monolayer were investigated by photoemission spectroscopy. Core-level photoemission results indicated that the stoichiometry of the nanodots was uniform at an intended ratio without Sn segregation. Quantum size effect was also proved by valence-band photoemission on the present GeSn nanodots.

Electron-spin-dependent 4He+ ion scattering on Bi surfaces

We studied low-energy (∼ 1.55 keV) electron-spin-polarized 4He+ ion scattering on a Bi(111) ultrathin film epitaxially grown on a Si(111) substrate. We observed that the scattered ion intensity differed between the incident He+ ions with up and down spins even though Bi is a non-magnetic element. To analyze the origin of this spin-dependent ion scattering (the spin asymmetry), we investigated the detailed relationship between the spin asymmetry and the incident angle, the azimuthal angle, the scattering angle, and the incident energy. All the data indicate that the spin asymmetry originates from the scattering cross section owing to the non-central force in the He+–Bi atom binary collision. The non-central force is most likely attributed to the spin–orbit coupling that acts transiently on the He+ 1s electron spin in the binary collision.