Hiroyuki Yamane

Does the molecular orientation induce an electric dipole in Cu-phthalocyanine thin films?

The effect of the molecular orientation on the molecular electronic structure is studied on the Cu-phthalocyanine∕graphite system via film thickness dependences of metastable-atom electron spectra and ultraviolet photoelectron spectra. We observed a decrease in the vacuum-level position and a corresponding band-bending-like shift in the highest occupied state only for thick films where the molecular tilt angle increases gradually with the film thickness. These shifts are explained by electric dipoles produced in the film by a gradient of the intermolecular electronic interaction along the surface normal due to the continuous increase in the molecular tilt angle. The result indicates that the change in the molecular orientation is an important origin of the band-bending-like shift in the molecular electronic states even if the molecule has no permanent electric dipole.

Very narrow photoemission bandwidth of the highest occupied state in a copper-phthalocyanine monolayer

Abstract We observed a very narrow bandwidth of the highest occupied molecular orbital (HOMO) state in ultraviolet photoemission spectra (UPS) of copper-phthalocyanine monolayer deposited on graphite. The HOMO band in UPS consists of three components which may originate from the vibrational coupling. The full width at half maximum of each component was found to be ∼150 meV at 295 K. This HOMO-bandwidth leads to an estimation that the lifetime of the HOMO hole should be at least longer than 2.2 fs, which may be dominated by the electron transfer rate from the substrate to the molecule.

Self-Assembled Nanowires with Giant Rashba Split Bands

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions.

PHOTODEGRADATION OF POLY(TETRAFLUOROETHYLENE) AND POLY(VINYLIDENE FLUORIDE) THIN FILMS BY INNER SHELL EXCITATION

Ion time-of-flight (TOF) mass spectra of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) thin films near fluorine and carbon K-edges were observed. For PTFE thin films, peaks corresponding to F+, CF+, and appeared, while for PVDF F+ and H+ were mainly observed. They indicate that for PTFE the polymer chain (C–C bonds) as well as C–F bonds are broken by irradiation of photons near fluorine and carbon K-edges, while for PVDF the bond scission occurs mainly at the C–F and C–H bond. Partial ion yields (PIY) of these ions for PTFE and PVDF thin films show strong photon energy dependencies near fluorine and carbon K-edges. The excitation from fluorine 1s to σ(C–F)* is specially efficient for F+ ion production for both PTFE and PVDF.

Systematic Study of Soft X-ray Spectra of Poly(Dg)·Poly(Dc) and Poly(Da)·Poly(Dt) DNA Duplexes

In the present work, we have undertaken a combined experimental and theoretical study of X-ray spectroscopies for DNA base pairs, more precisely near-edge X-ray absorption, X-ray emission, and resonant inelastic X-ray scattering applied to poly(dG).poly(dC) and poly(dA).poly(dT) DNA duplexes. We have derived several conclusions on the nature of these X-ray spectra: the stacking of pairs has very little influence on the spectra; the spectra of a DNA composed of mixed Watson-Crick base pairs are well reproduced by linear combinations of GC and AT base pairs involved; the amine and imine nitrogens show noticeable differences as building blocks in the absorption, emission, and resonant emission spectra. The calculated spectra are in good agreement with experimental results. The ramifications of these conclusions for the use of X-ray spectroscopy for DNA are discussed.

Low-energy electron transmission through organic monolayers: An estimation of the effective monolayer potential by an excess electron interference

In low-energy-electron transmission spectra of monolayer films of various organic-semiconductor molecules deposited on MoS2 and graphite surfaces, we found that the energy positions of spectral minima are proportional to (n+1/2)2, where n is positive integer and 0, independent of molecules and substrates. Despite the complex structure of each molecule, the (n+1/2)2 rule can be simply explained by the interference of an excess electron passing through the potential of the monolayer on the substrate. Using these results, we estimated the effective potential, the potential width and depth, of the monolayer felt by the injected excess electron.

Band alignment of a carbon nanotube/n-type 6H-SiC heterojunction formed by surface decomposition of SiC using photoelectron spectroscopy

Energy band alignment at the interface between carbon nanotubes (CNTs) and n-type 6H-SiC formed by the surface decomposition of SiC was investigated using high-resolution photoelectron spectroscopy (PES). Valence band spectra of the CNT films showed a Fermi edge, indicating metallic character. PES results revealed that a Schottky barrier was formed at the interface and the barrier height was 1.38 eV. Current-voltage measurements of the interface showed rectifying behavior, which was consistent with the PES results.

Low Energy Electron Transmission Study of Indium/(perylene-3,4,9,10-tetracarboxylic dianhydride) System

Low-energy electron transmission (LEET) experiments were performed for the indium/perylene-3,4,9,10-tetracarboxylic dianhydride (In/PTCDA) system prepared by a sequential deposition of PTCDA and In on a MoS2 single crystal surface. The dependence of LEET spectra and the work function change (Δ=film-substrate) on the deposition amount of In on the PTCDA monolayer showed that the organic-metal compound of In4PTCDA was formed at the In/PTCDA interface. The result is in good agreement with that previously obtained using metastable atom electron spectroscopy. Furthermore, from the temperature dependence of LEET spectra and Δ for the In/PTCDA system, we propose that in the as-grown In4PTCDA at room temperature In atoms are located out of the PTCDA plane, and after heat treatment, it becomes a planer structure.

Realization of a Strained Atomic Wire Superlattice.

A superlattice of strained Au-Si atomic wires is successfully fabricated on a Si surface. Au atoms are known to incorporate into the stepped Si(111) surface to form a Au-Si atomic wire array with both one-dimensional (1D) metallic and antiferromagnetic atomic chains. At a reduced density of Au, we find a regular array of Au-Si wires in alternation with pristine Si nanoterraces. Pristine Si nanoterraces impose a strain on the neighboring Au-Si wires, which modifies both the band structure of metallic chains and the magnetic property of spin chains. This is an ultimate 1D version of a strained-layer superlattice of semiconductors, defining a direction toward the fine engineering of self-assembled atomic-scale wires.

Hybridized electronic states in potassium-doped picene probed by soft x-ray spectroscopies

The electronic structure of the unoccupied and occupied states of potassium (K)-doped and undoped picene crystalline films has been investigated by using the element-selective and bulk-sensitive photon-detection methods of X-ray absorption and emission spectroscopies. We observed the formation of the doping-induced unoccupied and occupied electronic states in K-doped picene. By applying the inner-shell resonant-excitation experiments, we observed the evidence for the orbital hybridization between K and picene near the Fermi energy. Furthermore, the resonant X-ray emission experiment suggests the presence of the Raman-active vibronic interaction in K-doped picene. These experimental evidences play a crucial role in the superconductivity of K-doped picene.