Width-dependent band gap of arm-chair graphene nanoribbons formed on vicinal SiC substrates by MBE.

Formation and electronic states of graphene nanoribbons with arm-chair edges (AGNR) are studied on the SiC(0001) vicinal surfaces toward the [11-00] direction. The surface step and terrace structures of both 4H and 6H-SiC substrates are used as the growth templates of one-dimensional arrays of AGNRs, which are prepared using the carbon molecular beam epitaxy followed by hydrogen intercalation. A band gap is observed above theπ-band maximum by angle-resolved photoelectron spectroscopy (ARPES) for the both samples. The average widths of the AGNRs are 6 and 10 nm, and the estimated average band gaps are 0.40 and 0.28 eV for the 4H- and 6H- substrates, respectively. A simple and phenomenological inverse relation between the energy gap and AGNR width works in the analyses of the ARPES data.

Variation of the 4 × 1 to 8 × 2 structural transition temperature of quasi-one-dimensional indium chains upon carrier doping from Si(111) substrates.

We investigated the 4 × 1 to 8 × 2 structural transition temperature of quasi-one-dimensional indium chains on the (111) surface of Si substrates possessing various carrier concentrations via low-energy electron diffraction. The transition temperature was found to decrease from 120 K to below 77 K with increasing carrier concentration on both n- and p-type Si(111) substrates. This decrease in the transition temperature was found to be proportional to the shift of the Fermi level, which was numerically evaluated using a one-dimensional charge transfer model of the interface. The obtained results demonstrate that doping of the surface state with both electrons and holes can be readily controlled by judicious selection of Si substrates with appropriate carrier type and concentration.

Surface electronic states of Au-induced nanowires on Ge(0 0 1).

The electronic states of Au-induced atomic nanowires on Ge(0 0 1) (Au/Ge(0 0 1) NWs) have been studied by angle-resolved photoelectron spectroscopy with linearly polarized light. We have found three electron pockets around the [Formula: see text] line, where the Fermi surfaces are closed in a surface Brillouin zone (SBZ). The results indicate 2D Fermi surfaces of Au/Ge(0 0 1) NWs whereas the atomic structure is 1D. On the basis of the polarization-dependent spectra, the relation between SBZ and the direction of the atomic NW, and the symmetry of the surface state are clarified. These are very useful for further studies on the atomic structure of NWs.

Modulation of Electron-Phonon Coupling in One-Dimensionally Nanorippled Graphene on a Macrofacet of 6H-SiC.

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

Selective doping in a surface band and atomic structures of the Ge(111) (√3 × âˆš3)R30°-Au surface.

Atomic and electronic structures of the Ge(111) (√3 × âˆš3)R30°-Au surface with two metallic bands are studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). The bias-voltage-dependent periodic structure observed by STM is consistent with the electronic structure calculated for an optimized conjugate honeycomb chained trimer (CHCT) model. Electrons are selectively doped to the electron-like surface metallic band by excess Au atoms, which form triangle structures with the Au trimers of the CHCT model. The discrepancy for the bottom energy of the electron-like band between the ARPES results and those of the calculation is attributed to the doping. The triangle structure is mobile at room temperature, but stable at 80 K. Both Au and Ge atoms deposited at room temperature on the Ge(111) (√3 × âˆš3)R30°-Au surface dope electrons to the electron-like surface metallic band. Moreover, the Au atoms increase the spin-orbit interaction at the surface, and thus make the splitting of the spin-polarized band due to the interaction larger than that before the deposition.

Local vibrational excitation through extended electronic states at a germanium surface.

Atomic motion through excitation of extended surface electronic states on Ge(001) is studied using extraction of electrons by scanning tunneling microscopy and density functional theory. Single-electron excitation into the surface states nonlocally alters the tilting orientation of the surface Ge dimer, and the change rate depends on the excitation energy. Theoretical investigations identify the excited electronic states for the dimer motion, and clarify the strong coupling between the surface state electrons and a local vibrational mode of the dimer for changing the tilting orientation.

Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface.

Hydrogen-gas etching of a 6H-SiC(0001) surface and subsequent annealing in nitrogen atmosphere leads to the formation of a silicon oxynitride (SiON) epitaxial layer. A quantitative low-energy electron diffraction analysis revealed that the SiON layer has a hetero-double-layer structure: a silicate monolayer on a silicon nitride monolayer via Si-O-Si bridge bonds. There are no dangling bonds in the unit cell, which explains the fact that the structure is robust against air exposure. Scanning tunneling spectroscopy measured on the SiON layer shows a bulk SiO2-like band gap of approximately 9 eV. Great potential of this new epitaxial layer for device applications is described.

Self-assembled MnN superstructure.

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.

Direct observation of strain-induced change in surface electronic structure.

We have observed a novel modification of a surface state due to a local strain field induced by a nanopattern formation. N adsorption on the Cu(100) surface induces a nanoscale grid pattern, where the clean Cu regions remain periodically. The lattice is contracted on the clean region by adjacent c(2 x 2)N domains, which have a larger lattice constant. On this patterned surface, we have investigated the Tamm-type surface state at M by means of angle-resolved ultraviolet photoelectron spectroscopy. The binding energy of the Tamm state shifts toward the Fermi level continuously with increasing N coverage, i.e., the intensity of the strain field. This behavior due to the strain field is completely different from that caused by electron confinement observed on vicinal surfaces. The Brillouin zone extension corresponding to the lattice contraction was also detected.