A flat-lying dimer as a key intermediate in NO reduction on Cu(100).

The reaction of nitric oxide (NO) on Cu(100) is studied by scanning tunneling microscopy, electron energy loss spectroscopy and density functional theory calculations. The NO molecules adsorb mainly as monomers at 64 K, and react and dissociate to yield oxygen atoms on the surface at ∼70 K. The temperature required for the dissociation is significantly low for Cu(100), compared to those for Cu(111) and Cu(110). The minimum energy pathway of the reaction is via (NO)2 formation, which converts into a flat-lying ONNO and then dissociates into N2O and O with a considerably low activation energy. We propose that the formation of (NO)2 and flat-lying ONNO is the key to the exceptionally high reactivity of NO on Cu(100).

Atomic-scale study of the formation of sodium-water complexes on Cu(110).

We observed individual sodium (Na) atoms and their complexes with water molecules on Cu(110) with scanning tunneling microscopy at 6 K. We induced the reaction of a Na adatom with one or two water molecules, which yielded two kinds of Na-water complexes. Density functional theory calculations were performed to study the structure of the complexes, which revealed that the water molecules are bonded to a Na atom along the [11[combining macron]0] direction via an oxygen atom with the hydrogen atoms pointing toward the Cu atoms of the surface. The 1 : 1 Na-water complex is stablized by 225 meV upon bond formation, and the ligand water moves back and forth around the Na atom. The complex can accommodate another water molecule to yield a 1 : 2 Na-water complex with an energy gain of 214 meV. The atomic-scale identification of the alkali-water complexes would give fundamental insights into the hydration process of alkali cations and their specific adsorption onto metal electrodes.

Adsorption and reaction of H2S on Cu(110) studied using scanning tunneling microscopy.

Using low-temperature scanning tunneling microscopy (STM), the adsorption and reaction of hydrogen sulfide (H2S) and its fragments (SH and S) on Cu(110) are investigated at 5 K. H2S adsorbs molecularly on the surface on top of a Cu atom. With voltage pulses of STM, it is possible to induce sequential dehydrogenation of H2S to SH and S. We found two kinds of adsorption structures of SH. The short-bridge site is the most stable site for SH, while the long-bridge site is the second. Density functional theory calculations show that the S-H axis is inclined from the surface normal for both species. The reaction of H2S with OH and O was directly observed to yield SH and S, respectively, providing a molecular-level insight into catalyst poisoning.

Metallic conduction through van der Waals interfaces in ultrathin [Formula: see text] films.

While the van der Waals (vdW) interface in layered materials hinders the transport of charge carriers in the vertical direction, it serves a good horizontal conduction path. We have investigated electrical conduction of few quintuple-layer (QL) [Formula: see text] films by in situ four-point probe conductivity measurement. The impact of the vdW (Te-Te) interface appeared as a large conductivity increase with increasing thickness from 1 to 2 QL. Angle-resolved photoelectron spectroscopy and first-principles calculations reveal the confinement of bulk-like conduction band (CB) state into the vdW interface. Our analysis based on the Boltzmann equation showed that the conduction of the CB has a long mean free path compared to the surface-state conduction. This is mainly attributed to the spatial separation of the CB electrons and the donor defects located at the Bi sites.

Identification of single-layer metallic structure of indium on Si(1 1 1).

We have studied the In/Si(1 1 1) ([Formula: see text])-hex and striped phases by low-energy electron diffraction (LEED) and angle-resolved photoelectron spectroscopy (ARPES). The two phases are formed by different processes, and hence the different names have been used conventionally. We, however, found that LEED I-[Formula: see text] curves of the two phases agree with each other, indicating that they have an identical atomic structure. Our ARPES measurement revealed that the ([Formula: see text])-hex phase has metallic surface states. The observed Fermi surface was found to be very similar to the theoretical one calculated for an indium single-layer model.

Controlled switching of single-molecule junctions by mechanical motion of a phenyl ring.

Mechanical methods for single-molecule control have potential for wide application in nanodevices and machines. Here we demonstrate the operation of a single-molecule switch made functional by the motion of a phenyl ring, analogous to the lever in a conventional toggle switch. The switch can be actuated by dual triggers, either by a voltage pulse or by displacement of the electrode, and electronic manipulation of the ring by chemical substitution enables rational control of the on-state conductance. Owing to its simple mechanics, structural robustness, and chemical accessibility, we propose that phenyl rings are promising components in mechanical molecular devices.

Structure determination of Pb/Ge(111)-ß-(√3 × âˆš3)R30° by dynamical low-energy electron diffraction analysis and first-principles calculation.

We have determined the atomic structure of the Pb/Ge(111)-ß-(√3 × âˆš3)R30° surface, which was shown to exhibit a large Rashba spin splitting in a metallic surface state by dynamical low-energy electron diffraction analysis. The Pb coverage for the optimized atomic structure is 4/3 with one Pb atom located at every third H(3) site of the bulk-truncated Ge(111) surface and the other three near the T(1) sites but slightly displaced towards the T(4) sites. The determined atomic structure agrees well with the energetically optimized one obtained from the first-principles calculation. The calculation also revealed that the potential for the Pb atoms on the H(3) sites is very soft along the surface normal, suggesting that their vertical position is distributed within a range of about 0.2-0.3 Å. The previously proposed phase transition associated with the surface melting is discussed.

Large Rashba spin splitting of a metallic surface-state band on a semiconductor surface.

The generation of spin-polarized electrons at room temperature is an essential step in developing semiconductor spintronic applications. To this end, we studied the electronic states of a Ge(111) surface, covered with a lead monolayer at a fractional coverage of 4/3, by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES and first-principles electronic structure calculation. We demonstrate that a metallic surface-state band with a dominant Pb 6p character exhibits a large Rashba spin splitting of 200 meV and an effective mass of 0.028 m(e) at the Fermi level. This finding provides a material basis for the novel field of spin transport/accumulation on semiconductor surfaces. Charge density analysis of the surface state indicated that large spin splitting was induced by asymmetric charge distribution in close proximity to the nuclei of Pb atoms.

Structure determination of Bi/Ge(111)-[Formula: see text] by dynamical low-energy electron diffraction analysis and scanning tunneling microscopy.

We have determined the atomic structure of the Bi/Ge(111)-[Formula: see text] surface by dynamical low-energy electron diffraction (LEED) analysis and scanning tunneling microscopy (STM). The optimized atomic structure consists of Bi atoms which are adsorbed near the T(1) sites of the bulk-truncated Ge(111) surface and form triangular trimer units centered at the T(4) sites. The atomically resolved STM image was consistent with the LEED result. The structural parameters agree well with those optimized by a first-principles calculation which supports the interpretation of the electronic band splitting on this surface in terms of the giant Rashba effect.