Structural transition in atomic chains driven by transient doping.

A reversible structural transition is observed on Si(553)-Au by scanning tunneling microscopy, triggered by electrons injected from the tip into the surface. The periodicity of atomic chains near the step edges changes from the 1×3 ground state to a 1×2 excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first-principles density-functional calculations it is shown that the 1×2 phase is created by temporary doping of the atom chains. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing lifetimes in the millisecond range.

Confined doping on a metallic atomic chain structure.

On Si(111)-(5×2)-Au it is shown that metallic sections of quantum wires between two doping adatoms establish a local electronic structure which is primarily defined by the section length. Such confined doping is a direct consequence of reduced dimensionality and is not observed in higher dimensions. Within a chain segment, the effect of a spatially independent charge-carrier concentration is superimposed by a Coulomb-like interaction due to the positively charged dopants. This offers a natural explanation for the relatively broad photoemission features and the complex appearance in scanning tunneling microscopy and spectroscopy images.

Design and capabilities of an experimental setup based on magnetron sputtering for formation and deposition of size-selected metal clusters on ultra-clean surfaces.

The design and performance of an experimental setup utilizing a magnetron sputtering source for production of beams of ionized size-selected clusters for deposition in ultra-high vacuum is described. For the case of copper cluster formation the influence of different source parameters is studied and analyzed. Size-selected clusters are deposited on substrates and the efficiency of an electrostatic quadrupole mass selector is tested. Height analysis using atomic force microscopy (AFM) demonstrates relative standard size deviations of 7%-10% for the particles of various sizes between 6 nm and 19 nm. Combined analysis by AFM and transmission electron microscopy reveals that the clusters preserve almost spherical shape after the deposition on amorphous carbon substrates. Supported nanoparticles of a few nanometres in diameter have crystalline structure with a face-centered cubic (fcc) lattice.

Molecular nanostructures with strong dipole moments on the Si(111) 5 × 2-Au surface.

Switchable organic molecules adsorbed on a silicon surface combine the flexibility and the low cost of molecular electronic devices with the sophistication of modern silicon technology. The first step towards creating such hybrid devices is the formation of regular, ordered patterns of molecules on a silicon surface. A stepped Si surface passivated by a sub-monolayer of gold is found to provide a useful substrate for forming ordered molecular patterns. Molecules with strong dipole moments, such as fluorophenols, form a one-dimensional molecular array on such a substrate by adsorbing on top of the step edges. Local barrier height measurement by scanning tunneling spectroscopy demonstrates the possibilities to detect the direction of the dipole moment of an individual molecule. Polarization-dependent x-ray absorption spectroscopy reveals an oriented adsorption in both the azimuthal and polar directions.

Experimental evidence for spin-split bands in a one-dimensional chain structure.

Gold atom chains on vicinal Si(111) surfaces exhibit an unusual doublet of half-filled bands, whose origin has remained uncertain. The splitting is identified by angle-resolved photoemission as a spin splitting induced by the spin-orbit interaction (Rashba effect), in agreement with a theoretical prediction by Sánchez-Portal, Riikonen, and Martin. This interaction leads to a characteristic pattern of avoided band crossings at a superlattice zone boundary. Two out of four crossings are avoided, with a minigap E_{G}=85 meV and a k offset of 0.05 A;{-1}.

Electron-phonon interaction at the Si(111)-7 x 7 surface.

It is shown that electron-phonon interaction provides a natural explanation for the unusual band dispersion of the metallic surface states at the Si(111)-(7 x 7) surface. Angle-resolved photoemission reveals a discontinuity of the adatom band at a binding energy close to the dominant surface phonon mode at h(omega0) = 70 meV. This mode has been assigned to adatom vibrations by molecular dynamics calculations. A calculation of the spectral function for electron-phonon interaction with this well-defined Einstein mode matches the data. Two independent determinations of the electron-phonon coupling parameter from the band dispersion and from the temperature-dependent phonon broadening yield similar values of lambda = 1.09 and lambda = 1.06.

Confined Shockley surface states on the (111) facets of gold clusters.

Combining low-temperature scanning tunneling microscopy and spectroscopy with high-resolution ultraviolet photoemission, we have revealed a confined Shockley surface state on the (111) facets of gold clusters with about N=10(4) atoms grown in nanopits on highly oriented graphite. With tunneling spectroscopy, we observed energy dependent nodal patterns in the dI/dV maps, which are in quantitative agreement with the two-dimensional confinement of the surface state within the hexagonal facet area. The results indicate that the lattice of the ionic cores influences the electronic properties of the clusters significantly.