A. Arnau

Interaction of slow multicharged ions with solid surfaces

Abstract The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.

Formation of dispersive hybrid bands at an organic-metal interface

An electronic band with quasi-one-dimensional dispersion is found at the interface between a monolayer of a charge-transfer complex (TTF-TCNQ) and a Au(111) surface. Combined local spectroscopy and numerical calculations show that the band results from a complex mixing of metal and molecular states. The molecular layer folds the underlying metal states and mixes with them selectively, through the TTF component, giving rise to anisotropic hybrid bands. Our results suggest that, by tuning the components of such molecular layers, the dimensionality and dispersion of organic-metal interface states can be engineered.

Potential energy landscape for hot electrons in periodically nanostructured graphene.

We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy; unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at +3  eV appears on some areas of the surface. First-principles calculations explain the observations and permit us to conclude that the system behaves as a self-organized periodic array of quantum dots.

Dichotomous array of chiral quantum corrals by a self-assembled nanoporous kagomé network

The confinement of surface-state electrons by a complex supramolecular network is studied with low-temperature scanning tunneling microscopy and rationalized by electronic structure calculations using a boundary element method. We focus on the self-assembly of dicarbonitrile-sexiphenyl molecules on Ag(111) creating an open kagomé topology tessellating the surface into pores with different size and symmetry. This superlattice imposes a distinct surface electronic structure modulation, as observed by tunneling spectroscopy and thus acts as a dichotomous array of quantum corrals. The inhomogenous lateral electronic density distribution in the chiral cavities is reproduced by an effective pseudopotential model. Our results demonstrate the engineering of ensembles of elaborate quantum resonance states by molecular self-assembly at surfaces.

Electronic Stopping Power in LiF from First Principles

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of approximately 0.2 a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is approximately 2.4 at velocities slightly above the threshold (v approximately 0.4 a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local.

Characterization of single-molecule pentanedithiol junctions by inelastic electron tunneling spectroscopy and first-principles calculations

Received 18 December 2009; revised manuscript received 12 January 2010; published 4 February 2010 We study pentanedithiol molecular junctions formed by means of the break-junction technique with a scanning tunneling microscope at low temperatures. Using inelastic electron tunneling spectroscopy and firstprinciples calculations, the response of the junction to elastic deformation is examined. We show that this procedure makes a detailed characterization of the molecular junction possible. In particular, our results indicate that tunneling takes place through just a single molecule.

Exploring the Tilt-Angle Dependence of Electron Tunneling across Molecular Junctions of Self-Assembled Alkanethiols

Electronic transport mechanisms in molecular junctions are investigated by a combination of first-principles calculations and current-voltage measurements of several well-characterized structures. We study self-assembled layers of alkanethiols grown on Au(111) and form tunnel junctions by contacting the molecular layers with the tip of a conductive force microscope. Measurements done under low-load conditions permit us to obtain reliable tilt-angle and molecular length dependencies of the low-bias conductance through the alkanethiol layers. The observed dependence on tilt-angle is stronger for the longer molecular chains. Our calculations confirm the observed trends and explain them as a result of two mechanisms, namely, a previously proposed intermolecular tunneling enhancement as well as a hitherto overlooked tilt-dependent molecular gate effect.

Probing the site-dependent Kondo response of nanostructured graphene with organic molecules.

TCNQ molecules are used as a sensitive probe for the Kondo response of the electron gas of a nanostructured graphene grown on Ru(0001) presenting a moiré pattern. All adsorbed molecules acquired an extra electron by charge transfer from the substrate, but only those adsorbed in the FCC-Top areas of the moiré show magnetic moment and Kondo resonance in the STS spectra. DFT calculations trace back this behavior to the existence of a surface resonance in the low areas of the graphene moiré, whose density distribution strongly depends on the stacking sequence of the moiré area and effectively quenches the magnetic moment for HCP-Top sites.

Stopping Power for Protons in Aluminum.

The electronic stopping power of aluminum metal for protons has been calculated with explicit account taken of the different charge states of the photon inside the medium. The fraction of negative ions (H-) neutral atoms (H0) and bare protons (H+) in the beam as a function of ion speed are derived from the capture and loss cross-sections. The total stopping power is then obtained by weighting the calculated partial stopping powers with the appropriate charge state fractions. The energy loss per unit path length due to electronic exchange processes is also evaluated. Our calculations show that the relative contribution to the stopping power from capture and loss processes is of the order of 15% for the case of protons moving in aluminum. Good agreement with experimental data is found.

A charge state approach to the stopping power of ions in solids

Abstract A continuous capture and loss picture is used to calculate the stopping power of ions in solids at intermediate velocities. The total stopping power is obtained by weighting the appropriately calculated stopping powers of each charge state with the respective charge state fraction and adding the energy loss per unit path length due to electronic exchange processes. Linear response theory in the dielectric formalism and atomistic models are combined with many-body techniques to obtain the charge state distribution of ions with inclusion of solid state effects. The different mechanisms responsible for the charge exchange processes and the way of calculating the stopping for each charge state are also discussed. Results are presented for the particular case of He projectiles in solid Al targets, showing good agreement with the available experimental data.