1) Quantum nature of transport through atomic size metal contacts: We investigated the origin of discrete jumps in the conductance of a metal contact formed between the STM tip and Au(111) surfaces as the junction is elongated (A1). With the combination of STM experiments and theory we found that the discrete changes of conductance arise due to reorganisation of the atomic structure of the contact as it is elongated (A2). However, behind this effect, the elongation of the contact to form nanowires with lengths larger than the electron Fermi wavelength (~1 nm) leads to the confinement of one-dimensional-like quantum channels for transport (A3). We observed the appearance of diffusive transport and localisation as the wire length increases beyond the electronic coherence length in the nanowire (A2), and predicted the fingerprint of non-linear transport for a non-finite applied potential between the leads (A4).
2) Molecular adsorption on metals and semiconductors: We analysed the structure and electronic configuration of C60 on silicon and noble metal surfaces. The highest occupied molecular orbital (HOMO) was found responsible of the molecular structure and transport on the Si(111) surface (A5) denoting the covalent coupling with silicon dangling bonds. The tendency of forming a strong SiC sigma bond should be extended to other hydrocarbon systems on molecules. As a characteristic of weak chemisorption, the first unoccupied orbital (LUMO) dominated molecular transport around the Fermi level on noble metal surfaces (A6). The role of adsorption sites, step edges, and molecular orientation was resolved by the combination of STM and STS (A6, A7). Overall, we found the electronic structure of the molecule/surface chemical contact modulates the tunnel transport in the STM configuration, thus producing rectification behaviour in current-voltage characteristics (see figure 1). We found that structural factors like molecular chirality can be so strong in self-assembly processes that the shape of a metal surface can be mesoscopically changed (A8).
3) IETS-STS and electron induced chemistry: Single molecule vibrational spectroscopy was used to investigate the vibrational structure of several model molecular systems (Fig. 2) with the aim of understanding possible selection rules governing excitation
and detection of inelastic tunnel processes by molecular vibrations (A7, A9). Our measurements revealed the importance of local adsorption parameters for the detection of vibrations in the spectra. The intermolecular interaction (A7), proximity of step edges (A9), and molecular orientation (A10) do affect the measurements causing detectable shifts in the vibrations, or even the activation /deactivation of hidden/active modes. We demonstrated the use of a mode-selective strategy for controlling the motion of an ammonia molecule on a copper surface, based on controlling the tunnel electron energy and current, to select the excitation of a specific mode (A11). These investigations revealed the existence of effective intramolecular quenching pathways governing the chemistry at the single molecule scale (A12).
4) Electronic structure of metal surfaces: We applied real space maps of tunnel junction differential conductivity, and their Fourier transformation to resolve in energy and in reciprocal space the local density of states of metal surfaces using a low temperature STM. We explore the LDOS in a wide energy range, even above the vacuum level, and in the whole Brillouin zone (A13), allowing a complete reconstruction of surface state topology, as well as bulk bands (A16). We applied this technique to investigate relevant physical problems of surface science. We found that the transition between electronic confinement and superlattice electronic states in vicinal surfaces depends on the disorder of their terrace structure (A14). We also demonstrated that electrons with opposite spin do not interfere (A15) by using a model substrate, Bi(110), where the strong spin-orbit coupling leads to non-spin degenerate surface states in the proximity of the Fermi level.
2.2.1 List of our publications related to this proposal
2.2.1.1 Quantum transport though atomic size contacts:
A1 Quantum contact in gold nanostructures by scanning tunneling microscopy.
J.I. Pascual, J. Méndez, J. Gómez-Herrero, A.M. Baró, N. García, and Vu Thien Binh.
Physical Review Letters 71, 1852 (1993).
A2 Properties of metallic nanowires: from conductance quantization to localization.
J.I. Pascual, J. Méndez, J. Gómez-Herrero, A.M.Baró, N. García, U. Landman, W.D. Luedtke, E.N. Bogachek, and H.P. Cheng.
Science 267, 1793 (1995).
A3 Theory of conduction through quantum necks.
J.A. Torres, J.I. Pascual, and J.J. Sáenz.
Physical Review B 49, 16581 (1994).
A4 Non-linear Ballistic Conductance in Atomic-scale Metallic Contacts.
J.I. Pascual, J.A. Torres, and J.J. Sáenz.
Physical Review B (Rapid Communications) 55, R16033 (1997).
2.2.1.2 Adsorption and electronic spectroscopy of molecules
A5 Seeing molecular orbitals.
J.I. Pascual, J. Gómez-Herrero, C. Rogero, A.M. Baró, D. Sánchez-Portal, E. Artacho, P. Ordejón, und J.M. Soler.
Chemical Physics Letters 321, 78 (2000)
A6 Resolution of site-specific bonding properties of C60 on Au(111).
C. Rogero, J.I. Pascual, J. Gómez-Herrero, und A.M. Baró.
Journal of Chemical Physics 116, 832 (2002)
A7 Adsorption and growth of benzene on Ag(110).
J.I. Pascual, J.J. Jackiw, Z. Song, P.S. Weiss, H. Conrad, and H.-P. Rust.
Surface Science, 502-503, 1 (2002).
A8 Mesoscopic chiral step faceting of Ag(110) induced by the organic molecule PVBA
J.I. Pascual, J.V. Barth, G. Ceballos, G. Trimarchi, A. de Vita, K. Kern, H.-P. Rust.
Journal of Chemical Physics 120, 11367 (2004).
2.2.1.3 Inelastic tunneling spectroscopy and electron-induced processes
A9 Adsorbate-substrate vibrational modes of benzene on Ag(110) resolved with scanning tunneling spectroscopy.
J.I. Pascual, J.J. Jackiw, Z. Song, P.S. Weiss, H. Conrad, und H.-P. Rust.
Physical Review Letters 86, 1050 (2001)
A10 Vibrational spectroscopy on single C60 molecules: The role of molecular orientation
J.I. Pascual, J. Gómez-Herrero, D. Sánchez-Portal, und H.-P. Rust.
Journal of Chemical Physics 117, 9531 (2002)
A11 Selectivity in vibrationally mediated single-molecule chemistry
J.I. Pascual, N. Lorente, Z. Song, H. Conrad, und H.-P. Rust.
Nature 423, 525 (2003)
A12 Mode-specific strategy for controlling single-molecule reactions
N. Lorente, und J.I. Pascual.
Philosophical Transactions 362, 1227 (2004)
2.2.2 List of other relevant publications
A13 Visualization of surface electronic structure: dispersion of surface states of Ag(110).
J.I. Pascual, Z. Song, J.J. Jackiw, K. Horn, and H.-P. Rust.
Physical Review B (Rapid Communication) 63, 241103(R) (2001).
A14 Scanning tunneling spectroscopy study of Cu (554) : Confinement and dimensionality at a stepped surface.
M. Hansmann, J. I. Pascual, G. Ceballos, H.-P. Rust, and K. Horn.
Physical Review B (Rapid Communication) 67, 121409(R) (2003).
A15 Role of spin in quasiparticle interference
J. I. Pascual, G. Bihlmayer,Yu. M. Koroteev, H.-P. Rust, G. Ceballos, M. Hansmann, K. Horn, E. V. Chulkov, S. Blügel, P. M. Echenique, and Ph. Hofmann:
Physical Review Letters 93, 196802 (2004).
A16 Bulk electronic structure resolved with scanning tunnelling microscopy
J.I. Pascual, A. Dick, M. Hansmann, H.-P. Rust , J. Neugebauer, and K. Horn
submitted to Physical Review Letters (2004).