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The light emission from a tunneling junction induced by tunneling electrons has been studied around the cutoff at hν=eVt. The emitted photons are found to exceed the excitation energy provided by the energy of the tunneling electrons. The experiments have been performed by a low- temperature scanning tunneling microscope at 80 K for an Ag(111) surface and an Ag-covered PtIr tip. A detailed analysis of the emission spectra reveals that the findings cannot be explained by the thermal broadening of the electron Fermi distribution alone. However, a correct description is found if a finite lifetime of the excited states in the range of 30–80 fs is included.

If a current of electrons flows through a normal conductor (in contrast to a superconductor), it is impeded by local scattering at defects as well as phonon scattering. Both effects contribute to the voltage drop observed for a macroscopic complex system as described by Ohm's law. Although this concept is well established, it has not yet been measured around individual defects on the atomic scale. We have measured the voltage drop at a monatomic step in real space by restricting the current to a surface layer. For the Si(111)-( [see text]3 x [see text]3)-Ag surface a monotonous transition with a width below 1 nm was found. A numerical analysis of the data maps the current flow through the complex network and the interplay between defect-free terraces and monatomic steps.

By using scanning tunnelling potentiometry we characterized the lateral variation of the electrochemical potential µec on the gold-induced Ge(001)-c(8 × 2)-Au surface reconstruction while a lateral current flows through the sample. On the reconstruction and across domain boundaries we find that µec shows a constant gradient as a function of the position between the contacts. In addition, nanoscale Au clusters on the surface do not show an electronic coupling to the gold-induced surface reconstruction. In combination with high resolution scanning electron microscopy and transmission electron microscopy, we conclude that an additional transport channel buried about 2 nm underneath the surface represents a major transport channel for electrons.

The rates of a hindered molecular rotation induced by tunneling electrons are evaluated using scattering theory within the sudden approximation. Our approach explains the excitation of copper phthalocyanine molecules (CuPc) on Cu(111) as revealed in a recent measurement of telegraph noise in a scanning tunneling microscopy experiment. A complete explanation of the experimental data is performed by computing the geometry of the adsorbed system, its electronic structure, and the energy transfer between tunneling electrons and the molecule's rotational degree of freedom. The results unambiguously show that tunneling electrons induce a frustrated rotation of the molecule. In addition, the theory determines the spatial distribution of the frustrated rotation excitation, confirming the striking dominance of two out of four molecular lobes in the observed excitation process. This lobe selectivity is attributed to the different hybridizations with the underlying substrate. © 2013 American Physical Society., J.S., A.S., C.A.B., and R.M. gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft through the SFB616 ‘Energy Dissipation at Surfaces.’ N.L. is supported by the ICT-FET Integrated Project AtMol (http://www.atmol.eu). M.C.C. thanks the Studienstiftung desdeutschen Volkes.

Here we present two techniques which give insight on transport phenomena with atomic resolution. Ballistic electron emission microscopy is used to study the ballistic transport through layered heterogeneous systems. The measured ballistic fraction of the tunneling current provides information about lossless transport channels through metallic layers and organic adsorbates. The transport characteristics of Bi(111)/Si Schottky devices and the influence of the organic adsorbates perylene tetracaboxylic dianhydride acid and C60 on the ballistic current are discussed. Scanning tunneling potentiometry gives access to the lateral transport along a surface, thus scattering processes within two-dimensional electron systems for the Bi(111) surface and the Si(111)(?3 × ?3)-Ag surface could be visualized.

A scanning tunneling microscope with several tips is ideally suited to analyze the electronic transport through objects on the nanoscale. Two different configurations will be discussed. The lateral transport of electrons may be studied by using two tips to drive a current parallel to the surface. A third tip enables to map the corresponding electrochemical potential μ ec. Measurements for a 2D conducting layer will be discussed. To analyze the transport perpendicular to the surface, a thin metallic layer is placed on a semiconducting surface. At the interface a Schottky barrier is formed, which can only be overcome by electrons of sufficient energy. This may be used to split the tunneling current coming from the tip of the microscope, into the ballistic electrons and the electrons which underwent inelastic scattering processes. This technique has been applied to study the ballistic transport of electrons through a thin epitaxial Bi(111) layer as well as through individual molecules.

In this work we report on ballistic electron emission spectroscopy (BEES) studies on epitaxial layers of silver grown on silicon surfaces, with either a Si(111)-(7 × 7) or Si(100)-(2 × 1) surface reconstruction. The experiments were done at low temperature and in ultra-high vacuum (UHV). In addition, BEES measurements on polycrystalline Ag films grown on hydrogen-terminated H:Si(111)-(1 × 1) and H:Si(100)-(2 × 1) surfaces were performed. The Schottky barrier heights were evaluated by BEES. The results are compared to the values for the barrier height reported for macroscopic Schottky diodes. We show that the barrier heights for the epitaxial films substantially differ from the values measured on polycrystalline Ag films, suggesting a strong effect of the interface on the barrier height.

DIPC, 20018 San Sebastin/Donostia, Basque Country, Spain(Received 15 December 2008; published 1 April 2009)We report on ballistic electron emission microscopy and spectroscopy studies on epitaxial (3–5 nmthick) Bi(111) films, grown on n-type Si substrates. The effective barrier heights of the Schottky barrierobservedare 0.58 eV for the Bi=Sið100Þ-ð2 1Þ and 0.68 eV for the Bi=Sið111Þ-ð7 7Þ. At the step edgesof theepitaxialfilmsastrongincrease oftheballisticelectron emissionmicroscopy currentis observedforBi=Sið111Þ-ð7 7Þ,whilenoincreaseoccursforBi=Sið100Þ-ð2 1Þ.Theseobservationscanbeexplainedby the conservation of the lateral momentum of the electron at the metal-semiconductor interface.

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C 60 and for a planar organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride), the signals revealed characteristic submolecular patterns that indicated where ballistic transport was enhanced or attenuated. The transport was associated to specific electronic molecular states. At electron energies of a few electron volts, this “scanning near-field electron transmission microscopy” method could be applied to various adsorbates or thin layers.

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry. (C) 2011 American Institute of Physics. [doi:10.1063/1.3536528]

Recently the ballistic transport through organic molecules could be analyzed with submolecular resolution by an extension of ballistic electron emission microscopy. In this work we compare the results of ballistic transport of electrons and holes through C₆₀ molecules deposited onto a Bismuth/Silicon Schottky diode. The study of hole transmission also exhibits molecular a resolved pattern in the transmission images showing the molecular periodicity of the C₆₀ layer. © 2008 IOP Publishing Ltd. CA Bobisch