Scanning tunneling potentiometry (STP) is a powerful tool to analyze the conductance through thin conducting layers with lateral resolution in the nanometer range. In this work, we show how a commercial ultrahigh vacuum multiprobe system, equipped with four independent tips, can be used to perform STP experiments. Two tips are gently pushed into the surface applying a lateral current through the layer of interest. Simultaneously, the topography and the potential distribution across the metal film are measured with a third tip. The signal-to-noise ratio of the potentiometry signal may be enhanced by using a fourth tip, providing a reference potential in close vicinity of the studied area. Two different examples are presented. For epitaxial (111) oriented Bi films, grown on a Si(100)-(2 x 1) surface, an almost constant gradient of the potential as well as potential drops at individual Bi-domain boundaries were observed. On the surface of the Si(111)(3 x 3)-Ag superstructure the potential variation at individual monoatomic steps could be precisely resolved.

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.

The initial growth of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) was analysed. Ultrathin films with coverages of up to two layers were prepared on a (111) orientated copper single crystal by means of vapour deposition in an ultrahigh-vacuum chamber. The films were characterized in situ by scanning tunnelling microscopy (STM). Within the first layer, two different structures were found. Both exhibit a herringbone-like arrangement of the molecules, which is also found in the (102) plane of the α and β bulk phases. The twodimensional unit cell is given by two molecules which are rotated by about 90 • . As an effect of the interaction with the substrate, a voltage-dependent moiré pattern was observed for one of these phases. For the second layer, a herringbone phase was found that is denser than the phases of the first layer but less dense than the bulk phases.

The two planar organic molecules copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) are found to form an ordered mixed monolayer on Cu(111). The layers have been prepared by exposing the surface to an equivalent of a little bit more than half of a monolayer of CuPc and the same amount of PTCDA followed by thermal annealing. The investigations by scanning tunneling microscopy reveal regular patterns with a commensurate unit cell which contains one CuPc and two PTCDA molecules.

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C60 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.

We report on ballistic electron emission microscopy and spectroscopy studies on epitaxial (3-5 nm thick) Bi(111) films, grown on n-type Si substrates. The effective barrier heights of the Schottky barrier observed are 0.58 eV for the Bi=Sið100Þ-ð2 Â 1Þ and 0.68 eV for the Bi=Sið111Þ-ð7 Â 7Þ. At the step edges of the epitaxial films a strong increase of the ballistic electron emission microscopy current is observed for Bi=Sið111Þ-ð7 Â 7Þ, while no increase occurs for Bi=Sið100Þ-ð2 Â 1Þ. These observations can be explained by the conservation of the lateral momentum of the electron at the metal-semiconductor interface. DOI: 10.1103/PhysRevLett.102.136807 PACS numbers: 73.23.Ad, 73.20.At, 73.30.+y, 73.40.Àc Since the invention of ballistic electron emission microscopy (BEEM) by Bell and Kaiser two decades ago [1,2], studies on the conservation of lateral electron momentum at the interface formed between metals and semiconductors have remained puzzling. Schowalter and Lee [3,4] studied ballistic electron emission spectroscopy (BEES) for Au films grown on Si substrates with different crystallographic orientations. Assuming that the lateral electron momentum at the metal-semiconductor interface is conserved [2] the BEEM currents were expected to differ dramatically due to different projections of the conduction band minima of the Si on the (111) and (100) plane. However, almost identical BEEM currents were measured. These results were later confirmed by Weilmeier et al. [5,6]. In the case of Pd on Si, Ludeke and Bauer [7] attributed the observed equality of transmission across (111) and (100) to interface scattering randomizing the electron momentum. As discussed by García-Vidal et al.[8] and Bell [9], the results found on Au samples may be explained without additional electronic scattering processes, because the electronic band structure of the transition metals (Au, Ag, Pd, etc.) exhibits a band gap in the [111] direction, requiring a minimal lateral component of the electronic momentum. Taking into account the matching conditions at the metal-semiconductor interface, they predicted a similar onset for the BEEM current, but a higher intensity for Au=Sið111Þ in agreement with the experiments.A different situation is found with CoSi 2 =Sið100Þ versus CoSi 2 =Sið111Þ. The band structure of CoSi 2 allows the propagation of electrons at k k ¼ 0, and due to the excellent quality of the epitaxial layers there is very little diffuse scattering of electrons. The latter is confirmed by the high resolution of detail at the metal-semiconductor interface obtained by BEEM imaging [10], indicating that the electrons are limited to a very small cone within the CoSi 2 . A theoretical description by García-Vidal et al. [8,[11][12][13][14] could explain the high spatial resolution for BEEM and has predicted that the onset for the BEEM current should be shifted for the (111) versus the (100) substrate, due to the conservation of the lateral momentum at the interface. However, the experimental results for the BEEM current are contra...

The well-known Au/n-Si(111) Schottky interface is modified by a discontinuous pentacene film (∼1.5 nm thick) and studied using spatially resolved ballistic electron emission spectroscopy (BEES). The pentacene film introduced subtle changes to the interface which cannot be definitively detected by current-voltage measurements or a standard BEES analysis of the barrier height. In contrast, analyzing the BEES results in a dual-parameter (transmission attenuation and barrier height) space allows the effect of the pentacene film on the Au/n-Si(111) interface to be clearly demonstrated. We found that the pentacene film behaves like a tunneling barrier and increases the distribution of local barrier heights with a tendency toward lower values. Our results highlight the potential of the dual-parameter BEES analysis for understanding local interface modification by molecules.

In this work we report on the growth of high quality bismuth films with a layer thickness of 3–4 nm on a (100)-oriented silicon surface. We present a combined STM and LEED study to determine the best possible growth conditions regarding, for example, film flatness, grain size and low surface roughness. The deposition of bismuth was performed at a low temperature of about 130 K followed by moderate annealing to an ambient temperature or to temperatures slightly above. The result is an epitaxial Bi film with low surface roughness.