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We report on a reversible structural phase transition of a two-dimensional system that can be locally induced by an external electric field. Two different structural configurations may coexist within a CO monolayer on Cu(111). The balance between the two phases can be shifted by an external electric field, causing the domain boundaries to move, increasing the area of the favored phase controllable both in location and size. If the field is further enhanced new domains nucleate. The arrangement of the CO molecules on the Cu surface is observed in real time and real space with atomic resolution while the electric field driving the phase transition is easily varied over a broad range. Together with the well-known molecular manipulation of CO adlayers, our findings open exciting prospects for combining spontaneous long-range order with man-made CO structures such as >molecule cascades> or >molecular graphene>. Our new manipulation mode permits us to bridge the gap between fundamental concepts and the fabrication of arbitrary atomic patterns in large scale, by providing unprecedented insight into the physics of structural phase transitions on the atomic scale., ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). R.R. and N.L. acknowledge financial support from Spanish MINECO (Grant No. MAT2012-38318-C03-02 with joint financing by FEDER Funds from the European Union). P.A. acknowledges the MINCyT (project PICT Bicentenario 1962), CONICET (project PIP 0272), and UNR (project PID 19/I375) and the CCT-Rosario Computational Center. C.A.B. acknowledges financial support through the German research council (DFG).

et al., The dehydrogenation and dechlorination of FeOEP-Cl on Cu(111) has been studied in detail by scanning tunneling microscopy. Although, it is not possible to follow the reaction of an individual molecule, the complete pathway of the reaction with 22 inequivalent intermediate states and the rates of the involved processes are revealed. This is achieved by combining the analysis of a large data set showing thousands of molecules in the different stages of the reaction with numerical simulations., C. A. Bobisch acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).

We studied the electronic transport properties of a 4 QL thin Bi 2 Se 3 film in the hybridized phase on Si(111) by scanning tunneling potentiometry. When a transverse voltage is applied, the film exhibits a homogeneous electric field on the nm scale. In addition, thermovoltage signals with lateral nm variations are found which result from sample heating by the transverse current. The thermovoltage signals are directly correlated to morphological structures on the surface, i.e. step edges, and indicate a lateral variation of the local density of states at the Bi 2 Se 3 surface. No discernible voltage drops appear at the surface so that the whole film serves as a current carrying medium and scattering at surface defects is less important.

We analyze the light emission induced by the tunneling current flowing in a scanning tunneling microscopy experiment. In particular, we study the influence of organic ad-layers on the light emission on the initial monolayer of bismuth (Bi) on Cu(111) in comparison to the well-known case of organic ad-layers on Ag(111). On the Bi/Cu(111)-surface, we find that the scanning tunneling microscopy-induced light emission is considerably enhanced if an organic layer, e.g., the fullerene C60 or the perylene derivate perylene-tetracarboxylic-dianhydride, is introduced into the tip-sample junction. The enhancement can be correlated with a peculiarly weak interaction between the adsorbed molecules and the underlying Bi/Cu(111) substrate as compared to the Ag(111) substrate. This allows us to efficiently enhance and tune the coupling of the tunneling current to localized excitations of the tip-sample junction, which in turn couple to radiative decay channels.

We analyse the electron transport in a 16 quintuple layer thick stepped Bi

Upgrade of a commercial ultra-high vacuum four-probe scanning tunneling microscopy system for atomic resolution capability and thermal stability is reported. To improve the mechanical and thermal performance of the system, we introduced extra vibration isolation, magnetic damping, and double thermal shielding, and we redesigned the scanning structure and thermal links. The success of the upgrade is characterized by its atomically resolved imaging, steady cooling down cycles with high efficiency, and standard transport measurement capability. Our design may provide a feasible way for the upgrade of similar commercial systems.

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.

Silver Emission Follows Golden Rule Electrons emitted from the tip of a scanning tunneling microscope (STM) can be used in certain cases to excite optical emission from surface structure. Such methods can be used to characterize the “particle-in-a-box” states created in chains of varying lengths of metal atoms on surfaces. Chen et al. (p. 981 ) were able to spatially resolve photon emission for silver chains up to 10 atoms long, assembled on a nickel-aluminum alloy surface. The emission maxima correlated with the nodes seen in the derivative of STM current with voltage. This correlation follows from Fermi's golden rule, which connects the initial and final states of the radiation transition through the momentum operator and leads to a better understanding of the light-emission process.

Continuous, atomically flat, and epitaxial Bi(1 1 1) films could be grown on Si(0 0 1). The inherent strain of 2.3% between the Bi(1 1 1) and Si(0 0 1) lattices is relieved by the formation of a grating like one-dimensional misfit dislocation array at the heterointerface. The lattice distortions around each dislocation give rise to a pronounced height depression Δ h = 0.12 nm of the surface, which results in a spot splitting in low-energy electron diffraction and a height contrast in scanning tunneling microscopy (STM). Using STM surface profiles across these depressions, the Burgers vector of the underlying isolated non-interacting dislocations is estimated to be 0.377 nm. For thicker Bi films the ordering of the dislocation network is increased. This reflects an increase of repulsive interaction between neighboring dislocations.

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 growth of organic crystallites formed by 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) has been investigated by scanning tunneling microscopy on Cu(110), Cu(111) and Au(111). For the chosen parameters PTCDA exhibits Stranski-Krastanov growth. If the average coverage exceeds about 3-5 monolayers the formation of crystalline islands begins. Working at very low tunneling currents it has been possible to obtain molecular resolved images of crystallites formed by up to 100 molecular layers, while the overall coverage has a value of about 10-20 monolayers. Since the side walls typically exhibit a moderate slope in the range of 14-20degrees their structure may be as well analyzed as the flat top layer. As will be presented it is not only possible to determine the unit cell within the molecular layer but also their relative displacement between subsequent layers. A comparison between the 3D structures of several crystals will be given. (C) 2004 Elsevier B.V. All rights reserved.

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 analyse the electron transport in a 16 quintuple layer thick stepped Bi2Se3 film grown on Si(1 1 1) by means of scanning tunnelling potentiometry (STP) and multi-point probe measurements. Scanning tunnelling microscopy images reveal that the local structure of the Bi2Se3 film is dominated by terrace steps and domain boundaries. From a microscopic study on the nm scale by STP, we find a mostly linear gradient of the voltage on the Bi2Se3 terraces which is interrupted by voltage drops at the position of the domain boundaries. The voltage drops indicate that the domain boundaries are scatterers for the electron transport. Macroscopic resistance measurements (2PP and in-line 4PP measurement) on the µm scale support the microscopic results. An additional rotational square 4PP measurement shows an electrical anisotropy of the sheet conductance parallel and perpendicular to the Bi2Se3 steps of about 10%. This is a result of the anisotropic step distribution at the stepped Bi2Se3 surface while domain boundaries are distributed isotropically. The determined value of the conductivity of the Bi2Se3 steps of about 1000 S cm−1 verifies the value of an earlier STP study.

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.

To be or not to be chlorinated: When octaethylporphyrin iron(III) chloride (FeOEP-Cl) molecules are sublimated onto Cu(111) surfaces, two different molecular species are observed through scanning tunneling microscopy, showing either a protrusion or a depression at the center. In combination with van der Waals-corrected density functional calculations, our experiments reveal that one species corresponds to FeOEP-Cl molecules with the chlorine atom pointing away from the surface, whereas the other species has been dechlorinated., Financial support is acknowledged from the Deutsche Forschungsgemeinschaft through SFB616: Energy Dissipation at Surfaces.

The electronic structure at the surface of Bi(111) enables us to study the effect of defects scattering into multiple channels. By performing scanning tunneling spectroscopy near step edges, we analyze the resulting oscillations in the local density of electronic states (LDOS) as function of position. At a given energy, forward and backward scattering not only occur simultaneously but may contribute to the same scattering vector Δk. If the scattering phase of both processes differs by π and the amplitudes are almost equal, the oscillations cancel out. A sharp dip in the magnitude of the Fourier transform of the LDOS marks the crossover between forward and backward scattering channels.

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.

Although noise is observed in many experiments, it is rarely used as a source of information. However, valuable information can be extracted from noisy signals. The motion of particles on a surface induced, for example, by thermal activation or by the interaction with the tip of a scanning tunnelling microscope may lead to fluctuations or switching of the tunnelling current. The analysis of these processes gives insight into dynamics on a single atomic or molecular level. Unfortunately, scanning tunnelling microscopy (STM) is not a useful tool to study dynamics in detail, as it is an intrinsically slow technique. Here, we show that this problem can be solved by providing a full real-time characterization of random telegraph noise in the current signal. The hopping rate, the noise amplitude and the relative occupation of the involved states are measured as a function of the tunnelling parameters, providing spatially resolved maps. In contrast to standard STM, our technique gives access to transiently populated states revealing an electron-driven hindered rotation between the equilibrium and two metastable positions of an individually adsorbed molecule. The new approach yields a complete characterization of copper phthalocyanine molecules on Cu(111), ranging from dynamical processes on surfaces to the underlying electronic structure on the single-molecule level. © 2013 Macmillan Publishers Limited. All rights reserved., Financial support by the Deutsche Forschungsgemeinschaft through the SFB 616 Energy Dissipation at Surfaces is gratefully acknowledged. M.C.C. and A.S. thank the Studienstiftung des deutschen Volkes for support.

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.

Scanning tunnelling microscopy was used to study the morphological changes of the surface of a single-crystalline silver nanowire caused by a lateral electron current. At current densities of about 1.5 × 10(7) A cm(-2), surface atoms are extracted from step edges, resulting in the motion of surface steps, islands and holes with a thickness or depth of one monolayer. Upon current reversal the direction of the material transport can be altered. The findings are interpreted in terms of the wind force.

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.

We report on the molecular self-assembly of a highly ordered mixed molecular structure of copper–phthalocyanine (CuPc) and 3,4,9,10–perylene–tetracarboxylic–dianhydride (PTCDA) on Cu(1 1 1) starting from dense PTCDA domains. Low temperature scanning tunneling microscopy (LT-STM) data reveal the surface structure as the CuPc coverage is stepwise increased. A highly ordered mixed phase as well as a disordered mixed phase can be found, where the highly ordered mixed phase seems to be thermodynamically favored. Low energy electron diffraction (LEED) measurements aid the STM analysis. Tentative models for the formation of the highly ordered mixed phase are given.

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.

We report the spatial imaging of the photon transition probability of a single molecule at submolecular resolution. Photon imaging of a ringlike pattern is further resolved as two orthogonal vibronic transitions after incorporating spectral selectivity. A theoretical model and the calculated intensity images reveal that the transition probability is dominated by the symmetry of the positions of the tip and the transition dipole moment. This imaging technique enables the probing of the electronic and optical properties in the interior of a single molecule.

E-Journal of Surface Science and Nanotechnology 7, 175408 (2009). doi:10.1380/ejssnt.2009.441, Published by Surface Science Society of Japan, Tokyo

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.

Smooth and epitaxial thin bismuth (Bi) films with low defect density were grown on Si(001) by molecular beam epitaxy. The film quality is characterized by in situ spot profile analysis low-energy electron diffraction and scanning tunneling microscopy, and ex situ atomic force microscopy and X-ray diffraction. The complete process is accomplished in three steps. Firstly, a template of a strained 6 nm Bi(111) film is grown at 150 K. Secondly, during annealing to 450 K the strain is relieved by the formation of an ordered array of misfit dislocations at the interface. Finally, additional Bi is deposited at 450 K up to the desired thicknesses of the Bi film. The film consists of 90° rotated and twinned μm size crystallites with a terrace size larger than 100 nm and an overall roughness of only 0.6 nm. A 25 nm thick Bi film is relaxed to Bi bulk lattice constant which is confirmed by X-ray diffraction.

Homoepitaxial growth of Bi(111) at temperatures between 80--300 K has been studied using spot profile analyzing low-energy electron diffraction (SPA-LEED) and scanning tunneling microscopy (STM). From the intensity oscillations of the (00)-spot with Bi coverage and the STM topography of two-dimensional (2D) islands at low coverage, a pure 2D nucleation followed by a quasi bilayer-by-bilayer growth mode has been confirmed. The oscillation amplitude decays slowly with coverage, indicating a slow kinetic roughening due to a weak Ehrlich-Schwoebel step edge barrier. From the Arrhenius behavior of the average island separation an intraterrace diffusion barrier of ${E}_{d}=0.135\text{ }\text{eV}$ is estimated. Regularly ordered quasidendritic shape islands reflect an asymmetry in adatom diffusion along the steps and the corners of the islands.

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.

Despite their large lattice mismatch of 18 %, the lattices of Bi(111) and Si(001) fit surprisingly well. A remaining compressive strain in the Bi film of 2.3 % along the direction is accommodated by the formation of a periodic array of edge-type misfit dislocations confined to the interface. The strain fields surrounding each dislocation interact with each other, producing a quasi-periodic nanopattern of grating-like periodic height undulations on the surface. The separation and the amplitude of the height undulations have been derived by spot profile analyzing LEED and STM surface height profiles. The observed undulations agree well with elasticity theory.

The paper describes a simple scheme enabling the real-time characterization of fluctuations, e.g., of the conductance in scanning tunneling microscopy. The technique can be used in parallel to other data acquisition, evaluating the rate, the amplitude, and the duty cycle of telegraphic noise in the tunneling current. This kind of scanning probe microscopy allows to evaluate the noise parameters as a function of the average tunneling current, the electron energy, and the lateral position. Images of the noise with Ångstrom spatial resolution are acquired simultaneously to the topographic information providing a direct correlation between the structural information and the noise. The method can be applied to a large variety of systems to monitor dynamics on the nanoscale, e.g., the localization of tunneling current induced switching within a single molecule. Noise spectroscopy may reveal the involved molecular orbitals, even if they cannot be resolved in standard scanning tunneling spectroscopy. As an example we present experimental data of the organic molecule copper phthalocyanine on a Cu(111) surface [J. Schaffert, M. C. Cottin, A. Sonntag, H. Karacuban, C. A. Bobisch, N. Lorente, J.-P. Gauyacq, and R. Möller, Nature Mater. 12, 223-227 (2013)].

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]

In this article [1], one of the coauthors, B. Krenzer, is missing in the author list by mistake. The correct author list is the same as the one shown in this erratum.

Epitaxial Bi(111) films on the Si(100)−(2×1) surface were studied by two different scanning probe techniques, to obtain information on the buried interface. Ballistic electron emission microscopy reveals that the transmission across the Schottky barrier depends on the type of substrate terrace. The thermovoltage in scanning tunneling microscopy exhibits alternating signals for substrate step edges, which can be related to SA and SB steps, characteristic for the uncovered Si(100)−(2×1) surface. In addition to information about the growth mode of Bi, it was found that typical features of the Si(100)−(2×1) surface reconstruction are maintained.

Organic molecules often show very complex thermal desorption spectra. If there is an ordered structure the activation energy for desorption Edes will decrease for the first few layers because of the decreasing van der Waals interaction of the layers to the substrate. This is especially true for the system 3,4,9,10 perylene-tetracarboxylic-dianhydride on Cu(111). The spectra consist of three different signals which can be attributed to the second layer, the multilayer and a crystal phase. The numerical algorithm sketched in this paper gives a desorption energy of 2.35eV for the second layer and 2.2eV for the multilayer.

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

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 two-dimensional 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 moire 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.

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.