Nonlocal dissociative chemistry of adsorbed molecules induced by localized electron injection into metal surfaces

We present a novel approach to surface chemistry studies using scanning tunneling microscopy (STM), where dissociation of molecules adsorbed on metal surfaces is induced nonlocally in a 10-100 nm radius around the STM tip by hot electrons that originate from the STM tip and transport on the surface. Nonlocal molecular excitation eliminates the influence of the STM tip on the outcome of the electron-induced chemical reaction. The spatial attenuation of the nonlocal reaction is used as a direct measure of hot-electron transport on the surface.     

Model calculations of STM imaging and manipulation of oxygen on Pt(111)

We study tip-adsorbate–substrate interactions in scanning tunneling microscopy (STM) manipulation and imaging, and the influence of impurities on the images. Thence, we perform molecular dynamics simulations and calculate qualitative STM images for oxygen on Pt(1 1 1) surface. The adsorption site of the oxygen molecule is found to be in accordance with ab initio calculations. The calculated STM image has a good resemblance to the experimental ones. The contamination of the tip by oxygen or water alters the STM image strongly. Molecular dynamics simulations on manipulations of oxygen on the surface reveal several mechanisms of how molecular oxygen can be either produced or decomposed with STM tip. Finally, we find out that transfer of oxygen from the surface to an STM tip is not very probable.     

Stochastic switching of subphthalocyanine arrays triggered by scanning tunneling microscopy

Single-molecular switching phenomena in monolayer arrays of subphthalocyanine adsorbed on Cu(1 0 0) surface were investigated by scanning tunneling microscopy (STM) under ultrahigh vacuum. The molecules evaporated on the surface arranged in a square lattice taking the Cu(1 0 0)SubPc(5 × 5) epitaxy. During continuous STM imaging at fixed tunneling conditions the topography of the individual molecules spontaneously changed between the high and low states. This topographic change was attributed to orientational switching between the upward and downward adsorption of the axial Cl atom of the molecule on the Cu surface. Molecular energy calculations and statistical thermodynamic evaluation concluded that the tip-triggered disturbance in the close-packed molecular array induced the molecular rearrangement accompanied with the stochastic orientational switching.     

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.     

The influence of the geometry of the tip on the STM images

The theoretical studies presented in this paper concern the influence of the atomic structure of the tip on the tunneling of electrons between the tip and the sample in STM. This problem has been discussed for the system formed by Al(001) surface and aluminium tip, where different geometries of the tip were taken into consideration. The obtained results have shown how various geometries of the tip induce considerably different STM images of the surface.     

Preparing and regulating a bi-stable molecular switch by atomic manipulation

We present a scanning tunneling microscopy (STM) investigation into the influence of the STM tip on the adsorption site switching of polychlorinatedbiphenyl (PCB) molecules on the Si(111)-7?×?7 surface at room temperature. From an initially stable adsorption configuration, atomic manipulation by charge injection from the STM tip prepared a new bi-stable configuration that switched between two bonding arrangements. No switching rate bias dependence was found for +?1.0 to +?2.2?V. Assuming a thermally driven switching process we find that the measured energy barriers to switching are influenced by the exact location of the STM tip by more than 10%. We propose that this energy difference is due the dispersion interaction between the tip and the molecule.     

Distance-dependent STM-study of the W(110)/C-R(15×3) surface

The W(110)/C-R(15×3) reconstructed surface has been studied by STM at variable tip-surface separation controlled by the tunneling gap resistance. A pronounced dependence of the STM image contrast as a function of tip height has been observed which is explained by the suppression of higher Fourier components, i.e. small wavelength features, with increasing tip height and an additional spatial dependence of the decay length of the surface wavefunction. As an important implication of our study we have found that STM images of non-trivial surface structures can depend critically on the tunneling gap resistance.     

Selective analysis of molecular states by functionalized scanning tunneling microscopy tips

Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the pi states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.     

Deposition and fabrication of alkanethiolate gold nanocluster films on TiO2(1 1 0) and the effects of plasma etching

Deposition and fabrication of films of Au nanoclusters protected by alkanethiolate ligands are attempted on a TiO₂(1 1 0) surface and the structures of films are observed by a scanning tunneling microscope (STM). Effects of oxygen and hydrogen-plasma etching in addition to UV irradiation on the structure and chemical composition of the films are also investigated by using STM and X-ray photoelectron spectroscopy. Alkanethiolate Au nanoclusters are produced using a modified Brust synthesis method and their LB films are dip-coated on TiO₂(1 1 0). Alkanethiolate Au nanoclusters are weakly bound to the substrate and can be manipulated with an STM tip. Net-like structures of alkanethiolate Au nanoclusters are formed by a strong blast of air. Oxygen-plasma etching removes alkanethiolate ligands and simultaneously oxidizes Au clusters. At room temperature, prolonged oxygen-plasma etching causes agglomeration of Au nanoclusters. UV irradiation removes ligands partly, which makes Au nanoclusters less mobile. The net-like structure of alkanethiolate Au clusters produced by a blast of air is retained after oxygen and hydrogen-plasma etching.     

Scanning tunneling microscopy with spin-polarized electrons

We present a short outline of the first STM experiments with spin-polarized electrons performed in ultrahigh vacuum by using ferromagnetic CrO₂ tips and a Cr(001) single crystal surface. A clear distinction can be made between topographic STM line scans obtained with a non-magnetic tungsten tip and those obtained with a ferromagnetic CrO₂ tip, which are modified due to an additional contribution from spin-dependent vacuum tunneling. STM therefore has the potential to measure the local electron spin polarization of the free surface as well as the spatial distribution of spins on the atomic scale.     

Signature of tip electronic states on tunneling spectra

The influence of STM tip electronic states on the electron transport through an atomic object on a surface is studied both experimentally and theoretically. We present scanning tunnelling spectroscopy (STS) experimental results on Ag islands with two, blunt and sharp, STM tips. The data taken with the sharp tip have an additional peak at positive bias which corresponds to the tip apex atom state. We show that sudden tip sharpness variation and corresponding I(V) characteristic change may help to differentiate between electronic states of the tip and the sample. The experimental data are discussed and compared with theoretical calculations performed for two different tips. The current and differential conductance calculations are carried out by means of the Green's function technique and a tight-binding Hamiltonian.     

STM tip-enhanced photoluminescence from porphyrin film

Tunneling electrons-induced molecular fluorescence in organic film is enhanced by the surface plasmons. The plasmon enhancement can be expected not only by the plasmons of the substrate but also by the noble metal tip of scanning tunneling microscope (STM). In this report we investigate the tip effect in photoluminescence of meso-tetrakis(3,5-di-tertiarybutyl-phenyl)porphyrin (H₂TBPP) film on indium tin oxide (ITO) combined with a STM. The experimental result shows the PL of molecules is enhanced by an Ag tip. This enhancement factor is evaluated larger than 2000.     

Jump to contact and neck formation between Pb surfaces and a STM tip

We present a study of the interaction of a W STM tip and the (110) and (111) surfaces of Pb. Atomic resolution has been obtained at room temperature on Pb(110) and up to 330 K on Pb(111). At higher temperatures the surfaces can jump to mechanical contact with the STM tip, resulting in the formation of a connecting neck of Pb between tip and surface. As the tip is retracted, the neck elongates and finally breaks. The dependence of the maximum neck size on the temperature and the tip retraction speed indicates that surface diffusion is responsible for the neck build-up. When the surface is partially oxidized the maximum neck size is reduced. We derive a scaling relation between the maximum neck size, the retraction speed and the surface diffusion coefficient. With this relation and the temperature dependence of the maximum neck size we obtain activation energies for the neck build-up of 1.3 and 0.9 eV respectively for necks on Pb(110) and Pb(111). When a neck breaks, either a crater or a hillock is left on the surface.     

Optical selection rules in light emission from the scanning tunneling microscope

It is reported that optical selection rules still apply in light emission from the scanning tunneling microscope (STM). Linear polarization of isochromat light emitted from the tunneling gap between a STM tip made of tungsten (W) and a silicon (Si) sample with a (001) clean surface strongly depends on the bias voltage between tip and sample. The results show that pi* and sigma* surface states, for example, of the Si(001) sample contribute to emission of p- and s-polarized light, respectively, in accordance with optical selection rules.     

Recording intramolecular mechanics during the manipulation of a large molecule

The technique of single atom manipulation by means of the scanning tunneling microscope (STM) applies to the controlled displacement of large molecules. By a combined experimental and theoretical work, we show that in a constant height mode of manipulation the STM current intensity carries detailed information on the internal mechanics of the molecule when guided by the STM tip. Controlling and time following the intramolecular behavior of a large molecule on a surface is the first step towards the design of molecular tunnel-wired nanorobots.     

Image charge effect on the light emission of rutile TiO_2(110) induced by a scanning tunneling microscope

The plasmon-enhanced light emission of rutile TiO_2(110) surface has been investigated by a low-temperature scanning tunneling microscope(STM). We found that the photon emission arises from the inelastic electron tunneling between the STM tip and the conduction band or defect states of TiO_2(110). In contrast to the Au(111) surface, the maximum photon energy as a function of the bias voltage clearly deviates from the linear scaling behavior, suggesting the non-negligible effect of the STM tip on the band structure of TiO_2. By performing differential conductance( dI/dV) measurements, it was revealed that such a deviation is not related to the tip-induced band bending, but is attributed to the image charge effect of the metal tip, which significantly shifts the band edges of the TiO_2(110) towards the Femi level(E_F) during the tunneling process. This work not only sheds new lights onto the understanding of plasmon-enhanced light emission of semiconductor surfaces, but also opens up a new avenue for engineering the plasmon-mediated interfacial charge transfer in molecular and semiconducting materials.     

室温下单个甘氨酸分子在Cu(111)表面的操纵研究

先用低能电子衍射(LEED)证明了甘氨酸(NH₂-CH₂-COOH)能在室温下在Cu单晶表面产生比较稳定的吸附，然后用扫描隧道显微镜(STM)进一步研究了其吸附情况，看到单个甘氨酸分子在Cu(111)面上吸附稳定并至少有三种吸附状态.分子操纵研究结果表明，甘氨酸分子是被针尖“推着”移动的，它在Cu(111)面有固定的吸附位，并且移动时其吸附状态可以不变.研究结果表明，甘氨酸适合做室温下小分子的可控操纵研究，并且也说明室温下小分子的可控操纵是可能的. 关键词：     

Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference

It has been shown that by distorting a CoPc molecule adsorbed on a Au(111) surface a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), with two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads, the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface and between the molecule lobes.     

Scanning tunnelling microscopy imaging and modification of hydrogen-passivated Ge(100) surfaces

Scanning tunnelling microscopy (STM) study and modification of hydrogen (H)-passivated Ge(100) surfaces have been investigated. Thermal oxidation procedures were used to minimise surface roughness. Ge samples were passivated in HF solution after thermal oxidation. STM and atomic force microscope (AFM) imaging showed that, using HF etching after thermal oxidation, we can obtain a natural H-passivatedtopographically and chemically flat Ge(100) surface. The root-mean-square (rms) roughness ofa H-passivatedGe(100) surface measured both by STM and AFM is less than 2 ?. Electric properties of H-passivatedGe(100) surfaces were studied by scanning tunnelling spectroscopy (STS) in nitrogen ambient. STS showed that the H-passivated Ge surfaces were not pinned. Modification on H-passivated Ge(100) surfaces was carried out using STM by applying an electric voltage between the sample and tip in air. Modified features were characterised by STM and AFM imaging. On the H-passivated Ge(100) surfaces, stable, low-voltage, nanometer-scale modified features can be produced.