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.     

Probing quasiparticle states bound by disparate periodic potentials

Thin films of Ag(111) with two-dimensional crystallinity of large lateral coherence grow on Ge(111), free of in-plane registry with the underlying substrate. Ag s-p electrons forming two-dimensional quantum well states scatter coherently at the buried interface potential, resulting in an unexpected set of new quasiparticle states, as observed by angle-resolved photoemission. These new features originate from interactions among Ag quantum well bands, gaining a momentum equivalent to a reciprocal vector of the substrate lattice.     

Observation and investigation of graphite superlattice boundaries by scanning tunneling microscopy

In this article, we report on scanning tunneling microscopy (STM) observations of several different kinds of superlattice boundaries on highly-oriented pyrolytic graphite (HOPG) including an array of bead-like structures, a monolayer deep trench, a zig-zag shaped termination, and a plain boundary without features. Results of a simulation model show that a top rotated graphite layer with a straight boundary does not necessarily lead to the zig-zag shaped boundary of the resulting superlattice as has been previously claimed. The formation of the bead-like, trench, and zig-zag shaped boundaries is explained from the energetic point of view. Our study also shows evidence for the superlattice-mediated observation of a low-angle grain boundary with a varying tilt angle. A relationship between the periodicity of the boundary dislocations and the periodicity of the superlattice across the boundary is derived. The result of this work is important for an understanding of superlattices on graphite whose origin is not yet completely understood.     

High-resolution scanning tunneling spectroscopy of magnetic impurity induced bound states in the superconducting gap of Pb thin films

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity-induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale.     

Electronic excitation of ice monomers on Au(111) by scanning tunneling microscopy

Scanning tunneling microscopy, inelastic tunneling spectroscopy, and electron induced manipulation are used to investigate electronic excitation of D₂O monomers and small clusters adsorbed at the elbows of the Au(111) reconstruction. Diffusion of molecules, dissociation of clusters, and rearrangement of the reconstruction is induced by electronic excitation. Threshold energies of between 200 and 250 meV and of 446 meV are explained by combined vibrational modes of D₂O molecules. External vibrational modes of D₂O molecules on Au(111) are identified by inelastic tunneling spectroscopy at ≈18, 30, and 41 meV.     

Electronic structure of MgO-supported Au clusters: quantum dots probed by scanning tunneling microscopy

We investigate via density functional theory (DFT) the appearance of small MgO-supported gold clusters with 8 to 20 atoms in a scanning tunneling microscope (STM) experiment. Comparison of simulations of ultrathin films on a metal support with a bulk MgO leads to similar results for the cluster properties relevant for STM. Simulated STM pictures show the delocalized states of the cluster rather than the atomic structure. This finding is due to the presence of s- derived delocalized states of the cluster near the Fermi energy. The properties of theses states can be understood from a jellium model for monovalent gold.     

Study of resonant tunneling in Au nanoclusters on the surface of SiO2/Si thin films using the combined scanning tunneling microscopy and atomic-force microscopy technique

The morphology and electronic properties of Au nanoclusters on the surface of SiO₂ thin films on n ⁺-Si substrates are studied using the combined scanning tunneling microscopy (STM) and atomic-force microscopy (AFM) technique. The peaks associated with the resonant tunneling of electrons from the states of the valence band of the probe material to the states of the conduction band of the substrate material through Au nanoclusters are observed on the current-voltage characteristics for the contact of a p ⁺-Si AFM probe with Au nanoclusters. Experimental results are interpreted by calculating the tunnel transparency of the SiO₂/Au/SiO₂ double barrier structure in a strong electric field.     

Coulomb-blockade-induced bound quasiparticle states in a double-island structure

We determine the low temperature shape of the Coulomb-blockade staircase in a superconducting double-island device. For an odd number of electrons, in the ground state the intrinsic quasiparticle is bound to the tunneling contact. For a single channel contact the gap between the ground state and the continuum of excited states is of the order of the Josephson energy E(J). The temperature dependence of the Coulomb-blockade step width is nonmonotonic, with the minimal width occurring at T(i) approximately E(J)/ln(square root DeltaE(J)/delta), where Delta and delta are, respectively, the superconducting gap and mean level spacing in the island. For an even number of electrons, the Coulomb enhancement of the Josephson energy is shown to be significantly stronger than that for a single grain coupled to a lead. If the electrostatic energy favors a single broken Cooper pair, the resulting quasiparticles are bound to the contact at T=0.     

Observation of anthraquinone compounds using low temperature scanning tunneling microscopy

Three derivatives of anthraquinone deposited on Cu(1 1 1) have been observed by STM at liquid nitrogen temperature. Adsorption of the three molecules is found to be strongly dependent on the end group (phenyl, thienyl and pyridyl) surrounding the anthraquinone central part. While adsorption of the bi-pyridyl compound yields single molecules on the copper surface, evaporation of the phenyl derivative leads to the formation of stable dimers. The bi-thienyl anthraquinone has an intermediate behaviour with dimer formation and single molecule adsorption. These findings are assumed to result from the polar character of the molecules and from the bond strength of the different substituents with the metallic surface.     

Nanoscale structuring of SrRuO3 thin film surfaces by scanning tunneling microscopy

Surface modifications through line etching of SrRuO₃ thin films have been carried out using a scanning tunneling microscope under ambient conditions. The line etching is found to be dependent on both bias voltage and scan speed for a given number of scan repetitions. We observe that an applied voltage above a threshold value is required for successful line etching. The depth of the etched lines is increasing with increasing bias voltage and scan repetitions as well as with decreasing scan speed. Moreover, sub-50 nm laterally confined mesa structures could be reproducibly etched on the SrRuO₃ thin film surfaces.     

Ordered thin films of alkyloxy-substituted benzaldehydes on a graphite surface studied by scanning tunneling microscopy

The molecular assembly of three different trialkyloxy-substituted benzaldehydes on graphite has been studied using scanning tunneling microscopy (STM). It is found that these benzaldehydes, which do not have intermolecular hydrogen bonding, could form a lamella arrangement in which the headgroups are aligned side by side. The effect due to the alkyl chain length on the lamella structure is also presented. Received: 21 January 2002 / Accepted: 22 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-10/6255-7908     

Evidence of hole-electron quasiparticle interference in ErSi2 semimetal by Fourier-transform scanning tunneling spectroscopy

The semimetallic ErSi2 layer grown on Si(111) substrates provides an ideally confined 2D electron and hole gas that reflects in complex standing wave pattern at 77 K. The quasiparticles exist in a wide energy range from -800 to 300 meV without mixing with silicon bulk excitations. By comparing high resolution Fourier transform of dI/dV maps, with joint density of states calculations, we are able to determine the 2D band structure. We also clearly demonstrate that hole-hole and hole-electron quantum interferences dominate over electron-electron ones.