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Showing papers on "Scanning tunneling spectroscopy published in 2004"


Journal ArticleDOI
TL;DR: In this article, an accurate determination of surface-state linewidth by scanning tunneling spectroscopy, photoemission and directly in the time domain by two-photon photo-emission is presented.

370 citations


Journal ArticleDOI
TL;DR: The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme and two types of inelastic contributions are discussed.
Abstract: We study inelastic electron tunneling through a molecular junction using the nonequilibrium Green’s function formalism. The effect of the mutual influence between the phonon and the electron subsystems on the electron tunneling process is considered within a general self-consistent scheme. Results of this calculation are compared to those obtained from the simpler Born approximation and the simplest perturbation theory approaches, and some shortcomings of the latter are pointed out. The self-consistent calculation allows also for evaluating other related quantities such as the power loss during electron conduction. Regarding the inelastic spectrum, two types of inelastic contributions are discussed. Features associated with real and virtual energy transfer to phonons are usually observed in the second derivative of the current I with respect to the voltage Φ when plotted against Φ. Signatures of resonant tunneling driven by an intermediate molecular ion appear as peaks in the first derivative dI/dΦ and may show phonon sidebands. The dependence of the observed vibrationally induced lineshapes on the junction characteristics, and the linewidth associated with these features are also discussed.

280 citations


Journal ArticleDOI
TL;DR: In this article, the authors review the latest advances in the production and state-of-the-art characterization of B and N-doped carbon nanotubes and nanofibers and briefly discuss different approaches to producing these novel doped nano-systems.

255 citations


Journal ArticleDOI
TL;DR: Intrinsic molecular fluorescence from porphyrin molecules on Au(100) has been realized by using a nanoscale multimonolayer decoupling approach with nanoprobe excitation in the tunneling regime, suggesting an excitation mechanism via hot electron injection from either tip or substrate.
Abstract: Intrinsic molecular fluorescence from porphyrin molecules on Au(100) has been realized by using a nanoscale multimonolayer decoupling approach with nanoprobe excitation in the tunneling regime. The molecular origin of luminescence is established by the observed well-defined vibrationally resolved fluorescence spectra. The molecules fluoresce at low ``turn-on'' voltages for both bias polarities, suggesting an excitation mechanism via hot electron injection from either tip or substrate. The excited molecules decay radiatively through Franck-Condon ${\ensuremath{\pi}}^{*}\mathrm{\text{\ensuremath{-}}}\ensuremath{\pi}$ transitions.

211 citations


Journal ArticleDOI
TL;DR: A combination of angle-resolved photoemission and scanning tunneling microscopy is used to explore the possibilities for tailoring the electronic structure of gold atom chains on silicon surfaces as mentioned in this paper.
Abstract: A combination of angle-resolved photoemission and scanning tunneling microscopy is used to explore the possibilities for tailoring the electronic structure of gold atom chains on silicon surfaces. It is shown that the interchain coupling and the band filling can be adjusted systematically by varying the step spacing via the tilt angle from Si(111). Planes with odd Miller indices are stabilized by chains of gold atoms. Metallic bands and Fermi surfaces are observed. These findings suggest that atomic chains at stepped semiconductor substrates make a highly flexible class of solids approaching the one-dimensional limit.

196 citations


Journal ArticleDOI
TL;DR: Spin-averaged and spin-polarized scanning tunneling spectroscopy at low temperature was performed on nanometer-scale triangular Co islands grown epitaxially on Cu(111) in the submonolayer coverage regime to allow a separation of spectral contributions arising from different island stacking or from opposite magnetization states.
Abstract: Spin-averaged and spin-polarized scanning tunneling spectroscopy at low temperature was performed on nanometer-scale triangular Co islands grown epitaxially on Cu(111) in the submonolayer coverage regime. Two structurally different island types can clearly be distinguished by their spin-averaged electronic structure. Spin-polarized measurements allow a separation of spectral contributions arising from different island stacking or from opposite magnetization states, respectively. In an applied magnetic field, both island types are found to be magnetized perpendicular to the surface, with large values of saturation field, remanence, and coercivity.

133 citations


Journal ArticleDOI
TL;DR: A simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au is introduced and excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated.
Abstract: Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.

132 citations


Journal ArticleDOI
TL;DR: In this paper, a single-electron tunneling through a quantum dot is detected by means of a radio-frequency singleelectron transistor, and Poisson statistics of single electron tunneling events are observed from frequency domain measurements, and individual tunneling event are detected in the time-domain measurements.
Abstract: Single-electron tunneling through a quantum dot is detected by means of a radio-frequency single-electron transistor. Poisson statistics of single-electron tunneling events are observed from frequency domain measurements, and individual tunneling events are detected in the time-domain measurements. Counting tunneling events gives an accurate current measurement in the saturated current regime, where electrons tunnel into the dot only from one electrode and tunnel out of the dot only to the other electrode.

130 citations


Journal ArticleDOI
TL;DR: Analysis of the chain-localized electron dynamics shows that the dispersion is fully described within a 1D tight-binding approach, and DFT calculations confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.
Abstract: The quantum confinement in surface-supported atomic-scale metal chains was demonstrated, characterized by a pseudogap in the projected bulk band structure of the substrate. Low-temperature scanning tunneling spectroscopy and density functional theory (DFT) calculations were used for the study. The chain-localized electron dynamics showed that the dispersion was fully described within a ID tight-binding approach. The results confirm the confinement of unoccupied states to the chain in the relevant energy range, along with a significant extension of these states into the vacuum region.

128 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields.
Abstract: We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the 3He pot of an entirely UHV compatible 3He refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the energy resolution is about 100 μeV.

113 citations


Journal ArticleDOI
TL;DR: In this article, a review of the state-of-the-art results on ultrathin MgO films on Ag(001) surfaces has been presented, showing that the band gap of about 6 eV at the surface develops within the first 3 ML confirmed by local density of states (LDOS) calculations.
Abstract: Considerable progress has been made recently, using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS) and local density functional theory (DFT), in examining the atomic structure and electronic properties of ultrathin insulating films. This article reviews pertinent results to date with special emphasis on ultrathin MgO films on Ag(001) surfaces. Using STS, the layer-by-layer resolved electronic structure up to 3 ML shows that the band gap of about 6 eV at the MgO surface develops within the first 3 ML confirmed by local density of states (LDOS) calculations. Using model calculations, the atomic species observed in STM on the MgO film are unambiguously identified. These results underline the importance of a combination of local spectroscopy, scanning probe techniques and local density of states calculations for the understanding of matter on the microscopic level.

Journal ArticleDOI
TL;DR: In this paper, band-to-band tunneling was studied in ion-implanted P/N junction diodes with profiles representative of present and future silicon complementary metal-oxide-silicon (CMOS) field effect transistors.
Abstract: Band-to-band tunneling was studied in ion-implanted P/N junction diodes with profiles representative of present and future silicon complementary metal–oxide–silicon (CMOS) field effect transistors. Measurements were done over a wide range of temperatures and implant parameters. Profile parameters were derived from analysis of capacitance versus voltage characteristics, and compared to secondary-ion mass spectroscopy analysis. When the tunneling current was plotted against the effective tunneling distance (tunneling distance corrected for band curvature) a quasi-universal exponential reduction of tunneling current versus, tunneling distance was found with an attenuation length of 0.38 nm, corresponding to a tunneling effective mass of 0.29 times the free electron mass (m0), and an extrapolated tunneling current at zero tunnel distance of 5.3×107 A/cm2 at 300 K. These results are directly applicable for predicting drain to substrate currents in CMOS transistors on bulk silicon, and body currents in CMOS transistors in silicon-on-insulator.

Journal ArticleDOI
TL;DR: In this article, tunneling spectroscopy of Bi2Se3 and Bi2Te3 layered narrow gap semiconductors reveals finite in-gap density of states and suppressed conduction in the energy range of high valence-band states.
Abstract: Scanning tunneling spectroscopy of Bi2Se3 and Bi2Te3 layered narrow gap semiconductors reveals finite in-gap density of states and suppressed conduction in the energy range of high valence-band states. Electronic structure calculations suggest that the surface effects are responsible for these properties. Conversely, the interlayer coupling has a strong effect on the bulk near-gap electronic structure. These properties may prove to be important for the thermoelectric performance of these and other related chalcogenides.

Journal ArticleDOI
TL;DR: In this paper, the inelastic electron tunneling spectrum (IETS) of pyrolitic graphite has been measured with scanning tunneling spectroscopy at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles.
Abstract: The inelastic electron tunneling spectrum ~IETS! of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy ~STS! at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles. We discuss the enhancement of certain phonon modes by phonon-assisted tunneling in STS based on the restrictions imposed by the electronic structure of graphite. We also demonstrate the local excitation of surface plasmons in IETS, which

Journal ArticleDOI
TL;DR: A model in which the dehydrogenation of the benzene molecule is induced by the formation of the temporal negative ion due to the trapping of the electrons at the unoccupied resonant states formed by the pi orbitals is proposed.
Abstract: We have investigated the mechanism of the chemical reaction of the benzene molecule adsorbed on Cu(110) surface induced by the injection of tunneling electrons using scanning tunneling microscopy (STM). With the dosing of tunneling electrons of the energy 2–5 eV from the STM tip to the molecule, we have detected the increase of the height of the benzene molecule by 40% in the STM image and the appearance of the vibration feature of the ν(C–H) mode in the inelastic tunneling spectroscopy (IETS) spectrum. It can be understood with a model in which the dissociation of C–H bonds occurs in a benzene molecule that induces a bonding geometry change from flat-lying to up-right configuration, which follows the story of the report of Lauhon and Ho on the STM-induced change of benzene on the Cu(100) surface. [L. J. Lauhon and W. Ho, J. Phys. Chem. A 104, 2463 (2000)]. The reaction probability shows a sharp rise at the sample bias voltage at 2.4 V, which saturates at 3.0 V, which is followed by another sharp rise at ...

Journal ArticleDOI
TL;DR: In this article, epitaxial trilayer films of cobalt-phthalocyanine (CoPc) were studied on Au(1/1/3) 22×3 surfaces using a scanning tunneling microscope at 78 K.

Journal ArticleDOI
TL;DR: Scanning tunneling spectroscopy can directly measure the Mn-Mn interaction as a function of distance and local spin-polarized resonances within the valence bands significantly enhance the LDOS near the band edge.
Abstract: We present the spin and orbitally resolved local density of states (LDOS) for a single Mn impurity and for two nearby Mn impurities in GaAs. The GaAs host is described by a sp(3) tight-binding Hamiltonian, and the Mn impurity is described by a local p-d hybridization and on-site potential. Local spin-polarized resonances within the valence bands significantly enhance the LDOS near the band edge. For two nearby parallel Mn moments the acceptor states hybridize and split in energy. Thus scanning tunneling spectroscopy can directly measure the Mn-Mn interaction as a function of distance.

Journal ArticleDOI
TL;DR: The observed two standing waves caused by separate spin and charge bosonic excitations are found to constitute direct evidence for a Luttinger liquid.
Abstract: Electronic standing waves with two different wavelengths were directly mapped near one end of a single-wall carbon nanotube as a function of the tip position and the sample bias voltage with high-resolution position-resolved scanning tunneling spectroscopy. The observed two standing waves caused by separate spin and charge bosonic excitations are found to constitute direct evidence for a Luttinger liquid. The increased group velocity of the charge excitation, the power-law decay of their amplitudes away from the scattering boundary, and the suppression of the density of states near the Fermi level were also directly observed or calculated from the two different standing waves.

Journal ArticleDOI
TL;DR: A novel iterative experimental-theoretical technique which can identify the atomic structure of defects in many-atom nanoscale materials from scanning tunneling microscopy and spectroscopy data and is used to identify a defect responsible for the electronic properties of a carbon nanotube intramolecular junction.
Abstract: We have developed a novel iterative experimental-theoretical technique which can identify the atomic structure of defects in many-atom nanoscale materials from scanning tunneling microscopy and spectroscopy data. A given model for a defect structure is iteratively improved until calculated microscopy and spectroscopy data based on the model converge on the experimental results. We use the technique to identify a defect responsible for the electronic properties of a carbon nanotube intramolecular junction. Our technique can be extended for analysis of defect structures in nanoscale materials in general.

Journal ArticleDOI
TL;DR: In this article, the molecular organization and morphology of vacuum deposited thin films of tris (8-hydroxyquinoline) aluminum (Alq3) from metal-organic planar light-emitting nanocomposites consisting of gold island films and VD Alq3 film were studied.

Journal ArticleDOI
TL;DR: In this paper, an electron transport in a single-molecule magnet metal was studied, where the conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis.
Abstract: An electron transport is studied in the system that consists of a scanning tunneling microscopy, single-molecule magnet metal. Because of quantum tunneling of magnetization in a single-molecule magnet, linear response conductance exhibits stepwise behavior with increasing longitudinal field, and each step is maximized at a certain value of field sweeping speed. The conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis. A rigorous theory is presented that combines the exchange model with the Landau-Zener model.

Journal ArticleDOI
TL;DR: A spin-polarized self-consistent density functional theory (DFT) method for calculation of the electronic structure and transport properties of a system under a finite bias voltage is presented in this article.
Abstract: We present a spin-polarized self-consistent density functional theory (DFT) method for calculation of the electronic structure and transport properties of a system under a finite bias voltage. This method is implemented within the layer Korringa-Kohn-Rostoker approach. We calculate the tunneling conductance and the magnetoresistance (MR) of Fe/FeO/MgO/Fe tunneling junctions as functions of bias voltage. We find that the change in the electronic structure is minimal as a function of bias. The effective capacitance is consistent with the dielectric constant of MgO. The tunneling conductance is highly nonlinear. At low biases the TMR ratio is greatly reduced due to the large contribution to the tunneling current from interface resonance states. This contribution diminishes as the bias voltage increases, leading to an increase of the TMR ratio as a function of bias.

Journal ArticleDOI
TL;DR: In this article, a method to produce superconducting tips to be used in scanning tunneling microscopy/spectroscopy experiments was presented, and these tips were used to investigate the evolution of the electronic density of states of NbSe 2 from 0.3 K up to its critical temperature (7.2 K).
Abstract: We present a method to produce superconducting tips to be used in scanning tunneling microscopy/spectroscopy experiments. We use these tips to investigate the evolution of the electronic density of states of NbSe 2 from 0.3 K up to its critical temperature (7.2 K). The use of a superconducting tip (Pb) as counterelectrode provides an enhancement of the different features related to the DOS of NbSe 2 in the tunneling conductance curves, along all the studied thermal range. The analysis of the experimental results gives evidence of the presence of multiband superconductivity in NbSe 2 .

Journal ArticleDOI
TL;DR: In this paper, the authors present scanning tunneling spectroscopy measurements on the heavy-fermion superconductor and show that the superconducting gap opens over a large part of the Fermi surface.
Abstract: We present scanning tunneling spectroscopy measurements on the heavy-fermion superconductor ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}.$ Our results show that the superconducting gap opens over a large part of the Fermi surface. The deviations from isotropic BCS s-wave behavior are discussed in terms of a finite distribution of values of the superconducting gap.

Journal ArticleDOI
TL;DR: It is shown that the broadening of the impurity level leads to an additional and important contribution to the Fano resonance around the Fermi surface, especially in the mixed valence regime.
Abstract: We present a general theory for the Fano resonance in Anderson impurity systems. It is shown that the broadening of the impurity level leads to an additional and important contribution to the Fano resonance around the Fermi surface, especially in the mixed valence regime. This contribution results from the interference between the Kondo resonance and the broadened impurity level. Being applied to the scanning tunneling microscopic experiments, we find that our theory gives a consistent and quantitative account for the Fano resonance line shapes for both Co and Ti impurities on Au or Ag surfaces. The Ti systems are found to be in the mixed valence regime.

Journal ArticleDOI
TL;DR: Systematic, quantitative comparisons between scanning tunneling microscopy experiments and first principles simulations of O(2 x 2)/Ru(0001) have been performed and after adsorption of oxygen on the STM tip the authors observe a contrast reversal on the surface.
Abstract: Systematic, quantitative comparisons between scanning tunneling microscopy (STM) experiments and first principles simulations of $\mathrm{O}(2\ifmmode\times\else\texttimes\fi{}2)/\mathrm{Ru}(0001)$ have been performed. The shape of the atomic adsorbates in the images depends strongly on the tunneling resistance and changes reversibly from circular (high resistance) to triangular (low resistance). In addition, after adsorption of oxygen on the STM tip we observe a contrast reversal on the surface, confirmed by extensive numerical simulations.

Journal ArticleDOI
TL;DR: This work explains the counterintuitive result by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.
Abstract: Water adsorbed on Ag(111) at 70 K forms circular clusters that consist of six molecules. In scanning tunneling microscopy, this cyclic hexamer is imaged as a protrusion for voltages below V(SS)=-93 meV and as a depression for voltages above V(SS). The electronic density of states, however, increases around V(SS). We explain this counterintuitive result with the aid of calculated images by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.

Journal ArticleDOI
TL;DR: It is suggested that self-assembled phenylene ethynylene oligomer monolayers on Au(111) surfaces grow as thiols rather than asThiolates, and the kinetics of film growth is studied.
Abstract: In this paper, we report the self-assembly, electrical characterization, and surface modification of dithiolated phenylene-ethynylene oligomer monolayers on a Au(111) surface. The self-assembly was accomplished by thiol bonding the molecules from solution to a Au(111) surface. We have confirmed the formation of self-assembled monolayers by scanning tunneling microscopy (STM) and optical ellipsometry, and have studied the kinetics of film growth. We suggest that self-assembled phenylene ethynylene oligomers on Au(111) surfaces grow as thiols rather than as thiolates. Using low-temperature STM, we collected local current-voltage spectra showing negative differential resistance at 6 K.

Journal ArticleDOI
TL;DR: In this paper, the energetic position of the Au(111) surface state is compared before and after adsorbing different rare gas monolayers (Ar, Kr, and Xe).
Abstract: The energetic position of the Au(111) Shockley surface state is compared before and after adsorbing different rare gas monolayers (Ar, Kr, and Xe). We used ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) in combination to get more complete information by using the advantages of both methods. For determining the energetic position and the effective mass of the surface state in UPS an analytic mathematical method is used, which takes the finite angular resolution of the analyzer into account. We performed STS scans for the pure Au(111) surface as well as covered with a monolayer Kr and Xe. For an accurate analysis it is possible to use an extended Kronig-Penney model to take into account the influence of the $23\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ reconstruction. We found that the first monolayer of a rare gas induces shifts of around $50--150\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ increasing with the gas atomic number, whereas a second monolayer has only a small influence of about $3--18\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. Using an image potential model it is possible to characterize these shifts qualitatively. For a semiquantitative analysis the phase accumulation model is applied. Within this model we can describe the experimental data roughly with a Coulomb potential changing in dependence of the electron affinity and the dielectric constant of the rare gas.

Journal ArticleDOI
TL;DR: The surface electronic structure of Ge(001) was studied by scanning tunneling spectroscopy and unequivocally reveal the presence of a metallic state on the (2 x 1) domains, which is absent on the c(4 x 2) domains.
Abstract: The surface electronic structure of Ge(001) was studied by scanning tunneling spectroscopy. The measured surface densities of states unequivocally reveal the presence of a metallic state on the (2×1) domains, which is absent on the c(4×2) domains. This metallic state, so far observed only in integral measurements, is attributed to the flip-flopping dimers that constitute the (2×1) domains. Our data also reveal a set of previously unresolved surface states, in perfect agreement with published theoretical predictions