Showing papers on "Scanning tunneling spectroscopy published in 1995"

Theory of Tunneling Spectroscopy of d-Wave Superconductors.

Abstract: A tunneling theory for a normal metal--insulator-- $d$-wave superconductor junction is presented. In contrast to the $s$-wave superconductor, the tunneling conductance spectra strongly depend on the tunneling direction relative to the crystalline axes, and do not always represent the bulk density of states. Zero-bias conductance peaks are expected in $\mathrm{ab}$-plane tunneling. The present theory systematically explains various experimental results in the tunneling spectroscopy of high- ${T}_{c}$ superconductors.

Controlled manipulation of nanoparticles with an atomic force microscope

Abstract: We report on the application of the atomic force microscope (AFM) to manipulate and position nanometer‐sized particles with nanometer precision. The technique, which can be regarded as a nanometer‐scale analogy to atomic level manipulation with the scanning tunneling microscope, allowed us to form arbitrary nanostructures, under ambient conditions, by controlled manipulation of individual 30 nm GaAs particles. A whole new set of nanodevices can be fabricated particle‐by‐particle for studies of quantum effects and single electron tunneling. We also demonstrate a method, based on the AFM manipulation, to determine the true lateral dimensions of nano‐objects, in spite of the tip‐sample convolution.

Direct vortex lattice imaging and tunneling spectroscopy of flux lines on YBa2Cu3O7- delta.

Abstract: We report the observation of the flux line lattice in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ by scanning tunneling microscopy. The measurements were carried out at 4.2 K and in a magnetic field of 6 T applied along the $c$ axis. The vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77\ifmmode^\circ\else\textdegree\fi{}. We also report local tunneling spectroscopy into a vortex core which reveals two peaks separated by about 11 meV. The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure.

Room-temperature Coulomb blockade from a self-assembled molecular nanostructure.

Abstract: Nanometer-size Au clusters deposited from a cluster beam onto a thin dithiol film were studied at room temperature using an ultrahigh vacuum scanning tunneling microscope (STM). The dithiol molecules tether the deposited Au clusters to the underlying gold substrate and repeatable STM scans of the Au clusters were achieved. Data of the tunneling current as a function of applied voltage yield reproducible evidence for single electron tunneling at room temperature. By fitting the measured I(V) data to a Coulomb blockade model, estimates for the electrical resistance of a single dithiol molecule are also obtained.

A fascinating new field in colloid science: small ligand-stabilized metal clusters and possible application in microelectronics

Abstract: Small metal clusters, like Au55(PPh3)12Cl6, which fall in the size regime of 1–2 nm are colloidal nanoparticles with quantum properties in the transitional range between metals and semiconductors. These chemically tailored quantum dots show regarding the Quantum Size Effect (QSE) a level splitting between 20 and 100 meV, increasing from small particle sizes to the molecular state. The organic ligand shell surrounding the cluster acts like a dielectric “spacer” generating capacitances between neighboring clusters down to 10−18 F. Therefore, charging effects superposed by level spacing effects can be observed. The ligand-stabilized colloidal quantum dots in condensed state can be described as a novel kind of artificial solid with extremely narrow mini or hopping bands depending on the chemically adjustable thickness of the ligand shell and its properties. Since its discovery, the Single Electron Tunneling (SET) effect has been recognized to be the fundamental concept for ultimate miniaturization in microelectronics. The controlled transport of charge carriers in arrangements of ligand-stabilized clusters has been observed already at room temperature through Impedance Spectroscopy (IS) and Scanning Tunneling Spectroscopy (STS). This reveals future directions with new concepts for the realization of simple devices for Single Electron Logic (SEL).

Vacuum tunneling spectroscopy and asymmetric density of states of Bi 2 Sr 2 CaCu 2 O 8 + δ

Abstract: This paper reports very detailed low-temperature vacuum tunneling spectroscopy investigations of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (BSCCO) single crystals. For clean vacuum junctions formed between a cleaved BSCCO single crystal and the normal-metal tip of a scanning tunneling microscope, we obtain stable c-axis vacuum tunneling conditions that allow very reproducible low-temperature electron tunneling spectroscopy. A vacuum junction is identified by tunneling spectra which neither depend on tip/sample spacing nor change as a function of time and position on the sample in the Meissner state. In contrast to the frequently reported linear or parabolic increase with increasing bias voltage, the background conductance of such spectra is largely constant with a slight decrease up to \ifmmode\pm\else\textpm\fi{}300 meV bias. The normal-state conductance inferred from this background has a local maximum at negative sample bias, indicating a pileup below the Fermi level in the (ab)-plane normal-state density of states of BSCCO. Vacuum junctions at 4.8 K show a well developed superconducting gap with large peaks at the gap edges and a finite density of quasiparticle excitations filling the gap. These characteristics are not consistent with an isotropic BCS-like gap parameter. Outside the superconducting gap, the differential conductance curves are very asymmetric, with a striking dip which appears at negative sample bias only. This dip contributes a substantial amount of states to satisfy the conservation of states we find between normal and superconducting BSCCO.

A fascinating new field in colloid science: small ligand-stabilized metal clusters and their possible application in microelectronics: Part II: Future directions

Abstract: Small metal clusters, like Au 55 (PPh 3 ) 12 Cl 6 , which fall in the size regime of 1 - 2 nm are colloidal nanoparticles with quantum properties in the transitional range between metals and semiconductors These chemically tailored quantum dots show by the Quantum Size Effect (QSE) a level splitting between 20 and 100 meV, increasing from small particle sizes to the molecular state The organic ligand shell surrounding the cluster acts like a dielectric «spacer» generating capacitances between neighboring clusters down to 10 -18 F Therefore, charging effects superposed by level spacing effects can be observed The ligand-stabilized colloidal quantum dots in condensed state can be described as a novel kind of artificial solid with extremely narrow mini or hopping bands depending on the chemically adjustable thickness of the ligand shell and its properties Since its discovery, the Single Electron Tunneling (SET) effect has been recognized to be the fundamental concept for ultimate miniaturization in microelectronics The controlled transport of charge carriers in arrangements of ligand-stabilized clusters has been observed already at room temperature through Impedance Spectroscopy (IS) and Scanning Tunneling Spectroscopy (STS) This reveals future directions with new concepts for the realization of simple devices for Single Electron Logic (SEL) Part II presents models and connections between microscopic and macroscopic level, regardless of whether there already exist suitable nanoscale metal cluster compounds, and is aimed at the ultimate properties for a possible application in microelectronics

Shot-Noise Suppression in the Single-Electron Tunneling Regime

Abstract: Electrical current fluctuations through tunnel junctions are studied with a scanning-tunneling microscope. For single-tunnel junctions classical Poisson shot noise is observed, indicative for uncorrelated tunneling of electrons. For double-barrier tunnel junctions, formed by a nanoparticle between tip and surface, the shot noise is observed to be suppressed below the Poisson value. For strongly asymmetric junctions, where a Coulomb staircase is observed in the current-voltage characteristic, the shot-noise suppression is periodic in the applied voltage. This originates from correlations in the transfer of electrons imposed by single-electron charging effects.

Data evaluation technique for electron-tunneling spectroscopy.

Abstract: A systematic study of local-density-of-states (LDOS) deconvolution from tip-surface tunneling spectra is reported. The one-dimensional WKB approximation is used to simulate the process. A technique for DOS deconvolution from the electron-tunneling spectroscopy data is proposed. The differential conductivity normalized to its fit to the tunneling probability function is used as a method of recovering sample DOS. This explicit procedure does not use unconstrained parameters and reveals a better DOS deconvolution in comparison with other techniques. The advantage of this method is its feasibility for extracting two important physical parameters from experimental tunneling spectra: (i) local surface potential, and (ii) tip-sample distance. These values are the parameters used in the proposed fitting procedure. The local surface potential and the tip-sample distance retrieval are demonstrated by means of numerical simulations. Comparative scanning tunneling spectroscopy is proposed as an approach to eliminate the influence of the tip condition on the surface LDOS recovery. \textcopyright{} 1996 The American Physical Society.

Electronic structure, total energies, and STM images of clean and oxygen-covered Al(111).

Abstract: A set of density-functional calculations for clean and O-covered Al(111) are presented. At low O coverages the potential energy surface (PES) of chemisorbed O is investigated. The PES indicates large barriers (0.8 eV) against O diffusion and a large corrugation of the equilibrium O height over the Al(111) while only a moderate energy gain (5 eV per atom) is found upon ${\mathrm{O}}_{2}$ dissociation over the surface. The possible existence of ``hot'' O adatoms after ${\mathrm{O}}_{2}$ dissociation is discussed on the basis of the presented PES and existing dynamical simulations on model potentials. At high O coverages an attractive O-O interaction is identified together with an enhancement in the dipole moment induced per O atom. Finally, Tersoff-Hamann-type scanning tunneling microscopy (STM) topographs are derived based on the calculated one-electron wave functions and spectra. For the clean Al(111) a theoretical STM height corrugation compatible with the experimentally observed one is obtained if the tunneling conductance is assumed dominated by contributions from orbitals of atomic p character centered on the tip. For the O-covered Al(111) the theoretical topographs agree well with the observed ones.

Atomic resolution in photon emission induced by a scanning tunneling microscope.

Abstract: A low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) is used to excite photon emission from Au(110) surfaces. In the detected photon intensity the (1 \ifmmode\times\else\texttimes\fi{} 2) reconstruction of the Au surface is clearly resolved. This first observation of atomic resolution in STM-induced photon emission is interpreted in terms of local variations of the electromagnetic interaction of tip and sample occurring at constant tunneling current. Similar effects are expected to affect other scanning probe methods, in particular those involving photons.

Effects of vibrational excitation on multidimensional tunneling: General study and proton tunneling in tropolone

Abstract: Tunneling energy splittings of vibrationally excited states are calculated quantum mechanically using several models of two‐dimensional symmetric double well potentials. Various effects of vibrational excitation on tunneling are found to appear, depending on the topography of potential energy surface; the symmetry of the mode coupling plays an essential role. Especially, oscillation of tunneling splitting with respect to vibrational quantum number can occur and is interpreted by a clear physical picture based on the semiclassical theory formulated recently [Takada and Nakamura, J. Chem. Phys. 100, 98 (1994)]. The mixed tunneling in the C region found there allows the wave functions to have nodal lines in classically inaccessible region and can cause the suppression of the tunneling. The above analysis is followed by the interpretation of recent experiments of proton tunneling in tropolone. Ab initio molecular orbital calculations are carried out for the electronically ground state. A simple three‐dimensio...

Reconstruction of the GaAs (311)A surface.

Abstract: We study the reconstruction of the GaAs (311)A surface with reflection high-energy electron-diffraction (RHEED) and scanning tunneling microscopy (STM). The surface observed in situ with RHEED during molecular-beam epitaxy is distinguished by a lateral periodicity of 3.2 nm perpendicular to the [2\ifmmode\bar\else\textasciimacron\fi{}33] direction. This periodicity is confirmed employing in situ STM. High-resolution STM images furthermore reveal a surface reconstruction characterized by a dimerization of the surface As atoms. We show how the reconstruction can be formed, applying a simple electron-counting model. The excellent agreement with the experimental results further support this model, which was already found to explain the reconstructions of the (100) and the (111) surfaces. In order to form a semiconducting surface, the three uppermost layers are involved in the reconstruction process, giving rise to a depth modulation of 0.34 nm.

Identification of metals in scanning tunneling microscopy via image states.

Abstract: An oscillatory reversal of the contrast between Cu and Mo is observed with scanning tunneling microscopy (STM), using sample bias voltages of $+$5 V and higher. It is attributed to tunneling via a series of discrete states that are induced by a combination of the image potential and the applied field. They are offset in energy due to the different work functions of Cu and Mo. This effect provides a generally applicable mechanism for elemental contrast in STM.

Scanning Tunneling Microscopy, Tunneling Spectroscopy, and Photoelectrochemistry of a Film of Q-CdS Particles Incorporated in a Self-Assembled Monolayer on a Gold Surface

Abstract: Films of AOT-capped (AOT = dioctyl sulfosuccinate) cadmium sulfide nanoparticles (Q-CdS) prepared by incorporation into a self-assembled monolayer (SAM) of hexanethiol on Au were prepared. These were imaged at negative substrate bias in air by scanning tunneling microscopy, and Q-CdS particles were shown to cover most of the area on the SAM. Continuous scanning of the tip over the substrate removed Q-CdS particles to the outside of the scanning area because of tip-substrate interactions. Scanning tunneling spectroscopy was carried out in air with the tip held over the thin CdS film (100 nm) and over individual Q-CdS particles in the layer. The i vs V and dildV vs V curves indicated that the energy band gap of the Q-CdS particles is wider than that of the thin CdS films. The results of photoelectrochemistry also indicated that the onset photopotential is more negative for smaller Q-CdS particles and is related to the level of conduction band.

Photon‐assisted tunneling in a resonant tunneling diode: Stimulated emission and absorption in the THz range

Abstract: The dc I–V characteristic of a triple‐barrier resonant tunneling diode (RTD) integrated in a bowtie antenna and driven by THz radiation displays up to five additional resonant tunneling channels. These channels appear as additional peaks in the I–V characteristic whose voltage positions vary linearly with frequency in the investigated range between ν=1.0 and 3.4 THz. We attribute these peaks to photon‐assisted tunneling processes corresponding to absorption and stimulated emission of up to three photons. The experiments suggest that such a device can be utilized to detect and generate THz radiation.

Spatially varying energy gap in the CuO chains of YBa2Cu3O7-x detected by scanning tunneling spectroscopy.

Abstract: Current-imaging tunneling spectroscopy (CITS) was performed on cold-cleaved single crystals of YBa2Cu307, at 20 K. CITS data include 1(V) curves taken simultaneously with a topographic scanning tunneling microscopic image. 1(V) curves taken on CuO chains show an energy gap of about 20 meV which disappears near oxygen vacancies. We explain several features of large-junction 1(V) measurements, photoemission spectroscopy, and single-point 1(V) spectroscopy in terms of local effects detected by our CITS measurements. Finally, we consider the possibilities that this energy gap is due to either a charge-density wave or proximity-coupled superconductivity from the CuOz planes.

Differentiating Functional Groups with the Scanning Tunneling Microscope

Abstract: Scanning tunneling microscopy (STM) images of l-docosanol, 1-docosanethiol, didocosyl disulfide, and 1-chlorooctadecane on graphite are compared. The images of the I -docosanethiol and didocosyl disulfide show bright spots which are attributed to the positions of the S-H and S-S functional groups. The STM images of the l-docosanol and l-chlorooctadecane do not show such bright spots. Thb fact that both the S-H and S-S groups appear bright in the STM images indicates that the presence of an S atom on graphite results in a higher tunneling current when the tip scans over it compared to the tunneling current over a C, O. or Cl atom. The diff'erent behavior of the S atoms conrpiired to the O. C. and Cl atoms is discussed in terrns of the interactions between these atoms and the underlving graphite substrate. The persistent brightness of S atoms in the images of molecular adsorbates sugsests that sult'ur nrav serve as a useful "chromophore" for molecules imased bv STM.

Structure and electronic states on reduced SrTiO3(110) surface observed by scanning tunneling microscopy and spectroscopy

Abstract: Structure and electronic states on the (100) surface of reduced BaTiO3 single crystals were observed by scanning tunneling microscopy and spectroscopy. After annealing at 1150 °C, the direction of the edges ordered to run along the equivalents of [011] and [001] directions. All the observed step edges were of unit‐cell height. On each terrace atomlike corrugation with the periodicity multiples of the √5×√5 R26.6° superlattice was observed in the filled state images. A straightforward method to deduce the local density of states from the scanning tunneling spectroscopy data was introduced, based on multiplication of an inverted triangular transfer matrix, and was applied to the scanning tunneling spectroscopy data on the terraces. The amplitude of the local density of states at EF−1 eV varied in phase with the atomlike corrugation on the terrace, which was consequently ascribed to electronic states at the spatially ordered Ti 3d–oxygen vacancy complexes on the surface.

Tunneling potential barrier dependence of electron spin polarization.

Abstract: Scanning tunneling microscopy experiments reveal that the degree of spin polarization of electrons tunneling from Ni into a semiconductor increases with decreasing potential barrier thickness. The results show that the highly polarized $3d$ bands as well as the low-polarized $4sp$ bands contribute to the tunneling current and that the ratio of their tunneling probabilities depends on the potential barrier thickness and height. Furthermore, the tunneling potential barrier for the $3d$-like levels is estimated to be $\ensuremath{\sim}$1 eV higher than for the $4sp$ contribution.

Resonant tunneling bands and electrochemical reduction potentials

Abstract: The recent observations of resonant tunneling through unoccupied orbitals of molecular species imbedded in Metal-Insulator-Metal tunnel junctions are discussed in terms of transient reduction of the molecular species. Electrochemical reduction potentials for the solution phase molecular systems are compared to the orbital mediated tunneling spectroscopy (OMTS) data and a strong correlation is observed. A simple model is proposed that accounts for this correlation. This model also explains previous observations of spontaneous and permanent reduction of certain compounds in the tunnel junction environment. The model should be equally applicable to resonant tunneling and to spontaneous reduction observed in the scanning tunneling microscope. Based on the view presented here, orbital mediated tunneling spectroscopy is a new method for providing reduction potentials and affinity levels in adlayers and thin solid films.

Surface modification mechanism of materials with scanning tunneling microscope

Abstract: The surface modification mechanism with scanning tunneling microscope (STM) is investigated. Experiments in both ultrahigh vacuum and air are reported, using several kinds of materials to understand the mechanism systematically. Threshold voltages (Vt’s), which are defined as the voltages above which modification is possible under the STM tip, have linear dependence on the binding energies of the materials. Thus, the STM surface modification mechanism is attributed to the local sublimation induced by tunneling electrons. For the modification in air, it is also ascribed to the chemical reaction induced by tunneling electrons with adsorbed water, and the Vt’s also fit on this line by taking the reaction energy into consideration. Therefore, the process is a direct consequence of the high flux of low‐energy electrons incident on the surface from the STM tip.