Showing papers on "Scanning tunneling spectroscopy published in 2008"
TL;DR: Real space measurements by scanning tunneling spectroscopy reveal the existence of electron pockets at the higher parts of the ripples, as predicted by a simple theoretical model.
Abstract: We grow epitaxial graphene monolayers on Ru(0001) that cover uniformly the substrate over lateral distances larger than several microns. The weakly coupled graphene monolayer is periodically rippled and it shows charge inhomogeneities in the charge distribution. Real space measurements by scanning tunneling spectroscopy reveal the existence of electron pockets at the higher parts of the ripples, as predicted by a simple theoretical model. We also visualize the geometric and electronic structure of edges of graphene nanoislands.
599 citations
TL;DR: In this paper, the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS) was studied.
Abstract: Epitaxial growth of graphene on SiC surfaces by solid state graphitization is a promising route for future development of graphene based electronics. In the present work, we have studied the morphology, atomic scale structure, and electronic structure of thin films of few-layer graphene (FLG) on SiC(0001) by scanning tunneling microscopy and spectroscopy (STS). We show that a quantitative evaluation of the roughness induced by the interface is a tool for determining the layer thickness of FLG. We present and interpret thickness dependent tunneling spectra, which can serve as an additional fingerprint for the determination of the layer thickness. By performing spatially resolved STS, we find evidence that the charge distribution in bilayer graphene is inhomogeneous.
349 citations
TL;DR: By spatially mapping the spin-flipping channels with submolecular precision, this work is able to explicitly identify the specific molecular orbitals that mediate the superexchange interaction between molecules.
Abstract: The superexchange mechanism in cobalt phthalocyanine (CoPc) thin films was studied by a low temperature scanning tunneling microscope. The CoPc molecules were found to form one-dimensional antiferromagnetic chains in the film. Collective spin excitations in individual molecular chains were measured with spin-flip associated inelastic electron tunneling spectroscopy. By spatially mapping the spin-flipping channels with submolecular precision, we are able to explicitly identify the specific molecular orbitals that mediate the superexchange interaction between molecules.
238 citations
TL;DR: The combination of nanographenes and STM/STS, with the PAHs self-assembled in oriented molecular mono- or bilayers at the interface between an organic solution and the basal plane of graphite and contacted by the STM tip, is a simple, reliable, and versatile system for developing the fundamental concepts of molecular electronics.
Abstract: Single-molecule electronics, that is, realizing novel electronic functionalities from single (or very few) molecules, holds promise for application in various technologies, including signal processing and sensing. Nanographenes, which are extended polycyclic aromatic hydrocarbons (PAHs), are highly attractive subjects for studies of single-molecule electronics because the electronic properties of their flat conjugated systems can be varied dramatically through synthetic modification of their sizes and topologies. Single nanographenes provide high tunneling currents when adsorbed flat onto conducting substrates, such as graphite. Because of their chemical inertness, nanographenes interact only weakly with these substrates, thereby preventing the need for special epitaxial structure matching. Instead, self-assembly at the interface between a conducting solid, such as the basal plane of graphite, and a nanographene solution generally leads to highly ordered monolayers. Scanning tunneling spectroscopy (STS) a...
233 citations
TL;DR: Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads.
Abstract: Low-temperature spin-polarized scanning tunneling microscopy is employed to study spin transport across single cobalt-phthalocyanine molecules adsorbed on well-characterized magnetic nanoleads. A spin-polarized electronic resonance is identified over the center of the molecule and exploited to spatially resolve stationary spin states. These states reflect two molecular spin orientations and, as established by density functional calculations, originate from a ferromagnetic molecule-lead exchange interaction.
199 citations
TL;DR: Calculations show that the inherent spin magnetic moment of the molecule is only weakly affected by the interaction with the surface and suggest that the SMM character might be preserved.
Abstract: The electronic structure of isolated bis(phthalocyaninato) terbium(III) molecules, a novel single-molecular-magnet (SMM), supported on the Cu(111) surface has been characterized by density functional theory and scanning tunneling spectroscopy. These studies reveal that the interaction with the metal surface preserves both the molecular structure and the large spin magnetic moment of the metal center. The 4f electron states are not perturbed by the adsorption while a strong molecular/metal interaction can induce the suppression of the minor spin contribution delocalized over the molecular ligands. The calculations show that the inherent spin magnetic moment of the molecule is only weakly affected by the interaction with the surface and suggest that the SMM character might be preserved.
189 citations
TL;DR: It is demonstrated that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures and the generality of the approach is demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.
Abstract: The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.
188 citations
TL;DR: 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.
Abstract: 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.
175 citations
TL;DR: Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect.
Abstract: We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.
166 citations
TL;DR: The Coulomb potential of a single donor at the semiconductor-vacuum interface is mapped by utilizing the controlled manipulation of individually addressable impurities in semiconductor material.
Abstract: The charge state of individually addressable impurities in semiconductor material was manipulated with a scanning tunneling microscope. The manipulation was fully controlled by the position of the tip and the voltage applied between tip and sample. The experiments were performed at low temperature on the {110} surface of silicon doped GaAs. Silicon donors up to 1 nm below the surface can be reversibly switched between their neutral and ionized state by the local potential induced by the tip. By using ultrasharp tips, the switching process occurs close enough to the impurity to be observed as a sharp circular feature surrounding the donor. By utilizing the controlled manipulation, we were able to map the Coulomb potential of a single donor at the semiconductor-vacuum interface.
158 citations
TL;DR: It is illustrated that the magnitude of the tunneling enhancement depends on the initial redox state of HS6V6 (V(2+) or V(+*)), and a clear discrimination between the redox-mediated enhanced and the off-resonance tunneling currents I(enh) respective I(T).
Abstract: We report on the construction of an asymmetric tunneling junction between a Au STM tip and a Au(111)-(1 x 1) substrate electrode modified with the redox-active molecule N-hexyl-N'-(6-thiohexyl)-4,4'-bipyridinium bromide (HS6V6) in an electrochemical environment. The experiments focused on the reversible one-electron transfer reaction between the viologen dication V(2+) and the radical cation V(+*). Employing the concept of "electrolyte gating" we demonstrate transistor- and diodelike behavior based on in situ scanning tunneling spectroscopy at constant or variable bias voltages. We derived criteria and verified that the experimental data could be represented quantitatively by a model assuming a two-step electron transfer with partial vibrational relaxation. The analysis illustrates that the magnitude of the tunneling enhancement depends on the initial redox state of HS6V6 (V(2+) or V(+*)). Characteristic parameters, such as reorganization energy, potential drop, and overpotential across the tunneling gap were estimated and discussed. We present a clear discrimination between the redox-mediated enhanced and the off-resonance tunneling currents I(enh) respective I(T) and distinguish between electron transfer in symmetric and asymmetric Au | redox-molecule | Au configurations.
TL;DR: A break junction method using a scanning tunneling microscope has been applied to electrical conductance measurement of newly designed oligothiophene molecules terminated with a thiocyanate group, showing tunneling conduction was evident from an exponential decay of the conductance as a function of the molecular length.
Abstract: A break junction method using a scanning tunneling microscope has been applied to electrical conductance measurement of newly designed oligothiophene molecules terminated with a thiocyanate group. The tunneling conduction was evident from an exponential decay of the conductance as a function of the molecular length up to ca. 6 nm. The tunneling decay constant was estimated to be 0.1 A (-1). The pre-exponential factor was 1.3 x 10 (-6) S, which was smaller than that observed for alkanedithiols.
TL;DR: Transport simulations explain heating and heat dissipation of a single C(60) molecule in the junction of a scanning tunneling microscope by measuring the electron current required to thermally decompose the fullerene cage.
Abstract: We study heating and heat dissipation of a single \c60 molecule in the junction of a scanning tunneling microscope (STM) by measuring the electron current required to thermally decompose the fullerene cage. The power for decomposition varies with electron energy and reflects the molecular resonance structure. When the STM tip contacts the fullerene the molecule can sustain much larger currents. Transport simulations explain these effects by molecular heating due to resonant electron-phonon coupling and molecular cooling by vibrational decay into the tip upon contact formation.
TL;DR: In this article, the electronic properties of single molecules by means of low-temperature scanning tunneling spectroscopy (STS) were investigated using C60 molecules deposited on a Au(111) surface at different substrate temperatures and mixed with two different hydrocarbons.
Abstract: In this paper we investigate the electronic properties of single molecules by means of low-temperature scanning tunneling spectroscopy (STS) We focus on C60 molecules deposited on a Au(111) surface at different substrate temperatures and mixed with two different hydrocarbons In this way we change the fullerene interaction with the surface and/or the dipolar response of the molecular neighborhood to charging events We explore the dependence of the energy level alignment on the molecular surroundings The results confirm an already established picture in photoelectron spectroscopy
Journal Article•
TL;DR: This work shows how individual superparamagnetic iron nanoislands with typical sizes of 100 atoms can be addressed and locally switched using a magnetic scanning probe tip, thus demonstrating current-induced magnetization reversal across a vacuum barrier combined with the ultimate resolution of spin-polarized scanning tunneling microscopy.
TL;DR: The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope, suggesting that quantum tunneling is involved in the process.
Abstract: The dynamics of water dimers was investigated at the single-molecule level by using a scanning tunneling microscope. The two molecules in a water dimer, bound on a Cu(110) surface at 6 K, were observed to exchange their roles as hydrogen-bond donor and acceptor via hydrogen-bond rearrangement. The interchange rate is $\ensuremath{\sim}60$ times higher for $({\mathrm{H}}_{2}\mathrm{O}{)}_{2}$ than for $({\mathrm{D}}_{2}\mathrm{O}{)}_{2}$, suggesting that quantum tunneling is involved in the process. The interchange rate is enhanced upon excitation of the intermolecular mode that correlates with the reaction coordinate.
TL;DR: A combination of low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure observations reveals how the metal substrate induces a conformational adaptation into a distorted saddle-shaped geometry.
Abstract: We present a molecular-level study of the geometric and electronic properties of Co(II) tetraphenylporphyrin molecules adsorbed on the Cu(111) surface. A combination of low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure observations reveals how the metal substrate induces a conformational adaptation into a distorted saddle-shaped geometry. By scanning tunneling spectroscopy we identified the discrete energy levels of the molecule and mapped their spatial electron-density distributions. These results, along with a simple theoretical description, provide a direct correlation between the shape of frontier molecular orbitals and intramolecular structural features.
TL;DR: Inelastic Electron Tunneling Spectroscopy (IETS) has re-emerged as a premier analytical tool in the understanding of nanoscale and molecular junctions.
TL;DR: The modulation of the adsorption properties within the surface Moiré cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.
Abstract: We present a combined experimental (STM/scanning tunneling spectroscopy) and theoretical (density functional theory) study on the deposition of Au and Pd metal atoms on FeO/Pt(111) ultrathin films. We show that while the Pd atoms are only slightly oxidized, the Au atoms form positive ions upon deposition, at variance to a charge transfer into the Au atoms as observed for MgO/Ag(100). The modulation of the adsorption properties within the surface Moire cell and the charging induce the formation a self-assembled array of gold adatoms on FeO/Pt(111), whereas Pd atoms are randomly distributed.
TL;DR: In this article, a theory for interband tunneling in semiconducting carbon nanotube and graphene nanoribbon p−n junction diodes was developed for 1D carbon structures.
Abstract: A theory is developed for interband tunneling in semiconducting carbon nanotube and graphene nanoribbon p−n junction diodes. Characteristic length and energy scales that dictate the tunneling probabilities and currents are evaluated. By comparing the Zener tunneling processes in these structures to traditional Group IV and III–V semiconductors, it is proved that for identical bandgaps, carbon-based one-dimensional (1D) structures have higher tunneling currents. The high tunneling current magnitudes for 1D carbon structures suggest the distinct feasibility of high-performance tunneling-based field-effect transistors.
TL;DR: In this article, a vibrationally resolved intramolecular photon emission from a single molecule was induced by tunneling electrons in a scanning tunneling microscope (STM), and the resonant excitation of the emission was observed by tuning the bias voltage between the STM tip and the sample.
Abstract: A vibrationally resolved intramolecular photon emission from a single molecule was induced by tunneling electrons in a scanning tunneling microscope (STM). The resonant excitation of the emission was observed by tuning the bias voltage between the STM tip and the sample. The emission intensity of an individual vibronic peak goes through a maximum. The intrinsic linewidth ($\ensuremath{\sim}3.0\text{ }\text{meV}$, full width at half maximum) obtained from the emission spectra and the measured quantum yield ($\ensuremath{\le}1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{photon}$ per tunneling electron) reveal the effects of different processes on the intramolecular photon emission. The comparisons between STM-induced photon emission and other techniques were made.
TL;DR: Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy.
Abstract: Superconductivity of nanosized Pb-island structures whose radius is 0.8 to 2.5 times their coherence length was studied under magnetic fields using low-temperature scanning tunneling microscopy and spectroscopy. Spatial profiles of superconductivity were obtained by conductance measurements at zero-bias voltage. Critical magnetic fields for vortex penetration and expulsion and for superconductivity breaking were measured for each island. The critical fields depending on the lateral size of the islands and existence of the minimum lateral size for vortex formation were observed.
TL;DR: A combination of charge-transfer and lattice-matching properties for TCNE across substrates results in a strong variation of molecule-molecule and molecule-substrate interactions, which has significant implications for future organic/inorganic nanoscopic devices incorporating molecule-based magnetism.
Abstract: We have studied the structural and electronic properties of tetracyanoethylene (TCNE) molecules on different noble-metal surfaces using scanning tunneling spectroscopy and density functional theory. Striking differences are observed in the TCNE behavior on Au, Ag, and Cu substrates in the submonolayer limit. We explain our findings by a combination of charge-transfer and lattice-matching properties for TCNE across substrates that results in a strong variation of molecule-molecule and molecule-substrate interactions. These results have significant implications for future organic/inorganic nanoscopic devices incorporating molecule-based magnetism.
TL;DR: Zhang et al. as mentioned in this paper studied the real space properties of the wave functions involved in the tunneling process by means of ab initio theory and presented a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasifree electron states at the Brillouin zone center.
Abstract: Recent scanning tunneling spectroscopy experiments on graphene reported an unexpected gap of about $\ifmmode\pm\else\textpm\fi{}60\text{ }\text{ }\mathrm{meV}$ around the Fermi level [V. W. Brar et al., Appl. Phys. Lett. 91, 122102 (2007); Y. Zhang et al., Nature Phys. 4, 627 (2008)]. Here we give a theoretical investigation explaining the experimentally observed spectra and confirming the phonon-mediated tunneling as the reason for the gap: We study the real space properties of the wave functions involved in the tunneling process by means of ab initio theory and present a model for the electron-phonon interaction, which couples the graphene's Dirac electrons with quasifree-electron states at the Brillouin zone center. The self-energy associated with this electron-phonon interaction is calculated, and its effects on tunneling into graphene are discussed. Good agreement of the tunneling density of states within our model and the experimental $dI/dU$ spectra is found.
TL;DR: In this article, a novel tetraphenyl-porphyrin (H2−TPP) species is formed that can be clearly identified in scanning tunneling microscopy data resolving intramolecular features.
Abstract: By exposing tetraphenyl-porphyrin (H2−TPP) molecules anchored on a Ag(111) surface to a beam of cerium atoms under vacuum conditions, a selective interaction of the lanthanide with the porphyrin macrocycle is achieved. A novel Ce−TPP species is formed that can be clearly identified in scanning tunneling microscopy data resolving intramolecular features. The electronic structure of the Ce−TPP was characterized by scanning tunneling spectroscopy, identifying molecular resonances that are related to those of coadsorbed Co−TPP. The inhomogenous local electronic density distribution of the dominating Ce−TPP and Co−TPP occupied states was visualized by tunneling spectroscopy mapping. With complementary density functional theory calculations, the geometry and energetics of the Ce-porphyrin bonding was determined.
TL;DR: In this article, a combination of scanning tunneling microscopy, noncontact atomic force microscopy and theory is applied to a variety of oxide systems including Al2O3, NiO, ferroelectric BaTiO3 and tungstates and molybdates.
Abstract: The intersection of two fields, oxide surface science and scanning probe microscopy (SPM), has yielded considerable insight on atomic processes at surfaces. Oxide surfaces, especially those containing transition metals, offer a rich variety of structures and localized physical phenomena that are exploited in a wide range of applications. Nonlinear optics, superconductivity, ferroelectricity and chemical catalytic activity are but a few. Furthermore, the challenges and solutions associated with the chemistry of these surfaces and particularly the solutions to these problems have led to important understanding of tip–surface interactions that can inform SPM studies of all materials. Here, the development of understanding of the model systems TiO2 and SrTiO3 are considered in detail, to demonstrate the role of nonstoichiometry in surface structure evolution and the approach to interpreting structure at the atomic level. Then a combination of scanning tunneling microscopy, noncontact atomic force microscopy and theory are applied to a variety of oxide systems including Al2O3, NiO, ferroelectric BaTiO3, tungstates and molybdates. Recently developed sophisticated probes of local properties include spin-polarized tunneling, Fourier mapping of charge density waves, band gap mapping of superconductors and ultra fast imaging of atomic diffusion. The impact of these studies on our understanding of the behavior of oxides and of tip–surface interactions is summarized.
TL;DR: Unique electronic properties of self-organized Au atom chains on Ge(001) in novel c(8 x 2) long-range order are revealed by scanning tunneling microscopy, which represents an outstandingly close approach to a one-dimensional electron liquid.
Abstract: Unique electronic properties of self-organized Au atom chains on Ge(001) in novel c(8 x 2) long-range order are revealed by scanning tunneling microscopy. Along the nanowires an exceptionally narrow conduction path exists which is virtually decoupled from the substrate. It is laterally confined to the ultimate limit of single atom dimension, and is strictly separated from its neighbors, as not previously reported. The resulting tunneling conductivity shows a dramatic inhomogeneity of 2 orders of magnitude. The atom chains thus represent an outstandingly close approach to a one-dimensional electron liquid.
TL;DR: The interaction between single apex atoms in a metallic contact, using the break junction geometry, is studied, and the interaction between the two apex atoms on both electrodes, observed as a change of slope in the tunneling regime, is accounted for.
Abstract: We study the interaction between single apex atoms in a metallic contact, using the break junction geometry. By carefully training our samples, we create stable junctions in which no further atomic reorganization takes place. This allows us to study the relation between the so-called jump out of contact (from contact to tunneling regime) and jump to contact (from tunneling to contact regime) in detail. Our data can be fully understood within a relatively simple elastic model, where the elasticity k of the electrodes is the only free parameter. We find 5
TL;DR: High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C -H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopic data.
Abstract: We report inelastic electron tunneling spectroscopy (IETS) of a ${\mathrm{C}}_{8}$ alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.
TL;DR: Basic STM operation and image interpretation, techniques developed to manipulate single atoms and molecule with the STM to measure functional properties of surfaces, local spectroscopies used to characterize atoms and molecules at the single-molecule level, and surface perturbations affecting surface coverage and surface reactions are reviewed.
Abstract: Invented as a surface analytical technique capable of imaging individual atoms and molecules in real space, scanning tunneling microscopy (STM) has developed and advanced into a technique able to measure a variety of structural, functional, and spectroscopic properties and relationships at the single-molecule level. Here, we review basic STM operation and image interpretation, techniques developed to manipulate single atoms and molecules with the STM to measure functional properties of surfaces, local spectroscopies used to characterize atoms and molecules at the single-molecule level, and surface perturbations affecting surface coverage and surface reactions. Each section focuses on determining the identity and function of chemical species so as to elucidate information beyond topography with STM.