Current-distance-voltage characteristics of electron tunneling through an electrochemical STM junction

We have studied the electron tunneling process through an electrochemical scanning tunneling microscopic (STM) junction formed by a gold tip and a gold electrode immersed in an inert NaClO₄ solution. Current-distance-voltage characteristics of the tunneling process are examined by simultaneous measurement of tunneling current, voltage, and distance. The results indicate that the tunneling voltage across the junction changes with tunneling distance; however, tunneling conductance is an inverse exponential function of distance over the entire investigated range of tunneling current, voltage, and distance. The results provide clear evidence for the validity of a one-dimensional tunneling model for the aqueous tunneling process. Implications of the observation are mentioned with regard to the distance-dependent STM imaging and the origin of a low tunneling barrier height.     

Simultaneous observation of the triangular and honeycomb structures on highly oriented pyrolytic graphite at room temperature: An STM study

Scanning tunneling microscopy (STM) was used to study the surface structures of dry-prepared and di-chloroethane-treated HOPG samples. Both triangular and honeycomb structures were simultaneously observed with the same tip at room temperature around a strand (grain boundary) on the HOPG surface. This observation did not support the tip effect in STM imaging explanation for HOPG in literature. A general layer-sliding model was utilized to explain the experimental results: sliding of the HOPG topmost layer was used to explain the origins of the triangular and honeycomb structures, and molecule intercalation into inter-layer spacing between the first and second layers of HOPG induced inhomogeneous deformation of the HOPG topmost layer that accordingly generated the Moiré patterns of the HOPG sample in di-chloroethane.     

Local tunneling barrier height observations of NiAl(1 1 0)

Local work functions at individual atoms on a compound metal surface have been experimentally examined. The simultaneously obtained STM and local tunneling barrier height (LBH) images of NiAl(1 1 0) show that LBH at geometrically lower Ni sites is much higher than that at geometrically higher Al sites, indicating that each individual atom on the NiAl(1 1 0) surface has a specific potential barrier for relevant electrons.     

Electronic states on the surface of graphite

Graphite consists of graphene layers in an AB (Bernal) stacking arrangement. The introduction of defects can reduce the coupling between the top graphene layers and the bulk crystal producing new electronic states that reflect the degree of coupling. We employ low temperature high magnetic field scanning tunneling microscopy (STM) and spectroscopy (STS) to access these states and study their evolution with the degree of coupling. STS in magnetic field directly probes the dimensionality of electronic states. Thus two-dimensional states produce a discrete series of Landau levels while three-dimensional states form Landau bands providing a clear distinction between completely decoupled top layers and ones that are coupled to the substrate. We show that the completely decoupled layers are characterized by a single sequence of Landau levels with square-root dependence on field and level index indicative of massless Dirac fermions. In contrast weakly coupled bilayers produce special sequences reflecting the degree of coupling, and multilayers produce sequences reflecting the coexistence of massless and massive Dirac fermions. In addition we show that the graphite surface is soft and that an STM tip can be quite invasive when brought too close to the surface and that there is a characteristic tip-sample distance beyond which the effect of sample-tip interaction is negligible.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

A direct observation on the surface assembling and ordering of coumarin derivatives on the graphite surface

The 2D monolayer structures of two coumarin derivatives, 4-heptadecyl-7-hydroxycoumarin (HHC) 1 and 3-(2-benzo thiazolyl)-7-octadecyloxy-coumarin (BOC) 2, have been studied by STM and different sensitivity to thermal annealing for these two systems has been observed. The results show that the arrangements of these two molecules are obviously different in the self-assembled monolayer at 20 °C on the graphite surface. Furthermore, only BOC is sensitive to thermal annealing. The assembly of BOC becomes more stable and ordered after annealing at 65 °C for 1 h.     

Effect of bias voltage and electrodes size on the Pd conductance histograms

Conductance histograms of palladium nanocontacts in ultra high vacuum (UHV) were experimentally studied at room temperature using scanning tunneling microscope (STM). Our results show that the resolution of the pure Pd peaks, at 1.8 and around 3G₀, in the histograms, depends on the bias voltage and the electrodes size. The size of the electrodes should be as small as possible to achieve the higher extraction of hydrogen from them and therefore preventing the diffusion of H from the bulk to the nanocontacts during the conductance measurements, particularly at low bias voltage. This could explain why peaks have not been observed previously in the Pd histograms, using STM techniques in UHV.     

Morphology and electronic structure of bcc Co(1 1 0) and fcc/hcp Co(1 1 1) on Fe(1 1 0) investigated by STM and STS

We report on the growth of ultrathin epitaxial Co films on Fe(1 1 0) examined by scanning tunneling microscopy and spectroscopy (STM and STS). At room temperature Co forms pseudomorphic, ideally ordered body-centered cubic (bcc) layers for the first two monolayers as confirmed by atomically resolved STM images. This is in contrast to the related case of Co/Cr(1 1 0) where a superstructure occurs in the second layer. The third monolayer forms a close-packed structure and causes a transformation of the buried second monolayer into a close-packed structure. The Fe(1 1 0) substrate strongly influences the electronic structure of the first Co monolayer as concluded from the dI/dU spectra. This influence is less important for the second monolayer. The measured local density-of-states function for the bcc Co double layer is in agreement with theoretical predictions for bcc Co.     

STM/STS characterization of platinum silicide nanostructures grown on a Pt(1 1 1) surface

Formation of the platinum silicides nanostructures and their electronic properties have been studied using scanning tunneling microscopy and scanning tunneling spectroscopy. The investigated structures have been grown by solid state epitaxy upon deposition of the Si atoms (coverage about 0.2 ML) and sequential annealing at temperature range 600-1170 K. The formation of the Pt₂Si and PtSi islands was investigated until the Si atoms embedded into the Pt substrate at the 1170 K. The images of the silicides structures and Pt substrates with atomic resolution have been recorded. The evolution of the spectroscopic curves both for substrates and nanostructures, corresponding to the structural and sizes changes, have been shown.     

Chemisorption of atomic oxygen on Pd(1 1 1) studied by STM

Atomic oxygen resulting from the dissociation of O₂ on Pd(1 1 1) at low coverage was studied in a variable temperature scanning tunneling microscope (STM) in the range from 30 to 210 K. Oxygen atoms, which typically appear as 30-40 pm deep depressions on Pd(1 1 1), occupy fcc hollow sites and form ordered p(2 × 2) islands upon annealing above 180 K. The mobility of the atoms diminishes rapidly below 180 K, with an approximate diffusion barrier of 0.4-0.5 eV. Oxygen atom pairs produced by thermal dissociation of O₂ at 160 K occupy both fcc and hcp hollow sites. The atoms travel approximately 0.25 nm after dissociation, and the distribution of pairs is strongly influenced by the presence of subsurface impurities within the Pd sample. At much lower temperatures, the STM tip can dissociate oxygen molecules. Dissociation occurs at sample bias voltages exceeding approximately 0.1 V. Following tip-induced dissociation, the product atoms occupy only fcc hollow sites. Oxygen atoms can be manipulated via short range repulsive interactions with the STM tip.     

Electronic superstructures on the graphite surface studied by first-principles calculations

We present results of a first-principles study of the graphite surface in the presence of defects. Our calculations, based on density functional theory, show superstructures of periodicity ( ) in the electronic structure of the surface. In good agreement with STM experiments, these superstructures show a variety of patterns with their intensity decaying away from the defect. Two kind of defects were considered: metallic adatoms absorbed on graphite, and vacancies in the surface lattice. Similar results were found in both cases. Our results give strong support to the idea that these superstructures are due to purely electronic effects, and do not correspond to any atomic reconstruction of the graphite surface.     

Observation of double- to single-domain transition on the Eu/Si(1 0 0) surface by LEED and STM

The orientational phase diagram and morphology of the Eu-adsorbed Si(1 0 0) surface miscut by 0.4° have been studied by low-energy electron diffraction and scanning tunneling microscopy. We demonstrate that the original double-domain configuration with single-layer steps on the Si(1 0 0) substrate can be drastically broken at 0.4 monolayer (ML) of Eu. At this coverage, the ordered domain pattern formed by topographically non-equivalent terraces with Eu-induced 2 × 3 and “2 × 1” (so-called “wavy” structure) reconstructions is found, while no orthogonal 3 × 2 and “1 × 2” domains are observed. A model of the single-domain surface is proposed. The origin of the double- to single-domain transition found for the Eu/Si(1 0 0) system is discussed.     

One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM

We have performed low-temperature scanning tunneling microscopy (STM) experiments on the cold-cleaved surface of YBa₂Cu₃O_y single crystals to study the nanoscale electronic order in high-T_c superconductors. STM images measured at low-bias voltage below ∼50 meV show the one-dimensional (1D) electronic modulation along the Cu-O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range ∼2a-4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO₂ plane.     

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe_1−xTe_x, at doping levels of x=0.5 and 0.7, with critical temperatures . Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within . These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above T_c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T_c, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe_1−xTe_x, and in terms of its characteristic band structure.     

Defects analysis of Al/Si artificial nanocluster with moiré fringes

In this paper, it is demonstrated the defects play a very important role in determining the quality of artificial nanoclusters grown. The surface strain (stress) distribution around defects in Al artificial nanocluster is analyzed by Moiré fringes pattern. The moiré fringes generated by scanning lines in monitor and nanocluster array can be used as a “magnifier” to study surface imperfections with scanning tunneling microscopy (STM). As moiré fringes’ exist in many ordered nanostructures and adsorbate systems, the method is expected to have wide applications.     

STM/STS observation of ferrocene derivative adduct to C60 on HOPG

The C₆₀ONCFn cycloadduct (Fn=ferrocene) was prepared in the reaction between C₆₀ and ferrocene oxime, the ferrocene derivative was bound to C₆₀ at the 6–6 bond by a heterocyclic oxygen–nitrogen–carbon ring; the compound was stable in air. The compound dissolved in dichloroethane was deposited on HOPG and observed by UHV STM/STS methods. The molecules of C₆₀ONCFn formed several-microns-long straight chains with clearly visible adducted groups pointing to one side of the chain. The STM/STS observations are discussed within the terms of semiempirical quantum chemical molecular modeling.     

Epitaxial structures of self-organized, standing-up pentacene thin films studied by LEEM and STM

Growth of pentacene (Pn) thin films has been studied in situ by means of low-energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). A very low nucleation density of Pn grains has been observed on Bi(0 0 0 1)/Si(1 1 1) template, resulting in formation of large, monolayer-high Pn grains with diameter exceeding several hundreds of micrometers. We determined that formation of self-organized, standing-up Pn epitaxial layers was stabilized by a weak interaction between the substrate and Pn molecules and by the presence of the commensurate structure between the oblique Pn lattice and trigonal substrate surface lattice. The ‘point-on-line’ commensurability has been found along a-axis of Pn and one of the primitive vectors of substrate surface lattice. Strong ‘point-on-line’ commensurability in Pn/Bi(0 0 0 1)/Si(1 1 1) system resulted in a bulk-like epitaxial thin film growth, starting from the first layer. The presence of twins, often having a mirror line parallel to the direction of the ‘point-on-line’ matching, has been also detected using an asymmetric dark-field imaging mode in LEEM experiments, which, we believe, is the first LEEM demonstration of molecular tilt imaging.     

STM and chemistry: a qualitative molecular orbital understanding of the image of CO on a Pt surface

Theoretical calculations of STM images for isolated carbon monoxide on Pt(111) have been performed. These simulations are directly comparable to experimental data from Eigler's group. They also confirm a strong site dependence of the STM molecular contrast and assign the two experimental STM forms of CO with two distinct binding sites (the top and the bridge sites) which have already been well-characterised by other surface techniques. Moreover, the approach used here allows a detailed understanding of the electronic origin of the STM molecular pattern. Hence, we demonstrate that the CO STM pattern results from the superposition, with interference effects, of two current contributions. The direct through space tunneling process between the tip and the metal surface is decreased by the presence of the adsorbate molecule, but this depression in the current is more or less compensated, depending on the binding site, by the tunnel current mediated by the molecular levels of the adsorbate. The frontier π orbitals of CO have a small contribution to the current and the σ framework has the major influence with the 5σ lone pair (HOMO of CO) and also the low-lying 3σ. These two MO contributions, however, show a destructive interference effect, and this, together with the absence of π current, is responsible for the weak amplitude of the CO STM image.     

Exact calculation of the current in a one-dimension model of a scanning tunneling microscope

Electric current as a function of applied voltage have been calculated between two one-dimensional semi-infinite atomic chains. A tight-binding two band model and the modified tight-binding equations technique have been used. The current derivatives are compared with the surface local density of states. The comparison shows that the current derivatives reproduce the local density of states with a significant distortion. It occurs due to wave vector dependence of the transmission coefficient of electronic waves falling on the interface between the chains.     

Growth of a single layer gold stripe and investigation of the preferable growth direction on reconstructed Au(1 1 1) surface using STM

We have investigated the growth of nanometer-scale gold stripes on reconstructed Au(1 1 1) surface using scanning tunneling microscopy (STM). The experiment was carried out under the conditions of ultrahigh vacuum and room temperature. The stripes were grown by the scanning motion of the STM tip over the area containing more than one step edge with the tunnel resistance less than several tens of mega ohms (MΩs). Unlike the previous reports [J.C. Heyraud, J.J. Metoris, Surf. Sci. 100 (1989) 519; V.M. Hallmark, S. Chiang, J.F. Rabolt, J.D. Swalen, R.J. Wilson, Phys. Rev. Lett. 59 (1987) 2879], we found, by directly comparing the direction of the stripes and the orientation of the underlying lattice, that the gold stripes grow preferentially along [1,−1,0] direction and its threefold symmetric directions at (1 1 1) surface of fcc structure. We also found that the scanning direction of the STM tip does not affect the direction of the stripe growth although the growth rate is suppressed remarkably when the scanning direction is close to [1,1,−2] direction of Au(1 1 1) surface.