Downloads & Free Reading Options - Results

The properties of monolayer films of organic materials are important for a variety of technologies. We have employed the STM and AFM to study Langmuir-Blodgett films of a variety of polymers on substrates of graphite, molybdenum sulfate, and gold (111) on mica. The polymers were poly(octadecyl acrylate) (PODA), atactic and syndiotactic poly(methyl methacrylate) (PMMA) and poly(2-methyl 1-pentene sulfone) (PMPS). One striking feature was the degree of order observed; a second was the morphological difference between films of submonolayer thickness (long thin fibrils) and those of at least monolayer thickness (lumpy structures arranged in domains). By pulsing the STM bias voltage to values in excess of 4 V, we were able to bring about local modification of the polymer morphology.

The atomic and nanoscale structures of high-index gold surfaces in aqueous perchloric acid electrolyte as revealed by in-situ scanning tunneling microscopy (STM) under electrode potential control are reported with the objective of ascertaining systematically the terrace-step morphology and superstructures as a function of the crystallographic orientation. Six faces, Au(221), (331), (533), (311), (210), and (410), two each lying in the three fcc crystallographic zones, were selected to investigate the role of the step orientation and terrace width for non-vicinal faces. Data for the low-index surface Au(110) are also included for comparison with Au(331) and (221), since all three feature formally a n(111)-(111) terrace-step structure. Measurements of the double-layer capacitance as a function of the electrode potential, E, in dilute (10 mM) perchloric acid were also undertaken in order to evaluate the potential of zero charge (E sub pzc) for each surface and to check the potential-dependent surface stability. The two surfaces in the (111)-(100) zone, Au(533) and (311), both display essentially (1 x 1) (i.e bulk-termination) atomic structures at positive electrode charges (i.e. for E E sub pzc), yet exhibit significant surface relaxation at negative charges involving edge-atom depression and row buckling. For the surfaces in the (111)-(110) zone, Au(221) and (331), however, such surface relaxation is seen even at positive electrode charges. This behavioral difference can be understood on the basis of the differing step structures present in the two zones. Moreover, Au(331) undergoes a reversible (1 x 2) reconstruction at negative charges, involving row pairing.

Scanning tunneling microscopy images of the (0001) plane of highly oriented pyrolytic graphite show defect regions consisting of an extensive network of partial dislocations that form extended and contracted nodes. The partial dislocations in hexagonal graphite enclose triangular regions (about 1000 nm on a side) of faulted material comprised of rhombohedral graphite. Tunneling spectroscopy shows differences in the dependence of the tunneling current on voltage between the faulted and unfaulted regions. Electronic and elastic interactions of the tip with the HOPG surface are proposed to explain the observed image contrast between hexagonal and rhombohedral graphite. (ttl)

In-situ atomic-resolution scanning tunneling microscopy (STM) has been utilized on ordered Au(lll) under electrode potential control in acidic aqueous solution to examine iodine adlayer structures formed by iodide electrooxidation as well as electrosorption. In the potential region, -0. 3 to 0. 4 V vs SCE, below where electrooxidation of solution iodide occurs, several distinct iodine adlayer structures were observed. At the most negative potentials, structures close to the hexagonal ((check)3 x (check)3) R30 deg (Theta = 0.33) pattern are evident. The registry between the adlayer and the substrate lattice was deduced in part from composite domain images created by stepping the electrode potential so to alter markedly the adsorbate coverage during acquisition of a given STM image. At potentials between -0.2 and 0.2 V, adlayer patterns closer to the (5 x (check)3) (Theta = 0.4) structure became increasingly prevalent. This features a diminution in the iodine spacing along two of the three iodine rows, with a corresponding ca 5 deg shift from the R30 deg direction. Above 0.2 V, the STM images indicated the predominant presence of a more complex higher-coverage adlayer (Theta = 0.44) featuring long-range (19+/-lA) Z corrugations rotated by 8-10 deg from the hexagonal iodine adlattice. These corrugations arise from periodic alterations in the iodine binding site, necessitated by I-I distances that approach the van der Waals diameter. Above 0. 3 V, however, poly-iodide chains were also observed, featuring shorter (2.8 - 3. 2 A) I-I distances compatible with adsorbate-adsorbate chemical bonding; these increasingly distort the monomeric iodine superlattice structure.

Using a scanning tunneling microscope and light from a He-Ne laser, we have measured the surface photovoltage of the Si(111)-7X7 surface with coverages of Ag up to 1 monolayer. The data agree with a model for the photovoltage based on the recombination of photoexcited minority carriers with majority carriers thermally excited over the surface Schottky barrier. The surface Fermi level positions derived from these data are consistent for n and p type Si decreasing from 0.60 eV above the conduction band maximum for the clean surface to 0.40 eV near 1 monolayer coverage. Although our photovoltage data are spatially resolved on an atomic scale, we do not observe any spatial variation in the photovoltage on the clean surface or on surfaces partially covered by Ag islands.

Atomic resolution images of clean Si(001)-(2x1) and the mono-hydride phase, Si(001)-(2xl)H were investigated using scanning tunneling microscopy at various sample-tip bias voltages. At a sample-tip bias of -1.9 V, each dimer of the monohydride phase show two protrusions 3.3 A apart separated by a minimum 0. 11 A deep for a sample bias of -1.9 Volt, while clean dimers show a single protrusion per unit cell. Monohydride dimers appear lower than clean dimers, with apparent height differences ranging from 1.9 A at -1.6 V to 0.65 A at -3.0 V sample bias. An analysis of the apparent height and spatial distribution of tunneling current within each dimer can be used to unambiguously discriminate between clean dimers, monohydride dimers, and vacancy defects. This methodology is applied to study the distribution using disilane, revealing segregation of the monohydride into nearly isotropic islands.

Our research group is involved in the use of a millikelvin Scanning Tunneling Microscope (mk-STM), developed in previous grant periods.

The extraction of adatom-adatom interaction energies from STM and FIM images is discussed. For the case of comparing experimental data with a solved model (e.g. Monte-Carlo simulations), an expression for the expected precision is derived. It is demonstrated that in the absence of any Monte-Carlo calculations the interaction energies can be extracted if measurements are made at more than one temperature. The-relative uncertainty in the determination of the interaction energies for the case of a solved vs. an unsolved model as a function of system size and temperature is presented.

This work examines the electronic structure of new fullerene compounds by the combination of in situ transport measurements as a function of temperature and photoemission spectroscopy. In particular, we have grown thin films of cerium fullerides, Ce(x)C60, 0.5<x<l3, by co-evaporation of multiple molecular beam sources. Resistivity as a function of temperature showed activated transport as a function of temperature at all compositions between 9 and 300 kelvins. Core level photoemission spectroscopy showed weak hybridization of Ce 4f orbitals with the valence band for x<11 and strong hybridization at high cerium concentration. Valence band photoemission spectroscopy shows electronic correlation effects. jg p.1

The potential-dependent adsorption of sulfate on ordered Au(111) from acidic aqueous electrolytes has been examined in-situ by means of infrared reflection-absorption spectroscopy (IRAS) and by atomic-resolution scanning microscopy (STM) in order to explore further the nature of the adsorbate bonding and the structural changes attending the formation of the ordered adlayer at high potentials, as observed recently by using STM (ref. 8). Solution conditions that encompassed aqueous sulfuric acid, sulfuric acid/sulfate electrolytes of varying pH, and dilute sulfate in excess perchloric acid were selected in order to facilitate comparisons with recent adsorbate compositional data extracted from chronocoulometric and radiotracer measurements which utilized the last type of electrolyte (ref. 9). Essentially the same ordered adlayer structures were deduced by STM to form at suitably high potentials ( 0.8 V vs. SCE) in both sulfuric acid and the mixed sulfate/excess perchloric acid media. The adlayer, which exhibits a (sq. rt.3 x sq. rt.7) symmetry, involves a fractional sulfate coverage of 0.2, in accordance with chronocoulometic and radiotracer data (ref. 9). The possibility that the ordered sulfate adlayer incorporates coadsorbed hydronium cations is discussed; such coadsorption is suggested by the presence of additional tunneling maxima in the STM images. The IRAS data display a prominent S-0 stretching band (,vso) at 1155-1220 cm-l, the potential-dependent intensity of which correlates with the sulfate surface concentrations reported in ref. 9. The appearance of this vso feature is also insensitive to the electrolyte conditions, including pH, consistent with its assignment to adsorbed sulfate, rather than to bisulfate.

The focus of research in the last year has been STM(Scanning Tunneling Microscopy) studies of electrode/solution interfaces. STM, and its derivatives (e.g. AFM, Atomic Force Microscopy) are the only surface analytic methods capable of yielding real-space, atomic resolution, electronic and structural maps of immersed electrode surfaces. To achieve this goal, in the previous contract period we have developed STM tip preparation methodologies that enable atomic resolution imaging of electrode surfaces in solutions containing high (0.1 M) concentrations of electro donor and acceptor species. These techniques have significantly extended the applicability of STM to electrochemical investigations. Recent STM investigations of electrochemical systems have focused on elucidating the structure of adsorbate monolayers and of single strands of conducting polymers. Recently we have also used these ultra small STM tips to perform measurements of very rapid heterogeneous electron transfer rate constants at electrodes as small as 1 nanometer in radius.

The Scanning Tunneling Microscope has demonstrated unique capabilities for the examination of electrode topography, the vibrational spectroscopic imaging of surface adsorbed species, and the high resolution electrochemical modification of conductive surfaces. Here we discuss recent progress in electrochemical STM. Included are a comparison of STM with other ex situ and in situ surface analytic techniques, a discussion of relevant STM design considerations, and a semi-quantitative examination of faradaic current current contributions for STM at solution-covered surfaces. Applications of STM to the ex situ and in study of electrode surfaces are presented.

Application of the two-wavelength X-ray reflectometry to exploration of Ge/Si(001) hereostructures with dense chains of stacked Ge quantum dots is presented

We report a reproducible technique for the fabrication of sharp superconducting Nb tips for scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. Sections of Nb wire with 250 $\mu$m diameter are dry etched in an SF$_6$ plasma in a Reactive Ion Etcher. The gas pressure, etching time and applied power are chosen to produce a self-sharpening effect to obtain the desired tip shape. The resulting tips are atomically sharp, with radii of less than 100 nm, and generate good STM images and spectroscopy on single crystal samples of Au(111), Au(100), and Nb(100), as well as a doped topological insulator Bi$_2$Se$_3$ at temperatures ranging from 30 mK to 9 K.

The one-dimensional reconstruction of Au/Ge(001) was investigated by means of autocorrelation functions from surface x-ray diffraction (SXRD) and scanning tunneling microscopy (STM). Interatomic distances found in the SXRD-Patterson map are substantiated by results from STM. The Au coverage, recently determined to be 3/4 of a monolayer of gold, together with SXRD leads to three non-equivalent positions for Au within the c(8x2) unit cell. Combined with structural information from STM topography and line profiling, two building blocks are identified: Au-Ge hetero-dimers within the top wire architecture and Au homo-dimers within the trenches. The incorporation of both components is discussed using density functional theory and model based Patterson maps by substituting Germanium atoms of the reconstructed Ge(001) surface.

We have used a low-temperature scanning tunneling microscope (STM) to study the surface of heavily doped semiconductor InAs crystals. The crystals are cleaved in situ along the (110) plane. Apart from atomically flat areas, we also observe two major types of atomic scale defects which can be identified as S dopant atoms and as As vacancies, respectively. The strong bias voltage dependence of the STM image of the impurities can be explained in terms of resonant tunneling through localized states which are present near the impurity.

The local geometric and electronic structures of boron-induced reconstructions produced by thermal decomposition of diborane and decaborane on Si(001) has been investigated using scanning tunneling microscopy. STM images show that boron induces several related reconstructions, arising from ordered arrangements of simple structural subunits. These boron-induced atomic rearrangements order even at very low boron exposures, leading to a striking spatial segregation of boron on the surface. Similar reconstructions are observed using diborane and decaborane as boron precursors. Annealing at 1000 Kelvin for 90 seconds substantially improves the surface ordering, without significant diffusion of boron from the surface to the bulk.

The local geometric and electronic structures of boron-induced reconstructions produced by thermal decomposition of diborane and decaborane on Si(001) has been investigated using scanning tunneling microscopy. STM images show that boron induces several related reconstructions, arising from ordered arrangements of simple structural subunits. These boron-induced atomic rearrangements order even at very low boron exposures, leading to a striking spatial segregation of boron on the surface. Similar reconstructions are observed using diborane and decaborane as boron precursors. Annealing at 1000 Kelvin for 90 seconds substantially improves the surface ordering, without significant diffusion of boron from the surface to the bulk.

High--quality single crystals of the heavy fermion superconductors CeCoIn$_5$ and CeIrIn$_5$ have been studied by means of low--temperature Scanning Tunneling Microscopy. Methods were established to facilitate \textit{in-situ} sample cleaving. Spectroscopy in CeCoIn$_5$ reveals a gap which persists to above $T_c$, possibly evidencing a precursor state to SC. Atomically resolved topographs show a rearrangement of the atoms at the crystal surface. This modification at the surface might influence the surface properties as detected by tunneling spectroscopy.

The structures of halogen monolayers (I, Br, Cl, and F) and mixed halogen monolayers on Ag(111) surfaces are measured by scanning tunneling microscopy (STM). Adatom radii determined from nearest-neighbor bond distances are in excellent agreement with literature values of the radii of halides, indicating that the adsorbed halogen atoms have an essentially ionic character and are adsorbed as a surface Ag-halide complex. STM images of mixed halogen adlayers comprised of either coadsorbed I and Cl, I and F, or Cl and F, exhibit several structural features, e.g. a low frequency moire pattern and a large atom vacancy density, that are not observed in single-atom adlayers and that are ascribed to coulombic forces within the ionic adlayer. STM has also been used to monitor variations in adlayer structures following ion exchange of chemically distinct halogen species. STM images and data obtained from Auger electron spectroscopy indicate that the chemical stability of the surface Ag halide complexes in aqueous solutions decreases with increasing halogen electronegativity. The results suggest that the energy of solvation of the halide ions determines the relative stability of the Ag-halide surface bond similar to that observed for bulk Ag halide crystals. STM, Tunneling microscopy, Ag halide monolayers

Scanning tunneling microscopy and theory were combined to create novel depressive quantum dots at room temperature on the (110) surfaces of InSb --- dots which merit further exploration as potential nanopixels for tiny-device lithography; develop a strained-layer superlattice model of high-temperature superconductivity; image, understand, and make models of single-atom-high steps on III-V surfaces; invent and exploit a new kind of spectroscopy of surface states of semiconductors, called tipology; develop phenomenological models of variety of surface phenomena.

The technological impact of silicon carbide, SiC, covers two distinctly diverse areas: (a) as a high-strength structural ceramic, and (b) as a high-temperature wide bandgap semiconductor. In both of these applications, metallization of the SiC surface is a necessary and important procedure. From the structural point of view, a large complex shape can be produced by joining together parts of small size and simple geometries via a metallic interlayer. From the electronic point of view, good ohmic and Schottky contacts with metals are prerequisites for SiC-based device technology. For both cases, the Ti/SiC system appears to provide desirable results. When the Ti/SiC system is annealed in the temperature range 600- 1200 deg C, phases such as Ti5Si3, TiSi2, TiC and Ti3SiC2, are reportedly formed (1-6). Mechanically, these reaction products provide good bonding characteristics to yield a joint of adequate strength; while electronically, the silicide and carbide phases are metallic and thus exhibit rectifying behavior as expected from Schottky barrier diodes.

Scanning tunneling microscopy(STM) images of the(001) face (ab face) of the organic conductors (TMTSF)2X(TMTSF = tetramethyltetraselenafulvalene; X = C104-, ReO4-) reflect the surface molecular corrugation and anisotropic stacking of TMTSF molecules along the 100 direction. The lattice constants of the(001) face determined from STM for(TMTSF)2ClO4 are a 7.3 +/- 0.2 b = 7.8 +/- 0.2 A, and y = 69.50 + or - 20, and for (TMTSF)2ReO4 are 7.5 +/- 0.2 A, b 8.1 +/- 0.2 and y = 70 - or + 20, in excellent agreement with the known crystal structure. Height profile analysis of the tunneling current assigned to localized TMTSF electronic states are in agreement with the angle between the(001) face and the direction of the short axis of the TMTSF molecules. The STM images also are consistent with the high degree of electronic anisotropy of (TMTSF)2CIO4, with a broad electronic state density associated with the psuedo one-dimensional TMTSF stacks and a highly localized state density associated with Se-Se contacts between stacks along the 110 direction. The results are corroborated by STM of isomorphous(TMTSF)2Rel4 which exhibits negligible tunneling current associated with the Se-Se states, in agreement with the slightly larger Se-Se interstack distances in this compound. The lack of tunneling current in the interstack region of(TMTSF)2ReO4 signifies reduced electronic interaction transverse to the stacking direction, consistent with previously reported electronic properties of(TMTSF)2ReO4. The results indicate that STM can detect subtle differences in electronic structure that are responsible for bulk electronic properties.

We demonstrate the combination of scanning force microscopy and scanning tunneling spectroscopy in a local probe microscope operating at very low temperature (60 mK). This local probe uses a quartz tuning fork ensuring high tunnel junction stability. We performed the spatially-resolved spectroscopic study of a superconducting nano-circuit patterned on an insulating substrate. Significant deviations from the BCS prediction are observed.

Explicit predictions for Scanning Tunneling Microscopy (STM) on interacting one-dimensional electron systems are made using the Luttinger liquid formalism. The STM current changes with distance from an impurity or boundary in a characteristic way, which reveals the spin-charge separation and the interaction strength in the system. The current exhibits Friedel-like oscillations, but also carries additional modulated behavior as a function of voltage and distance, which shows the spin-charge separation in real space. Moreover, very close to the boundary the current is strongly reduced, which is an indication of the interaction strength in the system.

Explicit predictions for Scanning Tunneling Microscopy (STM) on interacting one-dimensional electron systems are made using the Luttinger liquid formalism. The STM current changes with distance from an impurity or boundary in a characteristic way, which reveals the spin-charge separation and the interaction strength in the system. The current exhibits Friedel-like oscillations, but also carries additional modulated behavior as a function of voltage and distance, which shows the spin-charge separation in real space. Moreover, very close to the boundary the current is strongly reduced, which is an indication of the interaction strength in the system.

We have studied the structure of MBE-grown InAs(001)-(2 x 4) surfaces exposed to low Sb2 fluxes by scanning tunneling microscopy (STM) and ab initio density functional theory (DFT). Experimentally, we observe an Sb-terminated alpha 2(2 x 4) phase over a wide range of temperatures (400 - 510 deg C) for low Sb2 flux (< 0.1 ML/s), whereas temperature and As2 flux must be carefully controlled to achieve the same As-terminated surface structure. At lower temperatures, we observe indications of an Sb-terminated (2 x 8) symmetry surface phase, and we report briefly on its proposed structure and stability, as well as its possible role in subsequent formation of the Sb-terminated (1 x 3) phase found at typical Sb2 fluxes used during heterostructure growth.

The instrumentation needed for combining a new imaging technique, Scanning Tunneling Microscopy (STM), with standard surface analytical methods has been developed. The surface analytical capabilities of a new system have demonstrated by study of the Barium catalyzed oxidation of Nickel. The operation of the STM has been demonstrated by a detailed study of the imaging of the graphite surface. The changes in graphite imaging with tunneling voltage have been measured and compared with theoretical predictions of the effect of surface change density and surface deformation on imaging via STM. The combination of STM with the surface analytical probes has been demonstrated in a comparison of LEED and STM measurements of a stepped Si surface. It is shown that LEED is rather insensitive to structural changes that are readily apparent using STM.

The utilization of infrared reflection-absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) to extract atomic-resolution information for ordered metal-solution interfaces in a related (and relatable) fashion to metal-ultrahigh vacuum (UHV) surfaces is illustrated by means of some recent results from our laboratory. Two specific topics are addressed. The first involves the potential-dependent properties of saturated carbon monoxide adlayers on low-index platinum and rhodium electrodes in aqueous and nonaqueous media. The central role of the surface potential in controlling the CO adlayer structure is discussed on the basis of in-situ IRAS data, especially in comparison with the properties of corresponding metal-uhv interfaces. The application of in-situ atomic-resolution STM in tandem with IRAS for elucidating real-space adsorbate structures is noted for saturated CO adlayers on Rh(111) and Rh(110) electrodes. The second topic concerns the application of in-situ STM to probe potential induced reconstruction at gold-aqueous interfaces. All three low-index gold surfaces are seen to undergo reconstruction at potentials corresponding to small negative electrode charges.

The utilization of infrared reflection-absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) to extract atomic-resolution information for ordered metal-solution interfaces in a related (and relatable) fashion to metal-ultrahigh vacuum (UHV) surfaces is illustrated by means of some recent results from our laboratory. Two specific topics are addressed. The first involves the potential-dependent properties of saturated carbon monoxide adlayers on low-index platinum and rhodium electrodes in aqueous and nonaqueous media. The central role of the surface potential in controlling the CO adlayer structure is discussed on the basis of in-situ IRAS data, especially in comparison with the properties of corresponding metal-uhv interfaces. The application of in-situ atomic-resolution STM in tandem with IRAS for elucidating real-space adsorbate structures is noted for saturated CO adlayers on Rh(111) and Rh(110) electrodes. The second topic concerns the application of in-situ STM to probe potential induced reconstruction at gold-aqueous interfaces. All three low-index gold surfaces are seen to undergo reconstruction at potentials corresponding to small negative electrode charges.

This research program improved our understanding of how surface chemical reactions, surface stress, and atomic level microstructure influence materials oxidation/corrosion processes. Central to this effort was the construction of a new scanning probe microscopy facility which now houses a UHV-STM and a combined STM/AFM for operation in both air and electrochemical environments. It was demonstrated that applied stress fields can influence the electrochemical pitting corrosion chemistry of pure aluminum and 2024-T3 alloy. The role that surface defects play in oxidation reactions involving graphitic surfaces was also examined. Here it was shown that surface defects, including ones occurring on the atomic level, can dominate the reactivity and morphological evolution of such surfaces. Finally, high-impedance STM imaging was used to elucidate the structure and domain boundary characteristics for adsorbed self- assembling molecular films, providing information that will be needed if such films are to be used for interface passivation. The basic science opportunities addressed in this program were quite significant, and have technological implications given current interest in corrosion prevention and developing an improved fundamental understanding for stress-corrosion cracking, a topic of obvious importance to the aircraft industry.

This research program improved our understanding of how surface chemical reactions, surface stress, and atomic level microstructure influence materials oxidation/corrosion processes. Central to this effort was the construction of a new scanning probe microscopy facility which now houses a UHV-STM and a combined STM/AFM for operation in both air and electrochemical environments. It was demonstrated that applied stress fields can influence the electrochemical pitting corrosion chemistry of pure aluminum and 2024-T3 alloy. The role that surface defects play in oxidation reactions involving graphitic surfaces was also examined. Here it was shown that surface defects, including ones occurring on the atomic level, can dominate the reactivity and morphological evolution of such surfaces. Finally, high-impedance STM imaging was used to elucidate the structure and domain boundary characteristics for adsorbed self- assembling molecular films, providing information that will be needed if such films are to be used for interface passivation. The basic science opportunities addressed in this program were quite significant, and have technological implications given current interest in corrosion prevention and developing an improved fundamental understanding for stress-corrosion cracking, a topic of obvious importance to the aircraft industry.

Rate parameters are reported for the electroreduction of eight CoIII(NH3)5X complexes at ordered Pt(111) surfaces coated with iodine adlayers whose structures are characterized by scanning tunneling microscopy (STM) in order to explore possible correlations between the outer-sphere electron- transfer kinetics and the spatially resolved adlattice properties as revealed by STM. The sixth ligands: acetates, and three cyclic organic carboxylates, were selected so to vary the reactant charge, and hence the magnitude of electrostatic double-layer effects, and to examine the effect of potential organic mediators. The ordered Pt(111) surfaces were prepared by flame annealing, followed by cooling in a stream of nitrogen over iodine crystals.

Rate parameters are reported for the electroreduction of eight CoIII(NH3)5X complexes at ordered Pt(111) surfaces coated with iodine adlayers whose structures are characterized by scanning tunneling microscopy (STM) in order to explore possible correlations between the outer-sphere electron- transfer kinetics and the spatially resolved adlattice properties as revealed by STM. The sixth ligands: acetates, and three cyclic organic carboxylates, were selected so to vary the reactant charge, and hence the magnitude of electrostatic double-layer effects, and to examine the effect of potential organic mediators. The ordered Pt(111) surfaces were prepared by flame annealing, followed by cooling in a stream of nitrogen over iodine crystals.

This article is from Nanoscale Research Letters , volume 7 . Abstract We have found that Co-2 × 2 islands grown on an Ag/Ge(111)-√3 × √3 surface have hcp structure with the (11-20) orientation. The island evolution involves transformation of the unit cell shape from parallelogram into rectangular, which is accompanied by the island shape transformation from hexagonal into stripe-like. Identified are two crystallographic directions for the island growth, the pseudo-[0001] and the pseudo-[1-100]. We have observed the occurrence of a lateral shift between the topmost and the underlying bilayers in the case of the island growth along the pseudo-[0001] direction. In contrast, the topmost and the underlying bilayers are unshifted for the growth along the pseudo-[1-100] direction.

The focus of research in the last year has been STM (Scanning Tunneling Microscopy) studies of electrode solution interfaces. STM, and its derivatives (e.g. AFM, Atomic Force Microscopy) are the only surface analytic methods capable of yielding real-space, atomic resolution, electronic and structural maps of immersed electrode surfaces. To achieve this goal, in the previous contract period we have developed STM tip preparation methodologies that enable atomic resolution imaging of electrode surfaces in solutions containing high (0.1 M) concentrations of electron donar and acceptor species. These techniques have significantly extended the applicability of STM to electrochemical investigations. Recent STM investigations of electrochemical systems surface under varied media.

X-ray standing-wave measurements and tunneling microscopy have been combined to solve the atomic geometry of the (square root of 3) X (square root of 3R 30 degrees) honeycomb phase of Bi on Si(111). The standing-wave measurements utilize three different diffracting planes to triangulate the surface position of Bi atoms. The unoccupied surface sites required to completely determine the structure can be deduced from Rutherford-backscattering coverage and low-energy electron-diffraction symmetry arguments. These arguments are completely confirmed by a tunneling-microscope study, which is free of the ambiguities of previous studies. The final result is a (2/3-ML) (square root of 3) X (square root of 3R 30 degrees) structure with Bi atoms in the T1 sites directly above first-layer Si atoms.

This article is from Beilstein Journal of Nanotechnology , volume 2 . Abstract Terthiophene (3T) molecules adsorbed on herringbone (HB) reconstructed Au(111) surfaces in the low coverage regime were investigated by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra-high vacuum conditions. The 3T molecules adsorb preferentially in fcc regions of the HB reconstruction with their longer axis oriented perpendicular to the soliton walls of the HB and at maximum mutual separation. The latter observation points to a repulsive interaction between molecules probably due to parallel electrical dipoles formed during adsorption. Constant-separation (I-V) and constant-current (z-V) STS clearly reveal the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, which are found at −1.2 eV and +2.3 eV, respectively. The HOMO–LUMO gap corresponds to that of a free molecule, indicating a rather weak interaction between 3T and Au(111). According to conductivity maps, the HOMO and LUMO are inhomogeneously distributed over the adsorbed 3T, with the HOMO being located at the ends of the linear molecule, and the LUMO symmetrically with respect to the longer axis of the molecule at the center of its flanks. Analysis of spectroscopic data reveals details of the contrast mechanism of 3T/Au(111) in STM. For that, the Shockley-like surface state of Au(111) plays an essential role and appears shifted outwards from the surface in the presence of the molecule. As a consequence, the molecule can be imaged even at a tunneling bias within its HOMO–LUMO gap. A more quantitative analysis of this detail resolves a previous discrepancy between the fairly small apparent STM height of 3T molecules (1.4–2.0 nm, depending on tunneling bias) and a corresponding larger value of 3.5 nm based on X-ray standing wave analysis. An additionally observed linear decrease of the differential tunneling barrier at positive bias when determined on top of a 3T molecule is compared to the bias independent barrier obtained on bare Au(111) surfaces. This striking difference of the barrier behavior with and without adsorbed molecules is interpreted as indicating an adsorption-induced dimensionality transition of the involved tunneling processes.

The adequate interpretation of scanning tunneling microscopy (STM) images of the clean Si(001) surface is presented. We have performed both STM observations and {\it ab initio} simulations of STM images for buckled dimers on the clean Si(001) surface. By comparing experimental results with theoretical ones, it is revealed that STM images depend on the sample bias and the tip-sample separation. This enables us to elucidate the relationship between the corrugation in STM images and the atomic structure of buckled dimers. Moreover, to elucidate these changes, we analyze details of the spatial distributions of the $\pi$, $\pi^{\ast}$ surface states and $\sigma$, $\sigma^{\ast}$ Si-Si bond states in the local density of states which contribute to STM images.

We have performed the most realistic simulation to date of the operation of a scanning tunneling microscope. Probe-sample distances from beyond tunneling to actual surface contact are covered. We simultaneously calculate forces, atomic displacements, and tunneling currents, allowing quantitative comparison with experimental values. A distance regime below which the probe becomes unstable is identified. It is shown that the real distance differs substantially from previous estimates because of large atomic displacements on the surface and at the probe-tip.

We report a simple method for the fabrication of Niobium superconducting (SC) tips for scanning tunnelling microscopy which allow atomic resolution. The tips, formed in-situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature Tc=9.2+-0.3 K as deduced from Superconductor Insulator Normal metal (NIS) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1 T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex

Adventitious defects within self-assembled monolayers of 1- octadecanethiol confined to Au substrate have been studied by a new method which takes advantage of the high spatial resolution of scanning tunneling microscopy (STM) and the molecular specificity of electrochemistry. The method permits direct visualization of the defect density and provides information about the chemical and structural nature of the defects. CN- was used to electrochemically etch Au from surface regions near defects. This leads to the formation of triangular etch pits, which exhibit a uniform in-plane orientation. A point- defect model is proposed to explain the orientation of the triangular pits. The model also predicts that the organomercaptan molecules occupy particular three- fold hollow sites.

Scanning tunneling microscopy (STM) has been used to image and modify the surfaces of III-V, II-VI and group IV semiconductors. A tip-simulator based on a photocode was developed. The simulator allows the development of ultra- sensitive electronics for controlling STM tip movement. Various forms of 'nano- machining,' including chiselling, sanding, and sweeping of atoms on a surface, were developed. An STM design was modified to allow bending of long thin samples of Si(100) in UHV to permit the study of surface strain. A variety of studies were conducted on Au (in air) CdTe (in air), Hg(sub 1-x)Mn(sub x)Te (under glycerin), and Hg(sub 1-x)Cd(sub x)Te (in air and under glycerin).

Scanning tunneling microscopy (STM) has been used to image and modify the surfaces of III-V, II-VI and group IV semiconductors. A tip-simulator based on a photocode was developed. The simulator allows the development of ultra- sensitive electronics for controlling STM tip movement. Various forms of 'nano- machining,' including chiselling, sanding, and sweeping of atoms on a surface, were developed. An STM design was modified to allow bending of long thin samples of Si(100) in UHV to permit the study of surface strain. A variety of studies were conducted on Au (in air) CdTe (in air), Hg(sub 1-x)Mn(sub x)Te (under glycerin), and Hg(sub 1-x)Cd(sub x)Te (in air and under glycerin).

We investigate vortices in LiFeAs using scanning tunneling microscopy/spectroscopy. Zero-field tunneling spectra show two superconducting gaps without detectable spectral weight near the Fermi energy, evidencing fully-gapped multi-band superconductivity. We image vortices in a wide field range from 0.1 T to 11 T by mapping the tunneling conductance at the Fermi energy. A quasi-hexagonal vortex lattice at low field contains domain boundaries which consist of alternating vortices with unusual coordination numbers of 5 and 7. With increasing field, the domain boundaries become ill-defined, resulting in a uniformly disordered vortex matter. Tunneling spectra taken at the vortex center are characterized by a sharp peak just below the Fermi energy, apparently violating particlehole symmetry. The image of each vortex shows energy-dependent 4-fold anisotropy which may be associated with the anisotropy of the Fermi surface. The vortex radius shrinks with decreasing temperature and becomes smaller than the coherence length estimated from the upper critical field. This is direct evidence of the Kramer-Pesch effect expected in a clean superconductor.

Graphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational Moire superlattice of CVD graphene on Cu (111), characterized by a periodicity of $1.5 \pm 0.05$ nm and corrugation of $0.15 \pm 0.05$ $\hbox{\AA}$ , as measured by Scanning Tunneling Microscopy. To understand the observed superlattice we have developed a newly parameterized Tersoff-potential for the graphene/Cu (111) interface fitted to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed CMD provides superlattices in good quantitative agreement with the experimental results, for a misorientation angle of $10.4 \pm 0.5,^{\circ}$ without any further parameter adjustment. Furthermore, the CMD simulations predict the existence of two non-equivalent high-symmetry directions of the Moir\'e pattern that could also be identified in the experimental STM images.

We studied experimentally and theoretically the electronic local density of states (LDOS) near single step edges at the surface of exfoliated graphite. In scanning tunneling microscopy measurements, we observed the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures extending over 3$-$4 nm both from the zigzag and armchair edges. Calculations based on a density-functional derived non-orthogonal tight-binding model show that these superstructures can coexist if the two types of edges admix each other in real graphite step edges. Scanning tunneling spectroscopy measurements near the zigzag edge reveal a clear peak in the LDOS at an energy below the Fermi energy by 20 meV. No such a peak was observed near the armchair edge. We concluded that this peak corresponds to the "edge state" theoretically predicted for graphene ribbons, since a similar prominent LDOS peak due to the edge state is obtained by the first principles calculations.

We studied experimentally and theoretically the electronic local density of states (LDOS) near single step edges at the surface of exfoliated graphite. In scanning tunneling microscopy measurements, we observed the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures extending over 3$-$4 nm both from the zigzag and armchair edges. Calculations based on a density-functional derived non-orthogonal tight-binding model show that these superstructures can coexist if the two types of edges admix each other in real graphite step edges. Scanning tunneling spectroscopy measurements near the zigzag edge reveal a clear peak in the LDOS at an energy below the Fermi energy by 20 meV. No such a peak was observed near the armchair edge. We concluded that this peak corresponds to the "edge state" theoretically predicted for graphene ribbons, since a similar prominent LDOS peak due to the edge state is obtained by the first principles calculations.