Scanning Tunneling Microscopy Results Research Articles

Visualization of Various Supramolecular Assemblies of Oligo(para-phenylenevinylene)-Melamine and Perylene Bisimide

A melamine derivative has been covalently equipped with two oligo(para-phenylenevinylene) (OPV) chromophores. This procedure yields a bifunctional molecule with two hydrogen-bonding arrays available for complementary binding to perylene bisimide derivatives. Depending on the solvent, hydrogen-bonded trimers, tetramers, and dimers on a graphite surface are observed for pure OPV-melamine by using scanning tunneling microscopy (STM). Upon the addition of perylene bisimide, linear tapes of perylene bisimide, 12-membered rosettes that consist of alternating hydrogen-bonded OPV-melamine and perylene bisimide moieties are visualized. These results provide direct evidence for the possible modes of hydrogen bonding within a supramolecular co-assembly in solution. Subsequently, the optical properties of pure OPV-melamine and co-assemblies with a perylene bisimide derivative were characterized in solution. In an apolar solvent, OPV-melamine self-assembles into chiral superstructures. Disassembly into molecularly dissolved species is reversibly controlled by concentration and temperature. Complementary hydrogen bonding to a perylene bisimide derivative in an apolar solvent yields multicomponent, pi-stacked dye assemblies of enhanced stability that are characterized by fluorescence quenching of the constituent chromophores. Titration experiments reveal that a mixture of hydrogen-bonded oligomers is present in solution, rather than a single discrete assembly. The solution experiments are consistent with the STM results, which revealed various supramolecular assemblies. Our system is likely not to be optimally programmed to obtain a discrete co-assembled structure in quantitative yield.

Competing order scenario of two-gap behavior in hole-doped cuprates

Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole-doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature ${T}_{c}$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a nonsuperconducting origin in a competing phase. Our analysis indicates that, near optimal doping, superconductivity can quench the competing order at low temperatures, and that some of the key differences observed between the STM and ARPES results can give insight into the superlattice symmetry of the competing order.

Substrate-termination and H 2O-coverage dependent dissociation of H 2O on Fe 3O 4(1 1 1)

The reaction of Fe 3O 4(1 1 1) with water vapour has been studied with scanning tunnelling microscopy (STM) and with X-ray and UV-photoemission as a function of water partial pressure and temperature. The photoemission results point to dissociation to form surface hydroxyls at a partial pressure of 10 −6 mbar H 2O and a substrate temperature of about 200 K. At 298 K it is known that dissociation occurs at around 10 −3 mbar [Kendelewicz et al., Surf. Sci. 453 (2000) 32]. This difference suggests that an intermolecular mechanism of dissociation is involved. It also suggests that the pressure dependence arises from a coverage term rather than differences in the Gibbs Free Energies of the oxide and hydroxide, as previously proposed. The STM results indicate that dissociation takes place on a termination of Fe 3O 4(1 1 1) thought to contain a 1/4 monolayer (ML) of Fe 3+ ions on top of a close-packed oxygen monolayer.

Formation of an ordered pattern of Bi nanolines on InAs(100) by self-assembly

Self-assembly of uniform patterns of nanolines over large surface areas has been proven to be difficult. The authors report that bismuth (Bi) adsorbate self-assembles into an ordered pattern of Bi nanolines separated by 4.3nm on the Bi-stabilized InAs(100)(2×1). The resulted nanoline surface is studied by scanning tunneling microscopy (STM) and low-energy electron diffraction. The plausible atomic models for the Bi nanolines are proposed on the basis of the STM results. The Bi lines are suggested to consist of two chains of adjacent Bi dimers positioned parallel to the chain and parallel to the Bi dimers of the (2×1) substrate.

Formation of highly crystalline C60 molecular films on a Bi(0001)/Si(111) surface

The results of scanning tunneling microscopy (STM) investigation of controllable growth of C60 adsorption on a Bi(0001)/Si(111) surface are reported. With the use of UHV STM, it has been shown that the most favorable sites for the initial stage of C60 adsorption are the double steps and domain boundaries. At ∼1 monolayer C60 coverage, the modulation pattern caused by the epitaxial relation between C60 and Bi unit cells has been observed. An increase in the C60 coverage up to several monolayers results in the formation of a highly crystalline molecular film.

Studies of Chemisorbed Tetracene on Si(111)-7×7

The chemisorption of tetracene on the Si(111)-7x7 surface was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. On the basis of the STM results and dimension analysis, two types of binding configurations were proposed. One of the configurations involves the di-sigma reaction between two C atoms of an inner ring with an adatom-rest atom pair on the substrate to give rise to an unsymmetrical butterfly structure. Tetracene in another configuration possesses four C-Si bonds that are formed via di-sigma reactions between the C atoms at the terminal rings with two center adatom-rest atom pairs within one-half of the surface unit cell. Besides, two other binding modes were proposed based on the dimension compatibility between the tetracene C and the substrate Si dangling bonds even though their identifications through the STM images are nonexclusive. Structural modeling and adsorption energies calculations were carried out using the DFT method. Factors affecting the relative thermodynamic stabilities based on the calculation results and the relative populations of tetracene in the different binding configurations as observed experimentally were discussed.

Combined Scanning Tunneling Microscopy and Synchrotron X-Ray Photoemission Spectroscopy Results on the Oxidative CuI Film Formation on Cu(111)

The initial stage of oxidative CuI film formation on Cu(111) has been studied in an electrochemical environment by means of cyclic voltammetry (CV), in situ scanning tunneling microscopy (STM) and ex situ synchrotron X-ray photoemission spectroscopy (SXPS). Cyclic voltammetric studies indicate a significant acceleration of copper oxidation in the presence of iodide. The reason for that is the iodide-mediated stabilization of cuprous species resulting in a downward shift of the onset potential for copper oxidation. Reactive sites for the copper oxidation followed by iodide complexation are exclusively defects such as substrate step edges. It is the surface-confined supersaturation of mobile CuI species that leads to the two-dimensional (2D) CuI film formation via nucleation and growth of a Cu/I bilayer on top of the preadsorbed iodide phase. In an advanced stage of copper oxidation, however, terraces are directly transformed into the 2D CuI film at the reactive boundary between metallic copper and the growing 2D CuI film. Structurally, this 2D CuI film is closely related to the (111) plane of crystalline CuIbulk (zinc blende type). There is no significant passivation of the copper surface against the oxidative dissolution reaction in the presence of the 2D CuI film. Copper dissolution in the presence of the 2D CuI film proceeds also via an inverse step flow mechanism involving the concerted receding of four atomic layers. A model of this process will be discussed on the basis of STM results. The transition from 2D to a three-dimensional (3D) CuI growth mode is observed for an advanced stage of copper oxidation.

Epitaxial growth, magnetic properties, and lattice dynamics of Fe nanoclusters on GaAs(001)

Epitaxial bcc-Fe(001) ultrathin films have been grown at $\ensuremath{\sim}50\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ on reconstructed $\mathrm{Ga}\mathrm{As}(001)\text{\ensuremath{-}}(4\ifmmode\times\else\texttimes\fi{}6)$ surfaces and investigated in situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and $^{57}\mathrm{Fe}$ conversion electron M\ossbauer spectroscopy (CEMS). For ${t}_{\mathrm{Fe}}=1$ ML (monolayer) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With increasing ${t}_{\mathrm{Fe}}$ the Fe clusters first connect along the $[\ensuremath{-}110]$, but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at ${t}_{\mathrm{Fe}}=4$ ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature-dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures ${T}_{B}$ of $62\ifmmode\pm\else\textpm\fi{}5$, $80\ifmmode\pm\else\textpm\fi{}10$, and $165\ifmmode\pm\else\textpm\fi{}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for ${t}_{\mathrm{Fe}}=1.9$, 2.2, and 2.5 ML, respectively. At $Tl{T}_{B}$, freezing of superparamagnetic clusters is inferred from the observed quasilinear $T$ dependence of the mean hyperfine magnetic field $⟨{B}_{hf}⟩$. By combining the STM and CEMS results, we have determined a large magnetic anisotropy constant of $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ and $\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{3}$ at ${t}_{\mathrm{Fe}}=1.9--2.2$ and 2.5 ML, respectively. For ${t}_{\mathrm{Fe}}\ensuremath{\leqslant}2.5$ ML, our uncoated ``free'' Fe clusters exhibit intrinsic magnetic ordering below ${T}_{B}$, contrary to literature reports on metal-coated Fe clusters on GaAs. Our present results demonstrate that the nature of the percolation transition for free Fe nanoclusters on GaAs(001) in UHV is from superparamagnetism to ferromagnetism. From the M\ossbauer spectral area, a very low Debye temperature ${\ensuremath{\Theta}}_{D}$ of $196\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is deduced for these uncoated Fe nanoclusters in UHV, indicating a strong phonon softening in the clusters.

Self-Organization of Gold-Containing Hydrogen-Bonded Rosette Assemblies on Graphite Surface

The self-organization of supramolecular structures, in particular gold-containing hydrogen-bonded rosettes, on highly oriented pyrolytic graphite (HOPG) surfaces was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning tunneling microscopy (STM). TM-AFM and high-resolution STM results show that these hydrogen-bonded assemblies self-organize to form highly ordered domains on HOPG surfaces. We find that a subtle change in one of the building blocks induces two different orientations of the assembly with respect to the surface. These results provide information on the control over the construction of supramolecular nanoarchitectures in 2D with the potential for the manufacturing of functional materials based on structural manipulation of molecular components.

Effects of hyperthermal proton bombardment on alkanethiol self-assembled monolayer on Au(1 1 1)

The effects of hyperthermal proton bombardment on alkanethiol self-assembled monolayer (SAM) on Au(1 1 1) are studied with scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The STM and XPS results show that proton bombardment with proton energy as low as 2 eV can induce cross-linking of the adsorbed alkanethiols and transform the original ordered SAM lattice to an array of nanoclusters of the cross-linked alkanethiols. For a bombardment at 3 eV with a fluence of 3 × 1 0 15 cm −2, the typical cluster size is about 5 nm. In addition, the cluster size distribution is narrow, with no cluster larger than 8 nm. The cluster growth can be promoted by increasing the fluence at a fixed bombardment energy or increasing the energy at a fixed fluence. This indicates that surface diffusion of alkanethiols and cluster growth can be harnessed by the control of the bombardment energy and fluence.

Bias voltage dependent shift of the atomic-scale structure of a Ge(111)-(2 × 1) reconstructed surface measured by low temperature scanning tunneling microscopy

The results of low temperature scanning tunneling microscopy (STM) investigations of a clean Ge(111) surface are presented. Bias dependent shifts of the atomic-scale structure caused by the (2 × 1) reconstruction of the Ge(111) surface are observed. A detailed comparison of experimental data with theoretical predictions based on the π-bonded chain model indicates that inelastic tip-sample interaction plays a significant role in STM imaging of the Ge(111)-(2 × 1) reconstructed surface.

Growth studies of Ta‐based films on Si with STM and XPS

AbstractThe aim of the experiments was the characterization of the first stages of film growth on silicon substrates with scanning tunneling microscopy (STM) and angle‐resolved X‐ray photoelectron spectroscopy (ARXPS). Thin Ta films were deposited in situ by magnetron sputtering onto Si in the thickness range from less than one monolayer up to 10 nm. The results of the experiments show a formation of tantalum silicide at the interface to the substrate. The local current‐voltage characteristics obtained by scanning tunneling spectroscopy (STS) measurements show that the first stages of tantalum silicide growth are characterized by island formation. The in situ STM results are in good agreement with those of the ARXPS investigations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scanning tunneling microscopy of the formation, transformation, and property of oligothiophene self-organizations on graphite and gold surfaces

Two alkyl-substituted dual oligothiophenes, quarterthiophene (4T)-trimethylene (tm)-octithiophene (8T) and 4T-tm-4T, were used to fabricate molecular structures on highly oriented pyrolytic graphite and Au(111) surfaces. The resulted structures were investigated by scanning tunneling microscopy. The 4T-tm-8T and 4T-tm-4T molecules self-organize into long-range ordered structures with linear and/or quasi-hexagonal patterns on highly oriented pyrolytic graphite at ambient temperature. Thermal annealing induced a phase transformation from quasi-hexagonal to linear in 4T-tm-8T adlayer. The molecules adsorbed on Au(111) surface in randomly folded and linear conformation. Based on scanning tunneling microscopy results, the structural models for different self-organizations were proposed. Scanning tunneling spectroscopy measurement showed the electronic property of individual molecules in the patterns. These results are significant in understanding the chemistry of molecular structure, including its formation, transformation, and electronic properties. They also help to fabricate oligothiophene assemblies with desired structures for future molecular devices.

The energy-resolved STM results, their apparent inhomogeneityand the true bulk behaviour of the HTSC cuprates, from anegative-U perspective

An attempt is made to reconcile the results of the prime experiments on thehigh temperature superconducting cuprates from Loram and coworkers (2006Preprint cond-mat/0609305) and Davis and co-workers (2005 Phys. Rev. Lett. 94 197005;2005 Science 309 1048), relating respectively to electronic specific heat and toenergy-resolved scanning tunnelling microscopy (STM), over the question of electronicinhomogeneity. I see the latter, at the appropriate level, as being essential to the evolutionof HTSC in these materials, and try to bring the above key works into alignment, not onlywith each other but with a wealth of related work. This is undertaken around anegative-U scenario that I have long advanced, a reassessment being advocated of the way in which theSTM and angle resolved photoemission spectroscopy (ARPES) results are currently beinginterpreted in regard to the ‘coherence peaks’.

Atom-by-Atom Growth of CdS Thin Films by an Electrochemical Co-deposition Method: Effects of pH on the Growth Mechanism and Structure

Thin films of cadmium sulfide (CdS) were prepared on Au(111) by a new electrochemical co-deposition method. The appropriate electrodeposition potentials based on the underpotential deposition potentials (upd) of Cd and S have been determined by the cyclic voltammetric measurements. The films were grown at room temperature from an aqueous solution of CdSO4, ethylenediaminetetraacetic acid (EDTA), and Na2S at various pHs. The thin films were characterized by atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and UV−visible spectroscopy techniques. The Cd/S ratio of electrodeposited thin films was found to be 1:1 determined by electron dispersive spectroscopy (EDS). XRD, STM, and AFM results revealed that the electrosynthesized thin films deposited at −0.520 V (vs Ag/AgCl) have a highly preferential orientation along (110) directions for hexagonal crystal and (111) directions for cubic crystal in the solutions with pH = 4 and pH = 5, respectively. The phase transitio...

Growth and Energy Bandgap Formation of Silicon Nitride Films in Radical Nitridation

The surface profiles and energy bandgap of sub-nanometer-thick silicon nitride layers, which were formed by radical nitridation, were analyzed by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). According to the STM results, the surface roughness of the nitride layers depends only on substrate temperature regardless of radio frequency (RF) power, which indicates that the growth mode of the silicon nitride layer in radical nitridation is attributed to the surface silicon atom migration but does not depend on the types of nitrogen radical. In contrast, STS spectra show that the energy bandgap of the silicon nitride layer is significantly changed with not only substrate temperature but also RF power. The densities of nitrogen radicals were analyzed by absorption and emission spectroscopy, which suggests that the contribution of the excited-state nitrogen atoms to radical nitridation increases as RF power increases. The monolayer-thick nitride layer with both an atomically flat surface and a wide energy bandgap can be formed under appropriate conditions, because they are limited by different thermaly activated features.

From nanowires to nanoislands: Morphological evolutions of erbium silicide nanostructures formed on the vicinal Si(001) surface

The growth behavior and morphology evolution of erbium silicide nanostructures are studied on the vicinal Si(001) surface with a 4° miscut angle towards the [110] direction as the functions of annealing temperature, annealing time, and Er coverage. Three kinds of nanostructures can be observed on the surface: nanowires, rectangular nanoislands and square nanoislands. The experimental results reveal that nanowires in AlB2-type structure and the nanoislands in ThSi2-type structure can be formed at 600–650 and 700–750°C, respectively. At the final growth stage, the nanowires of erbium silicide undergo Ostwald ripening. At high coverage of two monolayers, many AlB2-type nanoislands will appear in a rectangular shape on the sample surface. According to the AlB2-type crystalline structure of ErSi2 and the scanning tunneling microscopy results obtained on the flat and vicinal substrate surfaces, a model is suggested to describe the ErSi2∕Si(001) interfacial structure. This model explains the spatial orientation relationship between erbium silicide nanowire and Si dimer row, and is consistent with Er-induced (2×3) reconstruction at the initial stage of Er growth on the Si(001) surface.

Nanostructure characterization of (SmS) 1.19TaS 2 by means of STM/STS

Layered transition-metal dichalcogenide (LTMD) materials have been studied extensively. These materials can be easily intercalated with different elements due to their layered structure. This report shows the results of scanning tunneling microscopy and spectroscopy (STM/S) investigations of (SmS) 1.19TaS 2 misfit-layer compound. This compound is composed of two trigonal triple layers of S–Ta-S separated from one another by the SmS double layer. The structure of SmS layers exhibits square symmetry contrary to the trigonal TaS 2 host structure. The observed lattice symmetry suggests that the SmS layer was exposed due to cleavage process made before measurements. The I– V characteristics measured above the sample surface showed metallic-like behavior of this material.

Modulation structure in the STM images of the non-metallic misfit-layer compound (PbS) 1.12VS 2

This paper presents the scanning tunneling microscopy (STM) results of the misfit-layer compound (PbS) 1.12VS 2, which is constructed of alternately stacking of PbS (Q) and VS 2 (H) layers. Temperature dependent resistivity measurements show a semiconducting behavior with small activation energies. Unlike the metallic 1Q/1H type of compounds we have succeeded to take both the STM images of a Q layer and a H layer, because electron tunneling from the underlying H layer is suppressed when intermediate positive bias voltage ( V b) is applied to a tip. At V b = 0.15 V the image shows pseudo-tetragonal arrays of bright spots, although it is obscure with decreasing bias voltage and disappears at less than 10 mV. A modulation structure is found on the H layer of a stepped surface on which surface atoms are undulated in a period being twice the V–V interatomic distance in the [1 0] H or the [1 1] H direction.

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880–920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.