Atomic-resolution Scanning Tunneling Microscopy Research Articles

Investigation of the 4H–SiC surface

The silicon carbide (SiC) surface is more complex than that of silicon and can be carbon-terminated or silicon-terminated, and can exist as several reconstructions. Investigations of the surface structure as a function of temperature, under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED), are presented. The 4H–SiC surface can be passivated using a silicon deposition/evaporation technique to reconstruct the surface. This has a significant effect on the electrical behaviour of metal contacts to the silicon carbide surface, critical in any electronic device. Atomic resolution STM studies of the 4H–SiC surface have revealed step features and micropipe defects in unprecedented detail. STM has also been used to image clusters of metal deposited on the 4H–SiC surface. The effect of annealing on the behaviour of these nickel clusters is also presented. The surface of the silicon carbide is extremely important in the fabrication of silicon carbide electronic devices and this paper presents a discussion of the SiC surface with particular reference to its impact on SiC device applications in power electronics.

Site-Specific Chemistry of Ethylene on Si(114)-(2 × 1)

Si(114)-(2 x 1) is a stable high-index surface with a single domain-like surface reconstruction composed of parallel rows of 3-fold coordinated rebonded atoms (resembling adatoms), dimers, and tetramers (dimers combined with non-dimerized atoms in diene-like structures). The distinct chemical properties expected for the oriented array of different structural motifs make this surface an interesting template for direct, comparative studies of structure-dependent reactions. We have studied the room-temperature adsorption of ethylene on rebonded atoms, dimers, and tetramers on Si(114)-(2 x 1) by atomic-resolution scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations of structure, energetics, and simulated STM images. Ethylene chemisorbs in four distinct geometries on Si(114), all of which are di-σ-bonded structures, with all adsorption sites occupied in a well-ordered monolayer at saturation coverage. In order of decreasing adsorption probability, the ethylene-Si reaction products were ethylene-bridged rebonded atoms, dimer [2 + 2] adducts, tetramer [4 + 2] adducts on one side of the tetramer, and (at high coverages only) tetramer [2 + 2] adducts on the dimer component of the tetramer. The calculations indicate that adsorption is kinetically limited at room temperature, with one energetically favorable reaction on the tetramer inhibited by an adsorption barrier. A particularly interesting aspect of the Si(114)-(2 x 1) surface demonstrated by this work is that it can act as both a dienophile (dimer sites) and a diene (tetramer sites) in cycloaddition adsorption reactions.

Lattice-work structure of a TiO2(001) surface studied by STM, core-level spectroscopies and DFT calculations

Abstract The lattice-work structure of a rutile TiO 2 (0 0 1) surface, which is faceted to {1 1 4} plane, was studied by combining scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) and density functional theory (DFT) calculations. Synchrotron XPS and NEXAFS spectra taken for the faceted surface indicated that this surface possesses a stoichiometric composition. We propose a new stoichiometric structure model on the basis of XPS, NEXAFS, and atomically-resolved STM observations. DFT calculations revealed that the present stoichiometric model is energetically more favorable compared to the previously proposed models.

Mixed layer formation of copper overlayers on Ni(1 1 0) surface

Copper overlayer formation on the Ni(1 1 0) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Atom-resolved STM images showed that initially deposited Cu is replaced with surface Ni atoms forming atom-size depressions on the Ni(1 1 0) terraces and a Ni-rich quasi-one-dimensional island along the [ 1 1 ¯ 0 ] direction. Further Cu deposition yields a mosaic structure on the islands, indicating Cu/Ni mixed layer formation. From the quantitative measurement of the Cu/Ni ratio on the substrate and the islands, impinging Cu will be replaced with surface Ni whereas expelled Ni and directly impinging Cu to the island form the mixed island. The number of Cu atoms in the islands, however, more than the directly impinging Cu, indicate significant Cu/Ni replacement at the periphery of the island.

An atomic resolution scanning tunneling microscope that applies external tensile stress andstrain in an ultrahigh vacuum

We have developed an ultrahigh vacuum scanning tunneling microscope with an in situexternal stress application capability in order to determine the effects of stress and strainon surface atomistic structures. It is necessary to understand these effects becausecontrolling them will be a key technology that will very likely be used in futurenanometer-scale fabrication processes. We used our microscope to demonstrate atomicresolution imaging under external tensile stress and strain on the surfaces of wafers ofSi(111) and Si(001). We also successfully observed domain redistribution induced byapplying uniaxial stress at an elevated temperature on the surface of a wafer of vicinalSi(100). We confirmed that domains for which an applied tensile stress is directed along thedimer bond become less stable and shrink. This suggests that it may be feasible tofabricate single domain surfaces in a process that controls surface stress and strain.

A cryogenic Quadraprobe scanning tunneling microscope system with fabrication capability for nanotransport research

We describe the development and the capabilities of an advanced system for nanoscale electrical transport studies. This system consists of a low temperature four-probe scanning tunneling microscope (STM) and a high-resolution scanning electron microscope coupled to a molecular-beam epitaxy sample preparation chamber. The four STM probes can be manipulated independently with subnanometer precision, enabling atomic resolution STM imaging and four-point electrical transport study of surface electronic systems and nanostructured materials at temperatures down to 10 K. Additionally, an integrated energy analyzer allows for scanning Auger microscopy to probe chemical species of nanostructures. Some testing results are presented.

Complex superstructure patterns near defect sites of carbon nanotubes and graphite

Atomic resolution scanning tunneling microscope (STM) images of native and artificially created defect sites on graphite and carbon nanotubes were compared. The presence of position-dependent coexisting superstructure patterns was identified on all of the investigated samples. The results indicate that superstructure patterns are mainly determined by the available scattered states of the system rather than the detailed structure of the defect site. We propose an interference model, which can explain the presence of coexisting superstructures both on graphite and carbon nanotubes. The model predicts reduced corrugation amplitude for the case of graphite as compared to carbon nanotubes due to the wave-vector averaging on the Fermi circle.

Selective silver atom interaction atβ−SiC(100)surfaces: From anisotropic diffusion to metal atomic wires and stripes

Silver (Ag) atom interaction on beta-SiC(100) surface reconstructions is investigated by atom-resolved scanning tunneling microscopy. On the 3x2 (Si-rich) reconstruction, the adsorbate-adsorbate interaction is dominant with no surface wetting, leading to Ag cluster formation. In contrast, on the c(4x2) Si-terminated reconstruction, almost equivalent Ag-Ag and Ag-surface interactions allow selective one dimensional nano-object formation including Ag atomic wires and stripes following the substrate registry. Their orientation is mediated by anisotropic Ag atom diffusion occurring along Si-dimer rows at 25 degrees C and perpendicularly to them at elevated temperatures, suggesting dimer flipping as diffusion barrier. These metal nanowires potentially open up cross-wiring capability in massively parallel Si atomic lines network.

Towards hybrid silicon-organic molecular electronics: The stability of acetone on the Si(0 0 1) surface

We present the results of a combined study using scanning tunneling microscopy (STM) and density functional theory (DFT) of the interaction of acetone [(CH 3) 2CO] with the Si(0 0 1) surface. Three distinct adsorbate features were observed using atomic-resolution STM. One of the features appears as a bright protrusion located above a Si–Si dimer, while the other two are asymmetric about the dimer row and involve a second neighboring Si–Si dimer. One of the two asymmetric features has a protrusion located between the two dimers, while the other has a protrusion which is located at the site of a single dimer and exhibits a dimer sized depression on the adjacent dimer. DFT calculations have been performed for two structures; the four-membered ring structure and dissociation structure. Our calculations show that the bright single-dimer sized feature observed in the STM images could be attributed to either of these two calculated structures. However, neither of the two calculated structures can explain the appearance of the two-dimer wide asymmetric features observed in the experiment.

Clean Up on Graphene

APPLIED PHYSICS Graphene, which comprises exfoliated sheets of graphite that are often supported on a dielectric substrate, can display unusual electronic properties that arise through two-dimensional confinement, and in device configurations, biasing of the dielectric can be used to control its conductivity. Although these materials are often assumed to be nearly ideal, in practice the dielectric layer could have trapped charges, and materials used in processing could remain on the graphene. Ishigami et al. present atomic-resolution scanning tunneling microscopy studies of graphene devices supported on silica. The as-processed graphene surfaces are covered with photoresist and could not be atomically imaged, even after solvent cleaning. Exposure to an argon-hydrogen atmosphere at 400°C removes the photoresist layer and reveals the influence of the dielectric layer: In addition to the expected hexagonal patterns, a triangular lattice is observed, either from film curvature or the effect of trapped charges. The graphene layer exhibits corrugations that follow the underlying substrate's roughness. — PDS Nano Lett. 7 , 10.1021/nl070613a (2007).

Location and coordination of promoter atoms in Co- and Ni-promoted MoS2-based hydrotreating catalysts

In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to investigate the atomic-scale structure of the active Co- or Ni-promoted MoS2 nanoclusters in hydrotreating catalysts. Co-promoted MoS2 nanoclusters (Co–Mo–S) are found to adopt a hexagonal shape, with Co atoms preferentially located at (1¯010) edges with a 50% sulfur coverage. The first atom-resolved STM images of the Ni-promoted MoS2 nanoclusters (Ni–Mo–S) reveal that the addition of Ni also leads to truncated morphologies, but the degree of truncation and the Ni sites are observed to depend on the nanocluster size. Larger clusters (type A) are structurally similar to Co–Mo–S exposing fully Ni-substituted (1¯010) edges with a 50% S coverage. Smaller clusters (type B) show dodecagonal shapes terminated by three different edges, all of which contain Ni-promoter atoms fully or partially substituting the Mo atoms. The findings may shed more light on the different selectivities observed for the Co- and Ni-promoted hydrotreating catalysts.

Atomic Structure of Graphene on SiO2

We employ scanning probe microscopy to reveal atomic structures and nanoscale morphology of graphene-based electronic devices (i.e., a graphene sheet supported by an insulating silicon dioxide substrate) for the first time. Atomic resolution scanning tunneling microscopy images reveal the presence of a strong spatially dependent perturbation, which breaks the hexagonal lattice symmetry of the graphitic lattice. Structural corrugations of the graphene sheet partially conform to the underlying silicon oxide substrate. These effects are obscured or modified on graphene devices processed with normal lithographic methods, as they are covered with a layer of photoresist residue. We enable our experiments by a novel cleaning process to produce atomically clean graphene sheets.

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

Scanning-tunnelling spectra of cuprates

The study of bosonic modes that couple to the charge carriers is a key element in understanding superconductivity. Using atomic-resolution scanning-tunnelling microscopy (STM) to extract the spectrum of these modes in the high-temperature superconductor Bi2Sr2CaCu2O(8+delta), Lee et al. find a mode whose frequency does not depend on doping but that changes on isotopic substitution of 16O with 18O. From this, they infer a role for lattice modes (phonons). However, examination of their data reveals a weaker, but distinct, feature that has all the characteristics of the magnetic excitation identified as the bosonic mode in other competing experiments. We therefore suggest that the lattice mode seen by Lee et al. is not relevant to superconductivity and is due to inelastic tunnelling through the insulating oxide layer.

In-situ examination of the selective etching of an alkanethiol monolayer covered Au{111} surface

An examination of the selective etching mechanism of a 1-alkanethiol self-assembled monolayer (SAM) covered Au{111} surface using in-situ atomic force microscopy (AFM) and molecular resolution scanning tunnelling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au{111} surface and typically contains nanoscale non-uniformities such as pinholes, domain boundaries and monatomic depressions. During etching in a ferri/ferrocyanide water-based etchant, selective and preferential etching occurs at SAM covered Au(111) terrace and step edges where a lower SAM packing density is observed, resulting in triangular islands being relieved. The triangular islands are commensurate with the Au(111) lattice with their long edges parallel to its [11¯0] direction. Thus, SAM etching is selective and preferential attack is localized to defects and step edges at sites of high molecular density contrast.

Electron scattering in a multiwall carbon nanotube bend junction studied by scanning tunneling microscopy

The atomic resolution scanning tunneling microscopy investigation of a multiwall carbon nanotube bend junction is reported. Atomic resolution images taken at the junction region revealed position-dependent modulation of the electronic density of states, with a period larger than but commensurate to the underlying atomic lattice, attributed to the scattering of electrons on defect sites present in the junction region. We propose an interference model, suitable to interpret the experimentally observed electron density patterns by considering electronic states near the bands crossing points involved in the scattering processes. The model predicts that complex charge density oscillations present near defects are tunable by varying the applied bias potential.

Morphological evolution in oxygen-induced faceting ofRe(123¯1)

We have studied oxygen-induced faceting of the atomically rough $\mathrm{Re}(12\overline{3}1)$ surface by means of Auger electron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy (STM). In contrast to previous faceting studies on other refractory metal surfaces, where simple morphologies of the facets were reported, we find a coverage-dependent morphological evolution of the facets ranging from long sawtooth ridges to complex structures exposing four different facets. The faceting occurs only when oxygen coverage $(\ensuremath{\theta})$ exceeds 0.5 monolayer (ML) and the surface is annealed at $\ensuremath{\ge}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low oxygen coverage $(0.5\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, the $\mathrm{O}∕\mathrm{Re}(12\overline{3}1)$ surface becomes partially faceted upon annealing; further increasing of oxygen coverage $(0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$ causes the surface to become completely faceted, forming long sawtooth ridges along the $[\overline{2}113]$ direction with typical dimensions of $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in width and $g50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in length upon annealing at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The size of the ridges grows with annealing temperature and annealing time, and the distance between the ridges is quite uniform. The two sides of each ridge have $(01\overline{1}0)$ and $(11\overline{2}1)$ orientations, and atomic-resolution STM images reveal that the edge of the ridge is atomically sharp. For $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, a third set of facets, identified as $(10\overline{1}0)$, emerges and truncates the original ridges. With the surface fully covered by oxygen $(\ensuremath{\theta}=1\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, a fourth facet $(01\overline{1}1)$ also becomes prominent upon annealing. This morphological evolution is accompanied by a reduction of the average ridge length along $[\overline{2}113]$, indicating that the $(11\overline{2}1)$ facet is metastable. Our work demonstrates that even in a simple adsorbate/substrate system, the adsorbate-induced modification of the anisotropy of surface free energy can induce a complex sequence of changes in the surface morphology. The faceted Re surfaces may be model systems to study structure sensitivity in catalytic reactions, and may also provide promising templates to grow nanostructures.

Self-assembly of ultrafine nanolines upon Ho reaction with the Ge(001) surface

The reaction of the rare earth metal Ho with the Ge(001) surface at 440°C has been studied by scanning tunneling microscopy (STM). The self-assembly of ultrafine nanolines growing along substrate ⟨110⟩ directions has been observed, and based on atomic resolution STM images, the authors propose a model of the nanolines and comment on their relationship to the very initial stages of growth of a hexagonal germanide structure. The authors further report the presence of nanoscale trenches associated with well-ordered lines of missing dimer defects and discuss the relationship of these to the nanolines. Their results have possible applications involving interconnects or templating in nanoscale devices, and additionally, may provide insight into the nucleation mechanism of coarser nanowires.

Ordering of TiO2-Based Nanostructures on SrTiO3(001) Surfaces

A class of nanostructured surface phases on SrTiO3(001) is reported and characterized through atomic-resolution scanning tunneling microscopy and Auger electron spectroscopy. These surface phases are created via argon ion sputtering and UHV annealing and form close-packed domains of highly ordered nanostructures. Depending on the type of nanostructures present, the domain ordering exhibit either (6 x 2), (9 x 2), (12 x 2), (6 x 8), or (7 x 4) surface patterning. The nanostructures are composed of TiO2-derived complexes surrounded by a TiO2 surface termination. Such surface ordering phenomena introduce another level of complexity in the chemistry of perovskite oxide surfaces and provide a basis from which potential photocatalytic and molecular-ordering applications may be developed.

Scanning-tunneling-microscopy of the formation of carbon nanocaps on SiC(0 0 0 −1)

Formation of nanosized cap structures on a thermally treated 6H–SiC(0 0 0 −1) substrate was investigated using atomic-resolution ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). After removal of surface oxides by heating under UHV, clusters of carbon particles 1–2 nanometer in diameter were formed on the SiC surface at 1150 °C. Upon heating above 1150 °C, the carbon nanoparticles in the clusters merged, forming nanosized cap structures, which subsequently transformed into carbon nanotubes (CNTs). The clusters of nanosized carbon particles are thought to be the precursors of carbon nanocaps, and their sizes are strongly related to the sizes of CNT caps forming on the SiC surface.