Atomic-Level Characterization of Cubic Silicon Carbide Surfaces—A Review

Scanning tunneling microscopy (STM) images of the cubic β-SiC (100) and β-SiC (111) surfaces are taken after annealing to 1200°C to eliminate the surface oxide. Low-energy electron diffraction (LEED) patterns of the β-SiC (111) surface show a 6 √ 3 × 6 √ 3 geometry, while STM images show a 6 × 6 geometry. Contrast reversal is observed as tunneling voltage bias is reversed. Spectroscopic I/V measurements indicate the presence of a graphite layer on the top surface. A model of the surface is proposed where an incommensurate graphite monolayer is grown over a (1 × 1) Si-terminated β-SiC (111) surface. This model helps to explain the discrepancy between the 6 √ 3 × 6 √ 3 LEED pattern and the 6 × 6 geometry observed in STM images. Charge transfer between certain carbon atoms and silicon atoms gives rise to the 6 × 6 geometry and the contrast reversal.     

Revealing the atomic structure of the buffer layer between SiC(0 0 0 1) and epitaxial graphene

On the SiC(0 0 0 1) surface (the silicon face of SiC), epitaxial graphene is obtained by sublimation of Si from the substrate. The graphene film is separated from the bulk by a carbon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling the buffer layer from the SiC(0 0 0 1) substrate by means of hydrogen intercalation. Atomic resolution STM images of the buffer layer reveal that, within the periodic structural corrugation of this interfacial layer, the arrangement of atoms is topologically identical to that of graphene. After hydrogen intercalation, we show that the resulting QFMLG is relieved from the periodic corrugation and presents no detectable defect sites.     

Investigation of h-BN/Rh(111) nanomesh interacting with water and atomic hydrogen

Recent STM experiments show that by exposing h-BN/Rh(111) nanomesh to water or atomic hydrogen interesting phenomena can be observed. We investigated by Density Functional Theory (DFT) the structure of bare nanomesh as well as in the presence of water clusters and atomic hydrogen. Our simulations allow the correct interpretation of the observed modifications of the STM topography under different tested conditions. For example, we could determine that the frequently observed three protrusions within the pore appearing in STM images obtained after dosing small amounts of water, are most likely determined by water hexamers. We also could confirm that the flattening of the h-BN overlayer after dosing atomic hydrogen is determined by the intercalation of the latter between BN and metal, which prevents the effective binding between N and Rh.     

Scanning tunneling microscopy observation of circular electronic superstructures on multiwalled carbon nanotubes functionalised by nitric acid

We have performed STM measurements on nitric acid treated, functionalised multiwalled carbon nanotubes. Atomic resolution STM images around defects created by the acid treatment show circular √3 × √3 R30° type superstructures. We have used a simple scattering model to reproduce the experimentally observed STM images and show that such superstructures can be obtained by considering only the contribution of β site carbon atoms, known to dominate the STM image of graphite and multiwalled carbon nanotubes (MWCNTs).     

The Nature of the Catalytic Sites for H2 Dissociation

Scanning Tunneling Microscopy (STM) can reveal the nature of active sites on the surface of heterogeneous catalysts. This is shown for the case of the dissociation of molecular hydrogen on Pd(111), which has been studied recently both experimentally and theoretically. STM can image in real time to generate movies of adsorbed atoms diffusing on the catalyst surface and forming aggregates. Of particular interest is the behavior near saturation coverage, a situation that is common when catalysts operate under the gas pressures typical of many industrial reactions. Under these conditions, active catalyst sites are formed as a result of density fluctuations that free atoms at the catalyst surface of adsorbates, so that they become available for new reactions. Little is known about the structure of the sites generated in this process. While the end state of a dissociative adsorption of a diatomic molecule requires at least two empty sites to accommodate the reaction products, the initial state where the molecule adsorbs and dissociates, might be more complicated and its nature is unknown. The review shows how STM can provide an improved understanding of the nature of these initial sites.     

Formation and Geometry of a High-Coverage Oxygen Adlayer on Ru(001), the p(2 × 2)-3O Phase

A detailed low-energy electron diffraction (LEED)-IV analysis, complemented by scanning tunneling microscopy (STM) observations, was carried out for the apparent (2 × 2) structure of the oxygen-covered Ru(001) surface at a coverage of 0.75 ML. We present STM images of incomplete layers which allow one to define the symmetry of the ordered layer, in particular of the novel high density p(2 × 2)-3O phase. In the LEED-IV analysis we have tested 28 model structures; the results can be used for conclusions about the discrimination of this type of geometry determination. Our quantitative LEED analysis in connection with the STM results corroborates the model proposed before and shows that all of the oxygen atoms sit in the hcp sites with an averaged vertical distance to the outermost Ru layer of d^⊥_Ru–O = 1.22 Å. This value falls into the general trend of increasing d^⊥_Ru–O with oxygen coverage observed for the other ordered structures of adsorbed oxygen on Ru and is also predicted by recent total energy calculations. The O–Ru bonding distance of about 2.0 Å is essentially unchanged compared to the other structures. Considerable lateral and vertical displacements of both the O and the Ru atoms are found, with the O atoms being slightly displaced towards the fcc hollow site located in the center of three oxygen atoms. The two uppermost substrate layers are buckled; in the first layer three out of four Ru atoms of the (2 × 2) unit cell are shifted away laterally from their bulk positions. These shifts, globally as well as locally, can be understood in terms of local electron density changes induced by the adsorbed oxygen atoms.     

Modifying the STM tip for the ' ultimate ' imaging of the Si(111)-7×7 surface and metal-supported molecules

We report on high-resolution STM measurements with modified probe tips. First, both the rest atoms and adatoms of a Si(111)-7×7 surface are observed simultaneously. The visibility of rest atoms is dependent upon the sample bias voltage (less than -0.7 V) and is enhanced by sharpening the tip, which is rationalized by first-principles calculations. Second, a tip with a perylene molecule adsorbed at its apex is used to discriminate the molecular states and the metal states of the underlying Ag(110) surface, which is attributable to a mismatch between the energy levels of the functionalized tip and the adsorbates on silver. Lastly, high-resolution images of iron phthalocyanine (FePc) and zinc phthalocyanine (ZnPc) molecules on Au(111) are obtained by using an O(2)-terminated tip, and the images reveal rich intramolecular features arising from molecular orbitals that are not observed when using clean metallic tips.     

Graphene growth on Pt(111) and Au(111) using a MBE carbon solid-source

In this work we present a Molecular Beam Epitaxy (MBE) growth method to obtain graphene on noble metals using evaporation of carbon atoms from a carbon solid-source in ultra-high vacuum conditions. We have synthesized graphene (G) on different metal surfaces: from a well studied substrate as platinum, to a substrate where it can only be formed using innovative methods, as is the case of gold. For the characterization of the graphene layers we have used in situ surface science techniques as low energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM).One of the main advantages of our methodology is that low surface temperatures are required to form graphene. Thus, by annealing Pt(111) and Au(111) substrates up to 650 °C and 550 °C respectively during carbon evaporation, we have obtained the characteristic LEED diagrams commonly attributed to graphene on these surfaces. STM results further prove the formation of graphene. For the case of G on Pt(111), STM images show a long range ordering associated with moiré patterns that correspond to a monolayer of graphene on (111) platinum surface. On the other hand, G/Au(111) STM results reveal the formation of dendritic islands pinned to atomic step edges. This method opens up new possibilities for the formation of graphene on many different substrates with potential technological applications.     

Scanning tunneling spectroscopy of molecules on insulating films

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope (STM) to study the electron transport properties in the weak-coupling limit. The electronic decoupling provided by the films allows the direct imaging of the unperturbed molecular orbitals, as will be demonstrated in the case of individual pentacene molecules. The coupling between electronic states localized on the adsorbate and optical phonons in a polar insulator has two important implications: Peaks in conductance spectra resulting from resonant tunneling into electronic states of the molecules are significantly broadened by the presence of the insulator. Second, the ionic relaxations in a polar insulator may lead to an interesting charge bistability in atoms and molecules. STM-based molecular manipulation has been used to form a metallo-organic complex as well as to switch the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules.     

In situ STM study of surface catalytic reactions on TiO2(110) relevant to catalyst design

This paper briefly summarizes our recent work on the characterization of atom-resolved surface images of TiO₂(110) composed of Ti atoms, O atoms, defects and hydroxyls by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM). The paper also presents new kinetic aspects in the catalytic dehydration and dehydrogenation of formic acid on the characterized surface. Switchover of the reaction path from the dehydration to the dehydrogenation occurred by the presence of formic acid undetectable at the surface, where acidic formic acid molecules opened the basic catalysis. In situ STM observation revealed that the dehydrogenation reaction at 400–450 K was strongly suppressed in the vicinity of single-atom height steps. The suppressive effect of step ranged over 2.4 nm into terrace. It is likely that the catalyst with flat surfaces larger than 5 nm is active for the dehydrogenation of formic acid.     

New scanning tunneling microscopy technique enables systematic study of the unique electronic transition from graphite to graphene

A series of measurements using a novel technique called electrostatic-manipulation scanning tunneling microscopy were performed on a highly-oriented pyrolytic graphite (HOPG) surface. The electrostatic interaction between the STM tip and the sample can be tuned to produce both reversible and irreversible large-scale vertical movement of the HOPG surface. Under this influence, atomic-resolution STM images reveal that a continuous electronic reconstruction transition from a triangular symmetry, where only alternate atoms are imaged, to a honeycomb structure can be systematically controlled. First-principles calculations reveal that this transition can be related to vertical displacements of the top layer of graphite relative to the bulk. Detailed analysis of the band structure predicts that a transition from parabolic to linear bands occurs after a 0.09 nm displacement of the top layer.     

Atom-resolved noncontact atomic force microscopic and scanning tunneling microscopic observations of the structure and dynamic behavior of CeO2(1 1 1) surfaces

Atomic-scale structures and dynamic behaviors of CeO₂(1 1 1) surfaces were imaged by noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). Hexagonally arranged oxygen atoms, oxygen point vacancies, multiple oxygen vacancies, and hydrogen adatoms at the surfaces were visualized by atom-resolved NC-AFM observations. Multiple defects were stabilized by displacement of the surrounding oxygen atoms around the multiple defects, which gave enhanced brightness in the NC-AFM image due to a geometric reason. Multiple defects without reconstruction of the surrounding oxygen atoms were reactive and were healed by exposure to O₂ gas and methanol at RT. Successive NC-AFM and STM measurements of slightly reduced CeO₂(1 1 1) surfaces revealed that hopping of surface oxygen atoms faced to the metastable multiple defects was thermally activated even at room temperature (RT) and more promoted at higher temperatures. Heterogeneous feature of the reactivity of surface oxygen atoms with methanol was imaged by successive NC-AFM observations. These observations gave a new insight for understanding the surface structures and behavior of CeO_2−x with the facile oxygen reservoir and oxidation–reduction properties related to the unique catalysis.     

Nickel-containing nano-sized islands grown on Ge(111)-c(2 × 8) and Ag/Ge(111)-(√3 × √3) surfaces

The formation of nano-islands on both a Ge(111)-c(2 × 8) surface and an Ag/Ge(111)-(√3 × √3) surface evaporated with 0.1 ML Ni was investigated by scanning tunneling microscopy (STM). We have noticed that at temperatures lower than 670 K, the reaction between Ni and the individual substrate surfaces proceeds to form different structures: flat-topped islands with a 2√7 × 2√7 or a 3 × 3 reconstruction on the Ni/Ge(111)-c(2 × 8) surface vs. islands with a 7 × 7 reconstruction on the Ni/Ag/Ge(111)-(√3 × √3) surface. From this we have inferred that within a temperature range between room temperature and 670 K, the intermediate Ag layer retards mixing between Ni and Ge atoms. As a result, the grown islands are composed of pure Ni atoms. Within a temperature range from 670 to 770 K, most islands produced on the Ag/Ge(111)-(√3 × √3) surface are identical with those formed on the Ni/Ge(111)-c(2 × 8) surface, suggesting that above 670 K, Ni atoms are likely to bind with Ge atoms. However, an essential difference between STM images of the surfaces under study exists in the appearance of large elongated islands on the Ni/Ag/Ge(111)-(√3 × √3) surface. The formation of the latter is explained in terms of a difference in energy for Ni diffusion on the Ge(111)-c(2 × 8) and Ag/Ge(111)-(√3 × √3) surfaces.     

Carbon-tuned bonding method significantly enhanced the hydrogen storage of BN-Li complexes

Through first-principles calculations, we found doping carbon atoms onto BN monolayers (BNC) could significantly strengthen the Li bond on this material. Unlike the weak bond strength between Li atoms and the pristine BN layer, it is observed that Li atoms are strongly hybridized and donate their electrons to the doped substrate, which is responsible for the enhanced binding energy. Li adsorbed on the BNC layer can serve as a high-capacity hydrogen storage medium, without forming clusters, which can be recycled at room temperature. Eight polarized H(2) molecules are attached to two Li atoms with an optimal binding energy of 0.16-0.28 eV/H(2), which results from the electrostatic interaction of the polarized charge of hydrogen molecules with the electric field induced by positive Li atoms. This practical carbon-tuned BN-Li complex can work as a very high-capacity hydrogen storage medium with a gravimetric density of hydrogen of 12.2 wt%, which is much higher than the gravimetric goal of 5.5 wt % hydrogen set by the U.S. Department of Energy for 2015.     

Alloy formation during the electrochemical growth of a Ag-Cd ultrathin film on Au(1 1 1)

The electrodeposition of a Ag/Cd ultrathin film on a Au(1 1 1) surface and the formation of a surface alloy during this process have been studied using classical electrochemical techniques and in situ Scanning Tunneling Microscopy (STM). The films were obtained from separate electrolytes containing Ag⁺ or Cd²⁺ ions and from a multicomponent solution containing both ions. First, the polarization conditions were adjusted in order to form a Ag film by overpotential deposition. Afterwards, a Cd monolayer was formed onto this Au(1 1 1)/Ag modified surface by underpotential deposition. The voltammetric behavior of the Cd UPD and the in situ STM images indicated that the ultrathin Ag films were uniformly deposited and epitaxially oriented with respect to the Au(1 1 1) surface. Long time polarization experiments showed that a significant Ag-Cd surface alloying accompanied the formation of the Cd monolayer on the Au(1 1 1)/Ag modified surface, independent of the Ag film thickness. In the case of an extremely thin Ag layer (1 Ag ML) the STM images and long time polarization experiments revealed a solid state diffusion process of Cd, Ag, and Au atoms which can be responsible for the formation of different Ag-Cd or Au-Ag-Cd alloy phases.     

Restructuring of Hydrogenation Metal Catalysts Under the Influence of CO and H2

CO and H₂ structure and reactivity on single-crystal transition metal surfaces (platinum, rhodium, and palladium) were examined by surface-sensitive techniques including scanning tunneling microscopy (STM) and sum frequency generation (SFG) in high-pressure surface science studies. The studies indicated that ordered CO structures not observed in ultrahigh vacuum (UHV) can form at high pressure (10^-6–10³ torr). In addition, CO and H₂ induce metal atom mobility and restructure the surface. On platinum, CO dissociates at high temperature (≥ 500 K), and a platinum carbonyl precursor is implicated. Concerning catalytic reactions, structure sensitive CO dissociation plays an important role in the ignition of CO oxidation, whereas CO poisons olefin hydrogenation, which becomes CO desorption limited. Lastly, solid-state hydrogen atoms are more active for hydrogenation than surface hydrogen atoms. These results suggest that spatially and temporally resolved techniques would permit molecular studies of reaction intermediates of CO and H₂ in the future.     

A Combined XPS/STM and TPD Study of the Chemisorption and Reactions of Methyl Mercaptan at a Cu(110) Surface

Despite the structural similarities between methanol and methyl mercaptan (CH₃SH), replacing the oxygen atom by sulfur has a profound effect on their chemistry at copper surfaces. In a combined STM, XPS and TPD study of the reaction of methyl mercaptan with clean and partially oxidised Cu(110) surfaces we have found that unlike methanol, the scission of the SH bond (both in the presence of and in the absence of oxygen) to give adsorbed mercaptide (CH₃S(a)) results in a restructuring of the surface. Low concentrations of adsorbed mercaptide (< 1 × 10¹⁴ cm^-2) result in a severe degradation of the STM image due to the high mobility of the surface adlayer. A stable surface can be regained by increasing the concentration of mercaptide or the presence of another adsorbate such as oxygen. The reconstructed surface is characterised by very narrow terraces (typically 10-15Å wide) orientated mainly in the 1¯10 direction with a zig-zag structure. Higher resolution images of the terraces reveal an atomic scale structure with a c(2 × 2) unit cell, each cell containing (in total) two bright features. The XPS data confirm that mercaptide is present and show that the concentration at which islands of mercaptide become visible in the STM images is approximately 3 × 10¹⁴ cm^-2. At a surface concentration of 5.1 × 10¹⁴ cm^-2 the c(2 × 2) structure is seen to be complete, consistent with the c(2 × 2) unit cell containing two mercaptide species. On heating to 450K the mercaptide dissociates to give chemisorbed sulfur adatoms and methane. The latter implies that the hydrogen formed when the methyl mercaptan dissociates remains chemisorbed at the surface until removed by reaction with the methyl groups. At pre-oxidised surfaces where chemisorbed hydrogen is removed as water, mercaptide decomposition leads to ethane desorption with minor methane and ethane components, the latter indicating that methyl dehydrogenation is possible at the copper surface. STM shows that following the decomposition of the mercaptide adlayer the copper surface regains its original structure of broad terraces (typically 100-200Å wide), although a monolayer of chemisorbed sulfur is now present.Oxygen chemisorption is completely inhibited by mercaptide concentrations of 5 × 10¹⁴ cm^-2 but occurs at lower concentrations. The STM images show that the mercaptide and oxygen adsorbates form separate islands, in contrast to the coadsorption of hydrogen sulfide and oxygen. The influence of oxygen on the thermal desorption of mercaptide is discussed in the light of the structural data.     

Stable molecular orientations of a C60 dimer in a photoinduced dimer row

We present a theoretical study on stable orientations of photoinduced C₆₀ dimers in a C₆₀ monolayer film, on the basis of first-principles calculations within the framework of the density functional theory. Using Tersoff and Hamann’s tunnel current theory, current images of scanning tunneling microscopy (STM) for a C₆₀ dimer in several different orientations are reproduced from the spatial distribution of the wave functions of an isolated C₆₀ dimer. Comparing the resultant current images with experimentally obtained STM images, in consequence, two stable orientations of C₆₀ dimers are determined: one is that a C-C double bond shared by two adjacent hexagons faces to vacuum, and the other is that a C-C single bond shared by a hexagon and an adjacent pentagon faces to vacuum. These orientations are supported by empirical total-energy calculations.     

Structural Understanding of Self-Assembled Rare Earth Disilicide Nanostructures Via Scanning Probe Microscopy and First Principles Studies

We performed systematic experimental and computational studies to investigate the adsorption geometries of Y atoms on the Si(001) surface. This paves a way for understanding and eventually controlling the growth of rare earth disilicide wires on the Si(001) substrate that are promising for various applications. For a single Y atom, the interrow_dn site was found to be at least 400 meV lower in energy than other possible binding sites. The emulated STM images are in good agreement with experimental results of Er on Si(001). The strong bias and coverage dependence indicates the need for theoretical guidance for the correct interpretation of experimental data. We elucidate the Y-Si binding mechanism and provide insights toward the onset of formation of hexagonal rare earth disilicide wires.     

The electronic and magnetic properties of functionalized silicene: a first-principles study

ABSTRACT: Based on first-principles calculations, we study the structural, electronic, and magnetic properties of two-dimensional silicene saturated with hydrogen and bromine atoms. It is found that the fully saturated silicene exhibits nonmagnetic semiconducting behavior, while half-saturation on only one side with hydrogen or bromine results in the localized and unpaired electrons of the unsaturated Si atoms, showing ferromagnetic semiconducting or half-metallic properties, respectively. Total energy calculations show that the half-hydrogenated silicene exhibits a ferromagnetic order, while the half-brominated one exhibits an antiferromagnetic behavior.