An STM study of the adsorption of Pb on Cu(100): formation of an ordered surface alloy

The adsorption of Pb on Cu(100) from 0 to 1 ML was investigated by UHV scanning tunneling microscopy. We obtained atomic resolution images of the different superstructures which appear at 300 K with increasing coverage (c(4 × 4), c(2 × 2) and c( √2)R45°). We confirm recent results and propose, partly on the basis of low temperature studies, new arguments in favour of an incorporation of lead atoms in the surface layer of copper for low coverage. We demonstrate that the c(4 × 4) superstructure corresponds to an ordered surface alloy of Pb₃Cu₄ composition, by investigating separately the alloying and de-alloying transitions. De-alloying occurs during the first-order transition between the c(4 × 4) and c(2 × 2) superstructures.     

Probing dynamics of a phase transition of two-dimensional nano-domains with STM imaging and manipulation

A reversible, temperature-driven structural surface phase transition of Pb/Si(1 1 1) nano-domains is studied with a variable-temperature scanning tunneling microscope (STM). Finite-size effects of the transition are clearly demonstrated. Most importantly, structural fluctuations in the low-temperature phase can be induced by the direct interaction between the tip atoms and the surface atoms. The structural changes reveal dynamics in the low-temperature phase. Amazingly, the largest size of the domains that can be manipulated decreases with decreasing sample temperature.     

Diffusive atom transport along step edges on Ag(1 1 1) at 295 K

The entropy-driven relaxation of a unique, non-equilibrium step edge configuration on the Ag(1 1 1) surface was observed using time-resolved STM imaging at room temperature. Using the Gibbs–Thomson relation, the relaxation process is quantitatively described as diffusive mass transport in terms of a gradient in the chemical potential along the monoatomic step edge. The STM data directly show that mass transport on Ag(1 1 1) is dominated by step edge diffusion at 295 K, and allow an estimate of the corresponding effective energy barrier. We obtain E_eff=0.49±0.05 eV and compare this value with recent results on island diffusion studies.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

The adsorption of C6H5Cl on Si(111)7 × 7 studied by STM

The adsorption of chlorobenzene on Si(111)7 × 7 at room temperature was studied by scanning tunneling microscopy (STM). Selective chemisorption was observed at different adatom sites. It was found that the center adatoms were more reactive than the corner adatoms, and the faulted half of the unit cell was more reactive than the unfaulted. The mechanism is discussed in terms of the electronic and atomic structures in Si(111)7 × 7. Both preferences indicate that chlorobenzene was present initially in a mobile precursor state.     

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb-Si interfacial structure.     

Diffusion of Pb adatom and ad-dimer on Si(1 0 0) from ab initio studies

The diffusion pathways of Pb adatoms and ad-dimers on Si(1 0 0) are investigated by first-principles calculations. Pb adatoms are found to diffuse on top of the Si(1 0 0) dimer row with an energy barrier of 0.31 eV. However, Pb dimers are energetically more stable. Pb dimers on top of the dimer row have a high energy barrier (0.95 eV) to rotate from the lowest energy configuration to the orientation parallel to the underlying Si(1 0 0) dimer row. Once the ad-dimer is oriented parallel to Si(1 0 0) dimer row, they can diffuse along the dimer row with an energy barrier of only 0.32 eV.     

STM study of low pressure adsorption of silane on Si(111)7 × 7

We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH₂ species by chemisorption and involves the breaking of Si---Si backbonds of the adatom.     

Observation of new two-dimensional periodic structures in the confined regions of the reconstructed Pt(100) surface

New 2D periodic structures have been observed by STM in the regions of the reconstructed Pt(100) surface which are confined by domain boundaries or lattice steps. These structures can be seen only in a very narrow energy window near the Fermi level, and they are strongly correlated to the original atomic arrangements of the surface. These structures arise most probably from a modification in the distribution of the electronic density of states which is strongly coupled to the ion cores of the surface to produce a periodic shift in the atomic rearrangements in order to minimize the strain and free energy of the surface.     

A reversible reaction forming (---Cu---O---) strings and (Cu)6-clusters on Ag(110) shown by STM

An artificial new surface of (---Cu---O---) chains grown on Ag(110) surface was prepared by reacting a surface with Cu atoms, where the (---Cu---O---) chains grow in the [1 0] direction and are self-assembled on the Ag(110) surface in a (2 x 2)-p2mg structure. When the Cu---O/Ag(110) surface was heated in vacuum, the (---Cu---O---) chain decomposed to uniform cluster dots arranged along the [1 0] direction, where the cluster dots were composed of six Cu atoms. When the Ag(110) surface with the Cu---cluster dots was exposed to O₂, the (---Cu---O---) lines were redrawn along the [1 0] direction by reacting a s in the [1 0] direction with O₂. This is a reversible chemical reaction in one dimensional regime proved in atomic resolution.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Surface defects created by low energy (20 < E < 240 eV) ion bombardment of Ge(001)

Surface defects created on Ge(001) exposed to low energy Xe ions are characterized by in situ scanning tunneling microscopy (STM). The temperature of the sample during ion bombardment is 165 C and ion energies range from 20 to 240 eV. The ion collisions create defects (vacancies and adatoms) which nucleate and form vacancy and adatom islands. For fixed total vacancy creation, the vacancy island number density increases with increasing ion energy: the vacancy island number density is 1.6 × 10⁻²⁰ cm⁻² for 40 eV ion bombardment and increases to 4.4 × 10⁻²⁰ cm⁻² for 240 eV ion bombardment. The increased nucleation rate for vacancies is attributed to clustering of defects. The sputtering yield of Ge(001) is also measured by STM. The sputtering yield for 20 eV ions is approximately 10⁻³ per ion but the net yield for surface defects (sum of adatoms and vacancies) is an order of magnitude higher, 10⁻², due to adatom-vacancy pair creation.     

First-principles study of oxidized Nb(1 0 0) surface structures

The atomic positions of the oxygen-induced c(2 × 2)-O, (3 × 1)-O and (4 × 1)-O surface structures on Nb(1 0 0) are determined by first-principles electronic structure calculations within the density functional theory comparing experimentally observed scanning tunneling microscopy (STM) images. STM images of these surfaces are calculated on the basis of the theory of Tersoff and Hamann. The theoretical and experimental STM images of the oxygen-chemisorbed c(2 × 2)-O structural model agree well. However, only the oxide-covered (3 × 1)-O and (4 × 1)-O structural models with two layers of NbO and contraction of the unit length along longitudinal 〈1 0 0〉 direction by 10% result in the theoretical STM images that agree with the experimental ones.     

Self-regulated Ni cluster formation on the TiO2(1 1 0) terrace studied using scanning tunneling microscopy

We have studied the growth mode and morphology of Ni clusters on a TiO₂(1 1 0) surface with a wide terrace using scanning tunneling microscopy (STM) at a low coverage (less than 3 atoms nm⁻²). The Ni clusters are formed on the terrace at the low coverage of 0.2 atoms nm⁻². Their average dimensions are constant in three directions up to 1 atoms nm⁻². The Ni clusters have an oval shape with average sizes of 1.8 nm (along [0 0 1]) × 1.4 nm (along (in the [1 1 0] directions). Above the coverage of 1.0 atoms nm⁻², an increase in the cluster height occurs, retaining an almost constant lateral size. It is proposed that the interaction of the Ni cluster and the support surface regulates the Ni cluster size.     

A single h-BN layer on Pt(1 1 1)

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(1 1 1) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, X-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(1 1 1), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge X-ray absorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1×10^-6–1×10^-8 at 800 °C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(1 1 1) is proposed.     

High resolution images of Mo2C(0001)- structure by scanning tunneling microscopy

A C-terminated structure on Mo₂C(0001) was observed by scanning tunneling microscopy. The structure was observed as a honeycomb structure with dark depressions corresponding to C atoms which make up the lattice. High resolution imaging was possible at low tunneling resistance less than 1 MΩ. Each C atom appears as a shallow sombrero protrusion predicted by theoretical calculations of C atoms on a metal substrates. It is concluded that the C atoms occupied threefold hollow sites of the (1×1) Mo layer of the substrate and a model for the structure is proposed.     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

Adsorption of pyrimidine molecules on Pd(110) observed by scanning tunneling microscopy

Adsorbed states of pyrimidine molecules on Pd(110) have been studied by a scanning tunneling microscope (STM). The pyrimidine molecules are preferentially adsorbed on terraces, not at steps. The isolated pyrimidine molecule shows a 0.6 nm × 0.6 nm rectangular shape with two parts of elongated protrusions. Two adsorption sites are observed: on-top site of the Pd[1 0] row and the midway between two [1 0] rows. Pyrimidine molecules show a strong tendency to form dimers even at a low coverage (0.01 ML), indicating that there is an attractive interaction between two adsorbed molecules.     

Mixed PbSi dimer chains on Si(1 0 0): a first-principles study

It has been a common belief that the one-dimensional structures observed by STM at low coverage of Pb on Si(1 0 0) are buckled Pb-Pb dimer chains. However, using first-principles density functional calculations, we found that it is energetically favorable for Pb adatoms to intermix with Si atoms to form mixed dimer chains on Si(1 0 0), instead of Pb-Pb dimer chains as assumed in previous studies. Up to a Pb coverage of 0.125 ML, mixed PbSi dimer chain is 0.19 eV per Pb atom lower in energy than Pb dimer chain.     

Comment on ‘Step structure of vicinal Ge(001) surfaces’ by B.A.G. Kersten, H.J.W. Zandvliet, D.H.A. Blank and A. van Silfhout

We have investigated detailed structures of monatomic steps on Ge(001) at room temperature by using high-resolution scanning tunneling microscopy. Our conclusions are different from those of Kersten et al. [Surf. Sci. 322 (1995) 1] for the same system. The most crucial difference is that we have not observed the nonbonded SB step. We have pointed out a possible reason for the different conclusions.