Three-Ni-atom cluster formed by sulfur adsorption on Ni(1 1 1)

Sulfur adsorption on Ni(1 1 1) at room temperature has been studied by scanning tunneling microscopy. Irregularly-shaped islands, which are aggregates of small particles, appear with troughs on (1 1 1) terraces. Estimating the number of small particles in the irregular islands and that of Ni atoms ejected through the formation of troughs, we find that each small particle contains three Ni atoms. A three-Ni-atom cluster is proposed, where three S atoms are adsorbed on 4-fold hollow sites formed on the sides of the cluster. The Ni₃S₃ cluster can assume two orientations on the (1 1 1) surface. We show that the apparently-irregular distribution of small particles is well understood by assuming the alternate orientation of neighboring Ni₃S₃ clusters and the slight energy difference between the two orientations of a single Ni₃S₃ cluster. Real-time observation of a Ni(1 1 1) surface under H₂S ambience reveals that small islands composed of three or four Ni₃S₃ clusters change their positions rapidly, preventing the simple growth of islands at the initially-nucleated positions.     

Tetra-σ bonding adsorption of pyridine on Si(1 0 0)-2 × 1

We studied adsorption of pyridine on Si(1 0 0) at room temperature using high resolution photoemission spectroscopy (PES) and near edge X-ray adsorption fine structure (NEXAFS) in the partial electron yield (PEY) mode. The Si 2p, C 1s, N 1s spectra of pyridine on Si(1 0 0) showed that pyridine is chemisorbed on Si(1 0 0)-2 × 1 through the formation of the tetra-σ-bonded structure with the N atom and three C atoms. NEXAFS was conducted to characterize the adsorption geometry of pyridine on Si(1 0 0). The π* orbital of CC bond showed a good angle dependence in C K-edge NEXAFS spectra, and we were able to estimate the adsorption angle between chemisorbed pyridine of CC bond and the Si(1 0 0) surface using an analytical solution of NEXAFS intensity. We find the coexistence of two different tight bridges with the adsorption angles 42 ± 2° and 45 ± 2° with almost equal abundance.     

Coverage-dependent chemisorption of Cl on Si(1 1 4)

The dissociative chemisorption of Cl₂ on Si(1 1 4) was studied at room temperature using scanning tunneling microscopy. Filled and empty state imaging revealed a multitude of possible adsorption configurations with adsorption preferring rebonded atom and dimer sites over tetramer sites. The dissociation and subsequent adsorption processes were sufficiently exothermic that Cl could interrogate the potential energy landscape and find local energy minima. Annealing revealed configurations that preserved the strongest π-bonds of the (1 1 4) reconstruction.     

C60 adsorption on Cu(110) studied by optical spectroscopy

The formation of the interface between C₆₀ thin films and the Cu(110) surface has been investigated in situ using reflectance difference spectroscopy (RDS). The electronic interaction between C₆₀ molecules in the first monolayer and the substrate inhibits low‐energy intramolecular transitions, whereas the C₆₀ molecules above the first monolayer are effectively decoupled from the substrate. The morphology of C₆₀ thin films prepared at room temperature is thermally stable up to 500 K. Above this threshold, optical spectroscopy and low energy electron diffraction (LEED) indicate the formation of rather large three dimensional C₆₀ islands on a one monolayer thick wetting layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of a √3 × √3 surface on Si/Ge(1 1 1) studied by STM and LEED

We have performed a detailed study of the formation and the atomic structure of a √3 × √3 surface on Si/Ge(1 1 1) using both scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Both experimental methods confirm the presence of a √3 × √3 periodicity but unlike the Sn/Ge(1 1 1) and the Sn/Si(1 1 1) surfaces, the Si/Ge(1 1 1) surface is not well ordered. There is no long range order on the surface and the √3 × √3 reconstruction is made up of double rows of silicon atoms separated by disordered areas composed of germanium atoms.     

Pt-induced structures on Si(1 1 0) studied by STM

The structures induced by platinum (Pt) adsorption on Si(1 1 0) surface have been studied by scanning tunneling microscopy (STM) for coverage up to 2 monolayers (ML). Three surface phases have been found to form: “5×4”, “13×2” and “6×5” for Pt coverages 0.3, 0.5 and 1 ML respectively. All structures are formed by one-dimensional rows aligned along the direction. At the coverage >1 ML islands of, probably, Pt silicide start to form in form of 1D nanowires.     

Topologically induced surface electron state on Si(1 1 1) surfaces

First-principle electronic structure calculation reveals the appearance of a new class of surface state on hydrogenated and clean Si(1 1 1) surfaces. The states are found to exhibit different characteristics to conventional surface electron states in terms of the peculiar distribution of the wavefunction depending on the wavenumber. In addition, the state results in flat dispersion bands in a part of the surface Brillouin zone having energy of about 8 eV below the top of the valence band. An analytic expression based on the tight-binding approximation corroborates the surface electron state results from the delicate balance of the electron transfer among the atoms situated near the surface. The obtained results give a possible extension and generalization of the edge state in graphite ribbons with zigzag edges.     

Energy surfaces of rare-earth silicide films on Si(1 1 1)

We report on angle-resolved photoelectron spectroscopy results of thin dysprosium-silicide layers formed on Si(1 1 1), taken with a toroidal analyzer allowing to image the energy surfaces in k_||-space. At monolayer dysprosium coverages, where hexagonal DySi₂ grows with a 1×1 superstructure, electron pockets are observed at the points with highly anisotropic effective masses, and around a hole pocket at the point also an anisotropic dispersion is found. The band filling of these two bands amounts to one, indicating an even number of electrons assigned to the surface unit cell. Similar features are found for multilayer coverages, where hexagonal Dy₃Si₅ layers are formed with a superstructure. Here, the influence of zone folding effects due to the reconstructed layers in the bulk silicide is only weak because of the high surface sensitivity of the experiments.     

Initial stage of CdTe on Si(1 0 0) grown by MBE

The initial stage of CdTe growth on silicon has been investigated using angle-resolved photoemission and scanning tunneling microscopy (STM). In order to study initial stage of CdTe on Si, we have desorbed CdTe by annealing at 600 °C so that only one monolayer of Te remains on the Si(1 0 0) substrate. Te/Si(1 0 0)2×1 superstructure has been observed by LEED. Photoemission spectra indicate that Te atoms bond with the Si dangling bond. Atomically resolved STM images reveal that the Te atoms form dimers. It is observed that buckling direction of Te-dimer changes and the dimmers are broken in the site of some dimmer rows. It can be explained that the large lattice mismatch cause the switching of the buckling direction and the breaking of Te-dimer resulted surface relaxation.     

Adsorption of Ag on Si(1 0 0) surface

The chemisorption of one monolayer Ag atoms on an ideal Si(1 0 0) surface is studied by using the self-consistent tight-binding linear muffin-tin orbital method. The adsorption energies (E_ad) of different sites are calculated. It is found that the adsorbed Ag atoms are more favorable on C site (fourfold site) than on any other sites on Si(1 0 0) surface, the polar covalent bond is formed between Ag atom and surface Si atom, a Ag and Si mixed layer does not exist and does form an abrupt interface at the Ag–Si(1 0 0) interface. This is in agreement with the experiment results. The layer-projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated. Comparing with the Au/Si(1 0 0) system, the interaction is weaker between Ag and Si than between Au and Si.     

The origin of inter-dimer-row correlated adsorption for NH3 on Si(0 0 1)

We discuss the interaction between adsorbing ammonia molecules and pre-adsorbed ammonia fragments on the Si(0 0 1) surface, searching for experimental evidence of a H-bonded precursor state predicted by modelling. While correlations along dimer rows have already been identified, these mix substrate-mediated effects due to dimer buckling with ammonia–adsorbate effects. Correlations between fragments on neighbouring dimer rows are not affected by substrate effects (in this system), allowing an analysis of direct ammonia–adsorbate effects. We present an analysis of cross-row correlations in existing high-coverage STM data which shows significant correlations between NH₂ groups on neighbouring dimer rows over a significant range, providing evidence for the H-bonded precursor state with a range of around 10 Å. We discuss implications for the interpretation of STM images of ammonia on Si(0 0 1).     

Magnetization reorientation in Ni (1 1 1) ultrathin films studied by low-temperature hysteresis measurements

Surface magneto-optical Kerr effect (SMOKE) magnetometry in the temperature range 10–300 K was exploited to investigate the magnetic properties of high-quality Cu/Ni/Cu/Si(1 1 1) epitaxial heterostructures with thickness of the Ni layer, d_Ni, between 10 and 60 Å. For a fixed temperature, the equilibrium direction of the magnetization is parallel or perpendicular to the film surface, depending on the Ni thickness, because of the competition among shape anisotropy, magnetoelastic anisotropy and interface anisotropy. No reorientation of the magnetization could be observed as a function of temperature, for any of the specimens analyzed, while a large variation of the loop squareness and coercivity was found. This last variation has been qualitatively explained using a theoretical model based on a Green's function technique, valid for a monodomain film with a coherent rotation of the magnetization.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Characterization of InN epilayers grown on Si(1 1 1) substrates at various temperatures by MBE

InN films have been grown by plasma-assisted molecular beam epitaxy (PAMBE) and characterized by various technologies. It was found that the structural, optical and electrical properties can be drastically improved by raising growth temperature from 440 to 525 °C. Grainy morphology was found in the grain size was found in atomic force microscope images. The large grain size was about 360 nm for a film grown at 525 °C. These films exhibited Wurtzite structure with a c/a ratio ranging from 1.59 to 1.609. The dislocation densities estimated by X-ray diffraction techniques closely agreed with those analyzed by plan-view transmission electron microscopy. Photoluminescence (PL) studies confirmed near band-to-band transitions and the narrowest low-temperature PL peak width was found to be 24 meV at 0.666 eV. Carrier concentrations decreased from 1.44×10¹⁹ to 1.66×10¹⁸ cm⁻³ and Hall mobility increased from 226 to 946 cm² V⁻¹ s⁻¹ as the growth temperature is progressively increased from 440 to 525 °C. Raman spectra also indicated improved crystal quality as the growth temperature was raised.     

DFT calculations of adenine adsorption on coin metal (1 1 0) surfaces

Density functional theory is used to analyze in detail the adsorption of the adenine molecule on the (1 1 0) surfaces of Cu, Ag, and Au. While the adsorption configurations are similar in all three cases – the molecule bonds via two nitrogen atoms to the substrate – the details like charge transfer or local strain a rather different. The molecule–substrate interaction in case of Cu is stronger than for the more noble metals Ag and Au. Longe-range dispersion forces stabilize the adsorption configuration in dependence on the specific adsorption geometry. In case of Ag and Au, relativistic effects are found to be important.     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

Theoretical studies of Sm/Si(1 1 1)–n × 1 (n = 5, 7) and Sm/Si(1 1 1)–(8 × 2) structures

In this study we employ a state-of-the-art pseudopotential method to perform local density of states (LDOS) calculations of n × 1 (n = 5, 7) and (8 × 2) reconstructions induced by the adsorption of rare-earth samarium (RE) in the submonolayer range. We conducted a full comparison between images from scanning tunneling microscopy (STM) and theoretical LDOS. Images taken of both filled and empty states show the effects induced by honeycomb chains and Seiwatz chains. We conclude that LDOS calculations are consistent with the assignment of features observed experimentally by STM.