Fabrication of <1 1 0> oriented tungsten nano-tips by field-assisted water etching

We report the field-assisted H₂O etching that enabled us to fabricate nano-tips from polycrystalline <1 1 0> oriented tungsten wires at room temperature. We optimized the sharpening procedure in order to obtain field emissions (FEs) with high collimation. The typical tip apex was composed of a large base and a nano-protrusion with a radius of curvature less than 3.5 Å. The narrowest opening angle (full width at half maximum) of the FE was 4.3° at 150 pA. We prepared two types of tips using two different applied bias voltages during the H₂O etching. The electron microscope images and the analysis of Fowler-Nordheim (FN) plots revealed that the sizes of the individual bases depended on the fixed bias voltages during the H₂O etching and affected their FE properties. In addition, we could confirm that the FE current from the nano-tip was more stable than that of the normal tip.     

Non-contact AFM images of a C60 molecule adsorbed on the Si(0 0 1) surface: An ab initio method

A theoretical non-contact atomic force microscope (nc-AFM) image is simulated for an individual C₆₀ molecule adsorbed on the clean Si(0 0 1) surface. The ability to identify the intramolecular features of the molecule though nc-AFM imaging would allow many of the different configurations the molecule may take when adsorbed, to be identified. This would be an important preliminary step in the manipulation of the molecule along the surface, as the initial configuration that the molecule is in will affect the periodicity of the tip trajectory during the manipulation event (see D.L. Keeling et al., Phys. Rev. Lett. 94 (2005) #146104). Presented in this paper are simulated images and force curves above important sites for key stable configurations of the C₆₀ molecule. It is shown that in principle it is possible to distinguish between the different configurations of the molecule using nc-AFM. An ab initio DFT method was used to accurately describe the chemical interaction between the adsorbate and the Si tip, which has a single dangling bond at its apex.     

A density functional theory study of ethylene adsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models and Ni13 nanocluster

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian’03 software. It was found that ethylene has adsorbed molecularly on all clusters with π adsorption mode. Relative energy values were calculated to be −50.86 kcal/mol, −20.48 kcal/mol, −32.44 kcal/mol and −39.27 kcal/mol for Ni₁₃ nanocluster, Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) surface cluster models, respectively. Ethylene adsorption energy is inversely proportional to Ni coordination number when Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) cluster models and Ni₁₃ nanocluster are compared with each other.     

GaN grown on Si(1 1 1) with step-graded AlGaN intermediate layers

The authors report the growth of crack-free GaN on Si(1 1 1) substrate with step-graded AlGaN intermediate layers all grown at 1120 °C. By preparing all these layers at high-temperature, we can simplify the growth proceduce and minimize the growth time. Using X-ray diffraction and transmission electron microscopy, it was found that the high-temperature step-graded AlGaN intermediate layers can effectively reduce the tensile stress on GaN epitaxial layers. Photoluminescence and Raman measurements also indicate that we can improve the crystal quality of GaN by inserting the step-graded AlGaN intermediate layers.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.     

ARPES measurements on Si(1 1 1) hole subband induced by Pb and Ga adsorption

The subband dispersions in the Si(1 1 1) p-type inversion layers induced by Pb and Ga adsorbed surface structures were measured by angle-resolved photoemission spectroscopy (ARPES). The surface structures used here were and Si(1 1 1)6.3 × 6.3-Ga. is a new surface phase found in this study. Because it is significant in our study to investigate potential effects of surface superstructures on the hole subband dispersion, we investigated the subband energy levels quantitatively comparing them with those calculated using the triangular approximation. It was found that the energy separation of the adjacent subband quantum levels in the inversion layers induced by gallium adsorption does not follow the triangular approximation. The possible band bending shape was proposed to explain the quantum level spacing of the subbands in Ga-induced inversion layers.     

Ab initio calculations of structural, electronic and dynamical properties of BeSe(1 1 0) surface

We use an ab initio pseudopotential method within the local-density approximation to determine the structural and electronic properties of the BeSe(1 1 0) surface. The relaxed geometry of this surface shows tilted cation-anion chains, with the anions being raised. The general pattern of the electronic structure of this surface is similar to that on other II-VI(1 1 0) surfaces. The phonon spectrum and corresponding surface density of states are also calculated using a linear response approach based on the density functional perturbation theory. In our calculations, we have found two localized phonon modes in the acoustic-optical gap region. The atomic displacement patterns of these surface phonon modes are presented and discussed.     

The structure of a stoichiometric TiO2 nanophase on Pt(1 1 1)

The structure of a rectangular TiO₂ nanophase grown epitaxially on a Pt(1 1 1) substrate has been investigated by a combined experimental-theoretical approach. It is found that such nanophase is stoichiometric, incommensurate to the substrate and has the structure of a lepidocrocite layer. The film is weakly bound to the metal surface via the O atoms of the oxide layer and consequently it does not have a fully wetting behavior. Two almost iso-energetic structures have been found based on first principles DFT calculations, one characterized by a short and one by a long interface distance, this latter being energetically slightly preferred. However, when the strain due to lattice mismatch is accommodated on the Pt(1 1 1) substrate instead of the TiO₂ film, only the long interface structure is found. The analysis of measured and computed valence band spectra and STM images supports the long interface, weakly interacting model.     

Perpendicular magnetic anisotropy of V/Co(0 0 1)

Based on the full-potential linearized augmented plane wave (FLAPW) calculations, various magnetic properties of ultra thin face centered cubic (fcc) Co(0 0 1) film and V adsorbed systems on Co(0 0 1) surface are explored. It was found that the V film grown on fcc Co(0 0 1) surface has large induced magnetic moment and the direction of magnetization is antiparallel to that of Co atom in the submonolayer coverage. Very interestingly, we found that the surface alloy and 0.5 ML adsorbed V/Co(0 0 1) systems have perpendicular magnetocrystalline anisotropy and the magnitude of anisotropy energy in 0.5 ML V on fcc Co(0 0 1) surface is greatly larger than that of surface alloy, while we observed in-plane magnetization in pure fcc Co(0 0 1) film. It was found that the spin-orbit interaction through spin-flip process cannot be ignored, therefore the simple relation with orbital anisotropy is not applicable in the interpretation of magnetocrystalline anisotropy.     

Nano-faceting of fcc(1 1 0) surfaces controlled by adsorbates and atom deposition or removal

(1 1 0) surfaces of most fcc metals are only marginally stable against faceting into (1 1 1) orientations. Trace concentrations of adsorbates (surfactants) that prefer (1 1 1) over (1 1 0) facets can tip the balance and favor faceting of all or part of the (1 1 0) surface. The growth of such facets is impeded by island and vacancy-island nucleation barriers. However, during atom deposition or etching these barriers are reduced. Growth or removal conditions and facet stabilizing surfactant concentrations control the evolving faceted nano-patterns. Recent observations of hut-shaped nanocrystals formation on Al(1 1 0) are consistent with this model.     

Chemical bonding of PTCDA on Ag surfaces and the formation of interface states

We present a detailed investigation of the interface bonding of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Ag(1 1 1) and Ag(1 1 0) surfaces by a combination of structural and electronic techniques (SPA-LEED, STM, TPD, UPS, HR-XPS, and NEXAFS) thus obtaining a consistent picture of the adsorption behaviour of PTCDA/Ag in the monolayer regime. The interaction with silver is strong and leads to the formation of new common hybrid orbitals in the monolayer, which are interface states for PTCDA films on Ag, involving at least LUMO, HOMO, and HOMO-1, and the Ag 5s- and 4d-states. This chemisorption is based on a covalent interaction between metal and molecular states, and can unambiguously be distinguished from mere van-der-Waals bonding.     

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO2

RuO₂(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O₂ and high reaction temperatures. Subsequently the RuO₂ was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO₂ gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO₂ displays high reactivity at relatively low temperatures (?450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O₂ species. RuO₂ deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.     

Growth kinetics of metastable (3 3 1) nanofacet on Au and Pt(1 1 0) surfaces

A theoretical epitaxial growth model with realistic barriers for surface diffusion is investigated by means of kinetic Monte Carlo simulations to study the growth modes of metastable (3 3 1) nanofacets on Au and Pt(1 1 0) surfaces. The results show that under experimental atomic fluxes, the (3 3 1) nanofacets grow by 2D nucleation at low temperature in the submonolayer regime. A metastable growth phase diagram that can be useful to experimentalists is presented and looks similar to the one found for the stationary growth of the bcc(0 0 1) surface in the kinetic 6-vertex model.     

On the formation of OH ordered layers by dissociation of H2O on an oxygen covered Ag(1 1 0) surface: An STM investigation

We present results of an STM investigation of water interaction with an oxygen covered Ag(1 1 0) on the example of the O(4 × 1) reconstructed surface. In agreement with numerous previous experimental works, using diffraction techniques, we found that a structure of OH(1 × 2) type, displaying rows in the [1 −1 0] direction, is formed. The new features revealed by this local probe study, is the presence of quasi rectangular islands evenly distributed across the terraces, with a density of 0.22 ± 0.03 and a mean area of 90 ± 15 nm² at 220 K. They are imaged at an apparent height of 0.14 nm. It is remarkable that the same OH row structure is present on the whole terrace “on top” and “in between” the islands. These features are attributed to silver islands of mono-atomic height, formed by clustering of silver ad-atoms released during reaction of the O atoms with the water molecules. These findings point to a more complex behaviour of the reaction dynamics than previously described. They emphasise the key role of the silver ad-atoms, present in the added rows of the initial Ag(1 1 0)-O(4 × 1) surface, in the formation of the nanostructures. In turn it is concluded that the rows evidenced by this STM and previous diffraction studies, are formed by OH chains.     

High resolution X-ray photoelectron spectroscopy study on initial oxidation of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface

An initial oxidation dynamics of 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface has been studied using high resolution X-ray photoelectron spectroscopy and supersonic molecular beams. Clean 4H-SiC(0 0 0 1)-(√3 × √3)R30° surface was exposed to oxygen molecules with translational energy of 0.5 eV at 300 K. In the first step of initial oxidation, oxygen molecules are immediately dissociated and atomic oxygens are inserted into Si-Si back bonds to form stable oxide species. At this stage, drastic increase in growth rate of stable oxide species by heating molecular beam source to 1400 K was found. We concluded that this increase in growth rate of stable oxide is mainly caused by molecular vibrational excitation. It suggests that the dissociation barrier is located in the exit channel on potential energy hypersurface. A metastable molecular oxygen species was found to be adsorbed on a Si-adatom that has two oxygen atoms inserted into the back bonds. The adsorption of the metastable species is neither enhanced nor suppressed by molecular vibrational excitation.     

Growth of silver structures on silicon surfaces observed in vivo by scanning tunneling microscopy

Early stages of growth of silver thin films on oriented silicon surfaces Si(1 0 0)2 × 1 and Si(1 1 1)7 × 7 were studied directly during deposition at room temperature by the scanning tunneling microscopy. Single Ag atoms deposited on the Si(1 0 0)2 × 1 surface diffuse too fast on the surface to be imaged by the microscope. Nucleation on C-type defects of the Si(1 0 0)2 × 1 reconstruction has been observed. During further growth, the defects represent stable terminations of silver chains. Ag nanoclusters growing on the Si(1 1 1)7 × 7 surface have been studied as a system with low diffusivity at room temperature. On this surface, presence of effective interaction between Ag clusters and individual Ag atoms in neighboring cells of the reconstruction has been identified. The interaction results in lowering the barrier for Ag atom hopping to an adjacent unit cell occupied by an Ag cluster. Unique possibilities arising from scanning the surface directly during growth are demonstrated.     

Li ion neutralization on Ag layers grown on Cu(1 1 1)

Electron transfer processes in the neutralization of Li⁺ ions on Ag layers grown on Cu(1 1 1) are investigated in quest of quantum confinement effects. Neutralization probabilities in the scattering of Li⁺ for incident ion energies in the 300 eV to 2 keV range are reported for Ag coverages ranging from 0.15 ML to 5 ML. Results are compared to those for Ag(1 1 1) and Cu(1 1 1) surfaces of bulk crystals. Although existing studies of the characteristics of Ag layers on Cu(1 1 1) indicate significant differences in electronic structure as a function of film thickness, the electron transfer probabilities we measure are found to be very close to those for bulk Ag(1 1 1). These results are commented on the basis of existing models and earlier studies of Li ion neutralization on various metals.     

Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.     

Surface quantum well state at the striped Cu(1 1 0)(2 × 1)O surface studied by angle resolved photoemission spectroscopy

We have performed an angle resolved photoemission spectroscopy with high energy and high momentum resolutions and have observed the k dependent energy dispersion curves of the striped Cu(1 1 0)(2 × 1)O surface. It is found that the Shockley surface state electron is confined in the clean surface along the perpendicular direction to the stripes and forms a quantum well state (QWS). It has also been clearly observed that an electron of Cu-O antibonding state is confined within the oxygen covered surface.