Dynamics of NF3 in a condensed film on Au(1 1 1) as studied by electron-stimulated desorption

We report a low-temperature dynamics study of condensed layers of NF₃ on Au(1 1 1) by time-of-flight electron-stimulated desorption ion angular distribution (TOF-ESDIAD), temperature-programmed desorption (TPD) and low-temperature scanning tunneling microscopy (LT-STM). Upon adsorption at 30 K, molecular NF₃ adsorption occurs first at the step edges and at minor terrace defect sites with the formation of 2D islands. Within the islands, NF₃ is adsorbed in an upright conformation via the nitrogen lone pair electrons projecting fluorine atoms away from the surface as judged by the presence of only a sharp F⁺ central beam in the ESDIAD pattern. At higher coverages, 3D islands start to populate the surface. Electron bombardment of a thick NF₃ (∼6 ML) layer adsorbed on the Au(1 1 1) surface leads to emission of F⁺, N⁺, NF⁺, and ions as observed in the TOF-ESD distribution. Upon heating to ∼37 K, a sudden decrease of the and ion yield, which is not related to thermal desorption, is observed which reflects the surface migration of NF₃ molecules, leading to local thinning of the film. The thinning process occurs at the temperature of onset of molecular rotations and self-diffusion in the bulk NF₃ crystal. In this process, some NF₃ molecules move closer to the surface which results in higher efficiency for ion neutralization by the underlying metal surface. In the TPD spectra, the monolayer desorption is observed to begin at ∼65 K, exhibiting zero-order kinetics with an activation energy of 21 kJ/mol.     

Photoelectron spectroscopy study of Pb/Ag(1 1 1) in the submonolayer range

The vacuum deposition of Pb onto Ag(1 1 1) gives rise to two different surface structures depending on coverage and deposition temperature. At room temperature (RT), low energy electron diffraction (LEED) reveals a sharp reconstruction completed at 1/3 Pb monolayer (ML). Beyond, a close-packed Pb(1 1 1) incommensurate overlayer develops. At low temperature (LT, ∼100 K) the incommensurate structure is directly observed whatever the coverage, corresponding to the growth of close-packed two-dimensional Pb(1 1 1) islands. Synchrotron radiation Pb 5d core-level spectra clearly demonstrate that in each surface structure all Pb atoms have essentially a unique, but different, environment. This reflects the surface alloy formation between the two immiscible metals in the reconstruction and a clear signature of the de-alloying process at RT beyond 1/3 ML coverage.     

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.     

Co-induced nano-structures on Si(1 1 1) surface

The interaction of cobalt atoms with silicon (1 1 1) surface has been investigated by means of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Besides the Co silicide islands, we have successfully distinguished two inequivalent Co-induced reconstructions on Si(1 1 1) surface. Our high-resolution STM images provide some structural properties of the two different derived phases. Both of the two phases seem to form islands with single domain. The new findings will help us to understand the early stage of Co silicide formations.     

Adsorption structure of germanium on the Ru(0 0 0 1) surface

Coverage-dependent adsorption energy of the Ge/Ru(0 0 0 1) growth system and the geometrical distortions of the most stable adsorption structure are investigated through first-principles calculations within density functional theory. A local minimum in adsorption energy is found to be at a Ge coverage of 1/7 monolayer with a Ru(0 0 0 1)- symmetry. Based on this stale superstructure, the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) images are simulated by means of surface local-density of states (LDOS). The results are consistent well with the STM measurements on the phase for Ge overlayer on Ru(0 0 0 1). From this stimulation, the relations between the STM images and the lattice distortion are also clarified.     

Substantially low desorption barriers in recombinative desorption of deuterium from a Si(1 0 0) surface

We have studied desorption kinetics of deuterium molecules from a Si(1 0 0) surface by means of temperature-programmed desorption (TPD) spectra and isothermal desorptions.Three desorption components, denoted as β_1,A,β_1,B, and C, can be distinguished in semi-logarithmic plots of the TPD spectra.Their peak positions and intensities are strongly affected by the surface preparation methods employed, either with or without annealing to control the initial D coverage .Peak C appears at the leading edge of the TPD peak.It accounts for only about 5% of the TPD peak at and it diminishes rapidly with decreasing , vanishing at .In contrast, together the β_1,A and β_1,B peaks account for the whole TPD peak at any less than 1.0 ML. The maximum of the β_1,A peak is nearly constant at around the maximum temperature of the TPD peak.On the other hand, the β_1,B peak appears on the high-temperature side of the TPD peak and it systematically shifts to higher temperatures with decreasing .These results imply that first- and second-order kinetics are operating for the β_1,A and β_1,B desorptions, respectively.Isothermal desorption experiments confirm the above predicted kinetics for a limited region, namely .From the results for the rate curve analysis, the desorption barriers are evaluated to be 1.6 ± 0.1 eV and 1.8 ± 0.1 eV for the β_1,A and β_1,B desorptions, respectively.These values are substantially lower than the widely accepted value of ∼2.5 eV. To reproduce the measured TPD spectra we take the Arrhenius-type rate equation containing the first- and second-order rate terms for the β_1,A and β_1,B desorptions.The TPD spectra measured for can be reasonably fit with the proposed rate equation when the values given above for E_d,A and E_d,B are used. For , however, the TPD curves are not fit with the same values; rather, the best-fit curves require values for E_d,A and E_d,B larger than those given above. Combining the present kinetics results with those obtained by STM along with the studies, the β_1,A and β_1,B peaks may be attributed to desorption along the 2H path, while peak C may be attributed to desorption along the 4H path. The atomistic desorption mechanism as well as the energy relationship between the desorption barrier and isosteric heat of adsorption are discussed.     

High-resolution angle-resolved photoemission study of Ni(1 1 1) surface state

High-resolution angle-resolved photoemission spectroscopy (ARPES) has been conducted to study the Shockley state (SS) in ferromagnetic Ni(1 1 1) located at the point of the surface Brillouin zone. We have determined the Fermi wave vector and Fermi energy of the state with excitation photon energies of hν = 6.9-27.5 eV. On the basis of ARPES spectral shape analyses, we have found significant electron-electron interaction in the SS.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

A novel approach to measure the step line tension and the step dipole moment on vicinal Au(0 0 1) electrodes

In this paper we introduce a new experimental approach to determine the potential dependence of the step line tension on metal electrodes in contact with an electrolyte: (0 0 1) and (1 1 n) surfaces of single crystal gold electrodes were investigated by impedance spectroscopy in solutions containing weakly adsorbing anions, such as , F⁻ and . Within the limits of error the shift in the potential of zero charge is proportional to the step density of the vicinal surfaces indicative of a well-defined dipole moment per step length. The dipole moments per step atom are 6.8 ± 0.8, 5.2 ± 0.4, 5.8 ± 0.5 × 10⁻³ eÅ for , , and F⁻ containing electrolytes, respectively. Using the values for the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tension of the stepped and the step free surface. Our results represent the first experimental confirmation of a recent theoretical model proposing that in absence of specifically adsorbed ions the step line tension should decrease (roughly linear) with potential.     

Oxygen induced surface structure of Mo(1 1 0) studied by scanning tunneling microscopy

The oxygen induced surface structures formed on Mo(1 1 0) by oxygen exposure at 1300 K in UHV has been studied by scanning tunneling microscopy (STM). Two kinds of oxygen-adsorbed surface structures are observed. One consists of one-dimensional rows running along or directions at substrate molybdenum lattices, and another shows more complex structure including discrete arrangement of large protrusions and zig-zag alignments of small protrusions. This complex structure is probably a further oxygen-adsorbed structure than the well-known p(2 × 2) structure of 0.3 ML coverage. On the basis of STM image, an atomic model is proposed, where adsorbed oxygen atoms occupy both long-bridge and the quasi-threefold sites of molybdenum lattice (0.4 ML coverage). This structure is presumed to be a transient state during site-conversion with increase of oxygen exposure.     

Scanning tunneling microscopy investigation of clean (2 1 1) surface of Ta and Mo

Scanning tunneling microscopy was used to obtain images from clean (2 1 1) surfaces of Ta and Mo under the same ultrahigh vacuum conditions. In the case of tantalum the surface is very rough, and single extending atomic chains reach distances up to 100 Å. Those furrows are a surface reconstruction based on creation of (1 1 0) “micro-facets” along the [1 1 1] direction or (1 1 1)fcc micro-facets with a more dense structure than (1 1 0)bcc plane. In contrast, the (2 1 1) surface of Mo is smooth, and single atomic rows are visible on high resolution STM images. Step bunching occurs on the Mo surface, reducing the number of terrace edges.     

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 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.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Stable surface termination on vicinal 6H-SiC(0 0 0 1) surfaces

Ordered nanofacet structures on vicinal 6H-SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed.     

Direct STM evidence for Cu-benzoate surface complexes on Cu(1 1 0)

The chemisorption of benzoate on a Cu(1 1 0) crystal at room temperature was studied using low temperature scanning tunneling microscopy. STM images, obtained at 5 K for low benzoate coverage, show isolated surface species that consist of a single Cu adatom stabilizing two benzoate molecules in a flat orientation. These species are discussed in relation to other known metal-organic surface compounds. At higher coverage the overlayer, called the α-phase, was also observed at 5 K and found to contain features attributable to two Cu adatoms associated with two pairs of non-equivalent benzoate species. The observed topographic features are used to suggest refinements of the structural model of the ordered α-phase overlayer.     

STM study of l-serine adsorption on Cu(0 0 1)

The adsorption of l-serine on Cu(0 0 1) surface at 310 K was studied by scanning tunneling microscope (STM). l-serine molecules on the Cu(0 0 1) initially formed a domain of thick lines with a order structure along the direction on the terraces regardless of the coverage of serine. The thick lines were partly replaced by thin line along the direction, and completely disappeared in 2 h. It is considered that in these structures hydrogen bonds involved in hydroxymethyl group between adsorbates play some role in addition to intermolecular hydrogen bond between a hydrogen atom of amino group and an oxygen atom of carboxy group for alanine adsorption.     

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.     

Olefin bonding mechanism and growth to (1 0 0)(2 × 1):H diamond

The bonding and growth mechanism of photochemically attached olefin molecules to (1 0 0)(2 × 1):H diamond is characterized using atomic force (AFM) and scanning tunneling microscopy (STM) experiments in combination with molecular orbital calculations. To identify growth schemas, diamond surfaces after 10, 40 and 90 min of photo-chemically stimulated growth have been characterized. These data show clearly island formation which is discussed taking into account a growth model from silicon. The island growth shows no directional properties which are attributed to arrangement and geometrical properties of hydrogen terminated carbon bonds at the surface of (1 0 0) oriented (2 × 1) reconstructed diamond.