Anomalously large $g$-factor of single atoms adsorbed on a metal substrate
Jens WiebeAlexander A. KhajetooriansBruno ChilianR. WiesendangerSamir LounisA. T. CostaDouglas Mills
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The adsorption of cyclohexane on clean and oxygen-modified Ni{111} surfaces has been studied by reflection–absorption infrared spectroscopy (RAIRS). Adsorption on the clean surface proceeds via two-dimensional island growth in equilibrium with a random gas phase adsorbed on the bare surface and on top of the islands. The adsorbed cyclohexane exhibits a site symmetry of C3v which persists through to the multilayer regime. Adsorbed molecules in the first layer exhibit a broadened and downshifted νCH stretching vibrational band which has become the signature for cyclohexane on so many metal single-crystal surfaces, thought to arise from C—H⋯M interactions which may provide an important dehydrogenation channel. In order to determine the nature of the C—H⋯M interaction, adsorption on oxygen-modified Ni{111} surfaces was also investigated. The effect of the coadsorbed oxygen is strongly coverage dependent; adsorption of cyclohexane on the Ni{111}-(2 × 2-)-O surface results in further downshifting of the softened νCH stretching vibration, signalling the importance of charge transfer from the filled CH σ orbital to the metal in weakening the C—H bond. Adsorption on the Ni{111}-(√3 ×√3)R30°-O surface leads to total suppression of any C—H⋯M interaction, attributed to steric blocking of bare metal sites by the adsorbed adatoms.
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Thermal desorption spectroscopy
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Honeycomb
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The adsorption properties of metal atoms on the hydroxyl SiO2 surface have been studied by using first-principles calculation. It is found that In and Ga atoms are weekly bound to the surface, while Fe, Co and Ni atoms are bound to both Si and O atoms with strong chemical bonds. Potential energy surface and diffusion barrier calculations show that In (Ga) can easily diffuse on the surface because of a rather small diffusion barrier(0.1—0.3 eV). The obtained results are in close agreement with recent nano- synthesis experimental results.
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Surface diffusion
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Mulliken population analysis
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The theory of the evaporation of adsorbed atoms and ions of the alkali metals from a hot metal surface has been developed from somewhat different viewpoints by Langmuir and Kingdon and by Becker. These two treatments, which yield essentially the same results, are applicable only when equilibrium conditions obtain on the surface, so that the surface concentration is not changing. It is the purpose of the present note to extend Becker's treatment to take account of the rate of attainment of equilibrium on the surface, in order to lead to an interpretation of some experiments on the rate of evaporation of the alkali metals adsorbed on hot tungsten.
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We report a first-principles calculation that models the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100) surfaces. The adsorption of a W atom on a clean W(100) surface is compared to that of a W atom on a W(100) surface covered with a monolayer of Fe atoms. The total energy of the system is computed as the function of the height of the W adatom. Our result shows that the W atom first adsorbs on top of the Fe monolayer. Then, the W atom can replace one of the Fe atoms through a path with an energy barrier of 0.99 eV and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. This effect results in a more efficient adsorption and desorption of W atoms in the presence of Fe atoms. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms.
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A systematic density functional study of the adsorption and dissociation of O2 on the (001) surface of several transition metal carbides (TCMs; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) is presented. It is found that O2 may adsorb molecularly on two different sites with similar adsorption energy. At these sites, either O2 bridges two surface metal (M) atoms or it is placed directly on top of a M surface atom. A case apart is δ-MoC, where O2 adsorption on top of surface Mo atoms is far up in energy with respect to bridging two surface Mo atoms. The relative stability of O2 on these TMCs is dominated by the electron back-donation between the surface and O2 and the stabilization of the resulting partially charged molecule by the surface metal sites. Three reaction paths leading to O2 dissociation have been considered. The first reaction pathway starts from M−M bridge molecular adsorption and lead to O atoms on top of surface M atoms (TSM) and the second one (TSC) starts from on top molecular adsorption and lead to final states where O atoms are adsorbed on 3-fold hollow sites neighboring two M and one C surface atoms, while the third pathway (TSBC) starts from O on the M−M bridge and leads to TSC products. For each reaction path, transition state structures have been located and the corresponding energy barriers obtained. At low temperatures, O2 dissociation on group IV TMCs can only occur via the TSBC pathway whereas at high temperatures it may also take place starting through TSC. For the rest of the carbides, only TSC and TSM paths are possible. The calculated transition state theory rate constants reveal that TMCs of groups IV and V are easy to oxidize whereas this is especially difficult for δ-MoC. The rate constant trends follow the calculated energy barriers and explain the oxygen preference for carbon on group IV TMCs and δ-MoC, as well as the preference for metal atoms on group V TMCs.
Transition state
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The adsorption and recombination of H atoms at temperatures between 77 and 110 °K have been studied on various silicate glass surfaces consisting of fused silica, porous Vycor with surface adsorbed water, and porous Vycor with chemically altered surfaces achieved by replacement of the surface functional hydroxyl group by chlorine, fluorine, and methoxy substituents.The concentration of adsorbed H atoms was determined by a measurement of the e.s.r. signal and the kinetics of the disappearance of the hydrogen atoms were observed to be second order in mobile atoms, consistent with a diffusion controlled recombination process.The results require that there be at least two types of surface adsorbed atoms, namely, weakly physically adsorbed atoms that are mobile and immobile. Identification of these adsorbed atom types with a particular surface functional feature leading to the adsorption is proposed.The activation energy for the second order decay of hydrogen atoms at the surface was found to vary from 1.0 to 2.3 kcal/mol, depending upon the chemical nature of the surface. A correlation between this activation energy and an electronegativity function of the surface group is presented.
Electronegativity
Hydrogen atom
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