Measurement of the barrier height of the reflective W {110} plane boundaries in surface diffusion of single atoms

Plane boundaries are in general reflective to diffusing atoms. From about 2000 heating periods of direct field ion microscope observations we have studied adatom plane edge interaction and determined the barrier heights of the W {110} plane boundaries in single atom diffusion of W, Re, and Ir. Using equations we have derived recently, the barrier heights ΔE_b are found to be about 200 meV for these adatoms.     

Noble metals induced magnetic properties of graphene

The adsorption energies, stable configurations, electronic structures, and magnetic properties of the graphene with noble metal (NM=Pt, Ag, and Au) atom adsorption were investigated using first-principles density-functional theory. It is found that the bridge site is the most stable adsorption site for the Pt adatom; the Ag adatom can be stabilized almost equally at the bridge or the top site, while the Au adatom prefers to be adsorbed at top site. The Pt-graphene interaction is stronger than the interaction of Ag-graphene and Au-graphene, since the Pt atom has an unsaturated electronic d-shell (d⁹s¹). While there is no net magnetic moment for the Pt adatom, the Ag and Au adatoms still exhibit magnetic character on the graphene. The magnetic moments of the NM-graphene systems may be quenched (e.g., Pt-graphene), reduced (e.g., Ag-graphene) or not changed (e.g., Au-graphene) as compared with the values before adsorption. Therefore, the magnetic character of the adatom-graphene system can be turned by adsorbing different NM atoms on the graphene.     

Direct observation of adatom-substitutional atom interaction on dilute metal alloy surfaces

The field ion microscope is used to study interactions of a migrating tungsten adatom with substitutional rhenium atoms in the (110) plane of a W-3%Re alloy. By observing about 300 migration periods on a single (110) surface plane of a small field ion emittier, each position of the adatom with respect to deduced Re atom locations can be identified. The interaction is found to be attractive with a strength of 90 ± 7 meV at the closest equilibrium separation, and repulsive with a strength of at least 80 meV at the second closest separation. No interaction could be observed for larger separations indicating a strength of less than 10 meV. Results of a control experiment, diffusion of single W adatoms on the (110) plane of pure W, are also presented for comparison.     

Interfacial properties of Ce<Subscript>0.75</Subscript>Zr<Subscript>0.25</Subscript>O<Subscript>2</Subscript> supported noble metals (Pd,Pt) from first principles

First-principles electronic structure calculations of noble metals (NM=Pd, Pt)/Ce_0.75Zr_0.25O₂ systems are presented. It is found that: the NM adatoms do not prefer to stay at the atop or the bridge sites of the cations (Ce and Zr), but prefer to be adsorbed at or around the anion sites. The most preferable adsorption sites for both the Pd and Pt adatoms are the O-bridge sites neighboring the Zr dopant. The Pt adatom show much stronger interaction with the Ce_0.75Zr_0.25O₂(111) surface than does the Pd adatom. The interactions of the NM/Ce_0.75Zr_0.25O₂(111) interfaces are stronger than those of the corresponding NM/ceria(111) interfaces. There are some metal induced gap states (MIGS) appeared in the gaps of the NM/Ce_0.75Zr_0.25O₂(111) interfaces, which are important to catalytic properties of the NM/Ce_0.75Zr_0.25O₂(111) catalysts.     

Direct observations of the behavior of single silicon atoms on a metal surface

The behavior of single silicon adatoms on the W {110} plane has been successfully studied for the first time. Single atom diffusion parameters are found to be E_d = 0.70 ± 0.07 eV, and $\text{d}{}_0\text{= 3.08 × 10}{}^{\mathrm{?4}}\text{× 10}{}^{\mathrm{\pm 1.28}}\text{solcm}{}^2\text{s}$. The field desorption behavior of Si atoms is similar to that of metal adatoms. SiSi adatom-adatom interaction shows nonmonotonic distance dependence, but the repulsive region around 3.2 Å is much weaker than those found in metal adatom interactions.     

Atom binding at overlayers: Re on W(211)

Observations with the field-ion microscope have been made of the formation and structure of rhenium overlayers on W(211), as well as of the behavior of Re adatoms bound on top of the layer and at the layer edge. A well-ordered pseudomorphic rhenium film is formed by deposition and annealing at T > 500 K, and its structure has been established using a migrating Re adatom as a probe. Relative binding energies at a close-packed edge of the layer have been deduced from the frequency with which different sites are occupied by a rhenium adatom. The relative strength of binding varies markedly with the number of sites available at the edge. For a layer with 5 sites, binding is found to deviate dramatically from the predictions of the usual pairwise bonding schemes, in that interactions at the central site, where the coordination is highest, are weaker than at adjacent sites.     

A model for adatom structures

A model concerning adatom structures is proposed. Attractive nearest neighbour interactions, which may be of electronic nature lead to 2-dimensional condensation. Every pair bond causes and elastic dipole. The elastic dipoles interact via substrate strains with an anisotropics ^–3 power law. Different types of adatoms or sites are permitted and many-body effects result, from the assumptions. Electric dipole interactions of adatoms are included for comparison. The model is applied to the W(110) surface and compared with superstructures experimentally found in the W(110)-0 system. It is found that there is still lack for an additional next-nearest neighbour interaction.     

Adsorptions of metal adatoms on graphene-like BC_3 and their rich electronic properties: A first-principles study

Density functional calculations have been performed to investigate the adsorption of twenty two different kinds of metal adatoms on graphene-like BC_3. In contrast to the graphene adsorbed with adatoms, the BC_3 with adatoms shows many interesting properties.(1) The interaction between the metal adatoms and the BC_3 sheet is remarkably strong. The Li, Na, K, and Ca possess the binding energies larger than the cohesive energies of their corresponding bulk metals.(2)The Li, Na, and K adatoms form approximately ideal ionic bonds with BC_3, while the Be, Mg, and Ca adatoms form ionic bonds with BC_3 with slight hybridization of covalent bonds. The Al, Ga, In, Sn, and all transition metal adatoms form covalent bonds with BC_3.(3) For all the structures studied, there exhibit metal, half-metal, semiconducting, and spin-semiconducting behaviors. Especially, the BC_3 with Co adatom shows a quantum anomalous Hall(QAH) phase with a Chern number of -1 based on local density approximation calculations.(4) For Li, Na, K, Ca, Ga, In, Sn, Ti, V, Cr,Ni, Pd, and Pt, there exists a trend that the adatom species with lower ionization potential have lower work function. Our results indicate the potential applications of functionalization of BC_3 with metal adatoms.     

Adatom mobility on the surface planes of a simple metal

An approach using the Density Functional Approximation, which had in an earlier paper been applied to the binding of an adatom to a jellium metal (r_s = 4a₀), has been extended to a partially structured metal surface by introducing a layer of surface atoms to replace an equivalent layer of jellium. The model has been used to estimate the adatom motion energies over several quite close packed planes for a simple surface. An important preliminary step was to determine the general electron density contours for each surface by a simple variational method. The ensuing shielding charge distributions have an important bearing on the adatom motion energy. Adatom energies were calculated at three positions: (1) A above a surface atom, (2) B above a bridge position between two surface atoms and (3) C above a central position between three or four neighboring atoms. The motion energy was taken to be E_B ? E_C. As might have been expected this quantity was larger for the less closely packed planes, although it was always quite small due to the nature of the metal — large r_S, small ion core and typical s-p binding. To a rather surprising degree the strength of the shielding charge, the energies and the positions of the adatoms proved to be quite smooth functions of a parameter chosen to measure the close packing of the surface, namely the square of the interplanar distance divided by the surface area per atom.     

Surface resonant states and the adsorption of 4d atoms on Mo (110) and (100)

The adsorptive properties of Mo (110) and Mo (100) relative to 4d transition adatoms are investigated and compared using a tight-binding model. The surface resonant states existing on the (100) free surface are shown to have a drastic influence. On Mo (110) a Mo adatom retains a strong atomic character while on Mo (100) one approaches a surface molecule limit which is due to a strong interaction between the adsorbate atomic state and the free surface resonance. Consequently a large anisotropy of binding energy is found. The case of different 4d adatoms is also discussed.     

Diffusion of single adatoms of platinum,iridium and gold on platinum surfaces

Diffusion of single adatoms of platinum iridium and gold on platinum surfaces has been studied experimentally using field ion microscopy and theoretically using Morse potentials to describe the adatom bonding. The order of increasing activation energies for the surfaces studied was (111), (113), (011), (133), (001) and calculated activation energies agreed satisfactorily with the experimental values. On (113) and (011) platinum the activation energies for the adsorbates increased in the order Au, Pt, Ir. The calculations show that this variation arises largely from differences in the adsorbate binding energy, with the differences in adatom size having little effect. Diffusion of Au on (011) platinum occurred only along the direction of the surface channels, as is to be expected from the surface structure. In contrast, Pt and Ir adatoms were found to diffuse in two dimensions on (011). Since direct inter-channel jumps by adatoms should require large activation energies, it is proposed that the observed inter-channel adatom transfers proceed by a surface vacancy mechanism.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

Predicting the metal growth mode and wetting of noble metals supported on c-ZrO2

Employing periodic density-functional calculations, we predict the metal growth mode for ideal and defective interfaces of noble metals (Pd and Pt) deposited on top of cubic-ZrO₂. We discuss various energetic contributions and surface textures. We propose that isolated noble metal atoms are supported on both steps and terraces on the surfaces. Pt, on the other hand, shows a higher probability to decorate steps and other low oxygen coordinated sites, while Pd forms larger metal aggregates on both high oxygen coordinated sites (terraces) as well as low oxygen coordinated sites (steps). Oxygen defects on the support act as nucleation sites for the metal cluster growth. We also investigate metal wetting of Pd and Pt on the {1 1 1} surface of cubic-zirconia. Our calculations predict that the wetting angles are ca. 110-130° for 1 and 2 mono-layers of Pd and Pt, respectively, suggesting that metal wetting decreases as the metal loading increases.     

Comment on the “cross-channel diffusion mechanism on the Pt(110) surface” BY S.H. Garofalini and T. Halicioglu

It is known that the Pt(110) surface exhibits in its stable state a 1 × 2 reconstruction. It seems therefore not appropriate to compare theoretical surface diffusion coefficients calculated for the regular, non-reconstructed Pt(110) surface with the experimental data obtained by field ion microscopy. In particular, the process of cross-channel diffusion by atom exchange which is observed on the regular (110) surface by molecular dynamics simulations may have no relationship to the experimental findings of cross-channel adatom diffusion on reconstructed Pt(110) and Ir(110) surfaces. An alternate mechanism for easy cross-channel adatom diffusion on reconstructed (110)?1 × 2 surfaces of Pt and Ir is proposed.     

Photon-induced chemical vapor deposition of molybdenum on Pt(1 1 1)

Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO)₆. Mo(CO)₆ adsorbs molecularly on a Pt(1 1 1) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO)₆ undergoes decarbonylation to form surface Mo(CO)_x (x ? 5) under irradiation of ultraviolet light. The Mo(CO)_x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350-550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.     

A re-interpretation of the LEED structures formed by iodine on W(l10)

The early work done by Avery on the adsorption of I₂ on W(110) has been re-interpreted using adatom models originally developed for the Cl₂/Br₂/I₂/Fe (100) systems. The experimental coverages and LEED patterns are described precisely using variable, non-coincident nets of halogen atoms. It is shown that the movement of spots within the diffraction pattern arises from the movements of iodine atoms along simple crystallographic directions. The model assumes repulsive lateral interactions between iodine adatoms which is consistent with the desorption behaviour. The reasons for structural changes within the adlayer are discussed using the model, and the internuclear spacings and geometry of the adlayer are shown to be consistent with previous work on Fe(100) and W(100).     

Anchoring platinum on graphene using metallic adatoms: a first principles investigation

First principles calculations based on spin-polarized density functional theory were used to identify metallic adatoms that would strengthen the Pt(111)/graphene interface (with a low work of separation of 0.009 J m(-2)), when the adatom was placed between the Pt(111) and the graphene. It was shown that the strength of the Pt-adatom bond, which had a metallic character, increased with the amount of charge transferred from the adatom to the Pt. The carbon-adatom bond, on the other hand, had a mixed ionic and covalent character and was weaker than the Pt-adatom bond for each of the 25 elements considered. Consequently, the total Pt(111)/graphene interface strength and, hence, the anchoring effect of the adatom were controlled by the carbon-adatom bond strength. Metals with unfilled d orbitals increased the Pt/graphene interface strength to above 0.5 J m(-2). The carbon-adatom bond strength was proportional to the ratio between the charge transferred from the adatom to the graphene (ΔZ(C)) and the charge transferred to the Pt surface (ΔZ(Pt)); i.e., the ΔZ(C)/ΔZ(Pt) ratio defined the ability of an adatom to anchor Pt to graphene. For Ir, Os, Ru, Rh and Re, ΔZ(C)/ΔZ(Pt) > 1.0, making these elements the most effective adatoms for anchoring Pt to graphene.     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

氢在担载金属表面的吸附研究

应用格林函数方法在紧束缚近似下研究了氢在担载金属表面的吸附性质。采用自洽的Anderson-Newns吸附模型,对氢在Pt/ZnO,Cu/ZnO和Ni/ZnO三种担载式复合体系表面的吸附能△E、吸附态能级E_ad作了计算,并讨论了金属簿层在ZnO衬底上的沉积厚度及金属-衬底相互作用对氢在该类复合体系表面的吸附性质的影响。计算表明,金属-衬底相互作用越强,氢在Pt(Cu,Ni)/ZnO体系表面的吸附能及电荷转移量越小。金属-衬底相互作用抑制了氢在金属表面的吸附。衬底对金属表面吸附性质的影 关键词：