Two-fold Bridge Site Research Articles

Heterogeneous adatom diffusion on fcc(100) surfaces: Ni, Cu, Rh, Pd, and Ag

We present activation barriers and prefactors for the migration of heterogeneous adatoms on fcc(100) surfaces. Two mechanisms are considered in this paper: (1) hopping of the adatom from one four-fold hollow site to an adjacent four-fold hollow site via a two-fold bridge site; and (2) exchange of the adatom with an atom in the first surface layer. Twenty heterogeneous combinations of Ni, Cu, Rh, Pd, and Ag were treated using transition state theory, and select comparisons were made to the results of finite temperature molecular dynamics simulations. The interaction potentials were generated using the molecular dynamics/Monte Carlo corrected effective medium (MD/MC-CEM) theory throughout. We find that the final state energies differ due to the variation of metallic bonding with coordination for the different types of metal atoms. This variation with coordination is reflected in the surface energies of the two metals, and thus this macroscopic quantity can be used to correlate the amount of energy gained or released when the adatom displaces a surface atom. Due to the non-directional character of metallic bonding in the fcc metals, this difference in energetic stability of final configurations is also found to generally determine whether bridge hopping diffusion or atomic displacement is the dominant kinetic process in these heterogeneous systems.

Sensitivity of Electrochemical Adlayer Structure to the Metal Crystallographic Orientation: Potential-Dependent Iodide Adsorption on Au(100) in Comparison with Other Low-Index Surfaces

Abstract : The potential-dependent ordered atomic structures formed for iodide adsorption on Au(100) from dilute aqueous alkali iodide electrolytes as discerned by means of in-situ scanning tunneling microscopy (STM) are reported and compared to the corresponding behavior of the other two low-index faces, Au(111) and (110) . As in related studies, emphasis is placed on linking the microscopic structural information to the voltammetric and other macroscopic electrochemical response, including the use of 'potentiodynamic' STM tactics where real-space images are acquired during appropriate electrode potential steps or sweeps. While relatively low iodide coverages, theta sub I approx. 0.1, are sufficient to lift the hexagonal Au(100) reconstruction, yielding large ordered (1 x 1) domains by ca-0.6 V vs SCE, ordered iodide adlayers are formed only above -0.2 V. A compressible incommensurate (2 sq rt 2 x p sq rt 2) phase (theta sub I, approx. 0.46 - 0.49) is observed between ca -0.2 and 0 V, which reverts to a distinct (2 sq rt of 2 x sq rt of 2)R45 deg structure (theta sub I approx. 0. 5) featuring uniform binding in twofold bridging sites. Interestingly, while the latter structure is retained locally at higher potentials, it is interspersed with narrow (3-4 atom wide) strips having a rotated hexagonal pattern. The latter becomes increasingly dense, and eventually dominant, towards higher potentials. This microscopically nonuniform (or spatially modulated ) phase is noteworthy as well as unusual; the corresponding iodine phases on Au(111) and (110) display, as is common, structural uniformity at a given potential.

Entropy-controlled site occupation of CO adsorbed on Ni(100)

The adsorption of CO on a Ni(100) surface has been studied by FT-IRAS in the temperature range from 85 K to 300 K. At 300 K and for Θ=0.5, the CO molecules are predominantly adsorbed in on-top sites with only a minor fraction located at two-fold bridge sites. Measurements on a Ni(100) surface pre-covered with sulphur, oxygen and carbon indicate that the occupation of bridge sites may be caused by small amounts of surface impurities. The relative broadness of the infrared bands is explained by CO molecules occupying intermediate positions at domain walls. Upon lowering the temperature, the bridge sites are increasingly occupied at the expense of terminal sites. This process is completely reversible and is explained by a contribution of the hindered translations of the adsorbed CO molecules to the entropy. At 85 K and for low initial coverages, we observe an unusual high CO stretching frequency at 2205 cm−1 which cannot be explained at present.

Chemisorption on stepped metal surfaces: CO/vicinal Ni(100)

The chemisorption properties of carbon monoxide on two vicinal Ni(100) surfaces have been studied with surface infrared reflection–absorption spectroscopy and low energy electron diffraction. For coverages ≤0.50 monolayer, equilibrium adlayers are formed in which CO populates atop sites on the low-index (100) terrace, as well as twofold bridging sites along both the highly-kinked and close-packed step edges of the Ni[(100)-1.4°(01̄0)] and Ni[(100)-9°(01̄1̄)] surfaces investigated. Low energy electron diffraction (LEED) measurements confirm that all three long-range structures established on the (100) surface—c(2×2) at 0.50 ML, hexagonal at 0.61 ML, and compressed-hexagonal at 0.69 ML—are also formed on the Ni [(100)-1.4°(01̄0)] surface. On the Ni [(100)-9°(01̄1̄)] surface, however, only the ordered c(2×2) structure appears. A simple Arrhenius analysis of the relative population of step and terrace sites estimates a small binding energy preference for populating step sites. This weak preference is of comparable magnitude to the CO–CO interactions that produce long range structures. To evaluate quantitatively the binding energy difference between adsorption at step and terrace sites, step adsorption isotherms are measured as a function of total coverage at select temperatures over the 90–300 K window. The isotherms are modeled with simple Monte Carlo simulations of adsorption on stepped surfaces, which include a 1.0 kcal/mol binding energy preference for step sites. The data and simulations indicate that the primary role played by the steps in the chemisorption of CO is to serve as nucleation centers for island growth.

The redox behaviour of compressed bismuth overlayers irreversibly adsorbed on Pt(111)

Cyclic voltammetry has been used to characterize the redox behaviour of irreversibly adsorbed bismuth on Pt(111). In agreement with Clavilier and co-workers, we find that a single redox peak at 0.57 V (Pd/H) is observed for θ Bi < 0.33 corresponding to a two-electron transfer overall. However, at higher bismuth coverages (0.33 < θ Bi <0.56) a multiples of redox peaks centred at 0.57 V (Pd/H) is seen. Comparison of our results with previous work by Paffett and co-workers concerning the adsorption of bismuth on Pt(111) from the gas phase suggests that the multiplet structure is associated with a compressed bismuth overlayer in which a variety of different sites are occupied by the bismuth adatom. It is further concluded (from the potentials at which the multiplet redox peaks occur) that for θ Bi < 0.33, all bismuth adatoms are situated in the twofold bridge site. This unusual result is discussed in the light of recent surface science studies of alkali metal adsorption on f.c.c.(111) transition metal surfaces.

Adsorbate-induced de-reconstruction in the interaction of H2S with Ge(001)2*1

The room temperature adsorption of H2S on Ge(001)2*1 has been studied using surface-extended X-ray-absorption fine structure, low-energy electron diffraction and Auger electron spectroscopy. The Ge(001)2*1-S system formed by a saturation dose followed by an anneal to remove H has a S-Ge bond length of 2.36+or-0.05 AA, with S sitting in twofold bridge sites, consistent with breaking of the Ge-Ge dimer reconstruction. Further adsorption of H2S at low or elevated temperatures is kinetically unfavourable, and it is proposed that this is consistent with the surface Ge atoms sitting in non-ideal rather than bulk-terminated positions.

Threefold-coordinated hollow adsorption site for c(4 x 2)-NO/Ni(111): A surface-extended x-ray-absorption fine-structure study.

Surface-extended x-ray-absorption fine-structure studies on the c(4\ifmmode\times\else\texttimes\fi{}2)-NO/Ni(111) system yield a nearest-neighbor O-Ni distance of 2.68\ifmmode\pm\else\textpm\fi{}0.05 A\r{} and show that NO adsorbs in threefold-coordinated hollow sites. This result casts doubt on the generally used method of adsorption site determination via molecular vibrational frequencies, which for this system led to an assignment of a twofold bridge site.

Bonding at Cu-Al interfaces: Relevance to microcircuit packaging

the nature of bonding interactions at Cu-Al interfaces is investigated both theoretically and experimentally. Extended Hückel-type tight binding calculations were performed on a model for Al binding to a Cu (111) surface. Binding at a three-fold bridge site was most preferred, followed by binding at a two-fold bridge site, followed by binding directly atop a surface Cu atom. Cu-Al interactions at all sites are strongly attractive. These results suggest that the metals should form chemical bonds, even though the interface may be very well defined. Thus, any bonding technique which cleans the surfaces of oxides and allows intimate contact between the metals should produce strong, electrically conductive bonds. Interfacial bonding was probed experimentally by sectioning Cu samples to which Al wire had been ultrasonically bonded. After bonding, the sample was baked in vacuo to accelerate formation of intermetallic phases. The intermetallic phases were observed after only 2 h of annealing at 300°C.

Chemisorption of carbon monoxide on Cr(111) studied by XPS, UPS and HREELS

Two different bonding states of carbon monoxide, “side-on” type and terminally bonded CO, are identified on Cr(111) at 200 K using XPS, UPS and HREELS. The side-on type CO with a low C O stretching vibrational frequency, 1100 cm −1, increases with CO exposure up to ∼0.5 L and decreases above that. On the other hand, the terminally bonded CO with a frequency of 1980 cm −1 appears at ∼0.45 L and increases with CO exposure. The binding energies of the C 1s and O 1s lines are 285.0 and 531.9 eV for the former and 286.3 and 532.6 eV for the latter, respectively. For the UPS spectra, the photoemission bands are observed at ∼21.75 and 12 eV for the former and 8.0 and 11.3 eV for the latter. Co-chemisorption study of CO and hydrogen suggests that the side-on type CO occupies twofold bridge sites.

An infrared study of NO adsorption at defect sites on Pt(111)

Abstract The interaction of NO with three vicinal Pt(111) surfaces with different defect concentrations has been studied with infrared spectroscopy. The defect concentrations were calibrated using CO adsorption which has well-characterized IR bands associated with bonding at defect sites. The adsorption of NO on these platinum surfaces gives rise to numerous IR bands in the range 1320–1820 cm −1 . Two of the NO bands correspond to molecules at defect sites. Molecules bound at defect sites with a linear Pt-N-O configuration yield an IR band in the range 1820–1840 cm −1 while a bent Pt-N-O species at a defect site has a wavenumber of 1609–1634 cm −1 . We have directly confirmed our assignment of these bands by blocking the defect sites with atomic oxygen. Other IR bands are assigned to NO at terminal (1700–1720 cm −1 ) and two-fold bridge sites (1475–1500 cm −1 ) of the (111) terraces.

An infrared study of NO adsorption at defect sites on Pt(111)

The interaction of NO with three vicinal Pt(111) surfaces with different defect concentrations has been studied with infrared spectroscopy. The defect concentrations were calibrated using CO adsorption which has well-characterized IR bands associated with bonding at defect sites. The adsorption of NO on these platinum surfaces gives rise to numerous IR bands in the range 1320–1820 cm −1. Two of the NO bands correspond to molecules at defect sites. Molecules bound at defect sites with a linear Pt-N-O configuration yield an IR band in the range 1820–1840 cm −1 while a bent Pt-N-O species at a defect site has a wavenumber of 1609–1634 cm −1. We have directly confirmed our assignment of these bands by blocking the defect sites with atomic oxygen. Other IR bands are assigned to NO at terminal (1700–1720 cm −1) and two-fold bridge sites (1475–1500 cm −1) of the (111) terraces.

Orientation dependence of the hydrogen molecule's interaction with Rh(001).

First-principles calculational results, that conflict with conventional wisdom, are reported for the orientation dependence of the energy of H{sub 2}, a few bohrs above a Rh(001) surface. In geometric configurations near the saddle point where H-Rh bonds begin to form, the favored molecular axis orientation is along a {l angle}100{r angle} direction, corresponding to the attraction of each H atom to a twofold bridge site. This is true even though the final destinations of the H atoms are fourfold hollows, and even though the {l angle}100{r angle} orientation corresponds to relatively low symmetry, i.e., {ital C}{sub {ital s}} rather than {ital C}{sub 2{ital v}}.

A diffuse LEED study of the adsorption structure of disordered benzene on Pt(111)

Previous LEED structural studies of benzene chemisorption on transition metal surfaces have relied on the presence of coadsorbed carbon monoxide to induce ordering in the overlayer. To investigate the effect of CO on the benzene structure, the disordered pure benzene layer chemisorbed on Pt(111) is studied by diffuse LEED intensity analysis. The diffuse LEED intensity has been measured using a digital LEED apparatus and analysed using an extension of the beam set neglect method. The benzene is found to chemisorb in a two-fold bridge site rotated by 30° with respect to the site found for the ordered phase formed in the presence of coadsorbed CO. In addition, the benzene ring is found to be distorted. Parallel distortions are subject to large error bars, but perpendicular spacings have been more accurately determined. These include a buckling distortion not previously seen for benzene chemisorption on transition metal surfaces.

Influence of coadsorbed bismuth and copper on carbon monoxide adlayer structures at ordered low-index platinum-aqueous interfaces

The influence of predosed bismuth and underpotential deposited (UPD) copper on the adlayer structure of carbon monoxide on ordered Pt(111), (100), and (110) electrodes in 0.1 M HClO 4 has been examined by means of in situ FTIR spectroscopy in the CO stretching ( v CO) region. The irreversibly adsorbed and electroactive nature of the bismuth adlayers enabled the effect to be examined of known and adjustable metal coverages, θ Bi, upon the v CO spectra for various dosed CO coverages, θ CO. On Pt(111), increasing θ Bi yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in the CO site occupancy from twofold bridging to terminal coordination as deduced from the relative v CO, band intensities. At the “saturation” bismuth coverage, θ Bi(sat) = 0.21, the CO binding is exclusively terminal throughout the θ CO range, 0.05 ≲ θ CO < 0.37. These results are consistent with adsorption of bismuth chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. A similar effect of increasing θ Bi upon θ CO [up to θ Bi(sat) = 0.43] is seen on Pt(100); the bridging v CO feature is again preferentiall yet incompletely, removed by θ Bi(sat). These findings indicate that an intermixed layer is formed on Pt(100) with bismuth again present at twofold bridging sites. By contrast, saturated CO adlayers in the presence of UPD copper on Pt(111) form segregated domains as evidenced by v CO spectra. The latter results indicate the presence of compressed CO island structures even down to low θ CO. Corresponding data for CO and copper on Pt(100), however, are consistent with the presence of intermixed structures, as deduced from the marked decreases in the terminal v CO frequency for larger copper coverages. Spectra for CO on Pt(110) with either coadsorbed bismuth or copper are also indicative of the presence of microscopically intermixed adlayers. The contrasting occurrence of segregated and intermixed copper/CO adlayers on Pt(111), and on Pt(100) and Pt(110), respectively, are consistent with the more open structure of the latter faces (cf. ref. [18]). For comparison, spectral data are also reported for CO/Bi adlayers on polycrystalline platinum.

Coverage- and potential-dependent binding geometries of carbon monoxide at ordered low-index platinum- and rhodium-aqueous interfaces: comparisons with adsorption in corresponding metal-vacuum environments

Infrared spectra for carbon monoxide adsorbed at ordered low-index platinum- and rhodium-aqueous interfaces as a function of electrode potential, E, and CO coverage, θ coare compared with each other and with vibrational spectra obtained at corresponding metal-ultrahigh vacuum (UHV) surfaces in order to explore the influences of the doublelayer environment on the CO adlayer structure. Earlier results for Pt(111), (100), (110), and Rh(100) electrodes in aqueous perchloric acid are supplemented by additional data in neutral and alkaline electrolytes in order to expand the accessible potential range. New results are also reported for Rh(111) and Rh(110) for each of these conditions. The form of the electrochemical infrared spectra in the C-O stretching ( v CO) region, especially the relative intensities of the characteristic terminal and twofold bridging v CO bands, tends to be most similar to spectra obtained at the metal-UHV surfaces at high θ CO and when the former are evaluated at relatively positive potentials. Altering the potential in the negative direction on most electrodes at high θ CO favors increasingly CO binding in bridging rather than terminal sites, especially on the rhodium surfaces. This is consistent with the greater extent of dπ-2π ∗ metal-CO back donation expected under these conditions. Bridging CO coordination is also favored increasingly at most electrochemical interfaces towards lower θ CO, these effects of water and/or hydrogen coadsorption are compatible with the behavior of analogous UHV systems. The v CO frequency- E slopes at constant adsorption site occupancy vary substantially with the CO binding geometry, being ∼ 30–35, 40–45, and 50–60 cm −1 V −1 for terminal, twofold and threefold bridging sites, respectively, at high θ CO. These findings, together with the observed increases in d v CO/d E towards lower θ CO. are consistent with the anticipated variations in back bonding. At least at high θ CO, the observed smaller v CO frequencies for the electrochemical relative to the corresponding UHV interfaces are largely compatible with the lower surface potentials that characterize the former, along with differences in site occupancy.

Scanning-tunneling-microscopy studies of Ag on Si(100)-(2 x 1).

The adsorption and growth of Ag on Si(100) at room temperature have been studied by scanning tunneling microscopy. The initial adsorption of Ag occurs at the two-fold bridge site in between adjacent Si dimer rows. Ag atomic chains are formed with preferred lengths. Metallic Ag islands form at coverages {approx gt}0.5 ML, and develop into (111)- and (110)-oriented crystallites covering the entire surface at higher coverages.

Electronic effects of surface oxygen on the bonding of NO to Pt(111)

Changes in the bonding of NO on Pt(111) induced by the coadsorption of high coverages of oxygen atoms have been studied with temperature programmed desorption (TPD), vibrational spectroscopy using high resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). Modification of the electronic structure of surface Pt atoms by the strongly electron-withdrawing adsorbed oxygen atoms alters the relative stabilities of NO adsorption sites and the nature of the Pt-NO bond. Coadsorption of 0.25 ML of O (a) destabilizes the two-fold bridge site for NO adsorption that is energetically preferred on clean Pt(111) and causes preferential NO adsorption in the atop site initially. For this oxygen coverage, some population of the bridge site occurs at the highest NO coverages, but occupation of this site can be eliminated completely by preadsorption of 0.75 ML of oxygen. This high coverage of coadsorbed oxygen now induces a further change in the nature of the NO chemisorption bond for NO adsorbed in atop sites, forming bent NO rather than the linear NO species formed on clean Pt(111). The saturation coverage of bent NO is 0.15 ML on this 0.75 ML oxygen-precovered surface and the heat of adsorption is only 1–2 kcal/mol less than linear NO adsorbed in atop sites on clean Pt(111). By using the HREELS and UPS data to identify these three chemically distinct forms of NO (a), we are able to rationalize their formation (and subsequent properties) in different electronic environments by correlating bonding configurations with the charge-transfer capabilities of the Pt substrate. Finally, we note that despite the presence of large excesses of O (a), NO is never oxidized to form NO 2, unlike the analogous facile oxidation of CO on Pt. This contrast in oxidation energetics is readily explained in terms of the measured relative barrier heights for oxidation versus desorption.

Electrochemical infrared spectroscopy of carbon monoxide on ordered rhodium (111): Comparisons with vibrational spectra on Pt(111) and in related surface–vacuum environments

Surface infrared spectra are reported for carbon monoxide adsorbed on a well-ordered Rh(111) crystal in 0.1 M HClO4 , and in neutral and alkaline electrolytes as a function of CO coverage θ as well as electrode potential. The results are compared in detail with corresponding vibrational spectra obtained recently for CO at the Pt(111)–aqueous interface, and on Rh(111) and Pt(111) in ultrahigh vacuum (uhv) in the absence and presence of coadsorbed water, in order to explore the possible influences of the aqueous electrochemical environment upon the CO surface coordination geometry. The potential-dependent surface infrared spectra for high CO coverages in 0.1 M HClO4 exhibited a major C–O stretching band (νCO ), due to terminal (i.e., linearly bound) CO at 2025–2042 cm−1 over the potential range −0.25 to 0.25 V vs SCE; a weaker νCO band, attributed to CO bound to twofold bridging sites, was observed at 1792–1815 cm−1 under these conditions. The latter form undergoes electro-oxidation preferentially, as deduced from spectral sequences obtained during slow voltammetric sweeps. At CO coverages close to saturation, θ≊0.75, the linear/bridged site occupancy ratio is estimated to be roughly 2. At lower CO coverages within the hydrogen adsorption region, a band at about 1870–1875 cm−1 appears at the expense of the terminal νCO feature. Corresponding spectra obtained in neutral 0.1 M NaClO4 and alkaline electrolytes also yielded similar terminal and bridging νCO bands, downshifted in frequency from those in 0.1 M HClO4 to an extent (≲50 cm−1 ) commensurate with the more negative electrode potentials involved. Comparison of the electrochemical infrared spectra with vibrational spectra obtained for the analogous Rh(111)/CO system in uhv indicates that the presence of the aqueous environment yields only small (≲20 cm−1 ) downshifts in the νCO band frequencies. Essentially the same conclusion is obtained from a corresponding comparison between vibrational spectra obtained at the electrochemical and uhv Pt(111)/CO interfaces. Cognizance is taken in these analyses of the known (or anticipated) differences in the potentials of zero charge and work functions in the electrochemical and uhv environments, respectively. These findings are also compared with recent vibrational spectra obtained for CO coadsorbed with water on Rh(111) and Pt(111) in uhv at low temperatures.

The adsorption of atomic hydrogen on Cu(111) investigated by reflection-absorption infrared spectroscopy, electron energy loss spectroscopy and low energy electron diffraction

Hydrogen atoms were adsorbed on Cu(111) at 150 K. The vibrational spectrum of the adsorbed hydrogen was measured by both reflection-absorption infrared spectroscopy (RAIRS) and electron energy loss spectroscopy (EELS) in the temperature range 90–280 K. The saturated surface displayed a (3 × 3) LEED pattern which was converted to a (2 × 2) pattern on heating to 186 K. The vibrational spectrum observed by EELS was little affected by this structural transformation. The major energy loss peak at 1040 cm −1 was assigned to the symmetric stretching mode of hydrogen in a two-fold bridge site. The infrared spectrum revealed more detail, including a second sharp absorption band at 1150–1170 cm −1 which was assigned to the first overtone of the deformation mode of hydrogen in a two-fold bridge site. The intensity of the electron energy loss peak, excited by dipole scattering, and of the infrared absorption bands provided an estimate for the effective charge, e ∗, of the hydrogen adatom of 0.05–0.06 e which is similar to values previously estimated for the hydrogen adatom on W(100).

On the splitting of the CO 1? level in the chemisorption system CO/Ni(111): substrate or lateral interaction?

LCGTO Xα model cluster calculations have been carried out to rationalize the shape of the CO 1π band of the chemisorption system CO/Ni(111) observed in angular resolved photoemission. The splitting induced by substrate interaction at twofold bridging sites (0.4 eV) is much smaller than the value deduced from experiment (1.2 eV). From calculations on (CO) n clusters lateral adsorbate interaction is estimated to cause a sizable broadening of the 1π band (1.2eV, in satisfactory agreement with experiment), but essentially no splitting (<0.1 eV). Therefore, rehybridisation due to local interaction does not seem to suffice as an explanation for the observed shape of the CO 1π band.