Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped,platinum surfaces

Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm⁻¹ higher on a terrace site than on a step site.     

The effect of oxygen islanding on Co and H2 oxidation on Pt(111)

A Pt(111) surface partially covered with ¹⁶O islands was postdosed with ¹⁸O₂ to increase the island size under conditions where oxygen atom mobility is limited. Subsequently, these islands were titrated with CO and H₂. Preferential production of species containing ¹⁸O was not observed during any stage of the reaction. Assuming there is no extensive mixing of ¹⁶O and ¹⁸O isotopes within the islands, the results indicate no preferential reaction at island perimeters. The data are consistent with a model in which reaction proceeds uniformly over the whole island but the concentration of the mobile species (CO, H) inside the island is controlled by the island structure. This model qualitatively accounts for several apparent rate anomalies reported for CO and H₂ oxidation.     

Chemisorption and catalytic oxidation of CO on palladium

The adsorption of CO on clean, low index crystal faces of palladium is analyzed in terms of the fixed site and mobile models. Isotherms give evidence of adsorbed molecule interaction or heterogeneity, although in the case of Pd(111) the Clausius-Clapeyron heats do not, and partial mobility is indicated. The oxidation mechanism in the mixed-Langmuir formulation is analyzed in terms of the absolute rate theory, and in terms of the isolated domain or island model for mixed adsorption. The data, which are qualitatively similar on all crystal faces can be adequately accounted for on the basis of a non-equilibrium mixed-Langmuir adsorption on a surface with long-range heterogeneity.     

A reflection-absorption infrared study of carbon monoxide and nitric oxide adsorption on platinum (100)

The adsorption of NO, CO, and NO/CO mixtures, onto Pt(100), is studied by RAIRS. CO and NO are found to adsorb into islands at 300 K, but the islands breakup upon heating to 400 K. Dosing with a mixture of NO and CO at temperatures below 325 K is found to produce a mixed NO/CO island. There is a shift in the CO peak and NO peak during mixed island formation which is attributed to a strong chemical interaction between the adsorbed NO and CO. This interaction is found to produce an increase in the desorption temperature of CO. Autocatalytic behavior is found to arise because of an enhanced reactivity when CO enters a mobile state. The autocatalytic behavior could be responsible for the “surface explosion” reported by Lesly and Schmidt.     

The structure and mobility of carbon on Ru(001)

Interaction of ethylene with Ru(001) at temperatures above 300 K causes carbon to be deposited on the surface. Thermal desorption spectroscopy (TDS) of CO adsorbed on the precarburized surface shows that at carbon coverages below 0.5 ML the CO bond to the surface is weakened, but the adsorption capacity is only slightly diminished. At carbon coverages above 0.5 ML, CO adsorption sites are blocked. Temperature programmed oxidation (TPO) of the carbon layer shows a first-order peak at 570 K which indicates carbon-oxygen neighbors and a peak at ≈ 650K that requires mobility in the surface layer. CO TDS, TPO, Auger electron spectroscopy (AES), and SIMS all show that annealing the carbon layer causes growth of graphitic islands that starts at ≈ 600 K and is complete at ≈ 900 K. Temperatures above 1145 K cause dissolution of carbon into the bulk. The island formation, which requires a minimum carbon coverage, is indicated by increased CO uptake, a high-temperature TPO peak, loss of the AES carbidic feature, and restricted isotope mixing in the C₂⁻ ions in SIMS of ¹³C deposited on an annealed ¹²C surface. The SIMS experiments demonstrate the use of this technique for the study of proximity and structure in surface layers.     

Rate-ratio asymptotic analysis of non-premixed methane flames

The asymptotic structure of laminar, non-premixed methane flames is analysed using a reduced four-step chemical-kinetic mechanism. Chemical reactions are presumed to take place in two layers: the inner layer and the oxidation layer. In the inner layer the fuel reacts with radicals and the main compounds formed are the intermediate species CO and H₂. These intermediate species are oxidized in the oxidation layer. The structure of the oxidation layer is described by two second-order differential equations: one for CO and the other for H₂. Two limiting cases are considered. At one limit the global step CO+H₂?CO₂+H₂ is presumed to maintain partial equilibrium everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. At the other limit the steady-state approximation is introduced for H₂ everywhere in the oxidation layer except in a thin layer adjacent to the inner layer. This limit, called ‘slow CO oxidation’, has not been analysed previously. The structure of the inner layer is described by two second-order differential equations: one for the fuel and the other for the H radicals. This is a significant improvement over previous models in which either a steady-state approximation is introduced for the H radicals in the inner layer, or the reaction between the fuel and radicals is presumed to be very fast. The chain-breaking elementary reaction CH₃+H+M→CH₄+M is found to have a significant influence on the structure of the inner layer and on the scalar dissipation rates at extinction. The influence of this reaction was either neglected in previous models or was included as a perturbation to the principal elementary reactions taking place to the leading order in the inner layer. Using the results of the asymptotic analysis the scalar dissipation rates at extinction are calculated at a pressure of 1 bar. They are found to agree well with those calculated numerically using a chemical-kinetic mechanism made up of elementary reactions.     

Vanadium oxide nanostructures on Rh(1 1 1): Promotion effect of CO adsorption and oxidation

The adsorption of CO and the reaction of CO with pre-adsorbed oxygen at room temperature has been studied on the (2 × 1)ORh(1 1 1) surface and on vanadium oxideRh(1 1 1) “inverse model catalyst” surfaces using scanning tunnelling microscopy (STM) and core-level photoemission with synchrotron radiation. Two types of structurally well-defined model catalyst V₃O₉Rh(1 1 1) surfaces have been prepared, which consist of large (mean size of 50 nm, type I model catalyst) and small (mean size <15 nm, type II model catalyst) two-dimensional oxide islands and bare Rh areas in between; the latter are covered by chemisorbed oxygen. Adsorption of CO on the oxygen pre-covered (2 × 1)ORh(1 1 1) surface leads to fast CO uptake in on-top sites and to the removal of half (0.25 ML) of the initial oxygen coverage by an oxidation clean-off reaction and as a result to the formation of a coadsorbed (2 × 2)O + CO phase. Further removal of the adsorbed O with CO is kinetically hindered at room temperature. A similar kinetic behaviour has been found also for the CO adsorption and oxidation reaction on the type I “inverse model catalyst” surface. In contrast, on the type II inverse catalyst surface, containing small V-oxide islands, the rate of removal of the chemisorbed oxygen is significantly enhanced. In addition, a reduction of the V-oxide islands at their perimeter by CO has been observed, which is suggested to be the reason for the promotion of the CO oxidation reaction near the metal-oxide phase boundary.     

Insulator-metal transition in a conservative system: An evidence for mobility coalescence in island silver films

Aging, which manifests itself as an irreversible increase in electrical resistance in island metal films is of considerable interest from both academic as well as applications point of view. Aging is attributed to various causes, oxidation of islands and mobility of islands followed by coalescence (mobility coalescence) being the main contenders. The effect of parameters like substrate temperature, substrate cleaning, residual gases in the vacuum chamber, ultrasonic vibration of the substrate, suggest that the mobility coalescence is responsible for the aging in island metal films. Electron microscopy studies show evidence for mobility of islands at high substrate temperatures. The comparison of aging data of island silver films deposited on glass substrates in ultra high vacuum and high vacuum suggests that the oxidation of islands, as being responsible for aging in these films, can be ruled out. Further, under certain conditions of deposition, island silver films exhibit a dramatic and drastic fall in electrical resistance, marking the insulator-metal transition. This interesting transition observed in a conservative system — after the stoppage of deposition of the film — is a clear evidence for mobility coalescence of islands even at room temperature. The sudden fall in resistance is preceded by fluctuations in resistance with time and the fluctuations are attributed to the making and breaking of the percolation path in the film.     

Bistability in the shape transition of strained islands

The equilibrium shape of a monatomic strained island on a substrate depends on the step free energies and the difference in surface stress between the island and the substrate. For small island sizes the step free energies dominate, resulting in compact islands. Beyond a critical island size, however, the strain energy becomes dominant and the island maximizes its perimeter, resulting in elongated islands. Here we show that for strained islands with force monopoles pointing in opposing directions at neighboring steps, a regime exists near the critical island size where both compact and elongated shapes can coexist.     

Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

The interaction of CO and O2 with thin islands of Pd

The adsorption and coadsorption of CO and O₂ on islands of Pd about 7 monolayers thin was investigated by TPD and AES. It was found that CO decomposes much less on the Pd islands grown on W(110)-c(14 × 7)-0 than on small particles of Pd on bulk oxide substrates (mica, sapphire). Also, oxygen interacts unusually strong with the island surfaces at temperatures as low as about 600 K. These new results are attributed mainly to SMSI (strong metal/support interaction). Detailed studies of Pd island growth on W(110)-c(14 × 7)-O show that the island thickness can be varied systematically from ～ 3 to ～ 8 monolayers.     

Vibrational characterization of carbon monoxide oxidation on the Pt(111) surface

The surface reaction between coadsorbed carbon monoxide and atomic oxygen has been characterized using high resolution electron energy loss spectroscopy, coupled with temperature programmed reaction spectroscopy on a Pt(111) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Preferential oxidation of bridge bonded CO is not observed despite the fact that bridge bonded CO is adsorbed less vigorously than linearly bound CO. Saturation of the Pt(111) surface with one quarter of a monolayer of atomic oxygen completely suppresses the adsorption of bridge bonded CO. However, substantial coverages of bridge bonded CO can be coadsorbed if the Pt(111) surface is only partially saturated with atomic oxygen. The vibrational data for reaction of coadsorbed CO and atomic oxygen is consistent with a reaction mechanism involving reaction of mobile CO along oxygen island perimeters.     

Strongly-driven coarsening of height-selected Pb islands on Si(1 1 1)

A theoretical model is proposed to describe the rapid coarsening observed for Pb islands on a Si(1 1 1) surface where classical kinetics breaks down. In this system, quantum size effects produce mesa-like Pb islands with chemical potentials depending strongly on their heights, in addition to the usual dependence on the step curvature. Furthermore, a dense wetting layer enables fast mass transport between islands. Incorporating these features, our theoretical model predicts evolution of the island height distribution in good agreement with experiments.     

Nanowedge island formation on Mo(1 1 0)

The deposition of ultrathin Fe films on the Mo(1 1 0) surface at elevated temperatures results in the formation of distinctive nanowedge islands. The model of island formation presented in this work is based on both experiment and DFT calculations of Fe adatom hopping barriers. Also, a number of classical molecular dynamics simulations were carried out to illustrate fragments of the model. The islands are formed during a transition from a nanostripe morphology at around 2 ML coverage through a Bales-Zangwill type instability. Islands nucleate when the meandering step fronts are sufficiently roughened to produce a substantial overlap between adjacent steps. The islands propagate along the substrate [0 0 1] direction due to anisotropic diffusion/capture processes along the island edges. It was found that the substrate steps limit adatom diffusion and provide heterogeneous nucleation sites, resulting in a higher density of islands on a vicinal surface. As the islands can be several layers thick at their thinnest end, we propose that adatoms entering the islands undertake a so-called “vertical climb” along the sides of the island. This is facilitated by the presence of mismatch-induced dislocations that thread to the sides of the islands and produce local maxima of compressive strain. Dislocation lines also trigger initial nucleation on the surface with 2-3 ML Fe coverage. The sides of the nanowedge islands typically form along low-index crystallographic directions but can also form along dislocation lines or the substrate miscut direction.     

Mixing length scales: step meandering and island nucleation on vicinal surfaces

Simultaneous island nucleation and step flow growth on vicinal surfaces are studied by Monte Carlo simulations. Step edges experience meandering instability under growth conditions if there is a kink barrier suppressing adatom jumps around kink sites. This instability has a characteristic length scale, with different scaling properties from the island separation scale. We show that there is a coupling between island nucleation and step edge instability. The length scale associated with nucleation begins to couple with the wavelength of the step edge patterns when islands and steps coalesce. Only in the submonolayer regime step meandering is independent of island formation. In this regime the island separation has a cross-over scaling behaviour as terrace width is varied.Received: 9 April 2003, Published online: 19 November 2003PACS: 81.15.Aa Theory and models of film growth - 68.55.Ac Nucleation and growth: microscopic aspects - 81.16.Rf Nanoscale pattern formationM. Rusanen: Present address: Institut Français du Pétrole, Groupe de Modélisation Moléculaire, BP 311, 92852 Rueil-Malmaison, FranceJ. Kallunki: Present address: Laboratory of Physics, PO Box 1100, FIN-02015 HUT, Espoo, Finland     

Study of nanoclusters growth at initial stages of ultrathin film deposition by kinetic modeling

Comparative analysis of Au, Cu, Pt, Ni and Fe nanoclusters growth on amorphous carbon substrate by proposed kinetic model based on rate equations is present. Partial sticking coefficients introduced into the model let to discriminate elementary processes such as adatom adsorption and diffusion on bare substrate and on top of islands, nucleation and mobility of islands and its coalescence, 2-d and 3-d island growth modes. The quantitative fittings of experimental time dependencies of surface coverage, clusters density, cluster size are performed by solving model equations. From the best fittings the values of phenomenological coefficients defining elementary processes are found for different materials. Comparative analysis of those coefficients let to discover mechanisms of nanoclusters formation and growth of different materials. It is shown that clusterization for Cu and Au is more favorable than for Pt and Ni. Diffusivity for Pt and Ni on amorphous carbon (a-C) substrate is significantly less than for Au and Cu. In opposite, diffusivity on the top of islands for Ni and Pt is significantly higher than for Au and Cu. The mobility of islands for Au and Cu is much higher than for Ni and Pt. The fitting of experimental curves of Fe deposition on a-C at different temperatures showed that temperature mainly influences sticking process but not diffusion.     

Surface Restructuring, Kinetic Oscillations, and Chaos in Heterogeneous Catalytic Reactions

Kinetic oscillations in catalytic reactions on single-crystal surfaces often result from the interplay of the purely chemical reaction steps and adsorbate-induced surface restructuring. A classical example is CO oxidation on Pt(100). We survey evolution of the models used to simulate this reaction and show how it can be described self-consistently by employing Monte Carlo simulations combined with the lattice-gas model, taking into account substrate-substrate, substrate-adsorbate and adsorbate-adsorbate lateral interactions. Under the reactive conditions, this approach predicts formation of mesoscopic restructured well ordered islands with atomically sharp boundaries.     

Numerical study of growth and relaxation of small C60 nanoclusters

We investigate by means of kinetic Monte Carlo simulations the growth and thermal relaxation of small fullerene nanoclusters modelled by the Pacheco-Prates Ramalho pair potential. The activation barriers for diffusion processes are calculated on the fly by the dimer method. The elementary transitions which are likely to occur around room temperature are figured out. We study island growth on a perfect fullerene cluster and obtain a morphological transition of the island with increasing temperature. At T = 150 K, the islands are small and irregular. Around room temperature, elongated chain islands are obtained while at higher temperature, they are compact with an anti-Mackay stacking. These island morphologies have been shown to influence the character of the growth. Thus, growing fullerene clusters are disordered with rough surface below T = 300 K whereas at T = 450 K the growth occurs facet-by-facet within the growing shell.     

Au island growth on a Si(1 1 1) vicinal surface

Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges.     

原子表面扩散的对称破缺对二维岛形成的控制

文章阐述了当CO吸附改变时Pt/Pt(111)同质外延单层岛的的微观选择机制，即岛的取向由沉积原子的边－角扩散运动的不对称性惟一决定，对没有CO介入的理想真空生长情况，衬底温度不能改变三角形岛的取向。CO的吸附反转了边－角扩散牟不对称性，从而使岛的取向发生变化。