Adatom Concentration Research Articles

The Importance of Olefin Readsorption and H2/CO Reactant Ratio for Hydrocarbon Chain Growth on Ruthenium Catalysts

Abstract The synthesis of high molecular weight hydrocarbons on Ru catalysts requires the readsorption of primary α-olefin products. Such a readsorption step initiates a surface chain by reversing the β-hydrogen abstraction reactions: the chain termination step that forms an α-olelin. On Ru catalysts. 60 to 90% of C21+ products require at least one readsorption event. These readsorption steps become increasingly imptortant as chain size increases. This occurs because the diffusive removal of α-olefins from catalyst particles slows down significantly with increasing molecular size leading to long intraparticle residence times that favor secondary readsorption reactions. Chain growth probability and paraffin content therefore increase with molecular size. As a result, carbon number distributions do not obey simple Flory kinetics, an observation previously attributed to multiple chain growth sites and to the higher solubility of larger hydrocarbons in Fischer-Tropsch liquids. At low H2/CO reactant ratios, where termination to olefins should be favored, large chains terminate only as paraffins. This again reflects the importance of diffusion-enhanced olefin readsorption which not only dominates bed residence time effects for large hydrocarbons but also weakens the effects of changes in reactant concentration. High H2/CO reactant ratios lead to higher concentrations of hydrogen adatoms on Ru surfaces and thus favor chain termination to paraffins by hydrogen addition steps. Consequently, the contribution of olefin readsorption to chain growth and product molecular weight decreases markedly with increasing H2/CO ratio.

Adsorption of rare-earth metals (Tb,Dy,Er) on a Mo(110) surface

The adsorption of Tb, Dy and Er on a Mo(110) surface has been studied by means of AES, LEED, work function change measurements and quartz microbalance technique with the aim of understanding the evolution of the structure, structure-related properties and growth mode as a function of the adatom concentration, the adsorbate and the temperature. Although many similarities are found within investigated adsorbates some differences in details have been observed. The irreversible changes in work functions, submonolayer film structures and film growth modes, and a considerable decrease of the initial dipole moments of the adatoms observed at elevated substrate temperature, are probably determined by the change of the electronic states of the adsystems. The relation between the at present investigated initial dipole moments of RE atoms adsorbed on a Mo(110) surface and their electron structure is discussed.

STM study of single-crystal graphite

Freshly cleaved surfaces of single-crystal graphite (SCG) are shown to have very large expenses of step-free surface; however, the tunnelling signal is found to be considerably noisier than with HOPG. We speculate that the noise is due to carbon adatoms hopping onto the STM tip. Carbon adatoms are normally immobilized at steps. The lower step density on SCG results in an increased average surface concentration of carbon adatoms as compared with HOPG. Atomically resolved topographic images of the (0001) surface of SCG were also obtained. They show the same trigonal symmetry that characterizes the analogous images of HOPG, but with a two- to three-fold increase in corrugation depth. In addition to the low density of steps, SCG is relatively devoid of graphitic artifacts and pseudo-periodic features that have been mistaken for DNA and other genuine molecular structures, implying that SCG may be a useful substrate for STM studies of biological samples.

Effects of adatom concentration on Au/Si(111) and Si/Au interface formation

Effects of Au(Si) adatom concentration on Au/Si(Si/Au) interface formation were investigated by photoemission and partial-electron-yield spectroscopy using synchroton radiation, as well as by Auger-electron spectroscopy. Both interfaces are found to be reactive. The process of Au/Si(111) interface formation can be divided into three stages: for very low Au coverages, θ<2 Å, Au forms a rather unreacted adlayer, in the intermediate range, 2 < θ < 25 Å, a reacted layer with a stoichiometry close to Au3Si is found; the high-coverage region is governed by the formation of an unreacted metallic Au phase beneath an about one reacted monolayer. For Si/Au the formation of a reacted phase is observed from the first stage of Si deposition. A model for the formation of both interfaces in terms of Au spd/Si sp hybridization is presented.

Structure and stability of underpotentially deposited layers on gold(111) studied by optical second harmonic generation

Abstract : This report details changes in the second harmonic optical response from Au(111) during the underpotential deposition of Cu, Ag, Pb, and TI. The signal from Au(111) exhibits a strong anisotropy that is assigned to a coupling of the SH photons with interband transitions. Underpotential deposition is used to form overlayers of foreign metals with precise control of the surface concentration of adatoms, and the participation of the d-like electronic states in binding of metal adsorbates is evidenced in the optical response. We compare the results from the Au(111)/Cu and the Au(111)/Ag interface with those from the (111) surface of the single crystal adsorbates and find that the results are consistent with the known electronic structure of the metals and geometric structures of the overlayers. In the case of lead and thallium deposition, a strong perturbation of anisotropic second harmonic signal is observed and attributed to the inability of the large adatoms to lattice match with the substrate. The consideration of domain effects is important for the optical results to be consistent with recent structural determinations.

Adatom Concentration Profiles on Simulated Vicinal Surfaces During Epitaxial Growth

ABSTRACTConcentration profiles of surface atoms have been obtained from Monte Carlo simulations of epitaxial growth with anisotropic nearest-neighbor bonding on two different vicinal GaAs(001) surfaces which are misoriented toward [110] (denoted asA) and [110] (denoted asB). These profiles are compared with predictions based upon the original Burton-Cabrera-Frank (BCF) theory and with a modified BCF theory which accounts for the fact that all atoms, not simply free adatoms, can participate in surface diffusion. On both surfaces the modified BCF equation provides a more accurate description of the adatom concentration profiles than that obtained using only the free adatom concentration. This is due to step detachment processes which leads to an effective source of free adatoms that is not taken into account in the original BCF approach. Above the temperature where the growth becomes dominated by step advancement, the curvature of the concentration profile of potentially active atoms on both surfaces are similar but the concentrations differ due to the difference of the capture efficiency of the two types of step edges.

The effects of surface diffusion on the growth of thin film

A thin film growth model with the consideration of transient nucleation was proposed to simulate the vapor deposition process onto solid surface. This model is based on the mechanism of surface diffusion of adatoms, and the adatom concentration distribution is obtained by solving the diffusion equation around stable clusters The computation procedure is divided into a sequence of time stages in order to solve the time dependent problem. The cluster density and cluster size distribution can thus be obtained in every time stage. The results show that the transient stage can not be neglected for the case of completed condensation. The effect of adatoms consumption by stable clusters is important only in the later stage of the process when a significant number of stable clusters are formed. The effects of substrate temperature on the nucleation rate and the cluster growth are also discussed.

Analytical Representation of Charge-Density Profile at a Jellium Surface Covered with the Alkali Metal Submonolayers

Α simple trial function for the electron density profiles at the surface of ie bum covered by a submonolayerof alkafi adatoms is proposed. The values of work function changes due to adsorption, computed by use of this function, are in good agreement with experimental data. It is shown also that the proposed earlier trial functions for the electron density profiles at the metal surface covered by metallic adlayers, give a wrong sequence of surface concentration of adatoms, at which the minimum of work function for different adsorbates occurs.

Electron density near clean and alkali covered semiconductor surfaces

The electron density near clean and alkali-covered semiconductor surfaces is calculated using the exact relationship between the electrostatic potential of an adsystem and the exchange-correlation energy of the semiconductor substrate. The change in the work-function of a semiconductor surface as a function of the alkali adatom concentration is predicted on the basis of the calculated electron density. The inclusion of Coulomb correlation in this system makes it possible to obtain a more realistic description of the variation of the semiconductor surface properties with alkali adsorption.

Order-disorder transitions at the Ge(111) surface

Abstract Medium-energy ion scattering is used to study the order-disorder transitions occurring at the Ge(111) surface around 573 K and 1050 K. The measurements show that after the low-temperature transition the adatom concentration has been reduced to about 60% of the concentration at room temperature. In the high-temperature transition, a thin film of positionally disordered atoms is formed on the surface. The number of disordered atoms per unit area is found to be (0.7−1.1) × 10 15 atoms cm −2 , corresponding to 1 to 1.5 Ge(111) monolayers. The thickness of the film remains constant up to temperatures to within 25 K of the melting point. Our findings are in good agreement with the predictions of a thermodynamic model that is based on a theory due to Mikheev and Chernov.

Sb periodic doping in low-temperature Si-MBE

The Sb doping during the homoepitaxial growth on Si(111) and Si(100) surfaces has been studied at temperatures of 300–600°C. There are high-temperature and low-temperature growth regimes: for the (111) orientation the transition temperature is 500°C at growth velocity of 0.3nm/s. In the first range the exponential dependence of the Sb incorporation coefficient with an activation energy of about 1.2 eV is observed. The transition to the low-temperature regime is accompanied by the change of the surface structure from Si(111)-7×7 to 1×1. In this regime the Sb incorporation coefficient tends to unity and the activation energy is about 0.1 eV. The low-temperature doping is distinguished by the decrease of Sb surface segregation and can be used to create complicated epitaxial structures with large concentration gradients. The characteristics of the doping process observed are explained by the change of the Si adatoms concentration on the growing surface.

Order-disorder transitions at the Ge(111) surface

Medium-energy ion scattering is used to study the order-disorder transitions occurring at the Ge(111) surface around 573 K and 1050 K. The measurements show that after the low-temperature transition the adatom concentration has been reduced to about 60% of the concentration at room temperature. In the high-temperature transition, a thin film of positionally disordered atoms is formed on the surface. The number of disordered atoms per unit area is found to be (0.7−1.1) × 10 15 atoms cm −2, corresponding to 1 to 1.5 Ge(111) monolayers. The thickness of the film remains constant up to temperatures to within 25 K of the melting point. Our findings are in good agreement with the predictions of a thermodynamic model that is based on a theory due to Mikheev and Chernov.

On the surface coverage with adatoms during the initial nucleation stages of nickel deposition onto glassy carbon

The surface concentration of nickel adatoms during the initial stages of nucleation onto glassy carbon substrates has been determined by galvanostatic and potentiostatic pulse methods. It is established that nickel adatoms carry a partial positive charge of approximately 25%. The value of the diffusion coefficient of nickel adatoms is calculated to be D a ≈ 1.25 × 10 −10cm 2 s −1, and the value of the diffusion overvoltage is found to be 0.5–1.5% of the overall overvoltage. The resistance of the surface diffusion process of adatoms R d = 0.147 Ω cm 2. The results obtained lead to the conclusion that at high overvoltages the rate-determining stage of the overall reaction of crystal growth might be the surface diffusion of adatoms, which is suppressed by the adsorption of 2-butyn-1,4-diol.

Nucleation and growth of erbium clusters on amorphous carbon substrates. II. Rate equation approaches to nucleation kinetics

For pt.I see ibid. vol.1, no.2, p.145-51 (Oct. 1990). The correlation function and the distance distribution of nearest neighbours for erbium clusters deposited onto amorphous carbon surfaces are calculated, using (X,Y) coordinates of the clusters. Theoretical treatment of the processes of thin film nucleation and growth for both locally varying adatom concentration and mobile clusters suggested by Venables in 1973 are applied to the experimental results. The nearest neighbour distance distribution agrees well with the correlation function introduced by Venables, but Schmeisser's modification to Venables' correlation function proposed in 1974 does not produce a good fit to high number density distributions.

Nonlinear diffusion equation for crystal growth on stepped surfaces

An equation that includes both adatom diffusion and diatomic island formation is derived for describing growth on stepped surfaces. The equation is integrated numerically to obtain adatom and island concentration profiles along the terraces. Comparison of this solution with experimental measurements on vicinal GaAs(001) for a variety of Ga and As2 fluxes and with Monte Carlo simulations shows that inclusion of island formation in the growth equation is crucial in determining the temperature beyond which growth becomes dominated by step propagation.

Electron density near clean and alkali covered semiconductor surfaces

Abstract The electron density near clean and alkali-covered semiconductor surfaces is calculated using the exact relationship between the electrostatic potential of an adsystem and the exchange-correlation energy of the semiconductor substrate. The change in the work-function of a semiconductor surface as a function of the alkali adatom concentration is predicted on the basis of the calculated electron density. The inclusion of Coulomb correlation in this system makes it possible to obtain a more realistic description of the variation of the semiconductor surface properties with alkali adsorption.

The surface reaction of SO2 with adsorbed NO−3 on Ag metal microstructures: A surface enhanced Raman scattering experiment

Surface enhanced Raman scattering has been used in conjunction with mass spectrometry to study the in situ replacement reaction of adsorbed NO−3 on Ag metal powder with SO2 gas to form adsorbed SO=3 and SO=4 and gaseous NO2. Analysis of the intensity changes of the observed peaks lead to a model for surface reaction at low coverage. The time dependent background intensity, proportional to the adatom concentration which changes during the chemical reaction, is used to normalize the surface enhanced Raman scattering (SERS) peaks intensities. The resulting spectrum is proportional to the time dependent coverage and, if the reaction heat is small, the time independent electromagnetic enhancement. Examples are given demonstrating this effect.

Indirect interactions in clusters and submonolayers of noble metal adatoms on a jellium surface

The asymptotic form of the indirect pseudopotential interaction of noble metal adatoms is used to calculate the energy of clusters and submonolayers on a jellium surface. It is assumed that adatoms embedded into the nonuniform electron gas at the surface interact through the mean electron density which depends on the adatom-adatom and adatom-surface distances. The total energy of hexagonal submonolayers and surface clusters with linear, square and hexagonal structure is calculated. From the computations it follows that with increasing adatom concentration the growth of hexagonal close-packed islands is energetically more favourable than hexagonal or square submonolayers with regularly varying lattice constant.

Mass spectrometric studies of adsorption of silver on tungsten (110) surfaces

AbstractThe adsorption of silver on (110)tungsten was studied by a mass spectrometric molecular beam technique. The temporal behaviour of the flux of adatoms leaving the substrate was measured at fixed temperatures between 860 K and 1080 K for constant impinging rates over the range 0.2 · 1013 to 9.0 · 1013 cm−2 s−1.For moderate supersaturation the growth mode changed from the layer‐by‐layer growth of three monolayers to the growth of three‐dimensional nuclei (Stranski‐Krastanov mechanism). At equal rates of impinging and desorption flux, the equilibrium adatom concentration was determined and a set of adsorption isotherms was thus derived. The shape of the experimental isotherms was found to be broadly similar to that of the theoretical Fowler‐Guggenheim isotherm for a localized adsorption model. The binding energy of an Ag atom to the substrate, the lateral binding energy of a pair of atoms, the coordination number, and the vibration frequency of the adatoms for all three monolayers were obtained by suitable fitting procedures.

The kinetics of fast steps on crystal surfaces and its application to the molecular beam epitaxy of silicon

Crystal growth by molecular beam epitaxy (MBE) occurs under conditions of high supersaturation. The classic growth theory of Burton, Cabrera, and Frank (BCF) is based on the assumption that surface steps move slowly. Consequently, it requires modifications to be applicable to MBE because the velocities of surface steps may be large. In addition, because such steps are asymmetric structures, as observed experimentally by field ion microscopy, capture probabilities from above and from below a step must differ markedly. Hence the adatom concentration distribution cannot be at equilibrium at steps; there, it also suffers a discontinuity. We propose a model that treats surface step motion as a Stefan problem and that also respects its physical asymmetry. Calculations are presented which extend and complete recently published results that had imposed the restrictive condition of local equilibrium at steps. Step velocity is estimated as a function of supersaturation, degree of asymmetry, and step density. Concentration profiles are then computed; they are found to be generally skewed. In all cases, we show that the behavior of the growing crystal is convective rather than diffusive when the supersaturation is large. Consequently, we can understand the extraordinary insensitivity of the MBE of Si to changes in growth temperature and orientation.