Deposition Of Adatoms Research Articles

Photosensitive self-assembled nanoarchitectures containing surfactant-free Si nanocrystals produced by laser fragmentation in water

We report on a rapid and scalable method for the self-assembly of networks of photosensitive silicon nanocrystal (Si-nc) micrograin produced by nanosecond laser fragmentation in water. Electrical conductivity through the long range (>200 μm) self-assembled luminescent Si-nc based networks is shown. The process forms stable nanoarchitectures which are compatible with subsequent plasma and heat treatments allowing for the deposition of carbon adatoms on the self-assembled structure. This strategy is environmental-friendly and could be efficiently used to control the complex interactions at the Si-ncs surface promoting the growth of specific nanostructures that in turn would enable the formation of long-ranged assemblies.

Studies on hard TaN thin film deposition by R C-Mag technique

The physical and mechanical properties of pulsed rotating cylindrical magnetron sputter-grown tantalum nitride (TaN) thin films were studied. Initially, films were grown at ambient substrate temperature by varying the reactive (N2) to sputter (Ar) gas ratio (R) at a constant pulsing frequency of the target power (100kHz). The results were compared with planar magnetron-grown TaN samples. The R C-Mag. grown thin films have properties nearly similar to the high temperature (300°C) dc planar magnetron sputter deposited samples. In comparison to the planar magnetron deposition, the progression of the phase composition occurs over a wider range of R in the pulsed R C-Mag. deposition. These observed differences for R C-Mag. deposition are attributed to the increased glancing angle deposition of adatoms and pulsing of the target power. To study the effect of pulsing frequency of the target power in R C-Mag., the films were also grown at different frequencies at a fixed R (0.1). With the increase in frequency, the mechanical hardness increased up to 50kHz and started decreasing beyond 50kHz. The observed changes in the mechanical hardness are attributed to the increase in stress and to the formation of increased polycrystalline understoichiometric TaN phases.

Reactions of vinyl groups on a model chromia surface: Vinyl chloride on stoichiometric α-Cr 2O 3[formula omitted

The thermal reaction of vinyl chloride on nearly-stoichiometric α-Cr 2O 3 ( 1 0 1 ¯ 2 ) produces gas phase acetylene, ethylene, 1,3-butadiene, and dihydrogen, and results in the deposition of chlorine adatoms. No surface carbon or combustion products (CO, CO 2, H 2O) are observed. Thermal desorption and spectroscopic studies indicate that the surface reaction of vinyl chloride proceeds through C–Cl bond cleavage to form Cl adatoms and surface vinyl groups which dehydrogenate to acetylene, hydrogenate to ethylene, and couple to form butadiene. Cl adatoms affect surface reactivity in two ways: (1) by increasing the barrier to vinyl dehydrogenation from 145 to 160 kJ/mol, and (2) by blocking Cr 3+ sites which shuts down the surface chemistry. Selectivity to butadiene is dependent on Cl adatom coverage, where the increased stability of vinyl intermediates at lower Cl coverages gives rise to more butadiene coupling product. At higher Cl coverages, Cl appears to inhibit the mobility of surface vinyl and decreases the reaction probability for coupling. Photoemission and near edge X-ray absorption fine structure (NEXAFS) spectra show that a mixed monolayer of molecular and dissociated vinyl chloride is formed at 130 K. The polarization dependence of the NEXAFS indicates that vinyl chloride π-bonds with the molecular plane nominally parallel to the surface, while vinyl intermediates σ-bond at Cr centers with the molecular plane nominally perpendicular to the surface, in agreement with DFT predictions of the adsorption geometries.

Diffusion mechanisms taking place at the early stages of cobalt deposition onAu(111)

In the present work a detailed atomic-level analysis of some of the main diffusionmechanisms which take place during cobalt adatom deposition are studied within atomdynamics (AD) and the nudged elastic band (NEB) method. Our computer simulationsreveal a very fast exchange between Co and Au atoms when the deposit is a single cobaltadatom. However, when the nucleus size increases, a decrease in the exchange probability isobserved. Activation energies for different transitions are obtained using AD in combinationwith the NEB method.

Optimal conditions for submonolayer nanoisland growth in ion beam assisted deposition

We study the optimal conditions for nanoisland growth in ion beam assisted deposition (IBAD). This situation occurs when adatom islands remain small enough to prevent the onset of three-dimensional growth, while at the same time preventing ion-induced surface erosion. We develop a rate equation model, which includes continuous deposition of adatoms and creation of vacancies, recombination of vacancies at adatom island edges and recombination of adatoms at vacancy island edges. Using this model we demonsrate that at onset between the rough and smooth layer-by-layer growth regimes there is a simple exponential relation between the largest size of the vacancy islands and the external control parameters of the growth.

Anisotropic corner crossing barriers in nanorod growth

The growth of quasi 1-dimensional bismuth rods from graphite step edges is modeled using kinetic Monte Carlo simulations. An anisotropic growth model is developed and tested against experimentally observed behaviour. The model incorporates adatom deposition, diffusion, and irreversible attachment of material to the aggregate. It is found that the best agreement between experiments and simulations occurs when anisotropic corner crossing barriers are introduced to the model.

Evidence of Water Reorientation on Model Electrocatalytic Surfaces from Nanosecond-Laser-Pulsed Experiments

The behavior of water at the interface formed between a quasi-perfect Pt(111) single-crystal electrode and an aqueous electrolyte solution is studied by means of the laser-induced temperature jump method. This method is based on the use of nanosecond laser pulses to suddenly increase the temperature at the interface. The measurement of the response of the interface toward the laser heating under coulostatic conditions provides evidence on the net orientation of water at the interface. Especially interesting is the study of the effect on the interfacial water caused by the selective deposition of foreign metal adatoms, because these bimetallic systems usually exhibit appealing electrocatalytic properties. The T-jump methodology shows that the surface composition strongly affects the interaction of water with the surface. The most representative parameter to characterize this interaction is the potential where water reorientation occurs; this potential shifts in different directions, depending on the relative values of the electronegativity of the adatom and the substrate. These results are discussed in the light of available information about the effect of adatom deposition on the work function and the surface potential of the modified surface. Finally, some implications on the enhancement of the electrocatalytic activity are briefly discussed.

On the enhanced electrocatalytic activity of Pd overlayers on carbon-supported gold particles in hydrogen electrooxidation

Palladium-gold particles with varied composition were prepared by Pd electrochemical deposition on Au nanoparticles immobilized on model carbon support. Pd-Au/C catalysts were characterized ex situ by transmission electron microscopy, energy dispersive X-ray analysis and X-ray photoelectron spectroscopy, and in situ, by underpotential deposition of hydrogen and copper adatoms, and CO stripping. Hydrogen oxidation reaction on pristine and CO-poisoned Pd-Au/C particles was studied using rotating disk electrode (RDE) technique. It was found that the decrease of the effective Pd overlayer thickness below ca. two monolayers resulted in a two-fold increase of the exchange current density of the hydrogen oxidation reaction and in significant increase of CO tolerance.

Optimal conditions for submonolayer nanoisland growth in ion beam assisted deposition

We study the optimal conditions for nanoisland growth in ion beam assisted deposition (IBAD). This situation occurs when adatom islands remain small enough to prevent the onset of three-dimensional growth, while at the same time preventing ion-induced surface erosion. To this end, we develop a rate equation model of IBAD, which embodies continuous deposition of adatoms and creation of vacancies, recombination of vacancies at adatom island edges, as well as recombination of adatoms at vacancy island edges. These rate equations are solved by numerical simulations based on the particle coalescence method. To determine the optimal growth condition, we find the largest mean size of the vacancy islands leading to their survival. We show that at this onset between the rough and smooth layer-by-layer growth regimes there is a simple exponential relation between the largest size of the vacancy islands and the external control parameters of the growth.

Effects of short-range attraction in unstable metal (110) epitaxial growth: Molecular dynamics simulations for Cu, Ag, and Al

The results of molecular dynamics simulations of the process of adatom deposition near steps for Cu(110), Ag(110), and Al(110) surfaces are presented in order to determine the effects of short-range (SR) attraction on the surface current and selected mound angle during unstable epitaxial growth. Our results indicate that, in qualitative agreement with previous work on metal (100) and (111) surfaces, for [001] steps the short-range attraction of depositing atoms to step edges can significantly increase the selected mound angle for typical energies used in epitaxial growth. In contrast, for the case of deposition near $[1\overline{1}0]$ steps, due to geometric effects the effects of short-range attraction are significantly weaker. As a result the overall ``uphill funneling'' probability ${P}_{\mathrm{av}}^{1\overline{1}0}$ is essentially the same as in the absence of SR attraction. Predictions for the selected mound angles along [001] and $[1\overline{1}0]$ steps are also presented for the case of irreversible growth with a large Ehrlich-Schwoebel barrier. Our results are also compared with recent experimental results in which anisotropic hut clusters with (111) and (100) facets have been observed. While our results are in qualitative agreement with experimental observations that the selected mound slopes are typically larger for [001] steps than for $[1\overline{1}0]$ steps, they also indicate that the effects of short-range attraction are not sufficient to explain the large-angle facets observed in $\mathrm{Al}∕\mathrm{Al}(110)$ growth at $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, thus confirming that in this case other effects such as uphill diffusion at step edges must play an important role.

A new approach to structural study of underpotential deposition of adatoms on the electrode surface, platinum (111)

An in-situ x-ray diffraction study using surface differential diffraction (SDD) was carried out to investigate the structural arrangement of Cu adatoms on Pt (111) in the underpotential region. A potential dynamic mode of deposition was used and the adlayer spacing of the adsorbate atoms determined as a function of potential. The studies showed that the adlayer spacing points to an initial open structure with Cu atoms sitting on the 3-fold hollow sites. The unoccupied 3-fold hollow sites are filled on completion of a monolayer.

Underpotential deposition of cadmium adatoms on Te and CdTe

Underpotential deposition (UPD) of Cd adatoms onto the surface of Te and CdTe films in Cd 2+-containing solutions has been studied using electrochemical methods and AFM. The electrochemical deposition of Cd adatoms on Te and CdTe begins at potentials 400 mV more positive than the reversible potential of Cd 2+/Cd couple and proceeds irreversibly. A strong chemical interaction of Cd adatoms with the surface Te atoms is the driving force of the UPD process. The deposition of Cd adatoms on the tellurium surface is accompanied by their stepwise interaction with tellurium to give CdTe nanophase.

Morphological evolution of ion-sputtered Pd(001): Temperature effects

We have investigated the kinetic effects on the morphological evolution of an ${\mathrm{Ar}}^{+}$-ion sputtered Pd(001) by in situ, real-time x-ray reflectivity and grazing-incidence small angle x-ray scattering measurements at various substrate temperatures. We find that surface roughness $W$ and its associated growth exponent $\ensuremath{\beta}$ increase with $T$ up to a certain temperature ${T}_{m}$. Beyond ${T}_{m}$, however, they decrease with increasing $T$. For $T<{T}_{m}$, surface roughening and coarsening kinetics are mostly driven by the deposition and diffusion of sputter-induced adatoms as reported in molecular beam epitaxial growth. When $T$ is near ${T}_{m}$, however, vacancies rather than adatoms become the dominant surface species. Above ${T}_{m}$, the surface smoothing via adatom detachment and vacancy diffusion across step edges becomes effective.

Dependence of morphology of pulsed-laser deposited coatings on temperature: a kinetic Monte Carlo simulation

Considering deposition, dissociation, and diffusion of adatoms on the surface of substrates with hexagonal structure, we performed a kinetic Monte Carlo modelling to explore aspects of the thin film growth via pulsed laser deposition (PLD) within the submonolayer regime. First- and second-nearest-neighbor interactions calculated by the Morse potential are taken into account. The simulations show that the behavior of PLD at a relatively low substrate temperature is similar to that of molecular-beam epitaxy (MBE). However, with increasing temperature, there would be a crossover to PLD characterized by an increase of the island density with the pulse duration decreasing. Moreover, we show that the scaling function of the island size distribution for PLD and MBE at the temperatures T=250 and 550 K. Similarly, the scaling function for PLD resembles that of MBE at the relatively low temperature, and then, it is markedly different from MBE at the higher temperature. The physical origins involved in the growth behaviors above are proposed.

Si epitaxial growth on Br-Si(100): How steric repulsive interactions influence overlayer development

Scanning tunneling microscopy results show the consequences of Si adatom deposition onto Br-saturated $\mathrm{Si}(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$. Those adatoms undergo an exchange reaction with Br and they are immobile at room temperature. In the low coverage regime, annealing to $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ leads to dimerization, limited ordering, and the formation of short Si chains. Adatom capture by those chains produces features of even and odd numbers of atoms. Annealing at $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ eliminates the odd chains, but diffusion is highly constrained by Br site blocking. With subsequent Si depositions, there is further nucleation of chains and chain growth. The local patterning of the Si chains reveals the influence of the Br steric repulsive interactions as out-of-phase structures were favored over in-phase structures around any given chain. Eventually, $(3\ifmmode\times\else\texttimes\fi{}2)$ adlayer patches develop. Second layer chains appear after the deposition of $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, with layer-2 nucleation at antiphase domain boundaries. Bromine loss was observed, even at $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and it is probably tied to the weaker bonding to single isolated Si adatoms.

In Situ Surface Characterization of Preferentially Oriented Platinum Nanoparticles by Using Electrochemical Structure Sensitive Adsorption Reactions

A new method for in situ surface characterization of preferentially oriented platinum nanoparticles is presented. The method is based on the use of structure-sensitive surface reactions such as the redox behavior of underpotential and spontaneous deposition of adatoms. Examples of its use for surface characterization of Pt nanoparticles synthesized by different methods are given. The method has been also employed to rationalize the different behavior of these Pt nanoparticles for the ammonia electro-oxidation in a basic medium.

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4+ 0.01 M CuSO4Solution

The formation of a copper adatom layer on polycrystalline platinum in a copper sulfate solution is studied by cyclic voltammetry in different cycling ranges at 0.1 V s–1. The copper adatom deposition kinetics is controlled by the following factors. The substrate's top layer structure during the oxygen exit onto the surface may be unstable at anodic limits Ea = 0.90–1.35 V. The concentration of copper oxides (active centers) may be higher at Ea = 0.8–0.95 V. The balance between different adsorption sites differs in different cycling conditions. Of importance is the number of complexes Oss–Ptn–SO4 and Oss–Ptn–Oc, where Oss is subsurface oxygen and Oc is chemisorbed oxygen.

Effects of atomic hydrogen on the selective area growth of Si and Si1−xGex thin films on Si and SiO2 surfaces: Inhibition, nucleation, and growth

Supersonic molecular beam techniques have been used to study the nucleation of Si and Si1−xGex thin films on Si and SiO2 surfaces, where Si2H6 and GeH4 have been used as sources. A particular emphasis of this study has been an examination of the effects of a coincident flux of atomic hydrogen. The time associated with formation of stable islands of Si or Si1−xGex on SiO2 surfaces—the incubation time—has been found to depend strongly on the kinetic energy of the incident molecular precursors (Si2H6 and GeH4) and the substrate temperature. After coalescence, thin film morphology has been found to depend primarily on substrate temperature, with smoother films being grown at substrate temperatures below 600 °C. Introduction of a coincident flux of atomic hydrogen has a large effect on the nucleation and growth process. First, the incubation time in the presence of atomic hydrogen has been found to increase, especially at substrate temperatures below 630 °C, suggesting that hydrogen atoms adsorbed on Si-like sites on SiO2 can effectively block nucleation of Si. Unfortunately, in terms of promoting selective area growth, coincident atomic hydrogen also decreases the rate of epitaxial growth rate, essentially offsetting any increase in the incubation time for growth on SiO2. Concerning Si1−xGex growth, the introduction of GeH4 produces substantial changes in both thin film morphology and the rate nucleation of poly-Si1−xGex on SiO2. Briefly, the addition of Ge increases the incubation time, while it lessens the effect of coincident hydrogen on the incubation time. Finally, a comparison of the maximum island density, the time to reach this density, and the steady-state polycrystalline growth rate strongly suggests that all thin films [Si, Si1−xGex, both with and without H(g)] nucleate at special sites on the SiO2 surface, and grow primarily via direct deposition of adatoms on pre-existing islands.

A nonelectrochemical reductive deposition of ruthenium adatoms onto nanoparticle platinum: anode catalysts for a series of direct methanol fuel cells

The surface of Pt nanoparticles was cleaned and saturated with hydrogen by treatment first with a 3% aqueous solution of H 2O 2 and then with hydrogen gas under water at room temperature. Reaction between the surface hydrogen and aqueous RuCl 3 deposited 0.18 surface equivalents of Ru ad onto the Pt nanoparticles. The deposition was repeated several times, with each reaction depositing ∼0.18 surface equivalents more Ru ad onto the Pt–Ru ad nanoparticles. The resulting Pt–Ru ad nanoparticles were analysed using cyclic voltammetry, CO stripping voltammetry, and as catalysts for electrooxidation of MeOH in three-electrode experiments and in prototype direct methanol fuel cells. The optimum surface coverage ( θ Ru) for electrooxidation of MeOH was ∼0.33 under these conditions.

Electrochemical deposition and stripping of nickel at a polycrystalline gold film electrode studied by cyclic voltammetry and surface conductance

Adatom and bulk nickel deposition onto gold thin film electrodes from an electrolyte containing KSCN, was studied by simultaneous application of cyclic voltammetry and the surface conductance (SC) technique. The latter is sensitive only to surface impurities, which allows monitoring of the formation of nickel adatoms on the gold film surface. At low adatom coverages nickel deposition fulfils the surface Linde rule for surface impurities. The SC response during bulk nickel deposition shows a small decrease due to the increase of the sample thickness.