Coverage-dependent Sticking Probability Research Articles

The molecular dynamics of adsorption and dissociation of O2 on Pt(553).

Molecular adsorption and dissociation of O2 on the stepped Pt(553) surface have been investigated using supersonic molecular beam techniques and temperature programmed desorption. The initial and coverage-dependent sticking probability was determined with the King and Wells technique for various combinations of incident kinetic energy, surface temperature, incident angle, and surface coverage. A comparison with similar data for Pt(533) and Pt{110}(1 × 2) shows quantitatively the same high step-induced sticking at low incident energies compared to Pt(111). The enhancement is therefore insensitive to the exact arrangement of atoms forming surface corrugation. We consider energy transfer and electronic effects to explain the enhanced sticking. On the other hand, dissociation dynamics at higher incident kinetic energies are strongly dependent on step type. The Pt(553) and Pt(533) surfaces are more reactive than Pt(111), but the (100) step shows higher sticking than the (110) step. We relate this difference to a variation in the effective lowering of the barrier to dissociation from molecularly adsorbed states into atomic states. Our findings are in line with results from experimental desorption studies and theoretical studies of atomic binding energies. We discuss the influence of the different step types on sticking and dissociation dynamics with a one-dimensional potential energy surface.

A compact low-temperature single crystal adsorption calorimetry setup for measuring coverage dependent heats of adsorption at cryogenic temperatures.

Here we present the modification of an already existing Single Crystal Adsorption Calorimetry (SCAC) apparatus which has been extended by a compact cooling system to measure the coverage dependent heats of adsorption of gaseous compounds on thin metal substrates in a temperature range from 80 K to 430 K. The setup is characterized and its performance is tested by studying the adsorption of CO on Pt(111) at 150 K and 300 K. Coverage dependent sticking probabilities and heat of adsorption measurements are compared to previous experimental and theoretical studies proving the reliability of our compact low-temperature-SCAC setup.

Dynamics of water adsorption on Pt{110}-(1×2): A molecular dynamics study

Recent experimental studies of water adsorption on Pt{110}-(1x2) using supersonic molecular beams [F. R. Laffir et al., J. Chem. Phys. 128, 114717 (2008)] have revealed that the translational energy dependence of the initial sticking probability is a stepwise function with a threshold energy of 5 kJ/mol. The initial sticking probability increases sixfold from approximately 0.1 (at translational energies less than 5 kJ/mol) to approximately 0.64 (at translational energies greater than 10 kJ/mol). The aim of this work is to study the adsorption dynamics of water using classical molecular dynamics simulation in order to assess what physical factors are responsible for the observed behavior of the initial sticking probability. The simulations were performed using a purpose-designed code; water molecules were modeled using the well-known TIP4P water model, whereas the water-platinum potential energy function was determined using the ab initio density functional theory calculations. We conclude that the main factor controlling the initial sticking probability is a relatively weak energy transfer between the water molecule and the surface substrate during collision. This energy transfer is enhanced when the total energy of the water molecule increases. The assumption of an exponential increase of the probability of the energy transfer as a function of total energy of water molecule gives initial sticking probabilities very similar to those experimentally obtained. The same model was applied for the simulation of the coverage dependent sticking probability using a hybrid method comprising molecular dynamics and kinetic Monte Carlo approaches. We found a reasonable agreement between our results and the experimental data. The sticking probability as a function of coverage initially increases due to an increasing amount of the adsorbate island edges; it reaches a maximum and finally decreases as the islands merge together at high coverage. The saturation coverage was determined to be 2.8 ML at surface temperature 165 K, where water forms a puckered almost regular lattice with each water molecule having four nearest neighbors. At the studied temperature we did not observe the existence of stable water multilayers on the surface which is consistent with the experimental findings.

Coverage-dependent sticking probability and desorption kinetics of water molecules on Rh(111)

The adsorption and desorption kinetics of water molecules on Rh(111) were investigated using temperature programed desorption (TPD). Water molecules on Rh(111) show coverage-dependent sticking probability; the initial sticking probability is estimated to be 0.46. In the desorption process, a dilute gaslike phase and two-dimensional islands of water coexist on the surface. Based on the model proposed by Kreuzer and Payne [Surf. Sci.200, L433 (1988)], the apparent fractional-order TPD spectra can be interpreted as first-order desorption from the coexistence of two phases on which the sticking probabilities are different. Based on this, the previous estimation of pre-exponential factors assuming half-order desorption [A. Beniya et al., J. Chem. Phys.125, 054717 (2006)] should be revised.

Effect of background gas environment on oxygen incorporation in TiN films deposited using UHV reactive magnetron sputtering

The effect of the base pressure on the incorporation of oxygen into reactively magnetron-sputtered metal-nitride films has been investigated. A UHV sputtering system with a base pressure of less than 10 −6 Pa was used to examine the relationship between a deliberately introduced background pressure of oxygen and a measured oxygen content in the sputter-deposited TiN films. The results showed that with an oxygen partial pressure of 10 −4 Pa, the deposited TiN was found to include 10–20 at.% of oxygen when measured by the technique of X-ray photoelectron spectroscopy (XPS). When no oxygen was admitted into the system, no trace of oxygen could be detected in the deposited TiN films. The incorporation mechanism is discussed in terms of the coverage-dependent sticking probabilities of O 2 and N 2 on a Ti metal surface.

Direct Sticking and Differential Adsorption Heats as Probes of Structural Transitions: O2 on the Stepped Ni{211} Surface

Coverage-dependent heats of adsorption and sticking probabilities for oxygen on Ni[211] have been measured at 300 K using single-crystal adsorption calorimetry. The data are consistent with a switch from dissociative chemisorption at low coverage to oxide formation above 2 ML adatom coverage. The initial heat of adsorption is 620 kJ mol(-)(1), considerably higher than for any low Miller index nickel surface; this is attributed to the presence of low coordination step atoms that are preferably occupied up to 1 ML. As the coverage increases, the heat is found to drop very rapidly, indicating the presence of strong lateral adatom repulsions, which ultimately drive a transition from the chemisorption regime to oxide film formation at higher coverage. The shape of the coverage-dependent sticking probability is consistent with a direct adsorption mechanism at low coverage. At higher coverage, the transition between the chemisorption and oxidation regimes is relatively complex compared with low Miller index nickel surfaces. This is discussed in terms of the influence of the step sites on the [211] surface.

Modelling pattern formation in CO + O 2 on Pt{1 0 0}

We extend a detailed kinetic model for CO + O 2 on Pt{1 0 0} to describe pattern formation. The model includes: (i) a non-linear power law to describe the phase transition, (ii) trapping and untrapping processes explicitly considered, and (iii) experimentally determined coverage-dependent sticking probabilities and rate constants. This model is extended to include diffusion and gas global coupling. Diffusion is included through a mass-balance equation which couples the migration of CO with the phase transition. Gas global coupling is introduced considering realistic values of the pumping flow, the reactor volume and the size of the crystal.

Analytical modeling of silicon etch process in high density plasma

Plasma etching of silicon is one of the important etching processes used in modern integrated circuit manufacturing and micro-electro-mechanical systems fabrication. A good understanding of this process leads to better models which are the key to easier and less costly plasma etching process design. The main focus of this paper is on the simulation of the ion reflection from feature sidewalls and the resulting microtrenches. Pure Cl2 plasma was used for experiments because of the simple chemistry. SPEEDIE (Stanford etching and deposition profile simulator) was used in this work. Langmuir adsorption model was used for etching kinetics. Self-consistent calculations were done for fluxes using surface coverage dependent sticking probabilities. For ion reflection, it was assumed that the reflected ions come off with a distribution about the specular reflection angle. This distribution is modeled as cosnθ (θ is the deviation from the specular angle) and is important in getting the correct shape for microtrenches in simulations. A three-dimensional (3D) calculation of the reflection flux was done taking into account the 3D angular distribution of the incoming ions. The ion reflection efficiency was deducted from the silicon ion enhanced etching yield versus ion angle of incidence data. The simulation results match the experimental profiles fairly well.

The detailed kinetics of the adsorption of hydrogen on polycrystalline copper studied by reactive frontal chromatography

The coverage-dependent sticking probabilities of molecular hydrogen on polycrystalline copper supported on alumina have been determined in the temperature range 213–273 K by analysis of the hydrogen frontal adsorption line shape. In this temperature range the initial sticking probabilities increase from 8× 10−13 (213 K) to 1.3× 10−10 (273 K). The overall activation energy to adsorption has been found to be 42 kJ mol−1. The application of reactive frontal chromatography for the measurement of hydrogen sticking probabilities on copper is a novel variant of the N2O reactive frontal chromatographic method, developed for the measurement of copper surface areas. Its use here shows that reactive frontal chromatography may be applied generally to any adsorbate/adsorbent system involving activated adsorption and low sticking probabilities.

Coverage dependence of the kinetics for H 2 desorption from Rh(111)

Coverage-dependent sticking probabilities and second-order rate constants for recombinative desorption of hydrogen from Rh(111) have been measured using molecular beam relaxation spectroscopy (MBRS) and time-resolved specular helium scattering. The sticking probability follows second-order Langmuir coverage dependence, with s 0 equal to 0.01 ± 0.005. Under isothermal and nearly isosteric conditions over the coverage range 0.2–0.7 ML, the second-order rate constant for desorption is essentially independent of hydrogen coverage, in contrast to kinetic parameters determined from thermal desorption spectra.

Microcalorimetric study of ethylene adsorption on the Pt{111} surface

The reaction of C 2H 4 on Pt{111} at 300 K has been investigated by single crystal adsorption calorimetry. The initial heats of adsorption and the initial sticking probability are 174 kJ/mol and 0.67 respectively. The adsorption heat versus coverage data shows a remarkable minimum which serves as a basis for a mechanism involving ethylidene (CHCH 3) as an intermediate for the formation of ethylidyne (C 2H 3) on the Pt{111} surface. The coverage-dependent sticking probability strongly indicates that adsorption is precursor mediated and is well-fitted by the King and Wells expression. From the structure of the decomposition species known to form on the Pt{111} surface, the average CPt bond strength is found to be about 245 kJ/mol, although the actual value is found to be species-dependent.

Origin of spontaneous desorption observed in direct sticking probability experiments of weak molecular adsorption

Using two isotopes of ethane, we have investigatd the origin of spontaneous ethane desorption previously observed during direct adsorption probability measurements of molecular ethane adsorption onto a cold Pt(111) surface. We believe the spontaneous desorption may result from a phase transition in the ethane adlayer at coverages near monolayer saturation which shifts adsorbed ethane to a lower binding energy surface site and/or molecular configuration. We also provide conclusive evidence that desorption from the monolayer does not occur during our measurements of the coverage-dependent sticking probability.

Origin of spontaneous desorption observed in direct sticking probability experiments of weak molecular adsorption

Using two isotopes of ethane, we have investigated the origin of spontaneous ethane desorption previously observed during direct adsorption probability measurements of molecular ethane adsorption onto a cold Pt(111) surface. We believe the spontaneous desorption may result from a phase transition in the ethane adlayer at coverages near monolayer saturation which shifts adsorbed ethane to a lower binding energy surface site and/or molecular configuration. We also provide conclusive evidence that desorption from the monolayer does not occur during our measurements of the coverage-dependent sticking probability.

Oxidation of carbon monoxide catalysed by rhodium (110)

The oxidation of carbon monoxide on Rh(110) has been studied using a thermal molecular beam apparatus, with a flux of reactant at the crystal surface of ~ 0.01 ML s −1, in the temperature range 300–700 K. Results are reported for three types of experiments, (1) where the crystal is exposed to a mixed beam of CO and oxygen and reaction is allowed to reach steady state, (2) where the crystal is predosed with oxygen and then exposed to a beam of CO, and (3) where reaction is allowed to reach steady state at one temperature (usually 370 K), and the crystal temperature is then raised at ~ 10 K min −1. A kinetic model has been developed for the reaction that assumes Langmuir-Hinshelwood kinetics with coverage dependent sticking probabilities and activation energies, and that describes the experimental results well over all of the conditions studied. The reactivity of the rhodium crystal may be divided into two regimes, depending on oxygen coverage. At θ O > 0.6 ML, reaction becomes much slower than at lower oxygen coverages, especially at T > 400 K. At low temperatures reaction is inhibited by the presence of CO, but above T ≈ 400 K the steady state coverage of CO becomes small and reaction is then inhibited by adsorbed oxygen.

Oxidation of carbon monoxide catalysed by rhodium (110)

The oxidation of carbon monoxide on Rh(110) has been studied using a thermal molecular beam apparatus, with a flux of reactant at the crystal surface of ~ 0.01 ML s−1, in the temperature range 300–700 K. Results are reported for three types of experiments, (1) where the crystal is exposed to a mixed beam of CO and oxygen and reaction is allowed to reach steady state, (2) where the crystal is predosed with oxygen and then exposed to a beam of CO, and (3) where reaction is allowed to reach steady state at one temperature (usually 370 K), and the crystal temperature is then raised at ~ 10 K min−1. A kinetic model has been developed for the reaction that assumes Langmuir-Hinshelwood kinetics with coverage dependent sticking probabilities and activation energies, and that describes the experimental results well over all of the conditions studied. The reactivity of the rhodium crystal may be divided into two regimes, depending on oxygen coverage. At θO > 0.6 ML, reaction becomes much slower than at lower oxygen coverages, especially at T > 400 K. At low temperatures reaction is inhibited by the presence of CO, but above T ≈ 400 K the steady state coverage of CO becomes small and reaction is then inhibited by adsorbed oxygen.

An analysis of modulated molecular beam mass spectrometry applied to nonlinear systems

We have extended a formalism, developed previously to describe linear systems [H.-C. Chang and W.H. Weinberg, J. Chem. Phys. 66 (1977) 4176; Surface Sci. 65 (1977) 153], to the case of nonlinear reaction models in an analysis of modulated molecular beam mass spectrometry. The formalism is derived explicitly for the case of nonlinear surface reactions, adsorption with a coverage dependent sticking probability, and coupled bulk diffusion and nonlinear reaction. Specific examples illustrating how the formalism may be applied include the catalytic decomposition of formic acid on platinum and the reaction of fluorine with tantalum.