Thermal Energy Atom Research Articles

On the shape of the in-phase TEAS oscillations during epitaxial growth of Pt(111)

The growth of Pt(111) from its vapour phase is investigated by means of TEAS (thermal energy atom scattering) in a wide range of substrate temperatures: 100–800 K. The evolution of the in-phase He specular peak height during Pt deposition is studied in particular. At higher substrate temperatures ( T s > 500K), the in-phase peak height exhibits longlived temporal oscillations with a clearly asymmetric shape. This asymmetry, which increases with temperature, reveals substantial coarsening of the adatom islands during monolayer deposition, most likely due to Ostwald ripening processes.

Adsorption and coadsorption of CO and H 2 on Fe(111) probed by TEAS

Thermal energy atom scattering (TEAS), as developed by Comsa and coworkers, has been widely used to study a variety of processes on smooth, close packed metal surfaces such as Pt(111). The He beam specular reflectivity of these surfaces is very high, and absorbates or defects usually have a relatively large scattering cross section. For TEAS to have a wider applicability, studies on more corrugated surfaces should be undertaken. We report here studies of CO and H 2 adsorption and coadsorption on the open Fe(111) surface. The He specular intensity drops rapidly and smoothly with CO exposure on the Fe(111) surface. In contrast to CO/Pt(111), the diffuse scattering of He from CO on Fe(111) is not a negligible fraction of the intensity. The surface exhibits a finite reflectivity even at high CO coverages. An effective scattering cross section of 56Å 2 was derived from a fit of the reflectivity data to a model taking this diffuse scattering into account. For H 2 exposure to the Fe(111) surface, the He specular reflectivity decreases less abruptly, with distinct slope changes as coverage increases. Angular and temperature dependent measurements suggest distinct adsorption sites for the dissociatively adsorbed hydrogen. When CO and H 2 are coadsorbed, the order of adsorption affects the final composition and coverage in the overlayer. If the surface is first saturated with CO, H 2 will not adsorb, as evidenced by constant TEAS signal. When H 2 is adsorbed first, even to saturation coverage, CO will adsorb, displacing H 2 from the surface. Evidence is obtained for CO and H 2 segregation on the surface at intermediate coverages.

Simulation studies of the dissociation of H 2 on stepped surfaces

We have simulated the dissociation reaction of H 2 on stepped ideal honeycomb, square and hexagonal surfaces with a simple kinetic model based on the idea proposed by Poelsema and Comsa [Scattering of Thermal Energy Atoms from Disordered Surfaces, Springer Tracts in Modern Physics 115 (Springer, Berlin, 1989)] that dissociation on a stepped Pt(111) surface occurs exclusively on step sites. The observed sticking coefficient s H 2 of H 2 consists of two competing channels: (1) physisorbed molecules incident on terrace sites diffusing to the step, and (2) direct dissociation of molecules incident on or near a step site. The results support this picture qualitatively and indicate that the behaviour of the system can be separated depending on surface temperature into three domains in each of which a different mechanism dominates. (1) T < T 1: a geometric effect, i.e. the cluster size distribution of vacant adsorption sites on the surface, (2) T 1 < T < T 2: a geometric effect combined with nearest neighbour interactions due to ad atoms, and (3) T > T 2: direct adsorption near a step site. Of the three surfaces structures studied the honeycomb structure corresponds most closely to the observed H 2 dissociation.

7399. Epitaxial growth of metals studied with thermal energy atom scattering: S Ferrer and J J de Miguel, Vacuum, 41, 1990, 464–466

7399. Epitaxial growth of metals studied with thermal energy atom scattering: S Ferrer and J J de Miguel, Vacuum, 41, 1990, 464–466

Kinetic model for cooperative dissociative chemisorption and catalytic activity via surface restructuring

A kinetic model for catalytic reactions involving metal surface restructuring is proposed and discussed. The model is an extension of the equilibrium model for cooperative dissociative chemisorption [I. Oppenheim and R.D. Levine, Chem. Phys. Lett. 155 (1989) 168]. The model provides a possible explanation as to: (a) why strong binding sites (e.g. those at high surface roughness) can also exhibit a high turnover rate resulting in efficient catalytic activity, (b) why bond breaking occurs preferentially over a narrow temperature range and, (c) why this range is lower for more open surfaces. All these and other conclusions derive from the central result of the model that dissociative chemisorption of the physisorbed molecules is a cooperative process. It must, however, be emphasized that this behavior is due to the cooperative rearrangement of the substrate metal atoms upon the chemisorptive dissociation of the absorbate and not to the weaker forces that may operate among the adsorbates. The model is a kinetic one and depends on input from experiment. In particular, we take two general results as given. One is the well documented [G.A. Somorjai and M.A. Van Hove, Catal. Lett. 1 (1988) 433] restructuring of the surface upon bond breaking. The other, is that upon a bare, defect-free, smooth surface, the probability for dissociative chemisorption of an isolated physisorbed molecule, is low, [B. Poelsema and G. Comsa, Scattering of Thermal Energy Atoms (Springer, Berlin, 1989) p. 90].

Mechanisms for annealing of ion-bombardment-induced defects on Pt(111).

The morphology of a Pt(111) crystal during bombardment with 600-eV ${\mathrm{Ar}}^{+}$ ions has been investigated at constant temperatures ${\mathit{T}}_{\mathit{s}}$ with thermal-energy atom scattering. Below ${\mathit{T}}_{\mathit{s}}$\ensuremath{\lesssim}600 K the formation of three-dimensional (3D) structures is observed, while above ${\mathit{T}}_{\mathit{s}}$\ensuremath{\gtrsim}700 K the removal proceeds layer by layer. From the quantitative modeling of the data we infer that 3D structures develop when annealing proceeds only by intralayer diffusion of vacancies, while layer-by-layer removal is observed when interlayer mass transport also becomes effective. The latter process is believed to be related to the onset of meandering (roughening) of atomic steps.

Observation of sputtering damage on Au(111)

The morphology of a Au(111) surface has been observed with the STM (scanning tunneling microscope) after ion bombardment with 2.5 keV Ne + ions at about 400 K. Mostly triangular and hexagonal shaped vacancy islands are seen in the STM topographs. They are bounded by monatomic steps, oriented along the closed packed 〈110〉 directions. The general morphology confirms the conclusions inferred from TEAS (thermal energy atom scattering) measurements on ion bombarded Pt(111) surfaces. The observation of a propensity for the formation of {100} microfacetted 〈110〉 ledges is discussed.

Investigation by thermal energy atom scattering of the (2×1) clean Si(100) surface and of the C(8×8) and (2×8) reconstructions induced by copper

For many years it has been firmly established that the primary reconstruction of the Si(100) surface is the (2×1) induced by the formation of dimers. But numerous studies have recently shown that other reconstructions attributed to various arrangements of dimers and vacancies can be obtained: C(4×2), C(2×2), C(4×4), C(8×8), (2×8), (2×n)... The possible influence of Ni contamination at very low concentration has been also pointed out in the formation of the (2×n) phases but the C(8×8) is attributed to the clean surface. In our study by diffraction of He atoms the C(8×8) reconstruction is obtained (after Ar+ bombardment, annealing and a slow cooling 10°/mn) and the first order phase transition (T=875 K) associated to the emergence of this phase is observed. A second first order phase transition (T=1175 K) is also pointed out and is associated to the appearance of a (2×8) reconstruction after quenching. A characterization of the surface by SIMS and x‐ray fluorescence reveals a maximum copper contamination of 0.08 at. unit cell. Then the clean surface is studied and these experimental results are discussed.

The deuterium—oxygen reaction on Pt(111): Influence of atomic steps

We have investigated the deuterium—oxygen reaction on a Pt(111) surface, and the influence of atomic steps on this reaction at surface temperatures around 500 K and oxygen coverages Θ O >0.01. The kinetics of the reaction has been studied with molecular beam relaxation spectroscopy (MBRS), while the surface has been characterized with respect to oxygen coverage and step density by thermal energy atom scattering (TEAS). It appears that the reaction becomes slower in the presence of steps. This is explained by hydroxyl (OD) or deuterium becoming non-reactive when bound at step sites. A model, assuming a fast equilibrium between deuterium bound at step and terrace sites, describes the measurements remarkably well. From the measurements, we derive a value of 4.8 kcal/mol for the difference in binding energy of deuterium at step and terrace sites, in good agreement with previously reported values.

Epitaxial growth of metals studied with thermal energy atom scattering

The intensities of diffracted beams from a surface show, in some cases, pronounced temporal oscillations when an epitaxial film is growing from the vapour. This is easily understandable within the frame of the kinematical diffraction theory as being due to scattering from different surface levels. Analysis of the diffracted intensities allows quantitative values of the step densities of the growing film to be obtained. This is illustrated by the diffraction of neutral atoms (TEAS) from the surface of a Cu(100) crystal growing from the vapour phase. Also, it is shown that analysis of the diffracted intensity allows differentiation among the island nucleation vs step propagation growth regimes. The growth of cobalt on Cu(100) has also been studied with TEAS. It has been found that it grows pseudomorphically with an interplanar spacing of 1.74 ± 0.04 A°. The intensity vs deposition time curves show a peculiar shape that has been understood as being caused by the existence of two different step heights at the surface at low coverages: the substrate steps (1.80 A) and the cobalt steps (1.74 A).

Study by thermal energy atom scattering of the reconstructions of the Si(100) surface

The formation of aligned rows of dimers at the Si(100) surface produces the well known basic (2 × 1) reconstruction. However, during the last years many reconstructions (C(4 × 2), C(2 × 2), C(4 × 4), C(8 × 8), (2 × 8), (2 × n)) have been observed by diffraction techniques (LEED, TEAS). The (2 × n) family has been interpreted as induced by a very low contamination of Ni (LEED experiment, threshold of the ratio of the Ni/Si Auger peaks = 0.35%) but the C(8 × 8) structure has been attributed to the clean surface. In this study by He diffraction (TEAS) the C(8 × 8) and the (2 × 8) reconstructions are basically obtained after different thermal treatments and two critical temperatures associated to the appearance of these phases are observed but a SIMS analysis reveals a very slight copper contamination. On clean samples only the (2 × 1) is observed.

Morphology of a Pt(111) surface during its growth from the vapor phase

Thermal energy atom scattering was used to study the growth of a Pt(111) crystal via vapor deposition of platinum. At 650 K, temporal oscillations in the reflected helium signal provide evidence for a layer-by-layer growth mechanism via island nucleation, persisting during the deposition of over 150 monolayers. The damping of these oscillations results primarily from minute inhomogeneities of the platinum deposition rate over the area probed by the helium beam. The crystal growth mechanism is strongly temperature dependent: at 450 K 3-dimensional growth occurs, while at 900 K the crystal grows via step propagation

Epitaxial growth of metals: from monolayer to superlattice

The aim of this work is to show that, in order to produce high-quality metallic superlattices, the major requirements are to have an accurate control of the growth process and to use very flat, well ordered substrates. To this end, the growth of Co and Cu on Cu(100) has been studied from its very early stages, by means of Thermal Energy Atom Scattering (TEAS). Also, different Co Cu superlattices have been produced, and studied by neutron diffraction and SQUID magnetometry. They show good crystallinity, with the cobalt layers magnetized in plane and ordered antiferromagnetically. The magnetic moment of the cobalt atoms coincides with that of bulk fcc cobalt.

Epitaxy and magnetic properties of fcc cobalt films on Cu(100)

The epitaxial growth of fcc cobalt films on top of a Cu(100) substrate has been investigated by a variety of methods including Auger Electron Spectroscopy (AES), Medium Energy Electron Diffraction (MEED) and Thermal Energy Atom Scattering (TEAS). In a simultaneous AES/MEED experiment the growth mode was determined to be of the Frank-v.d. Merwe type. The magnetic properties of these films have been studied in situ by means of the Surface Magneto Optical Kerr Effect (SMOKE). We observed a strong thickness dependence of the Curie temperature leading to T c ≈ 300 K for a cobalt coverage of 2 atomic layers. The orientation of the magnetic moments, which was always found to be within the film plane, shows a distinctive in-plane magnetocrystalline anisotropy. In fcc cobalt films with a thickness of more than 3 monolayers we identified the 〈110〉 directions as the easy axes of magnetization.

Dissociative adsorption of O 2 on Ag(111)

Thermal energy atom scattering (TEAS) is used to determine the initial sticking probability S 0 for dissociative chemisorption of O 2 on Ag(111). Mixed beams of He and O 2 are used to study S 0 as a function of energy of the molecule (MBTEAS). At thermal energies S 0 is about 3.5×10 −6. It drops to immeasurably low values with increasing energy and rises to about 1×10 −5 on further increase of the incident energy (1.6 eV at 45°). S 0 seems to scale with E n. Two different mechanisms are responsible for sticking. At low energies physisorption-mediated sticking is dominant, and at high energies sticking via an activated precursor prevails. The adsorbed oxygen is removed in a first-order process with a decay time of 1000 s, as determined from the time dependence of the TEAS signal.

Surface phonon dispersion curves via thermal energy atom scattering: He atoms on RbCl(001) surface

A thermal energy atom scattering instrument, employing a time-of-flight technique to observe inelastically scattered He atoms, has been used to measure the surface phonon dispersion curves of the RbCl(001) surface. Data were collected over the entire Brillouin zone for a target temperature of 115 K and incident He wave vector of ki ≊7 Å−1. The results for the 〈100〉 and 〈110〉 high-symmetry directions are in good agreement with theoretical predictions based on a slab dynamics model calculation. Relaxation effects of the surface are negligible for this crystal.

Characterization of the growth processes and magnetic properties of thin ferromagnetic cobalt films on Cu(100)

Abstract The growth of thin films of fcc cobalt on Cu(100) has been characterized with medium energy electron diffraction (MEED) and thermal energy atom scattering (TEAS) in order to prepare films of reproducible structural perfection, interdiffusion profile and atomic concentration. The magnetic properties of thin films are shown to be strongly affected by these — usually difficult to control — parameters providing a clue to the possible origin of contradictory reports in this field. In particular, for carefully grown films, a clear thickness dependence of the Curie temperature, not previously reported for this system, has been found.

Epitaxial growth of metals: Experimental results and Monte Carlo simulation

Abstract Layer-by-layer growth of epitaxial systems has been studied with Monte Carlo (MC) simulations, and the results have been compared with those of thermal energy atom scattering (TEAS). This technique has proved to be purely kinematical and sensitive only to defects in the outermost layers, thus allowing for a direct comparison with MC results.

Characterization of the growth processes and magnetic properties of thin ferromagnetic cobalt films on Cu(100)

The growth of thin films of fcc cobalt on Cu(100) has been characterized with medium energy electron diffraction (MEED) and thermal energy atom scattering (TEAS) in order to prepare films of reproducible structural perfection, interdiffusion profile and atomic concentration. The magnetic properties of thin films are shown to be strongly affected by these — usually difficult to control — parameters providing a clue to the possible origin of contradictory reports in this field. In particular, for carefully grown films, a clear thickness dependence of the Curie temperature, not previously reported for this system, has been found.

Epitaxial growth of metals: Experimental results and Monte Carlo simulation

Layer-by-layer growth of epitaxial systems has been studied with Monte Carlo (MC) simulations, and the results have been compared with those of thermal energy atom scattering (TEAS). This technique has proved to be purely kinematical and sensitive only to defects in the outermost layers, thus allowing for a direct comparison with MC results.