Thermal Energy Atom Scattering Research Articles

Grazing incidence fast atom diffraction, similarities and differences with thermal energy atom scattering (TEAS).

Grazing incidence fast atom diffraction (GIFAD) at surfaces has made rapid progress and has established itself as a surface analysis tool where effective energy E⊥ of the motion towards the surface is in the same range as that in thermal energy atom scattering (TEAS). To better compare the properties of both techniques, we use the diffraction patterns of helium and neon atoms impinging on a LiF (001) surface as a model system. E-Scan, θ-scan, and φ-scan are presented where the primary beam energy E is varied between a few hundred eV up to five keV, the angle of incidence θi between 0.2 and 2° and the azimuthal angle φi around 360°. The resulting diffraction charts are analyzed in terms of high and low values of effective energy E⊥. The former provides high resolution at the positions of the surface atoms and the attached repulsive interaction potentials while the second is sensitive to the attractive forces towards the surface. The recent progress of inelastic diffraction is briefly presented.

Kinetics and dynamics in physisorption of CH3Cl on HOPG: surface temperature and molecular orientation dependence

We report a study of kinetics and dynamics in physisorption of CH(3)Cl on a highly-oriented pyrolytic graphite (HOPG). Thermal energy atom scattering (TEAS) was used to probe the kinetics of thermal CH(3)Cl adsorption on HOPG during the coverage evolution. The desorption energy of CH(3)Cl on HOPG changes from 0.25 to 0.30 eV with increasing surface coverage, suggesting the attractive interaction between CH(3)Cl molecules on the surface. On the other hand, the oriented molecular beam scattering was used to monitor the dynamical interaction of CH(3)Cl with HOPG at zero coverage, demonstrating that the CH(3)Cl scattering intensity depends on the molecular orientation of the incident CH(3)Cl. The observed steric preference is not sensitive to the surface temperature. These results suggest that the moderate anisotropy in the interaction potential induces the molecular-orientation dependence of energy dissipation during the transient trapping into the physisorption well.

Physisorption of CH3Cl on HOPG -Dynamics and Kinetics-

We report a study of dynamics and kinetics in physisorption of CH3Cl on an highly-oriented pyrolytic graphite (HOPG). The oriented molecular beam scattering was used to monitor the dynamical interaction of CH3Cl with HOPG in the zero coverage limit, demonstrating that the CH3Cl scattering intensity depends on the molecular orientation of the incident CH3Cl. The observed steric preference is not sensitive to the surface temperature. These results suggest that the moderate anisotropy in the interaction potential induces the molecular-orientation dependence of energy dissipation during the transient trapping into the physisorption well. On the other hand, the thermal energy atom scattering (TEAS) was used to probe the kinetics of thermal CH3Cl adsorption on HOPG during the coverage evolution. The desorption energy of CH3Cl on HOPG changes from 0.25 to 0.30 eV with increasing surface coverage, suggesting the attractive interaction between CH3Cl molecules on the surface.

Growth of Co and Fe on Cu(1 1 1): experiment and BFS based calculations

The structure and morphology of Co and Fe films grown on Cu(1 1 1) have been investigated by thermal energy atom scattering (TEAS) and low-energy electron diffraction (LEED). It has been found that the growth mode of Co and Fe can be greatly improved by using Pb as surfactant, although in the case of Fe this works only for the first bilayer. This shows that the two systems exhibit decisive differences already in the first stages of the growth process. In a second series of experiments, the effect of codepositing Co–Cu and Fe–Cu on the films quality was investigated. The results are very promising, and suggest that very flat, structurally ordered fcc Fe–Cu and Co–Cu films can be prepared by applying this technique together with the use of Pb as surfactant. These results were complemented by atomistic simulations based on the BFS method for alloys. Simulations performed in the low-coverage regime suggest that the early stages of growth are governed to a great extent by the affinity of Cu for Co and Fe. We have also performed temperature-dependent Monte Carlo simulations to determine the structure of superlattices formed by codeposition of Cu–Co and Cu–Fe.

Growth of thin Fe/Fe2O3 films on the Cu(110) surface

The growth of Fe/Fe-oxide double-layers on Cu(110) was studied with thermal energy atom scattering (TEAS), Auger electron spectroscopy, and low-energy electron diffraction (LEED). An iron film with a thickness of about 0.6 nm was evaporated at low temperature (130 K) on a smooth, well-ordered thin film of Fe2O3 prepared on Cu(110). This Fe film is disordered. Ordering of the film was observed at temperatures between 400 and 600 K. At 530 K, a structure corresponding to that of a well-ordered α-Fe(001) surface was observed with TEAS and LEED. Clear evidence was found for a mixing of the Fe and Fe2O3 layers at the interface, already beginning at the deposition temperature of 130 K. With increasing temperature, the mixing of the Fe and Fe2O3 layers became gradually more effective until, at around 600 K, it was essentially completed. Upon annealing the sample to 1000 K the structure of the film changes and a very thin (less than 2 ML) FeO film on top of the Cu(110) surface is obtained.

Deposition of mass-selected clusters studied by thermal energy atom scattering and low-temperature scanning tunneling microscopy: An experimental setup

We present an experimental setup for the investigation of the processes occurring during the deposition of mass-selected clusters on a well-defined surface. The sample is analyzed in situ by two complementary methods: thermal energy atom scattering (TEAS) and scanning tunneling microscopy (STM). TEAS is used to study the dynamical processes during the deposition and to gather statistical information about the resulting structures on the surface. Subsequent STM measurements allow us to investigate the collision outcome on an atomic scale. The setup is highly versatile and guarantees ultra-high-vacuum conditions and cryogenic temperatures (≈30 K) of the sample at all times even during sample transfer. Clusters are produced in a CORDIS-type cluster source. A new compact multichannel effusive He source in combination with a new Wien-filter-based He detector are used for TEAS measurements. The new low-temperature STM allows measurements in a temperature range between 8 and 450 K. Atomic resolution on the Pt(111) surface is regularly observed at Tsample=8 K. The performances of the setup are illustrated by STM images obtained after the deposition of Ag7+ clusters with Ekin=95 and 1000 eV on bare Pt(111) and by measurements made of the deposition of Ag7+ clusters with Ekin=20 eV in a Xe–rare-gas matrix adsorbed on Pt(111).

Co growth on Pt(997): from monatomic chains to monolayer completion

We have used the Pt(997) surface as a deposition template to grow high-density arrays (5×10 6 cm −1) of parallel Co nanowires. We demonstrate that we can routinely grow ordered arrays of single monatomic chains of Co with precise coverage monitoring by in situ thermal energy atom scattering (TEAS). Nanowires constituted by one or more atomic chains have been characterized as a function of the substrate temperature and coverage by TEAS and scanning tunneling microscopy (STM). We find that, due to slow edge diffusion at low temperature and interlayer mass transport at high temperature, periodic arrays of uniform smooth chains are obtained only between 250 and 300 K. Co deposition in this temperature range results in a uniform coverage of chains having the same average width that can be used to investigate the magnetic properties of one-dimensional structures. The structure of a single-layer Co film has also been investigated: we show that the Co monolayer develops a non-periodic dislocation network upon deposition at 250 K. There is evidence for moderate Co incorporation into the first Pt layer below room temperature.

熱エネルギー原子線散乱法による吸着脱離過程の測定

熱エネルギー原子線散乱法による吸着脱離過程の測定

Thermal desorption from ordered chemisorbed phases studied by helium scattering: Oxygen on Ag(110)

We relate the helium specular beam intensity during adsorbate desorption (He desorption curve) to the instantaneous surface coverage. In this way the He desorption curve is shown to provide a picture of the desorption process which can be fruitfully compared to the one coming from thermal desorption mass spectra (TDS), obtained under strictly comparable experimental conditions. We tested the combination of thermal energy atom scattering (TEAS) and TDS in the case of the associative desorption from long range ordered O(2×1)–Ag(110) phase. Using the so-called overlap approach and assuming intense adsorbate–adsorbate (and vacancy–vacancy) attractions along Ag–O–Ag rows we obtain an instantaneous coverage which is in good agreement with TDS results. He desorption curves confirm the extreme sharpness of the desorption transition further indicating that the tails of the TDS peak bear small contributions from oxygen which did not belong to the O(2×1) phase. Opportunities and limitations inherent to the use of the He desorption curve (and its first derivative) as a marker of the temperature position and sharpness of the desorption transition are also addressed.

Surface kinetics of a nonlinear oxygen-induced (1×5)→(1×1) phase transition on Ir{100}

The interaction of oxygen with the stable Ir{100}-(1×5) and the metastable (1×1) surfaces has been studied using supersonic molecular beams in the surface temperature range 200–1080 K. Starting from the clean (1×5) substrate, the adsorption kinetics are dominated by the adsorbate-induced lifting of the reconstruction. The formation of (1×1) islands occurs between two limiting oxygen surface coverages, as confirmed by helium scattering and low-energy electron diffraction (LEED) measurements. Two distinct temperature regimes are observed in the sticking probability measurements; between 350 and 600 K the local oxygen coverage on the (1×1) phase is about 0.28 monolayers (ML) during the prevailing phase transformation, whereas it is 0.20 ML in the temperature range 700–900 K. This “biphasic” behavior is explained by the enhancement of surface diffusion of adsorbed oxygen atoms at sample temperatures above 650 K and has been investigated further using thermal energy atom scattering (TEAS). In contrast to the (1×5) phase, TEAS measurements show that random adsorption of O2 takes place on the clean metastable (1×1) surface. At 1080 K a pronounced flux dependence of the sticking probability is observed due to a nonlinear growth law for the formation of (1×1) islands, r=c(θO1×5)4.5. Thermal desorption measurements accompanied by LEED show that the desorption rate is strongly influenced by the (1×1) to (1×5) surface phase transition; repulsive lateral interactions exist between adsorbed oxygen atoms on the (1×1) substrate. We present a mathematical model which takes these effects into account in reproducing the salient features of the temperature programmed desorption (TPD) spectra. Sticking probability, TEAS, and TPD data are all consistent with a defect concentration of 0.03 ML on the clean (1×5) surface annealed at 1400 K.

On the low-temperature growth of Pb on Cu (100)

The morphology and structure of Pb deposits on Cu(100) between 150 and 220 K is investigated using low-energy electron diffraction (LEED), auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). It is found that Pb grows along a 〈111〉 axis, with Pb〈110〉 parallel to Cu〈100〉. In the surface plane, this relationship of epitaxy induces tensile stress of 3.1% in the direction of the common axis and compressive stress of −0.8% in the perpendicular direction. Starting from the wetting monolayer made above room temperature, the growth proceeds by a three regime sequence. The growth of a complete bilayer is followed by a quasi layer-by-layer regime which switches, at a temperature-dependent critical coverage, into a three-dimensional pyramidal growth mode. LEED observations suggest that the stresses are fully relaxed by the bilayer. These results are shown to be in good agreement with published thermal energy atom scattering (TEAS) data obtained on the same system. It is shown that the transition from the quasi layer-by-layer regime to the three-dimensional pyramidal growth mode is triggered by the development of islands with a triangular shape, which results in a limitation of the mass transport between atomic Pb layers.

Characterisation and O2 titration of CO islands on Pt 100: helium atom scattering and reactive sticking probability measurements

Abstract Thermal energy atom scattering (TEAS) has been used simultaneously with molecular beam adsorption and thermal desorption to probe the lateral distribution of CO on a Pt 100 single-crystal surface. On the initial hex-R phase at 350 K, (1 × 1) island growth occurs at a constant local coverage ( θ (1 × 1) CO ) of 0.4 ML. However, during desorption the surface remains wholly in the (1 × 1) state until ( θ (1 × 1) CO ) ≈ 0.25 ML. Subsequent reconstruction to hex occurs at a constant local coverage of 0.19 ML in the remaining (1 × 1) domains. Hysteresis is not observed for the pre-prepared (1 × 1) surface, on which a random CO distribution is observed during adsorption. The scattering cross-section Σ for a single CO molecule on the hex-R and (1 × 1) surfaces at 350 K has been determined as 71 ± 10 and 130 ± 18A 2 respectively. The behaviour during oxidation of (1 × 1)-CO islands on the initial hex-R surface prepared by either adsorption or desorption is found to be consistent with the TEAS data. The first sticking probabilities for O 2 and CO on the CO ad -freed and O ad -freed (1 × 1) phase of Pt 100 are also reported.

Surface morphology after low coverage Pt deposition on Cu(110)

The morphology of a Cu(110) surface after low coverage Pt atom deposition from the vapour phase is studied with thermal energy atom scattering (TEAS) in the temperature range of 200–800 K. Both cross section measurements and diffraction analysis reveal different structures. At low temperatures, isolated defects containing a metal adatom are observed. Between 350 and 650 K, experimental results suggest that these aggregates are dissociated, leaving behind them an almost flat surface patch with an incorporated Pt atom. Above 650 K, bulk dissolution of Pt occurs.

The adsorption of H2 on Fe(111) studied by thermal energy atom scattering

The interaction of H2 with Fe(111) has been studied by thermal energy atom scattering (TEAS). The specularly scattered He intensity as a function of hydrogen coverage exhibits a concave drop in scattered He intensity up to 30% coverage, followed by a plateau and another drop in intensity at 80% coverage. A model has been developed to account for this data which assumes three adsorption sites for H on Fe(111), in analogy with the three desorption peaks seen in temperature-programmed desorption (TPD). The adsorption sites have not been definitely assigned, but are labeled ‘‘deep–hollow,’’ ‘‘shallow–hollow,’’ and ‘‘on-top.’’ Competitive adsorption into the deep-hollow and shallow-hollow sites is assumed to account for the initial concavity of the data. Adsorption into on-top sites only becomes significant at 80% coverage. Effective cross sections and reflectivities for the three sites have been obtained for θi=60° and 40°. When a H saturated surface is heated, the scattered He intensity decreases upon heating from 173 to 240 K, at which point desorption has already begun. This drop in intensity is not completely explicable by a Debye–Waller attenuation. It is proposed to be due to a shift in population of the adsorbed H to more exposed, on-top sites, in accord with a Boltzmann distribution of adsorption sites. The scattered He intensity increases upon further heating from 240 to 400 K, corresponding to the desorption of recombined H2 from the surface. Comparison of the He/H/Fe(111) system is made with the He/H/Pt(111) and He/H/Fe(110) systems.

He beam scattering studies of the interaction of CO with iron (111)

The interaction of CO with Fe(111) has been studied by thermal energy atom scattering (TEAS) at surface temperatures of 250 and 400 K. The behavior of He scattering from both the clean and CO covered Fe(111) surface is different from that on smooth, close-packed surfaces such as Pt(111). Although the specular He intensity from the CO covered surface is not negligible, as it is on smooth surfaces, CO is seen to be a perfectly diffuse scatterer. A model to describe the scattering intensity as a function of CO coverage was developed which takes into account the He intensity scattered from the clean surface and the intensity scattered from the CO covered surface, with no interference effects. Effective cross sections of 56 Å2 for He scattering from molecular CO and 93 Å2 per molecule for He scattering from dissociated CO were obtained. Comparison of the He/CO/Fe(111) system is made with He/CO/Pt(111) and He/H/Pt(111) systems and clear differences are seen. As the temperature of the CO covered surface is increased there are two increases in scattered He intensity at ∼360 and 750 K, corresponding to the desorption of molecular and recombined CO, respectively. This study suggests that, although the interpretation of the data is more complicated than for smooth surfaces, TEAS is a viable technique for application to studies of adsorption on more corrugated surfaces.

RHEED and TEAS characterization of thin-film growth

By means of a Monte Carlo simulation the author has studied the effect of different growth mechanisms on the intensity evolution of reflection high-energy electronic diffraction (RHEED) and thermal energy atom scattering (TEAS). The author found that due to the different sensitivities to the surface defects RHEED and TEAS give different results for the same surface evolution during the process of film growth.

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.

Scattering of Cl 2 from clean and Cl-covered Ag(111)

Supersonic molecular beam scattering is employed to investigate the interaction between hyperthermal (translational energy E i = 1−3 eV) Cl 2 and a clean or Cl-covered Ag(111) surface. The angular distributions of Cl 2 scattered from a Cl-covered Ag(111) are very broad. The ratio of the densities measured at the surface normal and at the maximum of the distribution scales with the momentum of the incoming molecules parallel to the surface. This might be caused by the large corrugation of the covered surface or by trapping desorption, as was recently suggested by DeLouise for Cl 2 scattering from Cl-covered GaAs(110). Measurements done at a clean surface (coverage ≤ 0.1 monolayer) show a much sharper distribution. Thermal energy atom scattering (TEAS) is used to determine the initial sticking probability S 0 to be between 0.3 (1 eV, Θ i = 70°) and 0.8 (1 eV, Θ i = 60°).

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.

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.