Comparison between Si(100) and Si(111) in the reaction with oxygen at high temperatures

In this Letter, both the dynamics and kinetics of the reaction of oxygen molecules on Si(100)p2 × 1 and Si(111)7 × 7 and 1× 1 surfaces are compared. In all three cases, two distinct adsorption channels were observed. For oxygen molecules with translational energies less than 0.08 eV, the initial sticking is not sensitive to the energy or the angle of incidence, but displays a high sensitivity to the surface structure. At higher energies, a second channel becomes effective. The initial sticking coefficient increases rapidly and scales with the normal component of the translational energy, but the dependence on surface structure is greatly diminished. The kinetics of SiO formation are qualitatively similar on all surfaces with slightly higher rates on Si(111).     

Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

Adsorption of oxygen on clean single crystal faces of aluminium

Auger electron spectroscopy and work function measurements have been used to study the interaction of clean Al(111) and Al(100) faces with oxygen at low pressure near room temperature. The results for the two faces differ strongly. Thus, the sticking probability of the (111) face decreases rapidly with coverage, while the work function increases slightly, by 0.1 eV at 200 L. In contrast, the sticking probability of the (100) faces goes through a maximum, whereas the work function decreases almost linearly with coverage, the total decrease at 200 L being 0.5–0.8 eV. The shape of the Al L_{2, 3}VV spectrum from oxidized Al(100) is independent of coverage, and it is in fact very similar to previously reported spectra from oxidized polycrystalline aluminium. The corresponding spectrum from Al(111) exhibits large changes with oxygen coverage and shows a previously unreported double peak at ~60 eV. The results are explained on the assumption that oxygen adsorbs randomly on the (111) face, and that thin (~5 Å) islands of Al₂O₃-like oxide form on the (100) face.     

Sticking and displacement channels in the COg + O2/Pt(111) reaction

A molecular beam of CO, impinging on a Ft surface saturated with molecular oxygen, causes displacement of O₂ molecules into the gas phase. The kinetics of the displacement and associated CO sticking have been measured for CO kinetic energies in the range 0.06-1.83 eV. At low kinetic energies the main displacement channel is associated with the sticking of CO, which by dynamic energy and momentum transfer causes O₂ molecules to leave the surface, with a probability of 0.09 per stuck CO molecule. At the highest CO kinetic energies an additional displacement channel is appearing, namely inelastic (non-sticking) scattering of CO molecules, which deposit enough energy to displace adsorbed O₂ into the gas phase.     

Photoemission yield spectroscopy of electronic surface states on germanium (111) surfaces

The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures.     

Surface photovoltage spectroscopy of electronic surface states on cleaved germanium(111) surfaces

Surface photovoltage (SPV) measurements on UHV cleaved Ge(111) surfaces at 100 K are reported for photon energies 0.4 < ?ω < 1 eV. The SPV spectra are sensitive to surface treatment. Upon annealing to temperatures above 200°C, which is accompanied by a reconstruction change from the (2 × 1) to an (8) superstructure, the SPV spectrum shows 2 shoulders below band gap energy with threshold energies near 0.4 and 0.45 eV. These structures are interpreted in terms of electronic transitions from the valence band into empty surface state levels which are related to the (8) superstructure. Adsorbed oxygen and water vapor both cause new similar transitions from the valence band into empty surface states at 0.08 eV below the bottom of the conduction band.     

What determines the sticking probability of water molecules on ice?

We present both experimental and theoretical studies of the sticking of water molecules on ice. The sticking probability is unity over a wide range in energy (0.5 eV-1.5 eV) when the molecules are incident along the surface normal, but drops as the angle increases at high incident energy. This is explained in terms of the strong orientational dependence of the interaction of the molecule with the surface and the time required for the reorientation of the molecule. The sticking probability is found to scale with the component of the incident velocity in the plane of the surface, unlike the commonly assumed normal or total energy scaling.     

Binding states of zinc on the polar faces of Zno

The desorption of zinc from cleaved polar faces was investigated in UHV. TDS (calibrated by evaporation of metallic Zn), surface conductivity, LEED and AES were used. The initial sticking coefficient is lower on the Zn face than on the O face, 0.2 and 1 respectively. On both faces two states of adsorption occur. The lower state on the Zn face has an activation energy of 35 kcal/mole(1.5 eV). The corresponding energy on the oxygen face is 16 kcal/mole (0.7 eV). The lower peak is connected with an increase in surface conductivity on both faces. On the Zn face this surface conductivity can be attributed to a surface donor 1.3 eV above the conduction band edge. Above one monolayer the formation of islands and an epitactical growing of a zinc lattice were found.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Steps and facets on annealed LaAlO3{100} and {110} surfaces

Lanthanum-aluminate (LaAlO₃) is one of the optimum substrates for epitaxic growth of thin oxide films. In this paper, the structures of the {100} and {110} surfaces of annealed LaAlO₃ are studied using reflection electron microscopy (REM). 010 steps have been observed on {100}, these are the lowest energy steps. The {100} surface is atomically flat, but the {110} surfaces exhibit high-density fine structures distributed on large surface terraces. These fine structures correspond to the formation of small width (100) and (010) facets on the (110) surface. A growth model is given to interpret the formation of large steps and large terraces on the {110} surfaces.     

Initial adsorption of O2 on Si(1 0 0): Non-adiabaticity originating both from a discrete and a continuous set of electronic excitations

The initial adsorption of O₂ on Si(1 0 0) is investigated by density-functional theory calculations. The potential energy surface shows strong corrugations which can be interpreted as precursor states, however, there are also large areas where adsorption proceeds without a barrier. Furthermore, the initial sticking probability as a function of translational energy using first-principles molecular dynamics is calculated. The result is in disagreement with measurements of sticking probability which vary from high-low-high values as the translational energy of the oxygen molecules increase. A simple non-adiabatic model is put-forth that explains not only the measured sticking probability, but also have a novel interpretation of the increased sticking probability owing to tensile stress. The model deals with non-adiabatic effects originating both from a discrete and continuous set of electronic excitations. The implications are general and can be applied to other systems.     

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH₃Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH₃Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH₃Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH₃Cl caused a significant work function decrease, Δ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH₃Cl at lower concentration, prevented the adsorption of CH₃Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.     

Measurement of polarized target asymmetry in γd → pn in the photon energy region between 0.3 and 0.7 GeV

The target asymmetry in γd → pn has been measured at proton c.m. angles of 70°, 100° and 130° in the photon energies between 0.3 and 0.7 GeV. Results show relatively small asymmetry values in contrast to large proton polarizations. A phenomenological analysis by Ikeda et al. does not reproduce the present data, especially in the lower energy region.     

Comparison of the CO and D2 oxidation reactions on Pd supported on MgO(100), MgO(110) and MgO(111)

Oxidation of D₂ and CO on oxygen pre-exposed 200 nm thick Pd films, epitaxially grown on MgO(100), MgO(110) and MgO(111), has been investigated in the temperature range 100–300°C. Oxygen initial sticking coefficients have been determined to be close to 1 for the 100 and 110 films, and around 0.8 for the 111 film. The sticking coefficient and reactive sticking coefficient for CO oxidation on Pd/MgO(100) is also close to 1, and the maximum reactive sticking coefficient for hydrogen oxidation is determined to be around 0.9 at temperatures above 200°C. It is shown that the reactivities for the different surfaces vary strongly with surface and oxygen coverage, and the consequence of this for supported particle catalysts is pointed out.     

Variation du travail de sortie du W (100) par adsorption d'oxyge`ne,de ce´sium et co-adsorption d'oxyge`ne et de ce´sium

The work-function determination by the Kelvin method is used to study the adsorption of oxygen, cesium and co-adsorption of oxygen and cesium on a (100) tungsten surface, at room temperature. The work-function change of the clean surface with the oxygen exposure is used to estimate the sticking coefficient and the dipole moment of adsorbed oxygen. During cesium deposition on the clean surface, a minimum at 1.58 eV and a plateau at 1.80 eV are obtained. Starting from the minimum obtained with cesium, oxygen adsorption leads to a decrease of the work function down to 1.17 eV, when cesium adsorption on a previously oxygenated surface gives a 1.12 eV minimum and an increase of the plateau up to 2.20 eV. This last variation is shown to be consistent with the observed increase of the dipole moment of cesium adsorbed on a partially oxygenated surface, which accounts also for the lowering of the work function minimum.     

The adsorption and decomposition of NO on Pd(110)

The sticking probability of nitric oxide (NO) on Pd(110) and the relative selectivity of the surface to nitrogen (N₂) and nitrous oxide (N₂O) production has been measured as a function of coverage and as a function of surface and gas temperatures using a molecular beam. It is found that, at low temperatures (<440 K), molecular adsorption occurs with an initial sticking probability of 0.40 ± 0.02, rising quickly to a maximum of about 0.48 ± 0.02 as coverage increases before falling towards saturation. Following adsorption at 170 K four distinct adsorption sites can be identified by subsequent TPD. Hence, if beaming occurs at a temperature above the TPD peak due to a given site, then that site cannot be populated and the saturation coverage is found to be reduced. At higher temperatures (440–650 K) the sticking probability is seen to decrease continuously as a function of coverage. At a given NO uptake, the sticking probability falls with temperature indicating that the rate of NO desorption is significant in this temperature range. In addition, dissociation occurs leading to the desorption of nitrogen and nitrous oxide leaving only oxygen adatoms on the surface. The oxygen adatoms poison further reaction but can be cleaned off, even at the lowest temperature at which dissociation occurs, by hydrogen or carbon monoxide. At the low temperature end of this range more nitrous oxide is produced than nitrogen but this ratio falls with temperature until, above 600 K, there is 100% selectivity to the production of nitrogen which we propose is due to the low lifetime of molecular NO on the surface. However, at such high temperatures, reaction only occurs on a few sites probably located at the few step edges present on the crystal.     

Dependence of the sticking probability on initial molecular orientation: NO on Ni(100)

The sticking probability of NO at Ni(100) was examined using a beam of orientated NO molecules. It is found to be higher for N-end collisions. The asymmetry of the sticking probability has been measured to be a linear function of the molecular degree of orientation. It was determined to be A = 0.7 ± 0.1 and nearly independent of coverage when normalized to the degree of orientation. The orientational dependence of the sticking probability as a function of coverage shows that the adsorption of the molecules cannot be described by a precursor model that neglects direct chemisorption.     

Surface properties of Si from photoelectric emission at room temperature and 80 °K

The position of the Fermi level with respect to the energy bands at a semiconductor surface as well as changes in the work function can be determined from the energy distributions of photoelectrically emitted electrons. Prior studies involved the photoelectric yield spectrum and required assumptions concerning the photoelectric threshold; the present method is free of such assumptions. Measurements at room temperature indicate that the Fermi level lies 0.23 eV and 0.41 eV above the top of the valence band for degenerate p and n-type materials, respectively. These results confirm those of Allen and Gobeli¹). Cooling to 80 °K increases the work function of p-type material by 0.025 eV while that of n-type Si remains unchanged; the results show that the electron and hole gases in the surface states are degenerate. The density of surface states lies between 7 × 10¹³ and 10¹⁵ eV⁻¹ cm⁻². On the cesiated surface, the Fermi level lies 0.16 eV below the conduction band at room temperature and coincides with its bottom at 80 °K.