Low energy electron diffraction studies of high index crystal surfaces of platinum

Several high Miller Index crystal surfaces of platinum have been examined by low energy electron diffraction (LEED) and have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The ordered stepped surfaces were found to be stable in ultra-high vacuum up to 1500 K, and the reasons for this remarkable stability are discussed. The presence of ordered atomic steps may be considered as a general structural property of high index surfaces regardless of the chemical bonding in the crystal. A nomenclature is suggested to identify the surface structures of stepped surfaces.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

Low energy electron diffraction and auger electron spectroscopy studies of the structure of adsorbed gases on solid surfaces

Low energy electron diffraction (LEED) studies of the structure of adsorbed molecules on crystal surfaces revealed that ordered surface structures predominate under most conditions of the experiments. In the absence of chemical reactions with the substrate, the degree of ordering depends on the heats of adsorption, ΔH_ads, and the activation energies for surface diffusion, ΔE_D1. Since ΔH_ads is usually markedly larger than ΔE_D1, small changes of substrate temperature facilitate ordering without appreciable increase in desorption rates. The surface structures of adsorbed gases that have been reported so far have been tabulated. For molecules whose size is compatible with the interatomic distance of the substrate, rules of ordering can be proposed that permit prediction of the structure of the adsorbed layer that is likely to form. These rules indicate close packing due to attractive interactions in the adsorbed layer, and that the rotational multiplicity of the substrate is likely to be maintained by the adsorbate structure. When molecules whose dimensions are larger than the substrate interatomic distance are adsorbed, the conditions that control ordering are more complex and simple rules may not be readily applicable.The surface structures of adsorbed gases have also been studied on high Miller Index substrate surfaces. These surfaces are characterized by ordered steps separated by terraces of low index surface orientation. Many gases have different ordering characteristics on stepped surfaces than on low index crystal faces due to the stronger substrate-adsorbate interactions in these surfaces. The dissociation of diatomic molecules at steps induces the formation of new types of surface structures (frequently one-dimensional) and the dehydrogenation of hydrocarbons at steps induces the formation of ordered carbonaceous surface structures that would not nucleate on low index substrate planes.So far, mostly work function changes upon adsorption gave indication of the magnitude of charge transfer upon adsorption and on forming of new surface chemical bonds. Most recently, chemical shifts of the Auger transitions of the substrate atoms and of the adsorbed molecules upon chemisorption, have been found to provide additional information on charge redistribution during adsorption.     

High energy electron diffraction at Si(001) surfaces

Elastic scattering of 10 keV electrons at Si(001) surfaces at grazing incidence was investigated. The intensity of the specularly reflected elastically scattered electrons as a function of the angle of incidence I_el(γ) was measured for different azimuthal angles and was compared with calculations using the dynamical diffraction theory. It turned out that the contribution of the elastically scattered electrons to the total intensity of the reflections strongly decreases with decreasing angle of incidence. Exciting the reflection (008) the elastic contribution is around 30%, decreasing to about 12% in the case of the reflection (004). In the calculations multiple beam effects, absorption, a smooth variation of the potential at the surface and a reduction of the topmost interlayer spacing were taken into account. There is satisfactory agreement between the structures of experimental and calculated intensity curves, I_el(γ) indicating a slight compression of the surface lattice to be probable. Quantitative agreement, however, for absolute intensities was not obtained.     

Low energy electron diffraction from Na(110) and Na2O(111) surfaces

Surfaces ofNa(110) are grown, investigated and oxidised to give Na₂O(111) surfaces. LEED spectra are taken for these surfaces and compared with theory to determine the surface composition of sodium oxide: the surface terminates the crystal in an integral number of electrically neutral NaONa sandwiches, with a bulk-like inter layer spacing. The effective Debye temperature for the Na(110) surface was found to be 107 K.     

Low energy electron diffraction study of the krypton (111) surface

Krypton single crystals of the (111) surface orientation have been epitaxially grown on the (100) face of iridium. LEED intensity-energy spectra have been obtained for the specular and first order non-specular beams of the (111) krypton surface. The temperature dependence of the reflected intensities has also been monitored. Analysis in the kinematic scattering formalism has yielded the krypton effective surface Debye temperature as a function of electron energy. Comparison of the measured effective Debye temperature with the results of a kinematic calculation has indicated probable applicability of the Lennard-Jones (6–12) interatomic potential to surface atoms of krypton.     

Low energy electron microscopy of surfaces

In low energy electron microscopy (LEEM) surfaces are imaged with LEED electrons. Either the (00) beam (bright field mode) or one of the other diffracted beams (dark field mode) can be used for producing a true (non scanning) image of the surface. One can also obtain the LEED pattern of the illuminated area (typically 5–10 μm diameter) on the final screen.     

Low energy electron diffraction observations on Au-enriched Ni-0.8%Au alloy(110) surfaces prepared by surface segregation

Au-enriched (110) surfaces with Au coverages θ up to 1 monolayer (ML) were prepared by heating a Ni-0.8%Au crystal at temperatures T > 400°C. Low energy electron diffraction (LEED) indicated the following domains at T ≈ 20°C: 5× 1 (prominent for θ ≈ 0.7 ML), c(2 × 2) (0.7−0.9 ML). 7 × 7 (0.8−1.0 ML). The 7 × 7 domain was shown to contain c(2 × 4) subunits. The observations are shown to be consistent with a nearly close-packed 7 × 7 overlayer structure in which each c(2 × 4) subunit is associated with a hexagonal formation of six Au atoms, with a seventh Au atom at the center. Ni(210)-1 × 1 facets and Au(321)-3 × 1 facets due to Au crystallites were present on θ ≈ 1 ML surfaces. The effect of increasing T on θ and on LEED intensities was observed for θ ≈ 1 ML surfaces. θ was found to decrease by 15% on increasing T from 300 to 800°C. A first-order 7 × 7 → 2 × 7 phase transition was observed at T = 340 ± 10°C. Specific nearly close-packed overlayer models with AuAu equilibrium interatomic distance 2.7 Å were proposed for both the 7 × 7 and 2 × 7 surfaces. The models were shown to explain the observed LEED patterns qualitatively. Other LEED observations were interpreted tentatively to indicate a transition to a disordered overlayer structure for T > 550°C, disappearance of facets for T > 600°C, and one-dimensional melting of Au overlayers for T ≈ 800°C.     

Impedance and electron diffraction studies on single crystals of BINIVOX

Single crystals of the nominal composition Bi₂V_0.9Ni_0.1O_5.35 were prepared. They were studied by TGA/DTA, impedance spectroscopy and electron diffraction. The impedance of the crystals with Pt electrodes was measured in two directions: parallel and perpendicular to the layers of the intergrowth structure in the frequency range 10⁻²−10⁷ Hz at temperatures between 300 and 940 K. Arrhenius type plots for both directions show two linear regions with different activation energies. In the low temperature region the conductivity in the direction parallel to the layers was by a factor of about 1000 larger than in the perpendicular direction. In the high temperature region the ratio of conductivities was about 100. A time dependence of the conductivity in the perpendicular direction was observed during heating and cooling in limited temperature ranges. The electron diffraction experiments gave evidence that the superlattice spots present in the diffraction patterns at low temperature, disappear reversibly at temperatures higher than 650 K. The transition temperatures observed in the DTA and in the electron diffraction experiments correspond to the temperature of changes of the activation energy. An order-disorder transition of oxygen ions is considered to be responsible for this phenomenon. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Low energy electron diffraction with position-sensitive detection: Intensity measurements and comparison with positron diffraction for NaF(001)

A novel low energy electron diffraction (LEED) system incorporating a position-sensitive detector has been used to measure beam intensities as a function of incident electron energy. The detector characteristics, and the control of the incident beam focus and current are described. The (0,0)-beam intensity for NaF(001) surface was measured over the incident energy range of 2–150 eV, and the results were compared with the corresponding results for low energy positron diffraction (LEPD) [A.P. Mills and W.S. Crane, Phys. Rev. B31 (1985) 3988] as well as earlier LEED results. The LEED plots were found to contain two types of peak not present in corresponding LEPD ones: (a) narrow peaks attributable to beam-threshold interferences, and (b) odd-order Bragg peaks. The absences of these types of peak in LEPD are attributed tentatively to (a) the large incident angular divergence in the LEPD measurement cited, and (b) the relatively weak scattering of low energy positrons by Na⁺ ions, respectively.     

Low energy electron diffraction and work function change studies of the adsorption of substituted aromatic molecules on the (111) and (100) crystal faces of platinum

The chemisorption of toluene, m-xylene, mesitylene, n-butylbenzene, t-butylbenzene, aniline, nitrobenzene and cyanobenzene were studied on the (111) and (100) crystal faces of platinum at low pressures (10^?9 to 10^?7 torr) and at temperatures of 20 to 300 °C by low energy electron diffraction and work function change measurements. After adsorption, reorientation of the molecules in the adsorbed layer is necessary to form the ordered structures. Molecules that have either higher rotational symmetry (mesitylene) or have only small size substituents on the benzene rings exhibit better ordering if the adsorption is carried out at low incident flux. The adsorbed layers are more ordered on the (111) crystal face than on the (100) crystal face of platinum. The work function changes upon adsorption rangs from ?1.4 eV for nitrobenzene to ?1.8 eV for aniline. Both the diffraction and work function change data indicate that, under the conditions of these experiments, all of the molecules chemisorb with their benzene ring parallel to the surface and interact with the metal surface primarily via the π-electrons in the benzene ring. The substituent groups play an important role in determining the ordering characteristics of the overlayers but do not markedly effect the strength of the chemical bond between the substrate and the adsorbate.     

Characteristics electron energy loss studies of molecular halogens adsorbed on Fe(100)

The electron energy loss spectra for several molecular Br₂ monolayers adsorbed on a chemisorbed bromine overlayer on Fe(1 0 0) show a sharp loss at about 3.0 to 3.8 eV. For one or more molecular layers of I₂ adsorbed on a chemisorbed iodine overlayer on Fe (1 0 0), a sharp electron loss feature is observed at 4.4 ± 0.2 eV. It is suggested that the electron energy losses for condensed Br₂ at 3.0 eV and for condensed I₂ at 4.4 eV are a result of a 1π_g to 2σ_u electron excitation.     

Low energy ion scattering and Auger electron spectroscopy studies of clean nickel surfaces and adsorbed layers

Auger electron spectroscopy (AES) and noble gas ion scattering (IS) are applied to surface analysis in an in situ comparison of both techniques. Examples are the sorption of S and O on Ni (111). It is shown that IS allows quantitative measurements if a suitable calibration is possible. From the energy dependence of the backscattered intensity and from the surface coverage calibrated by AES an estimate of the energy dependence of the surface neutralization process becomes possible. For S and O a shadowing effect is observed, which enhances the sensitivity of IS in this case. From this shadowing effect it is concluded that the position of the adatoms is above the surface.     

Auger electron spectroscopy and electron energy loss spectroscopy studies on carbonization of Si(100) and (111) surfaces with ethylene

The reactions of Si(100) and Si(111) surfaces at 700 °C (973 K) with ethylene (C₂H₄) at a pressure of 1.3×10⁻⁴ Pa for various periods of time were studied by using Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS). For a C₂H₄ exposure level, the amount of C on the (111) surface was larger than that on the (100) surface. The formation of β-SiC grain was deduced by comparing the C_KLL spectra from the sample subjected to various C₂H₄ exposure levels, and from β-SiC crystal.     

Spin polarization effect in the theory of magnetic scattering from antiferromagnetic NiO (111) surfaces by polarized low energy electron diffraction

Dynamical calculations are performed to determine the differential cross section of low energy electrons scattered from antiferromagnetic NiO (111) surfaces. We find that the spin-dependence of this quantity with respect to the incident electron polarization depends strongly on: (1) the magnetization of the topmost layer, (2) the exchange potential model used, and (3) the incident beam angle.     

Low energy electron diffraction beam profiles and phase transition at Si(111)-7 X 7 surface

Angular profiles of low energy electron diffraction (LEED) beams from Si(111)-7 × 7 are measured for various crystal temperatures T near the phase transition with apparent critical temperature T_c ≈ 1140 K. From analyses of the profiles it is concluded that (1) long range superstructure order persists for T up to at least 50 K above T_c and (2) with increasing T the correlation length characterizing the short-range order peaks for T ≈ T_c ? 100 K and decreases rapidly for T >T_c. Conclusion (1) is discussed with reference to a dislocation network model of Si(111)-7 × 7 reconstruction.     

Very low energy electron reflection at Cu(001) surfaces

Measurements of the coefficient of elastic reflection of very low energy electrons at Cu(001), Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N surfaces are reported. The measurements refer to normally-incident electrons with kinetic energies E in the range 0.5–22 eV. The elastic reflection coefficient R_el was determined from separate observations of the total reflection coefficient and of the energy distribution of reflected electrons. For Cu(001) R_el is 0.55 at E = 0.5 eV and drops monotonically to 0.03 with increasing E, the maximum slope being at E = 3 eV. Theoretical calculations of R_el are reported. The reflection amplitude of the substrate crystal was parameterized using existing results of accurate band structure calculations, and the surface scattering matrix was evaluated for assumed surface scattering potentials. It is shown that to fit the observed R_el it is necessary to take account of both the image potential and the extension of the imaginary part of the crystal scattering potential into vacuum. From the fit, the range of the imaginary potential is 1.0 Å. For Cu(001) (2 × 4)45°O and Cu(001)c(2 × 2)N the values of R_el at E = 0.5 eV were 0.35 and 0.15, respectively. The effect of adsorption in reducing R_el is especially marked for E < 2 eV. Adsorption of either O or N results in an additional peak in R_el near E = 12 eV.     

Structure determination of the (100) surface of rhodium by low energy electron diffraction

Elastic low-energy electron diffraction intensity data have been measured as a function of energy for two directions of incidence for the (100) surface of rhodium. The dynamical perturbation programs of Van Hove and Tong have been used for analysing these new experimental data, and it is concluded that the normal face-centered cubic registry is maintained to the surface layer. A preliminary comparison between measured and calculated I(E) curves indicates the topmost interlayer spacing to be 1.96 ± 0.10 Å, and therefore possibly slightly expanded from the bulk interlayer spacing of 1.90 Å.