Low-energy Electron Energy Loss Spectroscopy Research Articles

Low energy inelastic electron scattering from carbon monoxide: II. Excitation of the b3Σ+, j3Σ+, B1Σ+, C1Σ+ and E1Π Rydberg electronic states

In this second part of a two part paper (first part: Zawadzki et al (2020 J. Phys. B: At. Mol. Opt. Phys. 53 165201)) we present differential scattering cross sections for excitation of several Rydberg electronic states of carbon monoxide by electron impact. The first part concerned the low-lying valence states of CO. In the present study cross sections are obtained experimentally using low-energy electron energy-loss spectroscopy and theoretically using the R-matrix method. Incident electron energies range from near-threshold of 12.5 eV to 20 eV while the scattering angles range from 20° to 120°. The R-matrix calculations use three distinct close-coupling models and their results are compared to available experimental and theoretical cross sections. The overall comparison leads to significantly improved description of the excitation cross sections for this target.

Low energy inelastic electron scattering from carbon monoxide: I. Excitation of the a3Π, a′3Σ+ and A1Π electronic states

Differential scattering cross sections for electron excitation of the three lowest excited electron states of carbon monoxide are obtained experimentally using low-energy electron energy-loss spectroscopy and theoretically using the R-matrix method. The incident electron energies range from near-threshold of 6.3 eV to 20 eV. Experimental scattering angles range from 20° to 120°. The normalization of the experimental cross sections is made to available experimental elastic scattering data (Gibson et al 1996 J. Phys. B: At. Mol. Opt. Phys. 29 3197). The R-matrix calculations use three distinct close-coupling models and their results are compared to available experimental and theoretical cross sections. The overall comparison leads to significantly improved description of the excitation cross sections for this target.

Model catalysis studies of the oxidation of propane over VOx-based catalysts

VOx-based catalyst has been extensively used in catalysts for oxidation reactions, especially for the partial oxidation of alkanes. Our previous study showed that the VOx/Pt(111) surfaces are highly active for the complete oxidation of propane. In this article, SmOx was used as an additive for the VOx/Pt(111) model surfaces to improve the catalytic performance for the partial oxidation of C3H8. And Mo foil was used to replace Pt to examine the substrate effect on the VOx/SmOx. Ordered SmOx/Pt(111), VOx/SmOx/Pt(111), VOx/SmOx/Mo and VOx/Pt(111) were prepared under ultra-high-vacuum conditions and characterized using Auger electron spectroscopy, low-energy electron diffraction and high-resolution electron energy loss spectroscopy. The results show that the main product is CO2 with a small amount of CO on the Pt substrate. The SmOx layer on the Pt(111) surface or between VOx and Pt(111) suppresses the activities of both Pt and VOx/Pt, but enhances the selectivity to CO. Replacing Pt(111) with a Mo foil, VOx on SmOx/Mo shows a much lower activity, while achieves some selective product of propene.

Reversible hydrogenation of deuterium-intercalated quasi-free-standing graphene on SiC(0001)

Hydrogenation of deuterium-intercalated quasi-free-standing monolayer graphene on SiC(0001) is obtained and studied with low-energy electron diffraction and high-resolution electron energy loss spectroscopy. While the carbon honeycomb structure remains intact, it is shown that a significant band gap opens in the hydrogenated material. Vibrational spectroscopy evidences for hydrogen chemisorption on the quasi-free-standing graphene is provided and its thermal stability is studied.

Investigation on the orderly growth of thick zinc phthalocyanine films on Ag(100) surface

The growth of zinc phthalocyanine (ZnPc) on Ag(100) surface from monolayer to multilayer was investigated by low-energy electron diffraction, x-ray diffraction, and high-resolution electron energy loss spectroscopy (HREELS). At monolayer coverage, ZnPc molecules form an ordered film with molecular planes parallel to the substrate. The same structure is maintained as the film thickness increases. HREELS analysis shows that intermolecular π-π interaction dominates during the film growth from monolayer to multilayer. The π-d interaction between the adsorbates and the substrate is only applicable in the first adlayer. Stronger intermolecular-layer interaction is observed at higher coverages.

Plasmon damping below the Landau regime: the role of defects in epitaxial graphene

The sheet plasmon in epitaxially grown graphene layers on SiC(0001) and the influence of surface roughness have been investigated in detail by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). We show that the existence of steps or grain boundaries in this epitaxial system is a source of strong damping, while the dispersion is rather insensitive to defects. To the first order, the lifetime of the plasmons was found to be proportional to the average terrace length and to the plasmon wavelength. A possible reason for this surprisingly efficient plasmon damping may be the close coincidence of phase (and group) velocities of the plasmons (almost linear dispersion) with the Fermi velocity of the electrons. Therefore, uncorrelated defects like steps only have to act as a momentum source to effectively couple plasmons to the electron–hole continuum.

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.

Reply to “Comment on ‘Resonant inelastic x-ray scattering of MnO:L2,3edge measurements and assessment of their interpretation’ ”

In their Comment to our recent paper [Phys. Rev. B 73, 035111 (2006)], M\"uller and H\"ufner proposed a comparison of our $\text{Mn}\text{ }{L}_{2,3}$ resonant inelastic x-ray scattering (RIXS) data with low-energy electron energy-loss spectroscopy (EELS) measurements. The differences in the experimental spectra highlighted by the authors can be easily ascribed to the different cross sections in the two techniques, even when they probe the same subset of excited states, namely, the $dd$ excitations. One important difference is that EELS is limited to quartet final states only, whereas RIXS can lead to doublets, too. Thus our theoretical interpretation of the RIXS spectra, made within two different models, is compatible with the traditional crystal-field analysis of EELS. As the Sugano-Tanabe diagram usually do not include the $3d$ spin-orbit and the interatomic superexchange interactions, their accuracy is limited to hundreds of meV and they should be utilized only for rough assignments of the spectral features. On the contrary, by calculating the RIXS spectral shape we could address more sophisticated models with remarkable agreement to the experimental results. This is hardly possible in the case of low-energy EELS, for which reliable simulations of the spectra are much more difficult.

Adsorption of Metal-Free Phthalocyanine on InSb and InAs(001)-4 × 2/c(8 × 2) Surface

We have studied the adsorption of metal-free phthalocyanine on 4 x 2/c(8 x 2) surface reconstruction of both InSb(001) and InAs(001) surfaces, by means of low-energy electron diffraction and high-resolution electron energy loss spectroscopy. We show that the molecule-surface interaction is stronger in the case of the InAs substrate as compared to the InSb one. Indeed, we observed that at full monolayer coverage, when the molecules are adsorbed on the InAs, and only in that case, there are substantial energy shifts of both the substrate surface plasmon and the first valence exciton of the molecules. These shifts are evidence of charge transfer from the molecules toward the substrate responsible for a strong molecule-substrate interaction such as chemisorption. In spite of the difference between the two substrates, at monolayer completion, H 2 Pc on InAs and InSb adopts a common (4n x 3) surface arrangement. The same that was already reported for the nonplanar lead and tin phthalocyanines (respectively, PbPc and SnPc) adsorbed on InSb(001).

Composition and structure of chemically prepared GaAs(1 1 1)A and (1 1 1)B surfaces

The (1 1 1)A and (1 1 1)B surfaces of GaAs chemically treated in HCl-isopropanol solution (HCl-iPA) and annealed in vacuum were studied by means of X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). To avoid uncontrolled contamination, chemical treatment and sample transfer into UHV were performed under pure nitrogen atmosphere. The HCl-iPA treatment removes gallium and arsenic oxides, with about 0.5–3 ML of elemental arsenic being left on the surface, depending on the crystallographic orientation. With the increase of the annealing temperature, a sequence of reconstructions were identified by LEED: (1 × 1) and (2 × 2) on the (1 1 1)A surface and (1 × 1), (2 × 2), (1 × 1), (3 × 3), (√19 × √19) on the (1 1 1)B surface. These sequences of reconstructions correspond to the decrease of surface As concentration. The structural properties of chemically prepared GaAs(1 1 1) surfaces were found to be similar to those obtained by decapping of As-capped epitaxial layers.

Self-assembled monolayer of tin-phthalocyanine on InSb(0 0 1)-(4 × 2)/c(8 × 2)

We have herein studied the adsorption of tin-phthalocyanine on the indium rich 4 × 2/c(8 × 2) reconstruction of the InSb(0 0 1) surface, using scanning tunneling microscopy, low-energy electron diffraction and high-resolution electron energy loss spectroscopy. The tin-phthalocyanine molecules form a stable monolayer with a characteristic long-range one-dimensional order, imposed by the twofold symmetry of the substrate surface reconstruction. High-resolution electron energy loss studies of both the vibrational and the electronic eigenmodes of the molecules show that phthalocyanines lie flat on the surface. In that, they behave the same way as lead phthalocyanines, which leads us to suggest that a common adsorption configuration may be adopted by non-planar phthalocyanines on the InSb(0 0 1)-4 × 2/c(8 × 2) surface.

Low-energy electron energy loss spectroscopy of rutile and anatase TiO2 films in the core electron excitation regions

Abstract Low-energy electron energy loss spectra in the core electron excitation regions were successfully measured for TiO2 thin-films with rutile or anatase structure. Noticeable differences in the excitation spectra of both Ti 2p and O 1s core electrons were observed between rutile and anatase TiO2 films, which may be due to the difference in the local symmetry of conduction band states between these polymorphs. The present technique was applied to characterize the crystal structure of the TiO2/SnO2 stacked-layers in the near-surface region, which is important for the application of these layers to a novel multifunctional glazing with both photocatalytic and heat-insulating properties.

Structure of thinSiO2films grown on Mo(112)

Ultra-thin ${\mathrm{SiO}}_{2}$ films were prepared by evaporating Si onto a Mo(112) surface followed by oxidation and annealing up to 1200 K. The surface structure and film quality were investigated by low-energy electron diffraction (LEED), Auger spectroscopy (AES), and high-resolution electron energy loss spectroscopy (HREELS). A well-ordered, monolayer $\mathrm{Mo}(112)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}{\mathrm{SiO}}_{2}$ structure was characterized by HREELS and shown to exhibit unique phonon features compared to bulk ${\mathrm{SiO}}_{2}.$ The phonon features are assigned to Si-O-Mo rather than Si-O-Si species, and the surface structure determined to be $\mathrm{Mo}(112)\ensuremath{-}c(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{-}[{\mathrm{SiO}}_{4}]$ where each of the four oxygen atoms bonds to the substrate Mo atoms.

Surface structures of ultrathin vanadium oxide films on Pd( [formula omitted

The growth and the atomic structure of epitaxial vanadium oxide thin films on Pd(1 1 1) have been investigated by scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS), combined with ab initio density-functional theory (DFT) calculations. At submonolayer coverage a well ordered (4×4) oxide overlayer forms, which transforms into a porous oxide network with an internal (2×2) periodicity upon exposure to H 2 at room temperature. The (2×2) phase represents an interface-stabilised surface–V 2O 3 layer, which becomes compact upon mild annealing in vacuum exhibiting a (2×2) honeycomb structure. Between 0.5 and 1 monolayer equivalents (MLE) the growth of oxide islands with a zigzag stripe structure is observed along with the (2×2) layer. At 1 MLE two distinct VO 2-like phases are coexistent at the surface in the form of islands with rectangular and hexagonal structures, interestingly they are clearly different from the known bulk-type rutile VO 2 lattice. The detailed atomic structure and energetic stability of these monolayer V-oxide phases have been revealed by the DFT calculations. Above 2 MLE three-dimensional crystallites grow epitaxially on Pd(1 1 1) with the corundum structure, which is typical of the bulk-type V 2O 3. Two stable V 2O 3(0 0 0 1) terminations have been found in the STM images, which are due to bulk-type oxygen planes and terminal vanadyl species, as suggested by the DFT calculations and confirmed by high-resolution electron energy loss phonon spectra.

The growth of ultrathin Al2O3 films on Cu(1 1 1)

The growth of ultrathin films of Al2O3 on Cu(111) in the temperature range 300–1200K was investigated by using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). Eight monolayers of a mixture of nickel and aluminum (Ni:Al=1:2) were deposited on Cu(111) at 300K by simultaneous evaporation of both Ni and Al from NiAl crystal material. The bimetal layer was oxidized at 300K until saturation and annealed gradually to 1200K. During oxygen adsorption, only aluminum is oxidized. Annealing of the oxidized layer to 1200K leads to the formation of a well-ordered aluminum oxide. The HREEL spectra show the characteristic Fuchs–Kliever phonons of Al2O3 (410, 620 and 885cm−1). During annealing, Ni diffuses into the Cu(111) substrate. The LEED pattern of the ultrathin oxide layer has a hexagonal structure with a lattice constant of 3.1Å, which corresponds to the distance between two oxygen ions in the aluminum oxide.

Phase Transition of Silicon-Nitride Monolayer on Si(111) Surface Observed by Scanning Tunneling Microscopy

Phase transition of a silicon-nitride (SiN) monolayer on a Si(111) surface was observed by ultrahigh vacuum scanning tunneling microscopy (UHV-STM). The STM image showed an 8/3×8/3 regular array of dots on the Si(111)-8×8 surface. After annealing at 1030°C, the image changed to a 3/4×3/4 regular array of dots, which corresponds to the quadruplet surface. The rotation angle of one of the four quadruplet domains was directly measured and found to be about 10° with respect to the crystallographic structure of the Si(111) surface. Further annealing at 1050°C changed the 3/4×3/4 structure to the 7×7 structure, which means that the SiN monolayer decomposed and was removed from the surface completely. Overall annealing behavior observed in our STM experiments was consistent with low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS) data reported previously.

Electron-energy-loss spectroscopy of K xC 60 and K-halides: comparison in the K 3p excitation region

We have investigated the electronic structure of K x C 60 ( x=0–6) using low-energy electron energy loss spectroscopy (LEELS), especially focusing on the K 3p core-electron excitation spectra. It is found that the structure of the K 3p-excitation spectrum of K x C 60 quite differs from that of KCl. Furthermore, the K 3p-excitation LEELS of K 3C 60 has been revealed to be different from that of K 6C 60. K 3p electrons are excited into K 4s- and K 3d-derived empty states in both K x C 60 and KCl, but in the case of K x C 60 the K 3d-derived empty states have a rather complicated structure where several levels are not well separated. Consequently, K 3p-excitation LEEL spectra of K x C 60 show wide-spread plateau-like structures. The difference in the K 3p-excitation spectra of K 3C 60 and K 6C 60 is considered to originate from the different crystal field around K + cations in the C 60 molecular crystal.

Surface color centers on epitaxial NaCl films

The defect formation in epitaxial NaCl films (up to 5.6 nm thick) during low-energy (50--1500 eV) electron bombardment has been studied by a combination of low-energy electron diffraction and energy loss spectroscopy. At temperatures below 200 K two types of color center are generated at the surface, showing distinct electronic transition energies and different kinetics of formation. A rate equation analysis provides evidence for a random distribution of stable, immobile $F$ and $M$ centers on the surface. Their saturation density increases with the film thickness, indicating an indirect generation mechanism involving the whole film, presumably via self-trapping of excitons. The angular distribution of the inelastic electron scattering of excitons reveals their surface polariton character, providing the energy transport from film bulk to surface and within the surface. At room temperature the aggregation of Na clusters is observed. Patterns of high and low surface $F$- and $M$-center densities are potentially invaluable as templates for nanostructure devices on ionic crystal surfaces like NaCl(001), because of the chemical reactivity of surface color centers.

Hydrogen adsorption on Mo 1− xRe x(110) ( x=0–0.25) surfaces

The effects of adsorbed H on the Mo 1− x Re x (110), x=0, 0.05, 0.15, and 0.25, surfaces have been investigated using low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS). For the x=0.15 alloy only, a c(2×2) LEED pattern is observed at a coverage Θ∼0.25 ML. A (2×2) pattern is observed for H coverages around Θ∼0.5 ML from surfaces with x=0, 0.05, and 0.15. Both c(2×2) and (2×2) patterns are attributed to reconstruction of the substrate. At higher coverages, a (1×1) pattern is observed. For the alloy surface with x=0.25, only a (1×1) pattern is obtained for all H coverages. Two H vibrations are observed in HREELS spectra for all Re concentrations, which shift to higher energies at intermediate coverages. Both peaks exhibit an isotopic shift, confirming their assignment to hydrogen. For Re concentrations of x=0.15 and higher, a third HREELS peak appears at 50 meV as H (D) coverage approaches saturation. This peak does not shift in energy with isotopic substitution, yet cannot be explained by contamination. The intrinsic width of the loss peaks depends on the Re concentration in the surface region and becomes broader with increasing x. This broadening can be attributed to surface inhomogeneity, but may also reflect increased delocalization of the adsorbed hydrogen atom.

Adsorption of Ammonia and Its Influence on Coadsorbed Carbon Monoxide on Monolayer and Multilayer Palladium Epitaxially Grown on Mo(110)

The adsorption of ammonia on a monolayer of Pd on Mo(110), Pd1ML/Mo(110), and multilayer Pd(111)/Mo(110) surfaces has been studied using low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). A diffuse (2 × 2) LEED pattern has been observed for ammonia on the Pd1ML/Mo(110) surface. TPD measurements indicate a shift of the desorption peak maximum of ammonia to a lower temperature for monolayer palladium compared to multilayer palladium. However, both surfaces display virtually identical HREELS spectra with the symmetric deformation (umbrella mode) being the dominating feature, indicating an upright adsorption geometry on both surfaces. The coadsorption of NH3 and CO has been also studied with HREELS over a wide coverage range on both surfaces. A strong interaction between adsorbed NH3 and CO is indicated by the red shift of the CO stretching vibrational mode (as large as 400 cm-1) and a blue shift of the NH3 symmetric deformation (up to 100 cm-1). On Pd1ML/Mo(110), the coadsorption of NH3 induces a CO adsorption site change from the atop site to the three-hollow site. Evidence has been also found for a significantly inclined NH3 molecule in the coadsorbed layer with a relatively high CO coverage.