Low-energy Electron Diffraction Study Research Articles

Self-assembly of aluminium nano-crystals on the decagonal surface of Al–Co–Ni quasicrystal – a low-energy electron diffraction study

Self-assembly of aluminium nano-crystals on the decagonal surface of Al–Co–Ni quasicrystal – a low-energy electron diffraction study

An STM and LEED study of MOCVD-prepared P/Ge (1 0 0) to (1 1 1) surfaces

Certain device applications such as high-efficiency multijunction III–V solar cells require high-quality GaAs deposition on Ge substrates. Commercially, this is done using metal-organic chemical vapor deposition (MOCVD). If the GaAs is grown directly on Ge, pre-deposition etching of the Ge substrate by arsine can roughen the surface, producing poor GaAs/Ge epitaxy. If instead a GaAs/GaInP 2/Ge growth sequence is used, the template for growth is a phosphine-exposed P/Ge surface. Indeed, high-quality GaInP 2 and GaAs films are routinely grown using this method, yet little is known about the initial P/Ge surface. For this reason, we have conducted a survey of MOCVD-prepared P/Ge( mnn) surfaces, over the 54.75° range of surfaces between (1 1 1) and (1 0 0). Low-energy electron diffraction (LEED) and atomic-resolution scanning tunneling microscopy (STM) images reveal interesting new reconstructions on very flat surfaces. Almost all of the surfaces support ordered defect arrays, presumably to relieve P-induced surface stress.

A LEED and STM study of H(D) adsorption on C(0 0 0 1) surfaces

Adsorption of H (D) atoms on natural graphite (NG) single crystal and highly oriented pyrolytic graphite (HOPG) surfaces was studied with low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). No superstructure was observed in LEED, which suggests random adsorption of H on C(0001). Loss of diffraction at increasing H(D) coverage indicates loss of translational symmetry on the surface. STM data suggest that on D covered surfaces randomly distributed C atoms exist which pucker out of the basal plane. Puckered atoms are those which contribute to the STM profile of clean HOPG, i.e., which have no neighbour in the plane below.

X-ray photoelectron spectroscopy and low-energy electron diffraction study on the oxidation of NiAl(110) surfaces at elevated temperatures

We studied the growth of ultrathin alumina films on NiAl(110) substrates at elevated temperatures using LEED and XPS. We obtained a well-ordered alumina layer without additional annealing when we introduced oxygen at a pressure of 6.6×10 −6 Pa and a substrate temperature of 1020 K. The alumina layer thicknesses were calculated using a uniform overlayer model, and 10.9 Å of the alumina layer was obtained when the oxygen dose increased to 4000 L. The binding energies of Al 2 p and O 1 s for Al 2O 3 shifted to higher binding energy synchronously when the alumina layer increased in thickness during 1020 K oxidation.

Low-energy electron diffraction study of Xe adsorption on the ten-fold decagonal Al-Ni-Co quasicrystal surface

The adsorption of Xe onto the tenfold surface of decagonal Al-Ni-Co was studied using low-energy electron diffraction (LEED). LEED isobar measurements indicate that Xe grows in a layer-by-layer mode for at least the first two layers in the temperature range 60\char21{}80 K. The half-monolayer isosteric heat of adsorption was measured to be $250\ifmmode\pm\else\textpm\fi{}10\mathrm{meV}.$ No superlattice was observed for the first layer of Xe, which is therefore presumed either to have a quasicrystalline structure or to be disordered. Upon adsorption of the second layer, an ordered Xe bilayer forms, which has a structure consistent with domains of bilayer Xe(111) aligned along substrate symmetry directions. At higher Xe coverages (several Xe layers), the LEED pattern becomes more distinct and remains consistent with that from a Xe(111) surface.

Perylene tetracarboxylic diimide ultrathin film deposition on Pt(1 0 0): a LEED, AES, REELS and STM study

We have investigated the first stages of organic molecular-beam deposition at room temperature of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) ultrathin films on Pt(1 0 0) by means of various techniques: Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection electron energy loss spectroscopy (REELS) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). We show mainly that PTCDI grows on Pt(1 0 0) in the Stranski–Krastanov mode (as it is the case for perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) on Cu(1 1 0)). After the formation of a disordered first monolayer (ML), the formation of growing 3D crystalline islands of PTCDI was observed. Contrary to PTCDA deposited on Cu(1 1 0), annealing of the sample after deposition of the first monolayer did not lead to any molecular ordering. This could be due to a strong perpendicular interaction between platinum and adsorbed PTCDI molecules compared to molecular lateral ones. The EELS surface gap continuously opens to reach a value of 1.8 ± 0.2 eV for a deposit with an equivalent thickness of 20 ML, to be compared with 2.2 eV the expected highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of PTCDI.

AES and LEED study of well‐ordered oxide film grown on Cu–9at.%Al(111)

AbstractAn alloy of Cu–9at.%Al(111) has been oxidized in a low‐energy electron diffraction (LEED)/AES and a scanning AES instrument at elevated temperatures. Dosing with 1300 L of oxygen at 995 K gives rise to well‐ordered oxide layer formation on the Cu–9at.%Al alloy. The structure of the ordered oxide confirmed by LEED is ($7/\sqrt{3} \times 7/\sqrt{3}$) R30°. The chemical state of the oxide was Al2O3. The morphology of the surface observed with SEM in the scanning AES instrument revealed flat oxide growth with triangular defects of the same orientation. The possible epitaxy between the alloy substrate and alumina layer has been discussed. Copyright © 2003 John Wiley & Sons, Ltd.

Pronounced Odd−Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Based Thiol SAMs: A Combined STM and LEED Study

Self-assembled monolayers (SAMs) of ω-(4‘-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Preparation at elevated temperature results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other aromatic thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell containing 8 molecules is found which corresponds to a packing density reduced by 25% compared to that of BP3. The odd−even effect observed in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.

Structure determination of ordered and disordered organic molecules on a surface from the substrate diffraction spots in low energy electron diffraction: ( [formula omitted])R30°-C 2H 2 and disordered CH 3OH on Pd(1 1 1)

We examine the effects of chemisorbed organic molecules on the intensity versus beam energy ( I/ E) variation of the substrate (1 × 1) Bragg spots in low energy electron diffraction (LEED). We find that these integer-order spots alone can provide important information about the local geometries of the molecules on the surface, largely independent of the degree of long-range order of an adsorbate overlayer. This is demonstrated by comparing LEED studies of two different adsorbates on Pd(1 1 1), namely: C 2H 2 which forms an ordered ( 3 × 3 )R30° overlayer, and thus gives rise to superstructure (or fractional-order) Bragg spots, and disordered CH 3OH, which scatters diffusely into directions between the integer-order spots. This allows LEED to be used conveniently for the determination of the structures of adsorbates on surfaces as a continuous function of coverage, independent of the degree of long-range order of the adlayer.

CCl 4 chemistry on the reduced selvedge of a α-Fe 2O 3(0 0 0 1) surface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) were used to study the degradation of CCl 4 on the reduced selvedge of a natural single crystal α-Fe 2O 3(0 0 0 1) surface in ultrahigh vacuum. Before exposure to CCl 4, STM images indicate that approximately 85% of the reduced surface exhibits a Fe 3O 4(1 1 1) 2 × 2 termination, while the remaining 15% is terminated by 1 × 1 and superstructure phases. Images obtained after room temperature dosing with CCl 4 and subsequent flashing to 600 K reveal that chlorine atoms are adsorbed only on surface regions with the Fe 3O 4(1 1 1) 2 × 2 termination, not on 1 × 1 and superstructure regions. Chlorine atoms from dissociative adsorption of CCl 4 are observed to occupy two distinct positions located atop lattice protrusions and in threefold oxygen vacancy sites. However, in companion chemical labeling experiments, chlorine atoms provided by room temperature, dissociative Cl 2 adsorption on this surface are found to occupy sites atop lattice protrusions exclusively. The clear dissimilarity in STM feature shape and brightness at the two distinct chlorine adsorption sites arising from CCl 4 dissociation as well as the results of the Cl 2 chemical labeling experiments are best explained via reactions on a Fe 3O 4(1 1 1) 2 × 2 selvedge terminated by a 1/4 monolayer of tetrahedrally coordinated iron atoms. On this surface, adsorption atop an iron atom occurs for both the CCl 4 and Cl 2 dissociative reactions. A second adsorption site, assigned as binding to second layer iron atoms left exposed following surface oxygen atom abstraction resulting in the formation of phosgene (COCl 2), only appears in the case of reaction with CCl 4. The reaction mechanism and active site requirements for CCl 4 degradation on iron oxide surfaces are discussed in light of this evidence and in the context of our previously reported results from Auger electron spectroscopy (AES), LEED, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy studies.

Sb induced (7×7) to (1×1) surface phase transformation of the Si(111) surface

Adsorption of Sb at a very low flux rate results in an epitaxial layer-by-layer growth on Si(111) surface held at room-temperature. Band-bending is not observed for submonolayer Sb coverages while sharp changes in the photoemission features are observed for 1.0 monolayer (ML) Sb adsorption. Changes in the core level binding energy and width in X-ray photoelectron spectroscopy, surface related feature in Electron energy loss spectroscopy and spot intensity ratios in Low energy electron diffraction studies suggest a surface phase transition upon adsorption of 1.0 monolayer of Sb. A plausible model is proposed to explain the abrupt metal–semiconductor transformation at this critical coverage of 1.0 ML.

Morphology of GaN(0001) and GaN(0001̄) surfaces: Persistence of surface clusters

We report a scanning tunneling microscopy and low energy electron diffraction study of GaN(0001) and GaN(0001̄) surfaces. Surface preparation methods included anneal, N2+ sputter/anneal, and chemical etching with Br2. The onset of surface facet formation by thermal anneal is 1050 K, determined by electron diffraction, whereas the onset of facet formation by Br2 etching at elevated sample temperatures is 765 K. For all surfaces studied, facetted and unfacetted, nanosize clusters are observed in scanning tunneling microscopy images regardless of treatment. The presence and persistence of these clusters is discussed, as well as their potential impact on studies of GaN surface chemistry and surface electronic structure.

IRRAS and LEED studies of films of the long chain n-alkane n-C44H90 on Cu(1 0 0) and Cu(1 1 0)

The film structures of the long chain n-alkane n-C44H90 on Cu(100) and Cu(110) were investigated by infrared reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). The first layer on Cu(100) consists of the molecules oriented with the flat-on geometry where the carbon skeleton plane is parallel to the substrate. The molecular axes are arranged along [011] and [0-11] directions. On the other hand, the molecules in the first layer on Cu(110), which also adopt the flat-on geometry, are arranged with their molecular axes along only [1-10] direction, reflecting the in-plane anisotropy of the substrate surface. Noticeable difference in molecular orientation in the multilayers was not observed between the two substrates. The substrates affect only the first layers, and the surface symmetry restricts the molecular arrangement.

Electron-spectroscopy and -diffraction study of the conductivity of CVD diamond ( [formula omitted])2×1 surface

A chemical vapor deposition as-grown diamond (0 0 1) single-domain 2 × 1 surface was studied by electron-spectroscopy and electron-diffraction in ultrahigh vacuum (UHV). In order to change the surface conductivity (SC) of the diamond in UHV, three annealing stages were used; without annealing, annealing at 300 °C and annealing at 550 °C. From low energy electron diffraction and X-ray photoelectron spectroscopic (XPS) studies, an existence of SC was suggested for the first two stages of annealing and an absence of SC was suggested for the last stage of annealing. Changes in C KVV Auger electron spectroscopic spectra, C KVV Auger electron diffraction (AED) patterns and C 1s XPS peak positions were noticed between the annealing stages at 300 and 550 °C. These changes are interpreted as such that the state of hydrogen involvement in a subsurface of diamond (0 0 1)2 × 1 changes as SC changes. In particular, the presence of local disorder in diamond configuration in SC subsurface is pointed out from C KVV AED. From C 1s XPS peak shifts, a lower bound for the Fermi-level for SC layers from the valence band top is presented to be ∼0.5 eV.

High-resolution Si 2p core-level and low-energy electron diffraction studies of the Ca/Si(1 1 1)-(3 × 2) surface

We have investigated the surface structure of the Ca/Si(1 1 1)-(3 × 2) surface using low-energy electron diffraction (LEED) and high-resolution core-level photoelectron spectroscopy. Weak ×2 streaks were observed in LEED at 300 K. After cooling the sample to 100 K, ×2 spots, which originate from both (3 × 2) and c(6 × 2) periodicities, appeared. By considering the energy shift and intensity of each surface component observed in the Si 2p core-level spectra, we conclude that the structure of the (3 × 2) surface is basically the same as that of the honeycomb-chain-channel model with a Ca coverage of 1/6 ML. Further, we propose that the weak ×2 streaks at 300 K result from thermally induced disorder.

The surface of Sr 2Ru 0.9Mo 0.1O 4: a LEED and STM study

In the unconventional superconductor Sr 2RuO 4, substitution of a small amount of Mo for Ru destroys the superconducting state. We have used low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to study the cleaved (1 0 0) surface of the Mo-doped strontium ruthenate: Sr 2Ru 0.9Mo 0.1O 4. Excellent LEED patterns indicate a well-ordered surface similar to Sr 2RuO 4. The analysis of LEED- I( V) spectra revealed that the (Ru/Mo)O 6 octahedra at the surface are rotated alternating clockwise and counterclockwise by 8.8° ± 2.5° about the direction of the surface normal, which is identical to these on the surface of Sr 2RuO 4. But the high-resolution STM images of Sr 2Ru 0.9Mo 0.1O 4 show a surface fundamentally different from that of Sr 2RuO 4. The STM images are surprisingly inhomogeneous without clear atomic resolution. This indicates a local spatial roughness in the electronic structure induced by the Mo doping.

Studies of the facetting of the polished (100) face of CaF 2

The present paper deals with studies of the facetting of the polished (1 0 0) surface of CaF 2 during annealing and growth in UHV using low energy electron diffraction (LEED), atomic force microscopy (AFM), and transmission electron microscopy (TEM). First morphological modifications of the polished surfaces become visible at temperatures of T=874 K . Surfaces annealed at T=974 K exhibit a micro-roughening with pyramidal protrusions and corresponding depressions. LEED studies indicate the evolution of {1 1 1} facets. Reflexes from the (1 0 0) surface are not seen. After growth of about 660 monolayers of CaF 2 at T=1093 K and a saturation ratio S=33 from the vapor phase, larger pyramid-like or hip roof-like crystallites are developed. The results of AFM height profiles as well as of the LEED investigations indicate again the formation of {1 1 1} facets as proved by their angles of 54.7 o with the base (1 0 0) surface. This shows that the crystallites are homoepitaxially grown on the underlying CaF 2 substrate.

Structural determination of two-dimensionalYSi2epitaxially grown on Si(111)

We have used a combination of dynamical low-energy-electron diffraction and density functional formalism calculations to find a structural model for two-dimensional (2D) ${\mathrm{YSi}}_{2}$ layers epitaxially grown on Si(111). Both techniques show that the geometric structure of the yttrium silicide is quite similar to other 2D rare-earth silicides. The surface termination consists of a relaxed Si-bilayer and underlying Y atoms on ${T}_{4}$ sites [with respect to the Si(111) interface]. The low-energy electron diffraction study shows several occurrences of minima in the R factor. The analysis of diffracted beams measured at non-normal incidence allows us to discriminate the spurious minima.

Dynamical LEED study ofPd(111)−(3×3)R30°−Xe

A low-energy electron diffraction (LEED) study of $\mathrm{Pd}(111)\ensuremath{-}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}\mathrm{Xe}$ at 77 K indicates that the Xe adsorption site is on top of the Pd atoms. The Xe--Pd bond length is $3.07\AA{}\ifmmode\pm\else\textpm\fi{}0.06\AA{}.$ The substrate structure is essentially unrelaxed from the bulk structure. These results contrast with an earlier spin-polarized LEED study, which indicated that hollow sites are occupied in this structure. The low-coordination-site geometry for Xe on Pd(111) is discussed in the context of earlier experimental studies and recent density functional theory results for Xe adsorption on metal surfaces.

Morphological and electronic properties of ultrathin crystalline silica epilayers on a Mo(112) substrate

Ultrathin crystalline silica layers grown on a Mo(112) substrate have been shown to be a useful silica model oxide support in surface science model catalyst studies. As the oxide support material plays an important role in the catalytic process, a multitechnique surface science study is presented to characterize the morphological and electronic properties of the heteroepitaxial system ${\mathrm{SiO}}_{2}/\mathrm{Mo}(112).$ The long-range order of the silica epilayer which grows commensurate with a $c(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit mesh on the Mo(112) substrate is studied by low-energy electron diffraction (LEED). The defect structure of the silica epilayer is characterized in a spot profile analysis (SPA)-LEED study. Antiphase domain boundaries split the silica epilayer into an array of silica crystal grains whose average size and shape is determined. Aiming to prepare flat silica surfaces, the change in the surface roughness with progress in the film preparation is monitored in a combined SPA-LEED and scanning tunneling microscopy (STM) study and seen to influence also the Si-O stretching frequency in the infrared-reflection-absorption spectroscopy spectra. In STM images of the final silica film an average surface roughness of about 1 \AA{} is detected. It is possible to visualize the silica film unit cell periodicity. A combined anger electron spectroscopy and ultraviolet photoelectron spectroscopy valence band study confirms the silica film stoichiometry and the growth of a 4:2 coordinated silica polymorph on the Mo(112) surface. These various surface science studies allow us to propose models for the growth and structure of the silica epilayer on the Mo(112) surface.