Diffuse LEED Research Articles

Na Deposition on MnO(100)

Na deposition on the MnO(100) surface was investigated by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Na TPD and XPS results indicate that adsorbed Na interacts strongly with the MnO substrate to form an irreversibly-adsorbed, oxidic Na compound on the surface for coverages up to 1 monolayer (ML). This strongly-bound Na diffuses into the MnO subsurface and bulk at elevated temperatures above 500K. For Na coverages above 1 ML, metallic Na is present and desorbs from the surface below 500K. The deposition of Na on MnO(100) follows a Stranski–Krastanov (SK) growth mode, with the formation of metallic Na islands following completion of the first Na monolayer. After Na deposition, the surface exhibits a diffuse (1×1) LEED pattern, suggesting the formation of disordered Na overlayers. After heating to 1000K, the surface presents a (2×2) LEED pattern indicating that a surface reconstruction is induced by the diffusion of Na into the near surface region. CO2 can be used as a probe molecule in TPD to distinguish between metallic Na islands and oxidic Na in the first ML, and to indicate when Na that is still observable by XPS goes subsurface.

Alloy formation and chemisorption at Cu/Pt(111) bimetallic surfaces using alkali ISS, XPD, and TPD

Alloying and surface structures of Cu films evaporated onto a Pt(111) single-crystal substrate were studied by X-ray photoelectron diffraction (XPD), low-energy alkali ion scattering spectroscopy (ALISS), and low-energy electron diffraction (LEED). Alloying begins at temperatures above 500K, and increasing the annealing temperature of deposited films to 900K caused all Cu atoms to diffuse deep into the subsurface region of the Pt crystal. One particular Cu/Pt(111) bimetallic alloy surface was characterized in detail, and this surface was formed by depositing one monolayer of Cu onto the Pt(111) surface and then annealing to 550K. Only a diffuse (1×1) LEED pattern was observed from this Cu/Pt(111) alloy, which indicates that there was no long-range, ordered intermetallic compound created at the surface for these conditions or any others that we investigated. ALISS and XPS were used to determine that the Cu concentration in the topmost, surface layer of this alloy was 7 atomic percent. XPD and ALISS give consistent results showing that Cu in this alloy was present in the first, second, and third layers at the surface, forming a surface alloy. Cu atoms in the alloy are located at Pt atom lattice sites, and are coplanar with the topmost Pt atomic layer without significant corrugation or buckling. Temperature programmed desorption (TPD) measurements showed that both CO and NO are more weakly adsorbed and have smaller desorption energies on the Cu/Pt(111) surface alloy compared to the Pt(111) surface.

Outermost Surface Structures and Oxygen Reduction Reaction Activities of Co/Pt(111) Bimetallic Systems Fabricated Using Molecular Beam Epitaxy

We studied oxygen reduction reaction (ORR) activities for outermost surfaces of 0.3 nm thick Co deposited on Pt(111) (Co0.3 nm/Pt(111)) bimetallic systems fabricated using molecular beam epitaxy at various Co deposition temperatures. Results show that Co0.3 nm/Pt(111) fabricated at temperatures lower than 393 K displays extra low-energy electron diffraction (LEED) spots outside the integer ones, indicating incoherent epitaxial growth of Co. A new IR band that is attributed to linearly bonded carbon monoxide (CO) on the Pt site influenced by neighboring Co atoms emerges at 2052 cm–1 for 333 K fabricated Co0.3 nm/Pt(111), in addition to the CO–Pt and CO–Co bands. With increasing fabrication temperature, the new band shifts to higher frequencies and reaches 2082 cm–1 for 773 K fabricated Co0.3 nm/Pt(111), which has a diffuse (1×1) LEED pattern. We evaluated the dependence of the deposition temperature on the lattice parameters of the Co0.3 nm/Pt(111) and ascribed the band at 2082 cm–1 to adsorbed CO on a Pt-enriched topmost surface having 6-fold symmetry. Although the incoherent epitaxial Co layer was unstable in 0.1 M HClO4 aqueous solution, the Pt-enriched topmost surface is rather stable and the ORR activity is 10 times higher than that for clean Pt(111). The activities for Pt0.3 nm,0.6 nm/Co0.3 nm/Pt(111) artificial sandwich (superlattice) surfaces were also evaluated. The obtained results indicate that the Co atoms located at the second atomic layer strongly modify the electrocatalysis of the topmost surface.

Growth, structure, and magnetism of single-crystallineNixMn100−xfilms andNiMn∕Cobilayers on Cu(001)

The growth and structure of single-crystalline ${\mathrm{Ni}}_{x}{\mathrm{Mn}}_{100\ensuremath{-}x}$ films on Cu(001) were studied for concentrations of $x\ensuremath{\geqslant}13$ and thicknesses of 0--15 monolayer (ML). Medium energy electron diffraction (MEED) curves revealed a layer-by-layer growth mode at a substrate temperature of $T=300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for alloy films with $40<x\ensuremath{\leqslant}100$ and coverages up to 14 ML. A smooth transition to a three-dimensional growth is concluded from the decaying amplitude of the MEED oscillations as the Mn content increases. We find a characteristic $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure by low-energy electron diffraction (LEED) for alloy compositions in the vicinity of the equiatomic region. A kinematic analysis of specular LEED intensity curves was employed to determine the average interlayer spacing $d$, yielding a decreasing $d$ with Ni content from $1.90\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for $x=13$ to $1.73\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ for pure Ni on Cu(001). For equiatomic $c(2\ifmmode\times\else\texttimes\fi{}2)$ $\mathrm{Ni}\mathrm{Mn}∕\mathrm{Cu}(001)$ we propose a bulklike $L{1}_{0}$ crystal structure, which is characterized by an in-plane orientation of the bulk $c$ axis. We show that Co grows layer by layer and assumes a $p(1\ifmmode\times\else\texttimes\fi{}1)$ structure on equiatomic $c(2\ifmmode\times\else\texttimes\fi{}2)$ $\mathrm{Ni}\mathrm{Mn}∕\mathrm{Cu}(001)$. Using $\mathrm{Co}∕\mathrm{Cu}(001)$ as a substrate for equiatomic NiMn leads to a non-layer-by-layer growth of the alloy film and a diffuse LEED pattern with weak $c(2\ifmmode\times\else\texttimes\fi{}2)$ spots. The investigation of such bilayer structures by magneto-optical Kerr effect measurements (MOKE) indicates the presence of an antiferromagnetic (AFM) order in ${\mathrm{Ni}}_{x}{\mathrm{Mn}}_{100\ensuremath{-}x}$ films with $x$ close to the equiatomic composition and thicknesses above 8 ML at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, as concluded from the coercivity enhancement.

In situ cleaning and characterization of oxygen- and zinc-terminated, n-type, ZnO{0001} surfaces

A layer containing an average of 1.0 monolayer (ML) of adventitious carbon and averages of 1.5 ML and 1.9 ML of hydroxide was determined to be present on the respective O-terminated (0001̄) and Zn-terminated (0001) surfaces of ZnO. A diffuse low-energy electron diffraction pattern was obtained from both surfaces. In situ cleaning procedures were developed and their efficacy evaluated in terms of the concentrations of residual hydrocarbons and hydroxide and the crystallography, microstructure, and electronic structure of these surfaces. Annealing ZnO(0001̄) in pure oxygen at 600–650 °C±20 °C reduced but did not eliminate all of the detectable hydrocarbon contamination. Annealing for 15 min in pure O2 at 700 °C and 0.100±0.001 Torr caused desorption of both the hydrocarbons and the hydroxide constituents to concentrations below the detection limits (∼0.03 ML=∼0.3 at. %) of our x-ray photoelectron spectroscopy instrument. However, thermal decomposition degraded the surface microstructure. Exposure of the ZnO(0001̄) surface to a remote plasma having an optimized 20% O2/80% He mixture for the optimized time, temperature, and pressure of 30 min, 525 °C, and 0.050 Torr, respectively, resulted in the desorption of all detectable hydrocarbon species. Approximately 0.4 ML of hydroxide remained. The plasma-cleaned surface possessed an ordered crystallography and a step-and-terrace microstructure and was stoichiometric with nearly flat electronic bands. A 0.5 eV change in band bending was attributed to the significant reduction in the thickness of an accumulation layer associated with the hydroxide. The hydroxide was more tightly bound to the ZnO(0001) surface; this effect increased the optimal temperature and time of the plasma cleaning process for this surface to 550 °C and 60 min, respectively, at 0.050 Torr. Similar changes were achieved in the structural, chemical, and electronic properties of this surface; however, the microstructure only increased slightly in roughness and was without distinctive features.

Sulphur-induced electron accumulation on InAs: a comparison of the (0 0 1) and (1 1 1)B surfaces

The clean and sulphur-treated (0 0 1) and (1 1 1)B surfaces of InAs have been studied using high-resolution electron-energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The diffuse (2 × 4) low energy electron diffraction pattern observed on the atomic hydrogen cleaned InAs(0 0 1) surface disappeared after S-dosing from an electrochemical cell. Upon annealing at 350 °C, a sharp (2 × 1) reconstruction is observed which changes to a disordered (4 × 2) pattern following sulphur desorption at 400 °C. The clean InAs(1 1 1)B surface exhibits a (1 × 1) structure which also disappears upon S-dosing and returns after annealing at 450 °C. XPS from the (1 1 1)B surface indicates that sulphur is bonded to both indium and arsenic on the as-dosed surfaces, whilst it was only bonded to indium on the (0 0 1) surface. Annealing the (1 1 1)B surface to 200 °C removes all the S–As bonding, and annealing above 300 °C decreases the In:As ratio on both surfaces as a result of sulphur substitution for arsenic atoms. Semi-classical dielectric theory simulations of the HREELS spectra, together with solutions of the Poisson equation, have been used to determine the electron accumulation and band bending profiles as a function of surface treatment. Both the (0 0 1) and (1 1 1)B surfaces exhibit a dramatic increase in the observed downwards band bending and the surface state density after annealing the sulphur-terminated surfaces. The downwards band bending reaches 600 meV, with the surface state density increased by as much as an order of magnitude compared with the clean surfaces. The surface donor density corresponds to approximately 1% of the sulphur atoms at the surface, indicating that sulphur-induced ionised surface states are associated with surface defects.

Holographic diffuse LEED image reconstruction for simultaneous occupation of different oxygen adsorption sites on Ni(1 1 1)

We determined the local adsorption structure of disordered oxygen on the Ni(111) surface by means of diffuse holographic LEED. The measurements have been performed above the critical temperature (Tc=450 K) for the oxygen order–disorder phase transition at 500 K and at a coverage of Θ=0.25 ML. At this temperature we found, in agreement with a previous LEED-IV-analysis [Surf. Sci. 349 (1996) 185], that besides the fcc threefold sites also hcp sites are occupied. In addition, a small amount seems to be located also at top and bridge sites. Reconstructing the holographic wavefield information, the different oxygen adsorption geometries are superimposed in the real space image. Nevertheless, a spatial resolution of 0.5–1 Å was sufficient to clearly distinguish between them. The influence of algorithmic parameters on the image quality was tested.

Magnetic order at a single-crystal surface in the diffuse-scattering theory

A theoretical description of incoherent spin-dependent multiple scattering of electrons at a magnetically disordered single-crystal surface is reported. A formalism in which the spin operators specify the magnetic state of a surface atom is used for the description of magnetic order at the surface. The theory is based upon the concepts used in multiple scattering spin-dependent diffuse LEED theory (DSPLEED) theory. In the present considerations, this theory is extended to the case of magnetic materials by using the time-independent Dirac equation with an effective magnetic field. Thus, an expression for incoherent spin-dependent intensity for magnetic material is obtained. It depends on the Fourier transform on the surface lattice of the spin-pair correlation function and, as a consequence, on the magnetic properties of the surface. The equations for the description of magnetization and various correlation functions in the frame of effective field theory are derived and the results of the numerical calculations are presented for the particular case of Ni(1 0 0) surface. The spin–orbit induced and exchange asymmetries are calculated. It is found that the magnetic DSPLEED is sensitive to the properties of the surface characterized by the spin-pair correlation functions. Thus, it is demonstrated that the magnetic DSPLEED can be an effective method in the investigation of critical behaviour of magnetic surfaces.

Theoretical studies of spin-polarized DLEED rotation curves: disordered CO overlayer on Pt(111)

We present a series of theoretical spin-polarized diffuse low-energy electron diffraction studies which explore the information that may be extracted from the rotation curves. At selected beam energies, we report calculations of the angular variations of the relativistic incoherent intensity along with the spin–orbit induced incoherent scattering asymmetry. Both the polar angle of incidence and the azimuthal angle of incidence of a polarized beam are scanned consecutively. Through use of relativistic multiple scattering theory of diffuse LEED, we study the particular advantages of using DSPLEED rotation curves for the disorder surface structure analysis. We explore the CO/Pt(111) system, focusing on the sensitivity of features in the rotation curves to some aspects of adsorbate/substrate geometry. For example, the incoherent asymmetry parameters prove to be very sensitive to the adsorbate/substrate spacing.

Coverage-dependent diffuse spin-polarized electron diffraction analysis of CO on Pt( [formula omitted

The coverage dependence of the local adsorption structure of carbon monoxide on Pt(1 1 1) is investigated by means of diffuse spin-polarized electron diffraction (DSPLEED). We present the theoretical discussion of the scattering of spin-polarized electrons from the disordered CO layer adsorbed on the Pt(1 1 1) surface. The spin-dependent diffuse low-energy electron diffraction (LEED) intensity and the resulting diffuse scattering asymmetry parameter are calculated for different adsorption structures of CO connected with different coverages. We report the energy as well as the angular dependence of the diffuse relativistic intensity and of the spin–orbit asymmetry. We explore the information that may be extracted from the DSPLEED rotation curves which are calculated at a selected beam energy. The polar angle of incidence of a polarized beam is held fixed and the azimuthal angle is scanned. We are focusing on the sensitivity of features in the rotation curves to the coverage-dependent structure of the adlayer keeping other geometrical parameters of the CO/Pt(1 1 1) system fixed. We find the rotation curves to be very sensitive with respect to the correlation effects governing the adlayer formation while the energy-dependent DSPLEED spectra do not expose such strong dependency.

On the disordered adsorption of CO on Pt( [formula omitted]) by diffuse SPLEED

The spin-dependent diffuse LEED intensity and the resulting incoherent scattering asymmetry parameter from a single-crystal surface Pt( 1 1 1 ) partially covered with CO molecules are calculated for different values of parameters which characterize the adsorbate/substrate system. These parameters are divided into: geometrical parameters related to the local arrangement of molecules near chemisorption sites and order parameters related to the statistical distribution of the occupied chemisorption sites. The aim of this paper is to present in details the scheme of the spin-dependent DLEED intensity calculations and to show the practical use of the theory describing the spin-dependent back scattering of low-energy electrons at a single-crystal surface partially covered with a disordered overlayer of elements. It is demonstrated that the structural analysis of the disordered surface can be successfully based on the diffuse LEED experiment with polarized beam of incident electrons.

Structural transitions of perylene and coronene on silver and gold surfaces: A molecular-beam epitaxy LEED study

The growth of perylene and coronene up to a monolayer is investigated continuously by low-energy electron diffraction (LEED) on the metal surfaces Ag(110), Au(110) $1\ifmmode\times\else\texttimes\fi{}2,$ and Au(111) $22\ifmmode\times\else\texttimes\fi{}\sqrt{3}.$ Both molecules show [restricted on Au(110) $1\ifmmode\times\else\texttimes\fi{}2]$ the evolution from isotropic disordered structure in the submonolayer regime to a highly (substrate-dependent) ordered monolayer. Two-dimensional gas, fluid, and crystalline phases can be distinguished. On the rough Au(110) $1\ifmmode\times\else\texttimes\fi{}2$ surface, a periodic structure in the [001] direction can be observed, while in the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction, diffraction patterns arise from diffuse LEED intensity. Just before the monolayer is complete, structural transitions between highly ordered structures occur in all presented adsorbates. Leaving and reaching of commensurate structures show ``stick-slip'' behavior. The lateral ordering process of these molecules allows epitaxial growth without domain walls because crystallization does not start from islands in the submonolayer regime.

Structural transitions of chemisorbed iodine on Cu( [formula omitted

The adsorption of molecular iodine on Cu(1 0 0) was studied by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) in conditions of ultrahigh vacuum. Chemisorbed iodine exhibits several structural phase transformations with increasing coverage. At the coverage θ=0.25 a simple p(2×2) structure with one iodine atom per unit mesh forms. The coverage θ is determined as the ratio of adsorbate species to surface substrate atoms. Further iodine dosing leads to the disappearance of the p(2×2) phase and the formation of a disordered (liquid) phase characterised by a diffuse LEED pattern. For this phase, no modulations at the atomic scale were observed by STM. At a coverage of θ=0.33 the iodine layer solidifies into a new well-ordered phase with a c(6×2) periodicity. At θ>0.33 a new c(14×2) phase was detected on the surface coexisting with the c(6×2). After the complete covering of the surface with the c(14×2) structure ( θ=0.36) it continuously transforms into the more dense c(5×2) structure at saturation.

Lattice vibrations in diffuse LEED investigations

So far, as we considered the DLEED intensities within the theory based on the statistical approach, their temperature dependence was taken into account only via statistical distribution of adsorbate. The present approach includes the influence of temperature due to vibrations of elements at the substrate, which change the adsorption energy and the pair interaction potential, respectively. The above quantities are evaluated on the basis of the anharmonic potential leading to the temperature renormalization of the hamiltonian describing the adlayer properties. In this way we derive generalized correlation functions of element–element (atom or molecule) displacement at the surface. These functions form the effective temperature dependent potential which allows us to calculate the DLEED intensities, whose temperature dependence is exemplified by the observations of CO molecules adsorbed on 2D surface lattice of Pt(111).

Early stages of surface alloy formation: a diffuse LEED I( V) study

The technique of diffuse low energy electron diffraction (LEED) I( V) analysis is shown to be a useful quantitative probe of the structure of surface alloys in the early stages of growth before long-range order develops. The Cu(1 0 0)/Pd system has been studied in the Pd coverage range 0.10–0.55 ML. We demonstrate the Pd adsorbs at all coverages primarily by substitutional replacement of top layer copper atoms forming a two-dimensional Cu x Pd 1− x surface alloy. At low coverages a quasi-random substitutionally disordered two-dimensional alloy is formed with local order detected by LEED developing at Pd coverages above 0.25 ML.The surface geometry is not strongly coverage dependent with top layer Cu and Pd atoms almost coplanar and the first interlayer spacing slightly expanded (3%) with respect to the bulk value.

Composition and structure of the Co–Al interface for thin Co films deposited on Al(0 0 1) and Al(1 1 0) surfaces at room temperature

High-energy ion backscattering and channeling, combined with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) were used to characterize the interface thickness and composition for thin Co films deposited on Al(0 0 1) and Al(1 1 0) surfaces at room temperature. For the Al(0 0 1) surface, measurements of the backscattered ion yields from Al and Co show that substrate Al atoms are continuously displaced for Co coverages up to 3 ML (monolayer, ML), at which point Co metal begins to cover the mixed interface. Based on XPS intensity analysis, we conclude that a CoAl-like phase forms at the interface. A very diffuse LEED pattern with high background was observed after a deposition of 7.6 ML of Co. For the Al(1 1 0) surface, intermixing of Co and Al atoms was observed up to 5 ML of Co deposition, where Co metal begins to cover the interface. No LEED pattern was observed for any Co coverage exceeding 0.2 ML on the Al(1 1 0) surface. The interface thickness is about 10 Å for both surfaces.

H 2S adsorption and the effect of sulfur on the oxidation of Ni 3Al(111)

We report the formation of a (2×2) sulfur adlayer by the adsorption of H 2S on Ni 3Al(111) at room temperature, followed by annealing to ∼1100 K. From Auger electron spectroscopy (AES) measurements, this pattern is consistent with a sulfur coverage ( θ S) of ≤0.25 monolayers (ML). A diffuse low-energy electron diffraction pattern with streaks, corresponding to θ S=0.3 ML, was observed after sulfur adsorption at ∼800 K, but the (2×2) order was restored upon flash annealing to ∼1100 K. AES results rule out any aluminum enrichment or sulfide formation in the surface during exposure to H 2S or thermal treatment. The presence of sulfur on the Ni 3Al(111) surface retards its oxidation at room temperature. Sulfur is found to remain at the oxide/Ni 3Al(111) interface during oxidation at ∼300 K.

DIFFUSE LEED INTENSITY CALCULATIONS TAKING INTO ACCOUNT THE PAIR CORRELATION EFFECTS IN PARTIALLY ORDERED OVERLAYER

The elastic diffuse intensity of electrons scattered by a short-range-ordered adlayer depends on the structure factors and on renormalized t matrices for each kind of occupied bonding sites. In many cases, the computation of these structure factors is a crucial point for the application of the general DLEED theory. Their determination necessitates the knowledge of the site-occupancy pair correlation functions. Up to now two procedures removing pair correlation effects from the data have been used for DLEED intensity calculations. The first procedure, the Y-function method, can be applied only in the particular case where there is one kind of chemisorption site. The second one was proposed recently and is based on the sampling of diffuse LEED intensity distributions by conventional spot intensities. The aim of this paper is to study DLEED intensity distribution coming from the sampling method and from the general treatment of DLEED taking into account the pair correlation effects. We take as an example the Pt(111)/CO system. The site-occupancy pair correlation functions of CO adsorbed on Pt(111) are obtained by a Monte Carlo simulation. DLEED intensities are analyzed using the energy-dependent I–V curves.

Inversion of low-energy electron diffraction data with no pre-knowledge factor beyond optical holography

In this review, I describe how low-energy (typically below 500eV) electron diffraction spectra can be inverted to produce three-dimensional coordinates of atoms neighbouring a reference atom with no prior knowledge of what type or types of atom are present. The reference atom may be one of many equivalent near-surface atoms from which a core-level photoelectron is excited or, in the case of diffuse low-energy electron diffraction, one of many equivalent adsorbate atoms (lacking in long-range order) on the surface of a crystalline substrate. Other variants apply to low-energy electron diffraction, Kikuchi electron diffraction and time-reversed versions in which the wavenumber (energy) and direction of the incident beam are varied. I show that, for such low-energy electron diffraction spectra, the relative phases between the reference wave and scattered waves have a known geometric form if the spectra are taken from within a small angular cone in the near-back-scattering directions. By using the back-scattering small cone at each direction of interest, a simple algorithm is developed to invert the spectra and extract object atomic positions with no input of calculated dynamical factors. The article also reviews key ideas and works which led to the current understanding of this field.

Surface structures and electronic states of clean and (NH4)2Sx-treated InAs(111)A and (111)B

The surface structures, unoccupied and occupied electronic states, and chemical states of surface atoms for clean and (NH4)2Sx-treated InAs(111)A and (111)B surfaces have been studied using low-energy electron diffraction (LEED), inverse photoemission spectroscopy (IPES), and (x-ray and ultraviolet) photoemission spectroscopy (PES). Thermal stability of the treated surfaces upon annealing in an ultrahigh vacuum is also investigated. A diffuse (1×1) LEED pattern appears for the treated -(111)A and -(111)B surfaces annealed at 230 and 330 °C, respectively, Upon annealing the (111)B sample at 380 °C, the (1×1) structure remains and the LEED spots become clearer. For the (111)A annealed at 380 °C, the pattern changes to a clear (2×2) structure which is found for the first time for sulfurized (111) surfaces of III–V compounds. Sulfur is completely desorbed from both the (111)A and (111)B surfaces at 440 °C, exhibiting the (2×2) and (1×1) structures, respectively. IPES and PES results indicate that unoccupied and occupied dangling bonds disappear for the sulfur-adsorbed (111)A-(2×2) and (111)B-(1×1) surfaces, respectively. S 2p spectra show that sulfur is bonded to both indium and arsenic on the (111)B surface annealed at less than 340 °C and bonded only to indium at 420 °C. On the other hand, it is bonded solely to indium on the (111)A surface with and without annealing. Surface core-level shifts of In 4d and adsorption sites of sulfur are discussed.