Low-energy Electron Diffraction Structure Research Articles

Strain modulation in small molecule physisorption in two dimensions: LEED structure analysis and DFT modeling of the system.

The structure of the system was investigated experimentally by means of quantitative LEED I(V) analysis and computationally using dispersion corrected density functional theory (DFT-D). Three different structure models with four, five, and six molecules were considered. The lowest reliability factors and thus best agreement of measured and calculated I(V) curves was found for the structure model containing five molecules per surface unit cell. Essential features of the experimental best-fit adlayer structure are supported by DFT. A slight inclination and lateral shift of twofold coordinated molecules away from the on-top position over Na+ adsorption sites is interpreted as compensation of strain between substrate and adlayer.

LEED-I(V) Structure Analysis of the (7 × √3)rect SO4 Phase on Ag(111): Precursor to the Active Species of the Ag-Catalyzed Ethylene Epoxidation

According to a recently proposed mechanism, the silver-catalyzed industrial synthesis of ethylene oxide (EO) involves adsorbed SO4. The O atoms that are added to the ethylene molecules to give EO originate from SO4, which may solve the long-standing question about the active oxygen species in this reaction. Here, we report a low-energy electron diffraction structure analysis of an ordered phase of SO4 on the Ag(111) surface, forming a (7 × √3)rect structure and containing the oxygen species that before had been spectroscopically identified on the active catalyst. Using I(V) data from a low-energy electron microscope and an input model from density functional theory, the complex structure could be solved. It contains SO4 moieties on a reconstructed Ag(111) surface in which all four O atoms bind to Ag atoms. In the proposed ethylene epoxide reaction model, the structure represents the parent phase from which the active SO4 phase is formed by a lifting of the reconstruction.

Novel genetic algorithm search procedure for LEED surface structure determination

Low Energy Electron Diffraction (LEED) is one of the most powerful experimental techniques for surface structure analysis but until now only a trial-and-error approach has been successful. So far, fitting procedures developed to optimize structural and nonstructural parameters—by minimization of the R-factor—have had a fairly small convergence radius, suitable only for local optimization. However, the identification of the global minimum among the several local minima is essential for complex surface structures. Global optimization methods have been applied to LEED structure determination, but they still require starting from structures that are relatively close to the correct one, in order to find the final structure. For complex systems, the number of trial structures and the resulting computation time increase so rapidly that the task of finding the correct model becomes impractical using the present methodologies. In this work we propose a new search method, based on Genetic Algorithms, which is able to determine the correct structural model starting from completely random structures. This method—called here NGA-LEED for Novel Genetic Algorithm for LEED—utilizes bond lengths and symmetry criteria to select reasonable trial structures before performing LEED calculations. This allows a reduction of the parameter space and, consequently of the calculation time, by several orders of magnitude. A refinement of the parameters by least squares fit of simulated annealing is performed only at some intermediate stages and in the final step. The method was successfully tested for two systems, Ag(1 1 1)(4 × 4)-O and Au(1 1 0)-(1 × 2), both in theory versus theory and in theory versus experiment comparisons. Details of the implementation as well as the results for these two systems are presented.

Formation of a Buffer Layer for Graphene on C-Face SiC{0001}

Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in a Si-rich environment are studied using low-energy electron diffraction (LEED) and low-energy electron microscopy (LEEM). Upon graphitization, an interface with rt(43) x rt(43)-R7.6 degree symmetry is observed by in situ LEED. After oxidation, the interface displays rt(3) x rt(3)-R30 degree symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like "buffer layer" that forms between the graphene and the SiC, similar to that observed on Si-face SiC. From a dynamical LEED structure calculation for the oxidized C-face surface, it is found to consist of a graphene layer sitting on top of a silicate (Si2O3) layer, with the silicate layer having the well-known structure as previously studied on bare SiC(000-1) surfaces. Based on this result, the structure of the interface prior to oxidation is discussed.

Low-energy electron diffraction structure analysis of the Al13Co4(100) surface

Low-energy electron diffraction structure analysis of the Al₁₃Co₄(100) surface

Low-energy electron diffraction structure determination of an ultrathin CoO film on Ag(0 0 1)

The atomic structure of a four layer thick film of CoO on a Ag(0 0 1) substrate has been determined by comparing experimental low-energy electron diffraction (LEED) I(V) curves with multiple scattering calculations. The CoO film has been prepared using reactive evaporation of Co in an oxygen atmosphere leading to almost layer-by-layer growth. Contrary to the surface of CoO crystals an outward relaxation of the two outermost CoO layers as well as rumpling in the top layer has been found. The supposed driving force of this relaxation is the in-plane compressive stress, which results from the pseudomorphic growth of the CoO film on the Ag(0 0 1) substrate and the lattice mismatch of the two materials.

Experimental deduction of In/Si(1 1 1) 2D phase diagram and ab initio DFT modeling of 2√3 phase

We have carried out adsorption and residual thermal desorption experiments of Indium on Si (1 1 1) 7 × 7 reconstructed surface, in the submonolayer regime, in Ultra High Vacuum (UHV) using in situ probes such as Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). The coverage information from AES and the surface symmetry from LEED is used to draw a 2D phase diagram which characterizes each observed superstructural phases. The different superstructural phases observed are Si(1 1 1)7 × 7–In, Si(1 1 1)√3 × √3R30°–In, Si(1 1 1)4 × 1–In, Si(1 1 1)2√3 × 2√3R30°–In and Si(1 1 1)√7 × √3–In in characteristic temperature and coverage regime. In addition to the 1/3 ML, √3 × √3-In phase, we observe two additional √3 × √3-In phases at around 0.6 and 1 ML. Our careful residual thermal desorption studies yields the Si(1 1 1)2√3 × 2√3R30°–In phase which has infrequently appeared in the literature. We probe theoretically the structure of this phase according to the LEED structure and coverage measured by AES, assuming that the model for Si(1 1 1)2√3 × 2√3R30°–In is very proximal to the well established Si(1 1 1)2√3 × 2√3R30°–Sn phase, using ab initio calculation based on pseudopotentials and Density Functional Theory (DFT) to simulate an STM image of the system. Calculations show the differences in the atomic position and charge distribution in the Si(1 1 1)2√3 × 2√3R30°–In case.

Growth and structure of gold films on a Re() surface

We have deposited Au films in ultra-high vacuum onto a rhenium () surface in submonolayer and multilayer concentrations and studied them by means of low- andmedium-energy electron diffraction in the temperature range between 300 and 800 K. In thesubmonolayer range, Au forms several low-energy electron diffraction (LEED) phases, namely, a(1 × 3), a(1 × 4),a (1 × 5)and a (1 × 6) phase, consisting of one-dimensionally ordered Au chains in the [] direction, until the formation of a complete pseudomorphic monolayer is indicated by a(1 × 1) LEED phase. In themultilayer regime, a (1 × 8) LEED phase appears over a surprisingly wide coverage range from about two to atleast eight monolayers, which we interpret as a hexagonal uniaxially compressedreconstructed Au overlayer on pseudomorphically grown hexagonal close-packed goldlayers. In order to get access to absolute Au coverages, we have performed LEED(I,V) measurements and carried out a LEED structure determination for the(1 × 1) phase. We propose the formation of a full Au monolayer in which both the Re trough andtop-row sites are being covered by Au atoms. The data are discussed and compared withthose from previous studies on related systems.

Racemic versus Enantiopure Alanine on Cu(110): An Experimental Study

The adsorption of racemic alanine on the Cu(110) surface has been compared to that of enantiopure alanine using low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). No evidence of chiral resolution at the surface was observed for the racemic system, indicating that the formation of separate enantiopure areas is not preferred. Also, in contrast to the enantiopure system, no chirally organized phase was observed for the racemic system. LEED shows that both systems display a common (3 x 2) phase at high coverage. However, the pathway and kinetic barriers to this phase differ markedly for the racemic and the enantiopure systems, with the racemic (3 x 2) appearing at a temperature that is more than 100 K below that required for the enantiopure system. In addition, we report intriguing complexities for the (3 x 2) LEED structure that is ubiquitous in amino acid/Cu(110) systems. First, a common (3 x 2) pattern with a zigzag distortion can be associated with both the racemic and enantiopure systems. For the racemic system, the coverage can be increased further to give a "true" (3 x 2) LEED pattern, which is a transformation that is impossible to enact for the enantiopure system. Most importantly, STM images of the "distorted" and "true" (3 x 2) structures created in the racemic system show subtle differences with neither arrangement being fully periodic over distances greater than a few molecules. Thus, the (3 x 2) phase appears to be more complicated than at first indicated and will require more complex models for a full interpretation.

The transition from the adsorbed state to a surface alloy in the Sn/Ni(1 1 1) system

Conventional X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and high resolution photoelectron spectroscopy with synchrotron radiation (SRPES) were used in order to study the overlayer to surface alloy transition in the Sn/Ni(1 1 1) system, as a function of temperature and initial Sn coverage. The TPD of CO adsorbed at 150 K, shows complete blocking of CO adsorption on the perfect surface alloy (0.33 ML Sn), whereas for less than 0.33 ML Sn, CO adsorbs on the alloy-free regions with the same local saturation coverage as on the clean Ni(1 1 1) surface. For RT deposition of 0.1 to 2.6 ML Sn/Ni(1 1 1), only a new sharp c(4 × 2) LEED structure was observed at exactly 0.25 ML, whereas Sn in excess of 1 ML yielded a c(2 × 2) LEED structure, when flashed at 700 K, and the sharpest (√3 × √3) R30° alloy structure, when annealed at 1000 K. A series of high resolution Sn4d SRPE spectra with 50 eV photon energy taken upon short heating of 1.2 ML Sn/Ni(1 1 1) at increasing temperatures, showed the gradual transition from the overlayer (Sn4d 5/2 BE 24.00 eV) to the alloyed (Sn4d 5/2 BE 23.73 eV) state of Sn. The combination of LEED and SRPES results suggests that the overlayer-to-alloy transition occurs via Sn adsorption at the threefold hollow Ni(1 1 1) sites. A theoretical estimation of the Sn4d 5/2 chemical shift using data from ab initio electronic structure calculations yielded a value of −0.36 eV, in reasonable agreement with the experiment.

The surface geometries of the medium and high coverage carbon monoxide structures c(2 × 4)–(2CO) and [formula omitted]R19°–(4CO) on Ni{1 1 1}

The surface geometries of the p ( 7 × 7 ) R19°–(4CO) and c(2 × 4)–(2CO) layers on Ni{1 1 1} and the clean Ni{1 1 1} surface were determined by low energy electron diffraction structure analysis. For the clean surface small but significant contractions of d 12 and d 23 (both 2.02 Å) were found with respect to the bulk interlayer distance (2.03 Å). In the c(2 × 4)–(2CO) structure these distances are expanded, with values of d 12 = 2.08 Å and d 23 = 2.06 Å and buckling of 0.08 and 0.02 Å, respectively, in the first and second layer. CO resides near hcp and fcc hollow sites with relatively large lateral shifts away from the ideal positions leading to unequal C–Ni bond lengths between 1.76 and 1.99 Å. For the p ( 7 × 7 ) R19°–(4CO) layer two best fit geometries were found, which agree in most of their atomic positions, except for one out of four CO molecules, which is either near atop or between bridge and atop. The remaining three molecules reside near hcp and fcc sites, again with large lateral deviations from their ideal positions. The average C–Ni bond length for these molecules is, however, the same as for CO on hollow sites at low coverage. The average C–Ni bond length at hollow sites, the interlayer distances, and buckling in the first Ni layer are similar to the c(2 × 4)–(2CO) geometry, only the buckling in the second layer (0.08 Å) is significantly larger. Lateral and vertical shifts of the Ni atoms in the first layer lead to unsymmetric environments for the CO molecules, which can be regarded as an imprint of the chiral p ( 7 × 7 ) R19° lattice geometry onto the substrate.

The surface geometry of carbonmonoxide and hydrogen co-adsorbed on Ni{1 1 1}

A combination of photoelectron spectroscopy, temperature programmed desorption and low energy electron diffraction structure determinations have been applied to study the p(2 × 2) structures of pure hydrogen and co-adsorbed hydrogen and CO on Ni{1 1 1}. In agreement with earlier work atomic hydrogen is found to adsorb on fcc and hcp sites in the pure layer with H–Ni bond lengths of 1.74 Å. The substrate interlayer distances, d 12 = 2.05 Å and d 23 = 2.06 Å, are expanded with respect to clean Ni{1 1 1} with buckling of 0.04 Å in the first layer. In the co-adsorbed phase CO occupies hcp sites and only the hydrogen atoms on fcc sites remain on the surface. d 12 is even further expanded to 2.08 Å with buckling in the first and second layer of 0.06 and 0.02 Å, respectively. The C–O, C–Ni, and H–Ni bond lengths are within the range of values also found for the pure adsorbates.

Study of CO adsorption on La–Rh(1 0 0) surface alloys

Adsorption of carbon monoxide on La–Rh(1 0 0) surface alloys has been studied by photoelectron spectroscopy, low-energy electron diffraction (LEED) and temperature programmed desorption (TPD). The surface alloys were formed by depositing overlayers of La in the monolayer regime on a Rh(1 0 0) substrate, and by subsequent annealing to about 1350 K. Carbon monoxide (CO) was dosed to saturation at various temperatures. The LEED pattern of the dissociated CO overlayer shows a c(4×4) structure with respect to the Rh(1 0 0) substrate. The LEED structure for the sub-monolayer alloy takes on a ring structure superposed on a (1×1) structure. At higher temperatures an apparent (10×10) superstructure is superposed on the ring structure. The TPD results shows that four different peaks are present in the desorption traces. From the core level spectroscopic data it is argued that the two first peaks are a combination of direct CO desorption, CO dissociation and associative CO desorption. The remaining two peaks are assumed to be related to associative desorption from atomic carbon and oxygen in a combination of different bonding states and different adsorbate interactions. Rh 3d core level spectra shows that the two peaks related to the surface atoms are shifted towards the bulk peak for CO on the alloy. The La 5p core level spectra shows the splitting of the core level into two peaks for CO and dissociated CO on the alloy. The 4σ and 5σ/1π molecular levels of CO on the alloy are observed in the valence band spectra, with similar binding energies as for CO on a disordered La layer. In particular, a stronger rehybridisation of the CO 4σ state with the electronic states of the surface alloy as compared to CO on clean Rh(1 0 0), is argued to be responsible for dissociation of CO on the alloy at low temperatures.

Structure and stability of CN adlayers on Rh(110)

The formation and stability of CN adlayers on Rh(110), formed by dissociative adsorption of C 2N 2 at 373 K, have been studied as a function of coverage and temperature by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Two different CN adsorption states have been distinguished by their different C 1s and N 1s XPS core-level binding energies. The CN-I state is exclusively occupied up to a surface coverage of 0.5 monolayers (ML), where a well-ordered c(2×2) LEED pattern is observed. The CN-II state becomes additionally populated at higher coverages from 0.5 ML to the saturation coverage of 0.87 ML. At CN saturation, a c(4×2) LEED structure is formed. Desorption of CN as molecular C 2N 2 occurs only for surface coverages >0.5 ML and appears to be mainly derived from the CN-II state. The onset of CN bond rupture is indicated at ∼450–550 K, depending on the CN coverage; the resulting N ad desorbs at ∼580 K from the more crowded surface and in the range of ∼650–950 K, whereas C ad remains at the Rh surface and cannot be desorbed thermally.

The interaction of copper with a rhenium(0001) surface: structure, energetics, and growth modes

Thin Cu films were vacuum-deposited onto Re(0001) and investigated by means of temperature-programmed desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). TDS (performed with a heating rate of 7.7K/s) reveals three (coverage-dependent) Cu binding states β3, β2, and β1 between 1000 and 1200K associated with Cu monolayer and multilayer formation, respectively. While the second and multilayer states β2 and β1 exhibit clear zero-order kinetics, the first monolayer state desorbs with more complicated desorption kinetics due to attractive mutual Cu–Cu interactions. The activation energy for desorption varies between ∼200kJ/mol for vanishing Cu coverage and ∼320kJ/mol near the monolayer saturation. Cu grows first pseudomorphically and forms, between 2 and 3ML, an incomplete (14×14) LEED structure (only first- and second-order diffraction spots are visible). At larger coverages, the formation of genuine Cu(111) crystallites is indicated by a Cu(1×1) LEED pattern. AES and XPS suggest an incomplete Stranski–Krastanov growth mode, since the Re signals remain visible even after deposition of 12 nominal Cu layers. The absence of any Cu and Re core level shifts in XPS (to within the resolution of our instrument) points to a negligible chemical interaction between the two elements at the interface.

Nature, growth, and stability of vanadium oxides on Pd(111)

Thin films of vanadium oxides grown on a Pd(111) single crystal surface have been studied using high resolution x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The vanadium oxides have been prepared by reactive evaporation of vanadium in pO2=2×10−7 mbar at 250 °C from submonolayer to 5 monolayer (ML) coverages. As observed on other substrates, the stoichiometry of the oxide phase varies as a function of the coverage, VO/VO2-like at low coverages to V2O3 for thicker oxide layers as indicated by XPS V 2p core level spectra and the characteristic NEXAFS fingerprints at both V 2p and O 1s edges. The V2O3 oxide phase grows epitaxially on the Pd(111) surface in the form of small three-dimensional (3D) islands as revealed by LEED and STM. The thermal stability of the oxides is also coverage dependent: the decomposition onset temperatures range from 300 °C for submonolayer coverage to ⩾500 °C for 5 ML. The V 2p XPS data indicate that the 1 ML V2O3-like oxide disproportionates into higher (4+) and lower (2+) oxidation state phases on heating to 300 °C. This surface shows a complex LEED pattern, due to the coexistence of ordered 3D V oxide islands and bare Pd patches as revealed by STM. On heating to 350 °C a simple p(2×2) LEED structure evolves, originating from a well ordered two-dimensional overlayer, which entirely wets the Pd(111) surface. At higher temperatures (400–500 °C), the vanadium oxide reduces to metallic vanadium and V/Pd alloy phases, as demonstrated by XPS.

Reconstructed oxide structures stable in air: Silicate monolayers on hexagonal SiC surfaces

Ultrathin oxide layers on hexagonal SiC surfaces were studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). SiC(0001) and SiC(0001̄) samples were ex situ prepared using thermal hydrogen etching or a microwave powered hydrogen plasma treatment. A well ordered (3×3)R30° reconstructed surface is observed by LEED immediately upon introduction into vacuum. The samples contain oxygen of approximately one layer equivalent bonded to Si atoms as indicated by AES. From a full dynamical LEED structure analysis carried out for the SiC(0001̄) surface the crystallographic structure is determined: The silicon oxide is arranged as a silicate (Si2O3) layer on top of the SiC substrate, forming rings of (3×3)R30° periodicity with twofold coordinated oxygen atoms in the topmost position. The oxygen incorporation into the surface presumably proceeds via rapid oxidation in air of the well ordered topmost substrate bilayer. The extreme stability of the resulting surface reconstruction is caused by the absence of dangling bonds in the surface terminating silicate layer. On the SiC(0001) surface a similar silicate type layer terminated by its oxygen atoms is found by the LEED analysis. However, the adlayer and substrate are bonded via a linear Si–O–Si bond.

Combined EELS/STM study of the adsorption of nitric oxide and the formation of GaO xN y on CoGa(001)

The adsorption of NO and the formation of GaO x N y on CoGa(001) was studied by means of high-resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). The adsorption of NO at 300 K leads to the formation of an amorphous galliumoxinitride (am-GaO x N y ) film on CoGa(001). After annealing to 825 K, a well-ordered GaO x N y film is formed. The atomic ratio between the amount of oxygen and nitrogen on the surface was estimated to be O/N≈2/1. The well-ordered GaO x N y film is characterized by a (2×1) LEED structure with two domains oriented perpendicular to each other. The EEL spectrum of GaO x N y exhibits four Fuchs–Kliewer modes with frequencies at 250, 455, 695 and 775 cm −1. The band-gap was determined to be 4.1±0.2 eV and lies between the values of β-Ga 2O 3 (4.5 eV) and of GaN (3.5 eV).

LEED structures on Si(111) surfaces quenched from elevated-temperature cleavages

Silicon wafers were cleaved at various temperatures from room temperature to 720°C and observed by low-energy electron diffraction (LEED). The cooling rate after cleavage was rapid, most of the temperature drop occurring within one or two seconds, in order to freeze-in the surface structures. The (111)2×1 reconstruction alone was observed on surfaces cleaved at temperatures up to 417°C. At higher cleavage temperatures, very slight 5×5 and mostly 7×7 reconstructions co-existed with 2×1, which latter became unobservable after cleavage at 500°C. At cleavage temperatures above 550°C, only 1/7-order spots were visible, but a full 7×7 LEED pattern was not observed until cleavage above 710°C. The results provide evidence about surface structure stability and transitions.

Structural studies of WC(0001) and the adsorption of benzene

We report on studies dealing with the structure of WC(0001) and the adsorption of benzene on this surface. An I(V)-low-energy electron diffraction structure analysis has been performed to elucidate the surface structure of WC(0001). These studies indicate that the surface consists of a tungsten layer covered by carbon randomly distributed on the hcp sites with a coverage of 30% that of a full carbon layer. The distance between this carbon layer and the tungsten layer beneath is enlarged by 5% with respect to the spacing between carbon and tungsten layers in the bulk. Only a small deviation from the bulk value was found for the distance between the first tungsten layer and the carbon layer below. No indications of surface reconstruction have been observed. Benzene adsorption was studied on clear oxygen covered and oxidized WC(0001). The benzene multilayer desorbs at T≤200 K. On stoichiometric WC(00001), molecular benzene of (sub)monolayer coverage is found up to temperatures of T≈230 K. After desorption of this species, small signals of fragments are visible in the photoelectron spectra up to T≈1000 K. Above this temperature, a graphite covered surface remains. On a surface covered by a thin closed oxide phase (WO) only multilayer adsorption is found; above T≈200 K no adsorption takes place under UHV conditions. Weakly oxidized WC(0001) interacts more strongly with benzene in that strong photoemission signals of a (sub)monolayer species are visible up to a temperature of T≈340 K.