Low-energy Electron Diffraction Intensity Analysis Research Articles

LEED intensity analysis of the structure of Al on GaAs(110)

The atomic structure of 1/2 monolayer of Al on GaAs(110) is examined by low energy electron diffraction (LEED) intensity analysis. Room temperature deposition produces a disordered overlayer that does not modify the substrate structure. However, an ordered 1×1 structure results when the system is annealed at 450°C. The AES and LEED measurements suggest a reaction of Al on GaAs to form AlAs, in agreement with some of the photoemission data. As a preliminary result, the multiple scattering analysis indicates that a structure for which Al replaces the second layer Ga provides the most satisfactory description of the measured LEED intensities.

Metal-Surface Reconstruction Induced by Adsorbate:Fe(110)p(2×2)-S

Reconstruction of the Fe(110) surface by a quarter monolayer of S is shown by lowenergy electron-diffraction intensity analysis to correspond to a model in which hardsphere atoms displace to a unique jamming point. After reconstruction the S atoms sit in fourfold hollows of touching Fe atoms with S-Fe bond lengths of 2.17 and 2.36 \AA{}. A general construction for 2\ifmmode\times\else\texttimes\fi{}2 reconstructions with rectangular symmetry is given and applied to this case and to Ni(001)(2\ifmmode\times\else\texttimes\fi{}2)-C.

Atomic structure of Fe {111}

A clean Fe {111} surface was prepared and studied with LEED (low-energy electron diffraction) and AES (Auger electron spectroscopy). A LEED intensity analysis was carried out with a new computational scheme (THIN) specially designed for short interlayer spacings. The results are, for the fust interlayer spacing, d 12 = 0.70 ± 0.03 Å and for the inner potential V 0 = 11.1 ± 1.1 eV, the confidence intervals referring to 95% confidence level. Thus, the Fe {111} surface is contracted 15.4% with respect to the bulk (0.827 Å).

The surface reconstructions of the (100) crystal faces of iridium, platinum and gold: II. Structural determination by LEED intensity analysis

The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.

Atomic Structure and Chemical Composition of Compound Semiconductor Surfaces

Research during the last decade has led to the microscopic understanding of the atomic, electronic, and chemical (compositional) structures at semiconductor surfaces and interfaces. First, we present a brief review of LEED (Low energy Electron Diffraction) intensity analyses of the atomic geometry of the (110) surfaces of 3-5 and 2-6 compounds. The dynamical calculation proposes an atomic model of the (110) surface with top twolayer reconstructions for GaAs, InSb, InP, and probably the topmost singlelayer reconstruction for the more ionic ZnTe. Secondly, after a brief description of standard AES analyses of the chemical composition of the surfaces and interfaces, particular attention is focussed on soft Xray photoemission studies on the interface formed by MBE (Molecular Beam Epitaxy) techniques. Thirdly, theoretical band calculations are mentioned based on the LEED atomic model. Finally, to develop a model of the atomic geometry and the initial step of oxidation. we present recent ab initio first principle calculations using a technique of quantum chemistry which has been applied to the GaAs (110) surface.

LEED analysis of acetylene and ethylene chemisorption on the Pt(111) surface: Evidence for ethylidyne formation

The stable surface species formed from the chemisorption of acetylene (C2H2) or ethylene (C2H4) on the Pt(111) surface (T∼300–350 K) has been studied by a low-energy electron diffraction intensity analysis. High resolution electron energy loss spectra reported by Ibach et al. have been interpreted by comparison to infrared data on relevant model compounds. The surface species most consistent with these studies is ethylidyne ( C–CH3). The species is coordinated to a threefold surface site with the C–C axis normal to the surface within an uncertainty of ∼15°. A saturated C–C bond length of 1.50±0.05 Å and three equivalent Pt–C bond lengths of 2.00±0.05 Å are determined by the LEED analysis and are consistent with the reported structures of ethylidyne in organometallic clusters. The ethylidyne group forms readily upon exposure of C2H4 to the Pt(111) surface at T∼300 K with the loss of one hydrogen atom per ethylene. The complete conversion of C2H2 to ethylidyne requires the presence of hydrogen atoms and proceeds rapidly at T∼350 K. By comparison to reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene ( C=CH2) is an intermediate in both reactions.

Analysis of a photographic–vidicon camera method of LEED intensity measurements

Low-energy electron diffraction (LEED) has great need for a quick and efficient means of making intensity measurements (e.g., in studying reactive surfaces which quickly degrade, and in handling the enormous amounts of data needed for data averaging). The photographic-vidicon camera method fills this need. This paper describes a system and procedure for this method. We have pointed out advantages of this method over other methods of LEED intensity data collection and analysis, and have included comments on the advantages of our experimental system over other systems using this method. General properties of the photographic and vicdicon system are analyzed as well as specific tests done on the method. Estimates of probable error in spot intensity measurements due to numerous effects have been analyzed and I-V curves reduced from raw photographic data are compared. It is hoped that the description, comments, and analysis will facilitate the incorporation of the photographic-vidicon method of LEED analysis into research programs.

Random Occupation of Adsorption Sites in thec(2×2)Structure of CO on Fe{001}

A low-energy electron-diffraction intensity analysis of the Fe{001} $c(2\ifmmode\times\else\texttimes\fi{}2)$-CO structure confirms the results of photoemission studies by other workers suggesting dissociation of the CO molecule. The data are fitted well by a model with random occupation of fourfold symmetrical sites by carbon and oxygen atoms in a $c(2\ifmmode\times\else\texttimes\fi{}2)$ configuration, similar to that proposed earlier by Felter and Estrup and by Riwan, Guillot, and Paigne for Mo{001} $c(2\ifmmode\times\else\texttimes\fi{}2)$-CO.

The surface structure and bonding of (2 × 2) acetylene overlayers on platinum (111): LEED intensity analysis

We report the results of a low-energy electron diffraction intensity analysis of the structure of (2 × 2) arrays of acetylene (C 2H 2) on the Pt(111) surface. We have determined the structure of two different bonding states of the chemisorbed species (separated by a thermal activation barrier) which were recently identified by LEED and ultraviolet photoelectron spectroscopy. The more weakly bound (metastable) state involves coordination to basically one surface metal atom (Pt-C bond lengths of 2.5(1) Å) whereas attachment to three metal atoms is found for the more strongly bound (stable) state (Pt-C bond lengths of 2.2(1) Å and 2.6(1) Å). A lack of sensitivity to possible C-C bond elongation and CCH angle-bending for the adsorbed molecule is also reported and discussed.

Further evaluation of the transform-deconvolution method for surface-structure determination by analysis of low-energy electron-diffraction intensities

The conventional approach to surface structure determination by analysis of low-energy electron diffraction (LEED) consists of comparison between experimental intensities and intensities calculated on the basis of a model of the diffraction process and a trial model of the surface structure. Although this model calculation approach has been used with some success in the case of simple structures, it is unlikely that it can be easily extended to more complex systems unless a reasonable approximation to the structure is known or can be determined by another method. In this article we further describe and evaluate the transform-deconvolution method, which is in essence an adaption to LEED of the Patterson function method of x-ray crystallography, in which direct use is made of the experimental data without prior assumption of a model structure. Within the approximation of single-scattering theory it is shown that the Patterson functions of LEED intensities consist of sums of convolution products of structural and nonstructural terms from which the structural parameters can be recovered via a deconvolution process. Procedures are described for tackling the problem of nonuniqueness, which reslts from the limited amount of data available from a LEED experiment, and problems associated with the effects of multiple scattering. The results of application of the transform-deconvolution method to the analysis of LEED intensities from Cu(100), Ni(100), Al(111), and Al(100) are presented. Consistent values for the surface interlayer spacings are obtained from the analysis of a number of intensity spectra in each case.

The crystallography of clean surfaces and chemisorbed species as determined by low-energy electron diffraction

Low-energy electron diffraction (LEED) is a valuable technique for determining not only two-dimensional translational periodicities of surface atoms but also their detailed locations via an analysis of LEED intensities. We outline the experimental diffraction intensity data and various methods of theoretical analysis including dynamical calculations, data averaging, and data inversion methods needed for such structural determinations. The relative merits of the dynamical analysis of either azimuthal intensity distributions at fixed energy, I(φ), or the energy dependence of the intensity, I(E), to determine surface crystallography are examined via model calculations. Dynamical analysis of the I(E) spectra of the oxygen (2 × 1) overlayer and the hydrogen (1 × 2) overlayer structures on Ni(110) are presented. We determine that the oxygen atoms in the (2 × 1) structure reside in the twofold bridge sites on top of 〈1 10〉 rows of the Ni(110) surface and are located 1.92 ± 0.04 Å from the two adjacent Ni atoms. For the casee of hydrogen on Ni(110) we find that little agreement is found between calculated and experimental I(E) spectra for simple hydrogen adsorption models. A surface distortion model is found to best describe the experimental data where adjacent 〈1 1〉 rows of Ni atoms are alternately attracted or repelled 0.1 ± 0.05 Å together or apart and compressed into the bulk by 0.1 ± 0.05 Å. The chemical trends of these and other recent LEED determinations as well as the future directions of surface crystallography are briefly discussed.

Atomic underlayer formation during the reaction of Ti{0001} with nitrogen

Changes in the {0001} surface of Ti during exposure to nitrogen gas are monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Evidence is provided for the formation of a Ti{0001}1 × 1-N phase followed by a Ti{001} 3 × 3−30°-N structure. A LEED intensity analysis of the Ti{0001}1 × 1-N phase reveals that the N atoms penetrate into the octahedral interstitial holes underneath the first layer of Ti atoms. This is the first ordered monoatomic underlayer found in the earliest stages of any solid-gas interaction. The surface structure bears close resemblance to that of {111} planes in bulk TiN. We find a Ti-N bond length of 2.095 Å to be compared with the value of 2.120 A in bulk TiN. The analysis of the Ti{0001}1 × 1-N structure indicates that Ti{0001} 3 × 3−30°-N is not a low-coverage phase. The importance of recognizing the existence of 1 × 1 phases prior to the formation of superstructures is emphasized, and some procedures for extracting the information from experimental observations are discussed.

Surface structure determination by low-energy electron diffraction

The reflection of an incident «low-energy» (10 eV≲E≲≲500 eV) electron by a solid is dominated by the consequences of its interactions with the valence electrons in the solid. In particular, the high probability that the incident electron suffers an inelastic electron-electron collision renders the elastic electron-solid scattering cross-sections direct measures of the electronic structure and positions of ion cores within 10 A or less of the solid’s surface. We indicate the major features of the coherent reflection,i.e. diffraction, of such low-energy electrons from planar surfaces of single-crystal solids and of current theoretical models of this phenomenon. Emphasis is placed on those features of the electron-electron and electron-ion-core interactions which permit the extraction of the atomic geometry of surface species from measured low-energy electron diffraction (LEED) intensities. Analysis of the elastic low-energy electron diffraction intensities from the low-index faces of aluminum reveals the relatively modest influence on these intensities of the detailed shape of the one-electron optical potential and of the electronic structure of the ion cores. This result, in turn, is used to analyse the atomic structure of Ni(100)-c(2×2)-S, Ni(100)-c(2×2)-O and Cu(100)-c(2×2)-O adsorbed overlayers. Finally, we indicate the present status of applications of LEED intensity analyses to determine the atomic geometry of periodic surface structures.

Determination of the Structure of Solid Surfaces by Low-Energy Electron Diffraction

Major advances occurring during the past two years in the development of the theory of low-energy electron diffraction (LEED) are surveyed. The quantitative nature of mathematical models of LEED from periodic systems is indicated by reference to analyses of observed LEED intensities from low-index faces of aluminum, nickel and copper. Extensions of these models to describe elastic LEED from partially disordered adsorbed overlayers are noted. Applications of model calculations to ascertain the atomic geometry of adsorbed overlayers are described. Analyses of observed LEED intensities for c (2×2) overlayers of oxygen and sulphur on the (100) faces of nickel and copper serve as examples of such applications. The analysis of inelastic LEED intensities to determine the dispersion relation of surface plasmons is illustrated for Al (111).

Determination of Atomic Positions in theC(2×2)Oxygen Structure on a Nickel (100) Surface by a Dynamical Low-Energy Electron-Diffraction Method

The dynamical method of low-energy electron-diffraction intensity analysis has been applied to the system of $C(2\ifmmode\times\else\texttimes\fi{}2)$ oxygen on nickel(100). This system differs considerably both geometrically and chemically from the sodium-on-nickel system previously reported. The diffraction data are consistent with a model where oxygen sits in a layer displaced normal to the surface by $d=1.5\ifmmode\pm\else\textpm\fi{}0.1$ \AA{} from the center of the top layer of nickel atoms. Oxygen atoms sit above pyramidal hollows in the nickel layer below.