LEED Study Research Articles

Adsorption geometry of Cu( [formula omitted])-(12 × 2)-14Xe

Low-energy electron diffraction was used to study the adsorption geometry of Xe on Cu(1 1 0). At low temperatures, a higher-order commensurate (12 × 2) structure can be formed that consists of rows of Xe atoms aligned along the direction of the rows on Cu(1 1 0), and having a spacing between the rows equal to the Cu row spacing. A dynamical LEED study of this structure at 20 K indicates that the rows of Xe atoms are localized on top of the Cu substrate rows. The lateral registry of the Xe rows along the Cu rows includes 2 Xe atoms per (12 × 2) unit cell in atop positions above Cu atoms, with the others in lower-symmetry sites. The average Xe–Cu interlayer spacing is 3.3 ± 0.1 Å and there is a small degree (0.1 Å) of vertical rumple in both the overlayer and the top substrate layer. The substrate surface with adsorbed Xe has essentially the same relaxation as the clean surface. The analysis of the adatom vibrations in this structure indicates that the isotropically-averaged rms vibration amplitude is 0.2 Å for Xe, while that of the top-layer Cu atoms is 0.04 Å.

XPS and LEED study of Argon bombarded Al(1 1 1) surface

Argon ion bombardment on Al(1 1 1) leads to formation of adatoms and sub-surface bubbles. From X-ray photoemission spectroscopy study, we find that the Ar core-levels exhibit a systematic shift in binding energy and change in the line shape as function of ion incidence energy. This is related to the increase in size of the sub-surface bubbles created by argon ion bombardment where the Ar atoms are trapped. Low energy electron diffraction pattern for Al(1 1 1) exhibits broadened spots and enhanced background intensity after bombardment which is related to the formation of random steps and terraces on the surface.

An XPD and LEED study of highly strained ultrathin Ni films on Pd(1 0 0)

The epitaxial growth of ultrathin Ni films on the Pd(100) surface was studied by means of X-ray photoelectron diffraction (XPD) and LEED experiments. In excellent numerical agreement with the predictions of elasticity theory, the data indicate the formation of tetragonally strained Ni epitaxial layers, which subsequently turns into a bulk-like Ni structure as the thickness of approximately 12MLE is exceeded. This study demonstrates that LEED and XPD methodologies are rather complementary in order to have a detailed picture of the evolution of the overlayer structure in different thickness regimes.

HOLOGRAPHIC ANALYSIS OF KIKUCHI ELECTRON DIFFRACTION DATA FROM ${\rm Si}(111)(\sqrt{3}\times\sqrt{3}){\rm R}30^\circ{\rm -Al}$

Holographic analysis was applied to Kikuchi electron diffraction data collected from the [Formula: see text] structure to produce real-space images of the surface structure. Methods for calibrating the initial diffraction data are fully described. The images clearly show features caused by the Al atoms in the T4 absorption site. Measurements of the Al–Si bond length and the height of the Al above the top layer of Si agree with the values known from dynamical LEED studies.

STM studies of the N/Cu(111) system: surface structure, electron confinement and tip-induced modification

We have used scanning tunneling microscopy to examine N-terminated Cu(111) surfaces. For substaturation N coverages, elongated rectangular islands are formed, the internal structures of which are in good agreement with a model proposed in a previous LEED study. These islands sufficiently perturb the surface so as to reflect surface state electrons in the adjacent clean surface regions. This results in the observation of electron standing wave patterns. We have simultaneously observed both the electron wave patterns and atomic resolution. We have also shown that under conditions of high electric fields, the N atoms at the surface can be forced down into the second layer with Cu atoms liberated from the first layer to form single layer high islands.

Alkali Metal Modification of the Mo(100) Reconstruction

LEED studies reveal that the clean Mo(100) surface reconstruction is modified by small amounts of adsorbed K or Na in such a way that the long-range order is maintained but the surface periodicity changes continuously with coverage. The long-range order is also maintained with the adsorption of small amounts of Li, but no periodicity changes are observed. These findings are discussed in relation to driving mechanisms proposed for the clean reconstruction. Although the changes might be attributed to charge-density-wave formation in a qualitative sense, this mechanism appears unable to provide an explanation for the detailed behavior. On the other hand, it appears possible to describe the observed sequence of structures in terms of a local bonding model that includes alkali atom repulsions.

Surface structure of Si(100) with submonolayer coverages of C

We report a combined STM, LEED and AES study of Si(100) surfaces prepared by coevaporation from two MBE-sources with Si and SiC, respectively. The flux from the SiC source contains 10% C, enabling studies of deposited layers with C-concentrations in the range 0–10%. After room temperature deposition, the structures have been annealed at 600 °C to generate a c(4×4) reconstruction. This has previously been reported to contain from 0.0 to 0.5 monolayers (ML) of C. Annealing at 800 °C irreversibly transforms the c(4×4) surface to a 2×1-reconstructed surface that contains precipitates of SiC. Since only 0.07 ML of C is needed to have 98% of the surface covered with the c(4×4) reconstruction, we conclude that the c(4×4) reconstruction is impurity-induced rather than having C-atoms in well defined positions within each unit cell. The c(4×4) reconstruction is attributed to a basic structure containing buckled parallel ad-dimers, which in approximately 50% of the reconstructed unit cells is decorated with perpendicular dimers.

Combined LEED and STM study of PTCDA growth on reconstructed Au(111) and Au(100) single crystals

The growth of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) thin films on the reconstructed surfaces of Au(1 1 1) and Au(1 0 0) single crystals was investigated with low energy electron diffraction and scanning tunneling microscopy under ultrahigh vacuum conditions. The samples were prepared by means of the organic molecular beam epitaxy (OMBE) technique. Two phases with molecular arrangements resembling the (1 0 2) plane herringbone structure of the PTCDA bulk crystal are found on Au(1 1 1); one of them is observed at higher deposition rates only. Both herringbone phases grow homogeneously in large domains on top of the alternating domains of the well known (22×3) Au(1 1 1) surface reconstruction. We show that this surprising growth behavior can be explained using the point-on-line coincidence concept. On the reconstructed Au(1 0 0) surface, we observe various orientations of a herringbone phase. Again the different orientations of this phase are point-on-line coincident, this time with the almost hexagonal Au(1 0 0)hex surface reconstruction. Besides the herringbone phases we regularly observe a square phase of PTCDA on both substrate surfaces.

A tensor LEED study of an unusual cyclic hydrocarbon intermediate formed by benzene adsorption on [formula omitted

Tensor LEED has been used to determine the geometry of a p(3×1) phase formed by benzene adsorption to saturation coverage on Co(10 1 ̄ 0) at 300 K. In contrast to benzene overlayers previously subjected to quantitative surface structural analysis, adsorption occurs with the ring in a tilted geometry across the substrate close packed [12 1 ̄ 0] atomic rows in an off-centre bridge site with C s symmetry.

Ethylene hydrogenation on Pt thin films on Ni(1 0 0) surface

From XPS and LEED studies, a layer-by-layer growth of Pt thin films is indicated up to 8 ML coverage on Ni(1 0 0) surface while a (1×1) structure changes to a disordered structure above 2 ML of Pt film thickness. The 1 ML Pt overlayer, as a model (1 0 0) surface of Pt–Ni alloy single crystal, shows a considerable d-band narrowing probably due to the formation of an interface state with the Ni substrate. This induces weakly adsorbed hydrogen atoms and ethylene molecules and, as a result, the maximum evolution of ethane is observed around 200 K. A wide d-band extending to −6 eV binding energy is developed for Pt 6 ML film characteristic of bulk Pt electronic states, resulting in a similar surface chemistry of ethylene as on Pt(1 1 1) surface.

The Defect-Mediated Mechanism of the High-Temperature Oscillatory NO + CO Reaction on Pt{100} As Revealed by Real-Time in-situ Vibrational Spectroscopy

At a partial pressure of 10-7 mbar, the reaction between NO and CO on Pt{100} exhibits oscillatory behavior in two distinct temperature regimes. Oscillations in the high-temperature regime (380−411 K) are accompanied by a phase transition from the (1 × 1) surface to the hex surface. Using infrared reflection−absorption spectroscopy (IRAS) and a novel method of data acquisition, we show that during the oscillation cycle, the only molecular species present on the surface is atop CO, adsorbed on the (1 × 1) phase at very low coverage (∼0.03−0.007 ML). Furthermore, the minimum in the CO coverage coincides with the maximum reaction rate, as measured by the partial pressure of CO2. From a comparison of these data with previously published LEED and PEEM studies of the same system, it can be seen that the high-reaction-rate branch of the oscillatory cycle coincides with the maximum area of the surface in the hex phase. This is in contrast to previously proposed mechanisms, which assume that the (1 × 1) surface is...

Low-energy electron diffraction study of the surface geometry of Ni(100)–(3×3)-K+4O

A dynamical LEED study of the (3×3) structure which forms upon coadsorption of K and O on Ni(100) indicates that this structure is comprised of one K atom and four O atoms per (3×3) unit cell. The K atom is adsorbed in a substitutional site, and surrounded by four O atoms in hollow sites. The four O atoms are displaced by 0.35±0.08 Å from the centers of the hollows, and the average ONi distance is 1.92±0.17 Å. The KO distance is 2.93±0.15 Å and the KNi distance is 3.33±0.12 Å. We propose that the primary driving force for the creation of the vacancies is the reduction of energy when the K atoms move closer to the surface. At a later stage of oxidation, these vacancies may facilitate the formation of NiO by helping to alleviate the strain in the lattice mismatch between the NiO structure and Ni(100).

XPS, LEED and STM study of thin oxide films formed on Cr(110)

The growth, thickness, composition and structure of chromium oxide thin films formed by exposing Cr(110) single-crystal surfaces to gaseous oxygen at 300 and 625 K have been investigated by XPS, LEED and STM measurements. The oxide films formed at the two temperatures are significantly different. At 300 K, a granular and non-crystalline oxide is formed, which grows with a constant ∼Cr 2O 3 stoichiometry up to a limiting thickness of 0.9 nm. The film is hydrated with a water content of 10–20%, which decreases upon annealing. Nuclei of oxide with a lateral dimension of ∼0.7 nm and a height of ∼0.2 nm have been observed in the nucleation stage. These nuclei grow predominantly laterally and coalesce to fully cover the substrate surface prior to the thickening stage. At 625 K, a first stage of oxygen adsorption is observed in which stripes 1.5–2.3 nm wide and parallel to the Cr[001] direction are observed after annealing in UHV. They correspond to narrow segments of mixed and close-packed planes of O atoms and ions having a geometry and orientation similar to those of the anions planes in the oxide crystals. Rows of adatoms, possibly Cr 3+ ions of oxide nuclei, are observed above the stripes. Thickening at 625 K leads to the formation of a non-crystalline oxide, which grows up to a limiting thickness of 4.6 nm. The presence of Cr 3+ vacancies related to a significant cation transport through the oxide film in this temperature regime is detected. After UHV annealing at 825 or 925 K, the film is anhydrous. The Cr 3+ vacancies are accumulated at the metal/oxide film interface. The film crystallizes in epitaxy with the substrate in the following orientation: α-Cr 2O 3(0001)‖Cr(110) and α-Cr 2O 3[213̄0]‖Cr[001]. The STM measurements of the unit cell of the α-Cr 2O 3(0001) surface are consistent with a termination by a cation plane and show three tunneling sites assigned to the various possible locations of the Cr 3+ ions at room temperature due to surface diffusion.

LEED study of Ni (1 0 0) and (1 1 1) surface damage caused by Ar + ion bombardment with low energy and small doses

Surface damage formation on the Ni (1 0 0) and Ni (1 1 1) surfaces induced by Ar + ion bombardment at low energy (0.1–1.2 keV) and small doses (<5×10 15 ion/cm 2) was studied with LEED. The degradation of the diffraction spot intensities was measured directly during ion bombardment. Using the procedure of Jacobson and Wehner for description of the exponential intensity/dose function, the mean area of damage per incident ion was determined. The experimental results have confirmed that partial annealing of the surface damage takes place at room temperature during ion bombardment. An exponential relationship observed between point defect concentration and annealing time at higher temperature indicated a first-order reaction. The activation energies for Ni (1 0 0) and Ni (1 1 1) surfaces were determined.

Structural aspects of chemisorption at Cu(110) revealed at the atomic level

We illustrate the impact that scanning tunnelling microscopy (STM) has made on our understanding of chemisorption and catalysis at metal surfaces at the atomic level by considering four examples where information from surface sensitive techniques was also available. The advantages of STM and the limitations of some of the other experimental methods are discussed. (1) Through a combination of STM and X-ray photoelectron spectroscopy (XPS) we have established that a number of distinct oxygen chemisorbed states can exist at a Cu(110) surface. These are metastable and temperature dependent. Furthermore, the presence of chemisorbed sulphur is shown to promote a specific oxygen state – isolated oxygen strings – which are likely to be more chemically reactive than the oxygen overlayer present at Cu(110). In this sense the sulphur is a structural promoter. (2) The oxidation of ammonia under ammonia-rich conditions results in the growth of imide (NH) strings at a Cu(110) surface and this has been followed quantitatively by STM. The reactive surface oxygen state participates in an oxydehydrogenation reaction generating NH-radical species which undergo surface diffusion and result in string growth. (3) Nitric oxide dissociates at Cu(110) to generate a two-phase system of chemisorbed nitrogen and oxygen adatoms. The oxygen is present in a well ordered (2 × 1) structure and the nitrogen in a (3 × 2) structure. The limitations of an earlier LEED study are discussed. (4) Structural aspects of chemisorbed sulphur generated by the dissociative chemisorption of hydrogen sulphide and methyl mercaptan are discussed. In the latter case carbon–sulphur bond cleavage results in the formation of a sulphide overlayer at 450 K with complete removal of carbon as desorbed hydrocarbons. Various sulphur structures have been delineated over a wide temperature range.

Theoretical study of the termination of the Fe 3O 4 (111) surface

Ab-initio periodic Hartree–Fock calculations for the structure of the (111) surface of Fe 3O 4 (magnetite) are presented. The slabs that are derived by an ideal bulk truncation that leaves one or two iron monolayers outside oxygen layers are found to be more stable than others since they preserve most of the coordination of the surface atoms. The stability of the slabs that represent the surface layers depends on the overall composition, specifically on the deviation from stoichiometry, and on the dipole moment perpendicular to the surface. The symmetrical slab with the layer sequence Fe 2O 4Fe 3O 4Fe 2, terminated on each side by iron bilayers, is the best compromise since symmetry insures the neutrality of the dipole moment. This slab is oxygen-deficient. The energetically preferred structure relaxes so that one of the two outermost iron layers moves toward the slab center plane, exchanging sequence with the oxygen layer. The slab with the layer sequence FeO 4Fe 3O 4Fe, which is also symmetric, is terminated by iron single monolayers, would represent an excessive oxidation of the iron atoms. This slab may be reduced by hydrogenation; it is then strongly stabilized and the vertical displacement of the oxygen atoms agrees with the structure determined by LEED (the LEED study would not have detected hydrogen).

On the structural transformation of the reconstructed Pt(100) in electrolyte solutions

It is demonstrated by a combined electrochemical, LEED, RHEED and AES study that the reconstructed Pt(100) surface is stable in aqueous electrolytes under certain potential conditions. The reconstructed Pt(100) surface has higher stability ranges than Au(100) in both 0.01M H2SO4 (0.6V) and 0.01M HClO4 (1.1V). The reconstructed Au(100) surface is uniformly transformed into the (1×1) phase in both H2SO4 and HClO4 solutions, whereas with Pt(100) this happens only in H2SO4 at 1.0V. The latter surface is not uniformly deconstructed in 0.01M HClO4, even up to 1.1V. The stability range is drastically reduced in the presence of Cl− (0.01mM), and the (5×1) phase does not survive in a 0.1mM HCl solution. Electrochemical annealing such as at Au in the presence of Cl− was not found at Pt(100)-(1×1) after lifting of reconstruction, whereas a well-ordered array of <011> steps/rows with a mean spacing of 2.8nm (2×5-fold periodicity of a reconstructed Pt(100)) developed. This is an intermediate structure formed by the Pt phase transition (5×1)→(1×1) in electrolytes.

Co ultra-thin films on Pt(111) and Co-Pt alloying: a LEED, Auger and synchrotron x-ray diffraction study

Co ultra-thin films deposited on Pt(111) are analysed in detail, using LEED, Auger spectroscopy and x-ray diffraction. The growth is quasi layer-by-layer up to about 3 ML. Co grows as islands, in incoherent epitaxy with the Pt substrate; the corresponding satellites are observed even for coverages as low as 0.12 ML. The Co in-plane parameter is very close to its own bulk value. For thicker films, deposited at room temperature, face centred cubic (fcc), twin fcc and hexagonal close packed (hcp) fractions are present. Using x-ray diffraction, we followed in real-time the transformations of the deposited film upon annealing. The role of the Co film structure (which depends on the film thickness) is predominant. For `thick' films (6 ML) a sharp transition occurs around 670 K: below this temperature, the film is mainly hcp so that it allows little Pt incorporation, while above it turns to a very homogeneous fcc alloy of composition close to Pt60Co40. Segregation phenomena were analysed and found similar to those occurring at the surface of the corresponding bulk alloys.

LEED AND STM STUDY OF Cs ON Cu(211)

A low-temperature (25 K) STM study of Cs adsorption on Cu(211) indicates that Cs forms variable-density structures which align along the step edges of the Cu(211) surface. The density of the overlayer increases with Cs coverage, forming a quasihexagonal c(2×2) structure at a coverage of 0.17. A dynamical LEED study of that structure at 130 K indicates that the Cs atoms are adsorbed on top of the Cu atoms in the center of the terraces, with a Cs–Cu nearest-neighbor distance of 3.56± 0.04 Å. This structure is accompanied by a significant rumpling of the Cu(211) surface.

Interface and Schottky barrier formation in the Gd/GaAs(110) system

The interface formation upon deposition of Gd overlayers onto n-type GaAs(110) substrates was investigated at room and low (20 K, 100 K) temperatures by photoemission, UPS, AES, and LEED. Binding energy shifts of Ga 3 d and As 3 d levels at 300 K and 100 K were observed starting from the minimal Gd coverage (∼0.03 ML). It was found that band bending saturates only after metallization of the interface at 1–2 ML. It is argued that two different mechanisms are involved in the Schottky-barrier formation: Fermi level pinning by defect states and metal-induced gap states. The growth of Gd overlayer at low and room temperatures leads to reacted components showing up in Ga 3 d and As 3 d photoemission spectra at coverages ≥0.03 ML. LEED studies at T=20 300 K showed disordered interface structures at submonolayer and high coverage ranges. Temperature-induced changes in the interface formation were observed as well. At 300 K the overlayer is cluster-like even at 0.03 ML, while at 100 K and 20 K it is close to Stranski–Krastanov and layer-by-layer types, respectively.