Complex Low-energy Electron Diffraction Pattern Research Articles

New atomic-scale insights into the I/Ni(100) system: phase transitions and growth of an atomically thin NiI2 film.

We use a traditional surface science approach to create and study an atomically thin NiI2 film (a promising two-dimensional ferromagnetic material) formed on nickel substrate as a result of molecular iodine adsorption. The I/Ni(100) system was examined with scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and density functional theory calculations. We found out that the iodine adsorption on Ni(100) at 300 K leads to the formation of non-equilibrium phases, whereas the adsorption at elevated temperature (≥390 K) gives rise to the thermodynamically stable phases. In both cases, a simple p(2 × 2) structure is formed at 0.25 ML. As more iodine is adsorbed at 300 K, the p(2 × 2) phase is replaced by the small coexisting domains of c(3 × 2) and c(6 × 2) phases both corresponding to the coverage of 0.33 ML, while adsorption at elevated temperature results in the formation of only one c(3 × 2) phase. At further iodine adsorption the c(3 × 2) phase transforms into the c(5 × 2) one, while the c(6 × 2) phase - into the one both corresponding to the coverage of 0.40 ML. In addition to simple chemisorbed phases, a new shifted-row reconstruction of Ni(100) induced by iodine adsorption was discovered. At coverages exceeding 0.40 ML, we observed complex LEED patterns and superstructures in STM and assigned them to specific surface reconstructions. We also found that prolonged iodine dosing leads to the nucleation of nickel iodide islands and the growth of a 2D atomically thin iodide film partially exfoliated from the substrate.

Oxygen Adsorption on the Fe(110) Surface: The Old System – New Structures

Adsorption of oxygen on the (110) surface of epitaxial iron films on tungsten (110) was studied using low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), and Auger electron spectroscopy within an exposure range of 0–300 Langmuir (L). Selected oxygen adsorption structures on Fe(110) reported in the literature were critically compared and revised in reference to the present study. The initial adsorption of 1/4 oxygen monolayer resulting in the commonly observed (2 × 2) structure was followed by a structure that was frequently termed as (3 × 1). Its complex LEED pattern was ultimately resolved and interpreted as originating from two structural domains of a large oblique unit cell (eight times larger than the substrate unit cell) and 3/8 oxygen coverage. A new (3 × 2) structure was identified at a coverage of 2/3. The domain interpretation of last two structures was verified by LEEM and confirmed by density functional theory calculations. The onset of oxygen–iron bonding formation was recognized by the change in the symmetry of the LEED pattern and the shape of the iron AES signal. Finally, the formation of an iron oxide FeO(111) monolayer was evidenced at the oxygen exposure of ∼300 L.

Formation of graphene on SiC(0001¯) surfaces in disilane and neon environments

The formation of graphene on the SiC(000-1) surface (the C-face of the {0001} surfaces) has been studied, utilizing both disilane and neon environments. In both cases, the interface between the graphene and the SiC is found to be different than for graphene formation in vacuum. A complex low-energy electron diffraction pattern with rt(43) x rt(43)-R\pm7.6{\deg} symmetry is found to form at the interface. An interface layer consisting essentially of graphene is observed, and it is argued that the manner in which this layer covalently bonds to the underlying SiC produces the rt(43) x rt(43)-R\pm7.6{\deg} structure [i.e. analogous to the 6rt(3) x 6rt(3)-R30{\deg} "buffer layer" that forms on the SiC(0001) surface (the Si-face)]. Oxidation of the surface is found to modify (eliminate) the rt(43) x rt(43)-R\pm7.6{\deg} structure, which is interpreted in the same manner as the known "decoupling" that occurs for the Si-face buffer layer.

STEP REARRANGEMENT UPON LOW PRESSURE OXIDATION OF THE Pt3Ti(510) SURFACE: A STUDY BY SCANNING TUNNELING MICROSCOPY

The oxidation of the single crystal stepped Pt 3 Ti (510) surface at oxygen pressures below 10-5 mbar and at a temperature of 770 K was studied by means of X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) and low energy electron diffraction (LEED). Scanning tunneling microscopy (STM) was used to follow the evolution of the surface morphology on the atomic scale. The clean surface studied in ultrahigh vacuum conditions was found by LEIS to be composed of platinum only in the outermost surface plane. LEED and STM indicate that the clean Pt 3 Ti (510) surface consists of (100) terraces separated by double atomic steps. The exposure of the clean surface to oxygen at pressures in the range of 10-7–10-5 mbar leads to the growth of a titanium oxide layer (with a composition close to TiO) which covers completely the substrate surface. The TiO film has long range order and exhibits complex LEED patterns. The STM measurements indicate that the ordered array of steps is kept in the early stages of the oxide film growth, whereas a change of the step morphology and step orientation is observed during the oxidation process.

Photoelectron spectroscopy study of Mo 2C(0001)

The (0001) surface of α-Mo 2C has been studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy and angle-resolved photoelectron spectroscopy. When the ion-bombarded (1 keV of Ar +) surface is annealed at 1600 K, a C-rich MoC-like surface is formed. The surface gives a complex LEED pattern composed of (1×1) orthorhombic spots, (2×2)R15° spots, and (4/3×4/3)R5° spots. Photon energy dependence of the normal-emission spectra is measured and it is found that the spectrum in the valence band region consists of Mo 4d band, Mo 4d–C 2p hybrid band, and C 2s band. Any peaks associated with surface states are not found.

Photoelectron diffraction determination of the structure of ultrathin vanadium films on Cu(001)

X-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED) have been used to study the structural properties of V thin films on Cu(001). For room-temperature growth, submonolayer coverages result in $(1\ifmmode\times\else\texttimes\fi{}1)$ LEED patterns that evolve to exhibit very diffuse $(2\ifmmode\times\else\texttimes\fi{}1)$ structure at approximately 1 monolayer coverage. We do not observe any V forward-focusing enhancements for V films that exhibit either the $(1\ifmmode\times\else\texttimes\fi{}1)$ or $(2\ifmmode\times\else\texttimes\fi{}1)$ structure, suggesting that these structures are limited to the first 1--2 vanadium layers. At coverages above 1 monolayer, the V films display complex LEED patterns characteristic of four bcc(110) domains. This structure persists to V coverages as high as 100 ML, and the LEED and XPD angular scans suggest that V in these films retain the bulk V lattice constant. These results have important ramifications for predictions of magnetic order in vanadium thin films that typically assume pseudomorphic growth.

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.

The growth of magnesium chloride monolayer and multilayer structures on different transition metal (Pt, Pd, Rh) single crystals with varied orientations

The growth of magnesium chloride (MgCl 2) monolayer and multilayer structures on a variety of transition metal single crystal surfaces [Pd(111) and (100), Pt(111) and (100)-hex and Rh(111)] has been studied. Low energy electron diffraction (LEED) has been employed as a probe of the long range order at both monolayer and multilayer coverages while temperature programmed desorption (TPD) provided information on the strength of adsorbate-substrate interactions. LEED results indicated that MgCl 2 growth at both monolayer and multilayer coverages is dependent upon the magnitude of the monolayer adsorbate-substrate interactions which are themselves a function of the underlying single crystal template. LEED results indicated that, on each transition metal surface studied, the MgCl 2 monolayer structures act as a template for subsequent multilayer structures. On Pt(111) and Pd(111), identical, well ordered LEED structures at both monolayer and multilayer coverages, were produced. The commensurate (4 × 4) and (√13 × √13)R13.9° monolayer structures were associated with expansions and contractions in the MgCl 2 unit cell dimension, respectively. These structural changes were correlated with the strength of the monolayer adsorbate-substrate interaction, which on these surfaces lead to distinct monolayer TPD features. In contrast, MgCl 2 growth on Pt(100)-hex and Pd(100) exhibited more complex LEED patterns associated with various degrees of ordered and rotationally disordered domains. This was attributed to a reduction in the magnitude of monolayer adsorbate-substrate interactions, correlated by TPD measurements. Experiments on Rh(111) indicate that the factors responsible for ordered MgCl 2 growth on this surface differ from those observed on Pt and Pd single crystals. In general, better matching between the substrate and adsorbate unit cell dimensions leads to enhanced adsorbate-substrate interactions which are responsible for the degree of ordering at monolayer and subsequent multilayer coverages.

Formation of Ordered, Nonreconstructed CdTe ( 111 ) Surfaces Electrochemically: Studies by LEED and Auger Electron Spectroscopy

Surface treatments for single crystals have been examined to determine procedures which result in the formation of ordered stoichiometric surfaces, reflecting a truncation of the bulk crystal structure. Nonreconstructed surfaces were obtained in solution using a etching solution followed in sequence by a sulfuric acid rinse and electrochemical reduction. The etching procedure leaves a disordered Te layer and etch products such as . By rinsing in , the can be removed leaving only the Te layer. Subsequent electrochemical reduction converts the Te layer to telluride ions, which are soluble. Low energy electron diffraction (LEED) and Auger electron spectroscopy were used to characterize the surfaces. Ordered (1X1) LEED patterns and stoichiometric surfaces were observed following the procedure just described. Variations in the intensity distributions between spots in the (1X1) LEED patterns for the (111)Cd and (111)Te surfaces were significant enough to provide a clear means of differentiating the two faces. Vacuum studies where ion bombardment has been used to clean surfaces generally result in damaged surfaces, which can be repaired by subsequent annealing. The (111)Cd surface following annealing displays a (2X2) LEED pattern, the result of a surface reconstruction. The (111)Te displays a complex LEED pattern indicative of the formation of pyramid‐shaped facets. Another etching solution consisting of , , and was also investigated but proved too severe; the resulting Te layer was thicker than the one formed in and was difficult to remove. Surface heterogeneity appears to have been introduced by this etching process as well.

The interaction of oxygen and platinum on W(110)

Low-energy electron diffraction (LEED) and Auger electron spectroscopy have been used to characterize Pt overlayers coadsorbed with oxygen on W(110). Previous work has shown that multilayers of Pt alone will cluster into three-dimensional crystallites of bulk Pt when annealed, leaving a pseudomorphic monolayer of Pt covering the W(110) surface. When oxygen is present on the surface, the monolayer of Pt is no longer stable. Annealing a W(110) surface on which both Pt and oxygen are adsorbed causes nearly all of the Pt to agglomerate into clusters as the oxygen largely replaces the monolayer on the surface. The presence of the Pt clusters reduces the temperature required for desorption of oxygen, however, and the Pt spreads across the W surface as the oxygen is removed. The complex LEED patterns observed when coadsorbed oxygen and Pt are annealed and the changes in the temperature required for desorption of oxygen suggest that there is an interaction between the two adsorbates and that there is not complete phase separation; i.e., some Pt remains dispersed on the W surface within the oxygen phase.

Adsorption and reaction of chlorine with the titanium(0001) surface

The authors have studied the adsorption of chlorine on the Ti(0001) surface using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Chlorine overlayers were generated by using an electrochemical source of molecular chlorine at temperatures ranging from 30 to 600/sup 0/C. Adsorption is dissociative with an approximately constant sticking coefficient at low coverages. The adsorbed chlorine is sensitive to electron beam stimulated desorption (ESD) but does not thermally desorb below 750/sup 0/C. They did not observe any ordered LEED patterns unless a high-coverage surface was annealed to a temperature of 620/sup 0/C. The resulting complex LEED pattern suggests that the adsorbed chlorine forms a coincidence lattice on the titanium surface. The data is consistent with the formation of a pseudo-halide surface layer related to TiCl/sub 3/.

A new system for LEED intensity measurements using a real-time digital video processor

A new system for low-energy electron diffraction (LEED) intensity measurements has been developed using a video camera and digital processing of the video signal. Complete two-dimensional LEED patterns are digitized in real time with high resolution using a commercial video processor. Intensity-voltage (I-V) data on all beams in complex LEED patterns are collected simultaneously. A microcomputer analysis program automatically tracks the diffraction beams as a function of energy and calculates beam position, size, and integrated intensity, including a local background correction. Using a video tape recorder for intermediate data storage, a complete set of I-V curves can be collected in less than 100 s.

Initial stages of hydride formation: Hydrogen on Ce(001).

The interaction of hydrogen with a Ce(001) single-crystal surface has been investigated with use of angle-resolved uv photoemission spectroscopy (ARUPS), electron-energy-loss spectroscopy (ELS), and low-energy electron diffraction (LEED). Hydrogen uptake from the gas phase results in a cerium-hydrogen solid-solution phase, which is evidenced by a 4.2-eV structure in UPS and a 3-eV H-derived interband transition in ELS. After larger H/sub 2/ exposures (>500 L) a low-energy plasmon feature at 1.6-eV loss energy signals the formation of a surface hydride phase of dihydridelike stoichiometry (1 langmuir (L)equivalent10/sup -6/ TorrXc). The surface disorders as a result of hydride formation. It is suggested that hydrogen occupies tetrahedral sites in the solid solution and dihydridelike phases. Annealing of H/sub 2/-exposed Ce surfaces to 300--400XdeC induces the population of octahedral sites by hydrogen in an activated process, and a corresponding UPS band is detected at 3.4 eV. A H-induced hexagonal reconstruction of the Ce(001) surface with a complex LEED pattern is observed after annealing to >400/sup 0/C, and the characteristic features of the UPS of this reconstructed surface suggest hydrogen in both tetrahedral and octahedral sites near the surface.

The interaction of oxygen with Cu(111): Adsorption, incorporation and reconstruction

The room temperature interaction of oxygen with Cu(111) surfaces was studied at exposures of up to 10 6 L by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and surface potential (SP) measurements. Three distinct stages in the interaction were discernible from both AES and SP measurements, the first two being adsorption and incorporation of oxygen into the Cu lattice. The final stage was also characterised by the appearance of a |− 3 1 2 2 | structure in LEED, and is interpreted in terms of a surface reconstruction, the LEED results being fitted to three rotational domains of a distorted Cu(100)−( 2 × 2 ) R45° structure on the Cu(111) substrate. CO reduction of the oxidised surface produced somewhat different results on re-oxidation marked by the absence of the third stage SP change and the appearance of a complex LEED pattern at high exposures.

LEED and electron energy loss measurements of iron deposited on Ni(100)

The adsorption and growth of iron on Ni(100) has been studied using the combined techniques of Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS). Initially iron grows by a layer-by-layer mechanism for the first few layers. In this stage of growth, fcc iron is produced. High iron coverages result in the observation of complex LEED patterns with satellites around the main (1 × 1) diffraction spots. This is due to the formation of bcc Fe(110) crystallites arranged in domains with different orientations. The presence of fcc iron crystallites was also inferred for the thick films. EELS studies show the presence of three stages in the growth of iron on Ni(100): low-coverage, film-like and bulk-like. The changes in the EELS spectra of the Fe Ni(100) system with iron coverage are used to re-interpret some of the electron energy loss peaks of nickel and iron. The 9.7 eV peak of clean nickel is believed to be due to interband transitions while the 27.3 eV peak is identified as volume plasmons, as is the peak at 16.3 eV of iron.

Leed and electron energy loss measurements of iron deposited on Ni(100)

The adsorption and growth of iron on Ni(100) has been studied using the combined techniques of Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS). Initially iron grows by a layer-by-layer mechanism for the first few layers. In this stage of growth, fcc iron is produced. High iron coverages result in the observation of complex LEED patterns with satellites around the main (1 × 1) diffraction spots. This is due to the formation of bcc Fe(110) crystallites arranged in domains with different orientations. The presence of fcc iron crystallites was also inferred for the thick films. EELS studies show the presence of three stages in the growth of iron on Ni(100): low-coverage, film-like and bulk-like. The changes in the EELS spectra of the FeNi(100) system with iron coverage are used to re-interpret some of the electron energy loss peaks of nickel and iron. The 9.7 eV peak of clean nickel is believed to be due to interband transitions while the 27.3 eV peak is identified as volume plasmons, as is the peak at 16.3 eV of iron.

Partial phase diagram of the two-dimensional Ni(111)-S system from LEED and Auger electron spectroscopy

Using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) the ordered phases in the two-dimensional Ni(111)-S system have been examined. The results are presented in terms of a “temperature versus coverage” phase diagram. At relatively low coverages a well-ordered Ni(111) p(2 × 2)S structure was observed. This undergoes a reversible transformation as temperature is cycled. In an intermediate narrow coverage range, a two-phase structure was observed for the first time in this system. This structure consists of Ni(111) p(2 × 2)S and a complex structure similar to the one reported by other investigators. The temperatures of these reversible structural transformations were determined. At high coverages only a complex LEED pattern was observed. The structure producing this pattern undergoes a reversible transformation at high temperatures (i.e. ≳ 570 K).

Interaction of nitrogen with iron surfaces: I. Fe(100) and Fe(111)

The adsorption of N 2 on clean Fe(100) and Fe(111) single-crystal surfaces was studied in the temperature range 140–1000 K by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), thermal-desorption spectroscopy (TDS) and work-function measurements (Δφ). Above room temperature, only dissociative adsorption takes place, leading to increases in work function of 0.33 and 0.25 eV on Fe(100) and (111), respectively, and is mainly identified with UPS by the appearance of a chemisorption level derived from N2p-states at about 5 eV below the Fermi level. At 500 K, the initial rate of adsorption is faster by about a factor of 20 on the (111) plane, the initial sticking coefficient, however, being very small (10 −7–10 −6) on both surfaces. The initial activation energies for adsorption are about 5 and 0 kcal/mole on Fe(100) and Fe(111), respectively, and increase with coverage in both cases. The mean activation energies for desorption were estimated to be 58(100) and 51 kcal/mole (111), so that nearly equal values for the strength of the MN bond result. A simple ordered c2 × 2 structure is formed on Fe(100) which is completed at θ = 0.5 and for which a model is proposed wherein the N atoms are located in fourfold sites on the unreconstructed Fe(100) surface, leading to a configuration similar to that in the (002) plane of (fcc) Fe 4N. Several independent observations strongly indicate that the Fe(111) surface reconstructs. A whole series of complex LEED patterns (depending on N bulk and surface concentrations and on the conditions of heat treatment) is formed with this plane which are interpreted in terms of the formation of hexagonal layers of “surface nitrides” which have a thickness of about 2 atomic layers and most probably are related to the (111) plane of Fe 4N. Desorption of N 2 (being found to be a first-order rate process) is regarded as equivalent to the decomposition of the “surface nitrides.” The close similarity to the kinetics of decomposition of (bulk) Fe 4N indicates identical mechanisms for both processes. Although the bulk solubility of N is very small under the chosen experimental conditions, this process interferes with the adsorption and desorption measurements and was analyzed in some detail, mainly by 14 N 15 N isotopic exchange. Evidence for the existence of a weakly bound (probably molecular) species was found with Fe(111) only at the lowest temperatures (140 K) and under a steady-state pressure of 4 × 10 −4 Torr of N 2. This species causes a decrease in the work function and is rapidly pumped off. Its adsorption energy is estimated to be in the range between 5 and 10 kcal/mole.

Adsorption of oxygen on W(100): Adsorption kinetics and structure

The adsorption of oxygen on W(100) single crystal surfaces is studied by Auger electron spectroscopy (AES), flash desorption, low-energy electron diffraction (LEED) and retarding field work function measurements with the aim of obtaining a better understanding of the adsorption kinetics and of the structures of the adsorbed layer. The AES results reveal step-wise changes of the sticking coefficients in the coverage range 0 to 1, and activated adsorption at higher coverages. Upon room temperature adsorption a series of complex LEED patterns is observed. In layers adsorbed at 1050 K and cooled to room temperature, the well-known p(2 × 1) structure is the first ordered structure observed. This structure shows a reversible order-disorder transition between 700 K and 1000 K and is characterized by a work function which is lower than that of the clean surface. Heating room temperature adsorbates changes their structure irreversibly. At temperatures below 750 K some new structures are observed. Combining the results obtained in this study with other published work leads to a considerable revision of the previously accepted model of the adsorption of oxygen on W(100).