X-ray Photoelectron Diffraction Data Research Articles

The growth of iron oxide films on Pt(111): a combined XPD, STM, and LEED study

The three complementary surface structure probes of X-ray photoelectron diffraction (XPD), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED) have been combined in a single instrument. This experimental system has been utilized to study the growth of iron oxide films on Pt(111) over the thickness range from 0.75 to 3.00 monolayers (ML). Each film was formed by first depositing an overlayer of pure Fe with a certain coverage in ML (ranging from 0.75 ML to 3.00 ML) and then thermally oxidizing the Fe at a temperature of 980 K and in an oxygen pressure of 4×10 −6 Torr. For films up to ∼1 ML in thickness, a bilayer of Fe and O similar to those in bulk FeO parallel to a (111) plane formed. In agreement with a prior STM and LEED study by Galloway et al., we find this bilayer to be an incommensurate oxide film forming a lateral superlattice or Moiré structure with short- and long-range periodicities of ∼3.1 and 26.0 Å. From the XPD data, in addition, it can be concluded that the topmost oxygen layer is highly relaxed inward by ∼0.6 Å as compared to the bulk FeO (111) interplanar spacing, and that the stacking of the topmost O atoms with respect to the underlying Pt is dominated by one of two structurally very similar possibilities. It is furthermore necessary to consider interactions over at least four atomic layers (O, Fe, and the first two Pt layers) to explain this dominance of one stacking type. For thicker iron oxide films from 1.25 to 3.0 ML, the growth mode is essentially Stranski–Krastanov: iron oxide islands form on top of the FeO(111) bilayer mentioned above. For iron oxide films of 3.0 ML thickness, X-ray photoelectron spectroscopy (XPS) yields an Fe 2p 3/2 binding energy and an Fe:O stoichiometry consistent with the presence of Fe 3O 4. XPD data further prove this overlayer to be Fe 3O 4(111)-magnetite in two almost equally populated domains with a 180° rotation between them. The structural parameters for this Fe 3O 4 overlayer generally agree with those of a previous LEED study, except that we do not find a terminating partial monolayer of Fe and arrive at a significant difference in the first Fe–O interplanar spacing. Overall, this work demonstrates the considerable benefits to be derived by using this particular set of complementary surface structure probes in such epitaxial growth studies.

Direct structure analysis of W(110)-(1×1)-O by full solid-angle X-ray photoelectron diffraction with chemical-state resolution

The atomic structure of the W(110)-(1×1)-O surface with an additional (1×12) periodicity has been determined using full solid-angle X-ray photoelectron diffraction (XPD) patterns with chemical-state resolution. The W 4f binding energy of atoms directly bonded to oxygen is 0.73 eV higher than that of “bulk” W atoms. The full solid-angle XPD data for this shifted peak show strong forward focusing peaks, making it possible to estimate the adsorbate position directly. The oxygen is found to be in a triply-coordinated site, with a height of about 1.1 Å above the first-layer W atoms, and a lateral distance between the oxygen atom and the top of the triangle of W atoms of l=1.5 Å, which is shifted from the centered position of l=1.67 Å. This position of the oxygen atoms was then used as a starting point for a more refined structure analysis by fitting theoretically calculated patterns to the observed XPD data. This gives a more precise position of z=0.84 Å and l=1.52 Å, in good agreement with the XPD estimate.

Evidence for the suppression of incident beam effects in Auger electron diffraction

Auger electron diffraction (AED) of the Cu(100) surface has been studied through the anisotropy of the elastic backdiffused beam electrons, the L 2,3M 4,5M 4,5 (LVV) and the M 2,3M 4,5M 4,5 (MVV) transitions in polar scan along the two main directions [001], [011] and in azimuth scan at normal emission. The intensity anisotropies of the low and high kinetic energy Auger lines are in antiphase to each other as in experiments in which these transitions are excited by X-ray photons. This behaviour has been exploited to single out the origin of the physical mechanisms accompanying the diffraction of the emitted electrons. Incident beam effects appear to be sizeable only when the collection of the AED spectra are made with an angle integrating electron analyser (cylindrical mirror analyser or low electron energy diffraction apparatus), but they appear negligible when electron collection is performed through a small solid-angle detector. The conclusions reached by our measurements are supported by good agreement with experimental and theoretical X-ray photoelectron diffraction data and demonstrate that, when the incident beam energy is sufficiently higher than the kinetic energy of the Auger electron detected, the influence of the incident beam on AED is negligible.

Structure and composition of the titanium oxide layers formed by low-pressure oxidation of the Ni 94Ti 6(110) surface

The structure and composition of the titanium oxide phases obtained by low-pressure oxidation at ca 800 K of the Ni 94Ti 6(110) surface have been investigated by a combination of surface sensitive techniques. We found that upon oxygen exposure in these conditions layers of oxide consisting mainly of TiO 2 grow on the alloy surface. The TiO 2 phase formed in the very early stages of oxidation has a quasi-hexagonal unit cell, as shown by low-energy electron diffraction, and its structure has no equivalent in any of the stable phases of TiO 2. The structure of this ultrathin oxide film consists of a layer of titanium atoms between two atomic planes of oxygen atoms. Upon increasing the oxygen exposure, islands of epitaxial rutile (110) form on the surface, as shown by the X-ray photoelectron diffraction data.

High-resolution inverted x-ray photoelectron diffraction studies of Si(100)

An inversion of the x-ray photoelectron diffraction (XPD) experiment has been performed in which incident electrons are diffracted to reach different sites within a crystal with differing relative probabilities reflecting those for escape of photoelectrons from the same sites in the corresponding XPD experiment. The diffracted-electron flux arriving at each site was monitored by using a Si(Li) detector to measure the intensity of the characteristic x-ray emission following core ionization by the incident electrons. This new technique, termed `inverted' x-ray photoelectron diffraction (iXPD), has been initially applied to Si(100) for rotation about , and the expected close parallels between XPD and iXPD data have been observed. Although the intensity of the x-ray signal increased rapidly as the incident-beam energy was increased beyond the threshold value for core ionization, the extent of the angular anisotropy correspondingly decreased, and there was little advantage in terms of signal:noise ratio in the iXPD pattern in increasing the energy to more than 10 - 15 eV above the threshold. The reduction in anisotropy resulted from a progressive degradation of the directional integrity of the electron beam by inelastic energy-loss processes. The angular resolution in iXPD is governed by the incident-beam divergence, which can easily be as small as , much less than the angular acceptance of typical electron energy analysers. High-resolution iXPD data taken above the threshold showed up to 38% angular anisotropy, much more than for conventional XPD at angular resolution, while iXPD data taken as much as 15 - 30 eV above the threshold still revealed splittings of all of the principal forward-scattering maxima of . Similar splittings have been reported for high-resolution XPD of other systems, but not for Si(100). Their origins are briefly discussed.

Stabilization of a face-centered-cubic Mn structure with the Ag lattice parameter

We have studied the structure of epitaxial Mn sandwiches made of ∼ 1 monolayer (ML) Ag/3 ML Mn/Ag(100) by using photoemission methods and low-energy electron diffraction (LEED). Epitaxial films of 3 ML Mn grown on Ag(100) at room temperature (RT) are found to have a body-centered-tetragonal (bct) structure. Yet, upon deposition of ∼ 1 ML Ag, remarkable changes occur in the Mn film. X-ray photoelectron diffraction data reveal that the Mn layer relaxes towards a face-centered-cubic (fcc) structure with the lattice parameter of Ag. This implies an exceptionally large atomic volume for a 3d transition metal as well as unusual electronic and magnetic properties. Actually, an evolution towards more atomic-like behaviour is clearly evidenced by valence band and Mn3s core level photoemission data.

Long-range ordered acetate structure on Cu(110)

A chemisorbed layer prepared by exposing a clean Cu(110) surface to acetic acid, CH 3COOH, at room temperature was studied by LEED and X-ray photoelectron diffraction (XPD). LEED shows a well ordered pattern with an oblique 4 −4 1 1 unit cell. The equivalent oxygen coverage is 0.75 corresponding to three acetate species per unit cell. Polar angle O 1s and C 1s intensity distributions measured in four different azimuths for this layer show a large number of peaks which can be correlated with an ordered array of acetate species of two different configurations. A structure model for the acetate layer is proposed on the basis of LEED and XPD data. Single scattering model calculations carried out for this model show good agreement with experiment.

Initial stages of growth of Mn on Ag(100) studied by X-ray photoelectron diffraction and valence band photoemission

The growth mode of Mn on Ag(100) in the monolayer range was studied using X-ray photoelectron diffraction (XPD), angle-resolved valence band photoemission and low energy electron diffraction. Our results show that MnAg intermixing takes place. The thickness of the intermixed films depends critically on the growth temperature. At room temperature the XPD data clearly indicate that Mn is located in the first and second topmost atomic layers. The growth at higher temperatures, i.e. for 1 ML deposited at 460 K, or 1 ML deposited at RT and then annealed at 460 K, results in enhanced interdiffusion and Ag segregation.

Epitaxy and structure of the chloride phase formed by reaction of chlorine with Cu(100). A study by X-ray photoelectron diffraction

Chloride layers of thickness of several atomic layers were prepared by low temperature reaction of Cl 2 with a clean Cu(001) surface. The structure of the chloride layer was examined by X-ray photoelectron diffraction (XPD). The results of the comparison of experimental XPD data and calculations carried out using the single scattering cluster-spherical wave (SSC-SW) model show that the chloride grows as epitaxial fcc, zincblende CuCl, with the (111) plane parallel to the substrate surface. The results also indicate that the (111) “chlorine” termination is the actual termination of the chloride phase, out of the two possible terminations of the polar zincblende structure.

Initial growth and structure of Mn on Ag(100): Formation of a superficial alloy

We have investigated the crystallographic structure of ultra-thin Mn films deposited at room temperature on a clean Ag(100) single crystal, using angle-resolved ultra-violet photoemission, X-ray photoelectron diffraction (XPD) and low-energy electron diffraction. The evolution of the Ag 4d valence band shows deviations from simple layer by layer growth. These results, combined with the XPD data, indicate that by ∼1–1.5 monolayers of Mn a two layers thick Mn-rich epitaxial MnAg alloy is formed that continues the face centered cubic lattice of the Ag substrate.

Complete k-Space Visualization Of X-Ray Photoelectron Diffraction

AbstractA highly detailed x-ray photoelectron diffraction data set has been acquired for crystalline Cu(001). The data set for bulk Cu 3p emission encompasses a large k-space volume (k=3–10 Å-1) with sufficient energy and angular sampling to monitor the continuous variation of diffraction intensities. The evolution of back-scattered intensity oscillations is visualized by energy and angular slices of this ‘volume’ data set. Large diffractyyion data sets such as this will provide rigorous experimental tests of real-space reconstruction algorithms and multiple-scattering simulations.

Epitaxy of thin Cr layers on Cu(001)

Thin Cr films (6–30 monolayers) have been evaporated in ultra-high vacuum onto a Cu(001) single crystal kept at room temperature and have been studied by low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD), and angle resolved ultra-violet photoemission (ARUPS). The LEED diagrams obtained for these layers reveal the formation of epitaxial body-centered cubic (bcc) Cr domains with Cr(110)‖Cu(001) and Cr[11̄1̄]∥Cu[110]. The Cu(001) substrate symmetry results in four different orientations of these domains. This conclusion is confirmed by the XPD data. Indeed the modulations of the Cr 2p 3 2 core level intensity as a function of emission angle are in agreement with single scattering simulations performed for a superposition of equivalent bcc Cr(110) domains on a Cu(001) substrate. Moreover, ARUPS results at normal emission also indicate that the Cr film exposes a (110) surface.

Surface Core-Level Photoelectron Diffraction of Surface Reconstructions

AbstractExperimental surface core-level shifts (SCLSs) aid in understanding the roles of altered electronic and screening properties at reconstructed semiconductor surfaces.1 Because of unresolved theoretical issues, the assignment of SCLSs has often remained controversial even though the surface geometry has been completely determined with traditional probes.Angular- and energy-dependent x-ray photoelectron diffraction (XPD) of each chemically resolved surface core-level electron can be used to discriminate the structure around each chemically shifted species. However, this technique requires tunability, high energy resolution to separate the SCLSs from the bulk core-levels, and high photon flux in order to gather large amounts of data in a reasonable time. Using the newly commissioned spectromicroscopy beamline 7.0 at the Advanced Light Source, we have acquired chemically-resolved XPD data for Si(111) 7×7 for several photon energies, and by examination of forward-focusing peaks we can draw tentative conclusions about the atomic origins of each core-level shifted peak.

X-ray photoelectron diffraction and low-energy electron diffraction study of the interaction of oxygen with the Ni(001) surface: c(2 × 2) to saturated oxide

We have carried out a combined X-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED) study of the interaction of oxygen with Ni(001) at ambient temperature from the c(2 × 2) structure up to the saturated oxide. Several structural conclusions are possible based on an R-factor comparison of an extensive series of azimuthal- and polar-O 1s XPD data to theoretical simulations based on a single-scattering cluster (SSC) model with spherical wave scattering. A new method for normalizing experimental and theoretical intensities for XPD R-factor analyses is also used. For the c(2 × 2) structure, we find that the oxygen sits in the fourfold hollows site, at a vertical distance ( z) of approximately 0.75 Å above the first Ni plane, in excellent agreement with the most recent LEED determination. There is also strong evidence from the XPD results that oxide nucleation occurs very early in the chemisorption region (much earlier than observable by LEED) and this effect could explain the proposals for in-plane bonding or pseudobridge bonding previously reported in the literature. The saturated oxide that forms at ambient temperatures is found from LEED and XPD to form a strained superlattice that is expanded by 1 6 with respect to the underlying Ni(001). We also find that the saturation oxide coverage is much larger than that previously reported in the literature of 2–3 monolayers (ML), and that it is in fact 4–5 ML. Other conclusions concerning the nature of the strain in the oxide are also possible, including a vertical expansion in lattice constant from the first to second plane of 0.1–0.2 Å.

Theoretical photoelectron diffraction simulations for K/Si(001)2 × 1

A strong debate exists in the literature concerning the adsorption sites of K atoms on the Si(001)2 × 1 surface at different K coverages. X-ray photoelectron diffraction (XPD) is a powerful tool for the study of local surface structures. Azimuthal K2p XPD simulations discussed for several positions and coverages of K on the Si(001)2 × 1 surface. Whereas in the standard calculation of XPD data the small scattering centre approximation is used, a curved wave theory is applied to the final state of photoelectrons in this paper. The study on a single-domain K/Si(001)2 × 1 surface should improve the knowledge on this surface essentially.

Electron diffraction from ordered and disordered AuCu3(001)

We present X-ray photoelectron diffraction (XPD) data from AuCu3(001) at 293 and 823 K. i.e. below and above the well known order-disorder transition of the bulk. Azimuthal XPD data of Au4f and Cu 3p show a rather weak dependence on temperature. They are compared with layer-type multiple scattering calculations and it is concluded that the first two layers do not mix up to 823 K. In agreement with other authors the ordered alloy is found to have 50% Au and 50% Cu in the first layer while the second layer contains 100% Cu.

Surface structures for the O/Ni(001) system from c(2×2)O to saturated NiO

The use of x-ray photoelectron diffraction (XPD) for the structural study of oxide growth on Ni(001) from c(2×2)O up to the saturated oxide of approximately two-monolayers thickness is described. Some rather unique structural information is obtained from XPD. This includes the site type and position of the c(2×2) overlayer and the geometry and lattice parameters of the saturated oxide. The XPD data are also in excellent agreement with single-scattering cluster calculations at the spherical wave level.

Temperature dependence of x-ray photoelectron diffraction from copper: Surface and bulk effects.

The temperature dependence of x-ray photoelectron diffraction (XPD) effects in azimuthal distributions of Cu 2${p}_{3/2}$ and Cu 3p core-level intensities from a clean copper (001) surface has been measured in the range from 298 to 1010 K and for two polar angles of emission relative to the surface of theta=7\ifmmode^\circ\else\textdegree\fi{} (highly surface sensitive) and theta=45\ifmmode^\circ\else\textdegree\fi{} (more bulk sensitive). Measurements have been performed with typical angular resolutions of \ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}--6\ifmmode^\circ\else\textdegree\fi{}, although some data obtained with a much higher resolution of \ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}0.6\ifmmode^\circ\else\textdegree\fi{} are also discussed. Very-high-angular-resolution azimuthal scans of Cu 2${p}_{3/2}$ intensity for theta values near 45\ifmmode^\circ\else\textdegree\fi{} show much more fine structure and sensitivity to polar-angle variation than analogous results obtained at lower angular resolution.The diffraction-produced anisotropies as measured by \ensuremath{\Delta}I/${I}_{\mathrm{max}}$=(${I}_{\mathrm{max}}$-${I}_{\mathrm{min}}$)/ $_{\mathrm{max}}$ undergo a much greater relative decrease with increasing temperatures at theta=7\ifmmode^\circ\else\textdegree\fi{} than at theta=45\ifmmode^\circ\else\textdegree\fi{}. Single-scattering cluster (SSC) calculations including Debye-Waller factors predict very well the observed diffraction patterns and their qualitative temperature dependence, as well as the variation of anisotropy with temperature at theta=7\ifmmode^\circ\else\textdegree\fi{}. At theta=45\ifmmode^\circ\else\textdegree\fi{}, theory significantly underestimates the decrease in anisotropy with temperature, probably due to multiple scattering effects occurring for emission along low-index directions with a high density of scattering atoms. Double scattering calculations were performed to estimate these effects and they are compared to experiment and the results of SSC calculations. The effective surface Debye temperatures as deduced from theory and experiment at theta=7\ifmmode^\circ\else\textdegree\fi{} are 202 and 240 K for Cu 2${p}_{3/2}$ and Cu 3p emission, respectively, in good agreement with prior determinations. Finally, the Cu 2${p}_{3/2}$ XPD data for theta=7\ifmmode^\circ\else\textdegree\fi{} gave no evidence for short-range surface melting effects on Cu(001) at temperatures up to T/${T}_{\mathrm{melting}=0.74}$.

ＸＰＤ法で調べた半導体初期界面構造

Application of X-ray photoelectron diffraction (XPD) to the study of the structures of submonolayer semiconductor interfaces is described. A simple kinematical formalizm is basically valid for the analysis of XPD data. This is demonstrated for the Ge (111) lattice. Determination of adsorbate geometries of Ge (111) √3 × √3-Sn and Si (111) √3 × √3 -Ag surfaces is explained as examples of XPD analysis. A brief description of future trend and prospect of photoelectron diffraction in general is made. [J. Cryst. Soc. Jpn. 28, 218 (1986) ] .

X-ray photoelectron diffraction study of oxygen adsorption on the stepped copper surfaces (410) and (211)

We report the first structural investigation of adsorption on stepped metal surfaces using core-level azimuthal X-ray photoelectron diffraction (XPD). The specific systems studied are oxygen adsorption on Cu(410) and Cu(211). Both surfaces show enhanced oxygen adsorption rates compared to the simple low-index surfaces of the terraces, suggesting adsorption sites on the steps. Kinematical LEED analyses verify the step structure of both clean Cu(410) and Cu(211), and also indicate a reconstruction of Cu(211) for exposures ≳ 3 L into a surface with diatomic step heights. At higher exposures of ≳ 75 L, Cu(211) is further observed to facet. Cu(410) is by contrast very stable in structure up to 1200 L exposure. Detailed azimuthal XPD data were obtained for O 1s emission from both surfaces at a low and a high exposure to oxygen. Pronounced diffraction features are seen; these are observed to be sensitive to both the polar angle of emission and the degree of exposure. By comparing the experimental data with single-scattering cluster calculations for different possible adsorption geometries, the most likely adsorption sites for atomic oxygen were deduced. Oxygen is found to lie predominantly on the steps for low exposures of both surfaces: for 0.75 L exposure on Cu(410) it occupies a two-fold site at the top of the step and for 5 L exposure on reconstructed Cu(211), a four-fold site at the bottom of the step. Vertical positions are determined with an estimated accuracy of ± 0.2 Å. At higher exposures ≳ 40 L for the Cu(410) surface, both step and (100) terrace sites are found to be occupied in a modified c(2 × 2) structure. O 1s polar-scan data for Cu(410) also verify that the initial adsorption site is on the step. Such angle-resolved XPS measurements thus have considerable promise for stepped-surface structural studies.