Normal Incidence X-ray Standing Wave Research Articles

Normal incidence X-ray standing wave analysis of thin gold films

Normal incidence X-ray standing wave (NIXSW) analysis has been successfully performed on epitaxial gold films on mica substrates using reflection from the (1 1 1) planes parallel to the surface. We show that NIXSW can be used to monitor the decrease in order within the gold film caused by annealing, and the position of sulfur within a monolayer of methyl thiolate (CH 3S–) on the surface. The Au–S layer spacing was found to be 2.54 ± 0.05 Å, in close agreement with previous work on a single crystal system.

Structural characterisation of ultra-thin VO x films on TiO 2(1 1 0)

The normal incidence X-ray standing wave (NIXSW) technique has been applied to investigate the structure of ultra-thin VO x films grown on TiO 2(1 1 0) and pre-characterised by core level photoemission. For a film composed of a sub-monolayer coverage of V deposited in ultra-high vacuum the local structure of two coexistent species, labelled ‘oxidic’ and ‘metallic’, has been investigated independently through the use of chemical-shift-NIXSW. The ‘oxidic’ state is shown to be consistent with a mixture of epitaxial or substitutional sites and chemisorption into sites coordinated to three surface O atoms. The metallic V atoms also involve a mixture of chemisorption and second-layer sites above the substrate surface consistent with the formation of small V clusters. VO x films up to ∼6 atomic layers were also grown by post-oxidation (sequential V deposition and annealing in oxygen) and by reactive evaporation in a partial pressure of oxygen. While films of around one monolayer or less are consistent with epitaxial VO 2 growth, the film quality deteriorates rapidly with increasing thickness and is worse for reactive evaporation. A possible interpretation of the NIXSW data is increasing contributions of V 2O 3 crystallites. The inferior quality of the reactively evaporated films may be due to an insufficient supply of oxygen.

Structure Investigation of Ag(111)(√7×√7)R19°-SCH3 by X-ray Standing Waves: A Case of Thiol-Induced Substrate Reconstruction

The structure of the Ag(111)(radical7x radical7)R19 degrees-CH3S surface phase, formed by interaction of Ag(111) with gas-phase dimethyl disulfide, (CH3S)2, has been investigated by normal-incidence X-ray standing wave (NIXSW) analysis, using (111), (11), and (200) Bragg reflections. The resulting NIXSW structural parameter values clearly exclude any simple overlayer adsorption model on an Ag unreconstructed surface. A reconstructed surface model is proposed that is consistent with the NIXSW measurements and with previous scanning tunneling microscopy results. This comprises a near-hexagonal Ag surface layer with an Ag density of only 3/7 that of the underlying substrate layers; the methanethiolate molecules are adsorbed into 3-fold coordinated hollow sites on this open layer. The results are discussed in the context of the very limited published studies of longer alkyl chain thiolates on Ag(111).

XSW feasibility study of C [formula omitted]Ph [formula omitted]–OH on Ag(1 1 1): combining hard X-rays, low Z and, low coverage

We have undertaken a ”proof of principle” Normal Incidence X-ray standing wave (NIXSW) experiment on a monolayer of C 60 Ph 5 –OH, a functionalised fullerene, on Ag(111) and show that it is indeed possible to use XSW in case of extremely low coverage of low Z elements such as oxygen. We find a tentative coherent fraction f co of 30% and a coherent position D co of 2.35Å.

The local structure of SO 2 and SO 3 on Ni(1 1 1)

The normal incidence X-ray standing wave (NIXSW) technique, supported by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS), has been used to determine the local adsorption geometry of SO 2 and SO 3 on Ni(1 1 1). Chemical-state specific NIXSW data for coadsorbed SO 3 and S, formed by the disproportionation of adsorbed SO 2 after heating from 140 K to 270 K, were obtained using S 1s photoemission detection. For adsorbed SO 2 at 140 K the new results confirm those of an earlier study [Jackson et al., Surf. Sci. 389 (1997) 223] that the molecule is located above hollow sites with its molecular plane parallel to the surface and the S and O atoms in off-atop sites; corrections to account for the non-dipole effects in the interpretation of the NIXSW monitored by S 1s and O 1s photoemission, not included in the earlier work, remove the need for any significant adsorption-induced distortion of the SO 2 in this structure. SO 3, not previously investigated, is found to occupy an off-bridge site with the C 3v axis slightly tilted relative to the surface normal and with one O atom in an off-atop site and the other two O atoms roughly between bridge and hollow sites. The O atoms are approximately 0.87 Å closer to the surface than the S atom. This general bonding orientation for SO 3 is similar to that found on Cu(1 1 1) and Cu(1 0 0) both experimentally and theoretically, although the detailed adsorption sites differ.

Normal incidence standing wave analysis of the Cu(111)-(√3×√3)R30°-Mn structure: evidence for stacking faults or large-scale alloying?

Abstract The interaction of Mn with the Cu(1 1 1) surface has been investigated using the normal incidence X-ray standing wave (NIXSW) method. Analysis of the NIXSW was complicated by the apparent large shift in position of many layers of the Cu atoms relative to their expected bulk positions indicated by a change in the coherent position. Quantitative analysis of the NIXSW shows that there are at least two but possibly many sites for each element in the altered layer.

Atop adsorption site of sulphur head groups in gold-thiolate self-assembled monolayers

Using normal incidence X-ray standing wave (NIXSW) analysis, we show that the sulphur head group occupies the atop site for methyl-, butyl- and octyl-thiolates on Au(1 1 1), with a Au–S distance of 2.50 ± 0.05 Å, and not the hollow or bridging sites favoured by theoretical calculations. Based on the results of recent theoretical calculations and NIXSW experiments for decyl-thiolate (C 10H 21S–), it is argued that thiolates may reconstruct the Au surface in such a way as to create a structure in which the increasing intermolecular interactions with increasing chain length may lead to head group displacements away from atop sites.

Does an Encapsulated Atom ‘feel' the Effects of Adsorption?: X-ray Standing Wave Spectroscopy of Ce@C82 on Ag(111)

A comparison of normal incidence X-ray standing wave (NIXSW) spectra from bulk films and monolayers of Ce@C82 on Ag(111) clearly shows that the Ce atom does not adopt a preferred intramolecular bonding site as a result of adsorption. NIXSW profiles measured at room temperature and 100 K are each associated with a coherent fraction of zero, indicating a high level of static (rather than vibrational) disorder in the intramolecular Ce positions.

Observation of an anti-phase domain structure in the Cu{1 0 0}/Sn surface alloy system

Low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and the normal incidence X-ray standing wave (NIXSW) technique have been used to determine the structure of the first surface phase formed on Cu{1 0 0} upon deposition of 0.21 monolayers (ML) of tin. At this coverage the LEED images exhibits a modified p(2 × 2) structure with split fractional order diffraction spots. A double scattering LEED pattern simulation consistent with both the Sn surface coverage and the complex split beam LEED pattern suggests an ordered phase based on a p(2 × 2) structure with light anti-phase domain walls. Possible adsorption sites are identified from NIXSW data by triangulation of the distances between the Sn atoms and the (2 0 0) and (1 1 1) planes. The results indicate that Sn atoms adsorb in a fourfold site but does not distinguish between overlayer and substitutional sites. The STM data reveal small domains of p(2 × 2) structure formed by the Sn atoms being adsorbed in substitutional sites. These domains typically contain 16 Sn atoms arranged in a square 4 × 4 array, however larger and smaller rectangular domains are also observed with sides containing three, five and six Sn atoms. The width of the domain wall is found to vary with rows of one, three or five copper atoms separating the domains in both the [0 1 1] and [ 0 1 1 ̄ ] directions. The STM images indicate that the Cu atoms in the domain walls are vertically displaced from their expected positions. This is attributed to elastic strain due to the large metallic diameter of Sn (0.162 nm) relative to Cu (0.127 nm).

Orientation and constraints of endohedral lanthanum inLa@C82molecules adsorbed on Cu(111)

The orientation of the endohedral La atom in $\mathrm{La}@{\mathrm{C}}_{82}$ adsorbed on Cu(111) has been determined using the normal-incidence x-ray standing wave (NIXSW) technique. The NIXSW measurements were carried out using both the normal 〈111〉 and 70.5\ifmmode^\circ\else\textdegree\fi{} off-normal $〈1\ifmmode\bar\else\textasciimacron\fi{}11〉$ Bragg reflections. The La $〈1\ifmmode\bar\else\textasciimacron\fi{}11〉$ profiles of the $\mathrm{La}@{\mathrm{C}}_{82}$ monolayer and submonolayers are found to be similar to the 〈111〉 profile of the multilayer, while the 〈111〉 profiles of the monolayer and submonolayers are distinctly different with much lower peak height. This indicates significant ordering of the La atoms in the monolayers with respect to height above the Cu surface but little lateral ordering. The results of curve fits to the monolayer profiles show that the La atoms are regionally located at 1.62 (\ifmmode\pm\else\textpm\fi{}0.77) \AA{} above an extended scattering plane from the crystal. Cooling the monolayers to a temperature of 170 K increases ordering of the La atoms with respect to height above the surface. The NIXSW results demonstrate that the endohedral molecules adopt a restricted orientation upon adsorbing on the surface and the La atom resides on either the upper or lower half of the cage. Theoretical calculations show that the most stable La positions are located in the top half of the fullerene cage, which supports the La upper configuration. The results are explained in terms of electrostatic interactions between the endohedral atom, the fullerene cage, and the substrate.

Normal incidence X-ray standing wave study of Fe on Cu( [formula omitted

The structure of annealed ultra-thin Fe films on Cu(1 1 1) has been investigated using normal incidence X-ray standing wave (NIXSW). Films of between 0.5 and 5.3 monolayer equivalence (MLE) have been deposited in situ and briefly annealed to 513 K. NIXSW measurements of the (1 1 1) and ( 1 ̄ 1 1) reflections using 2p 3/2 photoemission show that the film is pseudomorphic up to 2 MLE, with near complete occupation of the fcc hollow sites. For this regime, the value of the (1 1 1) coherent position is equivalent to a film spacing relative to the lattice of 2.00±0.03 Å. For the 5.3 MLE film, there is a drastic reduction in the ( 1 ̄ 1 1) coherent fraction and shift in the ( 1 ̄ 1 1) coherent position that is consistent with the transformation of the entire film to a Kurjumov-Sachs oriented bcc(1 1 0) structure.

A structural study of the interaction of methanethiol with Pt [formula omitted] using X-ray standing waves

In combination with surface characterisation by synchrotron radiation X-ray photoelectron spectroscopy, the normal incidence X-ray standing wave (NIXSW) technique has been applied to a determination of the structure of surface phases formed on Pt(1 1 1) by reaction with methanethiol. On surfaces heated to ⩾ ≈500 K, producing only coadsorbed atomic S and C, the S atoms are found to occupy fcc hollow sites (directly above Pt atoms in the third layer) in a geometry essentially identical to that of simple ordered S overlayer phases on Pt(1 1 1) with a S–Pt layer spacing of 1.67 Å, but with possible fractional co-occupation of a complex S phase. On a surface annealed to ≈223 K only a surface methanethiolate (CH 3S–) species is believed to be present, the favoured model involves a tilted off-atop bonding such that the S atoms are located offset from the fcc hollow sites, with frustrated rotational vibrations of large amplitude, although an alternative model based on co-occupation of atop and fcc hollow sites is also consistent with the NIXSW data.

A NIXSW structural investigation of the low temperature silyl phase formed by SiH 4 reaction with Cu [formula omitted

The structure of the Cu(1 1 1) -(3×3) -SiH x surface formed by reaction of silane (SiH 4) with Cu(1 1 1) at 140 K, has been determined using normal incidence X-ray standing wave (NIXSW) analysis. The (3×3) phase, thought to comprise a mixture of SiH 3 (2/9 ML) and SiH or Si species (1/9 ML), has all the surface silicon atoms located in hcp 3-fold hollow sites at a distance of 1.98±0.04 A ̊ from the surface copper scatterer planes.

1-Chloro-2-fluoroethane Adsorption on Cu(111): Structure and Bonding

The adsorption of 1-chloro-2-fluoroethane on Cu(111) has been studied using low-energy electron diffraction, Auger electron spectroscopy, UV photoelectron spectroscopy, line of sight temperature-programmed desorption, and normal incidence X-ray standing wave (NIXSW) analysis. The molecule adsorbs and desorbs molecularly. The activation energy for desorption of the first layer increases linearly from 30.8 to 32.8 kJ mol-1 as the coverage increases from zero to full monolayer coverage with a preexponential of 1012 s-1. The activation energy for the multilayer is found to be 27.1 kJ mol-1 for the same preexponential value. The NIXSW structural study shows that the molecule bonds via its chlorine end, in an atop site, with a Cu···Cl distance of 3.06 ± 0.06 Å. The fluorine end, at a distance of 3.02 ± 0.1 Å from the surface copper plane, is also located approximately in an atop site. This orientates the molecule with its Cl···F axis parallel to the surface. These results are compared with similar ones for 1,2-dichloroethane and 1-bromo-2-chloroethane on Cu(111). All three molecules have similar adsorption geometries, bonding via a single halogen (the heaviest) in an atop site, while the bond strength is determined, predominantly, by this halogen.

A NIXSW structural investigation of the (√3×√3) R30°-Cu 2Si surface alloy phase formed by SiH 4 reaction with Cu(1 1 1)

The structure of the Cu(1 1 1)–(√3×√3) R30°-Cu 2Si surface alloy has been determined using normal incidence X-ray standing wave (NIXSW) analysis. It comprises two equally populated, monolayer thick (√3×√3) R30°-Cu 2Si domains. In one domain the silicon and copper atoms both reside in the fcc type threefold hollows, in the other they both reside in the hcp type threefold hollows. For both domains the silicon atoms are 2.04±0.04 Å from the substrate Cu(1 1 1) scatterer plane.

Structure of ultrathin films of Co on Cu(111) from normal-incidence x-ray standing wave and medium-energy ion scattering measurements

Applications of the techniques of normal-incidence x-ray standing wave (NIXSW) and medium-energy ion scattering (MEIS) to the elucidation of the structure of an ultrathin metallic film, Co on Cu(111), are reported. NIXSW and MEIS are shown to yield valuable and complementary information on the structure of such systems, yielding both the local stacking sequence and the global site distribution. For the thinnest films of nominally two layers, the first layer is of entirely fcc registry with respect to the substrate, but in the outermost layer there is significant occupation of hcp local sites. For films up to 8 monolayers (ML) thick, the interlayer spacing of the Co layers is $0.058\ifmmode\pm\else\textpm\fi{}0.006\AA{}$ smaller than the Cu substrate (111) layer spacing. With increasing coverage, the coherent fraction of the $(1\ifmmode\bar\else\textasciimacron\fi{}11)$ NIXSW decreases rapidly, indicating that the film does not grow in a fcc continuation beyond two layers. For films in this thickness range, hcp-type stacking dominates fcc twinning by a ratio of 2:1. The variation of the $(1\ifmmode\bar\else\textasciimacron\fi{}11)$ NIXSW coherent fraction with thickness shows that the twinning occurs close to the Co/Cu interface. For thicker films of around 20 ML deposited at room temperature, medium-energy ion scattering measurements reveal a largely disordered structure. Upon annealing to 300 \ifmmode^\circ\else\textdegree\fi{}C the 20-ML films order into a hcp structure.

X-ray standing wave study of wet-etch sulphur-treated InP(100) surfaces

Normal incidence X-ray standing wave (NIXSW) measurements are presented for sulphur terminated InP(100) surfaces, prepared by wet-etch in (NH 4) 2S solution followed by UHV anneal. Standing wave profiles for sulphur 1s photoyield in three planes (220, 311 and 31 1 ) indicate that sulphur is close to the phosphorus site. Coherent positions and fractions are compared with the predictions of a number of models of the surface, including novel 2×2 structures. The experimental data can be reproduced by a general two-dimer model, but it is more likely that the data are reflecting significant disorder.

Chemical-shift X-ray standing wave studies: coadsorption site determination of PFx fragments on Ni(111)

Abstract Using P 1s photoemission as a spectral fingerprint, the X-ray-induced fragmentation of PF 3 adsorbed on Ni(111) has been characterised at both room temperature and 140 K. At room temperature only a single fragment is seen, but at low temperature three different phosphorus states are seen, corresponding to two different fragments, in addition to intact PF 3 . Full normal-incidence X-ray standing wave (NIXSW) {111} site triangulation has been performed on each of the chemically shifted P 1s photoemission signals in order to establish the local adsorption sites, and to aid the species identification. At room temperature the single fragment is proposed to be atomic P, which occupies fcc hollow sites with a P–Ni nearest neighbour distance consistent with a phosphide. At 140 K the three P states are assigned to PF 2 occupying bridge sites, and the two inequivalent P atoms of a P 2 F x species, bonded to the surface through one P atom that occupies fcc hollow sites. A scheme of surface reactions is proposed to account for these conclusions and to reconcile the results with those of earlier structural and spectroscopic studies of this system.

X-ray Studies of Self-Assembled Monolayers on Coinage Metals. 2. Surface Adsorption Structures in 1-Octanethiol on Cu(111) and Ag(111) and Their Determination by the Normal Incidence X-ray Standing Wave Technique

The adsorption structures of self-assembled monolayers (SAMs) formed at saturation coverages of 1-octanethiol SAMs, prepared under ultrahigh vacuum (UHV) conditions on Cu(111) and Ag(111) substrates, were measured using a normal-incidence X-ray standing wave (NIXSW) technique. A thorough and pedagogical exposition of the relationship between the structural parameters determined in NIXSW and a number of simple adsorption structures is presented. In the two SAMs studied, the NIXSW data indicated the formation of monolayers which induced a reconstruction of the outermost layer of the substrate. SAMs on Cu and Ag were indistinguishable using the NIXSW technique, but given the different steric requirements imposed by each surface it is proposed that a different adsorption geometry is adopted in each case, which is either incommensurate with the underlying substrate or commensurate on a large mesh. In the case of Ag(111) the NIXSW data are consistent with the formation of a distorted (√7×√7)R19.1° structure, proposed by other workers. In the case of Cu(111), it is proposed that the adsorbate structure is most likely a significant distortion of the (√3×√3)R30° structure observed in SAMs of alkanethiols on Au(111) and we speculate on the existence of an overlayer of CH3−(CH2)7−S−Cu3 clusters.

The structure of PF 3 adsorbed on Cu(111)

The structure of PF 3 adsorbed on Cu(111) at 110 K has been determined using both near edge X-ray absorption fine structure (NEXAFS) and normal incidence X-ray standing wave (NIXSW) methods. Two X-ray reflection conditions were used, the (111) and ( 1 ̄ 11) , to determine the atomic positions of the phosphorus and fluorine atoms by triangulation using NIXSW, whereas NEXAFS was used to determine the alignment of the molecule. PF 3 adsorbs at atop sites with the fluorine atoms away from the surface and its C 3 axis aligned along the surface normal. The Cu–P distance is 2.25±0.04 Å, and the distance between the copper and the fluorine layers along the C 3 axis is 3.05±0.04 Å. The adsorbed molecule maintains its gas-phase geometry and undergoes either free rotation about the C 3 axis, or is azimuthally orientated such that the projections of the P–F bonds on to the surface point towards the next nearest neighbour copper atoms. The NIXSW analysis for the fluorine photoemission data was carried out using a backward/forward asymmetry parameter to compensate for a breakdown in the dipole approximation for photoemission that occurs for fluorine 1s photoemission under the NIXSW conditions used here.