X-ray Photoelectron Diffraction Measurements Research Articles

Zn Vacancies as Hydrogen Trap Sites in Polar Surfaces: A New Stabilization Mechanism for the ZnO(0001)-(2×2) Surface Reconstruction

The (2×2) reconstruction of the ZnO(0001) surface has been investigated by X-ray photoelectron diffraction (XPD). Comparing the XPD measurements with multiple-scattering simulations, the single Zn vacancy per (2×2) surface unit cell model is confirmed, and structures with O adatoms are ruled out. The analysis indicates an outward relaxation of the topmost Zn layer, in contrast to the usually reported results by density-functional theory (DFT) calculations. On the basis of DFT, we describe a new stabilizing mechanism of the polar ZnO surface through surface reconstruction where the Zn vacancies are occupied by three hydrogens atoms. The DFT surface relaxation of the proposed model is in excellent agreement with the XPD findings. Our DFT simulations also strongly indicate that the migration of hydrogens atoms to the surface, coming from the bulk, may influence the desorption of the surface Zn atom to create the vacancy.

Time-resolved X-ray photoelectron diffraction using an angle-resolved time-of-flight electron analyzer

X-ray photoelectron diffraction (XPD) provides atomic resolution, element sensitive local structure information about the surfaces and interfaces of materials. In the work reported in this paper, a two-dimensional angle resolved time-of-flight (2DARTOF) system is used to perform time-resolved XPD experiments on epitaxial silicene. The two Si 2p peak components from the different atomic sites in the silicene layer allow extraction of the individual XPD patterns. Time-resolved measurements captured small angle shifts in the forward focusing peak, indicating laser-induced changes in the silicene structure. At 10 ns, the XPD patterns appear to relax back to the equilibrium state. This work demonstrates that 2DARTOF systems are well suitable for time-resolved XPD measurements.

Correlation between Charge Transfer and Adsorption Site in CoPc Overlayers Adsorbed on Ag(100)

In this paper we investigate the local adsorption geometry of CoPc films adsorbed on a Ag(100) surface and its effect on the molecule–substrate interactions. Using high-resolution electron energy loss spectroscopy we demonstrate that the charge-transfer mechanism at the organic–metal interface depends on the structural properties of the CoPc film. We discuss these findings in terms of the molecular adsorption sites, as determined by X-ray photoelectron diffraction measurements and multiple scattering simulations. We show that the distance between the central Co atom and the topmost Ag layer is of the order of 0.3 nm. In addition, we demonstrate that the CoPc molecules adsorb on different atomic sites depending on the superlattice symmetry, as varied by changing the coverage. This is further confirmed by low-energy electron diffraction and scanning tunneling microscopy measurements, emphasizing that, depending on the molecular superstructure adopted by the molecules, there are one or two molecules per unit...

Synthesis of nitrogen-doped epitaxial graphene via plasma-assisted method: Role of the graphene–substrate interaction

Functionalization of graphene by substitution of carbon with nitrogen atoms is a promising way to tailor its electronic properties, but a good control over the heteroatomic configuration in the graphene network is most often a difficult task. In this paper, the synthesis of N-doped graphene by nitrogen plasma treatment of graphene/Ir(111) is presented. The formation of substitutional, pyrrolic and pyridinic nitrogen is analyzed by means of X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The graphene–Ir interaction is suggested to control the variation in the relative concentration of the nitrogen species. Annealing of the sample also leads to modifications of the nitrogen species incorporated in the graphene layer. Furthermore, the connection of the substitutional nitrogen arrangement with its corresponding spectroscopic fingerprint is unequivocally confirmed by XPD measurements, which give also a direct insight on the local geometry of the nitrogen atoms incorporated in the carbon network.

Ordered Oxide Surfaces on Metals: Chromium Oxide

We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of ordered chromium oxide ultrathin films prepared on a Pd(111) single-crystal surface. The films where grown by thermal evaporation of Cr under an oxygen atmosphere and sample temperature of 600 K. The ordered films produced are strongly dependent on the film thickness and annealing treatment. Films with thickness below 5 A, produced at low coverages, display a p(2 × 2) structure relative to Pd(111). Thicker films (thickness > 10 A) always have a \( \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } \) structure. The photoemission measurements were done using conventional X-ray sources as well as synchrotron radiation taken at the Laboratorio Nacional de Luz Sincrotron. The XPD data were interpreted through the use of a multiple scattering calculation approach combined with a genetic algorithm for surface structure optimization. Combining the information from XPS, low energy electron diffraction and XPD measurements we have determined the surface structure of the\( \left( {\sqrt 3 \times \sqrt 3 } \right)R30^{ \circ } \) phase. Elsewhere, we report on the structure of the p(2 × 2) phase.

Si epitaxial growth on LaAlO 3(0 0 1)

We report on the growth of Si on c(2 × 2) reconstructed LaAlO 3(0 0 1) surfaces at high substrate temperature (700 °C) by molecular beam epitaxy. An initial Volmer–Weber mode is evidenced using reflection high energy electron diffraction (RHEED), X-ray photoelectron diffraction (XPD) and atomic force microscopy. After the deposition of a few monolayers, the islands coalesce. Using X-ray photoelectron spectroscopy, we demonstrate that Si islands exhibit an abrupt interface with the LaAlO 3 substrate without formation of silicate or silica. Finally, combined RHEED and XPD measurements show the epitaxial growth of Si with a unique Si(0 0 1)//LaAlO 3(0 0 1) and Si<1 0 0>//LAO<1 1 0> relationship.

Growth and structure of thin MnO films on Ag(0 0 1) in dependence on film thickness

We have investigated the structure of thin MnO films grown on a single-crystal Ag(0 0 1) surface in dependence on layer thickness by combination of low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) methods. The analysis of LEED profiles shows that the in-plane lattice constant of the epitaxial grown MnO film on Ag(0 0 1) depends strongly on the thickness of the film. The analysis of LEED data leads to the assumption that the lattice mismatch between MnO and Ag may be released by a tetragonal distortion of the film in comparison to bulk-MnO. In addition, XPD measurements in forward-scattering conditions are considered. Polar-angle scans have been recorded and analysed for Mn2p and O1s photoelectrons excited by Al Kα radiation to get detailed structural parameters. It is shown that, in contrast to thin films of simple metals, the forward-scattering model should be applied very carefully in the interpretation of XPD data.

Multiple scattering investigation of the 1T-TaS2 surface termination

Multiple scattering theory based on a cluster model is used to simulate full hemispherical X-ray photoelectron diffraction measurements on a 1T-TaS2(0001) surface. Key points to determine the surface termination are discussed. As the commonly applied single scattering simulations do not give satisfying results, a multiple scattering approach has to be used to accurately simulate the full hemispherical photoelectron diffraction patterns. Differences and similarities between calculations of Ta and S terminated surfaces are presented along with experimental results at room temperature using both, the single and the multiple scattering approaches. We find that the surface is S terminated and that the quantitative difference between the calculations for both terminations permits to show the limits of the single scattering approach for solving surface termination problems. Moreover, by generalizing the results obtained using the multiple scattering approach, we discuss the application of this method to other similar systems.

X-ray photoelectron diffraction study of Cu(1 1 1): Multiple scattering investigation

Multiple scattering theory based on a cluster model is used to simulate full-hemispherical X-ray photoelectron diffraction measurements in order to verify how state of the art multiple scattering simulations are able to reproduce the experiment. This approach is applied to the Cu(1 1 1) surface for two different photoelectron kinetic energies. Differences and similarities between single and multiple scattering are discussed in comparison with experimental results. We find that the present approach gives very good results despite some limitations.

Development of photoelectron spectro‐holography apparatus

AbstractWe improved the photoelectron spectro‐holography apparatus for high‐speed and high‐energy photoelectron detection. The combination of an aperture for ±1.4° acceptance angle at the diffraction plane and a 64‐channel detector realized the high‐speed XPS and rapid x‐ray photoelectron diffraction (XPED) measurements Furthermore, high‐energy XPS and XPED measurements excited by Cu Kα have been performed. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Improvement of angle‐resolved electron energy analyser for high‐performance x‐ray photoelectron diffraction measurements

AbstractFor more precise and rapid x‐ray photoelectron diffraction (XPED) measurements, we have improved an angle‐resolved electron energy analyser developed a ±1.4° diffraction plane aperture mode, and a novel high‐speed 64‐channel detector. In this study, a large number of photoelectrons from a newly developed diffraction plane aperture mode were detected by a novel high‐speed detector, facilitating rapid and precise XPED measurements. The Ti 2p XPED patterns from an SrTiO3(001) surface also revealed the performance of this improved analyser. In addition, high‐speed XPS measurements from an Ag polycrystal were performed for several milliseconds. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Highly angular resolved x‐ray photoelectron diffraction measurements from a Ge(111) surface

AbstractUsing a high angle‐resolving electron energy analyser with a novel input lens system, we measured Ge 3p x‐ray photoelectron diffraction (XPED) patterns from a Ge(111) surface excited by Al Kα with changing angular resolution from ±0.04° to ±2°, and considered the angular resolution dependence of XPED patterns from experiments and theory. Highly angular resolved XPED patterns containing fine features such as Kikuchi‐like bands were obtained. These patterns were reproduced by theoretical calculations. We also performed the theoretical calculations for further investigation of highly angular resolved XPED. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Estimating soft-mode frequencies of surface overlayers by means of photoelectron diffraction: The(2×2)surface-V2O3/Pd(111)

The $(2\ifmmode\times\else\texttimes\fi{}2)$ surface-${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ layer, an interface-mediated vanadium oxide phase observed on Pd(111) in the submonolayer coverage range, has been investigated by means of angle scanned x-ray photoelectron diffraction (XPD), which gives a direct experimental confirmation of the model derived by scanning tunnelling microscopy (STM) and density functional calculations (DFT), with a quantitative determination of the V-O interlayer spacing. In addition, XPD measurements compared to single scattering cluster--spherical wave (SSC-SW) simulations revealed a peculiar broadening of V-O forward scattering (FS) maxima that is limited to azimuthal scans and that cannot be accounted for by isotropic Debye-Waller attenuation of the diffraction features. However, the existence of a soft phonon mode in the overlayer, associated with substantial in-plane displacements from equilibrium of O scatterers with respect to V emitters, could explain the experimental observation. The existence of such a soft mode has been confirmed by DFT calculations. It consists of an in-plane quasirotation around the V emitter of the three nearest-neighbor O atoms, and the estimated DFT frequency amounts to $15{\mathrm{cm}}^{\ensuremath{-}1}.$ The XPD data have been analyzed by means of SSC-SW simulations wherein a harmonic oscillator model has been employed to approximate the effect of the soft phonon mode on XPD curves. As a result, an experimental determination of the frequency of the mode has been obtained $(40\ifmmode\pm\else\textpm\fi{}25{\mathrm{cm}}^{\ensuremath{-}1}),$ which is of the same order of magnitude as the DFT predicted frequency. Moreover, the sensitivity of XPD scans to the correlation of soft-mode atomic displacements has been studied, leading to the estimate of a ``soft-mode XPD coherence length'' for the system under investigation. This work therefore explores an application of XPD as a surface spectroscopy sensitive to vibrational soft modes.

Development of a Novel Instrument for X-Ray Photoelectron Diffraction and Holography

For X-ray photoelectron diffraction (XPED) and holography measurements we developed a novel laboratory instrument with the multienergy high power X-ray source and the high energy and high angular resolution photoelectron spectrometer system. The photon intensities of Al–Kα, Cr–Lα and Cu–Kα were estimated at 4.6 × 1011 cps, 7.5 × 1010 cps and 7.2 × 1010 cps, respectively. Ag3d XPS also revealed that the energy resolutions of Al–Kα and Cr–Lα sources were 0.9 eV and 3.1 eV, respectively. XPS and XPED of h-BN/Ni(111) excited by Al–Kα and Cr–Lα elucidated the potential and the validity of the XPED and holography analysis by using this novel instrument. Cu–Kα excitation XPS and Ti1s XPED measurements of the SrTiO3 (001) surface have also been performed.

CO adsorption on Pt(1 1 0) investigated by X-ray photoelectron diffraction

The adsorption of CO on a Pt(1 1 0) surface was studied by X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), and low energy electron diffraction (LEED). CO exposure during cooling the sample from 600 K to several final temperatures, T f, resulted in CO layers of increasing coverage. For T f>350 K, a disordered CO layer was obtained, characterized by a (1×1) LEED pattern, but also by an increasing intensity of inter-molecular scattering peaks in C 1s XPD with increasing CO coverage. This behavior appears to indicate a rising degree of short range order in the adlayer. A well-ordered (2×1)-CO pattern at a coverage of 1.0 was obtained for final temperatures below 300 K. XPD measurements provided evidence of 22° tilted CO molecules, with the projected tilt parallel to the [0 0 1] azimuth of the Pt(1 1 0) surface. The latter is consistent with a p2mg symmetry of the (2×1)-CO structure. A second well-ordered c(8×4)-CO pattern was prepared upon adsorption at T<240 K, which was characterized by an O 1s doublet, assigned to on-top and bridge adsorbed CO, at a total coverage of 1.09. The corresponding XPD C 1s intensity distribution, measured over a large solid angle, indicated tilted and perpendicular CO, and inter-molecular scattering peaks distinctly different from those of the (2×1)p2mg-CO layer. A structure model for the c(8×4)-CO layer was developed. Single scattering cluster calculations were performed for real space models of CO layers at low coverage, and for the high coverage (2×1)p2mg and c(8×4) layers. Good consistency between the experimental and theoretical C 1s angular intensity distributions was found, involving both tilted and perpendicular CO.

Surface and subsurface alloy formation of vanadium on Pd(111)

We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By means of low-energy ion spectroscopy (LEIS), Auger electron spectroscopy (AES) scanning tunnelling microscopy (STM), X-ray photoelectron diffraction (XPD) and ab initio local density functional calculation we find that, at room temperature, deposited vanadium atoms substitute surface palladium atoms. In addition, islands are formed on the surface that consist mostly of the substituted palladium atoms. At higher temperatures vanadium diffuses into subsurface layers and, at a temperature of 300°C, only a small amount of vanadium is observed in the topmost layer. STM revealed the formation of a ( 3 × 3 ) R30° superstructure and XPD measurements demonstrated that this structure is due to vanadium atoms incorporated in the second layer. This finding is confirmed by ab initio calculations. A model for the ( 3 × 3 ) R30° structure based on the experiments and the ab initio calculations is given.

Texture control of PbTiO3 and Pb(Zr,Ti)O3 thin films with TiO2 seeding

The nature and the role of 1 to 5 nm thick TiO2 seed layers for the growth of textured PbTiO3 and Pb(Zr,Ti)O3 thin films on textured Pt(111) thin film substrates have been studied. Under otherwise identical in situ sputter deposition process conditions, the PbTiO3 texture could be turned from (100) to (111) orientation by adding the seed layer. This is demonstrated by patterning the TiO2 film. Auger electron spectroscopy and x-ray photoemission spectroscopy showed that the seed layer was a continuous TiO2 film. X-ray photoelectron diffraction measurements revealed epitaxial ordering in the seed layer. As there is no azimutal order among the Pt grains, the reduced information of azimutally averaged polar cuts is obtained. These give strong evidence for a strained rutile (110) structure. Various deposition experiments indicated that the TiO2 is effective only when it is ordered before the PbTiO3 nucleation starts. The epitaxial relationship between PbTiO3(111) and Pt(111) is thus mediated by the intermediate, epitaxial TiO2 film, which is dissolved or transformed to PbTiO3 afterwards. The observed growth behavior is discussed in terms of surface and interface energies.

COMPOSITIONAL OSCILLATIONS AND STRAIN EFFECTS AT THE ANNEALED Co/Pd(100) INTERFACE: A STUDY BY X-RAY PHOTOELECTRON DIFFRACTION AND LOW ENERGY ION SCATTERING

The properties of the interface obtained by cobalt deposition on the Pd(100) surface and successive annealing have been studied by X-ray photoelectron diffraction (XPD) and low energy ion scattering (LEIS). The "as deposited" cobalt layer shows considerable lattice strain (about 10%) for coverages up to approximately 20-monolayer equivalents. Annealing at temperatures of the order of 700 K leads to intermixing with the formation of a surface alloy and the partial release of the strain. The combination of LEIS and XPD measurements shows that cobalt in the surface phase is preferentially located in the second outermost layer from the surface.

X-Ray Photoelectron Diffraction Measurements of Hexagonal GaN(0004) Thin Films

ABSTRACTWe report on the first scanned-angle x-ray photoelectron diffraction measurements on GaN(0001) in the wurtzite structure, as grown on sapphire substrates using MOCVD. These as-grown samples reveal forward scattering peaks in agreement with a theoretical calculation using a single scattering cluster calculation. The surface contamination by O and C does not exhibit any clear structure. From the combination of experiment and theoretical calculation and from a simple intensity ratio argument the surface termination for these samples could be determined to be N. The data also indicate that C is on average closer to the GaN surface than O.

Hcp and bcc Cu and Pd Films.

Epitaxial hcp and bcc Cu and Pd films were grown on W(100). The epitaxial relation of the hcp films is determined by reflection high energy electron diffraction (RHEED) as $(11\overline{2}0)\ensuremath{\parallel}(100)$ and $[0001]\ensuremath{\parallel}[011]$. The growth of hcp Cu and Pd is attributed to the much smaller misfit for this hcp orientation in either the [ $1\overline{1}00$] or [0001] direction, respectively, than for any fcc orientation. This misfit difference overcomes the energy difference between hcp and fcc crystal structure. The atomic diameter of Pd is near that of W, which also gives a good fit for bcc pseudomorphic Pd. The energy difference between hcp and bcc determines the structure. X-ray photoelectron diffraction measurements show a bcc Pd film, transmission RHEED hcp islands.