X-ray Photoelectron Diffraction Patterns Research Articles

Circular dichroism in hard X-ray photoelectron diffraction observed by time-of-flight momentum microscopy

X-ray photoelectron diffraction (XPD) is a powerful technique that yields detailed structural information of solids and thin films that complements electronic structure measurements. Among the strongholds of XPD we can identify dopant sites, track structural phase transitions, and perform holographic reconstruction. High-resolution imaging of kll-distributions (momentum microscopy) presents a new approach to core-level photoemission. It yields full-field kx-ky XPD patterns with unprecedented acquisition speed and richness in details. Here, we show that beyond the pure diffraction information, XPD patterns exhibit pronounced circular dichroism in the angular distribution (CDAD) with asymmetries up to 80%, alongside with rapid variations on a small kll-scale (0.1 Å−1). Measurements with circularly-polarized hard X-rays (hν = 6 keV) for a number of core levels, including Si, Ge, Mo and W, prove that core-level CDAD is a general phenomenon that is independent of atomic number. The fine structure in CDAD is more pronounced compared to the corresponding intensity patterns. Additionally, they obey the same symmetry rules as found for atomic and molecular species, and valence bands. The CD is antisymmetric with respect to the mirror planes of the crystal, whose signatures are sharp zero lines. Calculations using both the Bloch-wave approach and one-step photoemission reveal the origin of the fine structure that represents the signature of Kikuchi diffraction. To disentangle the roles of photoexcitation and diffraction, XPD has been implemented into the Munich SPRKKR package to unify the one-step model of photoemission and multiple scattering theory.

Surface and interface analysis of a low-dimensional Au–Si surface alloy on Au(110) by means of XPS and XPD

The chemical and structural characteristics of a low-dimensional Au–Si surface alloy are presented in this work. Alloy formation was obtained by deposition of a sub-monolayer Si on Au(110). This preliminary phase to Si nano-ribbons is being investigated, as the transition from clean Au(110) to a silicon nano-ribbon coated surface is not yet understood. A multiple technique study has been carried out for detailed atomic structure determination and chemical investigation. Particular attention is paid to the clarification of the structural arrangement at the surface and at the interface. Using low-energy electron diffraction, the periodicity of the structure on long-range order could be examined. By means of high-precision photoemission measurements using synchrotron radiation, the electronic and atomic structure of the alloy can be presented. The investigation by photoelectron spectroscopy (XPS) using soft x-rays for a high surface sensitivity showed different chemical environments in the high-resolution spectra. The x-ray photoelectron diffraction (XPD) measurements, which are sensitive to the local atomic order, gave an approach to the structural configuration of the alloy. A new structural arrangement was found simulating both Au and Si XPD patterns. The results are compared to former proposed structure models. A deconvolution of the Si 2p XPD pattern revealed the origin of two chemically shifted XPS components.

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.

Processing and analysis of X-ray photoelectron diffraction data using IGOR Pro

A software package is presented for nearly real-time display of diffractograms during X-ray photoelectron diffraction (XPD) data acquisition and for processing and analysis after an experiment. During the experiment, the package is able to automatically read X-ray photoelectron spectroscopy (XPS) data, perform initial data processing and project intensity values as XPD diffractograms. Four display modes are supported. After the experiment, the package is able to open, process and analyze XPD patterns. The processing functions include rotation, cropping, creating a full 2π pattern using symmetry operations, smoothing and converting a pattern to an image suitable for publication. The analysis functions include displaying polar angles, azimuthal angles, intensity, the core level spectrum at a selected angular data point, and azimuthal and radial profiles. The package also integrates fitting functions for core level spectra. The package is developed using the IGOR Pro scripting language. A graphical user interface has been developed to allow all the operations just by mouse clicking. The package is designed to interface directly with an XPD system at the Saclay center of the French Atomic Energy and Alternative Energies Authority, but the algorithms are generally applicable and can be readily adapted to other XPD systems.

Atomic characterization of nano-facet nitridation at SiC (11¯00) surface

Nitridation effectively reduces the density of defects at SiO2/SiC interfaces. The SiC (11¯00) face, which appears at the nano-trench structure on the (0001) and (0001¯) surfaces, is composed of Si- and C-facets. To understand the nitridation mechanism of this face, we investigate the atomic arrangement of N at SiC (11¯00) surface with high temperature annealing in N2 ambient using x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). The N 1s spectrum indicates that N atoms cover about 5/6 of the monolayer and are bonded to Si atoms. The N 1s XPD pattern significantly differs from bulk Si 2p and the C 1s patterns, suggesting that the majority of N atoms occupy C sites at the topmost layer of the 4H-SiC substrate. Based on simulations of the N 1s spectrum and XPD pattern, we propose a local structure where N atoms replace C atoms at four different sites of the SiC (11¯00) surface and bond to three Si atoms.

Sensitivity of photoelectron diffraction to conformational changes of adsorbed molecules: Tetra-tert-butyl-azobenzene/Au(111).

Electron diffraction is a standard tool to investigate the atomic structure of surfaces, interfaces, and adsorbate systems. In particular, photoelectron diffraction is a promising candidate for real-time studies of structural dynamics combining the ultimate time resolution of optical pulses and the high scattering cross-sections for electrons. In view of future time-resolved experiments from molecular layers, we studied the sensitivity of photoelectron diffraction to conformational changes of only a small fraction of molecules in a monolayer adsorbed on a metallic substrate. 3,3′,5,5′-tetra-tert-butyl-azobenzene served as test case. This molecule can be switched between two isomers, trans and cis, by absorption of ultraviolet light. X-ray photoelectron diffraction patterns were recorded from tetra-tert-butyl-azobenzene/Au(111) in thermal equilibrium at room temperature and compared to patterns taken in the photostationary state obtained by exposing the surface to radiation from a high-intensity helium discharge lamp. Difference patterns were simulated by means of multiple-scattering calculations, which allowed us to determine the fraction of molecules that underwent isomerization.

An Ordered Mixed Oxide Monolayer Formed by Iron Segregation on Rutile-TiO2(011): Structural Determination by X-ray Photoelectron Diffraction

Iron impurities in the rutile-TiO2(011) surface can result in the formation of an ordered, ternary iron–titanium oxide monolayer. Here the FeTi2O5 mixed oxide monolayer predicted by DFT simulations is confirmed by synchrotron based angle scanned X-ray photoelectron diffraction (XPD) studies. The ternary oxide monolayer has been synthesized on rutile-TiO2(011) substrate via two different experimental pathways: first, by segregation of Fe impurities from the bulk by annealing a clean TiO2(011) substrate in 1 × 10–7 mbar of oxygen at ∼450 °C and, second, by physical vapor deposition of Fe on clean TiO2(011) in 1 × 10–7 mbar of oxygen at ∼450 °C. For both preparation procedures an intermixed surface oxide is formed with Fe and Ti in 2+ and 4+ charge states, respectively. The surface is characterized by high-resolution and fast X-ray photoelectron spectroscopy (XPS), XPD, and low energy electron diffraction (LEED). Multiple electron scattering simulations implemented in the electron diffraction in atomic clusters (EDAC) package were performed for comparing experimental XPD patterns with structural models. The reliability of the approach was tested on XPD pattern of reconstructed clean TiO2(011)-2 × 1 surface, for which a structural model exists. The XPD pattern of the monolayer ternary iron titanium oxide is compared to structural models proposed by density functional theory (DFT)-based models.

Photoelectron diffraction from laser-aligned molecules with X-ray free-electron laser pulses.

We report on the measurement of deep inner-shell 2p X-ray photoelectron diffraction (XPD) patterns from laser-aligned I2 molecules using X-ray free-electron laser (XFEL) pulses. The XPD patterns of the I2 molecules, aligned parallel to the polarization vector of the XFEL, were well matched with our theoretical calculations. Further, we propose a criterion for applying our molecular-structure-determination methodology to the experimental XPD data. In turn, we have demonstrated that this approach is a significant step toward the time-resolved imaging of molecular structures.

Following the molecular motion of near-resonant excited CO on Pt(111): A simulated x-ray photoelectron diffraction study based on molecular dynamics calculations

A THz-pump and x-ray-probe experiment is simulated where x-ray photoelectron diffraction (XPD) patterns record the coherent vibrational motion of carbon monoxide molecules adsorbed on a Pt(111) surface. Using molecular dynamics simulations, the excitation of frustrated wagging-type motion of the CO molecules by a few-cycle pulse of 2 THz radiation is calculated. From the atomic coordinates, the time-resolved XPD patterns of the C 1s core level photoelectrons are generated. Due to the direct structural information in these data provided by the forward scattering maximum along the carbon-oxygen direction, the sequence of these patterns represents the equivalent of a molecular movie.

Acquisition of photoelectron diffraction patterns with a two-dimensional wide-angle electron analyzer

An efficient and fast way to measure photoelectron diffraction data over the full 2π angular range with high data point density is presented, taking advantage of the massive parallel detection capabilities of modern two-dimensional electron detectors. We introduce generic routines for data binning and for the mapping of the detector signal onto emission angles. X-ray photoelectron diffraction patterns taken from Bi(111) with the new detection scheme are compared to data sets taken with a conventional hemispherical analyzer equipped with a channeltron detector. As a result, the data acquisition time can be reduced by roughly a factor of ten while obtaining comparable if not superior data quality. The sampling technique is extended to UV-excited angle-resolved photoelectron spectroscopy as illustrated by a mapping of the Fermi surface of Cu(111).

A simple sample treatment on insulating materials for X-ray photoelectron diffraction: C 1s XPD of diamond (0 0 1) single crystal

A simple method of sample treatment for angle-scan X-ray photoelectron diffraction (XPD) for insulating diamond is demonstrated. The resulting XPD patterns have shown that a quantitative measure of near-surface atomic order is obtained for the insulating diamond. This simple method may be applied in principle to any insulating material, which enables non-destructive characterization of near-surface order of insulating materials by XPD.

Structural defects on a 1T-TiSe2 surface: Experiment and model calculation of photoelectron diffraction

A 1T-TiSe2 surface is studied by means of scanning tunneling microscopy (SMT) and X-ray photoelectron diffraction (XPD). The diffraction patterns were modeled in the multiple electron scattering approximation using the EDAC code. Variants of the point and structural defects on the 1T-TiSe2 surface are considered. The experimental and theoretical XPD patterns are compared on the basis of a convergence analysis of their R factors.

Characterization of 1T-TiSe2 surface by means of STM and XPD experiments and model calculations

Scanning tunneling microscopy (STM) and X-ray photoelectron diffraction (XPD) are applied to study the surface of layered dichalcogenide 1T-TiSe2. XPD pattern simulation for the 1T-TiSe2 surface is performed in the approach of electron multiple scattering within the EDAC code: considered are models of structural defects in the 1T-TiSe2 lattice, relaxation contraction (expansion) of surface layers and van der Waals gap, and deviation of the 1T-TiSe2 surface geometry from the basal plane (001). The atomic structure of 1T-TiSe2 surface layers is reconstructed from the XPD pattern on Se(LMM) and Ti2p core level using the photoelectron holography scattering pattern extraction algorithm with maximum entropy method (SPEA-MEM). The results of the 3D reconstruction are in agreement with the XPD pattern simulation data. In both cases, the TiSe2 surface corresponds to 1T polytype; an increase is observed in the parameter a0 and in the van der Waals gap between two surface slabs. It is assumed that similar structural distortions of the 1T-TiSe2 lattice lead to the formation of an energy gap between the valence band and the conduction band of titanium diselenide, which was observed earlier by photoemission spectroscopy and follows from the theoretical calculations.

Surface Investigation of Intermetallic PdGa(1̅ 1̅ 1̅)

The intermetallic PdGa is a highly selective and potent catalyst in the semihydrogenation of acetylene, which is attributed to the surface stability and isolated Pd atom ensembles. In this context PdGa single crystals of form B with (111) orientation were investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED) to study the electronic and geometric properties of this surface. UPS and thermal desorption spectroscopy (TDS) were used to probe the chemisorption behavior of CO. The PdGa(111) surface exhibits a (1 × 1) LEED and a pronounced XPD pattern indicating an unreconstructed bulk-truncated surface. Low-temperature STM reveals a smooth surface with a (1 × 1) unit cell. No segregation occurs, and no impurities are detected by XPS. The electronic structure and the CO adsorption properties reveal PdGa(111) to be a bulk-truncated intermetallic compound with Pd-Ga partial covalent bonding.

Observation and simulation of hard x ray photoelectron diffraction to determine polarity of polycrystalline zinc oxide films with rotation domains

X ray photoelectron diffraction (XPD) patterns of polar zinc oxide (ZnO) surfaces were investigated experimentally using hard x rays and monochromatized Cr Kα radiation and theoretically using a cluster model approach and a dynamical Bloch wave approach. We focused on photoelectrons emitted from the Zn 2p3/2 and O 1s orbitals in the analysis. The obtained XPD patterns for the (0001) and (0001¯) surfaces of a ZnO single crystal were distinct for a given emitter and polarity. Polarity determination of c-axis-textured polycrystalline ZnO thin films was also achieved with the concept of XPD, even though the in-plane orientation of the columnar ZnO grains was random.

Surface structure determination of Pd on W(100) using X-ray photoelectron diffraction

Ferromagnetic ordering in non-ferromagnetic transition metals induced by structural effects such as lattice expansion and stacking faults have been predicted by several theoretical studies. Different surfaces such as Nb(100), W(100) and C(0001) have been suggested as potential templates which could drive a Pd ultrathin film to a stable ferromagnetic state, depending on growth packing, film thickness and lateral lattice parameter. In this work we present an X-ray photoelectron diffraction (XPD) study of sub-monolayer to a few layers of Pd grown on W(100) and its surface structure determination. XPD patterns of Pd 3d and W 4f peaks were measured using synchrotron radiation with photon energy of 1810eV provided by the SXS beamline at the Brazilian Synchrotron Light Laboratory. Our results indicate that the first few monolayers of Pd grow on W(100) with bcc packing and lateral lattice parameter dramatically changed in comparison with the Pd(fcc) bulk values. Based on these results we also present a discussion on the magnetic properties of this system derived from spin-polarized DFT calculation.

Polarity determination of wurtzite-type crystals using hard x-ray photoelectron diffraction

The surface structure of a single-crystal ZnO wafer was studied by angle-resolved x-ray photoelectron spectroscopy (ARXPS) using synchrotron radiation. As a result, well-defined x-ray photoelectron diffraction (XPD) patterns were obtained for the (0001) and (0001¯) polar surfaces using the photoemission from the Zn 2p3/2 and O 1s core levels. The XPD patterns were indexed assuming forward scattering of photoelectrons by neighboring ions. Further, the XPD patterns for the (0001) and (0001¯) surfaces were different from each other, indicating the possibility for using the XPD technique for polarity determination.

X-ray Photoelectron Diffraction Study of bias-treatment for the growth of 1-inch-diameter Hetero-Epitaxial diamond (001) thick films

Bias-treatment (BT) is widely used as an initial seeding process of chemical vapor deposition (CVD) growth of diamonds. In a BT, substrates are negatively biased by an order of hundred volts so that positive ions in the plasma of CVD gas source bombard the substrate to form 'seeds' of diamond growth. Bias-treated 1-inch-diameter (ϕ) Ir(001)/MgO(001) substrates were used as the substrates of the CVD growth of 1-inch- ϕ diamond (001) thick films. XPS (X-ray Photoelectron Spectroscopy) spectra and C 1s and Ir 4f XPD (X-ray Photoelectron Diffraction) patterns were measured for the BT 1-inch- ϕ samples. 5 typical portions on the sample were measured; one at the central portion and the other 4 were peripheral portions, 90° apart from each other and ~4 mm from the edge of the sample. The uniformity of the result over the 5 portions was confirmed. The coverage of carbon layers on the sample after BT was evaluated to be ~ 50 ML in the unit of monolayer of Ir(001)1x1 surface. A quantitative measure of the effect of BT, A45° (the degree of variation of XPD pattern as defined as Imax−Imin/Ia, where Imax, Imin and Ia are the maximum, minimum and average intensities of photoelectrons in an azimuthal diffraction pattern at the polar angle of 45°, respectively), was examined for C 1s and Ir 4f core-levels for the 1-inch- ϕ BT samples. The A45° values appeared to be ~3% and ~17% for C 1s and Ir 4f, respectively. The results were compared with those for other bias-treated smaller samples reported so far. The A45° value of ~3% for C 1s is found to be very small compared to the corresponding values for the smaller samples reported so far. Thus, the method of BT for the present 1-inch- ϕ sample was found to have a large margin of improvement in terms of A45° for C 1s XPD.

Structure of the (010) surface of the orthorhombic complex metallic alloyT-Al3(Mn,Pd)

The atomic and electronic structures of the (010) surface of the $\mathrm{T}{\text{-Al}}_{3}$(Mn,Pd) complex metallic alloy is investigated by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), x-ray photoelectron diffraction (XPD), and ab initio calculations. While structural imperfections are observed at the surface and out of the various possible terminations, the puckered P2 layer is identified as the only surface termination, thus pointing out the existence of a well-defined minimum in the surface energy landscape. The measured step heights correspond to distances between identical planes along the [010] direction in the bulk model, i.e., $b/2$. A bias dependency of the STM topography is found. The XPD and LEED patterns confirm the pseudotenfold symmetry of the sample. XPS and UPS show a more metallic signature of the $\mathrm{T}$ phase compared to Al-based quasicrystalline phases.

Structure investigation of the (100) surface of the orthorhombicAl13Co4crystal

We present a detailed study of the (100) surface of the orthorhombic ${\text{Al}}_{13}{\text{Co}}_{4}$ crystal using both experimental and ab initio computational methods. This complex metallic alloy is an approximant of the decagonal Al-Ni-Co quasicrystalline phase. After sputter-annealing preparation of the surface at 1073 K, the low-energy electron diffraction pattern recorded exhibits a pseudotenfold symmetry with lattice parameters consistent with those of the bulk model. At this stage, scanning tunneling microscopy (STM) measurements reveal two different types of surface terminations. A comparison between these two surface structures and the bulk planes indicate that the terminations correspond either to an incomplete puckered layer $(P)$ or to an incomplete flat layer $(F)$. At 1173 K, the majority of the surface consists of $P$ layer terminations. STM images calculated from our proposed surface model are in good agreement with experimental images. X-ray photoelectron diffraction patterns and single scattering cluster calculations further confirm that the local atomic arrangements present in the bulk model are preserved within the near-surface region.