X-ray Photoelectron Diffraction Data Research Articles

Occupancy of lattice positions probed by X-ray photoelectron diffraction: A case study of tetradymite topological insulators

Occupancy of different structural positions in a crystal lattice often seems to play a key role in material properties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.

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.

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.

Photoelectron diffraction from single oriented molecules: Towards ultrafast structure determination of molecules using x-ray free-electron lasers

We provide a molecular structure determination method, based on multiple-scattering x-ray photoelectron diffraction (XPD) calculations. This method is applied to our XPD data on several molecules having different equilibrium geometries. Then it is confirmed that, by our method, bond lengths and bond angles can be determined with a resolution of less than 0.1 \AA{} and ${10}^{\ensuremath{\circ}}$, respectively. Differently from any other scenario of ultrafast structure determination, we measure the two- or three-dimensional XPD of aligned or oriented molecules in the energy range from 100 to 200 eV with a $4\ensuremath{\pi}$ detection velocity map imaging spectrometer. Thanks to the intense and ultrashort pulse properties of x-ray free-electron lasers, our approach exhibits the most probable method for obtaining ultrafast real-time structural information on small to medium-sized molecules consisting of light elements, i.e., a ``molecular movie.''

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.

Multiple scattering x-ray photoelectron diffraction study of the SrTiO3(100) surface

The atomic surface structure of SrTiO3(100) after annealing at 630 °C in vacuum is investigated by x-ray photoelectron diffraction (XPD) using the Sr 3d5/2 core level. The photoelectron diffraction peaks are successfully assigned by considering the forward scattering of photoelectrons by the atomic potential near the emitter atom in the lattice. The strongest diffraction peaks are aligned along the single crystal internuclear axes. We compare the results of photoelectron multiple scattering calculations (MSC) of SrO and TiO2 terminated SrTiO3(100) surfaces, including surface relaxation and rumpling, with the experimental data. For TiO2 and SrO terminated SrTiO3(100) surfaces, all top-layer cations relax inward, whereas second-layer atoms relax outward. The surface rumpling for SrO- and TiO2-terminated surfaces agrees well with low-energy electron diffraction results. Using a genetic algorithm the best agreement of MSC to the experimental XPD data is obtained for a SrO terminated surface with a 30% coverage of 3 ML SrO(100) islands.

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.

Structural investigations of epitaxial InN by x-ray photoelectron diffraction and x-ray diffraction

The authors investigated a 1μm thick molecular beam epitaxy–grown InN film by means of full hemispherical x-ray photoelectron diffraction and high resolution x-ray diffraction. While x-ray diffraction reveals that this nominally hexagonal InN layer contains roughly 1% of cubic phase InN, a comparison between measured and simulated x-ray photoelectron diffraction data allowed them to directly determine the polarity of the crystal. Furthermore, the data indicate that the InN surface consists of a mosaic of domains oriented at an azimuth of 180° to each other, where the azimuth corresponds to the rotation angle around the [0001] axis.

Surface Chirality of CuO Thin Films

We present X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) investigations of CuO thin films electrochemically deposited on an Au(001) single-crystal surface from a solution containing chiral tartaric acid (TA). The presence of enantiopure TA in the deposition process results in a homochiral CuO surface, as revealed by XPD. On the other hand, XPD patterns of films deposited with racemic tartaric acid or the "achiral" meso-tartaric acid are completely symmetric. A detailed analysis of the experimental data using single scattering cluster calculations reveals that the films grown with l(+)-TA exhibit a CuO(1) orientation, whereas growth in the presence of d(-)-TA results in a CuO(11) surface orientation. A simple bulk-truncated model structure with two terminating oxygen layers reproduces the experimental XPD data. Deposition with alternating enantiomers of tartaric acid leads to CuO films of alternating chirality. Enantiospecifity of the chiral CuO surfaces is demonstrated by further deposition of CuO from a solution containing racemic tartaric acid. The pre-deposited homochiral films exhibit selectivity toward the same enantiomeric deposition pathway.

Growth of Cr-doped TiO 2 films in the rutile and anatase structures by oxygen plasma assisted molecular beam epitaxy

As part of a search for new spintronic materials, highly ordered films of Cr x Ti 1− x O 2 in both the rutile and anatase structures and for several Cr concentrations ranging from x = 0.02 to 0.16 were grown by oxygen plasma assisted molecular beam epitaxy. X-ray photoelectron diffraction data of the Cr 2p level exhibit the same patterns and the same modulation amplitudes as those observed for Ti 2p, providing a strong indication that a large fraction of the Cr atoms occupy substitutional lattice sites in both structures. The Cr 2p core-level spectra as well as a Cr 3d related dopant signal above the valence band of TiO 2 are characteristic of Cr 3+ ions. At room temperature, Cr-doped anatase films exhibit ferromagnetic order with a saturation magnetization of ∼0.6 μ B per Cr atom and strong in-plane anisotropy.

Pure and cobalt-doped SnO 2(101) films grown by molecular beam epitaxy on Al 2O 3

Pure and Co-doped epitaxial SnO 2 films grown by oxygen plasma assisted molecular beam epitaxy on r-cut α-alumina substrates were investigated by electron diffraction, X-ray photoelectron spectroscopy (XPS), and X-ray photoelectron diffraction (XPD). On hot alumina substrates (∼ 800 °C) only a submonolayer amount of Sn adsorbs, indicating a strong adhesion of the first monolayer of tin on the alumina surface. SnO 2 films grown at ∼ 400–600 °C substrate temperature exhibit a SnO 2(101)[010]‖Al 2O 3(-1012)[12-10] epitaxial relationship. Subtle differences in the XPD data of SnO 2 films compared to measurements on SnO 2(101) single crystal surfaces are consistent with the presence of a high density of stoichiometric antiphase domain boundaries in the film. These planar defects are introduced in the SnO 2 film to compensate for the more than 10% lattice mismatch between the SnO 2 films and the alumina substrate along the SnO 2[-101] direction. Co x Sn 1− x O 2 films with a Co-cation concentration of 5–15% were also grown. XPS indicates that Co is in a 2+ oxidation state and XPD shows that tin is replaced substitutionally by Co.

Reactive growth of NiO ultrathin films on Pd(1 0 0): a multitechnique approach

Reactive growth of NiO ultrathin films on Pd(1 0 0) has been performed by evaporating metallic Ni in an oxygen atmosphere. The evolution of the ultrathin film is followed by means of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD) and scanning tunnelling microscopy (STM). The first monolayer (ML) of the deposited oxide develops as a completely wetting 2D overlayer with a c(4 × 2) periodicity, as shown by sharp LEED patterns and atomically resolved STM images. Further NiO deposition results in 3D NiO(1 0 0) islands formation on-top the c(4 × 2) superstructure, as shown by STM images and XPD data. Two layers thick islands are shown to be pseudomorphic to the substrate, i.e. characterised by in-plane compressive strain and interlayer expansion. LEED, XPD and STM give independent yet converging evidence that partial strain relaxation occurs within the third monolayer.

Epitaxial TiO2 nanoparticles on Pt(111): a structural study by photoelectron diffraction and scanning tunneling microscopy

Angle-scanned X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM) are used to characterise the structure of TiO2 nanoparticles grown on a Pt(111) single crystal surface. The nanoparticles grow over a well-ordered oxide interfacial layer that displays a (square root 43 x square root 43) - R7.6 degrees superstructure with a unit cell (18.2 x 18.2 A), as demonstrated by STM and low-energy electron diffraction (LEED). Our XPS Ti 2p core level spectra suggest a significant contribution from reduced titanium ions within the interfacial layer. On the contrary, according to XPS binding energy data, the nanoparticles are mostly composed of Ti(IV) ions. During the initial stage of the growth, the nanoparticles are on the average 2 nm high and about some tens of nm wide, and show a flat on-top surface, while the interparticle region show the structure of the ordered interfacial layer. During later stages of the deposition, the particles become larger and they show a more irregular, globular-like shape as well as coalescence. But, even at this stage of the growth, large interparticle regions are present. Moreover, the nanoparticles produce a distinct XPD pattern which demonstrates that they grow with a preferential azimuthal orientation with respect to the substrate surface. A simple geometrical analysis of the XPD data in terms of forward scattering events suggests that the particles crystallize in the rutile TiO2 structure and expose the (100) surface. This hypothesis is supported by means of multiple scattering simulations of the XPD patterns.

Reactive deposition of NiO ultrathin films on Pd(1 0 0)

NiO ultrathin films have been grown on Pd(1 0 0) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4 × 10 −6 mbar. The electronic and structural evolution of the resulting NiO(1 0 0) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(1 0 0) films even ∼50 equivalent monolayers thick can be grown with good long-range order, as shown by (1 × 1) LEED images.

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.

Composition and structure of ultrathin vanadium oxide layers deposited on SnO 2( [formula omitted

Ultrathin layers of vanadium oxides on SnO 2(1 1 0) were prepared by evaporation of metallic vanadium in oxygen atmosphere (partial pressure of O 2: 1×10 −5 Pa) at room temperature (RT). Vanadium oxide layers were deposited on the (4×1) reconstructed surface of SnO 2(1 1 0). The substrate surface was prepared by cycles of sputtering and annealing in vacuum. X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS) results show that the growth mechanism of the oxide layers involves the migration of tin atoms from the substrate onto the surface of the film already at RT. Although no LEED pattern is observed, the X-ray photoelectron diffraction (XPD) measurements reveal that the vanadium oxide films grow epitaxially on the substrate surface. The similarity between the Sn 3d XPD pattern measured for the clean SnO 2(1 1 0) surface and the V 2p pattern measured for the oxide layers suggests that these films have a structure close to that of the substrate (rutile-type lattice). The present results can be explained with a formation of a vanadium oxide of the series V n O 2 n−1 with 4⩽ n⩽8. These so called Magnéli phases of vanadium oxides have structures similar to that of rutile VO 2. This interpretation was confirmed by comparison of multiple scattering calculations with the experimental XPD curves. The vanadium oxide films are stable and the formation of mixed vanadium–tin oxide does not occur after annealing in vacuum at ≈800 K. The XPD data seem to indicate that tin atoms are present at the surface as a two-dimensional phase. STM images collected for the annealed phases reveal a morphology consistent with the XPS and LEIS results.

An X-ray photoelectron diffraction structural characterization of an epitaxial MnO ultrathin film on Pt(1 1 1)

An epitaxial ultrathin MnO/Pt(1 1 1) film (13 Å thick) has been grown by reactive UHV deposition of Mn 2(CO) 10 on Pt(1 1 1) at 200°C in the presence of water. Angle scanned X-ray photoelectron diffraction (XPD) and LEED have been used to structurally characterize the film. The observed 1×1 LEED pattern is in accord with the bulk lattice parameter of a MnO(1 1 1) surface (3.14 Å) and demonstrates that the film is ordered in the long range. Full hemispherical O1s and Mn2p XPD plots have been obtained and analysed on the basis of multiple scattering calculations. The XPD data confirm that the MnO(1 1 1) surface (as a single domain) is exposed and a best fit procedure based on a R-factor analysis provides a direct evidence for a relaxation of the outermost double layer, whose values are similar to those found in other similar systems (CoO and FeO).

Ultrathin vanadium films on Cu(001): structure and magnetism

X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and magnetic linear dichroism in angular distributions of photoelectrons (MLDAD) have been used to study the structural and magnetic properties of V thin films on Cu(001). For room-temperature growth, XPD and LEED data indicate that below 1 monolayer (ML), equivalent V films exhibit a (2×1) structure that evolves to four domains of bcc (110) at coverages above 1 ML. This multi-domain structure produces a LEED pattern often referred to as ‘(3×1)’. This structure persists to V coverages as high as 100 ML, and the LEED and XPD angular scans indicate that V in these films retains the bulk V lattice constant. MLDAD results for 1–5 ML V films at room temperature and at 150 K provide no evidence for in-plane magnetic ordering. Annealing to 450° C results in V clustering at low coverages accompanied by Cu diffusion. For thick V films, the primary affect of annealing is interdiffusion of Cu and V.

X-ray photoelectron diffraction study of an anatase thin film: TiO 2(001)

An anatase thin film, TiO 2(001), has been grown by metal–organic chemical vapor deposition on a SrTiO 3(001) substrate. The electronic and structural properties of the anatase thin film were investigated by X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Comparisons of the core-level binding energies for O 1s and Ti 2p 3/2 photoelectrons was made between anatase and rutile, and were found to be identical for both polymorphs. Structurally, the surface gave a (1×1) LEED pattern with a high background, indicating regions of good long-range order. Comparison between experimental XPD data and single-scattering cluster calculations indicate that there is good agreement with an anatase crystal termination, and poor agreement with a rutile crystal termination. Furthermore, good agreement was found at low takeoff angles for experiment and calculations suggesting that the thin film has good short-range order at the surface. This study indicates that anatase is stable and does not undergo a transformation to the rutile polymorph when exposed to atmosphere after film growth or after heating to low temperatures in ultrahigh vacuum.

Direct observation of subsurface oxygen on Rh(111)

A Rh(111) surface has been exposed to oxygen under conditions that favour the formation of a subsurface oxygen species. After exposure to 10 5 L O 2 at a surface temperature of 470 K X-ray photoelectron diffraction data of O 1s emission reveal that (5±2)% of a monolayer of oxygen atoms occupy octahedral interstitial sites just underneath fcc adsorption sites. The occupation of such a site forces the oxygen atoms adsorbed in its close vicinity to switch from the stable fcc site to the hcp adsorption site. Implications of this finding for the interpretation of temperature-programmed desorption spectra, as well as for the catalytic behaviour of the oxidized Rh(111) surface, are discussed.