Experimental X-ray Photoelectron Diffraction Research Articles

Unravelling the surface structure of β-Ga2O3 (100).

The present work is on a comprehensive surface atomic structure investigation of β-Ga2O3 (100). The β-Ga2O3 single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the β-Ga2O3 surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the β-Ga2O3 surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

Surface structure characterization of a (√5 × √5)-R26.6° reconstruction of strontium titanate (001) by X-ray photoelectron diffraction

The surface composition of a (√5 × √5)-R26.6° (Rt5) reconstruction of a strontium titanate (001) single crystal surface was characterized by X-ray photoelectron spectroscopy (XPS), and its atomic surface structure was determined by a combination of low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the analyzed Rt5 reconstruction of the SrTiO3 (001) surface has two possible terminations: 40% of SrO and 60% of TiO2.

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.

Surface structure characterization by X-ray photoelectron diffraction of Sn ultra-thin films deposited on Pd(111)

The formation of surface alloys obtained by annealing ultra-thin films of Sn deposited on a Pd(111) surface was characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Annealing the surface at 600 K produced a (3x3)R30° LEED pattern, and the comparison between experimental and theoretical XPD results indicated the formation of a corrugated bi-dimensional Pd2Sn surface alloy, with Sn atoms being present not only on the outmost layer but also at least on the second internal layer.

Surface Characterization of NbO Islands Formed on Nb(100) by X-Ray Photoelectron Diffraction

An ordered thin film of niobium oxide (NbO) was formed on the Nb(100) surface by heating the niobium single crystal at 1023 K for 2 min under an oxygen atmosphere. The surface composition was analyzed by X-ray photoelectron spectroscopy (XPS), and the surface structure was characterized by low energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) generated by synchrotron radiation. Annealing at 1223 K in ultra-high vacuum yielded a reconstructed (3 × 1) LEED pattern. XPS measurements indicated that the surface comprised of metallic and oxidized niobium species. The comparison between experimental and theoretical XPD results indicated that 60% of the Nb(100) surface was covered by two-layered nanoislands of NbO, and 40% had bare patches.

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.

Forward scattering in hard X-ray photoelectron spectroscopy: Structural investigation of buried Mn–Ga films

X-ray photoelectron diffraction (XPD) in combination with hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the structure of buried layers in thin multilayer films. A detailed layer-by-layer investigation was performed using the element-specific, local-probe character of XPD. In the present work, angular-resolved HAXPES at a photon energy of 7.94 keV photon energy was used to investigate a Cr/Mn62Ga38/Mg/MgO multilayer system. Differences in the angular distributions of electrons emitted from Mn and Ga atoms revealed that the structure of Mn62Ga38 changes from L10 towards D022 for increasing annealing temperatures. A c/a ratio of 1.81 ± 0.06 was determined for the buried Mn62Ga38 layer in a D022 structure from the XPD experiment. The improvement of the structural order of the Mn62Ga38 layer is accompanied by an improvement of the structure of the overlying MgO layer.

Probing the surface atomic structure of Au/Cr2O3/Pd(111) by photoelectron diffraction

A detailed investigation concerning the surface atomic structure of the Au/Cr2O3 model catalyst deposited on a Pd(111) single crystal surface is presented. The system was prepared by molecular beam epitaxy (MBE) and characterized in situ by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The element-specific short-range order information was obtained from XPD experiments supported by a comprehensive multiple scattering calculation diffraction approach. Based on the experiments, we have strong evidence of Au island formation on the Cr2O3 surface. The experiments indicated that the islands are constructed of two Au monolayers and formed by the important structural relaxations in the three outermost atomic layers of the Au/Cr2O3 surface. Such a surface structure could explain the particular catalytic reactivity displayed by catalysts based on Au nanoparticles dispersed on several oxide matrices.

Preparation and investigation of the A-site and B-site terminated SrTiO3(001) surface: A combined experimental and theoretical x-ray photoelectron diffraction study

We have investigated surface composition, structure, and termination of SrTiO3(001) samples (STO) following typical steps of standard ex situ and in situ preparation procedures by performing x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD) experiments, and multiple scattering cluster (MSC) calculations. XPS has confirmed the presence of Sr2+, Ti4+, and O2-related to STO. Well-developed diffraction features have been observed in all XPD polar scans demonstrating the good structural ordering of the investigated surfaces. In order to permit a detailed comparison of XPD experiment and theory, we have especially taken care of effects due to the angular dependent instrument function by applying a new ratio procedure. Thus, we achieved a very good agreement between results of MSC calculations and experimental angular distributions. A fingerprint region has been identified in Sr3d polar scans, which has exhibited both evident dependencies on surface preparation steps in the experiments and clear sensitivity to surface termination in the theoretical modeling. In this manner, we have been able to describe as-received samples and in situ-treated samples as being TiO2- and SrO-terminated, respectively. Standard buffered hydrofluoric acid-treated samples turned out to be mostly TiO2-terminated but with a non-negligible admixture of different, likely SrO-type termination.

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.

Characterization of Ultra-Thin Films of Pd Deposited on Au(111)

Ultra-thin films (1 and 3 monolayers) of Pd were deposited on the Au(111) surface and then characterized by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger spectroscopy (XAES), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For the 1 ML Pd film annealed at 450 °C, XPS and XAES results indicated that Pd had diffused into the Au substrate. For the 3 ML Pd film deposited at room temperature, the comparison between experimental and theoretical XPD results indicated approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.

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.

Crystallographic structure of ultra-thin films of Pd on Ni(1 1 1) and Ni on Pd(1 1 1) studied by photoelectron diffraction

We report studies of ultra-thin films of Pd and Ni deposited on Ni(1 1 1) and Pd(1 1 1) surfaces, respectively, using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For a 1.5 ML Pd film deposited on Ni(1 1 1) at room temperature, XPS indicates that Pd grows in a layer-by-layer fashion and does not diffuse on the Ni substrate, whereas LEED exhibits a reconstructed pattern, which can be attributed to a distribution of bi-dimensional islands on the surface with a lateral lattice parameter different from that of Ni(1 1 1). Annealing the film at 650 °C produces a (1 × 1) LEED pattern, which suggests Pd diffusion and alloy formation. By using a systematic XPD analysis, we were able to determine that Pd diffused at least to the fourth layer into the Ni(1 1 1) substrate in low concentrations (10–20%), and that 75% of the surface remained covered by Pd bi-dimensional islands. The complementary system, Ni on Pd(1 1 1), presented similar LEED and XPS results. The comparison between experimental and theoretical XPD results indicated that the surface was partially covered by Ni islands (50–60%), and the other part was formed by random Ni x Pd 100− x surface alloy.

Surface composition and structure of nickel ultra-thin films deposited on Pd(111)

Ultra-thin nickel films deposited on the Pd(111) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). Up to 3 ML coverage, a LEED (1 × 1) pattern with a diffuse background due to a random distribution of Ni atoms on the surface is observed. Annealing at 600 °C reduced the background drastically and sharp (1 × 1) spots appeared on the screen, but XPS showed no presence of nickel on the surface, indicating diffusion into the bulk. Annealing at 300 °C for 30 min yielded also a sharp (1 × 1) LEED pattern, and the XPS Ni/Pd intensity ratio decreased with annealing time. The comparison between experimental and theoretical XPD results indicated that the surface was covered partially by Ni islands and partially by a random Ni–Pd surface alloy.

Structure analysis of the system Hafnium/Silicon [formula omitted] by means of X-ray photoelectron spectroscopy and X-ray photoelectron diffraction (XPD)

Due to the ongoing miniaturization of semiconductor devices new gate dielectrics are required for future applications. In this work we investigated hafnium silicide as a pre-system for hafnium oxide, one of the most promising candidates. One of the major problems of HfO 2 -films on silicon is the formation of hafnium silicide at the HfO 2 / Si interface. Therefore, ultrathin films of the system HfSi on Si ( 1 0 0 ) with a systematic varied thickness from 3 to 30 Å were prepared. Measurements were conducted by means of X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). Also full 2 π X-ray photoelectron diffraction (XPD) patterns with high spectral resolution were recorded. Against other reports related to thicker films, several heating cycles showed no phase transitions of the ultrathin films. However, above temperatures of 630 ∘ C an island formation is strongly indicated. The experimental XPD patterns are compared to simulated patterns of model structures. For the first time we present a modification of the C49 structure a possible structure for ultrathin HfSi 2 -films on bulk Si. As an outlook possibilities for preparing the system HfO 2 / Si ( 1 0 0 ) are introduced.

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.

Surface composition and structure of palladium ultra-thin films deposited on Ni(1 1 1)

Ultra-thin palladium films deposited on the Ni(1 1 1) surface were characterized by X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD). For low coverage, LEED shows a (1 × 1) pattern similar to that of the substrate. For intermediate coverage, the LEED pattern displays extra spots around the main (1 × 1) spots, resembling a Moiré coincidence pattern, probably associated with the formation of Pd bi-dimensional islands oriented in different directions on the Ni(1 1 1) surface. The results obtained by XPS and XPD corroborate this finding. The LEED pattern displays this structure up to 500 °C. Annealing at 650 °C brings back the (1 × 1) pattern, which is associated with a Pd island coalescence and alloy formation by Pd diffusion in the first atomic layers of the Ni(1 1 1). In this paper we present a detailed study of this surface structure via a comparison between XPD experiment and theory.

Structural investigation of the Ca/Si(111)-(3*2) surface using photoelectron diffraction

The atomic structure of the Ca induced Si(111)-(3×2) surface at 300 K has been investigated using X-ray photoelectron diffraction (XPD). After confirming the quality of the single-domain (3×2) surface by LEED, in which weak ×2 streaks were observed likewise the patterns reported in the literature, we have measured the Ca 2p core-level spectra within an azimuthal angle range of ±66°, where 0° corresponds to the direction perpendicular to the Ca chain, and at takeoff angles from 9° to 15° using the Mg Kα line. The experimental XPD patterns showed good agreement with the simulated XPD patterns of the T4 site model, while the simulated XPD patterns of the H3 model do not agree with the experimental results. Taking these results into account, we conclude that Ca atoms are adsorbed on the T4 site at 300 K and thus that the weak ×2 streaks do not result from the presence of two adsorption sites as proposed in the literature. [DOI: 10.1380/ejssnt.2006.166]