Crystal Truncation Rod Measurements Research Articles

Real-time observation of liquid-gallium ordering on epitaxially-grown GaN(0001) by X-ray scattering measurements

X-ray crystal truncation rod (CTR) measurements are commonly used to analyze surface or interfacial atomic structures quantitatively. However, it is difficult to expand the measurement range to determine three-dimensional atomic structures in real time. We developed a method for CTR measurements that involves the simultaneous intensity measurements of X-rays and electron diffraction under molecular beam epitaxial conditions. Using the proposed method, we observed the formation of liquid gallium ordering on the GaN(0001) surface and determined the ordered structure within each layer.

Polarity‐Driven Atomic Displacements at the 2D Mg2TiO4‐MgO (001) Oxide Interface for Hosting Potential Interlayer Excitons

AbstractInterlayer excitons in solid‐state systems have emerged as candidates for realizing novel platforms ranging from excitonic transistors and optical qubits to exciton condensates. Interlayer excitons have been discovered in 2D transition metal dichalcogenides, with large exciton binding energies and the ability to form various van der Waals heterostructures. Here, an oxide system consisting of a single unit cell of Mg2TiO4 on MgO (001) is proposed as a platform for hosting interlayer excitons. Using a combination of density functional theory (DFT) calculations, molecular beam epitaxy growth, and in situ crystal truncation rod measurements, it is shown that the Mg2TiO4‐MgO interface can be precisely controlled to yield an internal electric field suitable for hosting interlayer excitons. The atoms in the polar Mg2TiO4 layers are observed to be displaced to reduce polarity at the interface with the non‐polar MgO (001) surface. Such polarity‐driven atomic displacements strongly affect electrostatics of the film and the interface, resulting in localization of filled and empty band‐edge states in different layers of the Mg2TiO4 film. The DFT calculations suggest that the electronic structure is favorable for localization of photoexcited electrons in the bottom layer and holes in the top layer, which may bind to form interlayer exciton states.

Solid state reduction of nickelate thin films

The square-planar nickelates are a class of superconductors analogous to the cuprates and promise to provide insight into the pairing mechanism in high-temperature superconducting oxides. The parent phase of doped superconducting films is ${\mathrm{NdNiO}}_{2}$, which is prepared by reducing ${3}^{+}$ Ni in ${\mathrm{NdNiO}}_{3}$ films. In this paper, we develop an ultrahigh vacuum reduction method using aluminum deposited on top of the ${3}^{+}$ nickelates and monitor the reduction process in real time through in situ crystal truncation rod measurements and diffraction x-ray absorption near edge spectroscopy measurements across the Ni $K$ edge. We establish a relation between Ni valence and the lattice constant of $\mathrm{NdNi}{\mathrm{O}}_{3\ensuremath{-}x}$ and show that the process can precisely control the oxygen content in the films. Finally, we extend the reduction process to quintuple square-planar nickelates.

Reversal of Anomalous Hall Effect and Octahedral Tilting in SrRuO3 Thin Films via Hydrogen Spillover.

The perovskite SrRuO3 (SRO) is a strongly correlated oxide whose physical and structural properties are strongly intertwined. Notably, SRO is an itinerant ferromagnet that exhibits a large anomalous Hall effect (AHE) whose sign can be readily modified. Here, a hydrogen spillover method is used to tailor the properties of SRO thin films via hydrogen incorporation. It is found that the magnetization and Curie temperature of the films are strongly reduced and, at the same time, the structure evolves from an orthorhombic to a tetragonal phase as the hydrogen content is increased up to ≈0.9 H per SRO formula unit. The structural phase transition is shown, via in situ crystal truncation rod measurements, to be related to tilting of the RuO6 octahedral units. The significant changes observed in magnetization are shown, via density functional theory (DFT), to be a consequence of shifts in the Fermi level. The reported findings provide new insights into the physical properties of SRO via tailoring its lattice symmetry and emergent physical phenomena via the hydrogen spillover technique.

A combined rotating disk electrode-surface x-ray diffraction setup for surface structure characterization in electrocatalysis.

Characterizing electrode surface structures under operando conditions is essential for fully understanding structure-activity relationships in electrocatalysis. Here, we combine in a single experiment high-energy surface x-ray diffraction as a characterizing technique with a rotating disk electrode to provide steady state kinetics under electrocatalytic conditions. Using Pt(111) and Pt(100) model electrodes, we show that full crystal truncation rod measurements are readily possible up to rotation rates of 1200 rpm. Furthermore, we discuss possibilities for both potentiostatic as well as potentiodynamic measurements, demonstrating the versatility of this technique. These different modes of operation, combined with the relatively simple experimental setup, make the combined rotating disk electrode-surface x-ray diffraction experiment a powerful technique for studying surface structures under operando electrocatalytic conditions.

Adsorption, surface relaxation and electrolyte structure at Pt(111) electrodes in non-aqueous and aqueous acetonitrile electrolytes.

In situ electrochemical surface X-ray diffraction was employed to investigate the atomic scale structure of the electrochemical double layer and the relaxation at the Pt(111) electrode surface in non-aqueous and aqueous acetonitrile electrolytes under potential control. The X-ray measurements provide insight into the potential-dependence of the interface structure by combining potentiodynamic measurements (X-ray voltammetry) with potentiostatic measurements (crystal truncation rod data) to probe both the metal and electrolyte sides of the interface. The crystal truncation rod measurements are consistent with the potential dependent reorientation of acetonitrile in the absence of water and a parallel arrangement in the presence of water. As acetonitrile concentration increases, the electron density closest to the electrode surface also increases. Finally, Pt surface relaxation in a range of aqueous and non-aqueous solvents is discussed in general with regards to the structure of the electrochemical double layer.

High-energy x-ray diffraction from surfaces and nanoparticles

High-energy surface-sensitive x-ray diffraction (HESXRD) is a powerful high-energy photon technique (E g 70 keV) that has in recent years proven to allow a fast data acquisition for the 3D structure determination of surfaces and nanoparticles under in situ and operando conditions. The use of a large-area detector facilitates the direct collection of nearly distortion-free diffraction patterns over a wide $q$ range, including crystal truncation rods perpendicular to the surface and large-area reciprocal space maps from epitaxial nanoparticles, which is not possible in the conventional low-photon energy approach ($E=10\ensuremath{-}20\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$). Here, we present a comprehensive mathematical approach, explaining the working principle of HESXRD for both single-crystal surfaces and epitaxial nanostructures on single-crystal supports. The angular calculations used in conventional crystal truncation rod measurements at low-photon energies are adopted for the high-photon-energy regime, illustrating why and to which extent large reciprocal-space areas can be probed in stationary geometry with fixed sample rotation. We discuss how imperfections such as mosaicity and finite domain size aid in sampling a substantial part of reciprocal space without the need of rotating the sample. An exact account is given of the area probed in reciprocal space using such a stationary mode, which is essential for in situ or operando time-resolved experiments on surfaces and nanostructures.

Bayesian inference of metal oxide ultrathin film structure based on crystal truncation rod measurements

Monte Carlo (MC)-based refinement software to analyze the atomic arrangements of perovskite oxide ultrathin films from the crystal truncation rod intensity is developed on the basis of Bayesian inference. The advantages of the MC approach are (i) it is applicable to multi-domain structures, (ii) it provides the posterior probability of structures through Bayes' theorem, which allows one to evaluate the uncertainty of estimated structural parameters, and (iii) one can involve any information provided by other experiments and theories. The simulated annealing procedure efficiently searches for the optimum model owing to its stochastic updates, regardless of the initial values, without being trapped by local optima. The performance of the software is examined with a five-unit-cell-thick LaAlO3 film fabricated on top of SrTiO3. The software successfully found the global optima from an initial model prepared by a small grid search calculation. The standard deviations of the atomic positions derived from a dataset taken at a second-generation synchrotron are ±0.02 Å for metal sites and ±0.03 Å for oxygen sites.

Electrochemical Oxidation of Smooth and Nanoscale Rough Pt(111): An In Situ Surface X-ray Scattering Study

The potential dependent surface structure of Pt(111) electrodes in electrochemical environment was studied by in situ crystal truncation rod measurements and in operando grazing incidence small angle X-ray scattering. Determination of the interface structure in the oxidation region reveal a continuous increase of the coverage of place exchanged atoms toward more positive potentials. The resulting oxide consists of an oxygen rich outer and a Pt-rich inner atomic layer. With increasing potential the oxide approaches a more uniform stoichometry. While place exchange on atomically smooth terraces only occurs above 1.05 V, structural changes of 3D Pt islands, grown by repetitive oxidation/reduction cycles, already commence at 0.8 V, which is assigned to oxidation at Pt steps. Evolution of the 3D island distribution occurs in all stages of the cycles: Whereas Pt oxidation leads to narrowing of the lateral size distribution, vertical island growth occurs during the subsequent reduction.

In situx-ray scattering study of Ag island growth on Si(111)7×7

We report on the epitaxial relationship between Ag and the Si(111)$7\ifmmode\times\else\texttimes\fi{}7$ substrate where the wetting layer and the emergence of islands was investigated using in situ x-ray scattering with a combination of grazing incidence diffraction, specular reflectivity, and crystal truncation rod measurements. The atomic-scale structure of the wetting layer evolves continuously with coverage until a transition where it ceases to change its structure concomitantly with the appearance of islands. The islands are observed to reside on the Si(111)$7\ifmmode\times\else\texttimes\fi{}7$ and, although the minimum average island height is three atomic layers of face-centered-cubic Ag, the average island height depends on the coverage and temperature. The majority of the Ag islands are oriented along the symmetry-equivalent Si crystallographic axes and a minority population of islands are rotated by $15.{7}^{\ensuremath{\circ}}$. A coincidence-site lattice model is used to show that kinetic considerations lead to the observed island orientations.

An in-situ X-ray diffraction study on the electrochemical formation of PtZn alloys on Pt(1 1 1) single crystal electrode

The electrochemical formation and dissolution of the oxygen reduction reaction (ORR) PtZn catalyst on Pt(111) surface is followed by in-situ X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements. When the crystalline Pt surface is polarized to sufficiently negative potential values, with respect to an Ag/AgCl|KCl reference electrode, the electrodeposited zinc atoms diffuse into the bulk and characteristic features are observed in the X-ray patterns. The surface structure and composition during deposition and dissolution is determined from analysis of XRR curves and measurements of crystal truncation rods. Thin Zn-rich surface layer is present during the alloy formation while a Zn-depleted layer forms during dissolution.

X-ray Reflectometry and Related Surface Near X-ray Scattering Methods

Abstract Surface sensitive X-ray scattering methods are mostly non-destructive tools which are frequently used to investigate the nature of thin films, interfaces and artificial near surface structures. Discussed here are diffraction based methods, namely reflectometry and the related techniques grazing incidence diffraction and crystal truncation rod measurements. For the experiment, an X-ray beam is diffracted from surface near structures of the sample and detected by adequate detectors. To analyze the data the according X-ray scattering theory has to be applied. The full theory of surface sensitive X-ray scattering is complex and based on general considerations from wave optics. However, instructive insights into the scattering processes are provided by the Born-approximation which in many cases yields sufficient results. The methods are applied to solve the structure of a mercury-electrolyte interface during a chemical reaction and to determine the strain distribution in surface near SiGe quantum dots.

A new x-ray interface and surface scattering environmental cell design for in situ studies of radioactive and atmosphere-sensitive samples

We present a novel design of a purpose-built, portable sample cell for in situ x-ray scattering experiments of radioactive or atmosphere sensitive samples. The cell has a modular design that includes two independent layers of containment that are used simultaneously to isolate the sensitive samples. Both layers of containment can be flushed with an inert gas, thus serving a double purpose as containment of radiological material (either as a solid sample or as a liquid phase) and in separating reactive samples from the ambient atmosphere. A remote controlled solution flow system is integrated into the containment system that allows sorption experiments to be performed on the diffractometer. The cell's design is discussed in detail and we demonstrate the cell's performance by presenting first results of crystal truncation rod measurements. The results were obtained from muscovite mica single crystals reacted with 1 mM solutions of Th(IV) with 0.1 M NaCl background electrolyte. Data were obtained in specular as well as off-specular geometry.

Reversal of chloride-induced Cu(001) subsurface buckling in the electrochemical environment: Anin situsurface x-ray diffraction and density functional theory study

The interface of Cu(001) electrode surfaces in 10 mM HCl solution was studied by in situ surface x-ray diffraction and density functional theory, focusing on the precise structure of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ Cl adlayer formed at positive potentials. Crystal truncation rod measurements in this adsorbate phase at a potential of $\ensuremath{-}0.20\text{ }{\text{V}}_{\text{Ag}/\text{AgCl}}$ reveal distinct differences to corresponding data by Tolentino et al. [Surf. Sci. 601, 2962 (2007)] for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ Cl structure formed at the Cu(001)-vacuum interface. Although in both environments, the atoms in the second Cu layer exhibit a small vertical corrugation, the sign of this corrugation is reversed. Furthermore, also the Cu-Cl bond distance and the average Cu interlayer spacings at the surface differ. Ab initio calculations performed for this adsorbate system reproduce these effects---specifically the reversal of the subsurface second-layer buckling caused in the presence of coadsorbed water molecules and cations in the outer part of the electrochemical double layer. In addition, studies at more negative potentials reveal a continuous surface phase transition to a disordered Cl adlayer at $\ensuremath{-}0.62\text{ }{\text{V}}_{\text{Ag}/\text{AgCl}}$, but indicate a substantial Cl coverage even at the onset of hydrogen evolution.

Hydrophilicity transition of the clean rutile TiO 2 (1 1 0) surface

We present contact angle measurements of water on single-crystal rutile TiO 2 (1 1 0) surfaces, exposed to ambient air, or protected in dry air. Our measurements indicate that the surfaces exposed to ambient air are hydrophobic, with a contact angle of θ = 61(5)°. However, the well-protected dry surface also exhibits some hydrophobic tendency, with θ = 32(5)°. It is known that UV irradiation transforms both surfaces superhydrophilic, with θ = 0° [R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431–432]. We also present preliminary X-ray crystal truncation rod measurements on the hydrophobic TiO 2 (1 1 0) surface, and of the effect of UV illumination on the surface.

Crystal Truncation Rod Measurements from Buried Quantum Dots

We present a method of modelling the X-ray diffraction (XRD) from buried quantum dots (QDs). Both diffuse scattering and crystal truncation rods can be modelled by calculating the scattering atomistically and adding a contribution from the bulk crystal. Because there are a large number of atoms, their positions cannot be fitted independently and Keating energy minimisation is used to ensure that the strain is physically realistic. Calculated scattering is compared with experimental data, and some of the challenges presented by real QDs are considered.

Nitric-oxide adsorption and oxidation on Pt( [formula omitted]) in electrolyte solution under potential control

We studied the adsorption and oxidation of NO adlayers on Pt(1 1 1) prepared from different NO precursors in an electrochemical cell in situ by surface X-ray diffraction and ex situ by X-ray photoemission spectroscopy. We found that NO adsorbs molecularly, independent of the preparation method. There is no relaxation of the outermost Pt atomic layer induced by the presence of adsorbed NO. We observe a surface structure of the Pt(1 1 1) very similar to the clean surface prepared under ultra-high vacuum conditions. A surface mediated oxidation process of the NO towards NO 2 is initiated at a potential of around 0.95 V vs. normal hydrogen electrode. The in situ measurements of crystal truncation rods show that this process is accompanied by a significant roughening of the surface.

Structure of the clean NiAl(1 1 0) surface and the Al 2O 3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al 2O 3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al 2O 3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni–Al topmost surface with an amplitude value of R Ni/ Al =0.16±0.01 A ̊ for the clean surface and R Ni/ Al =0.18±0.02 A ̊ for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8% (outwards) and the Ni sites are contracted by −3.2% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3%) relative to their bulk positions has been found, while the Ni atoms remain (−0.9%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4%). This chemical disorder was not enhanced by the presence of the Al 2O 3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.

Oxidation of NiAl(1 0 0) studied with surface sensitive X-ray diffraction

In this article surface X-ray diffraction experiments on the oxidation of the NiAl(1 0 0) surface will be described. Crystal truncation rod measurements show that the clean, flash-annealed surface is Ni terminated with an inward relaxation of the first layer of 3%. After oxidation at 950°C and 1×10 −6 mbar O 2 for 1000 s epitaxial oxide growth of θ-Al 2O 3 with monoclinic structure was observed, as determined from the oxide superstructure rods. The oxide layer forms a commensurate (2×1) superstructure with two in-plane domains that are rotated 90° with respect to each other. In addition, for each in-plane domain a twinning along the growth direction was revealed from the reflection splitting on the oxide layer superstructure rods, leading to a total of four different domains.

Strain as driving force for interface roughening of δ-doping layers

The growth of smooth and abrupt heteroepitaxial semiconductor interfaces is limited by strain originating from the lattice mismatch of the different materials. Employing measurements of crystal truncation rods, we have performed an extensive study of the impact of intra-layer strain on structural properties of ultra-thin ( δ) doping layers. The use of molecular beam epitaxy and related methods, i.e., the use of surfactants and low growth temperatures with subsequent annealing (solid phase epitaxy) allows the preparation of Bi, Sb, Ge and Si 1− x C x δ layers with doping profiles of monolayer width and high peak layer concentration (>10%). We find a linear dependence of the interface roughness on the intra-layer strain.