Surface X-ray Diffraction Experiments Research Articles

Effect of TaN buffer layer on the sensitivity of ASIC-integrated AMR sensors

This study investigates the impact of a TaN buffer layer on the performance of ASIC-integrated Anisotropic Magnetoresistive (AMR) position sensors. Our findings demonstrate that the inclusion of a TaN buffer layer significantly enhances the sensitivity and stability of the sensors, with a measured sensitivity of 0.678 ± 0.03 Gauss/mV. In contrast, devices without the buffer layer exhibit a notable decline in sensitivity, decreasing to 0.165 ± 0.02 Gauss/mV after 450℃ annealing. In addition, AMR sensors fabricated on non-ASIC substrates showed no such deterioration. Magneto-optical Kerr effect (MOKE) measurements reveal that the change in sensitivity is primarily attributed to variations in the anisotropy field. We propose that the increased anisotropy field is a result of magnetostatic energy induced by alterations in the PECVD-deposited SiO2 surface granular structure, supported by surface morphology characterization and X-ray diffraction experiments. These findings highlight the critical role of the TaN buffer layer in optimizing the performance and stability of ASIC-integrated AMR sensors.

Electrochemical oxidation of Pt(111) beyond the place-exchange model

Oxide formation plays an important role in the degradation of Pt electrocatalysts. However, the exact oxide structure and reaction mechanism are not fully understood. Here, we used in situ surface X-ray diffraction experiments to resolve the oxide formation at a Pt(111) model electrode at potentials near the onset of the oxygen evolution reaction. Fast experiments are possible by using X-ray photons with a high kinetic energy in combination with a large 2D detector. By employing very low potential sweep rates we obtain a more ordered oxidized surface compared to literature data from potential step experiments. This demonstrates that the oxidation process is strongly governed by the reaction kinetics. The increased surface order enables us to disentangle two subsequent oxidation process; initially the place-exchange process, followed by the formation of a partially disordered oxide in which still 50% of the surface atoms reside on sites commensurate to the Pt(111) surface. The reduction experiments indicate that the place-exchange process is structurally reversible, whereas the disordered oxide causes the surface roughening observed during potential cycling. Despite the increased surface order, oxide superstructures are not observed. These results provide important insights in the oxidation and degradation process of Pt(111), which are valuable for the design of improved electrocatalysts and they rationalize operating procedures.

Processing and properties of translucent bismuth sodium titanate ceramics

Lead-free bismuth sodium titanate piezoceramics were processed by a solid-state route with a novel precursor approach. By carefully controlling the processing parameters and sintering atmospheres, translucent ceramics with pore-free, homogeneous microstructures with exceptionally low dielectric loss at elevated temperatures. The pore-free microstructure can influence the operating life of a dielectric device positively, since pores and other microstructural defects are usually responsible for material failures within devices. Synchrotron and surface X-ray diffraction experiments revealed a clear dependence of the crystal structure on sintering parameters and defect chemistry. Microstructural analysis showed a dependency of a secondary phase on the sintering atmosphere. The grain size could be adjusted using a shortened grinding process instead of using the common methods like increasing calcining or sintering temperature.

Real-time monitoring the growth of strained off-stoichiometric NixFe3−xO4 ultrathin films on MgO(001)

Ni x Fe 3 − x O4 thin films with varying Ni amount (0 ≤ x ≤ 1.5) were deposited on MgO(001) via reactive molecular beam epitaxy. The growth process was monitored during film deposition by means of X-ray diffraction. All prepared films exhibit a well-ordered structure with complete vertical crystallinity throughout the whole film growth and flat surfaces of the final films independent of the Ni amount. An enhancement of the vertical compression in the initial growth continuously decreases up to a film thickness of 8 nm. During further growth, all films exhibit residual and constant vertical compression with lateral adaption of the final films to the substrate lattice, as observed by high energy surface X-ray diffraction experiments. Hard X-ray photoelectron spectroscopy measurements of the final films reveal increasing Fe3+:Fe2+ ratios for higher Ni content and point to additional NiO agglomerations within the films exceeding the stoichiometric Ni amount of x = 1.

Surface Oxidation of Pt(111) Studied By Surface X-Ray Diffraction and Grazing-Incidence Small-Angle X-Ray Scattering

Electrochemical growth of Pt oxide is important because of its role in the oxygen-reduction reaction (ORR) and in Pt dissolution, which affect the performance and durability of PEM fuel cell Pt electrocatalysts. It has been known for a long time that potential cycling leads to a restructuring of the surface, in which Pt atoms move away from their lattice sites. The details of this process have not been fully resolved, though early surface X-ray diffraction (SXRD) studies confirmed the key role of a "place exchange" in which Pt and O (or OH) species exchange places [1]. Reported here are in-situ SXRD and Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) studies of Pt(111) electrodes in 0.1 M HClO4 carried out at the European Synchrotron Radiation Facility [2-6]. The atoms are located by a crystal-truncation rod analysis, which shows that the place-exchanged Pt is located directly above its original lattice site. Simultaneous cyclic voltammograms of current and the intensity of the (1,1,1.5) reflection associate the initiation of the place exchange with the "oxide" peak at 1.05 V vs RHE. The voltammogram is unchanged by cycling to 1.15 V, as the place exchange is reversed on the reduction cycle, albeit with significant hysteresis. Cycling to higher potentials leads to irreversible changes to the surface, which were studied in detail with GISAXS. GISAXS probes the evolution of nanoscale features on the surface. Progressive surface roughening occurs with cycling, to an extent that is dependent on the upper reversal potential (E up). The average spacing of the islands ranges from 40-70 Å for E up from 1.4-1.6 V, with smaller spacing for higher potentials. The spacing increases slightly with cycle number but is mainly determined after the first cycle. The islands grow in height and become more prominent and homogeneous in size with the number of cycles. These changes are similar to the evolution of Pt surfaces in UHV undergoing deposition of Pt or surface erosion, showing that the underlying processes such as vacancy coalescence are common for the UHV and electrochemical cases. SXRD experiments in the presence of oxygen show no effect on the place exchange during cycling. The oxygen reduction current decreases at potentials more negative than the initiation of the place exchange, showing that it is limited by the adsorbed intermediate(s) in the ORR (likely OH) and not Pt oxide formation. The authors thank the European Synchrotron Radiation Facility, Deutsche Forschungsgemeinschaft, and the Natural Sciences and Engineering Research Council of Canada for support. [1] H. You, D.J. Zurawski, Z. Nagy, R.M. Yonco, J. Chem. Phys., 100, 4699 (1994). [2] J. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, D.A. Harrington, Electrochim. Acta, 224, 220 (2017).[3] M. Ruge, J. Drnec, B. Rahn, F. Reikowski, D.A. Harrington, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, J. Am. Chem. Soc., 139, 4532 (2017).[4] M. Ruge, J. Drnec, B. Rahn, F. Reikowski, D.A. Harrington, F. Carla, R. Felici, J. Stettner, O.M. Magnussen, J. Electrochem. Soc., 164, H608 (2017).[5] J. Drnec, M. Ruge, F. Reikowski, B. Rahn, F. Carlà, R. Felici, J. Stettner, O.M. Magnussen, D.A. Harrington, Pt oxide and oxygen reduction at Pt(111) studied by SXRD, Electrochem. Comm., 84, 50 (2017).[6] J. Drnec, D.A. Harrington, O.M. Magnussen, Current Opinion in Electrochemistry, (2017), doi:10.1016/j.coelec.2017.09.021

Kinetic Monte Carlo simulations of GaN homoepitaxy on c- and m-plane surfaces.

The surface orientation can have profound effects on the atomic-scale processes of crystal growth and is essential to such technologies as GaN-based light-emitting diodes and high-power electronics. We investigate the dependence of homoepitaxial growth mechanisms on the surface orientation of a hexagonal crystal using kinetic Monte Carlo simulations. To model GaN metal-organic vapor phase epitaxy, in which N species are supplied in excess, only Ga atoms on a hexagonal close-packed (HCP) lattice are considered. The results are thus potentially applicable to any HCP material. Growth behaviors on c-plane (0001) and m-plane (011¯0) surfaces are compared. We present a reciprocal space analysis of the surface morphology, which allows extraction of growth mode boundaries and direct comparison with surface X-ray diffraction experiments. For each orientation, we map the boundaries between 3-dimensional, layer-by-layer, and step flow growth modes as a function of temperature and growth rate. Two models for surface diffusion are used, which produce different effective Ehrlich-Schwoebel step-edge barriers and different adatom diffusion anisotropies on m-plane surfaces. Simulation results in agreement with observed GaN island morphologies and growth mode boundaries are obtained. These indicate that anisotropy of step edge energy, rather than adatom diffusion, is responsible for the elongated islands observed on m-plane surfaces. Island nucleation spacing obeys a power-law dependence on growth rate, with exponents of -0.24 and -0.29 for the m- and c-plane, respectively.

Moiré beatings in graphene on Ru(0001)

The moir\'e superstructure of a single layer of carbon on ruthenium, where $25\ifmmode\times\else\texttimes\fi{}25$ graphene honeycombs sit on $23\ifmmode\times\else\texttimes\fi{}23$ substrate unit cells, is determined from first principles. The density functional theory (DFT) calculations predict two kinds of structural units, $\ensuremath{\Omega}$ and $Y$, in the supercell, which are identified as moir\'e beatings or moirons. The related topographic bucklings, or ``hills,'' have distinct carbon conformations and a height of 1.16 \AA{}. The different moirons are observed with scanning tunneling microscopy (STM), and surface x-ray diffraction (SXRD) also discriminates the two. This connects ab initio DFT calculations with STM and SXRD experiments in unit cells containing more than 4000 atoms.

Atomic-layer synthesis and imaging uncover broken inversion symmetry in La2−xSrxCuO4films

Materials with broken inversion symmetry are of great interest for their exotic electronic properties but are relatively rare. We show here that even if a material is inversion-symmetric in the bulk form, in epitaxial thin films of the same compound the crystallographic unit cell can be quite asymmetric. To demonstrate this, we have used atomic layer-by-layer molecular beam epitaxy to synthesize La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$ films with Sr-dopant atoms deposited above, below, or on both sides of each CuO${}_{2}$ layer. Surface x-ray diffraction experiments analyzed by the Coherent Bragg Rod Analysis (COBRA) method have been carried out in combination with energy-differential diffraction measurements to determine whether the actual Sr distribution coincided with the nominal one (that would be expected in absence of La-Sr interdiffusion). The differential approach minimizes the systematic errors, and the combination with COBRA is potentially capable to determine the concentration of atomic species on a monoatomic layer-by-layer basis. The results show that the concentration of Sr in La/Sr layers just above the CuO${}_{2}$ layers is much larger than in layers just below them, irrespective of the deposition sequence, and drastically breaking the inversion symmetry.

Stable Cation Inversion at theMgAl2O4(100)Surface

From an interplay of atom-resolved noncontact atomic force microscopy, surface x-ray diffraction experiments, and density functional theory calculations, we reveal the detailed atomic-scale structure of the (100) surface of an insulating ternary metal oxide, MgAl2O4 (spinel). We surprisingly find that the MgAl2O4(100) surface is terminated by an Al and O-rich structure with a thermodynamically favored amount of Al atoms interchanged with Mg. This finding implies that so-called Mg-Al antisites, which are defects in the bulk of MgAl2O4, become a thermodynamically stable and integral part of the surface.

Wurtzite-Type CoO Nanocrystals in Ultrathin ZnCoO Films

Surface x-ray diffraction experiments reveal that, in cobalt-doped ZnO films two to five monolayers thick, Wurtzite-type CoO nanocrystals are coherently embedded within a hexagonal boron-nitride- (h-BN)-type ZnO matrix, supporting the model of a phase separation. First-principles calculations confirm that, in contrast with ZnO, the formation of h-BN-type CoO is unfavorable in the ultrathin film limit. Our results are important for understanding magnetic properties of transition metal-doped semiconductors in general.

Structure of thePd8Ni92(110)catalytic surface from first principles

We have used spin-polarized density functional theory calculations to investigate the structure of the ${\mathrm{Pd}}_{8}{\mathrm{Ni}}_{92}(110)$ alloy surface, which has been extensively studied for its excellent catalytic properties. Our calculations have corroborated that this alloy has a quite complex ($N\ifmmode\times\else\texttimes\fi{}1$, $N\ensuremath{\sim}6$) structure, so-called ``sawtooth'' (ST) reconstruction. Moreover, we have accurately computed the metal-metal bond distances and the interlayer spacings. We have also proved that the inclusion of the spin-polarization effects is crucial for a good description of the energetics. Our results agree nicely with those arising from scanning tunneling microscopy and surface x-ray diffraction experiments. In addition, our model brings insights into the driving forces leading to the ST reconstruction.

Effect of Sodium Dodecyl Sulfate at Different Hydration Conditions on Dioleoyl Phosphatidylcholine Bilayers Studied by Grazing Incidence X-ray Diffraction

The effect of the surfactant content and hydration conditions in the phases of dioleoyl phosphatidylcoline (DOPC)/sodium dodecyl sulfate (SDS) mixtures was studied. To this end, surface X-ray diffraction experiments have been performed on bilayers of the mixtures deposited on hydrophobic silicon wafers by dip coating. To investigate the effect of relative humidity (RH) on bilayer organization, a humidity chamber with dry-wet air control was used, and RH values were fixed between 1 and 65%. Our results showed, in addition to the lamellar phase, a rhombohedral phase in mixtures at low hydration conditions (RH < 30%). The d spacing between lamellae increased with the RH and SDS content. This fact could be associated with a swelling effect that is probably due to the localization of water molecules between the polar headgroups of the DOPC and SDS forming the bilayers. The electron-density profiles calculated by Fourier reconstruction of the lamellar stacking for the different samples also confirmed this fact. In addition, the increase in d spacing could be related to the increase in the hydrophilic character of the mixture when the SDS content increases. The rhombohedral phase was more clearly observed in mixtures with high SDS content. Thus, the stalk structure of the rhombohedral phase could be facilitated because of the SDS contribution to inverse structures.

Spin Reorientation and Structural Relaxation of Atomic Layers: Pushing the Limits of Accuracy

The correlation between an ad-layer-induced spin reorientation transition (SRT) and the ad-layer-induced structural relaxation is investigated by combined in situ surface x-ray diffraction and magneto-optical Kerr-effect experiments on Ni/Fe/Ni(111) layers on W(110). The Fe-induced SRT from in-plane to out-of-plane, and the SRT back to in-plane upon subsequent coverage by Ni, are each accompanied by a small lattice relaxation of at most 0.002 A. Such a small strain variation excludes a magnetoelasticity driven SRT, and we suggest the interface anisotropy as a possible driving force.

Faster acquisition of structure-factor amplitudes in surface X-ray diffraction experiments

The traditional way of measuring intensities in surface X-ray diffraction experiments is by performing rocking scans. Owing to experimental time limitations, this rather slow procedure leads to incomplete data sets for large surface unit cells. This lack of information not only may affect adversely the accuracy of the refined atomic positions, but also hinders the application of three-dimensional direct methods of structure solution. Here the viability of the alternative data collection strategy proposed by Specht &amp; Walker [J. Appl. Cryst.(1993),26, 166–171] is investigated by using as test material a 50 Å thick NBCO thin film grown on an SrTiO3(001) substrate. This procedure, which is based on stationaryLscans,i.e.the sample is kept fixed during theLscan, shortens the measuring time by an order of magnitude. In this study, special attention has been paid to the validation of the predicted theoretical Lorentz factor, which is specific for this experimental setup.

Three-dimensional Strain Relaxation in the As/Si (001)-(2×1) Surface

Surface X-ray diffraction experiments and elastic-energy calculations have been performed to determine strain relaxation in the As/Si (001)-(2×1) surface. The result of the in-plane analysis shows that symmetric As dimers formed on the Si(001) substrate have a bond length of 2.54 Å, while the inward displacement of second-layer Si atoms is 0.19 Å in an irreducible (2×1) unit cell. From the subsequent out-of-plane analysis, we confirmed that the Si atoms in the third and fourth substrate layers are displaced upward (x=0.75) and downward (x=0.25) alternately. The observation is supported by an elastic-energy calculation using the Keating model. The determined structure includes in-plane and out-of-plane substrate Si atoms up to the sixth layer. A preliminary measurement of a fractional-order reflection intensity as a function of temperature has shown that the As/Si(001)-(2×1) surface becomes more ordered in the temperature range of 710 to 817 K. The intensity rapidly decreases above 817 K, which is interpreted to be due to the desorption of As2 molecules from the surface with an activation energy of 5.9±1.1 eV.

Oxidative etching of cleaved synthetic diamond {1 1 1} surfaces

In this study, three commonly used methods for oxidative etching of diamond {1 1 1} faces are compared: gas phase etching using `dry' oxygen, gas phase etching using an oxygen/water vapour mixture and liquid etching in molten potassium nitrate. The synthetic diamond surfaces are prepared by cleavage. The morphology of the surfaces is studied using atomic force microscopy and the kinetics of the reactions is determined by measuring the decrease in thickness of the diamond. The atomic arrangement of the {1 1 1} surfaces etched in oxygen/water is studied using surface X-ray diffraction. Upon dry oxygen etching, the {1 1 1} faces are roughened and become morphologically unstable. This observation conflicts with standard theory, which predicts {1 1 1} to be a stable F-face that should etch via a layer mechanism. A possible explanation for this is chemical roughening. The addition of water vapour to the oxygen has a dramatic effect on the etching mechanism of the {1 1 1} faces. Now etching proceeds via a layer mechanism involving monoatomic steps. Shallow etch pits are formed, of which the slope increases for increasing etching temperature. Surface X-ray diffraction experiments show that the surface is –OH terminated. For potassium nitrate etching, the {1 1 1} face etches also via a layer mechanism and triangular etch pits with rounded corners are formed, having point or flat bottoms. This etching technique appears to be the best method to reveal different types of defects ending on diamond {1 1 1} surfaces.

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.

Surface X-Ray Diffraction of 5-I-Uracil/Ag(111) and 2-S-Uracil/Ag(111)

We present surface X-ray diffraction experiments on ordered large organic monolayers, As model systems we have chosen two uracil derivatives, namely 5-I-uracil (IU) and 2-S-uracil (SU) which on Ag(111) form a uniaxially incommensurate and a commensurate superstructure, respectively. These are characterized by transformation matrices (263/1.78 7 1 ) for IU and ( 4 2 5 3 ) for SU. Although the measured reflection intensities are extremely weak (about 2 to 4 orders of magnitude weaker than those near the antiphase condition along the Ag crystal truncation rods), about 40 independent inplane reflections (hk0) could be measured in each case allowing the development of structure models. In the case of IU adsorption a surface reaction takes place where a 5-5'-di-uracil molecule is formed after separation of the I atom from the pyrimidine rings. The formation of ordered structures needs mild thermal activation (T - 400 K). The I atoms form a separate I/ Ag(111)-(√3 × √3) phase. In the case of SU no reaction is observed and the unit cell contains four molecules. The arrangement of the SU molecules in the surface unit cell is similar to the arrangement in bulk SU. The commensurability of the SU superstructure is explained by the strong S-Ag interaction.

Nature of the Low-Temperature3×3Surface Phase of Pb/Ge(111)

The structure of the low-temperature 3 X 3 surface phase of Pb/Ge(111) has been determined by surface x-ray diffraction experiments. Analysis of the data reveals that the Pb layer exhibits a vertical corrugation of similar to 0.4 Angstrom, affecting also the two top Ge layers. Significant distortions are found down to the third Ge layer. This structural model supports that the 3 X 3 ground state can be best described as a cooperative distortion stabilized by a gain in electronic energy. [S0031-9007(99)08749-9].

Ultrahigh vacuum/high pressure chamber for surface x-ray diffraction experiments

We describe an ultrahigh vacuum chamber that can be internally pressurized to several bars and that is designed to perform surface x-ray diffraction experiments on solid-gas interfaces. The chamber has a cylindrical beryllium window that serves as the entrance and exit for the x rays. The sample surface can be ion bombarded with an ancillary ion gun and annealed to 1200 K.