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The stacking sequence of hexagonal close-packed and related crystals typically results in steps on vicinal {0001} surfaces that have alternating A and B structures with different growth kinetics. However, because it is difficult to experimentally identify which step has the A or B structure, it has not been possible to determine which has faster adatom attachment kinetics. Here we show that in situ microbeam surface X-ray scattering can determine whether A or B steps have faster kinetics under specific growth conditions. We demonstrate this for organo-metallic vapor phase epitaxy of (0001) GaN. X-ray measurements performed during growth find that the average width of terraces above A steps increases with growth rate, indicating that attachment rate constants are higher for A steps, in contrast to most predictions. Our results have direct implications for understanding the atomic-scale mechanisms of GaN growth and can be applied to a wide variety of related crystals., The basal-plane surfaces of hexagonal close-packed crystals typically exhibit an alternating sequence of A and B steps with different atomic structures and growth kinetics. Here the authors demonstrate a method to determine whether A or B steps have faster kinetics under specific growth conditions.

Understanding surface dynamics during epitaxial film growth is key to growing high-quality materials with controllable properties. X-ray photon correlation spectroscopy (XPCS) using coherent X-rays opens new opportunities for in situ observation of atomic-scale fluctuation dynamics during crystal growth. Here, we present XPCS measurements of two-dimensional island dynamics during homoepitaxial growth in the layer-by-layer mode. Analysis of the results using two-time correlations reveals a new phenomenon—a memory effect in the arrangement of islands formed on successive crystal layers. Simulations indicate that this persistence in the island arrangements arises from communication between islands on different layers via adatoms. With the worldwide advent of new coherent X-ray sources, the experimental and analysis methods pioneered here will enable broad application of XPCS to observe atomic-scale processes on surfaces. In situ measurements based on coherent X-ray spectroscopy are performed during the epitaxial growth of gallium nitride films, revealing a memory effect in the arrangement of islands formed on successive crystal layers.

Miscut surfaces of layered crystals can exhibit a stair-like sequence of terraces having periodic variation in their atomic structure. For hexagonal close-packed and related crystal structures with an {\alpha}{\beta}{\alpha}{\beta} stacking sequence, there have been long-standing questions regarding how the differences in adatom attachment kinetics at the steps separating the terraces affect the fractional coverage of {\alpha} vs. {\beta} termination during crystal growth. To demonstrate how surface X-ray scattering can help address these questions, we develop a general theory for the intensity distributions along crystal truncation rods (CTRs) for miscut surfaces with a combination of two terminations. We consider half-unit-cell-height steps, and variation of the coverages of the terraces above each step. Example calculations are presented for the GaN (0001) surface with various reconstructions. These show which CTR positions are most sensitive to the fractional coverage of the two terminations. We compare the CTR profiles for exactly oriented surfaces to those for vicinal surfaces having a small miscut angle, and investigate the circumstances under which the CTR profile for an exactly oriented surface is equal to the sum of the intensities of the corresponding family of CTRs for a miscut surface., Comment: 15 pages, 15 figures, Supplemental with 9 tables specifying coordinates of the 9 reconstructions examined. v2 has edits for clarity, addition of supplemental. (arXiv admin note: substantial text overlap with arXiv:2007.05083)

Burton-Cabrera-Frank (BCF) theory has proven to be a versatile framework to relate surface morphology and dynamics during crystal growth to the underlying mechanisms of adatom diffusion and attachment at steps. For an important class of crystal surfaces, including the basal planes of hexagonal close-packed and related systems, the steps in a sequence on a vicinal surface can exhibit properties that alternate from step to step. Here we develop BCF theory for such surfaces, relating observables such as alternating terrace widths as a function of growth conditions to the kinetic coefficients for adatom attachment at steps. We include the effects of step transparency and step-step repulsion. A general solution is obtained for the dynamics of the terrace widths, assuming quasi-steady-state adatom distributions on the terraces. An explicit simplified analytical solution is obtained under widely applicable approximations. From this we obtain expressions for the full-steady-state terrace fraction as a function of growth rate. Fits of the theoretical predictions to recent experimental determinations of the steady-state and dynamics of terrace fractions on GaN (0001) surfaces during organo-metallic vapor phase epitaxy give values of the kinetic coefficients for this system. In Appendices, we also connect a model for diffusion between kinks on steps to the model for diffusion between steps on terraces, which quantitatively relates step transparency to the kinetics of atom attachment at kinks, and consider limiting cases of diffusion-limited, attachment-limited, and mixed kinetics., Comment: 22 pages, 11 fig, 5 tables, 3 appendices, and Supplemental. For clarity, arXiv:2007.05083 was split into two distinct papers focusing on separate subtopics. This paper focuses on developing BCF theory in presence of non-identical step-edges. The other paper arXiv:2010.06166, on the scattering experiments whose interpretations required the extensions to BCF

The high temperature growth behavior of epitaxial LaGaO3 thin films with and without SrO is determined with realtime x-ray scattering. We find SrO alters the thin film growth mode of LaGaO3, both when pre-deposited on a surface as well as when SrO and LaGaO3 are co-deposited. We also find that depositing a small amount of SrO on a LaGaO3 surface induces significant structural rearrangement in the film. We describe possible mechanisms under which these transformations may occur. The strong effect of SrO on the microstructure of La1-xSrxGaO3 likely has wider implications for other ionic conducting oxide materials.

Oxygen reduction and transport in electrochemical applications are strongly affected by the reactant gas composition, including impurities such as water. Understanding the relationship between oxygen reactivity and humidity is key for attaining stability and high efficiency of solid oxide fuel cells, which are especially attractive for transportation applications as low emission power generation devices. Both short- and long-term effects of moisture have previously been associated with use of ambient air. To better understand these effects, we study oxygen exchange kinetics at the cathode surface and the cathode/electrolyte interface using La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) epitaxial thin films as a model-surface system. In situ synchrotron X-ray techniques evaluate the oxygen reduction reaction (ORR) kinetics in response to environmental variables. The results suggest a clear enhancement of the ORR rate upon short term exposure of the perovskite-structured films to water. On the basis of these measurements, along with in situ X-ray characterization of Sr segregation and computational first-principles studies, we suggest a model for this increased ORR activity that may lead to further improvements by stabilizing active cathode surface configurations.

Methods are described to measure divergence by imaging the apparent source size using either slits to form a pinhole camera or a compound refractive lens. The values of brightness and coherent flux obtained are compared with those calculated for the undulator source, to evaluate the effects of beamline optics., In anticipation of the increased use of coherent X-ray methods and the need to upgrade beamlines to match improved source quality, here the coherence properties of the X-rays delivered by beamline 12ID-D at the Advanced Photon Source have been characterized. The measured X-ray divergence, beam size, brightness and coherent flux at energies up to 26 keV are compared with the calculated values from the undulator source, and the effects of beamline optics such as a mirror, monochromator and compound refractive lenses are evaluated. Diffraction patterns from slits as a function of slit width are analyzed using wave propagation theory to obtain the beam divergence and thus coherence length. Imaging of the source using a compound refractive lens was found to be the most accurate method for determining the vertical divergence. While the brightness and coherent flux obtained without a monochromator (‘pink beam’) agree well with those calculated for the source, those measured with the monochromator were a factor of three to six lower than the source, primarily because of vertical divergence introduced by the monochromator. The methods described herein should be widely applicable for measuring the X-ray coherence properties of synchrotron beamlines.

In situ synchrotron X-ray diffraction was used to investigate oxygen surface exchange behavior in La0.6Sr0.4Co0.2Fe0.8O3−δ/Gd2O3-doped CeO2/Y2O3-stabilized ZrO2 (LSCF/GDC/YSZ) epitaxial thin-film h...

We describe an instrument that exploits the ongoing revolution in synchrotron sources, optics, and detectors to enable in situ studies of metal-organic vapor phase epitaxy (MOVPE) growth of III-nitride materials using coherent x-ray methods. The system includes high-resolution positioning of the sample and detector including full rotations, an x-ray transparent chamber wall for incident and diffracted beam access over a wide angular range, and minimal thermal sample motion, giving the sub-micron positional stability and reproducibility needed for coherent x-ray studies. The instrument enables surface x-ray photon correlation spectroscopy, microbeam diffraction, and coherent diffraction imaging of atomic-scale surface and film structure and dynamics during growth, to provide fundamental understanding of MOVPE processes.

The A(n+1)B(n)O(3n+1) Ruddlesden-Popper homologous series offers a wide variety of functionalities including dielectric, ferroelectric, magnetic and catalytic properties. Unfortunately, the synthesis of such layered oxides has been a major challenge owing to the occurrence of growth defects that result in poor materials behaviour in the higher-order members. To understand the fundamental physics of layered oxide growth, we have developed an oxide molecular beam epitaxy system with in situ synchrotron X-ray scattering capability. We present results demonstrating that layered oxide films can dynamically rearrange during growth, leading to structures that are highly unexpected on the basis of the intended layer sequencing. Theoretical calculations indicate that rearrangement can occur in many layered oxide systems and suggest a general approach that may be essential for the construction of metastable Ruddlesden-Popper phases. We demonstrate the utility of the new-found growth strategy by performing the first atomically controlled synthesis of single-crystalline La3Ni2O7.

Proper bonding and assembly techniques are essential for fabrication of functional metal-based microdevices. Transient liquid phase (TLP) bonding is a promising technique for making enclosed metallic microchannel devices. In this paper, we report results of TLP bonding of Cu-based structures at temperatures between 823 K and 883 K (550 °C and 610 °C) with thin elemental Al foils as intermediate boding layers. In situ X-ray diffraction was utilized to examine the structure of Cu/Al interface in real time, resulting in a proposed sequence of structural evolution of the Cu/Al/Cu TLP bonding interface region. Three different types of bonding interface structures, the “γ 1 structure,” the “eutectoid structure” (“E structure”), and the “E/γ 1/E structure,” were observed through electron microscopy, and related to the proposed sequence of interfacial structural evolution. Tensile fracture tests were conducted on TLP-bonded Cu/Al/Cu coupon assemblies. Hardness of the various phases within the bonding interface region was probed with instrumented nanoindentation. Results of mechanical testing were correlated to the structure of the bonding interface region. The present results provide an understanding of the structural evolution within the Cu/Al/Cu TLP bonding interface region, and offer guidance to future bonding of Cu-based microsystems.

The interaction between X-rays and matter is an intriguing topic for both fundamental science and possible applications. In particular, synchrotron-based brilliant X-ray beams have been used as a powerful diagnostic tool to unveil nanoscale phenomena in functional materials. However, it has not been widely investigated how functional materials respond to the brilliant X-rays. Here, we report the X-ray-induced reversible resistance change in 40-nm-thick TiO2 films sandwiched by Pt top and bottom electrodes, and propose the physical mechanism behind the emergent phenomenon. Our findings indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging X-rays. We found that this effect, combined with the X-ray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a nonvolatile reversible resistance change. Understanding X-ray-controlled reversible resistance changes can provide possibilities to control initial resistance states of functional materials, which could be useful for future information and energy storage devices.

The high abundance and fast kinetics of select transition metal oxide catalysts are attractive features for many chemical and electrochemical device applications. However, the activity of such catalysts can be accompanied by phase instabilities that prevent their widespread usage. Furthermore, complexities associated with variations in phase behavior and oxygen stoichiometry have hindered studies on the true origins of catalytic activity. Here, we explore the interactions between activity, phase stability, and microstructure using in operando synchrotron X-ray techniques and gas chromatography/mass spectroscopy (GCMS) to probe the behavior of model SrCoO2.5+δ (SCO) catalysts. Pulsed laser deposition was used to prepare SCO thin films on (001) SrTiO3, (111) SrTiO3, and pseudocubic (001) DyScO3 substrates. The GCMS catalytic measurements were performed with a custom-built microreactor compatible with a synchrotron X-ray diffractometer at the Advanced Photon Source. The activity for carbon monoxide oxidation was determined as a function of temperature from 500 °C to 800 °C. We show that the SrCoO2.5+δ films are active for CO oxidation, most likely by direct reaction with lattice oxygen; consequently, the activity was observed to increase as the films become less stable, with the most active film being the one exhibiting the lowest surface and crystal quality. All films decompose at high temperatures, with in operando diffraction indicating the gradual formation of Sr-rich hexagonal and CoO phases. We find that real-time studies of model oxide systems with synchrotron X-rays is a powerful means of gaining insight into the varied processes taking place at catalytic surfaces.The high abundance and fast kinetics of select transition metal oxide catalysts are attractive features for many chemical and electrochemical device applications. However, the activity of such catalysts can be accompanied by phase instabilities that prevent their widespread usage. Furthermore, complexities associated with variations in phase behavior and oxygen stoichiometry have hindered studies on the true origins of catalytic activity. Here, we explore the interactions between activity, phase stability, and microstructure using in operando synchrotron X-ray techniques and gas chromatography/mass spectroscopy (GCMS) to probe the behavior of model SrCoO2.5+δ (SCO) catalysts. Pulsed laser deposition was used to prepare SCO thin films on (001) SrTiO3, (111) SrTiO3, and pseudocubic (001) DyScO3 substrates. The GCMS catalytic measurements were performed with a custom-built microreactor compatible with a synchrotron X-ray diffractometer at the Advanced Photon Source. The activity for carbon monoxide oxidation...

Functional oxide superlattices offer new and exciting possibilities for the exploration of emergent properties at the nanoscale. While the behavior of La1−xSrxFeO3 films has been extensively investigated at low temperatures, few studies have been carried out at high temperatures, particularly for LaFeO3/SrFeO3 superlattice systems. Here, we investigate the transport behavior and optical properties of (LaFeO3)n/(SrFeO3)1 superlattices at 373 K and above. Using optical spectroscopy, we observe a low energy excitation at ∼1 eV, attributable to charge transfer between the O 2p and Fe 3d states of the δ-doped single SrFeO3 layer. From in-plane conductivity measurements on the superlattices, we determine activation energies that are much lower than those of alloyed samples and vary with the total number of SrFeO3 layers. This suggests that polaronic transport is confined near the SrFeO3 regions, permitting mobilities significantly enhanced over those in alloyed thin films.

The restricted volume above and below the sample on a six-circle diffractometer can limit the size and complexity of sample environments used in surface diffraction studies. An alternative configuration of the diffractometer, where the sample normal is aligned parallel to the χ axis, allows for ample space above and below the χ circle for instrumentation. The merits of this approach are outlined and angles are derived for the diffraction condition for constant-incident-angle, constant-sample-azimuthal-angle and specular geometries. Using a version of this code written forSPEC(http://www.certif.com/content/spec/), sample specular and nonspecular crystal truncation rods measured from a 5 nm-thick thin film are presented.

Resonant anomalous X-ray reflectivity (RAXR) is a powerful technique for measuring element-specific distribution profiles across surfaces and buried interfaces. Here, the RAXR technique is applied to characterize a complex oxide heterostructure, La0.6Sr0.4Co0.2Fe0.8O3−δ, on NdGaO3, and the effects of data sampling and model-dependent fitting procedures on the extracted elemental distribution profile are evaluated. The strontium profile through a 3.5 nm-thick film at 973 K and at an oxygen partial pressure of 150 Torr (1 Torr = 133.32 Pa) was determined from the measured RAXR spectra. The results demonstrate thatin situRAXR measurements can provide key insights into temperature- and environment-dependent elemental segregation processes, relevant, for example, in assessing the cathode performance of solid oxide fuel cells.

Oxygen vacancies in proximity to surfaces and heterointerfaces in oxide thin film heterostructures have major effects on properties, resulting, for example, in emergent conduction behaviour, large changes in metal-insulator transition temperatures or enhanced catalytic activity. Here we report the discovery of a means of reversibly controlling the oxygen vacancy concentration and distribution in oxide heterostructures consisting of electronically conducting In2O3 films grown on ionically conducting Y2O3-stabilized ZrO2 substrates. Oxygen ion redistribution across the heterointerface is induced using an applied electric field oriented in the plane of the interface, resulting in controlled oxygen vacancy (and hence electron) doping of the film and possible orders-of-magnitude enhancement of the film's electrical conduction. The reversible modified behaviour is dependent on interface properties and is attained without cation doping or changes in the gas environment., Oxygen vacancies near the interface in oxide heterostructures can lead to large changes in properties, including metal–insulator transition temperatures or catalytic activity. Here, the authors demonstrate a way to reversibly control the oxygen-vacancy concentration and distribution in oxide heterostructures.

Advances in synthesis techniques and materials understanding have given rise to oxide heterostructures with intriguing physical phenomena that cannot be found in their constituents. In these structures, precise control of interface quality, including oxygen stoichiometry, is critical for unambiguous tailoring of the interfacial properties, with deposition of the first monolayer being the most important step in shaping a well-defined functional interface. Here, we studied interface formation and strain evolution during the initial growth of LaAlO3 on SrTiO3 by pulsed laser deposition, in search of a means for controlling the atomic-sharpness of the interfaces. Our experimental results show that growth of LaAlO3 at a high oxygen pressure dramatically enhances interface abruptness. As a consequence, the critical thickness for strain relaxation was increased, facilitating coherent epitaxy of perovskite oxides. This provides a clear understanding of the role of oxygen pressure during the interface formation, and enables the synthesis of oxide heterostructures with chemically-sharper interfaces., Comment: 25 pages

The nucleation and growth of Cu2O and NiO islands due to oxidation of CuxNi1–x(001) films were monitored, at various temperatures, by in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM) and in situ synchrotron X-ray diffraction (XRD). In remarkable contrast to our previous observations of Cu and Cu–Au oxidation, irregular-shaped polycrystalline oxide islands formed with respect to the Cu – Ni alloy film, and an unusual second oxide nucleation stage was noted. In situ XRD experiments revealed that NiO formed first epitaxially, then other orientations appeared, and finally polycrystalline Cu2O developed as the oxidation pressure was increased. The segregation of Ni and Cu towards or away, respectively, from the alloy surface during oxidation could disrupt the surface and cause polycrystalline oxide formation.

The initial oxidation behavior of Cu0.5Au0.5 (0 0 1) thin film was investigated by in situ ultrahigh vacuum transmission electron microscopy to model nano-oxidation of alloys with one active component and one noble component. The formation of irregular-shaped octahedron Cu2O islands with cube-on-cube crystallographic orientation to the substrate film was observed at all temperature studied. The energetics of Cu2O nucleation for Cu and Cu0.5Au0.5 oxidation was compared. Cu0.5Au0.5 oxidation has lower nucleation activation energy due to the reduced mismatch strain between Cu2O and Cu0.5Au0.5 films. On the other hand, the reaction kinetics for Cu0.5Au0.5 alloy oxidation is slower due to the higher diffusion activation energy of Cu. � 2006 Elsevier B.V. All rights reserved.

Nanofluids, i.e. fluid suspensions of nanometer-sized solid particles and fibers, have been proposed as a route for surpassing the performance of heat transfer liquids currently available. Recent experiments on nanofluids have indicated significant increases in thermal conductivity compared with liquids without nanoparticles or larger particles, strong temperature dependence of thermal conductivity, and significant increases in critical heat flux in boiling heat transfer. Some of the experimental results are controversial, e.g. the extent of thermal conductivity enhancement sometimes greatly exceeds the predictions of well-established theories. So, if these exciting results on nanofluids can be confirmed in future systematic experiments, new theoretical descriptions may be needed to account properly for the unique features of nanofluids, such as high particle mobility and large surface-to-volume ratio.

In situ synchrotron X-ray scattering was used to observe both the growth of PbTiO3 films by metal-organic chemical vapour deposition and the behaviour of the ferroelectric phase transition as a function of film thickness. The dependences of growth mode and deposition rate on gas flows and substrate temperature were determined by homoepitaxial growth studies on thick films (50 nm). These studies facilitated the growth of thin coherently strained PbTiO3 films on SrTiO3 (001) substrates, with thicknesses ranging from 2 to 42 nm. Experiments on the ferroelectric phase transition as a function of film thickness were carried out in these films under controlled mechanical and electrical boundary conditions.

Using non-equilibrium molecular dynamics simulations in which a temperature gradient is imposed, we study how the ordering of the liquid at the liquid–solid interface affects the interfacial thermal resistance. Our simulations of a simple monoatomic liquid show no effect on the thermal transport either normal to the surface or parallel to the surface. Even for of a liquid that is highly confined between two solids, we find no effect on thermal conductivity. This contrasts with well-known significant effect of confinement on the viscoelastic response. Our findings suggest that the experimentally observed large enhancement of thermal conductivity in suspensions of solid nanosized particles (nanofluids) can not be explained by altered thermal transport properties of the layered liquid.

▪ Abstract Nanofluids, consisting of nanometer-sized solid particles and fibers dispersed in liquids, have recently been demonstrated to have great potential for improving the heat transfer properties of liquids. Several characteristic behaviors of nanofluids have been identified, including the possibility of obtaining large increases in thermal conductivity compared with liquids without nanoparticles, strong temperature-dependent effects, and significant increases in critical heat flux. Observed behavior is in many cases anomalous with respect to the predictions of existing macroscopic theories, indicating the need for a new theory that properly accounts for the unique features of nanofluids. Theoretical studies of the possible heat transfer mechanisms have been initiated, but to date obtaining an atomic- and microscale-level understanding of how heat is transferred in nanofluids remains the greatest challenge that must be overcome in order to realize the full potential of this new class of heat transfer fluids.

We have used in situ synchrotron x-ray scattering to investigate PbTiO3 films grown epitaxially on SrTiO3 (001) substrates. When the films are cooled below a thickness-dependent Curie temperature, the ferroelectric phase forms as 180° stripe domains with a well-defined period. With further cooling, the period changes fairly abruptly from a lower to a higher value, suggesting a transition between two stripe domain phases. The periods of both stripe phases are observed to depend on the square root of film thickness, in agreement with theory.

As part of a program to understand and control the structure of ferroelectric thin films grown by metal organic vapor phase epitaxy (MOVPE), we have used X-ray scattering to observe the surface structure of PbTiO 3 films during and following growth. Moderately high energy (24 keV) X-rays are used to penetrate the chamber walls for in situ measurements in the high-temperature, reactive MOVPE environment. Performing measurements in situ allows us to study the growth process in real time, to control the thickness of the films to sub-unit-cell accuracy, to observe the surface structure in equilibrium with the vapor, and to preserve film stoichiometry during high-temperature study by maintaining an overpressure of PbO. While the higher X-ray energy also permits a large volume of reciprocal space to be mapped, it presents challenges for surface scattering due to the small critical angle. Examples of results will be presented from studies of surface structure dynamics, crystal growth, and ferroelectric stripe domains in thin films.

The grain-size dependent thermal conductivity of nanocrystalline yttria-stabilized zirconia from 6–480K is reported. The thermal conductivity for a grain size of 10 nm is reduced to approximately half that of coarse-grained or single-crystal material at all measured temperatures. A method for determining the interfacial resistance to thermal transport in polycrystalline materials from measurements of grain-size-dependent thermal conductivity is described and applied. The results suggest a new strategy for identifying improved thermal barrier materials by choosing materials with large interfacial thermal resistance and reduced dimensionality or grain size, rather than by focusing on minimization of bulk thermal conductivity alone.

Recent measurements on nanofluids have demonstrated that the thermal conductivity increases with decreasing grain size. However, such increases cannot be explained by existing theories. We explore four possible explanations for this anomalous increase: Brownian motion of the particles, molecular-level layering of the liquid at the liquid/particle interface, the nature of heat transport in the nanoparticles, and the effects of nanoparticle clustering. We show that the key factors in understanding thermal properties of nanofluids are the ballistic, rather than diffusive, nature of heat transport in the nanoparticles, combined with direct or fluid-mediated clustering effects that provide paths for rapid heat transport.

Proton nuclear magnetic resonance spectra of nanocrystalline n-PdH 0.7 at room temperature reveal two distinct, motionally-narrowed peaks, attributed to crystallite interstitial and interfacial hydrogen populations. In addition, proton spin-lattice relaxation measurements show enhanced relaxation rates. Both results are qualitatively consistent with a previously-observed hydrogen concentration enhancement in grain boundaries of α-phase n-PdH x . Magnetization recovery data for n-PdH 0.7 also reveal that the separate populations of hydrogen do not exchange on time scales of up to order 0.1 s, a surprising result considering the correlation time associated with spin-lattice relaxation, of order 10 −8 s.

We report on the synthesis, stability, and local structure of In2O3 thin films grown via rf-magnetron sputtering and characterized by in-situ x-ray scattering and focused x-ray nanodiffraction. We find that In2O3 deposited onto (0 0 1)-oriented single crystal yttria-stabilized zirconia substrates adopts a Stranski–Krastanov growth mode at a temperature of 850 °C, resulting in epitaxial, truncated square pyramids with (1 1 1) side walls. We find that at this temperature, the pyramids evaporate unless they are stabilized by a low flux of In2O3 from the magnetron source. We also find that the internal lattice structure of one such pyramid is made up of differently strained volumes, revealing local structural heterogeneity that may impact the properties of In2O3 nanostructures and films.

Thin film In2O3/YSZ heterostructures exhibit significant increases in electrical conductance with time when small in-plane electric fields are applied. Contact resistances between the current electrodes and film and between the current electrodes and substrate are responsible for the behavior. With an in-plane electric field, different field profiles are established in the two materials, with the result that oxygen ions can be driven across the heterointerface, altering the doping of the n-type In2O3. A low frequency inductive feature observed in AC impedance spectroscopy measurements under DC bias conditions was found to be due to frequency-dependent changes in the contact resistance.

In developing cost-effective complex oxide materials for the oxygen evolution reaction, it is critical to establish the missing links between structure and function at the atomic level. The fundamental and practical implications of the relationship on any oxide surface are prerequisite to the design of new stable and active materials. Here we report an intimate relationship between the stability and reactivity of oxide catalysts in exploring the reaction on strontium ruthenate single-crystal thin films in alkaline environments. We determine that for strontium ruthenate films with the same conductance, the degree of stability, decreasing in the order (001)(110)(111), is inversely proportional to the activity. Both stability and reactivity are governed by the potential-induced transformation of stable Ru(4+) to unstable Ru(n4+). This ordered(Ru(4+))-to-disordered(Ru(n4+)) transition and the development of active sites for the reaction are determined by a synergy between electronic and morphological effects.

We present real-time X-ray scattering studies of the influence of silicon on the homoepitaxial growth mode of GaN grown by metal-organic vapor-phase epitaxy. Both annealing of Si-doped GaN and surface dosing of GaN with disilane are shown to change the mode of subsequent growth from step-flow to layer-by-layer. By comparing the growth behavior induced by doped layers which have been annealed to that induced by surface dosing, we extract an approximate diffusion coefficient for Si in GaN of 3.5 ×10 −18 cm 2 / s at 810°C.

Nominally 2{sup o} vicinal GaN(0001) surfaces exhibit monolayer-height steps at 990{sup o}C in the metal-organic chemical vapor deposition environment. Real-time x-ray scattering observations at 715--990{sup o}C indicate that there is a tendency for step bunching during growth. Below 850{sup o}C, step bunches nucleated during growth remain and coarsen after growth, while above 850{sup o}C, the surface reverts to monolayer-height steps after growth. Surfaces vicinal toward the {l_brace}1{bar 1}00{r_brace} and the {l_brace}11{bar 2}0{r_brace} planes exhibit similar behavior. We suggest a simple equilibrium surface orientational phase diagram for vicinal GaN(0001) that is consistent with these observations.

Out-of-plane structures of the GaN (0 0 0 1) surface in the metal-organic chemical vapor deposition (MOCVD) environment have been determined using in situ grazing-incidence X-ray scattering. We measured 11 2 l crystal truncation rod intensities at a variety of temperatures and ammonia partial pressures on both sides of the 1×1 to ( 3 ×2 3 ) R 30° surface phase transition. The out-of-plane structure of the ( 3 ×2 3 ) R 30° phase appears to be nearly independent of temperature below the transition, while the structure of the 1×1 phase changes increasingly rapidly as the phase transition is approached from above. A model for the structure of the 1×1 phase with a partially occupied top Ga layer agrees well with the data. The observed temperature dependence is consistent with a simple model of the equilibrium between the vapor phase and the surface coverage of Ga and N. In addition, we present results on the kinetics of reconstruction domain coarsening following a “quench” into the ( 3 ×2 3 ) R 30° phase field.

Oxide nanofluids were produced and their thermal conductivities were measured by a transient hot-wire method. The experimental results show that these nanofluids, containing a small amount of nanoparticles, have substantially higher thermal conductivities than the same liquids without nanoparticles. Comparisons between experiments and the Hamilton and Crosser model show that the model can predict the thermal conductivity of nanofluids containing large agglomerated Al2O3 particles. However, the model appears to be inadequate for nanofluids containing CuO particles. This suggests that not only particle shape but size is considered to be dominant in enhancing the thermal conductivity of nanofluids.

Vapor-phase processes such as chemical vapor deposition (CVD) and reactive ion etching are the primary methods for the production-scale synthesis and processing of many high-quality thin-film materials. For example, these processes are widely used in the microelectronics industry for synthesis and lithography of the various semiconducting, insulating, and conducting layers in devices. Understanding the means of controlling the microstructure and composition of these materials is of great technological interest. However a difficulty often encountered in developing vapor-phase processes is an undesirable dependence on trial-and-error methods for optimizing the many process parameters. These parameters include gas composition, flow rate, pressure, and substrate temperature, all of which are typically changing with time. This reliance on empirical methods can be attributed to the tremendous chemical and physical complexity of vapor-phase processes and the lack of appropriate in situ measurement techniques for the vapor-phase environment.We have initiated a program to apply synchrotron x-ray analysis techniques as real-time probes of film and surface structure during vapor-phase processing. X-rays have a combination of properties which makes them particularly well-suited for these studies. Unlike electrons, x-rays have a sufficiently low absorption to penetrate vapor-phase processing environments and chamber walls. Unlike visible light, x-rays have wavelengths and energies suitable for study of atomic-scale structure and chemistry. A growing number of in situ synchrotron x-ray investigations of film growth and processing demonstrate the power of these techniques.