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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.

How materials evolve at thermal equilibrium and under external excitations at small length and time scales is crucial to the understanding and control of material properties. X-ray photon correlation spectroscopy (XPCS) at X-ray free electron laser (XFEL) facilities can in principle capture dynamics of materials that are substantially faster than a millisecond. However, the analysis and interpretation of XPCS data is hindered by the strongly fluctuating X-ray intensity from XFELs. Here we examine the impact of pulse-to-pulse intensity fluctuations on sequential XPCS analysis. We show that the conventional XPCS analysis can still faithfully capture the characteristic time scales, but with substantial decrease in the signal-to-noise ratio of the g2 function and increase in the uncertainties of the extracted time constants. We also demonstrate protocols for improving the signal-to-noise ratio and reducing the uncertainties.

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

Solid-phase epitaxy (SPE) and other three-dimensional epitaxial crystallization processes pose challenging structural and chemical characterization problems. The concentration of defects, the spatial distribution of elastic strain, and the chemical state of ions each vary with nanoscale characteristic length scales and depend sensitively on the gas environment and elastic boundary conditions during growth. The lateral or three-dimensional propagation of crystalline interfaces in SPE has nanoscale or submicron characteristic distances during typical crystallization times. An in situ synchrotron hard x-ray instrument allows these features to be studied during deposition and crystallization using diffraction, resonant scattering, nanobeam and coherent diffraction imaging, and reflectivity. The instrument incorporates a compact deposition system allowing the use of short-working-distance x-ray focusing optics. Layers are deposited using radio-frequency magnetron sputtering and evaporation sources. The deposition system provides control of the gas atmosphere and sample temperature. The sample is positioned using a stable mechanical design to minimize vibration and drift and employs precise translation stages to enable nanobeam experiments. Results of in situ x-ray characterization of the amorphous thin film deposition process for a SrTiO3/BaTiO3 multilayer illustrate implementation of this instrument., Comment: 28 pages, 6 figures

Watching defects in heated thin films The response of materials to external conditions depends on small-scale features such as defects and grain boundaries. Yau et al. heated gold thin films and used coherent x-ray diffractive imaging to track how these microstructures developed during grain growth (see the Perspective by Suter). The technique allowed nondestructive visualization of the features in three dimensions. The method should help link external stimuli to material response through changes in microstructure, thereby allowing development of novel materials through microstructural engineering. Science , this issue p. 739 ; see also p. 704

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.

International audience; III-As nanowires are candidates for near infrared light emitters and detectors that can be directly integrated onto silicon. However, nanoscale to microscale variations in structure, composition, and strain within a given nanowire, as well as variations between nanowires, pose challenges to correlating microstructure with device performance. In this work, we utilize coherent nano-focused x-rays to characterize stacking defects and strain in a single InGaAs nanowire supported on Si. By reconstructing diffraction patterns from the 2110 Bragg peak, we show that the lattice orientation varies along the length of the wire, while the strain field along the cross-section is largely unaffected, leaving the band structure unperturbed. Diffraction patterns from the 0110 Bragg peak are reproducibly reconstructed to create three-dimensional images of stacking defects and associated lattice strains, revealing sharp planar boundaries between different crystal phases of wurtzite (WZ) structure that contribute to charge carrier scattering. Phase retrieval is made possible by developing multi-angle Bragg projection ptychography (maBPP) to accommodate coherent nanodiffraction patterns measured at arbitrary overlapping positions at multiple angles about a Bragg peak, eliminating the need for scan registration at different angles. The penetrating nature of x-ray radiation, together with the relaxed constraints of maBPP, will enable in operado imaging of nanowire devices.

The generation of two X-ray pulses with tunable nanosecond scale time separations has recently been demonstrated at the Linac Coherent Light Source using an accelerator based technique. This approach offers the opportunity to extend X-ray Photon Correlation Spectroscopy techniques to the yet unexplored regime of nanosecond timescales by means of X-ray Speckle Visibility Spectroscopy. As the two pulses originate from two independent Spontaneous Amplified Stimulated Emission processes, the beam properties fluctuate from pulse pair to pulse pair, but as well between the individual pulses within a pair. However, two-pulse XSVS experiments require the intensity of the individual pulses to be either identical in the ideal case, or with a accurately known intensity ratio. We present the design and performances of a non-destructive intensity diagnostic based on measurement of scattering from a transparent target using a high-speed photo-detector. Individual pulses within a pulse pair with time delays as short as 0.7 ns can be resolved. Moreover, using small angle coherent scattering, we characterize the averaged spatial overlap of the focused pulse pairs. The multi-shot average-speckle contrasts from individual pulses and pulse pairs are compared.

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 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 ${(0 0 0 1)}$ and m-plane ${(0 1 \overline{1} 0)}$ 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 m- and c-plane, respectively., 30 pages, 40 figures

The astrolabe is an ancient analogue astronomical computing device used for calculations relating to position and time of the observer’s location. In its most common form (the planispheric astrolabe), it consists of an engraved plate or series of plates held together and pinned in a housing, the assembly usually being made of brass. The present study describes the use of X-ray diffraction (XRD) and X-ray fluorescence (XRF) in a synchrotron to elucidate the composition of, and fabrication techniques used for, the major component parts of the astrolabe. The synchrotron XRF studies are compared to similar studies made with a handheld XRF instrument and the advantages and disadvantages of both approaches are discussed.

The multilayer Laue lens (MLL) is a novel diffractive optic for hard X-ray nanofocusing, which is fabricated by thin film deposition techniques and takes advantage of the dynamical diffraction effect to achieve a high numerical aperture and efficiency. It overcomes two difficulties encountered in diffractive optics fabrication for focusing hard X-rays: (1) small outmost zone width and (2) high aspect ratio. Here, we will give a review on types, modeling approaches, properties, fabrication, and characterization methods of MLL optics. We show that a full-wave dynamical diffraction theory has been developed to describe the dynamical diffraction property of the MLL and has been employed to design the optimal shapes for nanofocusing. We also show a 16 nm line focus obtained by a partial MLL and several characterization methods. Experimental results show a good agreement with the theoretical calculations. With the continuing development of MLL optics, we believe that an MLL-based hard x-ray microscope with true nanometer resolution is on the horizon.

Synchrotron X rays were used to perform nondestructive transmission diffraction and fluorescence experiments on a group of 24 European and Islamic astrolabes dated between 1350–1720 A.D. in order to determine their compositions. A group of six astrolabes produced in Lahore between 1601–1662 A.D. were found to contain a mixed α+β brass microstructure, proving that the brass was produced by a comelting technique rather than the traditional cementation process. The results also show evidence of dezincification, attributed to heavy annealing of the brass during astrolabe manufacture. This effect was so severe that an accurate analysis of the bulk Zn composition could not be determined from the fluorescence results alone; transmission X-ray diffraction gives a more accurate measurement of the bulk Zn composition.

The planispheric astrolabe occupies a special place among early scientific instruments because of its mathematical sophistication, its elegant appearance, and its usefulness to astronomers in pre-telescopic times. The operation of the astrolabe is based on the mathematical properties of stereographic projection, concepts that were understood by the time of Ptolemy in the second century a.d., although surviving instruments are from much later periods.

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.

We introduce a time-delayed beam splitting method based on the energy separation of x-ray photon beams. It is implemented and theoretically substantiated on an example of an x-ray optical scheme similar to that of the classical Michelson interferometer. The splitter/mixer uses Bragg-case diffraction from a thin diamond crystal. Another two diamond crystals are used as back-reflectors. For energy separation the back-reflectors are set at slightly different temperatures and angular deviations from exact backscattering. Because of energy separation and a minimal number (three) of optical elements, the split-delay line has high efficiency and is simple to operate. Due to the high transparency of diamond crystal, the split-delay line can be used in a beam sharing mode at x-ray free-electron laser facilities. The delay line can be made more compact by adding a fourth crystal.

A hard X-ray delay line device capable of splitting single FEL X-ray pulses into two adjustable fractions and recombining them with the goal of performing X-ray Photon Correlation Spectroscopy and pump - probe type studies was designed. The performance of the device has been verified at the XPP and XCS instruments of LCLS. The measured throughput of the device at 7.9 keV is 3.6%. The coherence properties of the LCLS beam passing through the delay line were investigated by analyzing speckle patterns produced by single LCLS pulses. A high speckle contrast of 0.69 was found, indicating the feasibility of performing coherence based experiments with the delay line.

Reaction pathways for complete decomposition of ammonia on the InN (0001) surface are investigated using first principles calculations. We show that while the initial NH(3) decomposition on this surface can proceed by H dissociation, its further decomposition is most favorable by H transfer. The calculated low diffusion barriers for the decomposed species on the surface imply that the metal-organic chemical vapor deposition growth of InN is a reaction-limited process rather than diffusion-limited at low adsorbate coverage.

The Hard X-ray Nanoprobe Beamline is a precision platform for scanning probe and full-field microscopy with 3–30 keV X-rays. A combination of high-stability X-ray optics and precision motion sensing and control enables detailed studies of the internal features of samples with resolutions approaching 30 nm., The Hard X-ray Nanoprobe Beamline (or Nanoprobe Beamline) is an X-ray microscopy facility incorporating diffraction, fluorescence and full-field imaging capabilities designed and operated by the Center for Nanoscale Materials and the Advanced Photon Source at Sector 26 of the Advanced Photon Source at Argonne National Laboratory. This facility was constructed to probe the nanoscale structure of biological, environmental and material sciences samples. The beamline provides intense focused X-rays to the Hard X-ray Nanoprobe (or Nanoprobe) which incorporates Fresnel zone plate optics and a precision laser sensing and control system. The beamline operates over X-ray energies from 3 to 30 keV, enabling studies of most elements in the periodic table, with a particular emphasis on imaging transition metals.

We show that light drives large-amplitude structural changes in thin films of the prototypical ferroelectric PbTiO3 via direct coupling to its intrinsic photovoltaic response. Using time-resolved x-ray scattering to visualize atomic displacements on femtosecond time scales, photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion of photogenerated free charges within the ferroelectric and can be simply explained by a model including both shift and screening currents, associated with the displacement of electrons first antiparallel to and then parallel to the ferroelectric polarization direction.

A systematic study of energies and structures of InN and GaN (0001) surfaces exposed to NH${}_{3}$ and its decomposition products was performed with first-principles methods. A phenomenological model including electron counting contributions is developed based on calculated DFT energies and is used to identify low-energy structures. These predictions are checked with additional DFT calculations. The equilibrium phase diagrams are found to contain structures that violate the electron counting rule. Densities of states for these structures indicate $n$-type conductivity, consistent with available experimental results.

Thermodynamic theory is developed for the ferroelectric phase transition of an ultrathin film in equilibrium with a chemical environment that supplies ionic species to compensate its surface. Equations of state and free energy expressions are developed based on Landau-Ginzburg-Devonshire theory, using electrochemical equilibria to provide ionic compensation boundary conditions. Calculations are presented for a monodomain PbTiO$_3$ (001) film coherently strained to SrTiO$_3$ with its exposed surface and its electronically conducting bottom electrode in equilibrium with a controlled oxygen partial pressure. The stability and metastability boundaries of phases of different polarization are determined as a function of temperature, oxygen partial pressure, and film thickness. Phase diagrams showing polarization and internal electric field are presented. At temperatures below a thickness-dependent Curie point, high or low oxygen partial pressure stabilizes positive or negative polarization, respectively. Results are compared to the standard cases of electronic compensation controlled by either an applied voltage or charge across two electrodes. Ionic surface compensation through chemical equilibrium with an environment introduces new features into the phase diagram. In ultrathin films, a stable non-polar phase can occur between the positive and negative polar phases when varying the external chemical potential at fixed temperature, under conditions where charged surface species are not present in sufficient concentration to stabilize a polar phase., 53 pages, 24 figures

Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin ${\mathrm{PbTiO}}_{3}$ films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.

Polarization switching in ferroelectrics has been thought to occur only through the nucleation and growth of new domains. Here we use in situ synchrotron x-ray scattering to monitor switching controlled by applied chemical potential. In sufficiently thin PbTiO₃ films, nucleation is suppressed and switching occurs by a continuous mechanism, i.e., by uniform decrease and inversion of the polarization without domain formation. The observed lattice parameter shows that the electric field in the film during switching reaches the theoretical intrinsic coercive field.

Nature materials 8, 702-703 (2009). doi:10.1038/nmat2521, Published by Nature Publishing Group, Basingstoke

Time-resolved small-angle x-ray scattering was used to study the early stage of phase separation in a silicate system. Scattering patterns were acquired in real time as a sample was quenched {ital in} {ital situ} from a temperature {ital T}{sub 1} in the single-phase region directly to a temperature {ital T}{sub 2} in the two-phase region and held isothermally. This was made possible by using a high-intensity synchrotron x-ray source, a fast position-sensitive detector, a sample heater capable of rapid quenches, and a sample composition chosen to give good scattering contrast and relatively slow kinetics. The composition studied, 90 mol% SiO{sub 2}, 5 mol% BaO, and 5 mol% K{sub 2}O, undergoes subliquidus phase separation at temperatures below 1021 K to form alkali-oxide-rich and alkali-oxide-poor amorphous phases. The temperatures used were {ital T}{sub 1}=1074 K and {ital T}{sub 2}=1014, 995, 970, and 950 K. While the resultant data agree qualitatively with the linearized thermodynamics of the Cahn-Hilliard-Cook (CHC) theory of spinodal decomposition (i.e., the structure factor changes exponentially with time at each wave number), they do not agree with the simple diffusive kinetics used in the CHC theory. The data are in very good agreement with a modified theory incorporating diffusion-induced-flow (DIF)more » kinetics, in which one considers both the generation of stress arising from unequal mobilities of the diffusing species and the relaxation of this stress through deformation. Values of the material parameters as a function of temperature obtained from fits of the DIF theory to the data are in reasonable agreement with independent measurements and estimates based on a regular-solution model.« less

We report a process to fabricate multilayer Laue lenses (MLL's) by sectioning and thinning multilayer films. This method can produce a linear zone plate structure with a very large ratio of zone depth to width (e.g., >1000), orders of magnitude larger than can be attained with photolithography. Consequently, MLL's are advantageous for efficient nanofocusing of hard x rays. MLL structures prepared by the technique reported here have been tested at an x-ray energy of 19.5 keV, and a diffraction-limited performance was observed. The present article reports the fabrication techniques that were used to make the MLL's.

We present a formalism of x-ray dynamical diffraction from volume diffractive optics with large numerical aperture and high aspect ratio, in an analogy to the Takagi-Taupin equations for strained single crystals. We derive a set of basic equations for dynamical diffraction from volume diffractive optics, which enable us to study the focusing property of these optics with various grating profiles. We study volume diffractive optics that satisfy the Bragg condition to various degrees, namely flat, tilted and wedged geometries, and derive the curved geometries required for ultimate focusing. We show that the curved geometries satisfy the Bragg condition everywhere and phase requirement for point focusing, and effectively focus hard x-rays to a scale close to the wavelength., Comment: 18 pages, 12 figures

We report that the sign of the polarization of an epitaxial ferroelectric film can be determined from the interference between the x-ray scattering from the film and the substrate. X-ray scattering measurements of a 10 nm epitaxial PbTiO3 film grown by metal-organic chemical vapor deposition on a SrTiO3 substrate are presented. The scattering profile near the 001 peaks of the film and substrate shows clear evidence of the interference effects. Analysis indicates that this film is a single domain of specific polarity.

Understanding the suppression of ferroelectricity in perovskite thin films is a fundamental issue that has remained unresolved for decades. We report a synchrotron x-ray study of lead titanate as a function of temperature and film thickness for films as thin as a single unit cell. At room temperature, the ferroelectric phase is stable for thicknesses down to 3 unit cells (1.2 nanometers). Our results imply that no thickness limit is imposed on practical devices by an intrinsic ferroelectric size effect.

We measured static x-ray speckle contrast variation with the incident photon energy across sample-specific absorption edges. We propose that the variation depends strongly on the spectral response function of the monochromator.

We investigated the behavior of speckle contrast and size under various experimental conditions using 1.82 keV x-rays. In this paper, we report the comparison of two different setups for x-ray speckle experiments: one employing a focusing zone plate and one in which a pinhole selects the size of the coherent x-ray beam. We found a strong dependence of the speckle contrast and size on the type of setup. In general, the pinhole setup results in higher contrast but smaller speckle size. On the other hand, the zone plate setup allows one to target much smaller areas of interest in the sample, down to submicron dimensions, and also to adjust the speckle size. We anticipate that these results will be useful in future time-correlation spectroscopy experiments.

We have implemented in the undulator first-optics enclosure of the Massachusetts Institute of Technology-McGill University-IBM Corporation Collaborative Access Team Sector at the Advanced Photon Source an x-ray beamline and a spectrometer optimized for performing small-angle, wide- bandpass, coherent x-ray scattering experiments. We describe the novel features of this set-up. The performance of the beamline and the spectrometer has been characterized by measuring static x-ray speckle patterns form isotopically disordered aerogels. Statistical analysis of the speckle patterns has been performed from which we extract the speckle width sand contrast versus wave-vector transfer and sample thickness. The measured speckle widths and contrast are compared to direct numerical evaluations of the intensity correlation function. The calculated widths are in poor agreement with the measurements but the calculated contrast agrees well with the measured contrast.

We report a study of the oxidation process of individual PtNi nanoparticles (NPs) conducted with a novel scanning multi-layer Laue lens X-ray microscope. The elemental maps reveal that alloyed PtNi NPs were transformed into Pt/NiO core-shell NPs by thermal oxidation. The observations furthermore indicate that a coalescence of Pt/NiO core-shell NPs occurred during oxidation.

Hard x-ray microscopy with nanometer resolution will open frontiers in the study of materials and devices, environmental sciences, and life sciences by utilizing the unique characterization capabilities of x-rays. Here we report two-dimensional nanofocusing by multilayer Laue lenses (MLLs), a type of diffractive optics that is in principle capable of focusing x-rays to 1 nm. We demonstrate focusing to a 25 × 27 nm(2) FWHM spot with an efficiency of 2% at a photon energy of 12 keV, and to a 25 × 40 nm(2) FWHM spot with an efficiency of 17% at a photon energy of 19.5 keV.

We report a hard x-ray patterning capable of drawing lines with a width below 100 nm using x-rays at 0.165 nm. A specially prepared mask based on multilayer growth technology was used as an x-ray mask effectively. The x-ray Talbot effect in near field was investigated and utilized in the patterning. Since multilayers with a few nanometer layer spacing are readily available, the proposed hard x-ray nano patterning, free of the limit imposed by the Rayleigh criterion in optical range, can potentially be an ultimate optical lithography technique.

Homoepitaxial SrTiO3 thin films grown by molecular beam epitaxy are analyzed using high-resolution x-ray diffraction and transmission electron microscopy. Measured 00L x-ray scans from stoichiometric and nonstoichiometric films are compared with calculations that account for the effects of film thickness, lattice parameter, fractional site occupancy, and an offset between film and substrate at the interface. It is found that thickness fringes, commonly observed around Bragg reflections even in stoichiometric homoepitaxial SrTiO3 films, arise from a film/substrate interface offset. Transmission electron microscopy studies confirm the presence of strain at those homoepitaxial interfaces that show an offset in x-ray diffraction. The consequences for stoichiometry optimization of homoepitaxial films using high-resolution x-ray diffraction and the quality of regrown oxide interfaces are discussed.

We report improved results for hard x-ray focusing using a multilayer Laue lens MLL. We have measured a line focus of 16 nm width with an efficiency of 31% at a wavelength =0.064 nm 19.5 keV using a partial MLL structure with an outermost zone width of 5 nm. The results are in good agreement with the theoretically predicted performance. © 2008 American Institute of

This talk will describe proposed studies of atomic- and nanoscale dynamics in condensed matter which take advantage of the high coherent x-ray flux to be produced by future x-ray free-electron laser (FEL) sources, such as the Linac Coherent Light Source at Stanford. In particular, I will focus on the current status and future prospects for photon correlation spectroscopy using coherent x-rays (XPCS), and the use of the ultrashort pulse structure of the x-ray FEL to observe dynamics into the femtosecond range.

The theory of spinodal decomposition in amorphous systems is extended to include the effect of internal stress due to coherency strain. A quantitative treatment is developed for the kinetics of internal stress relaxation via plastic deformation. The general solution for the evolution of the composition and internal stress distributions during the early stage of spinodal decomposition is derived for a system obeying the visco-elastic constitutive relations of a Maxwell solid. A kinetic equation for the structure factor is obtained for the limiting case in which coherency strain is relatively large. In this case, it is found that plastic deformation to relieve internal stress can become the rate-limiting step in the evolution of Fourier components having large wavenumbers. The predicted shape of the structure factor as a function of wavenumber, time, and temperature can be very different from that obtained when the kinetics of internal stress relaxation are neglected. The theory is expected to be particularly applicable to amorphous systems in which the species of interest have very different mobilities, and can explain the discrepancies found in previous analyses of scattering data for the lead-aluminum-borate system and alkali-silicate systems.