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The structural and functional maturation of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is essential for application to pharmaceutical testing, disease modeling, and ultimately therapeutic use. Multicellular 3D-tissue platforms have improved functional maturation of hiPSC-CMs, but probing cardiac contractile properties remains challenging in a 3D environment, especially at depth and in live tissues. Using small angle X-ray scattering (SAXS) images, we show that hiPSC-CMs, matured and examined in a 3D environment, exhibit periodic spatial arrangement of the myofilament lattice, which has not been previously detected in hiPSC-CMs. Contractile force is found to correlate with both scattering intensity (R2=0.44) and lattice spacing (R2=0.46). Scattering intensity also correlates with lattice spacing (R2=0.81), suggestive of lower noise in our structural measurement relative to the functional measurement. Notably, we observe decreased myofilament ordering in tissues with a myofilament mutation known to lead to hypertrophic cardiomyopathy (HCM). Our results highlight the progress of human cardiac tissue engineering and enable unprecedented study of structural maturation in hiPSC-CMs., Comment: 12 pages, 6 figures, supplementary material available upon request to the lead author

Strontium-doped lanthanum manganite is widely used as a cathode material for solid oxide fuel cells (SOFCs). In this study, the oxygen diffusivity in A-site-deficient (La0.8Sr0.2)0.95MnO3±δ (LSM-20) was measured as a function of temperature (600°C, 700°C, and 800°C) and oxygen partial pressure (0.03 atm, 0.16 atm, and 0.30 atm). Tracer (O-18) concentration profiles were measured in single-crystal epitaxial thin films of LSM-20 by time-of-flight secondary-ion mass spectroscopy (TOF-SIMS). The profile was fit to a diffusion equation to obtain the diffusivity of oxygen in bulk LSM-20 as a function of temperature and oxygen partial pressure. A defect model was used in conjunction with the experimental data to calculate the activation energies of oxygen migration, Schottky reaction, redox reaction, and charge disproportionation reaction in LSM-20. These calculated data were then used to calculate the Brouwer diagram of (La0.8Sr0.2)0.95MnO3±δ at 800°C, a typical operating temperature of SOFCs.

Coherent grazing-incidence small-angle X-ray scattering is used to investigate the average kinetics and the fluctuation dynamics during self-organized nanopatterning of silicon by Ar$^+$ bombardment at 65$^{\circ}$ polar angle. At early times, the surface behavior can be understood within the framework of linear theory. The transition away from the linear theory behavior is observed in the dynamics through the intensity correlation function. It quickly evolves to exhibit stretched exponential decay on short length scales and compressed exponential decay on length scales corresponding the dominant structural length scale - the ripple wavelength. The correlation times also peak strongly at the ripple length scale. This behavior has notable similarities but also significant differences with the phenomenon of de Gennes narrowing. Overall, this dynamics behavior is found to be consistent with simulations of a nonlinear growth model., Comment: 10 pages, 12 figures

In this study, a strontium doped lanthanum cobalt ferrite thin film with 30% Sr on A-site, denoted as La 0.7 Sr 0.3 Co 0.2 Fe 0.8 O 3-δ or LSCF-7328, is investigated before and after annealing at 800 °C under CO 2 containing atmosphere for 9 h. The formation of secondary phases on surface of post-annealed LSCF-7328 is observed using atomic force microscope (AFM) and scanning electron microscope (SEM). The extent of Sr segregation at the film surface is monitored using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique. The bonding environment of the secondary phases formed on the surface is investigated by synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Scanning transmission electron microscopy (STEM) and related spectroscopy techniques are used for microstructural and quantitative elemental analyses of the secondary phases on surface. These studies reveal that the secondary phases on surface consist of SrO covered with a capping layer of SrCO 3 . The formation of Co-rich phases is observed on the surface of post-annealed LSCF-7328.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1–x SrxCo0.2Fe0.8O3−δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on t...

The ultra-high vacuum surface preparation of heteroepitaxial lanthanum strontium cobalt ferrite thin films has been studied using soft X-ray photoelectron spectroscopy. Specifically, the effect of annealing the films at low temperatures in low partial pressures of oxygen and argon has been investigated. We find that atmospheric surface carbon contamination of the films can be removed in select anneal temperature regimes in argon, but remains bound to the surface with oxygen annealing at any temperature. Irrespective of the gas used, an insulating phase transition occurs near 300 °C due to strontium segregation at the surface. The surface develops more insulating character if annealed with oxygen. Different species are proposed to be responsible for the discrepancy in insulating character.

a b s t r a c t Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common practice to improve the cathode performance in solid oxide fuel cells. A computational procedure is developed in this work to predict the equilibrium oxygen vacancy non-stoichiometric concentrations at experimen- tally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry and solid electrolyte coulometry measurements for the strontium-doped lanthanum cobalt iron oxides at different compositions. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures.

The surface topological evolution during the growth of indium nitride (InN) by plasma-assisted atomic layer epitaxy (ALEp) on gallium nitride (GaN) (0001) substrates was studied using in situ real-time grazing incidence small-angle x-ray scattering (GISAXS) for 180, 250, and 320 °C growth temperatures. The GISAXS data reveal that the ALEp growth of InN on GaN in this temperature range proceeds in a Stranski–Krastanov mode, in which the 2D–3D transition occurred after 2.3 monolayers for 180 °C, 1 monolayer for 250 °C, and 1.5 monolayers for 320 °C. The corresponding initial island center-to-center distances were 7.4, 11.6, and 11.7 nm. Additionally, island coarsening was observed to increase with temperature. After 200 growth cycles, the mean island diameters were 3.9, 5.6, and 7.0 nm, and the mean island center-to-center distances were 8.6, 13.7, and 17.1 nm for 180, 250, and 320 °C growth temperatures, respectively. For the 320 °C growth, the mean island shape was observed to gradually evolve from relatively mounded to cylindrical. These results are supported by atomic force microscopy and specular x-ray reflectivity.

In situ and in vacuo surface studies of in situ and ex situ GaN substrate preparation were conducted to advance fundamental understanding of GaN surface preparation for low temperature atomic layer epitaxial growth. Grazing incidence small angle x-ray scattering (GISAXS) information is complemented with in vacuo x-ray photoelectron spectroscopy and ex situ atomic force microscopy studies to assess different ex situ sample preparation methods to produce the most suitable GaN surface for epitaxy. The authors have determined that a UV-ozone exposure followed by an HF dip produces the cleanest and smoothest GaN surface. They have further determined with GISAXS that subjecting the optimum surface to the established low temperature emulated gallium flash-off atomic level process (ALP) eliminates the need for any nitridation ALP. These ex situ and in situ cleaning preparations result in clean, highly-ordered surfaces that should provide an ideal substrate for high quality crystalline epitaxial films.In situ and in vacuo surface studies of in situ and ex situ GaN substrate preparation were conducted to advance fundamental understanding of GaN surface preparation for low temperature atomic layer epitaxial growth. Grazing incidence small angle x-ray scattering (GISAXS) information is complemented with in vacuo x-ray photoelectron spectroscopy and ex situ atomic force microscopy studies to assess different ex situ sample preparation methods to produce the most suitable GaN surface for epitaxy. The authors have determined that a UV-ozone exposure followed by an HF dip produces the cleanest and smoothest GaN surface. They have further determined with GISAXS that subjecting the optimum surface to the established low temperature emulated gallium flash-off atomic level process (ALP) eliminates the need for any nitridation ALP. These ex situ and in situ cleaning preparations result in clean, highly-ordered surfaces that should provide an ideal substrate for high quality crystalline epitaxial films.

In situ synchrotron x-ray studies were employed to develop a fundamental understanding of the low temperature atomic level processes (ALPs) for GaN substrates to develop in situ methods for preparation of epitaxy ready surfaces. An emulated gallium flash-off (GFO) ALP, followed by a hydrogen clean ALP, and a subsequent nitridation ALP are studied as a function of temperature and number of cycles. The results demonstrate that ideal GFO ALP results are achieved at a higher temperature, 500 °C, and that only ten GFO ALP cycles are needed to remove the surface oxide and result in an ordered GaN surface. Continued GFO ALP cycles at 500 °C roughen the GaN surface. GFO ALP executed at 400 °C only roughens the surface, while executing the GFO ALP at 250 °C causes uneven surface features presumably due to the incomplete removal of the oxide. The hydrogen clean ALP generally roughens the surface at all three temperatures after 30 cycles of the GFO ALP. Further, the nitridation ALP executed after 30 cycles of the GFO ALP, at any of the above temperatures, has little effect since the surface of the GaN has been roughened beyond recovery. These results provide insight into optimal GaN substrate surface preparation at temperatures consistent with the low temperature atomic layer epitaxy process.

The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean square surface roughness and carbon impurity levels at or below atomic force microscopy and x-ray photoelectron spectroscopy sensitivity limits, respectively. At tp 30 s, it appears that trimethylindium precursor molecules start to decompose, resulting in higher carbon content in the film. Thus, the nature of GISAXS correlated length scale directly correlates with the material quality. Additional ex situ characterizations reveal an electron mobility of 6–31 cm2/V s for a 3–5 nm thick InN film on a-plane sapphire, which is similar to the reported value of 30 cm2/V s for a 1300 nm thick InN film grown by molecular beam epitaxy directly on sapphire. Thus, the combination of in situ synchrotron x-ray analysis and ex situ characterization is a powerful approach to develop understanding of the growth mechanisms of ALEp of III-N materials in order to improve the quality by reducing impurities and broaden material applications.The authors present an in situ study of the effect of nitrogen plasma pulse time on the temporal evolution of the surface morphology of InN growth on a-plane sapphire at 250 °C by plasma assisted atomic layer epitaxy (ALEp). The growth surface evolution was monitored in real-time using grazing incidence small angle x-ray scattering (GISAXS) measurements at an x-ray incidence angle of 0.8°. Nitrogen plasma pulse time (tp) was varied between 15 and 30 s in 5-s steps, and for all tp, the near specular scattering broadens and correlated peaks develop and evolve along the Yoneda Wing (YW). For tp ≥ 20 s, a YW with one correlated length scale evolves. At the end of the growth, the longest correlated length scale is 16.54 nm for tp = 25 s. Porod analysis of GISAXS intensity at high qy for tp = 25 s shows the formation of mounded shapes at the early stage of nucleation that transitioned to cylinders after about 3 unit cells of InN growth. Additionally, at tp = 25 s, the growth rate is highest with root mean squar...

Silicon surface evolution during room temperature low-energy (300, 500 and 1000 eV) normal incidence Ar + ion bombardment in the presence of Mo seed atoms has been studied with real-time grazing-incidence small-angle X-ray scattering and ex situ atomic force microscopy. When a small amount of Mo atoms was supplied to the Si surface during ion bombardment, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (“dots”) coarsened with time until they reached a constant spatial wavelength. The longer length scale corrugations associated with kinetic roughening, however, continued to grow in amplitude during bombardment. The overall roughness is dominated by different corrugations at different times in the kinetic evolution, showing a complex behavior. The evolution of the kinetic roughening can be described by the Family–Vicsek scaling hypothesis, but measured scaling exponents are not in agreement with those of existing models.

The kinetics of the disorder−order transition (ODT) in a triblock of polystyrene (PS) and poly(ethylene-co-butylene) (PEB), in mineral oil, a selective solvent for the middle PEB block, was followed by time-resolved SAXS using both temperature ramp and rapid temperature quench techniques. Data analysis, using the Percus−Yevick interacting hard-sphere model plus Gaussians for the Bragg peaks in the ordered body-centered-cubic (bcc) state, shows that the core radius Rc, the hard-sphere volume fraction φ, and their interaction radius Rhs increased with decreasing temperature in the disordered state. At the ODT, φ reaches a maximum and Rc and φ change on ordering. The kinetics exhibits two stages corresponding to temperature equilibration and supercooling of the micellar fluid followed by nucleation and growth of the ordered state. The induction time for the onset of the second stage shows a minimum around the glass transition of the polystyrene cores.

Atomic ordering in AlGaN films produced by plasma assisted molecular beam epitaxy (PA-MBE) was investigated with X-ray diffraction (XRD) and transmission electron microscopy selected area diffraction (TEM-SAD). Under nitrogen-rich growth conditions, films undergo 1×1 monolayer cation ordering along the [0001] growth direction. This type of ordering is stronger in N-polar films than in Ga-polar films and is sensitive to the growth ratio of group-III to group-V flux. These observations are in qualitative agreement with the first principal calculations by Northrup and co-workers. Under group-III rich growth conditions of Ga-polar films, the XRD data is consistent with the quasi-random stacking of spontaneously formed 14-ML and 12-ML superlattice structures. Reciprocal space mapping shows a small tilt (0.25°) between the crystallographic c-axis of the wurtzite structure and the superlattice axis. This result can be explained by the presence of c-axis oriented superlattice domains with a parallel step translation between neighboring domains. This result is in qualitative agreement with a recently proposed model by Venezuela and co-workers of self-organized superlattice formation during step flow growth of strained alloys on vicinal surfaces.

Atomic layer deposition (ALD) enables the conformal coating of porous materials, making the technique suitable for pore size tuning at the atomic level, e.g., for applications in catalysis, gas separation and sensing. It is, however, not straightforward to obtain information about the conformality of ALD coatings deposited in pores with diameters in the low mesoporous regime (

Texture development in (Ti,Ta)Si2, formed from Ti–Ta alloy (0–6 at. % Ta) thin films sputtered on Si(001) and poly-Si substrates, was studied using both in situ and ex situ x-ray diffraction. The addition of Ta fundamentally changes the C54 texture. At 6 at. % Ta, an effective composition for lowering the C49–C54 transformation temperature, films on both substrates develop a strong (010) texture normal to the film plane. Alloy films on Si(001) also exhibit an in-plane orientational relationship between the final C54 texture and the underlying silicon lattice. A pseudoepitaxial film relationship with the substrate may, therefore, be playing an important role in the C54 transformation process. For 32 nm films on single-crystal substrates, the in situ measurements show that significant evolution of the C54 texture occurs during the growth process, suggesting that those grains well oriented with respect to the Si substrate lattice have higher growth velocities. The possibility of a template effect by the C40 ...

We have used small-angle x-ray scattering (SAXS), and small-angle neutron scattering (SANS) to study the micelle structure of a polystyrene-block-poly(ethene-co-butene)-block-polystyrene triblock copolymer in dilute - semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane) Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus-Yevick interacting hard-sphere model The temperature dependence of the core radius, the hard-sphere interaction radius and the volume fraction of hard spheres were obtained We also used synchrotron-based time-resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus-Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl-Johnson-Avrami theory of nucleation.

The texture of blanket C49 and C54 phase TiSi2 films on Si (001) substrates was examined with x-ray pole figure analysis. Textures were studied both in films with initial Ti deposited by sputtering and by low-temperature, plasma assisted chemical vapor deposition. Detailed differences were observed between silicide films grown by subsequently annealing films deposited with the two different methods. However, in both cases, the dominant C49 phase orientational relationships develop with respect to the substrate (001) planes. In contrast, we find that the dominant crystallographic relationships of the C54 phases in both cases are not with the surface (001) plane of the substrate, but instead with the Si (111) planes. Texturing in the case of the sputtered films is particularly complex with one prominent orientation having C54 (11¯0)‖(1¯11) Si with C54 (331)‖(011) Si and a second having C54 (1¯03¯)‖(111) Si with C54 (040)‖(11¯0) Si. These orientations, which suggest local epitaxy of the disilicide with...

We present a detailed structural investigation of self-assembled indium gallium nitride nanodots grown on c-plane aluminum nitride templates by the droplet heteroepitaxy technique in a plasma-assisted molecular beam epitaxy reactor. Various growth parameters, including the total coverage of the metal species, relative and total metal effusion fluxes, and nitridation temperature were investigated. Analyses of in situ reflection high-energy electron diffraction patterns and comparison with simulations showed that the resulting crystal structure was a mixture of wurtzite and twinned zinc blende phases, with the zinc blende phase increasingly dominant for lower metal coverages and lower nitridation temperatures, and the wurtzite phase increasingly dominant for higher nitridation temperature. Studies by field emission scanning electron microscopy and atomic force microscopy revealed that the nanodots exhibit trimodal size distributions, with the dot morphologies of the intermediate size mode often resembling a...

Wide bandgap semiconducting nitrides have found wide-spread application as light emitting and laser diodes and are under investigation for further application in optoelectronics, photovoltaics, and efficient power switching technologies. Alloys of the binary semiconductors allow adjustments of the band gap, an important semiconductor material characteristic, which is 6.2 eV for aluminum nitride (AlN), 3.4 eV for gallium nitride, and 0.7 eV for (InN). Currently, the highest quality III-nitride films are deposited by metalorganic chemical vapor deposition and molecular beam epitaxy. Temperatures of 900 °C and higher are required to deposit high quality AlN. Research into depositing III-nitrides with atomic layer epitaxy (ALEp) is ongoing because it is a fabrication friendly technique allowing lower growth temperatures. Because it is a relatively new technique, there is insufficient understanding of the ALEp growth mechanism which will be essential to development of the process. Here, grazing incidence small...

In this study, we investigated phase separation and long-range atomic ordering phenomena in InGaN alloys produced by molecular beam epitaxy. Films grown at substrate temperatures of 700–750 °C with indium concentration higher than 35% showed phase separation, in good agreement with thermodynamic predictions for spinodal decomposition. Films grown at lower substrate temperatures (650–675 °C) revealed compositional inhomogeneity when the indium content was larger than 25%. These films, upon annealing to 725 °C, underwent phase separation, similar to those grown at the same temperature. The InGaN films also exhibited long-range atomic ordering. The ordering parameter was found to increase with the growth rate of the films, consistent with the notion that ordering is induced at the growth surface. The ordered phase was found to be stable up to annealing temperatures of 725 °C. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice...

This study investigates Sr surface segregation behavior and phase formation in La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF), a commonly used cathode material for solid oxide fuel cells (SOFCs). (100)-oriented LSCF thin films were deposited on (110)-oriented NdGaO3(NGO) substrates by Pulsed Laser Deposition (PLD). The samples were annealed in atmospheres with various CO2partial pressures at 800°C. Using the synchrotron technique of Total Reflection X-ray Fluorescence (TXRF), surface segregation in these thin films were quantified. The morphological changes at the surface were examined by AFM studies. The kinetics and thermodynamics of the segregation are discussed.

A limiting factor for Solid Oxide Fuel Cell (SOFC) performance is the oxygen reduction reaction (ORR) at the cathode surface. Changes in the surface composition and structure on annealing at 800°C has been studied, since these can influence the ORR. Idealized single crystals of cathode materials La0.8Sr0.2MnO3 (LSM) and La1-xSrxCoyFe1-yO3 (LSCF) were grown as heteroepitaxial thin films on lattice matched single crystal substrates. Changes upon heating the films to operating temperature and pressures were characterized using various synchrotron x-ray techniques. Total Reflection X-ray Fluorescence (TXRF) measurements, which probe compositional changes, were made at high temperature in real time. The LSM surfaces were found to develop manganese enrichment when heated. Highly strontium doped LSCF were found to develop strontium-rich surfaces. HArd X-ray PhotoElectron Spectroscopy (HAXPES) was used to investigate the electronic structure of the materials. Highly strontium doped LSCF precipitates a surface strontium phase that contains both oxide and carbonate contributions.

We investigated the effect of silicide formation on ion-induced nanopatterning of silicon with various ultrathin metal coatings. Silicon substrates coated with 10 nm Ni, Fe, and Cu were irradiated with 200 eV argon ions at normal incidence. Real time grazing incidence small angle x-ray scattering (GISAXS) and x-ray fluorescence (XRF) were performed during the irradiation process and real time measurements revealed threshold conditions for nanopatterning of silicon at normal incidence irradiation. Three main stages of the nanopatterning process were identified. The real time GISAXS intensity of the correlated peaks in conjunction with XRF revealed that the nanostructures remain for a time period after the removal of the all the metal atoms from the sample depending on the binding energy of the metal silicides formed. Ex-situ XPS confirmed the removal of all metal impurities. In-situ XPS during the irradiation of Ni, Fe, and Cu coated silicon substrates at normal incidence demonstrated phase separation and the formation of different silicide phases that occur upon metal-silicon mixing. Silicide formation leads to nanostructure formation due the preferential erosion of the non-silicide regions and the weakening of the ion induced mass redistribution. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4797480]

In-situ X-ray diffraction during rapid thermal annealing at rates from 3 to 20°C s −1 was used to study the formation of titanium silicide phases from a thin film of Ti and doped single-crystal or undoped polycrystalline silicon substrates. In all cases, a metal-rich silicide (identified as either Ti 5 Si 4 or Ti 5 Si 3 ) was the first crystalline phase to form between 500 and 550°C, followed by C49-TiSi 2 between 575 and 600°C. The formation of C49-TiSi 2 completely consumed the metal-rich silicide before transforming into the low-resistance C54-TiSi 2 phase at approximately 800°C. These results indicate that while there is sufficient thermodynamic driving forces to nucleate both a metal-rich silicide and C49-TiSi 2 the subsequent growth of C49-TiSi 2 , is kinetically favored over the growth of Ti 5 Si 4 or Ti 5 Si 3 .

Small-angle X-ray scattering was used to study semidilute polystyrene-cyclohexane solutions at temperatures from below the coexistence curve to above the Θ point. Deviations from Θ point concentration scaling were observed as the coexistence line was approached. A new scaling form is suggested which combines the concentration scaling near the Θ point with the temperature scaling near a spinodal; it collapses the experimental data. This approach assumes that the “bare” correlation length ge 0 scales as c −1 . The observed low-angle “excess scattering” is best described by the Debye-Bueche random heterogeneity model, rather than the attractive potential model of Medjahdi and co-workers. The “excess scattering” is virtually independent of temperature, suggesting that the inhomogeneities which cause it are not affected by the approaching phase transformation.

Synchrotron-based time-resolved small-angle X-ray scattering was used to measure the early-stage spinodal decomposition kinetics in a highly viscous semidilute solution of polystyrene in dioctyl phthalate. Although strong nonlinearities in the kinetics were observed, a linear theory analysis could fit the evolution of the structure factor at short times and low wavenumbers. Nonlinearities become significant earlier at higher wavenumbers, which is consistent with recent computer simulation studies. From the linear theory analysis, the exponential relaxation rate of the structure factor R(q) was found to be linear in q 2 in the low-wavenumber region of the experiments, in agreement with mean-field theory for spinodal decomposition in polymer solutions. The wavenumber-dependent Onsager transport coefficient A(q), which is the Fourier transform of the nonlocal mobility in a polymer system, was determined to scale as q -4 . This q dependence, which is stronger than that predicted by existing theory, may be related to entanglement effects.

We demonstrate the use of a synchrotron radiation source for in situ x-ray diffraction analysis during rapid thermal annealing (RTA) of 0.35 μm Salicide (self-aligned silicide) and 0.4 μm Polycide (silicided polysilicon) TiSi2 Complementary Metal Oxide Semiconductor (CMOS) gate structures. It is shown that the transformation from the C49 to C54 phase of TiSi2 occurs at higher temperatures in submicron gate structures than in unpatterned blanket films. In addition, the C54 that forms in submicron structures is (040) oriented, while the C54 that forms in unpatterned Salicide films is randomly oriented. Although the preferred oreintation of the initial C49 phase was different in the Salicide and Polycide gate structures, the final orientation of the C54 phase formed was the same. An incomplete conversion of C49 into C54-TiSi2 during the RTA of Polycide gate structures was observed and is attributed to the retarding effects of phosphorus on the transition.

Real time grazing incidence small angle x-ray scattering and x-ray fluorescence (XRF) are used to elucidate nanodot formation on silicon surfaces during low energy ion beam irradiation of ultrathin iron-coated silicon substrates. Four surface modification stages were identified: (1) surface roughening due to film erosion, (2) surface smoothing and silicon-iron mixing, (3) structure formation, and (4) structure smoothing. The results conclude that 2.5 x 10(15) iron atoms in a 50 nm depth triggers surface nanopatterning with a correlated nanodots distance of 25 nm. Moreover, there is a wide window in time where the surface can have correlated nanostructures even after the removal of all the iron atoms from the sample as confirmed by XRF and ex-situ x-ray photoelectron spectroscopy (XPS). In addition, in-situ XPS results indicated silicide formation, which plays a role in the structure formation mechanism. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773202]

Nanoscale surface topography evolution on Ge surfaces irradiated by 1 keV Kr+ ions is examined in both directions perpendicular and parallel to the projection of the ion beam on the surface. Grazing incidence small angle x-ray scattering is used to measure in situ the evolution of surface morphology via the linear dispersion relation. A transition from smoothing (stability) to pattern-forming instability is observed at a critical ion incidence angle of approximately 62◦ with respect to the surface normal. The linear theory quadratic coefficients which determine the surface stability/instability are determined as a function of bombardment angle. The Ge surface evolution during Kr+ irradiation is qualitatively similar to that observed for Ar+ irradiation of Si. However, in contrast to the case of Si under Ar+ irradiation, the critical angle separating stability and instability for Ge under Kr+ irradiation cannot be quantitatively reproduced by the simple Carter-Vishnyakov mass redistribution model.

In this paper we report detailed cathodoluminescence (CL) studies of deep UV emitting AlGaN multiple quantum wells (MQWs) embedded in AlN cladding layers to simulate a deep UV laser structure. These structures were produced by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC substrates. The AlGaN QWs were grown under excess gallium conditions, a growth mode consistent with liquid phase epitaxy rather than physical vapor phase epitaxy. This growth mode leads to AlGaN films with compositional inhomogeneities and thus band structure potential fluctuations. The degree of such compositional inhomogeneities depends on the amount of excess gallium and employment of indium as a surfactant during growth. The structure and microstructure of these MQWs were investigated by x-ray diffraction and TEM/STEM. Their optical properties were investigated with spatially-resolved CL spectroscopy and mapping. E-beam pumping experiments were performed by irradiating the top surface and collecting the luminescence from the cleaved edge. The observed superlinear dependence of the QW CL intensity on beam current together with linewidth narrowing strongly suggests light amplification by stimulated emission.

Atomic layer deposition is demonstrated to be greatly suitable to tune the pore size of ink-bottle mesopores at the atomic level. This is confirmed by quantitative electron tomography, which enable...

The nanoscale kinetics of surface topography evolution on silicon surfaces irradiated with 1 keV Ar${}^{+}$ ions is examined in both directions perpendicular and parallel to the projection of the ion beam on the surface. We use grazing incidence small angle x-ray scattering to measure in situ the evolution of surface morphology via the linear dispersion relation. We study the transition from surface ultra-smoothening at low angles of deviation from normal ion incidence to a pattern-forming instability at high incidence angles. A model based on the effects of impact-induced redistribution of those atoms that are not sputtered away explains both the observed ultra-smoothening at low angles from normal ion incidence and the instability at higher angles and accounts quantitatively for the measured two-dimensional dispersion relation and its dependence on incidence angle.

Heterogeneous microscale dynamics in the martensitic phase transition of cobalt is investigated with real-time x-ray scattering. During the transformation of the high-temperature face-centered cubic phase to the low-temperature hexagonal close-packed phase, the structure factor evolution suggests that an initial rapid local transformation is followed by a slower period during which strain relaxes. Coherent x-ray scattering measurements performed during the latter part of the transformation show that the kinetics is dominated by discontinuous sudden changes-avalanches. The spatial size of observed avalanches varies widely, from 100 nm to 10 μm, the size of the x-ray beam. An empirical avalanche amplitude quantifies this behavior, exhibiting a power-law distribution. The avalanche rate decreases with inverse time since the onset of the transformation.

The structure of GaN films grown by electron‐cyclotron‐resonance‐assisted molecular beam epitaxy on Si(111), Si(001), basal‐plane sapphire, a‐plane sapphire, and r‐plane sapphire substrates was studied with four‐circle x‐ray diffractometry. Phase content, domain size, inhomogeneous strain, and in‐plane and out‐of‐plane domain misorientations were measured and compared for films grown on each type of substrate. Wurtzite and zinc blende polymorphs were found to coexist in films grown on Si(111). The two structures grow in the (0002) and (111) orientations, respectively, so that they may transform into each other via stacking faults on close‐packed planes. Smaller amounts of zinc blende material were also found in predominately (0002) wurtzitic films on a‐plane sapphire and (1120) wurtzitic films on r‐plane sapphire.

We report the development of large 50 mm diameter free standing wafers of GaN. GaN layers up to 350 /spl mu/m thick have been grown by HVPE on lattice-matched LiAlO/sub 2/ substrates. The original oxide substrates were removed by wet chemical etching, and the defect structure of the GaN wafers has been investigated. The issue of stacking faults vs. polar inversion domains has been resolved in favor of ABABACAC stacking sequences. A UV-emitting LED (368 nm) has been developed on these substrates, using molecular beam epitaxy.

The authors have investigated the early stages of indium island formation and growth by vapor phase deposition on room temperature sapphire using real-time grazing incidence small angle x-ray scattering (GISAXS), followed by ex-situ atomic force microscopy and scanning electron microscopy. The results are consistent with the formation and coalescence of hemispherical islands, as described by Family and Meakin. Monte Carlo simulations of systems of coalescing islands were used to supplement and quantify the results of GISAXS, and a good agreement is seen between the data and the simulations.

We examine how the substrate temperature during Ti film sputter deposition influences the subsequent texture formation in TiSi2 thin films. Titanium films of 32 nm thickness were sputtered onto Si(001) at elevated substrate temperatures varying between 100 °C and 900 °C. After the depositions, in situ x-ray diffraction (XRD) measurements were performed to study the thin film reactions in real time, as the samples were annealed. The XRD results show that the substrate temperature significantly influences the texture of the initial Ti film as well as the texture of the resulting C54-phase TiSi2. The preferred Ti orientation gradually changes from (002) to (101) fiber texture as the deposition temperature increases up to 500 °C. Films deposited at 600 °C transformed into the C49 phase during deposition while films deposited at 700 °C and higher temperatures transformed into the C54 phase during deposition. The series of deposited films was annealed up to 1000 °C in He to complete the C54 phase formation while monitoring the texture evolution in situ using a position sensitive x-ray detector. The XRD results show that the final C54 phase texture changes from a dominant (311) orientation normal to the substrate to a (010) orientation for substrate temperatures between 600 °C and 700 °C. The C49-C54 phase transformation temperature is also lowered for these deposition temperatures. Ex situ pole figure analysis of the film deposited at 700 °C confirms the dominant C54 (010) texture and shows an in-plane orientation with C54 [001] ∥ Si [110]. For substrate temperatures between 800 °C and 900 °C, the C54 texture changes dramatically. In this case, θ - 2 θ scans do not show a preferred C54 orientation, but pole figure analysis indicates weak inplane orientations.

We have studied the formation of titanium silicides in the presence of an ultra-thin layer of Ta, interposed between Ti and Si. In-situ x-ray diffraction (XRD), resistance measurements and elastic light scattering were used to study the thin film reactions in real time during ramp anneals to 1000°C. On poly-Si substrates the Ta thickness was varied from 0 to 1.5 nm while the Ti thickness was held constant at ∼27 nm. The time-resolved XRD shows that the volume fraction of C40 and metal-rich silicide phases grows with increasing Ta layer thickness. Increased Ta layer thicknesses also delay the growth of the C49 disilicide phase to higher temperatures. Among the Ta thicknesses we examined, 0.3 nm is the most effective in lowering the C49-C54 transformation temperature. Films with Ta layers thicker than 0.5 nm do not completely transform into the C54 phase. The texture of the C54 phase is also sensitive to the Ta thickness. The C54 disilicide film is predominantly (010) textured for the Ti / 0.3 nm Ta sample. The final C54 texture is significantly different for Ta layers thinner or thicker than the optimal 0.3 nm. This suggests that the most effective thickness for lowering the C54 formation temperature is related to the development of a strong (010) texture. The possibility of a template effect by the C40 or metal-rich Ti5Si3 phases is also discussed on the basis of texture considerations.

Solid oxide fuel cells (SOFCs) are ideally suited for environmentally benign conversion of chemical energy in hydrocarbons to electricity. While medium scale SOFC power systems in the hundreds of kWe have been demonstrated, larger scale application of such systems have been hobbled by still too high cost. In order to bring down the cost of these systems, manufacturing costs have to be decreased while the power densities have to be improved. Achieving these twin goals will require the reduction of operating temperatures of the cells. On a single cell level, the largest source of performance loss at lower operating temperatures occurs in the cathode. Thus many different groups are presently investigating a slew of new cathode materials. While the performance of the new cathode materials are impressive under controlled operating conditions, it is presently not clear whether they will stand up to the rigorous and punishing operating environments of actual fuel cell stacks. In particular the interfacial and bulk stability of the new generation of cathode materials such as strontium doped lanthanum cobaltites, strontium doped lanthanum cobaltites , and lanthanum nickelates when exposed to anodic gases such as carbon dioxide and water vapor is presently unknown. Exposure to at least trace amounts of anodic gases during the lifetime of the stack is a likely occurrence and could arise from back diffusion, pinholes in the electrolyte and/or interconnection, or compromised seals. Thus, it is important to have a clear understanding of the thermodynamic and kinetic stability of both the interfaces and the bulk of the cathode material under exposure to the aforementioned anodic gases. In this paper, we present results from a study that combines electrochemical and x-ray measurements of the impact of carbon dioxide exposure on strontium doped lanthanum cobalt iron oxide (LSCF), as a function of strontium dopant concentration, gas composition, and temperature. These are combined with computational studies of the stability of the LSCF cathode to carbon dioxide exposure. The longer term goal of this study is to identify cathode compositions which have long term stability to exposure to trace anode side gases.

Atomic layer deposition (ALD) is a thin film deposition technique that has been studied with a variety of in situ techniques. By exploiting the high photon flux and energy tunability of synchrotron based x-rays, a variety of new in situ techniques become available. X-ray reflectivity, grazing incidence small angle x-ray scattering, x-ray diffraction, x-ray fluorescence, x-ray absorption spectroscopy, and x-ray photoelectron spectroscopy are reviewed as possible in situ techniques during ALD. All these techniques are especially sensitive to changes on the (sub-)nanometer scale, allowing a unique insight into different aspects of the ALD growth mechanisms.

It has been demonstrated, using synchrotron radiation, that at rapid thermal annealing rates (3°C/s) the formation of CoSi2 shifts to higher temperatures when a thin Ti interlayer is placed between Co and polycrystalline Si. It has also been shown that the Ti interlayer reduces the temperature range between the start of CoSi formation and CoSi2 formation (i.e. the range over which CoSi is present). 13 nm of Co deposited by physical vapor deposition on polycrystalline Si with and without either a 2 nm or 3.4 nm interlayer of Ti was analyzed in-situ by monitoring x-ray diffraction (XRD) peak intensity as a function of temperature using monochromatic radiation from a synchrotron beam line and by monitoring resistivity as a function of temperature in a rapid thermal annealing (RTA) system. The XRD analysis indicates that the phase formation proceeds from CoSi to CoSi2 in a temperature range that decreases from about 200°C to 140°C to 115°C with pure Co, Co/2 nm Ti and Co/3.4 nm Ti films respectively. The onset of the CoSi formation increases by about 135°C and 160° for Co/ 2 nm Ti and Co/3.4 nm Ti compared to pure Co. The CoSi temperature range decreases from about 75°C in pure Co to less than 50°C in Co/Ti. In-situ RTA resistance along with in-situ XRD analysis indicates that the onset formation temperatures for CoSi are about 440°C, 575°C and 600°C and the temperatures for the completion of CoSi2 formation are about 640°C, 715°C and 715°C for Co, Co / 2 nm Ti and Co / 3.4 nm Ti films respectively. The results are consistent with the Ti interlayer acting as a diffusion barrier during the initial stages of the Co-Si reaction.

Native and Mo-seeded nanostructuring of the Si(100) surface during Ar+ ion bombardment is investigated by means of real-time grazing-incidence small-angle x-ray scattering and atomic force microscopy. During off-axis bombardment at room temperature, the native early-stage growth kinetics of nanoripples on the surface is found to be in reasonable overall agreement with theoretical predictions, particularly when an ion impact induced lateral mass redistribution term is included. For normal-incidence bombardment at room temperature, a native short wavelength smoothing of the amorphized Si surface is observed, suggesting that ion impact induced lateral mass redistribution dominates the Bradley–Harper instability. During 5% Mo-seeded normal-incidence bombardment at temperatures up to 450 °C, nanodots form with heights decreasing as the substrate temperature increases. This trend is counter to that typically observed for the growth of large cone structures on metals and suggests that the primary effect of thermal energy here is in promoting surface smoothing, rather than increasing diffusion of seed atoms to form protective clusters. During seeded bombardment at 650 °C the surface remains crystalline and surface corrugations exhibit dynamic scaling characteristic of surface diffusion-driven instabilities. This is the same behavior as is found in the absence of seeding and its presence suggests that at this concentration seeding does not play a large role during normal-incidence bombardment of the Si surface at high temperatures.

Gallium adsorption and desorption on c-plane sapphire has been studied by real-time grazing incidence small-angle x-ray scattering and x-ray fluorescence as a function of substrate temperature (680–740°C) and Ga flux. The x-ray techniques monitor the surface morphology evolution and amount of Ga on the surface. During deposition, nanodroplets of liquid Ga are observed to form on the surface and coarsen. The growth of droplet size during continuous deposition follows dynamical scaling, in agreement with expectations from theory and simulations which include deposition-induced droplet coalescence. However, observation of continued droplet distance scale coarsening during desorption points to the necessity of including further physical processes in the modeling. The desorption rate at different substrate temperatures gives the activation energy of Ga desorption as 2.7eV, comparable to measured activation energies for desorption from Ga droplets on other substrates and to the Ga heat of vaporization.

The temperature dependence of the surface morphology evolution during 1000 eV Ar+ ion bombardment of Si(100) surfaces at normal incidence is studied in real time. At room temperature the surface is amorphized by the ion bombardment but remains smooth. Calculations suggest this may be due to ion impact induced lateral mass redistribution. However, at the fluxes used here, surface roughening occurs above 400 °C, and in the range of 400 °C to 500 °C a transition region from amorphous to crystalline surface is observed. Above 500 °C, the surface remains crystalline and the growing corrugations exhibit dynamic scaling with power law growth in amplitude and characteristic length scale. This behavior is characteristic of instabilities driven by surface diffusion processes.

Effects of seed atoms on the formation of nanodots on silicon surfaces during normal incidence Ar+ ion bombardment at room temperature are studied with real-time grazing-incidence small-angle x-ray scattering (GISAXS), real-time wafer curvature stress measurements and ex situ atomic force microscopy. Although Si surfaces remain smooth during bombardment at room temperature, when a small amount of Mo atoms is supplied to the surface during ion bombardment, the development of correlated structures (“dots”) is observed. Stress measurements show that initially a compressive stress develops during bombardment, likely due to amorphization of the surface and insertion of argon. However, seeding causes a larger tensile stress to develop with further bombardment, possibly due to the formation of higher density regions around the Mo seed atoms on the surface. Detailed fits of the GISAXS evolution during nanostructure growth show that the instability is larger than predicted by the Bradley-Harper theory of curvature...

Self-organized gallium nitride nanodots have been fabricated using droplet heteroepitaxy on c-plane sapphire by plasma-assisted molecular beam epitaxy at different substrate temperatures and Ga fluxes. Nanoscale Ga droplets were initially formed on the sapphire substrate at high temperatures by Ga deposition from an effusion cell in an ultrahigh vacuum growth chamber. Subsequently, the droplets were converted into GaN nanodots using a nitrogen plasma source. The process was monitored and controlled using real-time grazing-incidence small-angle x-ray scattering. The samples were examined postgrowth by in situ grazing incidence x-ray diffraction and reflection high-energy electron diffraction, which confirmed the epitaxial relationship between the GaN nanodots and the sapphire surface. X-ray diffraction indicated that the wurtzite phase was dominant at higher substrate temperature (710°C), but a mixture of wurtzite and zinc blende phases was present at a substrate temperature of 620°C. Ex situ atomic force ...

Real-time grazing-incidence small-angle x-ray scattering has been employed to study the adsorption and desorption of Ga on c-plane sapphire and Ga-polar GaN surfaces. Formation of self-organized liquid Ga nanodroplets has been observed on sapphire during Ga exposure from an effusion cell at high flux. Subsequent to the Ga deposition, the nanodroplets were nitridated in situ by a nitrogen plasma source, which converted the droplets into GaN nanodots. In addition to the droplet studies, at lower Ga flux, the adsorption and desorption of Ga have been studied in the predroplet regime. For identical processing conditions, significantly different Ga adsorption∕desorption rates were observed on sapphire and GaN surfaces.

The formation of self-organized Si nanostructures induced by Mo seeding during normal incidence Ar+ ion bombardment at room temperature is reported. Silicon surfaces without Mo seeding develop only power-law roughness during 1000eV ion bombardment at normal incidence, in agreement with scaling theory expectations of surface roughening. However, supplying Mo atoms to the surface during ion bombardment seeds the development of highly correlated, nanoscale structures (“dots”) that are typically 3nm high with a spatial wavelength of approximately 30nm. With time, these saturate and further surface roughening is dominated by the growth of long-wavelength corrugations.