Real-time Synchrotron Research Articles

Initial Growth Mode of GaN Film on Stepped Sapphire

The initial GaN growth mode on stepped sapphires by plasma enhanced metal organic molecular beam epitaxy (PEMOMBE) has been analyzed using in-situ, real time synchrotron x-ray diffraction and x-ray absorption. The sapphire substrate annealed at high temperature had flat terraces and regular atomic steps. The crystal quality and the vicinal angle of sapphire substrate had an effect on the width of terraces and the step arrangement. The initial growth mode of the GaN film on the regular atomic step (AS) surface was the layer-by-layer mode and changed to the 3D growth mode within 2 bilayer thickness. In the meanwhile, the growth mode of the GaN film grown on the sapphire with random roughness (RR) surface made the flat surface in the early stage and changed the 3D growth mode. As increasing the film thickness, the nucleation layer grows strain-free hexagonal GaN on stepped sapphires

Thickness Dependence of the Crystallization of α-Fe<sub>2</sub>O<sub>3</sub>/α-Al<sub>2</sub>O<sub>3</sub>(0001) Thin Films Grown by Sputtering

The crystallization of α-Fe2O3/α-Al2O3(0001) thin films has been studied using real-time synchrotron x-ray scattering and atomic force microscope. In the sputter-grown amorphous films of various thicknesses at room temperature, we find the coexistence of α-Fe2O3 and Fe3O4 interfacial crystallites (~50-Å-thick), well aligned to the α-Al2O3[0001] direction. The amorphous precursor is crystallized to the epitaxial α-Fe2O3 grains in three steps with annealing temperature; i) the growth of the well aligned α-Fe2O3 interfacial crystallites to approximately 200-Å-thick, together with the transformation of the Fe3O4 crystallites to the α-Fe2O3 crystallites (< 400°C), ii) the growth of the less aligned α-Fe2O3 grains on top of the well aligned grains (> 400°C), and iii) the nucleation of the different less aligned α-Fe2O3 grains directly on the α-Al2O3 substrate (> 600°C). The surface evolution of the amorphous precursor films after annealing is consistent with the microstructure evolution during the crystallization.

Evaporation of Epitaxial ZnO Films During Post-Deposition Annealing

The effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/Al2O3(0001) films during post-deposition annealing was examined. On the basis of real-time synchrotron X-ray scattering analysis and atomic force microscopy (AFM), we found that evaporation as well as grain growth played important roles on the surface morphology of the ZnO/Al2O3(0001) films. Facets with higher surface energies than the (0001) planes were formed during grain growth in the initial stage of annealing. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into a two-dimensional (2D) flat (0001) surface at 800 °C, which was confirmed by AFM. The real-time measurement of film thickness during annealing clearly supports the previous results that evaporation is directly related to surface morphology. The evaporation rate is high in the initial stage, which is caused by the preferential evaporation from high-energy facets but decreases after the transition to a flat (0001) surface.

Thermotropic Phase Behavior of a Liquid‐Crystalline Poly(ether ester) Derived from Hydroxydibenzoic Acid, 2‐Methyl‐1,3‐propanediol and R‐1,3‐Butanediol

AbstractThe thermotropic phase behavior of a liquid‐crystalline poly(ether ester) derived from 4′‐hydroxybiphenyl‐4‐carboxylic acid and two different spacer diols (2‐methyl‐1,3‐propanediol and R‐1,3‐butanediol) has been analyzed by differential scanning calorimetry, real time synchrotron X‐ray diffraction and solid‐state 13C NMR. It has been found that the polymer develops a smectic mesophase that presents order inside the layers. This mesophase has a slow rate of formation, in such a way that it is possible to obtain either an amorphous glass or a liquid‐crystalline glass. These two phases exhibit different and independent glass transition temperatures: 95 °C for the amorphous one and 85 °C for the mesophase. The diffractogram of this mesophase shows two clear diffractions in the wide‐angle region and a long spacing (of low intensity) in the small‐angle region. All those features are characteristic of a mesophase of intermediate order, presumably of the type SmI, in contrast to the low‐ordered SmCalt mesophase reported for the racemic polymer. Thus, the chirality of the spacer, which provides structural regularity, makes easier the packing of the chains, so that a more ordered mesophase is obtained. In addition, both the solid‐state 13C NMR line shapes and the value of $T_{1\rho }^H $ are practically the same for the quenched and the annealed samples, indicating that the “ordered” phase obtained by annealing is not a three‐dimensional crystal, thus confirming its assignment as mesophase, with a considerable degree of conformational disorder.magnified image

Growth mode in strained ZnO films on Al2O3(0001) during sputtering

We investigated the temperature dependence of growth mode in highly mismatched sputter-grown ZnO/Al2O3(0001) heteroepitaxial films using real-time synchrotron X-ray scattering. We find that the growth mode changes from 2 dimensional (2D) layer to 3D island in early growth stage with temperature (300∘C–500∘C). At around 400∘C, however, intermediate 2D platelets nucleate in early stage, act as nucleation cores of 3D islands and transform to misaligned state during further growth. The results of the strain evolution during growth suggest that the surface diffusion is a major factor in determining the growth mode in the strained ZnO/Al2O3(0001) heteroepitaxy.

Indium adsorption on GaN under metal-organic chemical vapor deposition conditions

Real-time synchrotron grazing-incidence x-ray fluorescence is employed to study indium adsorption on the GaN (0001) surface under typical process conditions for InGaN metal-organic chemical vapor deposition. An indium condensation boundary is mapped as a function of trimethylindium pressure, substrate temperature, and carrier gas composition. Below the condensation boundary, indium surface coverage reaches a maximum of ∼1∕4 ML. The addition of 8% H2 to the carrier gas is found to have a significant effect on both condensation and adsorption of indium.

Mechanisms of Gold Bioaccumulation by Filamentous Cyanobacteria from Gold(III)−Chloride Complex

The mechanisms of gold bioaccumulation by cyanobacteria (Plectonema boryanum UTEX 485) from gold(III)-chloride solutions have been studied at three gold concentrations (0.8,1.7, and 7.6 mM) at 25 degrees C, using both fixed-time laboratory and real-time synchrotron radiation absorption spectroscopy (XAS) experiments. Interaction of cyanobacteria with aqueous gold(III)-chloride initially promoted the precipitation of nanoparticles of amorphous gold(I)-sulfide at the cell walls, and finally deposited metallic gold in the form of octahedral (111) platelets (approximately 10 nm to 6 microm) near cell surfaces and in solutions. The XAS results confirm that the reduction mechanism of gold(III)-chloride to metallic gold by cyanobacteria involves the formation of an intermediate Au(I) species, gold(I)-sulfide.

Physically based modelling of phase transformations during welding of low-carbon steel

Austenitisation and reversion to ferrite in the heat affected zone (HAZ) of AISI1005 welded steel have been simulated using physically based derivations for interface mobility and the driving force for transformation in a multiphase phase-field methodology. The nucleation and growth of both austenite upon heating and ferrite upon cooling are presented. Pearlite is modelled as a pseudo-homogeneous phase to drastically reduce computational costs. Comparison of calculated phase fractions with in situ real-time synchrotron measurements in the literature yields a good agreement, both in temporal and spatial aspects.

Growth dynamics of pentacene thin films: Real-time synchrotron x-ray scattering study

Real-time synchrotron x-ray scattering in the anti-Bragg configuration was used to monitor the dynamics of pentacene film growth on inert substrates. A distributed-growth model, according to which pentacene molecules adsorbed on the nth layer can either nucleate and contribute to the growth of the (n+1)th layer or transfer downward and contribute to the growth of the nth layer, gave a good description of the data. For molecules adsorbed on the first and second layers, the probability of downward transfer was found to be dependent on the substrate, and independent of temperature within the range from 25 to 60 deg. C. For films grown on SiO{sub 2}, an Ehrlich-Schwoebel barrier of the order of 70 meV dominated downward transfer of pentacene molecules in layers away from the substrate. For films grown on an alkylated self-assembled monolayer, significant desorption of pentacene molecules from the substrate at elevated temperatures forced the growth mode toward the three-dimensional limit.

Real-Time Synchrotron SAXS and WAXD Studies on Annealing Behavior of Poly[(R)-3-hydroxybutyrate] Single Crystals

Poly[(R)-3-hydroxybutyrate] (P(3HB)) single crystals were grown isothermally at different crystallization temperatures (Tcs) in a dilute solution. Structural changes of the sedimented single-crystal mats on heating were followed by real-time synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques, and the dependence of the annealing behavior on Tc was examined. The two-dimensional (2D) SAXS pattern clearly demonstrated that the P(3HB) single crystal exhibits a discontinuous increase in the lamellar thickness at an annealing temperature. The onset of the discontinuous thickening depended on Tc or the initial lamellar thickness. The discontinuous thickening occurred at higher temperature as the initial lamellar thickness increased. In the transition region where the discontinuous thickening progressed, the new scattering peak intensity increased while the original one decreased. On the other hand, the crystallinity, estimated from the 2D WAXD patterns, first decrea...

Phase behaviour of a liquid crystalline polyetherester derived from 4′-hydroxy-1,1′-biphenyl-4-carboxylic acid and the ether-diol 4-(3-hydroxypropoxy)butan-1-ol

The phase behaviour of PBO3O4, a liquid crystalline polyetherester derived from 4′-hydroxy-1,1′-biphenyl-4-carboxylic acid and the ether-diol 4-(3-hydroxypropoxy)butan-1-ol, has been investigated by differential scanning calorimetry (DSC), real-time synchrotron X-ray diffraction, optical microscopy and solid-state 13C NMR. The results reveal an interesting polymesomorphism in PBO3O4, since, on cooling from the melt, a low-order smectic mesophase is obtained first, followed by a smooth transition into a slightly more ordered mesophase, and a final transition to a phase with a relatively high degree of order, stable at room temperature. The subsequent melting shows enantiotropic behaviour but, interestingly, the more ordered phase presents a considerably low undercooling. The 13C NMR results in the solid-state of the ordered phase show that a single T 1 ρ H relaxation time (and a single line shape for each particular carbon signal) is exhibited by PBO3O4. This fact, plus the low undercooling, suggests the formation of a highly ordered mesophase, instead of a three-dimensional crystal, which is the case of the corresponding polyester with the same spacer, or of the polyetherester with an all-methylene spacer. Additional experiments about the phase behaviour of the starting monomer for the synthesis of PBO3O4, reveal that a highly ordered mesophase is also obtained for the monomer, with a diffractogram rather similar to that exhibited by the ordered mesophase of the polymer.

Structural analysis of activated Mg(Nb)H 2

A metastable niobium hydride, believed to trigger rapid H desorption from the MgH 2 matrix, was detected in the course of real time synchrotron diffraction studies of the desorption kinetics of hydrogen in MgH 2 with about 5% NbH nanoparticles. In order to completely characterize this metastable phase, we performed in situ neutron diffraction experiments during hydrogenation/dehydrogenation of a magnesium–niobium nanocomposite. At 250 ° C (under 1 bar of H pressure) a hydride phase NbH x appears. Unfortunately, the available low pressure together with the low q-range did not allow to propose a structural arrangement for this phase. We thus switched to standard neutron diffraction (that is at constant pressure and room temperature) of ball-milled Mg–Nb–H nanocomposites for which various NbH hydrides were detected including the solid solution. It turns out that the formation of these niobium hydrides (metastable or not) is closely related to the milling time and depends on the quality of the starting metal.

Real time synchrotron analysis of the initial stages of the galvanization process in Al containing Zn baths

A real time synchrotron analysis of the breakdown of the inhibition effect of the Fe2 Al5 layer is reported. Both δ and δ−ζ Fe-Zn phases grow simultaneously when inhibition is suppressed until the Zn layer is consummated. Then the δ phase grows at the expense of the δ−ζ phase. The same mechanism is observed both below and above the Zn melting point. Phase transformations kinetics obey an Arrhenius type law that is not affected by the liquid or solid state of the Zn layer.

Effect of evaporation on surface morphology of epitaxial ZnO films during postdeposition annealing

We investigated the effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/Al 2O 3(0 0 0 1) films during postdeposition annealing using real time synchrotron X-ray scattering and atomic force microscopy (AFM). We find that evaporation as well as grain coalescence play crucial roles on the surface morphology of the ZnO/Al 2O 3(0 0 0 1) films. Grain growth occurring in initial stage of annealing forms facets with higher surface energies than the (0 0 0 1) planes. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 °C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface.

Synchrotron Ultrasmall-Angle X-ray Scattering Studies on Tensile Deformation of Poly(1-butene)

Real-time synchrotron ultrasmall-angle X-ray scattering measurements were performed during the tensile deformation of crystalline phase I poly(1-butene) (P1B). For comparison, high-density polyethylene (HDPE) was also studied. During tensile deformation, the main contribution to the scattering intensity is due to platelike cavities in the case of P1B, whereas very rarely such cavity scattering is observed in the case of HDPE. At small deformations, the platelike cavities were found to occur only in those lamellae whose normals are perpendicular to the stretching direction. This effect is due to the blocky substructure of the crystalline lamellae. At large orientations these cavities reorient turning their normal gradually perpendicular to the stretching direction; this effect is due to the preferential orientation of the polymer chains. Nevertheless, a fraction of the cavities with their normal parallel to the stretching direction are preserved at large deformation. It was found that there exists a critical thickness of cavities below which no stable cavity could be observed. The strain of the cavities with their normal parallel to the stretching direction is proportional to the macroscopic strain but with smaller values.

Effect of ultra-thin buffer on the structure of highly mismatched epitaxial ZnO during sputter growth

We investigated the microstructural evolution of ZnO/Al 2O 3(0 0 0 1) films with and without an ultra-thin (4 nm) ZnO buffer that was grown at a low temperature (LT) of 300 °C using real time synchrotron X-ray scattering, atomic force microscopy, and high resolution electron microscopy. It is shown that the ultra-thin two-dimensional (2D) layers play a critical role for improving the ZnO layer quality, by inducing 2D growth mode instead of 3D mode at 500 °C in early stage. The ZnO films grown on the ultra-thin buffer exhibited structural coherence between the surface and the interface in the substrate normal direction in early stage. The great enhancement of the structural quality was attributed to the strain accommodation by the 2D ultra-thin buffer.

Observation of Two Glass Transitions in a Thermotropic Liquid-Crystalline Polymer

DSC and real-time variable-temperature synchrotron experiments have been performed on a thermotropic poly(etherester), PH31B32, with biphenyl units as mesogens and spacers with methyl substituents. It has been found that PH31B32 develops a low-ordered SmCalt mesophase with a rather slow rate of formation in such a way that the isotropic melt of this polymer can be easily quenched into the glassy amorphous state at the usual cooling rates of the calorimeter. Considerably high annealing times at temperatures above Tg are necessary to develop such a mesophase. The extent of transformation and symmetry of the mesophase have been determined by means of calorimetric and X-ray diffraction experiments. The DSC results show that the glass-transition temperatures of the amorphous and liquid-crystalline states are clearly different: 95 and 84 °C, respectively. Moreover, the results show that the isotropization temperature increases linearly with the annealing temperature (the temperature at which the mesophase has been formed). The synchrotron experiments show that the mesophase layer spacing is slightly lower for the higher annealing temperatures, indicating a slightly more compact structure. Although the WAXS profiles of the two phases (mesophase and amorphous) are rather similar, the mesophase has noticeably smaller most probable intermolecular distances and a smaller width of the WAXS broad peak. The temperature coefficients of these intermolecular distances are also different for the various phases, and clear discontinuities are observed at the glass transitions. The corresponding values of Tg obtained from these results are 94 and 84 °C for the amorphous and liquid-crystalline phases, respectively, in perfect agreement with the DSC results. The correlation lengths estimated from the widths of the mesophase layer peaks are of the order of the extended chain length of the polymer, so that no extensive chain folding seems to be present in the mesophase of PH31B32. This fact will favor the attainment of high degrees of liquid crystallinity. Moreover, the higher correlation lengths deduced for the higher annealing temperatures point to a segregation of defects from the smectic structure in a greater extent, which may explain the corresponding higher isotropization temperatures.

Thermotropic Behavior of a Liquid Crystalline Polybibenzoate with an Asymmetric Oxymethylenic Spacer

The thermotropic phase behavior of P3O4B, a polybibenzoate with an asymmetric oxymethylenic spacer, has been analyzed by DSC, real-time synchrotron X-ray diffraction, and optical microscopy. The results show that the following phases are found on cooling: isotropic melt, SA mesophase, SC mesophase, and three-dimensional crystal. The subsequent melting reveals the enantiotropic character of P3O4B since the inverse phase sequence is found. Additionally, a clear recrystallization process is observed on melting, prior to the crystal−SC transition. The thermotropic properties of P3O4B are compared with those of its analogue polyester with an all-methylene spacer, poly(octamethylene p,p‘-bibenzoate), P8MB, and other polybibenzoates with oxymethylenic spacers. Compared with P8MB, significantly lower transition temperatures are found for P3O4B due to the higher flexibility and asymmetry of the spacer. The lowering is especially important for the formation of the final crystal structure, so that the mesophase temperature window is considerably enlarged in P3O4B, allowing to observe the transition SA to SC. Moreover, the comparison of the thermotropic properties of P3O4B with those of other polybibenzoates with oxymethylenic spacers reveals that P3O4B is the only one able to crystallize at the usual cooling rates of the calorimeter.

Cumulative mechanical moments and microstructure deformation induced by growth shape in columnar solidification.

The dynamical interaction between columnar interface microstructure and self-stress, resulting in unforeseen mechanical deformation phenomena, is brought to light by means of in situ and real-time synchrotron x-ray topography during directional solidification of dilute aluminum alloys. Beyond long-known local mechanical stresses, global mechanical constraints are found to be active. In particular, column rotation results from deformation caused by the mechanical moments associated with the very growth shape, namely, the cumulative torque acting together with the cumulative bending moment under gravity. A basic model allowing for a qualitative explanation of the observed distinctive features of the self-stress effects on microstructure dynamics is proposed.

Twin formation in sputter-grown ZnO∕Al2O3(0001) epitaxial film: A real time x-ray scattering study

Twin formation and strain evolution in highly mismatched sputter-grown ZnO∕Al2O3(0001) hereroepitaxial films were investigated using real time synchrotron x-ray scattering measurements and high resolution electron microscopy (HREM). We reveal the existence of a critical thickness at which the twin of the 30° rotated domains starts to nucleate within the ZnO films, followed by gradual strain relaxation. Twin growth above the critical thickness stops as the strain almost fully relaxes. A HREM image shows that the twin nucleates 5–10nm away from the interface as isolated domains, not as types of layers.