L. B. Lurio

Investigation of dehydrogenation mechanism of MgH2–Nb nanocomposites

Nanocomposites of composition MgH2–V and MgH2–Nb have been shown to have very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium, which eases hydrogen penetration into the material, and by the particular microstructure of this nanocomposite. In this paper, we report a systematic structural study of the nanocomposite MgH2–Nb5at.%. To see the effect of hydrogenation/dehydrogenation process on the crystal structure, X-ray diffraction under hydrogen pressure was carried out at different temperatures. Crystallite size was evaluated by X-ray powder diffraction peak broadening. In order to see the evolution of crystal phases during the dehydrogenation process, real time X-ray investigation of dehydrogenation in MgH2–Nb nanocomposite was carried out using synchrotron radiation. It was found that a niobium hydride metastable phase, closely related to the low temperature e-NbH phase, acts as a gateway for hydrogen. Activation energies of the hydrogenation and dehydrogenation processes are also estimated.

Area detector based photon correlation in the regime of short data batches: Data reduction for dynamic x-ray scattering

A method for reducing time sequences of raw scattering images to intensity time-autocorrelation functions is presented. The procedure is based on the use of a charge coupled device (CCD) area detector, and optimized for operating in the regime of short data batches. Its application to x-ray photon correlation spectroscopy (XPCS) measurements is described in detail. Using a slow-scan CCD, we explain how to achieve data acquisition on a 30 ms or faster time scale, while simultaneously acquiring data from many coherence areas in parallel. The statistical uncertainties of the acquired XPCS data are quantified experimentally, and compared to the theoretically expected noise levels of the correlation functions.

Pore size distributions in nanoporous methyl silsesquioxane films as determined by small angle x-ray scattering

Small angle x-ray scattering (SAXS) measurements were performed on nanoporous methyl silsesquioxane films that were generated by the incorporation of a sacrificial polymeric component into the matrix and subsequently removed by thermolysis. The average pore radii ranged from 1 to 5 nm over a porosity range of ∼5–50%. The distribution in pore size was relatively broad and increases in breadth with porosity. The values and observations obtained by SAXS are in good agreement with field emission scanning electron microscopy.

Design and characterization of an undulator beamline optimized for small-angle coherent X-ray scattering at the Advanced Photon Source

An undulator beamline and small-angle-scattering spectrometer have been implemented at the Advanced Photon Source. The beamline is optimized for performing small-angle wide-bandpass coherent X-ray scattering measurements, and has been characterized by measuring static X-ray speckle patterns from isotropically disordered samples. Statistical analyses of the speckle patterns have been performed from which the speckle widths and contrast are extracted versus wavevector transfer and sample thickness. The measured speckle widths and contrast are compared with an approximation to the intensity correlation function and found to be in good agreement with its predictions.

Optimizing the signal-to-noise ratio for X-ray photon correlation spectroscopy

An analysis is presented of how to optimize the experimental beamline configuration for achieving the best possible signal-to-noise ratio (SNR) in X-ray photon correlation spectroscopy experiments using area detectors. It is shown that there exists an optimum detector distance; namely, the highest SNR is achieved by matching the angular pixel size with the angular source size. Binning several pixels together can increase the SNR by permitting to match the shape of a detector pixel to the shape of the source. It is also shown that collimating slits several times wider than the effective transverse coherence length are optimal; further, it is demonstrated that the energy dependence of the SNR is dictated by the energy dependence of detector efficiency and source brilliance. Ultimately the effects of focusing and low longitudinal coherence are discussed.

X-ray Photon Correlation Spectroscopy Studies of Surfaces and Thin Films

The technique of X-ray Photon Correlation Spectroscopy (XPCS) is reviewed as a method for studying the relatively slow dynamics of materials on time scales ranging from microseconds to thousands of seconds and length scales ranging from microns down to nanometers. We focus on the application of this technique to study dynamical fluctuations of surfaces, interfaces and thin films. We first discuss instrumental issues such as the effects of partial coherence (or alternatively finite instrumental resolution) and optimization of signal-to-noise ratios in the experiments. We then review what has been learned from recent XPCS studies of capillary wave fluctuations on liquid surfaces and polymer films, of nanoparticles used as probes to study the interior dynamics of polymer films, of liquid crystals and multilamellar surfactant films, and of metal surfaces, and magnetic domain wall fluctuations in antiferromagnets. We then discuss studies of non-equilibrium dynamics described by 2-time correlation functions. Finally, we briefly speculate on possible future XPCS experiments at new synchrotron sources currently under development including studies of dynamics on time scales down to femtoseconds.

Combining x-ray and optical spectroscopies in the study of dilute semiconductor nanoparticle composites

We discuss a methodology for the cooperative analysis of optical and x-ray spectroscopies to deduce the thermophysical properties of dilute suspensions of semiconductor nanoparticles in a wide band gap host. X-ray spectroscopy is used to determine concentration and bonding of selected elements. Resonant Raman spectroscopy establishes limits on composition and strain in particles. Analysis of optical absorption, with constraints provided by x-ray and Raman measurements, yields the particle size distribution and concentration. As an example of this approach, we study borosilicate glasses doped with ∼0.1 wt% CdS and heat treated to produce nanoparticles.

Demonstration of Feasibility of X-Ray Free Electron Laser Studies of Dynamics of Nanoparticles in Entangled Polymer Melts

The recent advent of hard x-ray free electron lasers (XFELs) opens new areas of science due to their exceptional brightness, coherence, and time structure. In principle, such sources enable studies of dynamics of condensed matter systems over times ranging from femtoseconds to seconds. However, the studies of “slow” dynamics in polymeric materials still remain in question due to the characteristics of the XFEL beam and concerns about sample damage. Here we demonstrate the feasibility of measuring the relaxation dynamics of gold nanoparticles suspended in polymer melts using X-ray photon correlation spectroscopy (XPCS), while also monitoring eventual X-ray induced damage. In spite of inherently large pulse-to-pulse intensity and position variations of the XFEL beam, measurements can be realized at slow time scales. The X-ray induced damage and heating are less than initially expected for soft matter materials.

X-ray speckle visibility spectroscopy in the single-photon limit.

The technique of speckle visibility spectroscopy has been employed for the measurement of dynamics using coherent X-ray scattering. It is shown that the X-ray contrast within a single exposure can be related to the relaxation time of the intermediate scattering function, and this methodology is applied to the diffusion of 72 nm-radius latex spheres in glycerol. Data were collected with exposure times as short as 2 ms by employing a resonant shutter. The weak scattering present for short exposures necessitated an analysis formalism based on the spatial correlation function of individual photon charge droplets on an area detector, rather than the usual methods employed for intensity correlations. It is demonstrated that this method gives good agreement between theory and experiment and thus holds promise for extending area-detector-based coherent scattering methods to the study of faster dynamics than previously obtainable.

Growth and dissolution of CdS nanoparticles in glass

We report and analyse x-ray absorption spectroscopy of the Cd K edge of CdS-doped glass during growth and dissolution of CdS nanoparticles. Average particle size is deduced from a combination of x-ray and optical spectroscopies. We propose a simple thermodynamic model in which the limiting reactant is in only two environments, either dissolved in the glass or in CdS particles. In this model, the time- and temperature-dependent concentration of Cd dissolved in the glass is related to the total concentrations of Cd and S, the equilibrium solubility of the limiting reactant, and the average CdS particle size.