Experimental 3D coherent diffractive imaging from photon-sparse random projections.

The routine atomic resolution structure determination of single particles is expected to have profound implications for probing structure-function relationships in systems ranging from energy-storage materials to biological molecules. Extremely bright ultrashort-pulse X-ray sources - X-ray free-electron lasers (XFELs) - provide X-rays that can be used to probe ensembles of nearly identical nanoscale particles. When combined with coherent diffractive imaging, these objects can be imaged; however, as the resolution of the images approaches the atomic scale, the measured data are increasingly difficult to obtain and, during an X-ray pulse, the number of photons incident on the 2D detector is much smaller than the number of pixels. This latter concern, the signal 'sparsity', materially impedes the application of the method. An experimental analog using a conventional X-ray source is demonstrated and yields signal levels comparable with those expected from single biomolecules illuminated by focused XFEL pulses. The analog experiment provides an invaluable cross check on the fidelity of the reconstructed data that is not available during XFEL experiments. Using these experimental data, it is established that a sparsity of order 1.3 × 10-3 photons per pixel per frame can be overcome, lending vital insight to the solution of the atomic resolution XFEL single-particle imaging problem by experimentally demonstrating 3D coherent diffractive imaging from photon-sparse random projections.

X-ray nanotomography of coccolithophores reveals that coccolith mass and segment number correlate with grid size.

Coccolithophores of the Noëlaerhabdaceae family are covered by imbricated coccoliths, each composed of multiple calcite crystals radially distributed around the periphery of a grid. The factors that determine coccolith size remain obscure. Here, we used synchrotron-based three-dimensional Coherent X-ray Diffraction Imaging to study coccoliths of 7 species of Gephyrocapsa, Emiliania and Reticulofenestra with a resolution close to 30 nm. Segmentation of 45 coccoliths revealed remarkable size, mass and segment number variations, even within single coccospheres. In particular, we observed that coccolith mass correlates with grid perimeter which scales linearly with crystal number. Our results indirectly support the idea that coccolith mass is determined in the coccolith vesicle by the size of the organic base plate scale (OBPS) around which R-unit nucleation occurs every 110-120 nm. The curvation of coccoliths allows inference of a positive correlation between cell nucleus, OBPS and coccolith sizes.

Hard X-rays as pump and probe of atomic motion in oxide glasses.

Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.

Strong infrared photoluminescence in highly porous layers of large faceted Si crystalline nanoparticles.

Almost all physical processes in solids are influenced by phonons, but their effect is frequently overlooked. In this paper, we investigate the photoluminescence of large silicon nanoparticles (approximately 100 nm size, synthesized by chemical vapor deposition) in the visible to the infrared detection range. We find that upon increasing laser irradiance, an enormous photoluminescence emission band appears in the infrared. Its intensity exhibits a superlinear power dependence, increasing over four orders of magnitude in the investigated pump power range. Particles of different sizes as well as different shapes in porous layers are investigated. The results are discussed taking into account the efficient generation of phonons under high-power pumping, and the reduced capability, porosity dependent, of the silicon nanoparticles to exchange energy with each other and with the substrate. Our findings are relevant for heat management strategies in silicon.

Silica nanoparticles as tracers of the gelation dynamics of a natural biopolymer physical gel.

The gelation of methylcellulose in water has been studied by X-ray photon correlation spectroscopy, electrophoresis and rheological measurements by looking into the dynamics of silica nanoparticles as tracers in the polymer matrix. The temperature and scattering vector dependence of the structural relaxation time is investigated at the nanometric length scale during the formation of the strong gel state. We find a stress-dominated dynamics on approaching the gel state, characterized by a hyper-diffusive motion of the silica particles. These results support the idea of a unifying scenario for the dynamics of complex out of equilibrium soft materials.

Three-dimensional coherent diffractive imaging on non-periodic specimens at the ESRF beamline ID10.

The progress of tomographic coherent diffractive imaging with hard X-rays at the ID10 beamline of the European Synchrotron Radiation Facility is presented. The performance of the instrument is demonstrated by imaging a cluster of Fe2P magnetic nanorods at 59 nm 3D resolution by phasing a diffraction volume measured at 8 keV photon energy. The result obtained shows progress in three-dimensional imaging of non-crystalline samples in air with hard X-rays.

Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: dynamics and scaling of the intermediate scattering function.

Intermediate scattering functions are measured for colloidal hard sphere systems using both dynamic light scattering and x-ray photon correlation spectroscopy. We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behavior found by Segré and Pusey [Phys. Rev. Lett. 77, 771 (1996)] but challenged by Lurio et al. [Phys. Rev. Lett. 84, 785 (2000)]. We observe a scaling behavior over several decades in time but not in the long-time regime. Moreover, we do not observe long-time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of long-time diffusion coefficients at these scattering vectors.

Photon correlation spectroscopy with high-energy coherent x rays.

We performed x-ray photon correlation spectroscopy on a model suspension of colloidal particles using x rays of three different energies, namely, 8 keV, 13.5 keV, and 19 keV. The observed reduction in the degree of coherence with increasing x-ray energy, as measured by the contrast of the correlation functions, is consistent with theoretical estimates. We show that it is well possible and under certain circumstances even advantageous to perform experiments with coherent x rays at these higher energies. We argue that the reduced absorption may not only allow for thicker samples but also for longer acquisition times because of the reduced radiation damage, thus outweighing in many cases the effect of the reduced coherent flux. The use of higher energy x rays for photon correlation spectroscopy can therefore lead to a substantial increase in the signal-to-noise ratio and constitutes a promising option for future experiments on samples of polymeric or biological origin.

Microfocusing of hard X-rays with cylindrically bent crystal monochromators.

High-energy X-ray focusing with bent-crystal monochromators is known to be hampered by so-called depth or crystal-thickness aberrations. A theoretical model of focus broadening based on the geometrical theory of X-ray diffraction in slightly deformed crystals is presented and compared with experimental data. First, it is shown that depth broadening can be avoided in the Laue geometry by an appropriate choice of asymmetry angle. Based on this finding, a monochromator for high-pressure diffraction experiments has been designed and a source-size-limited focal spot below 10 microns is observed. As a consequence of the box-shaped rocking curve of bent Laue crystals, the focus is free of long-ranging tails. Diffraction patterns of standard powder samples were recorded on imaging plates and a theoretical description of the energy-dispersion-related peak broadening is given. Finally, diffraction patterns of N2 at 180 kbar demonstrate the excellent data quality achievable with this monochromator.

Laue orientation imaging.

Advantage was taken of the highly focused X-ray beam (10-30 microm) and the broad white spectrum of synchrotron X-rays at the ESRF for automatic recording of Laue patterns from polycrystals and extraction of orientation information. The procedure used is similar to that applied for electron-backscattering patterns in the scanning electron microscope and provides data for local orientation mapping used in texture analysis. Laue patterns are obtained from a thin slice of material in transmission and recorded with a CCD detector. The Laue geometry is converted into a gnomonic projection in which co-zonal reflections lie on straight lines. On applying the Hough transform these lines are merged into a single point, which is recognized by the computer and assigned zone indices [uvw] by comparison with a table of interzonal angles. From the angular positions of several [uvw] the crystal orientation is calculated. The method is illustrated for the orthorhombic magnesium silicate olivine.

New features of dislocation images in third-generation synchrotron radiation topographs.

Some aspects of the dislocation contrast observed at third-generation synchrotron radiation set-ups are presented. They can be explained by taking into account angular deviation effects on the beam propagation, which are visible because of the ;almost plane-wave' character of these sources. In particular, we show how the evolution of the direct image width of a dislocation as a function of the sample-to-film distance can allow a complete determination of the Burgers vector, i.e. in sign and modulus. In addition, experimental results obtained in monochromatic beam topography are compared with simulated images calculated assuming plane-wave illumination and are demonstrated to show a satisfactory agreement. The utility of the weak-beam technique in enhancing the spatial resolution is demonstrated and a criterion for the selection of experimental conditions depending upon the required spatial resolution, signal-to-noise ratio and exposure time is presented.