Xiaowen Shi

Differential stress induced by thiol adsorption on facetted nanocrystals

Polycrystalline gold films coated with thiol-based self-assembled monolayers (SAM) form the basis of a wide range of nanomechanical sensor platforms. The detection of adsorbates with such devices relies on the transmission of mechanical forces, which is mediated by chemically derived stress at the organic-inorganic interface. Here, we show that the structure of a single 300-nm-diameter facetted gold nanocrystal, measured with coherent X-ray diffraction, changes profoundly after the adsorption of one of the simplest SAM-forming organic molecules. On self-assembly of propane thiol, the crystals flat facets contract radially inwards relative to its spherical regions. Finite-element modelling indicates that this geometry change requires large stresses that are comparable to those observed in cantilever measurements. The large magnitude and slow kinetics of the contraction can be explained by an intermixed gold-sulphur layer that has recently been identified crystallographically. Our results illustrate the importance of crystal edges and grain boundaries in interface chemistry and have broad implications for the application of thiol-based SAMs, ranging from nanomechanical sensors to coating technologies.

Synthesizing skyrmion bound pairs in Fe-Gd thin films

We show that properly engineered amorphous Fe-Gd alloy thin films with perpendicular magnetic anisotropy exhibit bound pairs of like-polarity, opposite helicity skyrmions at room temperature. Magnetic mirror symmetry planes present in the stripe phase, instead of chiral exchange, determine the internal skyrmion structure and the net achirality of the skyrmion phase. Our study shows that stripe domain engineering in amorphous alloy thin films may enable the creation of skyrmion phases with technologically desirable properties.

Soft x-ray ptychography studies of nanoscale magnetic and structural correlations in thin SmCo5 films

High spatial resolution magnetic x-ray spectromicroscopy at x-ray photon energies near the cobalt L3 resonance was applied to probe an amorphous 50 nm thin SmCo5 film prepared by off-axis pulsed laser deposition onto an x-ray transparent 200 nm thin Si3N4 membrane. Alternating gradient magnetometry shows a strong in-plane anisotropy and an only weak perpendicular magnetic anisotropy, which is confirmed by magnetic transmission soft x-ray microscopy images showing over a field of view of 10 μm a primarily stripe-like domain pattern but with local labyrinth-like domains. Soft x-ray ptychography in amplitude and phase contrast was used to identify and characterize local magnetic and structural features over a field of view of 1 μm with a spatial resolution of about 10 nm. There, the magnetic labyrinth domain patterns are accompanied by nanoscale structural inclusions that are primarily located in close proximity to the magnetic domain walls. Our analysis suggests that these inclusions are nanocrystalline Sm2...

Evaluation of partial coherence correction in X-ray ptychography

Coherent X-ray Diffraction Imaging (CDI) and X-ray ptychography both heavily rely on the high degree of spatial coherence of the X-ray illumination for sufficient experimental data quality for reconstruction convergence. Nevertheless, the majority of the available synchrotron undulator sources have a limited degree of partial coherence, leading to reduced data quality and a lower speckle contrast in the coherent diffraction patterns. It is still an open question whether experimentalists should compromise the coherence properties of an X-ray source in exchange for a higher flux density at a sample, especially when some materials of scientific interest are relatively weak scatterers. A previous study has suggested that in CDI, the best strategy for the study of strong phase objects is to maintain a high degree of coherence of the illuminating X-rays because of the broadening of solution space resulting from the strong phase structures. In this article, we demonstrate the first systematic analysis of the effectiveness of partial coherence correction in ptychography as a function of the coherence properties, degree of complexity of illumination (degree of phase diversity of the probe) and sample phase complexity. We have also performed analysis of how well ptychographic algorithms refine X-ray probe and complex coherence functions when those variables are unknown at the start of reconstructions, for noise-free simulated data, in the case of both real-valued and highly-complex objects.

Elastic relaxation in an ultrathin strained silicon-on-insulator structure

Coherent x-ray diffraction was used to study the relaxation in single ultrathin strained silicon structures with nanoscale accuracy. The investigated structure was patterned from 20 nm thick strained silicon-on-insulator substrate with an initial biaxial tensile strain of 0.6%. Two-dimensional maps of the post-patterning relaxation were obtained for single 1 × 1 μm2 structures. We found that the relaxation is localized near the edges, which undergo a significant contraction due to the formation of free surfaces. The relaxation extent decreases exponentially towards the center with a decay length of 50 nm. Three-dimensional simulations confirmed that over-etching is needed to explain the relaxation behavior.

Coherent X-ray diffraction investigation of twinned microcrystals

Coherent X-ray diffraction has been used to study pseudo-merohedrally twinned manganite microcrystals. The analyzed compositions were Pr(5/8)Ca(3/8)MnO(3) and La(0.275)Pr(0.35)Ca(3/8)MnO(3). The prepared loose powder was thermally attached to glass (and quartz) capillary walls by gentle heating to ensure positional stability during data collection. Many diffraction data sets were recorded and some of them were split as expected from the main observed twin law: 180° rotation around [101]. The peak splitting was measured with very high precision owing to the high-resolution nature of the diffraction data, with a resolution (Δd/d) better than 2.0 × 10(-4). Furthermore, when these microcrystals are illuminated coherently, the different crystallographic phases of the structure factors induce interference in the form of a speckle pattern. The three-dimensional speckled Bragg peak intensity distribution has been measured providing information about the twin domains within the microcrystals. Research is ongoing to invert the measured patterns. Successful phase retrieval will allow mapping out the twin domains and twin boundaries which play a key role in the physical properties.

Radiation-induced bending of silicon-on-insulator nanowires probed by coherent x-ray diffractive imaging

Coherent x-ray diffractive imaging (CXDI) is a powerful technique to study the bending of silicon-on-insulator nanowires. Radiation-induced bending causes the nanowires to exhibit highly distorted diffraction patterns in reciprocal space, from which a strain distribution is inferred. To confirm this, we simulate diffraction patterns using finite-element-analysis (FEA) calculations, which show excellent agreement with the experimental measurements. Our findings provide details of how silicon-on-insulator MOSFET devices might become strained during radiation damage of the underlying oxide.

Mechanical breakdown of bent silicon nanowires imaged by coherent x-ray diffraction

We have developed a method of coherent x-ray diffractive imaging to surmount its inability to image the structure of strongly strained crystals. We used calculated models from finite–element analysis to guide an iterative algorithm to fit experimental data from a series of increasingly bent wires cut into silicon-on-insulator films. Just before mechanical fracture, the wires were found to contain new phase structures, which are identified as dislocations associated with crossing the elastic limit.

The imaging and coherence beamline I13L at DIAMOND (Conference Presentation)

The Diamond Beamline I13L is designed to imaging on the micron- and nano-lengthsale with X-rays of energies between 6 and 30 keV [1]. Two independently operating branchlines and endstations have been built at distance of more than 200m from the source for this purpose. The imaging branch is dedicated for imaging in real space, providing In-line phase contrast imaging and grating interferometry with micrometre resolution and full-field transmission microscopy with 50nm spatial resolution. On the coherence branch coherent diffraction imaging techniques such as ptychography, coherent X-ray diffraction (CXRD) and Fourier-Transform holography are currently developed. Because of the large lateral coherence length available at I13, the beamline hosts numerous microscopy experiments. The coherence branchline in particular contains a number of unique features. New instrumental designs have been employed such as a robot arm for the detector in diffraction experiments and a photon counting detector for diffraction experiments. The so-called ‘mini-beta’ layout in the straight section of the electron storage ring permits modulating the horizontal source size and therefor the lateral coherence length. We will present the recent progress in coherent imaging at the beamline and the sciences addressed with the instrumental capabilities. Reference: [1] C. Rau, U. Wagner, Z. Pesic, A. De Fanis Physica Status Solidi (a) 208 (11). Issue 11 2522-2525, 2011, 10.1002/pssa.201184272

Bragg projection ptychography on niobium phase domains

Bragg projection ptychography (BPP) is a coherent x-ray diffraction imaging technique which combines the strengths of scanning microscopy with the phase contrast of x-ray ptychography. Here we apply it for high resolution imaging of the phase-shifted crystalline domains associated with epitaxial growth. The advantages of BPP are that the spatial extent of the sample is arbitrary, it is nondestructive, and it gives potentially diffraction limited spatial resolution. Here we demonstrate the application of BPP for revealing the domain structure caused by epitaxial misfit in a nanostructured metallic thin film. Experimental coherent diffraction data were collected from a niobium thin film, epitaxially grown on a sapphire substrate as the beam was scanned across the sample. The data were analyzed by BPP using a carefully selected combination of refinement procedures. The resulting image shows a close packed array of epitaxial domains, shifted with respect to each other due to misfit between the film and its substrate.