Biomedical subjects

U Weierstall

Publications and source records attributed to U Weierstall.

6 recordsLinked to original sources

PubMed2003

Experimental lensless soft-X-ray imaging using iterative algorithms: phasing diffuse scattering.

Images of randomly placed two-dimensional arrays of gold balls have been reconstructed from their soft-X-ray transmission diffraction patterns. An iterative hybrid input-output (HiO) algorithm was used to solve the phase problem for the continuous distribution of diffuse X-ray scattering. Knowledge of the approximate size of the clusters was required. The images compare well with scanning electron microscope (SEM) images of the same sample. The use of micrometre-sized silicon nitride window supports is suggested, and absorption filters have been used to allow collection of low spatial frequencies often obscured by a beam stop. This method of phasing diffuse scattering may have application to scattering from individual inorganic nanostructures or single macromolecules.

Journal Article↗

PubMed2002

Phase recovery and lensless imaging by iterative methods in optical, X-ray and electron diffraction.

Thomas Young's quantitative analysis of interference effects provided the confidence needed to revive the wave theory of light, and firmly established the concept of phase in optics. Phase plays a similarly fundamental role in matter-wave interferometry, for which the field-emission electron microscope provides ideal instrumentation. The wave-particle duality is vividly demonstrated by experimental 'Young's fringes' using coherent electron beams under conditions in which the flight time is less than the time between particle emission. A brief historical review is given of electron interferometry and holography, including the Aharonov-Bohm effect and the electron Sagnac interferometer. The simultaneous development of phase-contrast imaging at subnanometre spatial resolution has greatly deepened our understanding of atomic processes in biology, materials science and condensed-matter physics, while electron holography has become a routine tool for the mapping of electrostatic and magnetic fields in materials on a nanometre scale. The encoding of phase information in scattered farfield intensities is discussed, and non-interferometric, non-crystallographic methods for phase retrieval are reviewed in relationship to electron holography. Examples of phase measurement and diffraction-limited imaging using the hybrid input-output iterative algorithm are given, including simulations for soft X-ray imaging, and new experimental results for coherent electron and visible-light scattering. Image reconstruction is demonstrated from experimental electron and visible-light Fraunhofer diffraction patterns. The prospects this provides for lensless imaging using particles for which no lenses exist (such as neutrons, condensates, coherent atom beams and X-rays) are discussed. These new interactions can be expected to provide new information, perhaps, for example, in biology, with the advantage of less damage to samples.

Algorithms↗

PubMed2002

Ordering energies and occupancies in doped TiAl, and dedicated ALCHEMI instrumentation.

Alloy design has been a lifelong interest of Gareth Thomas, and modern design algorithms include atomistic parameters which are obtainable from new electron microscope techniques such as ALCHEMI. In this paper, we discuss the relevance of ALCHEMI site occupancy measurements to intermetallic alloys, and summarize prior work. The results are found to lie in regions of a site-occupancy diagram (SOC) relating ordering energies to occupancy, as predicted by the Bragg-Williams theory of short-range order. These predictions also explain previous inconsistencies in the ALCHEMI measurements. A diffraction camera and X-ray detector system of novel design is proposed for dedicated ALCHEMI analysis for substitutional and interstitial dopant site-occupancy measurement, and details of the design given. Using this novel hardware design, the data-collection times for two-dimensional ALCHEMI patterns should be reduced by an order of magnitude or more, and the full data collection process automated. The resulting occupancy information can provide essential input parameters for atomistic alloy design algorithms, and can provide entirely new information on interstitial occupancies in minerals, ceramics, semiconductors, and alloys.

Algorithms↗

PubMed2001

Image reconstruction from electron and X-ray diffraction patterns using iterative algorithms: experiment and simulation.

The hybrid input-output iterative algorithm, which solves the phase problem for scattering from non-periodic objects, is reviewed for application to X-ray and electron diffraction data. Desirable convex constraints, including the sign of the scattering potential for electrons, and compact support, are discussed. The cases of complex and real exit-face wavefunctions, strong and weak phase objects, various supports, and the use of coherent focussed radiation are reviewed. Reconstruction of general complex objects requires accurate knowledge of the support, which should consist of two holes or a triangle in an opaque mask. The support boundaries should be as sharp as possible. Strong phase objects without absorption can be recovered if the support consists of one hole, is accurately known and has sufficiently sharp boundaries. Real and weak phase objects with absorption can be recovered without accurate knowledge of the support area if the support boundaries are sufficiently sharp and the support consists of one or more holes. A sign constraint on the scattering potential is used to recover weak phase objects. The experimental realization of theoretically desirable support conditions is discussed. A two-stage method of finding the support for complex objects is proposed. Experimental results from applying the Gerchberg-Saxton-Fienup HiO-algorithm to coherent electron diffraction patterns are presented, using specially made e-beam lithographed support structures. Images with a resolution of about 5 nm are thus recovered from the intensities alone in coherent electron diffraction patterns from non-periodic objects. Limitations of the present experiments are identified and suggestions made for development of both X-ray and electron work.

Journal Article↗

PubMed2000

Surface structural sensitivity of convergent-beam RHEED: Si (0 0 1) 2 x 1 models compared with dynamical simulations

The aim of this work is to evaluate the sensitivity of convergent-beam RHEED for the refinement of surface atomic structure. We have compared experimental and theoretical convergent-beam RHEED patterns from the silicon (0 0 1) reconstructed surface. The experiment was carried out in a custom designed UHV diffraction camera,using a micron sized probe. Both experimental and theoretical CB-RHEED patterns show complex details, highly sensitive to the surface structure. The multiple scattering simulations were based on two experimental structural models which make different assumptions for dimer tilt, one derived from X-ray diffraction results, and another from LEED data. The simulated CB-RHEED patterns using the X-ray model were found to be in closer agreement with our experiment than the LEED model. However, the agreement is not entirely satisfactory, suggesting that further improvement on this model is necessary.

Journal Article↗

PubMed2000

Transmission Electron Diffraction at 200 eV and Damage Thresholds below the Carbon K Edge.

Transmission electron diffraction patterns from ultra-thin aromatic and aliphatic organic films at beam energies of 200 eV-1 keV have been recorded in a custom low energy electron transmission (LEET) chamber. A significant reduction of the molecular damage cross-section, measured by fading of diffraction spots, was found for thin films of the aromatic perylene when the beam energy was reduced from 400 to 200 eV. The corresponding measurements for the aliphatic tetracontane showed a smaller "threshold energy" and the differences are discussed. Electron beam damage from other aromatic materials has also been studied at low energy. Comparison of the carbon K shell ionization cross-section and the measured damage cross-sections show that carbon K-shell ionization is strongly correlated with the damage observed in aromatics at beam energies higher than 284 eV. Calculation of the minimum number of unit cells needed for imaging a single molecule, and comparison of calculated elastic with measured damage cross-sections both indicate new possibilities for imaging biomolecules with low energy electrons.

Journal Article↗