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P Rez

Publications and source records attributed to P Rez.

12 recordsLinked to original sources
PubMed2000

Can high-angle annular dark field scattering be represented by a local operator?

High-angle annular dark field imaging has become an invaluable technique for recording atomic resolution STEM images. Many analyses of high-angle annular dark field images assume that the signal is the result of a local scattering operator and can be represented as a simple convolution of a probe function with a set of atomic columns. The apparent simplicity of the technique and the straightforward increase in signal with atomic number have lead to the belief that it is possible to quantify impurity concentrations at atomic column resolution. The limitations in these assumptions are examined on the basis of approximations starting from a complete theory for high-angle scattering based on multi phonon excitations. Not surprisingly, the accuracy of the local scattering operator approximation improves as the inner cut-off angle is increased.

Journal Article↗
PubMed1999

Schemes to determine the crystal potential under dynamical conditions using voltage variation.

Charge densities and crystal structures can be determined routinely from X-ray diffraction as X-ray scattering is relatively weak and single scattering can be assumed. The strong dynamical diffraction of high-energy electrons has prevented electron diffraction from being used in the same way. Dynamical diffraction describes both the propagation of the Bragg diffracted wave in the crystal and the scattering by the crystal potential. The balance between these two processes changes as a function of voltage due to relativistic effects. The difference in diffracted intensities recorded at two voltages is shown to be directly proportional to the crystal potential. This is confirmed by calculations using first-order perturbation theory which show negligible differences compared to exact calculation. It should therefore be possible to use differences in intensity measured as a function of voltage to determine the crystal potential directly. If the full complex wave function is available, then there is a particularly simple procedure to recover the potential, even under dynamical conditions.

Journal Article↗
PubMed1996

Evidence for aggregation in oxalate stone formation: atomic force and low voltage scanning electron microscopy.

PURPOSE: The aim of this investigation was to differentiate between aggregation and crystal growth by studying the structure of oxalate stones at high spatial resolution using recently developed microscopy techniques. MATERIALS AND METHODS: Sections from 6 complete human oxalate stones and 4 stone fragments were prepared by ultramicrotomy and examined by both low voltage scanning electron microscopy and atomic force microscopy. RESULTS: The scanning electron microscopy showed lamellar structures up to 10 microns. in size, consistent with previous results, and provided evidence that these structures were composed of smaller particles. The atomic force microscopy clearly showed arrays of the small particles, whose size varied between 500A and 2800A. CONCLUSION: Our images suggest that an ordered aggregation of small crystallites is responsible for oxalate stone formation.

Humans↗
PubMed1992

The thickness determination of organic crystals under low dose conditions using electron energy loss spectroscopy.

In the 3-dimensional (3-D) reconstruction of protein crystals with variable thicknesses the electron images and diffraction patterns can only be merged if the crystal thickness is known. Measurement of the thickness using the ratio of the number of inelastically scattered electrons to the number of electrons in the zero loss peak can be accomplished with parallel electron energy loss spectrometry (PEELS). A theoretical analysis of the accuracy of the technique on paraffin crystals of different thicknesses is presented. Our experimental studies with paraffin crystals show the feasibility of measuring a single layer of 47A with good accuracy under low dose and low temperature conditions. A simple experimental apparatus is proposed to obtain thicknesses from small regions of unstained protein crystals prior to collecting the 3-D data sets from the unexposed area of the same crystal.

Crystallography↗