58 research outputs found
FOX: A friendly tool to solve nonmolecular structures from powder diffraction
Structural characterization from powder diffraction of compounds not containing isolated molecules but three-dimensional infinite structure (alloys, intermetallics, framework compounds, extended solids) by direct space methods has been largely improved in the last 15 years. The success of the method depends very much on a proper modeling of the structure from building blocks. The modeling from larger building blocks improves the convergence of the global optimization algorithm by a factor of up to 10. However, care must be taken about the correctness of the building block, like its rigidity, deformation, bonding distances, and ligand identity. Dynamical occupancy correction implemented in the direct space program FOX has shown to be useful when merging excess atoms, and even larger building blocks like coordination polyhedra. It also allows joining smaller blocks into larger ones in the case when the connectivity was not a priori evident from the structural model. We will show in several examples of nonmolecular structures the effect of the modeling by correct structural unit
Coherent Diffraction Imaging of Single 95nm Nanowires
Photonic or electronic confinement effects in nanostructures become
significant when one of their dimension is in the 5-300 nm range. Improving
their development requires the ability to study their structure - shape, strain
field, interdiffusion maps - using novel techniques. We have used coherent
diffraction imaging to record the 3-dimensionnal scattered intensity of single
silicon nanowires with a lateral size smaller than 100 nm. We show that this
intensity can be used to recover the hexagonal shape of the nanowire with a
28nm resolution. The article also discusses limits of the method in terms of
radiation damage.Comment: 5 pages, 5 figure
Large and uniform optical emission shifts in quantum dots externally strained along their growth axis
We introduce a method which enables to directly compare the impact of elastic
strain on the optical properties of distinct quantum dots (QDs). Specifically,
the QDs are integrated in a cross-section of a semiconductor core wire which is
surrounded by an amorphous straining shell. Detailed numerical simulations show
that, thanks to the mechanical isotropy of the shell, the strain field in a
core section is homogeneous. Furthermore, we use the core material as an in
situ strain gauge, yielding reliable values for the emitter energy tuning
slope. This calibration technique is applied to self-assembled InAs QDs
submitted to incremental tensile strain along their growth axis. In contrast to
recent studies conducted on similar QDs stressed perpendicularly to their
growth axis, optical spectroscopy reveals 5-10 times larger tuning slopes, with
a moderate dispersion. These results highlight the importance of the stress
direction to optimise QD response to applied strain, with implications both in
static and dynamic regimes. As such, they are in particular relevant for the
development of wavelength-tunable single photon sources or hybrid QD
opto-mechanical systems
Fast computing of scattering maps of nanostructures using graphical processing units
Scattering maps from strained or disordered nano-structures around a Bragg
reflection can either be computed quickly using approximations and a (Fast)
Fourier transform, or using individual atomic positions. In this article we
show that it is possible to compute up to 4.10^10 $reflections.atoms/s using a
single graphic card, and we evaluate how this speed depends on number of atoms
and points in reciprocal space. An open-source software library (PyNX) allowing
easy scattering computations (including grazing incidence conditions) in the
Python language is described, with examples of scattering from non-ideal
nanostructures.Comment: 7 pages, 4 figure
Coherent x-ray wavefront reconstruction of a partially illuminated Fresnel zone plate
International audienceA detailed characterization of the coherent x-ray wavefront produced by a partially illuminated Fresnel zone plate is presented. We show, by numerical and experimental approaches, how the beam size and the focal depth are strongly influenced by the illumination conditions, while the phase of the focal spot remains constant. These results confirm that the partial illumination can be used for coherent diffraction experiments. Finally, we demonstrate the possibility of reconstructing the complex-valued illumination function by simple measurement of the far field intensity in the specific case of partial illumination
Boosting spatial resolution by incorporating periodic boundary conditions into single-distance hard-x-ray phase retrieval
A simple coherent-imaging method due to Paganin et al. is widely employed for
phase-amplitude reconstruction of samples using a single paraxial x-ray
propagation-based phase-contrast image. The method assumes that the
sample-to-detector distance is sufficiently small for the associated Fresnel
number to be large compared to unity. The algorithm is particularly effective
when employed in a tomographic setting, using a single propagation-based
phase-contrast image for each projection. Here we develop a simple extension of
the method, which improves the reconstructed contrast of very fine sample
features. This provides first-principles motivation for boosting fine spatial
detail associated with high Fourier frequencies, relative to the original
method, and was inspired by several recent works employing empirically-obtained
Fourier filters to a similar end
Interface-driven phase separation in multifunctional materials: the case of GeMn ferromagnetic semiconductor
We use extensive first principle simulations to show the major role played by
interfaces in the mechanism of phase separation observed in semiconductor
multifunctional materials. We make an analogy with the precipitation sequence
observed in over-saturated AlCu alloys, and replace the Guinier-Preston zones
in this new context. A new class of materials, the phases, is proposed
to understand the formation of the coherent precipitates observed in the GeMn
system. The interplay between formation and interface energies is analyzed for
these phases and for the structures usually considered in the literature. The
existence of the alpha phases is assessed with both theoretical and
experimental arguments
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