Electrode-induced lattice distortions in GaAs multi-quantum-dot arrays

Copyright © Materials Research Society 2019. Increasing the number of quantum bits while preserving precise control of their quantum electronic properties is a significant challenge in materials design for the development of semiconductor quantum computing devices. Semiconductor heterostructures can host multiple quantum dots that are electrostatically defined by voltages applied to an array of metallic nanoelectrodes. The structural distortion of multiple-quantum-dot devices due to elastic stress associated with the electrodes has been difficult to predict because of the large micrometer-scale overall sizes of the devices, the complex spatial arrangement of the electrodes, and the sensitive dependence of the magnitude and spatial variation of the stress on processing conditions. Synchrotron X-ray nanobeam Bragg diffraction studies of a GaAs/AlGaAs heterostructure reveal the magnitude and nanoscale variation of these distortions. Investigations of individual linear electrodes reveal lattice tilts consistent with a 28-MPa compressive residual stress in the electrodes. The angular magnitude of the tilts varies by up to 20% over distances of less than 200 nm along the length of the electrodes, consistent with heterogeneity in the metal residual stress. A similar variation of the crystal tilt is observed in multiple-quantum-dot devices, due to a combination of the variation of the stress and the complex electrode arrangement. The heterogeneity in particular can lead to significant challenges in the scaling of multiple-quantum-dot devices due to differences between the charging energies of dots and uncertainty in the potential energy landscape. Alternatively, if incorporated in design, stress presents a new degree of freedom in device fabrication

Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction

The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts dynamics. We report the facet resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment dependent chemical composition. We find that the initially Pt terminated nanoparticle surface gets Rh enriched under CO oxidation reaction conditions. The local composition is facet orientation dependent and the Rh enrichment is non-reversible under subsequent CO reduction. Tracking facet resolved strain and composition under operando conditions is crucial for a rational design of more efficient heterogeneous catalysts with tailored activity, selectivity and lifetime.Comment: 15 pages, 4 figures, 32 reference

Spatially resolved fluorescence of caesium lead halide perovskite supercrystals reveals quasi-atomic behavior of nanocrystals

We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters

Cylindrical Reflex Triode Warm X-Ray Source

Cylindrical reflex triodes (CRTs) driven by a pulsed power generator can produce relatively low-endpoint X-ray spectra (~250 keV) that can be suitable for certain radiation-matter interaction studies. CRTs have several advantages over other reflex triode configurations, the most significant being the ability to operate multiple concentric CRTs, connected either in parallel or in series. Such configurations can result in increased X-ray output without increasing the endpoint or changing the pulsed-power generator. This article describes results from, and modeling of, experiments employing a single CRT and two CRTs in a series configuration. The modeling includes an analytic, physics-based model of CRT electrical operation; measured and calculated radiation distributions and spectra; and predicted effects of several configuration variations

Raw data repository for manuscript "Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction"

Raw data depository for https://arxiv.org/abs/2103.01573. Details with file information are given in the file description-of-datasets.pd

Developing Safe System Projects in Africa

When dealing with road safety in Africa, one should bear in mind that road safety problems in Africa must be seen in their own context as the solutions proposed to address them. While it is relevant to take into account international good practices, African stakeholders should become owners of the interventions addressing their problems and take the responsibility for developing and implementing the appropriate solutions, taking advantage of suitable technical assistance, if needed. Based on these considerations, in this paper a presentation is made of the process used in the European research project "SaferAfrica - Innovating dialogue and problems appraisal for a safer Africa" to develop Safe System road safety intervention proposals for Africa. SaferAfrica aims at supporting policy makers and stakeholders with evidence on critical risk factors, related actions and good practices drawn from high quality data and knowledge. This project also serves as a platform to foster effective cooperation in road safety and to propose possible next road safety steps and identify possible funding sources in those countries were capacity review is carried out. In the project, road safety and traffic management capacity reviews at the country level were carried out in four countries (Cameroon, Burkina Faso, Tunisia and Kenya), following the World Bank guidelines. After conducting such a capacity review, these guidelines recommend the preparation and implementation of Safe System projects, “stand-alone, multisector initiatives targeting high-risk corridors and areas, with outcomes large enough to be reliably measured.” In SaferAfrica, this approach aims at facilitating the implementation of Safe System projects in the considered countries, by identifying detailed short-term improvement plans, and producing contextualized Terms of Reference for a number of interventions per selected country. These interventions are remedial in nature, they address high-priority concerns and demonstrate the viability of high potential gains within current administrative and legislative frameworks. In order to design interventions suitable to the existing context, a transferability assessment tool is adopted within a “participative” process, involving all possible interested parties, from the institutions to NGOs. The tool will indicate which immediate enabling actions are required to overcome legislative, regulatory, organisational, institutional and other barriers that may prevent measures or actions from being implemented. Results from the process are presented and discussed

Towards a quantitative determination of strain in Bragg Coherent X-ray Diffraction Imaging: artefacts and sign convention in reconstructions

Bragg coherent X-ray diffraction imaging (BCDI) has emerged as a powerful technique to image the localdisplacement field and strain in nanocrystals, in three dimensions with nanometric spatial resolution.However, BCDI relies on both dataset collection and phase retrieval algorithms that can induce artefactsin the reconstruction. Phase retrieval algorithms are based on the fast Fourier transform (FFT). Wedemonstrate how to calculate the displacement field inside a nanocrystal from its reconstructed phasedepending on the mathematical convention used for the FFT. We use numerical simulations to quantifythe influence of experimentally unavoidable detector deficiencies such as blind areas or limited dynamicrange as well as post-processing filtering on the reconstruction. We also propose a criterion for theisosurface determination of the object, based on the histogram of the reconstructed modulus. Finally,we study the capability of the phasing algorithm to quantitatively retrieve the surface strain (i.e., thestrain of the surface voxels). This work emphasizes many aspects that have been neglected so far inBCDI, which need to be understood for a quantitative analysis of displacement and strain based on thistechnique. It concludes with the optimization of experimental parameters to improve throughput andto establish BCDI as a reliable 3D nano-imaging technique

Variable‐Wavelength Quick Scanning Nanofocused X‐Ray Microscopy for In Situ Strain and Tilt Mapping

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