Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging

Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this article we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. The article also discusses the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.Comment: 18 pages, 6 figures Submitted to New Journal of Physic

Flexible Photodiodes Based on Nitride Core/Shell p-n Junction Nanowires

International audienceA flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane trans- ferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p−n junction nanowires containing InGaN/ GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications

Quantum Impurity Entanglement

Entanglement in J_1-J_2, S=1/2 quantum spin chains with an impurity is studied using analytic methods as well as large scale numerical density matrix renormalization group methods. The entanglement is investigated in terms of the von Neumann entropy, S=-Tr rho_A log rho_A, for a sub-system A of size r of the chain. The impurity contribution to the uniform part of the entanglement entropy, S_{imp}, is defined and analyzed in detail in both the gapless, J_2 <= J_2^c, as well as the dimerized phase, J_2>J_2^c, of the model. This quantum impurity model is in the universality class of the single channel Kondo model and it is shown that in a quite universal way the presence of the impurity in the gapless phase, J_2 <= J_2^c, gives rise to a large length scale, xi_K, associated with the screening of the impurity, the size of the Kondo screening cloud. The universality of Kondo physics then implies scaling of the form S_{imp}(r/xi_K,r/R) for a system of size R. Numerical results are presented clearly demonstrating this scaling. At the critical point, J_2^c, an analytic Fermi liquid picture is developed and analytic results are obtained both at T=0 and T>0. In the dimerized phase an appealing picure of the entanglement is developed in terms of a thin soliton (TS) ansatz and the notions of impurity valence bonds (IVB) and single particle entanglement (SPE) are introduced. The TS-ansatz permits a variational calculation of the complete entanglement in the dimerized phase that appears to be exact in the thermodynamic limit at the Majumdar-Ghosh point, J_2=J_1/2, and surprisingly precise even close to the critical point J_2^c. In appendices the relation between the finite temperature entanglement entropy, S(T), and the thermal entropy, S_{th}(T), is discussed and and calculated at the MG-point using the TS-ansatz.Comment: 62 pages, 27 figures, JSTAT macro

Evolutionary diversity and developmental regulation of X-chromosome inactivation

X-chromosome inactivation (XCI) results in the transcriptional silencing of one X-chromosome in females to attain gene dosage parity between XX female and XY male mammals. Mammals appear to have developed rather diverse strategies to initiate XCI in early development. In placental mammals XCI depends on the regulatory noncoding RNA X-inactive specific transcript (Xist), which is absent in marsupials and monotremes. Surprisingly, even placental mammals show differences in the initiation of XCI in terms of Xist regulation and the timing to acquire dosage compensation. Despite this, all placental mammals achieve chromosome-wide gene silencing at some point in development, and this is maintained by epigenetic marks such as chromatin modifications and DNA methylation. In this review, we will summarise recent findings concerning the events that occur downstream of Xist RNA coating of the inactive X-chromosome (Xi) to ensure its heterochromatinization and the maintenance of the inactive state in the mouse and highlight similarities and differences between mammals

GIXRD of nanoscale strain patterning in wafer bonding

International audienceSynchrotron grazing incidence X-ray diffraction is used to study tuneable lateral dislocation superlattices obtained by Si wafer bonding. Deformation satellites peaks around the {2 2 0} reflections are measure for pure twist and mixed (twist–tilt) grain boundaries. The twist of the two crystals induces a square network of screw dislocations whose features are calculated by continuum elasticity theory and kinematical approximation. The tilt of the crystals is accommodated by alternated mixed dislocations and the interactions between these two networks is discussed from the X-ray diffraction measurements

Au-Ni solid solutions studied by numerical relaxation

A semi-empirical potential based on the tight-binding second moment approximation is used to study Au-Ni solid solutions at 0 K. The energy of formation, the topological disorder, and the elastic constants are analysed within the whole concentration range. The results coincide very well with available experiments and may be explained by a frustration of the interatomic bondings by the fcc lattice. The size effect predominates over the chemical one and the analysis of interstitial sites shows that the local chemical surroundings lead to a splitting of the fcc lattice tetrahedral volumes.Un potentiel semi-empirique, issu de la méthode des liaisons fortes dans l'approximation du second moment, est utilisé pour étudier la solution solide Au-Ni à 0 K. L'énergie de formation, le désordre topologique et les constantes élastiques sont analysées pour toute l'étendue de concentration. Les résultats sont en très bon accord avec les données expérimentales disponibles et peuvent être interprétés par une frustration des liaisons atomiques par le réseau cfc. L'effet de taille est prépondérant devant l'effet chimique et l'analyse des sites interstitiels montre que l'environnement chimique local conduit à une levée de dégénérescence des volumes tétraédriques du réseau cfc

Ordering of Ge quantum dots with buried Si dislocation networks

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