Structure of CdTe/ZnTe superlattices

The structure of CdTe/ZnTe superlattices has been analyzed through θ/2θ x‐ray diffraction, photoluminescence, and in situ reflection high‐energy electron diffraction (RHEED) measurements. Samples are found to break away from Cd_(x)Zn_(1−x)Te buffer layers as a consequence of the 6% lattice mismatch in this system. However, defect densities in these superlattices are seen to drop dramatically away from the buffer layer interface, accounting for the intense photoluminescence and high‐average strain fields seen in each of our samples. Observed variations in residual strains suggest that growth conditions play a role in forming misfit defects. This could explain discrepancies with calculated values of critical thickness based on models which neglect growth conditions. Photoluminescence spectra reveal that layer‐to‐layer growth proceeded with single monolayer uniformity, suggesting highly reproducible growth. Our results give hope for relatively defect‐free Cd_(x)Zn_(1−x)Te/Cd_(y)Zn_(1−y)Te superlattices with the potential for applications to optoelectronics offered by intense visible light emitters

Electrical determination of the valence-band discontinuity in HgTe-CdTe heterojunctions

Current-voltage behavior is studied experimentally in a Hg0.78Cd0.22Te-CdTe-Hg0.78Cd0.22Te heterostructure grown by molecular beam epitaxy. At temperatures above 160 K, energy-band diagrams suggest that the dominant low-bias current is thermionic hole emission across the CdTe barrier layer. This interpretation yields a direct determination of 390±75 meV for the HgTe-CdTe valence-band discontinuity at 300 K. Similar analyses of current-voltage data taken at 190–300 K suggest that the valence-band offset decreases at low temperatures in this heterojunction

Bending strain-tunable magnetic anisotropy in Co2FeAl Heusler thin film on Kapton

Bending effect on the magnetic anisotropy in 20 nm Co$_{2}$FeAl Heusler thin film grown on Kapton\textregistered{} has been studied by ferromagnetic resonance and glued on curved sample carrier with various radii. The results reported in this letter show that the magnetic anisotropy is drastically changed in this system by bending the thin films. This effect is attributed to the interfacial strain transmission from the substrate to the film and to the magnetoelastic behavior of the Co$_{2}$FeAl film. Moreover two approaches to determine the in-plane magnetostriction coefficient of the film, leading to a value that is close to $\lambda^{CFA}=14\times10^{-6}$, have been proposed.Comment: 4 pages, 4 figure

Rational Topological Design for Fluorescence Enhancement upon Aggregation of Distyrylfuran Derivatives

A series of 2,5-distyrylfuran derivatives bearing pentafluorophenyl- and cyanovinyl units have been synthesized for aggregation-induced emission (AIE). The effect of the type and extent of the supramolecular connections on the AIE of the furan derivatives were examined and correlated with their X-ray crystal structures. It was found that the simultaneous presence of cyano and perfluorophenyl units strongly enhances the fluorescence upon aggregation. Single-crystal X-ray diffraction analysis confirmed that C — H⋅⋅⋅F, F⋅⋅⋅F, C — H⋅⋅⋅nitrile, Ar⋅⋅⋅ArF (Ar=aryl, ArF=fluoroaryl), and nitrile⋅⋅⋅ArF intra- and intermolecular interactions drive the topology of the molecule and that solid-state supramolecular contacts favor AIE of the furan derivatives

Distyrylfuranes: Synthesis and optical properties

Date du colloque : 05/2013</p

Median solution and noise sorting, a reconstruction algorithm for emission tomography with large hole collimator

SPECT (Single Photon Emission Computerized Tomography) presents poor performances in term of sensitivity and spatial resolution. The bottleneck of the process is due to the use of thin hole collimators. Collimators with large and long holes have been proposed (CACAO), but they involve a difficult inverse problem. OSEM (Ordered Subset Expectation Maximization) has been extensively used with success in conventional Thin Hole Collimator SPECT (THC-SPECT). Unfortunately this algorithm presents some limitations, when applied to the CACAO problem. We developed an algorithm based on the search for a median solution. The first publication describing this algorithm was limited to unrealistic small size problems (6×6 pixels). We present here a dualisation approach, which allows us to solve larger problems. Results on simulations for a 64×64 pixels size problem are given. Comparisons of the same problems treated by the OSEM approach are given

Topological and packing mode modification for solid-state emission enhancement of bis(perfluorostyryl)furan derivatives

An unsymmetric bis(perfluorostyryl)furan with a cyanovinyl unit was prepared (F1). Its crystallographic structure was resolved along with its cyano vinylene (1) and unsubstituted (2) aldehyde precursors. F1 was found to form ribbons involving C–H⋯F interactions, while also having multiple intermolecular contacts, including C–F⋯Fπ and π–π interactions. These contacts also occurred when F1 aggregated in 9 : 1 water/THF mixtures. When the supramolecular contacts are engaged, the emission intensity of F1 increases, with absolute emission yields of 9 and 25% occurring in aggregates and powder, respectively

Raman scattering determination of strain in CdTe/ZnTe superlattices

The strain configuration in CdTe/ZnTe strained-layer superlattices has been measured by Raman scattering near resonance. The ZnTe-like longitudinal optical phonon energy in the superlattice is significantly shifted from the bulk value to lower energies and the shift increases with increasing superlattice CdTe fraction. The observed shifts agree with calculations of strain shifts based on a free-standing strain distribution