Optical properties of single ZnTe nanowires grown at low temperature

Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer^{2}. The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires

New approach for gas identification using supervised learning methods (SVM and LVQ)

This article proposes a new approach for gas identification, this approach relies on applying supervised learning methods to identify a single gas as well as a mixture of two gases. The gas is trapped in a gas discharge tube, it is then ionized at a relatively low pressure using an HV transformer. The images captured after the ionization of each single gas is then captured and transformed into a database after being treated in order to be classified. The obtained results were very satisfying for SVM as well as for LVQ. For the case of identification of a single gas, the learning rate as well as the validation rate for both methods were 100%. However, for the case of mixture of two gases, a Multi-Layer Perceptron neural network was used to identify the gases, the learning rate as well as the validation rate were 98.59% and 98.77% respectively. The program developed on MATLAB takes the captured image as an input and outputs the identified gases for the user. The gases used in the experiments are Argon (Ar), oxygen (O2), Helium (He) and carbon dioxide (CO2)

Extraction of the homogeneous linewidth of the spectrally diffusing line of a CdSe/ZnSe quantum dot embedded in a nanowire

International audienceWe present a simple method to extract the homogeneous linewidth of a single photon emitter line exhibiting fast (down to 1 ns) spectral diffusion (SD). It is based on a recently developed technique using photon correlation measurements on half of the line. Here we show that the SD induced bunching depends on the ratio between the width of the homogeneous line and the spectral diffusion amplitude. Using this technique on a CdSe/ZnSe quantum dot, we investigate the temperature dependence of its fast SD amplitude and its homogeneous excitonic linewidt

Insertion of CdSe quantum dots in ZnSe nanowires: Correlation of structural and chemical characterization with photoluminescence

International audienceZnSe nanowires with CdSe quantum dot insertions were grown by molecular beam epitaxy using gold as a catalyst. Structural, chemical, and optical properties of the wires and quantum dots were characterized using electron microscopy and photoluminescence spectroscopy. We determined the crystalline structure, the chemical composition, and the size of the quantum dot and established a correlation between quantum dot size and luminescence. As expected, a blueshift of the luminescence was observed for decreasing quantum dot size. The comparison of calculated photoluminescence energy and experimental data seems to indicate that the quantum dots consist of a ZnxCd1-xSe ternary alloy rather than pure CdSe

Insertion of CdSe quantumdots in ZnSe nanowires : MBE growth and microstructure analysis

International audienceZnSe nanowire growth has been successfully achieved on ZnSe (100) and (111)B buffer layers deposited on GaAs substrates. Cubic [100] oriented ZnSe nanowires or [0001] oriented hexagonal NWs are obtained on (100) substrates while [111] oriented cubic mixed with [0001] oriented hexagonal regions are obtained on (111)B substrates. Most of the NWs are perpendicular to the surface in the last case. CdSe quantum dots were successfully incorporated in the ZnSe NWs as demonstrated by transmission electron microscopy, energy filtered TEM and high angle annular dark field scanning TEM measurements

Effect of extended defects on AlGaN QDs for electron-pumped UV-emitters

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like defects starting at the first AlN buffer/superlattice interface and propagating vertically. These defects are associated with a dislocation that produces strong shear strain, which favors the formation of 30{\deg} faceted pits. The cone-like structures present Ga enrichment at the boundary facets and larger QDs within the defect. The bimodality is attributed to the differing dot size/composition within the defects and at the defect boundaries, which is confirmed by the correlation of microscopy results and Schr\"odinger-Poisson calculations

Exciton-phonon coupling efficiency in CdSe quantum dots embedded in ZnSe nanowires

International audienceExciton luminescence of a CdSe quantum dot (QD) inserted in a ZnSe nanowire is strongly influenced by the dark exciton states. Because of the small size of these QDs (2-5 nm), exchange interaction between hole and electron is highly enhanced and we measured large energy splitting between bright and dark exciton states (ΔE∈[4,9.2] meV) and large spin-flip rates between these states. Statistics on many QDs showed that this splitting depends on the QD size. Moreover, we measured an increase of the spin-flip rate to the dark states with increasing energy splitting. We explain this observation with a model, taking into account the fact that the exciton-phonon interaction depends on the bright to dark exciton energy splitting, as well as on the size and shape of the exciton wave function. It also has consequences on the exciton line intensity at high temperature

Pseudo-square AlGaN/GaN quantum wells for terahertz absorption

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Terahertz absorbing AlGaN/GaN multi-quantum-wells: Demonstration of a robust 4-layer design

International audienceWe report on AlGaN/GaN multi-quantum-well structures displaying intersubband absorption in the THz spectral range. First, we theoretically analyze the weaknesses of the state-of-the-art GaN-based step-quantum-well architecture from an optoelectronic standpoint. We then propose a modified geometry with improved structural robustness considering the uncertainties associated to the growth. This later structure, consisting of 4-layer quantum wells, has been grown by plasma-assisted molecular-beam epitaxy and characterized structurally and optically. Low temperature absorption of samples with different Si doping levels confirms intersubband transitions in the far-infrared, centred at 28 μm

Nouvelles observations sur l'habitat gallo-romain à Vaison-la-Romaine (Vaucluse)

Researches recently carried out on three spots of the site of Vaison (La Villasse Nord, Propriété Perret and Pommerol), yielded more or less significant information (but pieces of information are especially interesting concerning the south of the Villasse area). Several stages were distinguished in La Villasse and in Pommerol. Among other results, the issue concerning the boundaries of the ancient town is emphasized more precisely, and an even town planning is attested as early as the Augustan epoch (houses were built with equal modules around courtyards).Bellet Michel-Edouard, Boccacino Catherine, Borgard Philippe, Bouillot Jocelyne. Nouvelles observations sur l'habitat gallo-romain à Vaison-la-Romaine (Vaucluse). In: Revue archéologique de Narbonnaise, tome 23, 1990. pp. 71-94