Spatial modulation of above-the-gap cathodoluminescence in InP nanowires

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Nanometer Scale Spectral Imaging of Quantum Emitters in nanowires and Its Correlation to Their Atomically Resolved Structure

International audienceWe report the spectral imaging in the UV to visible range with nanometer scale resolu-tion of closely packed GaN/AlN quantum discs in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as one nanometer and separated from each others by only few nanometers, and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disc size and emission wavelength allows us to evidence the quantum confined Stark effect leading to an emission below the bulk GaN band gap for discs thicker than 2.6 nm. Helped with simula-tions, we show that the internal electric field in the studied quantum discs is smaller than what is expected in the quantum well case. We evidence a clear dispersion of the emission wave-lengths of different quantum discs of identical size but different position along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire, and is thus attributed to the related strain variations along the wire. The present work opens the way both for fundamental studies of quantum confinement in closely packed quan-tum emitters and for characterizations of optoelectronic devices presenting carrier localization on the nanometer scale

On the measurement of the fractal dimension of aggregated particles by electron microscopy : experimental method, corrections and comparison with numerical models

Digital annular dark-field images of aggregated iron polydisperse particles are obtained using a computer-controlled STEM. Different methods are discussed on how to extract the fractal dimension of the aggregates, and the sources of error are analysed. The results are compared with computer simulations on a polydisperse version of the cluster-cluster aggregation model. Simulations show that polydispersity does not affect the fractal dimension. The experimental result for the fractal dimension ( D =1.9 ± 0.1) is consistent with the cluster-cluster model with linear trajectories.Des images digitalisées en fond noir annulaire d'agrégats polydispersés de billes de fer sont obtenues avec un microscope électronique piloté par ordinateur. On discute des différentes méthodes capables d'extraire la dimension fractale des agrégats et on analyse les différentes sources d'erreur. Les résultats sont comparés avec des simulations à l'ordinateur utilisant une version polydisperse du modèle d' agrégation par collage d'amas. Les simulations montrent que la polydispersité n'affecte pas la valeur de la dimension fractale. Le résultat expérimental (D =1,9 ± 0,1 ) est consistant avec un modèle d'agrégation par collage d'amas avec trajectoires linéaires

CACAO a collimation means well suited for pixellated detectors.

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The structure of carbon encapsulated NiFe nanoparticles

Carbon encapsulated NiFe nanoparticles (NiFe@C) have been prepared by high-temperature methane encapsulation of the bare bimetallic particles on alumina. High-resolution transmission electron microscopy pictures show that about 6-nm thick carbon layers encapsulate 10-20-nm diameter NiFe nanoparticles. The NiFe nanoparticles are shown to be single-crystalline and no carbide is found at the NiFe-C interface. This is confirmed by the electron energy-loss spectroscopy (EELS) measurements that in addition show that both Ni and Fe have a zero (metal) valence and that only graphite is present. EELS also shows that the nickel-to-iron ratio is exactly unity for all particles studied. Metallic Pd nanoparticles with a diameter of 1-2 nm can be anchored on the carbon layers, which creates a Pd/NiFeC&C type of catalyst that could be used for liquid phase reactions. The EELS analysis reveals that part of the nanoparticles present are not Pd but other oxidic carbon encapsulated nanoparticles. (C) 2001 Academic Press