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Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from ``indirect'', finite-momentum, excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed., Main: 6 pages and 4 figures, Supplementary: 6 pages and 7 figures

International audience

International audience; Equivalent-spherical models are widely used in lidar and remote-sensing for inferring microphysical properties of various aerosols. The concept of fractal aggregates is introduced and applied to calculate accurate backscattering coefficients of black carbon particles. The purpose of this work is to assess the performance of equivalent-spherical models to model the backscattering cross-sections of irregularly shaped particles such as fresh black carbon particles. The use of equivalent spherical models is discussed in regard to lidar measurements.

International audience; Materials science of carbon nanotubes (CNT) lies at the intersection of various paradigms from fundamental to applied physics and chemistry. Recognizing how the different concepts can be combined together to understand CNT formation still remains a challenge. The synthesis of CNTs with desired chirality and diameter is one of the most important challenges for the nanotubes science and achieving such selectivity requires a good understanding of their growth mechanism. There is an overwhelming body of evidence that catalytic growth is the most promising method for the CNTs synthesis, especially for reaching a chiral selectivity synthesis.[1,2] Although knowledge of the different stages of growth has advanced considerably[3] a full picture is still elusive. Mastering chirality control, however, requires a deeper understanding of the very early stage of CNT nucleation, when the symmetry type is set. A well-controlled synthesis of catalyst nanoparticles (composition, morphology, size) appears to be a mandatory condition for controlling the characteristics of the as-synthesized nanotubes.[4] Recent researches focused especially on the growth of multiwalled CNT[5,6] have shown that the catalyst (re)shaping is correlated with surface energy modification due to C adsorption. To the best of our knowledge previous works have been devoted to the understanding of the relationship between changes in catalyst particle morphology and CNT nucleation [7-11] but more quantitative description is greatly needed for the elaboration of SWCNTs with defined chirality.The aim of the present study is to complement the well-established ex-situ observation of SWCNTs growth [12, 13] with real-time, in-situ TEM observations of the real CVD growth reactions without losing significant resolution. The study presents the development of a unique approach that combines the HRTEM technique, with the implementation of radical-assisted CVD gas sources in a modified environmental transmission electron microscope (ETEM) equipped with a Cs image aberration-corrector. The new set-up, well adapted for carrying out real-time in-situ observations, allows one to keep a higher vacuum within the TEM chamber while bringing gas molecules to the sample, was used to analyze catalyst reshaping and its morphology dynamics during CNT nucleation/growth under controlled conditions. As applied to different new and controlled bimetallic nanoalloy catalysts types, such as CoRu, FeRu and NiRu catalyst [11], the emphasis is to understand the role played by the catalyst and put forward a growth mechanism essential to realized SWCNTs with specific and targeted chirality. In particular, we will discuss the role of the growth temperature and the chemical composition of the catalyst on the final SWCNTs structure.

Intraband transitions are optical transitions between confined states of the conduction band or the valence band. Spectroscopy of intraband transitions gives an accurate measurement of the confinement energies as well as the spatial symmetry of the wave functions. The main applications are photodetection and unipolar lasers. In 1D and 0D nanostructures, the lateral confinement of the carriers allows intraband transitions polarised in the layer plane. Samples studied in this work have been grown by molecular beam epitaxy. Strains, due to the lattice mismatch between InAs and InP, give rise to nanometric islands in the Stranski-Krastanov growth mode. Structural characterization reveals that it is possible to obtain different types of nanostructures by adjusting growth parameters. The weak lattice mismatch (3%) allows a full coverage of the InAlAs surface by wires, elongated dots or isotropic dots. The samples have been characterized by photoluminescence, photo-induced absorption and photocurrent spectroscopies. These experiment s reveal the presence of intense infrared absorptions (12-14µm), polarized in the layer plane and attributed to optical transition between the ground state and the first excited state confined along [110]. This absorption reaches 26% in 10 planes of n-doped (1012cm-2) elongated dots. We also observe other transitions at shorter wavelength (9-12µm) in the [110] and [1-10] polarisations attributed to transitions between excited states in the conduction band. The synthesis of the different spectroscopies allows the reconstruction of the energetic diagram. Results are in agreement with calculations based on a k.p model.; Dans les nanostructures 1D et 0D, le confinement latéral des porteurs autorise des transitions intrabandes polarisées dans le plan des couches. Ces structures présentent alors un intérêt pour la photodétection à incidence normale ou l'émission de lumière par la surface. Les échantillons sont réalisés par épitaxie par jets moléculaires. Les contraintes dues au désaccord de maille entre l'InAs et l'InP donnent naissance à des îlots de taille nanométrique en suivant le mode de croissance Stranski-Krastanov. La caractérisation structurale de ces îlots par AFM et TEM montre un profil en forme de pyramide tronquée et un allongement le long de la direction [1-10]. Le faible désaccord de maille (3%) conduit à une couverture complète de la surface d'InAlAs par des boîtes allongées ou des fils. Les échantillons ont été caractérisés par photoluminescence et par spectroscopie d'absorption photo-induite. Ces expériences ont permis de montrer la présence d'absorptions infrarouges remarquables (12-14µm), polarisées dans le plan des couches et attribuées à la transition optique entre l'état fondamental et le premier état excité confiné suivant la direction [110]. D'autre part, nous mettons également en évidence la présence de transitions moins intenses à plus courte longueur d'onde (9-12µm) dans les polarisations [110] et [1-10] dont l'intensité est fortement dépendante de la température. Nous attribuons ainsi ces résonances à des transitions entre niveaux excités de la bande de conduction. Des mesures de spectroscopie de photocourant complètent l'étude optique et montrent les performances remarquables de ce système pour la photodétection à incidence normale. Les résultats issus de simulations, tenant compte de la non-parabolicité des bandes, montrent un accord satisfaisant avec les expériences et autorisent l'identification des raies observées. La synthèse des différentes spectroscopies conduit à la reconstruction de la structure énergétique des boîtes.