Near Band Edge excitation in 2D materials by Transmission Electron Microscopy

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Spectroscopy on Black Phosphorus exfoliated down to the monolayer

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Fe-Ti-O based catalyst for large-chiral-angle single-walled carbon nanotube growth

International audienceCatalyst selection is very crucial for controlled growth of single-walled carbon nanotubes (SWNTs). Here we introduce a well-designed Fesingle bondTisingle bondO solid solution for SWNT growth with a high preference to large chiral angles. The Fesingle bondTisingle bondO catalyst was prepared by combining Ti layer deposition onto premade Fe nanoparticles with subsequent high-temperature air calcination, which favours the formation of a homogeneous Fesingle bondTisingle bondO solid solution. Using CO as the carbon feedstock, chemical vapour deposition growth of SWNTs at 800 °C was demonstrated on the Fesingle bondTisingle bondO catalyst. Nanobeam electron diffraction characterization on a number of individual SWNTs revealed that more than 94% of SWNTs have chiral angles larger than 15°. In situ environmental transmission electron microscopy study was carried out to reveal the catalyst dynamics upon reduction. Our results identify that the phase segregation through reducing Fesingle bondTisingle bondO catalyst leads to the formation of TiOx-supported small Fe nanoparticles for SWNT growth. The strong metal-support interactions induced by partial reduction of TiOx support promote the wettability of Fe nanoparticle, accounting for the preferential growth of large-chiral-angle SWNTs. This work opens a new avenue for chiral angle selective growth of SWNTs

Suivi de la cinétique associée à la phase gamma' dans le superalliage N18 en utilisant des mesures de résisitivité électrique in situ

International audienceIn nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ' precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ' precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ' phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes

Distinguishing different stackings in layered materials via luminescence spectroscopy

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.Comment: Main: 6 pages and 4 figures, Supplementary: 6 pages and 7 figure

Probing structural and electronic properties of h-BN by HRTEM and STM

International audienceAfter the discovery of graphene and its consequences in the field of nanoscience and nanomaterials, there has been a growing interest in 2D materials and also their vertical stacking due to unique properties and potential applications.[1] For instance, it was shown the transport properties of exfoliated graphene supported by hexagonal boron nitride (h-BN) could approach the intrinsic graphene limits.[2] Nevertheless, studying the structural properties of 2D materials and 2D heterostructures is crucial to understand their physical and chemical properties. Our motivations have been to exploit state of the art aberration-corrected high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM) to study the structure and electronic properties of graphene (G), h-BN and G/h-BN heterostructures. HRTEM analyses were conducted with a JEOL ARM microscope equipped together with a cold FEG, an aberration corrector for the objective lens and a One view camera (Gatan). Notably, we used high-speed atomic-scale imaging to study with unprecedented dynamics (up to 25 fps) the nucleation and growth mechanisms of triangular holes in h-BN under beam irradiation (Figure 1). The direct observation of B and N atom sputtering and surface reconstruction processes allow understanding how the triangular shape and orientation of holes are maintained during the growth. Interestingly, by studying the effects of the electron dose and the number of BN layers, we demonstrate that these atomic-scale processes are simultaneously driven by kinetic and thermodynamic effects. Further works are in progress to study the stability of h-BN/G stacking under electron-beam irradiation. STM analyses were carried out with low temperature STM at 4 K, on 2D heterostructures that consist in a few layers of graphene doped with nitrogen on thick exfoliated flakes of BN deposited on SiO 2. Remarkably, we show that STM allows identifying and characterizing ionization defects within the BN flakes below the graphene layers (Figure 2). This study opens new avenues to probe the electronic interactions between this two stacked materials

Confinement of Dyes inside Boron Nitride Nanotubes: Photostable and Shifted Fluorescence down to the Near Infrared

Fluorescence is ubiquitous in life science and used in many fields of research ranging from ecology to medicine. Among the most common fluorogenic compounds, dyes are being exploited in bioimaging for their outstanding optical properties from UV down to the near IR (NIR). However, dye molecules are often toxic to living organisms and photodegradable, which limits the time window for in vivo experiments. Here, it is demonstrated that organic dye molecules are passivated and photostable when they are encapsulated inside a boron nitride nanotube (dyes@BNNT). The results show that the BNNTs drive an aggregation of the encapsulated dyes, which induces a redshifted fluorescence from visible to NIR‐II. The fluorescence remains strong and stable, exempt of bleaching and blinking, over a time scale longer than that of free dyes by more than 104. This passivation also reduces the toxicity of the dyes and induces exceptional chemical robustness, even in harsh conditions. These properties are highlighted in bioimaging where the dyes@BNNT nanohybrids are used as fluorescent nanoprobes for in vivo monitoring of Daphnia Pulex microorganisms and for diffusion tracking on human hepatoblastoma cells with two‐photon imaging

Distinguishing Different Stackings in Layered Materials via Luminescence Spectroscopy

peer reviewedDespite 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

Advanced 1D heterostructures based on nanotube templates and molecules

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities

Spectroscopie infrarouge de fils et boîtes quantiques d'InAs/InAlAs/InP(001)

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