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Hexagonal boron nitride encapsulation is the method of choice for protecting graphene from environmental doping and impurity scattering. It was previously demonstrated that metal-organic vapor phase epitaxy (MOVPE) grows epitaxially ordered, uniform BN layers on epigraphene (graphene grown on SiC). Due to graphene non-wetting properties, h-BN growth starts preferentially from the graphene ledges. We use this fact here to selectively promote growth of high-quality flat h-BN on epigraphene by patterning epigraphene microstructures prior to BN growth. Thin h-BN films (down to 6 nm) grown by MOVPE show smooth and pleated surface morphology on epigraphene, while crumpled BN is observed on the SiC. Cross-sectional high-resolution transmission electron microscopy images and fluorescence imaging confirm the higher BN quality grown on the epigraphene. Transport measurements reveal p-doping as expected from hydrogen intercalation of epigraphene and regions of high and low mobility. This method can be used to produce structurally uniform high-quality h-BN/epigraphene micro/nano scale heterostructure.

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 figures

At the nanometric scale, the synthesis of a random alloy (i.e. without phase segregation, whatever the composition) by chemical synthesis remains a not easy task, even for simple binary type systems. In this context, a unique approach based on the colloidal route is proposed enabling the synthesis of face-centred cubic and monodisperse bimetallic, trimetallic, tetrametallic and pentametallic nanoparticles with diameters around 5 nm as solid solutions. The Fe-Co-Ni-Pt-Ru alloy and its subsets are considered which is a challenging task as each element has fairly different physico-chemical properties. Nanoparticles are prepared by temperature-assisted co-reduction of metal acetylacetonate precursors in the presence of surfactants. It is highlighted how the correlation between precursors' degradation temperatures and reduction potentials values of the metal cations is the driving force to achieve a homogenous distribution of all elements within the nanoparticles.

Even though ultrasonication is considered to be an effective method to disperse carbon nanotubes (CNTs), its devastating effects on the nanotubes are often neglected. Here, even mild ultrasonication is found to rapidly extract the inner single-wall CNTs (SWCNTs) from the outer shells of the double-wall CNTs (DWCNTs). As-synthesized DWCNTs are gently solubilized in a surfactant solution, strictly avoiding any ultrasonication, followed by two consecutive density gradient ultracentrifugation (DGU) steps to obtain a purified colloidal solution of isolated DWCNTs. The latter is carefully selected based on in situ resonant Raman (RRS) and fluorescence (PL) spectroscopy, measured as a function of depth directly in the ultracentrifuge tube after DGU. These purified DWCNTs are ultrasonicated in successive time steps while intermittently probing the sample via RRS and PL spectroscopy. These results unravel the very fast increasing yet saturating extraction mechanism that leads to the formation of fluorescing SWCNTs. A statistical high-resolution transmission electron microscopy study confirms the drastic increase in SWCNTs after ultrasonication, and evidences that ultrasonication forms SWCNTs from both the inner and outer shells of the DWCNTs. This study demonstrates how easily ultrasonication extracts SWCNTs from individually solubilized DWCNTs, unavoidably complicating any further spectroscopic studies on DWCNTs severely., Comment: 41 pages (64 incl. SI), 9 figures (19 SI figures)

Realizing high-performance nanoelectronics requires control of materials at the nanoscale. Methods to produce high quality epitaxial graphene (EG) nanostructures on silicon carbide are known. The next step is to grow Van der Waals semiconductors on top of EG nanostructures. Hexagonal boron nitride (h-BN) is a wide bandgap semiconductor with a honeycomb lattice structure that matches that of graphene, making it ideally suited for graphene-based nanoelectronics. Here, we describe the preparation and characterization of multilayer h-BN grown epitaxially on EG using a migration-enhanced metalorganic vapor phase epitaxy process. As a result of the lateral epitaxial deposition (LED) mechanism, the grown h-BN/EG heterostructures have highly ordered epitaxial interfaces, as desired in order to preserve the transport properties of pristine graphene. Atomic scale structural and energetic details of the observed row-by-row, growth mechanism of the 2D epitaxial h-BN film are analyzed through first-principles simulations, demonstrating one-dimensional nucleation-free-energy-barrierless growth. This industrially relevant LED process can be applied to a wide variety of van der Waals materials.

Resonant Raman spectroscopy is a widely used technique for single-walled carbon nanotube (SWCNT) characterization, in particular in the radial breathing mode (RBM) range which provides direct information on the structure of the nanotube in resonance. The RBM peak counting method, i.e. acquiring Raman spectrum grids on a substrate with a select set of discrete laser lines and counting RBM peaks as single nanotubes, is frequently used to characterize SWCNT growth samples, despite the many factors that can induce errors in the results. In this work, we cross-characterize the diameter distributions obtained through this methodology with diameter distributions obtained by counting SWCNT diameters in transmission electron microscopy (TEM) and discuss the different results and biases between the techniques. This study is performed on a broad diameter distribution sample, and on two chirality-enriched samples whose chirality distributions are determined by photoluminescence excitation spectroscopy (PLE) and statistical analysis of high resolution TEM (HRTEM) images. We show that the largest differences between the Raman peak counting and TEM diameter distributions stem from the chirality-dependence of SWCNT Raman cross-sections and the patchy vision offered by the use of only a few discrete excitation wavelengths. The effect of the substrate and TEM-related biases are also discussed.

Recent advances in structural control during the synthesis of SWCNTs have in common the use of bimetallic nanoparticles as catalysts, despite the fact that their exact role is not fully understood. We therefore analyze the effect of the catalyst' s chemical composition on the structure of the resulting SWCNTs by comparing three bimetallic catalysts (FeRu, CoRu and NiRu). A specific synthesis protocol is designed to impede the catalyst nanoparticle coalescence mechanisms and stabilize their diameter distributions throughout the growth. Owing to the ruthenium component which has a limited carbon solubility, tubes grow in tangential mode and their diameter is close to that of their seeding nanoparticle. By using as-synthesized SWCNTs as a channel material in field effect transistors, we show how the chemical composition of the catalysts and temperature can be used as parameters to tune the diameter distribution and semiconducting-to-metallic ratio of SWCNT samples. Finally, a phenomenological model, based on the dependence of the carbon solubility as a function of catalyst nanoparticle size and nature of the alloying elements, is proposed to interpret the results., Comment: 10 pages, 6 figures

Structural identification of double-walled carbon nanotubes (DWNT) is presented through a robust procedure based on the latest generation of transmission electron microscope, making possible a statistical analysis based on numerous nano-objects. This approach reveals that inner and outer tubes of DWNTs are not randomly oriented, suggesting the existence of a mechanical coupling between the two concentric walls. With the support of atomic scale modelisations, we attribute it to the presence of incommensurate domains whose structures depend on the diameters and helicities of both tubes, and where inner tubes try to achieve a local stacking orientation to reduce strain effects.

Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the $K$ edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results of quality comparable to those obtained from non resonant X-ray inelastic scattering, but with advantageous specificities such as an enhanced sensitivity at low q and a much higher simplicity and versatility that makes it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations against our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method.

Hexagonal boron nitride (hBN) regains interest as a strategic component in graphene engineering and in van der Waals heterostructures built with two dimensional materials. It is crucial then, to handle reliable characterization techniques capable to assess the quality of structural and electronic properties of the hBN material used. We present here characterization procedures based on optical spectroscopies, namely cathodoluminescence and Raman, with the additional support of structural analysis conducted by transmission electron microscopy. We show the capability of optical spectroscopies to investigate and benchmark the optical and structural properties of various hBN thin layers sources.