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.

Hyperspectral microscopy of boron nitride nanolayers using hybrid femto/picosecond coherent anti-Stokes Raman scattering.

The rise in interest in two-dimensional (2D) nanomaterials has been notable in recent years. In particular, hexagonal boron nitride (h-BN), recognized as an optimal substrate for enhancing graphene properties, holds promise for electronic applications. However, the widely employed spontaneous Raman microscopy, a gold standard for graphene study, faces strong limitations in h-BN due to its large bandgap and low cross section. In this Letter, high-resolution femto/picosecond coherent anti-Stokes Raman scattering (fs/ps-CARS) spectroscopy is used for hyperspectral imaging of nanometric h-BN layers. Our study establishes that CARS signal effectively enhances Raman signature related to in-plane ring vibrations, thus providing valuable quantitative insights into sample thickness and crystalline quality, also corroborated by additional AFM measurements.

η-Carbides (Co, Mo, or W) Nanoparticles from Octacyanometalates Precursors-Based Network.

This paper describes a simple, two-steps chemical pathway to obtain bimetallic carbide nanoparticles (NPs) of general formula MxM″yC, also called η-carbides. This process allows for a control of the chemical composition of metals present in the carbides (M = Co and M″ = Mo or W). The first step involves the synthesis of a precursor consisting of a network of octacyanometalates. The second step consists in a thermal degradation of the previously obtained octacyanometalates networks under neutral atmosphere (Ar or N2 ). It is shown that this process results in the formation of carbide NPs with diameter of ≈ 5nm, and the stoichiometries Co3 M'3 C, Co6 M'6 C, Co2 M'4 C for the CsCoM' systems.

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.

Molecular beam epitaxial growth of multilayer 2D-boron nitride on Ni substrates from borazine and plasma-activated nitrogen.

2D boron nitride (2D-BN) was synthesized by gas-source molecular beam epitaxy on polycrystalline and monocrystalline Ni substrates using gaseous borazine and active nitrogen generated by a remote plasma source. The excess of nitrogen atoms allows to overcome the thickness self-limitation active on Ni when using borazine alone. The nucleation density and the shape of the 2D-BN domains are clearly related to the Ni substrate preparation and to the growth parameters. Based on spatially-resolved photoemission spectroscopy and on the detection of the π plasmon peak, we discuss the origin of the N1s and B1s components and their relationship with an electronic coupling at the interface. After optimization of the growth parameters, a full 2D-BN coverage is obtained, although the material thickness is not evenly distributed. The 2D-BN presents a granular structure on (111) oriented Ni grains, showing a rather poor cristallographic quality. On the contrary, high quality 2D-BN is found on (101) and (001) Ni grains, where triangular islands are observed whose lateral size is limited to ∼20µm.

Momentum-Resolved Dielectric Response of Free-Standing Mono-, Bi-, and Trilayer Black Phosphorus.

Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin-layer BP is photo-oxidized under ambient conditions and the energy of its electronic states shifts in different dielectric environments. We prepared free-standing samples of few-layer BP under glovebox conditions and probed the dielectric response in a vacuum using scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS). Thresholds of the excitation energy are measured at 1.9, 1.4, and 1.1 eV for the mono-, bi-, and trilayer BP, respectively, and these values are used to estimate the corresponding optical band gaps. A comparison of our results with electronic structure calculations indicates that the variation of the quasi-particle gap is larger than that of the exciton binding energy. The dispersion of the plasmons versus momentum for one- to three-layer BP and bulk BP highlights a deviation from parabolic to linear dispersion and strong anisotropic fingerprints.

Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN.

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10 K comparable to that of direct band-gap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN. Ab initio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300±50 meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk h-BN.

Advanced synthesis of highly crystallized hexagonal boron nitride by coupling polymer-derived ceramics and spark plasma sintering processes-influence of the crystallization promoter and sintering temperature.

Hexagonal boron nitride nanosheets (BNNSs) are promising 2D materials due to their exceptional chemical and thermal stabilities together with their electrical insulation properties. A combined synthesis method involving the polymer-derived ceramics (PDCs) route and the spark plasma sintering (SPS) process is proposed, leading to well-crystallized and pure layered h-BN crystals, prone to be exfoliated into large BNNSs. Here we focus more specifically on the influence of two key parameters of the process to be optimized: the Li3N concentration (0-10 wt%) and the SPS temperature (1200 °C-1950 °C). The presence of Li3N, added as crystal promoter in the pre-ceramic powder, significantly improves the crystallinity level of the product, as evidenced by XRD, SEM and Raman spectrometry. SPS temperature strongly modifies the size of the resulting h-BN flakes. The influence of SPS temperature on both purity and crystallinity is studied using cathodoluminescence. h-BN flakes larger than 200 µm2 (average flake area) are obtained. Few-layered BNNSs are successfully isolated, through exfoliation process.

Photooxidation and quantum confinement effects in exfoliated black phosphorus.

Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

Atomic configuration of nitrogen-doped single-walled carbon nanotubes.

Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first-principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighboring local lattice distortion such as Stone-Thrower-Wales defects. We also show that the largest fraction of nitrogen amount is found in poly aromatic species that are adsorbed on the surface of the nanotube walls. The stability under the electron beam of these nanotubes has been studied in two different cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures.

Structural Size Effect in Capped Metallic Nanoparticles.

The surfactant used during a colloidal synthesis is known to control the size and shape of metallic nanoparticles. However, its influence on the nanoparticle (NP) structure is still not well understood. In this study, we show that the surfactant can significantly modify the lattice parameter of a crystalline particle. First, our electron diffraction measurements reveals that NiPt nanoparticles around 4 nm in diameter covered by a mixture of oleylamine and oleic acid (50:50) display a lattice parameter expansion around 2% when compared to the same particles without surfactant. Using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) techniques, we show that this expansion can not be explained by crystal defects, twinning, oxidation, or atoms insertion. Then, using covered NPs in the 4-22 nm size range, we show that the lattice parameter evolves linearly with the inverse of the NP size, as it is expected when a surface stress is present. Finally, the study is extended to pure nickel and pure platinum NPs, with different sizes, coated by different surfactants (oleylamine, trioctylphosphine, polyvinylpyrrolidone). The surfactants induce lattice parameter variations, whose magnitude could be related to the charge transfer between the surfactant and the particle surface.

Thickness dependent sensing mechanism in sorted semi-conducting single walled nanotube based sensors.

Single walled carbon nanotube (SWCNT) networks present outstanding potential for the development of SWCNT-based gas sensors. Due to the complexity of the transport properties of this material, the physical mechanisms at stake during exposure to gas are still under debate. Previously suggested mechanisms are charge transfer between gas molecules and SWCNT and Schottky barrier modulation. By comparing electrical measurements with an analytical model based on Schottky barrier modulation, we demonstrate that one mechanism or the other is predominant depending on the percolation of metallic carbon nanotubes. Below the metallic SWCNT percolation threshold, sensing is dominated by the modulation of the Schottky barrier, while above this threshold, it is only attributed to a charge transfer between SWCNT and gas molecules. Both mechanisms are discussed in terms of sensitivity and resolution leading to routes for the optimization of a gas sensor architecture based on highly enriched semiconducting carbon nanotube films.

Colloidal synthesis of nanoparticles: from bimetallic to high entropy alloys.

At the nanoscale, the synthesis of a random alloy (i.e. without phase segregation, whatever the composition) by chemical synthesis remains a difficult 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) is considered a challenging task as each element has fairly different physico-chemical properties. Particles 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 potential values of the metal cations is the driving force to achieve a homogeneous distribution of all elements within the nanoparticles.

Young modulus, mechanical and electrical properties of isolated individual and bundled single-walled boron nitride nanotubes.

The Young modulus of individual single-walled boron nitride nanotubes (SW-BNNTs) was determined using a high-resolution transmission-electron microscope (HRTEM)-atomic force microscope (AFM) set-up. The Young modulus and maximum stress for these NTs were deduced from the analysis of the stress-strain curves, and discussed as a function of the considered value for the shell thickness of an SW-BNNT. The elastic properties of bundles of SW-BNNTs were also investigated. All these experiments revealed that SW-BNNTs are very flexible. Furthermore, the electrical behavior of these SW-BNNTs was also analyzed employing a scanning tunneling microscope (STM) holder integrated with the same HRTEM. I/V curves were measured on individual tubes as well as on bundles of SW-BNNTs.

Pi-stacking functionalization of carbon nanotubes through micelle swelling.

We report on a new, original and efficient method for pi-stacking functionalization of single-wall carbon nanotubes. This method is applied to the synthesis of a high-yield light-harvesting system combining single-wall carbon nanotubes and porphyrin molecules. We developed a micelle-swelling technique that leads to controlled and stable complexes presenting an efficient energy transfer. We demonstrate the key role of the organic solvent in the functionalization mechanism. By swelling the micelles, the solvent helps the non-water-soluble porphyrins to reach the micelle core and allows a strong enhancement of the interaction between porphyrins and nanotubes. This technique opens new avenues for the functionalization of carbon nanostructures.

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.

Nickel platinum (Ni x Pt1-x ) nanoalloy monodisperse particles without the core-shell structure by colloidal synthesis.

We report a new and versatile colloidal route towards the synthesis of nanoalloys with controlled size and chemical composition in the solid solution phase (without phase segregation such as core-shell structure or Janus structure) or chemical ordering. The principle of the procedure is based on the correlation between the oxidation-reduction potential of metal cations present in the precursors and the required synthesis temperature to nucleate particles without phase segregation. The procedure is demonstrated via the synthesis of Face Centered Cubic (FCC) Ni x Pt1-x nanoparticles, which was elaborated by the co-reduction of nickel(ii) acetylacetonate and platinum(ii) acetylacetonate with 1,2-hexadecanediol in benzyl ether, using oleylamine and oleic acid as surfactants. The chemical composition and solid solution FCC structure of the nanoalloy are demonstrated by crosslinking imaging and chemical analysis using transmission electron microscopy and X-ray diffraction techniques. Whatever the chemical composition inspected, a systematic expansion of the lattice parameters is measured in relation to the respective bulk counterpart.

Highly Ordered Boron Nitride/Epigraphene Epitaxial Films on Silicon Carbide by Lateral Epitaxial Deposition.

The realization of 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 two-dimensional (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.

Tuning bimetallic catalysts for a selective growth of SWCNTs.

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 nanoparticles. By using the as-synthesized SWCNTs as a channel material infield 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.

Atomic-scale structural identification and evolution of Co-W-C ternary SWCNT catalytic nanoparticles: High-resolution STEM imaging on SiO2.

Recently, W-based catalysts have provided a promising route to synthesize single-walled carbon nanotubes (SWCNTs) with specific chirality, but the mechanism of the growth selectivity is vaguely understood. We propose a strategy to identify the atomic structure as well as the structure evolution of the Co-W-C ternary SWCNT catalyst. The key is to use a thin SiO2 film as the catalyst support and observation window. As the catalyst is uniformly prepared on this SiO2 film and directly used for the SWCNT synthesis, this method has an advantage over conventional methods: it creates an opportunity to obtain original, statistical, and dynamic understanding of the catalyst. As a technique, atomic-scale imaging directly on SiO2 serves as a powerful and versatile tool to investigate nanocrystals and high-temperature reactions; for the synthesis of SWCNTs, this work successfully visualizes the structure and evolution of the catalyst and illuminates the possible nucleation sites of the chirality-specific growth.