Boron and Nitrogen Isotope Effects on Hexagonal Boron Nitride Properties.

The unique physical, mechanical, chemical, optical, and electronic properties of hexagonal boron nitride (hBN) make it a promising 2D material for electronic, optoelectronic, nanophotonic, and quantum devices. Here, the changes in hBN's properties induced by isotopic purification in both boron and nitrogen are reported. Previous studies on isotopically pure hBN have focused on purifying the boron isotope concentration in hBN from its natural concentration (≈20 at% 10 B, 80 at% 11 B) while using naturally abundant nitrogen (99.6 at% 14 N, 0.4 at% 15 N), that is, almost pure 14 N. In this study, the class of isotopically purified hBN crystals to 15 N is extended. Crystals in the four configurations, namely h10 B14 N, h11 B14 N, h10 B15 N, and h11 B15 N, are grown by the metal flux method using boron and nitrogen single isotope (> 99%) enriched sources, with nickel plus chromium as the solvent. In-depth Raman and photoluminescence spectroscopies demonstrate the high quality of the monoisotopic hBN crystals with vibrational and optical properties of the 15 N-purified crystals at the state-of-the-art of currently available 14 N-purified hBN. The growth of high-quality h10 B14 N, h11 B14 N, h10 B15 N, and h11 B15 N opens exciting perspectives for thermal conductivity control in heat management, as well as for advanced functionalities in quantum technologies.

Preparation of Confined One-Dimensional Boron Nitride Chains in the 1-D Pores of Siliceous Zeolites under High-Pressure, High-Temperature Conditions.

Low-dimensional boron nitride (BN) chains were prepared in the one-dimensional pores of the siliceous zeolites theta-one (TON) and Mobil-twelve (MTW) by the infiltration, followed by the dehydrocoupling and pyrolysis of ammonia borane under high-pressure, high-temperature conditions. High-pressure X-ray diffraction in a diamond anvil cell and in a large-volume device was used to follow in situ these different steps in order to determine the optimal conditions for this process. Based on these results, millimeter-sized samples of BN/TON and BN/MTW were synthesized. Characteristic B-N stretching vibrations of low-dimensional BN were observed by infrared and Raman spectroscopies. The crystal structures were determined using a combination of X-ray diffraction and density functional theory with one and two one-dimensional zig-zag (BN)x chains per pore in BN/TON and BN/MTW, respectively. These 1-D BN chains potentially have interesting photoluminescence properties in the far ultraviolet region of the electromagnetic spectrum.

Dynamic Instability of Individual Carbon Nanotube Growth Revealed by In Situ Homodyne Polarization Microscopy.

Understanding the kinetic selectivity of carbon nanotube growth at the scale of individual nanotubes is essential for the development of high chiral selectivity growth methods. Here we demonstrate that homodyne polarization microscopy can be used for high-throughput imaging of long individual carbon nanotubes under real growth conditions (at ambient pressure, on a substrate) and with subsecond time resolution. Our in situ observations on hundreds of individual nanotubes reveal that about half of them grow at a constant rate all along their lifetime while the other half exhibits stochastic changes in growth rates and/or switches between growth, pause, and shrinkage. Statistical analysis shows that the growth rate of a given nanotube essentially varies between two values, with a similar average ratio (∼1.7) regardless of whether the rate change is accompanied by a change in chirality. These switches indicate that the nanotube edge or the catalyst nanoparticle fluctuates between different configurations during growth.

Anomalous Volume Changes in the Siliceous Zeolite Theta-1 TON due to Hydrogen Insertion under High-Pressure, High-Temperature Conditions.

High-pressure X-ray diffraction and Raman spectroscopy in a diamond anvil cell were used to study the insertion of the chemical hydrogen storage material, ammonia borane, in the one-dimensional pores of the zeolite theta-1 TON. Heating of this material up to 300 °C under pressures up to 5 GPa resulted in the release of a significant amount of hydrogen due to the conversion of ammonia borane confined in the channels of TON and outside the zeolite to polyaminoborane and then polyiminoborane chains. The filling of TON with hydrogen resulted in a much greater increase in unit cell volume than that corresponding to thermal expansion of normal compact inorganic solids. This process at high temperature is accompanied by a phase transition from the collapsed high-pressure Pbn21 form to the more symmetric Cmc21 phase with expanded pores. This material has a high capacity for hydrogen adsorption under high-temperature, high-pressure conditions.

Enhanced sieving from exfoliated MoS2 membranes via covalent functionalization.

Nanolaminate membranes made of two-dimensional materials such as graphene oxide are promising candidates for molecular sieving via size-limited diffusion in the two-dimensional capillaries, but high hydrophilicity makes these membranes unstable in water. Here, we report a nanolaminate membrane based on covalently functionalized molybdenum disulfide (MoS2) nanosheets. The functionalized MoS2 membranes demonstrate >90% and ~87% rejection for micropollutants and NaCl, respectively, when operating under reverse osmotic conditions. The sieving performance and water flux of the functionalized MoS2 membranes are attributed both to control of the capillary widths of the nanolaminates and to control of the surface chemistry of the nanosheets. We identify small hydrophobic functional groups, such as the methyl group, as the most promising for water purification. Methyl- functionalized nanosheets show high water permeation rates as confirmed by our molecular dynamic simulations, while maintaining high NaCl rejection. Control of the surface chemistry and the interlayer spacing therefore offers opportunities to tune the selectivity of the membranes while enhancing their stability.

Role of Sulfur Vacancies and Undercoordinated Mo Regions in MoS2 Nanosheets toward the Evolution of Hydrogen.

Low-dimensional materials have been examined as electrocatalysts for the hydrogen evolution reaction (HER). Among them, two-dimensional transition metal dichalcogenides (2D-TMDs) such as MoS2 have been identified as potential candidates. However, the performance of TMDs toward HER in both acidic and basic media remains inferior to that of noble metals such as Pt and its alloys. This calls for investigating the influence of controlled defect engineering of 2D TMDs on their performance toward hydrogen production. Here, we explored the HER activity from defective multilayered MoS2 over a large range of surface S vacancy concentrations up to 90%. Amorphous MoS2 and 2H MoS2 with ultrarich S vacancies demonstrated the highest HER performance in acid and basic electrolytes, respectively. We also report that the HER performance from multilayered MoS2 can be divided into two domains corresponding to "point defects" at low concentrations of surface S vacancies (Stage 1) and large regions of undercoordinated Mo atoms for high concentrations of surface S vacancies (Stage 2). The highest performance is obtained for Stage 2 in the presence of undercoordinated Mo atoms with a TOF of ∼2 s-1 at an overpotential of 160 mV in 0.1 M KOH which compares favorably to the best results in the literature. Overall, our work provides deeper insight on the HER mechanism from defected MoS2 and provides guidance for the development of defect-engineered TMD-based electrocatalysts.

Voltage-activated transport of ions through single-walled carbon nanotubes.

Ionic transport through single-walled carbon nanotubes (SWCNTs) is promising for many applications but remains both experimentally challenging and highly debated. Here we report ionic current measurements through microfluidic devices containing one or several SWCNTs of diameter of 1.2 to 2 nm unexpectedly showing a linear or a voltage-activated I-V dependence. Transition from an activated to a linear behavior, and stochastic fluctuations between different current levels were notably observed. For linear devices, the high conductance confirmed with different chloride salts indicates that the nanotube/water interface exhibits both a high surface charge density and flow slippage, in agreement with previous reports. In addition, the sublinear dependence of the conductance on the salt concentration points toward a charge-regulation mechanism. Theoretical modelling and computer simulations show that the voltage-activated behavior can be accounted for by the presence of local energy barriers along or at the ends of the nanotube. Raman spectroscopy reveals strain fluctuations along the tubes induced by the polymer matrix but displays insufficient doping or variations of doping to account for the apparent surface charge density and energy barriers revealed by ion transport measurements. Finally, experimental evidence points toward environment-sensitive chemical moieties at the nanotube mouths as being responsible for the energy barriers causing the activated transport of ions through SWCNTs within this diameter range.

Unveiling the Evolutions of Nanotube Diameter Distribution during the Growth of Single-Walled Carbon Nanotubes.

In situ and ex situ Raman measurements were used to study the dynamics of the populations of single-walled carbon nanotubes (SWCNTs) during their catalytic growth by chemical vapor deposition. Our study reveals that the nanotube diameter distribution strongly evolves during SWCNT growth but in dissimilar ways depending on the growth conditions. We notably show that high selectivity can be obtained using short or moderate growth times. High-resolution transmission electron microscopy observations support that Ostwald ripening is the key process driving these seemingly contradictory results by regulating the size distribution and lifetime of the active catalyst particles. Ostwald ripening appears as the main termination mechanism for the smallest diameter tubes, whereas carbon poisoning dominates for the largest ones. By unveiling the key concept of dynamic competition between nanotube growth and catalyst ripening, we show that time can be used as an active parameter to control the growth selectivity of carbon nanotubes and other 1D systems.

Elastic Constants, Optical Phonons, and Molecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3 Hybrid Perovskite.

Low frequency dynamics has been studied in a CH3NH3PbBr3 hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C44 shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation-rotation coupling associated with the cubic to tetragonal phase transition at Tc ≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to Tc. The critical dynamics at Tc ≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.

In situ Raman probing of graphene over a broad doping range upon rubidium vapor exposure.

We report in situ Raman scattering experiments on single-layer graphene (SLG) and Bernal bilayer graphene (BLG) during exposure to rubidium vapor. The G- and 2D-band evolutions with doping time are presented and analyzed. On SLG, the extended doping range scanned (up to about 10(14) electrons/cm(2)) allows the observation of three regimes in the evolution of the G-band frequency: a continuous upshift followed by a plateau and a downshift. Overall the measured evolution is interpreted as the signature of the competition between dynamic and adiabatic effects upon n-doping. Comparison of the obtained results with theoretical predictions indicates however that a substrate pinning effect occurs and inhibits charge-induced lattice expansion of SLG. At low doping, a direct link between electrostatic gating and Rb doping results is presented. For BLG, the added electrons are shown to be first confined in the top layer, but the system evolves with time toward a more symmetric repartition of the added electrons in both layers. The results obtained on BLG also confirm that the slope of the phonon dispersion close to the K point tends to be slightly reduced at low doping but suggest the occurrence of an unexpected increase of the phonon dispersion slope at higher electron concentration.

Supramolecular discrimination of carbon nanotubes according to their helicity.

Adsorption of specifically designed and geometrically constrained polyaromatic amphiphiles on single-walled carbon nanotubes (SWNTs) was found to be selective of the nanotube helicity angle. Starting from the same SWNT mixture, photoluminescence and resonant Raman spectroscopies show that a pentacenic-based amphiphile leads to the solubilization of armchair SWNTs and that a quaterrylene-based amphiphile leads to the solubilization of zigzag SWNTs. The results were predicted by the design of the two amphiphiles and are consistent with a supramolecular recognition of the nanotube graphene-type atomic structure by the aromatic part of the molecules through optimized pi-pi-stacking interactions.

Raman modes of index-identified freestanding single-walled carbon nanotubes.

Using electron diffraction on freestanding single-walled carbon nanotubes, we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3 nm. On the same freestanding tubes, we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7 nm), these measurements confirm previously established radial breathing mode frequency versus diameter relations and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.