Ming-Sheng Wang

Electron-Beam-Induced Substitutional Carbon Doping of Boron Nitride Nanosheets, Nanoribbons, and Nanotubes

Substitutional carbon doping of the honeycomb-like boron nitride (BN) lattices in two-dimensional (nanosheets) and one-dimensional (nanoribbons and nanotubes) nanostructures was achieved via in situ electron beam irradiation in an energy-filtering 300 kV high-resolution transmission electron microscope using a C atoms feedstock intentionally introduced into the microscope. The C substitutions for B and N atoms in the honeycomb lattices were demonstrated through electron energy loss spectroscopy, spatially resolved energy-filtered elemental mapping, and in situ electrical measurements. The preferential doping was found to occur at the sites more vulnerable to electron beam irradiation. This transformed BN nanostructures from electrical insulators to conductors. It was shown that B and N atoms in a BN nanotube could be nearly completely replaced with C atoms via electron-beam-induced doping. The doping mechanism was proposed to rely on the knockout ejections of B and N atoms and subsequent healing of vacancies with supplying C atoms.

Rapid and Direct Conversion of Graphite Crystals into High‐Yielding, Good‐Quality Graphene by Supercritical Fluid Exfoliation

Graphene has attracted a great deal of attention in recent years due to its unusual electronic, mechanical, and thermal properties. Exploiting graphene properties in a variety of applications requires a chemical approach for the large-scale production of high-quality, processable graphene sheets (GS), which has remained an unanswered challenge. Herein, we report a rapid one-pot supercritical fluid (SCF) exfoliation process for the production of high-quality, large-scale, and processable graphene for technological applications. Direct high-yield conversion of graphite crystals to GS is possible under SCF conditions because of the high diffusivity and solvating power of SCFs, such as ethanol, N-methyl-pyrrolidone (NMP), and DMF. For the first time, we report a one-pot direct conversion of graphite crystals to a high yield of graphene sheets in which about 90-95% of the exfoliated sheets are < 8 layers with approximately 6-10% monolayers and the remaining 5-10% are > or = 10 layers.

Tensile Tests on Individual Multi-Walled Boron Nitride Nanotubes

www.MaterialsViews.com C O M M Tensile Tests on Individual Multi-Walled Boron Nitride Nanotubes U N IC A By Xianlong Wei , * Ming-Sheng Wang , Yoshio Bando , and Dmitri Golberg * IO N Boron nitride nanotubes (BNNTs) are a structural equivalent of carbon nanotubes (CNTs) with alternating B and N atoms substituting for C atoms in a honeycomb lattice. To date, BNNTs have attracted a signifi cant research interest due to the following facts: (i) BN tubes are electrical insulators with a band gap of ∼ 5.5 eV independent of tube diameter and chirality, (ii) they have a comparable stiffness to CNTs and disputably possess strength higher than CNTs, (iii) they exhibit higher thermal and chemical stabilities compared to CNTs, (iv) BN tubes are piezoelectric and highly thermoconductive. [ 1 ] Among all these characteristics, outstanding mechanical properties paired with the electrical insulation are thought to be particularly important for the smart BNNT applications. BNNTs have been demonstrated to have comparable elastic modulus to CNTs, both theoretically and experimentally. Hernández et al. predicted a BNNT elastic modulus of ∼ 0.9 TPa, which is a bit smaller but comparable to that of CNTs ( ∼ 1.2 TPa). [ 2 ] Similar elastic modulus was also pointed out by several other groups. [ 3–5 ] The theoretical predictions agree well with the experimentally measured values. [ 6–8 ] In addition to rival elastic modulus of BN and C NTs, BNNTs were thought to have comparable or, likely, even higher yield strength than CNTs under a tensile load. Similar with CNTs, 5/7/7/5 dipoles (resulted from Stone-Wales transformations) were found to be the primary defect nuclei under tension in BNNTs. [ 9 , 10 ] At a high temperature, Bettinger et al. and Dumitricǎ et al. computed a higher yield strength of BNNTs compared to CNTs since the formation energy of primary defects in BNNTs was higher than in CNTs and remained positive at a larger strain. [ 9 , 10 ] However, due to lower activation energy of 5/7/7/5 defects in BNNTs, their yield strain under room temperature (and for a realistic strain rate) was calculated to be ∼ 15–19% dependent on chirality, which is ∼ 85% of the CNT yield strain. [ 11 ] Song et al. calculated the critical strain for Stone-Wales transformation to be 11.47% for (5, 5) arm-chair and 14.23% for (10, 0) zig-zag BNNTs. [ 12 ] Despite those theoretical estimates, to the best of the authors knowledge, the yield strength of BNNTs has never been measured in an experiment. In contrary, the yield strength of their C counterparts has reliably been determined during several tensile loading tests. [ 13–18 ] Furthermore, since B-N bonds are partially ionic, while C-C bonds are purely covalent, the extent of how this varying

Heterojunctions between metals and carbon nanotubes as ultimate nanocontacts

We report the controlled formation and characterization of heterojunctions between carbon nanotubes and different metal nanocrystals (Fe, Co, Ni, and FeCo). The heterojunctions are formed from metal-filled multiwall carbon nanotubes (MWNTs) via intense electron beam irradiation at temperatures in the range of 450–700 °C and observed in situ in a transmission electron microscope. Under irradiation, the segregation of metal and carbon atoms occurs, leading to the formation of heterojunctions between metal and graphite. Metallic conductivity of the metal–nanotube junctions was found by using in situ transport measurements in an electron microscope. Density functional calculations show that these structures are mechanically strong, the bonding at the interface is covalent, and the electronic states at and around the Fermi level are delocalized across the entire system. These properties are essential for the application of such heterojunctions as contacts in electronic devices and vital for the fabrication of robust nanotube–metal composite materials.

Mechanical Properties of Si Nanowires as Revealed by in Situ Transmission Electron Microscopy and Molecular Dynamics Simulations

Deformation and fracture mechanisms of ultrathin Si nanowires (NWs), with diameters of down to ~9 nm, under uniaxial tension and bending were investigated by using in situ transmission electron microscopy and molecular dynamics simulations. It was revealed that the mechanical behavior of Si NWs had been closely related to the wire diameter, loading conditions, and stress states. Under tension, Si NWs deformed elastically until abrupt brittle fracture. The tensile strength showed a clear size dependence, and the greatest strength was up to 11.3 GPa. In contrast, under bending, the Si NWs demonstrated considerable plasticity. Under a bending strain of <14%, they could repeatedly be bent without cracking along with a crystalline-to-amorphous phase transition. Under a larger strain of >20%, the cracks nucleated on the tensed side and propagated from the wire surface, whereas on the compressed side a plastic deformation took place because of dislocation activities and an amorphous transition.

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.

Post-synthesis carbon doping of individual multiwalled boron nitride nanotubes via electron-beam irradiation.

We report on post-synthesis carbon doping of individual boron nitride nanotubes (BNNTs) via in situ electron-beam irradiation inside an energy-filtering 300 keV high-resolution transmission electron microscope. The substitution of C for B and N atoms in the honeycomb lattice was demonstrated through electron energy loss spectroscopy, spatially resolved energy-filtered elemental mapping, and in situ electrical measurements. Substitutional C doping transformed BNNTs from electrical insulators to conductors. In comparison with the existing post-synthesis doping methods for nanoscale materials (e.g., ion implantation and diffusion), the discovered electron-beam-induced doping is a well-controlled, little-damaging, room-temperature, and simple strategy that is expected to demonstrate great promise for post-synthesis doping of diverse nanomaterials in the future.

Tensile tests on individual single-walled carbon nanotubes: linking nanotube strength with its defects.

www.MaterialsViews.com C O M M U Tensile Tests on Individual Single-Walled Carbon Nanotubes: Linking Nanotube Strength with Its Defects N IC A By Ming-Sheng Wang , * Dmitri Golberg , * and Yoshio Bando IO N Superb toughness of CNTs, seamless cylinders made of graphene sheets, has attracted prime attention of mechanical engineers. Theoretical calculations predict that an ideal single-walled carbon nanotube (SWNT) possesses an extremely high tensile strength of 75–135 GPa, depending on tube chirality. [ 1–3 ] However, most previous experimental results from several groups have typically been less sound. [ 4–7 ] Taking Yu et al.’s report as an example, tensile tests on 19 individual multi-walled nanotubes (MWNTs) pointed at the failure strengths of 11–63 GPa, with a mean value of only 28 GPa. [ 4 ] These theory-experiment discrepancies actually arise from the presence of structural defects in nanotubes introduced during their growth and/or post-growth treatments. Although the effects of such defects on tube fracture have only been computed via e.g. quantum mechanical or molecular mechanical calculations, [ 2 ] an experimental study clearly linking the nanotube defects to its fracture strength has not been available to date. Furthermore, the processes of defect nucleation, their propagation and relations to the tube failure at loading have also remained elusive and untested. We also noted that till now, only a few sets of CNT fracture measurements have been reported for any tube structure, either individual MWNTs or ropes of SWNTs. In addition, experimental data on such structures suffers from various uncertainties arising from e.g. interlayer (or intra-tube) load transfer, layer and/or tube sliding, and the estimation of an effective cross-sectional area. Also, the outermost layer of MWNTs usually has a much larger diameter than the model SWNTs used in most of theoretical calculations. This discrepancy brings other complications for the direct comparison between experimental results and theoretical simulations. The only direct and most reliable way is to take an individual SWNT and to conduct a direct strength measurement on it. Such experiment would be highly desirable for both experimentalists and theoreticians within the Nanotube community. [ 8 ] However, the extremely small dimensions, e.g. a diameter of a few nanometers, and a micrometer scale length impose a tremendous burden on an experimentalist. The diffi culties include visualizing, picking and placing of such tiny, fl exible individual SWNTs followed by nanoclamp fabrication within a force-sensor microdevice. Although some mechanical

Nanomaterial Engineering and Property Studies in a Transmission Electron Microscope

Modern methods of in situ transmission electron microscopy (TEM) allow one to not only manipulate with a nanoscale object at the nanometer-range precision but also to get deep insights into its physical and chemical statuses. Dedicated TEM holders combining the capabilities of a conventional high-resolution TEM instrument and atomic force -, and/or scanning tunneling microscopy probes become the powerful tools in nanomaterials analysis. This progress report highlights the past, present and future of these exciting methods based on the extensive authors endeavors over the last five years. The objects of interest are diverse. They include carbon, boron nitride and other inorganic one- and two-dimensional nanoscale materials, e.g., nanotubes, nanowires and nanosheets. The key point of all experiments discussed is that the mechanical and electrical transport data are acquired on an individual nanostructure level under ultimately high spatial, temporal and energy resolution achievable in TEM, and thus can directly be linked to morphological, structural and chemical peculiarities of a given nanomaterial.

Mechanical Properties of Bamboo-like Boron Nitride Nanotubes by In Situ TEM and MD Simulations: Strengthening Effect of Interlocked Joint Interfaces

Understanding the influence of interfacial structures on the nanoarchitecture mechanical properties is of particular importance for its mechanical applications. Due to a small size of constituting nanostructural units and a consequently high volume ratio of such interfacial regions, this question becomes crucial for the overall mechanical performance. Boron nitride bamboo-like nanotubes, called hereafter boron nitride nanobamboos (BNNBs), are composed of short BN nanotubular segments with specific interfaces at the bamboo-shaped joints. In this work, the mechanical properties of such structures are investigated by using direct in situ transmission electron microscopy tensile tests and molecular dynamics simulations. The mechanical properties and deformation behaviors are correlated with the interfacial structure under atomic resolution, and a geometry strengthening effect is clearly demonstrated. Due to the interlocked joint interfacial structures and compressive interfacial stresses, the deformation mechanism is switched from an interplanar sliding mode to an in-plane tensile elongation mode. As a result of such a specific geometry strengthening effect, the BNNBs show high tensile fracture strength and Youngs modulus up to 8.0 and 225 GPa, respectively.