Xianlong Wei

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.

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

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.

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.

Charge trapping at the MoS2-SiO2 interface and its effects on the characteristics of MoS2 metal-oxide-semiconductor field effect transistors

The field effect transistors (FETs) based on thin layer MoS2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxide-semiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO2-MoS2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS2 based metal-oxide-semiconductor FETs is adjusted from 4 V to −45 V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS2 sheet.

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.

New Insight in Understanding Oxygen Reduction and Evolution in Solid-State Lithium Oxygen Batteries Using an in Situ Environmental Scanning Electron Microscope

Via designing a facile microscale all-solid-state lithium-oxygen battery system constructed in an environmental scanning electron microscope, direct visualization of discharge and charge processes of the lithium-oxygen battery is achieved. Different morphologies of the discharge product are observed, including a sphere, conformal film, and red-blood-cell-like shape, with a particle size up to 1.5 μm; whereas upon charge, the decomposition initiates at their surface and continues along a certain direction, instead of from the contact point at the electrode. These new findings indicate that the electron and lithium ion conductivities of Li2O2 could support the growth and decomposition of the discharge product in our system. In addition, our results indicate that various morphologies of Li2O2 arise from the different current density and surface chemistry of CNT, and the growth and decomposition of the particle are related to the uneven distribution of the ionic and electronic conductivities of Li2O2.

Light coupling and modulation in coupled nanowire ring-Fabry-Pérot cavity.

CdS nanowire (NW) ring cavities were fabricated and studied for the first time. The rings with radii from 2.1 to 5.9 microm were fabricated by a nanoprobe system installed in a scanning electron microscope. Radius dependent whispering gallery modes (WGMs) were observed. A straight CdS NW with Fabry-Pérot (F-P) cavity structure was fabricated and placed by the side of a NW ring cavity to form a coupled ring-F-P cavity. When the NW ring was excited by a focused laser, a bright green light spot was observed at the output end of the straight NW, indicating that the latter had served as an effect waveguide to couple the light out from the ring cavity. The corresponding light spectrum showed that the WGMs had been modulated. We confirmed that the NW F-P cavity had served as a modulator. Such a coupled cavity has potential application in a nanophotonic system.

Electron Emission from Individual Graphene Nanoribbons Driven by Internal Electric Field

Electron emission from individual graphene nanoribbons (GNRs) driven by an internal electric field was studied for the first time inside a high resolution transmission electron microscope equipped with a state-of-art scanning tunneling microscope sample holder with independent twin probes. Electrons were driven out from individual GNRs under an internal driving voltage of less than 3 V with an emission current increasing exponentially with the driving voltage. The emission characteristics were analyzed by taking into account monatomic thickness of GNRs. While deviating from the two-dimensional Richardson equation for thermionic emission, they were well described by the recently proposed by us phonon-assisted electron emission model. Different from widely studied field electron emission from graphene edges, electrons were found to be emitted perpendicularly to the atomic graphene surfaces with an emission density as high as 12.7 A/cm(2). The internally driven electron emission is expected to be less sensitive to the microstructures of an emitter as compared to field emission. The low driving voltage, high emission density, and internal field driving character make the regarded electron emission highly promising for electron source applications.