Field emission and strain engineering of electronic properties in boron nitride nanotubes

The electrical properties of boron nitride (BN) nanostructures, particularly BN nanotubes (NTs), have been studied less in comparison to the counterpart carbon nanotubes. The present work investigates the field emission (FE) behavior of BNNTs under multiple cycles of FE experiments and demonstrates a strain-engineering pathway to tune the electronic properties of BNNTs. The electrical probing of individual BNNTs were conducted inside a transmission electron microscope (TEM) using an in?situ electrical holder capable of applying a bias voltage of up to 110?V. Our results indicate that in the first cycle a single BNNT can exhibit the current density of ～1?mA?cm(-2) at 110?V and the turn-on voltage of 325?V?μm(-1). However, field emission properties reduced considerably in subsequent cycles. Real-time imaging revealed the structural degradation of individual BNNTs during FE experiments. The electromechanical measurements show that the conductivity of BNNTs can be tuned by means of mechanical straining. The resistance of individual BNNTs reduced from 2000 to 769?MΩ and the carrier concentration increased from 0.35?×?10(17) to 1.1?×?10(17)?cm(-3) by straining the samples up to 2.5%.     

Nanococoon seeds for BN nanotube growth

A modification in the conventional arc-discharge method for synthesis of nanotubes is presented. By injecting pure nitrogen gas directly into the plasma we have greatly increased the amount of boron nitride nanotubes produced. Isolated nanotubes and bundles were characterized by TEM. The vast majority of the nanotubes were double-walled with outer diameter around 3 nm. The predominance of double-walled BN nanotubes is seen as a direct result of the distribution of the number of graphitic BN layers for the nanococoons, second major product of the synthesis. Detailed HRTEM examination of the ends of BN nanotubes indicates continuity between the graphitic BN layers that coat boron nanoparticles, that is nanococoon, and the nanotube. At the other end of the nanotubes a flat angular cap was observed. HRTEM images of nanotube ends give support to a root-based growth mechanism.     

Synthesis of multiwall boron nitride nanotubes dependent on crystallographic structure of boron

Synthesis and growth of multiwall boron nitride nanotubes (BNNTs) under the B and ZrO₂ seed system in the milling–annealing process were investigated. BNNTs were synthesized by annealing a mechanically activated boron powder under nitrogen environment. We explored the aspects of the mechanical activation energy transferred to milled crystalline boron powder producing structural disorder and borothermal reaction of the ZrO₂ seed particles on the synthesis of BNNTs during annealing. Under these circumstances, the chemical reaction of amorphous boron coated on the seed nanoparticles with nitrogen synthesizing amorphous BN could be enhanced. It was found that amorphous BN was crystallized to the layer structure and then grown to multiwall BNNTs during annealing. Especially, bamboo-type multiwall BNNTs were mostly produced and grown to the tail-side of the nanotube not to the round head-side. Open gaps with ∼0.3 nm of the bamboo side walls of BNNTs were also observed. Based on these understandings, it might be possible to produce bamboo-type multiwall BNNTs by optimization of the structure and shape of boron coat on the seed nanoparticles.     

The influence of the stacking orientation of C and BN stripes in the structure, energetics, and electronic properties of BC2N nanotubes

Carbon and boron nitride nanotubes present significant differences in their electronics. However, they have isoelectronic bonds and very similar geometrical structures that allow BCN nanotubes to be synthesized. These BCN nanotubes present properties that can vary according to their relative number of B, C, and N atoms, and their atomic distribution on the nanotube surface. Here we employ first-principles density functional theory to study BCN nanotubes with BC(2)N stoichiometry. These nanotubes are composed of pure BN and C stripes which are stacked (i) in parallel, (ii) perpendicularly, and (iii) forming helicoidal patterns along the nanotube axes. We found that the different strain energies of the curved C and BN arcs in the nanotubes with parallelly aligned stripes can lead to geometries that deviate significantly from the usual circular shape. A sinusoidal shape was predicted for a BC(2)N nanotube with a helicoidal arrangement of the C and BN stripes due to differences in the C-B and C-N bonds parallel to the tube axis. It was shown that the phase segregation is energetically favoured. Such structural preference and the relative stability of the BC(2)N nanotubes can be explained in terms of the ratio between the total number of bonds and the number of C-B and C-N bonds in the nanotubes. Finally, we found that one type of BC(2)N nanotube with helicoidal C and BN stripes, although having a zigzag structure, exhibits a metallic character.     

Buckling of carbon nanotubes: a literature survey

This survey paper comprises 5 sections. In Section 1, the reader is introduced to the world of carbon nanotubes where their structural form and properties are highlighted. Section 2 presents the various buckling behaviors exhibited by carbon nanotubes that are discovered by carbon nanotube researchers. The main factors, such as dimensions, boundary conditions, temperature, strain rate and chirality, influencing the buckling behaviors are discussed in Section 3. Section 4 presents the continuum models, atomistic simulations and experimental techniques in studying the buckling phenomena of carbon nanotubes. A summary as well as recommendations for future research are given in Section 5. Finally a large body of papers, over 200, is given in the reference section. It is hoped that this survey paper will provide the foundation knowledge on carbon nanotube buckling and inspire researchers to advance the modeling, simulation and design of carbon nanotubes for practical applications.     

Electro-thermo-torsional buckling of an embedded armchair DWBNNT using nonlocal shear deformable shell model

Electro-thermo-torsional buckling response of a double-walled boron nitride nanotube (DWBNNT) has been investigated based on nonlocal elasticity and piezoelasticity theories. The effects of surrounding elastic medium such as the spring constant of the Winkler-type and the shear constant of the Pasternak-type are taken into account. The van der Waals (vdW) forces are considered between inner and outer layers of nanotube. According to the relationship between the piezoelectric coefficient of armchair boron nitride nanotubes (BNNTs) and stresses, the first order shear deformation theory (FSDT) is used. Energy method and Hamilton’s principle are employed to obtain coupled differential equations containing displacements, rotations and electric potential terms. The detailed parameter study is conducted to investigate the effects of nonlocal parameter, elastic foundation modulus, temperature change, piezoelectric and dielectric constants on the critical torsional buckling load. Results indicate that the critical buckling load decreases when piezoelectric effect is considered.     

Selective growth of boron nitride nanotubes by plasma-enhanced chemical vapor deposition at low substrate temperature

Hexagonal boron nitride nanotubes (BNNTs) were synthesized at a low substrate temperature of 800?°C on nickel (Ni) coated oxidized Si(111) wafers in a microwave plasma-enhanced chemical vapor deposition system (MPCVD) by decomposition and reaction of gas mixtures consisting of B(2)H(6)-NH(3)-H(2). The 1D BN nanostructures grew preferentially on Ni catalyst islands with a small thickness only. In situ mass spectroscopic analysis and optical emission spectroscopy were used to identify the gas reactions responsible for the BNNT formation. The morphology and structural properties of the deposits were analyzed by SEM, TEM, EDX, SAD and Raman spectroscopy. The growth mechanism of the BNNTs was identified.     

Convenient synthesis of Fe-filled boron nitride nanotubes by SHS method

Fe-filled boron nitride (BN) nanotubes with high purity and good yield were conveniently synthesized by a novel ball-milling and self-propagation high-temperature synthesis (SHS) method at a low temperature (700 °C). The as-prepared product was characterized by XRD, FTIR, SEM, TEM and HRTEM. The results of XRD, FTIR and HRTEM reflect that the product is a hexagonal BN nanotube filled with Fe. The results of SEM and TEM reveal that the Fe-filled BN nanotubes have a diameter of 20-150 nm with the wall-thickness of about 20 nm and the length of more than 5 μm. The possible growth mechanism was also discussed.     

Functionalization of BN nanotubes with dichlorocarbenes

The geometric and electronic structures of dichlorocarbene (CCl(2)) functionalized BN nanotubes (BNNTs) were studied using density functional theory within the generalized gradient approximation. We found that the CCl(2) addition favors slanted B-N bonds in zigzag tubes, and the CCl(2)-attached BNNTs prefer open rather than closed three-membered-ring (3MR) structures in all the zigzag (n,0) BNNTs studied, whereas closed 3MR structure occurs in the CCl(2)-attached BN graphene layer. The binding energies decrease with increase of the CCl(2) coverage, but the electronic properties of BNNTs do not change significantly, irrespective of the CCl(2) coverage.     

Underlying mechanics of active nanocomposites with tunable properties

This paper proposes the development of a new class of nanotube-based piezoelectric polymeric composites with controllable bond expansion and contraction in the interface of nanotube and matrix for use in next-generation structural vibration control systems. It is theoretically shown that through applying external electrical field, the quality of adhesion between nanotube and the matrix at nanoscale can be imparted to result in novel engineered composites at macroscale with tunable mechanical properties ranging from stiffer structure to better damper, the attributes that are essential for structural vibration control. Along this line, two classes of nanotubes; namely, carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs), are investigated. It is demonstrated that BNNTs possess better tunability compared to CNTs due to their outstanding molecular crystalline structure. More specifically, it is shown that mechanical properties of BNNT-based structures are more sensitive to the variation of separation distance in the interphase zone, and the stick-slip mechanism, which is responsible for damping change, can easily occur in BNNT-reinforced piezoelectric polymers.     

Bulk synthesis, growth mechanism and properties of highly pure ultrafine boron nitride nanotubes with diameters of sub-10 nm

As a structural analogue of the carbon nanotube (CNT), the boron nitride nanotube (BNNT) has become one of the most intriguing non-carbon nanostructures. However, up to now the pre-existing restrictions/limitations of BNNT syntheses have made the progress in their research rather modest. This work presents a new route toward the synthesis of highly pure ultrafine BNNTs based on a modified boron oxide (BO) CVD method. A new effective precursor--a mixture of Li?O and B--has been proposed for the growth of thin, few-layer BNNTs in bulk amounts. The Li?O utilized as the precursor plays the crucial role for the present nanotube growth. The prepared BNNTs have average external diameters of sub-10 nm and lengths of up to tens of μm. Electron energy loss spectrometry and Raman spectroscopy demonstrate the ultimate phase purity of the ultrafine BNNTs. Property studies indicate that the ultrafine nanotubes are perfect electrical insulators exhibiting superb resistance to oxidation and strong UV emission. Moreover, their reduced diameters lead to a dramatically decreased population of defects within the tube walls and result in the observation of near-band-edge (NBE) emission at room temperature.     

Deformation-driven electrical transport of individual boron nitride nanotubes

In contrast to standard metallic or semiconducting graphitic carbon nanotubes, for years their structural analogs, boron nitride nanotubes, in which alternating boron and nitrogen atoms substitute for carbon atoms in a graphitic network, have been considered to be truly electrically insulating due to a wide band gap of layered BN. Alternatively, here, we show that under in situ elastic bending deformation at room temperature inside a 300 kV high-resolution transmission electron microscope, a normally electrically insulating multiwalled BN nanotube may surprisingly transform to a semiconductor. The semiconducting parameters of bent multiwalled BN nanotubes squeezed between two approaching gold contacts inside the pole piece of the microscope have been retrieved based on the experimentally recorded I-V curves. In addition, the first experimental signs suggestive of piezoelectric behavior in deformed BN nanotubes have been observed.     

Boron Nitride Nanotubes

The current status of research on boron nitride nanotubes (BNNTs)—carbon nanotube structural analogues—is discussed. Latest achievements in BNNT synthesis, morphology, and atomic structure analysis as well as physical, chemical, and functional property evaluations are reviewed. Similarities and differences between structural parameters and properties of BNNTs in comparison with conventional carbon nanotubes are particularly highlighted. Recent breakthroughs in BNNT filling, doping and functionalization, morphology, and electronic structure engineering are examined. Finally, prospective BNNT applications for fabricating field‐effect transistors, gas accumulators, and reinforcing polymer films are presented.     

Isotope effect on band gap and radiative transitions properties of boron nitride nanotubes

We have carried out an isotope study on the band gap and radiative transition spectra of boron nitride nanotubes (BNNTs) using both experimental and theoretical approaches. The direct band gap of BNNTs was determined at 5.38 eV, independent of the nanotube size and isotope substitution, by cathodoluminescences (CL) spectra. At lower energies, several radiative transitions were observed, and an isotope effect was revealed. In particular, we confirmed that the rich CL spectra between 3.0 and 4.2 eV reflect a phonon-electron coupling mechanism, which is characterized by a radiative transition at 4.09 eV. The frequency red shift and peak broadening due to isotopic effect have been observed. Our Fourier transform infrared spectra and density functional theory calculations suggest that those radiative transitions in BNNTs could be generated by a replacement of some nitrogen atoms with oxygen.     

In situ SEM observation of column-like and foam-like CNT array nanoindentation

Quantitative nanoindentation of nominally 7.5 and 600 μm tall vertically aligned carbon nanotube (VACNT) arrays is observed in situ within an SEM chamber. The 7.5 μm array consists of highly aligned and weakly interacting CNTs and deflects similarly to classically defined cylindrical columns, with deformation geometry and critical buckling force well estimated using the Euler-Bernoulli theory. The 600 μm array has a highly entangled foam-like morphology and exhibits sequential buckle formation upon loading, with a buckle first forming near the array bottom at approximately 2% strain, followed by accumulating coordinated buckling at the top surface at strains exceeding 5%.     

Controlled surface modification of boron nitride nanotubes

Controlled surface modification of boron nitride nanotubes has been achieved by gentle plasma treatment. Firstly, it was shown that an amorphous surface layer found on the outside of the nanotubes can be removed without damaging the nanotube structure. Secondly, it was shown that an oxygen plasma creates nitrogen vacancies that then allow oxygen atoms to be successfully substituted onto the surface of BNNTs. The percentage of oxygen atoms can be controlled by changing the input plasma energy and by the Ar plasma pre-treatment time. Finally, it has been demonstrated that nitrogen functional groups can be introduced onto the surface of BNNTs using an N(2) + H(2) plasma. The N(2) + H(2) plasma also created nitrogen vacancies, some of which led to surface functionalization while some underwent oxygen healing.     

Wrinkle-free nanomechanical film: control and prevention of polymer film buckling

For the first time, we report on methods to control and prevent polymer films from buckling. Buckled morphologies were created by thermally cycling or mechanically compressing a poly(dimethylsiloxane) substrate coated with a polyelectrolyte multilayer film. By variation of the dimensions of the surface topography relative to the buckling wavelength (e.g., pattern size is less than, equal to, and greater than the buckling wavelength), the orientation and the local morphology of the buckled films were controlled. On the basis of the information obtained, we demonstrate how to alleviate the unavoidable buckling by incorporating nanoparticles into the film. In addition, we studied the effect of the silica layer that results from oxygen plasma treatment and the critical temperature for permanent film buckling.     

Tunable Piezoelectricity of Multifunctional Boron Nitride Nanotube/Poly(dimethylsiloxane) Stretchable Composites

Boron nitride nanotubes (BNNT) uniformly dispersed in stretchable materials, such as poly(dimethylsiloxane) (PDMS), could create the next generation of composites with augmented mechanical, thermal, and piezoelectric characteristics. This work reports tunable piezoelectricity of multifunctional BNNT/PDMS stretchable composites prepared via co-solvent blending with tetrahydrofuran (THF) to disperse BNNTs in PDMS while avoiding sonication or functionalization. The resultant stretchable BNNT/PDMS composites demonstrate augmented Young's modulus (200% increase at 9 wt% BNNT) and thermal conductivity (120% increase at 9 wt% BNNT) without losing stretchability. Furthermore, BNNT/PDMS composites demonstrate piezoelectric responses that are linearly proportional to BNNT wt%, achieving a piezoelectric constant (|d₃₃|) of 18 pmV⁻¹ at 9 wt% BNNT without poling, which is competitive with commercial piezoelectric polymers. Uniquely, BNNT/PDMS accommodates tensile strains up to 60% without plastic deformation by aligning BNNTs, which enhances the composites’ piezoelectric response approximately five times. Finally, the combined stretchable and piezoelectric nature of the composite was exploited to produce a vibration sensor sensitive to low-frequency (≈1 kHz) excitation. This is the first demonstration of multifunctional, stretchable BNNT/PDMS composites with enhanced mechanical strength and thermal conductivity and furthermore tunable piezoelectric response by varying BNNT wt% and applied strain, permitting applications in soft actuators and vibration sensors.     

Computational modelling of a non-viscous fluid flow in a multi-walled carbon nanotube modelled as a Timoshenko beam

In the design of nanotube-based fluidic devices, a critical issue is the effect of the induced vibrations in the nanotube arising from the fluid flow, since these vibrations can promote structural instabilities, such as buckling transitions. It is known that the induced resonant frequencies depend on the fluid flow velocity in a significant manner. We have studied, for the first time, the flow of a non-viscous fluid in stubby multi-walled carbon nanotubes, using the Timoshenko classical beam theory to model the nanotubes as a continuum structure. We have obtained the variations of the resonant frequencies with the fluid flow velocity under several experimentally interesting boundary conditions and aspect ratios of the nanotube. The main finding from our work is that, compared to an Euler-Bernoulli classical beam model of a nanotube, the Timoshenko beam predicts the loss of stability at lower fluid flow velocities.     

Activation of boron nitride nanotubes and their polymer composites for improving mechanical performance

Boron nitride nanotubes (BNNTs) are inappropriate for further chemical derivatization because of their chemical inertness. We demonstrate covalent activation of chemically inert BNNTs by isophorone diisocyanate (IPDI) to form isocyanate group (NCO)-terminated BNNT precursors with an 'NCO anchor' ready for further functionalization. As identified by Fourier transform infrared spectroscopy, a number of molecules or polymers with -COOH, -OH or -NH? groups are readily attached to the activated IPDI-BNNTs. The IPDI-BNNT-involving polymer composites have shown mechanical properties are considerably improved due to the good dispersibility of IPDI-BNNTs in the polymer matrix and the strong interfacial interactions between BNNTs and polymers. The methodology reported here provides a promising method to promote the chemical reactivity of BNNTs and covalently modify polymer nanocomposites with improved mechanical performance.