单壁氮化硼纳米管的结构、对称性和晶格动力学

本文分析了氮化硼纳米管的结构对称性,并对其晶格振动模的对称性进行了分类。计算了单壁氮化硼纳米管的声子色散关系,给出了扶手椅管和锯齿管拉曼和红外活性模的频率随管径的变化规律。     

Probing the out-of-plane distortion of single point defects in atomically thin hexagonal boron nitride at the picometer scale

Crystalline systems often lower their energy by atom displacements from regular high-symmetry lattice sites. We demonstrate that such symmetry lowering distortions can be visualized by ultrahigh resolution transmission electron microscopy even at single point defects. Experimental investigation of structural distortions at the monovacancy defects in suspended bilayers of hexagonal boron nitride (h-BN) accompanied by first-principles calculations reveals a characteristic charge-induced pm symmetry configuration of boron vacancies. This symmetry breaking is caused by interlayer bond reconstruction across the bilayer h-BN at the negatively charged boron vacancy defects and results in local membrane bending at the defect site. This study confirms that boron vacancies are dominantly present in the h-BN membrane.     

Isotope effect on the thermal conductivity of boron nitride nanotubes

We have measured the temperature-dependent thermal conductivity kappa(T) of individual multiwall boron nitride nanotubes using a microfabricated test fixture that allows direct transmission electron microscopy characterization of the tube being measured. kappa(T) is exceptionally sensitive to isotopic substitution, with a 50% enhancement in kappa(T) resulting for boron nitride nanotubes with 99.5% 11B. For isotopically pure boron nitride nanotubes, kappa rivals that of carbon nanotubes of similar diameter.     

Field emission and current-voltage properties of boron nitride nanotubes

We have measured electrical transport properties of boron nitride nanotubes using an in situ manipulation stage inside a transmission electron microscope. Stable currents were measured in a field emission geometry, but in contact the nanotubes are insulating at low bias. At high bias, the nanotubes show stable, reversible breakdown current.     

Fluid synthesis and structure of a new polymorphic modification of boron nitride

A new previously unknown phase of boron nitride with a hardness of 0.41–0.63 GPa has been pre-pared by the supercritical fluid synthesis. The presence of a new phase is confirmed by the X-ray spectra and IR absorption spectra, where new reflections and bands are distinguished. The fundamental reflection of the X-ray diffraction pattern is d = 0.286–0.291 nm, and the characteristic band in the infrared absorption spectrum is observed at 704 cm^?1. The X-ray diffraction pattern and the experimental and theoretical infrared absorption spectra show that a new synthesized boron nitride phase can be a cluster crystal (space group 211) with a simple cubic lattice. Cage clusters of a fullerene-like morphology B₂₄N₂₄ with point symmetry O are arranged in lattice sites.     

轴压和扭转复合载荷作用下氮化硼纳米管屈曲行为的分子动力学模拟

采用分子动力学方法模拟了氮化硼纳米管在轴压和扭转复合荷载作用下的屈曲和后屈曲行为.在各加载比例下,给出了初始线性变形阶段和后屈曲阶段原子间相互作用力的变化,确定了屈曲临界荷载关系.通过对屈曲模态的细致研究,从微观变形机理上分析了纳米管对不同外荷载力学响应的差异.研究结果表明,扶手型和锯齿型纳米管均呈现出非线性的屈曲临界荷载关系,复合加载下的屈曲行为具有强烈的尺寸依赖性.温度升高将导致屈曲临界荷载的下降,且温度的影响随加载比例的变化而变化.无论在简单加载或复合加载中,同尺寸的碳纳米管均比氮化硼纳米管具有更强地抵抗屈曲荷载的能力.     

Optical transitions in single-wall boron nitride nanotubes

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.     

Electromechanical buckling analysis of boron nitride nanotube using molecular dynamic simulation

In this paper, the effect of electric field on axial buckling of boron nitride nanotubes is investigated. For this purpose, molecular dynamics simulation and continuum mechanics are used for the first time simultaneously. In molecular dynamics simulation, the potential between boron nitride atoms is considered as Tersoff and Timoshenko beam theory is used in continuum mechanics. In this paper, buckling of zigzag and armchair boron nitride nanotubes are investigated. Here, the effects of the electric field and the length of the boron nitride nanotube on the critical load are investigated and it is shown that the effect of the electric field is different with respect to the arrangement of atoms in the boron nitride nanotubes. In fact, the electric field creates axial and torsional loads on the zigzag and armchair nanotube, respectively. Axial buckling of the zigzag nanotube is dependent on the electric field, whereas in the armchair nanotubes, the electric field changes have no effect on the axial buckling. To better understand the impact of the electric field on axial buckling, these results are compared with the continuum mechanics.     

Atomic structure of boron nitride nanotubes with an armchair-type structure studied by HREM

Hexagonal networks of boron nitride (BN) nanotubes were investigated by high-resolution electron microscopy (HREM) and image simulation. From HREM images, lattice planes of {002} and hexagonal rings of a BN nanotube were confirmed. Asymmetrical layer arrangements were found, and a structure model for double-walled BN nanotube with an armchair-type structure has been proposed.     

First-principles study of nanotubes within the tetragonal,hexagonal and dodecagonal cycle structures

A systematic study has been done on the structural and electronic properties of carbon, boron nitride and aluminum nitride nanotubes with structure consisting of periodically distributed tetragonal (T ≡A₂X₂), hexagonal (H ≡A₃X₃) and dodecagonal (D ≡A₆X₆) (AX=C₂, BN, AlN) cycles. The method has been performed using first-principles calculations based on density functional theory (DFT). The optimized lattice parameters, density of state (DOS) curves and band structure of THD-NTs are obtained for (3, 0) and (0, 2) types. Our calculation results indicate that carbon nanotubes of these types (THD-CNTs) behave as a metallic, but the boron nitride nanotubes (THD-BNNTs) (with a band gap of around 4 eV) as well as aluminum nitride nanotubes (THD-AlNNTs) (with a band gap of around 2.6 eV) behave as an semiconductor. The inequality in number of atoms in different directions is affected on structures and diameters of nanotubes and their walls curvature.     

Nanotube phonon waveguide

We find that the high thermal conductivity of carbon nanotubes remains intact under severe structural deformations while the corresponding electrical resistance and thermoelectric power show compromised responses. Similar robust thermal transport against bending is found for boron nitride nanotubes. Surprisingly, for both systems the phonon mean free path exceeds the characteristic length of structural ripples induced by bending and approaches the theoretical limit set by the radius of curvature. The robustness of heat conduction in nanotubes refines the ultimate limit that is far beyond the reach of ordinary materials.     

Influence of growth duration on size and morphology of boron nitride nanotubes grown via chemical vapor deposition technique

Boron nitride nanotubes are synthesized on Si substrate via a chemical vapor deposition technique with different growth durations. Field emission scanning electron microscopy micrographs show a clear influence of growth duration on size and morphology of the synthesized nanotubes. It reveals that the diameter of the tubes decreases and length increases with an increase in growth duration. Total diameter of the tube has been reduced up to 31% and length increased up to 30% with an increase of 20 min growth duration. As a result, morphology of nanotubes has also been changed from curve like to straight. Transmission electron microscope confirms the tubular structure of the synthesized nanotubes with an interlayer spacing of 0.34 nm that corresponds to d₍₀₀₂₎ plane of hexagonal boron nitride and its crystalline nature. Energy dispersive X-ray spectroscopy indicates the presence of magnesium particles in the synthesized samples that refers to its catalytic growth. X-ray photoelectron spectroscopy confirms the elemental compositions of the sample. Raman spectra reveal a peak shift of 5.48 cm⁻¹ towards higher region of wavelength that corresponds to E_2g mode of vibration in hexagonal boron nitride. This result also confirms the structural change in the synthesized boron nitride nanotubes with respect to the growth duration.     

Growth and characterization of boron nitride nanotubes having novel morphologies using mechanothermal process

Journal of Nanoparticle Research - We report an effective approach to synthesize boron nitride (BN) nanotubes having novel morphologies employing a mechanothermal process. In this process, a...     

A density functional theory study on parameters fitting of ultra long armchair （n, n） single walled boron nitride nanotubes

Armchair(n,n) single walled boron nitride nanotubes with n = 2-17 are studied by the density functional theory at the B3LYP/3-21G(d) level combined with the periodic boundary conditions for simulating the ultra long model.The results show that the structure parameters and the formation energies bear a strong relationship to n.The fitted analytical equations are developed with correlation coefficients larger than 0.999.The energy gaps of(2,2) and(3,3) tubes are indirect gaps,and the larger tubes(n = 4-17) have direct energy gaps.Results show that the armchair boron nitride nanotubes(n = 2-17) are insulators with wide energy gaps of between 5.93 eV and 6.23 eV.     

Structural-diffraction analysis of nanocrystalline materials

We have studied the structure and phases of boron nitride and zirconium dioxide (both have a wide spectrum of crystalline sizes) using x-ray analysis and electron diffraction microscopy. We show that, even when the crystallites are of order 100 nm, their diffraction pattern is similar to that of other nanocrystalline materials. This pattern is dictated by the high degree of dispersion and the lattice distortions of the crystallites' periphery. The distortions in these materials are caused by internal stresses. In boron nitride the stress relaxes by polygonization of the peripheral regions. This destroys the long-range translational order and leads to the formation of intercrystallite regions due to incoherent binding between crystals. In zirconium oxide short-range order is destroyed by variations in the lattice constants in the regions near the crystal faces.Tomsk' State Architectural—Structural Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika No. 1, pp. 107–113, January, 1994.     

Modeling of electronic density of states for single-wall carbon and boron nitride nanotubes

The electronic density of states is calculated for all possible geometric configurations of single-wall carbon and boron nitride nanotubes. The calculation is based on the numerical differentiation of the two-dimensional dispersion relations for graphite and hexagonal boron nitride. The differentiation is performed for all allowed values of the wave vector using the π-electron approximation. For the particular carbon nanotubes chosen as examples, a good agreement is demonstrated between the calculated values of energy spacing of the symmetric van Hove singularities in the density of states and the experimental data obtained from the resonance Raman scattering study.     

Electronic structure of boron nitride nanostructures doped with a carbon atom

We have investigated, using first-principles calculations, the role of a substitutional carbon atom on the electronic properties of boron nitride monolayers, nanotubes, and nanocones. It is shown that electron states in the energy-gap are independent of the curvature, being the same for the monolayer, for the cone and for the tube. It is also found, that the presence of carbon in the boron nitride compounds induces a spin polarization, with magnetic moment of 1.0 μ_B, which does not depend on the curvature.     

Effects of structural defects on the compressive buckling of boron nitride nanotubes

In this paper, we examined the buckling of perfect and defective armchair boron nitride nanotubes with three types of vacancy defects, i.e. B- and N- single vacancy defects and B–N- double vacancy defect, using molecular dynamics simulations. To this end, all systems were modeled with a Tersoff-type potential, which is able to accurately describe covalent bonding of BN systems. We applied external uniaxial compressive forces to the nanotubes in vacuum and derived the critical buckling loads and strains, at room temperature in an NVT-ensemble. Our results showed significant differences between the critical buckling strengths of pristine and defective nanotubes. The resistance to axial buckling decreased with the introduction of one vacancy defect, and the B–N- double vacancy was the most seriously damaged structure, followed by B-vacancy and N-vacancy defects. Furthermore, the B-vacancy was shown to have the most significant effect on the decrease of the critical buckling strain. This can be attributed to the excessive asymmetries and perturbations induced in the structure of the nanotube and the local deformations around the defective site around the B-vacancy, even before loading. Moreover, results show that reduction in the buckling strength of the nanotube due to the presence of more than one B-vacancy defect depends on their distribution. If the two or three defects are close to each other, they act as a single point of weakness and the critical buckling load is only slightly reduced (similar to the existence of only one vacancy defect). However, if the defects are at more distant points, the critical buckling load may experience a higher decrease. Results show that vacancy defects play a critical role in the compressive buckling performance of boron nitride nanotubes and special attention must be paid to the presence of structural defects when designing members against buckling, especially for micro- and nano-electro-mechanical systems. On the other hand, defect engineering is a great means for tailoring the buckling strength of boron nitride nanotubes, in cases where the nanotube is expected to absorb energy through compressive buckling deformation and is not designed against, but for buckling.     

Effect of doping on electronic properties of double-walled carbon and boron nitride hetero-nanotubes

The effect of boron nitride (BN) doping on electronic properties of armchair double-walled carbon and hetero-nanotubes is studied using ab initio molecular dynamics method. The armchair double-walled hetero-nanotubes are predicted to be semiconductor and their electronic structures depend strongly on the electronic properties of the single-walled carbon nanotube. It is found that electronic structures of BN-doped double-walled hetero-nanotubes are intermediate between those of double-walled boron nitride nanotubes and double-walled carbon and boron nitride hetero-nanotubes. Increasing the amount of doping leads to a stronger intertube interaction and also increases the energy gap.     

Electric polarization of heteropolar nanotubes as a geometric phase

The threefold symmetry of planar boron nitride (BN), the III-V analog to graphene, prohibits an electric polarization in its ground state, but this symmetry is broken when the sheet is wrapped to form a BN nanotube. We show that this leads to an electric polarization along the nanotube axis which is controlled by the quantum mechanical boundary conditions on its electronic states around the tube circumference. Thus the macroscopic dipole moment has an intrinsically nonlocal quantum mechanical origin from the wrapped dimension. We formulate this novel phenomenon using the Berry's phase approach and discuss its experimental consequences.