封装钠的硼氮纳米管在垂直构型下场发射的第一性原理研究

采用密度泛函形式化的自洽方法模拟了纯的硼氮纳米管和封装钠的硼氮纳米管在垂直于管轴的电场作用下的场发射. 发现硼氮纳米管封装钠之后的近自由电子态参与场发射并表现出很强的发射特性.通过分析场发射总能量分布曲线的特征,探索近自由电子态对于场发射的作用和场发射电流对于外电场的响应. 发现在垂直管轴的发射构型下,场发射具有很灵敏的电场响应度,这是与硼氮纳米管中近自由电子态的特殊分布取向有关.     

Torsional buckling behavior of boron-nitride nanotubes using molecular dynamics simulations

Based on molecular dynamics simulations, the mechanical properties and buckling behavior of boron-nitride nanotubes under the action of torsional load are investigated. According to the results, the torsional properties of a boron-nitride nanotube are higher than those of its carbon counterpart. The effect of geometrical parameters on these parameters is also investigated. It is observed that by increasing the radius of nanotubes of the same length, unlike the critical shear strain, the critical torque considerably increases. The effect of chirality is also found to be negligible in the cases of critical shear strain and buckling mode, unlike the critical torque.     

Excitons in optical spectra of boron-nitride (BN) nanotubes

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the on-site interaction U=2t with the hopping integral t=1.1 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes, which show metallic and semiconducting properties depending on the chiralities.     

Breakdown of Fourier's law in nanotube thermal conductors

We present experimental evidence that the room temperature thermal conductivity (kappa) of individual multiwalled carbon and boron-nitride nanotubes does not obey Fourier's empirical law of thermal conduction. Because of isotopic disorder, kappa's of carbon nanotubes and boron-nitride nanotubes show different length dependence behavior. Moreover, for these systems we find that Fourier's law is violated even when the phonon mean free path is much shorter than the sample length.     

Electron-electron interaction effects on the optical excitations of semiconducting single-walled carbon nanotubes

We report correlated-electron calculations of optically excited states in ten semiconducting single-walled carbon nanotubes with a wide range of diameters. Optical excitation occurs to excitons whose binding energies decrease with increasing nanotube diameter, and are smaller than the binding energy of an isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of the second optical exciton polarized along the nanotube axis to that of the lowest exciton is smaller than the value predicted within single-particle theory. The experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes.     

Phonon transport in isotope-disordered carbon and boron-nitride nanotubes: is localization observable?

We present an ab initio study which identifies dominant effects leading to thermal conductivity reductions in carbon and boron-nitride nanotubes with isotope disorder. Our analysis reveals that, contrary to previous speculations, localization effects cannot be observed in the thermal conductivity measurements. Observable reduction of the thermal conductivity is mostly due to diffusive scattering. Multiple scattering induced interference effects were found to be prominent for isotope concentrations > or approximately 10%; otherwise, the thermal conduction is mainly determined by independent scattering contributions of single isotopes. We give explicit predictions of the effect of isotope disorder on nanotube thermal conductivity that can be directly compared with experiments.     

单壁BN纳米管和碳纳米管物理吸附储氢性能的理论对比研究

采用巨正则蒙特卡罗方法(GCMC)研究了单壁氮化硼纳米管(SWBNNTs)和单壁碳纳米管(SWCNTs)的物理吸附储氢性能,主要对比研究了纳米管的管径、温度和手性对二者物理吸附储氢量的影响. 研究结果表明：在低温下,SWBNNTs的物理吸附储氢性能优于相应的SWCNTs;但是随着温度的升高,二者的物理吸附储氢性能差别越来越小,在常温下,SWBNNTs不具备有比SWCNTs更强的物理吸附储氢性能,而是和相同条件下的SWCNTs相差不大,只是在高压下的物理吸附储氢量稍稍大于SWCNTs,并给出了合理的理论解释 关键词： 巨正则蒙特卡罗方法(GCMC) 单壁氮化硼纳米管(SWBNNTs) 单壁碳纳米管(SWCNTs) 储氢     

Modeling on ion rejection using membranes comprising ultra-small radii carbon nanotubes

In this paper, we investigate the complete ion rejection using carbon nanotube membranes comprising ultra-small radii nanotubes. Three acceptance radii for a water molecule, a sodium ion and a chloride ion are determined assuming the continuous approximation. Given the acceptance radii, we may confine the scope of the nanotube radius so that only water molecules can pass through but the heavier sodium and chloride ions are repulsed from the nanotube ends. We assume that the collective motion of water molecules inside a sufficiently long nanotube is driven by atomic vibrations so that classical phonon theory might be used to deduce the average water transit time inside the nanotube for ion rejection. We predict that for carbon nanotube membranes comprising nanotubes of radii lying between 3.4 and 3.9 ?, only water molecules will pass through, and sodium and chloride ions will not, which together using phonon theory, we deduce that the smaller the nanotube radius, the lower the average water transit time and the higher the efficiency of the membrane for ion rejection purposes. The present theoretical approach has the merit of rapid computational times and indicates those nanotube radii where future experimental work might be focussed.     

Dragging of polarizable nanodroplets by distantly solvated ions

We show by molecular dynamics simulations that ions intercalated in carbon and boron-nitride nanotubes can be solvated at distance in polarizable nanodroplets adsorbed on their surfaces. When the ions are driven in the nanotubes by electric fields, the adsorbed droplets are dragged together with them. We illustrate this phenomenon by dragging assemblies of 20-10,000 water molecules by individual Na+ and Cl- ions. This ion-facilitated dragging could be applied in molecular delivery, separation, and desalination.     

First-principle study of interaction of molecular hydrogen with BC<Subscript>3</Subscript> composite single-walled nanotube

The physisorption of molecular hydrogen in BC₃ composite single-walled nanotube, investigated using density functional theory, was compared with single-walled carbon nanotube. Both external and internal adsorption sites of these two nanotubes have been studied with the hydrogen molecular axis oriented parallel to the nanotube wall. The calculated results show that: ([see full textsee full text]) the physisorption energies of a H₂ molecule are larger for BC₃(8,0) composite nanotube than for C(8,0) nanotube at all adsorption sites examined. ([see full textsee full text]) For these two nanotubes, the physisorption energies are larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes. The different behavior between these two nanotubes is explained by the contour plots of electron density and charge-density difference of them. The present computations suggest that BC₃ nanotube may be a better candidate for hydrogen storage than carbon nanotube.     

Quantum-chemical calculations of the piezoelectric characteristics of boron nitride and carbon nanotubes

The piezoelectric characteristics of boron-nitride and carbon nanotubes were calculated. The electronic structure of nanotubes was studied using the quantum-chemical semiempirical MNDO method within a molecular-cluster model. The piezoelectric constants e _zzz, e _xzz, and e _xxx of boron nitride nanotubes of two structural modifications (n, n) (n = 5, 6, ..., 9) and (n, 0) (n = 6, 7, ..., 12) were calculated. The values of the piezoelectric constants e _zzz of tubular boron nitride are found to be close in order of magnitude to the respective values obtained using nonempirical calculation methods. The piezoelectric constant e _xzz of a (6, 6) carbon nanotube doped with substitutional point defects was calculated.     

Modeling of material properties after ultrashort laser and XUV excitation

By means of first principles calculations, we studied the possibility of manipulating structural properties of different materials via excitation with intense femtosecond laser or extreme ultraviolet (XUV) pulses. For silicon and boron-nitride nanotubes, we performed ab initio molecular dynamics simulations using the code CHIVES, developed in our group, to describe their laser-induced structural dynamics. For both materials, we determined the damage thresholds. We also investigated the structural response of magnesium and copper to ultrashort XUV excitation. For this purpose, we performed frozen-phonon calculations based on all-electron density functional theory and allowed the possibility of core-hole excitation. We found that Cu undergoes bond hardening and Mg bond softening upon creation of core holes and hot electrons, where we defined the bond strength by the vibrational frequencies.     

Effect of uniaxial tensile strength on the electrical properties of doped carbon nanotubes: Density functional theory

We investigated the effect of uniaxial tensile strength on a pristine carbon nanotube, boron-doped carbon nanotube, nitrogen-doped carbon nanotube and co-doped carbon nanotube with boron and nitrogen atoms. To achieve our goal, we performed our calculations with the aid of density functional theory. We studied the changes in the electrical properties after the atomic substitution of a carbon atom by boron, nitrogen, and boron and nitrogen in pristine carbon nanotubes. We also applied uniaxial tensile strength to doped structures as well as pristine one. In addition to studying the band gap, we studied the changes in the Fermi energy, valence bands, and conduction bands. We found that defects as well as stress and strain play a crucial rule on modifying the electrical properties of carbon nanotubes.     

碳纳米管复合吸波涂层微波吸收性能的模拟计算

如何利用碳纳米管复合吸波涂层的参数进行吸波性能优化是电磁屏蔽研究的热点之一.涂层参数对吸波性能影响的研究主要停留在实验探索阶段,而碳纳米管的结构参数对吸波性能影响的研究鲜有报道.因此,从微观结构层次研究涂层参数对吸波性能的影响有重要意义.基于多壁碳纳米管的等效电路,利用碳纳米管结构参数与等效电路各元件参数的关系,研究了碳纳米管损耗微波的机理,建立了碳纳米管结构参数与微波反射率的关系式.根据此关系式,利用Matlab软件模拟计算了碳纳米管管长、管径、涂层中碳纳米管的含量以及涂层厚度对微波反射率的影响.模拟计算结果表明:涂层的微波反射率随碳纳米管含量变化的模拟曲线与实验结果符合;碳纳米管含量和厚度是影响吸收峰位置和吸收强度的重要参量,而碳纳米管直径和长度是主要影响吸收峰强度的参量.     

Molecular dynamics study of the torsional vibration characteristics of boron-nitride nanotubes

In recent years, synthesizing inorganic nanostructures such as boron nitride nanotubes (BNNTs) has led to extensive studies on their exceptional properties. In this study, the torsional vibration behavior of boron-nitride nanotubes (BNNTs) is explored on the basis of molecular dynamics (MD) simulation. The results show that the torsional frequency is sensitive to geometrical parameters such as length and boundary conditions. The axial vibration is found to be induced by torsional vibration of nanotubes which can cause instability in the nanostructure. It is also observed that the torsional frequency of BNNTs is higher than that of their carbon counterpart. Moreover, the shear modulus is predicted by incorporating MD simulation numerical results into torsional vibration frequency obtained through continuum-based model of tubes. Finally, it is seen that the torsional frequency of double-walled boron-nitride nanotubes (DWBNNTs) is between the frequencies of their constituent inner and outer tubes.     

Raman spectrum of single-walled boron nitride nanotube

Using the spectral moments method, the calculations of the Raman spectra of single-walled boron nitride nanotubes (SW-BNNTs) were performed in the framework of the force constants model. Spectra were computed for chiral and achiral nanotubes for different diameters and lengths. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with group theory analysis was found. We show that the modes in the low frequency region are very sensitive to the nanotube diameter variation, whereas the ones associated to the tangential region are chirality dependent. The number of Raman active modes, their frequencies, and intensities depend on the length of the nanotube.     

Boron-nitride and boron-carbonitride nanotubes: synthesis,characterization and theory

We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.     

A Theoretical Study of a Single-Walled ZnO Nanotube as a Sensor for H_2 O Molecules

We have studied the property of single-walled ZnO nanotubes with adsorbed water molecules, and theoretically designed a new sensor for detecting water molecules using single-walled ZnO nanotubes using a combination of density functional theory and the non-equilibrium Green's function method. Details of the geometric structures and adsorption energies of the H 2 O molecules on the ZnO nanotube surface have been investigated. Our computational results demonstrate that the formation of hydrogen bonding between the H 2 O molecules and the ZnO nanotube, and adsorption energies of the H 2 O molecules on the ZnO nanotube are larger than the adsorption energies of other gas molecules present in the atmospheric environment. Moreover, the current-voltage curves of the ZnO nanotube with and without H 2 O molecules adsorbed on its surface are calculated, the results of which showed that the H 2 O molecules form stable adsorption configurations that could lead to the decrease in current. These results suggest that the single-walled ZnO nanotubes are able to detect and monitor the presence of H 2 O molecules by applying bias voltages.     

Interaction of alkanethiols with single-walled carbon nanotubes: First-principles calculations

We report the results of our first-principles study based on density functional theory on the interaction of alkanethiols with both defected and defect-free single-walled carbon nanotube (SWCNT). The adsorption energies are calculated for various configurations such as alkanethiol molecule approaching to defect sites heptagon, hexagon, and pentagon in defective tube, and another case where the alkanethiol approaching to hexagon in defect-free nanotube. The calculated results showed that alkanethiols are rather strongly bound to the outer surface of both the defected and defect-free carbon nanotubes with the binding energy of about −50.58 kcal/mol, consistent with the experimental result. We also find that alkanethiols prefer to be adsorbed on the hexagon ring site of defect-free nanotube. Furthermore, the effect of alkanethiols chain length on the adsorption of alkanethiols on carbon nanotubes has been investigated, and the obtained results reveal that the longer alkanethiols bind rather more strongly to the nanotube surface.     

Reshaping elastic nanotubes via self-assembly of surface-adhesive nanoparticles

Elastic sheets with macroscopic dimensions are easy to deform by bending and stretching. Yet shaping nanometric sheets by mechanical manipulation is hard. Here we show that nanoparticle self-assembly could be used to this end. We demonstrate that spherical nanoparticles adhering to the outer surface of an elastic nanotube can self-assemble into linear structures: rings or helices on stretchable nanotubes, and axial strings on nanotubes with high rigidity to stretching. These self-assembled structures are inextricably linked to a variety of deformed nanotube profiles, which can be controlled by tuning the concentration of nanoparticles, the nanoparticle-nanotube diameter ratio and the elastic properties of the nanotube. Our results open the possibility of designing nanoparticle-laden tubular nanostructures with tailored shapes, for potential applications in materials science and nanomedicine.