Molecular dynamics study of the destruction of single-walled carbon nanotubes under tension

The deformation and destruction of single-walled open carbon nanotubes are investigated in the framework of the molecular dynamics model of charges localized at bonds, which takes into account the atomic (ionic) and electronic degrees of freedom. This approach allows one to study excited electronic states induced by electronic transitions and to investigate both the ionic and electronic subsystems simultaneously. The structural transformations of nanotubes with (10, 2) chirality under tension are investigated, and the tensile stress-strain diagrams of these nanotubes in the temperature range 100–1000 K are calculated. It is established that, at low and moderate temperatures, the destruction of the nanotubes has a similar nature: the interatomic covalent bonds are broken in the normal cross section of the nanotube. At high temperatures, the nanotube undergoes a scission along the stretching axis.     

Molecular dynamics studies on the thermal conductivity of single-walled carbon nanotubes

We have studied the thermal conductivity of single-walled carbon nanotubes (SWCNTs) using the NEMD method. The results indicate that the thermal conductivity values are not profoundly influenced by the specific simulation-technique used in the MD simulations. Some possible reasons, which could be responsible for the discrepancy on thermal conductivity values of SWCNTs in the literatures, are discussed.      

单壁碳纳米管在石墨基底上运动的分子动力学模拟

采用分子动力学模拟方法研究单壁碳纳米管在石墨基底上的运动.首先碳纳米管在基底弛豫至平衡状态，然后对其施加一固定外力，撤去外力后，碳纳米管在基底上逐渐减速至停止.为了研究管径、手性角对运动方式的影响，本文选择了C(10,10)，C(10,9)，C(10,8)，C(10,5)，C(10,0)，C(8,8)六种单壁碳纳米管进行模拟.结果表明，碳纳米管在石墨基底上的运动方式由手性角决定，与管径无关.手性角等于30°时，碳纳米管与石墨基底之间为公度结构，碳纳米管的运动出现周期性的滑动和翻滚现象；手性角大于28.3°小于30°时，碳纳米管一边向前滑动一边滚动；手性角小于26.3°时，碳纳米管在基底上滑动.碳纳米管的手性角决定了它与石墨基底接触界面的微观构型，从而决定了碳纳米管的运动方式. 关键词： 分子动力学模拟 碳纳米管 动能 结构公度性     

Molecular dynamics simulation of silver bromide nanostructures in single-walled carbon nanotubes

Nanostructures formed upon filling single-walled carbon nanotubes of different diameters (ranging from 11.5 to 17.6 ?) with silver bromide have been investigated using the molecular dynamics method. The results of molecular dynamics computer simulation have demonstrated that, in such tubes, AgBr nanotubes in the form of rolled-up two-dimensional crystalline networks (including structures both with a trigonal coordination and with a tetragonal coordination of ions) can be produced as well as fragments of the NaCltype structure, which is typical of bulk AgBr crystals. In the initial stage of their filling, the carbon nanotubes in the silver bromide melt are deformed, on average, to a greater extent than those in a similar system with AgI. After taking out from the melt, the degree of deformation of the nanotubes decreases and, in the majority of cases, AgBr nanotubular structures based on a hexagonal network are formed inside them.     

Molecular junctions by joining single-walled carbon nanotubes

Crossing single-walled carbon nanotubes can be joined by electron beam welding to form molecular junctions. Stable junctions of various geometries are created in situ in a transmission electron microscope. Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds. The observations are supported by molecular dynamics simulations which show that the creation of vacancies and interstitials induces the formation of junctions involving seven- or eight-membered carbon rings at the surface between the tubes.     

Intersubband exciton relaxation dynamics in single-walled carbon nanotubes

We study exciton (EX) dynamics in single-walled carbon nanotubes (SWNTs) included in polymethylmethacrylate by two-color pump-probe experiments with unprecedented temporal resolution. In the semiconducting SWNTs, we resolve the intersubband energy relaxation from the EX2 to the EX1 transition and find time constants of about 40 fs. The observation of a photoinduced absorption band strictly correlated to the photobleaching of the EX1 transition supports the excitonic model for primary excitations in SWNTs. We also detect in the time domain coherent oscillations due to the radial breathing modes at approximately 250 cm(-1).     

Visible and near-infrared excited-state dynamics of single-walled carbon nanotubes

Excited-state lifetimes of isolated single-walled semiconducting carbon nanotubes (SWNTs) have been measured for the first time; these excited states, observed over the 400- to 1800-nm spectral domain, possess lifetimes that range from several ps to more than 100 ps. Sub-ps to ps decay components are assigned to relaxation in SWNT bundles. Interrogation of the samples with different SWNT mean diameters further confirms the dependence of the excited-state lifetime on roll-up vector. The ratio of fast and slow decaying component contributions in the first van Hove band can be viewed as a measure of the bundle content. PACS 78.67.Ch; 78.47.+p; 61.46.+w; 73.22.-f     

Interband recombination dynamics in resonantly excited single-walled carbon nanotubes

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H+ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.     

Molecular dynamics simulation of SnF2 nanostructures in the internal channels of single-walled carbon nanotubes

A molecular dynamics simulation of solid tin(II) fluoride nanostructures formed in internal channels of single-walled carbon nanotubes (SWCNTs) has been performed using two types of model potentials—without and with inclusion of the polarization of ions. For the potential taking into account the polarization of ions, an ordered SnF₂@SWCNT structure is reproduced: in SWCNT(10, 10), it has the form of the SnF₂ internal nanotube. At the same time, the SnF₂@SWCNT(11,11) structure is substantially disordered (glass-like). It has been found that heating of the SnF₂@SWCNT model system produces a superionic state characterized by a high mobility of fluorine ions without migration of tin ions. The model potentials disregard the covalent character of Sn-F bonds and the specific interactions of a lone electron pair of the Sn²⁺ ion. This makes it impossible to completely reproduce the properties of SnF₂ at normal pressures. However, some characteristics of the SnF₂ high-pressure modification can be reproduced if the polarization of ions is taken into account.     

Initial growth of single-walled carbon nanotubes on supported iron clusters: a molecular dynamics study

Molecular dynamics simulations were used to study the initial growth of single-walled carbon nanotubes (SWNTs) on a supported iron cluster (Fe₅₀). Statistical analysis shows that the growth direction of SWNTs becomes more perpendicular to the substrate over time due to the weak interaction between carbon nanotube and the substrate. The diameter of the nanotube also increases with the simulation time and approaches the size of the supported iron cluster.     

碳纳米管和碳化硅纳米管热导率的分子动力学研究

采用Tersoff势测试和研究了反向非平衡分子动力学中的Müller-Plathe法和Jund法在一维纳米管热传导中的应用.在相同的模拟步数中,Müller-Plathe法可以得到很好的结果,热导率在交换频率大于50时对参数的选择并不敏感.然而,Jund法并不能得到良好的线性温度梯度,其热导率在一定程度上依赖于选择的热流大小.在此基础上,运用Müller-Plathe法进一步研究了碳纳米管和碳化硅纳米管的长度、直径和温度对热导率的影响.结果表明,无论是碳纳米管还是碳化硅纳米管,其长度、直径和温度对热导率的影响是一致的.只要长度增加,纳米管的热导率相应增大,但增长速率不断降低.直径对热导率的影响很大程度上还取决于温度,在高温时,直径对热导率几乎没有影响.除此之外,纳米管的热导率随着温度的增加总体上也是不断降低的,但峰值现象的出现还受纳米管长度的影响.     

单壁碳纳米管的场发射特性研究

利用场发射显微镜(FEM)和四极质谱计分别研究了经过热处理的单壁碳纳米管的场发射图像和热处理过程中样品脱附的残气质谱.当热处理温度达到1000℃左右时得到了单壁碳纳米管的场发射像,此像可能是顶端开口的单根(16,0)锯齿形单壁碳纳米管的具有原子可分辨的场发射图像.四极质谱分析结果表明,在此温度范围W针尖晶粒间隙中有O原子和C原子释放出来.它们对单壁碳纳米管顶端的修饰是我们能观察到这些碳纳米管场发射像的可能原因. 关键词： 单壁碳纳米管(SWCNTs) 场发射显微镜(FEM) 四极质谱     

Resonance frequency of chiral single-walled carbon nanotubes using Timoshenko beam theory

This Letter develops a model that analyzes the resonant frequency of the chiral single-walled carbon nanotubes (SWCNTs) subjected to a thermal vibration by using Timoshenko beam model, including the effect of rotary inertia and shear deformation. The analytical solution is derived and the frequency equation is obtained. The results based on the beam model show that the frequency increases with decreasing the nanotube aspect ratio of length to diameter. In addition, the frequency obtained by Timoshenko beam model is lower than that calculated by Euler beam model. As the nanotube aspect ratio of length to diameter decreased, the discrepancy is more obvious. Furthermore, as the effect of thermal vibration increases, the frequency for chiral SWCNTs decreases.     

Conductivity of single-walled carbon nanotubes

In a system of N interacting single-level quantum dots (QDs), we study the relaxation dynamics and the current–voltage characteristics determined by symmetry properties of the QD arrangement. Different numbers of dots, initial charge configurations, and various coupling regimes to reservoirs are considered. We reveal that effective charge trapping occurs for particular regimes of coupling to the reservoir when more than two dots form a ring structure with the C_N spatial symmetry. We reveal that the effective charge trapping caused by the C_N spatial symmetry of N coupled QDs depends on the number of dots and the way of coupling to the reservoirs. We demonstrate that the charge trapping effect is directly connected with the formation of dark states, which are not coupled to reservoirs due to the system spatial symmetry C_N. We also reveal the symmetry blockade of the tunneling current caused by the presence of dark states.     

Diameter-dependent relaxation dynamics of 1D excitons in single-walled carbon nanotubes

We have studied 1D exciton relaxation dynamics in semiconducting single-walled carbon nanotubes (SWNTs) by femtosecond pump–probe experiments. The time evolution of change in transmittance ΔT/T induced by photo-excitation varies depending on the tube diameter. The decay time decreases with a decrease in the tube diameter. Pressure measurements have been conducted to explore the relaxation mechanism. The deformation potential estimated from the pressure dependence of photoluminescence spectra increases with decreasing tube diameter. This means that the exciton–phonon interaction becomes stronger in the smaller diameter tubes. The diameter dependences of decay time and deformation potential suggest that the exciton–phonon interaction plays an important role in exciton nonradiative relaxation process in semiconducting SWNTs.     

Hydrogenation of single-walled carbon nanotubes

Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process.     

Nucleation of single-walled carbon nanotubes

The nucleation pathway for single-wall carbon nanotubes on a metal surface is demonstrated by a series of total energy calculations using density functional theory. Incorporation of pentagons at an early stage of nucleation is energetically favorable as they reduce the number of dangling bonds and facilitate curvature of the structure and bonding to the metal. In the presence of the metal surface, nucleation of a closed cap or a capped single-wall carbon nanotube is overwhelmingly favored compared to any structure with dangling bonds or to a fullerene.     

单壁碳纳米管储氢的第一原理分子动力学模拟

本文用第一性原理平面波赝势方法模拟研究了手性单壁碳纳米管与氢分子的相互作用,考察了碳纳米管直径对储氢性能的影响。对单壁碳纳米管储氢的模拟结果表明: (1)物理吸附时,H2可以吸附在空腔内,也可以吸附在管与管之间的空隙中,纳米管内部的氢吸附力均高于管外,而“完好无损”的H2分子不能够穿过管壁而进入管内。(2)化学吸附时,碳纳米管对氢的吸附首先出现在管的边缘附近,碳纳米管局部会发生形变,SWCNTs的张力会随C-H键的增加而增大,系统不稳定。（3）随着直径的增加,纳米管内、外的氢吸附力差异减小。