In situ observation of thermal relaxation of interstitial-vacancy pair defects in a graphite gap

Direct observation of individual defects during formation and annihilation in the interlayer gap of double-wall carbon nanotubes (DWNT) is demonstrated by high-resolution transmission electron microscopy. The interlayer defects that bridge two adjacent graphen layers in DWNT are stable for a macroscopic time at the temperature below 450 K. These defects are assigned to a cluster of one or two interstitial-vacancy pairs (I-V pairs) and often disappear just after their formation at higher temperatures due to an instantaneous recombination of the interstitial atom with vacancy. Systematic observations performed at the elevated temperatures find a threshold for the defect annihilation at 450-500 K, which, indeed, corresponds to the known temperature for the Wigner energy release.     

Metastable Frenkel pair defect in graphite: source of Wigner energy?

The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.     

基于碳纳米管分子封装的一维电子波函数扫描

利用第一性原理,设计并研究了一类基于单臂碳纳米管的分子封装的分子体系．计算表明,半环葫芦脲类化合物可有效封装碳纳米管,引入微弱的分子间相互作用,对碳纳米管的电子态能级结构分布 仅带来微弱影响．半环葫芦脲分子与碳纳米管在管径方向的一维电子态波函数充分耦合,进而有效改变了一些前沿分子轨道的波函数在管径两头的分布以及相应的电子布居浓度．基于电子输运的模拟,发现半环葫芦脲分子在碳纳米管一维方向滑动时的某个电压下的电导变化可准确反映电子态波函数在相应分子导电通道上的一维分布信息．     

The influence of incorporating organic molecules or inorganic nanoparticles on the optical and electrical properties of carbon nanotube films

Organic molecules and inorganic nanoparticles were incorporated into transparent and conductive single- or double-wall carbon nanotube (SWNT or DWNT) films, and their electrical and optical properties were measured. When organic tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) molecules were incorporated into the nanotube films, sheet resistance was reduced to ∼50% of those from the pristine SWNT and DWNT films. Larger improvements were observed with Au nanoparticle decoration or HNO₃/SOCl₂ dipping processes. The sheet resistances were measured to be at 75% of transmittance for HNO₃/SOCl₂-treated DWNT films and at 77% for Au-incorporated DWNT films, making their electrical conductivities 200%-300% better than those of the pristine DWNT films. It was observed that DWNTs have better electrical/optical performance than SWNTs. The relative influence of various dopants, F₄TCNQ, Au, and HNO₃/SOCl₂ as well as microwave irradiation on the optical and electrical properties was identified by using Raman and UV-vis-NIR spectra.     

Determination of optical isomers for left-handed or right-handed chiral double-wall carbon nanotubes

The handedness relationship between adjacent layers in nested double-wall carbon nanotubes (DWNTs) has been investigated for the first time. Our high-resolution electron microscopy analysis on a series of specimen tilts can successfully tell the handedness of each constituent nanotube in a DWNT, and therefore the chiral indices (n, m) including their optical isomers [(n, m) or (m, n)] of inner and outer nanotubes can be uniquely determined. It is shown that right-handed and left-handed nanotubes are equally distributed for both the inner and outer nanotubes in the examined specimens and a preferable handedness relationship between the adjacent layers in DWNT may exist.     

Carbon nanotube electron windmills: a novel design for nanomotors

We propose a new drive mechanism for carbon nanotube (CNT) motors, based upon the torque generated by a flux of electrons passing through a chiral nanotube. The structure of interest comprises a double-walled CNT formed from, for example, an achiral outer tube encompassing a chiral inner tube. Through a detailed analysis of electrons passing through such a "windmill," we find that the current, due to a potential difference applied to the outer CNT, generates sufficient torque to overcome the static and dynamic frictional forces that exist between the inner and outer walls, thereby causing the inner tube to rotate.     

Molecular dynamic investigation of mechanical properties of armchair and zigzag double-walled carbon nanotubes under various loading conditions

Using molecular dynamic simulation (MDS), effects of chirality and Van der Waals interaction on Young's modulus, elastic compressive modulus, bending, tensile, and compressive stiffness, and critical axial force of double-walled carbon nanotube (DWCNT) and its inner and outer tubes are considered. Achieving the highest safety factor, mechanical properties have been investigated under applied load on both inner and outer tubes simultaneously and on each one of them separately. Results indicate that as a compressive element, DWCNT is more beneficial than single-walled carbon nanotube (SWCNT) since it carries two times higher compression before buckling. Except critical axial pressure and tensile stiffness, in other parameters zigzag DWCNT shows higher amounts than armchair type. Outer tube has lower strength than inner tube; therefore, most reliable design of nanostructures can be attained if the mechanical properties of outer tube taken as the properties of DWCNT.     

γ射线辐照下多壁碳纳米管结构转变过程研究

采用60Coγ射线辐照纯净的多壁碳纳米管,用高分辨透射电镜和拉曼光谱,研究了多壁碳纳米管由石墨结构向无定形结构转变的演化过程.发现在γ射线辐照下,碳纳米管的外部石墨层逐渐失去最初的有序结构而向无定形结构转变.而且,随着γ射线辐照剂量的增加,无定形结构不断推进,而石墨层结构则不断减小,直至使整个碳纳米管变为一个中空的无定形纳米线结构.用原子位移理论和溅射机理对这种转变过程进行了分析.γ射线轰击碳纳米管击出碳原子,碳原子停留在晶格的间隙位置上产生间隙原子,在它原来的平衡位置则留下一个空位.当轰击粒子动能足够大时导致碰撞级联效应,无序结构增加.多数空位和间隙原子可能相互复合而彼此退火,但仍有少数原子作为间隙原子而造成晶格进一步缺陷.辐射也可以引起碳原子的溅射,溅射出来的碳原子沉积在碳纳米管的外壁上形成一层无定形碳结构.     

Quantum interference in nanotube electron waveguides

We have calculated the quantum conductance of single-walled carbon nanotube (SWNT) waveguide by using a tight binding-based Greens function approach. Our calculations show that the slow conductance oscillations as well as the fast conductance oscillations are manifestations of the intrinsic quantum interference properties of the conducting SWNTs, being independent of the defect and disorder of the SWNTs. And zigzag type tubes do not show the slow oscillations. The SWNT electron waveguide is also found to have distinctly different transport behavior depending on whether or not the length of the tube is commensurate with a (3N+1) rule, with N the number of basic carbon repeat units along the nanotube length.     

Nonlinear vibration and stability analysis of a double-walled carbon nanotube under electrostatic actuation

In this paper, forced vibrations of a double-walled clamped–clamped carbon nanotube (DWNT) are studied. Two Euler–Bernoulli beams are used to model the inner and outer layers of the DWNT. An electrostatic actuation, which is comprised of DC and AC voltages is applied between the nanotubes and the electrode. In the system model, the nonlinear form of the interlayer van der Waals (vdW) force, and also, the mid-plane stretching are considered. The obtained equations are solved through Galerkin and multiple scales methods for primary and secondary resonances. The frequency response of the system is obtained as a function of some of the system parameters. A stability analysis of the response is conducted and bifurcation points are determined. The results demonstrate that the DWNT shows different behavior by changing the value of DC voltage. It is also observed that both layers of the DWNT vibrate with the same frequency under the primary and secondary resonance conditions.     

Dependence of in—tube doping on the radius and helicity of single—wall carbon nanotubes

Using the Lennard－Jones interaction potential between the impurity atom and carbon atom, we have studied the dependence of in-tube impurity doping on the radius of a single-wall carbon nanotube (SWNT), as well as its helicity. The obtained results show that the radius of the most stably doped SWNT is different for different kinds of impurity atoms. This is useful for producing the required doped SWNT. In addition, it is found that the helicity of tube has a strong effect on the potential energy of the atoms doped in the SWNT.     

Novel single-walled carbon nanotubes periodically embedded with four- and eight-membered rings

Based on experimental results, we obtain five types of single-walled carbon nanotube (SWNT) clusters with different chirality indices and diameters using density functional theory (DFT). We then obtain the corresponding SWNTs by using periodic boundary conditions. Studies of the stability and electronic properties show that the stability of the novel SWNTs is independent of the chirality index and relates only to the tube diameter; larger diameters correspond to more stable SWNTs. The electronic properties all show metallic characteristics independent of the chirality indices and tube diameters, thereby promoting the application of metallic-type SWNTs.     

Molecular dynamics study of effects of radius and defect on oscillatory behaviors of C60-nanotube oscillators

Using the micro-canonical ensemble, we investigate the oscillatory behaviors of some selected C60-nanotube oscillators by the classical molecular dynamics (MD) simulations method. The second-generation empirical bond-order potential and the van der Waals potential are used to describe bonding and nonbonding atomic interactions, respectively. In the process of simulation, two factors of the radius and vacancy defect of single-walled carbon nanotubes (SWCNTs) are discussed to investigate their effects on the oscillatory behaviors of C60-nanotube oscillators. The simulation results show that the energy dissipation of the C60-nanotube oscillator is sensitive to the radius and vacancy defect, and that the effect of the vacancy defect on the oscillatory behaviors of oscillator depends obviously on the radius of the outer tube. It is found that a single vacancy defect placed on the outer tube of the C60-(17,0) nanotube oscillator can significantly reduce energy dissipation. For C60-(18,0), C60-(19,0) and C60-(11,11) nanotube oscillators, however, the results show that an oscillator containing a vacancy defect is less stable than the one without defect.     

170keV质子辐照对多壁碳纳米管薄膜微观结构与导电性能的影响

碳纳米管具有优异的导电性, 是未来电子元器件的理想候选材料, 应用前景广阔. 针对碳纳米管在空间电子元器件的应用需求, 本文研究了170 keV质子辐照对多壁碳纳米管薄膜微观结构与导电性能的影响. 采用扫描电子显微镜(SEM)、拉曼光谱仪(Raman)、X射线光电子能谱仪(XPS)及电子顺磁共振谱仪(EPR)对辐照前后碳纳米管试样的表面形貌和微观结构进行分析; 利用四探针测试仪对碳纳米管薄膜进行导电性能分析. SEM分析表明, 170 keV质子辐照条件下, 当辐照注量高于5×10¹⁵ p/cm² (protons/cm²)时, 碳纳米管薄膜表面变得粗糙疏松, 纳米管发生明显弯曲、收缩及相互缠结现象. 目前, 质子辐照纳米管发生的收缩现象被首次发现. 基于Raman和XPS分析表明, 170 keV质子辐照后碳纳米管的有序结构得到改善, 且随辐照注量增加, 碳纳米管的有序结构改善明显. 结构的改善主要是由于170 keV质子辐照碳纳米管所产生的位移效应导致缺陷重组. EPR分析表明, 随着辐照注量的增加, 碳纳米管薄膜内的非局域化电子减少. 利用四探针测试分析表明, 碳纳米管薄膜的导电性能变差, 这是由于170 keV质子辐照导致碳纳米管薄膜中的电子特性及形态发生改变. 本文研究结果有助于利用质子辐照对碳纳米管膜结构和性能进行调整, 从而制备出抗辐射的纳米电子器件.     

Codoping of Potassium and Bromine in Carbon Nanotubes： A Density Functional Theory Study

We investigate the co-doping of potassium and bromine in single-walled carbon nanotubes （SWCNTs） and doublewalled carbon nanotubes （DWCNTs） based on density functional theory. In the co-doped （6,0） SWCNTs, the 4s electron of potassium is transferred to nanotube and Br, leading to the n-type feature of SWCNTs. When potassium is intercalated into inner tube and bromine is put on outer tube, the positive and negative charges reside on the outer and inner tubes of the （7,0）@（16,0） DWCNT, respectively. It is expected that DWCNTs would be an ideal candidate for p-n junction and diode applications.     

Demetalization of single-walled carbon nanotube thin films with microwave irradiation

The microwave irradiation effects on purified HiPCO and CoMoCat single-walled carbon nanotube (SWNT) thin films are investigated. The surface conductivities of the SWNT films are extracted from the measured THz transmission coefficients to provide a direct indication of the metallic content in the films. The observed drastic conductivity decrease indicates a significant metallic content reduction after the microwave irradiation. Two different laser excitations are applied for Raman spectroscopy to reveal the response of different nanotube species. The Raman spectra of both HiPCO and CoMoCat thin films confirm the decrease of metallic carbon nanotubes. The observed microwave-induced effects may potentially lead to a convenient scheme for demetalization of single-walled carbon nanotube mixtures.     

Ion irradiation effects on conduction in single-wall carbon nanotube networks

We have measured how irradiation by Ar⁺ and N⁺ ions modifies electronic conduction in single-wall carbon nanotube (SWNT) networks, finding dramatically different effects for different thicknesses. For very thin transparent networks, ion irradiation increases localization of charge carriers and reduces the variable-range hopping conductivity, especially at low temperatures. However, for thick networks (SWNT paper) showing metallic conductivity, we find a relatively sharp peak in conductivity as a function of irradiation dose. Our investigation of this peak reveals the important role of thermal annealing extending beyond the range of the irradiating ions, and shows the dependence on the morphology of the samples. We propose a simple model that accounts for the temperature-dependent conductivity. PACS 73.63.Fg; 61.80.-x     

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.     

Interaction of fragmented double-stranded DNA with carbon nanotubes in aqueous solution

Aqueous suspensions of ultrasonically fragmented double-stranded (fds-) DNA and single-walled carbon nanotubes (SWNTs) have been investigated by UV- and IR-absorption, NIR-emission and Raman spectroscopy. According to gel-electrophoresis, the lengths of the polymer fragments were 100–500 base pairs. Analysis of IR and UV data indicates the presence of both double-stranded (ds) and single-stranded (ss)-regions in the fragments. SWNT complex with DNA was revealed by NIR-emission and Raman spectroscopy. It turned out that fds-DNA is less efficient in holding nanotubes in the aqueous solution than ss-DNA. From the UV-data, the character of the helix-coil transition is seen to be like that for fds-DNA off and on nanotube, however, DNA thermostability increased in this latter case. The effective charge density on the DNA sugar-phosphate backbone of the fds-DNA:SWNT hybrid was less than that of DNA alone. Spectroscopic data can be explained by a model in which the formation of hybrids starts due to the interaction between untwisted ss-regions of DNA and the nanotube: the strands wrap on the tube and thus create an ‘anchor’ for the whole polymer. The ds-part of the polymer is located close to the nanotube.     

MD investigation of the collective carbon atom behavior of a (17, 0) zigzag single wall carbon nanotube under axial tensile strain

The collective dynamic behavior of carbon atoms of a (17, 0) zigzag single wall carbon nanotube is investigated under tensile strains by molecular dynamics (MD) simulations. The “slip vector” parameter is used to study the collective motion of a group of atoms and the deformation behavior in three different directions (axial, radial, and tangential) of a (17, 0) carbon nanotube. The variations of radial slip vectors indicate almost all carbon atoms of the (17, 0) carbon nanotube will stay on the cylindrical surface before the yielding of the single wall carbon nanotube (SWNT). Furthermore, the tangential vectors show kinking deformation for the (17, 0) zigzag tube only rarely appears when the crack occurs. Non-symmetrical deformation around a carbon atom along the axial direction also can be found. The variations in the slip vector values of each atom display a symmetrical crack along the horizontal direction and normal to the tube axis. Chain-like structures with 3–4 atoms can be observed, with the number of chain-like structures decreasing before the breakage of the SWNT. The mechanical properties and dynamic behavior of a (17, 0) zigzag SWNT under tensile strain are also compared with that of a (10, 10) armchair tube in our previous study (Weng et al. 2009).