First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

Gas adsorption on a single walled carbon nanotube-model simulation

We simulate the conduction variation of a gas-adsorbed carbon nanotube by a hybridization model, which has been previously used to simulate the gas adsorption on a nanographite ribbon. Two energy parameters, hybridization interaction and orbital energy level, are employed to simulate and distinguish the adsorbed gases. Two mechanisms, carrier localization and charge distribution, coexist in the gas adsorption process and provide a qualitative explanation for the current increase or decrease in gas adsorption experiments for the carbon nanotube.     

Orbital hybridization and charge transfer in carbon nanopeapods

We investigate the electronic structure of the fullerenes encapsulated inside carbon nanotubes, the so-called nanopeapods, using the first-principles study. The orbital hybridization of LUMO+1 (the state above the lowest unoccupied molecular orbital) of C60, rather than LUMO as previously proposed, with the nanotube states explains the peak at approximately 1 eV in recent scanning-tunneling-spectroscopy (STS) data. For the endohedral metallofullerenes nested in the strained nanotube, the charge transfer shifts the relative energy levels of the different states and produces a spatial modulation of the energy gap in agreement with another STS experiment.     

Flow-driven voltage generation in carbon nanotubes

The flow of various liquids and gases over single-walled carbon nanotube bundles induces an electrical signal (voltage/current) in the sample along the direction of the flow. The electrical response generated by the flow of liquids is found to be logarithmic in the flow speed over a wide range. In contrast, voltage generated by the flow of gas is quadratically dependent on the gas flow velocity. It was found that the underlying physics for the generation of electrical signals by liquids and gases are different. For the liquid, the Coulombic interaction between the ions in the liquid and the charge carriers in the nanotube plays a key role while electrical signal generation due to gas flow is due to an interplay of Bernoulli’s principle and Seebeck effect. Unlike the liquid case which is specific to the nanotubes, the gas flow effect can be seen for a variety of solids ranging from single and multi-walled carbon nanotubes, graphite and doped semiconductors.     

Nanotube actuators for nanomechanics

Carbon nanotubes are unique nanostructures with interesting properties that suit them to a range of diverse applications including nanoscale electronics, use in composites, as gas storage media and scanning probe tips. An exciting property of carbon nanotubes is their ability to efficiently convert electrical energy into mechanical energy (actuation). Nanotube actuation is caused by the geometrical expansion of the carbon–carbon covalent bond caused by charge transfer into the nanotube [Abstract American Chemical Society 22 (1999); Abstract American Chemical Society 20 (2000)]. This ability to actuate, in addition to their high strength (∼1 TPa), makes macro-scale sheets of nanotubes termed `bucky paper', ideal for artificial muscles [Science 284 (1999) 1340]. Carbon nanotube actuators based on bucky paper have been shown to generate an order of magnitude higher stresses than those observed for natural muscle. These promising results suggest that carbon nanotube actuators based on a single (or a few hundred) nanotubes will also lead to enhanced applications on the micro- or nano-scale in the biomedical or electronic fields. This paper provides an overview of carbon nanotube actuators, their exceptional properties, current research ideas and possibilities for future applications.     

碳纳米管阵列水渗透性质的研究

碳纳米管阵列组成的碳纳米管分子膜在生物学分子器件等方面有重要应用. 本文利用分子动力学方法计算研究水分子对(11, 11)碳纳米管阵列的渗透过程. 结果发现, 只有当阵列间隙面积大于57.91 Ų时, 水分子才能进入阵列间隙中, 并揭示了碳管内部、阵列间隙内水分子结构随相邻碳管间距变化的演化趋势以及管内外水分子电偶极矩的分布特性.     

Preparation and Characterization of Linear Low Density Polyethylene/Carbon Nanotube Nanocomposites

Linear low‐density polyethylene (LLDPE)/multiwalled carbon nanotube (MWNT) nanocomposites were prepared via melt blending. The morphology and degree of dispersion of nanotubes in the polyethylene matrix were investigated using scanning electron microscopy (SEM). Both individual and agglomerates of MWNTs were evident. The rheological behavior and mechanical and electrical properties of the nanocomposites were studied using a capillary rheometer, tensile tester, and Tera ohm‐meter, respectively. Both polyethylene and its nanocomposites showed non‐Newtonian behavior in almost the whole range of shear rate. Addition of carbon nanotubes increased shear stress and shear viscosity. It was also found that the materials experience a fluid‐solid transition below 1 wt% MWNT. Flow activation energy for the nanocomposites was calculated using an Arrhenius type equation. With increasing nanotube content, the activation energy of flow increases. A decrease of about 7 orders of magnitude was obtained in surface and volume resistivity upon addition of 5 wt% MWNT. In addition, a difference between electrical and rheological percolation thresholds was observed. The results confirm the expected nucleant effect of nanotubes on the crystallization process of polyethylene. A slight increase in Young's modulus was also observed with increasing MWNT content.     

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.     

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.     

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.     

界面羟基对碳纳米管摩擦行为和能量耗散的影响

本文采用分子动力学模拟研究了羟基对碳纳米管摩擦和能量耗散方式的影响.研究结果表明:由于界面间氢键的形成,碳纳米管所受的平均摩擦力明显增大;随着羟基比例的改变,界面间氢键的数量与摩擦力的变化趋势一致;碳纳米管的手性角对摩擦力有一定的影响,扶手椅型碳纳米管所受的摩擦力比其他类型的碳纳米管的大;直径对摩擦力的影响较大,直径越大界面间的摩擦力越大,其原因是大直径的碳纳米管底部变平导致界面接触面积增大;界面接枝羟基后,体系的声子态密度中出现羟基的振动峰;随羟基比例的增加,羟基的振动在能量耗散中起到更为重要的作用,当碳纳米管和硅基底的羟基比例为10%/20%时,体系能量耗散的主要途径由碳纳米管和硅基底的振动转变为羟基的振动.     

Transport properties of orbitally hybridized organic semiconductors

A microscopic theory based on the orbital hybridization model via single orbital approximation is developed to calculate the current variation in organic semiconductors that are coupled to the external orbits from the environment. The charge transfer resulted from the orbital hybridization between the environment and the organic semiconductor rebuilds the energy levels and eventually alters the transport properties of the organic semiconductor. Two parameters in our theory, the orbital energy level of the environment relative to the energy level of organic semiconductor and the orbital hybridization interaction, dominate the current variation in the organic semiconductors. Our results show that the suppression of atomic dimerization due to orbital hybridization gives rise to an increase of electrical conduction in organic semiconductor. Also, after coupling with the environment, the charge-donating organic semiconductors are more conductive than the charge-accepting ones.     

Electronic properties of oxidized carbon nanotubes

The effect of oxygenation on the electronic properties of semiconducting carbon nanotubes is studied from first principles. The O2 is found to bind to a single-walled nanotube with an adsorption energy of about 0.25 eV and to dope semiconducting nanotubes with hole carriers. Weak hybridization between carbon and oxygen is predicted for the valence-band edge states. The calculated density of states shows that weak coupling leads to conducting states near the band gap. The oxygen-induced gap closing for large-diameter semiconducting tubes is discussed as well. The influence of oxygen on the magnetic property is also addressed through a spin-polarized calculation and compared to experiment.     

A study on interaction of DNA molecules and carbon nanotubes for an effective ejection of the molecules

The ejection of DNA molecules from carbon nanotubes is reported from interaction energy perspectives by molecular dynamics simulations. The critical ejection energy, which is to be applied to a DNA molecule for a successful ejection from a carbon nanotube, is investigated based on a study on the friction and binding energy between the DNA molecule and the tube. An effective ejection is realized by subjecting a kinetic energy on the DNA molecule that is larger than the solved critical ejection energy. In addition, the relationship between ejection energies and sizes of DNA molecules and carbon nanotubes is investigated.     

Josephson current in carbon nanotubes with spin-orbit interaction

We demonstrate that curvature-induced spin-orbit coupling induces a 0-π transition in the Josephson current through a carbon nanotube quantum dot coupled to superconducting leads. In the noninteracting regime, the transition can be tuned by applying a parallel magnetic field near the critical field where orbital states become degenerate. Moreover, the interplay between charging and spin-orbit effects in the Coulomb blockade and cotunneling regimes leads to a rich phase diagram with well-defined (analytical) boundaries in parameter space. Finally, the 0 phase always prevails in the Kondo regime. Our calculations are relevant in view of recent experimental advances in transport through ultraclean carbon nanotubes.     

单壁碳纳米管:纳米发电机和纳米马达

简要回顾了单壁碳纳米管的发现及研究现状,介绍了一种新颖的悬空单壁碳纳米管的制备方法;在此基础上,通过新的一种四电极方法,用实验证明水分子可以进入两端开口的单壁碳纳米管内,由于水分子偶极子与碳纳米管中载流子的相互产生相互耦合作用,载流子的定向运动(电流)可以使水产生定向运动(纳米马达);同时,水的运动又会使碳纳米管中的载流子产生定向运动而产生一个电动势(纳米发电机).     

Stability and electronic properties of carbon nanotubes doped with transition metal impurities

We apply first-principles method to investigate the effect of the diameter on the stability and electronic properties of zigzag carbon nanotubes doped with iron, nickel and manganese impurity atoms. In this contribution we follow the evolution of the electronic and structural properties as a function of the nanotube diameter. As a general result, we found that the binding energy decreases with the increasing nanotube radius. Additionally, depending on the interaction of transition metal impurity with the tubular carbon structure, it is observed that the total magnetization varies with the tube diameter due to hybridization and confinement effects. It is also shown that such magnetization varies with the curvature radius, increasing for manganese impurity atoms and decreasing for iron and nickel.     

Field-modulated carrier transport in carbon nanotube transistors

We have investigated the electrical transport properties of carbon nanotube field-effect transistors as a function of channel length, gate dielectric film thickness, and dielectric material. Our experiments show that the bulk properties of the semiconducting carbon nanotubes do not limit the current flow through the metal/nanotube/metal system. Instead, our results can be understood in the framework of gate and source-drain field induced modulation of the nanotube band structure at the source contact. The existence of one-dimensional Schottky barriers at the metal/nanotube interface determines the device performance and results in an unexpected scaling behavior.     

Control of the motion of nanoelectromechanical systems based on carbon nanotubes by electric fields

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube. The electric dipole moments of carbon nanotubes with chemically modified ends are calculated by the molecular orbital method. These nanotubes can be set in motion under the effect of a nonuniform electric field. The possibility of controlling the motion of nanoelectromechanical systems with the proposed method is demonstrated using a nanotube-based gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator are analyzed, and its operation is simulated by the molecular dynamics method. The controlling parameters and characteristics corresponding to the controlled operating conditions at a constant frequency for the system under investigation are determined.     

First-principles study of electronic and elastic properties of Stone–Wales defective zigzag carbon nanotubes

The interaction and coupling between the electrical, mechanical properties and formation energy for SW defective (10,0) carbon nanotube is studied in density functional theory. The investigated configurations include the axial and circumferential orientations for single defect as well as four distribution types for double ones. The more stable defective configurations, namely, SW-I configurations for single SW defective carbon nanotube and II–II-(2) and I–I ones for double SW defective tubes are related to high symmetry distribution of the defects. Moreover, we found that the σ^?–π^* hybridization induced by curvature effect causes the semiconductor to metal transition for double axial SW defects case. Young's modulus reduction of SW defective carbon nanotube with respect to defect-free one is less than 8%. The energy bands and Young's moduli of double SW defective tubes are mostly affected by the defect distribution and concentration but insensitive to the circumferential distance between the double defects.