Plasmon‐enhanced Raman scattering by suspended carbon nanotubes

We report plasmon‐enhanced Raman scattering of the order of 10³ by a metallic carbon nanotube partially suspended inside a near‐field cavity. The tube is part of a small bundle, and is interfaced with an Au nanodisc dimer using a recently developed assembly scheme based on dielectrophoretic deposition. Spatially resolved Raman measurements with two excitation wavelengths and two orthogonal polarizations confirm that the enhancement arises from a 65 nm long suspended tube segment. We show that the orientation of the tube inside the cavity can be as effective for generating enhancement as placing the nanotube precisely in a plasmonic hotspot. Position and shape of the G‐peak show that the suspended part of the tube is free of strain and doped with a Fermi energy shift ≤40 meV. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

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.     

First-principles simulations of chemical reactions in an HCl molecule embedded inside a C or BN nanotube induced by ultrafast laser pulses

We show by first-principles simulations that ultrafast laser pulses induce different chemical reactions in a molecule trapped inside a nanotube. A strong laser pulse polarized perpendicular to the tube axis induces a giant bond stretch of an encapsulated HCl molecule in semiconducting carbon nanotube or in a BN nanotube. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different: either complete disintegration or rebonding of the HCl molecule. Radial motion of the nanotube is always observed and a vacancy appears on the tube wall when the HCl is perpendicular to the tube axis. Those results are important to analyze confined nanochemistry and to manipulate molecules and nanostructures encapsulated in organic and inorganic nanotubes.     

Multimode transport in Schottky-barrier carbon-nanotube field-effect transistors

We present a detailed study on the impact of multimode transport in carbon nanotube field-effect transistors. Under certain field conditions electrical characteristics of tube devices are a result of the contributions of more than one one-dimensional subband. Through potassium doping of the nanotube the impact of the different bands is made visible. We discuss the importance of scattering for a stepwise change of current as a function of gate voltage and explain the implications of our observations for the performance of nanotube transistors.     

Terahertz current oscillations in single-walled zigzag carbon nanotubes

We report time-dependent terahertz current oscillations on an n=10 single-walled zigzag carbon nanotube (CNT) that is 100 nm long. To obtain transport characteristics in this CNT, we developed an ensemble Monte Carlo (MC) simulator, which self-consistently calculates the electron transport and electrical potential. The ensemble MC simulations indicate that, under certain dc bias and doping conditions, the average electron velocity and concentration oscillate. This leads to current oscillations in space and time, on the tube, and at the contacts. We attribute this to accumulation and depletion of the CNT electrons at different locations on the tube, giving rise to low and high density electron regions. These local dipoles are a result of intra- and intersubband scatterings and different subband dispersion relations. This in turn forms propagating dipoles and current oscillations.     

Interplay of wetting and elasticity in the nucleation of carbon nanotubes

We use molecular dynamics and simple thermodynamic arguments to model the interaction between catalyst nanoparticles and carbon nanotube caps, and we illustrate how the competition between cap strain energy and adhesion plays a role in the lifting of these caps from the catalyst surface prior to tube elongation. Given a particular cap structure, we show that there is a lower bound on the catalyst size from which the cap can lift. This lower bound depends on the cap's spontaneous curvature and bending rigidity, as well as the catalyst binding strength, and it explains the mismatch between single-walled carbon nanotube and catalyst diameters observed in prior experiments. These findings offer new insight into the nucleation of carbon nanotubes, and they may lead to the design of catalysts that can better control nanotube structure.     

X-ray irradiation effect of double walled carbon nanotube

We have studied double-walled carbon nanotube (DWNT) irradiated by soft X-ray by Raman scattering spectroscopy and the spectral characteristics are compared to single-walled carbon nanotube (SWNT) irradiated under the same condition. We proved that DWNT is more stable for the X-ray induced defect formation than SWNT. Moreover, we found that the outer tube of DWNT was more sensitive on X-ray irradiation than the inner tube. The defect was recovered by annealing in Ar at lower temperature than that of SWNT. Based on these results, we inferred that X-ray irradiation leads to formation of interstitial-vacancy pairs, Frenkel defects, in carbon nanotube. The interstitial-vacancy separation on the inner tube of DWNT is conceivably shorter than that of the outer tube.     

介质中双缺陷电荷对碳纳米管场效应晶体管量子输运特性的影响

用非平衡格林函数理论和紧束缚模型近似计算长沟道弹道输运p型碳纳米管场效应管中电流强度.研究当场效应管介质（SiO₂）中存在两个带电缺陷时，载流子散射所引起的电流强度减小和栅极阈值电压偏移量与缺陷位置的关系.介质中两个缺陷所带电荷Q₁=Q₂=+e（-e为电子电荷），都靠近源极或者都靠近漏极，或者一个电荷靠近源极另一个电荷靠近漏极.在工作状态下，所引起的电流强度相对减小比介质中只存在单个正电荷Q=+e且靠近源极（或漏极）时所引起的电流强度相对减小大得多.如果两个正电荷都在沟道中央附近，随着两个电荷的轴向距离减小，栅极阈值电压偏移的绝对值明显增加.栅极阈值电压偏移可达到-0.35 V.     

Electronic properties of double-layer carbon nanotubes

The electronic spectra for double-wall zigzag and armchair nanotubes are found. The influence of nanotube curvatures on the electronic spectra is also calculated. Our finding that the outer shell is hole doped by the inner shell is in the difference between Fermi levels of individual shells which originate from the different hybridization of π orbital. The shift and rotation of the inner nanotube with respect to the outer nanotube are investigated. We found stable semimetal characteristics of the armchair DWNTs in regard of the shift and rotation of the inner nanotube. We predict the shift of k_F towards the bigger wave vectors with decreasing of the radius of the armchair nanotube.     

Charge transfer from ammonia physisorbed on nanotubes

We report the use of nanotube field-effect transistor devices for chemical sensing in a conducting liquid environment. Detection of ammonia occurs through the shift of the gate voltage dependence of the source-drain current. We attribute this shift to charge transfer from adsorbed ammonia molecules, with the amount of charge estimated to be as small as 40 electrons for the smallest shift detected. Using the concentration dependence of the response as an adsorption isotherm, we are able to measure the amount of charge transfer to be 0.04 electron per ammonia molecule.     

Possible role of phospholipid nanotubes in directed transport of membrane vesicles

We indicate that membrane nanotubes may have an important role in directed transport of membrane vesicles between different membrane-enclosed compartments in cells. We present experimental evidence that small blebs of phospholipid nanotube may travel along the nanotube and act as vehicles for transporting the enclosed solution. We have also observed similar small membrane blebs of a long membrane tube in red blood cells. In both cases the small vesicles seem to be a distended integral part of the membrane tube and not independent vesicles entrapped within the tube.     

Nanotube Electron Drag in Flowing Liquids

We show that electric current can be generated in metallic carbon nanotubes immersed in liquids flowing along them. Molecular layers of the liquid coat the nanotube, slip along its surface, and excite there a phonon wind, which drags free carriers in the tube. The induced electric current should allow building of nanoscale detectors or power cells.     

Effect of ion impinging on the microstructure and field emission of carbon nanotubes

Effects of ion impinging on the microstructure and field electron emission properties of screen-printed carbon nanotube films were investigated. We observed that the plasma treatment modified the microstructure of CNTs along with the remarkable increase of emission site density. With the prolongation of ion impinging time, the emission current falls down first, and then rises up to higher than that of the untreated films. It is proposed that the change of emission characteristics is due to the different emission mechanisms. After the treatment, electrons are emitted predominantly from the nano-nodes on the tube wall instead from the nanotube tips.     

纳米碳管阵列场发射电流密度的理论数值优化

以纳米碳管阵列为研究对象,利用镜像悬浮球模型及Fowler-Nordheim电流密度公式,对纳米碳管阵列的场发射电流密度进行计算,进而综合考虑场发射增强因子及场发射电流密度对纳米碳管阵列场发射性能进行定量优化.参考碳管阵列场发射电流密度最大值及场发射增强因子,表明当纳米碳管阵列间距为碳管高度十分之一时,纳米碳管阵列的场发射性能得到优化.与以前的理论估算结果相比,优化的阵列间距进一步减小.当纳米碳管间距过大,场发射增强因子增加,而场发射电流密度会在更大程度上减小;当纳米碳管密度较大时,场发射增强因子受到静电 关键词： 纳米碳管 场发射 增强因子 电流密度     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

Geometric constant defining shape transitions of carbon nanotubes under pressure

We demonstrate that when a single-walled carbon nanotube is under pressure it undergoes a series of shape transitions, first transforming from a circle to an oval and then from an oval to a peanut. Most remarkably, the ratio of the area of the tube cross sections at the second transition over that at the first transition appears as a constant, independent of the tube radius. Its accurate value is computed to be G=0.819 469, by formulating a variational geometry problem to represent single-walled carbon nanotubes with a family of closed plane curves of fixed length and minimum bending energy. The implications of such a geometric constant in designing nanotube electromechanical pressure sensors are discussed.     

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.     

Modeling electrostatically induced collapse transitions in carbon nanotubes

Molecular dynamics simulations demonstrate how a mechanically bistable single-walled carbon nanotube can act as a variable-shaped capacitor. If the voltage is tuned so that collapsed and inflated states are degenerate, the tube's susceptibility to diverse external stimuli--temperature, voltage, trapped atoms--diverges following a universal curve, yielding an exceptionally sensitive sensor or actuator. The boundary between collapsed and inflated states can shift hundreds of angstroms in response to a single gas atom inside the tube. Several potential nanoelectromechanical devices could be based on this electrically tuned crossover between near-degenerate collapsed and inflated configurations.     

Intrinsic and extrinsic effects in the temperature-dependent photoluminescence of semiconducting carbon nanotubes

The temperature dependence of the band gap of semiconducting carbon nanotubes was measured for ten different nanotube species. The unprecedented effectiveness in avoiding the effect of external strain, or any other effects originating from the surrounding environment, lead to an accurate measurement of the band gap temperature dependence, giving fundamental insight into the nanotube electron-phonon interaction. Small but reproducible energy shifts of the emission lines with temperature were observed, showing a moderate chirality dependence, well in agreement with recent theoretical calculations. In addition to the energy shift, a substantial narrowing of the emission lines was also observed. The removal of the temperature shift of the band gap allows the precise measurement of the effect of external strain on carbon nanotubes in different environments.