Efficient compact model for calculating the surface potential of carbon-nanotube field-effect transistors using a curve-fitting method

This paper presents an analytical method to compute the surface potential of ballistic metal-oxide semiconductor field-effect transistor (MOSFET)-like carbon-nanotube field-effect transistors (CNFETs). The proposed compact model considers the surface potential as functions of the carbon-nanotube diameter, gate insulator thickness, gate voltage and drain voltage. One of the advantages of this model is that there is no need to refer to the numerical model to recalculate the surface potential each time nanotube diameter or insulator thickness is changed. Instead of using a constant smoothing parameter regardless of the device size and applied bias voltages, a parameter calculated for the specific situations is employed to provide the simulation results with higher accuracy. The validity of the proposed model was verified by comparing the simulated output characteristics of three CNFETs with those of the numerical model and the previous compact model.     

A molecular dynamics simulation study on resonance frequencies comparison of tunable carbon-nanotube resonators

We investigated tunable gigahertz-resonators, which is based on the application of a telescoped double-walled carbon-nanotube that can be used repeatedly and operate at a single frequency or have a relatively narrow frequency range, via classical molecular dynamics simulations of a double-walled carbon-nanotube. Two types of telescoped double-walled carbon-nanotube resonators were compared with each other; one was bridge-type and another was cantilever-type, and one side was connected to a position controller in order to achieve a telescoped carbon-nanotube. The frequency bandwidth of our cantilevered type design can exceed that of the bridged type. Our simulations showed that such a system can tune it up its resonance frequency by controlling the length of oscillating carbon-nanotube resonator.     

基于碳纳米管场发射三电极显示器的设计和实验

提出了一种基于碳纳米管场发射三电极大面积全彩色平板显示器模型,其特点是栅极和阴极在同一个平面上,驱动电压低,而电子传输比可以高达29.3%(达到阳极的电子与从阴极碳管上发射出来的电子之比),实验结果和理论模拟的结果符合得很好.阴极和栅极之间的沟槽可以用激光打标机刻蚀形成,用做阴极发射体的碳纳米管浆料用水浴法形成,显示器制作工艺简单.     

Molecular dynamics study of effects of intertube gap on frequency-controlled carbon-nanotube oscillators

Model of frequency-controlled carbon-nanotube oscillators are proposed and their dynamic properties are investigated via classical molecular dynamic simulations. Their operation frequencies can be changed by engineering the intertube gap, which dominantly affects their operating frequencies in the improved bandwidths rather than those achieved by initial velocity engineering.     

Micropatterned vertically aligned carbon-nanotube growth on a Si surface or inside trenches

The good field-emission properties of carbon nanotubes coupled with their high mechanical strength, chemical stability, and high aspect ratio, make them ideal candidates for the construction of efficient and inexpensive field-emission electronic devices. The fabrication process reported here has considerable potential for use in the development of integrated radio-frequency amplifiers or field-emission-controllable cold-electron guns for field-emission displays. This fabrication process is compatible with currently used semiconductor-processing technologies. Micropatterned vertically aligned carbon nanotubes were grown on a planar Si surface or inside trenches, using chemical vapor deposition, photolithography, pulsed-laser deposition, reactive ion etching, and the lift-off method. This carbon-nanotube fabrication process can be widely applied for the development of electronic devices using carbon-nanotube field emitters as cold cathodes and could revolutionize the area of field-emitting electronic devices. Received: 30 August 2001 / Accepted: 3 September 2001 / Published online: 20 December 2001     

Anisotropy of sheared carbon-nanotube suspensions

We measure the anisotropy of sheared carbon-nanotube suspensions for a broad range of concentration, aspect ratio, and strain rate using a variety of methods. Our measurements highlight the importance of excluded-volume interactions in the semidilute regime, with scaling in terms of a dimensionless shear rate. Our results also suggest that such interactions might be exploited to fractionate carbon nanotubes by length in simple shear flow.     

Spin-flip mesoscopic transport through a carbon nanotube quantum dot system

The pure spin transport in an entire metallic single-wall carbon-nanotube (SWCN) interacting quantum dot (QD) system is investigated by using non-equilibrium Green's function (NEGF) technique. The novel spin current performance introduced by one constant and one rotating magnetic fields shows the unique four-fold degenerate electron shell structure which exists the SWCN QD sensitively. Spin transport properties can be designed by tuning the orbital and Zeeman configuration in the central resonant region, which are greatly influenced by the Coulomb interaction and the magnetic fields.     

The role of structural inhomogeneities in the temperature behavior of the thermopower in metallized nanotubes with impurities

High values of the thermopower and its derivative with respect to temperature in metallized carbon nanotubes at low temperatures are shown to result from the following factors: structural inhomogeneities and modification of the electronic structure both due to local structural regions of disrupted long-range order and to electron-electron interaction. The former factor plays an important role in carbon-nanotube bundles, whereas the latter is of substantial significance in single-walled nanotubes. Statistical inhomogeneities and all broken bonds are essential for electron scattering that gives rise to electron transport in metallized carbon nanotubes. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 31–41, March, 2009.     

含有五边形—七边形缺陷的单壁纳米碳管的输运性质研究

运用第一性原理的密度泛函理论结合非平衡格林函数研究了含有五边形—七边形拓扑缺陷的纳米碳管异质结的输运性质.结果发现：拓扑缺陷对碳管的输运性质有很大影响；另外，不同类型的碳管形成的异质结的输运性质也有明显的差异. 关键词： 纳米碳管 输运性质 异质结 透射系数     

脉冲磁场处理对碳纳米管掺杂MgB2线材临界电流密度的影响

对碳纳米管(CNT)掺杂MgB₂超导体磁场处理后的行为进行了研究. 结果表明，CNT掺杂MgB₂超导体经5T脉冲磁场处理后临界电流密度J_c(H)在低磁场下提高了2—3倍，高场下提高一个数量级以上，扫描电镜结果显示CNT沿着处理磁场方向规则排列并且成为MgB₂基体的形核中心和高效的磁通钉扎中心. 关键词： 2')" href="#">MgB₂ 碳纳米管 脉冲磁场处理     

Interaction-induced renormalization of Andreev reflection

We analyze the charge transport between a one-dimensional weakly interacting electron gas and a superconductor within the scaling approach in the basis of scattering states. We derive the renormalization group equations, which fully account for the intrinsic energy dependence due to Andreev reflection. A strong renormalization of the corresponding reflection phase is predicted even for a perfectly transparent metal-superconductor interface. The interaction-induced suppression of the Andreev conductance is shown to be highly sensitive to the normal-state resistance, providing a possible explanation of experiments with carbon-nanotube/superconductor junctions by Morpurgo et al. [Science 286, 263 (1999)].     

Electrically tunable spin polarization in a carbon nanotube spin diode

We have studied the current through a carbon-nanotube quantum dot with one ferromagnetic and one normal-metal lead. For the values of gate voltage at which the normal lead is resonant with the single available nondegenerate energy level on the dot, we observe a pronounced decrease in the current for one bias direction. We show that this rectification is spin dependent, and that it stems from the interplay between the spin accumulation and the Coulomb blockade on the quantum dot. The degree of resulting spin polarization is fully and precisely tunable using the gate and bias voltages.     

Charge-induced anisotropic distortions of semiconducting and metallic carbon nanotubes

To accommodate extra electrons or holes injected into a single-wall carbon nanotube, carbon-carbon bonds adjust their lengths. Resulting changes in carbon-nanotube length as a function of charge injection provide the basis for electromechanical actuators. We show that a key mechanism at low injection levels, modulation of electron kinetic energy, provides nanotube deformations that are both anisotropic and strongly dependent on nanotube structure. Nanotubes can exhibit both expansion and contraction, as well as nonmonotonic size changes. The magnitude of the actuation response of semiconducting carbon nanotubes may be substantially larger than that of graphite.     

Mobile ambipolar domain in carbon-nanotube infrared emitters

We spatially resolve the infrared light emission from ambipolar carbon-nanotube field-effect transistors with long-channel lengths. Electrons and holes are injected from opposite contacts into a single nanotube molecule. The ambipolar domain, where electron and hole currents overlap, forms a microscopic light emitter within the carbon nanotube. We can control its location by varying gate and drain voltages. At high electric fields, additional stationary spots appear due to defect-assisted Zener tunneling or impact ionization. The laterally resolved measurement provides valuable insight into the transistor behavior, complementary to electronic device characteristics.     

Kinetics and mechanism of proton transport across membrane nanopores

We use computer simulations to study the kinetics and mechanism of proton passage through a narrow-pore carbon-nanotube membrane separating reservoirs of liquid water. Free energy and rate constant calculations show that protons move across the membrane diffusively along single-file chains of hydrogen-bonded water molecules. Proton passage through the membrane is opposed by a high barrier in the effective potential, reflecting the large electrostatic penalty for desolvation and reminiscent of charge exclusion in biological water channels. At neutral pH, we estimate a translocation rate of about 1 proton per hour and tube.     

Interplay of magneto-elastic and polaronic effects in electronic transport through suspended carbon-nanotube quantum dots

We investigate the electronic transport through a suspended carbon-nanotube quantum dot. In the presence of a magnetic field perpendicular to the nanotube and a nearby metallic gate, two forces act on the electrons: the Laplace and the electrostatic force. They both induce coupling between the electrons and the mechanical transverse oscillation modes. We find that the difference between the two mechanisms appears in the cotunneling current.     

Four-electron shell structures and an interacting two-electron system in carbon-nanotube quantum dots

Low-temperature transport measurements have been carried out on single-wall carbon-nanotube quantum dots in a weakly coupled regime in magnetic fields. Four-electron shell filling was observed, and the magnetic field evolution of each Coulomb peak was investigated. Excitation spectroscopy measurements have revealed Zeeman splitting of single particle states for one electron in the shell, and demonstrated singlet and triplet states with direct observation of the exchange splitting at zero-magnetic field for two electrons in the shell, the simplest example of Hund's rule.     

Effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling through a finite-sized carbon-nanotube system

The effects of intradot electron–electron interaction on the photon-assisted Andreev tunneling of a superconductor/carbon-nanotube/superconductor system are studied by using nonequilibrium Green's function technique. The inverse supercurrent reflecting the π-junction transition emerges in the spin-split energy-levels regime polarized by the Coulomb interaction. For the positive tunneling case, the supercurrent reaches its maximum when the spin-degenerate energy-levels are nearest to the Fermi surface. Conversely, for the negative tunneling case, the supercurrent reaches its maximum when two split energy-levels are symmetric with respect of the Fermi surface. The sign and the amplitude of the Andreev tunneling depend distinctly on the energy-level spacing tuned by photon-assisted tunneling. In order to fully understand the transport characteristics, the current-carrying density of states are investigated, which clearly shows the enhancement, suppression or even reversion of the supercurrent.     

Quantum transport in molecules and nanotube devices

Quantum transport is usually cast as an "open" scattering problem, for which available computational methods have not achieved the accuracy of methods for conventional "closed" problems. Here we cast quantum transport as a closed problem and demonstrate fully converged currents for the prototype benzene-dithiolate system. We further report results for carbon-nanotube field-effect transistors, highlighting differences with Si-based devices, e.g., band mixing caused by the gate electric field. We also find that the source-drain current exhibits an intrinsic saturation as a function of the gate voltage.     

Features of the Radiation-Stimulated Hydrogenation of an Aluminum Surface

By the methods of reflection-absorption IR (infrared) spectroscopy, electrical measurement and AFM microscopy, the process of the radiation-stimulated hydrogenation of the aluminum surface in the Al/ads n-hexane system under γ-irradiation at room temperate is studied. On the basis of the dose dependence of the Al surface specific resistance, two stages of the hydrogenation process in the absorbed dose range (0.5?120 kGy) are revealed. It is established that the transition from the first to the second stage is accompanied by a decrease in the electrical conductivity of Al by 35 times and an increase in the hydride-nanolayer thickness by an order of magnitude. AFM studies of the aluminum surface relief showed that radiation-stimulated hydrogenation is accompanied by the formation of carbon-nanotube structures. A possible mechanism for the radiation-stimulated hydrogenation of aluminum is suggested.