Local control of electric current driven shell etching of multiwalled carbon nanotubes

We report on a novel method for local control of shell engineering in multiwalled carbon nanotubes (MWNTs) using Joule-heating induced electric breakdown. By modulating the heat dissipation along a nanotube, we can confine its thinning and shell breakdown to occur within localized regions of peak temperatures, which are distributed over one-half of the NT length. The modulation is achieved by using suitably designed nanomachined heat sinks with different degrees of thermal coupling at different parts of a current-carrying nanotube. The location of electric breakdown occurs precisely at the regions of high temperatures predicted by the classical finite-element model of Joule heating in the MWNT. The experiments herein provide new insight into the electric breakdown mechanism and prove unambiguously that shell removal occurs due to thermal stress, underpinning the diffusive nature of MWNTs. The method demonstrated here has the potential to be a powerful tool in realizing MWNT bearings with complex architectures for use in integrated nanoelectromechanical systems (NEMS). In addition, the breakdown current and power in the nanotubes are significantly higher than those observed in nanotubes without heat removal via additional heat sinks. This indicates future avenues for enhancing the performance of MWNTs in electrical interconnect and nanoelectronic applications. PACS  73.63.Fg; 65.80.+n     

Graphene/carbon nanotubes composites as a counter electrode for dye-sensitized solar cells

As an alternative platinum counter electrode in dye-sensitized solar cells (DSSCs), carbon materials based counter electrode were prepared using multi-walled carbon nanotubes (MWNTs)/graphene nano-sheets (GNS) composite by simple doctor blade method. We found that the photovoltaic performance was strongly influenced by the concentration of GNS in composite electrode. The composite electrode with 60% MWNTs and 40% GNS based DSSCs showed the maximum power conversion efficiency of 4.0% while sputter deposited platinum counter electrode based DSSCs showed a power conversion efficiency of 5.0%.     

输入电功率在大范围内变动时声致发光与电功率之间的关系

采用频率为１．１ＭＨｚ，声辐射面积为３ｃｍ＾２换能器，在输入电功率０－１２０Ｗ范围内研究了被空气自然饱和纯水中声致发光强度和输入电功率在大范围内变化时的关系，结果表明，声致发光在输入电功率为７０－９０Ｗ之间，不再随输入电功率的增加而增加，呈现出饱和现象，在饱和值的出现前，发光强度随输入电功率的增加呈不规则的阶状增加，在饱和值结束后，随输入电功率的增加，发光强度迅速减小，本文对实验结果进行了些定性的     

Gate-voltage dependence of zero-bias anomalies in multiwall carbon nanotubes

Temperature dependence of zero-bias conductance of the vanadium (V)/multiwall carbon nanotube (MWNT)/V structure is studied. As temperature is reduced, the conductance decreases with a functional form consistent with a power law. For the first time, we find that the exponent depends significantly on gate voltage. This exponent dependence cannot be explained by Luttinger-liquid theory for ballistic MWNTs. We interpret the obtained results within the framework of the nonconventional Coulomb blockade theory for strongly disordered MWNTs.     

Electrical properties of boron-doped MWNTs synthesized by hot-filament chemical vapor deposition

We have synthesized a large amount of boron-doped multiwalled carbon nanotubes (MWNTs) by hot-filament chemical vapor deposition. The synthesis was carried out in a flask using a methanol solution of boric acid as a source material. The scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy were performed to evaluate the structural properties of the obtained MWNTs. In order to evaluate the electrical properties, temperature dependence of resistivity was measured in an individual MWNTs with four metal electrodes. The Raman shifts suggest carrier injection into the boron-doped MWNTs, but the resistivity of the MWNTs was high and increased strongly with decreasing temperature. Defects induced by the plasma may cause this enhanced resistivity.     

Vertically aligned peapod formation of position-controlled multi-walled carbon nanotubes (MWNTs)

We have succeeded in making peapod structures with vertically aligned multi-walled carbon nanotubes (MWNTs), grown on a silicon substrate using hot filament chemical vapor deposition. Opening the tips of the MWNTs was performed at 600 C using oxygen and then C₆₀ molecules were deposited on them to fill their inner spaces. We confirmed that the MWNTs were open at the tips and filled with C₆₀ molecules using transmission electron microscopy. In addition, it has been determined that MWNTs can be filled with other materials, such as molybdenum oxide.     

Superconductivity in entirely end-bonded multiwalled carbon nanotubes

We report that entirely end-bonded multiwalled carbon nanotubes (MWNTs) can exhibit superconductivity with a transition temperature (T(c)) as high as 12 K, which is approximately 30 times greater than T(c) reported for ropes of single-walled nanotubes. We find that the emergence of this superconductivity is highly sensitive to the junction structures of the Au electrode/MWNTs. This reveals that only MWNTs with optimal numbers of electrically activated shells, which are realized by end bonding, can allow superconductivity due to intershell effects.     

New synthesis of multi-walled carbon nanotubes using an arc discharge technique under organic molecular atmospheres

We have synthesized multi-walled carbon nanotubes (MWNTs) using a DC arc discharge method under organic molecular atmospheres. This method allows us to synthesize about five times more MWNTs than are synthesized using the usual arc discharge method, using discharge conditions of 100 A and 20 V. We have examined the synthetic yield of MWNTs at various pressures under different organic atmospheres. The yield of MWNTs increases with the number of carbon atoms in the organic molecule. Received: 21 September 2000 / Accepted: 18 December 2000 / Published online: 26 April 2001     

Multiwalled carbon nanotubes mass-produced by dc arc discharge in He–H2 gas mixture

Uniform cathode deposits (longer than 15 mm), containing multiwalled carbon nanotubes (MWNTs) inside, were produced by dc arc discharge evaporation with a computer-controlled feeder of a pure-carbon electrode without a metal catalyst in a He–H₂ gas mixture. The purification of MWNTs was carried out to remove amorphous carbon and carbon nanoparticles. High-resolution transmission electron microscopy observations and Raman scattering studies show that the MWNTs possess a high crystallinity and a mean outermost diameter of ∼ ∼10 nm. It has been confirmed that the current density in the electron field emission from a purified MWNT mat can reach 77.92 mA/cm², indicating that the purified MWNTs are a promising candidate electron source in a super high-luminance light-source tube or a miniature X-ray source.     

Connection of silicon nanocrystals (Si-nc) with multi-walled carbon nanotubes

During the last decade silicon nanocrystals (Si-nc) have received widespread interest because of their high quantum efficiency for light emission at room temperature. However, the challenge still ahead is to study and apply these to single Si-ncoptoelectronics, i.e., solving problems linked with connection and manipulation. In this letter we report on connecting (wiring) single Si-nc with conducting multi-walled carbon nanotubes (MWNTs). We have been able to establish a strong mechanical connection by direct growth of MWNTs on Si-nc used as support of iron nanoparticles, by catalytic chemical vapor deposition (CCVD). To monitor the initial stage of the MWNTs growth process, we used a tapered element oscillating microbalance (TEOM). We compared the growth process on Si-nc coated by iron (Fe/Si-nc) to the standard process of growing MWNTs on alumina as support for iron (Fe/Al). The results showed that in the case of Fe/Si-nc catalyst, we obtained three times larger diameter of multi-walled CNTs compared to Fe/Al. This was mainly due to the Si-nc size. The diameter of the CNTs only depended on the size of the Si-nc particles that rested stuck on the tip of the MWNTs. The connected Si-nc kept their photoluminescence properties at room temperature. The present findings open new opportunities in the development of nanodevices for the optoelectronic application field. PACS 81.07.Bc; 81.07.Lk; 81.07.De     

Tomonaga-Luttinger liquid and Coulomb blockade in multiwall carbon nanotubes under pressure

We report that the conductance of macroscopic multiwall nanotube (MWNT) bundles under pressure shows power laws in temperature and voltage, as corresponding to a network of bulk-bulk connected Tomonaga-Luttinger liquids (LLs). Contrary to individual MWNTs, where the observed power laws are attributed to Coulomb blockade, the measured ratio for the end and bulk obtained exponents, approximately 2.4, can be accounted for only by LL theory. At temperatures characteristic of interband separation, it increases due to thermal population of the conducting sheets unoccupied bands.     

Raman spectroscopy and field-emission properties of CVD-grown carbon-nanotube films

Carbon-nanotube films are very efficient cathodes for field-emission devices. This study presents a comprehensive comparison between structural, spectroscopic and field-emission properties of films of aligned and non-aligned multi-wall nanotubes (MWNTs) which are grown by thermal chemical vapour deposition. Three types of films are investigated: vertically aligned MWNTs with clean and coated nanotube side walls as well as non-aligned MWNT films. Raman spectra taken on the aligned MWNT films consist of many lines of first-, second- and third-order signals. Several lines are reported here for the first time for MWNTs. The presence of the surface coating leads to a decrease and broadening of the higher-order signals as well as an increase in the disorder-induced contributions in the first-order regime. The aligned MWNT films have excellent field-emission properties with very high emission current densities and low turn-on and threshold fields. The presence of a surface coating has no impact on the efficiency of the field-emission process. Films of non-aligned MWNTs show considerably reduced electron-emission current densities and larger critical fields. Received: 25 April 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Conical surface structures on model thin-film electrodes and tape-cast electrode materials for lithium-ion batteries

A computationally-effective approach for calculating the electromechanical behavior of SWNTs and MWNTs of the dimensions used in nano-electronic devices has been developed. It is a mixed finite element–tight-binding code carefully designed to realize significant time saving in calculating deformation-induced changes in electrical transport properties of the nanotubes. The effect of the MWNT diameter and chirality on the conductance after mechanical deformation was investigated. In case of torsional deformation, results revealed the conductance of MWNTs to depend strongly on the diameter, since bigger MWNTs reach the buckling load under torsion much earlier, their electrical conductivity changes more easily than in small diameter ones. For the same outer diameter, zigzag MWNTs are more sensitive to twisting than armchair MWNTs since the hexagonal cells are oriented in such a way that they oppose less resistance to the buckling deformations due to torsion. Thus small diameter armchair MWNTs should work better if used as conductors, while big diameter zigzag MWNTs are more suitable for building sensors.     

Electron field emission from magnetic nanomaterial encapsulated multi-walled carbon nanotubes

The present work describes the field emission characteristics of nanoscale magnetic nanomaterial encapsulated multi-walled carbon nanotubes (MWNTs) fabricated over flexible graphitized carbon cloth. Ni/MWNTs, NiFe/MWNTs and NiFeCo/MWNTs have been synthesized by catalytic chemical vapor decomposition of methane over Mischmetal (Mm)-based AB₃ (MmNi₃, MmFe_1.5Ni_1.5 and MmFeCoNi) alloy hydride catalysts. Metal-encapsulated MWNTs exhibited superior field emission performance than pure MWNT-based field emitters over the same substrate. The results indicate that a Ni-filled MWNT field emitter is a promising material for practical field emission application with a lowest turn-on field of 0.6 V/μm and a high emission current density of 0.3 mA/cm² at 0.9 V/μm.     

Linear conductance of multiwalled carbon nanotubes at high temperatures

We have studied the electrical conductance of gas-desorbed multiwalled carbon nanotubes (MWNTs) at high temperatures, and found a peculiar linear temperature dependence of conductance over a wide temperature range from 100 to 800 K. We interpret this phenomenon by using a thermal activation picture of conduction channels below the gap in the vicinity of the Dirac points. The result also indicates a very short and temperature-independent electron mean free path in our MWNTs, and provides a way to determine the number of residual conduction channels in the MWNTs.     

电容式RF MEMS开关自热效应的多物理场协同仿真北大核心CSCD

随着信号输入功率的升高,电容式RF MEMS开关会发生自热效应使膜片变形,引起开关气隙高度的改变,导致开关驱动电压漂移,严重影响其可靠性。由于自热效应的失效机理涉及到复杂的多物理场耦合,因此提出了“电磁-热-应力”的多物理场协同仿真方法描述其失效模式,并分析其失效机理。首先利用HF-SS软件建立开关的电磁仿真模型,得到不同输入功率下膜片的耗散功率;再以此作为热源,利用ePhysics软件建立开关的热仿真模型,得到膜片上的温度分布;然后将温度梯度作为载荷,利用ePhysics软件建立开关的应力仿真模型,得到开关的形变行为;最后,根据膜片形变所致的气隙高度变化,得到驱动电压漂移的失效预测模型。以一种具有矩形膜片结构的典型电容式RFMEMS开关为例,利用该方法得到：矩形膜片表面电流密度主要分布在膜片的长边的边缘;温度沿膜片长边逐渐降低,且膜片中心处温度最高、锚点处温度最低;膜片的热应力变形呈马鞍面形,且最大形变点发生在膜片长边的边缘处,仿真还得到0~5 W输入功率下膜片的最大形变量;并拟合出了0~5W输入功率下的开关驱动电压-输入功率漂移曲线,该曲线具有线性特征并与文献实测数据极为吻合,由此证明了该方法的有效性。     

Effect of gases on the temperature dependence of the electric conductivity of CVD multiwalled carbon nanotubes

The influence of various gaseous media on the temperature dependence of the electric conductivity σ of multiwalled carbon nanotubes (MWNTs) synthesized using the method of catalytical chemical vapor deposition (CVD) has been studied. The σ(T) curves were measured in a temperature range from 4.2 to 300 K in helium and its mixtures with air, methane, oxygen, and hydrogen. The introduction of various gaseous components into a helium atmosphere leads to a significant decrease in the conductivity of MWNTs in the interval between the temperatures of condensation and melting of the corresponding gas. Upon a heating-cooling cycle, the conductivity restores on the initial level. It is concluded that a decrease in σ is caused by the adsorption of gases on the surface of nanotubes.     

Determination of the intershell conductance in multiwalled carbon nanotubes

We report on the intershell electron transport in multiwalled carbon nanotubes (MWNTs). To do this, local and nonlocal four-point measurements are used to study the current path through the different shells of a MWNT. For short electrode separations less, similar 1 mum the current mainly flows through the two outer shells, described by a resistive transmission line with an intershell conductance per length of approximately (10 kOmega)(-1)/microm. The intershell transport is tunnel type and the transmission is consistent with the estimate based on the overlap between pi orbitals of neighboring shells.     

电子回流对相对论速调管稳定工作的影响

模拟研究了电子束电压、电流及注入微波功率等参数对相对论速调管放大器中电子回流产生的影响,模拟发现注入微波过高、电子束电压过低及电子束电流过大都会导致电子回流。同时研究了回流电子对相对论速调管稳定工作的影响,研究发现,电子的回流会引起自激振荡,进而导致锁频锁相失败,器件不能稳定工作。     

Fermi-edge resonance and tunneling in nonequilibrium electron gas

Fermi-edge singularity changes in a nonequilibrium system, acquiring features that reflect the structure of energy distribution. In particular, it splits into several components if the energy distribution exhibits multiple steps. While conventional approaches, such as bosonization, fail to describe the nonequilibrium problem, an exact solution for a generic energy distribution can be obtained with the help of the method of functional determinants. In the case of a split Fermi distribution, the "open loop" part of the Greens function possesses power law singularities. At the same time, the resulting tunneling density of states exhibits broadened peaks centered at Fermi sublevels.