Direct laser interference patterning of multi-walled carbon nanotube-based transparent conductive coatings

Topographical structures were created on the surface of multi-walled carbon nanotube-based coatings deposited on borosilicate glass using the direct laser interference patterning (DLIP) technique. Films made by multi-walled carbon nanotubes (MWNTs) dispersed in antimony-doped tin oxide (ATO) matrix and networks of MWNTs with both low and high adherence to the substrates were irradiated with one single laser pulse. Due to the high absorption coefficient of ATO, the film was completely removed at the interference maxima positions leading to periodic arrays of high quality on macroscopic areas. Additionally, increase of the laser fluence has produced wider ablated regions. Irradiation of high adherent networks of MWNTs produced a periodic porous structure, what has been attributed to the presence of adherence promoters in the film. On the other hand, MWNT networks with low adhesion to the substrate were strongly removed at the interference maxima positions. In this case, however, the fabricated periodic structures presented several defects that result from the poor adherence of the film to the substrate.     

Negative differential resistance in a carbon nanotube-based molecular junction

By applying non-equilibrium Green's function formalism combined with first-principles density functional theory, we have investigated the electronic transport properties of a carbon nanotube-based molecular junction with different terminations (H-, C- and N-). The results show that the different terminations at the carbon nanotube ends strongly affect the transport properties of the junction. The current through the N-terminated carbon nanotube junction is significant larger than that through the H- and C-terminated junctions at low biases. Moreover, negative differential resistance behaviors can be observed in the N-terminated carbon nanotube junction, whereas not in the other two cases.     

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

Degradation of a carbon nanotube-based field-emission cathode during ion sputtering

The degradation rate of carbon nanotubes (CNTs) in an electron field emitter is calculated. The degradation mechanism is taken to be the sputtering of the CNT surface by the ions that result from the ionization of residual gas molecules by an electron impact. The degradation rate and the corresponding CNT lifetime are calculated as a function of the nanotube geometry, the applied voltage, the pressure and kind of a residual gas, the interelectrode gap, and the nanotube array density. The obtained strong dependence of the degradation rate on the applied voltage is caused by a sharp character of the I?CV emission characteristic determined by the Fowler-Nordheim relationship. The dependence of the degradation rate on the interelectrode gap is induced by the corresponding dependence of the probability of reaching the CNT surface.     

碳纳米管悬浮液的光限幅特性实验研究

通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液,测量了其对于1 064 nm,脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时,出射能量密度随入射能量密度的增加而线性增加;当入射能量密度为160 mJ/cm2时,出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值,约16 mJ,同时,对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量,对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射;另外,非线性折射对光限幅效果也有一定的作用。     

碳纳米管光学天线的有效波长和谐振特性

将经典金属自由电子气模型应用于金属型碳纳米管, 基于光学天线有效波长理论, 得出了金属型碳纳米管光学天线响应的有效波长与碳纳米管介电特性之间的普适关系. 在对碳纳米管介电特性进行第一性原理计算的基础上, 以金属型4 Å碳纳米管为例, 进一步研究了金属型碳纳米管光学天线响应的有效波长与入射波长之间的关系, 以及金属型碳纳米管光学偶极子天线的谐振特性. 通过将已有传统金属光学天线和碳纳米管天线有效波长的研究结果进行对比, 验证了本文理论的正确性. 结果表明, 碳纳米管光学天线响应的有效波长与入射波长呈近似线性关系, 与传统金属材料构成的同直径光学天线相比, 碳纳米管天线显示出了更强的波长压缩能力, 并且在可见光到红外波段内易于发生谐振. 该研究方法可为碳纳米管光学天线研究提供新的思路.     

碳纳米管悬浮液的光限幅特性实验研究

 通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液,测量了其对于1 064 nm,脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时,出射能量密度随入射能量密度的增加而线性增加;当入射能量密度为160 mJ/cm²时,出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值,约16 mJ,同时,对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量,对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射;另外,非线性折射对光限幅效果也有一定的作用。     

Thermal properties of carbon nanotubes and nanotube-based materials

The thermal properties of carbon nanotubes are directly related to their unique structure and small size. Because of these properties, nanotubes may prove to be an ideal material for the study of low-dimensional phonon physics, and for thermal management, both on the macro- and the micro-scale. We have begun to explore the thermal properties of nanotubes by measuring the specific heat and thermal conductivity of bulk SWNT samples. In addition, we have synthesized nanotube-based composite materials and measured their thermal conductivity. The measured specific heat of single-walled nanotubes differs from that of both 2D graphene and 3D graphite, especially at low temperatures, where 1D quantization of the phonon bandstructure is observed. The measured specific heat shows only weak effects of intertube coupling in nanotube bundling, suggesting that this coupling is weaker than expected. The thermal conductivity of nanotubes is large, even in bulk samples: aligned bundles of SWNTs show a thermal conductivity of >200 W/m K at room temperature. A linear K(T) up to approximately 40 K may be due to 1D quantization; measurement of K(T) of samples with different average nanotube diameters supports this interpretation. Nanotube–epoxy blends show significantly enhanced thermal conductivity, showing that nanotube-based composites may be useful not only for their potentially high strength, but also for their potentially high thermal conductivity. Received: 17 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

重频激光对多壁碳纳米管悬浮液光限幅特性的影响研究

在激光以不同重复频率入射下,研究了直径分布为10～20 nm的多壁碳纳米管悬浮液对波长为1064 nm激光的光限幅效应,定性分析了不同重复频率激光对多壁碳纳米管悬浮液的光限幅影响。结果表明:碳纳米管悬浮液对波长为532 nm和1064 nm的激光都具有很强的光限幅响应;在多脉冲激光入射时,碳纳米管悬浮液的光限幅效应比单脉冲激光入射时更显著。     

Transverse elasticity of multi-walled carbon nanotubes

A discrete shell model is proposed to describe the radial deformation of carbon nanotubes under a hydrostatic pressure and the radial Young's modulus of (single- or multi-walled) nanotubes is obtained. It is found that the radial modulus decreases with increasing tube diameter while increases with increasing number of layers. The computational results agree well with the previous results of SWNTs and indicate that the radial modulus of carbon nanotubes is independent of the Poisson's ratio.     

XPS study of fluorinated carbon multi-walled nanotubes

Arc-produced carbon multi-walled nanotubes (MWNTs) were fluorinated at 420 °C in a flow of diluted F₂ gas containing small admixture of HF gas. Fluorinated materials (F-MWNTs) with 10–55 wt.% fluorine content were studied by XPS. It was shown that fluorination begins at the external layers of nanotubes and the reaction front propagates inside the multi-layer particles in concert with structural deterioration of graphene layers. The C₂F stoichiometry still allows MWNT wall integrity, similar to known for SWNTs. The fluorine contents in the product can noticeably exceed this higher fluorine limit for tube stability. The position of the F 1s line at 688.2 eV does not depend on the fluorine concentration. Nearly covalent C–F bonds dominate the F-MWNT samples, with a small quantity (2–9%) of ionic bonds also present. Fluorinated carbon tends to spatially separate from non-fluorinated carbon.     

Resonances of sandwiched optical antenna

We calculate the resonant properties of the sandwiched triangle and bowtie antennas using finite difference time domain technique and compare with one-layer structures. The sandwiched antennas possess two tunable resonances corresponding to the symmetric and antisymmetric modes for dipole excitation, which can be understood by the hybridization of the plasmons supported by the two golden layers of the antennas. We obtain a giant field enhancement and a full width at half maximum as larger as 385 nm for the sandwiched bowtie antenna.     

Soft limiting circuit implementable with a single multi-walled carbon nanotube

Dispersed growth techniques are applied to grow carbon nanotubes between metal pads on silicon wafers. Process conditions are tuned to yield nanotube devices having only one multi-walled carbon nanotube connecting the metal pads. The nonlinear transfer characteristics of these devices are often accompanied with high impedance and low conduction current. These attributes can be utilized for a soft limiting circuit with impedance much higher than conventional silicon-based implementations.     

Guided waves characteristics of multi-walled carbon nanotubes

The theoretical analysis of propagation of guided waves in the multi-walled carbon nanotubes is presented within the framework of the classical electrodynamics. Electronic excitations of each wall of the system are modeled as an infinitesimally thin cylindrical layer of the π-electrons, whose dynamics are described by means of the fluid theory. General expressions of dispersion relations are obtained for the electromagnetic wave with the transverse magnetic and transverse electric modes, respectively, by solving Maxwell and fluid equations with appropriate boundary conditions.     

多壁碳纳米管光限幅特性的研究

用化学气相沉积法制备了多壁碳纳米管,并将其溶解在甲苯溶液中.用波长为1064nm的皮秒脉冲激光测量该样品的透过率,发现了非常明显的光限幅特性.当入射光强较小时,透射光强度随入射光强度的增大而增大,输出与输入为线性关系;随着入射光强的增大,透射光强增长的速度明显变慢,并逐渐趋于饱和.当入射光强度较小时,样品的透过率接近100%;而当入射光强为8GW/cm²时,非线性透过率达到30%.根据三光子吸收理论计算,理论拟合与实验结果非常符合,说明多壁碳纳米管的三光子吸收产生了光限幅效应.实验测 关键词： 多壁碳纳米管 光限幅 三光子吸收     

Geometrical influence of a deposited particle on the performance of bridged carbon nanotube-based mass detectors

A nonlocal continuum-based model is derived to simulate the dynamic behavior of bridged carbon nanotube-based nano-scale mass detectors. The carbon nanotube (CNT) is modeled as an elastic Euler-Bernoulli beam considering von-Kármán type geometric nonlinearity. In order to achieve better accuracy in characterization of the CNTs, the geometrical properties of an attached nano-scale particle are introduced into the model by its moment of inertia with respect to the central axis of the beam. The inter-atomic long-range interactions within the structure of the CNT are incorporated into the model using Eringen's nonlocal elastic field theory. In this model, the mass can be deposited along an arbitrary length of the CNT. After deriving the full nonlinear equations of motion, the natural frequencies and corresponding mode shapes are extracted based on a linear eigenvalue problem analysis. The results show that the geometry of the attached particle has a significant impact on the dynamic behavior of the CNT-based mechanical resonator, especially, for those with small aspect ratios. The developed model and analysis are beneficial for nano-scale mass identification when a CNT-based mechanical resonator is utilized as a small-scale bio-mass sensor and the deposited particles are those, such as proteins, enzymes, cancer cells, DNA and other nano-scale biological objects with different and complex shapes.     

Investigation of a nanostrip patch antenna in optical frequencies

This is the first report and investigation of a patch antenna in optical frequency range. Variety of plasmonic nanoantenna reported so far is good at enhancing the local field intensity of light by orders of magnitude. However, their far-field radiation efficiency is very poor. The proposed patch antenna emits a directional beam with high efficacy in addition to enhancing the intensity of near field. The nano-patch antenna (NPA) consists of a square patch of gold film of dimension 480 nm², placed on a substrate of dielectric constant \( \varepsilon_{\text{r}} \)  = 3.9 and thickness 150 nm with a ground plane of gold film of dimension 1,080 nm². The NPA resonates at 210 THz and has gain nearly 2 dB and radiation efficiency 45.18 %. The NPA might be useful in variety of applications such as optical communication, nano-photonics, biosensing, and spectroscopy.     

Transport properties of a nanotube-based transistor

Transport properties of doped nanotube-based double junctions forming a nanotransistor are investigated within the tight binding formalism. The effects of doping, gate length and gate-source hopping have been considered. It is found that in addition to the importance of rotational symmetry in determining transport properties, large gains can be achieved for semiconducting doped tubes. Received 30 November 2000     

卡塞格伦光学天线偏轴及性能分析

设计了具有抛物面结构的卡塞格伦光学天线.通过对偏轴下的卡塞格伦光学天线系统的分析,得到了不同偏转角所对应的接收光斑面积表达式和功率衰减曲线.讨论了偏轴下的接收天线增益与波长及偏转角的关系,仿真出在偏轴与轴对准两种情形下的增益曲线.仿真结果表明,〖JP2〗最大偏轴比轴对准情形增益降低了6.564dB.最后分别针对轴对准与某种偏轴情形下的系统做了光斑测试实验与天线耦合效率测试实验.结果为：偏轴下天线耦合效率降低了26.966%.这些研究为星间光通信中控制系统实现光轴的精确对准提供了理论依据,具有重要的实用价值 关键词： 偏轴 卡塞格伦光学天线 偏转角 增益