Multiscale simulation of carbon nanotube devices

In recent years, the understanding and accurate simulation of carbon nanotube-based devices has become very challenging. Conventional simulation tools of microelectronics are necessary to envision the performance and use of nanotube transistors and circuits, but the models need to be refined to properly describe the full complexity of such novel type of devices at the nanoscale. Indeed, many issues such as contact resistance, low dimensional electrostatics and screening effects, as well as nanotube doping or functionalization, demand for more accurate quantum approaches. In this article, we review our recent progress on multiscale simulations which aim at bridging first principles calculations with compact modelling, including the comparison between semi-classical Monte Carlo and quantum transport approaches. To cite this article: C. Adessi et al., C. R. Physique 10 (2009).     

Atomistic simulation of the laser induced damage in single wall carbon nanotubes: Diameter and chirality dependence

The effect of high energy laser pulses on single wall carbon nanotubes (SWNT) is studied by a non-equilibrium quantum mechanical model. For the studied laser parameters, we find ablation thresholds that vary between 1.9 eV/atom and 2.3 eV/atom. For zigzag tubes a linear increase of damage thresholds as function of diameter is observed. For armchair tubes, a stability maximum is found at the (10,10) SWNT. We find that below but close to the damage threshold the nanotubes show the presence of standing waves. PACS 78.70.-g; 52.38.Mf; 61.46.+w     

碳纳米管振荡的分子动力学模拟

运用分子动力学模拟方法,模拟了三种碳纳米管振荡器内管的振荡运动.结果显示：振荡器的内管越短,振荡的频率越大,且受到的轴向回复力的波动也越大.内管在沿着管轴振荡的同时,还绕着管轴旋转,转动的动能有明显涨落并与内管管长密切相关.该研究对于开发碳纳米管的相关应用技术有指导意义. 关键词： 分子动力学模拟 多壁碳纳米管 振荡     

Terahertz radiation from armchair carbon nanotube dipole antenna

This paper investigates the electromagnetic radiation characteristics of a metallic, large aspect ratio single walled carbon nanotube antenna in the terahertz frequency region below 12.5 THz. The key features of terahertz pulse have been revealed on the carbon nanotube antenna in comparison with conventional photoconductive switching. The terahertz waveforms, radiation power and their field distributions have been evaluated and are analysed. The Fourier transformed spectra over the whole frequency range demonstrate that the carbon nanotube antenna can be used as radiation source for broadband terahertz applications.     

Methane in carbon nanotube: molecular dynamics simulation

The behaviour of methane molecules inside carbon nanotubes at room temperature is studied using classical molecular dynamics simulations. A methane molecule is represented either by a shapeless super-atom or by a rigid set of five interaction centres localized on atoms. Different loadings of methane molecules ranging from the dense gas density to the liquid density, and the influence of flexibility of the CNT on structural and dynamic properties of confined molecules are considered. The simulation results show the decreases of the diffusion coefficient of methane molecules with density. At higher densities diffusion coefficient values are almost independent of molecular shape, but at low densities one observes faster motion of the super-atom molecule than that for the tetrahedral model of the molecule. For loadings of methane considered here the nanotube flexibility, introduced by the reactive empirical bond order (REBO) potential for interactions between carbon atoms of nanotube, does not have an effect on diffusivity of methane molecules, and its impact on the molecular structure is weak. It is found that methane molecules in the vicinity of the nanotube wall show tripod orientation with respect to the nanotube surface.     

Molecular dynamics simulation on mechanicalproperty of carbon nanotube torsional deformation

In this paper torsional deformation of the carbon nanotubes is simulated by molecular dynamics method. The Brenner potential is used to set up thesimulation system. Simulation results show that the carbon nanotubes can bear larger torsional deformation, for the armchair type (10,10) single wall carbon nanotubes, with a yielding phenomenon taking place when the torsional angle is up to 63$^{\circ}$(1.1rad). The influence of carbon nanotube helicity in torsional deformation is very small. The shear modulus of single wall carbon nanotubes should be several hundred GPa, not 1\,GPa as others reports.     

The dynamics simulation of Ne atom injected into single-wall carbon nanotube

The dynamical processes of Ne atom injected into single-wall carbon nanotube (SWCNT) are modeled with molecular dynamics simulations. The threshold energies to encapsulate rare-gas atoms in SWCNT are presented. The range of tube radius for stable oscillation is revealed, which is independent of the type of carbon nanotubes. And the oscillatory frequency is sensitive to the change in the diameter, the length and chirality of the tube.     

Atomistic simulations to characterize the influence of applied strain and PKA energy on radiation damage evolution in pure aluminum

Knowledge of defects generation, their mobility, growth rate, and spatial distribution is the cornerstone for understanding the surface and structural evolution of a material used under irradiation conditions. In this study, molecular dynamics simulations were used to investigate the coupled effect of primary knock-on atom (PKA) energy and applied strain (uniaxial and hydrostatic) fields on primary radiation damage evolution in pure aluminum. Cascade damage simulations were carried out for PKA energy ranging between 1 and 20 keV and for applied strain values ranging between ?2% and 2% at the fixed temperature of 300 K. Simulation results showed that as the atomic displacement cascade proceeds under uniaxial and hydrostatic strains, the peak and surviving number of Frenkel point defects increases with increasing tension; however, these increments were more prominent under larger volume changing deformations (hydrostatic strain). The percentage fraction of point defects that aggregate into clusters increases under tension conditions; compared to the reference conditions with no strain, these increases are around 13% and 7% for interstitials and vacancies, respectively (under 2% uniaxial strain), and 19% and 11% for interstitials and vacancies, respectively (under 2% hydrostatic strain). Clusters formed of vacancies and interstitials were both larger under tensile strain conditions, with increases in both the average and maximum cluster sizes. The rate of increase/decrease in the number of Frenkel pairs, their clustering, and their size distributions under expansion/compression strain conditions were higher for higher PKA energies. Overall, the present results suggest that strain effects should be considered carefully in radiation damage environments, specifically for conditions of low temperature and high radiation energy. Compressive strain conditions could be beneficial for materials used in nuclear reactor power systems.     

Atomistic simulation of the transition from atomistic to macroscopic cratering

Using large-scale atomistic simulations, we show that the macroscopic cratering behavior emerges for projectile impacts on Au at projectile sizes between 1000 and 10000 Au atoms at impact velocities comparable to typical meteoroid velocities. In this size regime, we detect a compression of material in Au nanoparticle impacts similar to that observed for hypervelocity macroscopic impacts. The simulated crater volumes agree with the values calculated using the macroscopic crater size scaling law, in spite of a downwards extrapolation over more than 15 orders of magnitude in terms of the impactor volume. The result demonstrates that atomistic simulations can be used as a tool to understand the strength properties of materials in cases where only continuum models have been possible before.     

Si表面间水平碳纳米管束的分子动力学模拟研究

本文采用分子动力学模拟方法研究了Si表面间单壁水平碳纳米管束SWCNT (10,10)的变形和摩擦特性.系统在弛豫平衡后,首先对碳纳米管束施加压力至碳纳米管或Si表面结构破坏.之后在无压力和高压力两种情况下使上表面沿水平方向做剪切运动以研究碳纳米管束的摩擦特性.结果表明,由于碳纳米管的柔韧性,碳纳米管束在加载过程中出现明显变形,但直至3.8 GPa高压下并无结构破坏.系统无压力时SWCNT (10,10)在原地轻微随机滚动,压力为3.8 GPa时,碳纳米管束出现了整体的轻微滑动,同时伴随无规律的轻微滚动, 关键词： 碳纳米管束 摩擦 分子动力学模拟     

碳纳米管从硅基板上剥离的拉伸分子动力学模拟研究

采用拉伸分子动力学方法研究了单壁碳纳米管(8, 8)在室温下从硅基板上被剥离的过程.当碳纳米管(CNT)在硅基底上被剥离时, 剥离距离和理想弹簧所测平均剥离力之间呈现一定规律的关系曲线,并出现了较大的正、负峰值. 比较了不同剥离速度下的平均剥离力,并拟合了其峰值与速度的关系. 拉伸分子动力学模拟结果显示,所需剥离力的最大值与速度之间呈现一定的线性关系, 模拟结果同生物物理学上类似的剥离实验结果符合较好,但相比于高分子, CNT和硅(Si)组成的界面吸附性能更强.讨论了碳纳米管长度、 半径及缺陷对剥离过程的影响,研究表明:所需最大的剥离力与CNT的长度无关, 但随CNT半径的增加,需要的最大剥离力线性增加; 5-7-7-5缺陷对剥离力最大值影响较小,而半径变化缺陷会削减最大剥离力. 在原子尺度对未来的试验进行了理论预测,为碳纳米管在硅微电子工业中的应用提供了理论基础.     

Atomistic simulation of slow grain boundary motion

Existing atomistic simulation techniques to study grain boundary motion are usually limited to either high velocities or temperatures and are difficult to compare to realistic experimental conditions. Here we introduce an adapted simulation method that can access boundary velocities in the experimental range and extract mobilities in the zero driving force limit at temperatures as low as ～0.2T(m) (T(m) is the melting point). The method reveals three mechanistic regimes of boundary mobility at zero net velocity depending on the system temperature.     

稀有气体原子注入缺陷性纳米碳管的分子动力学模拟

采用TLHT势和经典分子动力学方法研究了稀有气体原子(He，Ne，Ar，Kr，Xe)进入带缺陷的单壁纳米碳管(SWCNT)的动力学过程，计算出了稀有气体原子分别从管壁和管口入射时，它们能封装在SWCNT中的能量阈值E_k0，并与理想结构情形做了比较.结果表明：随着管壁缺陷半径r的增加，E_k0减小；当r<4.5 ?时，给定合适的初始动能，稀有气体原子能封装在纳米碳管中；而r=4.5 ?时，稀有气体原子不能封装在碳管中，且此时缺陷对Ar，Kr和Xe的输运特性有很大影响. 关键词： 纳米碳管 缺陷 稀有气体原子 分子动力学模拟     

Monte Carlo simulation of hydrogen physisorption in K-doped single walled carbon nanotube array

Properties of hydrogen physisorption in K-doped single walled carbon nanotube array (SWCNTA) are investigated in detail by grand canonical Monte Carlo simulation. The optimization of hydrogen storage capacity at 293 K and 10 MPa as a function of K-doping schemes, K atoms’ doped-sites, and SWCNTA configuration is discussed.     

Light radiation through a transparent cathode plate with single-walled carbon nanotube field emitters

In the conventional carbon nanotube backlight units (CNT-BLUs), light passes through the phosphor-coated anode glass plate, which thus faces closely the thin film transistor (TFT) backplate of a liquid crystal display panel. This configuration makes heat dissipation structurally difficult because light emission and heat generation occur simultaneously at the anode. We propose a novel configuration of a CNT-BLU where the cathode rather than the anode faces the TFT backplate by turning it upside down. In this design, light passes through the transparent cathode glass plate while heating occurs at the anode. We demonstrated a novel design of CNT-BLU by fabricating transparent single-walled CNT field emitters on the cathode and by coating a reflecting metal layer on the anode. This study hopefully provides a clue to solve the anode-heating problem which would be inevitably confronted for high-luminance and large-area CNT-BLUs.     

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.     

碳纳米管冷阴极脉冲发射特性及仿真模型研究

针对碳纳米管场致发射冷阴极在微波、毫米波电真空辐射源器件中的应用需求,采用2μs,20 kV的脉冲高压对碳纳米管场致发射冷阴极的脉冲发射特性进行了实验研究.通过改变阴阳极间距,对碳纳米管冷阴极发射电流特性及发生脉冲高压打火后的碳纳米管冷阴极发射特性进行了测试研究.在直径为4 mm的圆形平面碳纳米管冷阴极上获得最大发射电流16 mA,电流密度为127 mA/cm~2.以实验测试数据为基础,结合粒子模拟软件建立碳纳米管冷阴极场致发射仿真模型,给出了该仿真模型的相关参数,为下一步设计研制碳纳米管冷阴极电子光学系统及相关辐射源器件奠定基础.     

Transparent carbon nanotube coatings

Thin networks of carbon nanotubes (CNTs) are sprayed onto glass or plastic substrates in order to obtain conductive transparent coatings. Transparency and conductivity at room temperature of different CNT material are evaluated. CNT coatings maintain their properties under mechanical stress, even after folding the substrate. At a transparency of 90% for visible light we observe a surface resistivity of 1 kΩ/sq which is already a promising value for various applications.