Molecular dynamics investigation into the electric charge effect on the operation of ion-based carbon nanotube oscillators

The fabrication of nanoscale oscillators working in the gigahertz (GHz) range and beyond has now become the focal center of interest to many researchers. Motivated by this issue, this paper proposes a new type of nano-oscillators with enhanced operating frequency in which both the inner core and outer shell are electrically charged. To this end, molecular dynamics (MD) simulations are performed to investigate the mechanical oscillatory behavior of ions, and in particular chloride ion, tunneling through electrically charged carbon nanotubes (CNTs). It is assumed that the electric charges with similar sign and magnitude are evenly distributed on two ends of nanotube. The interatomic interactions between carbon atoms and van der Waals (vdW) interactions between ion and nanotube are respectively modeled by Tersoff-Brenner and Lennard-Jones (LJ) potential functions, whereas the electrostatic interactions between ion and electric charges are modeled by Coulomb potential function. A comprehensive study is conducted to get an insight into the effects of different parameters such as sign and magnitude of electric charges, nanotube radius, nanotube length and initial conditions (initial separation distance and velocity) on the oscillatory behavior of chloride ion-charged CNT oscillators. It is shown that, the chloride ion frequency inside negatively charged CNTs is lower than that inside positively charged ones with the same magnitude of electric charge, while it is higher than that inside uncharged CNTs. It is further observed that, higher frequencies are generated at higher magnitudes of electric charges distributed on the nanotube.     

Mechanical properties of sorbents depending on nanopore sizes

The effect of the nanopore size on the mechanical properties of a porous carbon material with the density of 1.4 g/сm³ is discussed. The atomistic models of porous carbon materials depending on the nanopore size are constructed. The numerical experiments are implemented with using the molecular mechanical method based on the Brenner potential. The Young’s moduli are evaluated for porous carbon structures at certain nanopore dimensions and are found to decrease with the enlarging nanopores.     

纳米孔壁面作用对蛋白质过孔影响的粗粒化分子动力学模拟

基于粗粒化分子动力学方法模拟电驱动蛋白质过孔过程,研究纳米孔-水/纳米孔-蛋白质相互作用对电泳迁移率的影响;用操控式分子动力学模拟分析蛋白质在不同相互作用下过孔摩擦系数和摩擦阻力.研究发现：蛋白质黏附纳米孔壁面对其过孔特性影响并不明显,而纳米孔-水相互作用对蛋白质过孔电泳迁移率和摩擦系数影响较大.随纳米孔-水相互作用增强,纳米孔壁面与蛋白质附近水分子运动差异显现,蛋白质过孔摩擦阻力显著增大,过孔摩擦系数随之增大,进而影响蛋白质过孔电泳迁移率.所得结果可为纳米孔材料设计提供理论指导.     

Water wettability in nanoconfined environment

Water confined in nanoscale space behaves quite differently from that in the bulk.For example,in biological aquaporins and in carbon nanotubes,the traversing water molecules form a single file configuration.Water would stay in vapor state in extremely hydrophobic narrow nanopores owing to the physicochemical interactions between the water molecules and the surface of the nanopore.A spontaneous wet-dry transition has been identified in both biological and artificial nanopores.The nanopore is either fulfilled with liquid water or completely empty.Based on this mechanism,the wetting and dewetting processes inside nanopores have been further developed into highly efficient nanofluidic gates that can be switched by external stimuli,such as light irradiation,electric potential,temperature,and mechanical pressure.This review briefly covers the recent progress in the special wettability in nanoconfined environment,water transportation through biological or artificial nanochannels,as well as the smart nanofluidic gating system controlled by the water wettability.     

Effect of Vibration on Water Transport through Carbon Nanotubes

We study the response of water permeation properties through a carbon nanotube on the time-dependent mechanical signals. It is found that there is a critical frequency of vibrating fc （about 1333 GHz） which plays a significant role in the water permeation properties. The total water flow, the net flux, the number of hydrogen bonds and the dipole flipping frequency of the single-file water chain inside the nanotube are almost unchanged for the frequency of vibrating f 〈 fc. Simulation results show that the nanotube can be effectively resistant to the mechanical noise. Such excellent effect of noise screening is attributed to the exceptional property of water molecules connected by strong hydrogen bonds with each other and forming a one-dimensional water chain inside the nanotube. Our findings are important for the understanding of why biological systems can achieve accurate information transfer in an environment full of fluctuations.     

Biophysical and electrochemical properties of Self-assembled noncovalent SWNT/DNA hybrid and electroactive nanostructure

DNA self-assembled hybrid nanostructures are widely used in recent research in nanobiotechnology. Combination of DNA with carbon based nanoparticles such as single-walled carbon nanotube (SWNT), multi-walled carbon nanotube (MWNT) and carbon quantum dot were applied in important biological applications. Many examples of biosensors, nanowires and nanoelectronic devices, nanomachine and drug delivery systems are fabricated by these hybrid nanostructures. In this study, a new hybrid nanostructure has been fabricated by noncovalent interactions between single or double stranded DNA and SWNT nanoparticles and biophysical properties of these structures were studied comparatively. Biophysical properties of hybrid nanostructures studied by circular dichroism, UV–vis and fluorescence spectroscopy techniques. Also, electrochemical properties studied by cyclic voltammetry, linear sweep voltammetry, square wave voltammetry, choronoamperometry and impedance spectroscopy (EIS). Results revealed that the biophysical and electrochemical properties of SWNT/DNA hybrid nanostructures were different compare to ss-DNA, ds-DNA and SWNT singly. Circular dichroism results showed that ss-DNA wrapped around the nanotubes through π-π stacking interactions. The results indicated that after adding SWNT to ss-DNA and ds-DNA intensity of CD and UV–vis spectrum peaks were decreased. Electrochemical experiments indicated that the modification of single-walled carbon nanotubes by ss-DNA improves the electron transfer rate of hybrid nanostructures. It was demonstrated SWNT/DNA hybrid nanostructures should be a good electroactive nanostructure that can be used for electrochemical detection or sensing.     

温度、应变率和空位缺陷对氮化硼纳米管压缩性能的影响

采用分子动力学方法,分别模拟了完好的和含有缺陷的氮化硼纳米管的轴向压缩过程。原子间的相互作用采用Tersoff多体势函数来描述。结果表明,同尺寸的锯齿型氮化硼纳米管的临界轴向压缩强度高于扶手型氮化硼纳米管,这与碳纳米管的研究结果一致。发现纳米管的压缩强度,如临界轴向内力在低温下受温度影响明显,并且和应变率的大小有关。然而,应变率对纳米管的弹性变形没有影响。另外,还发现空位缺陷降低了纳米管的力学性能。与完好的纳米管相比,含有缺陷的纳米管轴向压缩强度对于温度的影响并不敏感。     

Molecular understanding of osmosis in semipermeable membranes

We investigate single-file osmosis of water through a semipermeable membrane with an uncharged, a positively and a negatively charged nanopore. Molecular dynamics simulations indicate that the osmotic flux through a negatively charged pore (J_) is higher compared to the osmotic flux in a positively charged pore (J+) followed by the osmotic flux in the uncharged pore (J(0)), i.e., J_ > J+ > J(0). The molecular mechanisms governing osmosis, steady state osmosis, and the observed osmotic flux dependence on the nanopore charge are explained by computing all the molecular interactions involved and identifying the molecular interactions that play an important role during and after osmosis. This study helps in a fundamental understanding of osmosis and in the design of advanced nanoporous membranes for various applications of osmosis.     

Carbon nanotubes: opportunities and challenges

Carbon nanotubes are graphene sheets rolled-up into cylinders with diameters as small as one nanometer. Extensive work carried out worldwide in recent years has revealed the intriguing electrical and mechanical properties of these novel molecular scale wires. It is now well established that carbon nanotubes are ideal model systems for studying the physics in one-dimensional solids and have significant potential as building blocks for various practical nanoscale devices. Nanotubes have been shown to be useful for miniaturized electronic, mechanical, electromechanical, chemical and scanning probe devices and materials for macroscopic composites. Progress in nanotube growth has facilitated the fundamental study and applications of nanotubes. Gaining control over challenging nanotube growth issues is critical to the future advancement of nanotube science and technology, and is being actively pursued by researchers.     

Sequence dependence of DNA translocation through a nanopore

We investigate the dynamics of DNA translocation through a nanopore using 2D Langevin dynamics simulations, focusing on the dependence of the translocation dynamics on the details of DNA sequences. The DNA molecules studied in this work are built from two types of bases A and C, which have been shown previously to have different interactions with the pore. We study DNA with repeating blocks A(n)C(n) for various values of n and find that the translocation time depends strongly on the block length 2n as well as on the orientation of which base enters the pore first. Thus, we demonstrate that the measurement of translocation dynamics of DNA through a nanopore can yield detailed information about its structure. We have also found that the periodicity of the block sequences is contained in the periodicity of the residence time of the individual nucleotides inside the pore.     

Fabrication and Properties of Carbon Nanotube and Poly(vinyl alcohol) Composites

Dispersion of carbon nanotubes in a polymer matrix is one of the most critical issues in carbon nanotube/polymer composites. In this paper we discuss the considerable improvement in the dispersion of multiwalled carbon nanotubes (MWNTs) in poly(vinyl alcohol) (PVA) matrix that was attained through gum arabic treatment. The mechanical properties of these MWNT/PVA composites show that only 2 wt% nanotube loading increases the tensile modulus by more than 130%.     

Simulation of mechanical properties of carbon nanotubes with superlattice structure

The effect of radius and layer thickness on the mechanical properties of carbon nanotubes with ‘zigzag-armchair-zigzag’ superlattice structure (CNTSS) is investigated using molecular dynamics simulation method. The interactions between carbon atoms are modeled using the second-generation reactive empirical bond-order Brenner potential coupled with the Lennard-Jones potential. The results indicate that the Young's modulus of CNTSS shows a significant dependence on its radius and layer thickness. In contrast, the critical stress is insensitive to the layer thickness and radius of CNTSS. And the critical stress of CNTSS is close to that of its thicker carbon nanotubes segment. In addition, the damage modes of CNTSS depend on the connecting region due to the presence of 5–7 defects and the energy early concentrating in the junctions. The effects of the number of junctions on the mechanical properties of CNTSS are also discussed. The results indicate that the joints made in this way still have relatively high mechanical properties corresponding to that of the ideal single-walled carbon nanotube.     

Nanoelectromechanical devices with carbon nanotubes

Thanks to their excellent mechanical properties as well as interesting electrical characteristics, carbon nanotubes are among the most widely used materials for the study of electromechanical properties. This review paper presents the physical properties and the potential applications of carbon nanotube based nanoelectromechanical devices. We present an overview of fabrication methods followed by a discussion of the physical properties of CNT-NEMS. Finally some potential applications are discussed.     

单壁碳纳米管吸附氧分子的电子输运性质理论研究

用基于第一性原理的密度泛函理论和非平衡态格林函数方法对(4,4)单壁碳纳米管及其吸附氧气分子情况下的平衡态和非平衡态电导性质进行了研究. 发现在小于2 V的偏压下,系统对电压的增加呈现两种不同增长速率的电流响应,其中电压小于1.1 V时电流增加速率较大;而当电压大于该值后,电流对电压增加速率变缓. 吸附的氧分子提供双重的作用,一方面氧分子提供的能级有利于电子隧穿中心散射区;另一方面氧分子的电子态会破坏碳管的平移对称性,从而降低电子对系统的透射能力. 关键词： 单壁碳纳米管 氧分子吸附 电子输运 非平衡态格林函数     

Finite element simulation for estimating the mechanical properties of multi-walled carbon nanotubes

A finite element simulation technique for estimating the mechanical properties of multi-walled carbon nanotubes is developed. In the present modeling concept, individual carbon nanotube is simulated as a frame-like structure and the primary bonds between two nearest-neighboring atoms are treated as beam elements, the beam element properties are determined via the concept of energy equivalence between molecular dynamics and structural mechanics. As to the simulation of the interlayer van der Waals force which has intrinsic nonlinearity and complicated applying region, a simplifying method is proposed that the interlayer pressure caused by van der Waals force instead of the force itself is to be considered, and we make use of the linear part of the interlayer pressure near the equilibrium condition to avoid the nonlinearity in problem, then linear spring elements whose stiffness is determined by equivalent force concept can be utilized to simulate the interlayer van der Waals force such that significant modeling and computing effort is saved in performing the finite element analysis. Numerical examples for estimating the mechanical properties of nanotubes, such as axial and radial Young’s modulus, shear modulus, natural frequency, buckling load, etc., are presented to illustrate the accuracy of this simulation technique. By comparing to the results found in the literature and the possible analytical solutions, it shows that the obtained mechanical properties of nanotubes by the present method agree well with their comparable results. In addition, the relations between these mechanical properties and the nanotube size are also discussed.     

First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

Vibrational analysis of double-walled carbon nanotubes with inner and outer nanotubes of different lengths

In this study, the Euler-Bernoulli beam model is used to analyze the resonant vibration of double-walled carbon nanotubes (DWCNTs) with inner and outer nanotubes of different lengths. The resonant properties of DWCNTs with different inner and outer nanotube lengths are investigated in detail using this theoretical approach. The resonant vibration is significantly affected by the vibrational modes of the DWCNTs, and by the lengths of the inner and outer nanotubes. For an inner or outer nanotube of constant length, the vibrational frequencies of the DWCNTs increase initially and then decrease as the length of another nanotube increases. A design for nanoelectromechanical devices that operate at various frequencies can be realized by controlling the length of the inner and outer nanotubes of DWCNTs. This investigation may be helpful in applications of carbon nanotubes such as high frequency oscillators, dynamic mechanical analysis and mechanical sensors.     

Effect of modifiers on mechanical properties of polymer-based composite materials

In this paper, we present the experimental results on the study of mechanical properties of polymer-based nanocomposite materials with carbon nanotube or ultradisperse diamond inclusions. Tests are performed by nanoindentation methods. The results obtained for nanocomposites and a polymer used as a matrix in nanocomposites are compared.     

Synthesis and Properties of Polyacrylamide-Based Conducting Gels with Enhanced Mechanical Strength

A nanocomposite conducting hydrogel, polyacrylamide/MWNT/clay (abbreviated as PAM/MWNT/clay), prepared through in situ free radical aqueous polymerization and crosslinked by both clay, as a functional physical crosslinker, and N,N′-methylenebisacrylamide (MBA) as a chemical crosslinker, is reported. The morphology of the gels was characterized by scanning electron microscopy (SEM). The mechanical properties and electrical conductivity were also studied. The results show that the prepared hydrogels had the expected chemical components, with a highly porous structure, and the gels also showed high mechanical strength. The mechanical strength and electrical conductivity value increased with increasing content of multi-walled nanotube (MWNT), and decreased with increasing content of water.     

Quantitative analysis of electronic properties of carbon nanotubes by scanning probe microscopy: from atomic to mesoscopic length scales

A new multiscale approach to the quantitative interpretation of scanning probe microscopy data in terms of the local electronic properties of 1D systems such as carbon nanotubes is presented. The interactions between a probe and the system are treated using a combination of first-principles density functional calculations and continuum electrostatics modeling. Realistic tip size effects are included using an image charge model. It is shown that the local potential at a nanotube on a substrate due to a probe can be calculated quantitatively, allowing experimental data to be analyzed in terms of the electronic structure of defects.