Response Of Single-walled Carbon Nanotubes Research Articles

Various gradient elasticity theories in predicting vibrational response of single-walled carbon nanotubes with arbitrary boundary conditions

This article deals with the development and use of different gradient beam theories in order to predict the vibrational behavior of single-walled carbon nanotubes (SWCNTs). To address the problem of free vibration, the Euler–Bernoulli and Timoshenko beam theories in conjunction with the gradient elasticity theories including stress, strain and combined strain/inertia are implemented. The generalized differential quadrature method is employed to numerically solve the problem which can treat various boundary conditions. The results generated from the present gradient models are compared with those from molecular dynamics simulations as a benchmark of good accuracy and the proper values of small length scales used in the gradient models are proposed. This study shows prominent differences between various gradient models when the nanotube becomes very short (for aspect ratios of approximately lower than six). It is indicated that applying the strain gradient elasticity by incorporation of inertia gradients yields more reliable results especially for shorter length SWCNTs on account of two small-scale factors related to the inertia and strain gradients. Moreover, since with the reduction in the aspect ratio of nanotubes the effects of boundary conditions become dominant, a discussion is given to investigate the influence of end conditions on the vibrational characteristics of SWCNTs.

Nonlinear optical responses induced by Auger ionization in single-walled carbon nanotubes

We theoretically study the optical response of single-walled carbon nanotubes (SWNTs) under high-intensity laser irradiation. Due to the quasi-one-dimensional structure of SWNTs, the Coulomb interaction between excitons is enhanced relative to the bulk. As a result, excitons can be ionized to electrons and holes by the Auger ionization process under high-intensity laser irradiation. Taking into account the effect of Auger-ionized carriers, we calculate absorption spectra by solving the Bethe–Salpeter equation in the tight-binding approximation. We found that Auger-ionized carriers induce a band-gap renormalization and a bleaching of excitons, leading to a nonlinear optical response. Our calculation reveals the strong influence of Auger-ionized carriers on excitons under high-intensity laser irradiation, which may unravel the recently observed nonlinear behavior of photoluminescence emission spectra.

Nonlocal Continuum Modeling and Molecular Dynamics Simulation of Torsional Vibration of Carbon Nanotubes

This paper investigates the size effects in the dynamic torsional response of single-walled carbon nanotubes (SWCNTs) by developing a modified nonlocal continuum shell model. The purpose is to facilitate the design of devices based on CNT torsion by providing a simple, accurate, and efficient continuum model that can predict the frequency of torsional vibrations and the propagation speed of torsional waves. To this end, dispersion relations of torsional waves are obtained from the proposed nonlocal model and compared to classical models. It is seen that the classical and nonlocal models predict nondispersive and dispersive behavior, respectively. Molecular dynamics simulations of torsional vibrations of (6, 6) and (10, 10) SWCNTs are also performed, the results of which are compared with the classical and nonlocal models and used to extract consistent values of the nonlocal elasticity constant. The superiority and accuracy of the nonlocal elasticity model in predicting the size-dependent dynamic torsional response of SWCNTs are demonstrated.

Small scale effect on vibrational response of single-walled carbon nanotubes with different boundary conditions based on nonlocal beam models

The free vibration response of single-walled carbon nanotubes (SWCNTs) is investigated in this work using various nonlocal beam theories. To this end, the nonlocal elasticity equations of Eringen are incorporated into the various classical beam theories namely as Euler–Bernoulli beam theory (EBT), Timoshenko beam theory (TBT), and Reddy beam theory (RBT) to consider the size-effects on the vibration analysis of SWCNTs. The generalized differential quadrature (GDQ) method is employed to discretize the governing differential equations of each nonlocal beam theory corresponding to four commonly used boundary conditions. Then molecular dynamics (MD) simulation is implemented to obtain fundamental frequencies of nanotubes with different chiralities and values of aspect ratio to compare them with the results obtained by the nonlocal beam models. Through the fitting of the two series of numerical results, appropriate values of nonlocal parameter are derived relevant to each type of chirality, nonlocal beam model, and boundary conditions. It is found that in contrast to the chirality, the type of nonlocal beam model and boundary conditions make difference between the calibrated values of nonlocal parameter corresponding to each one.

A study of carbon-nanotube-based nanoelectromechanical resonators tuned by shear strain

This paper has investigated, using a classical molecular dynamics simulation method based on the Tersoff–Brenner potential, the resonance-frequency changes of single-walled carbon-nanotube resonators originating from the purely mechanical response of single-walled carbon nanotubes. The tension decreased with increasing rotation angle, so the resonance frequencies could be changed by controlling the rotation angles of the single-walled carbon nanotubes. The resonance frequencies decreased with increasing angle, and when the rotation angle was greater than 60°, the changes were marked. For nanotubes of similar length, the bandwidth for the (3, 3) single-walled carbon nanotube was higher than for the (5, 0) single-walled carbon nanotube. Because properties arising from the shear-strain-induced tension response can affect the electromechanical behavior of carbon nanotubes, the shear-strain-induced tension response should be given serious consideration in the application of embedded carbon nanotubes in nanoelectromechanical systems.

Total ionizing dose-hardened carbon nanotube thin-film transistors with silicon oxynitride gate dielectrics

We investigate the radiation response of single-walled carbon nanotube (SWCNT) thin-film transistors fabricated with 23 nm silicon oxynitride gate dielectric layers, for total ionizing doses (TIDs) of Co-60 gamma irradiation up to 2 Mrad(Si). Irradiations with ±1 MV/cm across the gate dielectric have little effect on the threshold voltage, yielding shifts of less than ±0.25 V and no detrimental effect on SWCNT mobility or maximum drain current. This illustrates the need to consider the total device material composition when investigating the radiation response of carbon nanoelectronics and substantiates the applicability of SWCNT-based nanoelectronics for use in high TID environments.

Magnetic Response of Single-Walled Carbon Nanotubes Induced by an External Magnetic Field

Using first-principles density functional calculations, magnetically induced currents are obtained for zigzag single-walled carbon nanotubes. Clear differences and trends in current flow are observed between the different nanotube families. In particular, for a magnetic field applied along the tube axis, the current response of the λ = 0 infinite nanotubes is paramagnetic, whereas for λ = 1 and 2 nanotubes, the response is diamagnetic. The results are used to predict and interpret the significant changes in NMR properties for small molecules encapsulated inside a tube.

Radiation Effects in Single-Walled Carbon Nanotube Thin-Film-Transistors

The fabrication, characterization, and radiation response of single-walled carbon nanotube (SWCNT) thin-film field effect transistors (SWCNT-TFTs) has been performed. SWCNT-TFTs were fabricated on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -Si substrates from 98% pure semiconducting SWCNTs separated by density gradient ultracentrifugation. Optical and Raman characterization, in concert with measured drain current I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ratios, up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , confirmed the high enrichment of semiconducting-SWCNTs. Total ionizing dose (TID) effects, up to 10 MRads, were measured in situ for a SWCNT-TFT under static vacuum. The results revealed a lateral translation of the SWCNT-TFT transfer characteristics to negative gate bias resulting from hole trapping within the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -SWCNT interface. Additional TID exposure conducted in air on the same device had the opposite effect, shifting the transfer characteristics to higher gate voltage, and increasing the channel conductance. No significant change was observed in the device mobility or the SWCNT Raman spectra following a TID exposure of 10 Mrad(Si), indicating extrinsic factors dominate the transfer characteristics in the SWCNT-TFT devices during irradiation. The extrinsic effects of charge trapping and the role that gas adsorption plays in the radiation response are discussed.

A resonance Raman study of carboxyl induced defects in single-walled carbon nanotubes

Changes in the vibrational response of single-walled carbon nanotubes (SWCNTs) resulting from the introduction of structural defects on their body were studied using resonance Raman spectroscopy. Structural defects were introduced on the SWCNTs by subjecting them to carboxylation for different intervals of time. Various Raman modes were observed, including the D, G +/G −, G' modes, and new defect induced modes were identified. Weaker vibrational modes corresponding to Stone–Wales defects with specific structures known as Haeckelites were identified. These modes were compared with the theoretically calculated modes and correlated to O 5,6,7 and R 5,7 Haeckelite structures.

Terahertz Spectroscopy of Single-Walled Carbon Nanotubes in a Polymer Film: Observation of Low-Frequency Phonons

We investigate the dielectric response of single-walled carbon nanotubes dispersed in poly(vinyl alcohol) matrix by using terahertz time domain spectroscopy. Frequency-dependent real and imaginary parts of the complex dielectric function are measured experimentally in the terahertz regime. The low-frequency phonons of carbon nanotubes, though predicted theoretically, are directly observed for the first time at frequencies 0.26, 0.60, and 0.85 THz. Further, a broad resonance is observed at 1.15 THz associated with the longitudinal acoustic mode of vibration of straight-chain segments of the long polymeric molecules in the film. The latter is observed at 1.24 THz for a pristine polymer film and has been used to derive the size of crystalline lamellae in the film.

Detection of amino acids encapsulation and adsorption with dielectric carbon nanotube

Calculations of the interaction energy and dielectric responses of single-walled carbon nanotubes to the presence of amino acid inside (encapsulation) or outside (adsorption) the nanotube are carried out. It is shown that the frequency shifts of selected nanotubes conveniently tailored to the size of the probed molecules and used in a resonator configuration can selectively detect the position (encapsulated or adsorbed in the tube) of different species. We demonstrate in particular that adsorption outside the nanotube is better detected than encapsulation whatever the amino acid tested. Without explicitly dealing with drug delivery, these calculations represent the first step toward the detection of amino acids encapsulation in potential drug vector.

Carbon Nanotube Terahertz Polarizer

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

The effects of UV/ozone treatments on the electrical transport behavior of single-walled carbon nanotube arrays

We investigated the electrical transport response of single-walled carbon nanotube (SWNT) arrays exposed to ultraviolet (UV) and ozone (O 3). Pure UV irradiation induced an increase in resistance due to molecular photodesorption on the nanotubes. The effect of the O 3 treatment on the resistance of SWNT arrays depended on the degree of O 3 due to different dominant mechanisms, including the transduction of molecular adsorption and O 3 doping on nanotubes as well as the creation of defects. The results suggest that the detailed control of UV/O 3 exposure could be useful in manipulating electrical devices using SWNT arrays.

Optical Absorption Spectra of Charge-Doped Single-Walled Carbon Nanotubes from First-Principles Calculations

We calculated the optical absorption spectrum response of single-walled carbon nanotubes under charge doping by using density functional theory. We find that the spectrum responses can be generally divided into two categories: one is similar to those obtained from the graphene zone-folding and rigid-band model, while the other deviates from the expectation and shows several special features. Our analysis reveals that the doping type and curvature effects play the primary role. Finally, we argue that the present results will probably prevail in more elaborate methods and other similar nanotubes.

Electron−Hole Interaction in Carbon Nanotubes: Novel Screening and Exciton Excitation Spectra

The optical response of single-walled carbon nanotubes is dominated by exciton states with unusually large binding energies. We show that screening in semiconducting tubes enhances rather than reduces the electron-hole interaction for separations larger than the tube diameter. This "antiscreening" region deepens the relative energy level of the higher exciton states yielding unconventional excitation spectra. The effect explains the discrepancy in the current experimentally extrapolated exciton binding energies (deduced using conventional model spectra) and those obtained from ab initio calculations on isolated tubes.

Dislocation onset and nearly axial glide in carbon nanotubes under torsion

The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,-1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps.

Universal Response of Single-Wall Carbon Nanotubes to Radial Compression

The mechanical response of single-wall carbon nanotubes to radial compression is investigated via atomic force microscopy (AFM). We find that the force F applied by an AFM tip (with radius R) onto a nanotube (with diameter d), rescaled through the quantity Fd;{3/2}(2R);{-1/2}, falls into a universal curve as a function of the compressive strain. Such universality is reproduced analytically in a model where the graphene bending modulus is the only fitting parameter. The application of this model to the radial Young's modulus E_{r} leads to a further universal-type behavior which explains the large variations of nanotube E_{r} reported in the literature.

Dynamical response of the non-linear vibration of single-walled carbon nanotubes

Dynamical response in single-walled carbon nanotubes (SWNTs) has been investigated through analytical and computational techniques. Analytically, an explicit expression for a formal control term has been estimated by using the Lie transforms. Computational studies reveal the suppression of chaos, which is in good agreement with the analytical investigations. And, our system follows regular patterns after the inclusion of control term.

High-pressure Raman response of single-walled carbon nanotubes: Effect of the excitation laser energy

We report high-pressure Raman experiments on the tangential vibrational modes of CarboLex bundled single-walled carbon nanotubes up to 6.5 GPa using two different excitation energies: 1.96 and 2.41 eV. We show through the curve-fitting technique, together with the modified interband transition energies versus diameter plot, how the nature of the resonant tubes is modified under the excitation energy, in particular under the 1.96 eV excitation. Having metallic and semiconducting tubes in resonance at ambient pressure, we find that only semiconducting tubes are in resonance at 3.5 GPa. We associate this loss of resonance from the metallic tubes to a redshift pressure response of the first $({E}_{11})$ transition energies from these tubes. Added to that, the change in the excitation energies leads to a change in the value of the transition pressure. This is simply associated with the fact of having different diameters in resonance under each excitation from the same sample.

Junction-Controlled Elasticity of Single-Walled Carbon Nanotube Dispersions in Acrylic Copolymer Gels and Solutions

Oscillatory shear rheometry is used to study the mechanical response of single-walled carbon nanotubes dispersed in solutions of acrylic diblock or triblock copolymers in 2-ethyl-1-hexanol. Thermal transitions in the copolymer solutions provide a route for the easy processing of these composite materials, with excellent dispersion of the nanotubes as verified by near-infrared photoluminescence spectroscopy. The nanotube dispersions form elastic networks with properties that are controlled by the junction points between nanotubes, featuring a temperature-dependent elastic response that is controlled by the dynamic properties of the matrix copolymer solution. The data are consistent with the formation of micelle-like aggregates around the nanotubes. At low temperatures the core-forming poly(methyl methacrylate) blocks are glassy, and the overall mechanical response of the composite does not evolve with time. At higher temperatures the enhanced mobility of the core-forming blocks enables the junctions to ach...