Bent Nanotubes Research Articles

Influence of curvature on the dynamical susceptibility of bent nanotubes

In this work we analyze the influence of curvature on the dynamic properties of the magnetization in bent nanotubes. Our results show that both the resonance frequencies and the number of resonant peaks have a strong dependence on the ground state of the magnetization created by the curvature. Our results can be understood from the analysis of the effective anisotropy and the Dzyaloshinskii–Moriya interaction (DMI) induced by curvature. The ability to control the dynamic properties of these curved ferromagnets makes them excellent candidates for developing applications based on resonant modes of spin waves.

Analytical study of symmetry breaking and frequency mixing effects in the oscillatory dynamics of fluids within bent nanotubes

Analytical study of symmetry breaking and frequency mixing effects in the oscillatory dynamics of fluids within bent nanotubes

Influence of Curvature on the Dynamical Susceptibility of Bent Nanotubes

Influence of Curvature on the Dynamical Susceptibility of Bent Nanotubes

Magnetic ground states for bent nanotubes

Magnetic nanotubes have been widely studied because they are promising candidates to be part of devices based on spintronic and magnonic technologies. However, the experimental techniques used to prepare such structures cannot guarantee the generation of perfect elements, and some geometric imperfections can appear. In this direction, the existence of a bent in a nanotube could change the magnetic properties of a device. In this work, we analyze the influence of curvature on the magnetic properties of a bent nanotube. Our results show a strong dependence of the magnetization ground state on the curvature, the thickness, and the length of the bent nanotubes. Also, we obtained the critical thickness where the transition from the so-called in-surface to the vortex state occurs. For thick tubes, we observe the in-surface state, and for thin structures, the vortex configuration is the lowest energy state. These results could be useful for the development of magnetic devices with bent nanotubes.

Phenomenological constitutive model for a CNT turf

Carbon nanotubes (CNT), grown on a substrate, form a turf – a complex structure of intertwined, mostly nominally vertical tubes, cross-linked by adhesive contact and few bracing tubes. The turfs are compliant and good thermal and electrical conductors. In this paper, we consider the micromechanical analysis of the turf deformation reported earlier, and develop a phenomenological constitutive model of the turf. We benchmark the developed model using a finite element implementation and compare the model predictions to the results two different nanoindentation tests.The model includes: nonlinear elastic deformation, small Kelvin–Voigt type relaxation, caused by the thermally activated sliding of contacts, and adhesive contact between the turf and the indenter. The pre-existing (locked-in) strain energy of bent nanotubes produces a high initial tangent modulus, followed by an order of magnitude decrease in the tangent modulus with increasing deformation. The strong adhesion between the turf and indenter tip is due to the van der Waals interactions.The finite element simulations capture the results from the nanoindentation experiments, including the loading, unloading, viscoelastic relaxation during hold, and adhesive pull-off.

Valley–spin blockade and spin resonance in carbon nanotubes

The manipulation and readout of spin qubits in quantum dots have been successfully achieved using Pauli blockade, which forbids transitions between spin-triplet and spin-singlet states. Compared with spin qubits realized in III-V materials, group IV materials such as silicon and carbon are attractive for this application because of their low decoherence rates (nuclei with zero spins). However, valley degeneracies in the electronic band structure of these materials combined with Coulomb interactions reduce the energy difference between the blocked and unblocked states, significantly weakening the selection rules for Pauli blockade. Recent demonstrations of spin qubits in silicon devices have required strain and spatial confinement to lift the valley degeneracy. In carbon nanotubes, Pauli blockade can be observed by lifting valley degeneracy through disorder, but this makes the confinement potential difficult to control. To achieve Pauli blockade in low-disorder nanotubes, quantum dots have to be made ultrasmall, which is incompatible with conventional fabrication methods. Here, we exploit the bandgap of low-disorder nanotubes to demonstrate robust Pauli blockade based on both valley and spin selection rules. We use a novel stamping technique to create a bent nanotube, in which single-electron spin resonance is detected using the blockade. Our results indicate the feasibility of valley-spin qubits in carbon nanotubes.

Using bent carbon nanotubes for the fabrication of electromechanical switches

An electromechanical switch based on bent carbon nanotubes was fabricated. The shape and structure of the bent carbon nanotubes allows one to produce a low cost and low working voltage switch. The fabrication process is free of any nanolithography. The electrical characteristics of the fabricated device were investigated, both experimentally and theoretically. Actuation of the fabricated device shows hysteresis behavior in the measured I–V curves depending on the structural parameters of the bent nanotubes. The relationship between the pull-in voltage and the morphology of the bent nanotubes was studied by the obtained hysteresis curves. A scanning electron microscope was used for structural analysis. This study introduced an easy way to fabricate electromechanical switches with controllable on/off states.

Temperature-dependent thermal conductivity of bent carbon nanotubes by molecular dynamics simulation

Molecular dynamics simulations were performed to evaluate temperature-dependent thermal conductivity of bent carbon nanotubes. Thermal conductivities of bent nanotubes are predicted to be smaller than those of straight nanotubes. This is due to the suppression of high frequency phonons from the density of states calculations. It was found that for the defect-free bent nanotubes, the ratio of thermal conductivity of bent nanotubes to that of the straight ones are temperature and diameter independent, while significantly relies on the bent characteristic size. The more is the nanotube bent, the smaller is thermal conductivity obtained. For the larger nanotubes, the buckled defects were observed after bending and the ratio decrease rapidly. The ratios of thermal conductivity of the buckled nanotubes to that of the straight ones increase with the increasing temperatures until a maximum is obtained.

Thermal fluorination effects on carbon nanotubes for preparation of a high-performance gas sensor

A high-performance NO gas sensor was prepared by inducing thermal fluorination of carbon nanotube semiconductors. Thermal fluorination of multi-walled carbon nanotubes (MWCNTs) was carried out at various temperatures (100 ∼ 1000 °C) to investigate the effects of the reaction temperature. The mechanism of high-performance NO gas sensor electrode was shown to depend on the fluorination temperature in a way that can be divided into three regions, separated at 400 and 1000 °C. In the first temperature region, the induction of fluorine functional groups onto MWCNTs showed the opposite trend in electrical resistance change comparing with traditional p-type MWCNTs. In the second temperature region, the induced fluorine functional groups were attenuated by generated fluorinated carbon gases resulting in the decomposition of MWCNTs and the recovery of traditional p-type gas sensor behavior. In the highest temperature region above 1000 °C, reoriented carbon structure was observed, showing bent nanotubes produced from destruction by fluorination and subsequent reorientation due to the high temperature. The gas sensing responsiveness was significantly improved by the thermal fluorination, which causes electrophilic attraction, creates adsorption sites for target NO gases and improve hydrophobicity for gas sensing stability in humid condition. In conclusion, a high-performance gas sensor was obtained by thermal-fluorination of MWCNTs.

Effects of magnetic and electric fields on the growth of carbon nanotubes using plasma enhanced chemical vapor deposition technique

Multiwalled carbon nanotubes are grown on nickel-seeded silicon substrates using plasma-enhanced chemical vapor deposition method at a temperature of 650 ° C utilizing a mixture of acetylene and hydrogen. Magnetic and electric fields were used to obtain well-oriented carbon nanotubes. The direction of growth was found to strongly depend on the directions and magnitudes of the applied fields. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) have been used to investigate the grown nanotubes. The SEM and TEM images of as grown nanotubes show that applying magnetic field during the growth process affects the growth direction of the nanotubes and, furthermore, bent nanotubes can be achieved by changing the direction of the applied electric field alone. Raman spectroscopy has been used to analyze the structure of the samples.

Mechanical behavior of a carbon nanotube turf

Carbon nanotubes grown on a substrate form a turf – a complex structure of intertwined nanotubes cross-linked by adhesive contact. We analyze the physical mechanism of deformation on the basis of: (i) standard and continuous stiffness nanoindentation; and (ii) micromechanical scaling analysis. At moderate strains, deformation is fully reversible, comprising small viscoelastic relaxation caused by the thermally activated sliding of contacts. The pre-existing strain energy of bent nanotubes produces a high initial tangent modulus, followed by an order of magnitude decrease in the tangent modulus.

Electrostatics of straight and bent single-walled carbon nanotubes

Response of a single-walled carbon nanotube to external electric field, F, is calculated analytically within the classical electrostatics. Field-induced charge density distribution is approximately linear along the axis of metallic nanotube and depends rather weakly, as ln(h/r), on the nanotube length, h, (here r is the nanotube radius). In a semiconducting nanotube with a gap, E_g, charge separation occurs as F exceeds the threshold value F_{th}=E_g/eh. For F>F_{th}, positively and negatively charged regions at the ends of nanotube are separated by a neutral strip in the middle. For bent nanotubes the number of neutral strips can be one or two depending on the direction of F.

In situ characterization of the field-emission behaviour of individual carbon nanotubes

The current–voltage (I–V) characteristics of carbon nanotubes (CNTs) during field emission were investigated byin situ imaging and field-emission (FE) measurement inside a field-emission scanningelectron microscope (FE-SEM). A primary electron of the FE-SEM could induce andenhance a large stimulated electron emission from a CNT which might be due to the stronglocal field on the tip of the CNT in the presence of an applied voltage. FE of bentnanotubes (BNTs) can initially occur after they are fully straightened or it can start in thebent state (during geometrical straightening) as the applied field increases. The FE from asingle CNT follows FN (Fowler–Nordheim) behaviour with a single linear slope inthe FN plot. The FE from two nanotubes with a geometrical change during FEshowed transition of the FN slope from the low voltage to the high voltage region,which could be due to interactions between two dynamic neighbouring BNTs.

Production of carbon nanotubes by self-propagating high-temperature synthesis

Carbon nanotubes are prepared by the method of self-propagating high-temperature synthesis for the first time. The initial components for this synthesis are carboniferous materials (soda, limestone, and Teflon) and reducers (magnesium, lithium, and sodium) with addition of a nickel or iron catalyst. The morphology of the nanotubes (straight multiwall nanotubes apparently free of a catalyst, bent nanotubes completely filled with a catalyst, and carbon nanofibers) is similar to that of nanotubes grown by chemical methods. The nanotubes account for 2–4 wt % of the product synthesized.

High-resolution Raman microscopy of curled carbon nanotubes

Patterned carbon nanotube assemblies with bent nanotube bundles were investigated with combined atomic force microscopy and confocal Raman imaging spectroscopy to identify conditions of carbon nanotubes in the bent state. We showed that the tangential G mode on Raman spectra systematically shifts downward upon nanotube bending as was predicted earlier. This lower frequency shift is attributed to the tensile stress, which results in the loosening of C–C bonds in the outer nanotube walls.

A non-linear analysis of the bending modulus of carbon nanotubes with rippling deformations

Recently, some experimental techniques involving bending of cantilevered nanotubes have been used to deduce their elastic bending modulus. Using these methods it was found that the value of elastic bending modulus decreases dramatically from about 1 TPa for D less than 10 nm to 100 GPa for D larger than 20 nm. This discovery defines usual belief that the bending modulus is a material’s intrinsic property, independent of the size of the nanotubes. This paper concerned with the non-linear moment–curvature relationship during bending of nanotubes, and the effect of ripple formation on the bending modulus. By using an advanced finite element analysis package, ABAQUS, a non-linear bending moment–curvature relationship of carbon nanotubes, which is the appearance of a rippling mode on the inner arc of the bent nanotubes, is simulated from the three-dimensional orthotropic theory of finite elasticity deformation. Utilizing the non-linear bending moment–curvature relationship, and a non-linear vibration analysis method, we capture that the rippling deformation can indeed result in effective bending modulus of carbon nanotubes decrease substantially with increasing diameter. The result carried out may be used to explain some experiment phenomena, and also guides further experiment investigating the bending behaviors of carbon nanotubes.

Electronic spectrum and ballistic transport in bent nanotubes

It is shown that bending of a nanotube leads to splitting of the electron energy levels due to breaking of the azimuthal symmetry. The bent section of the nanotube acts as a scatterer for ballistic carriers, resulting in qualitative changes in the dependence of conductance on the Fermi energy.

Effective bending modulus of carbon nanotubes with rippling deformation

This paper concerned with the relation of the bending moment to the bending curvature during bending of carbon nanotubes, and the relation between the rippling formation and the bending modulus. Based on the three-dimensional orthotropic theory of finite elasticity deformation, a non-linear bending moment–curvature relationship of carbon nanotubes which is the appearance of wavelike distortion on the inner arc of the bent nanotubes is simulated by using an advanced finite element analysis package, ABAQUS. Utilizing the non-linear bending moment–curvature relationship, the effective bending modulus of carbon nanotubes with different cross-sections are obtained by means of a bi-linear theory and a simplified vibration analysis method. The effective bending modulus of carbon nanotubes simulated in the paper is close to the measuring result presented in reference [Science 283 (1999) 1513].

Numerical simulation for bending modulus of carbon nanotubes and some explanations for experiment

Abstract The bending mechanical property of carbon nanotubes are numerically investigated in this paper. An advanced finite element analysis package, ABAQUS, is used to simulate the formation of rippling which is the appearance of wavelike distortion on the inner arc of the bent nanotubes, caused by the severe anisotropy of carbon nanotubes and a relatively large deformation. A non-linear bending moment–curvature relationship is obtained, which shows the tangential stiffness greatly decreases when rippling appears. This result can be used to explain the phenomenon and conclusion of the resonant experiment measuring the Young's modulus of carbon nanotubes, in which the Young's modulus calculated using linear theory is found to sharply decrease as the diameter increases [Science 283 (1999) 1513]. Here an analytical method is adopted to conduct a vibration analysis using a bi-linear bending constitution simplifying from the non-linear bending moment–curvature relationship, and the effective Young's modulus have been calculated for multi-walled carbon nanotubes of various sizes. The result carried out in the paper is similar to the measuring result is given by Poncharal et al. [Science 283 (1999) 1513].

Effect of bending instabilities on the measurements of mechanical properties of multiwalled carbon nanotubes

It has been reported that a multiwalled carbon nanotube in bending may exhibit an unusal elastic mode that corresponds to the wavelike distortion or ripple along the inner arc of the bent nanotube, called the rippling mode, which cannot be predicted by the linear theories. The present analysis shows that the rippling mode is permissible by the theory for highly anisotropic elastic materials of finite deformation and that the dependence of the bending moment upon the bending curvature can be well approximated by a bilinear relation, in which the transition from one linear branch to the other corresponds to the emergence of the rippling mode. With this bilinear relation, the authors show that the deflection response to a transverse force is consistent with the unusual behavior reported by Wong, Sheehan, and Liebert [Science 277, 1971 (1997)]. Furthermore, their analysis indicates that there exists a critical diameter, for given load and length, which corresponds to the emergence of rippling mode, and that the effective Young's modulus of multiwalled carbon nanotubes at the vibration resonance drops sharply as the diameter increases to surpass this critical value, confirming a phenomenon observed by Poncharal, Wang, Ugarte, and de Heer [Science 283, 1513 (1999)].