Intertube Coupling Research Articles

Thermal Rectification Modulation of Parallel Multiple Carbon/Boron Nitride Heteronanotubes.

Thermal rectification (TR) efficiency has always been an important concern for thermal rectifiers. However, in practical terms, the amount of heat conduction is equally not negligible. To get high values on both of them, the carbon nanotube arrays with high thermal conductivity and large heat conduction areas were considered, along with carbon/boron nitride heteronanotubes (CBNNTs) with excellent TR property. In our work, multiple CBNNT models are constructed, and the TR ratio under different conditions is investigated using nonequilibrium molecular dynamics, with double CBNNTs (D-CBNNTs) aligned in parallel as the main analytical object. It is shown that weakening the intertube coupling is an available way to enhance the TR ratio, and adjusting the heteronanotube length and spacing can also effectively regulate the TR. In the process of changing the coupling coefficient, we analyzed both phonon changes and atomic vibrations and got a good correspondence, and the BN region is more variable in D-CBNNTs. In addition, the covariation of phonon localization and intertube phonon exchange with the coupling coefficient results in an invariant backward heat flux in the D-CBNNT. Furthermore, by adjusting the carbon proportion and lowering the coupling coefficient in the model, an excellent TR ratio in four CBNNTs was obtained and its heat flux is even larger than the value at a carbon percentage of 50% in larger coupling. We fully utilized the phonon density of states, phonon participation rate, and mean square displacement. Our results demonstrate the possibility of multiple CBNNTs as thermal rectifiers and provide theoretical guidance for heteronanotube arrays to be applied.

Curvature-controlled band alignment transition in 1D van der Waals heterostructures

The effect of curvature on the band alignment of one-dimensional (1D) van der Waals (vdW) transition metal dichalcogenide (TMDC) heterostructures is studied by comprehensive first-principles calculations. We find that, as the diameter of a TMDC nanotube decreases, the combined effect of curvature-induced flexoelectricity and circumferential tensile strain causes a rapid lowering of the conduction band minimum, whereas the valence band maximum exhibits an initial lowering before rising. As individual TMDC nanotubes form coaxial heterostructures, the concerted effect of diameter-dependent band-edge levels and intertube coupling via flexovoltage can result in a transition of intertube band alignment from Type II to Type I in multiple heterostructural systems, including large-diameter MoSe2@WS2, MoTe2@MoSe2, and MoTe2@WS2 heterostructures. These results lay down a foundation for the rational design of 1D vdW heterostructures.

ZnO nanoparticles functionalized SWCNTs as highly sensitive SERS substrate for heavy metal ions detection

This report presents the most effective and novel approach to fabricate a three-dimensional (3D) Surface-Enhanced Raman Scattering (SERS) substrate for detection of heavy metal ions in the environment. Firstly, Single Wall Carbon Nanotubes (SWCNTs) are synthesized by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique at low temperature. Then, the surface of SWCNTs is functionalized by Zinc Oxide nanoparticles (ZnO NPs) using thermal evaporation technique to achieve ZnO@SWCNTs nanocomposite film as a SERS substrate. The ZnO@SWCNTs with 3D nanostructure increases the coverage of plasmonic nanostructure. The effect of intertube and interparticle coupling generates the multitude ‘hot spots’ with a strong SERS effect. Different characterization techniques- Field Emission Scanning Electron Microscope (FESEM), X-Ray Diffraction (XRD), UV–Vis and Raman spectroscopy were performed. For detection of heavy metal ions, the Raman spectra are recorded for samples of different metal ions- Pb2+, Hg2+, Cd2+, Cu2+, and As2+. The proposed substrate demonstrates high sensitivity, selectivity and very low Limit of detection of the order of 0.225 nM with high stability and repeatability for Pb2+. Therefore, the proposed ZnO@SWCNTs nanocomposite SERS substrate can be used as a powerful tool for the detection of heavy metal ion- Pb2+ in aqueous solutions for a green environment.

Neutron Scattering in Nanotubes

Abstract: Neutron scattering has got numerous applications in various fields of research like material research, crystallography etc. Thermal neutron scattering in randomly un-aligned multi walled carbon nanotubes is based on its anisotropic frequency distribution function. This frequency distribution function is obtained using a dynamical model which includes the presence of both the surface modes and intertube coupling. Experimentally measured values of specific heat for randomly un-aligned multi walled carbon nanotubes has been studied to detect the phonon frequency distribution function. Elastic scattering is used for analyzing structures. Inelastic scattering is applied for the study of atomic vibrations. The dynamical model has been used to find the scattering values. Both scattering values are calculated and compared. It was concluded that the difference in their values is large. This difference would affect the transport processes in randomly un-aligned multi walled carbon nanotubes Keywords: Specific heat, Elastic scattering, Inelastic scattering, Neutron Diffraction, Frequency Distribution Function

Thermal Neutron Scattering of Un-Aligned Multi-walled Carbon Nanotubes

Abstract: Neutron scattering is the scattering of free neutrons by matter. This process is used for the investigation of materials. Neutron scattering is an experimental technique which is applied in various areas of physics, physical chemistry, biophysics, crystallography and materials research. Neutron diffraction (elastic scattering) is used for determination of structures of materials. In this paper we attempted to study thermal neutron scattering in randomly un-aligned multi walled carbon nanotubes making use of an anisotropic dynamical model. This model includes the presence of both the surface modes and intertube coupling. Comparison of scattering cross section of un-aligned multiwalled carbon nanotubes has been done with fullerene and graphite. It was concluded that there is a significant difference between the values of scattering cross section for randomly un-aligned multiwalled carbon nanotubes and fullerene at higher values of energy. Keywords: Carbon nanotubes, Elastic scattering, Neutron Diffraction, Frequency distribution function, Specific heat

Modulation of intertube band dispersion relation of carbon nanotube bundles by symmetry and intertube wave function coupling

Based on density functional theory with a local density approximation and the simple tight-binding approximation, we investigated an electronic structure of carbon nanotube bundles in terms of mutual nanotube arrangement. The dispersion relation near the Fermi level along the intertube direction was found to be sensitive to the nanotube species and their mutual orientation within the bundles. Nanotube bundles with three-fold symmetry exhibited a substantial orientation dependence in the band dispersion relation along the intertube direction. The tight-binding calculation and wave function analysis revealed that this orientation dependence arises from the intertube wave function coupling whether a node exists between nanotubes.

Controlled exciton–plasmon coupling in a mixture of ultrathin periodically aligned single-wall carbon nanotube arrays

We study theoretically the in-plane electromagnetic response and the exciton–plasmon interactions for an experimentally feasible carbon nanotube (CN) film system composed of parallel aligned periodic semiconducting CN arrays embedded in an ultrathin finite-thickness dielectric. For homogeneous single-CN films, the intertube coupling and thermal broadening bring the exciton and interband plasmon resonances closer together. They can even overlap due to the inhomogeneous broadening for films composed of array mixtures with a slight CN diameter distribution. In such systems, the real part of the response function is negative for a broad range of energies (negative refraction band), and the CN film behaves as a hyperbolic metamaterial. We also show that for a properly fabricated two-component CN film, by varying the relative weights of the two constituent CN array components, one can tune the optical absorption profile to make the film transmit or absorb light in the neighborhood of an exciton absorption resonance on-demand.

(Invited) Intertube Coupling in Double-Walled Carbon Nanotubes Beyond Mechanical Interaction

Double walled carbon nanotubes (DWCNTs) are an intriguing example of a one-dimensional Moiré system, because they differ in diameter and chirality of the inner and outer tubes. For many years, DWCNTs have been described as two weakly coupled single walled carbon nanotubes (SWCNTs) without much electronic interaction. Advances in theoretical modeling, however, predicted diverse coupling possibilities when considering the electronic interactions between twisted layers of low dimensional materials. This coupling can induce the formation flat bands and strongly perturb the electronic states. Experimentally, this should manifest in a reduced bandgap in DWCNTs and a red-shift of the optical transition energies. Chasing this coupling effect, we used resonant Raman spectroscopy to measure the transition energies of semiconducting inner tubes in DWCNTs. The transition energies are indeed reduced, with varying red-shifts for the same inner wall chirality (50 and 150 meV). The shift depends on the electronic type of the host tube that we assign from based on the Radial breathing mode frequencies. The coupling increases from weak to strong by tightening intratube spacing in agreement with theoretical predictions. We discuss our observations in light of joint and localized vibrational and optical excitations in the two tubes making up the DWCNT.

Quantitative analysis of the intertube coupling effect on the photoluminescence characteristics of distinct (n, m) carbon nanotubes dispersed in solution

In this work, we quantitatively studied the intertube coupling of different (n, m)-sorted semiconducting single-wall carbon nanotubes (SWCNTs) on their photoluminescence (PL) efficiencies by precisely tuning the ratio of (9, 4) and (6, 5) SWCNTs in the mixture. A significant decrease in the PL intensity of (9, 4) SWCNTs was observed after mixing with (6, 5) species when fixing the (9, 4) concentration, which was confirmed to be caused by the absorption of incident photons and reabsorption of the emitted photons by the added (6, 5) species. By contrast, a similar decrease in the PL intensity of (6, 5) SWCNTs was also observed after mixing with the larger-diameter (9, 4) species. Different from that of (9, 4) SWCNTs, the PL decrease of (6, 5) SWCNTs was found to originate not only from photon absorption and reabsorption by the (9, 4) species but also from one-way exciton energy transfer (EET) from the (6, 5) SWCNTs to the larger-diameter (9, 4) SWCNTs. Both the experimental results and numerical simulations further demonstrated that increasing the concentration of mixed (9, 4) SWCNTs would enhance the effects of photon absorption and reabsorption and EET on the PL intensity of (6, 5) SWCNTs quantified by the decrease ratio of the (6, 5) PL intensity. Meanwhile, the influence of EET was found to be always weaker than that of photon absorption and reabsorption. We proposed that the observed EET between isolated SWCNTs in a surfactant solution is derived from their proximity due to Brownian motion.

Observation of Drastic Electronic-Structure Change in a One-Dimensional Moiré Superlattice.

We report the first experimental observation of a strong-coupling effect in a one-dimensional moiré superlattice. We study one-dimensional double-wall carbon nanotubes (DWCNTs) in which van der Waals-coupled two single nanotubes form a one-dimensional moiré superlattice. We experimentally combine Rayleigh scattering spectroscopy and electron beam diffraction on the same individual DWCNTs to probe the optical transitions of the structure-identified DWCNTs in the visible spectral range. Among more than 30 structure-identified DWCNTs examined, we experimentally observed and identified a drastic change of the optical transition spectrum in a DWCNT with chirality (12,11)@(17,16). The origin of the marked change is attributed to the strong intertube coupling effect in the moiré superlattice formed by two nearly armchair nanotubes. Our numerical simulation is consistent with the experimental findings.

Robust carbon nanotube composite fibers: Strong resistivities to protonation, oxidation, and ultrasonication

Carbon nanotube (CNT) fibers have great potential in the field of high performance fibers. However, poor inter-tube coupling between bundles resulting in low structural and mechanical stability under strong acid, ultrasonication and high-temperature oxidation limited the practical applications of CNT fibers in extreme environment. Here we report the preparation of robust carbon nanotube/carbon (CNT/C) composite fibers with highly aligned and dense structure utilizing an ultrafast Joule heating tension-annealing approach. CNT fibers prepared by floating catalytic chemical vapor deposition were infiltrated by polyacrylonitrile (PAN) solution, followed by programable tension annealing in argon for 10 s. Such a short process carbonized infiltrated PAN, resulting in the formation of CNT/C fibers within which CNTs were bonded by pyrolytic carbon. Due to the carbon-bonded structure, the composite fibers exhibited improved resistivity against structure damages when exposed to strong acid, ultrasonication, and high-temperature oxidation. Comparing with the pristine CNT fibers, such composite fibers showed 320% improvement in breaking load, 354% increase in strength (2.3 GPa) and 667% increase in modulus (60 GPa), respectively. Moreover, such composite fibers have low densities (1.48 g/cm3) and excellent flexibility and toughness. These combined features could broaden the application of the CNT/C composite fibers in many areas.

Mechanical and thermal properties of the coaxial carbon nanotube@boron nitride nanotube composite

The mechanical and thermal properties of a novel one dimensional nanocomposite, a coaxial (10,0) carbon nanotube inside a (19,0) boron nitride nanotube (CNT@BNNT), are studied by molecular dynamics (MD) simulations. Results show that the CNT@BNNT composites possess excellent mechanical strength and high thermal conductivity, both of which are comparable to that of individual CNT. The calculated Young's Modulus is about 0.856 TPa and the critical strain and the maximum stress can be increased by increasing the inter-tube coupling strength while decreased by increasing the tube length. Due to its larger tube densities, the overall thermal conductivity of the CNT@BNNT arrays is estimated to be 2.36 times that of the CNT arrays when used as the thermal interface materials (TIMs) and also exhibits better thermal stability at high temperatures owing to the presence of the BNNT sheath. Compression stress leads to a decrease in the thermal conductivity of the CNT@BNNT, and the thermal conductivity drops rapidly near the critical strain. However, when the compression strain exceeds the critical one, the thermal conductivity changes slowly with further increase of the compression strain. Phonon spectrum demonstrates that high frequency phonons play a dominate role in the thermal conductivity of the CNT@BNNT nanocomposite.

Photoluminescence Imaging of Polyfluorene Surface Structures on Semiconducting Carbon Nanotubes: Implications for Thin Film Exciton Transport.

Single-walled carbon nanotubes (SWCNTs) have potential to act as light-harvesting elements in thin film photovoltaic devices, but performance is in part limited by the efficiency of exciton diffusion processes within the films. Factors contributing to exciton transport can include film morphology encompassing nanotube orientation, connectivity, and interaction geometry. Such factors are often defined by nanotube surface structures that are not yet well understood. Here, we present the results of a combined pump-probe and photoluminescence imaging study of polyfluorene (PFO)-wrapped (6,5) and (7,5) SWCNTs that provide additional insight into the role played by polymer structures in defining exciton transport. Pump-probe measurements suggest exciton transport occurs over larger length scales in films composed of PFO-wrapped (7,5) SWCNTs, compared to those prepared from PFO-bpy-wrapped (6,5) SWCNTs. To explore the role the difference in polymer structure may play as a possible origin of differing transport behaviors, we performed a photoluminescence imaging study of individual polymer-wrapped (6,5) and (7,5) SWCNTs. The PFO-bpy-wrapped (6,5) SWCNTs showed more uniform intensity distributions along their lengths, in contrast to the PFO-wrapped (7,5) SWCNTs, which showed irregular, discontinuous intensity distributions. These differences likely originate from differences in surface coverage and suggest the PFO wrapping on (7,5) nanotubes produces a more open surface structure than is available with the PFO-bpy wrapping of (6,5) nanotubes. The open structure likely leads to improved intertube coupling that enhances exciton transport within the (7,5) films, consistent with the results of our pump-probe measurements.

Role of inter-tube coupling and quantum interference on electrical transport in carbon nanotube junctions

Due to excellent transport properties, Carbon nanotubes (CNTs) show a lot of promise in sensor and interconnect technology. However, recent studies indicate that the conductance in CNT/CNT junctions are strongly affected by the morphology and orientation between the tubes. For proper utilization of such junctions in the development of CNT based technology, it is essential to study the electronic properties of such junctions. This work presents a theoretical study of the electrical transport properties of metallic Carbon nanotube homo-junctions. The study focuses on discerning the role of inter-tube interactions, quantum interference and scattering on the transport properties on junctions between identical tubes. The electronic structure and transport calculations are conducted with an Extended Hückel Theory-Non Equilibrium Green's Function based model. The calculations indicate conductance to be varying with a changing crossing angle, with maximum conductance corresponding to lattice registry, i.e. parallel configuration between the two tubes. Further calculations for such parallel configurations indicate onset of short and long range oscillations in conductance with respect to changing overlap length. These oscillations are attributed to inter-tube coupling effects owing to changing π orbital overlap, carrier scattering and quantum interference of the incident, transmitted and reflected waves at the inter-tube junction.

Surface-enhanced Raman scattering properties of multi-walled carbon nanotubes arrays-Ag nanoparticles

A three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate is fabricated by decorating multi-walled carbon nanotubes (MWNTs) arrays with silver nanoparticles (AgNPs) on silicon, via magnetron sputtering and annealing method. The MWNTs-AgNPs hybrids with 3D structure increase the coverage of plasmonic nanostructures. The effect of inter-particle and inter-tube coupling creates a multitude of hot spots, with a strong SERS effect. The structure of MWNTs-AgNPs hybrid material is confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, Raman scattering from MWNTs, which is significantly enhanced in such hybrids, is systematically investigated in all samples. Surface-enhanced Raman scattering property is proved using 4-mercaptobenzoic acid (4-MBA) as standard analyte and the detection concentration level is down to 10−10 M. Additionally, the stability and recyclability of MWNTs-AgNPs substrates have been studied, positioning SERS mapping within an area of 10 × 10 μm2 is carried out to investigate the uniformity; time-course SERS mapping is performed to study the influence induced by molecule solution evaporation process. Finally, finite-difference time-domain (FDTD) method is utilized to simulate E-field distribution of the hybrid structures with multilayer AgNPs decorated on MWNTs. The results show a Raman scattering enhancement factor (EF) of 107.

Effects of inter-tube coupling on the electro-optical properties of silicon carbide nanotube bundles studied by density functional theory

The electronic and optical properties of bundled armchair and zigzag silicon carbide nanotubes (SiCNTs) are investigated by using density functional theory. The effects of inter-tube coupling on the electronic dispersions of SiCNT bundles are demonstrated. It was found that the band structure of (6,0) SiCNT bundle shows metallic feature. The calculated dielectric functions of the armchair and zigzag bundles are similar to that of the isolated tubes, except for the appearance of broadened peaks, small shifts of peak positions about 0.1eV and increasing of peak intensities. For (6,0) SiCNT with smaller radius, by considering interband and interaband transitions, the band structure coupling causes an extra peak at low energies.

Magnetic phase diagram of the coupled triangular spin tubes forCsCrF4

Using Monte Carlo simulations, we explore the magnetic phase diagram of triangular spin tubes coupled with a ferromagnetic intertube interaction for ${\mathrm{CsCrF}}_{4}$. The planar structure of the coupled tubes is topologically equivalent to the kagome-triangular lattice, which induces nontrivial frustration effects in the system. We particularly find that, depending on the intertube coupling, various ordered phases are actually realized, such as incommensurate order, ferromagnetic order, and cuboc order, which is characterized by the noncoplanar spin structure of the 12 sublattices accompanying the spin chirality breaking. We also discuss the relevance of the results to recent experiments on ${\mathrm{CsCrF}}_{4}$.

Electronic structure and optical properties of boron nitride nanotube bundles from first principles

The electronic and optical properties of bundled armchair and zigzag boron nitride nanotubes (BNNTs) are investigated by using density functional theory. Owing to the inter-tube coupling, the dispersions along the tube axis and in the plane perpendicular to the tube axis of BNNT bundles are significantly varied, which are characterized by the decrease of band gap, the splitting of the doubly degenerated states, the expansions of valence and conduction bands. The calculated dielectric functions of the armchair and zigzag bundles are similar to that of the isolated tubes, except for the appearance of broadened peaks, small shifts of peak positions about 0.1eV and increasing of peak intensities.

The Effect of Local Electric Field in the Middle Dielectric Wall on the Infrared Plasmonic Shift of the Concentric Gold Double Nanotubes.

The effect of inserted gold nanotube on the localized surface plasmon resonance (LSPR) shifting and local electric field enhancement of concentric gold double nanotubes has been studied by using the quasi-static electricity. Because of the combined effect from inter-surface and inter-tube plasmon coupling, the LSPR in gold double nanotubes could be easily tuned into the infrared wavelength region. The calculation results indicate the plasmon peak shifting is quite sensitive to the thickness of the inner gold tube and the gap between the inner and outer gold tubes. The physical mechanism has been attributed to the local field effect of the separate dielectric layer. Because of the additional attraction from the inner gold tube to the free electrons at outer gold tube, the decreasing electric field in the middle dielectric spacer layer reduces the restoring force and lead to intense red shift of LSPR.

Acoustic phonon spectrum of unaligned multi-walled carbon nanotubes and zero phonon Rayleigh Mössbauer scattering

Frequency distribution function of acoustic phonons in unaligned multi-walled carbon nanotubes has been obtained, for the first time, by unfolding experimentally measured temperature variation of its specific heat in the range of 1–200 K. We have used a dynamical model that not only takes into account surface modes of the graphene sheets that form carbon nanotubes, but also inter-tube coupling through the presence of three-dimensional modes. The unfolded frequency distribution function of dynamical modes yields temperature dependent specific heat that is in good agreement with corresponding experimental results—in most of the temperature range, deviation is ~5 % or less. The computed temperature dependent values of zero phonon Rayleigh Mossbauer scattering fraction of Sn-119 gamma-photons have been obtained by using the suggested dynamical model and the final unfolded frequency distribution function. It turns out that zero phonon Rayleigh Mossbauer scattering fraction varies with temperature markedly and can be rather easily measured.