Transport In Single-wall Carbon Nanotubes Research Articles

Effect of bundling on phonon transport in single-walled carbon nanotubes

Thermally conductive bulk materials composed of single-walled carbon nanotubes (SWCNTs), such as fibers and mats, are characterized by a complex network formed by entangled SWCNTs. The fundamental unit of this network is a bundle, in which SWCNTs are assembled through weak van der Waals interactions. Although the thermal conductivity of a bundle is conventionally described as the sum of the heat conduction in the constituent SWCNTs, recent experiments have demonstrated that bundling may reduce the high thermal conductivity of SWCNTs, which is counterintuitive considering the weak interactions between SWCNTs in a bundle. Herein, by performing spectral phonon transport analysis for bundles of SWCNTs with radii of approximately 3.4 Å, we explore the effect of the bundle size and chiral configuration on phonon transport in bundles. Results show that although the heat conduction properties of individual SWCNTs are maintained for bundles with a length of less than 1 μm around room temperature in both the ballistic and quasi-ballistic regimes, intertube interactions between constituent SWCNTs coupled with their structural low dimensionality may yield additional thermal resistance. In particular, the thermal conductivity of a bundle decreases when low-frequency phonons influenced by intertube interactions substantially contribute to the overall heat conduction, namely, either at low temperatures or for long bundles. Although the observed additional thermal resistance cannot fully explain the experimental results of reduced thermal conductivity, our findings on the effect of bundling can advance the understanding of thermal transport in bundles and the thermal management of SWCNT bulk materials.

Ultra-Permeable Single-Walled Carbon Nanotube Membranes with Exceptional Performance at Scale.

Enhanced fluid transport in single‐walled carbon nanotubes (SWCNTs) promises to enable major advancements in many membrane applications, from efficient water purification to next‐generation protective garments. Practical realization of these advancements is hampered by the challenges of fabricating large‐area, defect‐free membranes containing a high density of open, small diameter SWCNT pores. Here, large‐scale (≈60 cm2) nanocomposite membranes comprising of an ultrahigh density (1.89 × 1012 tubes cm−2) of 1.7 nm SWCNTs as sole transport pathways are demonstrated. Complete opening of all conducting nanotubes in the composite enables unprecedented accuracy in quantifying the enhancement of pressure‐driven transport for both gases (>290× Knudsen prediction) and liquids (6100× no‐slip Hagen–Poiseuille prediction). Achieved water permeances (>200 L m−2 h−1 bar−1) greatly exceed those of state‐of‐the‐art commercial nano‐ and ultrafiltration membranes of similar pore size. Fabricated membranes reject nanometer‐sized molecules, permit fractionation of dyes from concentrated salt solutions, and exhibit excellent chemical resistance. Altogether, these SWCNT membranes offer new opportunities for energy‐efficient nano‐ and ultrafiltration processes in chemically demanding environments.

Heat transport in carbon nanotubes: Length dependence of phononic conductivity from the Boltzmann transport equation and molecular dynamics

In this article, we address lattice heat transport in single-walled carbon nanotubes (CNTs) by a quantum mechanical calculation of three-phonon scattering rates in the framework of the Boltzmann transport equation (BTE) and classical molecular dynamics (MD) simulation. Under a consistent choice of an empirical, realistic atomic interaction potential, we compare the tube length dependence of the lattice thermal conductivity (TC) at room temperature determined from an iterative solution of the BTE and from a nonequilibrium MD (NEMD) approach. Qualitatively similar trends are found in the limit of short tubes, where an extensive regime of ballistic heat transport prevailing in CNTs of lengths $L\lesssim 1\,\rm{\mu m}$ is independently confirmed. In the limit of long tubes, the BTE approach suggests a saturation of TC with tube length, whereas direct NEMD simulations of tubes extending up to $L=10\,\rm{\mu m}$ are demonstrated to be insufficient to settle the question of whether a fully diffusive heat transport regime and an intrinsic value of TC exist for CNTs. Noting that acoustic phonon lifetimes lie at the heart of a saturation of TC with tube length as per the BTE framework, we complement the quantum mechanical prediction of acoustic phonon lifetimes with an analysis of phonon modes in the framework of equilibrium MD (EMD). A normal mode analysis (NMA) with an emphasis on long wavelength acoustic modes corroborates the BTE prediction that heat transport in CNTs in the long tube limit is governed by the low attenuation rates of longitudinal and twisting phonons.

Thermal transport in suspended SWCNTs at high heat fluxes

In recent years many reports have indicated that Fourier’s heat conduction law is violated at size scales comparable to the mean free path of the heat transfer carriers or time scales of femtoseconds. Another open question is whether Fourier’s heat conduction law still holds at extremely high heat fluxes. Here, we present an experimental study of temperature dependent thermal conductivities and spatially resolved temperature profiles in electrically heated single-walled carbon nanotubes (SWCNTs) using Raman spectroscopy. The experimental results yield evidence that thermal transport in SWCNTs obeys Fourier’s empirical law at high heat fluxes (q=1.4×1011W/m2) above 300K in vacuum.

Thermal Transport in Single-Walled Carbon Nanotubes Under Pure Bending

Keeping mesoscopic electronics cool is a significant challenge, for which carbon nanotubes (CNTs) are useful, in principle. However, so far there has been a discrepancy between experimental results and theoretical predictions of how CNTs transfer heat under bending deformation--an important issue in powering mobile devices with body heat, or cooling flexible electronics. Using objective molecular dynamics, the authors provide a comprehensive study of length- and curvature-dependent transport of phonons in bent CNTs, reconciling experiment and theory, with important implications for nanoscale thermal management and phononic devices.

Vibrational Excitation in Electron Transport through Carbon Nanotube Quantum Dots.

Electron transport in single-walled carbon nanotubes (SWCNTs) is extremely sensitive to environmental effects. SWCNTs experiencing an inhomogeneous environment are effectively subjected to a disorder potential, which can lead to localized electronic states. An important element of the physical picture of such states localized on the nanometer-scale is the existence of a local vibronic mainfold resulting from the localization-enhanced electron-vibrational coupling. In this Letter, scanning tunneling spectroscopy (STS) is used to study the quantum-confined electronic states in SWCNTs deposited on the Au(111) surface. STS spectra show the vibrational overtones identified as D-band Kekulé vibrational modes and K-point transverse out-of plane phonons. The presence of these vibrational modes in the STS spectra suggests rippling distortion and dimerization of carbon atoms on the SWCNT surface. The present study thus, for the first time, experimentally connects the properties of well-defined localized electronic states to the properties of their associated vibronic states.

Contactless probing of the intrinsic carrier transport in single-walled carbon nanotubes

Intrinsic carrier transport properties of single-walled carbon nanotubes are probed by two parallel methods on the same individual tubes: the contactless dielectric force microscopy (DFM) technique and the conventional field-effect transistor (FET) method. The dielectric responses of SWNTs are strongly correlated with electronic transport of the corresponding FETs. The DC bias voltage in DFM plays a role analogous to the gate voltage in FET. A microscopic model based on the general continuity equation and numerical simulation is built to reveal the link between intrinsic properties such as carrier concentration and mobility and the macroscopic observable, i.e. dielectric responses, in DFM experiments. Local transport barriers in nanotubes, which influence the device transport behaviors, are also detected with nanometer scale resolution.

Cushioning effect, enhanced localized plastic flow and thermal transport in SWCNT–lead silicate glass composite

Here we report enhanced hardness, fracture toughness and thermal conductivity of single walled carbon nanotubes (SWCNT) embedded lead silicate glass composite. The enhanced hardness of the composite was due to cushioning behavior of the agglomerated SWCNT bundles where as the increment in toughness is due to crack bridging and accumulation of SWCNT bundles around the crack zone. Interestingly the accumulation of SWCNT bundles which was hitherto not yet been observed was due to enhanced localized plastic flow in the composite. Moreover, moderate increment of thermal conductivity above room temperature was discussed in the context of interfacial thermal resistance.

Aggregation Kinetics and Transport of Single-Walled Carbon Nanotubes at Low Surfactant Concentrations

Little is known about how low levels of surfactants can affect the colloidal stability of single-walled carbon nanotubes (SWNTs) and how surfactant-wrapping of SWNTs can impact ecological exposures in aqueous systems. In this study, SWNTs were suspended in water with sodium dodecylsulfate (SDS) as a surface-active dispersing agent. The effect of SDS concentration on SWNT suspension stability was investigated with time-resolved dynamic light scattering (TRDLS) initial aggregation studies utilizing both monovalent (Na(+)) and divalent (Ca(2+)) cations. The critical coagulation concentration (CCC) values increased with SDS concentration for the Na(+) treatments, but the Ca(2+) treatments were less sensitive to SDS concentration changes. Longer term stability studies with SDS concentrations orders of magnitude below the SDS critical micelle concentration demonstrated that SWNTs remained suspended for over six weeks in a surface water. Transport studies in a freshwater sediment similarly showed a SDS concentration-dependent mobility of SDS-wrapped SWNTs in that SWNTs showed a relatively greater retention at lower SDS concentrations (0.001%-0.05% w/v) than at a higher SDS concentration (0.1%). It is hypothesized that the stability and mobility of SWNT suspensions is directly related to the surface coverage of SDS on the SWNT surface that simultaneously increases electrosteric repulsion and decreases surface chemical heterogeneity. Overall, these studies demonstrate that low levels of surfactant are effective in stabilizing and mobilizing SWNTs in environmental media.

Hydrogen transport in single-walled carbon nanotubes encapsulated by palladium

Hydrogen transport and loading into single-walled carbon nanotubes (SWCNT) encapsulated by thin Pd layers onto a massive Pd substrate were studied using a complex of vacuum thermal desorption, cyclic voltammetry and ESR methods. By adding SWCNT the hydrogen capacity of the Pd–SWCNT composite under electrochemical loading increases as much as 25% relative to Palladium metal alone. This provides moderate growth in the gravimetric capacity of the total composite based on a massive Pd substrate. The hydrogen binding energy in the SWCNT (eH=0.075eV/H-atom), estimated by studies of hydrogen transport in the Pd–SWCNT composite was lower than predicted for the Pd–SWCNT complex, but higher than the physisorption on the bare SWCNT. Using ESR we established that the Pd–Cx e-complexes formed at the wall of nanotube could be considered as hydrogen adsorption site, providing both high net gravimetric capacity and low hydrogen binding energy in the Pd encapsulated SWCNT. The results obtained provide an opportunity to probe a condensed hydrogen phase of nanometer scale confined in SWCNT, encapsulated by transition metals.

A nonequilibrium Green’s function study of thermoelectric properties in single-walled carbon nanotubes

The phonon and electron transport in single-walled carbon nanotubes (SWCNT) are investigated using the nonequilibrium Green’s function approach. In zigzag SWCNT (n,0) with mod(n,3)≠0, the thermal conductance is mainly attributed to the phonon transport, while the electron only has few percentage contribution. The maximum value of the figure of merit (ZT) is about 0.2 in this type of SWCNT. The ZT is considerably larger in narrower SWCNT because of enhanced Seebeck coefficient. ZT is smaller in the armchair SWCNT, where Seebeck coefficient is small due to zero band gap. It is found that the cluster isotopic doping can reduce the phonon thermal conductance obviously and enhance the value of ZT. The uniaxial elongation and compress strain depresses phonons in whole frequency region, leading to the reduction in the phonon thermal conductance in whole temperature range. Interestingly, the elongation strain can affect the phonon transport more seriously than the compress strain, because the high frequency G mode is completely filtered out under elongation strain ϵ>0.05. The strain also has important effect on the subband edges of the electron band structure by smoothing the steps in the electron transmission function. The ZT is decreased by strain as the reduction in the electronic conductance overcomes the reduction in the thermal conductance.

Effects of poly-l-tyrosine molecules decoration on the surface properties and electron transport of SWCNTs compared to the effects of DNA molecules

We investigated the effects of amino acid poly-l-tyrosine (PLT) molecule decoration on the surface properties and electron transport of single-walled carbon nanotubes (SWCNTs) compared to the effects of deoxyribonucleic acid (DNA) molecule decoration. Both types of molecules had a similar effect on CNT surfaces; both were applied to tubes as charges, and increased their zeta potential. From the FET characteristics, we found that PLT molecule and DNA molecule decoration have quite different effects on the conduction mechanism of CNTs, which indicates that they are induced by different binding mechanisms.

Electrical Connectivity in Single-Walled Carbon Nanotube Networks

Transport in single-walled carbon nanotubes (SWCNTs) networks is shown to be dominated by resistance at network junctions which scale with the size of the interconnecting bundles. Acid treatment, known to dope individual tubes, actually produces a dramatic reduction in junction resistances, whereas annealing significantly increases this resistance. Measured junction resistances for pristine, acid-treated and annealed SWCNT bundles correlate with conductivities of the corresponding films, in excellent agreement with a model in which junctions control the overall network performance.

Lattice thermal conductivity of single-walled carbon nanotubes: Beyond the relaxation time approximation and phonon-phonon scattering selection rules

We present a theoretical description of phonon thermal transport and intrinsic lattice thermal conductivity in single-walled carbon nanotubes (SWCNTs). An exact solution of the phonon Boltzmann equation is implemented using an efficient scheme that allows consideration of SWCNTs with a wide range of radii and chiralities. Our approach combines for the first time calculations of the full spectrum of anharmonic three-phonon scattering processes in SWCNTs with use of the correct selection rules that severely restrict the phase space of this scattering. In particular, we demonstrate that if only the acoustic phonon branches are considered, no umklapp scattering can occur: scattering of acoustic phonons by optic phonons is required to produce thermal resistance in SWCNTs. We also show that the commonly used relaxation time approximation gives a particularly poor description of thermal transport in SWCNTs because of the unusually weak phonon-phonon umklapp scattering in these systems.

Transport of Single-Walled Carbon Nanotubes in Porous Media: Filtration Mechanisms and Reversibility

Deposition of nanomaterials onto surfaces is a key process governing their transport, fate, and reactivity in aquatic systems. We evaluated the transport and deposition behavior of carboxyl functionalized single-walled carbon nanotubes (SWNTs) in a well-defined porous medium composed of clean quartz sand over a range of solution chemistries. Our results showthat increasing solution ionic strength or addition of calcium ions result in increased SWNT deposition (filtration). This observation is consistent with conventional colloid deposition theories, thereby suggesting that physicochemical filtration plays an important role in SWNT transport. However, the relatively insignificant change of SWNT filtration at low ionic strengths (< or = 3.0 mM KCl) and the incomplete breakthrough of SWNTs in deionized water (C/Co = 0.90) indicate that physical straining also plays a role in the capture of SWNTs within the packed sand column. It is proposed that SWNT shape and structure, particularly the very large aspect ratio and its highly bundled (aggregated) state in aqueous solutions, contribute considerably to straining in flow through porous media. We conclude that both physicochemical filtration and straining play a role at low (< 3.0 mM) ionic strength, while physicochemical filtration is the dominant mechanism of SWNT filtration at higher ionic strengths. Our results further show that deposited SWNTs are mobilized (released) from the quartz sand upon introduction of low ionic strength solution following deposition experiments with monovalent salt (KCl). In contrast, SWNTs deposited in the presence of calcium ions were not released upon introduction of low ionic strength solution to the packed column, even when humic acid was present in solution during SWNT deposition.

Light‐Induced Charge Transfer in Pyrene/CdSe‐SWNT Hybrids

On account of their nano-scale size, large aspect ratio and high conductivity, single-walled carbon nanotubes (SWNTs) have emerged as an attractive choice for conducting composite materials. Composites incorporating SWNTs show percolation dominated conductivity with a much lower volume threshold (volume fraction ≈ 10), compared to those with nanoparticles. Two-thirds of SWNTs are p-type semiconductors with holes as the charge carriers. Using the holeblocking nature of SWNTs, conducting polymers having SWNTs as dopants are effectively used as the hole buffering and electron transport layers in organic light-emitted diodes (OLEDs). For the active layer of OLEDs and organic photovoltaic (OPV) devices, incorporation of SWNTs enhances the charge separation and facilitates charge transport, hence improving the performance, i.e., the short circuit current, the filling factor and power conversion efficiency. However, since SWNTs are a mixture of metallic and semiconducting nanotubes with a small bandgap (≈ 0.6 eV), both electrons and holes in the composite matrix prefer to transfer onto and then be quenched on the SWNTs. Directly incorporating SWNTs into active layers of OLEDs and OPV devices does not facilitate the electron/hole separation, nor does it improve the performances of devices. On the other hand, semiconductor nanoparticles, possessing large and tunable bandgaps (1–2 eV), can be incorporated into conducting polymers to form hybrid solar cells. Holes are transported along the polymer chains and electrons hop along the nanoparticle network. However, nanoparticles with large volume fractions are needed because of their high percolation threshold (>≈30%). By contrast, the volume fraction for the conducting polymer is small, leading to low hole mobility and hence limiting the solar cell efficiency. Semiconductor nanorod-polymer hybrid solar cells benefit from the quasi-one-dimensional (1-D) electron transport along the rods, a mechanism which allows the use of a smaller volume fraction of nanorods and a larger volume fraction of polymer to conduct holes with higher mobilities. Although Alivisatos et al. have demonstrated that longer nanorods lead to higher energy conversion efficiency, nanorods of larger lengths and uniformly distributed diameters are difficult to fabricate. Semiconductor nanoparticle-SWNT hybrids have been the subject of recent interest as a consequence of the development of methods for the chemical modification of SWNTs. Such hybrids are well suited for use in optoelectronic devices, given the tunable bandgap of nanoparticles, quasione-dimensional (1-D) transport of SWNTs, and the ease of chemical fabrication. As part of a drive towards finding applications, an examination of the charge transfer (CT) between the various components of the hybrids is needed. By using such hybrid materials as photo-electrodes, efficient electron transfer from semiconductor nanoparticles, such as CdS, CdSe, and CdTe (donor) to SWNTs (acceptor) has been demonstrated by several groups to lead to increased photon generated current (Fig. 1). The CT also results in photolumiC O M M U N IC A IO N

Charge-injection-induced dynamic screening and origin of hysteresis in field-modulated transport in single-wall carbon nanotubes

Gate modulated transport in semiconducting single-wall carbon nanotubes shows significant hysteresis in their transfer characteristics. The origin of this hysteresis is generally attributed to the screening of the gate voltage due to the movement of mobile charges/ions in the inherent presence of a trapping/detrapping mechanism in the adjacent dielectric, as in conventional silicon metal-oxide-semiconductor field-effect transistors. However, recent works have conjectured that the screening charges may originate from the nanotube itself. From an extensive study of the temperature dependence of the hysteresis behavior in nanotube field-effect transistors the authors experimentally establish this alternative mechanism, in which the screening charges are injected from the nanotube itself into the surrounding dielectric. Any detailed trapping/detrapping mechanism does not appear to play a significant role, and all experimental results can be simply explained in terms of a capacitive charging of the surrounding...

Structure and Electrical Transport of Single-Walled Carbon Nanotubes with Intramolecular Junction and Defects

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Separating spin and charge transport in single-wall carbon nanotubes

We demonstrate spin injection and detection in single wall carbon nanotubes using a 4-terminal, non-local geometry. This measurement geometry completely separates the charge and spin circuits. Hence all spurious magnetoresistance effects are eliminated and the measured signal is due to spin accumulation only. Combining our results with a theoretical model, we deduce a spin polarization at the contacts of approximately 25 %. We show that the magnetoresistance changes measured in the conventional two-terminal geometry are dominated by effects not related to spin accumulation.

Resistance Evaluation and Growth of Carbon Nanotubes

We have investigated electronic transport in single-wall carbon nanotubes attached to multiple electrodes. Resistance measurement using a pair of electrodes with different gaps enabled separate evaluation of nanotube resistivity and contact resistance. We found that the resistivity depends on nanotube diameter. Electrodes with gold or palladium exhibit similar contact resistance with an order of 10kΩ. Contact resistance is insensitive to back gate voltage, contrary to the Schottky-barrier transistor model. We also demonstrated the top-down control of diameter and position of Fe catalysts by means of “lithographically anchored nanoparticle synthesis (LANS)” for nanotube growth. Chemical vapor deposition by using these patterned particles successfully produced single-wall carbon nanotubes.