Properties Of Individual Carbon Nanotubes Research Articles

Molecular-level network engineering of crosslinker towards high-performance carbon nanotube fiber

Intensive efforts are underway to integrate the distinct properties of individual carbon nanotubes (CNTs) into their assemblies and meet the growing demand for lightweight and strong materials. This study achieved CNT fibers (CNTFs) with high specific tensile strength (4.97 N/tex) and specific modulus (206.8 N/tex) by designing all-ring type bridges between CNTs. This reinforcement strategy involves suppressing the distortion of CNTs and promoting effective crosslinks within the fiber structure to increase the total inter-bundle interactions. The molecular-level network within the linking segment reduces vulnerability at the typically weak connection points and enhances load-transfer efficiency between crosslinked CNTs. In addition to improvements in mechanical properties, incorporating dopant groups in the bridge enhanced the electrical conductivity, reaching up to 2086 S m2/kg. This strategy provides straightforward access to multifunctional fibers beyond those of commercially accessible or literature-reported benchmark fibers.

(Invited) Toward Spectral Homogeneity in Guanine Functionalized SWCNTs

Different chiralities of single-walled carbon nanotubes (SWCNTs) have distinct electronic and optical properties, with a range of band gaps that make them appropriate for several opto-electronic applications. Due to the unusually high mobility of charge carriers in SWCNTs,1 semiconducting SWCNTs may find use as logic gates in future high-speed nano-electronic circuits. With this in mind, controlled modulation of the optical and electronic properties of individual carbon nanotubes may be valuable in optoelectronic and nano-bioelectronic applications.It was recently discovered that when SWCNTs coated with single-stranded DNA are exposed to singlet oxygen, the guanine nucleobases in the DNA coatings form covalent bonds to the nanotube sidewalls.2 This alters the nanotube electronic and optical properties, allowing spatial and energetic control through selection of the DNA base sequence. However, the functionalized products display undesired spectral broadening interpreted as excess inhomogeneity compared to the pristine SWCNT reactants.Here we will present the results of efforts to understand and reduce this inhomogeneity. Our study has used molecular dynamics simulations of ssDNA – SWCNT hybrid structures, as well as analysis of experimental data. We have explored the effects of varied reaction conditions, including temperature, ionic strength, and pH on product yields and spectra. An unexpected inverse dependence of functionalization density on reaction temperature was uncovered. We will also present photophysical results extracted from spectral data via statistical modeling, including the energy depth of localized exciton quantum wells created by functionalization. The analysis points to fluorescence red shifts that reflect the random number of functionalization sites felt by excitons. Dürkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Extraordinary Mobility in Semiconducting Carbon Nanotubes. Nano Lett. 2004, 4, 35-39.Zheng, Y.; Bachilo, S. M.; Weisman, R. B. Controlled Patterning of Carbon Nanotube Energy Levels by Covalent DNA Functionalization. ACS Nano 2019, 13, 8222-8228.

Visualization of Thermal Transport within and between Carbon Nanotubes.

Despite the excellent thermal properties of individual carbon nanotubes (CNTs), the thermal characteristics of macroscopic CNT assemblies are poor. This is probably due to the presence of numerous nontrivial intertube boundaries. Currently, clarity on the inherent difference between intra- and inter-CNT thermal conductivities is not well-established. Herein, we report an in situ nanoscale observation on the anisotropic thermal transport of a single bundle of CNTs by monitoring evaporated gold nanoparticles as "thermomarkers". The experimental results indicate that even a small bundle shows colossal thermal anisotropy due to the intertube boundaries. The results are validated by finite element analysis that estimates an anisotropic thermal conductivity ratio greater than 100. The estimated value is much greater than most of the reported values measured on macroscopic specimens and matches with that of highly ordered pyrolytic graphite. Our study reveals the intrinsic thermal anisotropy of bundled CNTs and aids in visualizing nanoscale thermal transport.

Probing electrical properties of individual carbon nanotubes filled with Fe3C nanowires

The electrical properties of individual multiwall carbon nanotubes (CNTs) filled with Fe3C nanowires (Fe-CNTs) grown by chemical vapor deposition were investigated. The individual Fe-CNTs were measured by two-probe configuration in a scanning electron microscope, in which one probe was used to contact one end of the nanotubes and the other varied its contact position to measure the resistance along the Fe-CNTs. The data suggest that the ferromagnetic nanowires and the CNTs were well connected into a conduction network, and the resistance of the individual Fe-CNTs decreased as the filling rate increased. Analysis shows that the encapsulated ferromagnetic nanowires played a profound part in determining the electrical behavior of individual Fe-CNTs. The results may be useful for understanding of electronic transport of individual Fe-CNTs and applications based on individual Fe-CNTs.

(Invited) Mechanical and Gas Transport Properties of Pristine and Cross-Linked Carbon Nanotube Materials

Carbon nanotubes (CNTs) have a unique combination of mechanical properties, including extremely high elastic modulus, large strength, and relatively low bending rigidity. The porous CNT materials also have a potential for application as membranes for gas separation, purification, and sensing. The load transfer in networks of pristine CNT, however, is weak and CNT materials exhibit much smaller stiffness and strength than it can be anticipated based on properties of individual CNTs. Weak mechanical properties prevent CNT materials from using in multiple emerging applications. Covalent cross-linking via irradiation by energetic ion or electron beams, as well as chemical functionalization, allow one to enhance stability of CNT materials and improve their elastic and inelastic properties. In this work, the mechanical and gas transport properties of pristine and cross-linked CNT materials are studies theoretically based on a mesoscopic computational model. This model is capable of predicting the mechanical loading of CNT films and aerogels and gas transport in these materials in large-scale simulations involving thousands of nanotubes. In the mesoscopic model, every nanotube is represented by a chain of cylindrical mesoscopic segments. The mesoscopic force field accounts for the stretching, bending, and buckling of nanotubes, van der Waals interaction between CNTs, and presence of cross-links. Discrete cross-links between nanotubes are described by a model based on the Morse potential function for an individual cross-link and a geometrical model of the effective cross-link bond. The molecular dynamics simulations of pulling-out of a CNT from a cross-linked CNT bundle are performed in order to characterize the effect of cross-links on the shear load transfer between CNTs. The obtained results in the form of force-displacement curves and rates of cross-link breaking and reformation are used to parameterize the mesoscopic model of cross-links. The developed atomistic and mesoscopic models as well as novel continuum-level model of cross-linked CNT bundles are first applied to study the shear lag effect during pulling-out of CNTs from long nanotube bundles. The results of pull-out tests obtained with these models agree with each other and reveal a strong shear lag effect, when the effective sheaf force is limited by stretching of individual nanotubes. This effect is found to be a factor that puts limits on improving the shear load transfer by covalent cross-links. The developed mesoscopic model is then applied to study mechanical properties of thin quasi-two-dimensional films and isotropic three-dimensional aerogels composed of single-walled nanotubes. The in silico generated material samples with the entangled network of bundles of nanotubes are prepared by self-assembly of initially dispersed CNTs in dynamic mesoscopic simulations. The equilibrated films and aerogels are subjected to compressive and stretching loading. The simulations show a few distinct modes of compression of CNT films, including the wrinkling mode, and reveal the mechanisms responsible for irreversible material deformation and fracture by means of cross-link breaking and re-arrangement of nanotubes. In addition to quasi-static loading, elastic wave propagation through the CNT films with a size up to a few micrometers is studied in order to calculate the acoustic speed in CNT network materials. The elastic moduli, ultimate strength, and acoustic speed in CNT films are found as functions of the material and cross-link densities, CNT length, and strain rate. The study of gas diffusion through CNT network materials is based on a hybrid model, including the mesoscopic model for CNTs and a molecular dynamics-type approach based on direct solution of equations of motion for individual gas molecules. This hybrid approach is used to study self-diffusion of argon inside CNT networks and simulate gas permeability of CNT materials placed in between low- and high-pressure gas reservoirs. The simulations reveal the strong effect of porous structure and density of the CNT materials on gas self-diffusivity and the material effective permeability. This work is supported by NASA through an Early Stage Innovations grant from NASA’s Space Technology Research Grants Program (project NNX16AD99G) and by NSF (CAREER project CMMI-1554589). Computational support is provided by the Alabama Supercomputer Center and NASA's Advanced Supercomputing (NAS) Division.

Iodine nanoparticle-enhancing electrical and thermal transport for carbon nanotube fibers

The excellent electrical and thermal transport properties of individual carbon nanotubes (CNTs) have allowed them to stand out in nanoenergy and nanoelectronics applications. However, scaling them up from nanometer-sized tubes to micrometer assembled fibers generally leads to considerable electrical and thermal transport deterioration. In pursuit of a new strategy for boosting thermal and electrical transport, we experimentally confirmed that the intercalation of iodine (I) molecules can significantly raise the inter-tube interfacial electrical and thermal transport (IET and ITT), and thus boost the overall electrical and thermal transport performance of the fibers. The underlying mechanisms from an energy carrier standpoint are also explored and discussed. It is demonstrated that the introduced I molecules can invoke electron extraction from CNTs, which enhances the IET. The formed (I3)− and (I5)− polyiodide chains act as additional heat transfer channels and induce low-frequency phonons of the interfacial carbon atoms, and the intercalation also improves the matching level of the vibrational density of states at the inter-tube interface. These three-fold effects contribute to the ITT. Our findings are one step forward on the path towards advanced CNT-based functional materials with high electrical and thermal transport.

High Performance Fibers from Carbon Nanotubes: Synthesis, Characterization, and Applications in Composites—A Review

A carbon nanotube fiber is formed by interlocking carbon nanotubes that are perfectly aligned to give a continuous long fiber. The production of neat carbon nanotube fibers has paved the way for realization of superior mechanical and physical properties of individual carbon nanotubes at macroscopic scale. In this review paper, we elucidate the state of the art advances in fabrication methods, characterization, modeling of mechanical properties, and applications of carbon nanotube fibers as next generation high performance fibers for composites. Recommendations have been made on the aspects of design and scale-up for large scale manufacturing of carbon nanotube fibers using (a) a stepwise protocol for the determination rate controlling step(s) and the estimation of overall rate of reaction, and (b) computational fluid dynamics. Finally, the challenges and opportunities in carbon nanotube fiber research have been clearly brought out, and suggestions have been made for future work.

Modulation of thermal and thermoelectric transport in individual carbon nanotubes by fullerene encapsulation.

The potential impact of encapsulated molecules on the thermal properties of individual carbon nanotubes (CNTs) has been an important open question since the first reports of the strong modulation of electrical properties in 2002. However, thermal property modulation has not been demonstrated experimentally because of the difficulty of realizing CNT-encapsulated molecules as part of thermal transport microstructures. Here we develop a nanofabrication strategy that enables measurement of the impact of encapsulation on the thermal conductivity (κ) and thermopower (S) of single CNT bundles that encapsulate C 60, Gd@C 82 and Er 2@C 82. Encapsulation causes 35-55% suppression in κ and approximately 40% enhancement in S compared with the properties of hollow CNTs at room temperature. Measurements of temperature dependence from 40 to 320 K demonstrate a shift of the peak in the κ to lower temperature. The data are consistent with simulations accounting for the interaction between CNTs and encapsulated fullerenes.

(Invited) Carbon Nanotubes from Synthesis, Assembly to Electrochemical Sensors

The unique physical properties of individual Carbon Nanotubes (CNTs) surpass the properties of many advanced materials available today. Due to their large surface area, chemical stability, and electrical conductivity, they are the most promising candidates for a large number of electroanalytical applications. A material for these applications requires high purity CNT assemblies like fibers and films, that have good electrical conductivity, in some case additional insulation coatings. This talk will report the synthesis of catalyst free CNTs, their assembly into ribbons at 15.9 m/s speed, fiber processing and polymer coating into a micro-cable format. CNT micro-cable was used for detection of toxic metals contaminants in H2O, that are considered as the most significant potential threats to human health by ATSDR and EPA. Polystyrene coated CNT fiber was used as the working electrode; bare CNT thread was used as the auxiliary electrode; and a pseudo-reference electrode was fabricated by electroplating CNT fiber with Ag that is subsequently anodized in chloride solution to form a layer of AgCl. The Ag/AgCl coated CNT fiber electrode provides stable potential comparable to the conventional liquid-junction type Ag/AgCl reference electrode, while bare CNT fiber auxiliary electrode provided a stable current comparable to a Pt wire auxiliary electrode. This all-carbon CNT fiber three electrode cell is evaluated for simultaneous detection of trace levels of heavy metal ions by anodic stripping voltammetry (ASV). Hg2+, Cu2+ and Pb2+ in H2O were detected successfully, and the detection limits are 1.05 nM, 0.53 nM and 0.57 nM for Hg2+, Cu2+ and Pb2+, respectively. These electrodes significantly reduce the dimensions of the conventional three electrode electrochemical cell and have the potential to become low cost and disposable sensor.

Influence of the microstructure on effective mechanical properties of carbon nanotube composites

Despite the exceptional mechanical properties of individual carbon nanotubes (CNTs), the effective properties of CNT-reinforced composites remain below expectations. The composite\'s microstructure has been identified as a key factor in explaining this discrepancy. In this contribution, a method for generating representative volume elements of aligned CNT sheets is presented. The model captures material characteristics such as random waviness and entanglement of individual nanotubes. Thus it allows studying microstructural effects on the composite\'s effective properties. Simulations investigating the strengthening effect of the application of a pre-stretch on the CNTs are carried out and found to be in very good agreement with experimental values. They highlight the importance of the nanotube\'s waviness and entanglement for the mechanical behavior of the composite. The presented representative volume elements are the first to accurately capture the waviness and entanglement of CNT sheets for realistically high volume fractions.

Quantitative analysis of carbon nanotube cross-linking reactions

Covalent cross-linking of carbon nanotubes (CNTs) is a useful way of transferring the unique properties of individual CNTs to macroscopic structures for a wide variety of applications. For elaborate engineering of the cross-linking reaction of CNTs, quantitative analysis of cross-linking reaction is imperative. We report here a universally applicable method to quantitatively analyze the cross-linking of CNTs by esterification. To distinguish the cross-linking reaction from the one-side reaction, where only one end of the linker reacts with a CNT, mass and molar balances were established based on thermogravimetric analysis data. This analysis revealed that approximately one in five linkers was involved in the cross-linking reaction. Qualitative characterizations such as Fourier transform infrared and Raman spectroscopy were also used to confirm the cross-linking reaction.

A predictive model of the tensile strength of twisted carbon nanotube yarns

Due to the outstanding mechanical properties of individual carbon nanotubes (CNTs) at the nanoscale, CNT yarns are expected to demonstrate high strength at the macroscale. In this study, a predictable model was developed to predict the tensile strength of twisted CNT yarns. First, the failure mechanism of twisted CNT yarns was investigated using in situ tensile tests and ex situ observations. It was revealed that CNT bundles, which are groups of CNTs that are tightly bound together, formed during tensile loading, leaving some voids around the bundles. Failure of the CNT yarns occurred as the CNT bundles were pulled out of the yarns. Two stresses that determined the tensile strength of the CNT yarns were identified: interfacial shear and frictional stresses originating from van der Waals interactions, and the lateral pressure generated by the twisted yarn structure. Molecular dynamics and yarn mechanics were used to calculate these two stresses. Finally, the tensile strength of CNT yarns was predicted and compared with experimental data, showing reasonable agreement.

Electrical property enhancement of carbon nanotube fibers from post treatments

Utilizing the superior properties of individual carbon nanotubes (CNTs), the production of continuous CNT fibers has paved the way for novel CNT macroscale applications. However, it is still a great challenge to create highly electrically-conductive CNT fibers due to the poor transfer of electrical property from nanoscale individual CNTs to macroscale CNT fibers. Here, we report a fast and effective post treatment method consisting of mechanical condensation and acid treatment to enhance the electrical conductivity of the CNT fiber by a factor of 9. Such great enhancement is attributed to the surface modification effects during the post treatments, resulting in the lower contact resistance and stronger inter-CNT interactions. Moreover, the maximum current density of the acid-treated CNT fiber is up to 66000A/cm2, competitive to the conventional copper wires. The promising results in this study enable the CNT fibers novel materials for electrical wiring applications.

Continuous Carbon Nanotube-Based Fibers and Films for Applications Requiring Enhanced Heat Dissipation.

The production of continuous carbon nanotube (CNT) fibers and films has paved the way to leverage the superior properties of individual carbon nanotubes for novel macroscale applications such as electronic cables and multifunctional composites. In this manuscript, we synthesize fibers and films from CNT aerogels that are continuously grown by floating catalyst chemical vapor deposition (FCCVD) and measure thermal conductivity and natural convective heat transfer coefficient from the fiber and film. To probe the mechanisms of heat transfer, we develop a new, robust, steady-state thermal characterization technique that enables measurement of the intrinsic fiber thermal conductivity and the convective heat transfer coefficient from the fiber to the surrounding air. The thermal conductivity of the as-prepared fiber ranges from 4.7 ± 0.3 to 28.0 ± 2.4 W m(-1) K(-1) and depends on fiber volume fraction and diameter. A simple nitric acid treatment increases the thermal conductivity by as much as a factor of ∼3 for the fibers and ∼6.7 for the thin films. These acid-treated CNT materials demonstrate specific thermal conductivities significantly higher than common metals with the same absolute thermal conductivity, which means they are comparatively lightweight, thermally conductive fibers and films. Beyond thermal conductivity, the acid treatment enhances electrical conductivity by a factor of ∼2.3. Further, the measured convective heat transfer coefficients range from 25 to 200 W m(-2) K(-1) for all fibers, which is higher than expected for macroscale materials and demonstrates the impact of the nanoscale CNT features on convective heat losses from the fibers. The measured thermal and electrical performance demonstrates the promise for using these fibers and films in macroscale applications requiring effective heat dissipation.

Electron Spectroscopy of Single Quantum Objects To Directly Correlate the Local Structure to Their Electronic Transport and Optical Properties.

Physical property of a single quantum object is governed by its precise atomic arrangement. The direct correlation of localized physical properties with the atomic structures has been therefore strongly desired but still limited in the theoretical studies. Here, we have successfully examined the localized electronic properties of individual carbon nanotubes by means of high-resolution electron energy-loss spectroscopy combined with high-resolution transmission electron microscopy. Well-separated sharp peaks at the carbon K(1s) absorption edge and in the valence-loss spectra are obtained from a single freestanding carbon nanotube with the local chiral index and unambiguously identified as the transitions between the van Hove singularities. The spectra features clearly vary upon the different areas even in the individual carbon nanotube. Variations in interband transitions, plasmonic behaviors, and unoccupied electronic structures are clearly attributed to the local irregular atomic arrangement such as topological defect and/or elastic bond stretching.

Investigations of electrical transport properties of individual carbon nanotubes with nanoprober

We investigated and discussed quantitatively the transport properties of individual multiwalled (MW) carbon nanotubes (CNTs) by four-terminal measurement using a nanoprobing system. The homogeneity of the CNT was visibly examined using the electron beam absorbed current function of the nanoprober. The observed ohmic properties of the current–voltage characteristics and metallic transport properties of the CNTs proved that reliable contact of four probes was achieved on the outermost shell of MWCNTs. The experimental methodology was validated for the intrinsic properties of individual CNTs. Lower resistance per unit length was evaluated for thicker CNT. The measured resistance per unit length was lower than those reported by other researchers, but higher than ideally expected.

Morphology-dependent load transfer governs the strength and failure mechanism of carbon nanotube yarns

The outstanding properties of individual carbon nanotubes (CNTs) have motivated interest in CNT fibers and yarns for composite materials, high-strength conductors and multifunctional textiles. However, despite advances in manufacturing, the strength of CNT yarns remains 10–100 fold less than individual CNTs. In light of the complex, multi-scale load transfer in CNT yarns, a hierarchical model taking into consideration the morphology-dependent mechanics is necessary to understand this limitation. We present a coupled analytical and finite element model of three-dimensional morphology and the full tensile behavior of CNT yarns. By incorporating load-induced changes in morphology, simulations of yarns in tension show different load paths such as fracture-type or stick–slip failure depending on the waviness and number density of CNTs. The strength of untwisted pristine CNT yarns is shown to be limited to <10% of the intrinsic CNT strength, even at practical limits to CNT packing density and alignment. Load-induced changes in CNT morphology are verified by tensile testing of CNT yarns along with in-situ X-ray scattering. In addition, the nominal structure of the yarn is shown to strongly influence the improvement in strength achieved by densified and/or cross-linking, and a sublinear relationship between CNT contact enhancement and yarn strength is predicted. Thus, this work provides a means to investigate the complex load transfer mechanisms in CNT yarns and other assemblies, to further study their process–structure–property relationships, and to understand potential property limits.

Strengthening and failure mechanisms of individual carbon nanotube fibers under dynamic tensile loading

Rate-dependent mechanical properties of individual carbon nanotube (CNT) fibers are essential for developing advanced anti-collision composites. In this work, a free-falling Hopkinson tension bar was designed to measure the dynamic tensile strength of CNT fiber. The strength of pure CNT fiber is found to be more sensitive to strain rate than that of composite CNT fiber. The dynamic failure morphologies of pure and composite CNT fiber were compared with their quasi-static responses. It was observed that the pure CNT fiber tends to break off at the weakest point, leading to unraveling of fiber along the twisted direction. On the other hand, the infiltrated polymer tends to change the failure mode and rate sensitivity of CNT fibers. Furthermore, high speed deformation analysis under dynamic loading revealed that pure CNT fiber tends to fail at the weakest point while the composite CNT fiber breaks into several fragments.

Shape-controlled carbon nanotube architectures for thermal management in aerospace applications

Abstract

Mechanical properties of suspended individual carbon nanotube studied by atomic force microscope

This paper is a short review on the mechanical properties of individual carbon nanotubes (CNTs), which were studied by atomic force microscope (AFM). AFM force–distance measurements were applied to three different nano structures based on the suspended CNTs, those are a straight suspended CNT, a coiled CNT, and a torsional CNT. Force–distance measurements were done on the pick and valley position of the coiled CNT and the estimated elastic moduli was compared with the straight one. Nanoscale metal plate was fabricated on the middle part of the suspended CNT structure using conventional lithography and etching procedure. The torsional modulus of the suspended CNT was estimated from the force–distance measurement using AFM manipulation on top of the metal plate. The paper covers synthesis and preparation method of various suspended CNT structure and the analysis of the force–distance measurements appropriate to each CNT structure. Prospects of the AFM force–distance measurements on emergent nano scale materials as well as CNT are suggested in the end of this paper.