Structure‐Dependent Electrical Properties of Carbon Nanotube Fibers

Spun carbon nanotube (CNT) fibers have great potential for conducting and sensing applications owing to their unique, tunable electrical properties. Here we report the electron transport properties of neat, well-aligned CNT fibers spun from arrays of millimeter-long CNTs. The conductivity of asspun CNT fibers is around 595.2 S cm at room temperature, and its variation with temperature shows a semiconductive behavior from 300 to 75.4 K. The electron transport was found to follow a three-dimensional (3D) hopping mechanism. Importantly, it was found that chemical treatments may significantly affect the conductivities of as-spun fibers. Oxidizing the CNT fibers in air or HNO3 increased the conductivities, while covalent bonding of Au nanoparticles to the CNT fibers remarkably improved conductivity and changed conduction behavior. Conversely, annealing CNT fibers in Ar+ 6% H2 at 800 °C or under the CNT array growth conditions at 750 °C led to a dramatic decrease in conductivity. Owing to their conjugated and highly anisotropic 1D structures, carbon nanotubes (CNTs) are a fascinating new class of electronic materials from both theoretical and applied standpoints. The excellent conductivities of CNTs and their ability to carry very high current density, along with their high thermal conductivity, chemical stability, and mechanical strength, make CNTs uniquely promising for a broad range of applications, including building blocks for nanoscale electronic devices, microsensors for bio-agents and chemicals, and power cables for space shuttles. The electrical resistivity q of individual CNTs has been measured under ballistic conductions to be as low as 10 X cm for single-walled and 3× 10 X cm for multiwalled CNTs, respectively, indicating that CNTs may be better conductors than metals such as copper at room temperature. However, in most cases, due to the presence of various defects or impurities formed during the CNT growth, the conductivities of individual CNTs are often much lower than those under ballistic conduction with nanotubes free of defects. The electron transport in CNT assemblies is different from that in individual nanotubes. It has been reported that singlewalled carbon nanotube (SWNT) fibers, either synthesized directly by vertical floating chemical vapor deposition (CVD) methods or extruded from a super-acid suspension, exhibit room-temperature resistivities in the range of 1 × 10 to 7 × 10 X cm, which is nearly 100 times higher than the resistivities of single nanotubes. The resistivities of multiwalled carbon nanotube (MWNT) fibers are typically one or two orders of magnitude higher than that of SWNT fibers. Such large differences between single nanotubes and fiber assemblies may arise from a high impurity content (such as amorphous carbon and catalytic particles) in the fibers, which may profoundly affect electron transport by causing significant scattering, and contact resistances between nanotubes. Therefore, two approaches can be used to improve the electrical conductivity of CNT fibers: 1) minimize the contact resistances between nanotubes by improving the alignment of CNTs and by increasing the lengths of individual tubes; 2) improve the conductivity of individual CNTs by post-synthesis treatments. Itwas the objective of the study reported here to use these two approaches to produce CNT fibers with high conductivity and to study the fundamental conduction mechanisms of the CNT fibers. Thin and clean CNT fibers (typically 3 lm in diameter) were spun from arrays of well-aligned, millimeter-long CNTs, which were synthesized using ethylene CVD on a Fe catalyst film. By measuring the resistance of CNT fibers at temperatures from 300 K to 75.4 K, we investigated the electronic properties of as-spun fibers and their possible conducting mechanisms. It was also found that the conductivity of CNT fibers could be tuned through mild post-treatments. The spun CNT fibers were post-treated with five different procedures: 1) Annealing in air at 480 °C for half an hour in an attempt to clean off the amorphous carbon, whose oxidation temperature is often around 400 °C. 2) Oxidizing in dilute 5 M HNO3 solution at 40 °C to cause a weak chemical C O M M U N IC A TI O N