Field electron emission cathodes were constructed from knitted fabrics comprised entirely of carbon nanotube (CNT) fibers. The fabrics consisted of a top layer array of ∼2 mm high looped structures and a bottom layer that was 1 mm thick with a flat underlying surface. Field emission (FE) experiments were performed on 25.4 mm diameter CNT fabric cathodes in both direct current (DC) and pulsed voltage (PV) modes, and the results were compared to those obtained from a CNT film cathode. The DC measurements were performed at a maximum voltage of 1.5 kV. The CNT fabric cathode emitted 20 mA, which was an 8× increase over the emission current from the CNT film cathode. The DC results were analyzed using the corrected form of the Fowler–Nordheim FE theory initially developed by Murphy and Good, which allows for the determination of the formal emission area and effective gap-field enhancement factor. The PV experiments resulted in Ampere level emission currents from both CNT fabric and CNT film cathodes. For a 25 kV, 500 ns voltage pulse, the CNT fabric cathode emitted 4 A, which was 2× more current than the CNT film cathode. Scanning electron microscopy imaging after PV testing revealed that the fibers remained intact after >5000 pulses. These results indicate that knitted CNT fabrics offer a promising approach for developing large area, conformable, robust FE cathodes for vacuum electronic devices.

The field electron emission properties of carbon nanotube (CNT) films composed of densely packed and highly aligned CNTs were investigated. The CNT films were produced by a continuous film casting process and are spooled into long lengths with the CNTs aligned lengthwise in the film. The anisotropic nature of the CNT film morphology was confirmed by performing specific conductivity measurements in directions both parallel and perpendicular to the aligned CNT microstructure. Field emission experiments were performed on 5 and 10 mm wide films that were mechanically cut into small samples and then vertically mounted so that the emission occurred from the film edge. The films were mounted with the aligned CNT microstructure oriented either parallel or perpendicular to the direction of the applied electric field. The highest emission currents were produced by films mounted in the parallel alignment configuration. Additional experiments were performed on films that were folded, which eliminated surface irregularities at the film edge due to the cutting process. SEM imaging performed at the ridge of the folded film before and after field emission (FE) experiments showed that films mounted in the parallel alignment configuration had minimal surface damage after FE, while films mounted in the perpendicular alignment configuration showed substantial damage. The effective emission area and field enhancement factor were extracted from the FE data using the orthodox Fowler–Nordheim theory. Folded CNT film cathodes mounted in the parallel alignment configuration produced the highest emission currents, while demonstrating a larger emission area and lower field enhancement factor.

The increasing interest in mobile and wearable technology demands the enhancement of functionality of clothing through incorporation of sophisticated architectures of multifunctional materials. Flexible electronic and photonic devices based on organic materials have made impressive progress over the past decade, but higher performance, simpler fabrication, and most importantly, compatibility with woven technology are desired. Here we report on the development of a weaved, substrateless, and polarization-sensitive photodetector based on doping-engineered fibers of highly aligned carbon nanotubes. This room-temperature-operating, self-powered detector responds to radiation in an ultrabroad spectral range, from the ultraviolet to the terahertz, through the photothermoelectric effect, with a low noise-equivalent power (a few ${\mathrm{nW}/\mathrm{Hz}}^{1/2}$) throughout the range and with a $ZT$-factor value that is twice as large as that of previously reported carbon nanotube-based photothermoelectric photodetectors. Particularly, we fabricated a $\ensuremath{\sim}1$-m-long device consisting of tens of ${p}^{+}\text{\ensuremath{-}}{p}^{\ensuremath{-}}$ junctions and weaved it into a shirt. This device demonstrated a collective photoresponse of the series-connected junctions under global illumination. The performance of the device did not show any sign of deterioration through 200 bending tests with a bending radius smaller than 100 $\ensuremath{\mu}\mathrm{m}$ as well as standard washing and ironing cycles. This unconventional photodetector will find applications in wearable technology that require detection of electromagnetic radiation.

Ahmed Zubair,1 Xuan Wang,1 Francesca Mirri,2 Dmitri E. Tsentalovich,2 Naoki Fujimura,3 Daichi Suzuki,3 Karuppasamy P. Soundarapandian,4 Yukio Kawano,3 Matteo Pasquali,2,5,6,* and Junichiro Kono1,5,7,† 1Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA 2Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, USA 3Quantum Nano-electronics Research Center, Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan 4Department of Physics and Nanotechnology, SRM University, Chennai, Tamil Nadu 603203, India 5Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA 6Department of Chemistry, Rice University, Houston, Texas 77005, USA 7Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

Dispersing carbon nanotubes (CNTs) using surfactants into water requires ultrasonication that supplies mechanical energy to debundle and exfoliate CNTs. However, sonication is known to damage CNTs and to cut them into short fragments. Also, the CNT concentration in water dispersion is typically limited to up to 1.0 wt %. Here, we show that by using a sulfuric acid pretreatment, we can enhance the debundling of CNTs and reduce subsequent sonication to achieve homogeneous dispersions without damaging CNTs. Additionally, using a progressive and controlled dialysis, we are able to increase the CNT concentration up to 1.8 wt %. We demonstrate that such highly concentrated dispersions can be used as spin dopes to fabricate continuous fibers. Our fibers have an electrical conductivity up to 580 kS/m, a tensile strength of similar to 1 GPa, and a Young's modulus of 123 GPa, exceeding the mechanical properties of related fibers made from conventional surfactant-stabilized dispersions of sonicated CNTs.

At the microscopic scale, carbon nanotubes (CNTs) combine impressive tensile strength and electrical conductivity; however, their macroscopic counterparts have not met expectations. The reasons are variously attributed to inherent CNT sample properties (diameter and helicity polydispersity, high defect density, insufficient length) and manufacturing shortcomings (inadequate ordering and packing), which can lead to poor transmission of stress and current. To efficiently investigate the disparity between microscopic and macroscopic properties, a new method is introduced for processing microgram quantities of CNTs into highly oriented and well-packed fibers. CNTs are dissolved into chlorosulfonic acid and processed into aligned films; each film can be peeled and twisted into multiple discrete fibers. Fibers fabricated by this method and solution-spinning are directly compared to determine the impact of alignment, twist, packing density, and length. Surprisingly, these discrete fibers can be twice as strong as their solution-spun counterparts despite a lower degree of alignment. Strength appears to be more sensitive to internal twist and packing density, while fiber conductivity is essentially equivalent among the two sets of samples. Importantly, this rapid fiber manufacturing method uses three orders of magnitude less material than solution spinning, expanding the experimental parameter space and enabling the exploration of unique CNT sources.

Individual carbon nanotubes (CNTs) are highly conducting (both electrically and thermally), mechanically strong, and ultralight. These characteristics make them promising for use in various applications, including renewable energy, electronics, and aerospace. However, in macroscopic form, i.e., in the form of CNT bundles, these outstanding properties mostly vanish. Recently, fibers consisting of iodine-doped aligned CNTs with conductivities close to that of copper have been produced. Many studies have assumed polyiodide ions, such as tri-iodide (I 3 - ) and penta-iodide (I 5 - ), to be the origin of doping, but the observed large conductivity values cannot be explained with only polyiodides. Here, we quantitatively determine the doping level due to iodine and estimate the number of activated conductive channels in the double-wall CNTs (DWCNTs) that form the fibers. The CNT fibers used in this study were produced by the wet-spinning process, during which the nanotubes were doped with chlorosulfonic acid. The fibers were annealed for dedoping and then were kept in an iodine vapor chamber for 24 hours for doping. To quantify the conductivity increase due to iodine doping, we measured the current carrying capacity of the fibers. To determine the doping level, Raman experiments were performed at room temperature with 568 nm excitation on both iodine-doped and dedoped fibers. The figure shows G-band Raman spectra for (a) an iodine-doped DWCNT fiber and (b) a dedoped DWCNT fiber. These Raman spectra were fit with four Lorentzian peaks using the decomposition method developed from multiwavelength Raman spectroscopy studies on doped samples and high-pressure Raman experiments. These four peaks represent the upper (G + ) and lower (G - ) branches of the inner (G i ) and outer (G o ) tubes, characteristics of the G-band of DWCNTs. For the iodine-doped fiber, G o - , G i + and G o + are upshifted compared to the dedoped fiber. The G-band shift associated with the inner tube is due to the lattice contraction of the outer tube. From these shifts, it is possible to estimate the charge transfer per carbon atom, f C , to be 0.029 and the average Fermi level shift to be 1 eV. Due to the high level of doping, the number of conductive channels increases. To estimate the number of conductive channels, rough estimation was possible using E i (eV) = 0.38 i / d (nm) (for metallic tubes i = 0, 3, 6, ..., and for semiconducting tubes i = 1, 2, 4, 5, 7, 8, ...). Before doping, a statistical analysis gives 1/3 metallic (2 channels) and 2/3 semiconducting (0 channel) for inner and outer tubes, which suggests that the average number of conductive channels for a DWCNT is 4/3. As the outer diameter is ~2 nm, by moving the average Fermi level by 1 eV, we obtain i = ~5, which indicates that the number of conductive channels for a doped DWCNT is 26/3 on average. This conductance improvement is significant. By first-principles calculations based on the density functional theory (DFT), using the Vienna ab initio simulation package (VASP), the structural and electronic properties of both CNT systems were obtained. The plane-wave basis set cutoff energy was set to be 400 eV with a Gaussian smearing method of 0.005 eV width, in order to assure well-converged total energy and force values. All the atoms were allowed to relax until the maximum of forces acting on them became smaller than 0.01 eV/Angstrom. After optimization, the charge transfer of isolated CNTs and CNT bundles, respectively, was determined using Henkelman's group’s program for Bader charge analysis. This calculation proves that two I 2 in contact with CNTs can interact to form metastable I 3 - and I - with a large charge transfer to the carbon nanotube, considerably increasing the doping to a level compatible with experimental observations. Finally, another set of samples were prepared with iodine inside the DWCNTs in order to clearly observe the arrangements of the iodine atoms. We observe, through transmission electron microscopy (TEM), isolated I adjacent to I 3 species. Thus, large charge transfer due to I - and detection of I - through TEM explains the high conductivity achieved through iodine doping. Figure 1

We have developed an ultrabroadband thermal light emitter based on carbon nanotube fibers driven either by continuous or pulsed current. Particularly, in pulsed mode, this emitter provides shorter and more intense pulses of terahertz radiation than conventional thermal emitters.

Although previous research has explored the underlying theory of high-frequency behavior of carbon nanotubes (CNTs) and CNT bundles for antennas, there is a gap in the literature for direct experimental measurements of radiation efficiency. These measurements are crucial for any practical application of CNT materials in wireless communication. In this letter, we report a measurement technique to accurately characterize the radiation efficiency of λ/4 monopole antennas made from the CNT thread. We measure the highest absolute values of radiation efficiency for CNT antennas of any type, matching that of copper wire. To capture the weight savings, we propose a specific radiation efficiency metric and show that these CNT antennas exceed copper's performance by over an order of magnitude at 1 GHz and 2.4 GHz. We also report direct experimental observation that, contrary to metals, the radiation efficiency of the CNT thread improves significantly at higher frequencies. These results pave the way for practical applications of CNT thread antennas, particularly in the aerospace and wearable electronics industries where weight saving is a priority.

Graphenide solutions in NMP have been prepared by dispersing potassium intercalated graphite with the assistance of 18-crown-6. The highest graphenide solubility achieved is 1.5 mg mL-1. Graphenide solutions have been applied to spin graphene/SWCNT hybrid fibers.

The origin of highly efficient iodine doping of carbon nanotubes is not well understood. Relying on first-principles calculations, we found that iodine molecules (I-2) in contact with a carbon nanotube interact to form monoiodide or/and polyiodide from two and three I-2 as a result of removing electrons from the carbon nanotube (p-type doping). Charge per iodine atom for monoiodide ion or iodine atom at end of iodine chain is significantly higher than that for I-2. This atomic analysis extends previous studies showing that polyiodide ions are the dominant dopants. Moreover, we observed isolated I atoms in atomically resolved transmission electron microscopy, which proves the production of monoiodide. Finally, using Raman spectroscopy, we quantitatively determined the doping level and estimated the number of conducting channels in high electrical conductivity fibers composed of iodine-doped double-wall carbon nanotubes.

We study how intrinsic parameters of carbon nanotube (CNT) samples affect the properties of macroscopic CNT fibers with optimized structure. We measure CNT diameter, number of walls, aspect ratio, graphitic character, and purity (residual catalyst and non-CNT carbon) in samples from 19 suppliers; we process the highest quality CNT samples into aligned, densely packed fibers, by using an established wet-spinning solution process. We find that fiber properties are mainly controlled by CNT aspect ratio and that sample purity is important for effective spinning. Properties appear largely unaffected by CNT diameter, number of walls, and graphitic character (determined by Raman G/D ratio) as long as the fibers comprise thin few-walled CNTs with high G/D ratio (above ∼20). We show that both strength and conductivity can be improved simultaneously by assembling high aspect ratio CNTs, producing continuous CNT fibers with an average tensile strength of 2.4 GPa and a room temperature electrical conductivity of 8.5 MS/m, ∼2 times higher than the highest reported literature value (∼15% of copper's value), obtained without postspinning doping. This understanding of the relationship of intrinsic CNT parameters to macroscopic fiber properties is key to guiding CNT synthesis and continued improvement of fiber properties, paving the way for CNT fiber introduction in large-scale aerospace, consumer electronics, and textile applications.

Highly aligned, packed, and doped carbon nanotube (CNT) fibers with electrical conductivities approaching that of copper have recently become available. These fibers are promising for high-power electrical applications that require light-weight, high current-carrying capacity cables. However, a microscopic understanding of how doping affects the electrical conductance of such CNT fibers in a quantitative manner has been lacking. Here, we performed Raman spectroscopy measurements combined with first-principles calculations to determine the position of the average Fermi energy and to obtain the temperature of chlorosulfonic-acid-doped double-wall CNT fibers under high current. Due to the unique way in which double-wall CNT Raman spectra depend on doping, it is possible to use Raman data to determine the doping level quantitatively. The correspondence between the Fermi level shift and the carbon charge transfer is derived from a tight-binding model and validated by several calculations. For the doped fiber, we were able to associate an average Fermi energy shift of ∼-0.7 eV with a conductance increase by a factor of ∼5. Furthermore, since current induces heating, local temperature determination is possible. Through the Stokes-to-anti-Stokes intensity ratio of the G-band peaks, we estimated a temperature rise at the fiber surface of ∼135 K at a current density of 2.27 × 108 A m-2 identical to that from the G-band shift, suggesting that thermalization between CNTs is well achieved.

Conventional, commercially available terahertz (THz) polarizers are made of uniformly and precisely spaced metallic wires. They are fragile and expensive, with performance characteristics highly reliant on wire diameters and spacings. Here, we report a simple and highly error-tolerant method for fabricating a freestanding THz polarizer with nearly ideal performance, reliant on the intrinsically one-dimensional character of conduction electrons in well-aligned carbon nanotubes (CNTs). The polarizer was constructed on a mechanical frame over which we manually wound acid-doped CNT fibers with ultrahigh electrical conductivity. We demonstrated that the polarizer has an extinction ratio of ∼−30 dB with a low insertion loss (<0.5 dB) throughout a frequency range of 0.2–1.1 THz. In addition, we used a THz ellipsometer to measure the Müller matrix of the CNT-fiber polarizer and found comparable attenuation to a commercial metallic wire-grid polarizer. Furthermore, based on the classical theory of light transmission through an array of metallic wires, we demonstrated the most striking difference between the CNT-fiber and metallic wire-grid polarizers: the latter fails to work in the zero-spacing limit, where it acts as a simple mirror, while the former continues to work as an excellent polarizer even in that limit due to the one-dimensional conductivity of individual CNTs.

We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φiso of CNT solutions and show that this transition follows Onsager-like scaling φiso ∼ d/L. We characterize the length distributions of CNT samples by combining the measurements of ...

Acid spun carbon nanotube (CNT) fibers were investigated for their field emission properties and performance was determined to be dependent on fiber morphology. The fibers were fabricated by wet-spinning of pre-made CNTs. Fiber morphology was controlled by a fabrication method and processing conditions, as well as purity, size, and type of the CNT starting material. The internal fiber structure consisted of CNT fibrils held together by van der Waals forces. Alignment and packing density of the CNTs affects the fiber's electrical and thermal conductivity. Fibers with similar diameters and differing morphology were compared, and those composed of the most densely packed and well aligned CNTs were the best field emitters as exhibited by a lower turn-on voltage and a larger field enhancement factor. Fibers with higher electrical and thermal conductivity demonstrated higher maximum current before failure and longer lifetimes. A stable emission current at 3 mA was obtained for 10 h at a field strength of <1 V μm−1. This stable high current operation makes these CNT fibers excellent candidates for use as low voltage electron sources for vacuum electronic devices.

We have examined the anisotropic terahertz response of highly aligned single-wall, double-wall, and multiwall carbon nanotubes and quantitatively characterized their performance as low-cost terahertz polarizers.

We present a flexible ultrabroadband (visible to terahertz), polarization-sensitive carbon nanotube fiber photodetector. This photothermoelectric-effect-based room-temperature photodetector fabricated using a unique technique has high responsivities, up to 2.2 mA/W in the terahertz regime.