Browsing by Author "Komatsu, Natsumi"
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Item Colors of Single-Wall Carbon Nanotubes(Wiley, 2021) Wei, Nan; Tian, Ying; Liao, Yongping; Komatsu, Natsumi; Gao, Weilu; Lyuleeva‐Husemann, Alina; Zhang, Qiang; Hussain, Aqeel; Ding, Er-Xiong; Yao, Fengrui; Halme, Janne; Liu, Kaihui; Kono, Junichiro; Jiang, Hua; Kauppinen, Esko I.Although single-wall carbon nanotubes (SWCNTs) exhibit various colors in suspension, directly synthesized SWCNT films usually appear black. Recently, a unique one-step method for directly fabricating green and brown films has been developed. Such remarkable progress, however, has brought up several new questions. The coloration mechanism, potentially achievable colors, and color controllability of SWCNTs are unknown. Here, a quantitative model is reported that can predict the specific colors of SWCNT films and unambiguously identify the coloration mechanism. Using this model, colors of 466 different SWCNT species are calculated, which reveals a broad spectrum of potentially achievable colors of SWCNTs. The calculated colors are in excellent agreement with existing experimental data. Furthermore, the theory predicts the existence of many brilliantly colored SWCNT films, which are experimentally expected. This study shows that SWCNTs as a form of pure carbon, can display a full spectrum of vivid colors, which is expected to complement the general understanding of carbon materials.Item Giant terahertz polarization rotation in ultrathin films of aligned carbon nanotubes(Optical Society of America, 2021) Baydin, Andrey; Baydin, Andrey; Komatsu, Natsumi; Tay, Fuyang; Ghosh, Saunab; Makihara, Takuma; Noe, G. Timothy; Kono, Junichiro; Kono, Junichiro; Kono, Junichiro; Kono, JunichiroFor easy manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin (∼45nm), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to ∼20∘ and ∼110∘ for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a “magic” angle at which the total rotation through transmission and reflection becomes exactly 90°. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices.Item Hall effect in gated single-wall carbon nanotube films(Springer Nature, 2022) Yomogida, Yohei; Horiuchi, Kanako; Okada, Ryotaro; Kawai, Hideki; Ichinose, Yota; Nishidome, Hiroyuki; Ueji, Kan; Komatsu, Natsumi; Gao, Weilu; Kono, Junichiro; Yanagi, KazuhiroThe presence of hopping carriers and grain boundaries can sometimes lead to anomalous carrier types and density overestimation in Hall-effect measurements. Previous Hall-effect studies on carbon nanotube films reported unreasonably large carrier densities without independent assessments of the carrier types and densities. Here, we have systematically investigated the validity of Hall-effect results for a series of metallic, semiconducting, and metal–semiconductor-mixed single-wall carbon nanotube films. With carrier densities controlled through applied gate voltages, we were able to observe the Hall effect both in the n- and p-type regions, detecting opposite signs in the Hall coefficient. By comparing the obtained carrier types and densities against values derived from simultaneous field-effect-transistor measurements, we found that, while the Hall carrier types were always correct, the Hall carrier densities were overestimated by up to four orders of magnitude. This significant overestimation indicates that thin films of one-dimensional SWCNTs are quite different from conventional hopping transport systems.Item Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films(AIP Publishing LLC, 2018) Fukuhara, Kengo; Ichinose, Yota; Nishidome, Hiroyuki; Yomogida, Yohei; Katsutani, Fumiya; Komatsu, Natsumi; Gao, Weilu; Kono, Junichiro; Yanagi, KazuhiroHow the morphology of a macroscopic assembly of nanoobjects affects its properties is a long-standing question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both theﾠp-type andﾠn-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.Item Macroscopic weavable fibers of carbon nanotubes with giant thermoelectric power factor(Springer Nature, 2021) Komatsu, Natsumi; Ichinose, Yota; Dewey, Oliver S.; Taylor, Lauren W.; Trafford, Mitchell A.; Yomogida, Yohei; Wehmeyer, Geoff; Pasquali, Matteo; Yanagi, Kazuhiro; Kono, Junichiro; Carbon HubLow-dimensional materials have recently attracted much interest as thermoelectric materials because of their charge carrier confinement leading to thermoelectric performance enhancement. Carbon nanotubes are promising candidates because of their one-dimensionality in addition to their unique advantages such as flexibility and light weight. However, preserving the large power factor of individual carbon nanotubes in macroscopic assemblies has been challenging, primarily due to poor sample morphology and a lack of proper Fermi energy tuning. Here, we report an ultrahigh value of power factor (14 ± 5 mW m−1 K−2) for macroscopic weavable fibers of aligned carbon nanotubes with ultrahigh electrical and thermal conductivity. The observed giant power factor originates from the ultrahigh electrical conductivity achieved through excellent sample morphology, combined with an enhanced Seebeck coefficient through Fermi energy tuning. We fabricate a textile thermoelectric generator based on these carbon nanotube fibers, which demonstrates high thermoelectric performance, weavability, and scalability. The giant power factor we observe make these fibers strong candidates for the emerging field of thermoelectric active cooling, which requires a large thermoelectric power factor and a large thermal conductivity at the same time.Item Macroscopically Aligned Carbon Nanotubes: Preparation and Flexible Thermoelectric Applications(2020-05-18) Komatsu, Natsumi; Kono, JunichiroEver since the discovery of carbon nanotubes (CNTs), it has long been a challenging goal to create macroscopically ordered assemblies, or crystals, of CNTs that preserve extraordinary properties of individual CNTs on a macroscopic scale. Recently, a method to fabricate wafer-scale aligned CNT films via controlled vacuum filtration and a method to produce fibers of aligned CNTs through solution spinning have been developed. We first discuss parameters behind CNT alignment formation during vacuum filtration. We found that parallel grooves pre-existing on the surface of the filter membrane dictates the direction of the resulting CNT alignment, and further developed a method to imprint periodically spaced parallel grooves. We then discuss the thermoelectric properties of aligned CNT fibers. Thanks to the ultra-high electrical conductivity, we observed the highest p-type power factor at room temperature. Our finding provides a route for a powerful, mechanically robust, light weight, and non-toxic wearable thermoelectric devices.Item Modulation-Doped Multiple Quantum Wells of Aligned Single-Wall Carbon Nanotubes(Wiley, 2017) Komatsu, Natsumi; Gao, Weilu; Chen, Peiyu; Guo, Cheng; Babakhani, Aydin; Kono, JunichiroHeterojunctions, quantum wells, and superlattices with precise doping profiles are behind today's electronic and photonic devices based on III–V compound semiconductors such as GaAs. Currently, there is considerable interest in constructing similar artificial 3D architectures with tailored electrical and optical properties by using van der Waals junctions of low-dimensional materials. In this study, the authors have fabricated a novel structure consisting of multiple thin (≈20 nm) layers of aligned single-wall carbon nanotubes with dopants inserted between the layers. This “modulation-doped” multiple-quantum-well structure acts as a terahertz polarizer with an ultra-broadband working frequency range (from ≈0.2 to ≈200 THz), a high extinction ratio (20 dB from ≈0.2 to 1 THz), and a low insertion loss (<2.5 dB from ≈0.2 to 200 THz). The individual carbon nanotube films—highly aligned, densely packed, and large (2 in. in diameter)—were produced using vacuum filtration and then stacked together in the presence of dopants. This simple, robust, and cost-effective method is applicable to the fabrication of a variety of devices relying on macroscopically 1D properties of aligned carbon nanotube assemblies.Item Thermoelectric and Electronic Transport Studies of Ultrahigh-Conductivity Carbon Nanotube Fibers(2022-04-20) Komatsu, Natsumi; Kono, JunichiroCarbon nanotubes (CNTs) are marvelous one-dimensional (1D) materials with extraordinary electronic, mechanical, thermal, magnetic, and optical properties. However, the 1D properties of individual CNTs are often lost in randomly ordered macroscopic assemblies. Recently, macroscopic fibers of aligned carbon nanotubes (CNTs) with ultrahigh conductivity (> 10 MS/m) have emerged. Understanding of transport processes in these ordered CNT assemblies is critical for further conductivity improvement, but systematic experimental investigations have not been performed to date. Here, we have studied thermoelectric and electrical properties of CNT fibers produced by the solution spinning method. We first measured thermoelectric quantities while tuning the Fermi energy and obtained a giant thermoelectric power factor. In probing the temperature and magnetic field dependence of the electrical conductivity of the fibers, we observed a metallic behavior in a wide temperature range (40-300 K) – i.e., the conductivity monotonically increased with decreasing temperature. At low temperatures (< 50 K), strongly temperature-dependent negative magnetoresistance appeared, which is a hallmark of the phenomenon of weak localization, suggesting that the electron transport at low temperatures is quantum coherent. In addition to macroscopic CNT fibers with diameters of ~10 μm, we also performed conductivity measurements on individual crystalline CNT bundles (with diameters ~ 20 nm and lengths ~ 10 μm) that constitute the fibers, to determine the dimensionality and coherence lengths of carriers.Item Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control(American Chemical Society, 2023) Wais, Michael; Bagsican, Filchito Renee G.; Komatsu, Natsumi; Gao, Weilu; Serita, Kazunori; Murakami, Hironaru; Held, Karsten; Kawayama, Iwao; Kono, Junichiro; Battiato, Marco; Tonouchi, MasayoshiThe one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles’ dynamics, rerouting the thermalization pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.Item Ultrahigh strength, modulus, and conductivity of graphitic fibers by macromolecular coalescence(AAAS, 2022) Lee, Dongju; Kim, Seo Gyun; Hong, Seungki; Madrona, Cristina; Oh, Yuna; Park, Min; Komatsu, Natsumi; Taylor, Lauren W.; Chung, Bongjin; Kim, Jungwon; Hwang, Jun Yeon; Yu, Jaesang; Lee, Dong Su; Jeong, Hyeon Su; You, Nam Ho; Kim, Nam Dong; Kim, Dae-Yoon; Lee, Heon Sang; Lee, Kun-Hong; Kono, Junichiro; Wehmeyer, Geoff; Pasquali, Matteo; Vilatela, Juan J.; Ryu, Seongwoo; Ku, Bon-Cheol; The Carbon HubTheoretical considerations suggest that the strength of carbon nanotube (CNT) fibers be exceptional; however, their mechanical performance values are much lower than the theoretical values. To achieve macroscopic fibers with ultrahigh performance, we developed a method to form multidimensional nanostructures by coalescence of individual nanotubes. The highly aligned wet-spun fibers of single- or double-walled nanotube bundles were graphitized to induce nanotube collapse and multi-inner walled structures. These advanced nanostructures formed a network of interconnected, close-packed graphitic domains. Their near-perfect alignment and high longitudinal crystallinity that increased the shear strength between CNTs while retaining notable flexibility. The resulting fibers have an exceptional combination of high tensile strength (6.57 GPa), modulus (629 GPa), thermal conductivity (482 W/m·K), and electrical conductivity (2.2 MS/m), thereby overcoming the limits associated with conventional synthetic fibers.