R-3 Repository :: Browsing by Author "Gao, Weilu"

Browsing by Author "Gao, Weilu"

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Band structure dependent electronic localization in macroscopic films of single-chirality single-wall carbon nanotubes
(Elsevier, 2021) Gao, Weilu; Adinehloo, Davoud; Li, Xinwei; Mojibpour, Ali; Yomogida, Yohei; Hirano, Atsushi; Tanaka, Takeshi; Kataura, Hiromichi; Zheng, Ming; Perebeinos, Vasili; Kono, Junichiro
Significant understanding has been achieved over the last few decades regarding chirality-dependent properties of single-wall carbon nanotubes (SWCNTs), primarily through single-tube studies. However, macroscopic manifestations of chirality dependence have been limited, especially in electronic transport, despite the fact that such distinct behaviors are needed for many applications of SWCNT-based devices. In addition, developing reliable transport theory is challenging since a description of localization phenomena in an assembly of nanoobjects requires precise knowledge of disorder on multiple spatial scales, particularly if the ensemble is heterogeneous. Here, we report an observation of pronounced chirality-dependent electronic localization in temperature and magnetic field dependent conductivity measurements on macroscopic films of single-chirality SWCNTs. The samples included large-gap semiconducting (6,5) and (10,3) films, narrow-gap semiconducting (7,4) and (8,5) films, and armchair metallic (6,6) films. Experimental data and theoretical calculations revealed Mott variable-range-hopping dominated transport in all samples, while localization lengths fall into three distinct categories depending on their band gaps. Armchair films have the largest localization length. Our detailed analyses on electronic transport properties of single-chirality SWCNT films provide significant new insight into electronic transport in ensembles of nanoobjects, offering foundations for designing and deploying macroscopic SWCNT solid-state devices.
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.
Continuous transition between weak and ultrastrong coupling through exceptional points in carbon nanotube microcavity exciton–polaritons
(Springer Nature, 2018) Gao, Weilu; Li, Xinwei; Bamba, Motoaki; Kono, Junichiro
Non-perturbative coupling of photons and excitons produces hybrid particles, exciton–polaritons, which have exhibited a variety of many-body phenomena in various microcavity systems. However, the vacuum Rabi splitting (VRS), which defines the strength of photon–exciton coupling, is usually a single constant for a given system. Here, we have developed a unique architecture in which excitons in an aligned single-chirality carbon nanotube film interact with cavity photons in polarization-dependent manners. The system reveals ultrastrong coupling (VRS up to 329 meV or a coupling-strength-to-transition-energy ratio of 13.3%) for polarization parallel to the nanotube axis, whereas VRS is absent for perpendicular polarization. Between these two extremes, VRS is continuously tunable through polarization rotation with exceptional points separating crossing and anticrossing. The points between exceptional points form equienergy arcs onto which the upper and lower polaritons coalesce. The demonstrated on-demand ultrastrong coupling provides ways to explore topological properties of polaritons and quantum technology applications.
Destabilization of Surfactant-Dispersed Carbon Nanotubes by Anions
(Springer, 2017) Hirano, Atsushi; Gao, Weilu; He, Xiaowei; Kono, Junichiro
The colloidal stability of surfactant-dispersed single-wall carbon nanotubes (SWCNTs) is determined by microscopic physicochemical processes, such as association, partitioning, and adsorption propensities. These processes can be controlled by the addition of solutes. While the effects of cations on the colloidal stability of SWCNTs are relatively well understood, little is known about the effects of anions. In this study, we examined the effects of anions on the stability of SWCNTs dispersed by sodium dodecyl sulfate (SDS) using sodium salts, such as NaCl and NaSCN. We observed that the intensity of the radial breathing mode Raman peaks rapidly decreased as the salts were added, even at concentrations less than 25 mM, indicating the association of SWCNTs. The effect was stronger with NaSCN than NaCl. We propose that the association of SWCNTs was caused by thermodynamic destabilization of SDS assemblies on SWCNT surfaces by these salts, which was confirmed through SWCNT separation experiments using aqueous two-phase extraction and gel chromatography. These results demonstrate that neutral salts can be used to control the colloidal stability of surfactant-dispersed SWCNTs.
Efficient Modulation of 1.55 μm Radiation with Gated Graphene on a Silicon Microring Resonator
(American Chemical Society, 2014) Qiu, Ciyuan; Gao, Weilu; Vajtai, Robert; Ajayan, Pulickel M.; Kono, Junichiro; Xu, Qianfan
The gate-controllability of the Fermi-edge onset of interband absorption in graphene can be utilized to modulate near-infrared radiation in the telecommunication band. However, a high modulation efficiency has not been demonstrated to date, because of the small amount of light absorption in graphene. Here, we demonstrate a ~40% amplitude modulation of 1.55 μm radiation with gated single-layer graphene that is coupled with a silicon microring resonator. Both the quality factor and resonance wavelength of the silicon microring resonator were strongly modulated through gate tuning of the Fermi level in graphene. These results promise an efficient electro-optic modulator, ideal for applications in large-scale on-chip optical interconnects that are compatible with complementary metal-oxide-semiconductor technology
Engineering chirality at wafer scale with ordered carbon nanotube architectures
(Springer Nature, 2023) Doumani, Jacques; Lou, Minhan; Dewey, Oliver; Hong, Nina; Fan, Jichao; Baydin, Andrey; Zahn, Keshav; Yomogida, Yohei; Yanagi, Kazuhiro; Pasquali, Matteo; Saito, Riichiro; Kono, Junichiro; Gao, Weilu; Carbon Hub; Smalley-Curl Institute
Creating artificial matter with controllable chirality in a simple and scalable manner brings new opportunities to diverse areas. Here we show two such methods based on controlled vacuum filtration - twist stacking and mechanical rotation - for fabricating wafer-scale chiral architectures of ordered carbon nanotubes (CNTs) with tunable and large circular dichroism (CD). By controlling the stacking angle and handedness in the twist-stacking approach, we maximize the CD response and achieve a high deep-ultraviolet ellipticity of 40 ± 1 mdeg nm−1. Our theoretical simulations using the transfer matrix method reproduce the experimentally observed CD spectra and further predict that an optimized film of twist-stacked CNTs can exhibit an ellipticity as high as 150 mdeg nm−1, corresponding to a g factor of 0.22. Furthermore, the mechanical rotation method not only accelerates the fabrication of twisted structures but also produces both chiralities simultaneously in a single sample, in a single run, and in a controllable manner. The created wafer-scale objects represent an alternative type of synthetic chiral matter consisting of ordered quantum wires whose macroscopic properties are governed by nanoscopic electronic signatures and can be used to explore chiral phenomena and develop chiral photonic and optoelectronic devices.
Extraordinary sensitivity enhancement by metasurfaces in terahertz detection of antibiotics
(Nature Publishing Group, 2015) Xie, Lijuan; Gao, Weilu; Shu, Jie; Ying, Yibin; Kono, Junichiro
We have detected trace amounts of molecules of antibiotics (kanamycin sulfate) dispersed on metasurfaces with terahertz (THz) spectroscopy. Utilizing the extraordinary optical transmission resonance of an array of square-shaped slits on a silicon substrate at ~0.3 THz, we were able to monitor varying concentrations of kanamycin sulfate as low as ~100 picogram/L. In contrast, the lowest detectable concentration of kanamycin sulfate on silicon without any metallic structure was ~1 gram/L. This dramatic ~1010 times enhancement of sensitivity is due to the near-field enhancement of THz electric fields by the metamaterial structure. This result thus demonstrates the power and usefulness of metamaterial-assisted THz spectroscopy in trace molecular detection for biological and chemical sensing as well as for food product quality and safety inspection and control.
Graphene Photonic Devices for Terahertz and Mid-Infrared
(2013-11-08) Gao, Weilu; Xu, Qianfan; Kono, Junichiro; Mittleman, Daniel M.
Graphene and other strictly two-dimensional materials are the rising stars on the horizon of material science, condensed matter physics and engineering. The richness of optical and electronic properties of graphene attract much interest due to the exceptional high crystal and electronic quality resulting in large carrier mobility at room temperature and easily electrical control of carrier density, which find its true potential in photonics and optoelectronics. Novel graphene based broadband modulators, polarizer, active plasmonic resonators, ultra-fast lasers and etc are proposed and implemented in many literatures. Despite ample demonstrations of the true potential of graphene in optoelectronic devices, there is still unexplored region. In this thesis, we investigate the graphene photonic properties and optoelectronic devices in different regions ranging from longer wavelength terahertz frequency (THz) to shorter wavelength telecommunication frequency to reveal the whole picture of graphene. The Drude-like intraband absorptoion (i.e. free carrier effect) in graphene plays an important role in THz region. However, the extinction ratio that can be obtained when THz waves passing through a single layer graphene is limited due to its one-atomic-layer thickness and the non-resonant nature of the intraband absorption. By incorporating resonate structures with graphene, the high extinction ratio of THz wave transmission will be achieved utilizing the high localized electric field near the graphene layer. Combining the electrically controlled carrier density in graphene, a graphene-based THz modulators with high modulation depth, fast speed can be built. High carrier mobility of graphene at room temperature makes it a new platform for plasmonics with strong light-matter interactions, which has been theoretically proved to be able to support surface plasmon polarions (SPPs) with lower loss and higher mode confinement compared with metals. Furthermore, the electrically controlled carrier density of graphene renders it new possibility to build active plasmonic devices. Although many efforts have been done by either shaping the graphene to excite localized plamons or using near-field method to excite SPPs in continuous graphene layer in spite of low efficiency, we theoretically propose and experimentally demonstrate to utilize silicon diffractive gratings underneath the graphene to excite graphene SPPs, which can be actively controlled via back-gating structure. The ac carrier dynamics of graphene have different contribution at different frequency range, which are investigated by incorporating graphene with resonators operating at different frequencies. In mid-infrared region the same structure as that in THz region is integrated with graphene that proves the almost complete transparency of graphene in mid-infrared region for intrinsically doping graphene while in the shorter wavelength of telecommnucations frequency, graphene is also integrated with silicon ring resonators, which shows large absorption. However, this large absorption is resulted from interband absorption that is quite different from what we have observed in THz region that is from intraband absorption. So in summary, the intraband and interband carrier dynamics of graphene will have different contributions in devices operating at different frequency region, which makes the various applications available.
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, Kazuhiro
The 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.
Intersubband plasmons in the quantum limit in gated and aligned carbon nanotubes
(Springer Nature, 2018) Yanagi, Kazuhiro; Okada, Ryotaro; Ichinose, Yota; Yomogida, Yohei; Katsutani, Fumiya; Gao, Weilu; Kono, Junichiro
Confined electrons collectively oscillate in response to light, resulting in a plasmon resonance whose frequency is determined by the electron density and the size and shape of the confinement structure. Plasmons in metallic particles typically occur in the classical regime where the characteristic quantum level spacing is negligibly small compared to the plasma frequency. In doped semiconductor quantum wells, quantum plasmon excitations can be observed, where the quantization energy exceeds the plasma frequency. Such intersubband plasmons occur in the mid- and far-infrared ranges and exhibit a variety of dynamic many-body effects. Here, we report the observation of intersubband plasmons in carbon nanotubes, where both the quantization and plasma frequencies are larger than those of typical quantum wells by three orders of magnitude. As a result, we observed a pronounced absorption peak in the near-infrared. Specifically, we observed the near-infrared plasmon peak in gated films of aligned single-wall carbon nanotubes only for probe light polarized perpendicular to the nanotube axis and only when carriers are present either in the conduction or valence band. Both the intensity and frequency of the peak were found to increase with the carrier density, consistent with the plasmonic nature of the resonance. Our observation of gate-controlled quantum plasmons in aligned carbon nanotubes will not only pave the way for the development of carbon-based near-infrared optoelectronic devices but also allow us to study the collective dynamic response of interacting electrons in one dimension.
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, Kazuhiro
How 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.
Modulation-Doped Multiple Quantum Wells of Aligned Single-Wall Carbon Nanotubes
(Wiley, 2017) Komatsu, Natsumi; Gao, Weilu; Chen, Peiyu; Guo, Cheng; Babakhani, Aydin; Kono, Junichiro
Heterojunctions, 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.
Phonon-Assisted Intertube Electronic Transport in an Armchair Carbon Nanotube Film
(American Physical Society, 2023) Adinehloo, Davoud; Gao, Weilu; Mojibpour, Ali; Kono, Junichiro; Perebeinos, Vasili; The Smalley-Curl Institute
The electrical conductivity of a macroscopic assembly of nanomaterials is determined through a complex interplay of electronic transport within and between constituent nano-objects. Phonons play dual roles in this situation: their increased populations tend to reduce the conductivity via electron scattering, while they can boost the conductivity by assisting electrons to propagate through the potential-energy landscape. We identified a phonon-assisted coherent electron transport process between neighboring nanotubes in temperature-dependent conductivity measurements on a macroscopic film of armchair single-wall carbon nanotubes. Through atomistic modeling of electronic states and calculations of both electronic and phonon-assisted junction conductances, we conclude that phonon-assisted conductance is the dominant mechanism for observed high-temperature transport in armchair carbon nanotubes. The unambiguous manifestation of coherent intertube dynamics proves a single-chirality armchair nanotube film to be a unique macroscopic solid-state ensemble of nano-objects promising for the development of room-temperature coherent electronic devices.
Science and applications of wafer-scale crystalline carbon nanotube films prepared through controlled vacuum filtration
(The Royal Society, 2019) Gao, Weilu; Kono, Junichiro
Carbon nanotubes (CNTs) make an ideal one-dimensional (1D) material platform for the exploration of novel physical phenomena under extremely strong quantum confinement. The 1D character of electrons, phonons and excitons in individual CNTs features extraordinary electronic, thermal and optical properties. Since their discovery in 1991, they have been continuing to attract interest in various disciplines, including chemistry, materials science, physics and engineering. However, the macroscopic manifestation of 1D properties is still limited, despite significant efforts for decades. Recently, a controlled vacuum filtration method has been developed for the preparation of wafer-scale films of crystalline chirality-enriched CNTs, and such films have enabled exciting new fundamental studies and applications. In this review, we will first discuss the controlled vacuum filtration technique, and then summarize recent discoveries in optical spectroscopy studies and optoelectronic device applications using films prepared by this technique.
Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy
(American Physical Society, 2016) Zhang, Qi; Wang, Yongrui; Gao, Weilu; Long, Zhongqu; Watson, John D.; Manfra, Michael J.; Belyanin, Alexey; Kono, Junichiro
We have performed time-resolved terahertz absorption measurements on photoexcited electron-hole pairs in undoped GaAs quantum wells in magnetic fields. We probed both unbound- and bound-carrier responses via cyclotron resonance and intraexciton resonance, respectively. The stability of excitons, monitored as the pair density was systematically increased, was found to dramatically increase with increasing magnetic field. Specifically, the 1s−2p− intraexciton transition at 9 T persisted up to the highest density, whereas the 1s−2p feature at 0 T was quickly replaced by a free-carrier Drude response. Interestingly, at 9 T, the 1s−2p− peak was replaced by free-hole cyclotron resonance at high temperatures, indicating that 2D magnetoexcitons do dissociate under thermal excitation, even though they are stable against a density-driven Mott transition.
Terahertz Faraday and Kerr rotation spectroscopy of Bi1−xSbx films in high magnetic fields up to 30 tesla
(American Physical Society, 2019) Li, Xinwei; Yoshioka, Katsumasa; Xie, Ming; Noe, G. Timothy; Lee, Woojoo; Marquez Peraca, Nicolas; Gao, Weilu; Hagiwara, Toshio; Handegård, Ørjan S.; Nien, Li-Wei; Nagao, Tadaaki; Kitajima, Masahiro; Nojiri, Hiroyuki; Shih, Chih-Kang; MacDonald, Allan H.; Katayama, Ikufumi; Takeda, Jun; Fiete, Gregory A.; Kono, Junichiro
We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of Bi1−xSbx, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition x increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed minicoil magnet, we conducted measurements in magnetic fields up to 30 T, observing distinctly different behaviors between semimetallic (x<0.07) and topological insulator (x>0.07) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blueshifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields (>20 T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi1−xSbx.
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, Masayoshi
The 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.
Vacuum Bloch–Siegert shift in Landau polaritons with ultra-high cooperativity
(Springer Nature, 2018) Li, Xinwei; Bamba, Motoaki; Zhang, Qi; Fallahi, Saeed; Gardner, Geoff C.; Gao, Weilu; Lou, Minhan; Yoshioka, Katsumasa; Manfra, Michael J.; Kono, Junichiro
A two-level system resonantly interacting with an a.c. magnetic or electric field constitutes the physical basis of diverse phenomena and technologies. However, Schrödinger’s equation for this seemingly simple system can be solved exactly only under the rotating-wave approximation, which neglects the counter-rotating field component. When the a.c. field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch–Siegert shift. Here, we report the vacuum Bloch–Siegert shift, which is induced by the ultra-strong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultra-high-mobility two-dimensional electron gas inside a high-Qterahertz cavity in a quantizing magnetic field revealed ultra-narrow Landau polaritons, which exhibited a vacuum Bloch–Siegert shift up to 40 GHz. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field.
Wafer-scale Films and Devices of Spontaneously Aligned Carbon Nanotubes
(2016-06-06) Gao, Weilu; Kono, Junichiro
One of the grand challenges in nanoscience and nanotechnology is how to create macroscopic devices by assembling nano-objects while preserving their extraordinary properties. For example, individual single-wall carbon nanotubes (SWCNTs) possess unique one-dimensional properties that have stimulated much interest in diverse disciplines, and worldwide efforts are in progress to produce large-scale architectures of aligned SWCNTs. Various methods have been proposed and/or demonstrated, including both direct-growth and post-growth schemes, but the current state of this field is that there is still no method available for producing large-area single-domain films of highly aligned, densely packed and chirality-enriched SWCNTs. In this thesis, we describe a new process of controlled differential pressure filtration (CDPF) for producing a wafer-scale (i.e., inch-size) ﬁlm of aligned SWCNTs. This method works for SWCNTs synthesized by various methods and can be scaled up in three dimensions (i.e., in lateral size and thickness). We extensively characterized the produced large-area ﬁlms with different microscopy, spectroscopy and transport methods to demonstrate perfect global alignment with extraordinary photonic and optoelectronic properties. We developed ideal terahertz/infrared polarizers using this approach. The strikingly high degree of alignment with nematic order parameter (S) ~1 and the scalability with thickness ~ 100 nm distinguish CDPF from both existing two-dimensional (2D) and three-dimensional (3D) post-growth assembly techniques. We investigated the underlying mechanisms based on a proposed model of 2D confinement induced phase transition. We identified factors affecting the degree of alignment, including filtration speed, SWCNT concentration, surfactant concentration, hydrophilicity of the filter membrane, SWCNT length, and SWCNT diameter, in order to optimize filtration conditions for an optimally aligned film. Furthermore, by combining CDFP with well-developed solution-based chirality separation techniques (the gel chromatography and aqueous two phase extraction methods), we succeeded in producing chirality-enriched aligned films. The globally aligned chirality-enriched SWCNT films are promising for optoelectronic and electronic device applications. We demonstrated polarized luminescent devices, polarization-sensitive photodetectors, and anisotropic thin-ﬁlm transistors using semiconducting SWCNTs. CDPF-produced SWCNT films will produce a range of new opportunities not only in fundamental research of physics, chemistry, and materials science but also applications in electronics, optoelectronics, sensing, imaging, and medicine.
Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes
(Springer Nature, 2016) He, Xiaowei; Gao, Weilu; Xie, Lijuan; Li, Bo; Zhang, Qi; Lei, Sidong; Robinson, John M.; Hároz, Erik H.; Doorn, Stephen K.; Wang, Weipeng; Vajtai, Robert; Ajayan, Pulickel M.; Adams, W. Wade; Hauge, Robert H.; Kono, Junichiro
The one-dimensional character of electrons, phonons and excitons in individual single-walled carbon nanotubes leads to extremely anisotropic electronic, thermal and optical properties. However, despite significant efforts to develop ways to produce large-scale architectures of aligned nanotubes, macroscopic manifestations of such properties remain limited. Here, we show that large (>cm2) monodomain films of aligned single-walled carbon nanotubes can be prepared using slow vacuum filtration. The produced films are globally aligned within ±1.5° (a nematic order parameter of ∼1) and are highly packed, containing 1 × 106 nanotubes in a cross-sectional area of 1 μm2. The method works for nanotubes synthesized by various methods, and film thickness is controllable from a few nanometres to ∼100 nm. We use the approach to create ideal polarizers in the terahertz frequency range and, by combining the method with recently developed sorting techniques, highly aligned and chirality-enriched nanotube thin-film devices. Semiconductor-enriched devices exhibit polarized light emission and polarization-dependent photocurrent, as well as anisotropic conductivities and transistor action with high on/off ratios.

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