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Bagsican FG, Wais M, Komatsu N, Gao W, Weber LW, Serita K, Murakami H, Held K, Hegmann FA, Tonouchi M, Kono J, Kawayama I, Battiato M. Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication. NANO LETTERS 2020; 20:3098-3105. [PMID: 32227963 PMCID: PMC7227006 DOI: 10.1021/acs.nanolett.9b05082] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/10/2019] [Revised: 03/09/2020] [Indexed: 05/26/2023]
Abstract
Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.
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Komatsu N, Nakamura M, Ghosh S, Kim D, Chen H, Katagiri A, Yomogida Y, Gao W, Yanagi K, Kono J. Groove-Assisted Global Spontaneous Alignment of Carbon Nanotubes in Vacuum Filtration. NANO LETTERS 2020; 20:2332-2338. [PMID: 32092275 DOI: 10.1021/acs.nanolett.9b04764] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Ever 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 the one-dimensional quantum properties of individual CNTs on a macroscopic scale. Recently, a simple and well-controlled method was reported for producing wafer-scale crystalline films of highly aligned and densely packed CNTs through spontaneous global alignment that occurs during vacuum filtration (Nat. Nanotechnol. 2016, 11, 633). However, a full understanding of the mechanism of such global alignment has not been achieved. Here, we report results of a series of systematic experiments that demonstrate that the CNT alignment direction can be controlled by the surface morphology of the filter membrane used in the vacuum filtration process. More specifically, we found that the direction of parallel grooves pre-existing on the surface of the filter membrane dictates the direction of the resulting CNT alignment. Furthermore, we intentionally imprinted periodically spaced parallel grooves on a filter membrane using a diffraction grating, which successfully defined the direction of the global alignment of CNTs in a precise and reproducible manner. These results are promising not only for developing novel devices based on macroscopically aligned CNTs but also for understanding the microscopic physical mechanism of the alignment process.
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Heller DA, Jena PV, Pasquali M, Kostarelos K, Delogu LG, Meidl RE, Rotkin SV, Scheinberg DA, Schwartz RE, Terrones M, Wang Y, Bianco A, Boghossian AA, Cambré S, Cognet L, Corrie SR, Demokritou P, Giordani S, Hertel T, Ignatova T, Islam MF, Iverson NM, Jagota A, Janas D, Kono J, Kruss S, Landry MP, Li Y, Martel R, Maruyama S, Naumov AV, Prato M, Quinn SJ, Roxbury D, Strano MS, Tour JM, Weisman RB, Wenseleers W, Yudasaka M. Banning carbon nanotubes would be scientifically unjustified and damaging to innovation. NATURE NANOTECHNOLOGY 2020; 15:164-166. [PMID: 32157238 PMCID: PMC10461884 DOI: 10.1038/s41565-020-0656-y] [Citation(s) in RCA: 50] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
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Prochaska L, Li X, MacFarland DC, Andrews AM, Bonta M, Bianco EF, Yazdi S, Schrenk W, Detz H, Limbeck A, Si Q, Ringe E, Strasser G, Kono J, Paschen S. Singular charge fluctuations at a magnetic quantum critical point. Science 2020; 367:285-288. [DOI: 10.1126/science.aag1595] [Citation(s) in RCA: 31] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2018] [Revised: 09/07/2019] [Accepted: 12/05/2019] [Indexed: 11/02/2022]
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Ichinose Y, Yoshida A, Horiuchi K, Fukuhara K, Komatsu N, Gao W, Yomogida Y, Matsubara M, Yamamoto T, Kono J, Yanagi K. Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes. NANO LETTERS 2019; 19:7370-7376. [PMID: 31498635 DOI: 10.1021/acs.nanolett.9b03022] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Semiconductors are generally considered far superior to metals as thermoelectric materials because of their much larger Seebeck coefficients (S). However, a maximum value of S in a semiconductor is normally accompanied by a minuscule electrical conductivity (σ), and hence, the thermoelectric power factor (P = S2σ) remains small. An attempt to increase σ by increasing the Fermi energy (EF), on the other hand, decreases S. This trade-off between S and σ is a well-known dilemma in developing high-performance thermoelectric devices based on semiconductors. Here, we show that the use of metallic carbon nanotubes (CNTs) with tunable EF solves this long-standing problem, demonstrating a higher thermoelectric performance than semiconducting CNTs. We studied the EF dependence of S, σ, and P in a series of CNT films with systematically varied metallic CNT contents. In purely metallic CNT films, both S and σ monotonically increased with EF, continuously boosting P while increasing EF. Particularly, in an aligned metallic CNT film, the maximum of P was ∼5 times larger than that in the highest-purity (>99%) single-chirality semiconducting CNT film. We attribute these superior thermoelectric properties of metallic CNTs to the simultaneously enhanced S and σ of one-dimensional conduction electrons near the first van Hove singularity.
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Gao W, Kono J. Science and applications of wafer-scale crystalline carbon nanotube films prepared through controlled vacuum filtration. ROYAL SOCIETY OPEN SCIENCE 2019; 6:181605. [PMID: 31032018 PMCID: PMC6458426 DOI: 10.1098/rsos.181605] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/27/2018] [Accepted: 02/06/2019] [Indexed: 05/26/2023]
Abstract
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.
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Green ME, Bas DA, Yao HY, Gengler JJ, Headrick RJ, Back TC, Urbas AM, Pasquali M, Kono J, Her TH. Bright and Ultrafast Photoelectron Emission from Aligned Single-Wall Carbon Nanotubes through Multiphoton Exciton Resonance. NANO LETTERS 2019; 19:158-164. [PMID: 30484322 DOI: 10.1021/acs.nanolett.8b03564] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Ultrashort bunches of electrons, emitted from solid surfaces through excitation by ultrashort laser pulses, are an essential ingredient in advanced X-ray sources, and ultrafast electron diffraction and spectroscopy. Multiphoton photoemission using a noble metal as the photocathode material is typically used but more brightness is desired. Artificially structured metal photocathodes have been shown to enhance optical absorption via surface plasmon resonance but such an approach severely reduces the damage threshold in addition to requiring state-of-the-art facilities for photocathode fabrication. Here, we report ultrafast photoelectron emission from sidewalls of aligned single-wall carbon nanotubes. We utilized strong exciton resonances inherent in this prototypical one-dimensional material, and its excellent thermal conductivity and mechanical rigidity leading to a high damage threshold. We obtained unambiguous evidence for resonance-enhanced multiphoton photoemission processes with definite power-law behaviors. In addition, we observed strong polarization dependence and ultrashort photoelectron response time, both of which can be quantitatively explained by our model. These results firmly establish aligned single-wall carbon nanotube films as novel and promising ultrafast photocathode material.
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Li X, Bamba M, Yuan N, Zhang Q, Zhao Y, Xiang M, Xu K, Jin Z, Ren W, Ma G, Cao S, Turchinovich D, Kono J. Observation of Dicke cooperativity in magnetic interactions. Science 2018; 361:794-797. [DOI: 10.1126/science.aat5162] [Citation(s) in RCA: 66] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/07/2018] [Accepted: 06/21/2018] [Indexed: 11/02/2022]
Abstract
The interaction ofNtwo-level atoms with a single-mode light field is an extensively studied many-body problem in quantum optics, first analyzed by Dicke in the context of superradiance. A characteristic of such systems is the cooperative enhancement of the coupling strength by a factor of N. In this study, we extended this cooperatively enhanced coupling to a solid-state system, demonstrating that it also occurs in a magnetic solid in the form of matter-matter interaction. Specifically, the exchange interaction ofNparamagnetic erbium(III) (Er3+) spins with an iron(III) (Fe3+) magnon field in erbium orthoferrite (ErFeO3) exhibits a vacuum Rabi splitting whose magnitude is proportional to N. Our results provide a route for understanding, controlling, and predicting novel phases of condensed matter using concepts and tools available in quantum optics.
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Sui C, Yang Y, Headrick RJ, Pan Z, Wu J, Zhang J, Jia S, Li X, Gao W, Dewey OS, Wang C, He X, Kono J, Pasquali M, Lou J. Directional sensing based on flexible aligned carbon nanotube film nanocomposites. NANOSCALE 2018; 10:14938-14946. [PMID: 30046774 DOI: 10.1039/c8nr02137f] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
The electrical behaviors under mechanical deformation of an aligned single-walled carbon nanotube (SWCNT) film nanocomposite have been systematically investigated in this work. Electrical signals along the CNT axis (‖) and perpendicular to the CNT axis (⊥) follow a specific pattern, which enables the mechanical motion to be determined by vector analysis of such signals. The unique electrical behaviors of the sandwiched nanocomposites originate from the anisotropic characteristics of the CNT films. By combining in situ mechanical investigation with a coarse-grained molecular dynamics simulation, the shearing effect between SWCNTs is found to play a key role in stress-transfer along the ‖ direction, resulting in arc-shape cracks, while the peeling effect is dominant along the ⊥ direction, leading to unifom SWCNT bar bridging at cracks. The fabricated CNT based sandwiched nanocomposite is believed to have great potential in building flexible all-direction sensors.
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Blancon JC, Stier AV, Tsai H, Nie W, Stoumpos CC, Traoré B, Pedesseau L, Kepenekian M, Katsutani F, Noe GT, Kono J, Tretiak S, Crooker SA, Katan C, Kanatzidis MG, Crochet JJ, Even J, Mohite AD. Scaling law for excitons in 2D perovskite quantum wells. Nat Commun 2018; 9:2254. [PMID: 29884900 PMCID: PMC5993799 DOI: 10.1038/s41467-018-04659-x] [Citation(s) in RCA: 301] [Impact Index Per Article: 50.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2017] [Accepted: 05/09/2018] [Indexed: 12/03/2022] Open
Abstract
Ruddlesden–Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A’n-1MnX3n+1, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n-value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free carriers) and the exciton reduced mass, and their scaling with quantum well thickness, which are critical for designing efficient optoelectronic devices, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modeling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with both exciton reduced masses and binding energies decreasing, respectively, from 0.221 m0 to 0.186 m0 and from 470 meV to 125 meV with increasing thickness from n equals 1 to 5. Based on this study we propose a general scaling law to determine the binding energy of excitons in perovskite quantum wells of any layer thickness. Hybrid 2D layered perovskites are solution-processed quantum wells whose optoelectronic properties are tunable by varying the thickness of the inorganic slab. Here Blancon et al. work out a general behavior for dependence of the excitonic properties in layered 2D perovskites.
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Yanagi K, Okada R, Ichinose Y, Yomogida Y, Katsutani F, Gao W, Kono J. Intersubband plasmons in the quantum limit in gated and aligned carbon nanotubes. Nat Commun 2018; 9:1121. [PMID: 29549341 PMCID: PMC5856781 DOI: 10.1038/s41467-018-03381-y] [Citation(s) in RCA: 45] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2017] [Accepted: 02/06/2018] [Indexed: 11/09/2022] Open
Abstract
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.
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Stier AV, Wilson NP, Velizhanin KA, Kono J, Xu X, Crooker SA. Magnetooptics of Exciton Rydberg States in a Monolayer Semiconductor. PHYSICAL REVIEW LETTERS 2018; 120:057405. [PMID: 29481196 DOI: 10.1103/physrevlett.120.057405] [Citation(s) in RCA: 75] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/14/2017] [Indexed: 06/08/2023]
Abstract
We report 65 T magnetoabsorption spectroscopy of exciton Rydberg states in the archetypal monolayer semiconductor WSe_{2}. The strongly field-dependent and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons permits their unambiguous identification and allows for quantitative comparison with leading theoretical models. Both the sizes (via low-field diamagnetic shifts) and the energies of the ns exciton states agree remarkably well with detailed numerical simulations using the nonhydrogenic screened Keldysh potential for 2D semiconductors. Moreover, at the highest magnetic fields, the nearly linear diamagnetic shifts of the weakly bound 3s and 4s excitons provide a direct experimental measure of the exciton's reduced mass m_{r}=0.20±0.01m_{0}.
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Du L, Li X, Lou W, Sullivan G, Chang K, Kono J, Du RR. Evidence for a topological excitonic insulator in InAs/GaSb bilayers. Nat Commun 2017; 8:1971. [PMID: 29215018 PMCID: PMC5719361 DOI: 10.1038/s41467-017-01988-1] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2017] [Accepted: 10/30/2017] [Indexed: 11/09/2022] Open
Abstract
Electron–hole pairing can occur in a dilute semimetal, transforming the system into an excitonic insulator state in which a gap spontaneously appears at the Fermi surface, analogous to a Bardeen–Cooper–Schrieffer (BCS) superconductor. Here, we report optical spectroscopic and electronic transport evidence for the formation of an excitonic insulator gap in an inverted InAs/GaSb quantum-well system at low temperatures and low electron–hole densities. Terahertz transmission spectra exhibit two absorption lines that are quantitatively consistent with predictions from the pair-breaking excitation dispersion calculated based on the BCS gap equation. Low-temperature electronic transport measurements reveal a gap of ~2 meV (or ~25 K) with a critical temperature of ~10 K in the bulk, together with quantized edge conductance, suggesting the occurrence of a topological excitonic insulator phase. Weakly bound electron–hole pairs may condensate in two-dimensional systems, but experimental evidence has been lacking. Here, Du et al. report optical spectroscopic and electronic transport evidences for the formation of an excitonic insulator gap in topological InAs/GaSb quantum wells.
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Hirano A, Gao W, He X, Kono J. Destabilization of Surfactant-Dispersed Carbon Nanotubes by Anions. NANOSCALE RESEARCH LETTERS 2017; 12:81. [PMID: 28138897 PMCID: PMC5280815 DOI: 10.1186/s11671-017-1850-1] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/26/2016] [Accepted: 01/16/2017] [Indexed: 05/29/2023]
Abstract
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.
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Noe GT, Katayama I, Katsutani F, Allred JJ, Horowitz JA, Sullivan DM, Zhang Q, Sekiguchi F, Woods GL, Hoffmann MC, Nojiri H, Takeda J, Kono J. Single-shot terahertz time-domain spectroscopy in pulsed high magnetic fields. OPTICS EXPRESS 2016; 24:30328-30337. [PMID: 28059309 DOI: 10.1364/oe.24.030328] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.
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Tristant D, Zubair A, Puech P, Neumayer F, Moyano S, Headrick RJ, Tsentalovich DE, Young CC, Gerber IC, Pasquali M, Kono J, Leotin J. Enlightening the ultrahigh electrical conductivities of doped double-wall carbon nanotube fibers by Raman spectroscopy and first-principles calculations. NANOSCALE 2016; 8:19668-19676. [PMID: 27858049 DOI: 10.1039/c6nr04647a] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
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.
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Zhang Q, Wang Y, Gao W, Long Z, Watson JD, Manfra MJ, Belyanin A, Kono J. Stability of High-Density Two-Dimensional Excitons against a Mott Transition in High Magnetic Fields Probed by Coherent Terahertz Spectroscopy. PHYSICAL REVIEW LETTERS 2016; 117:207402. [PMID: 27886470 DOI: 10.1103/physrevlett.117.207402] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/27/2016] [Indexed: 06/06/2023]
Abstract
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.
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He X, Gao W, Xie L, Li B, Zhang Q, Lei S, Robinson JM, Hároz EH, Doorn SK, Wang W, Vajtai R, Ajayan PM, Adams WW, Hauge RH, Kono J. Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes. NATURE NANOTECHNOLOGY 2016; 11:633-8. [PMID: 27043199 DOI: 10.1038/nnano.2016.44] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2015] [Accepted: 02/19/2016] [Indexed: 05/02/2023]
Abstract
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 (>cm(2)) 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 × 10(6) nanotubes in a cross-sectional area of 1 μm(2). 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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Razanoelina M, Bagsican FR, Kawayama I, Zhang X, Ma L, Murakami H, Vajtai R, Ajayan PM, Kono J, Tonouchi M. Probing low-density carriers in a single atomic layer using terahertz parallel-plate waveguides. OPTICS EXPRESS 2016; 24:3885-3893. [PMID: 26907041 DOI: 10.1364/oe.24.003885] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of ~2 × 10(11) cm(-2), which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits ~30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials.
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Stier AV, McCreary KM, Jonker BT, Kono J, Crooker SA. Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla. Nat Commun 2016; 7:10643. [PMID: 26856412 PMCID: PMC4748133 DOI: 10.1038/ncomms10643] [Citation(s) in RCA: 104] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2015] [Accepted: 01/07/2016] [Indexed: 12/22/2022] Open
Abstract
In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately -230 μeV T(-1) (g-factor ≃-4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ∼1.53 and ∼1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials.
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Erikson KJ, He X, Talin AA, Mills B, Hauge RH, Iguchi T, Fujimura N, Kawano Y, Kono J, Léonard F. Figure of Merit for Carbon Nanotube Photothermoelectric Detectors. ACS NANO 2015; 9:11618-11627. [PMID: 26512738 DOI: 10.1021/acsnano.5b06160] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Carbon nanotubes (CNTs) have emerged as promising materials for visible, infrared, and terahertz photodetectors. Further development of these photodetectors requires a fundamental understanding of the mechanisms that govern their behavior as well as the establishment of figures of merit for technology applications. Recently, a number of CNT detectors have been shown to operate based on the photothermoelectric effect. Here we present a figure of merit for these detectors, which includes the properties of the material and the device. In addition, we use a suite of experimental characterization methods for the thorough analysis of the electrical, thermoelectric, electrothermal, and photothermal properties of the CNT thin-film devices. Our measurements determine the quantities that enter the figure of merit and allow us to establish a path toward future performance improvements.
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Yuan J, Wu J, Hardy WJ, Loya P, Lou M, Yang Y, Najmaei S, Jiang M, Qin F, Keyshar K, Ji H, Gao W, Bao J, Kono J, Natelson D, Ajayan PM, Lou J. Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2015; 27:5605-5609. [PMID: 26293810 DOI: 10.1002/adma.201502075] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/30/2015] [Revised: 07/13/2015] [Indexed: 06/04/2023]
Abstract
A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.
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Li B, Shi G, Lei S, He Y, Gao W, Gong Y, Ye G, Zhou W, Keyshar K, Hao J, Dong P, Ge L, Lou J, Kono J, Vajtai R, Ajayan PM. 3D Band Diagram and Photoexcitation of 2D-3D Semiconductor Heterojunctions. NANO LETTERS 2015; 15:5919-5925. [PMID: 26280193 DOI: 10.1021/acs.nanolett.5b02012] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
The emergence of a rich variety of two-dimensional (2D) layered semiconductor materials has enabled the creation of atomically thin heterojunction devices. Junctions between atomically thin 2D layers and 3D bulk semiconductors can lead to junctions that are fundamentally electronically different from the covalently bonded conventional semiconductor junctions. Here we propose a new 3D band diagram for the heterojunction formed between n-type monolayer MoS2 and p-type Si, in which the conduction and valence band-edges of the MoS2 monolayer are drawn for both stacked and in-plane directions. This new band diagram helps visualize the flow of charge carriers inside the device in a 3D manner. Our detailed wavelength-dependent photocurrent measurements fully support the diagrams and unambiguously show that the band alignment is type I for this 2D-3D heterojunction. Photogenerated electron-hole pairs in the atomically thin monolayer are separated and driven by an external bias and control the "on/off" states of the junction photodetector device. Two photoresponse regimes with fast and slow relaxation are also revealed in time-resolved photocurrent measurements, suggesting the important role played by charge trap states.
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Titova LV, Pint CL, Zhang Q, Hauge RH, Kono J, Hegmann FA. Generation of terahertz radiation by optical excitation of aligned carbon nanotubes. NANO LETTERS 2015; 15:3267-3272. [PMID: 25879274 DOI: 10.1021/acs.nanolett.5b00494] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis.
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Lei S, Wen F, Ge L, Najmaei S, George A, Gong Y, Gao W, Jin Z, Li B, Lou J, Kono J, Vajtai R, Ajayan P, Halas NJ. An Atomically Layered InSe Avalanche Photodetector. NANO LETTERS 2015; 15:3048-55. [PMID: 25822539 DOI: 10.1021/acs.nanolett.5b00016] [Citation(s) in RCA: 98] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Atomically thin photodetectors based on 2D materials have attracted great interest due to their potential as highly energy-efficient integrated devices. However, photoinduced carrier generation in these media is relatively poor due to low optical absorption, limiting device performance. Current methods for overcoming this problem, such as reducing contact resistances or back gating, tend to increase dark current and suffer slow response times. Here, we realize the avalanche effect in a 2D material-based photodetector and show that avalanche multiplication can greatly enhance the device response of an ultrathin InSe-based photodetector. This is achieved by exploiting the large Schottky barrier formed between InSe and Al electrodes, enabling the application of a large bias voltage. Plasmonic enhancement of the photosensitivity, achieved by patterning arrays of Al nanodisks onto the InSe layer, further improves device efficiency. With an external quantum efficiency approaching 866%, a dark current in the picoamp range, and a fast response time of 87 μs, this atomic layer device exhibits multiple significant advances in overall performance for this class of devices.
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