Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y37: Carbon Nanotube Optical Properties |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Feng Wang, University of California, Berkeley Room: 705/707 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y37.00001: Electron temperature dependence of the electron-phonon coupling strength in double-wall carbon nanotubes Ioannis Chatzakis We apply Time-Resolved Two-Photon Photoemission spectroscopy to probe the electron-phonon (e-ph) coupling strength in double-wall carbon nanotubes. The e-ph energy transfer rate G(T$_{\mathrm{e}}$, T$_{\mathrm{l}})$ from the electronic system to the lattice depends linearly on the electron (T$_{\mathrm{e}})$ and lattice (T$_{\mathrm{l}})$ temperatures for T$_{\mathrm{e}}$ \textgreater $\Theta_{\mathrm{Debye}}$. Moreover, we numerically solved the Two-Temperature Model. We found: (i) a T$_{\mathrm{e}}$ decay with a 3.5 ps time constant and no significant change in T$_{\mathrm{l}}$; (ii) an e-ph coupling factor of 2 $\times$ 10$^{16}$ W/m$^{3}$; (iii) a mass-enhancement parameter, $\lambda $, of (5.4 $\pm$ 0.9) $\times$ 10$^{-4}$; and (iv) a decay time of the electron energy density to the lattice of 1.34 $\times$ 0.85 ps. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y37.00002: Possibility for exciton Bose-Einstein condensation in carbon nanotubes Igor Bondarev, Areg Meliksetyan We demonstrate theoretically a possibility for exciton Bose-Einstein condensation (BEC) in individual small-diameter ($\sim $1-2 nm) semiconducting carbon nanotubes [1]. The effect occurs under the exciton-interband plasmon coupling controlled by an external electrostatic field applied perpendicular to the nanotube axis. The effect requires fields $\sim $1 V/nm and temperatures below 100 K that are experimentally accessible. The effect offers a testing ground for fundamentals of condensed matter physics in one dimension and opens up perspectives to develop tunable highly coherent polarized light source with carbon nanotubes. Possibilities for achieving BEC in 1D and 2D systems are theoretically demonstrated in the presence of an extra confinement potential [2]. We show that the strongly correlated exciton-plasmon system in a carbon nanotube presents such a special case. We find the critical BEC temperature and the condensate fraction as functions of temperature and electrostatic field applied. We discuss how the effect can be observed experimentally.\\[4pt] [1] I.V.Bondarev, A.V.Meliksetyan, arXiv1304.2804 (submitted to PRB);\\[0pt] [2] V.Bagnato, D.Kleppner, PRA 44, 7439; W.-S.Dai, M.Xie, PRA 67, 027601. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y37.00003: Band Gap Renormalization in Semiconducting Carbon Nanotubes Nicholas Lanzillo, Neerav Kharche, Saroj Nayak We use first-principles density functional theory (DFT) in conjunction with the GW Approximation to show that the presence of a dielectric substrate is found to alter the electronic properties of semiconducting carbon nanotubes. In particular, a substrate-induced polarization effect stabilizes the correlation energy in both the valence band maximum and the conduction band minimum, resulting in a decrease in the electronic band gap. This effect is due to non-local dielectric screening which can be described accurately through the GW Approximation but is not captured in DFT alone. While similar band gap renormalization effects have been observed for isolated molecules and even for two-dimensional materials on substrates, this is the first prediction of such an effect in a strictly one-dimensional geometry. We find that the magnitude of the band gap reduction is on the order of 0.5 eV when deposited on a hexagonal boron nitride substrate. This type of band gap modulation is of great importance in developing future opto-electronic devices. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y37.00004: Spontaneous exciton dissociation in carbon nanotubes Masahiro Yoshida, Yusuke Kumamoto, Akihiro Ishii, Akio Yokoyama, Takashi Shimada, Yuichiro K. Kato Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs.\footnote{Y. Kumamoto \textit{et al.}, arXiv:1307.5159 (2013).} A simple model is constructed to consistently describe the excitation power and voltage dependence of the photoluminescence and photocurrent. Using this model, we find that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons for photoconductivity measurements in single-walled carbon nanotubes being straightforward despite the large exciton binding energies. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y37.00005: Binding energy of the trion complex in carbon nanotubes Areg Meliksetyan, Igor Bondarev We derive an analytical expression for the binding energy of the trion complex (charged exciton) in small diameter ($\sim $1nm) carbon nanotubes. We use the (asymptotically exact) Landau-Herring approach [1,2] that was previously implemented by one of us (Ref.[3]) to evaluate the biexciton binding energy in carbon nanotubes. Within this approach, we find the universal asymptotic relationship between the trion, biexciton and exciton binding energies in the same carbon nanotube. Particularly, the trion binding energy we obtained is estimated to be greater than the corresponding biexciton binding energy by a factor $\sim $1.5 for carbon nanotubes with diameters $\sim $1nm, which reasonably agrees with the latest non-linear optical spectroscopy measurements reported in Refs.[4] and [5] (1.46 for the (6,5) nanotube and 1.42 for the (9,7) nanotube, respectively).\\[4pt] [1] L.D.Landau, E.M.Lifshitz, Quantum Mechanics (Pergamon, Oxford, 1991);\\[0pt] [2] C.Herring, Rev. Mod. Phys. 34, 631;\\[0pt] [3] I.V.Bondarev, PRB 83, 153409;\\[0pt] [4] B.Yuma et al., PRB 87, 205412;\\[0pt] [5] L.Colombier et al., PRL 109, 197402. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y37.00006: Photoluminescence Imaging of Oxygen Doped Individual Single-Walled Carbon Nanotubes Sibel Ebru Yalcin, Hisato Yamaguchi, Charudatta Galande, Jared J. Crochet, Aditya D. Mohite, Gautam Gupta, Xuedan Ma, Han Htoon, Stephen K. Doorn Semiconducting single-walled carbon nanotubes (SWNTs) are attractive candidates for near-IR optoelectronic applications. But they show low fluorescence quantum yield. Recent oxygen doping studies have shown that the quantum yield of the excitons can be enhanced by an order of magnitude due to the formation of local 0D sites on the SWNT surface. However, these studies have been limited to ensemble measurements. Understanding the dopant site, exciton migration and trapping dynamics on individual SWNTs is critical for controllably tuning the photo-physical behavior. We have studied ozonated individual (6,5) nanotubes as a function of progressive ozonation. We spatially resolved the pristine and doped state using visible and NIR sensitive cameras. We demonstrate PL imaging as a probe of the emission dynamics as a function of dopant concentration. The spectral studies show the red-shifted emission in the PL of the NTs due to the ozonated site. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y37.00007: Charge Generation in Photoexcited Large-Diameter Semiconducting Single-Walled Carbon Nanotube/Fullerene Blends Kevin Mistry, Bryon Larson, Nikos Kopidakis, Garry Rumbles, Jeffrey Blackburn Semiconducting single-walled carbon nanotubes (s-SWCNTs) have a number of extraordinary electrical and optical properties including high charge mobilities and tunable band gaps that make them appealing for FETs and PV devices. Using narrow chiral distributions of large-diameter (d \textgreater 1.2 nm) s-SWCNTs can be beneficial to these devices through increased carrier mobility and reduced trapping due to energetic differences between different chiralities. Additionally, much of the visible and near-IR region of the solar spectrum can be covered by appropriately tuning the diameter range of these s-SWCNTs along with careful selection of the fullerene acceptor material. Time-resolved microwave conductivity (TRMC) was used to explore charge separation in such s-SWCNT:fullerene (donor:acceptor) blends. TRMC allows for sensitive monitoring of charge generation and decay in response to photoexcitation. We will report on charge carrier lifetime dynamics and changes to free carrier yield in blends using carefully tuned combinations of SWCNT diameters and fullerene acceptors. Furthermore, we will discuss how these results can be used to design enhanced s-SWCNT:fullerene active layers. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y37.00008: Accessing the Dark Exciton States in Semiconducting Single-Walled Carbon Nanotubes with Terahertz Pulses Liang Luo, Ioannis Chatzakis, Jigang Wang Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical pump, terahertz probe spectroscopy of (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 0.5-2.5 THz (2-10.5 meV) at low temperature down to 5 K with resonant and off-resonant excitation at the E$_{22}$ transitions of (6,5) and (7,5) nanotubes. These results reveal, for the first time, dynamics of lowest dark excitons and density-dependent renormalization of these many-particle states. The internal-excitonic spectroscopy with THz pulses represents a fundamentally new spectroscopy tools to study dark excitons and shine new lights on the correlation physics of excitonic ground states. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y37.00009: Photoluminescence microscopy on air-suspended carbon nanotubes coupled to photonic crystal nanobeam cavities R. Miura, S. Imamura, T. Shimada, R. Ohta, S. Iwamoto, Y. Arakawa, Y.K. Kato Because carbon nanotubes are room-temperature telecom-band emitters and can be grown on silicon substrates, they are ideal for coupling to silicon photonic cavities.\footnote{R. Watahiki et al., Appl. Phys. Lett. 101, 141124 (2012).}$ ^{,}$\footnote{S. Imamura et al., Appl. Phys. Lett. 102, 161102 (2013).} In particular, as-grown air-suspended carbon nanotubes show excellent optical properties, but cavity modes with large fields in the air are needed in order to achieve efficient coupling. Here we investigate individual air-suspended nanotubes coupled to photonic crystal nanobeam cavities. We utilize cavities that confine air-band modes which have large fields in the air. Dielectric mode cavities are also prepared for comparison. We fabricate the devices from silicon-on-insulator substrates by using electron beam lithography and dry etching to form the nanobeam structure. The buried oxide layer is removed by wet etching, and carbon nanotubes are grown onto the cavities by chemical vapor deposition. We perform photoluminescence imaging and excitation spectroscopy to find the positions of the nanotubes and identify their chiralities. For both types of devices, cavity modes with quality factors of $\sim$3000 are observed within the nanotube emission peak. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y37.00010: Alternating gate-voltage effects on photoluminescence of air-suspended carbon nanotubes M. Jiang, Y. Kumamoto, A. Ishii, M. Yoshida, Y.K. Kato Gate voltages cause quenching of photoluminescence in carbon nanotubes through phase-space filling and doping-induced exciton relaxation.\footnote{S. Yasukochi et al., Phys. Rev. B 84, 121409(R) (2011).} In this work, we apply square-wave voltages to partially gated nanotubes and find that such quenching can be eliminated at high frequencies. The devices are fabricated on a silicon-on-insulator substrate and we start by etching trenches through the top silicon layer into the buried oxide. The top silicon layer is thermally oxidized for use as the gate and we form an electrode on one side of the trench. From catalyst particles placed on the electrode, nanotubes are grown over the trench onto the gate. For square-wave voltages at low frequencies, photoluminescence quenching occurs as expected. When the frequency becomes higher, we observe that emission increases linearly and saturates above a threshold frequency. Time-averaging of the voltage cannot explain such an increase, as it also occurs when offset voltages are added to the square-wave. Furthermore, the threshold frequency increases as the excitation laser power is turned up. These observations could be explained by a model in which photocarriers are stored by the gate fields and voltage switching induces light emission. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y37.00011: Diffusion-related exciton decay processes in air-suspended single-walled carbon nanotubes studied by photoluminescence microscopy A. Ishii, M. Yoshida, Y.K. Kato In carbon nanotubes, exciton diffusion causes complex photoluminescence properties through end quenching and exciton-exciton annihilation. In order to clarify the effects of these processes in air-suspended carbon nanotubes, where they are isolated from the surroundings, we perform photoluminescence measurements on over a hundred individual nanotubes. Nanotube length dependence is investigated by measuring emission from nanotubes suspended over trenches with various widths\footnote{S. Moritsubo \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 247402 (2010).} and excitation power dependence is also investigated on each nanotube. We analyze the results by calculating the effects of end quenching as a function of the tube length using a first-passage approach.\footnote{M. D. Anderson \textit{et al.}, Phys. Rev. B \textbf{88}, 045420 (2013).} At low excitation powers where the exciton-exciton annihilation is negligible, this model gives intrinsic exciton diffusion lengths and relative values of photoluminescence action cross section. For higher excitation powers, Monte Carlo simulations are used to quantitatively evaluate the exciton-exciton annihilation rates and spatial profiles of the exciton density. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y37.00012: Atomic and Excitonic Stability in Dirac Materials: A White Dwarf Perspective Kirill Velizhanin Dirac materials - systems where the low-energy spectrum of electronic excitations can be understood via solving the Dirac equation - draw a great amount of attention of the scientific community lately due to their enormous application potential and interesting basic physics. Examples of such materials include carbon nanotubes, graphene and, more recently, single-layer transition metal dichalcogenides. One surprising application of Dirac materials is their use as a platform to simulate various atomic and high-energy physics ``on a chip.'' For example, graphene has been recently used to ``mimic'' an atomic collapse of superheavy atoms [Y. Wang et al, Science, 340, 734, 2013]. In this talk I will discuss an unexpected similarity between atomic and excitonic collapse in Dirac materials and the limit of stability of such exotic astrophysical objects as degenerate stars (e.g., white dwarfs, neutron stars). Various aspects of this similarity, e.g., an application of the concept of the Chandrasekhar limit to the exciton stability in transition metal dichalcogenides, will be discussed. [Preview Abstract] |
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