Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D28: Focus Session: Carbon Nanotube Optics II |
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Sponsoring Units: DMP Chair: Vasili Perebeinos, IBM Watson Room: Colorado Convention Center 302 |
Monday, March 5, 2007 2:30PM - 2:42PM |
D28.00001: Inelastic X-ray Scattering Studies of Plasmons in Carbon Nanotubes M.H. Upton, R.F. Klie, J.P. Hill, T. Gog, D. Casa, W. Ku, Y. Zhu, M.Y. Sfeir, J. Misewich, G. Eres, D. Lowndes We investigate the physical parameters controlling the low energy screening in carbon nanotubes via electron energy loss spectroscopy and inelastic x-ray scattering. Two plasmon-like features are observed, one near 9 eV (the so- called $\pi$ plasmon) and one near 20 eV (the so-called $\pi+\sigma$ plasmon). At large nanotube diameters, the $\pi+\sigma$ plasmon energies depend exclusively on the number of walls and not on the radius or chiral vector. This shift indicates a change of strength of screening and the effective interaction at inter-atomic distance, and thus suggests an alternative mechanism of tuning the properties of the nanotube in addition to the well-known control provided by chirality and tube diameter. [Preview Abstract] |
Monday, March 5, 2007 2:42PM - 2:54PM |
D28.00002: Ab-inito study of metallic and semi-conducting carbon nanotubes . Sumit Saxena, Trevor A. Tyson We present first principle calculations to study the metal -- semiconductor transitions with pressure in zigzag nanotubes using the Local density approximation. Spin restricted calculations for metallic (9, 0) and semi-conducting (10, 0) carbon nanotubes were performed using the full potential projected augmented wave (PAW) method and using ultra-soft pseudo potentials. Our calculations show qualitative agreement to the reported experimental density of states (DOS) for the semi-conducting (10, 0) nanotubes [1]. The band gap between the valence and the conduction band using the pseudo potential formalism is found to be very close to that predicted using PAW approach. We observe that the DOS obtained using pseudo potentials reproduces the essential features however the full potential approach reproduces most of the features of the experimentally reported results. The details of the calculations and other results will be presented. [1] T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber, J. Phys. Chem. B 104 2794 (2000) [Preview Abstract] |
Monday, March 5, 2007 2:54PM - 3:06PM |
D28.00003: Raman Study of Phonon Softening in Individual Metallic Single Wall Nanotubes Hootan Farhat, Hyungbin Son, Jing Kong We have studied the Breit-Wigner-Fano (BWF) lineshape and frequency of the G$^{-}$ Raman mode in individual metallic nanotubes as function of the Fermi level position. Single wall carbon nanotubes are grown from dispersed nanoparticles and are doped electrostatically by means of a polymer electrolyte gate. The frequency of the G$^{-}$ phonon in metallic tubes is very sensitive to the position of the Fermi level. As the Fermi level is tuned below and above the Fermi point, a semiconducting like G-band is recovered both in terms of frequency and linewidth. Near the Fermi point, the downshift of the G$^{-}$ frequency with respect to that of semiconducting tubes reaches a maximum of up to 50cm$^{-1}$. The doping and diameter dependence of the phonon softening are explained in terms of electron phonon coupling. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:18PM |
D28.00004: Controlled Screening of Excitons in Single, Suspended Carbon Nanotubes. Andrew Walsh, A. Nickolas Vamivakas, Yan Yin, Stephen Cronin, Bennett Goldberg, M. Selim Unlu, Anna Swan Recent measurements in carbon nanotubes (CNTs) have demonstrated that the optical transition energies are excitonic (e-h) in nature, with binding energies that are large fractions of an eV. The exciton energies in CNTs should be sensitive to screening by the environment, yet only small variations of the optical transition energies have been reported for widely varying dielectric environments. Here, we use resonant Raman spectroscopy to follow the change in the optical transition energy of single carbon nanotubes suspended across trenches in dry nitrogen, in high humidity, and after immersion in water. The transition energies are shown to red shift monotonically with increased screening, up to 33 meV. We develop a scaling relationship between the exciton binding energy and the external $\varepsilon $ to quantify the effect of screening on the e-e and e-h interaction energies. These energies are shown to change by hundreds of meV with screening but almost cancel, leading to the small observed shifts reported both here and in the literature. For the nanotubes measured here, the e-e energy is found to be about 25{\%} greater than the exciton binding energy in an unscreened environment. [Preview Abstract] |
Monday, March 5, 2007 3:18PM - 3:30PM |
D28.00005: Direct Measurement of Strain-induced Changes in Carbon Nanotube Bandstructure Mingyuan Huang, Yang Wu, Bhupesh Chandra, Yuyao Shan, Tony Heinz, James Hone The transition energies of single-walled carbon nanotubes under uniaxial strain were measured by Rayleigh scattering spectroscopy. The transitions display significant strain-induced shifts, as predicted by theory. In semiconducting tubes, successive transitions shift in opposite directions. In chiral metals, the split peaks merge with strain. We also observe small, but measurable shifts in the transitions of armchair tubes. The behavior is qualitatively consistent with theoretical predictions based on the trigonal warping effect in nanotube bandstructure. [Preview Abstract] |
Monday, March 5, 2007 3:30PM - 3:42PM |
D28.00006: Raman Spectroscopy of Axially Strain Carbon Nanotubes Rajay Kumar, Stephen Cronin We investigate resonant Raman scattering of carbon nanotube bundles on an elastomer substrate under axial strains as high as 15{\%}. Over the applied strain range, the $G_{+}$ band Raman frequency decreases for both metallic and semiconducting nanotubes. The $G_{-}$ band Raman spectra, however, respond differently to strain for metallic and semiconducting nanotubes, giving insight into the nature of the broad metallic $G_{-}$ band lineshape. The $G_{- }$band frequency downshifts with applied strain for semiconducting nanotubes, while the $G_{-}$ band frequency increases with strain for metallic nanotubes. The $G_{-}$ band linewidth of metallic nanotubes also becomes narrower with strain, making it appear more semiconductor-like. Surprisingly, this metal to semiconductor transition is not reversible with strain, which indicates that nanotube-nanotube coupling plays a role in the observed broad $G_{-}$ band lineshape of metallic nanotubes. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D28.00007: Optical Spectroscopy of Individual Carbon Nanotubes Invited Speaker: Single-walled carbon nanotubes (SWNTs) constitute a family of more than 100 one-dimensional structures. With properties varying significantly as a function of their precise atomic structure and environment, SWNTs provide a rich material system to study 1-dimensional physics. To unravel the wealth of different behavior in the SWNTs, which range from metallic to semiconducting, it is generally desirable, and often essential, to probe them \textit{individually}. In this talk, I will describe the development and application of three techniques for optical spectroscopy of individual SWNTs: Rayleigh scattering$^{1}$, multiphonon-Raman scattering$^{2}$, and absorption spectroscopy. We will illustrate the wide range of physical information attainable from these methods, including analysis of the excited electronic states of semiconducting and metallic nanotubes$^{1}$, nanotube-nanotube interactions$^{3}$, and electron-phonon coupling$^{2}$. In addition to their separate use, these spectroscopies can also be fruitfully combined with one another and with other complementary non-optical, single nanotube characterization methods. The correlation of Rayleigh scattering with multi-phonon Raman measurements provides, for example, direct information on the resonance enhancement of electron-phonon interaction$^{2}$. On the other hand, application of Rayleigh scattering in conjunction with single nanotube electron diffraction has permitted us to obtain electronic spectra of SWNTs of independently determined structure$^{4}$. These measurements have permitted verification of the underlying theoretical trends used in previous assignments of nanotube optical spectra. Work done in collaboration with: D. Cho, W. Liu, B. Kessler, A. Zettl, Y. R. Shen (UC Berkeley and LBNL), J. Schuck (LBNL) T. Beetz, J. A. Misewich, L. Wu, Y. Zhu, M. Y. Sfeir (Brookhaven National Lab), and Y. Wu, L. Huang, J. Hone, S. O'Brien, L. E. Brus, and T. F. Heinz (Columbia University). \newline \newline $^{1 }$M. Y. Sfeir*, F. Wang*\textit{ et al.}, Science \textbf{306}, 1540 (2004). \newline $^{2 }$F. Wang\textit{, et al.}, Phys. Rev. Lett., submitted (2006). \newline $^{3 }$F. Wang\textit{, et al.}, Phys Rev Lett \textbf{96} (2006). \newline $^{4 }$M. Y. Sfeir, T. Beetz, F. Wang \textit{ et al.}, Science \textbf{312}, 554 (2006). [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:30PM |
D28.00008: Direct Measurement of the Quantum Yield of Isolated Single Walled Carbon Nanotubes Lisa Carlson, Todd Krauss Owing to their unique optical properties, single walled carbon nanotubes (SWNTs) have received much recent attention. However, questions remain about whether the fluorescence quantum yield (QY) varies among SWNT ($n,m)$ structures and whether the QY for isolated nanotubes differs from the ensemble. With an ensemble QY of less than 0.1{\%}, it is surprising that single nanotube fluorescence can be detected with relatively high signal to noise. This important photophysical parameter potentially limits how SWNTs could be used for applications in biological sensing, telecommunications, displays, solar cells, and quantum optics. We will present measurements of the fluorescence QY of isolated nanotubes, measured relative to CdTe/ZnS quantum dots (QDs) using single molecule microscopy. CoMoCAT SWNTs were ultrasonically dispersed into micelles using sodium cholate surfactant in D$_{2}$O; dilute mixtures of SWNTs and QDs were then spin cast onto quartz and their fluorescence intensities were directly compared. By accounting for differences in the absorption cross sections between the systems, the SWNT QY was determined to be $\sim $2{\%}, nearly two orders of magnitude greater than the ensemble measurement. We will report on whether the measured QY represents an intrinsic nanotube property or if it depends upon other factors such as local environment, intertube interactions, and defects. [Preview Abstract] |
Monday, March 5, 2007 4:30PM - 4:42PM |
D28.00009: Low Temperature Micro-photoluminescence and Raman Spectroscopy of Single-Walled Carbon Nanotubes Ajit Srivastava, Erik Haroz, Yoichi Murakami, Junichiro Kono We report micro-photoluminescence (PL) and resonance Raman spectroscopy studies performed on single single-walled carbon nanotubes at low temperatures. At sufficiently low temperatures, where the thermal energy k$_{B}$T is smaller than the predicted dark-bright exciton splitting, PL is expected to be quenched as excitons populate only the dark ground state. However, we observe strong PL from single tubes with very sharp linewidths ($\sim $ 1 meV for 1 nm diameter tubes) even at temperatures as low as 5 K. We will discuss the origin of this emission. We also study the PL linewidth as a function of temperature in order to provide insight into the PL line-broadening mechanisms. Resonance micro-Raman spectroscopy of single tubes was also performed at cryogenic temperatures, scanning the wavelength of the excitation laser beam around the E$_{22}$ transition of the nanotubes, which revealed rich structure both in the vibrational spectrum and the excitation profile. The temperature dependence of various Raman features will be presented. [Preview Abstract] |
Monday, March 5, 2007 4:42PM - 4:54PM |
D28.00010: Photoluminescence from inter-tube carrier migration in single-walled carbon nanotube bundles O. N. Torrens, D. E. Milkie, M. Zheng, J. M. Kikkawa We detect new, dominant PL features from aqueous suspensions of single-walled carbon nanotubes (SWNTs) associated with energy transfer between semiconducting species in SWNT bundles.$^{1}$ In these bundles, excitons are resonantly photoexcited at the E22 excitonic transition of populous, large bandgap SWNTs ((6,5), (7,5), and (8,3)). Excited excitons then efficiently migrate to smaller bandgap SWNTs ((7,6), (8,4), and (9,2)) and radiatively relax by emitting photons resonant with the E11 excitonic transition of these less common species. These energy transfer (ET) emission peaks demonstrate efficient exciton coupling between different SWNT species within bundles. Aqueous SWNT solutions with low levels of metallic SWNTs prevent quenching of bundle PL, and linear dichroism measurements of SWNT magnetic alignment detect bundle formation. [1] O. N. Torrens, D. E. Milkie, M. Zheng, J. M. Kikkawa, Nano Lett. (in press). [Preview Abstract] |
Monday, March 5, 2007 4:54PM - 5:06PM |
D28.00011: Unique Optical and Electrical Properties of Almost-Isolated Vertically Aligned Single-Walled Carbon Nanotubes Shigeo Maruyama, Erik Einarsson, Masayuki Kadowaki, Zhengyi Zhang A new insight is gained on the structure of the vertically aligned single-wall carbon nanotubes (VA-SWNTs) generated by ACCVD technique. Our recent finding of the simple removal method using hot-water enabled us to transfer this film to various flat substrates. Transferring this film on TEM grid made it possible to directly observe the morphology of nanotubes from the top. To our surprise, the average number of nanotubes of a bundle is less than about 10. Electronic properties measured by EELS revealed that nanotubes are virtually electronically isolated. Then, the characteristic resonant Raman features are reconsidered. The high resolution Raman measurements show the sharp features for the RBM peak which have been assigned to cross-polarized resonance. The isolated and cross-polarized absorption resonance in Raman will be discussed based on the recent identification of the excitonic cross-polarized absorption through photoluminescence spectroscopy. [Preview Abstract] |
Monday, March 5, 2007 5:06PM - 5:18PM |
D28.00012: Absolute potential of the Fermi level of single-walled carbon nanotubes via hydrogenase complex formation. Timothy McDonald, Drazenka Svedruzic, Yong-Hyun Kim, Jeffrey Blackburn, Shengbai Zhang, Paul King, Michael Heben The absolute potential of the Fermi level of nanotubes as a function of nanotube type is not presently understood, and is important for many nanotube applications and sorting strategies. Here, we study complexes of recombinant [FeFe] hydrogenases and single-walled carbon nanotubes. We find evidence that novel charge-transfer complexes are formed and are stable, which enables further study and application of this system. The hydrogenase functions as a hydrogen electrode sensitizing the nanotubes to the redox half-reaction for hydrogen. Thus the potential can be altered by changing the molecular hydrogen concentration, and this tunability is utilized to bleach various semiconducting nanotube transitions. By observing which are bleached and which remain emissive, we determine the alignment of the potential of the Fermi level of semiconducting single-walled carbon nanotubes. The experimentally determined Fermi level alignment is confirmed theoretically by the first-principles DFT-PBE method. [Preview Abstract] |
Monday, March 5, 2007 5:18PM - 5:30PM |
D28.00013: $\iota -$Carrageenan as a Matrix for Carbon Nanotube Spectroscopy William Rice, Yoichi Murakami, Junichiro Kono We have developed films of individualized single-walled carbon nanotubes (SWNTs) for spectroscopic studies using $\iota $-carrageenan, a polysaccharide macromolecule with a double helix structure, which is extruded from red seaweed. SWNTs produced by both the HiPco and CoMoCAT methods were separated using sodium cholate surfactants and ultracentrifugation. We found that for both HiPco and CoMoCAT tubes, the introduction of $\iota $-carrageenan did not significantly affect the interband optical absorption spectrum, indicating that separation was largely maintained. Further, we show that the optical density of the film is low in the mid-infrared ($\sim $3.5 -- 6 $\mu $m). This transparency is observed at temperatures as low as 4.2 K, making this film a good candidate for temperature-dependent spectroscopic studies of nanotubes. In addition, we confirmed that the polymer film transmits in the terahertz regime (.2 -- .9 THz). [Preview Abstract] |
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