Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P18: Focus Session: Carbon Nanotubes: Opto-Electronics |
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Sponsoring Units: DMP Chair: Vasili Perebeinos, IBM T.J. Watson Research Center Room: Baltimore Convention Center 315 |
Wednesday, March 15, 2006 11:15AM - 11:51AM |
P18.00001: Electrically-Induced Infrared Emission from Carbon Nanotube Devices Invited Speaker: The optical properties of carbon nanotubes (CNTs) are currently the focus of intense study. CNTs are direct band gap materials and their optical spectra have long been attributed to transitions between free particle bands. We show that studies of electrically-excited infrared (IR) emission from single nanotube molecules provide new insights into the electron-hole interactions in quasi-1D systems. We demonstrate strongly-enhanced electroluminescence from a partially suspended CNTFET operated under unipolar transport conditions [1]. In our devices, carriers are generated locally, when a single type of carrier is accelerated under high local electric fields at intra-molecular junctions to energies sufficient to create strongly correlated e-h pairs (excitons). This excitation mechanism contrasts with emission from radiative recombination of carriers (electrons and holes) injected from the opposite ends (source and drain) of a CNTFET operated under ambipolar transport conditions. The new excitation mechanism is about 1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by one-dimensional confinement. We show that the light emission intensity increases exponentially with the drive current in partially suspended CNTFETs, while in 3D materials light emission is usually proportional to the product of the electron and the hole currents. The strong Coulomb interaction between electrons and holes in a 1D CNT creates bound excitons whose binding energies are more than an order of magnitude larger that those in 3D materials, preventing them from dissociating under electrical fields thus contributing little to drive current compared with that in 3D. Finally, the much higher exciton density achieved in our devices than that in typical photoluminescence experiments allows us to detect emission from higher excitation states in CNTs. \newline \newline [1] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, Ph. Avouris, Science 310, 1171 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P18.00002: Exciton Formation and Electroluminescence Quenching during 1D Impact Excitation of Carbon Nanotubes Field-Effect Transistors La\"{e}titia Marty, Elyse Adam, David M\'{e}nard, Richard Martel There are few studies addressing the influence of excitonic effects on the electro-optical response of carbon nanotube (CNT) devices. We present here near infra-red electroluminescence (EL) from unipolar single- wall carbon nanotube field effect transistors (CNFETs) at high drain-source voltages. The conditions for emission at high field reveal that a single carrier type induces EL in CNFETs through a mechanism involving 1D impact excitation. Well-resolved spectra show that the emission is assigned to the radiative recombination of the E$_{1,1}$ exciton. An emission quenching is also observed at high field and attributed to an exciton-exciton annihilation process and free carrier generation. Excitons binding energy in the order of 270 meV for 1.4 nm CNTs is inferred from the spectral features.. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P18.00003: Uni- and Ambipolar Light Emission From Inhomogeneous Carbon Nanotube FET's James Tsang, Marcus Freitag, Jia Chen, Phaedon Avouris Heterogeneities in the environment of CNTFETs can produce stationary, unipolar, electroluminescence, in addition to the normal ambipolar emission. We compare the unipolar emission with the ambipolar emission in the same device to characterize the unipolar emission process and show how the heterogeneities modify the electronic properties of the CNT. If a CNTFET is partially covered by a PMMA overlayer, changes in the IV characteristics are observed which correlate with discontinuities in the motion of the ambipolar emission at the PMMA boundary, and the generation of unipolar emission at the boundary. These PMMA induced changes show there is a step in the potential along the CNT at the boundary. Similarly, localized effects in both the ambi- and unipolar emission are observed in CNTFETs containing closed loops. The unipolar emission requires the junction between the portions of the single carbon nanotube that form the base of a loop must support the voltage drop needed to generate the light. Direct comparison of the ambipolar and unipolar emission in the same device demonstrates the efficiency of the unipolar processes. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P18.00004: Carbon Nanotube p-n Junction Diodes Ji Ung Lee We describe the formation of p-n junctions along individual single-walled carbon nanotubes (SWNTs) using electrostatic doping techniques. The electrostatic doping preserves the pristine nature of CVD grown SWNTs, and when suspended, these diodes can be described by the ideal diode equation. The low background leakage currents coupled with a built-in electric field region to transport the quasi particles also makes these diodes ideal for studying the optical responses of SWNTs. We will describe several characteristics of SWNT diodes such as the quantum efficiency, origin of the quasi-particles (electrons and holes) currents, and effects due to excitons. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P18.00005: Electronic Transport in Individual Carbon Nanotube P-N Junction Diodes Nathaniel Gabor, Ken Bosnick, Paul McEuen We have investigated electronic transport in single-walled carbon nanotube p-n junction diodes formed using gates to electrostatically dope the tube. Previous measurements [1] have shown that such diodes demonstrate nearly ideal turn-on behavior at room temperature and low biases, consistent with thermal activation over the junction barrier. We have performed measurements over a broad temperature range and have verified that the transport is by thermal activation. From the temperature dependence of the current-voltage characteristics, we can extract the nanotube band gap and the transmission coefficient through the p-n junction region. [1] J.U. Lee et al, App. Phys. Lett. \textbf{85}, 145 (2004) [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P18.00006: Negative differential conductance in suspended semiconducting carbon nanotubes Haibing Peng, Alex Zettl Suspended single-wall carbon nanotubes (SWCNTs) have been grown directly on metal electrodes with electrical contact by chemical vapor deposition. Extraordinary negative differential conductance was observed for the first time in suspended semiconducting SWCNTs. The current-voltage characteristics show an abrupt drop of conductance as the source-drain voltage is increased, followed by a constant differential conductance at higher voltage. The effect is qualitatively different from the recently reported negative differential conductance in metallic SWCNTs (\textit{Phys. Rev. Lett.} \textbf{95} 155505, 2005). We suggest that the observed negative differential conductance in semiconducting SWCNTs may be attributable to Schottky barriers between the as-grown suspended SWCNTs and the electrodes, instead of optical phonon scattering invoked in explaining the negative differential conductance in metallic SWCNTs. Our observations not only have potential applications for novel electronic devices, but also shed light on better understanding and manipulating SWCNTs transistors. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P18.00007: Nanotube Film Electrodes in Electro-Optic Devices Zhuangchun Wu, Jeremiah K. Mwaura, Maria Nikolou, Timothy Steckler, David B. Tanner, John R. Reynolds, Andrew G. Rinzler The interface between conjugated polymers and conducting electrodes is crucial for the operation of organic electronic devices such as light emitting diodes (LEDs), electrochromics and photovoltaics. Transparent electrodes in these devices have been based mostly on indium tin oxide (ITO). There have been efforts to develop conducting polymer electrodes, and some success has been realized with films based on poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS).$^{1 }$In most cases however, the polymer conductivity is too low for such applications. Pure nanotube thin films, demonstrated to have much higher conductivities while exhibiting good transparency in the visible and near to mid IR, provide attractive alternatives.$^{2}$ Here we describe fabrication and performance of two devices: (1) an MEH-PPV polymer LED using a carbon nanotube film as the hole injecting electrode and (2) an infrared transmissive/absorptive electrochromic cell that makes use of the superior IR transmittance of the nanotube films. 1. A. A. Argun, A.Cirpan, J. R. Reynolds, Adv. Mater. 15, 1338 (2003\textbf{)}. 2. Z. Wu, \textit{et al.}, Science 305, 1273 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P18.00008: Measurements of exciton binding energies in single wall nanotubes using field dependent photocurrent spectroscopy A.D. Mohite, J.-T. Lin, G.U. Sumanasekera, B.W. Alphenaar We have used electric field dependent photocurrent measurements to distinguish between band-to-band and excitonic transitions in the excitation spectrum of a single wall nanotube capacitor. The zero field photocurrent spectrum is limited to carriers excited into continuum states that can freely diffuse from the nanotubes and into the metal contact. Application of an applied field allows for the separation of the bound exciton states via field ionization. Near the E$_{11}$ resonance, both excitonic and band-to-band transitions are resolvable with a binding energy of 109 meV. This is in reasonable agreement with recent theory for 1.3 nm diameter nanotubes$^{1}$. Near the E$_{22 }$resonance, we observe only a single field independent peak in the photocurrent spectrum indicating a fast decay of the exciton into the lower energy continuum states. Surprisingly, we are also able to resolve an exciton resonance associated with metallic nanotubes. Theory shows that in metallic nanotubes, optical transitions between the overlapping states at the Fermi energy are disallowed, giving rise to a symmetry gap$^{2}$. We measure the binding energy of the metallic exciton to be 49 meV for 1.3 nm diameter tubes. \textbf{References} (1) Perebeinos, V.; Tersoff, J.; Avouris, Ph., \textit{Phys. Rev. Lett.} \textbf{2004}, 92, 257402. (2) Spataru, C.D.; Ismail-Beigi, S.; Benedict, L.X,; Louie, S.G. \textit{Phys. Rev. Lett.} \textbf{2004}, 92, 077402. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P18.00009: Optical Properties of Aligned Carbon Nanotube Mats for Photonic Applications G.L. Zhao, D. Bagayoko, L. Yang We studied the optical properties of the aligned carbon nanotube (16, 0), (10, 0) and (8, 4) mats for photonic device applications. We employed the ab-initio density functional calculations in the linear combination of atomic orbital formalism. We calculated the electronic structure of the carbon nanotube mats and the real and imaginary parts of the dielectric functions as functions of photon energy. The calculated dielectric functions of the aligned carbon nanotube mats show a strong anisotropy when the electric field of light is parallel or perpendicular to the tube axes. Especially, there are strong peaks in the imaginary part of the dielectric function near the absorption edges, when the electric field of light is parallel to the carbon nanotube axes. The unusual optical properties of the semiconducting carbon nanotube mats present a new opportunity for applications in new electro-optical devices in the infrared energy region. Acknowledgments: this work was funded in part by NSF (Award No. 0508245), NASA (Award No. NCC 2-1344), and ONR (Grant No: N00014-05-1-0009). [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P18.00010: Theory of Auger Recombination of Excitons in One-Dimensional Nanostructures Feng Wang, Yang Wu, Mark S. Hybertsen, Tony F. Heinz The effective Coulomb interaction is greatly enhanced in one-dimensional (1D) systems. As has been recently demonstrated for single-walled carbon nanotubes [1], this strong Coulomb interaction causes the formation of tightly bound exciton states upon optical excitation of semiconducting materials. The strength of the Coulomb interaction in 1D systems leads to a second consequence: Auger recombination of excitons, also known as exciton-exciton annihilation, can be very efficient. Here we investigate the 1D Auger process using a point-contact model for the Coulomb interaction. We show that the Auger process is essentially temperature independent, in contrast to the behavior of weakly bound excitons and free carriers in bulk semiconductors. We apply the explicit expression that we have derived to single-walled carbon nanotubes. We obtain an Auger rate of $\sim $0.6 ps$^{-1}\mu $m, comparable to the reported experimental value [2]. [1] F. Wang et al., Science \textbf{308}, 838 (2005); [2] F. Wang et al., Phys. Rev. B \textbf{70}, 241403 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P18.00011: Measurement of Optical Stark Effect in Semiconducting Single-Walled Carbon Nanotubes Daohua Song, Feng Wang, Gordana Dukovic, Louis E. Brus, Tony F. Heinz, M. Zheng, G.B. Onoa, E.D. Semke The optical Stark effect in quantum-confined systems, such as quantum wells, has been the subject of active interest for many years. In this paper we present the first measurement of the optical Stark effect in carbon nanotubes. In our experiment we used two-color femtosecond spectroscopy to probe the E$_{11}$ transition in the nanotubes while applying a strong optical pump beam at significantly lower photon energy (the large detuning limit). The sample was an aqueous suspension of (6,5)-enriched single-walled carbon nanotubes. An instantaneous shift in the absorption line by up to 1 meV was observed; the magnitude of the shift scaled linearly with the pump intensity. The nature of the optical Stark effect in the carbon nanotube system, with its strong excitonic transitions, will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P18.00012: Bandgap Modulation by Transverse Electric Fields in Single Wall Carbon Nanotubes J. M. Kinder, E. J. Mele We study the variation of the electronic bandgap of semiconducting carbon nanotubes in a static electric field perpendicular to the nanotube axis. We consider three models for the transverse field profile and find that the spectrum is sensitive to the spatial variation of the transverse field. For a uniform transverse field, we show the bandgap is fixed until the field strength exceeds a critical value, in agreement with previous theoretical work. In contrast, we find no critical behavior when the applied field is localized to a region of the nanotube much smaller than its length. An arbitrarily weak field produces bound states inside the unperturbed bandgap whose binding energy vanishes as the fourth power of the applied field strength. The field strengths required to reduce the gap by a few percent are the same order of magnitude as those commonly used in scanning tunneling microscopy. [Preview Abstract] |
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