Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y28: Focus Session: Magnetic and Doping Effects in Cojugated Organics |
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Sponsoring Units: DPOLY DMP Chair: Maria Nikolou, Cornell University Room: Baltimore Convention Center 325 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y28.00001: Anomalous Magnetoresistance Phenomena in Organic Semiconductors Jeremy D. Bergeson, Derek M. Lincoln, Ruth Shima Edelstein, Vladimir N. Prigodin, Arthur J. Epstein We report magnetoresistance (MR) phenomena with temperature and bias dependence in organic semiconductor thin films with either nonmagnetic or magnetic contacts through high field reaching 9T. For nonmagnetic organic thin films such as Alq$_{3}$ we find a low field MR up to 15\%. A similar magnetic field effect has been reported earlier$^{1}$ but, as noted, the mechanism remains unclear. We propose a model of the anomalous MR where charge transport is space-charge limited. The current is determined by the e-h recombination rate. The recombination rate is field dependent, analogous to the chemical yield for radical pairs$^{2}$. Using an organic- based magnetic semiconductor$^{3}$, V[TCNE]$_{x}$, and Co as magnetic contacts, with a nonmagnetic organic semiconductor ($\alpha $-6T) leads to an order-of-magnitude broader zero-centered MR peak superimposed on a spin-valve effect. Possible origins of this broader MR will be discussed. 1. Francis, et al., New J. Phys. \textbf{6} 185 (2004); Frankevich, et al., Phys. Rev. B \textbf{53} 4498 (1996) 2. Steiner and Ulrich, Chem. Rev. \textbf{89} 51 (1989) 3. Pokhodnya, et al., Adv. Mater. \textbf {12} 410 (2000); Prigodin, et al., Adv. Mater. \textbf{14} 1230 (2002); Shima Edelstein, et al., Mater. Res. Soc. Symp. Proc. \textbf{871E} I7.3 (2005) [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y28.00002: Preparation, Magnetism, and Applications of Thin Films of the Organic Semiconductor V[TCNE]$_{x\sim 2}$ R. Shima Edelstein, D.M. Lincoln, J.-W. Yoo, N.P. Raju, J.D. Bergeson, A.J. Epstein, J.B. Kortright Different aspects of the room-temperature (T$_{c}>$350K) molecular magnet vanadium tetracyanoethylene (V[TCNE]$_{x\sim 2})$ remain only partially understood, including its intra- and inter-molecular bonding and spin distribution, degradation mechanisms, and mesoscopic magnetic and chemical order. Previous studies have shown that control of the local order, structure, spin, and chemical composition determines the magnetic state achieved and its charge transport and dynamic properties$^{1}$. In this study, we focus on thin films ($\sim $0.05-0.5 $\mu $m) prepared by CVD$^{2,3}$ (room temperature conductivity 10$^{-4}$-10$^{-3 }$S/cm). Soft x-ray absorption and Magnetic Circular Dichroism (MCD) spectra of the atomic edges are presented, together with XPS, SQUID magnetometry, and EPR spectroscopy. The knowledge gained enables successful demonstration of spin valve devices and photoinduced magnetism in this molecule-based magnetic semiconductor. 1. Pokhodnya, et al, PRB \textbf{63}, 174408 (2001) 2. Pokhodnya, et al, Adv. Mater. \textbf{12}, 410 (2000) 3. Shima, et al, MRS Proc. \textbf{871E}, I7.3 (2005) Supported by DOE grants {\#}DE-FG02-86ER45271, DE-FG02- 01ER45931, DE-AC03-76SF00098, and AFOSR grant {\#}F49620-03-1-0175. NSF-DMR grant {\#}0114098 and Dr. Lisa Hommel are acknowledged for XPS studies. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y28.00003: Magnetoresistance and Ferrimagnetic Resonance (FMR) on Thin Films of Organic-based Magnetic Semiconductor V[TCNE]$_{x\sim 2}$ with T$_{c}$ above 350 K. N.P. Raju, R. Shima Edelstein, A.J. Epstein We present magnetoresistance and ferrimagnetic resonance (FMR) results on a CVD-prepared [1] thin films (about 0.5 micron thick) [2] of ferrimagnetic semiconductor V(TCNE)$_{x\sim 2}$. The temperature dependence of resistance, and the magnetoresistance variation with magnetic field and temperature show similar to the trends reported earlier. [3] FMR spectra reported here show marked differences from the earlier studies on thicker samples (up to 2 microns). [4] Earlier reported FMR shows several sharp peaks compared to only two in the present study. We report temperature dependence of linewidth and integrated intensity for each of these two peaks. The earlier study suggested that the porosity of the sample as one of the possible origins for many peaks. It appears that absence of many sharp FMR peaks in the present sample may reflect less porosity of the thin film which is important for the development of spin-valve devices. 1. K.I. Pokhodnya et. al., Adv. Mater \textbf{12}, 410 (2000). 2. Shima et. al., MRS Proc. \textbf{871E}, I7.3 (2005) 3. N.P. Raju et. al., J. Applied Physics \textbf{93}, 6799 (2003). 4. R. Plachy et. al., Phys. Rev. B \textbf{70}, 064411 (2004). [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 8:48AM |
Y28.00004: Semiconducting Organic Thin Film Devices with Large Magnetoresistance Y. Sheng, \"{O}. Mermer, G. Veeraraghavan, T.D. Nguyen, T.L. Francis, M. Wohlgenannt A comprehensive study on a recently discovered, large magnetoresistance (MR) effect in sandwich devices comprised of nonmagnetic electrodes and organic thin films is performed. Devices were fabricated from pi-conjugated polymers and small molecular weight compounds in combination with different electrode materials, and characterized extensively at different voltages, temperatures, and at weak magnetic fields from DC up to 100 kHz in frequency. The MR effect shows only weak temperature dependence and is independent of the sign and direction of the magnetic field. The effect reaches up to 10{\%} in a magnetic field of 10 mT at room temperature. To illustrate a potential application of the effect, we demonstrate a prototype organic LED (OLED) touchscreen using the MR effect. To the best of our knowledge, the discovered effect is not adequately described by any of the MR mechanisms known to date. [Preview Abstract] |
Friday, March 17, 2006 8:48AM - 9:00AM |
Y28.00005: Structure-property relationships of water-dispersible, conductive PANI-PAMPSA Joung Eun Yoo, Kwang Seok Lee, James Norman, Matthew Espe, Yueh-Lin Loo Polyaniline (PANI) is an attractive candidate for organic and polymer electronics because of its high electrical conductivity when doped with molecular acids. Its utility as functional components in electrical devices, however, has been severely restricted by processing and patterning limitations because molecular acid-doped PANI does not dissolve in any common solvents. To overcome this barrier, we have investigated polyaniline that is template polymerized in the presence of a polymer acid, poly(2-acrylamino-2-methyl-1-propanesulfonic acid), PAMPSA. In addition to doping polyaniline, the sulfonic acid groups along the polymer acid chain render water- dispersability to PANI-PAMPSA. The doped polymer is therefore not only electrically conductive, it can be easily processed and patterned from an aqueous medium. Solid-State NMR indicates that template polymerization results in linear, defect-free polyaniline. Both X-ray diffraction and electron microscopy experiments on PANI-PAMPSA synthesized at varying PAMPSA molecular weights reveal that PANI-PAMPSA crystallinity increases with decreasing PAMPSA molecular weight. The conductivities of PANI-PAMPSA increase accordingly with crystallinity. These results are corroborated by UV-vis-NIR experiments where the polaron peaks become progressively broader and red-shifted with decreasing PAMPSA molecular weights. [Preview Abstract] |
Friday, March 17, 2006 9:00AM - 9:12AM |
Y28.00006: The Optical Conductivity and Dielectric Constant of Polyaniline Nanofiber-based Film Oludurotimi O. Adetunji, Nan-Rong Chiou, Arthur J. Epstein We report the optical properties of polyaniline/HCl nanofiber (PANN) films with interconnecting nanofibers of average diameter 100 nm. The room temperature dc conductivity is in the range of 2-4 S/cm in the insulating regime of the disorder induced metal- insulator (M-I) transition [1]. PANN films were probed by reflectance spectroscopy at room temperature over a broad energy range 2meV-6eV. The reflectance ($<$4200$\ cm^{-1}$) increases monotonically as the frequency is lowered but has no indication of a plasma edge. Optical constants such as frequency dependent dielectric constant and frequency dependent conductivity have been derived via Kramers-Kronig (K-K) analysis of the reflectance data using appropriate extrapolations. The resulting optical conductivity extrapolated to near-zero frequency scales is in agreement with the measured dc conductivity of 2-4 S/cm [2]. A maximum of the frequency dependent conductivity is found at $\sim$ 2400$\ cm^{-1}$ while the K-K analysis shows no zero crossing of the dielectric constant between 40-50000$\ cm^{-1}$. We discuss these results in terms of roles of disorder and localization. This leads us to the conclusion that PANN films are on the insulating side of the disorder induced metal- insulator transition. [1]R.S. Kohlman, et al., PRL 77, 13 (1996) [2]N.-R. Chiou and A.J. Epstein, Adv. Mater. 17, 1679 (2005) Supported in part by NSF-IGERT Grant No DGE-0221678. [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y28.00007: Charge transport in conducting polymer nanofibers Natalya Zimbovskaya Here, we present theoretical analysis of electron transport in polyaniline based (PANi) nanofibers assuming the metalic state of the material. To build up this theory we treat conducting polymers as a special kind of granular metals, and we apply the quantum theory of conduction in mesoscopic systems to describe the transport between metallic-like granules. Our results show that the concept of resonance electron tunneling as the predominating mechanism providing charge transport between the grains is supported with recent experiments on the electrical characterization of single PANi nanofibers. By contacting the proposed theory with the experimental data we estimate some important parameters characterizing the electron transport in these materials [1]. Using the Buttiker dephasing model within the scattering matrix formalism we analyze dephasing effects, and we show that these effects could be reduced enough to allow the structure of the electron transmission function to be exposed in the experiments on the electronic transport through fibers [2]. Also, we discuss the origin of rectifying features observed in current-voltage characteristics of fibers with varying cross-sectional areas. 1. N. A. Zimbovskaya, A. T. Johnson, Jr., and N. J. Pinto, Phys. Rev. B 72, 024213 (2005). 2. N. A. Zimbovskaya, J. Chem. Phys. 123, 114708 (2005). [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y28.00008: Effects of confinement on the transport properties of CSA doped polyaniline Raul Perez, Neliza Leon, Idalia Ramos, Nicholas Pinto, Pawan Kahol Polyaniline doped with camphor sulfonic acid (CSA) and cast from $m$-cresol can exhibit metallic behavior depending on the conditions of preparation. Under standard methods of preparation the transport properties of cast films generally lie on the insulating side of the metal-insulator transition. We have confined CSA doped PANi into the cylindrical pores of a dielectrically inert porous matrix and measured the temperature dependence of the resistance. The resistance of the confined polymer is seen to have a weaker dependence at low temperatures than that of the cast film. Further analysis of the results show that the charge transport of the confined polymer has moved into the metallic regime while that of the cast film lies in the insulating regime. Reduced barriers to charge transport that result from the suppression of microphase separation of the non-dopable forms of polyaniline due to extreme confinement in the porous matrix are believed to be responsible for this crossover. [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y28.00009: Gas sensing using the microwave conductivity of conducting polymer nanofiber thin films. Alexey Kovalev, Lintao Cai, Theresa Mayer There are a variety of chemoresistive sensors based on conducting polymer thin films, and studies of conducting polymer nanofiber thin films and single nanofibers sensors are an active area of research because of their enhanced sensitivity. Traditional chemoresistive sensors monitor a change in DC resistance that arises when a low concentration of a chemical vapor is present. In this talk, we will discuss the use of conducting polymer nanofiber thin films as gas sensors that operate in the microwave frequency range. Polyaniline nanofiber films that were 1 - 10 $\mu$m thick were characterized measuring DC conductivity (four-point) and microwave conductivity in the range of 4 - 8 GHz. We found a direct correlation between changes in the DC and microwave conductivity during exposure to ammonia and hydrochloric acid at concentrations of 10 - 1000 ppm. These films show a significant response over this range of concentrations, thus opening the possibility of using conducting polymers thin films as remote wireless gas sensors. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y28.00010: Anomalous transmission through a periodic subwavelength hole array in heavily doped conducting polymer films* Tatsunosuke Matsui, Z. Valy Vardeny, Amit Agrawal, Ajay Nahata, Reghu Menon Since Ebbesen et al. reported the phenomenon of ``anomalous transmission'' through optically thick metallic films perforated with two-dimensional (2D) subwavelength hole array, numerous studies have been carried out to explore both fundamental issues and potential device applications. So far, studies on ``anomalous transmission'' were carried out using metals and semiconductors. We report here the observation of ``anomalous transmission'' in 2D hole array on films of another, more exotic class of conductors, namely heavily-doped organic conducting polymers. Specifically, the conductivity of conducting polymers can be controlled in situ by changing doping level using an electrochemical technique, so that we could tune the transmission characteristics by applied voltage. With this goal in mind we will report the ``anomalous transmission'' spectra of conducting polymer films at various doping levels. *supported in part by ARO. [Preview Abstract] |
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