Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P29: Transport and Electronic Structure of Organic Electronic Materials |
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Sponsoring Units: DPOLY FIAP Chair: Arthur Epstein, Ohio State University Room: LACC 504 |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P29.00001: Theory of Quantum Hopping In Metallic Polymers and Applications in Electronics Invited Speaker: The low frequency electromagnetic response of highly conducting polymers (e.g., polyaniline and polypyrrole) in a metallic state$^{1}$, when analyzed within the standard theory of metals, is provided by an extremely small fraction of the total number of available electrons $\sim $ 0.1 {\%} (in contrast to $\sim $ 100 {\%} for common metals such as Cu, Ag, or Ni) but with anomalous long scattering time $>$ 10$^{-13}$ s ($\sim $ 100 times longer than for common metals). We show that a chain-linked network of metallic grains (the polymer's crystalline domains) connected by resonance quantum tunneling through strongly localized states in surrounding disordered medium produces this behavior. The small fraction of electrons is assigned to the low density of resonance states and the long scattering time is related to the narrow width of energy levels in resonance. Recently a new interesting phenomenon, an electric field effect, was reported for the doped highly conducting polymers. Upon applying the gate voltage of a few volts the conductivity of the polymer film drops by a several orders of magnitude$^{2}$. This observation is in conflict with the fact that the electric field cannot penetrate into a conductor further that the `lattice constant', and therefore its effect on the polymer film of $\sim $ 100 nm thickness should be negligible. We suggest that the field effect in doped conducting polymers is an electric field induced conductor-nonconductor transition described by the chain-linked granular model in the presence of mobile ions. The ion motion under the gate voltage is breaking the interdot percolation network by removing critical hoping sites and as a result producing the conductor-nonconductor transition. The experimental evidences for the present mechanism of field effect in conducting polymers are presented. \begin{enumerate} \item R.S. Kohlman \textit{et al}.,\textit{ Phys. Rev. Lett}. \textbf{78}, 3915 (1997). \item A.J. Epstein \textit{et al}., \textit{Curr. Appl. Phys.}\textbf{ 2}, 339 (2002). \end{enumerate} [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P29.00002: Charge injection, transport and trapping in nanoparticle based memory devices J. Campbell Scott, Luisa Bozano, Ryan Chiechi, Jodi Iwata Blends of metallic nanoparticles in a semiconducting organic host show bistable electrical resistance at low ($\sim $1 V) reading voltage. When a layer of the blend, of order 100 nm thick, is sandwiched between electrodes, a low resistance (on) state is set by applying a voltage pulse of order 2 V to 3 V, and is switched to the high resistance (off) state by a pulse of about 6 V to 8 V. This behavior is observed for several different metals (e.g. Au, Ag, Al, Mg) and for both polymeric and small-molecule semiconductors. We interpret the switching and bistability in terms of charge trapping and storage on the nanoparticles. When trapped charge density is high, the resulting space-charge field inhibits charge injection, yielding the off-state, and vice versa. A simple model based on Fowler-Nordheim tunneling shows that particles of order 2 - 5 nm in diameter exhibit sharp discharge thresholds in the range of a few volts, as observed in the experiments. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P29.00003: Local EFM measurements of organic conducting materials at various temperatures Tse Nga Ng, William Silveira, John Marohn We have investigated the charge injection processes in regioregular poly-3-hexylthiophene (P3HT) and in a molecularly-doped polymer, triarylamine (TPD) dispersed in polystyrene (PS), by electric force microscopy under high vacuum and at various temperatures. This microscopic study unambiguously reveals space-charge limited conduction within TPD-PS, in contrast to bulk current-voltage measurements. The temperature dependence of charge injection will also be shown to compare with current theories on charge injection into organic semiconductors. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P29.00004: On the electronic transport in doped polyaniline/polyethylene oxide nanofibers prepared via electrospinning Natalya A. Zimbovskaya, Alan T. Johnson Jr., Nicholas J. Pinto We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide. Scanning conductance microscopy shows that fibers with diameter below 15 nm are electrically insulating. Single fiber I-V characteristics show that thin fibers conduct more poorly than thick ones and fibers with large asymmetry along their length between the electrical contacts show rectifying behavior [1]. A theoretical analysis of the conductance in the polymeric nanofibers is presented. The analysis is based on the model of a granulated metal [2], so the polymeric material is treated as a network of small metallic-like domains made out of densely packed polymeric chains embedded in an amorphous array of disordered chains. Assuming the electronic transport to be provided by the electron tunneling between the metallic grains through the intermediate resonance states, the theory of conductance in molecular wires [3] is employed to calculate the tunneling current. It is shown that nonlinear features in the I-V curves could appear when the coupling of the grains to the intermediate ``bridge'' is weak enough. Obtained results are in agreement with the experiments. [1]. Y. Zhou, M. Freitag, J. Hone, C. Staii, A.T. Johnson, N.J. Pinto and A.G. MacDiarmid, Appl.Phys.Lett., v.83, 3800 (2003). [2]. V.N. Prigodin and A.J. Epstein, Physica B, v.338, 310 (2003). [3]. S. Datta, Nanotechnology, v.15, S433 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P29.00005: Dispersion and Current-Voltage Characteristics of Helical Polyacetylene Single Fibers Y.W. Park, H.J. Lee, A.N. Aleshin, J.Y. Lee, Y.S. Kim, D.W. Kim, Z.X. Jin, M.J. Goh, K. Akagi To study the transport properties of individual helical polyacetylene (PA) fibers, we developed a method to extract a single fiber from tightly entangled ropes of helical PA bulk film. After a few minutes of sonication of a piece of helical PA bulk film in an organic solution containing surfactant, a droplet of solution is deposited on the pre-patterned electrode under argon atmosphere. AFM images show that extracted helical PA fibers are typically 10 $\mu $m in length and 100--200 nm in diameter. We found that the helicity of bulk materials is conserved. We present the temperature dependencies of current-voltage characteristics of individual helical PA fibers doped with iodine. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P29.00006: Characterization of the Porphyrin Molecule as an Electronic Component Sathish Thriuvengadam, Kim Lewis, Raghu Ramachandran, Royston Siow, Theda Daniels-Race Porphyrins have recently generated great interest as a potential ``electronic material'' for use in functions such as sensing (i.e.-carbon monoxide), biomolecular and medical physics applications (i.e. antiviral agents) and for computer memory capability. Fundamental to the latter of these three and to the realization of molecular based electronic devices is the phenomenon of self-assembly. In this work, we investigate the electronic characteristics of porphyrins, using directed self-assembly monolayer of n-alkanethiols. SAM of alkanethiols on gold surfaces has been shown to form stable surface structures. It is expected that the exchange of molecules is most active at SAM defective sites, substrate step edges and substrate vacancy islands. We measure the I-V characteristics of porphyrin molecules by directed self-assembly in the SAM defect sites. It is observed in ambient conditions using conductive probe atomic force microscopy (CPAFM). The conductivity of porphyrin molecules in alkanethiol SAM is discussed here. This experimental result is further enhanced by our group through ``AFM study of current transport through porphyrin based molecules.'' [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P29.00007: CP-AFM Study of Current Transport Through Porphyrin – Based Molecules Raghu Ramachandran, Kim Lewis, Sathish Thiruvengadam, Royston Siow, Theda Daniels-Race Conductive Probe Atomic Force Microscopy (CP-AFM) is used to study current transport through dithiolated porphyrin based molecules. Porphyrin molecules are inserted at defect sites into an alkanethiolate SAM on Au (111), and the exposed top terminal end of the porphyrin with thiol is attached to a gold nanoparticle. These gold nanoparticles (d$_{CORE}$ = 1 nm to 5 nm)$_{ }$ stabilized by phosphine ligands are introduced into solution where ligands are displaced by thiol groups of the porphyrin bound to Au surface. I(V) measurements are done with nanoparticles of varying sizes to determine the effect on transport properties. Measurements are done using CP-AFM, and contamination is reduced by immersing the sample in toluene. Complementary work from our group will also be presented as ``Characterization of Porphyrin as an Electronic Component'' at this meeting. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P29.00008: The injection barrier at a metal/organic interface D. H. Dunlap, Tianjian Lu The landscape for the thermionic injection of electrons from a metal into a molecularly doped polymer is energetically disordered as a result of inhomogeneous electric fields coming from the non-uniform charging of dopant molecules in the vicinity of the metal/organic interface. This comprises an electric dipole (double - ) layer. We have determined the equilibrium composition of the dipole layer by simulated annealing, in order to study its influence on injection. We find that an electron must surmount an energetic barrier in order to escape into the organic, even when the LUMO level of the dopants is aligned with the Fermi level of the metal. For typical dopant densities, this barrier is on the order of 0.5 eV. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P29.00009: Charge Injection into Cathode-Doped Amorphous Organic Semiconductors Benjie Limketkai, Marc Baldo We analyze electron injection at a wide variety of metal-organic semiconductor interfaces, and find remarkably universal characteristics at low temperature. The current voltage characteristics at T = 10 K follow power-law behavior, $J \quad \sim \quad V^{m}$, where $m $=~(20+/-1) for over 15 combinations of the metals Li, Mg, Al, Ag and Au and the organic semiconductors Alq$_{3}$, BCP, CBP, TAZ, and CuPC. The material independence of injection characteristics at low temperature is attributed to the effect of interface roughness on the image potential. We develop an analytic model to explain charge injection into organic semiconductors at disordered interfaces. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P29.00010: Spectroscopy and Imaging of Metal-Organic Interfaces using BEEM Cedric Troadec, Linda Kunardi, Natarajan Chandrasekhar Charge injection from metal electrodes to organics is a subject of intense scientific investigation for organic electronics. Ballistic electron emission microscopy enables spectroscopy and imaging of buried interfaces with nanometer resolution. Spatial non-uniformity of carrier injection is observed for an Ag-PPP and an Ag-MEHPPV interfaces. Possible reasons are discussed. BEEM current images are found to correlate only marginally with the surface topography of the Ag film. We also determine the transmission function of the Ag-PPP interface. The transmission function is inferred from the derivative of the measured BEEM spectrum, and compared with theory, and with the transmission of metal inorganic semiconductor (MIS) interfaces. For Ag-PPP, we find a curvature opposite to that of MIS interfaces. This agrees well with the theoretical calculations on metal-phenyl ring interfaces. We demonstrate that patches of low Schottky barrier can nucleate current filaments and are likely responsible for the switching behavior observed in metal-organics. [Preview Abstract] |
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