Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D18: Focus Session: Transport and Optical Properties of Conjugated Polymers and other Solution Processable Semiconductors |
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Sponsoring Units: DPOLY Chair: Anvar Zakhidov, University of Texas at Dallas Room: 319 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D18.00001: Microstructure and Charge Transport in pBTTT Thin Film Transistors Chenchen Wang, Alberto Salleo, Ludwig Goris, Iain McCulloch, Martin Heeney, Alexander Ziegler The present work focused on the morphology and charge carrier mobility of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT) films. In annealed pBTTT films on oxide functionalized with octyltrichlorosilane (OTS), TEM study shows that the large-scale terraces observed by AFM, which was believed to be the reason for high charge carrier mobility, are composed of smaller crystalline grains. Using the mobility edge model, we find that, compared with the film on oxide, the density of trap states at the band edge is reduced in the film on OTS, and it is about the same as the trap density in poly(3-hexylthiophene) (P3HT), which has lower carrier mobility. This result indicates that the higher room-temperature mobility of pBTTT (0.34 cm$^{2}$/Vs) compared to P3HT (0.02 cm$^{2}$/Vs) is due to a high quasi-free carrier mobility and not to a lower trap density as previously thought. [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D18.00002: Interfacial Charge Transfer in Nanoscale Polymer Transistors Jeffrey Worne, Rajiv Giridharagopal, Kevin Kelly, Douglas Natelson, John Anthony Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel (L < 100 nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero gate voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface. We have also begun to use these same techniques to investigate the nature of interfacial charge transfer between metal electrodes and pentacene. We use these data together with our previous results to develop a more complete picture of metal/organic interfaces. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D18.00003: Ab initio study of molecular packing of organic semiconducting materials Sefa Dag, Lin-Wang Wang The self-organizing and electronic properties of organic semiconducting material, poly(3-hexylthiophene) (P3HT), have been investigated in terms of Ab initio density functional calculations. We found that thiophene-thiophene interaction in adjacent layers has a strong influence to create stacked planar structures. Our calculations showed that P3HT chains tend to stack into planar structures, in which adjacent thiophene-thiophene rings along the stacking direction are $180^o$ rotated with respect to each other. Theoretical powder diffraction profile of this structure showed same structure with experimental reflection peaks. We also showed enhanced transport resulting from the organization of P3HT chains. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D18.00004: Electrical Noise in Individual Conducting Polymer Nanowires Alexey Kovalev, Yanyan Cao, Theresa Mayer, Thomas Mallouk Electrical property characterizations of conducting polymer nanostructures have been limited primarily to resistance measurements. Electrical noise is one aspect that is usually overlooked, yet critical to their device performance. Moreover, electrical noise is more sensitive to the polymer doping and microstructure than resistance, which makes it particularly interesting for sensor applications. In this talk, we will present the results on the electrical noise measurements of individual multisegmented electrodeposited nanowires based on Poly(3,4-ethylenedioxythiophene (PEDOT) [1]. The polymer was electrochemically doped with either poly(4-styrenesulfonic acid) (PSS) or perchlorate (ClO$_{4})$. The nanowires had gold contacts on both ends and were measured in four-point and two-point configurations. We found that the electrical noise behavior is typical of $1/f$ noise, with a spectral density that depends on the polymer structure and is affected by the ambient conditions. Our data show that the contact noise represents a significant contribution to the total noise level. We will discuss the interpretation of these results assuming that the polymer is a disordered conductor. [1] Cao \textit{et al.}, \textit{Nano Letters} \textbf{Article ASAP} [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D18.00005: Thin film morphology of organic electronic materials S. D. Hudson, R. J. Kline, D. M. Delongchamp, O. D. Jurchescu, D. J. Gundlach, L. J. Richter The crystal orientation and morphology of a polythiophene (pBTTT) and an anthradithiophene (diF-TEADT, a pentacene analog) in thin films have been explored by TEM, SEM, AFM, GISAXD, NEXAFS, polarized FTIR and ellipsometry. The orientation has a striking influence on the performance of thin film transistors. We show that solution casting and annealing conditions have a significant effect on the morphology of pBTTT. Correlations between film surface step morphology and crystal orientation are determined. Interfacial interactions with the substrate (gold, silica, or fluorinated sam) govern the crystal orientation and crystal aggregate morphology of diF-TESADT. Depending on this orientation, the carrier mobility spans from approximately 0.001 cm$^{2}$/Vs to 0.4 cm$^{2}$/Vs. Epitaxial relationships within crystal aggregates are observed. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D18.00006: Conjugated polymer/layered inorganic nanocomposites: solution processable route to enhanced thermoelectric performance Kevin See, Jeffrey Urban, Rachel Segalman In recent years, incorporation of nanostructuring has led to notable improvements in the performance of thermoelectric materials. At a given temperature T, the thermoelectric figure of merit ZT is given by $\frac{S^2\sigma T}{\kappa }$ , where S is the Seebeck coefficient, $\sigma $ the electrical conductivity and $\kappa $ the thermal conductivity. In most cases, improvement in ZT through nanostructuring has been realized via reduction in thermal conductivity $\kappa $ rather than increases in the power factor S$^{2}\sigma $. Here we utilize solution-based intercalation chemistry to create layered inorganic/conjugated polymer nanocomposites with designed nanoscale interfaces engineered to enhance the power factor by energy filtering. The layered inorganic material Sb$_{2}$Te$_{3}$ was intercalated with poly(3-hexylthiophene), and the resulting composite material was cast into thin films from solution. The resulting devices exhibit Seebeck coefficients with two-fold enhancement over those reported for bulk Sb$_{2}$Te$_{3 }$with known conductivities for solution-processed films. These results demonstrate the promise of these novel intercalated materials for high performance solution processable thermoelectric materials. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D18.00007: Revealing Transmission in Metal-Molecule Junctions Using Length Dependant Thermopower Measurements Jonathan A. Malen, Peter Doak, Kanhayalal Baheti, T. Don Tilley, Arun Majumdar, Rachel A. Segalman Conductance in metal-molecule junctions is known to trend with molecular endgroups, backbone, and length, but a more complete picture of the junction's transmission structure has been hitherto elusive. We now report complimentary trends in the junction's thermopower ($S)$ that reveal length dependent changes in molecular orbital alignment and coupling with contact states. Phenylenediamines, phenylenedithiols, and alkanedithiols trapped between gold contacts were examined. $S$ increases linearly with length for phenylenediames and dithiols while it decreases linearly in alkanedithiols. Comparison of this data suggests that the molecular backbone determines the length dependence of $S$, while the endgroup determines the zero-length, or contact $S$. Transport in phenylenes was dominated by the HOMO, which moves closer to the Fermi energy of the contacts as \textit{$\sim $1/L}, and broadens due to contact coupling as \textit{$\sim $e}$^{-L}$. In contrast, the decreasing trend in $S$ for alkanedithiols suggests that transmission is largely effected by gold-thiol gap states between the HOMO and LUMO. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D18.00008: Thermally-Induced Mesophase Transitions in Alkyl-Substituted Thienoacenes Charles M. Shaw, Xinnan Zhang, Lidaris San Miguel Rivera, Geetha G. Nair, Antal Jakli, Adam J. Matzger, David C. Martin Pentathienoacene (T$_{5}$) is an organic molecule---first synthesized in recent years---that is most succinctly described as the thiophene analog of pentacene. In this study, the solid-state structure and phase behavior of dioctyl- and didodecyl-substituted T$_{5}$ were examined \emph{via} differential scanning calorimetry (DSC), variable-temperature, polarized optical microscopy (VT-POM), variable-temperature X-ray diffraction (VT-XRD) and electron diffraction (ED). DSC reveals the presence of a number of phase transitions, while ED, VT-OM and VT-XRD reveal the details of the structural changes of these transitions. The first phase transition exhibited by both materials is a crystal--crystal transformation that involves the contraction of the unit cell along the long axis by nearly 25\%. This change has been attributed to the introduction of numerous gauche defects at elevated temperatures. Further heating causes both molecules to exhibit a Smectic C liquid crystalline phase, identified by VT-OM and VT-XRD. VT-XRD was also utilized to elucidate lattice parameters for these various phases. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D18.00009: Fullerene-based anchoring groups for molecular electronics Christian Martin, Dapeng Ding, Jakob Sorensen, Thomas Bjornholm, Jan van Ruitenbeek, Herre van der Zant We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum and at RT we have studied the electrical properties of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and forms molecular junctions with a low-bias conductance of $3 \cdot 10^{-4}$ G$_0$. Compared to 1,4-benzenedithiol it exhibits a considerably lower conductance spread. Statistical analyses of the breaking process confirm the stability of the fullerene-gold bond. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D18.00010: An organic nanoparticles transistor behaving as a spiking synapse Dominique Vuillaume, Fabien Alibart, Christophe Novembre, David Guerin, Stephane Pleutin, Kamal Lmimouni, Christian Gamrat We demonstrate that an organic transistor, made of metal nanoparticles (NP) embedded into an organic semiconductor channel, behaves as a spiking synapse. We demonstrate that this device exhibits the main behavior of a biological synapse. For instance, it can be programmed to work as an excitatory or inhibitory synapse; it exhibits shot-term plasticity as well as spike timing dependent plasticity. This behavior is obtained by virtue of the combination of two properties: the transconductance gain of the transistor and the memory effect due to charges stored in the NP. The gold NP are immobilized into the source-drain channel by using surface chemistry (self-assembled monolayers) and they were subsequently covered by a thin film of pentacene. In a biological synapse, the excitatory behavior means that an incoming signal with a given frequency and duty cycle induces a post-synaptic signal having an increasing trend, whereas in the case of an inhibitory synapse, the post-synaptic signal tends to decrease. This behavior is exactly what we demonstrated for the ONTS. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D18.00011: The role of the oxygen/water redox couple in suppressing electron conduction in field-effect transistors Pierre L. Levesque, Carla M. Aguirre, Matthieu Paillet, Fran\c{c}ois Lapointe, Benoit C. St-Antoine, Patrick Desjardins, Richard Martel Much like with organic semiconductor FETs, a characteristic of carbon nanotube based devices has been their almost exclusive p-type character in air. Electron transport can be observed only under certain conditions, for instance devices annealed in vacuum. We investigated the impact of the chemical nature of the substrate and of ambient adsorbates on the field-effect switching behavior of both nanoscale and thin-film FETs. Our study, using carbon nanotubes as the testbed, revealed that the intrinsic material properties are modified when an adsorbed water layer containing solvated oxygen is present on the SiO$_{2}$ surface and lead to the reduction of n-type conduction. This finding demonstrates that an electrochemical charge transfer reaction between the semiconducting channel and the aqueous oxygen redox couple is the underlying phenomenon governing the suppression of electron conduction in these devices. This effect should be considered when measuring the transport properties of nanostructures such as nanowires, organic materials, nanotubes, graphene conducted on SiO$_{2}$/Si substrates. [Preview Abstract] |
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