Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U25: Focus Session: Oligoacene Semiconductors |
Hide Abstracts |
Sponsoring Units: DPOLY DMP Chair: John Rogers, University of Illinois Room: Baltimore Convention Center 322 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U25.00001: Preparation and properties of substituted acenes for organic electronics: pentacene through heptacene John Anthony Acenes (such as pentacene and tetracene) and functionalized acenes (such as rubrene) have demonstrated remarkable electronic properties in thin film devices and in single crystals. Our research effort is directed toward the synthesis of new classes of acene-based compounds that are stable, soluble, and that can be used to probe the relationships between solid-state order and electronic properties. Careful consideration of the relationship between molecular substituent and crystal packing has led to the development of new, soluble materials with thin-film properties comparable to their unfunctionalized counterparts. Functionalization of the central ring of pentacene can lead to a change in solid-state order from the classic herringbone arrangement to the less-common face-to-face interactions, while functionalization of the outermost rings of pentacene leads to subtle variations in the herringbone arrangement. Similar approaches can be taken with heteroacenes, again leading to solid-state interactions that favor strong interactions between the aromatic pi-clouds. Furthermore, the added stability gained through functionalization has allowed for the first time the preparation and study of derivatives of higher acenes such as hexacene and heptacene. The detailed study of the optical and electronic properties of a variety of functionalized acenes has led to the development of new materials for use in photovolatics, light-emitting diodes and thin-film transistors. We are grateful to the Office of Naval Research for support of this research. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U25.00002: Conducting AFM and 2D GIXD Studies on Pentacene Thin Films Hoichang Yang, Chang Y. Ryu, Tae Joo Shin, Kilwon Cho, Mang-Mang Ling, Zhenan Bao 2D GIXD, TM-, and C-AFM analyses of pentacene films support the idea that the morphology of ultrathin layers plays a crucial role in determining mobility in OTFT. While 60-nm-thick pentacene films exhibited similar terrace-like multilayer structure with the long axis of pentacene perpendicularly oriented as determined from TM-AFM and 2D GIXD, its charge mobility in an OTFT was quite different, depending on the types of hydrophobic SAM surface treatment. This difference is related to the morphological difference of the first pentacene layer ``buried'' under the terrace-like multilayers. We found that the faceted islands on HMDS showed larger current flow than the dendritic islands on OTS using C- AFM. This trend in C-AFM current images correlated well with the charge carrier mobility measured in OTFTs. Such faceted morphology represents single crystal-like pentacene islands, which have fewer internal crystal defects and higher current flow than the dendritic islands. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U25.00003: Hall Effect Measurements in Organic Single-Crystal FETs Vitaly Podzorov, Etienne Menard, John Rogers, Michael Gershenson We have observed classical Hall effect in the single-crystal rubrene OFETs [1]. The mobility determined from the Hall measurements (mu{\_}H) represents intrinsic, i.e. trap independent mobility of the charge carriers. At high temperatures, Hall mobility coincides with the longitudinal mobility determined from the standard FET measurements. In the investigated temperature range T = 170-300 K, mu{\_}H monotonically increases with decreasing T, while the longitudinal mobility first increases at high T (intrinsic regime) and then decreases at low T (non-intrinsic regime), consistent with the previous observations [2, 3]. In the intrinsic regime, the density of mobile field-induced charge carriers extracted from the Hall measurements, n{\_}H, coincides with the density n calculated using the gate-channel capacitance, and becomes smaller than n in the trap-dominated regime. The Hall data strongly support a band-like nature of the charge carrier transport in this system. 1. Podzorov, et al., Phys. Rev. Lett 95, 226601 (2005); 2. V. C. Sundar, et al., Science 303, 1644 (2004); 3. V. Podzorov, et al., Phys. Rev. Lett. 93, 086602 (2004); [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U25.00004: Hall effect in organic single-crystal field-effect transistors J. Takeya, Koichi Yamada, K. Tsukagoshi, Y. Aoyagi, T. Takenobu, Y. Iwasa Hall effect is detected in organic field-effect transistors at room temperature, using appropriately shaped rubrene (C$_{42} $H$_{28}$) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The observation of the normal Hall effect means that the accumulated surface charge is well extended in space over molecules, so that the external magnetic field can provide a transverse electromotive force. The result is consistent with band-like transport of the surface carriers rather than consecutive hopping processes occurring in response to the source-drain voltage in the organic transistors. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U25.00005: Control of Channel Conductivity of Rubrene Single Crystal Field Effect Transistors. Christian Kloc, Magnus Wikberg, Woo-Young So, Dave V. Lang, Theo Siegrist, Arthur P. Ramirez Carrier mobility higher than 1 cm$^{2}$/Vs has been measured in numerous organic single crystal FETs, making them interesting for microelectronic applications. The understanding why some organic pi-electron systems show high mobility and others, very similar molecules, show much lower mobility is crucial for design of efficient and robust organic semiconductor devices. It seems that transistor properties measured on FETs are extrinsic properties limited by technology used for transistor fabrication. However, to evaluate the applicability of organic semiconductors, intrinsic properties need to be assessed. We have carried out a program to purify and grow low defect density single crystals and fabricate FETs on their surfaces. Using graphite as electrodes and parylene as an insulator we measured maximal mobility in rubrene of 13 cm$^{2}$/Vs and significant anisotropy of transport properties. To control the transistor properties, we chemically modify the channel area and measured the conductivity of transistor channels before covering it with dielectrics and gate electrode. We found that the channel area of rubrene is very sensitive on reduction and oxidation and that the transistor properties may be modified by performing chemical reactions on the crystal surfaces before finishing transistor structure.. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U25.00006: Thermal Expansion and Molecular Motion in Rubrene and Tetracene Theo Siegrist, Christian Kloc, Magnus Wikberg, Simon Haas The closely related molecules rubrene (tetra-phenyl-tetracene) and tetracene are model systems for organic semiconductor materials. Very high carrier mobilities have been observed in rubrene crystals. Single crystals of rubrene and tetracene, produced by vapor phase growth at elevated temperatures, show clear differences in their quality. The thermal expansion/contraction coefficients for crystals of the two molecules have been measured using X-ray diffraction. The triclinic symmetry of tetracene crystals is reflected in the strongly anisotropic thermal expansion observed. In the case of rubrene, relatively small thermal expansion coefficients are found. Libration/translation values obtained using the rigid molecule model will be presented for both systems. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U25.00007: A Systematic Study of Metal Contacts on Single Crystalline Rubrene Woo-young So, A. Mike Sergent, Christian Kloc, Arthur Ramirez The performance of semiconductor devices is critically dependent on the metal- semiconductor heterojunction. In organics, despite the technological importance of such interfaces, little is known about the fundamental mechanisms that govern their performance in real devices. We have studied a series of metal contacts on rubrene single crystals and find systematic dependence of the transport barrier on the metal workfunction. These data provide insight into surface states that strongly influence the contact resistance. In the process of the study of metal-rubrene heterojunctions, we have realized an efficient single-crystal rubrene diode employing a hole-blocking metal contact. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U25.00008: Negative Thermal Expansion in Pentacene S. Haas, T. Siegrist, P. Pattison, B. Batlogg The molecules in organic semiconductors, such as the oligoacenes, are held together by weak van-der-Waals (v-d-W) forces. As a result, one observes low dissociation temperatures, and large thermal expansion coefficients. Surprisingly, we have found a negative expansion coefficient in particular crystal directions. We have performed a complete X-ray structure analysis of pentacene and tetracene single crystals in a temperature range from 100--380 K. The anisotropic thermal parameters are analyzed in terms of librations and translations of the rigid molecules. Interestingly, we find upon increasing temperature a near-zero thermal expansion along $a$ in tetracene, and a distinct contraction in pentacene. Upon close inspection of the full expansivity tensor and the thermal parameters we find a consistent explanation assuming that the v-d-W forces tend to minimize the relative shift along the long axis of adjacent molecules. This is further supported by the observation of an unusually large thermal expansion perpendicular to the layers, and the indications in our data of a distinctly anharmonic potential for the sliding motion along the long axis. These v-d-W forces leading to negative thermal expansion are expected to be larger in longer molecules and also to be the driving force behind the well known polymorphism of pentacene. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U25.00009: Charge transport in single-crystals of pentacene studied with temperature dependent THz time-domain spectroscopy. H.A. van Laarhoven, M. Koeberg, E. Hendry, M. Bonn, C.F.J. Flipse We study the charge dynamics and transport mechanisms in single crystals of pentacene by investigating the frequency dependent complex conductivity using THz time-domain spectroscopy. Such measurements on this material show generation of THz radiation from the sample as well as ultra-fast charge decay. Our measurements have been corrected for these effects, giving us the ability to see the true frequency dependent behaviour of the conductivity. Two different pump energies were used (266 and 400 nm) in the experiments. In the low temperature regime the mobile charges give a different response when being photo-excited at different wavelength. This disparity in charge transfer will be discussed and compared to theoretical models. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U25.00010: Optical emission and vibrational modes of uniform pentacene monolayers (*) Rui He, Nancy Tassi, Graciela Blanchet, Aron Pinczuk Pentacene monolayers are probed by photoluminescence and resonant Raman spectroscopies below 10K. Monolayers grown on polymeric substrate of poly-alpha-methyl-styrene (PAMS) exhibit high uniformity within micron size clusters. These films show sharp exciton luminescence bands, and the energy of the exciton optical emission displays a red-shift as the average film thickness increases. The large resonance enhancements of Raman scattering intensities enable the measurements of low-lying (40- 200cm-1) optical lattice vibrations from these monolayers. These experiments demonstrate that luminescence and resonant Raman scattering from single pentacene monolayers are venues for probing 2D properties, studies of interface effects, and thin film characterization. (*) Supported primarily by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF Award No. CHE-0117752 and by the New York State Office of Science, Technology, and Academic Research (NYSTAR), and by a research grant of the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U25.00011: Sensitivity of 1/f Noise to Chemical Constituents in Pentacene Thin Film Transistors W. X. Yan, E. Gomar-Nadal, Masa Ishigami, M. S. Fuhrer, E. D. Williams This study systematically investigates the sensitivity threshold of 1/f noise for use as a device diagnostic tool, with pentacene thin-film transistors (TFTs) as a model. When pentacene in TFTs was mixed with an incremental series of the oxidative impurity 6,13-pentacenequinone (PQ), 1/f noise power rose proportionate to increasing impurities with a preliminary sensitivity threshold of $\geq$ 0.6 $\%$ PQ, coupled with a decreasing hole mobility. The result and further theoretical interpretation can supplement current quality assessments and help better understand the innate deficiencies in organic electronics, thus potentially improving their quality. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U25.00012: Interface and Contact Formation in Pentacene Monolayer Transistors Byoung-nam Park, Soonjoo Seo, Fan Zheng, Franz Himpsel, Paul Evans Understanding charge transport in the accumulation layer of organic field effect transistors is crucial to improving their performance. Our in-situ electrical measurements during the deposition of pentacene onto a bottom-contact transistor structure with a silicon dioxide gate dielectric allowed us to study the formation of a transistor channel at the single- molecular-layer scale. At pentacene coverages near a percolation threshold the monolayer-high islands come into contact and current begins to flow through the channel. Using pauses in the deposition we have extracted transistor characteristic parameters with well-defined submonolayer island structures. Van der Pauw sheet resistance measurements show that the small field effect mobilities of monolayer transistors are associated with the formation of contacts rather than with the mobility of carriers within the semiconductor layer. Both near edge x-ray absorption fine structure measurements and atomic force microscopy show that the morphology and crystal structure of the pentacene layers changed as the total amount deposited onto the sample increased through the few-monolayer regime. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U25.00013: Growth and structure of pentacene thin films Boquan Li During the last few years, there has been an explosion of interests in exploring organic thin-films of small conjugated molecules, like pentacene (C22H14), as active semiconductor devices. The full potential of organic thin-film semiconductor devices for applications will not be realized until the growth of highly ordered organic thin films can be optimized and well controlled. Thin films of pentacene are known to crystallize in different polymorphs, which have layered structures characterized by their different interlayer spacings, d(001). How to grow highly ordered single phase molecular thin films remains a challenging subject. Motivated by this, we have investigated the growth of pentacene thin films on hydrogen-terminated Si (111) and clean Si (111) surfaces in an effort to gain a comprehensive understanding of the molecule-substrate interactions and explore strategies to suppress the formation of polymorphism and therefore to grow highly ordered pentacene thin films. Electron diffraction is used to investigate the in-plane molecular structure. The crystallographic results are correlated with growth. The detailed structural analysis results will be presented. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U25.00014: Electron and hole transport in organic single crystals J. Pflaum, A.K. Tripathi, S. Meyer Transport measurements reported in literature often classify the semiconducting behavior in organic thin films and single crystals to be whether p-type or n-type. Moreover, the majority of polyaromatic hydrocarbons such as pentacene in combination with technically relevant contact materials such as Au are found to preferentially show hole conductivity. In this presentation, we will discuss these key aspects of the charge carrier transport in case of highly-ordered organic crystals, the latter grown by diferent methods from previously purified material. By time-of-flight spectroscopy it will be demonstrated for various compounds that the chemical purity rather than the growth conditions determine the resulting semiconducting behavior. E.g. anthracene single crystals grown by sublimation provide as good electron as hole mobilities of about 1$\ cm^{2}$/Vs if the material is purified by zone- refinement. For crystals made of sublimation purified anthracene, no electron transport could be detected. Finally, comparing various structural motifs we will discuss possible concepts of molecular design enabling stability against photo- oxidation and allowing for the formation of highly-ordered thin films. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U25.00015: Oxygen and water defect formation processes in pentacene. Leonidas Tsetseris, Sokrates Pantelides Organic thin-film devices have emerged as promising candidates for novel electronic applications. Unlike traditional solid-state devices, the weak intermolecular non-covalent bonding of organic thin films leads to flexibility and possible pattern formation. The same mechanism, however, is responsible for the appearance of a variety of defects that may interfere with film growth and are potentially detrimental to the desired transport properties. Here we use first-principles calculations to study defect formation processes in the prototypical system of pentacene. In particular, we report on defect configurations of oxygen and water molecules in bulk pentacene and ultra-thin films on Si-based substrates. The results show that several stable configurations of such defects exist. Their presence has a direct bearing on growth processes and transport properties through strong covalent bonding and induced molecular distortions in their vicinity. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700