Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S16: Focus Session: Molecular Materials: Electronic Transport and Growth |
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Sponsoring Units: DMP DCP Chair: Douglas Natelson, Rice University Room: LACC 404A |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S16.00001: Ultrathin epitaxial graphite layers : 2D electron gas properties and a route towards graphene based nanoelectronics Claire Berger, Zhimin Song, Tianbo Li, Xuebin Li, Asmerom Ogbazghi, Rui Feng, Zhenting Dai, Alexei Marchenkov, Edward Conrad, Phillip First, Walt de Heer Nanopatterned ultrathin epitaxial graphite structures have been produced by thermal decomposition on single crystal SiC and conventional lithographic techniques. The films, composed of less than 5 graphene sheets, show remarkable 2D electron gas properties. Large positive magnetoconductances and large magnetoconductance anisotropy, indicate that orbital effects dominate the magnetotransport. Shubnikov de Haas oscillations have been observed as well as a pronounced zero-bias anomaly in low-temperature current versus voltage spectra. The films have been gated by applying potentials to gate electrode structures. These results and the control of the nanographite structure suggest nanoelectronic device applications, and a route towards realizing the device potential of nanographite, predicted to be comparable to carbon nanotubes. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S16.00002: Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite Yuanbo Zhang, Joshua Small, Philip Kim We use a unique micromechanical method to extract extremely thin graphite crystallites from bulk highly oriented pyrolitic graphite samples. Electric field effect devices are subsequently fabricated for galvanomagnetic measurements. Strong modulation of magneto- resistance and Hall resistance as a function of gate voltage is observed as the sample thickness approaches the screening length of graphite. Electric field dependent Landau level formation is detected from Shubnikov de Haas oscillations in magneto-resistance. The effective mass of and hole carriers has been measured from the temperature dependent behavior of these oscillations. Extending similar experimental methods to other layered materials will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S16.00003: Electric Field Effect in Planar Single-Layer Graphene Andre Geim, Kostya Novoselov, Da Jiang, Yaun Zhang, Tim Booth, Irina Grigorieva, Sergey Morozov, Anatoly Firsov, Sergey Dubonos We describe free-standing single-layer crystals of graphene, which are one carbon atom thick but extend over many microns laterally. This two-dimensional fullerene macromolecule is obtained by mechanical exfoliation and allows standard microfabrication procedures, as described in our earlier paper in Science \textbf{306}, 666 (2004). We have found graphene to be stable under ambient conditions, conductive and of remarkably high quality. Using graphene films, we have fabricated transistor-like devices and studied their properties from room to liquid-helium temperatures. Graphene exhibits a strong ambipolar electric-field effect with room-temperature mobilities of electrons and holes up to $\approx $10,000 cm$^{2}$/Vs, which implies ballistic transport over submicron distances. At low temperatures, we have observed pronounced Shubnikov-de Haas oscillations and well-developed plateau-like features, indicating onset of the quantum Hall effect. Graphene is found to be a zero-gap 2D semiconductor. Analysis of the quantum oscillations also indicates the linear, Dirac-like spectrum of its carriers. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S16.00004: Electrical Properties of Metallic and Semiconducting Transition-Metal Dichalcogenide Nanopatches. Enrique Cobas, Anthony Ayari, Ololade Ogundadegbe, Michael Fuhrer Metallic and semiconducting nanopatches of MoS$_2$, TaS$_2$ and WSe$_2$ dichalcogenide crystals on SiO$_2$ substrates were fabricated. The crystals were synthesized by a chemical vapor transport method or obtained in natural form and cleaved by mechanical or chemical exfoliation techniques to thicknesses as small as 2nm. Electrical contact to the nanopatches was established via lithographically defined metal leads to allow measurement of electrical properties, including charge carrier mobility and Hall mobility, in field-effect transistor geometries at various temperatures. These nanopatches represent progress toward studies of electron behavior in self-assembled two-dimensional systems. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S16.00005: X-ray topography and high resolution diffraction of single-crystal rubrene B.D. Chapman, R. Pindak, T. Siegrist, C. Kloc Assessing the fundamental limits of the charge carrier mobilities in organic semiconductors is an important step in optimizing organic-based electronic devices. Rubrene is an interesting organic semiconductor material with high charge carrier mobility. Improved characterization of the crystalline quality of rubrene is expected to lead to a better understanding of the role of defects on charge transport. Here, we present x-ray topography and high resolution diffraction measurements of high mobility single-crystal rubrene. The topographs reveal many features commonly found in self-nucleated inorganic crystals and they provide important information about the character and distribution of crystallographic defects. In addition, we find significant differences in the topographs of organic crystals that appear equivalent under optical polarization analysis. We demonstrate that x-ray topography is a suitable tool for the optimization of organic crystal growth. We also present complementary AFM and preliminary surface x-ray diffraction measurements of single-crystal rubrene. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S16.00006: Reciprocal Space Mapping in Organic Electronic Materials Theo Siegrist, Christian Kloc, Charley Chi Semiconductor materials have been traditionally studied using X-ray diffraction to asses stress/strain. However, X-ray rocking curves often do not provide a good separation of the mosaic structure and stress induced shifts in the peak positions. To further study the quality of organic semiconductor materials, the mosaic structure needs to be deconvolved from stress/strain induced peak shifts and peak broadening. Reciprocal space map scans were carried out for pentacene crystals obtained from different growth procedures. Large mosaic spreads were observed, however, individual grains are well crystallized with little strain present. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S16.00007: Charge transport and trap spectroscopy in organic molecular crystals Invited Speaker: Key issues in the field of organic semiconductors are the determination of the intrinsic transport properties and of the distribution and the role of the trap states. We report on the use of temperature-dependent space-charge limited current spectroscopy and of field-effect measurement techniques to determine the bulk and surface transport properties and the distribution of trap states in the bulk of organic molecular crystals (OMCs). Systematic studies of structurally and chemically pure rubrene crystals reveal deep states with a density as low as 10$^{15}$~cm$^{-3}$ and an exponentially increasing density of shallow tail states near the mobility edge. Furthermore, we are able to intentionally generate additional traps, determine their density and energetic position within the band gap, and their influence on the bulk transport. Charge transport at the surface of rubrene, tetracene, pentacene and alpha-sexithiophene crystals measured e.g. by a gated four-terminal ``flip-crystal'' FET technique indicates significant charge trapping with an estimated interface state density of 10$^{11}$ to 10$^{12}$~cm$^{-2}$ in high purity OMCs. Being able to measure the energetic distribution of trap states, to create and characterize defect states, and to study their influence on charge transport in OMCs constitutes significant progress in understanding crystalline organic semiconductors on a microscopic level. In collaboration with C. Krellner, S. Haas, K. P. Pernstich, A. Kloke, D. J. Gundlach, and B. Batlogg. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S16.00008: Study of defects in pentacene single crystals Oana D Jurchescu, Jacob Baas, Thomas T.M. Palstra Our research focuses on the growth of the pentacene single crystals with a high degree of purity, and the investigation of their morphology and physical properties. We are able to reduce the number of traps by two orders of magnitude compared with conventional methods. This is reflected in the value of hole mobility of 35 cm2/Vs at room temperature increasing to 58 cm2/Vs at 225 K. These high mobilities result from a purification of the material, that consists of removal of 6,13-pentacenequinone, as the major impurity, using vacuum sublimation under a temperature gradient. We further study the influence of air exposure on the electronic properties of pentacene single crystals. Our observations show that gases can diffuse reversibly in/out the crystals, and influence the electronic properties. We discern two competing mechanisms that modulate the electronic transport. On the one hand stands the presence of water from ambient air that intercalates into the crystal lattice and forms trapping sites for injected charges. On the other hand, the presence of oxygen increases the hole conduction. The latter effect is enhanced by the presence of visible light. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S16.00009: Thin Bismuth Film as a Template for Growth of Highly Ordered Pentacene: STM and LEEM study Jerzy T. Sadowski, Tadaaki Nagao, Yasunori Fujikawa, Shin Yaginuma, Abdullah Al-Mahboob, Toshio Sakurai, Gayle E. Thayer, Ruud M. Tromp Considering that the atomic bonding in bulk Bi can be described as intermediate between “covalent” and “metallic,” one may expect a unique growth mechanism for thin Bi films, different from that observed in the case of metals or semiconductors. In this talk, the results of the scanning tunneling microscope (STM) and low-energy electron microscope (LEEM) investigations will be used to show that thin Bi film undergoes an unique and unexpected structural transformation of the quasi-cubic, {012} oriented film into a hexagonal Bi(001) film [1]. Subsequently, the possibility of using annealed, well ordered Bi(001)/Si(111) films as the templates for the growth of organic thin films with excellent crystallinity will be demonstrated on the example of pentacene – one of the most promising organic semiconductors. Pn nucleates on Bi(001) into highly ordered, crystalline layer, with pentacene molecules “standing up” on the Bi surface, with (001) plane on the growth front. Pn layer is aligned with the Bi(001) surface, having “point-on-line” commensurate relation with the substrate. Moreover, a bulk-like structure is observed in Pn/Bi(001) film, directly from the first Pn layer. [1] T. Nagao et al., Phys. Rev. Lett. 93 (2004) 105501 [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S16.00010: Submonolayer Pentacene Thin Film Growth on Hydrogen-Passivated Si(111) substrate Weijie Huang, Boquan Li, Jian-min Zuo Exploding interests to implement organic semiconductors in novel electronic and optical applications urge the growth of organic thin film with large single crystalline grain and structural homogeneity. By tailoring growing parameters, such as incident flux,substrate surface chemistry and substrate temperature, we succeeded growing pentacene monolayer on a hydrogen-passivated Si(111) substrate, with average grain diameter of 15 micron, by low flux Organic Molecular Beam Deposition(OMBD). We observe that, using atomic force microscope, the morphology of the islands evolve from independent fractal shapes to correlated compact shapes, as the coverage increases, which can be explained by capture-zone growth model. Dynamic scaling behavior of the 2 dimensional pentacene islands is studied, from which critical island size for nucleation on H-Si(111) surface is extracted. Pentacene's large island size and layer-by-layer growth mode on H-Si(111) allow us to study fundamental processes of metal deposition on pentacene,which include metal-carbon bonding formation,crystal structure of the metal cluster on pentacene layers,and the pentacene-step-mediated growth of metal clusters. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S16.00011: Structures and phase transitions in the growth of tetracene on silicon Jun Shi, Xiaorong Qin We report a study on the evolution of the morphology of tetracene thin films during vapor deposition. The film growth has been carried out at room temperature in vacuum on H-passivated silicon substrates. We investigate the surface structures of the films under different growth conditions with an ex-situ atomic force microscopy. The surface structures have been found sensitive to the kinetic parameters such as the coverage and the deposition rate, and in favor of 3D-island formation. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S16.00012: Morphology of benzene and pentacene self-assembled monolayers and gold-molecule-gold junctions. Leonidas Tsetseris, Sokrates Pantelides The structural properties of self-assembled monolayers of organic molecules or nanostructures on metallic or semiconductor surfaces are critical in determining the electronic and transport properties of such configurations. Here we report results obtained by first-principles density-functional calculations on a number of systems. We first describe the morphology of benzene-dithiolate films on gold surfaces. Special emphasis is given on the behavior of the ultrathin film during the deposition of the second electrode, particularly on the kinetics of cleavage of hydrogen and formation of covalent S-Au bonds. We also discuss the work of Xu and Tao\footnote{B. Xu and N. J. Tao, Science {\bf 301}, 1221 (2003)} on formation of molecular junctions, and we analyze the distinct structural phases that can relate to quantum conductance observed in these experiments. Finally, we present results on the growth of pentacene films on Si and SiO$_2$\ surfaces, examining the possibility of forming chemisorbed structures. [Preview Abstract] |
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S16.00013: Growth of pentacene films for transistor applications George Malliaras, Alex Mayer, Randall Headrick Organic thin film transistors (OTFTs) are being developed in academic and industrial labs for ``disposable'' plastic electronics such as smart identification tags. Among the most promising organic semiconductors is pentacene, which yields transistors with performance similar to that of amorphous silicon. The interfaces of pentacene films with dielectric materials (gate oxide) and conductors (electrodes) play a major role in determining OTFT performance. A combination of synchrotron x-ray diffraction and atomic force microscopy was used to probe these interfaces and help optimize pentacene growth. By varying the growth conditions we were able to obtain polycrystalline films with crystallite sizes of the order of tens of microns. The connection between growth, morphology and OTFT performance will be discussed. [Preview Abstract] |
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