Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T20: Focus Session: Magnetic Properties of Organic Semiconductors/ Surface Characterization of Organic Materials |
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Sponsoring Units: DPOLY Chair: C.D. Frisbie, University of Minnesota Room: 321 |
Wednesday, March 18, 2009 2:30PM - 3:06PM |
T20.00001: Magnetoresistance and magnetic-field-effects in organic semiconductor devices. Invited Speaker: |
Wednesday, March 18, 2009 3:06PM - 3:42PM |
T20.00002: Photo-induced Magnetism and Spintronics in Organic Semiconductors Invited Speaker: Recent years have witnessed growing attention on manipulating spins in organic species. One of the interesting phenomena in organic-based magnets is controlling magnetic properties by optical stimulus, a property not exhibited in metallurgical magnets. Three classes of known phenomena and mechanism will be discussed: i) manipulation of number of spins by optically induced charge transfer in cyano-bimetallic complexes [1], ii) optical control of exchange coupling in Mn(TCNE)$_2$ compound [2], iii) light-induced change of magnetic anisotropy in the magnetic semiconductor V(TCNE)$_x$ [3]. The second part of this talk will be devoted to ongoing research on transferring spin polarized carriers through organic semiconductors. Recently, there have been lively activities as well as controversies on the application of organic semiconductors for transporting spin information. However, the understanding of spin injection and transport in organic semiconductors is still limited. We will address detailed mechanisms for spin injection and transport in organic semiconductor film of our rubrene (C$_{42}$H$_{28}$)-based spin valve and potential applications of organic-based spintronics. \\[4pt] [1] O. Sato, T. Iyoda, A. Fujishima, and K. Hashimoto, Science \textbf{272}, 704 (1996).\\[0pt] [2] D. A. Pejakovic', C. Kitamura, J. S. Miller, and A. J. Epstein, Phys. Rev. Lett. \textbf{88}, 057202 (2002).\\[0pt] [3] J. W. Yoo et al., Phys. Rev. Lett. \textbf{97}, 247205 (2006); \textbf{99}, 157205 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T20.00003: Magnetic properties of TCNQF4 reacted with Ni(cod)$_{2}$ Ian Terry, Kimberly Quinn, Marek Szablewski Recently it was reported that a room temperature ferromagnetic material (Tc$>$400K), Ni$_{2}$TCNQ, was synthesized by reacting the organic acceptor tetracyanoquinodimethane (TCNQ) with bis(1,5 cyclooctadiene) nickel (Ni(cod)$_{2})$[1]. In the present work we report the magnetic properties of a material which was synthesized following the same chemical route as that of Ni$_{2}$TCNQ, except tetrafluoro-tetracyanoquinodimethane (TCNQF4) was used instead of TCNQ. The new metal-organic compound shows qualitatively similar magnetic properties to Ni$_{2}$TCNQ, with ferromagnetic behavior being observed at room temperature. The specific magnetic properties can be described by assuming that there is both a paramagnetic and ferromagnetic phase in the material, with the ferromagnetic phase having a measured Curie temperature of about 620K, close to that of nickel. TEM and XRD data provide evidence for the existence of nickel nanoparticles within the material. We conclude that nickel nanoparticles are produced during the synthesis and are probably responsible for ferromagnetic properties observed at room temperature. 1.R. Jain \textit{et al}, Nature \textbf{445}, 291, (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T20.00004: Magnetic and surface studies of transition metal complexes for molecular spintronics Patrick Truitt, Raman Talwar, Ezekiel Johnston-Halperin, Norbani Abdullah, Carly Reed, Namrata Singh, Chandrani Chatterjee, Malcolm Chisholm We have synthesized organometallic complexes consisting of a transition metal ion chelated by amphiphilic ligands. This talk will focus on efforts to assess the suitability of these molecules for the creation of magnetically active monolayers via the Langmuir-Blodgett technique. The paramagnetic nature of the molecules is probed by SQUID magnetometry and EPR spectroscopy, demonstrating that the spin magnitude can be varied by chemical substitution of the transition metal ion. To study monolayer formation ability, the molecules are spread on a Langmuir trough and pressure-area isotherms are recorded under compression. Attempts to deposit monolayers onto substrates and to make electrical contact for transport measurements will also be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T20.00005: Magneto-Transport in Polyaniline Nanofiber Network K. Deniz Duman, N.-R Chiou, V.N. Prigodin, A.J. Epstein We report large magnetoresistance (up to 12{\%} at 8 T and 3 K) for polyaniline nanofiber network composed of nanofibers with an average diameter of about 80 nm. The polyaniline nanofiber networks were synthesized via chemical oxidative polymerization [1] and were studied at low and high electric and magnetic fields for temperatures 2 K- 250 K for their magneto-transport behavior. A transition from positive MR (temperatures 75 K and below) to negative MR (temperatures 100K and above) is observed. The MR may be explained by possible competing mechanisms; shrinkage of the hopping wavefunction and quantum interference effect in the applied magnetic field. It is also noted that applied electric field affects MR. In the positive MR regime an increase in MR is observed as the applied electric field decreases. Detailed results of various polyaniline nanofiber samples and possible mechanisms responsible for the magneto-transport behavior will be discussed.\\[0pt] [1] N.-R Chiou, A. J. Epstein, Adv. Mater. \textbf{17}, 1679 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:54PM |
T20.00006: Tuning the ionization energy of organic semiconductor films: The role of intramolecular polar bonds Invited Speaker: While an isolated individual molecule has only one ionization energy (IE), multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical conjugated organic compounds combined with first-principles calculations and electrostatic modeling reveal the existence of a surface dipole built into molecular layers. Its origin lies in intramolecular polar bonds (IPBs) of the individual molecules, and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the IE of one molecular species over up to 1 eV via control over layer morphology. Furthermore, mixing of differently terminated molecules (different IPBs) on a molecular length scale allows continuously tuning the IE of thin organic films between the limiting values of the two pure materials. Surface engineering of organic semiconductors via adjusting the polarity of intra-molecular bonds represents thus a viable alternative for controlling the energetics at organic/(in)organic interfaces. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T20.00007: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T20.00008: Perpendicular interaction between donor and accepter molecules on Au (111) U.G.E. Perera, R. Mishima, S-Wai Hla The capability to modify the electronic properties of materials by the interaction between donor and acceptor molecules plays a significant role in molecular electronics. Formation of molecular charge transfer complexes have been observed for different donor acceptor system in a lateral configuration. Here, we present the structural and electronic properties of decamethylmanganocene (Mn(C5Me5)2) and 7,7,8,8- tetracyanoquinodimethane (TCNQ ) molecules on a Au(111) surface at 4.6K using low temperature scanning tunneling microscopy (STM) to investigate the perpendicular interaction between the molecules. The molecular complexes were formed by depositing Mn(C5Me5)2 onto predeposited TCNQ on Au(111). The TCNQ formed a well ordered self-assembled clusters on Au(111) and Mn(C5Me5)2 adsorbed either on TCNQ layer or on bare Au(111) surface. Perpendicular interaction between the Mn(C5Me5)2 and TCNQ were determined by means of conductance tunneling spectroscopy. This work provides an important step for manipulating and tuning charge state of molecules using donor-acceptor molecular systems. The research is supported by United States Department of Energy BES grant number DE-FG02-02ER46012. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T20.00009: STM study of energy-storing photoactive organometallic molecules Jongweon Cho, Luis Berbil-Bautista, Niv Levy, Steve Meier, K. Peter C. Vollhardt, Michael F. Crommie (Fulvalene)tetracarbonyldiruthenium (FvRu$_{2}$(CO)$_{2})$ molecules store light energy through photoisomerization. UV illumination of molecules in solution or in the solid state results in a conformational change to a high-energy photoisomer. Upon mild heating the molecule reverses to its original structure, liberating $\sim $1.3 eV. Many potential future applications of this molecule involving light energy storage requires understanding its switching behavior in a device geometry, i.e., at a surface. We have investigated self-assembly and switching behavior of FvRu$_{2}$(CO)$_{2}$ molecules on Au(111) using scanning tunneling microscopy at cryogenic temperatures and we will report on these studies. [Preview Abstract] |
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