### Session Y36: Organic and Hybrid Surfaces and Interfaces

 Friday, March 20, 2009 8:00AM - 8:12AM Y36.00001: Diffusivity control in molecule-on-metal systems using electric fields Y.Y. Zhang , N. Jiang , S.X. Du , H.-J. Gao , Matthew J. Beck , Sokrates T. Pantelides Electronic devices constructed from molecule-on-metal systems are actively being explored for applications in logic and memory devices, sensors, fuel cells, and solar cells. The implementation of practical molecular electronic devices requires molecule-on-metal systems in which the arrangement of active molecules is fixed or can be controlled and the contact-molecule-contact system exhibits desirable electronic properties. Fe (II) Phthalocyanine (FePc) on Au (111) exhibits a number of promising electronic properties, but diffuses rapidly at room temperature. Using scanning tunneling microscopy and density functional theory calculations we show that applied electric fields can be employed to enhance or retard the diffusivity of FePc molecules on Au (111) independent of temperature. These results demonstrate the possibility of dynamic field-guided patterning of molecule-on-metal systems. Friday, March 20, 2009 8:12AM - 8:24AM Y36.00002: Oxygen-related traps in pentacene thin films: Energetic position and implications for transistor performance Wolfgang Kalb , Kurt Mattenberger , Bertram Batlogg We studied the influence of oxygen on the electronic trap states in a pentacene thin film kept under highly controlled conditions. This was done by temperature-dependent gated four-terminal measurements on pentacene thin-film transistors prior to and after controlled oxygen exposure. We developed and used a scheme that allows for the calculation of the essential transport parameters, such as the trap DOS, in an unambiguous way. The results are free from parasitic contact artifacts. Oxidation of pentacene in light leads to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of the order of 10$^{18}$ cm$^{-3}$. The measurements reveal how these traps affect the key device parameters, i.e. subthreshold performance and field-effect mobility. The study supports the assumption of a mobility edge for charge transport, and contributes to a detailed understanding of an important degradation mechanism of organic field-effect transistors. Friday, March 20, 2009 8:24AM - 8:36AM Y36.00003: Grazing-incidence X-ray Diffraction of Tetracene Thin Films on Hydrogenated Si(001) Substrate De-Tong Jiang , Andrew Tersigni , Chang-Yong Kim , Jun Shi , Robert Gordon , Ning Chen , Xiaorong Qin Ex situ grazing-incidence X-ray diffraction (GIXD) and wide angle Bragg diffraction have been performed on UHV epitaxially grown tetracene thin films on H/Si(001)-2x1 substrates. The in-plane lattices of the crystalline films were characterized by 2D reciprocal space imaging of the in-plane (11), (12) and (20) GIXD diffraction spots and the out-place lattices were characterized by the wide angle Bragg diffraction. The thickness of the tetracene films ranged from 1.2 monolayer (ML) to 15 ML. H/Si(001)-2x1 substrates with different surface roughness were used. The results indicate that the film structure characteristics are strongly influenced by the substrate conditions and under favorable conditions the homogeneous thin-film phase could dominate the growth up to about 8 ML. The implications of the results to the growth mechanisms and to thin film electronics applications will be discussed. Friday, March 20, 2009 8:36AM - 8:48AM Y36.00004: Nanoscale conductivity measurements on organic thin films and interfaces Justin Wells , Fei Song , Philip Hofmann Despite the importance of conductance measurements to bulk solid state physics, there is poor understanding of surface and nano-scale conductance - despite the relevance to smaller devices and the development of novel concepts for electronics. Of particular interest in this respect are self-organized organic nano-structures, which offer a virtually unlimited design freedom. Electron delocalization and transport in such systems is of great current interest. In this work, we utilise a recently developed nanoscale multi-contact mono-cantilever probe with a minimum spacing of 250 nm. We measure the conductivity of the Si(111)($\sqrt{3}\times\sqrt{3}$)Ag surface, and the corresponding changes in the conductivity which occur when organic absorbates are present. By increasing the coverage from sub-monolayer to multilayer, it is possible to see doping of the underlying Ag layer, as well as conduction through the organic film. These measurements are supported by PES and NEXAFS studies, and thus can be interpreted in terms of charge transfer and geometric structure. Friday, March 20, 2009 8:48AM - 9:00AM Y36.00005: Density Functional Theory of Transition Metal Phthalocyanines Noa Marom , Leeor Kronik Metal phthalocyanines (MPc's) are a family of highly stable molecules that, as a molecular solid, form organic semiconductors. They have been used in a broad range of applications, e.g., light emitting diodes, solar cells, gas sensors, thin film transistors, and even as single molecule devices. Here, we present a systematic density functional theory (DFT) study of the electronic structure of selected transition metal Pc's: CuPc, NiPc, CoPc, MnPc, and FePc. We critically assess the performance of several semi-local and hybrid exchange-correlation functionals for these systems, and compare the results to experimental photoemission data. For the low-spin systems CuPc, NiPc, and CoPc, we show that semi-local functionals fail qualitatively, primarily because of under- binding of localized orbitals due to self-interaction errors. For the intermediate-spin systems, MnPc and FePc, we show that DFT calculations are extremely sensitive to the choice of functional and basis set with respect to the obtained electronic configuration and to symmetry breaking. However, interestingly, all simulated spectra are in good agreement with experiment despite the differences in the underlying electronic configurations. Friday, March 20, 2009 9:00AM - 9:12AM Y36.00006: Cascade and Accumulation of Spin at CuPc/GaAs (100) interface Huanjun Ding , Irfan Irfan , Yongli Gao , Mirko Cinchetti , Marina Sanchez-Albaneda , Jan-Peter Wustenberg , Oleksiy Andreyev , Michael Bauer , Martin Aeschlimann We have investigated the spin dynamics in organic semiconductor, copper phthalocyanine (CuPc), with spin and time resolved two photon photoemission spectroscopy (STR-2PPE). Spin polarized electrons are generated optically from GaAs substrate, and injected into the unoccupied states of CuPc film. The apparent spin relaxation time is observed to have strong energy dependence. The spin polarization at high energy levels decreases much faster than that of the low energy levels. The experimental results are then explained by a cascade model. The calculation suggests that the spin information of the hot electrons can be well preserved during the energy relaxation process. Friday, March 20, 2009 9:12AM - 9:24AM Y36.00007: Electronic structure of MoO3 insertion layer at the interface between organic semiconductor and indium tin oxide (ITO). Huanjun Ding , Irfan Irfan , Yongli Gao , Frank So We have investigated the electronic structure of the interfaces formed by inserting thin layer of MoO$_{3}$ in between indium tin oxide (ITO) and different organic semiconductors, such as aluminium phthalocyanine chloride (AlPcCl) and copper phthalocyanine (CuPc), with photoemission and inverse photoemission spectroscopy (PES and IPES). The presents of MoO$_{3}$ layer at the interface increases the workfunction dramatically. As a result, the organic HOMO is almost aligned with the Fermi level (E$_{f})$ at the AlPc-Cl/MoO$_{3}$ interface. For thicker AlPc-Cl layers, gradual band bending is observed. However, the recovery of the HOMO is incomplete for AlPc-Cl thickness of 200 {\AA}, leading to a great reduction of the hole injection barrier compare to the case without MoO$_{3}$. Similar situation is found in case of CuPc/MoO$_{3}$, although the energy levels are almost fully recovered for CuPc film thicker than 200 {\AA}. The energy level alignment of these interfaces will be discussed to explain the improvement induced by MoO$_{3}$ layer in device performance. Friday, March 20, 2009 9:24AM - 9:36AM Y36.00008: HPLC and Semi-Prep Scale Fractionations of Poly(3-alkyl thiophenes) Scott LeFevre , Heungyeol Choi , Taihyun Chang , Chang Ryu Molecular weight and polydispersity play a crucial role in crystal formation for thin-films of conducting polymers and subsequently their charge mobility. In order to obtain well defined conducting polymer samples, high performance liquid chromatography (HPLC) analysis and separation of poly(3-alkyl thiophene) (P3AT) systems has been explored. In particular a precipitation-redissolution technique has been employed for the fractionation of P3ATs. Both solvent composition and temperature have been manipulated to tune the solvent quality on both hydrophobic and hydrophilic type stationary phases in order to obtain more well-defined samples of these electrically conducting polymers. In addition to HPLC separations on an analytical scale, semi-prep scale LC separations in the regime of 10mg to 100mg has also been carried out as proof of concept. Finally the interplay between solvent quality and the surface energy has been investigated using HPLC stationary phases consisting of either bare silica or C18 bonded silica. Friday, March 20, 2009 9:36AM - 9:48AM Y36.00009: Tailored Assembly of Organic Molecular Nanofibers into Advanced Donor-Acceptor Architectures Volodimyr Duzhko , Michael J. Kelley , Kenneth D. Singer Non-covalent self-assembly of organic molecules in organic solvents provides a multi-functional approach toward producing organic semiconducting nanostructures having versatile, well-ordered, architectures\footnote{ V. Duzhko and K.D. Singer, J. Phys. Chem. C 2007, 111, 27.} that are potentially integrable into useful electronic, optoelectronic and photonic device architectures. Aiming at molecular-scale tailoring of electron donor-acceptor blend architectures and rational engineering of their functionality for photovoltaic applications, we discuss our approach of solvent-based, electric-field-assisted\footnote{ V. Duzhko, J. Du, C.A. Zorman, and K.D. Singer, J. Phys. Chem. C 2008, 112, 12081.} integration of self-assembled donor (phthalocyanine) nanofibers into an acceptor (perylene diimide) matrix (or vice versa). We present results of our systematic spectroscopic, X-ray diffraction and scanning electron microscopy studies revealing the structure and morphology of neat fibers and fiber blends in various phases. Friday, March 20, 2009 9:48AM - 10:00AM Y36.00010: The substitution effect on the reorganization energy of metal free phthalocyanine Choongkeun Lee , Karl Sohlberg Many discotic (disk like) materials such as phthalocyanine are of interest for use in organic electronic devices because of their high charge mobility. The mobility of various discotic materials has been studied using the Marcus formalism. In the Marcus formalism, charge mobility is depends on two parameters, reorganization energy and coupling matrix constant. Of these two parameters the reorganization energy has more influence on the charge hopping rate. A small change in reorganization energy leads to a large change of charge mobility. We have employed electronic structure methods to describe substitution effects on the reorganization energy of phthalocyanine. The substitutions on the external phenyl rings have almost no influence on reorganization energy, but the substitutions on the internal nitrogen in phthalocyanine have strong influence on reorganization energy. The detailed relation between reorganization energy and substitution will be presented. Friday, March 20, 2009 10:00AM - 10:12AM Y36.00011: Rational design of the shape and periphery of discotics: a synthetic way towards high charge carrier mobilities Denis Andrienko , Xinliang Feng , Valentina Marcon , Wojciech Pisula , James Kirkpatrick , Ferdinand Grozema , Kurt Kremer , Klaus M{\"u}llen Conjugated materials offer the revolutionary prospect of producing semiconductor devices at low cost. The best to date discotics are built around the coronene unit and possess six fold symmetry. In the discotic phase six fold symmetric molecules stack with an average azimuthal twist of 30 deg, whereas the angle which would lead to the greatest electronic coupling and hence highest charge mobility is 60 deg. Here, a molecule with three fold symmetry and alternating hydrophilic/hydrophobic side chains is synthesized and X-ray scattering is used to prove the formation of the desired helical microstructure. Pulse radiolysis time resolved microwave conductivity measurements show that the material has indeed a very high mobility in the plastic crystalline phase, in the range of $0.1-0.2\, \rm cm^2 / Vs$. The physical structure of the assemblies of molecules are simulated using molecular dynamics. This, together with quantum chemical techniques, allows the computation of charge mobilities without fitting parameters. The calculations prove that mobility is still limited by structural defects and that a defect free assembly would lead to mobilities in excess of $10\, \rm cm^2 / Vs$. Friday, March 20, 2009 10:12AM - 10:24AM Y36.00012: Anisotropic phases in ferromagnetic ultrathin films from multipolar interactions Daniel Barci , Daniel Stariolo We present a model to describe complex phases observed at mesoscopic scales in ultrathin magnetic films with perpendicular anisotropy. The model is based on the interaction between magnetic dipolar as well as quadrupolar moments. This model has a very rich phase diagram. In the special case of films with strong perpendicular anisotropy, a nematic phase, characterized by orientational (stripe-like) but not translational order, is predicted. The isotropic-nematic transition belongs to the Kosterliz-Thouless type in the thermodynamic limit. However, we find that in actual experimental scales the fluctuations of the nematic order parameter are regularized by the sample size, and real orientational order, as predicted by mean field, should be observable. The transition may be characterized experimentally from measurements of the magnetic structure factor, from which the nematic order parameter is derived. Friday, March 20, 2009 10:24AM - 10:36AM Y36.00013: ABSTRACT WITHDRAWN