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Surface Diffusion of C60 on Crystalline Pentacene Using Molecular Dynamics

BackgroundMotivationGoalsMethodTINKER softwareSimulation systemsResultsC60-pentacene (3x3x2)Pentacene-pentacene (3x3x2)C60-pentacene (4x4x2)Conclusion. Outline. PentaceneHigh electron mobilityHigh degree of crystallinityUnit cell: 2 pentacene moleculesElectron donorBuckminsterfulle

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Surface Diffusion of C60 on Crystalline Pentacene Using Molecular Dynamics

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    1. Surface Diffusion of C60 on Crystalline Pentacene Using Molecular Dynamics Rebecca Cantrell MAE 715 - Professor Zabaras Atomistic Modeling of Materials Final Project Presentation May 7, 2007

    2. Background Motivation Goals Method TINKER software Simulation systems Results C60-pentacene (3x3x2) Pentacene-pentacene (3x3x2) C60-pentacene (4x4x2) Conclusion Outline

    3. Pentacene High electron mobility High degree of crystallinity Unit cell: 2 pentacene molecules Electron donor Buckminsterfullerene (“buckyballs”) High electron mobility Electron acceptor Background Pentacene better than other organic molecules because of its crystalline structurePentacene better than other organic molecules because of its crystalline structure

    4. C60-pentacene organic films have recently studied as flexible organic solar cells C60-pentacene film in between indium tin oxide coated with a conducting polymer as the anode and CsF/Al as the cathode Solar power conversion efficiency increases after annealing a C60 organic layer on top of pentacene layers Molecular ordering increases conversion efficiency! Motivation

    5. Study surface diffusion of C60 on pentacene Optimum temperature for surface diffusion? Is there a site hopping energy barrier? Molecular dynamics simulation C60-pentacene (3x3x2) system Compare to pentacene-pentacene (3x3x2) system Compare to C60-pentacene (4x4x2) system to determine effects of periodic boundary size Goals

    6. Molecular dynamics and molecular mechanics software used mainly for organic molecules Files used to run TINKER: .xyz, .key, .nbs Newton’s equations of motion velocity Verlet integration method Constant temperature Nosé-Hoover algorithm Thermalization (canonical ensemble, constant NVT) ? Full simulation (micro-canonical ensemble, constant NVE) TINKER Software

    7. Must specify an interaction potential to solve the equations of motion Extension of mm2 potential; mm3 better for multi-ringed structures Incorporates the stretching, bending, and tortional energies as well as the van der Waal interaction energies based on empirical parameters mm3 Potential

    8. Three systems considered C60-pentacene (3x3x2) Pentacene-pentacene (3x3x2) C60-pentacene (4x4x2) Fixed bottom layer, second layer allowed to vibrate Periodic boundary conditions Pressure: 1 atm; Temperature: 225 K – 400 K Simulation Systems

    9. xyz coordinates of the center of mass of the C60 molecule moving on the 3x3 layer of pentacene, collapsed onto one unit cell C60-Pentacene (3x3x2): Collapsed Unit Cell

    10. C60-Pentacene (3x3x2): Determining Diffusion Coefficients imply an increase in diffusion coefficient with increasing temperature, which is expected. However, there seems to be a local minimum for the diffusion coefficient at around 275 K. imply an increase in diffusion coefficient with increasing temperature, which is expected. However, there seems to be a local minimum for the diffusion coefficient at around 275 K.

    11. According to the Arrhenius equation, the diffusion versus temperature graph should follow an exponential curve The prefactor D0 contains the transition state information according to the following relationship based on transition state theory Plotting ln(D) vs. 1/T gives: slope = -Ea/kB ? Ea = 0.076 eV y-int = ln(D0) ? D0 = 2.27 Ų/ps C60-Pentacene (3x3x2): Site Hopping Activation Energy

    12. xyz coordinates of the center of mass of the C60 molecule moving on the 3x3 layer of pentacene, collapsed onto one unit cell Pentacene-Pentacene (3x3x2): Collapsed Unit Cell

    13. Pentacene-Pentacene (3x3x2): Determining Diffusion Coefficients

    14. Again, according to the Arrhenius equation, the diffusion versus temperature graph should follow an exponential curve The prefactor D0 contains the transition state information according to the following relationship based on transition state theory Plotting ln(D) vs. 1/T gives: slope = -Ea/kB ? Ea = 0.039 eV y-int = ln(D0) ? D0 = 5.81 Ų/ps Pentacene-Pentacene (3x3x2): Site Hopping Activation Energy

    15. 4x4x2 simulation cells significantly increased computation cost Different results than 3x3x2 simulation cell Strange oscillation occurring No clear trend for D vs. T Possible reasons: simulation time too short or cell size still not big enough to determine accuracy of results C60-Pentacene (4x4x2): Determining Diffusion Coefficients

    16. Useful information about the previously unknown surface diffusion of a C60 molecule on crystalline pentacene. D vs. T trends not as smooth as hoped, but comparing data to pentacene-pentacene data still insightful D for the C60-pentacene (3x3x2) system was overall lower than for pentacene-pentacene (3x3x2) system Ea of site hopping was higher for the C60-pentacene (3x3x2) system (~0.076 eV) than for the pentacene-pentacene (3x3x2) system (~0.039 eV) Periodic boundary size analasis revealed unexpected deviatations; further investigation necessary Future work would include also investigating the diffusion properties of multiple C60 molecules on the surface of crystalline pentacene to determine whether they tend to attract or repel Conclusion

    17. Questions

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