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Surface Diffusion of C60 on Crystalline Pentacene Using Molecular Dynamics

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. Outline. Background Motivation Goals Method TINKER software Simulation systems Results

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

  3. Background • Pentacene • High electron mobility • High degree of crystallinity • Unit cell: 2 pentacene molecules • Electron donor • Buckminsterfullerene (“buckyballs”) • High electron mobility • Electron acceptor S. Yoo, B. Domercq, B. Kippelen. Efficient thin-film organic solar cells based on pentacene/C60 heterojunctions. Applied Physics Letters. 2004. Vol 85, p5427. R. C. Haddon, T. Siegrist, R. M. Fleming, P. M. Bridenbaugh and R. A. Laudise. Band structures of organic thin-film transistor materials. Journal of Materials Chemistry. 1995, Vol 5, p 1719.

  4. Motivation • 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! A. C. Mayer, M. T. Lloyd, D. J. Herman, T. G. Kasen, G. G. Malliaras. Postfabrication annealing of pentacene-based photovoltaic cells. Applied Physics Letters. 2004. Vol 85, No. 25.

  5. Goals • 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

  6. TINKER Software • 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)

  7. mm3 Potential • 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 N. L. Allinger, Y. H. Yuh, J. H. Lii. Molecular Mechanics: The MM3 Force Field for Hydrocarbons. Journal of the American Chemical Society. 1989. Vol 111, No 23.

  8. Simulation Systems • 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

  9. T = 250 K T = 325 K T = 225 K T = 275 K T = 300 K T = 375 K T = 400 K T = 350 K C60-Pentacene (3x3x2):Collapsed Unit Cell • xyz coordinates of the center of mass of the C60 molecule moving on the 3x3 layer of pentacene, collapsed onto one unit cell The coordinate units are in Angstroms. Each dot corresponds to a time step of 1 ps. The red corresponds to the position of the center of mass of the C60 molecule; the green corresponds to the positions of the top hydrogen atoms of one pentacene molecule in the unit cell; and the blue corresponds to the positions of the top hydrogen atoms of the other pentacene molecule in the unit cell.

  10. C60-Pentacene (3x3x2):Determining Diffusion Coefficients • MSD vs. time vaguely implies an increase in diffusion coefficient with increasing temperature, which is expected • In two dimensions, D is given by: • Local minimum for the D around 275 K?

  11. C60-Pentacene (3x3x2):Site Hopping Activation Energy • 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 K. D. Dobbs, D. J. Doren. Dynamics of molecular surface diffusion: Origins and consequences of long jumps. J. Chem. Phys. 1992. Vol 97, No 5.

  12. T = 250 K T = 225 K T = 300 K T = 275 K T = 325 K T = 350 K T = 375 K T = 400 K Pentacene-Pentacene (3x3x2):Collapsed Unit Cell • xyz coordinates of the center of mass of the C60 molecule moving on the 3x3 layer of pentacene, collapsed onto one unit cell The coordinate units are in Angstroms. Each dot corresponds to a time step of 1 ps. The red corresponds to the position of the center of mass of the C60 molecule; the green corresponds to the positions of the top hydrogen atoms of one pentacene molecule in the unit cell; and the blue corresponds to the positions of the top hydrogen atoms of the other pentacene molecule in the unit cell.

  13. Pentacene-Pentacene (3x3x2):Determining Diffusion Coefficients • MSD vs. time clearly implies an increase in diffusion coefficient with increasing temperature, which is expected • Again, D is given by: • Overall greater diffusion coefficients than C60-pentacene (3x3x2) due to less sharing of electrons with the surface

  14. Pentacene-Pentacene (3x3x2):Site Hopping Activation Energy • 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

  15. C60-Pentacene (4x4x2):Determining Diffusion Coefficients • 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

  16. Conclusion • 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

  17. Questions

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