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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 Å²/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 Å²/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