Paper SS2+EM+TF-FrM5
Mechanistic Aspects of Organic Thin Film Growth from Energetic Sources: Insights from Experiment and Molecular Dynamics Simulations

Friday, October 19, 2007, 9:20 am, Room 611

We have used a combination of experiments and molecular dynamics simulations to investigate the fundamental molecular mechanisms contributing to the evolution and final morphology of thin films of pentacene deposited at hyperthermal incident kinetic energies (E_i = 1-10 eV). Experimentally, using supersonic molecular beam scattering techniques and atomic force microscopy we have characterized the adsorption probability as a function of both E_i and the angle of incidence (θ_i). Interestingly, we observe differences in the dynamics of adsorption for pentacene interacting with a clean SiO₂ substrate (submonolayer growth) vs. a SiO₂ substrate covered by a pentacene thin film (multilayer growth). Specifically, for E_i greater than ~ 1-2 eV, contribution of a new mechanism for pentacene adsorbing on pentacene is implicated. To determine the nature of this mechanism, we have used the non-reactive empirical MM3 potential to model the collision of pentacene molecules with a pentacene thin film using molecular dynamics. Our simulation cell consists of ca. 100 molecules, and includes an upper terrace of 4 x n unit cells, bounded by (010) step edges. Accounting for impacts both near the middle of the terrace and near the step edge, our results from simulation for the probability of adsorption compare very well with those measured experimentally. In particular, adsorption is found to decrease with increasing E_i, and, in general, with increasing θ_i. More importantly, the simulations give us insight into nature of the events that occur at high incident kinetic energies. For thermal incident kinetic energies we observe mostly simple trapping (molecular adsorption), with near unit probability. At higher E_i of 1-5 eV, a significant fraction of molecules (~ 30%) are found to directly insert into the upper terrace, whereas a higher fraction (~ 90%) of molecules impacting near the step edge in this same energy regime also end up incorporating into the upper terrace. Indeed, direct molecular insertion into the pentacene crystal structure is the dominant interlayer process when both the molecule’s orientation and incident angle are aligned normal to the surface, which leads to the formation of interstitials for the time scale of these simulations.