(121c) Computationally Derived Rules for the Persistence of C60 Nanowires On Recumbent Pentacene Bilayers

Research in organic electronic devices has seen an increasing focus on designing molecularly scaled materials. This has been driven by the exploitation of quantum size confinement effects of nanoparticles, nanotubes and nanowires that lead to different electronic, optical and other properties. In this study, we have considered the structural properties of all-organic heterojunctions, in particular, the C₆₀ /pentacene interface, as a prototypical planar p-n junction. Achieving one-dimensional C₆₀ structures on recumbent pentacene layers is the goal of this study, and we have discovered ways to significantly improve the likelihood of C₆₀ nanowire persistence using fully atomistic Molecular Dynamics (MD) for explicit all-atom models of the two components. We report three main effects that affect C₆₀ nanowire persistence sensitively: the angle that the recumbent pentacene molecules make with the surface normal, the amount of initial tensile strain applied to the nanowire, and the presence of surface step edges. At room temperature, C₆₀ nanowires oriented along the pentacene short axes are stable within the inherent timescale constraints of MD and are more likely to occur if they reside between, or within, pentacene rows for tilt angles, φ≤∼60^◦, with increasing likelihood of persistence the smaller the off-normal tilt angle. Nanowires oriented along the long axes of pentacene molecules (denoted N-S), are unlikely to form. The stability of nanowires to withstand thermal fluctuations was tested by raising the temperature from room temperature to 400 K, just below the temperature at which pentacene desorption occurs. Nanowires located between pentacene rows withstood this thermal inducement to disorder. However, C₆₀ nanowires located initially within pentacene rows are only stable in the range of tilt angles, φ₁=30–50^◦. Increasing temperatures leads to a higher likelihood of losing 1D order, or at least creating indecisive switching of the location of the nanowire from between to within pentacene rows. Flatter pentacene surfaces, characterized by tilt angles above about 60^◦, induce C₆₀ molecules to "burrow" into the pentacene surface creating considerable disorder. This result is also suggested by experimental STM observations by Dougherty et al. [2009]. An initial strain of 5% applied to the C₆₀ nanowires significantly decreases the likelihood of finding nanowires. In contrast, any appreciable surface roughness, even half a monolayer high, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C₆₀ molecules to step edges.