Computationally Derived Rules for Persistence of C60 Nanowires on Recumbent Pentacene Bilayers

The tendency for C-60 nanowires to persist on two monolayers of recumbent pentacene is studied using molecular dynamics (MD) simulations. A review of existing experimental literature for the tilt angle adopted by pentacene on noble metal surfaces shows that studies cover a limited range from 550 to 90, motivating simulation studies of essentially the entire range of tilt angles (10 degrees-90 degrees) to predict the optimum surface tilt angle for C-60 nanowire formation. The persistence of a ID nanowire depends sensitively on this tilt angle, the amount of initial tensile strain, and the presence of surface step edges. At room temperature, C-60 nanowires oriented along the pentacene short axes persist for several nanoseconds and are more likely to occur if they reside between, or within, pentacene rows for phi <= similar to 60 degrees. The likelihood of this persistence increases the smaller the tilt angle. Nanowires oriented along the long axes of pentacene molecules are unlikely to form. The limit of stability of nanowires was tested by raising the temperature to 400 K. Nanowires located between pentacene rows survived this temperature rise, but those located initially within pentacene rows are only stable in the range phi(1) = 30 degrees-50 degrees. Flatter pentacene surfaces, that is, tilt angles above about 60 degrees, are subject to disorder caused by C-60 molecules "burrowing" into the pentacene surface. An initial strain of 5% applied to the C-60 nanowires significantly decreases the likelihood of nanowire persistence. In contrast, any appreciable surface roughness, even by half a monolayer in height of a third pentacene monolayer, strongly enhances the likelihood of nanowire formation due to the strong binding energy of C-60 molecules to step edges.