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Here we a present a density functional theory study of pentacene ultra-thinfilms on flat [(111)] and vicinal [(455)] Au surfaces. We have performedcrystal and electronic structure calculations by using PBE and optB86b-vdWfunctionals and investigated the effects of long range Van der Waalsinteractions for different coverages starting from a single isolated moleculeup to 4 monolayers of coverage. For an isolated molecule both functionals yieldthe hollow site as the most stable one with bridge-60 site being very close inenergy in case of optB86b-vdW. Binding strength of an isolated pentacene on thestep edge was found to be much larger than that on the terrace sites. Differentexperimentally reported monolayer structures were compared and the (6 x 3) unitcell was found to be energetically more stable than the (2 x 2 $\sqrt{7}$) and(2 x $\sqrt{31}$) ones. For one monolayer films while dispersion correctedcalculations favored flat pentacene molecules on terraces, standard (PBE)calculations either found tilted and flat configurations to be energeticallysimilar (on (111) surface) or favored the tilted configuration (on (455)surface). PDOS calculations performed with optB86b-vdW functional showed largerdispersion of molecular orbitals over the Au states for the (455) surface whencompared with the (111) surface, indicating an enhanced charge carriertransport at the pentacene-gold interface in favor of the vicinal surface.Starting with the second monolayer, both functionals favored tiltedconfigurations for both surfaces. Our results underline the importance of thedispersion corrections for the loosely bound systems like pentacene on gold andthe role played by step edges in determining the multilayer film structure andcharge transfer at the organic molecule-metal interface.

Dispersion Corrected DFT Study of Pentacene Thin Films on Flat and Vicinal Au Surfaces