The interplay of topography and energy dissipation in pentacene thin films

Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(001) and (root 3 x root 3)R30 degrees-Ag/Si(111). After an optical excitation of electrons by a lambda = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(001), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (root 3 x root 3)R30 degrees-Ag/Si(111), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay. (C) 2012 Elsevier B.V. All rights reserved.