Silver and indium were deposited onto molecular layers of two perylene derivatives, viz. 3,4,9,10-perylene-tetracarboxylic dianhydride ͑PTCDA͒ and N,NЈ-dimethylperylene 3,4,9,10-dicarboximide ͑DiMe-PTCDI͒. The interaction between the metals and the organic semiconductors was probed in situ by Raman spectroscopy. The molecular structure is found to be preserved when Ag or In are deposited onto PTCDA and DiMe-PTCDI layers. For In, this is in contrast to previous suggestions of strong reactivity with PTCDA and covalent bond formation between In and O atoms in PTCDA. However, the molecules having direct contact with the metal are involved in a ground state dynamical charge transfer with different strength for Ag and In, resulting in a breakdown of vibrational selection rules. A significant enhancement of Raman internal vibrational modes is observed both for Ag and In deposition as a result of metal-induced surface enhanced Raman scattering ͑SERS͒. The enhancement factors observed for the internal modes reflect a rough morphology of the metal films which is influenced by the morphology of the underlying organic film. Moreover, conclusions regarding the indiffusion of the two metals can be drawn from the comparison of the breakdown of the selection rules for the internal modes and the spectral evolution of the external molecular modes.

PACS 68.35.Ja, 78.30.Jw Metal/organic interfaces play an important role in the performance of organic based devices. In this work the interface formation between Mg and N,N'-DiMethyl-3,4,9,10-Perylene Tetra Carboxylic DiImide (DiMePTCDI) films grown on sulphur-passivated GaAs (100) substrates is investigated in situ by Raman spectroscopy. The Raman spectra are taken in a backscattering geometry at room temperature under resonance conditions with the 488 nm (2.54 eV) Ar + laser line. They reveal that when Mg is deposited onto a 15 nm DiMePTCDI layer the external phonon modes are preserved up to large metal coverages. Since these modes are a fingerprint of the molecular crystal, their preservation indicates a low diffusion of Mg into the DiMePTCDI layer. Concerning the internal molecular modes, the Mg deposition induces a breakdown of selection rules which is proposed to originate from a dynamical charge transfer between the DiMePTCDI molecules and the metal. Above 0.3 nm nominal thickness of Mg the line-shape of the molecular breathing mode at 221 cm -1 develops an asymmetric tail towards higher frequencies. A similar effect is observed for the bands that occur at 1291 cm -1 and 1606 cm -1 but the asymmetry appears at the low frequency side. This line-shape asymmetry is likely to be related to a Fano resonant coupling between the molecular vibrational modes and the electronic continuum of states of metallic clusters formed above 0.3 nm Mg coverage.

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