Interaction between metals and organic semiconductors studied by Raman spectroscopy

Abstract

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.

Is It Possible to Observe Surface-Enhanced Raman Scattering from Buried Molecules?

“…We have corroborated the results of Tsang et al, 28 of the Zahn group, 18,19 and of reference 22. However, we could observe this only for a limited number of compounds (4,4′-bipyridine and 4-aminothiophenol).…”

Surface enhanced Raman scattering in organic thin films covered with silver, indium and magnesium

“…In Ref. [8] the modes activated by the SERS effect were identified as normal modes of the PTCDA molecule that are usually infrared active. The modes enhanced are labelled according to their symmetry in Fig.…”

“…The only reports on the external mode behavior in the vicinity of a metal surface are related to PTCDA and DiMe-PTCDI [8,9] where no enhancement of the external modes was detected. The lack of enhancement of the external modes can be partly due to the different resonant conditions compared to the internal modes and partly because of structural distortions caused by the metal atom diffusion into the organic crystalline grains.…”

“…The deposition of some metals like Ag and In onto organic layers of PTCDA gives rise to an enhancement of the intensity of the normally Raman-active internal vibrational modes, accompanied by the activation of normally infrared-active modes [8,9,12]. This effect is known as surface enhanced Raman scattering (SERS).…”

“…A few recent studies were dedicated to the investigation by Raman spectroscopy of the perylene derivative 3,4,9,10-perylene tetra-carboxylic dianhydride (PTCDA) forming interfaces with different metals [8,9]. The vibrational spectrum is a material fingerprint.…”