Optical absorption spectra of ultrathin PTCDA films on gold single crystals: Charge transfer beyond the first monolayer Roman Forker, Christian Golnik 1 , Giovanni Pizzi 2 , Thomas Dienel 3 , Torsten Fritz * Institut für Angewandte Photophysik, Technische Universität Dresden, D-01062 Dresden, Germany article info Article history: Received 9 June 2009 Received in revised form 6 August 2009 Accepted 7 August 2009 Available online 13 August 2009 PACS: 78.20.Ci 78.40.Me 78.66.Qn Keywords: PTCDA Au(1 1 1) Au(1 0 0) Differential reflectance spectroscopy Complex dielectric function Cations abstract We report on the optical absorbance behavior of ultrathin 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) films epitaxially grown on Au(1 1 1) and Au(1 0 0). Although PTCDA on Au(1 1 1) has been classified before as weakly interacting without noticeable metal–organic charge transfer (CT), we present clear optical evidence for ionized PTCDA at the interface. Unexpectedly, the charging of PTCDA is not restricted to the layer of contact, but extends into the second monolayer at least. A new feature in the optical spectra is identified as PTCDA cation, and we reason that the second monolayer is (partly) positively charged. We propose the CT to be related to the interface dipole formation between the first and sec- ond PTCDA monolayers on Au. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction The electronic (and thus optical) properties of organic dyes are markedly affected by the proximity of metal sur- faces, depending on the strength of the metal–organic interaction [1–4]. In particular, the electronic spectra of highly ordered 3,4,9,10-perylenetetracarboxylic dianhy- dride (PTCDA, see Fig. 1) thin films differ noticeably for various metal substrates ranging from weakly to highly interacting [5,6]. Furthermore, they are a function of film thickness, especially when passing from a monolayer (ML) to films comprising aggregates a few nanometers thick. The system PTCDA/Au(1 1 1) has been categorized before as weakly interacting with an induced density of charge transfer (CT) states below detection threshold, if present at all [6]. Here, we report in situ optical differential reflectance spectroscopy (DRS) measurements characteriz- ing the absorbance behavior of PTCDA films ranging from submonolayers to several MLs on Au(1 1 1) and Au(1 0 0) surfaces. We found clear distinctions in the behavior of the PTCDA contact layer with respect to further layers. Most importantly, we observe spectral features originating from ionized PTCDA, suggesting that CT indeed occurs at the interface. To the best of our knowledge, this constitutes the first observation of charged molecular species in the solid state on metal surfaces by means of optical spectros- copy. The charging of PTCDA is surprisingly not constrained 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.08.005 * Corresponding author. Tel.: +49 (0)351 463 34902; fax: +49 (0)351 463 37065. E-mail addresses: forker@iapp.de (R. Forker), fritz@iapp.de (T. Fritz). 1 Present address: Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan. 2 Permanent address: NEST-CNR-INFM and Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy. 3 Present address: Leibniz-Institut für Festkörper- und Werkstofffor- schung, Helmholtzstr. 20, D-01069 Dresden, Germany. Organic Electronics 10 (2009) 1448–1453 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel