The electronic band alignment on nanoscopically patterned substrates G. Koller * , B. Winter, M. Oehzelt, J. Ivanco, F.P. Netzer, M.G. Ramsey Institut fu ¨ r Physik, Karl-Franzens-Universita ¨ t, Universita ¨ tsplatz 5, A-8010 Graz, Austria Received 9 June 2006; received in revised form 6 November 2006; accepted 7 November 2006 Available online 8 December 2006 Abstract The electronic level alignment of the organic semiconductor para-sexiphenyl on a nanoscopically patterned substrate was investigated with ultraviolet photoemission spectroscopy, work function measurements and scanning tunnelling microscopy. The results show that for increasing coverage on inhomogeneous surfaces, shifts in electronic level alignment occur, which are due to the change from local to average band alignment. The Cu(1 1 0)–(2 · 1)O stripe phase, used as a model substrate consists of alternating stripes of bare and oxygen passivated copper, with stripe widths of 35 A ˚ compa- rable to the sexiphenyl molecular length. In the first molecular layer the electronic bands are clearly aligned to the local surface potential of the specific stripe, resulting in the superposition of two photoemission spectra offset by 1 eV. Beyond two monolayers the valence band spectra clearly indicate a single electronic level alignment, which is determined by the average interface dipole. Ó 2006 Elsevier B.V. All rights reserved. PACS: 73.20.At; 73.63.Rt; 73.22.-f; 68.43.Bc Keywords: Band alignment; Photoemission; Work function; Sexiphenyl; Copper; Nanostructures 1. Introduction The worldwide interest in organic semiconduct- ing materials is driven by the wealth of their possible technological applications. At present light emitting and simple electronic devices are on the threshold of being commercially realised, while new applications, such as solar cells and chemical sensors are being explored. Historically, the field of active molecular systems has been evolving from ‘‘plastic electronics’’ through ‘‘organic electronics’’ to ‘‘molecular elec- tronics’’; that is polymers, better defined as oligo- meric films, to the holy grail of single molecule devices. Clearly, an understanding of the electronic level alignment on the molecular scale is vital for molecular electronics, but it can also be important in present day devices as their contact interfaces are unlikely to be uniform on this scale, an intricacy, which may lead to charge trapping at the interface and reduced device performance. The area of such defects can also be on the nanoscale and thus in the order of the here investigated substrate nanostructures. 1566-1199/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2006.11.001 * Corresponding author. Fax: +43 316 380 9816. E-mail address: georg.koller@uni-graz.at (G. Koller). Organic Electronics 8 (2007) 63–68 www.elsevier.com/locate/orgel