Journal of Electron Spectroscopy and Related Phenomena 174 (2009) 22–27 Contents lists available at ScienceDirect Journal of Electron Spectroscopy and Related Phenomena journal homepage: www.elsevier.com/locate/elspec The electronic structure of pentacene revisited S. Berkebile a , G. Koller a , A.J. Fleming a , P. Puschnig b , C. Ambrosch-Draxl b , K. Emtsev c , T. Seyller c , J. Riley d , M.G. Ramsey a,∗ a Institute of Physics, Karl-Franzens University Graz, Universitätsplatz 5, 8010 Graz, Austria b Chair of Atomistic Modelling and Design of Materials, University of Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria c Lehrstuhl für Technische Physik, Universität Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany d Department of Physics, La Trobe University, VIC 3086, Australia article info Article history: Available online 14 April 2009 Keywords: Pentacene Photoemission Band dispersion Electronic structure abstract Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its disper- sion. In various works, evidence for HOMO band dispersion as high as 0.5eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momen- tum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2). © 2009 Elsevier B.V. All rights reserved. Ultraviolet photoemission spectroscopy (UPS) has an important place in the study of organic films and their interfaces, not the least because of the pioneering work of K. Seki and W. Salaneck which have emphasized the direct information it yields regarding the valence band all important to the opto-electronic proper- ties of these materials. Here, with the example of pentacene, we illustrate some of the pitfalls in interpreting UPS and then demon- strate the capabilities of modern angle-resolved instrumentation. The electronic structure of pentacene (5A) has recently attracted considerable interest particularly because of the high mobilities reported in 5A thin films. Consequently, there have been reports of the valence band photoemission of films grown on various sub- strates with particular emphasis on the HOMO and its dispersion [1–6]. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. Recently, we have reported the intra- and intermolecular band structure of 5A measured on a well characterised film with a single polymorph (the polymorph determined by Holmes et al. [7,8]) and single crystalline orientation, 5A(0 2 2). In these films, the intermolecular band width of the HOMO, in a direction per- pendicular to the molecular axis, was shown to be surprisingly small (50 meV) [6], in apparent contradiction to the previously ∗ Corresponding author. E-mail address: ramsey@uni-graz.at (M.G. Ramsey). reported dispersions [1]. Here, we first present the momentum distribution of the HOMO of a 5A(0 2 2) film. Then the devel- opment of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of momentum maps of the HOMO, we show that indeed a reasonably large disper- sion, in agreement with band structure calculations, does exist in 5A(0 2 2). The pentacene films were grown and measured in situ in ultra high vacuum (UHV). The p(2 × 1) oxygen reconstructed Cu(1 1 0) substrate surface was prepared by cleaning the copper crystal by repeated cycles of Ar + -ion bombardment and annealing at 800 K. The clean crystal was exposed to 40 L (1 L = 1 × 10 -6 Torr · s) oxy- gen at 600 K. The pentacene (Fluka) was deposited in situ from a thoroughly degassed evaporator such that the pressure in the sys- tem remained in the 10 -10 mbar range during film growth. Nominal growth rates of 2–5 Å min -1 , as monitored by a quartz microbal- ance assuming a density of 1.33 g cm -3 , were used. The geometric and electronic structure and the morphology of these films has been fully characterised with UV- and X-ray photoemission, X- ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD, both /2 scans and pole figure analysis), low energy electron diffraction and reflectance difference spectroscopy. Unless otherwise stated, films were grown at 100K and measured at room temperature (300 K). 0368-2048/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2009.04.001