Standing and flat lying α-6T molecules probed by imaging photoelectron spectroscopy

Photoelectron-emission microscopy (PEEM) is a very versatile tool for studying dynamic processes like growth and phase transitions on surfaces. But besides the real time characterization of the morphology it can provide local spectroscopic information on a sub-mu m scale. To this end we have combined a conventional PEEM setup with a He-discharge lamp delivering high energy photons for electron excitation. The setup thus allows probing the electronic structure of individual crystallites of alpha-sexithiophene on Ag(110). Upon growth of this organic semiconductor on the surface held at 370 K, needles consisting of flat lying molecules and platelets containing near upright standing molecules are formed. Both types of crystallites can not only be distinguished by their shape but also by their photoelectron emission spectra. We also identify a wetting layer consisting of flat lying molecules which have essentially the same spectroscopic signature as the molecules in the needles. (C) 2010 Elsevier B.V. All rights reserved.