Thin organic molecular films have been widely attracted an attention as they have promising electronic and optical properties for potential electronics applications [1]. The planar perylene derivative molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) has been used extensively as a model compound in studies of semiconducting organic films on various substrates. Well-ordered films of flat lying molecules can be formed on weakly interacting surfaces like Au and HOPG. For surfaces with intermediate interaction strength true epitaxial growth with site recognition can be realized. One prominent example is Ag where the PTCDA molecules on the Ag(111) surface have a herringbone structure with two molecules per unit cell [2]. Another interesting example is Ag terminated Si(111), which is a surface with an expected intermediate surface-molecule interaction strength [3]. Recently, we have used scanning tunneling microscopy (STM) to study PTCDA growth on a Ag/Si(111)-√3×√3 substrate. At submonolayer coverage the PTCDA molecules appear to be quite mobile on the surface. They assemble mainly in monolayer-thick well-ordered islands, preferably positioned next to a defect or a substrate domain border. Two different phases have been found on these islands with a commensurate relationship to the substrate. One is herringbone phase, known from several other low and medium interacting surfaces. Another has a bit unusual quadratic unit cell, a so-called square phase. Both phases have directions and unit cell size that matches the substrate lattice. For thicker films the molecules also form a herringbone-like phase similar to the previously reported bulk β-phase [1]. The electronic structures of various PTCDA phases have been investigated in detail by scanning tunneling spectroscopy (STS). Two types of interactions, i.e. molecule reacts with the substrate and molecule reacts with each other, will be discussed in connection to the mediated HOME and LUMO states of these PTCDA films.
[1]. S. R. Forrest, Chem. Rev. 97, 1793 (1997).
[2]. M. Eremtchenko, J.A. Schaefer, F.S. Tautz, Nature 425, 602 (2003).
[3]. P. Guaino, A. A. Cafolla, D. Carty, G. Sheerin, G. Hughes, Surf. Sci. 540, 107 (2003).
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