NEW INSIGHT IN THE OPTICAL-PROPERTIES OF ULTRATHIN OLIGOTHIOPHENE FILMS BY EPITAXIAL PREPARATION

Interesting optical properties are obtained if highly ordered interfac es and thin films of large organic molecules are prepared on an atomic ally flat and clean substrate. Then the optical spectra are very diffe rent from those of conventional films and reveal a variety of fine str uctures. On the basis of one example, quaterthiophene on Ag(111), it i s demonstrated that long-range ordered monolayers with commensurate su perstructures can be prepared on a well-defined surface. Even epitaxia l growth can be achieved, although this is much more complicated due t o the extremely different properties of both components. The optical s pectra then reveal a variety of fine structures which represent the co upling to different vibrations and can be assigned in detail. The 0-0 transitions become very sharp (40 cm(-1)) and show no Stokes shift. Th e absorption spectra indicate a likely Davydov splitting which leads t o a gradual reduction of the PL intensity of the 0-0 band for increasi ng film thickness. (C) 1998 Elsevier Science B.V. All rights reserved.