VIBRATIONAL AND GEOMETRIC STRUCTURE OF QUATERTHIOPHENE ON AG(111)

The adsorption of quaterthiophene (4T) on Ag(111) has been studied using TPD, XPS and FT-IRRAS. TPD measurements allow the monolayers and multilayers to be distinguished. The FT-IRRAS results show that in the monolayer the 4T molecule is oriented parallel to the surface and bonded to Ag via the pi-electron system. Also in the next few layers (similar to 5), 4T is planar in the trans-planar conformation (C(2h) symmetry) with predominantly parallel orientation to the surface. With increasing thickness, 4T forms a disordered phase with a twisted conformation which causes a symmetry change from the C(2h) to the C(2) point group. This twisted phase is metastable and changes into a crystalline phase for > 30 layers as a result of intermolecular interaction, of overlap of p(z) electrons and of steric hindrance. Such a growth behaviour of oligomer layers has several. consequences for applications.