[15p-2N-1~17] 12.2 Characterization and Materials Physics

[15p-2N-11] Influence of Substrate Surface Roughness to Polar Molecular Orientation of
Vacuum-Evaporated Alq₃ Thin Films

〇Takashi Isoshima¹ (1.RIKEN)

Keywords:Alq3 giant surface potential,spontaneous polar molecular orientation,surface roughness

Giant surface potential of a vacuum-evaporated Alq₃ in dark condition originates from noncentrosymmetric (polar) molecular orientation which spontaneously appears during evaporation. We have elucidated this polar orientation using the asymmetric dice model, in which a molecule is regarded as an asymmetric polyhedral dice rolled on a table (substrate surface): due to asymmetric form of the dice (molecule), molecular dipole moment cannot be completely cancelled and some residual remains as a finite averaged dipole moment. In this model, the substrate is assumed to be completely flat, but actual surface presents both nanoscopic and macroscopic roughness which might significantly affect to the averaged dipole moment. In this work, influence of surface roughness is theoretically investigated, deriving weighted average of dipole moment with certain distributions of surface tilt angle. It is shown that, although there can be some reduction, more than half of the dipole moment remains even on a very rough surface.