Variable-angle ellipsometry and molecular orientation in monoclinic organic semiconductors

Variable-angle ellipsometry has been used to characterize oligothiophene crystals with principal axes inclined at an angle to the surface normal. The results allow deducing with high sensitivity the orientation of the molecules in these anisotropic and absorbing organic solids, thus allowing one to follow the evolution of the molecular orientation after inducing the transition of the crystallographic structure between the two polymorphs of quaterthiophene. This material is representative of a large class of solids with technological applications in either crystal or thin-film form, where different structural properties and molecular orientation may arise from different preparation conditions.