Alternately deposited heterostructures of α-sexithiophene–para-hexaphenyl on muscovite mica(001) surfaces: crystallographic structure and morphology

Abstract

Multi-component systems of para-hexaphenyl (p-6P) and α-sexithiophene (α-6T) molecules show great promise for tuning the fluorescence colour of optically active films. As the opto-electronic properties of rod-like molecules in thin films strongly rely on their anisotropic orientation, a technique for preparation of well-defined, anisotropic multicomponent systems is required. We demonstrate that a p-6P film of less than two nanometer thickness grown on muscovite mica(001) substrates acts as an efficient alignment layer for epitaxial growth of α-6T crystallites. On top of such a p-6P alignment layer, multilayer heterostructures of alternately deposited p-6P and α-6T molecules were grown. Combined X-ray diffraction and transmission electron microscopy studies show that molecules forming α-6T crystallites align parallel to those in the p-6P crystallites leading to the perfect adoption of their herring-bone structures. This alignment is desirable for optical applications and we show that it is preserved for heterostructures composed of up to 120 alternately deposited p-6P (0.8 nm) and α-6T (3.4 nm) nominal layers (120 cycles). Although for co-evaporated α-6T–p-6P molecules formation of a mixed crystal polymorph is reported, we show that in periodically deposited α-6T–p-6P heterostructures phase separation occurs and both molecules crystallize in their well-known equilibrium structures.