Structural and morphological modifications in double layer heterostructures containing H2Pc, perylene-3, 4, 9, 10-tetracarboxylic dianhydride and Alq3

Abstract

The morphological, structural and spectroscopic properties of molecular double layers grown by organic molecular beam deposition have been investigated using atomic force microscopy, Nomarski microscopy, powder x-ray diffraction and electronic absorption spectroscopy. The structures, based on metal-free phthalocyanine (H2Pc), perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) and aluminum-tris-quinolate (Alq3), were deposited on glass substrates, with individual layer thicknesses ranging from 7 to 380 nm. Alq3 has no influence on the subsequent growth of either H2Pc or PTCDA, and its properties are not affected by a first molecular layer. By contrast, significant structural modifications are observed in double layers based on the two crystalline materials H2Pc and PTCDA. H2Pc deposited onto a PTCDA first layer departs from its herringbone structure, and adopts a layered structure with an interplanar spacing of 3.33 Å and an intermolecular shift of 1.7 Å. The structure of a PTCDA film deposited onto a α-H2Pc layer is completely disrupted. When PTCDA is deposited onto a β-H2Pc first layer, a morphological templating is observed and this annihilates any structural disruption. This is consistent with the growth of a microcrystalline PTCDA film on top of a α-H2Pc first layer.