Tuning the molecular packing structure of metal-phthalocyanines on graphene by inserting interfacial copper nano grains

Graphene has been widely used in various organic devices as either electrodes or templates to obtain better device performance. Usually, organic molecules with planar backbone prefer to lie down on graphene leading to molecular π−π stacking perpendicular to graphene substrate. However, for organic field-effect transistors, parallel π−π stacking is superior to the perpendicular ones since it enhances the charge carrier's mobility between source and drain. This triggers studies on tuning the crystalline structures of organic thin film on graphene. For metal-phthalocyanines (MPc) which are widely used in organic field-effect device as active layer, so far MPc thin film with parallel molecular π−π stacking on graphene hasn't been reported. In this study, we report a method to fabricate MPc films with single crystalline structure and molecular π−π stacking parallel to graphene surface. Copper nano grains (CNG) were deposited onto graphene surface prior to the deposition of MPc films. MPc films with parallel π−π stacking mode was obtained on graphene substrate which was covered by CNG with nominal thickness of 1 nm. This work provides a conceptually new approach to induce parallel π−π stacking of MPcs on graphene in that no functionalization of graphene is needed.