Self-organized thin films of hydrogen-bonded phthalocyanines: Characterization of structure and electrical properties on nanometer length scales

We present the structural and electrical property characterization of solution-deposited thin films of a self-organizing phthalocyanine 2,3,9,10,16,17,23,24-octa(2-(4-octylbenzamide ethyl-sulfanyl) copper(II) phthalocyanine, Pc (1). Self-organization into columnar aggregates occurs mainly as a result of hydrogen bonding interactions between the benzamide groups in the side chains. These H-bonding interactions lead to ordered films of Pc (1), with the plane of the Pc parallel to the substrate plane, on gold surfaces modified with amide or amine functionalities, and on freshly cleaved highly order pyrolitic graphite (HOPG). Atomic force microscopy (AFM) confirms layer-by-layer growth for these Pc films up to coverages of ∼10 monolayers, when deposition occurs from dilute solutions of the Pc. Conductive-tip AFM (C-AFM) measurements of these ordered Pc films, on modified Au and HOPG substrates, confirm the layered nature of these Pc films and show that the conductance decreases incrementally with the number of Pc monolayers being probed. On HOPG substrates, these measurements lead to an estimation of the conductance per Pc monolayer, and the Pc molecule-HOPG contact resistance of 8 Mω, a value which is comparable with previously explored organic semiconductor materials.