Quasi-Layer-by-Layer Growth of Pentacene on HOPG and Au Surfaces

High-resolution scanning tunneling microscopy (STM) is a promising method for characterizing organic semiconductors to obtain a deep understanding of organic semiconductor physics. However, organic films on conductive single-crystal substrates, which are required for STM, usually present different growth behaviors than the films on inert substrates such as SiO2. Here, we reported a simple modification method for modulating the organic semiconductor film growth on the highly oriented pyrolytic graphite (HOPG) and Au(111) substrates and investigated the detailed morphology evolution. Self-assembled monolayers (SAMs) fabricated from vacuum deposition and solution processing were introduced on these conductive substrates. Pentacene, a prototypical organic semiconductor, presented quasi-layer-by-layer growth on HOPG or Au(111) sufaces modified with solution-processed alkane monolayer. The pentacene film resembled the upright packing and terraced morphology but with larger grain size than that of thin-film phase on SiO2. The introduced n-dotriacontane layer decreased the interaction between pentacene adsorbates and the active substrate and provided a lower surface energy which supported the upright orientation of pentacene. Modification of the substrates with alkanes provides a feasible approach to grow high-quality organic thin films that are suitable for characterization down to the molecular level. Additionally, this approach is effective for two-dimensional substrate materials such as graphene and is not limited to single-crystal substrates.