Comparing interactions in three-fold symmetric molecules at solid–air interface

Abstract

Drop-casted ultra-thin films of Benzene-1,3,5 -tricarboxaldehyde (TCA) and Trimesic acid (TMA) are studied using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to understand the molecular as well as the global nature of molecular assembly on highly oriented pyrolytic graphite (HOPG). We compare the self-assembly of TMA with that of TCA, which offers relatively weak inter-molecular interaction while keeping the same symmetry. While TMA molecules assemble into a porous chicken-wire pattern through strong hydrogen bonding, TCA forms compact assembly driven by a balance of weak hydrogen bonding interactions and high packing density. Major growth patterns in TMA and TCA are isotropic in nature due to same inter-molecular interaction along all molecular compact directions of their adlayer. In addition to the isotropic growth, we also observe anisotropic growth patterns, where the inter-molecular interactions have different magnitude along different molecular compact directions. A substantial amount of amorphous growth is also observed for both molecules. The diversity in the growth behavior for both molecules is understood using inter-molecular interactions for TMA/TCA.