Scanning tunneling microscopy investigation of ordered thin films of bis(4-dioctadecylamino-phenyl) squaraine on highly oriented pyrolytic graphite

Scanning tunneling microscopy (STM) is used to investigate the two-dimensional (2D) ordered structures of bis(4-dioctadecylamino-phenyl) squaraine (referred to as 18-18HSQ), adsorbed from phenyloctane onto the (0001) surface of highly oriented pyrolytic graphite (HOPG). Nucleation and growth of the 2D ordered structures could be observed on the STM time scale for adsorption from lower concentrations of 18-18HSQ in phenyloctane. A number of polymorphs and multiple-layer structures were formed from both low and high deposition solution concentrations; however, single-layer structures were only observed to form from low concentrations. Growth of layers up to three molecules deep could be observed with STM. Although 18-18HSQ is an achiral molecule, 2D chiral polymorphs were observed. The unit cells of the 2D structures of 18-18HSQ were determined to be coincident with the HOPG lattice.