Two-dimensional molecular self-assemblies on surfaces studied by low-temperature scanning tunneling microscopy

Abstract

Supramolecular self-assembly on surfaces is a promising bottom-up approach to fabricate two-dimensional (2D) molecular nanostructures over macroscopic areas. In our studies, a wide range of 2D molecular self-assemblies on surfaces and their formations of regular supramolecular arrays were demonstrated by low-temperature scanning tunneling microscopy (LT-STM) in ultra-high vacuum (UHV) environments. Intensive effort had been devoted to construct mono- and bi-component organic molecular networks via various intermolecular interactions and molecule-substrate interactions. A comprehensive understanding of the underlying mechanisms that control the surface self-assemblies was also aimed. Epitaxial growth of organic p-conjugated molecular films on solid surfaces was investigated initially, including copper hexadecafluorophthalocyanine (F16CuPc) and di-indenoperylene (DIP) ultra-thin films on graphite and/or SiO2. To increase the tunability and functionality of the surface-supported nanostructures, robust 2D binary molecular networks, whose overall arrangements could be tuned by varying molecular intermixing ratios as well as molecular building blocks, were constructed by co-adsorption of F16CuPc with copper phthalocyanine (CuPc), p-sexiphenyl (6P), pentacene, and DIP respectively. The structural stability of the binary molecular arrays is enhanced by multiple intermolecular hydrogen bonds. Some of the rigid hydrogen-bonded networks can be used to adsorb incoming guest molecules at specific adsorption sites and facilitate the formation of regular patterns on the underlying templates. Finally, the chloroaluminum phthalocyanine (ClAlPc) molecule with electric dipole moment perpendicular to its molecular p-plane, was studied. 1D molecular dipole chain arrays were formed in the binary molecular system of ClAlPc and DIP on graphite. We also succeeded in reversibly manipulating the ClAlPc molecular configuration by STM tip.