Assemblies and polymerizations - Exploring the electrical and mechanical properties of organic molecules on metal surfaces

Understanding the interaction of molecules and metal surfaces is of paramount importance in research and engineering of new microchips, solar cells and nanobots for medical application. Progress in these fields requires minimizing mechanical wear and energy dissipation to increased stability of the components used. This work aims to investigate the mechanisms underlying molecular self-assembly and its resulting electronic properties along with adsorption and desorption of molecules. To this extend scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultra-high vacuum at low temperature are utilized and complemented with molecular dynamics (MD) simulations as well as density functional theory (DFT) calculations.
Two distinct quantum dot arrays alpha and beta, self-assembled from pyrene derivatives on Ag(111), are characterized by high-resolution AFM with a CO-modified tip, energy dissipation measurements and scanning tunneling spectroscopy (STS). The energy bands formed by interaction of the respective quantum dots of the alpha- and beta-arrays were lower in the beta-network compared to alpha and the interdot-tunneling rates and quantum-capacitances were reduced.
To investigate the mechanical behavior of poly-pyrenylene and poly-anthracenylene chains lifting and sliding experiments of pyrene- and anthracene-polymers were performed by AFM unveiling the rotational mechanics of single monomers as well as a snakelike motion during lifting of the pyrenylene chain. Sliding experiments further revealed a total change in mechanical behavior within the lifting distance of one monomer due to the intrinsic flexibility of the chain. A much stiffer structure, a pyrene-based graphene nanoribbon (pGNR), was then manipulated showing a a different sliding characteristic. This illustrated the importance of polymer degrees of freedom versus sliding behavior. Finally a polymer of sterically frustrated anthracene derivatives was peeled from the surface and gently redepositioned for stabilization of rotational conformers. Complementary MD simulations were used to investigate the influence of polymer length and pinning strength of the polymer-end on the desorption behavior.
This work unveils new aspects of quantum dot arrays self-assembled from organic molecules and characterizes the behavior and mechanical stabilization of organic polymers on metal surfaces in unprecedented detail to further technological advances.