Morphology and Growth Mechanisms of Self-Assembled Films on Insulating Substrates: Role of Molecular Flexibility and Entropy

We studied the effect of molecular flexibility on the morphology and growth mechanisms of self-assembled films on an insulating substrate using a combination of experimental and theoretical methods. 1,3,5-Tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and 1,4-bis(cyanophenyl)-2,5-bis(decyloxy) benzene (CDB) molecules were deposited on a KCl (001) surface and imaged using noncontact atomic force microscopy (NC-AFM). Both molecules were designed to contain the same anchoring groups and benzene ring structures, yet CDB self-assembled structures were observed to grow from step edges, while TCB self-assembled structures grew as islands on the clean terrace and from step edges. Density functional theory (DFT) and atomistic molecular dynamics simulations were performed to understand this qualitative difference in growth modes. Calculations of free energies of dimer formation and step adhesion provide an insight into the role played by entropy loss in the morphology and growth modes of self-assembled films.