Understanding How Aromatic Molecules Seed the Anisotropic Chemical Vapor Deposition of Graphene Nanoribbons

Graphene nanoribbons (GNRs) are promising materials for next-generation nanoelectronics, but challenges such as achieving narrow widths, precise orientation, and pristine armchair edges on technologically relevant substrates hinder their practical application. These difficulties are addressed by leveraging substrate-graphene interactions to drive anisotropic growth kinetics during CH₄ chemical vapor deposition (CVD), where the growth rate is directionally dependent. However, GNRs grown solely from CH₄ suffer from heterogeneous widths due to irregular nucleation. Here, we implement a novel two-step approach for GNR growth by (i) more uniformly seeding GNR synthesis using polycyclic aromatic hydrocarbons (PAHs), such as benzene and PTCDA, onto Ge(001) prior to (ii) CH₄ exposure. Scanning tunneling microscopy (STM) and density functional theory (DFT) reveal that PAH molecules diffuse and cluster at Ge step edges, leading to polydisperse seeds that disrupt GNR uniformity. By extrapolating reaction order for different PAH seeds based on CVD experiments, we demonstrate that reducing surface diffusivity (e.g., by using larger PAH molecules) can suppress clustering, leading to more monodisperse seeds and improved uniformity in GNR formation on the technologically relevant Ge surface.