Evolution of PTCDA-derived seeds prior to graphene nanoribbon growth on Ge(001)

Abstract

The synthesis of graphene nanoribbons (GNRs) can be realized via CH4 chemical vapor deposition (CVD) on substrates such as Ge(001) that promote highly anisotropic growth. Small polycyclic aromatic hydrocarbons (PAHs) are first sublimed onto Ge at relatively low temperature to form graphene-like seeds that subsequently initiate GNR growth with CH4 exposure at 1173 K. The behaviors of PAHs between their sublimation onto Ge and GNR growth are unclear. Here, we study an archetypical PAH – perylene-3,4,9,10-tetracarboxyl acid dianhydride (PTCDA) – on Ge(001) using both scanning tunneling microscopy (STM) and density functional theory (DFT) to characterize PAH configuration, surface diffusivity, and clustering. PTCDA becomes mobile above 673 K, consistent with a DFT diffusion barrier of 1.74 eV. The mobile PTCDA molecules meet, cluster, and fuse at Ge step edges, dehydrogenating at higher temperatures. These clusters have a height of 0.4 nm, similar to small graphene islands, and grow laterally with increasing temperature – reaching 1.2–2.1 nm in width at 1173 K, consistent with extrapolated CVD experiments. These results provide a plausible picture for how PTCDA forms seeds for anisotropic GNR CVD and show that PAHs with reduced surface diffusivity and inter-molecular reactivity are needed to enable more monodisperse PAH-seeded GNR synthesis.