Real-Time Monitoring of Growth and Orientational Alignment of Pentacene on Epitaxial Graphene for Organic Electronics

The interaction between a graphene layer and pentacene (PEN) molecules leads to the formation of a lying-down phase, which can improve charge transport for organic vertical field effect transistors and enhance the optical absorption for increased light harvesting in organic solar cells. Here, we present a comprehensive study of PEN growth on epitaxial graphene on silicon carbide (SiC). Simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) were used in situ for real-time monitoring of the PEN crystal growth with millisecond time resolution to identify two distinct anisotropic growth stages after the nucleation of the first monolayer (ML). In the first stage up to 1.5 nm, we observe rapid growth of pentacene domains along the (010) and (001) facets. This growth behavior is saturating after 1.5 nm. In a second stage, this is followed by continuous lateral crystal growth in only one in-plane direction (100) forming needle-shaped domains. In the second stage, an uninterrupted linear growth of the lying-down PEN phase is found based on the (001) diffraction up to 15 nm. Ex situ atomic force microscopy and polarized confocal Raman microscopy were used to further support the real-time observations of aligned PEN films on graphene.