Structural analysis of PTCDA monolayers on epitaxial graphene with ultra-high vacuum scanning tunneling microscopy and high-resolution X-ray reflectivity

Epitaxial graphene, grown by thermal decomposition of the SiC (0001) surface, is a promising material for future applications due to its unique and superlative electronic properties. However, the innate chemical passivity of graphene presents challenges for integration with other materials for device applications. Here, we present structural characterization of epitaxial graphene functionalized by the organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). A combination of ultra-high vacuum scanning tunneling microscopy (STM) and high-resolution X-ray reflectivity (XRR) is used to extract lateral and vertical structures of 0, 1, and 2 monolayer (ML) PTCDA on epitaxial graphene. Both Fienup-based phase-retrieval algorithms and model-based least-squares analyses of the XRR data are used to extract an electron density profile that is interpreted in terms of a stacking sequence of molecular layers with specific interlayer spacings. Features in the STM and XRR analysis indicate long-range molecular ordering and weak π-π* interactions binding PTCDA molecules to the graphene surface. The high degree of both lateral and vertical ordering of the self-assembled film demonstrates PTCDA functionalization as a viable route for templating graphene for the growth and deposition of additional materials required for next-generation electronics and sensors.