Thermodynamic Stability and Site-Specific Distribution of Graphitic and Pyridinic Nitrogen in Graphene Moiré on Ru(0001)

Graphene-like materials can be viewed as promising storage media for hydrogen as they are lightweight, durable, and scalable. For practical use, doping is required to overcome the kinetic limitations for diffusion and recombination on surfaces due to the required rehybridization of atoms. We studied the synthesis of nitrogen-doped graphene on Ru(0001) by chemical vapor deposition (CVD) of pyridine and N-doping through ion irradiation. Using the combination of scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and density functional theory (DFT) we unambiguously identify the structure and location of the N species within the graphene moiré. The pyridine CVD leads only to a low concentration (<0.1 at%) of N-related sites. Higher concentrations of N-dopants (>10 at%) are subsequently introduced by low-energy ion irradiation. The concentration of graphitic (GN) and pyridinic nitrogen (PN) can be tuned by varying the ion dose and annealing temperature. DFT calculations provide detailed information about the relative thermodynamic stability of GN and PN within the graphene moiré. Measured and simulated STM images of GN and PN yield an excellent agreement, allowing us to confidently establish that GN is preferentially located near the center of the Atop region of the graphene moiré, while PN is located at the boundary between the FCC and HCP region, which are defined by the registry between C and Ru atoms. This report explicitly confirms the site assignments and provides a foundation for the site synthesis and analysis of their structural and electronic properties that drive the thermodynamic stability and reactivity of N-doped graphene.