Modeling Reversible Gate-Assisted NO₂ Doping of Graphene

Recent experiments in our group have demonstrated gate-assisted programmable molecular doping of epitaxial graphene devices exposed to nitric acid vapor. [1] This addresses the high intrinsic n-doping in graphene grown epitaxially on SiC. In our top-gated devices, the Al₂O₃ gate dielectric acts as a capping layer to trap the dopants from the nitric acid vapor between graphene and Al₂O₃.  We perform DFT calculations to understand the doping effects of NO₂, the main constituent of nitric acid vapor, on graphene. The effect of an applied gate in these devices is modeled as an electric field. We explore the charge transfer, adsorption geometry, adsorption energy and density of states with and without an out-of-plane electric field. We find that NO₂ adsorption/desorption on graphene can be enhanced or suppressed depending on the sign of the electrostatic gate. This helps explain the mechanism underlying the reversible gate assisted control of the molecular doping of our devices. We predict our mechanism works in a limited range of electric field, but this limit is outside the range of gate voltages explored in our experiments. This approach to programmable molecular doping can be applied more generally to other systems.

[1] Y Liu et al. Gate-assisted programmable molecular doping of epitaxial graphene devices. Submitted.