Critical assessment of charge transfer estimates in non-covalent graphene doping

Non-covalent doping by pure charge transfer complexes is one possible solution to tune at low-cost electronic properties of carbon-based nanostructures, more specifically to enhance their conductivity. Here, we present a thorough density functional theory-based study of charge transfer estimates, by comparing available integration/partitioning scheme of the electronic density in periodic boundary conditions, as well as the influence of the exchange-correlation term, the cornerstone of DFT by testing various exchange-correlation functionals. Our test case is made of a freestanding graphene monolayer in interaction with two prototypical donor/acceptor molecules: TTF and TCNE. These results illustrate the role played by the exact exchange in the description of charge transfer processes, as well as the difference between the density-based and wavefunction-based partitioning schemes used in this study. When using hybrid functionals, charge transfer are usually smaller than when using standard generalized gradient approximations, especially for the donor molecule. In terms of electronic density partitioning schemes, both strategies provide quite similar charge transfers; however, each intra-molecular decomposition presents very distinct features, making the discussion of atomic charge reorganization on the electron/donor molecule highly dependent on the selected partitioning scheme.