Polypyrrole on graphene: A density functional theory study

•Density functional theory results assert that polypyrrole (PPy) physisorbs on the graphene surface.•PPy physisorption on graphene occurs at a distance of 3.5 Å and a tilt angle of 23°.•The PPy/graphene nanocomposite band structure does not have an energy band gap opening at the Dirac point.•PPy/graphene physisorption yields an electronic density rearrangement along the polymer backbone only.•A second graphene layer on top of PPy does not affect the above results. [Display omitted] Polypyrrole/graphene composite films are gaining importance for designing technologically interesting materials and for exploring fundamental properties of novel nanointerfaces. However, the adsorption mechanism of the polymer on the graphene surface is not well understood. In this work, the stability, molecular structure, and electronic structure of neutral polypyrrole (PPy) chains adsorbed on graphene are investigated with density functional theory (DFT). Energetically stable structures are attained when the polymer adsorbs forming a 60° angle with the graphene surface. Three graphene adsorption sites are considered for the 60° direction: top, hollow, and bridge. The polymer adsorbs on these sites at a distance of 3.5 Å and a 23° tilt angle. The adsorption binding energy of the nanofilm is exclusively due to the van der Waals corrections of DFT. Based on these first-principles results, we assert that PPy physisorbs on graphene. Importantly, the physisorbed PPy chains do not affect significantly the band structure of graphene and do not open an energy band gap at the Dirac point. Adding a second layer of graphene on top of the polymer chains was also considered.