First-principles study of the stability of calcium-decorated carbon nanostructures

In view of the interest in calcium-decorated carbon nanostructures motivated by potential biotechnological and nanotechnological applications, we have carried out a systematic and thorough first-principles computational study of the energetic and structural properties of these systems. We use density-functional theory (DFT) and ab initio molecular dynamic simulations to determine minimum energy configurations, binding energy profiles and the thermodynamic stability of Ca-decorated graphene and carbon nanotubes (CNT) as function of doping concentration. In graphene, we predict the existence of an equilibrium (root 3 x root 3) R30 degrees commensurate CaC6 monolayer that remains stable without clustering at low and room temperatures. For carbon nanotubes, we demonstrate that uniformly Ca-decorated zigzag (n <= 10, 0)CNT become stable against clustering at moderately large doping concentrations while Ca-coated armchair (n, n) CNT exhibit a clear thermodynamic tendency for Ca aggregation. In both Ca-doped graphene and CNT systems, we estimate large energy barriers (similar to 1 eV) for atomic aggregation processes, which indicates that Ca clustering in carbon nanosurfaces may be kinematically hindered. Finally, we demonstrate via comparison of DFT and Moller-Plesset second-order perturbation calculations that DFT underestimates significantly the weak interaction between a Ca dopant and a coronene molecule, and also that the Ca-coronene system is not physically comparable to Ca-doped graphene due to lack of electronic pi-d orbitals hybridization near the Fermi energy level.