Adsorption of titanium and titanium dioxide on graphene: n and p-type doping

Ab initio calculations within the density-functional theory formalism are performed to investigate the ground state, electric charge doping, and electronic properties of titanium and titanium dioxide monolayers adsorbed on a graphene surface. A new ground state structrure of Ti monolayer adsorbed on graphene is reported which is shown to be stable up to T = 500 K. Effects due to lower and higher Ti adatoms coverage are studied. We find that the adsorbed Ti provides a strong n-type doping which supports recent experimental observations. On the other hand, TiO_2 can induce both p- and n-type doping in the carbon monolayer depending on whether oxygen or titanium atoms are closest to the substrate. We identify the structures which are responsible for the experimentally observed autocompensation mechanism that leads to the reversion of adsorbate effects after oxidation of the adsorbed Ti.