  In recent years, various doping methods for epitaxial graphene have been
demonstrated through atom substitution and adsorption. Here we observe by
angle-resolved photoemission spectroscopy (ARPES) a coupling-induced Dirac cone
renormalization when depositing small amounts of Ti onto epitaxial graphene on
SiC. We obtain a remarkably high doping efficiency and a readily tunable
carrier velocity simply by changing the amount of deposited Ti.
First-principles theoretical calculations show that a strong lateral
(non-vertical) orbital coupling leads to an efficient doping of graphene by
hybridizing the 2pz orbital of graphene and the 3d orbitals of the Ti
adsorbate, which attached on graphene without creating any trap/scattering
states. This Ti-induced hybridization is adsorbate-specific and has major
consequences for efficient doping as well as applications towards
adsorbate-induced modification of carrier transport in graphene.
