  In this paper, the work function of graphene doped by different metal adatoms
and at different concentrations is investigated. Density functional theory is
used to maximize the reduction of the work function. In general, the work
function drops significantly before reaching saturation. For example in the
case of Cs doping, the work function saturates at 2.05 eV with a modest 8 %
doping. The adsorption of different concentrations on metal adatoms on graphene
is also studied. Our calculations show that the adatoms prefer to relax at
hollow sites. The transfer of electron from metallic dopants to the graphene
for all the studied systems shifts the Fermi energy levels above the
Dirac-point and the doped graphenes become metallic. The value of Fermi energy
shifts depends on the type of metallic dopants and its concentrations. A detail
analysis of the electronic structure in terms of band structure and density of
states, absorption energy, and charge transfer for each adatom-graphene system
is presented.
