  In this paper, we report first principles calculations based on density
functional theory to reveal the effects of selected adatoms (Li, Na, K, Be, Mg,
Ca, B, C, N, O, Al, Si, P, Ga, Ge, As, Se and S) adsorption on GaX ( where X=S,
Se) monolayers. It is found that all adatoms adsorbed on GaX monolayers can
form strong chemisorption bonds except for Mg atom due to weak bonding nature
of Mg atom with early 3d transition metals. Most of the adatoms of the same
group elements of the periodic table are bound to GaX substrates at the same
adsorption sites. All adatoms stayed above the upper layer of GaX sheets except
for Be adatom on GaSe substrate and overall hexagonal geometry has been
retained. The electronic structures are modified by locally through the
supercell calculations. Specific adatoms, such as C, Si, Ge, N, P and As give
rise to spin polarization and attain integer magnetic moments and hence can
contribute half metallic character to the system. Our results can serve as a
basis for future experimental and theoretical studies of adsorption on GaX.
