  We report the chemical reaction of single-layer graphene with hydrogen atoms,
generated in situ by electron-induced dissociation of hydrogen silsesquioxane
(HSQ). Hydrogenation, forming sp3 C-H functionality on the basal plane of
graphene, proceeds at a higher rate for single than for double layers,
demonstrating the enhanced chemical reactivity of single sheet graphene. The
net H atom sticking probability on single layers at 300 K is at least 0.03,
which exceeds that of double layers by at least a factor of 15. Chemisorbed
hydrogen atoms, which give rise to a prominent Raman D band, can be detached by
thermal annealing at 100~200 degrees C. The resulting dehydrogenated graphene
is "activated" when photothermally heated it reversibly binds ambient oxygen,
leading to hole doping of the graphene. This functionalization of graphene can
be exploited to manipulate electronic and charge transport properties of
graphene devices.
