  The chemical functionalization of graphene enables control over electronic
properties and sensor recognition sites. However, its study is confounded by an
unusually strong influence of the underlying substrate. In this paper, we show
a stark difference in the rate of electron transfer chemistry with aryl
diazonium salts on monolayer graphene supported on a broad range of substrates.
Reactions proceed rapidly when graphene is on SiO_2 and Al_2O_3 (sapphire), but
negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces. The
effect is contrary to expectations based on doping levels and can instead be
described using a reactivity model accounting for substrate-induced
electron-hole puddles in graphene. Raman spectroscopic mapping is used to
characterize the effect of the substrates on graphene. Reactivity imprint
lithography (RIL) is demonstrated as a technique for spatially patterning
chemical groups on graphene by patterning the underlying substrate, and is
applied to the covalent tethering of proteins on graphene.
