Not surprisingly, properties of a two-dimensional material, being all surface, may be exquisitely susceptible to contamination. (See “Experimenting with monolayers” and links therein.) This general observation has been recently confirmed in MoS2 as well.
On the other hand, this susceptibility can be exploited for band engineering, doping, etc. Two recent preprints on the arXiv make interesting reading. See arXiv:1308.4924, discussing organometallic complexes of graphene, as well as arXiv:1308.1645, discussing the reversible grafting of naphthylmethyl radicals to graphene.