Researchers from Columbia University say that the electronic properties of graphene are very sensitive to the local environment. Using scanning tunneling microscopy, the researcher studied graphene films exfoliated onto cleaved mica (an ultra flat material that contains surface electric dipoles).
It turns out that graphene remains undoped immediately above the water molecules but becomes p-doped on the mica surface surrounding them. On the other hand, graphene becomes n-doped near the potassium ion regions. There are two types of n-doping in graphene: immediately above the potassium plateau there is a lot more electron doping then areas on flat regions.
The experiments showed that graphene above the same flat silicate network is both electron- and hole-doped depending on its surface environment. This means that it's possible to either hole-dope or electron-dope graphene, or create specific n-type and p-type regions in the material that could allow for phenomena such as exciton (electron-hole) dissociation (useful for photovoltaics applications).