A method comprising: a deposition step comprising depositing a layer of graphene oxide; a deposition step comprising selectively exposing a region of the deposited graphene oxide layer to electromagnetic radiation to form a region of reduced graphene oxide adjacent to a neighbouring region of unexposed graphene oxide, the graphene oxide and adjacent reduced graphene oxide regions forming a junction therebetween to produce a graphene oxide-reduced graphene oxide junction layer; and repeating the deposition and exposure steps for one or more further respective layers of graphene oxide, over an underlying graphene oxide-reduced graphene oxide junction layer, to produce an apparatus in which the respective junctions of the graphene oxide-reduced graphene oxide layers, when considered together, extend in the third dimension.