Electric double-layer capacitance between an ionic liquid and few-layer graphene

Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance C(g). However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance C(EDL) between the ionic liquid and graphene involves the series connection of C(g) and the quantum capacitance C(q), which is proportional to the density of states. We investigated the variables that determine C(EDL) at the molecular level by varying the number of graphene layers n and thereby optimising C(q). The C(EDL) value is governed by C(q) at n < 4, and by C(g) at n > 4. This transition with n indicates a composite nature for C(EDL). Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor.