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.
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