Direct Write, Read, and Erase of a Vertical Heterostructure of Graphene-Monolayer Electrolyte-h-BN Using Electric...

Electric double layer (EDL) gating effectively modulates charge density in two dimensional (2D) crystals; however, the electrolyte is typically deposited on top of the 2D crystal, controlling charge most strongly in the 2D material it contacts directly. Here, a monolayer-thick electrolyte is positioned between two, 2D materials - hexagonal BN (h-BN) and multi-layer graphene - in a vertical heterostack. Ions in the electrolyte are bistable, meaning they can be moved by field-effect closer to either of the 2D crystals and retain doping after the voltage is removed. Electric force microscopy (EFM) is used to write, read, and erase the heterostacks, and map the graphene surface charge with 12 × 12 nm resolution. With graphene as the top layer of the heterostack, ions in the monolayer electrolyte dope the 2D crystal from underneath, inducing sheet densities of 5 × 10¹² $c{m}^{-2}$ , more than half of which is retained for 25 min after the voltage is removed, confirming bistability of the monolayer electrolyte. While the doping can also be erased, the data suggest that the energy barrier to erasing is higher than to writing, resulting in partial erasure at a voltage equivalent to the write voltage but opposite in polarity. This observation is consistent with prior device measurements of the monolayer electrolyte on 2D crystal field-effect transistors (FET).