  Ionic gating is a powerful technique for tuning the physical properties of a
material via electric field-induced charge doping, but is prone to introduce
extrinsic disorder and undesired electrochemical modifications in the gated
material beyond pure electrostatics. Conversely, reversible, volatile and
electrostatic modulation is pivotal in the reliable design and operation of
novel device concepts enabled by the ultrahigh induced charge densities
attainable via ionic gating. Here we demonstrate a simple and effective method
to achieve reversible and volatile gating of surface-sensitive ultrathin
niobium nitride films via controlled oxidation of their surface. The resulting
niobium oxide encapsulation layer exhibits a capacitance comparable to that of
non-encapsulated ionic transistors, withstands gate voltages beyond the
electrochemical stability window of the gate electrolyte, and enables a
fully-reversible tunability of both the normal-state resistivity and the
superconducting transition temperature of the encapsulated films. Our approach
should be transferable to other materials and device geometries where more
standard encapsulation techniques are not readily applicable.
