Plasma treatment is a powerful tool to tune the properties of two-dimensional materials.Previous studies have utilized various plasma treatments on two-dimensional materials.We find a new effect of plasma treatment.After controlled oxygen-plasma treatment on field-effect transistors based on two-dimensional SnSe2,the capacitive coupling between the silicon back gate and the channel through the 300hm SiO2 dielectric can be dramatically enhanced by about two orders of magnitude (from 11 nF/cm2 to 880nF/cm2),reaching good efficiency of ionliquid gating.At the same time,plasma treated devices show large hysteresis in the gate sweep demonstrating memory behavior.We reveal that this spontaneous ion gating and hysteresis are achieved with the assistance of a thin layer of water film automatically formed on the sample surface with water molecules from the ambient air,due to the change in hydrophilicity of the plasma treated samples.The water film acts as the ion liquid to couple the back gate and the channel.Thanks to the rich carrier dynamics in plasma-treated two-dimensional transistors,synaptic functions are realized to demonstrate short-and long-term memories in a single device.This work provides a new perspective on the effects of plasma treatment and a facile route for realizing neuromorphic devices.