Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating NiS2 single crystals

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator NiS2. Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an electrochemical gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional electrostatic gate effect previously seen in pyrite FeS2. We attribute this stark difference in electrochemical vs electrostatic gating response in NiS2 and FeS2 to the much larger S diffusion coefficient in NiS2. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric H2O provides an O reservoir. The present study of NiS2 thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.