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

Abstract

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${\mathrm{NiS}}_{2}$. 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 $\mathit{electrochemical}$ 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 $\mathit{electrostatic}$ gate effect previously seen in pyrite ${\mathrm{FeS}}_{2}$. We attribute this stark difference in electrochemical vs electrostatic gating response in ${\mathrm{NiS}}_{2}$ and ${\mathrm{FeS}}_{2}$ to the much larger S diffusion coefficient in ${\mathrm{NiS}}_{2}$. 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 ${\mathrm{H}}_{2}\mathrm{O}$ provides an O reservoir. The present study of ${\mathrm{NiS}}_{2}$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.