Ionic gating exploits electrolytes to control electrostatically the properties of semiconductors, in transistor devices with very large gate capacitance, in excess of 50 uF/cm^2 (three orders of magnitude larger than the capacitance of a commonly used, 300 nm thick SiO2 gate dielectric). Such a large capacitance allows charge densities up to 5 10^14 cm^-2 to be accumulated at the surface of different semiconductors, and causes new phenomena to appear, among which gate-induced superconductivity is possibly the best-known example.

As the level of control and understanding of ionic gating continues to improve, new applications of ionic gated devices emerge. Here I will provide a rapid overview of new results obtained using ionic gated transistors based on atomically thin, semiconducting transition metal dichalcogenides. I will start by rapidly touching on gate induced superconductivity in MoS2, to show how tunneling experiments can be performed on ionic gated transistors, despite the presence of the ionic liquid covering the MoS2 crystals. I will only mention the most interesting experimental observation, namely that the superconducting state is not fully gapped, as the DOS is found to vanish only linearly as the electron energy approaches the Fermi level. I will then proceed to the main part of my talk and discuss two issues. First, I will show how the very large gate capacitance of ionic gated devices allows quantitative energy spectroscopy of band edges in 2D semiconductors, allowing precise measurements of band gaps and band alignment. Second, I will discuss double-gated ionic devices allowing extremely large values of electric field to be applied perpendicularly to atomically thin semiconducting layers, so strong to enable band gaps as large as 1.5 eV to be fully quenched. I will illustrate this result with systematic measurements performed on few layer WSe2 double gated device, in which we succeeded in quenching the gap of tri-layer and thicker WSe2 crystals.