Ionic liquid gating - amazing results

I've mentioned ionic liquid gating a couple of times (here, here) before.  Ionic liquids are basically organic salts (molecular anion; molecular cation, though there are some that use, e.g., alkaline metal ions instead) that are liquid at room temperature.  The concept is simple enough:  use the ionic liquid as an electrolyte in a capacitor comprising a wire employed as a gate electrode and the surface of a sample of interest as the counterelectrode (helpfully contacted by source and drain electrodes).   Setting the bias of the gate relative to the surface drives the ions to move, building up (ionic) surface charge densities at the sample surface.   The material responds to screen what would otherwise be an enormous electric field penetrating the surface by accumulating mobile charge carriers of the appropriate species in a layer at the interface.  This approach, adopted by multiple groups, has been pushed hardest by the group of Iwasa.  The real advantage of this approach is that it allows access to electrostatically gated charge densities comparable to what can be achieved in chemical doping - on the order \( 10^{15} \) per cm² in that top layer of material.  (This is the regime that He Who Shall Not Be Named fraudulently claimed to access, but this time it's real!)

Particular highlights have included:

• Gating SrTiO₃ into metallicity and then superconductivity.
• Gating ZnO into metallicity.
• Gating ferromagnetic response in GaMnAs and Co-doped TiO₂.
• Gate-induced superconductivity in KTaO₃.

A new paper appears in Science this week with yet another impressive result.  Iwasa's team has used ionic liquid gating to push an exfoliated flake of MoS₂ into superconductivity.  Indeed, they see what looks a lot like a superconducting "dome" as a function of gated charge, with \(T_{c}\) actually decreasing above an optimal gated carrier density.   Wild.

The results just keep coming.  There are some real subtleties to the technique, though.  It's extremely important to make sure that the ionic liquid is really acting as a chemically inert electrolyte, and not inducing electrochemistry at the material surface or any other kind of chemical doping.

Anyway, this whole area is clearly one to watch in condensed matter.  Anytime you can push into a previously inaccessible regime, Nature tends to offer up surprises.