Tuning the electronic and transport properties of 3D and 2D materials via electric-double-layer field effect and gate-driven protonation

The ionic gating technique, introduced in the late 2000s, exploits the ultrahigh electric fields that develop in the electric double layer (EDL) at the interface between a solid sample and an electrolyte under the application of a low polarization voltage VG. The sub-nanometric thickness of the EDL allows for electric fields up to two orders of magnitude stronger than those achievable with conventional solid dielectrics, enabling a substantial surface charge-density modulation Δn2D, which can reach 1014–1015 cm−2. This level of modulation is sufficient to induce dramatic changes in the surface electronic properties of low- and moderate-carrier-density materials, such as 2D-like few-layer single crystals of band-gap insulators and semiconductors. These changes include phase transitions from insulating to metallic states, and, at higher VG, to superconducting states.

In addition to this electrostatic use of ionic gating, a more electrochemical application has emerged in recent years. This involves leveraging the intense electric field at the sample surface to inject ions into the material - a form of true doping. In our case, we used this technique to inject H+ ions into 3D materials or into the van der Waals gaps of 2D materials.

In this talk, following a brief introduction to the various techniques we use in our lab-ionic gating, gate-driven protonation, low-temperature electric transport measurements, and Andreev-reflection spectroscopy - I will summarize the main results of more than 14 years of research. We have worked on a wide variety of materials: from normal metals to the metallic superconductor NbN; from few-layer graphene to few-layer MoS2; from ultrathin films of iron-based superconductors to doped epitaxial diamond films; and from protonation-induced two-gap superconductivity in TiSe2 to heavily H-doped Pd. Throughout, the focus will remain on the remarkable potential of these techniques to probe and control electronic and transport properties at the nanoscale.