Anomalous metallic phases induced via ionic gate-driven intercalation of hydrogen and hydrogen-rich ions

The recent discovery of high-pressure (HP), near-room(NR)-temperature (T) superconductivity (SC) has been a milestone in fundamental research, but due to the HP involved (more than 200 GPa) and its instability in ambient conditions, any practical applications are currently unfeasible. Consequently, one of the most significant challenges is discovering a new class of technologically-relevant materials presenting ambient-pressure SC and displaying a transition at high T even if not necessarily at NR-T. Therefore, to assess the potential of hydrogen(H)-rich compounds to host new high T conventional superconductors at ambient pressure conditions, it is necessary to identify an alternative and cheap technique to investigate a wide range of materials. Ion intercalation is a method used to alter the properties of materials by modifying their crystal and electronic structures and can be performed via the ionic gating (IG) technique. During this process, carried out at room T and in ambient atmosphere, the material under study constitutes the active channel of an electrochemical transistor and is thus separated from a gate counter-electrode by an electrolyte. When a gate voltage (VG) is applied across the electrodes, intense electrostatic fields are generated through the dense accumulation of the solvated ions in the electrolyte on the surface of the channel, forming the so-called electric double layer. So, depending on the gating conditions and the nature of the material, non-volatile charge doping of macroscopic bulk of specimens of crystals is attainable via gate-driven intercalation of either H or organic ions which derive from the electrolyte. Exploiting the ultra-high electric field generated by the polarized electrolyte, it is possible to drive the accumulated ions in and out of the crystal lattice (CL) once sufficiently large VGs are applied. Furthermore, if the gating process is carried out at ambient conditions, the application of a positive VG in excess of the hydrolysis potential splits the water absorbed by the electrolyte into OH− and H+ ions. Then, the free H+ ions can be driven into the CL. Then, in the case of stable intercalation, the H+ or H-rich organic ions insertion is non-volatile, allowing several post-gating measurements. The original aim of this thesis project was to search for SC phases in several compounds via protonation, i.e. direct H intercalation, by means the IG method under optimized gating conditions. Indeed, H+ or H-rich organic ions intercalation was used to induce e−−doping and/or structure modifications, which are two possible ways to trigger the emergence of SC. The analyzed materials were Palladium (Pd), Palladium-Copper Alloy (Pd−Cu Alloy) and Molybdenum Disulphide (MoS2). The experiments in turn allowed us to affect the ground state of these materials and induce structural (Pd), charge-densitywave (MoS2), and/or insulator-to-metal (MoS2) phase transitions. Shortly, in the case of Pd, the emergence of an anomaly in the T dependence of resistivity was highlighted, which in literature is associated with a structural transition in the occupancy of H interstitial sites. H concentration, found from the comparison with literature, was approximately equal to the atomic ratio H/Pd = 0.74. In contrast, the resistivity of the Pd − Cu Alloy proved to be insensitive to the H levels obtained. Concerning the MoS2, T-dependent resistivity measurements showed the emergence of an anomalous metallic phase induced by the gate-driven intercalation of H-rich ions.