PhD Thesis Defense: Engineering Superconductivity in TMDs via Organic Intercalation

Intercalation—the process of introducing guest species into the spaces between the atomic layers of a host material—dramatically alters the electronic and structural properties of the intercalated material. In this doctoral thesis, we demonstrate how it is possible to induce and modify a superconductive state in molybdenum disulfide (MoS₂) and tantalum disulfide (TaS₂) through intercalation with organic compounds. Using a set of advanced characterization techniques—including X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and scattering-type scanning near-field optical microscopy (s-SNOM), together with transport measurements at temperatures below that of liquid helium—we have gained a detailed understanding of the intercalation process and its effects on superconductivity in these host materials. Our main results include the first demonstration of air-stable superconducting MoS₂ intercalated with organic molecules. The molecular structure of the guest species, tetraethylammonium (TEA+) and cetyltrimethylammonium (CTA+) ions, plays a critical role in determining the superconducting properties. Our experiments reveal that bulk crystals of TEA-intercalated MoS₂ exhibit a superconducting transition with a critical temperature (Tc) near 4 K. However, exfoliated micrometer-sized flakes did not achieve a fully superconducting state, due to uneven doping, as evidenced by our multi-technique characterization. To achieve a zero-resistance state in micrometer-thick flakes, we propose the intercalation of TaS₂, a natural superconducting compound. Our results show that, by adjusting the chemical intercalation environment, it is possible to produce superconducting micrometer-sized flakes with a Tc of 2.9 K, significantly higher than the natural TaS₂ Tc of 0.8 K. Lastly, we include a brief outlook in which we point at key areas of future potential research, indicating promising ways to leverage unique properties of guest organic species which, in our view, will grant access to novel physical phenomena.