Maheswar and co-authors found superconductivity in new, pure 2D 1T′-WS2

April 2021: Nature Materials

https://www.nature.com/articles/s41563-021-00971-y

New and pure metastable 1T′-phase transition metal dichalcogenide 2D crystals are discovered, and their electrical transport properties are published in Nature Materials. The collaborators, Prof. Zhang Hua’s research group, succeeded in synthesizing highly pure metastable semi-metallic 2D layered materials by a newly developed closed-system gas-solid method. The purity of phase in these crystals is key, as it enables very high intrinsic carrier concentrations. The IMRE team, Maheswar and Kedar, elucidated the physics behind topological 2D superconductivity with parallel and perpendicular magnetic field measurements, by performing thickness dependent electrical transport which revealed the key features. These semi-metallic 2D materials hold promise for future low-power efficient energy harvesting and low-power spintronic devices.

Figure: (a) SEM image of 1T′-WS2 crystals, (b) Fast Fourier transform filtered HAADF-STEM image (first half) and Simulated 1T′-WS2 structure (second half), (c) Superconducting transition 1T′-WS2 devices with different thicknesses

Wu Jing uses vacancy-induced Kondo hybridization to tune the thermoelectric transport in 2D MoS2

June 2020 - Proceedings of National Academy of Sciences

https://www.pnas.org/content/117/25/13929.short

In a careful and seminal study that took 3 years, a very careful study and analysis, we show that new physics such as Kondo scattering can enhance thermoelectric properties substantially!

The study of correlated phenomena in 2D semiconductors opens up new pathways toward understanding and engineering material functionalities (such as thermoelectrics) in easily accessible van der Waals solids. Local structural defects such as vacancies inevitably exist in natural as well as synthetic TMD crystals and have been predicted to serve as magnetic impurities capable of enhancing the strongly correlated effect. Herein we discover unusual thermoelectric behavior in sulfur vacancy-enriched MoS2 by rationally selecting h-BN as the substrate. We demonstrate that the thermoelectric transport properties can be strongly manipulated by vacancy-induced Kondo hybridization. A significant enhancement of thermoelectric power factor by two orders of magnitude is achieved in the MoS2/h-BN device.

Hong Kuan's work on lithium-intercalated molybdenum disulfide (LixMoS2)

Layered transition metal dichalcogenides (TMDCs) intercalated with alkali metals exhibit mixed metallic and semiconducting phases with variable fractions. Thermoelectric properties of such mixed-phase structure are of great interest because of the potential energy filtering effect, wherein interfacial energy barriers strongly scatter cold carriers rather than hot carriers, leading to enhanced Seebeck coefficient (S). Here, we study the thermoelectric properties of mixed-phase LixMoS2 as a function of its phase composition tuned by in situ thermally driven deintercalation. We find that the sign of Seebeck coefficient changes from positive to negative during initial reduction of the 1T/1T′ phase fraction, indicating crossover from p- to n-type carrier conduction. These anomalous changes in Seebeck coefficient, which cannot be simply explained by the effect of deintercalation-induced reduction in carrier density, can be attributed to the hybrid electronic property of the mixed-phase LixMoS2. Our work shows that careful phase engineering is a promising route toward achieving thermoelectric performance in TMDCs.