Controlling superconducting phase transition on a two-dimensional (2D) material is of great fundamental and technological interest from the viewpoint of making 2D resistance-free electronic circuits. Here, we demonstrate that a 1T-to-2H phase transition can be induced on the topmost monolayer of bulk (<100 nm thick) 1T-TaS 2 by thermal annealing. The monolayer 2H-TaS 2 on bulk 1T-TaS 2 exhibits a superconducting transition temperature (T c ) of 2.1 K, which is significantly enhanced compared to that of bulk 2H-TaS 2 . Scanning tunneling microscopy measurements reveal a 3 × 3 charge density wave (CDW) in the phase-switched monolayer at 4.5 K. The enhanced T c is explained by the suppressed 3 × 3 CDW and a charge-transfer doping from the 1T substrate. We further show that the monolayer 2H-TaS 2 could be switched back to 1T phase by applying a voltage pulse. The observed surface-limited superconducting phase transition offers a convenient way to prepare robust 2D superconductivity on bulk 1T-TaS 2 crystal, thereby bypassing the need to exfoliate monolayer samples.

Two‐dimensional transition metal dichalcogenides (TMDs) play host to a wide range of novel topological states, such as quantum spin Hall insulators, superconductors, and Weyl semimetals. The rich polymorphism in TMDs suggests that phase engineering can be used to switch between different charge order states. Intercalation of atoms or molecules into the van der Waals gap of TMDs has emerged as a powerful approach to modify the properties of the material, leading to phase transition or the formation of substoichiometric phases via compositional tuning, thus broadening the electronic and optical landscape of these materials for a wide range of applications. Here, we review the current efforts in the preparation of intercalated TMD. The challenges and opportunities for intercalated TMDs to create a new device paradigm for material science are discussed.

Chirality generates spontaneous symmetry breaking and profoundly influences the topology, charge, and spin orders of materials. The chiral charge density wave (CDW) exhibits macroscopic chirality in the achiral crystal during the spontaneous electronic phase transitions. However, the mechanism of chiral CDW formation is shrouded in controversy. In this work, we report that two-dimensional H-phase TaS2 synthesized by molecular-beam epitaxy (MBE) shows a predominantly chiral CDW phase. Scanning tunneling microscopy (STM) imaging of the CDW reconstruction spots reveals a clockwise or anticlockwise intensity variation along the STM-imaged spots. First-principles calculations further show that the rotational symmetry of the momentum-dependent electron-phonon coupling is broken, giving rise to chirality. Our work provides new insights into the physical origin of the chiral charge-ordered states, shedding light on a general ordering rule in chiral CDWs.