Reversible Semimetal–Semiconductor Transition of Unconventional-Phase WS₂ Nanosheets

Phase transition with band gap modulation of materials has gained intensive research attention due to its various applications, including memories, neuromorphic computing, and transistors. As a powerful strategy to tune the crystal phase of transition-metal dichalcogenides (TMDs), the phase transition of TMDs provides opportunities to prepare new phases of TMDs for exploring their phase-dependent property, function, and application. However, the previously reported phase transition of TMDs is mainly irreversible. Here, we report a reversible phase transition in the semimetallic 1T'-WS₂ driven by proton intercalation and deintercalation, resulting in a newly discovered semiconducting WS₂ with a novel unconventional phase, denoted as the 1T'_d phase. Impressively, an on/off ratio of >10⁶ has been achieved during the phase transition of WS₂ from the semimetallic 1T' phase to the semiconducting 1T'_d phase. Our work not only provides a unique insight into the phase transition of TMDs via proton intercalation but also opens up possibilities to tune their physicochemical properties for various applications.