Exfoliation of Quasi-Stratified Bi₂S₃ Crystals into Micron-Scale Ultrathin Corrugated Nanosheets

There is ongoing interest in exploring new two-dimensional materials and exploiting their functionalities. Here, a top-down approach is used for developing a new morphology of ultrathin nanosheets from highly ordered bismuth sulfide crystals. The efficient chemical delamination method exfoliates the bulk powder into a suspension of corrugated ultrathin sheets, despite the fact that the Bi₂S₃ fundamental layers are made of atomically thin ribbons that are held together by van der Waals forces in two dimensions. Morphological analyses show that the produced corrugated sheets are as thin as 2.5 nm and can be as large as 20 μm across. Determined atomic ratios indicate that the exfoliation process introduces sulfur vacancies into the sheets, with a resulting stoichiometry of Bi₂S_2.6. It is hypothesized that the nanoribbons were cross-linked during the reduction process leading to corrugated sheet formation. The material is used for preparing field effect devices and was found to be highly p-doped, which is attributed to the substoichiometry. These devices show a near-linear response to the elevation of temperature. The devices demonstrate selective and relatively fast response to NO₂ gas when tested as gas sensors. This is the first report showing the possibility of exfoliating planar morphologies of metal chalcogenide compounds such as orthorhombic Bi₂S₃, even if their stratified crystal structures constitute van der Waals forces within the fundamental planes