Synthesis of ultrathin two-dimensional nanosheets and van der Waals heterostructures from non-layered γ-CuI

Abstract Two-dimensional (2D) nanosheets have attracted considerable recent interest for their atomically thin geometry and unique thickness-dependent electronic properties. The 2D nanosheets studied to date are generally limited to intrinsically layered materials, in which the covalently bonded atomic layers are held together by weak van der Waals forces and can be readily exfoliated to single or few-atom thick nanosheets. To prepare 2D nanosheets from non-layered materials can greatly expand the scope of 2D materials, but is much less straightforward. Here, we report the successful synthesis of ultrathin nanosheets from non-layered γ-CuI on SiO₂/Si substrate using a facile physical vapor deposition process. The resulting γ-CuI nanosheets display a triangular and hexagonal geometry with the lateral dimension up to 5 μm and thickness down to 1 nm. Raman spectroscopy, X-ray diffraction, and transmission electron microscopy studies demonstrate the resulting nanosheets retain single-crystalline γ-CuI phase. Additionally, we further show the γ-CuI nanosheets can be readily grown on other 2D materials (e.g., 2D-WSe₂, 2D-WS₂) to form van der Waals heterostructures (vdWHs). Optical microscopy images and Raman intensity mappings confirm the formation of γ-CuI/WS₂ and γ-CuI/WSe₂ vertical heterostructures. The electrical transport studies show that γ-CuI nanosheets exhibit a low resistivity of ~0.3 Ω cm and γ-CuI/WS₂ vertical heterostructures display a p-n diode behavior with distinct current rectification. The synthesis of γ-CuI nanosheets and heterostructures open a pathway to ultrathin nanosheets and van der Waals heterostructures from non-layered materials and could open up exciting opportunities in electronics and optoelectronics.