Structure Characterizations of 3D spiral WSe2. (a, b) Schematic illustration of a spiral WSe2 structure (a) and the corresponding basal planes stacking order (b). (c) AFM image of a WSe2 flake. Scale bar: 2 μm. Inset: high-resolution AFM image of the region marked with red box. (d) STEM image of a typical spiral WSe2. Scale bar: 2 μm. Inset: the corresponding SAED pattern. (e) HAADF image of the region marked with red box shown in (d). Scale bar: 2 nm.