Fig. 2. (a) Schematic diagram of the experimental setup, inset: digital photograph of the real device. (b) XRD spectra of the Bi₂Se₃ nanosheets. (c) Raman spectrum of Bi₂Se₃ nanosheets. (d) Low-magnification SEM images of the liquid-exfoliated Bi₂Se₃ nanosheets, inset: high-magnification SEM images of the liquid-exfoliated Bi₂Se₃ nanosheets. (e) SEM images of pencil-drawn electrodes.