Fig. 1. Edge photocurrent response in WTe₂.

a Schematic diagram of scanning photocurrent measurement of a WTe₂ field effect device. b, c Crystal structure of WTe₂. Top view of the crystal structure of $\widehat{\mathbf{a}}-\widehat{\mathbf{b}}$ plane (b) and cross-section view of the interlayer stacking structure (c) of T_d- WTe₂. d, e Scanning reflection image (d) and scanning photocurrent response (e) of a typical 6-nm thick WTe₂ device. The measurement was performed with 633-nm light filtered from white light super-continuum and the power shine on the sample was about 15 μW. The excitation light was focused using a 100× objective lens and the focused spot size was 0.85 μm. The inset of Figure e is the optical microscope image and the white arrow marks the crystallographic $\widehat{\mathbf{a}}$-axis. The scale bars are all 2 μm.