Fig. 3: Evidence for the surface origin of the photocurrent response in Ag₂Te.

a–c, Schematic structure of the hBN(hexagonal born nitride)-Ag₂Te-hBN device (a), its false-color optical image with scheme of the measurement (b), and photocurrent response mapping (c). Outlines of Ag₂Te, electrodes, and top-hBN have been marked with dotted lines. d–k, The turn-over experiment of Ag₂Te. d Schematic of the turn-over process. The pink and blue arrows represent the opposite responses generated from surfaces A and B, respectively. The direction of the total response is determined by the upper surface, which would be reversed if the sample is turned over. e–g False-color optical images of the sample. One Ag₂Te nanoplate on polydimethylsiloxane (PDMS) (e) was mechanically cut into fragments and parts of them (recolored as blue) were lifted up and consequently turned over to B side up (g) by another PDMS. The rest parts (recolored as pink) keep A side up (f) the same as the initial nanoplate shown in (e). h–k, False-color optical images and photocurrent mapping results of the two devices fabricated from (f) and (g), respectively. The red dashed lines in f–k outlines the samples through the fabrication procedures. The yellow and green bars in e–i mark the two edges of the nanoplate, e1 and e2. White arrows in e–i indicate the same crystallographic direction along e1. Red arrows in j, k indicate the photocurrent directions. The laser wavelength is 690 nm and the power is ~80 μW in c, j, k. The scale bars are all 5 μm.