Fig. 2: Bi incorporates into the surface layer of GaAs NWs.
From: Self-selective formation of ordered 1D and 2D GaBi structures on wurtzite GaAs nanowire surfaces
a, d shows STM images of the Zb {110}-type surface and of the \(\left\{11\bar{2}0\right\}\)-type surface, respectively, including surface steps along different directions between the top terrace and a one monolayer lower terrace. The lower terraces appear black here due to contrast enhancement, the height scale bars are shown on the right side. Different contrast settings of the same STM image can be found for comparison in Supplementary Fig. 6. The many small bright protrusions correspond to Bi atoms. Red arrows in (a) and (d) point at atomic vacancies. The inset in (d) indicates a 2D GaBi island near a vacancy in the middle of a flat terrace. b, e illustrate the GaAs 3D crystal structure for (b) Zb and (e) Wz phase, overlaid with the NW shape. c, f show 2D models corresponding to the surfaces seen in (a) and (d). The red and blue areas represent the top terrace and the second atomic layer, respectively, of the NW surface, separated by surface steps. An arbitrary shape is chosen for the top terrace, but the possible directions of the surface steps are determined by the crystal structure and correspond to those visible in (a) and (d). Incorporation of Bi atoms (yellow dots) is illustrated, occurring at vacancies and via atomic step edges facing 〈111〉A/B directions on the Zb{110}-type surface (c) and step edges facing the [0001] B growth direction on the Wz \(\left\{11\bar{2}0\right\}\)-type surface (f), as motivated by the positions of the Bi atoms observed in (a) and (d). The direction indexes with red underline in (a) and (c) present the projections of <111 > -type directions in {110} plane. In (b), (c), (e), and (f), pink (green/yellow) spheres depict Ga (As/Bi) atoms. VT = −4.4 V, IT = 100 pA for all STM images.
