Fig. 2: Changes in NiAs_x interfacial layer during in situ TEM heating.

a A set of BF-TEM images of the stack of Si/SiO₂ substrate, b-As film, deposited polycrystalline Ni, and the interfacial NiAs_x layer showing a gradual expansion of the NiAs_x layer with an increase of temperature going from T_R = 22 °C to T = 292 °C. Grain growth in the NiAs_x layer is also visible. Each image is accompanied by a diffraction pattern (below) obtained from the stack at the same temperature. The layer stack is oriented along [101] zone axis of b-As. Scale bars are 30 nm and 1 nm⁻¹. The arced spots in the diffraction pattern at T_R correspond to polycrystalline NiAs_x (marked orange) and Ni layers (marked yellow). Progressive heating leads to the shortening of the arced spots from the polycrystalline NiAs_x layer and the appearance of two strong spots. b ADF-STEM images of the stack after reaching T = 292 °C and cooling it down to T_R. A High-resolution ADF-STEM image of the layer shows atomic columns of Ni₁₁As₈ crystal in [031] zone axis. Scale bars are 100 and 1 nm, respectively. c Atomic-resolution ADF-STEM image of the layer from another heating experiment showing the unique pattern of atomic columns of Ni₁₁As₈ crystal viewed along [100] zone axis. Scale bar is 1 nm. Insets in (b, c) are simulated ADF-STEM images of Ni₁₁As₈ in the same directions. d ADF-STEM image of the stack in (b) with EDX elemental maps of O (blue), As (green), and Ni (yellow) showing the complete transformation of the b-As film into Ni₁₁As₈. Light oxidation of the metal layers remains there even after the formation of Ni₁₁As₈. Scale bar is 100 nm.