Fig. 2

STM images (86 nm × 86 nm, sample bias voltage 1.0 V, tunneling current 1 nA) of the single crystal Ru(0001) sample at different stages of oxide film growth. a The initial atomically flat metallic substrate; b–f the oxide film grown at 630 K in O₂ at 3 × 10⁻⁵ mbar during b 30 min, c 40 min, d 50 min, e 1 h, and f 1.5 h. All images share the same gray scale ruler given to the right. The height–distance cross-sections along the dashed lines are presented in the lower part of the figure. The insets in a and f are 17.2 nm × 17.2 nm images with atomic resolution