Fig. 3. Revealing local structures of Hf-MOLs using HAADF-STEM.

a HAADF-STEM image of MOLs shows the missing-cluster regions. The inset is the enlarged region of the yellow frame, which shows one cluster missing from the center, as indicated by the white arrow. The area indicated by the yellow arrow was caused by the electron beam irradiation for >10 s. b Enlarged region of the white frame in a, where a row of clusters disappeared. c HAADF-STEM image of MOLs shows connected clusters and local disorder areas within the bulk of Hf-MOLs. Different layers of Hf-MOLs stack to form the yellow polygonal area, and some disorder structures (yellow square) are generated at the interface. d A zoomed-in area of the white square region in c. e HAADF-STEM image shows the boundary between Hf-MOLs. f Enlarged region of the white frame in d. g HAADF-STEM image of Hf-MOLs shows the step-edge site surface structure. The inset shows the enlarged region of the white square, and the white arrows indicate the position to add a new cluster. h Schematic illustration of the loop structure contributes to epitaxial growth of MOLs. The blue arrows show the connection site to add a new cluster, where the new cluster could form three rings (similar to the position indicated by the black arrow) with neighboring clusters. Scale bars: a, c, e, g 10 nm; b 5 nm; d, f 2 nm.