Fig. 3: Design of DL structure.

Single NC layer: (a) DF-STEM image of the interface (indicated by the white arrows) between Cu seed layer and as-prepared NC-6L. b SIM and corresponding inserted BF-STEM images of NC-6L after annealing at 100 °C for 30 min showing the abnormal grain growth in region α indicated by the yellow dotted line; the boundary of region α is marked by the white dashed lines. c SIMS analysis of the region α indicated in (b) showing a sharp increase in impurity content (Cu: orange, C: dark grey, S: yellow, Cl: cyan-green, and Si: cyan-blue); the dashed-line refers to the white dashed lines in (b). d SEM observation of a single NC-6L layer after annealing at 200 °C for 5 min showing a large number of defects near the Cu seed layer. NC-CG DL structure: (e) SIM image of the as-prepared structure of DL (CG + NC-6L); the original boundary between CG and NC-6L is marked by the white dashed line. f SIM image showing the crystal protrusion (yellow dotted line) from the original boundary (white dashed line) in DL after annealing at 100 °C for 10 min. SIMS analysis of as-prepared (solid lines) and annealed (dashed lines) DL: (g) Cl and (h) C and S. Colours refer to the characteristics listed in (c) and the grey dashed lines indicate the original boundary between CG and NC-6L. i SEM observation of the first CG layer after annealing at 200 °C for 5 min. Note that the white dashed lines indicate the boundary between CG and Cu seed layer in (e), (f), and (i). Source data are provided as a source data file.