The missing nano-architecture found in the barrier layer of porous anodic alumina

• Hexagrams were clearly revealed on the surface of the barrier layer.
• Initial breaching begins with six small holes around the center of each cell.
• The small holes merge subsequently into a primary circular opening.
• The primary opening diameter has a lower limit dependent on pore diameter.

Hexagrams were revealed on the barrier layer of porous anodic alumina during chemical etching and ion milling, indicating non-uniform etching. Six-membered ring structure holes appear firstly around the center of each cell and subsequently merge into a primary circular opening. A new pore-opening mechanism is proposed, and the possible origin of the non-uniform etching rate is also discussed.

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