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[Display omitted] • Our study demonstrates a novel strategy for the ultrathin ALD-Al 2 O 3 film depositing onto polyethylene terephthalate via polydopamine modification (PDA@PET). • Earlier nucleation, more consecutive growth, and more conformal film are observed during the ALD onto PDA@PET. • The presence of the PDA layer narrowed the region of TMA infiltration into the PET, resulting in the formation of a conformal film on the surface. • An ultrathin ALD-Al 2 O 3 film on PDA@PET shows excellent water vapor blocking performance. Atomic layer deposition (ALD) has drawn considerable attention recently for depositing functional films on polymers. However, most polymers are inert and thermally fragile, thus posing many challenges during deposition. This study presents a novel strategy for ALD of an ultrathin Al 2 O 3 film on alkali-treated polyethylene terephthalate (PET) via polydopamine (PDA) modification. The tightly adherent PDA layer exhibited numerous robust active sites (phenolic hydroxyl groups), which are highly desirable for precursor chemisorption during ALD. The modification process was systematically characterized. The PET substrate showed many active sites and a smooth surface after 24 h of PDA deposition. The island growth mode was found during the ALD of an Al 2 O 3 film on the samples. The PDA layer absorbed a large quantity of the precursor (trimethylaluminum [TMA]) during the initial deposition, contributing to the early nucleation and continuous growth of the Al 2 O 3 film. In addition, the presence of the PDA layer narrowed the region of TMA infiltration into the PET, resulting in the formation of a conformal film on the surface. Transmission electron microscopy indicated that an Al 2 O 3 film with a thickness of about 10 nm was formed on the PDA-modified PET substrate after 100 ALD cycles, and exhibited excellent water vapor blocking characteristics. These findings suggest that the added PDA layer has tremendous potential to act as a bonding layer between polymers and ALD functional films. [ABSTRACT FROM AUTHOR]