, has been utilized as capacitor dielectrics and gate oxides in memory devices owing to its high dielectric constant, very low permeability, and high thermal conductivity.[1±3] In nanotube-based electronics, the importance of Al 2 O 3 nanotubes will increase, for these tubes will be useful in the fabrication of nanocables, nanojacks, [4] and nanocapacitors.[5] Al 2 O 3 nanotubes have recently been prepared by etching a porous alumina membrane template, [6] by using a carbon nanotube template, [7] or via an electrochemical anodization of Al/Si in dilute sulfuric acid using porous alumina films. [4,5] Nevertheless, these preparation methods have some difficulties in controlling and achieving uniformity of the thickness of the Al 2 O 3 nanotubes. In the study reported here, an easy and convenient route to fabricate Al 2 O 3 nanotubes with conformally uniform thickness using ZnO nanowires as templates was developed. First, ZnO/Al 2 O 3 core/shell nanofibers were prepared by depositing Al 2 O 3 on ZnO nanowires by atomiclayer deposition (ALD), then amorphous Al 2 O 3 nanotubes were fabricated by wet-etching the ZnO core nanowires in the ZnO/Al 2 O 3 core/shell nanofibers. The thickness of the Al 2 O 3 nanotubes was precisely controlled by the number of ALD reaction cycles. A scanning electron microscopy (SEM) image of bare ZnO nanowires, and transmission electron microscopy (TEM) images of a ZnO/Al 2 O 3 core/shell nanofiber and an Al 2 O 3 nanotube are presented in Figure 1. The SEM image of Figure 1a shows a side view of the ZnO nanowires synthesized on a Si substrate by a thermal evaporation procedure; the diameter of the nanowires is in the range of 20±200 nm, and the length is 20±30 lm. In this procedure, a thick polycrystalline ZnO layer was first formed on the Si substrate by thermal evaporation of ZnO powder, and the ZnO nanowires were then COMMUNICATIONS 422