ZnO nanotubes were epitaxially grown on a Si(I 1 1) substrate by plasma-assisted molecular beam epitaxy using a two-step process. First, a low-temperature ZnO layer was deposited on the Si substrate at a temperature of 300 degrees C, which was treated by an O-2(+) -plasma for 30 min. Second, a ZnO layer that showed tubular-like nanostructure was grown on this treated layer at 550 degrees C using the same flow rate of the 02 and zinc. No catalyst or templates were required for the formation of the ZnO nanotubes. Field-emission scanning electron microscopy (SEM) and X-ray power diffraction (XRD) experiments showed that the ZnO nanotubes were single crystals of pure hexagonal wurtzite structure growing along the [0 0 0 2] direction. The diameters of the nanotubes were in a range of 10-90 nm, and the average size of the nanotubes was 50 nm. Room-temperature photoluminescence (PL) measurements of the ZnO nanotubes showed ultraviolet peaks at 3.290 eV with a full-width at half-maximum (FWHM) of 104 meV, which was comparable to those found in high-quality ZnO films. The PL spectrum at 81 K showed a sharp free-exciton emission with an FWHM of 22.1 meV. The SEM, XRD and PL results indicated that the prepared ZnO nanotubes have potential applications in optoelectronic devices. Possible reasons for the growth of ZnO nanotubes were discussed. (c) 2005 Elsevier B.V. All rights reserved.