Hydrothermal synthesis and optical properties of ZnO single-crystal hexagonal microtubes

ZnO microtubes were successfully synthesized by employing zinc acetate dihydrate as the starting material under hydrothermal condition. The ZnO microtubes were characterized by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that the as-prepared ZnO microtubes were hexagonal and single crystal in nature and grew on Si (1 1 1) substrates along the [0 0 0 2] direction, which have diameters of about 3 μm, lengths of 10-20 μm, and wall thickness of 80 nm. Optical properties of the ZnO microtubes were investigated by temperature-dependent photoluminescence (PL) spectroscopy. The PL spectrum at 82.2 K shows a strong UV emission from the near band-edge with a negligible visible emission from deep level, and donor bound exciton (D0X), free exciton (FX) and its phonon replica were observed in the UV-region. The PL spectra were dominated by the FX emission throughout the whole temperature range, and the energies of FX transitions and D0X transitions are close to that of the bulk values of ZnO.