||Oxide semiconductors, especially ZnO, have been a subject of considerable research interest due to their interesting optoelectronic properties. Recent researches have shown that one-dimensional ZnO nanowires have characteristics of high stability, good luminescence efficiency, low critical voltage, high radiate current density and durability. In this study photoluminescence (PL) spectroscopy and Raman scattering spectroscopy were utilized to explore the optical properties of ZnO nanowires.|
The ZnO nanowires were grown on a-plane sapphire substrates by a simple vapor phase transport method without metal catalysts. Such a catalyst-free synthesis can avoid the metal remnants in the nanowires. Room temperature PL measurement showed that the intensity of ultraviolet (UV) luminescence increases as the average diameter of ZnO nanowires decreases. Such an observation is quite different from the reported PL data of nanowires grown with the use of catalysts. Moreover, an “anomalous” redshift of the UV peak position with diminished wire diameter was observed. We attribute this redshift to the effects caused by the laser heating.
The full-width at half maximum (FWHM) of the UV luminescence from the ZnO nanowires was found to decrease with better uniformities of wire distribution, alignment, and diameter. In addition, the ratio of UV to green emission integrated intensities becomes higher as the FWHM of the UV peak decreases. Thus the FWHM of the UV luminescence seems to be a measure of the uniformity and crystallinity (defects) of ZnO nanowires.
Temperature-dependent PL and Varshni relation fitting results show the center position of UV luminescence is 3.29 eV for ZnO nanowires, and 3.24 eV for ZnO buffer layer structures. The Raman spectroscopy and SEM studies showed that the samples with randomly oriented nanowire structures exhibit the A1(LO) and E1(LO) vibrating modes.