Phonon confinement in individual titanium dioxide nanowires

Size effects on the phonon modes have been unambiguously observed on single individual titanium dioxide (TiO2) nanowire using Raman spectroscopy. A template-free oxidation process using ethanol vapor was developed to control the growth and the density, length, and size distribution of the TiO2 nanowires. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction were used to characterize the morphology, crystalline phases and microstructures. Using a confocal laser probe, Raman spectroscopy revealed in the individual rutile TiO2 nanowire, the Eg, A1g vibration modes which are confined in the radial directions. Along the single-crystalline nanowire axis, the systematic size-dependent variations in the center-shift, broadening as well as the shape asymmetry of the Raman peaks can be well explained in terms of the phonon confinement model.