nanowires were solid inside. A high-resolution transmission electron microscopy (HRTEM) image (Fig. 4) indicates that the nanowires form from the stacking of ( 001) sheets (see the inset) along the radial directions, which are not rolled as in the case of carbon nanotubes. It can be clearly seen that the growth direction (fiber direction) is along [120]. The photoluminescence (PL) spectrum (as shown in Fig. 5) of the GaN nanowires excited by 290 nm wavelength radiation reveals the existence of a strong and broad band centered at 342 nm. The blue-shift of the bandgap emission compared with the 365 nm band of bulk GaN [16] might be ascribed to the quantum confinement effect since a considerable fraction of the nanowires have diameters less than the Bohr exciton radius for GaN, 11 nm. [17] Another strong and broad peak, centered at 470 nm and with a number of small structures, might be ascribed to the existence of defects or surface states. However, further work is needed to clarify the underlying mechanism for the PL spectrum of the GaN nanowires.
This work demonstrates the feasibility of fabricating straight and smooth GaN nanowires using a nanoparticle catalyst and directed flow of carrier gas. The method is efficient and template-independent. Our preliminary experiments reveal that it will be straightforward to extend our method to fabricate nanowires of other nitrides and oxides.