which further confirmed the formation of GaN nanowires in our experiment. Several reports have indicated that the defect band emission of bulk GaN crystals may be attributed to the Ga or N vacancies or a related complex. [10] We describe a method for obtaining large quantities of high-purity crystalline GaN nanowires via VLS growth using indium metal as a catalyst. The resulting nanowires have a preferred [100] growth direction. The strong photoluminescence of the nanowires in the UV region suggests possible applications in nanotechnological optoelectronic devices. Further studies on optimization of the synthetic parameters, such as heating rates and heating times to control the diameter of the nanowires and highly oriented selective area growth, are underway.
Experimental
A silicon wafer or a quartz plate was used as a substrate for the growth of gallium nitride nanowires. The substrates were cleaned by a standard treatment in piranha solution (30 % H 2 O 2 / 70% H 2 SO 4 ) and rinsed with deionized water before use. Molten gallium (0.5 mL, 99.9999 %, Stream Chemicals) was placed on the substrates. A suspension (0.15 mL) was prepared by adding powdered indium (20 mg, 99.9999 %, 60 mesh, Stream Chemicals) to toluene (2 mL). This mixture was sonicated and then immediately dispersed onto the substrate. After toluene was evaporated, the substrate was transferred into a quartz tube placed in a furnace. The quartz tube was degassed under vacuum and purged with ammonia. The temperature of the furnace was increased to 910 C from room temperature at a rate of 50Β± 100 C/min and kept at 910 C for 12 h under a constant flow (18 sccm) of ammonia. After the furnace was cooled to room temperature, gray-black material was found on the surface of the substrate. The morphologies and crystal structures of the resulting materials were characterized using SEM (Hitachi, S2400 instrument) and XRD (Toshiba, A-40-Cu). Further structural and elemental analyses of a single nanowire were performed using TEM (Zeiss 10C at 100 kV), HRTEM (JEOL-400 EX at 400 kV), selected-area electron diffraction, and EDX. A chopped He-Cd laser (325 nm, Kimmon, IK Series) with an output power of 10 mW was used as an excitation source to acquire emission spectra at low temperature. The emission signal was collected by a single monochromator (Acton, SpectraPro-150) and detected by a lock-in amplifier coupled (Stanford Instrument, SR830) with a photomultiplier tube (Hamamatsu, R928).