Self-assembly and growth mechanism of N-polar knotted GaN nanowires on c-plane sapphire substrate by Au-assisted chemical vapor deposition

The knotted GaN nanowires with huge surface-to-volume ratio and special nanoknots structure have broad application prospects for hydrogen gas sensors, nanoscale piezotronic memristor and optoelectronics. The N-polar GaN nanowires comparing with Ga-polar nanostructures, have higher crystallinity, excellent optoelectronic properties and more sensitive sensing properties, etc. The synthesis of GaN nanowires with the simultaneous integration of the knotted and N-polar have both scientific significance and practical application prospects. Herein, we demonstrate a strategy for constructing high-quality N-polar knotted GaN nanowires on the c-plane sapphire substrate coated with Au layer via a convenient chemical vapor transport approach. The Au-assisted vapor transport initializes the GaN nucleation. The N-polar is acquired by nitridating the surface of substrate through NH3, and the knotted nanostructures are then obtained by controlling the reaction of Ga with N to modulate the growth pattern, and thus vapor-solid growth promotes the formation of the knotted nanowires with multiple single GaN pyramids. The morphology, structure, composition and optical properties of the as-synthesized nanowires were investigated through field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray diffraction, energy dispersive spectroscopy mapping, X-ray photoelectron spectroscopy, Raman spectrum and photoluminescence spectroscopy. The characterization results confirm that the knotted GaN nanowires have high quality and grow along the polar [0001‾] direction, and the widths of knotted hexagonal cross-sections are 80–320 nm and their average length is approximately 6 μm. The N-polar knotted GaN nanowires with unique nanostructures grown on the substrate can be promising candidates for high-performance nanoscale optoelectronic and photocatalytic devices and hydrogen gas sensors.