Effect of high temperature annealing on strain and band gap of GaN nanoparticles

Black-coloured GaN nanoparticles with an average grain size of 50 nm have been obtained by annealing GaN nanoparticles under flowing nitrogen at 1200 ^\circC for 30 min. XRD measurement result indicates an increase in the lattice parameter of the GaN nanoparticles annealed at 1200 ^\circC, and HRTEM image shows that the increase cannot be ascribed to other ions in the interstitial positions. If the as-synthesised GaN nanoparticles at 950 ^\circC are regarded as standard, the thermal expansion changes nonlinearly with temperature and is anisotropic; the expansion below 1000 ^\circC is smaller than that above 1000 ^\circC. This study provides an experimental demonstration for selecting the proper annealing temperature of GaN. In addition, a large blueshift in optical bandgap of the annealed GaN nanoparticles at 1200 ^\circC is observed, which can be ascribed to the dominant transitions from the C(\Gamma₇) with the peak energy at 3.532 eV.