We investigated the effect of substrate temperature on nanowire (NW) flower GaN epitaxial layers grown on catalyst-free Si (111) through physical vapor deposition via the thermal evaporation of GaN powder at 1150 degrees C in the absence of NH3 gas. The NW flowers were grown at various substrate temperatures from 1000 degrees C to 1100 degrees C for 60 min in N-2 ambient. The surface morphology as well as the structural and optical properties of GaN NW flowers were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence (PL). The results showed that the increase in substrate temperature resulted in a variation in crystal quality and surface morphology. SEM showed that the substrate temperature has a stronger effect on NW density and growth rate with respect to time. The average length of GaN flowers is estimated to be longer than 300 mu m after 1 h at 1100 degrees C, which corresponds to a fast growth rate of more than 200 mu m h(-1) at all substrate temperatures. The PL measurements showed strong near-band-edge (NBE) emission with a weak deep level emission. The green-yellow emission (GYE) can be attributed to N vacancies or to the V-Ga-O-N-complexes. The NBE peak exhibited a redshift with increasing substrate temperature, which results from the increase in strain level. The growth mechanism of the polycrystalline GaN NWs was also discussed. (C) 2013 Elsevier B.V. All rights reserved.