Ammonia-based molecular beam epitaxy (NH3-MBE) was used to grow catalyst-assisted GaN nanowires on (1 (1) over bar 02) r-plane sapphire substrates. Dislocation free [11 (2) over bar0] oriented nanowires are formed with pentagon shape cross-section, instead of the usual triangular shape facet configuration. Specifically, the cross-section is the result of the additional two nonpolar {10 (1) over bar0} side facets, which appear due to a decrease in relative growth rate of the {10 (1) over bar0} facets to the {10 (1) over bar1} and {10 (1) over bar1} facets under the growth regime in NH3-MBE. Compared to GaN nanowires grown by Ni-catalyzed metal-organic chemical vapor deposition, the NH3-MBE grown GaN nanowires show more than an order of magnitude increase in band-edge to yellow luminescence intensity ratio, as measured by cathodoluminescence, indicating improved microstructural and optical properties. (C) 2015 Elsevier B.V. All rights reserved.