We have grown GaN on patterned Si (1 1 0) substrates prepared using a nano-imprinting technique and investigated the influence of growth temperature and the nano-pattern on the morphology of the nanostructure. Although (1 (1) over bar 0 2) facets were preferentially formed as sidewalls at a growth temperature of 650 degrees C, {1 (1) over bar 0 0) facets became dominant at substrate temperatures above 700 degrees C. We found that closely packed hexagonal GaN nanostructures, which are quite promising for future high efficiency light emitting devices, can be formed by the correct choice of not only the alignment between the pattern and the in-plane crystalline orientation of the substrate but also the period of the triangular lattice array of Si nano-pillars. The formation of this unique structure can most probably be attributed to the self-inhibited growth of GaN on the sidewall facets. (C) 2011 Elsevier B.V. All rights reserved.