ZnO thin films were synthesized by radio-frequency (RF) magnetron sputtering of high purity ZnO solid targets on sapphire substrates. Depositions were carried out at selected temperatures between 293 K and 1173 K, and post-deposition annealing was performed at 1173 K for 3 min. in an O2 atmosphere. Samples for electron microscopy investigations were prepared by lift-out technique in a multi-beam FIB/SEM instrument. The ZnO thin films show generally uniform thickness (about 1µm), determined by transmission electron microscopy (TEM) imaging. Irrespective of the deposition temperature, the ZnO thin films are polycrystalline, with individual grains exhibiting columnar morphology with the long axis oriented perpendicular to the ZnO/sapphire interface. The grain size varies with the deposition temperature, and a direct correlation between grain size and photoluminescence has been observed. Analyses performed using low-temperature photoluminescence spectroscopy measurements at 12 K revealed luminescence peaks at 3.361, 3.317, 3.218 and 3.115 eV. The intensity of the luminescence peak at 3.317 eV decreased with increasing deposition temperature. The films deposited at lower temperatures also exhibited a higher density of stacking faults as observed from the atomic resolution TEM. The crystallographic imperfections/photoluminescence relationship is not clear. The purpose of this study is to quantify the observed crystallographic imperfections and understand their effect on the photoluminescence of undoped ZnO thin films deposited on sapphire substrates.