<p>Highly c-axis oriented nanocrystalline ZnO thin films were grown on Si substrates using rf magnetron sputter deposition technique. These thin films were irradiated with argon ions having an energy of 1.8 million electron volts (MeV) at fluences in the range from 1 × 10¹⁵ ions/cm² to 7 × 10¹⁵ ions/cm². Structural studies of the pristine and irradiation thin films revealed that the average crystallite size of wurtzite ZnO nanocrystals varied in the 10–19&#xa0;nm range and that the strain has decreased in the irradiated films. Fourier transform infrared spectroscopy (FT-IR) studies of the films show that argon ion irradiation has caused changes in the intensity, width and position of Zn–O stretching vibration band. Surface morphology studies revealed that the surface roughness decreased by 20–26% due to irradiation in the fluence range from 3 × 10¹⁵ to 5 × 10¹⁵ ions/cm². Current–Voltage (I–V) studies of the films performed under dark condition show that argon ion irradiation has caused a decrease in dark current upto 4 orders of magnitude. The results of the study indicate that coupled effects produced by 1.8&#xa0;MeV argon ions in nanocrystaline ZnO thin films can modify the structural, surface and dark current properties in a controlled manner, which may be useful to improve the sensitivity and signal-to-noise ratio of sensors.</p>