Multicolour Luminescence from Undoped ZnO Nanoparticles: An Exciting Outcome of Controlling the Annealing Atmosphere in a Facile Manner.

ZnO nanoparticles are synthesized employing a simple wet chemical method using zinc chloride and sodium hydroxide as precursors. After synthesis, the nanoparticles are annealed at 200 °C, 250 °C, 300 °C, and 350 °C for 2 h in air medium, nitrogen medium, and vacuum. All the samples are characterized for their optical, microstructural, compositional, and photoluminescence properties. The photoluminescence spectra of ZnO nanoparticles show interesting and remarkable changes with varying annealing atmospheres. Accordingly, red, green, and blue luminescence is obtained from undoped ZnO nanoparticles covering the entire visible range. This fascinating observation of generating the primary colours can be attributed to the fact of getting control over the surface defects of undoped ZnO nanoparticles after annealing in different atmospheres. The facile technique adopted in this work will open up an easy way to produce luminescence throughout the entire visible range from ZnO nanoparticles even without doping them. Therefore, ZnO nanoparticles may have potential applications as optical materials.