Structural and optical investigations in undoped and rare-earth doped zinc oxide nanoparticles for photon upconversion based applications

The present work reports comprehensive investigations of the physical properties of undoped and rare-earth (RE3+) doped ZnO based upconversion nanoparticles. X-ray diffraction analysis of these nanoparticles reveals the hexagonal wurtzite structure. The induced-strain value of the RE3+ doped ZnO nanoparticles increased by almost one order of magnitude, and their mean particle size reduced from 36 nm to 17 nm. The scanning electron microscopic images showed that the particles retained the spherical shape in the undoped and doped conditions. The elemental maps confirmed the uniform distribution of the RE3+ dopants. The recorded absorbance spectrum for the doped nanoparticles showed significant absorption in ultraviolet and near infra-red regions owing to the band-band transitions and the deep defect level transitions that are induced due to intrinsic and extrinsic defect states. A detailed analysis of the defect states of these samples is described based on the Urbach energy studies. An intense peak observed at 980 nm suggests the possibility of photon upconversion in the doped nanoparticles. A simplified energy diagram (as depicted below) has demonstrated the multi-photon absorption and possible upconversion mechanisms. The optical absorption spectrum of RE3+ doped ZnO nanocrystals encourages the promising application of these materials in diverse fields, particularly in biomedical and photovoltaic applications.