“…Oxide-wide-bandgap semiconductors are ideal materials for doping RE ions due to their availability at low cost, ease of synthesis, chemical stability, and, above all, due to the tunability of electrical and optical properties. ,, Among the semiconducting oxide host materials for REs, ZnO is a well-investigated and widely accepted semiconductor with its ∼3.37 eV bandgap energy at room temperature and exciton binding energy of 60 meV, favorable for laser and other optoelectronic applications mainly in the ultraviolet regime. For the past few decades, the properties of europium (Eu) doped ZnO have been studied extensively. – Eu-doped ZnO thin films and nanopowders have been shown as potential candidates for gas sensing, photocatalytic activity, thermal sensing, bioimaging, antibacterial applications, etc. – Besides, depending on the synthesis routes, europium doped in ZnO coexists in its Eu 2+ and Eu 3+ ionic states and is very promising in its 3+ state to produce red light via intra-4f transitions. , The structural characteristics of Eu-doped ZnO thin films, nanostructures, and ZnO-based glasses have been investigated in relation to their excitation–emission mechanisms and the host-to-dopant energy transfer processes. – Electroluminescence via hot-hole or hot-electron impact excitation is another functional application of Eu-doped ZnO. , However, ZnO reveals defect-related visible emission in the green–yellow–orange region, which depends on the synthesis methods and other parameters such as chemical reagents, the temperature of synthesis, post-synthesis annealing treatment, etc., thus questioning the reproducibility of the emission in the visible range. – An advantage of the defect states is that they can also act as a pathway for energy transfer to intentional dopants, promoting desirable emission from the dopants. , Enhanced emission from Eu 3+ states is observed in ZnO when energy transfer occurs mediated via defect centers , or directly via the transfer of exciton recombination energy to Eu ions …”