On the potential of Transparent Rare-Earth-Free ZnAl2O4 Ceramics targeted at the UV-C to UV-B emission

Materials capable of emitting in the ultraviolet UV-C to UV-B spectral range have received considerable attention due to their potential applications in biophotonics, plant lighting, optical cleaning, sterilization, and disinfection. However, progress in the development of phosphors that emit in this short-wavelength range has been relatively sluggish. Most UV-C phosphors developed to date are based on hosts doped with the rare-earth cation Pr3+ due to its bright UV luminescence originating from the 4f15d1 -> 4f2 interconfigurational transitions. In this study, we explore how the UV emission can be tailored to different wavelength ranges by developing rare-earth-free materials. Specifically, we demonstrate that UV-C to UV-B emission can be achieved in transparent ZnAl2O4 - based ceramics. For the first time, such transparent ceramics have been successfully produced with the help of reactive spark plasma sintering with SiO2 or LiF additives, which modified the process of grain growth resulting in an increase in the average grain size from 250 nm to 4 mu m. Moreover, these additives also improved the optical transmission properties, with in-line transmission measurements at a wavelength of 550 nm reaching values of 50 % and nearly 65 % in the infrared region. Depending on the addition of SiO2 and LiF, the excitation by photons with energy exceeding 6.2 eV has generated the deep UV emission peaks in the range from 230 to 300 nm. Based on the analysis of the results of luminescence spectroscopy and EPR studies, the observed UV-C emission is suggested to originate from the excitons localised near oxygen vacancies. The number of vacancies is the highest in undoped ZnAl2O4 ceramics, whereas adding SiO2 and LiF suppresses their number and modifies their local environment due to changes in charge compensation, which in turn allows tuning the spectral position of the UV-C band. The quantum yield of the UV-C emission reached 5 % for undoped ZnAl2O4 ceramics. The sintered ZnAl2O4 transparent ceramic shows a versatile solution for next-generation rare-earth-free deep-UV devices.