Tuning the emission color and enhancement of the NIR emission of ZnO:Ca phosphors with the Ca concentration

The development of luminescent materials with tunable color emission, low cost and free of rare earths is of current interest to produce more efficient white light emitting devices. In this regard, we report the luminescent and morphological properties of ZnO nanoparticles doped with Ca concentrations from 6 to 32 mol%. The analysis by X-ray diffraction demonstrated that all the ZnO:Ca samples presented a hexagonal phase and a small content (<3%) of CaO. The morphological analysis by SEM revealed that ZnO:Ca nanoparticles are grains of irregular shape and have average sizes of 84-323 nm. In addition, the crystallite size of the nanoparticles was calculated by the Scherrer's equation and found sizes in the range of 18-24 nm. Furthermore, the optical band gap was estimated for the ZnO:Ca and it increased from 3.14 to 3.23 eV after raising the Ca concentration from 6 to 32 mol%. The ZnO:Ca nanoparticles were excited with UV light (265 and 365 nm) and produced, blue, green, yellow, red and NIR emission at the same time. The color emission was tuned from blue to red as the Ca content increases from 6 to 32 mol% (under excitation at 365 nm). If the ZnO:Ca samples are excited with 265 nm, the main emission was blue. Furthermore, the ZnO:Ca sample synthesized with 25 mol% of Ca emitted white light with CIE coordinates of (0.34,0.32) under excitation of 365 nm. Interestingly, a NIR emission centered at 705-715 nm was observed in all the ZnO:Ca samples after UV excitation and was 1-3 more intense than the visible emissions in the sample synthesized with 16 mol% of Ca (made without TEA). Thus, the results demonstrate that the ZnO:Ca is a able to produce various emission colors for lighting applications and the NIR emission could be useful for biomedical applications.