Single red emission from upconverting ZnGa2O4:Yb,Er nanoparticles co-doped by Cr3+†
Near infrared (NIR) to visible upconversion has been the focus in the optical research community considering bioimaging applications mainly because NIR excitation has a deep penetration depth, low autofluorescence and high damage threshold to tissues. The Yb3+–Er3+ sensitizer–activator combination has been the preferred choice among upconversion nanophosphors (UCNPs) since it efficiently upconverts 980 nm light to green (550 nm) and red (660 nm) emissions. However, the green emission from this Yb/Er combination is undesirable because its escape from tissues is extremely difficult, and thus may cause damage to cell/tissues, which decreases the overall bioimaging sensitivity. Also, realizing manganese-free single emission in Yb–Er-based UCNPs is of great relevance since the multiple oxidation states of manganese can sometimes disturb the sensitizer-to-dopant energy transfer (ET) kinetics. Moreover, Cr3+ has a higher absorption coefficient for green color compared to manganese ions. Therefore, in this study, we first synthesized ZnGa2O4:Yb3+,Er3+,Cr3+ (ZGO-YEC) nanoparticles (NPs) via a hydrothermal method at 120 °C with judicious Cr3+ co-doping. Through further high temperature annealing, wherein a large fraction of doping ions is included inside the host lattice, these ZGO-YEC NPs emit a bright single red emission under 980 nm excitation. This process is facilitated by the absorption of green emission via the Er3+ → Cr3+ ET, followed by back ET to the 4F9/2 level of Er3+. They also directly emit NIR emission under 254 nm excitation. This luminescing ability can only be deciphered when dual NIR and UV excitations exist simultaneously, which makes illegal imitation very difficult to implement, thus improving the level of anticounterfeiting. This work provides an effective approach to obtain manganese-free UCNPs with bright red UC luminescence for the first time, which may be suitable for biological imaging, security applications, and optical probes for magnetic resonance imaging.