Codoping of Yttria (Y2O3): Ho–Yb Nanoparticles with Li Increase Emitted Green Light Intensity for Security Ink and Bioimaging

Rare earth-doped up-conversion materials have attracted considerable attention because of their various applications in solid-state lasers, color displays, bioimaging, and so forth. The emission of most up-conversion materials does not have enough intensity. However, this intensity can be increased by doping of sensitizers. Ho3+-doped compounds show green emission which have various applications. Here, we have reported how the intensity of green emission can be increased by doping of sensitizers Yb3+ and Li+. In this work, we have chosen Y2O3 as the host, Ho3+ as the activator, and Yb3+ as the sensitizer and have studied their up-conversion properties under 980 nm laser. The metal citrate-gel decomposition method provides a homogeneous solid solution formation. Green light intensity increases significantly by the increase in the Yb3+ concentration as well as codoping of Li+. The green light intensity of Ho3+ for Y2O3:10 at % Yb3+–2 at % Ho3+–5 at % Li+ system is 9 times more than that of the Y2O3:10 at % Yb3+–2 at % Ho3+ system. Also, we have carried out down-conversion phenomena of Ho3+ by excitation at 448 nm. Luminescence intensity of Ho3+ in the visible range decreases with doping of Yb3+ or Li+ due to energy transfer from Ho3+ to Yb3+. Interestingly, luminescence peaks corresponding to Yb3+ (1000 nm) and Ho3+ (1200 nm) in the NIR range are also observed under 448 nm excitation. Peak observed at 1000 nm is related to the quantum cutting phenomenon. The quantum efficiency is found to be 180% from calculations. Its applications are extensively shown in different areas such as security ink, consignment identification, and bioimaging of muscle and bone through the NIR light up-conversion process.