Insight into the Upconversion Luminescence of Highly Efficient Lanthanide-Doped Bi2O3 Nanoparticles

A series of Bi2O3 nanoparticles doped with Yb3+ and Ln3+ (Ln3+ = Er3+, Ho3+, Tm3+) ions were prepared by means of a Pechini-type sol–gel synthesis in order to develop novel approaches for the realization of high-performing upconverting nanophosphors, with controlled chromaticity output and enhanced emission efficiency. The overall upconversion mechanism originating the observed luminescence spectra is strongly influenced by the narrow bandgap of the Bi2O3 matrix (about 2.6 eV when doped at 10–12 at %) since the occurrence of optical band-to-band transitions sets such an upper energy threshold to the activation of those upconversion features characterizing the spectrum of the different Yb3+–Ln3+ systems. Moreover, as emerging from diffuse reflectance analysis performed on a series of Yb3+, Er3+ codoped samples with Yb content in the 0–20 at % range, the Bi2O3 energy gap can be properly tuned by varying the overall dopant concentration. This evidence suggests a strategy to achieve (i) chromaticity output control and (ii) the realization of single-band emitters. Concerning the last point, important results were achieved for Yb3+–Er3+ and Yb3+–Tm3+ codoped samples that behave nearly monochromatic in NIR-to-red and NIR-to-NIR upconverters under 980 nm light exposure, respectively, with significant damping of those radiative components in the blue-green part of the visible spectrum. Furthermore, the emission mechanism for the investigated systems is characterized by a remarkable quantum efficiency value, a fundamental parameter in view of possible application in bioimaging or anticounterfeiting fields.