Up-Conversion Tuneability in Sol–Gel Derived SiO<SUB>2</SUB>–KYF<SUB>4</SUB> Nano-Glass-Ceramics Doped with Yb<SUP>3</SUP><SUP>+</SUP>–Ho<SUP>3</SUP><SUP>+</SUP>–Tm<SUP>3</SUP><SUP>+</SUP> | Publicación