The influence of Nb2O5 crystallization on the infrared-to-visible upconversion in Er3+/Yb3+ co-doped SiO2-Nb2O5 nanocomposites

This study reports on how the niobium content influences the infrared-to-visible upconversion (UC) in Er3+/Yb3+ co-doped SiO2-Nb2O5 nanocomposites produced by an alternative sol-gel route. Raman spectroscopy, Transmission electron microscopy (TEM), and Photoluminescence spectroscopy (PL) helped to evaluate the structural and luminescent properties. The Nb2O5 content impacted the nanocrystallization process-different Nb2O5 polymorphs emerged, which affected the UC luminescence. An outstanding color tunability effect arose due to the unique structural features of the nanocomposites. Different green and red emissions appeared depending on the excitation power and on the Nb content. The Nb content of 30 mol% afforded luminescence of similar intensity in the green and red range. For higher than 40 mol% of Nb concentration, the luminescence was more intense in the red range. The green and red emissions indicated that an Energy Transfer Upconversion (ETU) process with Yb3+→ Er3+ energy transfer occurred efficiently. In addition, cross-relaxation process between Er3+ ions is also accounted for the red emission.