Figure 14.

a) UC emission spectra (300–450 nm) of CaF₂: x%Yb³⁺,y%Cu²⁺ upon excitation of 978 nm laser at RT. Inset is the photo of UC luminescence. b) Schematic diagram of the energy transfer from Yb³⁺‐trimer to one Cu²⁺. Reproduced with permission.52 Copyright 2016, The Royal Society of Chemistry.