Figure 2.

a) UCL spectra of NaYF₄:Yb,Er (red), LiYF₄:Yb,Er (black), and Na_xLi_yYF₄:Yb,Er (green) NCs excited by a 980 nm laser. Reproduced with permission.48 Copyright 2009, American Chemical Society. b) UCL spectra of KYb₂F₇‐based NCs excited by a 980 nm laser. c) Proposed four‐photon UC mechanism in KYb₂F₇:Er (2%) NCs following excitation with a 980 nm laser. d) Proposed excitation energy clustering in the Yb tetrad clusters of orthorhombic‐phase KYb₂F₇. Reproduced with permission.14 Copyright 2013, Nature Publishing Group. e) UCL spectra of KMnF₃‐based NCs: KMnF₃:Yb,Er, KMnF₃:Yb,Ho, and KMnF₃:Yb,Tm under excitation of a 980 nm laser. Reproduced with permission.38