Concentration-controlled emission in LaF3:Yb3+/Tm3+ nanocrystals: switching from UV to NIR regions

The homogeneous LaF3:Yb3+/Tm3+ nanocrystals (NCs) with an average diameter of 5 nm and different doping concentrations (Yb3+: 2-20%, Tm3+: 0.2-1.5%) were synthesized by a simple solvothermal approach. Their emission spectra, luminescent dynamics, and power-dependence of up-conversion (UC) photoluminescence were investigated in detail. Under 980 nm excitation, as-synthesized NCs exhibited interesting emission properties:a broad range of photoluminescence spanning from the ultraviolet (UV, similar to 330 nm) to near infrared (NIR, similar to 1700 nm) region, as well as the tunable relative intensity of different excited states. These unique UC or down-conversion (DC) emission behaviors were strongly influenced by the doping concentration of Tm3+ and Yb3+ ions within LaF3 NCs. With the increase of the concentration ratio between Yb3+ and Tm3+ ions, the NIR emissions were highly suppressed, but the UV and visible emissions increased quickly. The result demonstrated that such concentration effect was mainly originated from two mechanisms:one was the competition mechanism among the different intermediate exciting levels such as F-3(4), H-3(4), (1)G(4) and D-1(2), and the other was the cross-relaxation mechanism between the Tm3+ ions. By optimizing the concentration of Tm3+ and Yb3+ ions, multi-photon UC or DC luminescence properties of LaF3:Yb3+/Tm3+ NCs could be facilely tuned from UV to NIR, which will be helpful for their further applications in the field of sensing, imaging, labeling, lighting and displays, etc.