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

Abstract

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, ∼330 nm) to near infrared (NIR, ∼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 ³F₄, ³H₄, ¹G₄ and ¹D₂, 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.