Emission color tuning of core/shell upconversion nanoparticles through modulation of laser power or temperature

Upconversion nanoparticles (UCNPs) are an excellent choice to construct security features against counterfeiting, owing to their unique NIR-to-VIS upconversion luminescence (UCL) characteristics. However, the application of upconversion materials is limited, due to their single and invariant emission colors. Herein, the temperature-dependent UCL properties of NaGdF₄:Yb/Ho (or Tm) UCNPs in the solid state have been investigated. An anomalous UCL enhancement at higher temperatures has been demonstrated for these small-sized (<10 nm) UCNPs and the underlying mechanism is discussed herein. Meanwhile, effective UCL with tunable multicolor emissions has been realized by the rational incorporation of Ho³⁺ and Tm³⁺ emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F₄ UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm³⁺ and Ho³⁺ emitters to the excitation power density and temperature. The power- and temperature-responsive color shifts of these Ho/Tm co-doped UCNPs are favorable for immediate recognition by the naked eye, but are hard to copy, offering the possibility of designing more secure anti-counterfeiting patterns.