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 NaGdF: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 Ho and Tm emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm and Ho 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.