Luminescence Manipulation And Applications Of NaREF₄-Based Upconversion Materials

Rare earth(RE)upconversion nanoparticles(UCNPs)are luminescent materials with anti-Stokes shift phenomenon,which have been applied in display and anti-counterfeiting.However,due to the abundant energy levels of RE3+,UCNPs have typical multiple-emission peaks,decreasing their purity of emission colors.Limited by the type and concentration of doping ions,UCNPs usually have a fixed output color that does not change with the external stimulus,which is adverse for multicolor displays.In addition,the multiphoton process of upconversion luminescence makes UCNPs have weak luminescence intensity,limiting its application in the field of anti-counterfeiting.In this work,based on changing the relative intensities between the different emission peaks of UCNPs,the pure and controlled color-changing luminescence of UCNPs was obtained by adjusting the energy transfer distance and path inside UCNPs or introducing photonic crystal(PC)structure from outside to manipulate luminescence.Meanwhile,luminescence enhancement and angle-dependent luminescence were achieved by integrating UCNPs with PC.The traditional Na YF4 host was changed to Na Yb F4 to prepare UCNPs from the perspective of the structural composition of upconversion materials.A uniform β-Na Yb F4:Er3+(2 mol%)@Na YF4 nano-disk structure was prepared using the phase transition process of the thermal decomposition method to overcome the uncontrolled crystal morphology caused by the fast growth of β-Na Yb F4.Compared with the traditional red-green mixed light of Na YF4:Yb3+,Er3+(20:2 mol%)@Na YF4 generated by the two-photon upconversion process,a bright red emission was appeared in Na Yb F4:Er3+(2 mol%)@Na YF4 structure through a three-photon process of Er3+ due to the enhanced the energy transfer efficiency.Meanwhile,based on the power-dependent properties of multiphoton processes of Er3+ and back-energy-transfer characteristics of Er3+ to Yb3+,the luminescence color of the UCNPs gradient changed from green to red in the power density range of 0.8-12.9 W/cm2(980 nm).The prepared materials were encapsulated in NIR-LED,and a LED array was designed to realize the dynamic display of the green “walking” pattern and the red “standing” pattern under different current intensities.In addition,Na Yb F4:Er3+@Na Yb F4:Tm3+@Na YF4 was prepared by introducing the Tm3+,which can generate blue light,into the red-green color-changing system.White and adjustable emission color was achieved in the appropriate power density.UCNPs/PS PC composite structure was prepared by assembling polystyrene(PS)spheres on the surface of Na YF4:Yb3+,Er3+(20:2 mol%)UCNPs,PC endows UCNPs with angle-dependent luminescence properties.Based on the prohibition effect of photonic stop band(PSB)for the propagation of light with specific wavelengths,PCs with different PSBs can “filter” the green(540 nm)and red(654 nm)light of UCNPs respectively to obtain pure red andgreen luminescence.According to the angle-dependent properties of PSB,the blue-shifted PSB decreased the inhibitory effect of PC on the emission light,realizing the angle-dependent luminescence of UCNPs.Furthermore,UCNPs/PS-PDMS PC composite structures with matched PSB were prepared by infiltrating PDMS into the gaps between PS spheres.The angle selectivity of the upconversion luminescence was also realized due to the filtering effect of the PSB.In addition,the narrow full width at half maxima(FWHM)of the reflection spectra caused by the small refractive index difference between PS and PDMS makes the PS-PDMS PC an identifiable angle-dependent structural colors.Angle-dependent luminescence and structural colors endow the structure with excellent dual anticounterfeiting capabilities.Bilayer PC/UCNPs structure was prepared by depositing Na YF4:Yb3+,Nd3+,Er3+(30:1:0.5 mol%)@ Na YF4:Nd3+(20 mol%)(UCNPs)on the surface of photonic crystal with double PSBs to enhance upconversion luminescence using the reflection effect of PC.The traditional PC/UCNPs composite structure usually has a single PSB and requires a hard substrate such as a glass slide as a supporting material,which limits its application in anti-counterfeiting.Based on the layer-by-layer assembly of Si O2 microspheres,an opal PC/UCNPs composite structure with double PSBs was fabricated by the dip-coating method,whose PSB matched with the excitation(808 nm)and emission light(540 nm)of UCNPs synchronously.Compared with the blank sample,a 15.0-fold enhancement for the green light was achieved by the synergistic effect of the double PSBs.Furthermore,a flexible inverse opal PC/UCNPs structure with matched PSBs was prepared by filling the photocurable resin into the Si O2 opal PC/UCNPs template,which could be peeled off from the substrate.Meanwhile,the different structural colors were observed on the two sides of the flexible film: the reflection structural colors from green to blue in the specular angles on the front of the film,scattering colors from red to blue under non-specular angles on the back.The “Janus” structural colors and enhanced luminescence demonstrates excellent triple anticounterfeiting performance.