Upconversion nanocrystals (UCNCs) hold promise for bioimaging, solar cells, photocatalysis and volumetric displays. However, their upconversion luminescence intensities are usually low due to the weak and narrowband near-infrared absorption of lanthanide ions. Herein, we introduce and validate a strategy to hugely enhance upconversion luminescence intensity by using an organic near-infrared dye as an antenna to sensitize core/shell UCNCs. The dye can increase absorptivity and broaden the absorption spectrum of the UCNCs. Such dye sensitization, in combination with a core/shell structure, can tremendously enhance the upconversion luminescence (UCL) intensity of the UCNCs. The UCL intensity of dye-sensitized UCNCs excited at 820 nm is 800-folds higher than that of pure UCNCs excited at 980 nm. Further enhancement can be obtained by optimization of the dye emission and UCNC absorption spectral overlap. Moreover, the proposed approach can be extended to cover any part of the solar spectrum by using a set of dyes. This work provides new insights into the efficient enhancement of upconversion luminescence of the UCNCs and facilitates their applications.

Upconversion nanocrystals have a lot of advantages over other fluorescent materials. However, their applications are still limited due to their comparatively low upconversion luminescence (UCL). In the present study, a novel nanocomposite of Ag/graphene@SiO2-NaLuF4:Yb,Gd,Er for enhancing UCL was fabricated successfully, and its morphology, crystalline phase, composition, and fluorescent property were investigated. It is interesting to find that the Ag/graphene@SiO2-NaLuF4:Yb,Gd,Er and Ag@SiO2-NaLuF4:Yb,Gd,Er nanocomposites showed high UCL enhancements of 52- and 10-fold compared to the control of Ag-free nanocomposite SiO2-NaLuF4:Yb,Gd,Er, respectively. The enhancement of 52-fold is greater than those reported in our previous studies and some papers. Moreover, the measured life times of the Ag-presented nanocrystals were longer than that of Ag-absent counterparts. These enhancements of UCL can be ascribed to the effect of metal-enhanced fluorescence, which is caused by the enhancement of the local electric field. The UCL intensity of Ag/graphene@SiO2-NaLuF4:Yb,Gd,Er was 5.2-fold higher than that of Ag@SiO2-NaLuF4:Yb,Gd,Er, indicating that graphene presented in the fabricated nanocomposite structure favors metal-enhanced UCL. The small-sized Ag nanoparticles anchored on the graphene sheet mutually enhanced each other's polarizability and surface plasmon resonance, resulting in a big metal-enhanced UCL. This study provides a new strategy for effectively enhancing the UCL of upconversion nanocrystals. The enhancement potentially increases the overall upconversion nanocrystal detectability for highly sensitive biological, medical, and optical detections.

A novel nanocomposite photocatalyst NaLuF4:Gd,Yb,Tm@SiO2@Ag@TiO2 was developed for the first time. This composite material has a sandwich structure, including a NaLuF4:Gd,Yb,Tm upconversion nanocrystals (UCNCs) core, a media shell of amorphous SiO2 decorated with Ag nanoparticles, and an outer shell of anatase TiO2. The designed new structure takes advantage of the synergetic effect of UCNCs, Ag nanoparticles and TiO2. The UCNCs absorb near-infrared (NIR) light and transfer energy to TiO2, which extends the light responsive range of TiO2 to the NIR region. Ag nanoparticles not only enhance upconversion luminescence of the UCNCs but also enhance light harvesting and improve charge separation of TiO2. The results of photocatalytic applications show that the as-prepared catalyst has high photocatalytic activity. This study provides new insights into the fabrication of TiO2-based nanocomposite photocatalysts with high catalytic efficiency through effective integration of upconversion material, noble metal and TiO2 into a hetero-composite nanostructure.

A novel double-shell-structured β-NaLuF 4 : Gd,Yb,Tm@SiO 2 @TiO 2 :Mo nanocomposite photocatalyst has been developed for the first time. The nanocomposite consists of uniform β-NaLuF 4 : Gd, Yb, Tm upconversion nanocrystals as core, a media shell of SiO 2 , and a lot of small sized anatase TiO 2 nanoparticles as the outer shell. The TiO 2 shell is modified by Mo-doping, which can narrow the band-gap of TiO 2 and act as electron traps resulting in enhancement of light absorption and reduction of recombination rate of the photogenerated carriers. The upconversion nanocrystals can convert NIR into UV and visible lights which are overall absorbed by the modified TiO 2 shell. Photocatalytic activities of the prepared products are evaluated through photocatalytic degradation of RhB under the irradiations of simulated solar and NIR lights. The results show that the as-prepared nanocomposite displays a high photocatalytic activity which is significant higher than that of commercial P25 and pure TiO 2 . This work provides new insights into the fabrication of TiO 2 -based composites as high performance photocatalysts and facilitate their application in environmental protection issues using solar light.

Upconversion nanocrystals (UCNCs) have a lot of advantages over other fluorescent materials. However, their applications are still limited due to the relatively low upconversion luminescence (UCL). In the present study, a novel core/shell structural upconversion nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄:Yb, Pr was prepared successfully with a facilely solvothermal method and the properties of the nanocrystal were investigated. It was interesting to find that the as-prepared nano-crystal showed excellent UCL. Its UCL intensity was 65, 44, and 20 times higher than those of core nanocrystal NaYF₄:Gd, Yb, Er, core/inert-shell nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄, and core/active-shell nanocrystal NaYF₄:Gd, Yb, Er@NaYF₄:Yb, respectively. Moreover, the measured UCL lifetime of the as-prepared nanocrystal was longer than those of the controls. The mechanism of the UCL enhancement of the product was discussed in detail. The high UCL of the as-prepared product could be ascribed to the function of the shell, energy transfer from the active shell to the core, and Pr³⁺-doping in the shell. This study provided a new insight into the fabrication of core– shell structural upconversion nanocrystals with high UCL. The high UCL potentially increases the overall upconversion nanocrystal detectability for highly sensitive biological, medical, and optical detections.