Upconversion Luminescence Performance Of Rare Earth Materials Enhanced By Surface Plasmon Of Cuprous Sulfide

Due to its good photostability,large anti-Stokes shift,low biotoxicity,narrow emission bands,and stable physicochemical properties,rare earth doped upconversion materials are widely used in display lighting,solid-state lasers,sensing and detection,solar cells,and bioluminescence.However,the disadvantages of low absorption cross-section and low luminous efficiency have limited its further applications.So,based on the localized surface plasmon resonance（LSPR）effect of semiconductor cuprous sulfide,an in-depth exploration has been carried out on how to achieve efficient upconversion luminescence of rare earth-doped materials by the regulation of local electromagnetic field.The Gap type composite structure（mainly refers to the nanoscale distance between metal nanostructures and rare earth upconversion materials,which is generally realized by using metal oxide,SiO₂or air as the isolation layer）,a hybrid structure of semiconductor-insulator-semiconductor,and a core-shell composite structure were successively prepared for upconversion luminescence.Through structural design and optimization,the local electromagnetic field of rare earth ions in upconversion luminescence was regulated to realize efficient upconversion luminescence of rare earth materials.Besides,high-performance rare-earth-doped fluoride luminescence materials were prepared,and the effect of cuprous sulfide nanostructures on upconversion luminescence of rare earth materials was systematically studied.Furthermore,the upconversion luminescence enhancement mechanism of the above composite structures was also discussed.The main contents were shown as follows:1.A Gap type composite structure was constructed by combining cuprous sulfide nanoparticles with cubic NaYF₄:Er/Yb nanoparticles for upconversion luminescence.Based on the Gap type composite structure,the electromagnetic field around NaYF₄:Er/Yb nanoparticles was enhanced,and the upconversion luminescence intensity of NaYF₄:Er/Yb nanoparticles was improved.The details were listed as follows:（1）The optical absorption characteristics of cuprous sulfide nanoparticles with different copper-sulfur ratios were summarized and analyzed.Based on the Gap type composite structure,Cu₈S₅nanoparticles were optimized to enhance the upconversion luminescence of rare earth materials.（2）The luminescent performance of the Gap type composite structure have been fully studied.The results showed that compared with that of NaYF₄:Er/Yb-SiO₂nanoparticles,the fluorescence intensity of the Gap type composite structure at red light was increased by 5.8 times.（3）The electromagnetic field distribution of the Gap type composite structure was obtained by the finite difference time domain method（FDTD）,and the upconversion luminescence enhancement mechanism in the composite structure was systematically investigated.2.On the basis of the Gap type composite structure,a self-assembled cuprous sulfide film was introduced,and a new semiconductor-insulator-semiconductor（i.e.,cuprous sulfide nanoparticles-rare earth upconversion materials-cuprous sulfide film）composite structure was further constructed.The internal optical field of the composite structure was amplified by excitation of the local effect of the cuprous sulfide film on the incident light and the LSPR effect of the cuprous sulfide nanoparticles,thus the fluorescence enhancement of the rare earth upconversion material was further achieved.The details were shown below:（1）A semiconductor-Er₂O₃-semiconductor composite structure was constructed by combining cuprous sulfide nanoparticles and the Er₂O₃-SiO₂particles for upconversion luminescence.The results showed that the fluorescence intensity of the semiconductor-Er₂O₃-semiconductor composite structure was 16.2 times higher than that of Er₂O₃-SiO₂particles at red emission band of ~4F_9/2-~4I_15/2.（2）NaYF₄:Er/Yb particles with high-quality hexagonal phase were synthesized,and NaYF₄:Er/Yb was substituted for Er₂O₃to construct a semiconductor-Nay F₄-semiconductor composite structure for upconversion luminescence.The results showed that compared with that of NaYF₄:Er/Yb-SiO₂particles,the luminescence intensity of the semiconductor-NaYF₄-semiconductor composite structure in the green emission peak was increased by 12.5 times,and the corresponding quantum yield reached to 0.89%.3.Because the Gap type composite structure and the hybrid structure of semiconductor-insulator-semiconductor both have high scattering loss to excited light,a core-shell composite structure with cuprous sulfide nanoparticles as the core and rare earth upconversion materials as the outer layer was constructed.By optimizing the structure and taking full advantage of the absorption and scattering of incident light by cuprous sulfide nanoparticles,the further amplification of the internal optical field in the rare earth upconversion materials was successfully achieved,and finally the efficient upconversion luminescence of rare earth materials was realized.The details were shown below:（1）The core-shell-shell structure of Cu₈S₅-SiO₂-Er₂O₃composite was synthesized for upconversion luminescence.The results showed that the internal optical field of the core-shell structure can be effectively amplified based on the LSPR effect of cuprous sulfide nanoparticles.（2）An embedded composite structure with multiple Cu₈S₅-SiO₂nanoparticles as the core and upconversion luminescent material as the outer layer was prepared,and the effect of multiple cuprous sulfide nanoparticles on the internal optical field of the composite structure was studied.The results showed that under the irradiation of excited light,highly dense hotspot structures（small area with strong local electromagnetic field）were generated between cuprous sulfide nanoparticles by inducing the LSPR effect of cuprous sulfide nanoparticles,thereby effectively amplifying the internal optical field of the composite structure,making the fluorescence enhancement factor of the embedded composite structure reach 45 times.（3）A multi-core embedded composite structure with multiple cuprous sulfide nanoparticles and NaYF₄:Er/Yb particles as the core and amorphous silicon oxide as the outer layer was constructed for upconversion luminescence.The results showed that compared with that of NaYF₄:Er/Yb-SiO₂particles,the fluorescence intensity of the multi-core embedded composite structure in the green emission peak was increased by 42 times,and the corresponding quantum yield reached to 1.32%.This was significantly better than the reported homogeneous core-shell structure of m Cu_2-xS@SiO₂@Y₂O₃:Yb³⁺/Er³⁺nanocomposites（its fluorescence enhancement factor and the corresponding quantum yield are 35-folds and 0.085%,respectively）.In addition,due to its unique structure,the multi-core embedded composite structure exhibited the advantages of low toxicity,high-efficiency upconversion luminescence properties,and photothermal properties,which suggested that the composite structure should be promising in a wide variety of biological application.