Studies Of Noble Metal Nanoparticles Sensitized Up-conversion Fluorescence Enhancement

Rare-earth（RE） doped upconversion nanophosphors（UCNPs） are currently attracting considerable interests due to special nonlinear processes and broad application potentials in the fields of in vivo fluorescence imaging, bio-sensing, infrared photo-dynamical therapeutics, infrared detection, anticounterfeiting, fingerprint identification, solar cells and etc,which has been an important branch in the field of luminescence. In recent years, the development of nanotechnology has poured into novel activity and motivation for this field. The traditional methods for solving this problem are controlling the size, structure and surface of materials and altering the crystal field of host to enhance the upconversion luminescence（UCL） signals. However, due to lower UCL efficiency and smaller excitation cross-sections for the 4f-4f transitions of RE-doped UCNPs, the practical applications have been largely limited. In this work we reported the modulation of noble metal Au on the UC broadband emission of rare earth oxides Nd2O3, and the modulated local optical field of Na YF4:Yb³⁺,Tm³⁺ utilizing the surface Plasmon of Au-Ag alloy nanostructures. In addition, we revealed the physical essential of local field coupling with upconversion nanophosphors in depth. On this basis, we applied these composite films to the fingerprint identification. We aslo investigated the transparent Li YF4:Yb³⁺,Er³⁺ single crystal as an independent luminescent upconverter to improve the power conversion efficiency（PCE） of perovskite solar cells. The mian results are listed as follows:[1] Nd2O3/Au nanocomposites were prepared by a co-precipitation process, with excitation bands ranging of 780-980 nm, and UCL of a white broadband. The coupling of surface plasmon of Au nanorods with the UCL of Nd2O3 suppressed the threshold power of generating broadband and improved the UCL intensity. And more, the UCL enhancement was strongly dependent on the doped concentration of Au nanorods and the excitation power of 980 nm and 808 nm laser diodes. The optimum UCL enhancements are respectively, 11-fold and 9-fold under 980 nm and 808 nm excitation. In addition, the upconversion broadband emission and UCL enhancement mechanism of Nd2O3/Au nanocomposites were investigated.[2] The “island” Au-Ag alloy films were synthesised through a PMMA template method and the size of the alloy nanoparticles was controlled through the ratio of Ag NO3 to HAu Cl4 concentration（100-1000nm）. It was observed that the UCL enhancenment factor increased with the increase of alloy size and diffuse reflection, with the decrease of Na YF4 size, and the excitation power density, and with the increased order of multi-photon process. The UCL enhancement maintly originated from the coupling of the local surface plasmon resonant with the excitation electromagnetic field. The optimal enhancement factor was obtained as high as 180 folds. The Au-Ag/Na YF4:Yb,Tm composite film was applied to the high resolution patterns of the fingerprint identification.[3] Li YF4: Yb³⁺, Er³⁺ single crystal was prepared by an improved Bridgman method and applied to improve the PCE of perovskite solar cells as an independent luminescent upconverter. Its internal quantum efficiency was determined to be 5.72% under 6.2 W cm-2 980 nm excitation. When the single crystal was placed on front of the perovskite solar cells, the PCE was enhanced by 7.9% under the irradiation of simulated sunlight with 7-8 solar constants.