ENHANCED TWO-PHOTON PHOTOLUMINESCENCE IN THE COUPLED NOBLE METAL NANOPARTICLES

Abstract

This dissertation is the result of four years research in the field of photoluminescence in plasmonic nanoparticles (NPs). Coupled plasmonic nanoparticles were found to show greatly enhanced two-photon photoluminescence due to strong local field enhancement by plasmon coupling. In this thesis, this phenomenon was systematically studied both in the fundamentals and their potential applications. Firstly the interactions between noble metal nanoparticles and conjugated polymers were investigated. The positively charged polymers induced the coupling of Ag and Au NPs. The NPs effectively quenched the fluorescence of the polymers. Meanwhile, the two-photon photoluminescence of the NP themselves was dramatically enhanced by the plasmon coupling with an enhancement factor of 46 folds. Then TPPL in coupled Au nanocubes (NCs) induced by cysteine was studied. The coupling selectively enhanced the TPPL from X band transition with an enhancement of 60 folds due to the selectively enhanced emission quantum yield while the L band emission remains nearly unchanged. The band selective enhancement was thus demonstrated as a novel sensing method for cysteine detection. In section 3, we systematically studied the coupling enhanced TPPL in the single particle level. The TPPL intensity significantly increases from Au nanosphere monomer to trimer. A highest enhancement of 1.2 × 105 folds was obtained for the linear trimer. The TPPL spectra closely resemble their corresponding scattering spectra with a cos4 dependence on the excitation polarization to the coupling axis, suggesting the strong coupling of two-photon excitation to the longitudinal SPR of coupled NPs. In section 4, we discovered strong two-photon white light continuum in the thermally annealed metal ink film made of 5nm oleylamine capped Au NPs. Owing to dramatically enhanced excitation efficiency and emission quantum yield induced by the plasmon coupling, a 3350-fold TPWLC was obtained in the annealed film. The annealing could also be achieved by the photothermal effect of high power lasers, allowing us to sinter nanoscale structures in microscope. The recorded information could then be read out by the two-photon raster scanning. In addition, the nanoscale circuit is highly conductive, showing appealing application potential in the nanoscale data storage and electronic device fabrication.