Photoluminescence excitation enhancement

Spectroscopic properties of [Ru(bpy)3] " ", and the effects of varying the diimine ligands in [Ru(bpy)3 L ] + (L = diimine) on the electronic spectra and redox properties of these complexes have been reviewed. The properties of the optical emission and excitation spectra of [Ru(bpy)3] +, [Ru(bpy)2(bpy-d )] + and [Ru(bpy-d )3] " " and of related Os, Rh , and Pt and Os species have been analyzed and trends arising from changes in the metal d or MLCT character in the lowest triplet states have been discussed. A study of the interligand electron transfer and transition state dynamics in [Ru(bpy)3] " " has been carried out. The results of X-ray excited optical luminescence and XANES studies on a fine powder film of [Ru(bpy)3][C104]2 show that C and Ru localized excitation enhances the photoluminescence yield, but that of N does not. 

In summary, the photoluminescence of CdSe quantum dots can be strongly enhanced by nearby metal nanoparticles, where most of the enhancement results from excitation effects. We observed that the shape of the PLE spectra of the quantum dots near a metal nanoparticle is significantly altered for both gold and Ag nanoparticles, and shows a new PLE peak coincident with the LSPR peak of the metal nanoparticle. Although the absolute enhancement factor varies from one metal nanoparticle to another, the wavelength dep>endence of the total enhancement factor still mirrors the line shape of the metal nanoparticle s scattering spectrum. There may be a small offset in the maximum excitation enhancement from the nanoparticle s scattering peak (as was described for the total fluorescence in Section 4.3 above), but at present our experiments have not had sufficient spectral resolution to identify any such shift. 

Buckminsterfullerene (C50) has appeared to be an efficient photosensitizer for the PPV derivatives (61a) and (6ab). Ultrafast (> 1 ps) photoinduced electron transfer from PPV derivatives to C6o results in complete chaige separation, quenching of photoluminescence and enhancement of the photoconductivity by several orders of magnitude [429,435]. The photo-excited exciton-polaron is speculated to migrate along 50-100 monomer units of the polymer chain during its lifetime and to dissociate on encountering C o [429]. 

It is reported that the band structure of ZnS doped with transition metal ions is remarkably different from that of pure ZnS crystal. Due to the effect of the doped ions, the quantum yield for the photoluminescence of samples can be increased. The fact is that because more and more electron-holes are excited and irradiative recombination is enhanced. Our calculation is in good correspondence with this explanation. When the ZnS (110) surface is doped with metal ions, these ions will produce surface state to occupy the valence band and the conduction band. These surface states can also accept or donate electrons from bulk ZnS. Thus, it will lead to the improvements of the photoluminescence property and surface reactivity of ZnS. 

Figure 14.2 Evolution of the photoluminescence spectra from the QDs and Cy3 dyes in the QD-MBP-Cy3 assemblies versus increasing dye-to-QD ratio n (a), along with the corresponding fractional donor loss, acceptor enhancement, donor-based efficiency, and a fit of Equation (8) versus n (b). Spectra shown were corrected for direct excitation and deconvoluted. Case of 510 nm emitting QDs is shown. Adapted from reference 28 and reprinted by permission of the American Chemical Society.

Long-lived photoluminescence, at 826 nm, is reported (t 15 (is) for thin films of the processable, -conjugated polymer, poly(3-hexylthio-phene) (93JA8447). Excitation of the n—n transition with 518 nm light (So >S ) yields only very weak luminescence of 826-nm light, even at 18 K. The emission is enhanced, to point where it can be observed at room temperature, when the excitation wavelength is 250 nm, but it is completely quenched by oxygen. Prompt fluorescence decays within... 

Again, the question arises why do we see different degree of photoluminescence enhancements from the same particle as we vary the excitation wavelength Unlike the previous experiments with variety of organic fluorc hores shown in Figure 4.10, here we only use one kind of fluorc hore CdSe quantum dots... 

Figure 4.13 Enhancement of quantum dots photoluminescence near a nancqiarticle as a function of excitation wavelength for several single metal nani iarticles. Black dots are the ratio of the photoliuninescence near a nanc article to photoluminescence ir from a nanoparticle. The trace in each spectra is the LSPR...

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

As noted above, observations of large enhancements of the photoluminescence are insufficient to guarantee utility for application of plasmon-enhanced emission in OLEDs where the excited state is not photogenerated. In principle, increases in photoluminescence observed exfierimentally could be completely due to absorption enhancement. Even observation of reduced excited state lifetimes in conjunction with increased emission is insufficient to prove radiative rate enhancement since the lifetime reduction could be due to excited state quenching by the metallic surface and compensated by large absorption enhancements. 

Figure 19.5 Schematic diagram showing decomposition of total phosphorescence enhancement of PtOEP on silver films into absorption enhancement E X. ) and emissive rate enhancement E (%.2) based on the photophysical model described in the text and data from steady state and transient spectroscopy of PtOEP films with various thicknesses and excitation wavelengths as labeled. The lines represent the possible combinations that could explain the experimentally observed changes in photoluminescence where each position on the line represents a different choice of fQ, the fraction of the excited states that are quenched nonradiatively by interactions between the molecule and the metallic surface. The blue shaded region on the vertical axis is the range of possibilities allowed by constraints from extinction and excitation spectra as explained in the text. The dotted oval is what we believe to be the most likely decomposition for the 6 nm films characterized in Figure 19.4 as discussed in the text. Reprinted from reference 45 with permission of the American Chemical Society.

Figure 19.8 Photoluminescence enhancement of rhodamine monolayers covalently attached to glass with various amounts of nanotextured silver on top. (A) Extinction spectra of substrates with no silver (a) and increasing amounts of silver from (b) to (e). (B) Resulting photoluminescence from these subtrates for excitation at 450 nm. Reprinted from reference 49 with permission of the American Chemical Society.

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