Quantum Efficiency and Enhancement

In two more recent papers on single molecule fluorescence near thin metallic layers, Enderlein reminds us that the quenching by metal nanoparticles shortens the lifetime of the excited state and by doing this, increases the number of excitation cycles that the molecule can survive before it is photobleached. This is an extremely important point, especially when dealing with single molecules, as fluorescence quantum yields are not nearly as important as the number of photons that are emitted before photobleaching occurs. 

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FIGURE 10.14 PEDT PSS as hole injection layer for polymeric emitters enhanced luminance, enhanced quantum-efficiency, and enhanced power-efficiency. 

Figure 14,13 PEDOT PSS as hole injection layer for polymeric emitters enhanced luminance, enhanced quantum efficiency and enhanced power efficiency. From 5. Kirchmeyer and K. Reuter, Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophenes), J. Mater. Chem. 15, 2077 2088 (2005). Reproduced by permission of The Royal Society of Chemistry...

The dyes used most frequently as dopants are rubrene (61) and the dicyano-methylene dyes (62). Rubrene is able to convert the Alq3 fluorescence to yellow (570 nm), DCM 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-477-pyran, (62a) and its derivatives, 62b-d [149] to orange (600 nm). The quantum efficiencies are enhanced strongly to values near 100% upon doping with the dyes. For rubrene in Alq3, a quantum efficiency of 95 5% was reported [139]. 

A fluorescence decay of the ensemble of many CV molecules on a PMMA film is shown in Fig. 12. The decay was not fitted to a single exponential function and a stretched exponential function, but was well fitted to a biexponential function I(t) = Af exp(-t/xf) + Asexp(—t/ts), where tf and ts are time constants and Af and As are pre-exponential factors. We obtained tf = 0.43 ns and ts = 1.76 ns, and the ratio As/Af = 1.14. Compared with the excited state lifetime of CV (2-3 ps) in methanol and ethanol [5-8,58-68], the fluorescence lifetime of CV on a PMMA film increased more than two orders of magnitude [9-12] thus, so did the fluorescence quantum efficiency. The enhancement of the fluorescence efficiency of CV on a PMMA film made it possible to observe single CV molecules. Figure 13 shows fluorescent spots on a PMMA film on which a drop of 1-nM CV in methanol was spin-coated. The number of fluorescent spots in an image linearly increased with increasing concentration of a CV methanol... 

The detected fluorescence can be significantly enhanced, however, by exploiting the plasmonic enhancement which can occur when a metal nanoparticle (NP) is placed in the vicinity of a fluorescent label or dye [1-3]. This effect is due to the localised surface plasmon resonance (LSPR) associated with the metal NP, which modifies the intensity of the electromagnetic (EM) field around the dye and which, under certain conditions, increases the emitted fluorescence signal. The effect is dependent on a number of parameters such as metal type, NP size and shape, NP-fluorophore separation and fluorophore quantum efficiency. There are two principal enhancement mechanisms an increase in the excitation rate of the fluorophore and an increase in the fluorophore quantum efficiency. The first effect occurs because the excitation rate is directly proportional to the square of the electric field amplitude, and the maximum enhancement occurs when the LSPR wavelength, coincides with the peak of the fluorophore absorption band [4, 5]. The second effect involves an increase in the quantum efficiency and is maximised when the coincides with the peak of the fluorophore emission band [6]. 

One possible explanation of the data presented here is that fluorescence normally observed in the absence of aluminum is quenched as AF" complexes are formed. Simultaneously a new fluorescence peak for the complex may appear at slightly shifted wavelengths. The complex may have a higher quantum efficiency and therefore show greatly enhanced fluorescence. This hypothesis may be useful in... 

Liu, C.-y., Bard, A. J. (1998). Enhanced quantum efficiencies and short-circuit photocurrents in solid porphyrin thin film cells by internal electric lields,./. Am. Chem. Soc., 120 5575. 

Because of the relatively high regularity in the micellar core some asymmetric reactions occur with an enhanced stereoselectivity. Other typical effects of organized surfactants are the alteration of chemical and photophysical pathways, of quantum efficiency and ionization potentials, of oxidation and reduction properties, and finally of products and charges [6]. The morphology of micelles can change to rod-like structures at increasing concentrations of surfactants. The suitability of micellar... 

The use of an extrinsic photoconductor with direct injection has the advantage that dc gain can enhance I and therefore g, but gain saturation due to sweepout will limit ac gain to 1/2 at frequencies near f [6.30]. The capacitance C of an extrinsic photoconductor can be an order of magnitude lower than for a photodiode, which will lead to a higher /. With an extrinsic photodetector, crosstalk problems must be considered for the detector thicknesses necessary to provide reasonable quantum efficiency, and even for the 3-5 pm window operating temperatures will tend to be below 50 K. 

Gelloz B, Nakagawa T, Koshida N (1998b) Enhancement of the quantum efficiency and stability of electroluminescence from porous silicon by anodic passivation. Appl Phys Lett 73(14) 2021-2023... 

A decrease in exposure wavelength translates to new types of photoresists that will be stable upon contact with higher energy radiation. Whereas the DNQ photoresist system is effective in the MUV range, photoresists that feature chemical amplification (CAM) moieties are used exclusively for DUV and shorter wavelengths.The enhancement of quantum efficiency and sensitivity results... 

In this chapter, a brief theoretical overview is provided that discusses, among other things, EM enhancement of emission, enhanced absorption, quenching to metal surfaces, the distance, coverage, and temperature dependence of SEF, and the effects of quantum efficiencies on enhancement. Also discussed, is the preparation and characteristics of several different nanoparticle metal substrates that have been employed in the collection of SEF, and the surface-enhanced fluorescence of Langmuir-Blodgett (LB) monolayers. Finally, a summary of these concepts is presented, and the future of SEF is discussed. 

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