Strong resonant coupling between light and plasmons in noble metal particles of nanometer dimensions leads to a number of striking and technologically important optical effects, among them surface enhanced Raman scattering (SERS) [1] and the enhancement of fluorescence from nearby molecules. [2] While each of these show great promise for the development of highly sensitive biochip detectors, [3,4] fluorescence is the technique of choice for many biological assays. Significant enhancement here would greatly enhance the sensitivity of these assays to a host of target biomolecules. To date, the maximum enhancement available in fluorescence has not been established. This is largely due both to difficulties in controlling the size and shape of the particles, and to the multiplicity of contributing factors: increased radiative decay rate and enhanced electric fields at resonance, ''hot spots'', i.e. regions of high field between closely spaced particles. The substrate is known to play a role as well; in particular there have been suggestions that certain substrates might play an active role in light-plasmon coupling rather than merely shifting the resonance frequency, as would a conventional glass substrate. Here we report observations of just such an effect, in which the size dependence of the enhancement of fluorescence from monodisperse silver nanoparticles is profoundly altered by the use of a Si substrate. Comparing fluorescence measurements with calculations of the response of the silver nanoparticles to incident light, we show that unlike what is commonly assumed, the variation of the fluorescence enhancement with nanoparticle diameter does not simply follow that of plasmon excitation as measured by the optical extinction. Instead we find that it is the generation of regions of high electrical field intensity near the particle which dominates the fluorescence enhancement we observe, and that the silicon substrate plays an active role in this regard: sweeping these regions out from beneath the particles as their size approaches the optimum for fluorescence. ResultsIn Figure 1 we compare scanning electron microscopy images and fluorescence microscopy images from size-selected, fluorescently-tagged silver nanoparticles on a silicon substrate over a range of particle diameters. In Figure 1(a)-(e) the SEM images are shown in red, and fluorescence images for an excitation wavelength of 514 nm (exciting the Cy3 fluorophore), scanned from precisely the same regions are superposed in green. The SEM images show that particles are spherical, and randomly distributed. There are rare occasions where two or more particles are in contact; this occurs more frequently on samples with larger average particle diameter, as shown in Figure 1(e). In general, however, individual particles are well separated from their neighbors. The fluorescence intensity shows a strong and systematic variation from image to image, i.e. as the average diameter of the Ag nanoparticles is varied. Highest intensities are obtained from samples with partic...

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

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Abstract:We report on the effects of enhanced absorption/scattering from arrays of Au nanopillars of varied size and spacing on the spectral response of a P3HT:PCBM bulk heterojunction solar cell. Nanopillar array-patterned devices do show increased optical extinction within a narrow range of wavelengths compared to control samples without such arrays. The measured external quantum efficiency and calculated absorbance, however, both show a decrease near the corresponding wavelengths. Numerical simulations indicate that for relatively narrow nanopillars, the increased optical extinction is dominated by absorption within the nanopillars, rather than scattering, and is likely dissipated by Joule heating.