The Aspect Ratio Dependence of the Fluorescence of Gold Nanorods: An Experimental and Theoretical Study
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The Aspect Ratio Dependence of the Fluorescence of Gold Nanorods: An Experimental and Theoretical Study Susie Eustis and Mostafa A. El-Sayed Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Tech, Atlanta, GA 30332 ABSTRACT Experimental observations and theoretical treatments are carried out for the band shape and relative intensity of the emission from gold nanorods of various aspect ratios in the range between 2.6 (1.5 theory) and 6.3 (9 theory). The calculation of the fluorescence spectra requires knowledge of the nanorod size distribution, the enhancement factors and the shape of the un-enhanced fluorescence spectrum. The comparison between the observed and calculated fluorescence band shapes is found to be good. The calculated changes in the relative intensities with aspect ratios are found to be much greater than that observed experimentally. This is due to the fact that for the observed emission of all the rods studied, nonradiative processes dominate the relaxation mechanism of the excited state, a fact that was not included in the theoretical treatments. Experimental results and theoretical treatments will be presented. INTRODUCTION Gold nanoparticles have attracted a lot of attention due to the strong surface plasmon resonance responsible for optical properties.[1, 2] The free d-electrons in the metal are able to oscillate in resonance with the wavelength of light governed by the shape, size and dielectric constant of the particle as well as the dielectric constant of the surrounding material. Gold nanorods have two plasmon resonances, the transverse plasmon at 520nm along the short axis of the rod, and the longitudinal plasmon at longer wavelengths that is very sensitive to aspect ratio (length divided by width). Bulk gold fluorescence, first observed by Mooradian in 1969,[3] is very weak with a quantum yield of 10-10. Recent research has produced a number of reports of fluorescence enhancement in nanoparticles[4-16] as well as a background emission believed to be fluorescence in SERS.[17-22] Gold nanoclusters fluoresce in the visible and near IR with quantum yields[4-8] of up to 10-3. Emission from gold clusters was first observed[4] in 1998, by Wilcoxon et. al. where small gold clusters were found to fluoresce with quantum efficiencies in the 10-4 - 10-5 range. Recently emission from a single species of gold clusters has been identified.[8, 9] The emission wavelength changes with the size of the cluster, with Au8 emitting light below 500nm, and Au28 emitting light above 800nm.[8] The emission of gold nanorods is attributed to same mechanism as observed in bulk gold, a recombination between the electrons and holes in the interband. Previous studies[15] of our group found large enhancement in fluorescence emission for electrochemically synthesized rods, but not spheres. The quantum yields of the nanorod emission were 10-3 –10-4. [15] Theoretically the enhancement of the emission from nanorods was modeled using a lighting rod effect[23]
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