Photoluminescent Enhancement of Ruthenium Complex Monolayers by Surface Plasmon Resonance of Silver Nanoparticles
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Photoluminescent Enhancement of Ruthenium Complex Monolayers by Surface Plasmon Resonance of Silver Nanoparticles
Shanlin Pan, Zhenjia Wang and Lewis J. Rothberg* Chemistry Department, University of Rochester, Rochester, NY 14627, U.S.A. Abstract The photoluminescence intensity from a bis (2,2'-bipyridine)-(5–isothiocyanato-phenanthroline) Ruthenium (RuBICP) monolayer covalently bound to a glass substrate is found to be enhanced up to 20 times when silver nanoparticles are deposited on top. The emission spectra are blue shifted by interactions with the silver nanoparticles. Field enhancement is found to be able to enhance the radiative decay rate by over three orders of magnitude but fewer than 2 % of the molecules in the sample experience this large enhancement. Increased absorption rates also increase the luminescence but to a much smaller degree. Introduction
Glass
Surface plasmon resonance of metallic Ag Ag RuB ICP nanoparticles has been of great research interest for developing novel nano-optical devices and (3-mercaptopropyl) trim ethoxysilane probes to explore and understand important biological processes. It has been widely accepted Glass N that enhancement of optical processes at metallic S Ru (bpy)2 (II) surfaces [1-15] is primarily due to O C O Si NH N NH electromagnetic properties of metal O nanoparticles, although there is also some evidence for chemical enhancement of Raman Figure 1. RuBICP monolayer preparation scheme processes. In the case of increased luminescence and scheme with silver nanoparticle assembled on near metals, increases in absorption due to large top. local field enhancements, modifications of emission rates by coupling to surface plasmons and quenching of the excited state by charge and energy transfer are all relevant [3-5]. Quenching tends to dominate when dyes are attached to metal surfaces or only a few atomic distances away. If the dye and metal particle are separated further, the emission intensity can be increased if we photo-excite the dye-particle system at energies where both metal plasmon resonance and dye absorption can be triggered [6]. We will refer to concentration of the incident field by the plasmon scattering from the surface of metal as "absorption enhancement” and the increased radiative decay rate due to coupling of the molecular excited state with the metal plasmons as "emission *
Corresponding author, email: [email protected]
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enhancement" [7]. Emission enhancement plays an especially central role for photoluminescent enhancement of molecules with low emission quantum yield [7-8]. While the mechanism is complex, one can imagine the particles serve as an antenna to cause radiation to occur faster relative to non-radiative decay by borrowing oscillator strength of the resonating dipoles from the metal nanoparticles. Emission from weakly photoluminescent substances can be increased if they are positioned at an appropriate distance from a metal surface or colloid. Since most materials are only weakly luminescent, this has potential
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