Enhancement of the decay rate by plasmon coupling for Eu ³⁺ in an Au nanoparticle model system

Abstract

-1Lanthanides are an interesting class of ions for applications in a variety of lightemitting materials because of their rich energy level structure. For many applications, control over the transition rates between specific energy levels is desirable, to be able to tune the emission output and the absorption strength. One method to achieve this is by bringing lanthanide ions close to a metal nanoparticle and use plasmon coupling to modify transition rates. In this letter we present measurements on a model system consisting of silica-coated gold nanoparticles for which lanthanide complexes have been incorporated in an amphiphilic layer surrounding the silica. We varied the thickness of the silica layer between 7.6 and 19.1 nm and incorporated the Eu(TTA) 3 complex in the surrounding amphiphilic layer. For the 5 D0 emission of Eu 3+ we measured almost no decay rate enhancement with a 20 nm silica shell surrounding the Au particle, while for the thinnest shells we measured a decay rate enhancement up to a factor of five. Comparison with classical electromagnetic theory shows a good agreement between the experimentally observed enhancement and theory. Copyright c EPLA, 2011Introduction. -Lanthanides are an interesting group of elements because of their unique optical and magnetic properties, arising from the partially filled 4f -shell. The lanthanides usually occur as trivalent ions which have a rich 4f n energy level structure. Since the 4f electrons are shielded by the 5s and 5p electrons, there is only a weak influence from the environment on the position of the energy levels. This leads to sharp atomic-like transitions. Because of their interesting optical properties, lanthanides can be used for a wide range of applications, like light harvesting in solar cells [1,2], lighting [3], fluorescent lamps, white-light LEDs and displays, optical-signal processing [4] and bio-labeling [5]. For many of these applications, control over the luminescent properties is important. In spectral conversion processes for example, the transition rate between specific energy levels determines the efficiency for generating light of the desired wavelength. In the last decade it has been shown that noble-metal nanoparticles strongly interact with emitters and either enhance or quench their emission and

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