Enhancement of Phosphorescence by Surface-Plasmon Resonances in Colloidal Metal Nanoparticles: The Role of Aggregates

There has been strong interest in recent years in techniques enabling one to modify the rate of light emission by a chromophore. These methods, known as “radiative-decay engineering”, have been employed in biosensor applications, lightemitting diodes, and in the development of luminophores with improved performance. This technique relies on the increase of the spontaneous emission rate in emitters situated in the vicinity of metal structures where the density of photonic states is higher than in a homogeneous medium. The enhancement can be especially high if the optical transition frequency is resonant with the surface-plasmon (SP) oscillations in the metal. Increased rates of spontaneous emission can lead to enhancement of the chromophore’s emission quantum yield and significant reduction of the excited-state lifetime. SP-related enhancement of the emission rate has been observed on corrugated metal surfaces, noble-metal nanoparticles (NPs), and fractal-like structures. Enhancement of the spontaneous emission rate is a nearfield effect and is maximized in close proximity to the metal surface. Numerical modeling indicates that it decays significantly at distances from a NP’s surface comparable with the NP’s diameter. Besides the emission-rate acceleration, interactions with the metal surface may introduce non-radiative losses associated with Förster type energy transfer to the metal. The rate of this energy-transfer process generally scales as f/R, where f is the chromophore’s oscillator strength, R is the distance between the emitter and the metal surface, and m depends on geometric factors. For chromophores adjacent to subwavelength-sized metal NPs, the energy transfer has a 1/R dependence, unlike the 1/R dependence for adjacent point dipoles (for a two-dimensional metal surface one gets the well-known cubic dependence). A chromophore molecule placed in the vicinity of a metal particle experiences both emission-rate enhancement and quenching effects and the net balance of these phenomena is difficult to predict accurately in actual materials. The introduction of a dielectric, optically transparent spacer between the NP surface and the chromophore can be used to minimize emission quenching by preventing the emitter from approaching the metal surface. At the same time, spatial separation between the chromophore and the metal may nullify the emission-rate-enhancement effect if both emission enhancement and quenching occur on the same length scale. Triplet emitters may have a significant advantage in this situation, because their oscillator strength is significantly smaller than those of singlet emitters and therefore the radius of efficient energytransfer quenching is correspondingly smaller. The ratio of oscillator strengths for typical singlet and triplet emitters can be estimated as a ratio of their radiative lifetimes. Considering values of 1 ns and 10 ls, the ratio of the transition moments can be estimated to be of the order of 10 000. Since the energy-