Excitation of multiple dipole surface plasmon resonances in spherical silver nanoparticles

Abstract

We observe the appearance of multiple dipole surface plasmon resonances in spherical Ag nanoparticles when embedded in an organic semiconductor that exhibits a highly dispersive permittivity. Comparing the absorption spectra of thin-films with and without Ag nanoparticles reveals the presence of two plasmon peaks. Numerical simulations and calculations based * To whom correspondence should be addressed † imec, Kapeldreef 75, B-3001 Leuven, Belgium ‡ Department of Electrical Engineering (ESAT), Katholieke Universiteit Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium ¶ Department of Information Technology (INTEC), Photonics Research Group, Ghent University-imec, StPietersnieuwstraat 41, 9000 Ghent, Belgium 1 on an electrostatic model allow to attribute both peaks to dipole resonances, and show that the strong dispersion of the organic permittivity is responsible for this behavior. The presence of these two plasmon resonances was found to enhance the absorption of the organic semiconductor over a broad wavelength range, and in particular in the red absorption tail.Metal nanoparticles (NPs) possess unique optical properties that arise from the collective oscillations of their free electrons when they are excited by an electromagnetic wave. The strength and frequency of this so-called localized surface plasmon resonance (LSPR) does not only depend on the metal NP size and shape, but is also highly sensitive to changes in the dielectric environment. [1][2][3][4][5] These properties give metal NPs potential applications in sensors, 6-8 photovoltaic cells, 9-13 light emitting devices, 14,15 and surface-enhanced Raman scattering spectroscopy. 16,17 When a spherical metal NP is irradiated with light of a wavelength much larger than the particle radius R, it can be assumed that the NP is exposed to a spatially constant electromagnetic field and therefore only dipole LSPRs are excited. 18,19 According to this quasi-static approximation, the NP can be described as an ideal dipole with polarizability α(ω) given bywhere ε 0 is the vacuum permittivity and ω the frequency of the incident electromagnetic wave, and ε NP (ω) and ε m (ω) denote the relative permittivity of the metal NP and the embedding medium, respectively. In general, both relative permittivities are complex numbers with ε(ω) = ε 1 (ω) + iε 2 (ω). From Eq. 1, it follows that resonances can be excited, given that at the resonance frequencies the embedding medium absorbs weakly (i.e. ε 2,m (ω) << 1), and ε 2,NP (ω) or ∂ ε 2,NP (ω)/∂ ω are small. These resonances can then occur ifFor a spherical metal NP in most embedding media, this dipole resonance condition is typically fulfilled once, either in the ultraviolet, visible or near-infrared wavelength range. NPs has to our knowledge not been considered before. We propose its application for enhancing the absorption in organic photovoltaic cells and photodetectors.Thin-films were deposited on glass (Corning Eagle XG) or Si/SiO 2 substrates by thermal evaporation at a chamber pressure below 2 × 10 −6 ...

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