Interactions of Organic Fluorophores with Plasmonic Surface Lattice Resonances
It is common knowledge that metals, alloys and pure elements alike, are lustrous and reflective, the more so when a metal surface is flat, polished, and free from oxidation and surface fouling. However, some metals reflect visible light, in the 380 nm to 740 nm range of wavelengths, much more strongly than others. In particular, some metals reflect wavelengths in certain portions of the ultraviolet (UV), visible, and near-infrared (NIR) regime, let us say 200 nm to 2000 nm, while absorbing light strongly in other segments of this range. There are several factors that account for this difference between various metals. For a particular metal, its absorbance and reflectance (and, for thin films, transmittance) at various wavelengths are a function of the metal electronic band structure. Metals that have both an abundance of mobile (“free”) valence electrons and an abundance of vacant states in the valence band are able to support propagating oscillations in the free-electron density, much like sound waves in air, which are referred to as plasma oscillations, which are essentially pressure waves in the “plasma” or gas of free electrons in the solid metal. When described as quasiparticles in a manner analogous to phonons (lattice vibrations, propagating oscillations in the positions of atomic nuclei), these electron-density oscillations are referred to as plasmons, specifically volume or bulk plasmons when they exist and propagate in a continuous three-dimensional metal space. In principle, all metals have free electrons and partially occupied valence bands, so all metals ought to support plasmons to some extent, but the strength of the oscillations is greater in some metals (metals with large valence band occupancy and a high density of states at the Fermi level) than in others, and the metals with the strongest plasma oscillations are known as “plasmonic” metals. In terms of pure elements, the most plasmonic metals are, canonically, copper, silver, and gold. But there are several others that are known to be plasmonic, namely aluminum, gallium, and indium, as well as the alkali metals, magnesium, and nickel. Depending on the definition of plasmonic that one chooses, other metals would also qualify, but these listed metals are broadly considered “plasmonic”, and are accordingly known to strongly reflect visible light (barring surface oxidation, of course). The plasmonicity of these metals contributes to the high reflectivity of their bulk surfaces and films, which explains, in part, the fiery glint of gold-plated statues in sunlight, the faithful image reflection from silvered-glass mirrors, the use of a burnished speculum (roughly 70% copper, 30 % tin, by weight) alloy plate in the first reflecting telescope, and the use today of aluminum films in electric lamps, including car headlights, where reflecting the light out with minimal light loss due to absorption is key to their function. Of course, in all of these cases, the resistance to surface oxidation (especially for gold) or, for silver, the discovery that it could be deposited onto glass chemically, also contribute to their visible luster and to their historical use in mirrors and reflectors. But the plasmonicity of these metals can have very different effects when they are not shaped into smooth surfaces. In particular, when nanoparticles roughly 5 nm to 200 nm wide are made from plasmonic metals, they exhibit intense light scattering and absorption in the ultraviolet-visible-near-infrared regime. These intense optical effects are due, at least in large part, to the localized surface plasmon resonances (LSPRs) of the particles, and in particles in the 5 nm to 200 nm width range, the dipolar LSPRs, in which incident light excites an oscillating electric dipole within the particle, is often the fundamental mode that gives rise to their light scattering and absorbing properties, although higher order LSPRs (quadrupole, sextupole, octupole, etc.) may also play a significant role, especially for larger particles. Nanoparticles supporting LSPRs have many applications in colloidal suspension and in amorphous films and solids, in which they can act as a powerful pigment, as light-to-heat converters, as anti-reflection coatings, and as optical tags for biomolecules. This dissertation is not primarily concerned with the LSPRs of colloids and disordered films of plasmonic particles. Rather, this dissertation focusses on a different kind of plasmonic resonance that requires plasmonic particles to be arranged in an ordered one-dimensional or two-dimensional lattice, and embedded in a transparent dielectric (electrically insulating or semiconducting) medium. This new plasmonic mode, formed by the mutual radiative coupling of the dipolar LSPRs of the individual plasmonic nanoparticles constituting the lattice, is known as a surface lattice resonance (SLR), sometimes described as a plasmonic SLR to distinguish it from lattice resonances supported by lattices of Mie-scattering dielectric particles or excitonic nanoparticles. SLRs are another plasmonic mode, in addition to surface plasmons polaritons and LSPRs, through which visible light can be absorbed or scattered. In addition, SLRs in combination with various excitonic materials have been used to support a variety of exotic phenomena, including Bose-Einstein condensation of exciton-SLR-polaritons1–3, directional fluorescence4,5 and lasing from thin films (~200 nm thick) of dye-doped polymer in the direction normal to the plane of the lattice and the film6–8, and enhanced light absorption in thin-film amorphous silicon solar cells, which was shown to improve the photonic efficiency and overall performance of a solar cell based on a 100-nm-thick amorphous silicon active layer at the angles-of-incidence and wavelengths that were resonant with the SLR9,10. This last report is significant, in that it represents one system in which an SLR-supporting lattice was used to increase the photonic efficiency (portion of the incident light absorbed) in a solar cell. Meanwhile, a variety of reports demonstrates how the mobility and transferability of excitons can be increased by coupling the excitons to nanostructure-based plasmonic and photonic modes. These developments motivated the research presented in this dissertation. Specifically, the original research detailed in this dissertation consists of a series of experiments with SLRs and Frenkel-exciton supporting materials (organic dye-doped polymers) that were done to investigate whether SLRs might be able to increase the quantum efficiency of excitonic solar cells, in addition to increasing their photonic efficiency. Organic and excitonic solar cells are limited by the short exciton diffusion lengths (typically < 10 nm for disordered organic semiconductors) and short charge carrier diffusion lengths (typically < 10 nm for electrons or holes, depending on the material) and numerous practical and theoretical reports11 indicating that increasing the diffusion length or exciton-energy-transferability in the active layers of excitonic solar cells will increase their quantum efficiency by allowing more photogenerated excitons to reach an electrode, p-n junction, or another type of interface where they can be separated into charge carriers. Therefore, using photonic and plasmonic modes, such as the resonant photon modes in optical microcavities, to increase the mobility of these excitons, or to increase the rate at which their energy can be transferred over a distance, is a plausible means by which one could improve excitonic solar cells. While increased exciton diffusion lengths and increased energy transfer rates have been observed for excitons coupled to optical microcavities and various other modes, and while there are certainly reports indicating similar effects for SLR-coupled excitons (e. g. SLR-coupled excitons exhibit spatially coherent fluorescence over several microns at least) a clear report of enhanced energy transfer, or a quantitively defined increase in the energy transfer radius or diffusion length of excitons in an SLR-coupled dye film still has not been reported. It was with the aim of investigating whether excitons can indeed be made more mobile or transferable by coupling them to and SLR that the research presented here was undertaken. Accordingly, experiments were done to fabricate and characterize SLR-supporting lattices, and to correlate the structure and composition of the lattices to the optical properties and dispersions of the SLRs they support. These include the fabrication methods and angle-resolved transmission, reflection, and fluorescence spectra reported in Chapter 1 for lattices made of Au and Al nanoparticles, and also in Chapter 2 for dimeric Al nanoparticles, and Chapter 3 for Ag nanoparticle lattices. The spectra and other data presented in Chapter 1 demonstrate that the lattices that were fabricated did indeed support SLRs across the visible range, and that these SLRs could be successfully coupled to the excitons of a small molecule laser dye when a film of a transparent polymer, poly(methyl methacrylate), doped with the laser dye was coated onto the SLR-supporting nanoparticle lattice. The lingering ambiguity of the true nature of this coupling, despite the ample literature on this topic, is discussed in light of these original results in Chapter 1. In Chapter 2, the effects of exciton-SLR coupling on the transferability of the dye excitons was examined by measuring the effect of the SLRs on the efficiently of Förster resonance energy transfer (FRET) between two different small molecule laser dyes. The results did not show any increase in the FRET efficiency due to coupling of the donor dye exciton to the SLRs in the aluminum nanoparticle lattices. However, it did show a significant increase in the fluorescent emission of the donor dye, such that SLRs at a certain wavelength effectively outcoupled the donor dyes emission from the film and appeared increase the rate at which the donor dye excitons decayed radiatively, likely due to the increase in the local density of optical states that the SLRs furnished, which would increase the radiative decay rate of the donor dye exciton per Fermi’s golden rule. Therefore, while this result did not clearly corroborate or negate our hypothesis that exciton-SLR coupling can increase exciton diffusion length and FRET radius, it did provide interesting example of how SLR-enhanced fluorescence can decrease FRET efficiency, not by changing the FRET rate, but by increasing the radiative decay rate of the donor to make it the primary channel by which the donor dye exciton decays in films in which FRET to the acceptor is the primary channel for donor dye exciton decay without SLR-coupling. Chapter 3 explores the potential for SLRs to enhance and direct the emission from single-photon emitters, specifically fluorescent atomic lattice defects in few-atomic-later films of hexagonal boron nitride (hBN), a two-dimensional material and inorganic, insulator/wide-bandgap semiconductor. The results indicate that the SLRs hosted by the plasmonic lattice of Ag nanocylinders does indeed spatially filter (direct) the hBN defect fluorescence, such that the hBN fluorescent emission follows the wavelength-angle of emission relationship dictated by the SLR dispersion. Thus, the defects in hBN couple to the lattice in manner similar to organic dyes in a transparent polymer. Furthermore, the SLRs lattices supporting SLRs of certainly wavelengths (e. g. around 500 nm) increased by a factor of 6 the intensity of the hBN defect emission, and increased the emission from defects located both direction on the Ag nanocylinders, and defects in the film between the Ag nanocylinders, indicating that SLRs can activate defects by optical field effects, in addition to the mechanical strain induced in the hBN by the Ag pillars, which is known to increase the emissivity of the defects based on a previous study with hBN draped over non-plasmonic SiO2 nanopillars12,13. (1) Hakala, T. 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Atomic, Molecular and Optical Physics, Condensed Matter Physics, Inorganic Chemistry, Materials Chemistry, Optics, Organic Chemistry, Quantum Physics, Excitons, Plasmonics, Optics and Photonics, Photocatalysis, Photovoltaics, Energy Transfer
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