Correlated Structure and Optical Property Studies of Plasmonic Nanoparticles

Nanoparticles have been the subject of intense research over the last two decades. A large variety of shapes (from “spheres” to complex anisotropic particles), sizes (from a few to > 100 nm), and materials (metals, semiconductors) can now be synthesized, thus broadening the range of the physical properties accessible, which can significantly differ from their bulk counterparts. 3 In particular, metal nanoparticles have attracted special attention from synthetic and physical chemists. Numerous chemical and physical methods for the synthesis and fabrication of nanoparticles of controlled shape and size have been reported. 13 At the same time, experimental and theoretical investigations of the physical (such as optical and magnetic) properties of nanoparticles have highlighted how they relate to the structure. 17 Materials such as Ag, Cu, Au, and Al with a negative real and small positive imaginary dielectric constant over a range of wavelengths are capable of supporting a surface plasmon resonance (SPR) when driven with UV visible NIR electromagnetic radiation. The SPR can be propagating or localized. Propagating surface plasmons are observed on thin metallic films, whereas localized surface plasmons are observed on nanoscale structures. Such materials are designated as plasmonic. In the case of metal nanoparticles smaller than the wavelength of visible light, the surface plasmon resonance is localized. The physical origin of this lies in the conduction electrons that oscillate coherently with the frequency of the applied radiation field. A theoretical description of linear optical properties like extinction and scattering of a small spherical particle was proposed by Mie in 1908 for spherical particles. The extinction (absorption plus scattering) of a sphere of radius a with dimensions smaller than the wavelength of light can be expressed as