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Concept
Localized surface plasmon
Summary
A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. When a small spherical metallic nanoparticle is irradiated by light, the oscillating electric field causes the conduction electrons to oscillate coherently. When the electron cloud is displaced relative to its original position, a restoring force arises from Coulombic attraction between electrons and nuclei. This force causes the electron cloud to oscillate. The oscillation frequency is determined by the density of electrons, the effective electron mass, and the size and shape of the charge distribution. The LSP has two important effects: electric fields near the particle's surface are greatly enhanced and the particle's optical absorption has a maximum at the plasmon resonant frequency. Surface plasmon resonance can also be tuned based on the shape of the nanoparticle. The plasmon frequency can be related to the metal dielectric constant. The enhancement falls off quickly with distance from the surface and, for noble metal nanoparticles, the resonance occurs at visible wavelengths. Localized surface plasmon resonance creates brilliant colors in metal colloidal solutions.
For metals like silver and gold, the oscillation frequency is also affected by the electrons in d-orbitals. Silver is a popular choice in plasmonics, which studies the effect of coupling light to charges, because it can support a surface plasmon over a wide range of wavelengths (300-1200 nm), and its peak absorption wavelength is easily changed. For instance, the peak absorption wavelength of triangular silver nanoparticles was altered by changing the corner sharpness of the triangles. It underwent a blue-shift as corner sharpness of the triangles decreased. Additionally, peak absorption wavelength underwent a red-shift as a larger amount of reducing agent (HAuCl4) was added and porosity of the particles increased.
Official source
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