Plasmonic Nanoparticles
Noble metal nanoparticles support surface plasmons (oscillations of the conduction electrons at the nanoparticle surface) and have unique and highly useful properties. This plasmonic enhancement effect results in materials that have very large scattering and absorption cross sections across visible and near-infrared wavelengths.
The basis for the effect is the plasmon resonance of the free electrons in the metal nanoparticle, which can be understood by studying the polarizability (the ease with which charges, such as the conduction electrons on the metal nanoparticle surface, undergo charge distribution and form partial dipoles). For a spherical nanoparticle, the quasi-static polarizability of the nanoparticle is given by:
Where ε1 is the wavelength dependent dielectric function of the nanoparticle and ε2 is the dielectric function of the medium which remains roughly constant regardless of wavelength. When the condition Re{ ε1}=-2 ε2 is satisfied, the particle is driven into resonance resulting in a strong increase in the absorption and/or scattering at that wavelength.
Because the resonance condition depends on the wavelength dependent dielectric function of the nanoparticle as well as the dielectric function of the medium, nanoparticle optical properties are highly dependent on material composition, size, and the medium in which the particles are embedded. Both the shape and peak resonance wavelength of the nanoparticle plasmon resonance is influenced by the local refractive index. When particles are in water (n=1.33) the peak resonance of 80 nm Ag particles predicted by Mie Theory is ~445 nm. In air (n=1.00) the peak wavelength of the Plasmon resonance is predicted to be ~380 nm, a blue-shift of 65 nm. However, if the silver particle is coated with a thick shell of a material that has the same refractive index as water, the peak Plasmon wavelength will be very close to 445 nm. If the shell is very thin, the peak resonance will be near 380 nm. By adjusting the thickness of the shell, the peak resonance of coated nanoparticles can be tuned to any wavelength between these two extremes. To observer this effect for other particle sizes, compositions, refractive indices, see our Online Mie Theory Simulator (OMTS) which allows you to calculate and view the theoretical optical response of gold and silver nanoparticles.
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| Extinction spectra of Ag spheres of varying size. |
Mie Theory simulations of 80 nm Ag spheres with silica shells of varying thickness in air. |
Nanoparticle optical properties are also sensitive to the proximity of other plasmonic materials. When two or more plasmonic nanoparticles are near each other (roughly with edge-to-edge separations of one particle diameter or less) their surface plasmons couple as the conduction electrons on each particle surface begin to oscillate together. This effect is similar to molecular orbital theory in that plasmon coupling results in the oscillating electrons assuming the lowest energy state, causing the plasmon resonance wavelength of the coupled particles to red-shift to longer wavelengths (lower energies).
We also offer a UV-Visible spectroscopy service for plasmonic nanoparticles.