Silver Nanoparticle Optical Properties

There is growing interest in utilizing the optical properties of silver nanoparticles as the functional component in various products and sensors. Silver nanoparticles are extraordinarily efficient at absorbing and scattering light and, unlike many dyes and pigments, have a color that depends upon the size and the shape of the particle.
 
The strong interaction of the silver nanoparticles with light occurs because the conduction electrons on the metal surface undergo a collective oscillation when excited by light at specific wavelengths. Known as a surface plasmon resonance (SPR), this oscillation results in unusually strong scattering and absorption properties. In fact, silver nanoparticles can have effective extinction (scattering + absorption) cross-sections up to ten times larger than their physical cross-section. The strong scattering cross-section allows for sub 100 nm nanoparticles to be easily visualized with a conventional microscope.
When 60 nm silver nanoparticles are illuminated with white light they appear as bright blue point source scatterers under a dark field microscope. The bright blue color is due to an SPR that is peaked at a 450 nm wavelength. A unique property of spherical silver nanoparticles is that this SPR peak wavelength can be tuned from 400 nm (violet light) to 530 nm (green light) by changing the particle size and the local refractive index near the particle surface. Even larger shifts of the SPR peak wavelength out into the infrared region of the electromagnetic spectrum can be achieved by producing silver nanoparticles with rod or plate shapes.
 
(Left) Surface plasmon resonance where the free electrons in the metal nanoparticle are driven into oscillation due to strong coupling with a specific wavelength of the incident light.
(Right) Darkfield microscopy image of 60 nm silver nanoparticles.