Topics in Particle and Dispersion Science

Plasmon resonance tuning: Surrounding medium

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Plasmon resonances of metallic particle suspensions can be tuned by changing the medium surrounding the particles, thus changing the relative refractive index, m, of the particles. As explained in Plasmon resonances of metal nanoparticles, the resonance condition is nearly satisfied for spheres when the imaginary part of the dielectric function of the particle is approximately zero and the real part is close to -2ε_m where ε_m is the dielectric function of the medium. In Plasmon resonance of metal nanoparticles we discussed the optical properties of the particle in air where ε_m = 1. Clearly, changing the medium will have a profound effect on the optical resonance.

As the refractive index of the medium surrounding the nanoparticles increases, the absorption of light by the nanoparticle dispersion increases dramatically and then drops off as shown in Figure 1 for gold spheroids. While it may be difficult to achieve such high refractive indices of the surrounding media, the figure clearly shows that the plasmon resonance for gold can be maximized by optimizing the refractive index of the medium. Similar conclusions can be reached for other nanoparticles (Barber PW et al 1983 - attenuation spectra for silver spheroids as a function of particle size in air, water, and cyclohexane). More recently, Jensen TR et al 1999 have published data on periodic arrays of silver nanoparticles as a function of shape and dielectric medium. They found that oblate ellipsoids are least sensitive to the dielectric medium and truncated tetrahedral particles are most sensitive.

The effect of the external medium on plasmon resonance extends beyond the nanoparticle optics. Saito Y and Verma 2009 have recently pointed out that, for a optical nano-probe utilizing the plasmon resonance, it may be more convenient to coat the tip of the nano-probe to tune the resonance to the laser wavelength than to change the laser wavelength to match the resonance spectrum of the nano-probe.

See plasmon resonance tuning by changing the particle size, particle shape/structure/orientation, and other factors. See also plasmon resonance for additional references.

CITATION: Swanson N. L. 2008. Plasmon resonance tuning: Surrounding medium (www.tpdsci.com/Tpc/PlasmResNptTunExtMed.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 25-Oct-2008 Modified: 12-Jul-2009 Peer-reviewed: PENDING

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