Optical Properties of Metal Nanoparticles

Sriharsha Karumuri

Introduction

Why nanoparticles are different from bulk materials?

• Nanoparticles are different from bulk materials and isolated molecules because of their unique optical, electronic and chemical properties.

• As the dimensions of the material is reduced the electronic properties change drastically as the density of states and the spatial length scale of the electronic motion are reduced with decreasing size.

• Closely related to size-induced changes in the electronic structure are the optical properties of nanoparticles.

1) Gold nanoparticles were used as a pigment of ruby-colored stained glass dating back to the 17th century. Figure.1 shows a picture of the Rose Window of the Cathedral of Notre Dame. The bright red and purple colors are due to gold nanoparticles.

2) Lycurgus cup: It appears green in reflected light, but appears red when light is shone from inside, and is transmitted through the glass.

Surface plasmon resonance

Figure: Schematic of plasmon oscillation for a sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei.

When a nanoparticle is much smaller than the wave length of light, coherent oscillation of the conduction band electrons induced by interaction with an electromagnetic field. This resonance is called Surface Plasmon Resonance (SPR).

Literature review

Michael Faraday was first to report the study of the synthesis and colors of colloidal gold.

In 1908, Mie explained this phenomenon by solving Maxwell’s equation.

Mie theory predicted optical extinction of homogenous spherical particles 2R<<λ for very small particles as (extinction = scattering + absorption)

2

22

1

22

3

)(]2)([

)(...9

m

mext c

V

Where as V is the particle volume, ω is the angular frequency of the exciting light, and c is the speed of light. εm and ε (ω) = ε1 (ω)+ ε 2 (ω) are the dielectric functions of the surrounding medium and the metal, respectively

Synthesis processes

Wet chemical process Mechanical process Form in phase Gas phase synthesis Electroless deposition

Size dependence The changes gold–blue–purple–red

are largely geometric ones that can be explained with Mie theory, which describes light-scattering by a sphere.

When the metal nanoparticle is larger than the ~30 nm, the electrons oscillating with the light is not perfectly in phase. Some electrons get behind; this phenomenon is called retardation effect or phase retardation.

The subsequent changes, reddish - brown to orange to colorless, are due to quantum size effects.

Mulvaney, MRS Bulletin 26, 1009 (1996)

Surrounding medium The surface plasmon resonance

peak changes with its own dielectric properties and those of its local environment including the substrate, solvent, and adsorbates.

This principle that the high sensitivity of the surface plasmon resonance spectrum of noble metal nanoparticles to adsorbate-induced changes in the dielectric constant of the surrounding nanoenvironment used in chemosensing and biosensing.

Spectral shift for individual blue (roughly spherical) silver nanoparticles. Typical blue particle spectrum as it is shifted from (a) air to (b) 1.44 index oil, and successive oil treatments in 0.04 index incremental increases.

Jack J. Mock, David R. Smith, and Sheldon Schultz, Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles, Nano letters 2003 Vol. 3 No. 4 485-491.

Particle density Beginning from the left

“glass is doped with gold nanoparticles” and spacing between them is large.

In the right side figure ‘the bulk gold is doped with glass’. As the spacing is reduced, dipole interactions become increasingly important.

(a) Transmitted colors of the same Au@SiO2 films. (b) The reflected color of the films after deposition from a ruby red gold sol as a function of the silica shell thickness. Top left going across: 15 nm gold particles coated with silica shells of thickness 17.5, 12.5, 4.6, 2.9, and 1.5 nm.

Thearith Ung, Luis M. Liz-Marzan, and Paul Mulvaney, optical Properties of Thin Films of Au@SiO2 Particles, J. Phys. Chem. B 2001, 105, 3441-3452.

Applications

These differences in properties of nanoparticles are used in microelectronics, quantum dot lasers, chemical sensors, data storage, and a host of other applications.

Possible future applications of nanoparticles include the areas of ultrafast data communication and optical data storage, solar energy conversion, and the use of metallic nanoparticles as catalysts because of their high surface-to-volume ratios and different shapes.

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