Optical Properties of Gold Nanoparticles

• Introduction

• Size Dependence

• Shape Dependence

• Temperature Dependence

• Encapsulating Layers

• Conclusion

Outline:

• New optical properties which are observed neither in molecules (2 nm and smaller) nor in bulk metals.

• Presence of a strong band in the visible region of the absorbance (for spherical gold nanoparticles at about 520 nm).

• It attributed to surface plasmon oscillation modes of the conduction electrons (6s-electorons for gold).

Introduction:

Plasmon

A Plasmon is quantized oscillation in the density of an electron gas. It is due to electron-electron interactions.

Surface plasmon: Two-dimensional fluctuations at the surface.

• It can be explained by combining the classical Drude model with the Mie theory for light absorption and scattering by spheres.

• Sensitive to:– Size Dependence – Shape Dependence – Encapsulating Layers– Temperature Dependence– …

Introduction:

• Plasmon bandwidth– λ >> 2R, 2R < 25 nm Intrinsic size effects,

Dipole approximation

– 2R > 25 nm Extrinsic size effects, higher order terms contribute

• λmax – Intrinsic size effects

– Extrinsic size effects

Size Dependence:

λmax

Diameter[nm]

Mean Standard

Deviation [%]

λmax[nm]

8.913517

14.812520

21.79521

48.311533

99.311575

Summery of the size of the gold nanoparticles

• Family of alumina membranes that contain Au nanoparticles.

• Aspect ratio (length divided by diameter)– a/b < 1 Oblate or pancake-like– a/b = 1 Spheroid – a/b > 1 Prolate or needle-like

Shape Dependence:

Coulombs of gold plated

Particle length (nm)

Particle aspect ratio

Membrane color

0.05200.38Turquoise

0.065240.46Sky Blue

0.05280.54Blue Purple

0.10400.77Magenta

0.10701.3Violent

0.201402.7Ruby Rod

0.404007.7Red

0.7060012Scarlet

1.33120023Ferrari Red

Aspect ratio of the resulting Au nanoparticles, and color of the Nanoparticle-containing membrane

• Dephasing of the surface plasmon electron motion is a result of electron-electron repulsion rather than electron-phonon interactions.

Temperature Dependence:

Encapsulating Layers:

• Au@SiO2