Optical%20Properties%20of%20Gold%20Nanoparticles

1
Optical Properties of Gold Nanoparticles
2
Outline

• Introduction
• Size Dependence
• Shape Dependence
• Temperature Dependence
• Encapsulating Layers
• Conclusion

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Introduction

• 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).

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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.

5
Introduction

• 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

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Size Dependence

• Plasmon bandwidth
• ? gtgt 2R, 2R lt 25 nm Intrinsic size effects,
  Dipole approximation
• 2R gt 25 nm Extrinsic size effects, higher
  order terms contribute
• ?max
• Intrinsic size effects
• Extrinsic size effects

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?max
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Summery of the size of the gold nanoparticles
?max nm Mean Standard Deviation Diameter nm
517 13 8.9
520 12 14.8
521 9 21.7
533 11 48.3
575 11 99.3
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Shape Dependence

• Family of alumina membranes that contain Au
  nanoparticles.
• Aspect ratio (length divided by diameter)
• a/b lt 1 Oblate or pancake-like
• a/b 1 Spheroid
• a/b gt 1 Prolate or needle-like

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Aspect ratio of the resulting Au nanoparticles,
and color of the Nanoparticle-containing membrane
Membrane color Particle aspect ratio Particle length (nm) Coulombs of gold plated
Turquoise 0.38 20 0.05
Sky Blue 0.46 24 0.065
Blue Purple 0.54 28 0.05
Magenta 0.77 40 0.10
Violent 1.3 70 0.10
Ruby Rod 2.7 140 0.20
Red 7.7 400 0.40
Scarlet 12 600 0.70
Ferrari Red 23 1200 1.33
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Temperature Dependence

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

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Encapsulating Layers

• Au_at_SiO2