1. Optical Properties of Gold Nanoparticles

2. Outline: Introduction Size Dependence Shape Dependence
Temperature Dependence
Encapsulating Layers
Conclusion

3. 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).

4. 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 …

6. Size Dependence: Plasmon bandwidth λmax
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

7. λmax

8. Mean Standard Deviation [%]
Summery of the size of the gold nanoparticles
λmax
[nm]
Mean Standard Deviation [%]
Diameter
517 13
8.9
520 12
14.8
521 9
21.7
533 11
48.3
575
99.3

9. Shape Dependence:
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

10. 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
Magenta
0.77 40
0.10
Violent
1.3 70
Ruby Rod
2.7
140
0.20
Red
7.7
400
0.40
Scarlet 12
600
0.70
Ferrari Red 23
1200
1.33

11. Temperature Dependence:
Dephasing of the surface plasmon electron motion is a result of electron-electron repulsion rather than electron-phonon interactions.

12. Encapsulating Layers: