Optical Properties of Gold Nanoparticles
Outline: Introduction Size Dependence Shape Dependence Temperature Dependence Encapsulating Layers Conclusion
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).
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.
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 …
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
λmax
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
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
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
Temperature Dependence: Dephasing of the surface plasmon electron motion is a result of electron-electron repulsion rather than electron-phonon interactions.
Encapsulating Layers: Au@SiO2