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A key goal in metal nanocrystal research is to understand how and why the shape of conducting nanocrystals is so strongly shape dependent. For particles much smaller than the wavelength of light, the interaction with electromagnetic radiation is primarily through dipole coupling. However, the depolarization factor which determines the resonance frequency of the conduction electron oscillations (surface plasmon modes) is complex and can only be solved analytically for spheres and ellipsoids. Furthermore, the surface plasmon band is damped by shape dependent surface scattering and radiation damping [1].
The first images shows gold rods of different aspect ratios, illustrating that the colours of gold are many and extraordinarily dependent on small changes in aspect ratio. The second image shows how the growth of gold to form nanorods can be achieved in aqueous solution [2].
Finally when gold nanocrystals are assembled further optical effects occur due to the dipole coupling between the particles - see third image. This can be modelled using effective medium theories [3,4].
References:
C. Novo et al. PCCP 3540 (2006).
J. Perez-Juste et al. AFM 14, 571 (2004).
P. Mulvaney MRS Bulletin (2001) 1009-1014.
T. Ung 105, 3441 (2001).
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