Why do shape and size have the greatest effect on the optical properties of metal nanoparticles?
Maybe the attached papers helps to answer your question!?
Here, STEM-EELS has been used to measure size-dependent the energy band gap and surface plasmon resonances.
Joerg
Plasmonic particles are understood as Mie Scatterers. If you excite a particle at its plasmon resonance, the particle effectively becomes an Hertzian electric dipole due to its high polarizability. However, if the particles are not spherical, you may need to expand the radiation field into a multipole. Higher orders, however, may reduce the plasmonic resonance.
Maybe you can think of the nanoparticle as a carrior of a plasmonic wave. The nanoparticle will absorb energys that create a plasmonic wave that closes a circle around the entire nanoparticle. Of course, bigger particles would have a wave with a longer wavelength and therefore lower energy. Different shapes can have different closed circles around it. That is a very simple model, and it is not entirely correct, but it helps understanding.
Please consult the following two relevant books. I also attach a conference proceedings, edited by Wolfram Hergert and Thomas Wriedt (the editors of the second of the above-mentioned two books) , dedicated to Gustav Mie and the Mie theory. For completeness, I also attach the link to the original paper by Mie (in German) whose text is made available to the general public by Wiley Online Library.
http://link.springer.com/book/10.1007%2F978-3-642-17194-9
http://link.springer.com/book/10.1007%2F978-3-642-28738-1
http://www.scattport.org/files/mie-halle/Hergert-Mie-Theory-1908-2008-Booklet_Uni-Bremen-2008.pdf
http://onlinelibrary.wiley.com/doi/10.1002/andp.19083300302/abstract
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