1 23 Plasmonics ISSN 1557-1955 Plasmonics DOI 10.1007/s11468-012-9395-8 Plasmon Spectroscopy for Subnanometric Copper Particles: Dielectric Function and Core–Shell Sizing J. M. J. Santillán, F. A. Videla, L. B. Scaffardi & D. C. Schinca