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Metal nanoshells, which consist of nanometer-scale dielectric cores surrounded by thin metallic shells, have been designed and studied for their linear optical responses. The plasmon resonance of metal nanoshells displays geometric tunability controlled by the ratio of shell thickness either to the core radius or to the total radius of the particle. Using Mie theory the surface plasmon resonance (SPR) of metallic nanoshells (Au, Ag, Cu) is studied for different geometries and physical environments. Considering a final radius of about 20 nm, the SPR peak position can be tuned from 510 nm ͑2.43 eV͒ to 660 nm ͑1.88 eV͒ for Au, from 360 nm ͑3.44 eV͒ to 560 nm ͑2.21 eV͒ for Ag, and from 553 nm ͑2.24 eV͒ to 655 nm ͑1.89 eV͒ for Cu, just by varying the ratio t / R Shell and the environments inside and outside. With the decrease of the t / R Shell ratio the SPR peak position gets redshifted exponentially and the shift is higher for a higher refractive index surroundings. The plasmon linewidth strongly depends on the surface scattering process and its FWHM increases with the reduction of shell thickness.

Formation of Au core−Ag shell bimetallic nanoparticles in silica matrix is demonstrated through sequential implantation of Ag and Au ions and subsequent thermal annealing. Formation of core−shell structures is verified through optical absorption spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and simulated optical extinction spectra. A mechanism for the formation of such unusual structures in ion-implanted silica is proposed. By controlling the implantation energy of the two ions properly and keeping the implantation sequence Ag first and then Au, it is possible to create Au core−Ag shell nanoparticles in the silica matrix with homogeneous distribution.

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