Synthesis, linear extinction, and preliminary resonant hyper-Rayleigh scattering studies of gold-core/silver-shell nanoparticles: comparisons of theory and experiment Youngjin Kim, Robert C. Johnson, Jiangtian Li, Joseph T. Hupp * , George C. Schatz 1 Department of Chemistry, Materials Research Center and Center for Nanofabrication and Molecular Self-Assembly, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA Received 10 July 2001; in final form 7 December 2001 Abstract Gold-core/silver-shell nanoparticles featuring core diameters of 13 or 25 nm were prepared in aqueous solution. Silver-coated gold nanoparticles have two distinct plasmon absorption bands whose relative intensities depend on the shell thickness – features that are captured well by classical Mie theory. The core/shell spectra are not simply linear combinations of the pure-component spectra. Theory shows that shell dielectric effects upon the core can cause broadening, shifting, and, in some cases, damping of dominant extinction spectral features. Preliminary studies show that core–shell particles are reasonably efficient hyper-Rayleigh scatterers under conditions of two-photon resonance or near resonance with a high-energy plasmon band. Ó 2002 Published by Elsevier Science B.V. 1. Introduction Nanoscale metal particles have attracted sig- nificant recent attention because of their unusual size-dependent optical and electronic properties and because of their potential for application in novel electronic devices, nonlinear optical devices, chemical and biochemical sensors, and catalysis [1–10]. Nanoparticles of free-electron metals, es- pecially silver, gold, and copper, have been of particular interest spectroscopically because of their intense visible-region extinction – a property largely due to surface plasmon absorption (col- lective excitation of conduction-band electrons) [11]. The intense absorption and accompanying excitation translate into greatly enhanced local electromagnetic fields – which in turn can enhance otherwise weak multi-photon processes such as Raman scattering [12–15] and hyper-Rayleigh scattering (HRS) [8,16–20]. We have begun investigating metal-core/metal- shell particles from the point of view of optimizing ‘hyper-photonic’ behavior, especially HRS. We note that the c-irradiation based preparation, and 6 February 2002 Chemical Physics Letters 352 (2002) 421–428 www.elsevier.com/locate/cplett * Corresponding author. Fax: +1-847-491-7713. E-mail addresses: jthupp@chem.nwu.edu (J.T. Hupp), schatz@chem.nwu.edu (G.C. Schatz). 1 Also corresponding author. 0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII:S0009-2614(01)01506-8