Reduced Symmetry Metallodielectric Nanoparticles: Chemical Synthesis and Plasmonic Properties † Clarence Charnay, ‡ Allen Lee, ‡ Shi-Qing Man, ‡ Cristin E. Moran, § Corey Radloff, § R. Kelley Bradley, | and Naomi J. Halas* ,‡,§ Department of Electrical and Computer Engineering and Department of Chemistry, Center for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas, and Nanospectra L.L.C., Houston, Texas ReceiVed: February 5, 2003; In Final Form: April 22, 2003 We report a general chemical strategy for producing reduced-symmetry metallodielectric nanoparticles, nanocups, and nanocaps that combines nanoscale masking techniques and nanoparticle-seeded electroless plating. Using this approach, silica nanoparticles with a gold cup-shaped shell and, alternatively, a gold cap, are obtained. The plasmon response of both nanostructures is a sensitive function of orientation of the nanostructure with respect to the direction and polarization of incident light. This orientation dependence is examined experimentally by studying the extinction spectra of oriented nanocups and nanocaps on transparent substrates, and is also evaluated theoretically using a three-dimensional finite difference time domain (FDTD) method. Introduction Metal nanostructures have been studied extensively in the field of nanoscience; their robust synthetic and functionalization chemistry, in combination with their interesting physical proper- ties, make them ideal structures for fundamental research and applications. In particular, nanostructures made from the noble metals, (e.g. Au and Ag) with their associated strong plasmon resonance have generated great interest. The fact that the plasmon response is a sensitive function of nanostructure geometry, coupled with synthetic advances that allow for controlled and systematic variations in nanostructure geometries, is leading to a dramatic increase in interest in this topic. This renaissance is resulting in a new field called “plasmonics”, associated with the design and fabrication of nanooptical components that focus and manipulate light at spatial dimensions far below the classical diffraction limit. New applications of plasmonics, such as metal nanostructure-based strategies for chemical sensing, 1,2 electromagnetic wave transport, 3,4 and the development of new optically responsive materials 5,6 have recently been reported. This is also stimulating an increased theoretical interest in the electronic and electromagnetic proper- ties of mesoscopic metal structures. From the variety of nanoscale geometries that have stimulated interest in plasmonics, a particular geometry of significant practical interest is a nanoshell: a metallodielectric nanoparticle where Au or Ag forms a uniform shell around a dielectric core. It has been shown that metallodielectric, core-shell nanopar- ticles possess a tunable plasmon resonance that can be controlled by changing the ratio of the core radius to the shell thickness. 7,8 The core/shell ratio of a nanoshell controls its far-field optical properties, so that its color can be tuned across the electromag- netic spectrum. It also controls the intensity of the optical field at the surface of the nanoparticle, enabling the control and optimization of Raman scattering enhancements at the nanoshell surface. 9,10 For Au-silica nanoshells, the nanoshell particle is constructed by first attaching small gold nanoparticles onto the surface of a silica nanoparticle. This is followed by the reduction of gold from solution onto the core utilizing the chemisorbed gold nanoparticles as nucleation sites. The resulting nanoshell possesses a frequency agile plasmon resonance that can be tuned from the visible to the infrared region of the electromagnetic spectrum. 11-13 Several other methods for the synthesis of core- shell nanostructures have been reported, which include growth of a metal shell onto core materials other than silica, 14,15 variations in reductant chemistry, 16 and the synthesis of hollow crystalline shells by templating on block copolymers. 17 Reducing symmetry for the core-shell geometry does interesting things to the plasmon resonance of this nanostruc- ture: the plasmon response becomes a sensitive function of the orientation of the nanostructure with respect to incident light. Both the plasmon frequency and the cross section of the response show a strong and dramatic orientation dependence. This unique property should make possible the orientation and manipulation of these nanostructures by applied electric and electromagnetic fields, which in turn should give rise to new plasmonic devices and applications. Recent articles have reported the fabrication of reduced- symmetry nanoshells at an approximately 50% metal coverage by evaporation of metal onto dielectric nanoparticles deposited onto a substrate. 18 In this article we report a chemical strategy for synthesis of reduced-symmetry nanoparticles of two types: nanocups, with approximately 70-80% metal coverage, and nanocaps, the inverse structure, with approximately 20-30% metal coverage. Our approach combines nanoparticle deposition onto a substrate that provides a site for partial chemical passivation of the nanoparticle surface, followed by subsequent nanoshell nucleation and deposition chemistry to selectively coat a specific section of the nanostructure surface with metal. We also investigate the plasmon response of both nanocups and nanocaps as a function of nanoparticle orientation with respect to incident light and polarization angle. These results are † Part of the special issue “Arnim Henglein Festschrift”. * Corresponding author: halas@rice.edu. ‡ Department of Electrical and Computer Engineering. § Department of Chemistry. | Nanospectra L.L.C. 7327 J. Phys. Chem. B 2003, 107, 7327-7333 10.1021/jp034309r CCC: $25.00 © 2003 American Chemical Society Published on Web 06/17/2003