Plasmonic nanoshell arrays combine surface-enhanced vibrational spectroscopies on a single substrate.

The collective oscillations of free electrons in metallic materials—known as surface plasmons—have properties determined by the structure of the metal, and can ultimately be tailored to impart new optically-induced functionalities into materials for specific practical uses. For subwavelength metallic structures, the geometry of the structure controls the resonance frequencies of its surface plasmons. These structured metallic materials, in turn, provide new and unique methods for manipulating light, giving rise to a whole set of new properties and applications. The enhancement by a nearby metallic structure of molecular spectroscopies, such as Raman scattering, infrared absorption, UV/Vis absorption, and fluorescence, is a particularly interesting and useful phenomenon. The optical excitation of plasmon resonances supported by metallic nanostructures gives rise to strongly enhanced electromagnetic fields at their surfaces, which are largely responsible for the observed spectroscopic enhancements. Enhancements of Raman spectroscopy and infrared absorption spectroscopy are of particular interest due to the usefulness of these methods for elucidating molecular structures. Because of their virtually complementary selection rules, it would, in fact, be far preferable to use these two spectroscopies in combination with each other when possible, since together they can provide a complete “chemical fingerprint” for the identification of unknown molecules. The past decade has witnessed a dramatic resurgence of interest in surface-enhanced spectroscopies, fueled largely by the remarkable discovery that single-molecule sensitivity in surface-enhanced Raman spectroscopy (SERS) indeed appears achievable for molecules dispersed within random aggregates of metallic nanoparticles. 17–20] This enormous Raman enhancement has since been identified as arising from molecules positioned in the junctions between directly adjacent nanoparticles, a geometry which gives rise to huge field intensities between the two particles when illuminated, a configuration also known as a “hot spot”. 18, 20–22] In contrast to SERS, surface-enhanced infrared absorption (SEIRA) spectroscopy has not received nearly the same attention. This is primarily for two reasons: compared to SERS, more modest enhancements are anticipated, since SEIRA depends quadratically on the local field, not quartically as does SERS; moreover, the excitation of large local fields on metallic substrates across the broad range of mid-infrared frequencies needed for SEIRA has not previously been achieved. The plasmon hybridization picture, an analogy between interacting plasmons and the wave function hybridization of molecular orbital theory, provides a simple conceptual framework for the design of plasmon resonant structures that can be useful as substrates for surface-enhanced spectroscopies. For example, directly adjacent pairs of nanoparticles can be regarded as plasmonic “dimers”, where excitation of hybridized “bonding” dimer plasmons gives rise to the strong interparticle field enhancement that results in the large SERS enhancements of the hot-spot geometry. Ordered arrays of metallic nanospheres with sub-10-nm interparticle gaps have been shown to provide SERS enhancements by the same mechanism. Metallic nanoparticles with geometrically tunable plasmon resonances, such as metallic nanoshells, can also serve as optimized substrates for SERS when the Raman excitation laser wavelength is within the linewidth of plasmon resonance of the individual nanoparticles. 34] For metallic nanoshells, the field enhancement essentially arises from strongly interacting plasmons on the inner and outer surfaces of the metallic shell layer. 35] The integrated SERS enhancement of an individual nanoshell approaches that observed for hot spots of adjacent solid metallic nanoparticle pairs. Herein we report a specifically designed, subwavelengthstructured metallic substrate that simultaneously enhances two complementary vibrational spectroscopies, namely, Raman scattering and infrared absorption spectroscopy, by introducing plasmon resonances in the two diverse frequency regions required for both spectroscopies. Our strategy is based on the assembly of near-infrared-resonant nanoshells into a 2D periodic array with sub-10-nm interparticle gaps. The resulting nanoshell array is a unique structure which has hot spots in the interparticle junctions that enhance SERS at near-infrared wavelengths, and simultaneously provide broadband mid-infrared hot spots for SEIRA at precisely the same locations on the substrates. [*] H. Wang, J. Kundu, Prof. N. J. Halas Department of Chemistry, Department of Electrical and Computer Engineering, and the Laboratory for Nanophotonics Rice University 6100 Main Street, Houston, TX 77005-1892 (USA) Fax: (+ 1)713-348-5686 E-mail: halas@rice.edu Homepage: http://www.ece.rice.edu/~halas [**] We thank Peter Nordlander for insightful discussions on this subject. This work is supported by the Air Force Office of Scientific Research Grant F49620-03-C-0068, the National Science Foundation (NSF) grants EEC-0304097 and ECS-0421108, the Texas Institute for Bio-Nano Materials and Structures for Aerospace Vehicles funded by NASA Cooperative Agreement No. NCC-1-02038, the Robert A. Welch Foundation Grants C-1220, and the Multidisciplinary University Research Initiative (MURI) Grant W911NF-04-01-0203. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Communications