Published: October 18, 2011 r2011 American Chemical Society 22271 dx.doi.org/10.1021/jp2081269 | J. Phys. Chem. C 2011, 115, 22271–22275 ARTICLE pubs.acs.org/JPCC Enhanced Plasmonic Behavior of Incomplete Nanoshells: Effect of Local Field Irregularities on the Far-Field Optical Response Ovidio Pe~ na-Rodríguez* ,†,‡ and Umapada Pal § † Centro de Microan  alisis de Materiales (CMAM), Universidad Aut onoma de Madrid (UAM), Cantoblanco, E-28049 Madrid, Spain ‡ Instituto de  Optica, Consejo Superior de Investigaciones Científicas (IO-CSIC), C/Serrano 121, E-28006 Madrid, Spain § Instituto de Física, Benem erita Universidad Aut onoma de Puebla, Apartado Postal J-48, Puebla, Puebla 72570, Mexico ’ INTRODUCTION Plasmonic behavior of metallic nanoparticles (NPs) is cur- rently the subject of intense research because of the potential applications of such materials in many fields, including nonlinear optics, catalysis, chemical and biological sensing, and surface- enhanced Raman scattering (SERS). 1À6 Among plasmonic na- nostructures, metallic nanoshells, 7 consisting of a dielectric core surrounded by a thin metal shell, have received wide attention because of their high stability; ease of preparation; 8 and superior performance in biological applications such as cancer treatment, 9,10 medical diagnostics, 11 and immunoassay. 12,13 Commonly, metal nanoshells are synthesized over silica particles of uniform sizes grown by the St€ ober method. 14 Using successive chemical steps, the monodisperse silica cores are decorated with metal nano- particles. The metal NPs are then allowed to grow until they intersect with each other, coalesce, and form a continuous metallic shell over the dielectric core. 8,15,16 Although considerable effort has been devoted to fabricating such dielectric-core@metallic-shell nanostructures, it is still un- clear how a complete nanoshell and its optical properties evolve from the primary metallic NPs to form a complete coreÀshell structure. This transition is often monitored through the linear optical response of the composite nanostructures, and the obtained optical responses are associated with the geometrical features of a continuous metallic nanoshell. However, it is very unlikely that a perfect and continuous shell layer can be formed through the coalescence of metal NPs, and some researchers argue that the obtained optical response in certain cases is produced not by a continuous layer but, rather, by clusters of metallic nanoparticles. 17À19 Arguments aside, there remains a reasonable question: Is it valid to use Mie theory to simulate the optical response of these nanostructures? It is striking that, despite these concerns, Mie theory is widely used to simulate the optical properties of metallic nanoshells, and the results are quite satisfactory. In this work, we have simulated the optical response of metallic nanoshells at different stages of formation to elucidate how these systems evolve from the formation of discrete and isolated metal NPs over a dielectric core through the formation of a complete coreÀshell structure. In other words, we have investi- gated how the morphology of a metallic shell affects its optical properties. Bulk values of Au dielectric functions reported by Johnson and Christy 20 were used to calculate the optical res- ponses, after application of the usual size correction. 21 ’ PROCEDURE The studied system, depicted in Figure 1, consists of a silica core with a radius of 27.0 nm, over which spherical gold NPs of 5.0-nm radius were gradually added, until a complete shell formed. Initially, the particles were added with the criterion of maintaining maximum separation between them to avoid over- lapping. With the selected radii of the silica and metal NPs, one can add up to 130 nonoverlapping Au spheres ( f ≈ 0.52), and in principle, all of the resulting configurations can be simulated by means of the T-matrix method. 22,23 However, we used this method to add only up to 100 spheres ( f ≈ 0.4), as recent experimental evidence 24 suggests that, for a ratio of center-to- center separation to effective diameter, (R A + R B + d)/(R A + R B ) (where R A and R B are the radii of two contiguous metallic NPs and d is the gap between them), below 1.05 the plasmon coupling does not continue to intensify with decreasing interparticle sepa- ration. From this point onward, the simulations were performed Received: August 23, 2011 Revised: October 14, 2011 ABSTRACT: Plasmonic behavior of coreÀshell-type nanostructures is currently the focus of intense research, particularly because of the tunable optical response of such materials suitable for emerging applications such as laser-induced interstitial thermotherapy, surface-enhanced Raman scattering, and biosensing. In this work, we report on the plasmonic behavior of incomplete nanoshells, providing insight into their evolution and growth. During the initial stages of formation, well-separated, noninteracting metallic nanoparticles at the surface of a dielectric core behave like isolated particles, but irregular and intense local electric fields (hot spots) are created when the number of metallic spheres increases, dramatically affecting the far-field optical response of the integral structure. Under the action of these local fields, the position of the surface plasmon resonance (SPR) suffers a dramatic red shift up to 50% higher than that of a complete nanoshell. The presented results open up the possibility of fabricating metallic nanoshells that are more efficient than the conventional ones for biological applications.