Formation and Plasmonic Response of Self-Assembled Layers of Colloidal Gold Nanorods and Branched Gold Nanoparticles K. Marvin Schulz, Sabine Abb, Rute Fernandes, Martina Abb, Antonios G. Kanaras,* and Otto L. Muskens* SEPnet and Physics and Astronomy, Faculty of Applied and Physical Sciences, University of Southampton, Southampton SO17 1BJ, United Kingdom * S Supporting Information ABSTRACT: The plasmonic properties of self-assembled layers of rod- and branched- shaped gold nanoparticles were investigated using optical techniques. Nanoparticles were synthesized by a surfactant-guided, seed-mediated growth method. The layers were obtained by gradual assembly of nanoparticles at the interface between a polar and a nonpolar solvent and were transferred to a glass slide. Polarization and angle-dependent extinction measurements showed that the layers made of gold nanorods were governed by an effective medium response. The response of the layers made by branched gold particles was characterized by random light scattering. Microscopic mapping of the spatial mode structure demonstrates a uniform optical response of the nanoparticle layers down to a submicrometer length scale. ■ INTRODUCTION Plasmonic nanoparticles are the cornerstone of a large number of research studies in recent years. 1 Several groups pursue to understand how the properties of nanoparticles can be tuned to the benefit of physical and biomedical applications. 2 Toward this aim, a large number of synthetic routes have been reported describing chemical methods that result in successful control of the morphology of nanoparticles. Nanorods, 3 triangular nanoplates, 4 nanocages, 5 and branched particles 6 are only few of the complex shapes that are readily available nowadays. Particles of different morphologies show distinct optical responses due to the collective oscillations and localization of conduction electrons giving rise to localized surface plasmon (LSP) modes. For example, the optical response of isolated branched gold nanoparticles was found to be governed by LSP modes confined in the tips and the core of the particles. 7 Tuning of nanoparticle morphology as well as variations in the local dielectric environment could influence the behavior of the plasmon band. 7-9 Such manipulation of nanoparticle optical responses are of tremendous interest in sensing applications and optics. Compared to the single-particle characteristics, understand- ing the optical properties of extended layers of particles is of equal importance if one wants to realize the fabrication of mesoscale materials with preprogrammed properties. Toward this direction, several studies have been conducted on layers of small spherical nanoparticles assembled using Langmuir- Blodgett or layer-by-layer methods. In such systems, the plasmonic coupling between adjacent spherical particles results in a red-shifted and broadened LSP band. 10,11 The LSP can be further tuned using substrates which respond to temperature or mechanical stress. 12,13 Under high compression, spherical nanoparticle films exhibit an insulator-to-metal transition, resulting in a collapse of the plasmonic response and appearance of a response similar to that of a metal film. 12 In addition, nanoparticle coupling can be engineered through the use of capping and/or cross-linking molecules, allowing the control of tunnelling and conductive interactions between nanoparticles. 14 Although the spectroscopic characteristics of layers made from spherical nanoparticles are well-investigated, those results cannot be directly translated to other types of layers made from morphologically more complex nanoparticle geometries. Layers of particles consisting of anisotropic shapes, such as nanorods, and branched nanoparticles are of particular interest in optical sensing due to the multiple ways of electron localization within the assemblies. In recent work, films of gold nanorods showed a plasmon redshift resulting from dipole-dipole coupling. 15 On the other hand, the optical characteristics of layers of branched nanoparticles is expected to be more complicated and has not been studied so far. Here, we present a study of the optical properties of mono- and multilayered nanoparticle assemblies formed of either gold nanorods or gold branched nanoparticles. The layers were obtained by the gradual assembly of nanoparticles at the interface between a polar and a nonpolar solvent and were transferred to a glass slide. Thus, in contrast to previous studies on gold nanorods, the assemblies employed in this study are free from additional organics. Both types of nanoparticle assemblies are separated by only a thin coating of cetyl trimethylammonium bromide (CTAB) surfactants. The layers were characterized with SEM and their optical characteristics Special Issue: Colloidal Nanoplasmonics Received: January 13, 2012 Revised: February 24, 2012 Published: March 8, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 8874 dx.doi.org/10.1021/la300199j | Langmuir 2012, 28, 8874-8880