Electron Energy Loss Spectroscopy Investigation into Symmetry in Gold Trimer and Tetramer Plasmonic Nanoparticle Structures

We present a combined scanning transmission electron microscopy−electron energy loss spectroscopy (STEM−EELS) investigation into the mode symmetries of plasmonic nanoparticle trimer and tetramer structures. We obtain nanometer-resolved energy loss spectra for both trimer and tetramer structures and compare these to boundary element method simulations. We show that EELS, in conjunction with eigenmode simulations, offers a complete characterization of the individual superstructures, and we trace the evolution of both optically dark and bright modes and identify multipolar mode contributions. We then apply this technique to tetramer structures that exhibit an expanded range of mode symmetries for two-dimensional and three-dimensional selfassembled geometries. These findings provide a comprehensive experimental account of the available photonic states in self-assembled nanoparticle clusters.University of Melbourne (MATS Scholarship), Ernst and Grace Matthaei (Research Scholarship), Gates Cambridge (Scholarship), Royal Society (Newton International Fellowship scheme), European Research Council under the European Union’s Seventh Framework Program (Grant IDs: FP7/2007-2013)/ERC Grant Agreement No. 291522-3DIMAGE, Integrated Infrastructure Initiative (Reference No. 312483-ESTEEM2), Australian Research Council (ARC) (Grant IDs: LF 100100117, FT110100545, DP120101573), Natural Sciences and Engineering Research Council (NSERC), Canada (Discovery grant)This is the author accepted manuscript. The final version is available from the American Chemical Society via http://dx.doi.org/10.1021/acsnano.6b0379