Examining Substrate-Induced Plasmon Mode Splitting and Localization in Truncated Silver Nanospheres with Electron Energy Loss Spectroscopy Guoliang Li, †,¶ Charles Cherqui, ‡,¶ Yueying Wu, § Nicholas W. Bigelow, ‡ Philip D. Simmons, ∥ Philip D. Rack, §,⊥ David J. Masiello,* ,‡ and Jon P. Camden* ,† † Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry, University of Washington, Seattle, Washington 98195, United States § Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: Motivated by the need to study the size dependence of nanoparticle−substrate systems, we present a combined experimental and theoretical electron energy loss spectroscopy (EELS) study of the plasmonic spectrum of substrate-supported truncated silver nanospheres. This work spans the entire classical range of plasmonic behavior probing particles of 20−1000 nm in diameter, allowing us to map the evolution of localized surface plasmons into surface plasmon polaritons and study the size dependence of substrate-induced mode splitting. This work constitutes the first nanoscopic characterization and imaging of these effects in truncated nanospheres, setting the stage for the systematic study of plasmon-mediated energy transfer in nanoparticle−substrate systems. L ocalized surface plasmon resonances (LSPRs) are the coherent and collective oscillations of conduction band electrons at the surface of metallic nanoparticles. LSPRs are known to localize far-field light to a subdiffraction-limited length scale, yielding an intense electric field at the particle surface. This effect has been harnessed to dramatically enhance light-matter interactions, leading to a variety of applications such as surface-enhanced Raman spectroscopy (SERS), 1,2 subwavelength waveguides, 3 plasmonically enhanced photo- voltaics (PV), 4,5 and photocatalysis. 6,7 The broad field of interest that employs the use and study of LSPRs is known as plasmonics, which is expected to be a critical technology in the merging of photonics and electronics toward nanoscale dimensions. 8 Despite several important milestones, 9−13 the potential of LSPRs in enhancing solar energy-harvesting efficiency has yet to be realized. 14 Though a myriad of reasons exist, the most significant obstacle is the incomplete description of energy transfer between the LSPR and the neighboring environment. Plasmon-mediated energy transfer can be accomplished via two often-competing pathways: (1) plasmon-induced electron transfer and (2) plasmon-induced resonance energy trans- fer, 15−18 and it is challenging to distinguish them from each other in experiment. Recently, it was shown that the efficiency of plasmon-mediated energy transfer can be controlled by varying the parameters of the system, such as the particle geometry or the optoelectronic properties of the sub- strate. 15,17−21 Electron energy loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) has also revealed that substrate-induced LSPR localization can be manipulated to tune the efficiency of energy transfer in nanocube−substrate systems. 15 These studies indicate that energy transfer is highly dependent on the interplay between LSPRs and the electronic structure of the substrate. Because the spatial and energetic profiles of LSPRs can be tuned by varying the particle size, 22−24 it is tempting to conduct a full-size study of the nanocube-substrate system to gain a deeper understanding of plasmon-mediated energy transfer. However, because nanocubes are only available in a limited size range (<200 nm in cube edge length), 25,26 it is necessary to pursue alternative systems which exhibit similar LSPR localization effects but do not suffer from particle-size restrictions. Another system which exhibits strong interface local- ization 19,27,28 and has generated significant interest is the Received: May 8, 2015 Accepted: June 16, 2015 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 2569 DOI: 10.1021/acs.jpclett.5b00961 J. Phys. Chem. Lett. 2015, 6, 2569−2576