Three-dimensional imaging of localized surface plasmon resonances of metal nanoparticles

Abstract

The remarkable optical properties of metal nanoparticles are governed by the excitation of localized surface plasmon resonances (LSPRs). The sensitivity of each LSPR mode, whose spatial distribution and resonant energy depend on the nanoparticle structure, composition and environment, has given rise to many potential photonic, optoelectronic, catalytic, photovoltaic, and gas- and bio-sensing applications. However, the precise interplay between the three-dimensional (3D) nanoparticle structure and the LSPRs is not always fully understood and a spectrally sensitive 3D imaging technique is needed to visualize the excitation on the nanometre scale. Here we show that 3D images related to LSPRs of an individual silver nanocube can be reconstructed through the application of electron energy-loss spectrum imaging, mapping the excitation across a range of orientations, with a novel combination of non-negative matrix factorization, compressed sensing and electron tomography. Our results extend the idea of substrate-mediated hybridization of dipolar and quadrupolar modes predicted by theory, simulations, and electron and optical spectroscopy, and provide experimental evidence of higher-energy mode hybridization. This work represents an advance both in the understanding of the optical response of noble-metal nanoparticles and in the probing, analysis and visualization of LSPRs.

Aluminum Nanocrystals

“…For the representative 165 nm diameter Al nanocrystal studied here, three prominent modes were found: the lowest energy at 3.3 eV and intermediate energy modes at 5.5 and 7.1 eV (Figure 4a). EELS images were obtained at spectral windows corresponding to these three mode energies (Figure 4b), which provided sufficient bandwidth to allow the use of non-negative matrix factorization, a powerful and assumption-free modal decomposition approach, to generate the nanocrystal image 23,24 . The nanocrystal images consistently display 3-fold symmetry, consistent with the CL images.…”

“…The field intensity for the lowest energy dipolar 3.3 eV plasmon mode (Figure 4a, top) is strongly localized at the tips of the particle, similar to the lower energy modes of a nanocube. 23,24 The 5.5 eV mode (Figure 4a, center) is a quadrupolar surface plasmon resonance, with the field intensity localized at the faces of the structure. The 7.1 eV mode (Figure 4a, bottom) is also localized at the nanoparticle surface and is consistent with a dark higher order octupolar plasmon mode that is slightly redshifted due to the presence of the thin oxide layer at the surface of the nanocrystal.…”

The Substrate Effect in Electron Energy-Loss Spectroscopy of Localized Surface Plasmons in Gold and Silver Nanoparticles

“…3 accessible, allowing LSPs to be probed and studied with sub-nanometer spatial resolution 6,8,[22][23][24][25][26] . TEM requires specimens thin enough to be electron transparent (typically below 100 nm) and therefore, plasmonic nanoparticles studied by EELS are typically supported on thin membranes 6,8,27 or buried in a thin embedding material 28 .…”

Extremely confined gap surface-plasmon modes excited by electrons

“…In contrast, the use of swift electrons to excite SPs offers a powerful spectroscopic technique known as electron energy-loss spectroscopy (EELS) 7 . Performing EELS in state-ofthe-art transmission electron microscopes (TEMs) equipped with a monochromator and aberration corrector offers unmatched simultaneous spatial and spectral resolution 7,8 . Ritchie predicted the excitation of SPs using electrons 9 , and, more recently, lowloss EELS has become an increasingly popular technique in plasmonics, especially in the study and mapping of localized SP resonances 8,[10][11][12][13][14][15] .…”

“…Performing EELS in state-ofthe-art transmission electron microscopes (TEMs) equipped with a monochromator and aberration corrector offers unmatched simultaneous spatial and spectral resolution 7,8 . Ritchie predicted the excitation of SPs using electrons 9 , and, more recently, lowloss EELS has become an increasingly popular technique in plasmonics, especially in the study and mapping of localized SP resonances 8,[10][11][12][13][14][15] . Although the majority of plasmonic EELS studies have been focused on localized SP resonances, EELS has also been used to study propagating SPs (that is, waveguide modes) in, for example, metal thin films 16 and nanowires [17][18][19] , where standing waves are formed by forward-and backwardpropagating waveguide modes.…”