In this Letter, we exploit recent breakthroughs in monochromated aberration-corrected scanning transmission electron microscopy (STEM) to resolve infrared plasmonic Fano antiresonances in individual nanofabricated disk-rod dimers. Using a combination of electron energy-loss spectroscopy (EELS) and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasi-discrete and quasi-continuum localized surface plasmon resonances where infrared plasmonic Fano antiresonances appear. This work illustrates the capability of STEM instrumentation to experimentally observe nanoscale plasmonic responses that were previously the domain only of higher resolution infrared spectroscopies. arXiv:1908.01395v2 [cond-mat.mes-hall]
Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1−10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.
The Au−Al alloy system was investigated via a combinatorial thin film sputtering method for its potential as a plasmonic material. Au x Al 1−x combinatorial libraries were cosputtered from Au and Al elemental targets and the composition, phase, and dielectric function of a ∼350 nm film was determined using energy dispersive spectroscopy (EDS), grazing incidence X-ray diffraction (GIXRD), and spectroscopic ellipsometry, respectively. The phase evolution and optical properties were analyzed after annealing various compositions under a vacuum. The phases present matched the expected phases based on the published Al−Au binary phase diagram at all compositions. Interestingly, the mixed phase Al-AuAl 2 region showed the most optical tunability, where a maximum in the real part of the dielectric function progressively shifted to higher energy for increasing gold concentration. For almost pure AuAl 2 , the imaginary component is largely reduced in the visible range and is comparable to that of pure Al in the UV region. A 20-nm-thick film with composition Au 0.74 Al 0.26 was studied using a (scanning) transmission electron microscope with an in situ laser heating system. The structures of the as-deposited and laser annealed films were determined using selected area diffraction and the bulk plasmon of AuAl 2 and Al realized with electron energy loss spectroscopy. Last, the Au-rich solid solution region was investigated as a surface enhanced Raman spectroscopy (SERS) substrate using the benezenethiol (BT) molecule. Good SERS intensity was maintained up to 30% Al addition where enhancements of 10 5 to 10 7 were still observed.
Carrier-doped semiconductor nanocrystals (NCs) offer strong plasmonic responses at frequencies beyond those accessible by conventional plasmonic nanoparticles. Like their noble metal analogues, these emerging materials can harness free space radiation and confine it to the nanoscale but at resonance frequencies that are natively infrared and spectrally tunable by carrier concentration. In this work we combine monochromated STEM-EELS and theoretical modeling to investigate the capability of colloidal indium tin oxide (ITO) NC pairs to form hybridized plasmon modes, providing an additional route to influence the IR plasmon spectrum. These results demonstrate that ITO NCs may have greater coupling strength than expected, emphasizing their potential for near-field enhancement and resonant energy transfer in the IR region.
Understanding resonant coupling in plasmonic nanoassemblies is a challenging scientific endeavor, especially for particles with complex nanoarchitectures. Our ability to both model and measure this optical behavior, however, has rapidly developed in the last 20 years via a confluence of fabrication, spectroscopy, and theoretical analysis. Here, we precisely nanofabricate, characterize, and model the coupling and infrared optical responses of different plasmonic nanorhombus assemblies. Ranging from a monomer to a pentamer ensemble, experimental and simulated point spectra, spectrum images, and near-field maps agree well with the results of an analytical coupled normal mode model developed here. The analytical model reveals that the infrared optical responses of the nanorhombus systems can be explained by the coupling of the major and minor axis dipole and a quadrupole localized surface plasmon modes arising from the individual nanorhombus monomers. This model can be used to predict the plasmonic behavior of more complicated systems, and it elucidates the role that short-, intermediate-, and far-field coupling effects play in extended plasmonic assemblies more generally. This work also highlights how localized electron probes in the new generation of monochromated aberration-corrected scanning transmission electron microscopes can be used to study the optical responses of nanofabricated assemblies in the infrared spectral regime.
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