Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals

Abstract

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.

Imaging Infrared Plasmon Hybridization in Doped Semiconductor Nanocrystal Dimers

“…Localized surface plasmons (LSPs) concentrate optical energy into spatial regions significantly smaller than the diffraction limit. − While well understood at visible frequencies, ,, plasmonic responses in the infrared (IR) region are more difficult to achieve, despite being of considerable interest for applications in communications, , energy harvesting, − molecular sensing, , and surface-assisted photocatalysis. , Carrier-doped semiconductors offer a new class of nanomaterials with native IR LSP resonances that are tunable by controlling the free carrier concentrations via chemical doping strategies. − As a result, the IR response properties of these semiconductor nanomaterials and nanocrystals (NCs) are the subject of intense recent efforts. ,− …”

“…When in close proximity, the LSPs of individual particles can mix and form new hybridized plasmons with energies and spatial distributions dictated by the coupling strength and detuning of the individual particle responses. , Furthermore, LSPs can be simultaneously coupled to different and competing energy dissipation pathways or hybridize with other resonant modes in the local environment. − This variety of interactions presents a challenge for understanding how energy transfer occurs in nanoscale plasmonic materials. Although plasmon hybridization is well studied in noble metal nanoparticles, ,− it is unclear if NC particles meet the conditions necessary to undergo equivalent hybridization and coupling. ,, Particularly, no previous studies have explored how the range of LSP energies and dielectric parameters of NCs , influence the coupling strength and mixing parameters in few-particle systems. While recent works have examined plasmons of NC bulk aggregates , and the IR LSPs of NC clusters, , a clear picture of the effects of these parameters, as well as doping effects, has yet to emerge.…”

“…The electromagnetic field induced by a focused STEM electron beam behaves like an evanescent white light source, relaxing common far-field optical selection rules and enabling the excitation of the complete set of LSP modes of a plasmonic particle. , Importantly, the resulting plasmon excitations cause electron scattering interactions that can be energy-separated and measured in EELS to determine the resonance features of a nanomaterial. Recent advancements in electron energy monochromation have pushed EELS measurements into the far-IR, providing new utility for the measurement of IR plasmonic materials. − As a result, recent STEM-EELS experiments have characterized the spectral tunability of IR LSPs in doped NCs at the single particle level , and revealed that NCs exhibit geometric LSP modes with independent energy signatures that are analogous to metals. ,, Additionally, hybrid modes of semiconductor NCs have been extracted from EELS maps of 3–4 particle fluorine–ITO clusters using multivariate analysis; however, to date no studies have examined the hybrid modes of spherical NC dimers.…”

“…Figures b and c show the experimentally measured STEM annular dark field (ADF) images and corresponding EEL spectra of an ITO NC monomer and dimer with 10% tin doping. The monomer spectrum (red) has a single peak at 770 meV corresponding to the three degenerate dipolar LSPs . The aloof spectrum acquired at the dimer end (green) exhibits features at 565 and 800 meV.…”

“…The same probe positions are used for the spectra in Figure e as in Figure c. At the 2% Sn 4+ dopant level, the bulk plasmon is lower in energy when compared to the 10% Sn 4+ , and this difference directly contributes to the difference in the surface plasmon energies observed . The aloof spectra of the 2% ITO monomer (red) has a feature at 440 meV that corresponds to the dipolar LSP, which is ∼300 meV lower in energy compared to the same dipolar LSP mode of the 10% ITO monomer.…”