Advances in ultrahigh-energy resolution EELS: phonons, infrared plasmons and strongly coupled modes

Abstract

Nowadays, sub-50 meV atom-wide electron probes are routinely produced for electron energy loss spectroscopy in transmission electron microscopes due to monochromator technology advances. We review how gradual improvements in energy resolution enabled the study of very low-energy excitations such as lattice phonons, molecular vibrations, infrared plasmons and strongly coupled hybrid modes in nanomaterials. Starting with the theoretical framework needed to treat inelastic electron scattering from phonons in solids, we illustrate contributions in detecting optical surface phonons in photonic structures. We discuss phonon mapping capabilities in real and reciprocal space, and the localized phonon response near nano-/atomic-scale structural features. We also survey the progress of aloof spectroscopy in studying vibrations in organic materials and applications in measuring local temperature and photonic density of states in single nanostructures using phonon scattering. We then turn towards studies on infrared plasmons in metals and semiconductors. Spectroscopy analyses now extend towards probing extremely complex broadband platforms, the effects of defects and nanogaps, and some far-reaching investigations towards uncovering plasmon lifetime and 3D photonic density of states. In doped semiconductors, we review research on the use of the electron probe to correlate local doping concentration and atomic-scale defects with the plasmonic response. Finally, we discuss advances in studying strong coupling phenomena in plasmon–exciton and plasmon–phonon systems. Overall, the wealth of information gained extends our knowledge about nanomaterial properties and elementary excitations, illustrating the powerful capabilities of high-energy resolution scanning transmission electron microscopy–electron energy loss spectrometry.

Infrared Plasmons in Single La-doped BaSnO3 Nanocrystals Revealed by Monochromated STEM-EELS

“…Electron energy-loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) has been widely used for probing surface plasmons with high spatial resolution [1]. With the recent improvement in energy resolution [2], infrared plasmons in conducting oxides [3,4] and metal nanostructures [5,6] have also been studied in greater detail.…”

High Resolution Electron Energy-Loss Spectroscopy for the Study Organic and Biological Systems in the Scanning Transmission Electron Microscope

“…This has been achieved with the development of high-performance electron monochromators, improved spectrometer electron optics and introduction of new detectors with a high detector quantum efficiency (DQE). [1] For example, using a STEM equipped with a thermally assisted field emission source enables EELS to be performed with an energy resolution (E) in the range 0.5-0.8eV as measured by the full width half maximum (FWHM) of the zero-loss peak in the spectrum. Introduction of a commercially available monochromated instrument at the turn of the century enabled E<0.2eV.…”

“…[3] This groundbreaking design initially delivered E in 10-20meV range, but improvements in electron optics and detectors have enabled 4-6 meV to achieved on a number of instruments around the world. [1] This level of performance is opening up new opportunities for the study of both electronic and vibrational excitations using high resolution EELS in the scanning transmission electron microscope (STEM). One of the most exciting, and challenging, research opportunities for high resolution STEM-EELS is to probe the spatial distribution of functional groups in organic/ biological systems.…”

Multi-Sun EELS: Ultra-High Energy Resolution combined with High Spatial Resolution and High Beam Current

“…Examples include distinguishing between 13 C vs 12 C in an amino acid (L-alanine) and mapping their distribution with ~30nm resolution [2], detecting the vibrational signal from a single defect in a solid [3], and recording the vibrational spectrum of a single atom [4]. Other examples can be found in a recent review of the progress [5]. The field continues to advance rapidly, facilitated by continuing advances in the monochromator and the spectrometer [6], and by the development of new operating modes.…”