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Why EELS?

EELS is a powerful technique that provides compositional and chemical information from sub-nanometer areas in the sample. Due to the large interaction cross-sections and narrow angular distribution of the scattered electrons, EELS is a very sensitive and highly localized technique. The benefit of core-level EELS is its sensitivity to all elements in the periodic table (except atomic hydrogen). The deexcitation processes are competitive, resulting in near zero x-ray yield for low Z elements and near zero Auger yield for high Z elements. These processes reach parity (50/50 yield) around germanium (atomic number 32). In addition, EELS can probe the local density of states, making it uniquely suited to measure changes in local bonding at near-atomic resolution.



High-energy electrons interacting with the sample produce a myriad of signals. The elastic (including BSE) and inelastic interactions are the primary signals, while the other signals are secondary events caused by the sample relaxation after the inelastic scattering event. The EELS signal is strongly peaked in the forward direction, making near 100% collection efficiency possible.

Comparison Table

  EELS EDS XPS Auger CL
Probe → Signal e- → e- e- → \(\gamma\) \(\gamma\) → e- e- → e- e- → \(hv\)
Spatial resolution Å – µm nm – mm µm – mm µm – mm nm – mm
Compositional sensitivity
  • He – U
  • 0.1 atm% typical
  • B – U
  • 0.1 atm% typical
  • Li – U
  • 0.01 atm% submonolayer
  • Li – U
  • 0.1 atm% submonolayer
  • Sensitive to material properties rather than composition
Chemical sensitivity
  • Via edge shape and chemical shift
  • None
  • Via peak shape
  • Limited
  • Indirectly; linked to material properties
Ease of use (technique)
  • Requires setup of electron optics
  • Minimal setup
  • Ultra-high vacuum (UHV) handling
  • UHV handling
  • Methods to reduce charging
  • Requires setup of spectrometer
Strengths
  • High signal-to-noise ratio (SNR)
  • Chemical and composition data
  • Atomic-level localization
  • Unlimited thickness
  • High signal-to-background ratio (SBR)
  • Detects traces
  • Detailed chemical information
  • Excellent low-Z detection
  • SEM level spatial resolution
  • Very sensitive to mineral phases and doping
  • Only process to probe optical emission
Limitations
  • Requires thin samples
  • Some elements overlap
  • Non-local fluorescence
  • Low Z- limits
  • Limited thin-film SNR
  • Surface only
  • Requires UHV
  • >10 µm resolution
  • Surface only
  • Requires UHV
  • Conductive samples
  • Specific use cases
Note: e- indicates electrons (~5 – 200 keV), \(\gamma\) indicates high-energy photons (UV – x-ray), and indicates optical photons (IR – Vis).

For more information on the cathodoluminescence technique, please visit WhatIsCL.info, an educational site.