Electron Energy Loss Spectroscopy (EELS) is a technique performed with Scanning Transmission Electron Microscopes (STEM). The STEM technique uses a focused electron beam which is raster scanned over an electron-transparent sample. As the beam transmits through the sample, electrons will interact with and scatter from the sample atom: either passing without energy disturbance (elastic scattering) or losing energy through interactions with electrons bound to the sample atoms.

When an electron in the beam interacts with a core electron in a sample atom, it can transfer enough energy to induce excitation, resulting in a characteristic drop in the probe electron’s kinetic energy. The energy loss can be measured by an electron spectrometer and mapped to the specific element and energy level of the sample electron. EELS spectra contain features called Core Loss Edges that indicate the elements present in the scanned area of the sample, as well as their concentrations (in Atomic %). Beyond composition analysis, the chemical state and bonding of atoms in the material can be determined by analyzing the energy loss near edge structure (ELNES).

By evaluating the inelastically scattered electrons with these characteristic energy losses, EELS produces spectra that can be used to interpret elemental composition, sample thickness, valence electron density, optical response properties, and band structure.

Energy Dispersive Spectroscopy (EDX / EDS) can be performed in both scanning electron microscopes (SEM) and scanning transmission electron microscopes (STEM). In STEM, the electron beam is rastered over an electron transparent sample.

When electrons in the beam excite core electrons in the sample atoms, the subsequent relaxation of the core electrons produces characteristic x-ray emissions. These x-ray photons are then collected and analyzed according to their energies, producing an EDS spectrum. The energies of peaks in the spectrum correspond to the elements present in the scanned area of the sample, and the count of each kind of X-rays is proportional to the concentration of the corresponding element. Because a complete spectrum is collected at every pixel analyzed in the STEM image, elemental composition can both be quantified and mapped across the scanned region in the sample.

Quad-EDS detectors on Covalent’s Talos S/TEM system have a large solid angle and a symmetric orientation to collect the X-ray signal free from sample shadowing. This provides accurate quantification of elemental composition for all elements from Carbon to Uranium.