In transmission electron microscopy (TEM), a beam of accelerated electrons is transmitted through a selected region of an electron-transparent sample. By using Diffraction mode, the high-energy electrons in the beam are treated as waves, with wavelengths on the order of 1 / 1000^th of a nanometer. At this scale, atoms in the solid sample act as a diffraction grating, scattering the beam electrons at angles corresponding to the crystalline structure of the illuminated region of the sample. The interactions of the electrons (as waves) in the transmitted beam create an electron diffraction pattern, which can be used to measure lattice parameters, identify crystalline phase, and even interpolate the crystal structure and orientation of the target area of the sample.

Selected Area Electron Diffraction (SAED) collects the electrons scattered from a specific region, targeted with nanoscale precision using a specialized aperture inserted in the electron beam’s path before it reaches the sample. This gives SAED a spatial resolution up to several hundred times greater than X-ray diffraction techniques.

Like SEM systems, STEM instruments use a narrow, focused electron beam spot to probe the sample, scanning it in a raster pattern over an analytical area of interest. The output image is produced by detecting the scattered signal intensity at each pixel as the beam scans.

In a STEM – as in a TEM – the detector is mounted underneath the sample and picks up electrons which are transmitted through it. As such, STEM can only be used when the sample is sufficiently thin to be electron-transmissive.

In Covalent’s (S)TEM-enabled instruments, STEM offers improved spatial resolution compared to an SEM, and improved spatial correlation (for element mapping) compared to normal TEM.

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.

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.