Basic idea of how scanning electron microscope work
Basic concept of a scanning electron microscope
Imagine you're on a rescue boat searching for survivors in the water at night. It's pitch black out, but you have a searchlight. Rather than just pointing the searchlight in one direction and hoping the survivors happen to swim into view, you sweep the beam back and forth across the waters. At any single moment, you only see the spot where the beam is pointing as the light rays fly from the searchlight to the water, and then back into your eye. However, you create in your mind a complete picture of the scene by mentally stitching that mosaic of spots together into a whole. An SEM works just like this, except:
Instead of a searchlight shining photons of light, an SEM shines a beam of electricity (electrons).
Instead of just light reflecting off the target, an SEM reflects some electrons and also has other interactions that shine back toward you.
Just like the searchlight, the SEM sweeps the electron beam back and forth in a systematic scanning pattern.
Just like the searchlight, the SEM only sees one spot at a time.
Instead of light-detecting eyes, an SEM has electron and x-ray detecting detectors.
Instead of a human mind, an SEM has a computer that compiles the mosaic spots into a big picture...that gets interpreted by a human mind!
This diagram illustrates the process in a step-by-step manner:
Electron gun creates electron beam (kind of like a light bulb)
Condenser lens tidies beam (kind of like a flashlight's shiny reflector)
Aperture trims edges of beam so not fuzzy
Beam booster slows electrons to desired energy level
Magnetic lens and
Electrostatic lens focuses beam so forms cone-shape instead of cylinder
Scan coils push beam back/forth so they sweep across sample
Electron beam reflects/etc. off sample to send signal to detectors.
The letters in the diagram designate the different kinds of detectors.
How does the electron beam interact with the sample?
This is a classic diagram that summarizes the different ways electrons interact with the atoms in a sample. Much more complex that reflecting a searchlight beam, eh? Different electron microscopes are configured to detect different kinds of interactions, depending on what the researchers are trying to study.
Our electron microscope is configured to detect:
Secondary electrons (SE) are electrons that are part of the sample that get knocked out of the sample by the electron beam. This mainly happens to atoms at the topmost surface of the sample, so secondary electron images give us a view of the shapes on the surface of samples. Detectors 🅐🅑🅒
Please click here to see examples of secondary electron imaging.
Backscatter electrons (BSD) are electrons from the original beam itself that "bounce" back in a special way called "backscattering" so that they don't lose any energy. Because materials with more protons in their atomic nuclei are better at "bouncing" beam electrons back, backscatter electron imaging enables us to distinguish between things with different chemical compositions. Detectors 🅓🅔
Please click here to see examples of backscatter electron imaging.
Each element on the periodic table glows a characteristic x-ray energy. The energy dispersive spectrometer (EDS) analyzes the precise chemical composition of any point on the sample by identifying what energies of x-rays glow from the sample (tells us what elements are present), and how brightly each energy x-ray glows (tells us concentrations of each element present). Detector 🅕
Please click here to see examples of x-ray spectral analysis and chemical mapping.
Transmitted electrons (STEM) pass all the way through the sample. Some electrons run into atoms along the way and so do not make it all the way through the sample to the electron detector underneath. The STEM detector is useful for studying organelles inside cells, viruses, and nanoparticles. Detector 🅖
Please click here to see examples of transmitted electron imaging.