A scanning electron microscope, or SEM, is one of the most popularly used instrumental methods for examining and analyzing the microstructure and morphology of solid materials. Basically, it operates using the same principles as a light microscope with one main difference: It uses focused electron beams instead of photons in magnifying an object.
In an SEM, a beam of highly energetic electrons is focused on the sample material to create an image. As the electrons in the beam enter and interact with the sample, it produces multiple interactions, which may include the emission of secondary electrons, backscattered electrons, photons, and X-rays, either from or near the sample surface. Under certain conditions, it may also result in the excitation of phonons and diffraction.
Scanning Electron Microscope: How Does It Work?
The basic SEM is composed of several key components:
- Electron source (i.e., tungsten filament, solid-state crystal, and field emission gun or FEG)
- A series of condenser lenses
- Sample chamber
- Electron detectors
- Computer for viewing the images
After the electrons are produced at the source (located at the top of the column) through thermionic heating, they are accelerated down the column and are forced to pass through a series of condenser lenses to produce a focused beam of electrons. Once the beam focuses on the surface of the sample (which is mounted on a stage in the chamber area), it is deflected in the X and Y axes by the scanning coils located above the objective lens.
The electron beam can penetrate the surface of the sample up to a few microns deep and produce several interactions or signals from different penetration depths. The signals are then collected and mapped by one or more detectors to generate images on the computer screen.
Listed below are some of the most common detectors used in a scanning electron microscope.
- Secondary electron detector (SED). This detector is capable of producing high-resolution images (1-2 nm) from inelastically scattered electrons near the sample surface.
- Backscattered electron detector (BSD). The BSD spots elastically scattered electrons that have higher energy compared with atoms below the sample surface. Since BSD is capable of operating at lower vacuum levels, you can significantly reduce sample preparation requirements and consequently prevent beam damage by using it.
- Energy dispersive spectroscopy (EDS). Analyzing three-dimensional images of the microscopic structure may not be enough when you’re into elemental analysis. EDS addresses this problem by detecting the x-rays emitted as the electron beam displaces an inner shell electron that is replaced by an outer shell electron. Since different elements produce a unique energy difference between their inner and outer electron shells, the EDS data obtained can help you identify the different elements in a specimen.
Applications of SEM in the Biosciences
- In cell and molecular biology, SEM is used to explore the 3D structure (shape, size, structure, and form) of cells and tissues.
- In microbiology, this instrumental method allows for the investigation of viruses and bacteria and their interactions in a biological context. Additionally, it serves as an indispensable tool in studying the effects of antibiotics in bacterial morphology.
- SEM plays a vital role in medicine, particularly in exploring the nature and mechanisms of a disease to facilitate accurate diagnosis and formulate appropriate medications.
- SEM can also be used to analyze trace evidence (e.g., hairs, fingerprints, blood and other biological substances, gunshot residue, fibers, etc.) and other factors to discover more about the events surrounding a particular crime and provide solid evidence for law purposes.
Aside from being one of the most flexible instruments and magnification tools in materials science, SEM can be used in combination with other instruments such as a scanning probe microscope to provide even greater control and higher precision in studying nanostructures.