SEM stands for scanning electron microscope, an electron microscope that scans a specimen’s surface with a focused beam of electrons to produce high‑resolution images.
Explanation
The scanning electron microscope (SEM) is a type of electron microscope that produces images by rastering a finely focused electron beam over a specimen and collecting signals produced by electron–sample interactions. When the beam strikes the surface, it generates secondary and backscattered electrons, as well as characteristic X‑rays, which are detected to build a detailed map of the sample. Because electrons have much shorter wavelengths than visible light, SEMs achieve much higher resolution than light microscopes【130993524274552†L72-L85】. Modern instruments can resolve structures to the nanometre scale and provide depth‑of‑field images that reveal surface topography of microorganisms, cells, and materials【316925961926531†L60-L67】.
Samples for SEM must be dehydrated and typically coated with a thin conductive layer (gold, platinum, or carbon) to prevent charging in the vacuum chamber. The specimen is placed in a high‑vacuum environment where electromagnetic lenses focus and deflect the electron beam. Detectors collect emitted electrons to form an image displayed on a monitor. The SEM is widely used in microbiology, materials science, nanotechnology, and failure analysis to examine surface morphology, cell structures, viruses, and biofilms. Unlike transmission electron microscopy, which transmits electrons through ultrathin sections, SEM provides a three‑dimensional view of surfaces.
Applications and Features
- Uses a focused electron beam and detectors to map surface topography and composition.
- Requires sample preparation: fixation, dehydration, and conductive coating to withstand vacuum and electron bombardment.
- Produces images with nanometre resolution and large depth of field, making it ideal for viewing bacteria, viruses, spores, pollen, and engineered materials.
- Employed in quality control, forensic investigations, semiconductor inspection, and environmental studies.
- Limitations include the need for non‑living specimens and potential artefacts from preparation and coating.
The scanning electron microscope is a cornerstone tool in modern microscopy, providing detailed surface images that cannot be obtained with light microscopes. Its ability to reveal the ultrastructure of microorganisms and materials has made it indispensable in research and industrial laboratories.
Related Terms: Electron microscope, Transmission electron microscope, Micrograph, Magnification, Resolution.