Ascanning electron microscope(SEM) scans a focused electron beam end a surface to produce an image. The electrons in the beam interact with the sample, developing various signals that can be supplied to achieve information around the surface topography and also composition.

You are watching: What does sem stand for in biology


*

Why usage electrons rather of light in a microscope?

Given enough light, the human being eye can differentiate two points 0.2 mm apart, without the help of any additional lenses. This street is dubbed the fixing power or resolution of the eye. A lens or one assembly the lenses (a microscope) deserve to be provided to magnify this distance and enable the eye to see points even closer together than 0.2 mm.

A modern-day light microscope has actually a preferably magnification of about 1000x. The addressing power the the microscopic lense was no only minimal by the number and quality of the lenses but additionally by the wavelength the the light offered for illumination. White light has wavelengths native 400 to 700 nanometers (nm). The median wavelength is 550 nm which outcomes in a theoretical border of resolution (not visibility) the the light microscope in white light of about 200 – 250 nm. The figure below shows 2 points in ~ the borders of detection and also the two individual spots deserve to still be distinguished. The right picture shows the 2 points therefore close together that the main spots overlap.

*
Two points reflecting the borders of detection

The electron microscope was occurred when the wavelength ended up being the limiting factor in irradiate microscopes. Electrons have actually much much shorter wavelengths, enabling far better resolution.

Compare one Optical microscopic lense vs a Scanning Electron Microscope

As dimensions are shrinking because that materials and also devices, many structures can no much longer be identified by light microscopy. Because that example, to identify the truth of a nanofiber layer for filtration, as presented here, electron microscopy is forced to characterize the sample.


*
Optical microscope picture of nanofibers
*
Scanning electron microscope photo at 4000x magnification the the exact same nanofibers

How a Scanning Electron microscope Works

The mainSEM componentsinclude:

Sourceof electronsColumndown which electrons travel v electromagnetic lensesElectrondetectorSamplechamberComputerand display screen to see the images

Electrons are developed at the peak of the column, sped up down and passed through a combination of lenses and also apertures to develop a focused beam of electrons which access time the surface of the sample. The sample is an installed on a phase in the chamber area and, uneven the microscope is designed to run at short vacuums, both the column and also the chamber space evacuated by a combination of pumps. The level that the vacuum will count on the architecture of the microscope.

*
Schematic the a Scanning Electron Microscope

The position of the electron beam top top the sample is controlled by scan coils situated over the target lens. These coils enable the beam to be scanned over the surface ar of the sample. This beam rastering or scanning, as the surname of the microscope suggests, allows information around a defined area on the sample to be collected. As a result of theelectron-sample interaction, a number of signals space produced. This signals room then detect by proper detectors.

Sample-Electron Interaction

The scanning electron microscope (SEM) produces images by scanning the sample v a high-energy beam that electrons. Together the electrons interact with the sample, castle produce second electrons, backscattered electrons, and characteristic X-rays. These signals are built up by one or much more detectors to kind images which are then presented on the computer screen. When the electron beam access time the surface of the sample, that penetrates the sample come a depth of a few microns, depending on the speeding up voltage and also the density of the sample. Countless signals, like secondary electrons and also X-rays, are developed as a an outcome of this communication inside the sample.

*
Schematic the electron beam interaction.

See more: Difference Between Convex And Concave Polygon S: Different Kinds Of Polygons

The maximum resolution derived in one SEM relies on multiple factors, favor the electron point out size and interaction volume of the electron beam with the sample. While the cannot administer atomic resolution, some SEMs can accomplish resolution below 1 nm. Typically, modern full-sized SEMs provide resolution in between 1-20 nm whereas desktop systems can provide a resolution that 20 nm or more.