Scanning electron microscopy
Scanning electron microscopy (SEM) is a precise and fast technique for studying the microscopic structures and topology of a material’s surface. SEM has many applications in research and product development, such as imaging the surface morphology of particles or coatings. Because of its flexibility, SEM analysis can be used to solve a range of problems in the production process.
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The basic operation principle of SEM
A good scanning electron microscope (SEM) can have a resolution of less than 1 nm. With SEM, the detailed surface structures of the sample can be seen with high accuracy, which means that a precise image of its surface morphology and even topography can be obtained. There are many different SEM machines, additional detectors, and techniques available for different demands, but all these devices follow the same basic operating principle.
In SEM, a beam of electrons systematically wipes the sample's surface. Electrons are accelerated from a source of electrons and directed through multiple electromagnetic lenses and apertures before hitting the sample. The electrons interact with the surface of the sample and produce different signals when they deviate from their original direction. After the interaction has happened, an electron detector detects the electrons.
Some of the electrons that hit the surface of the sample backscatter, after which they are detected by the BSE (backscatter electron) detector. This provides information about the distribution of different elements in the sample, as electrons scatter more from heavier elements, which can be observed as a difference in contrast between different areas of the sample.
The beam of electrons also releases secondary electrons (SE) from the sample, which come off and scatter from its surface. Only such secondary electrons which are very close (less than 10 nm) to the surface can come free and be detected by the SE detector. The signal of the scattered electrons is stronger and brighter when the sample is located close to the detector. Therefore, the differences in the surface topography appear clearly in the final picture of the sample.
The formation of the picture
The detector is connected to a computer that turns the information into a picture. The picture of the sample is formed scan by scan and pixel by pixel on the screen of the computer. In addition to electrons, some SEM machines can detect light, too. When electrons interact with the surface of the sample, they make the sample produce cathodoluminescence. Cathodoluminescence happens when electrons interact with a luminescent material such as phosphor and cause the material to emit photons which can be seen with the naked eye as light if they have wavelengths in the visible spectrum.
Samples used in SEM must be dry, and their surfaces must conduct electricity. If not (for example in the case of biological samples), they must be pretreated before SEM. Usually, samples that do not conduct electricity must be coated with a conducting material before analysis. Suitable coatings include platinum, gold, palladium, and carbon.
The difference between SEM and traditional microscopy
The resolution of a picture taken with a microscope - that is, the shortest possible distance between two different targets with which the targets can still be distinguished from each other - depends on the wavelength of the electromagnetic radiation (for example light). Because electrons have a shorter wavelength than photons, electrons can be used instead of light to get a higher resolution. Therefore, the smallest details of the sample can be seen more closely with an electron microscope than with a traditional optical microscope. If an even higher resolution is needed, transmission electron microscopy (TEM) may be used.
SEM can also be combined with EDX (energy-dispersive X-ray spectroscopy) to perform SEM-EDX analysis. With this method combination, it is possible to determine the elemental composition of the sample in addition to its surface structures.
Suitable sample matrices
- Solid samples
Ideal uses of SEM
- Product development and quality control, for example failure analyses
- Material examination, for instance observation and measurement of the tiny detailed structures of the material’s surface and analysis of its breaking mechanisms
- Study of complex environmental and biological samples to find out the microscopic structures of their surfaces
Frequently asked questions
One common application of SEM is in particle size distribution analysis. The advantage of SEM compared with other particle analysis methods is its ability to detect particle shape as well as size.
Scanning electron microscopy is also used to screen solid surfaces for cracks or impurities, detect nanoparticles in food samples, and analyze the thickness of semiconductive thin films.
In the SEM-EDX method, an EDX detector (also called EDS) is connected to a standard SEM machine. SEM-EDX can be used for identifying elements and determining their distribution and concentrations in the sample. When electrons interact with the surface of the sample, they make the sample produce X-rays which can be detected with an EDX detector. Because every element has its own kind of X-ray spectrum, the elements and compounds in the sample along with their concentrations can be discovered with SEM-EDX.
In addition to EDX, an X-ray detector (XRF) can also identify elements even to the nearest micrometer based on their X-ray spectra when used together with SEM.
With SEM, only the surface structures of the sample can be examined. Even though cross-sectioning of the sample can be used to see the inner structures, this method is not an option if the sample and its inner parts are to be kept in one piece and undamaged (especially with biological samples).
If the sample is too large for the microscope, it may need to be cut before the analysis. This is often done using focused ion beam technology. Other sample preparation techniques are usually needed if the sample is dirty, wet, or does not conduct electricity.
The elements present on the surface of the sample cannot be identified with SEM alone. Instead, SEM-EDX and XRF are suitable methods for this purpose.
Solid samples are suitable for SEM analysis. Samples must be dry and their surfaces electricity-conducting for most SEM techniques to work. If the sample does not meet these requirements, it must often be pretreated before imaging.
Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.
When you contact us through our contact form or by email, one of our specialists will take ownership of your case and answer your query. You get an offer with all the necessary details about the analysis, and can send your samples to the indicated address. We will then take care of sending your samples to the correct laboratories and write a clear report on the results for you.
Samples are usually delivered to our laboratory via courier. Contact us for further details before sending samples.