Scanning electron microscopy
Scanning electron microscopy (SEM) is a precise and fast technique for studying the microscopic structures of material’s surfaces. It also allows to perceive the topology of the material.
SEM has many applications in research and product development, such as imaging the surface morphology of the produced particles or coatings. Because of its flexibility, SEM can be used to solve many different problems occurring in the production process.
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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) used. 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 light microscope. The higher the resolution, the more high-quality images can be produced.
The basic operation principle of SEM
A good scanning electron microscope (SEM) can have a resolution less than 1 nm. With a scanning electron microscope the detailed surface structures of the sample can be seen with high accuracy in which case 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 SEM machines follow the same basic operating principle. In SEM a beam of electrons wipes the surface of the sample systematically. Electrons are accelerated from a source of electrons and directed through multiple electromagnetic lenses and apertures before hitting the sample. 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, and those electrons are detected by the BSE (backscatter electron) detector. This provides information about the distribution of different elements in the sample, because electrons scatter more from heavier elements which can be seen 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 a 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 which turns the information into a picture. The picture of the sample forms scan by scan of the beam and pixel by pixel of the screen to 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, for example phosphor, and cause the material to emit photons which can be seen with naked eye as light if they have wavelengths in the visible spectrum.
Samples used in SEM must be dry and their surfaces electricity conducting. 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 are for example platinum, gold, palladium and carbon.
Suitable sample matrices
- Solid samples
- Different materials such as metals or polymers
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
A large selection of needs both in science and industry can be met with SEM, especially when additional detectors are attached to the microscope. However, SEM technique alone is also very useful for all kinds of research. Large and heavy samples can be examined with SEM and an excellent image quality with challenging materials and very small details can be obtained. SEM analyses of different materials, for example discovering what the microscopic surface structures of a metal look like, can be very useful for the research and development work. Finding out the lengths of the spaces between different particle level components in the material as well as its breaking mechanisms and reasons can help to optimize the production processes, too.
Because a lot of accessories can be added to the microscope, SEM can also be used for other purposes than imaging. In addition to visualizing detectors (BSE and SE detectors), various analytical detectors such as EDX (also called EDS), WDS and EBSD together with SEM provide different types of information about the sample of interest. Both EDX and WDS can be used for analysing the composition of the sample by identifying elements and measuring their concentrations. EBSD is a good tool for examining the crystalline structure of the material for example in industrial manufacturing or materials science. With the help of these detectors particle identification and structural analysis can be performed with SEM to ensure for example the quality of the product.
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.
Due to SEM-EDX, it can be found out if the produced material has some additional substances and at which point of the manufacturing process they have ended up in it. Elemental mapping, where different elements are marked with different colors in the picture of the sample, can be performed with SEM-EDX for example to find out the cause of corrosion or some other chemical reaction in the sample. More information about SEM-EDX can be found from our SEM-EDX page.
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 wanted to keep in one piece and undamaged (especially with biological samples).
If the sample is too large for the microscope, it may also need some cutting before the analysis. Other sample preparation techniques are usually needed if the sample is dirty, wet or does not conduct electricity.
The elements present in the surface of the sample cannot be identified with SEM alone. Instead, SEM-EDX and XRF are suitable methods for this purpose.
Solid samples can be analysed with SEM. Samples must be dry and their surfaces electricity conducting for most of the SEM techniques to work. If the sample does not meet these requirements, it must often be prepared before SEM.
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.