Scanning transmission electron microscopy

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"STEM study" redirects here. For investigations involving stem cells, see Stem cell research.
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A STEM equipped with a 3rd-order spherical aberration corrector
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Inside the corrector (hexapole-hexapole type)

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is [stem] or [esti:i:em]. As with any transmission illumination scheme, the electrons pass through a sufficiently thin specimen. However, STEM is distinguished from conventional transmission electron microscopes (CTEM) by focusing the electron beam into a narrow spot which is scanned over the sample in a raster.

The rastering of the beam across the sample makes these microscopes suitable for analysis techniques such as mapping by energy dispersive X-ray (EDX) spectroscopy, electron energy loss spectroscopy (EELS) and annular dark-field imaging (ADF). These signals can be obtained simultaneously, allowing direct correlation of image and quantitative data.

By using a STEM and a high-angle detector, it is possible to form atomic resolution images where the contrast is directly related to the atomic number (z-contrast image). The directly interpretable z-contrast image makes STEM imaging with a high-angle detector appealing. This is in contrast to the conventional high resolution electron microscopy technique, which uses phase-contrast, and therefore produces results which need interpretation by simulation.

Usually a STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors and needed circuitry; however, dedicated STEMs are also manufactured.

History

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Microscope schematic

In 1925, Louis de Broglie first theorized the wave-like properties of an electron, with a wavelength substantially smaller than visible light.[1] This would allow the use of electrons to image objects much smaller than the previous diffraction limit set by visible light. The first STEM was built in 1938 by Baron Manfred von Ardenne,[2][3] working in Berlin for Siemens. However, at the time the results were inferior to those of transmission electron microscopy, and von Ardenne only spent two years working on the problem. The microscope was destroyed in an air raid in 1944, and von Ardenne did not return to his work after WWII.[4]

The technique was not developed further until the 1970s, when Albert Crewe at the University of Chicago developed the field emission gun[5] and added a high quality objective lens to create a modern STEM. He demonstrated the ability to image atoms using an annular dark field detector. Crewe and coworkers at the University of Chicago developed the cold field emission electron source and built a STEM able to visualize single heavy atoms on thin carbon substrates.[6]

Aberration-corrected STEM was demonstrated with 1.9 angstrom resolution in 1997[7] and soon after in 2000 with roughly 1.36 angstrom resolution.[8] Aberration-corrected STEM provided the added resolution and beam current critical to the implementation of atomic resolution chemical mapping with spectroscopic techniques.


Aberration correction

The addition of an aberration corrector to electron microscopes enables electron probes with sub-ångström diameters to be used. This has made it possible to identify individual atoms columns with unprecedented clarity.

Room environment

High resolution scanning transmission electron microscopes require exceptionally stable room environments. In order to obtain atomic resolution imaging the room must have a limited amount of room vibration, temperature fluctuations, electromagnetic waves, and acoustic waves.

Biological application

The first application of this method to the imaging of biological molecules was demonstrated in 1971.[9] The motivation for STEM imaging of biological samples is particularly to make use of dark-field microscopy, where the STEM is more efficient than a conventional TEM, allowing high contrast imaging of biological samples without requiring staining. The method has been widely used to solve a number of structural problems in molecular biology.[10][11][12]

Low-voltage electron microscope

A low-voltage electron microscope (LVEM) is operated at relatively low electron accelerating voltage between 5-25 kV. Some of these can be a combination of SEM, TEM and STEM in a single compact instrument. Low voltage increases image contrast which is especially important for biological specimens. This increase in contrast significantly reduces, or even eliminates the need to stain. Resolutions of a few nm are possible in TEM, SEM and STEM modes. The low energy of the electron beam means that permanent magnets can be used as lenses and thus a miniature column that does not require cooling can be used.[13][14]

Electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) as a STEM measurement technique made possible with the addition of an electron spectrometer. The high-energy convergent electron beam in STEM provides local information of the sample, even down to atomic dimensions. With the addition of EELS, elemental identification is possible and even additional capabilities of determining electronic structure or chemical bonding of atomic columns. The low-angle inelastically scattered electrons used in EELS compliments the high-angle scattered electrons in ADF images by allowing both signals to be acquired simultaneously. EELS is a technique popular to STEM microscopists.

See also

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References

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  4. ↑ D. McMullan, SEM 1928 - 1965
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  9. ↑ Wall, J.S., 1971 "A high resolution scanning electron microscope for the study of single biological molecules" PhD thesis, University of Chicago
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  14. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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