Scanning transmission electron microscopy
Encyclopedia
A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). As with any transmission illumination scheme, the electron
s pass through a sufficiently thin specimen. However, STEM is distinguished from conventional transmission electron microscope
s (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 STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors and needed circuitry; however dedicated STEMs are manufactured also.
, working in Berlin
for Siemens
. However, the results were inferior to that of TEM at the time, 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 the field after WWII.
The technique did not become developed until the 1970s, with Albert Crewe at the University of Chicago
developing the field emission gun and adding a high quality objective lens to create the modern STEM, and demonstrated the ability to image atoms using ADF.
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.
, temperature fluctuations, electromagnetic waves, and acoustic waves.
(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.
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s pass through a sufficiently thin specimen. However, STEM is distinguished from conventional transmission electron microscope
Conventional transmission electron microscope
Conventional transmission electron microscope is a rarely used abbreviation referring to transmission electron microscope and high resolution transmission electron microscope . It is used to distinguish those parallel illumination techniques from convergent beam scanning transmission electron...
s (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
Energy-dispersive X-ray spectroscopy
Energy-dispersive X-ray spectroscopy is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on the investigation of an interaction of a some source of X-ray excitation and a sample...
(EDX) spectroscopy, electron energy loss spectroscopy
Electron energy loss spectroscopy
In electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...
(EELS) and annular dark-field imaging
Annular dark-field imaging
Annular dark-field imaging is a method of mapping samples in a scanning transmission electron microscope . These images are formed by collecting scattered electrons with an annular dark-field detector....
(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
Atomic number
In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
(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
High Resolution Transmission Electron Microscopy
High-resolution transmission electron microscopy is an imaging mode of the transmission electron microscope that allows the imaging of the crystallographic structure of a sample at an atomic scale. Because of its high resolution, it is an invaluable tool to study nanoscale properties of...
technique, which uses phase-contrast, and therefore produces results which need interpretation by simulation.
Usually STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors and needed circuitry; however dedicated STEMs are manufactured also.
History
In 1925, Louis de Broglie first theorized the wave-like properties of an electron, with a wavelength substantially smaller than visible light. 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 ArdenneManfred von Ardenne
Manfred von Ardenne was a German research and applied physicist and inventor. He took out approximately 600 patents in fields including electron microscopy, medical technology, nuclear technology, plasma physics, and radio and television technology...
, working in Berlin
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
for Siemens
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....
. However, the results were inferior to that of TEM at the time, 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 the field after WWII.
The technique did not become developed until the 1970s, with Albert Crewe at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
developing the field emission gun and adding a high quality objective lens to create the modern STEM, and demonstrated the ability to image atoms using ADF.
Crewe and coworkers at the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
developed the cold field emission electron source and built a STEM able to visualize single heavy atoms on thin carbon substrates.
Aberration correction
Aberration corrected machines have provided electron probes with sub-angstrom dimensions. This has enabled a new regime of atomic imaging.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 vibrationVibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
, 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. 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.Low-voltage electron microscope (LVEM)
The low-voltage electron microscope (LVEM) is a combination of SEM, TEM and STEM in one instrument, which operated at relatively low electron accelerating voltage of 5 kV. 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. Sectioned samples generally need to be thinner than they would be for conventional STEM (20–70 nm). Resolutions of a few nm are possible in TEM, SEM and STEM modes.Electron energy loss spectroscopy
Electron energy loss spectroscopyElectron energy loss spectroscopy
In electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...
(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
- Electron beam induced depositionElectron beam induced depositionElectron beam-induced deposition is a process of decomposing gaseous molecules by electron beam leading to deposition of non-volatile fragments onto a nearby substrate...
- Electron diffractionElectron diffractionElectron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern...
- Electron energy loss spectroscopyElectron energy loss spectroscopyIn electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...
(EELS) - Electron microscopeElectron microscopeAn electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
- Energy filtered transmission electron microscopyEnergy filtered transmission electron microscopyEnergy-filtered transmission electron microscopy is a technique used in Transmission electron microscopy, in which only electrons of particular kinetic energies are used to form the image or diffraction pattern...
(EFTEM) - High-resolution transmission electron microscopy (HRTEM)
- Low-voltage electron microscopy (LVEM)
- Scanning confocal electron microscopyScanning confocal electron microscopyScanning confocal electron microscopy is an electron microscopy technique analogous to scanning confocal optical microscopy . In this technique, the studied sample is illuminated by a focussed electron beam, as in other scanning microscopy techniques, such as scanning transmission electron...
- Scanning electron microscopeScanning electron microscopeA scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
(SEM) - Transmission Electron Aberration-corrected MicroscopeTransmission Electron Aberration-corrected MicroscopeTransmission Electron Aberration-Corrected Microscope is a collaborative research project between four US laboratories and two companies. It is based at the Lawrence Berkeley National Laboratory in Berkeley, California and involves Argonne National Laboratory, Oak Ridge National Laboratory and...