What makes an electron microscope

However, transmission electron microscopes are capable of studying defects in crystals at the atomic scale. A hybrid of these two instruments is the scanning transmission electron microscope STEM , developed in by Albert Crewe at the University of Chicago. By scanning a focused electron beam that passes through a thin sample with little spreading, the STEM has excellent local chemical-analysis capabilities like the scanning electron microscope because only a small part of the sample is illuminated by the small focused spot.

And like transmission microscopes, it also has good enough spatial resolution to produce images of atoms. In this device, a large electric field is applied to a sharp metal tip in a low pressure gas such that the gas atoms arriving at the tip are ionized and then accelerated away until they hit a screen.

Because this process is more likely to occur at certain places on the surface of the tip, such as at steps in the atomic structure, the resulting image represents the underlying atomic structure of the sample. Although the STM is an electron microscope, it does not depend on an electron beam in a vacuum like the transmission or scanning electron microscope.

Instead, it relies on the quantum-mechanical tunnelling of electrons between a sharpened tip and a conducting surface. Binnig, Rohrer and Ruska shared the Nobel Prize for Physics for the development of these devices. So why has it taken almost seven decades of research to reach a resolution that is still 50 times worse than the fundamental limit imposed by the de Broglie wavelength?

In order to achieve a resolution of 78 pm using electrons with a wavelength of 2 pm, the aperture of the lens that controls the width of the beam must therefore subtend an angle of more than 1. And for accurate focusing, the lens needs to shape the wavefront over this distance with a precision better than one quarter of the wavelength i. In units that are slightly easier to comprehend, this is equivalent to engineering a surface the width of the US with no variations in height greater than 3 cm!

The challenge appears even more difficult when you consider that the only tool available to shape the electron beam is a magnetic or electric field. This prevents us from making a field with precisely the right shape and results in aberrations that limit the performance of the microscope.

Indeed, soon after the invention of the transmission electron microscope, the German Otto Scherzer showed that a perfect, time-invariant, circularly symmetric lens can never be formed by an electric or magnetic field in free space: there will always be inherent spherical aberration. The term spherical aberration was first applied in light optics and arises from the blurring that is seen when lenses or mirrors with spherical surfaces are used.

In an electron lens, spherical aberration causes rays at large angles to come to a premature focus figure 2. Spherical aberration is a third-order effect, which means that angular deflection of a ray with respect to the optic axis depends on the cube of the angle.

Realizing that round electron lenses always suffer from spherical aberration, he asked a simple question: why must the lenses be round? The use of non-circularly symmetric lenses has been the key to correcting spherical aberration in the electron microscope.

Indeed, Scherzer had already realized in the s that the necessary third-order correction could be provided using a combination of octupole and quadrupole fields. An octupole has eight alternating poles: positive and negative potentials in the case of an electrostatic octupole, and north and south poles in the case of a magnetic octupole figure 2. The resulting field lines have a profile with fourfold rotational symmetry, but most importantly the field strength increases as the cube of the distance from the centre of the octupole.

As a result, electrons are deflected away from the beam axis in two perpendicular directions but towards it in the directions in between, therefore adding to the existing spherical aberration. This can be corrected by placing a quadrupole before the octupole. If this line matches up with one of the divergent axes of the octupole, then we will get the desired third-order correction in that direction. This combination of lenses actually leaves the round beam looking a bit square, but this can be taken care of using a third octupole.

Indeed, the image in figure 1 is possible thanks to a Nion device based on three octupoles and four quadrupoles. Nion was founded in by Ondrej Krivanek and Niklas Dellby, building on research carried out while they were researchers at Cambridge University in the UK. So far, the company has concentrated on retrofitting existing STEM instruments with aberration correctors based on a design originally developed by Crewe and co-workers.

However, it is currently testing an entirely new STEM instrument in which a cutting-edge aberration corrector comes as standard.

CEOS, which was founded in by Maximilian Haider and Joachim Zach, has built on substantial prior experience with aberration correctors, particularly on the work of Harald Rose of the University of Technology in Darmstadt. The company has concentrated on supplying sextupole-based correctors to existing TEM manufacturers, most of which now offer aberration correctors with their top-end microscopes.

The correctors developed by Nion and CEOS have increased the resolution of electron microscopes from about 0. However, given that Scherzer derived the principle of the multipole correctors in the s, you may wonder why it took so long to achieve this. Once again, the reason lies in the precision required to shape the electron beam correctly. If the octupoles that generate the negative spherical aberration are not perfectly aligned, large parasitic aberrations will arise that may overwhelm any benefits from the corrector.

The fields must be aligned better than 0. This problem was recognized from the outset by both CEOS and Nion, and the solution has been to add extra multipole fields such as dipoles in order to steer the beam accurately through the corrector. You can imagine the horror of a microscope user if they were confronted with a further 33 knobs on their console, each of which had to be precisely adjusted in order to achieve a high-resolution image.

The key to making this technology usable has been the development of desktop computers powerful enough to correct the parasitic aberrations automatically, much like the use of adaptive optics in astronomy.

Indeed, given that the electron microscope itself has been key in developing the electronic devices leading to this computing power, this is a real case of technologies in a symbiotic relationship.

When researchers want to test materials, they often construct small batteries called coin cells to see how the ensemble performs. Seaweed-like iron oxide nanodendrites growing on the membrane of a liquid cell in a transmission electron microscope. Credit: Matthew R. Others have trained the electron microscope on more-fundamental systems. At Eindhoven University of Technology in the Netherlands, Nico Sommerdijk and his colleagues have explored the formation of fluid-filled structures that resemble the vesicles in cells.

In work yet to be published, the researchers have imaged a two-sided polymer as it self-assembles in liquid to form an artificial vesicle. And with a team led by Jim de Yoreo at the Pacific Northwest National Laboratory in Richland, Washington, Sommerdijk has studied how a polymer can bind to calcium, a process that could provide insight into how marine creatures grow the iridescent material known as nacre or mother-of-pearl.

Liquid-cell research has challenges. One of the biggest, says de Yoreo, is that electrons can wreak havoc when they hit water or an organic solvent, creating charged radicals that can destroy samples, shift pH or generate reducing agents that cause unintended reactions.

It is also difficult to measure quantities such as pH and temperature inside the microscope. But others are heartened by the latest research on the effect of electron beams. The change has been spurred in large part by collaborations with researchers who focus on studying materials affected by nuclear radiation. In the past few years, Abellan and others have explored how additives can control the growth of particles and alter pH, and how solvents other than water, such as toluene, might limit the effect of electron beams on samples in liquid 6.

Advances in electron microscopy have also come from improving the electron beams themselves. Devices called monochromators have allowed researchers to narrow the range of energies for electrons that reach the sample. Researchers are starting to use that tighter spread of energy, along with spectrometers and other instruments, to reach beyond the basic structure and composition of materials and map more-sophisticated properties at ever-finer resolutions.

One such target is phonons — vibrations in the atomic lattice of materials. Earlier this year, physicist Toma Susi at the University of Vienna and his colleagues used a STEM electron beam to move a silicon atom from site to site inside a hexagonal graphene lattice 7. Electron microscopes are capable of higher-energy work. A silicon impurity is moved around inside a hexagonal graphene lattice using the focused electron beam of a scanning transmission electron microscope.

Atoms could be manipulated at the rate of around four jumps per minute. CC BY. At the University of Antwerp in Belgium, Johan Verbeeck is looking to make electrons into a more-sophisticated probe, by passing them through plates that can alter their phase.

Instead of doing a comprehensive scan, a microscope could hit a subset of points in the sample. Done right, such sparse sampling could still generate a large amount of useful data.

But the field is moving fast, he says. Correction 21 November : An earlier version of the caption for the image showing manipulation of a silicon impurity erroneously called the device a scanning tunnelling electron microscope. Jiang, Y. Nature , — PubMed Article Google Scholar. Tate, M. Zhu, Y. Nature Mater. Williamson, M. Yuk, J. Science , 61—64 Abellan, P.

Langmuir 32 , — Tripathi, M. Nano Lett. Hudak, B. The detector relays signals to an electronic console, and the image appears on a computer screen. Sometimes x-rays are detected and used to display the atomic elements within specimens. This can be very useful in analyzing the cellular or sub-cellular elemental content of tissues. Recent developments in slicing very thin sections of tissues, and imaging the face of the block of tissue, have enabled high resolution sub-cellular 3D images to be obtained.

Veterans Crisis Line: Press 1. Complete Directory. If you are in crisis or having thoughts of suicide, visit VeteransCrisisLine. Quick Links. The basic steps involved in all EMs: A stream of high voltage electrons usually KeV is formed by the Electron Source usually a heated tungsten or field emission filament and accelerated in a vacuum toward the specimen using a positive electrical potential.