It is believed that the first microscopes were invented in the 17th century. Then they represented an uncomplicated system of lenses, which only hinted at the presence of the microworld. Now, a spatial resolution race allows researchers to look at the atoms themselves. In addition to the contemplative experience, it helps in the creation of new materials, such as organometallic frameworks (MOF).
After that, he moved to the electron microscopy group at the Berkeley National Laboratory (academic path), where the development of an electron microscope with subangstromic resolution had just begun. There I was invited to the FEI company - recently Thermo Fisher. It is one of the world leaders in the production of transmission microscopes. While working for the FEI (industrial path), I participated in the installation and launch of several, at that time the most advanced, electron microscopes for the KAUST laboratory. Later, I visited this laboratory more than once to train personnel. So when I was invited to work at Core Lab as a leading microscopist, it was a natural choice.
How is TEM different from other techniques? For example, from atomic force microscopy?
By and large, TEM is the reincarnation of shadow theater. Only instead of paper figures - micro- and nano-objects, and the role of a light source is played by a coherent (homogeneous in space and time) flow of high-energy electrons. The rest is almost the same: we look at micro- or nano-objects and evaluate them by the shadow they cast. The function of the wall is performed by a fluorescent screen, photographic film or, increasingly, CCD cameras (about the same as in digital cameras). There may be other detectors as well.
In the classic version, TEM is implemented as a "vertical pipe with handles". Air is pumped out of the pipe, an electron gun is placed on top, which "shoots" electrons along the pipe. The test material is inserted into the middle of the tube, and an electron detector is placed under it. There is also a flip-flop: the gun is screwed on from the bottom, and the detector is on top. There are few such microscopes, but they are some of the best.
This is in general. In practice, nuances arise. First, to generate a coherent stream of electrons flying at a high and stable (with a spread of no more than 0,0003 percent) high-precision equipment is needed. Most existing microscopes accelerate electrons up to 300 kilovolts (about 80 percent of the speed of light). Meanwhile, once, before the advent of smartphones, there were systems that accelerate particles to a record 1.5 megavolts (97 percent of the speed of light). The second feature is that electrons are well absorbed in air. They need a vacuum to reach the sample. Therefore, TEM is also a vacuum equipment that requires a lot of energy and heats up. This involves the operation of cooling systems. Thirdly, the object under study must be translucent for electrons - only then it is possible to establish not only its shape by the silhouette of the shadow, but also its internal structure. The latter condition imposes a limitation on the thickness of the sample - as a rule, from 10 to 100 nanometers. In this case, the entire material is "translucent". The ability to examine all the "insides" distinguishes transmission microscopy from atomic force, in which only the surface is studied.
Finally, the microscope in TEM must be carefully insulated: the result is influenced by vibrations, acoustics, electromagnetic interference, even fluctuations in air temperature above 0.5 degrees Celsius. When it comes to angstrom scales, the smallest details matter.
One of the microscopes with which I happened to work in Mumbai did not have proper isolation and was located only 300 meters from the Indian Ocean. From the pictures from this microscope, I could accurately determine the beginning of the surf. The opposite example is the German laboratory Jülich Forschungszentrum, which is located in an active coal mining area. Every day, 100-meter combines dig into the rock and spread vibration for tens of kilometers around. Such vibration is invisible to a person, but not to an electron microscope. Therefore, they build separate buildings with super-stable temperatures and a "jammer" of cellular communications, and the microscopes themselves are placed on huge concrete blocks suspended on air cushions. In such places, the magic begins: we see atoms separated by only half an angstrom.
The cost and maintenance of TEM systems, especially those with subangstrom resolution, is skyrocketing. The price of a microscope alone can be several million dollars. At the same time, the scope of applications of the technology is wide: wherever you need to find out the internal structure, up to atomic levels. That is, it is also biology - the study of the structure of cells, viruses, proteins, DNA; and the entire electronics industry; and the petrochemical industry - a huge layer of research related to the development and analysis of catalysts.
In 2016, KAUST worked with other scientists to find a way to shine through MOFs without destroying them. How it works? Does your lab only use TEM to study MOF?
Our laboratory is quite versatile and can cover most of the previously mentioned areas and many others. MOF is one of the "fashionable" materials these days. The whole world is now actively studying organometallic frameworks. Naturally, KAUST does not stand aside.
The main difficulty in learning MOF lies in the name. In their structure, there is an organic matter that does not like electron irradiation very much. Under its action, MOFs are easily destroyed. You have to work not only quickly, but instantly. Therefore, when "transmitting" organometallic frameworks using TEM, it is necessary to limit the number of electrons falling on the sample. It is necessary to quickly and almost blindly (without irradiation) find a suitable place on its surface, orient it correctly and record a picture on which almost nothing is visible. Then, from "almost nothing" to restore the structure. This is the essence of the "low dose" technique described in the article. Highly sensitive digital cameras that can record the flight of a single electron are indispensable. Such cameras have recently appeared in our country.
Nearly all microscopy techniques are represented at Core Lab. Although the differential phase contrast (DPC) method was not available until recently, it is well known. If an internal magnetic or electric field is present in the sample under study, then, in comparison with the sample without a field, the "translucent" electrons will slightly change their trajectory. To register the deviation, several detectors (at least four) are needed, located so that electrons deflected by the magnetic or electric field of the sample fall mainly on only one detector. Then, comparison of the signal with other detectors will allow not only visualizing, but also measuring the magnetic or electric field. In practice, not four independent detectors are used, but one segmented one, and the readings of each are recorded separately. The concept is simple and works great. One thing: most of the old and even modern TEMs do not have such segmented detectors. Or there is no software to record four separate signals.
Since I became interested in microscopy, I have had the opportunity to "twist the knobs" on many TEMs, two hundred, maybe three. The DPC method worked on only one. It was not available in KAUST either. After starting work at Core Lab, I showed that DPC can be implemented on any TEM that has at least one standard detector, without additional modification. Now we are actively using the approach I proposed to study magnetic samples, in particular, nanoscale Ni / Co wires, the controlled movement of magnetic domain walls in which allows them to be converted into information carriers.
Recently, chemists have been able to directly measure the strength of bonds between individual hydrogen atoms. They used an atomic force microscope. TEM can do that?
No, TEM has other tasks. Namely: the structure of the material, properties, charge states, the electrical activity of individual atoms or defects in the crystal lattice inside the material, and so on.
Tell us about Titan Themis Z. If I understand correctly, we are talking about software development?
Rather, hardware. The Titan transmission microscope is an FEI product and was introduced 12-13 years ago. In fact, it is a platform on the basis of which you can build various more highly specialized or universal systems. Let's recall the "vertical pipe" scheme: in the case of Titan, if the pipe is cooled to the temperature of liquid nitrogen (–195, 75 degrees Celsius), we get Titan Krios - TEM for studying biomaterial. Shock freezing in ice helps organics retain their structure longer under the electron beam. Such microscopes are now booming.
If you add a little gas to the vacuum in the pipe, you get Titan ETEM (Environmental). It allows you to observe chemical processes in real time. And if we take the highest modification of this tube with a superstable electron gun, correctors of spherical aberrations, equip it with an X-ray detector (electrons flying through the sample generate it in a huge amount), screw the electron energy loss spectrometer from below and place the entire structure in an insulating box, it will turn out Titan Themis Z. Z stands for Atomic Number. Its benefit is that with additional detectors we can not only get a black-and-white picture, in which bright points correspond to atoms, but also "color" it. For each atom, set the type, often - describe the electronic properties of the material, for example, whether it is a dielectric or a conductor.
The Themis Z modification was introduced last year. KAUST acquired it for the first time in the world, adding a sixth to the last generation Titan TEM line of five. KAUST is now the only place with so many high-end transmission microscopes.
Has Themis Z already helped you get some results?
There are results, but publications are only planned. The microscope is still at the stage of commissioning.
KAUST collaborates extensively with other research groups. And with your Russian colleagues and partners?
While working at the FEI, I have visited many organizations - within the framework of fine tuning of equipment, joint experiments, training, lectures. He also came to Russia: to Novosibirsk, Yekaterinburg, St. Petersburg, Moscow. Local scientific centers have quite advanced research groups. Separately, I would like to note the Kurchatov Institute and Alexander Vasiliev's laboratory, where two Titan TEMs are installed at once, including Krios (and the neighbors, across the street, have another Titan). I still maintain close contact with the guys from this group.
Many Russian scientists with whom I work are scattered around the world. For example, the NanoGune scientific center in the Basque Country, in Spain: I have been working with the head of its TEM department, Professor Andrey Chuvilin for more than ten years. The result of our friendship is not only publications, but also developments in the field of TEM-techniques or "attachments". One example is a monochromator for an electron gun, which improves the quality and resolution of electron loss spectroscopy, opening up access to the study of low-energy quasiparticles: excitons, plasmons, phonons.
The monochromator is not a new device. But we recently demonstrated that it can provide an energy resolution almost ten times higher than that claimed by the manufacturer (a related note will appear in Nature Communication soon).In addition to studying quasiparticles, the discovery makes it possible, for example, to measure the band gap - one of the key characteristics of semiconductors - with extreme precision. And thanks to TEM, in this case, a spatial resolution is achieved that is not available with other methods.