Until 150 years ago, scientists doubted the existence of atoms, but modern electron microscopy techniques like TEM-EELS played a key role in confirming their existence. By analyzing the trajectories and energies of electrons to visualize their atomic-level structure, this technique helped establish atomism.
Today, we take it for granted that all matter is made up of atoms, but 150 years ago, people doubted that atoms really existed. Indirect evidence for the existence of atoms had already piled up by then, but scientists were still skeptical, simply because they had never seen one. The smallest things they could see with a magnifying glass at the time were bacteria, and to see an atom, they would have to magnify it hundreds of thousands of times more. Naturally, scientists couldn’t easily accept the existence of such an absurdly small substance. They wanted to confirm the existence of atoms with their own eyes.
So what was it that finally convinced scientists of the existence of atoms? The improved magnifying glass that gave them a direct view of atoms was the electron microscope (EM). EM can be divided into several types depending on the mechanism of operation. The one that is gaining attention for its greatest resolution is TEM-EELS (Transmission Electron Microscope-Electron Energy Loss Spectroscopy). As the name implies, TEM-EELS is divided into two instruments: a TEM and an EELS.
A TEM is an instrument that projects electrons onto an object to be analyzed and then analyzes the trajectories of the refracted electrons. The atoms that make up an object are divided into positively charged nuclei and negatively charged electrons. Therefore, electrons fired from the TEM equipment are subjected to attraction by the nucleus and repulsion by the electrons around the nucleus as they pass through the interior of the object. If the electron passes near the nucleus, the attraction of the nucleus causes the projected electron trajectory to be deflected. This refraction of the electron is called elastic scattering. On the other hand, the projected electrons can also be deflected by other electrons around the nucleus, far away from the nucleus. Because the electrons around the nucleus are widely spread out in space, the only time there is significant refraction in the electron-to-electron scale is when two electrons come close enough to collide. When a projected electron comes very close to an electron around the nucleus, the projected electron is deflected in the other direction by the strong force of the force of attraction. The refraction caused by this mechanism is called inelastic scattering. The degree to which electrons are deflected by elastic scattering is usually greater than by inelastic scattering. Therefore, a TEM can look at the trajectory of the refracted electron and determine whether it came from elastic scattering (atomic nuclei) or inelastic scattering (electrons). The TEM repeatedly transmits electrons with the same direction and speed at different points on the object, recording where the nucleus is located and where the electrons are located. Combining these data gives us a picture of how the atoms are arranged throughout the object.
However, there is one problem with this method. For example, suppose you projected electrons into the TEM around the nucleus of an atom with a small positive charge. Because the positive charge is small, the attraction between the nucleus and the launched electron is reduced, so the degree to which the electron is refracted is also weakened. When this happens, the TEM can’t tell whether the weakly refracted electrons come from elastic or inelastic scattering. That’s why scientists added a device called an EELS to the TEM. The EELS is a device that records the trajectory of the transmitted electrons as the TEM records their energy at the same time. As mentioned earlier, if the projected electrons undergo elastic scattering, they simply change direction due to the attraction of the atomic nucleus. However, if the electrons undergo inelastic scattering, their velocity is dramatically increased because they are bounced back as they approach close enough to collide with electrons around the nucleus. Therefore, even though elastic and inelastic scattering bend the projected electrons to a similar degree, determining the energy of the electrons with an EELS can help determine which scattering the electrons were bent by. For example, if we find an electron in a trajectory that has a velocity or kinetic energy that is so large that an elastically scattered electron could not possibly have, we can determine that the electron is refracted due to inelastic scattering.
With the above mechanism, TEM-EELS is a magnifying glass for modern scientists and is utilized throughout their research. Atomic-scale experiments are essential to maximize the physical and chemical properties of modern devices. Without TEM-EELS, it would be cumbersome and difficult to determine the atomic arrangement or type of atoms in a material using other indirect methods. For example, the semiconductor production process currently commercialized in Korea uses 10 nm-scale units, and 10 nm contains only 100 atoms. If even one of these 100 atoms is misplaced, the semiconductor will not function properly. Without seeing the atomic structure, it is not even possible to determine what is wrong with the semiconductor. This is where the importance of a good magnifying glass called a TEM-EELS comes into play.
It’s worth taking a deeper look at how scientists in the past thought about atoms. The idea of the atom can be traced back to the ancient Greek philosophers. Democritus thought that matter was composed of the smallest unit that could not be further divided, and he called that unit the atomos (ἄτομος). However, his theory was based strictly on philosophical reasoning and was not supported by scientific experiments or evidence. For this reason, for many years, the atom remained a hypothesis.
As science became increasingly systematized in the 19th century, atomism gained renewed attention. John Dolton, while studying the chemical reactions of gases, reintroduced the concept of atoms to explain the phenomenon of matter reacting in certain proportions. His work was an important first step in providing scientific support for the possibility that atoms actually existed, but it was still not possible to prove their existence through direct observation.
Until substantial evidence for atoms was presented, scientists remained cautious about atomism. Physicists at the time believed that if atoms really existed, they needed experimental evidence to explain their properties. In this context, the discovery of Brownian motion provided an important clue to the existence of atoms. Robert Brown had observed microscopic pollen grains moving irregularly in water. Brownian motion opened up the possibility that this phenomenon could be explained by the thermal motion of atoms and molecules, but even so, many scientists still did not fully accept the existence of atoms due to the lack of direct visual evidence.
In this context, the invention and development of the electron microscope marked an important turning point in the history of science and played a crucial role in the establishment of atomism, which forms the basis of modern science. More than just a tool, instruments like the TEM-EELS have deepened our understanding of the nature of matter by opening up a world we could not see with our eyes.
