In this blog post, we will examine how scientists became convinced of the existence of atoms and how TEM-EELS technology supported this discovery.
Today, we take it for granted that all matter is composed of atoms, but just 150 years ago, people were still skeptical about the existence of atoms. In fact, by that time, there was already an overwhelming amount of indirect evidence supporting the existence of atoms. However, scientists could not let go of their doubts simply because they had never seen an atom with their own eyes. The smallest things scientists could see with a magnifying glass at the time were bacteria, but to see atoms, they would have needed to magnify them tens of thousands of times. Naturally, scientists found it difficult to accept the existence of such incredibly small substances. They wanted to see atoms with their own eyes.
So what was the decisive factor that led scientists to confirm the existence of atoms? The improved magnifying glass that directly showed them the existence of atoms was the EM (electron microscope). EMs can be classified into various types depending on their operating mechanisms. Among them, the one attracting the most attention for its high resolution is TEM-EELS (Transmission Electron Microscope-Electron Energy Loss Spectroscopy). As the name suggests, TEM-EELS consists of two devices: TEM and EELS.
TEM is a device that projects electrons onto an object to be analyzed and then analyzes the trajectories of the electrons that are refracted. The atoms that make up an object are divided into positively charged atomic nuclei and negatively charged electrons. As a result, the electrons emitted from the TEM device experience an attractive force from the atomic nuclei and a repulsive force from the electrons surrounding the atomic nuclei as they pass through the object. If an electron passes near an atomic nucleus, its trajectory is deflected by the attractive force of the nucleus. This deflection of electrons is called elastic scattering. On the other hand, an electron that has been projected may also be deflected by other electrons surrounding the nucleus when it is far from the nucleus. Since electrons around the atomic nucleus are widely dispersed in space, significant deflection due to the repulsive forces between electrons occurs only when two electrons come close enough to collide. If the projected electron approaches very close to an electron around the atomic nucleus, the projected electron is deflected in a different direction due to the strong repulsive force. This type of scattering is called inelastic scattering. Typically, the degree of scattering caused by elastic scattering is greater than that caused by inelastic scattering. Therefore, TEM can determine whether the scattered electrons originated from elastic scattering (atomic nuclei) or inelastic scattering (electrons) by analyzing the trajectories of the scattered electrons.
TEM repeatedly passes electrons with the same direction and speed through multiple points of an object, recording where atomic nuclei are located and where electrons are located. By combining this data, it is possible to determine how atoms are arranged throughout the entire object. However, there is one issue with the above method. For example, consider projecting TEM electrons around an atomic nucleus with a small positive charge.
Since the magnitude of the positive charge is small, the attractive force between the atomic nucleus and the emitted electrons decreases, resulting in weaker electron deflection. As this phenomenon intensifies, the TEM can no longer distinguish whether the weakly deflected electrons originated from elastic scattering or inelastic scattering. To address this issue, scientists added an EELS device to the TEM. EELS is a device that records the energy of electrons as they pass through the TEM while simultaneously tracking their trajectories. As mentioned earlier, when electrons undergo elastic scattering, their direction of travel is simply altered by the attractive force of the atomic nucleus. However, when electrons undergo inelastic scattering, they collide with electrons around the atomic nucleus and are deflected, causing their speed to increase dramatically. Therefore, even if elastic and inelastic scattering deflect the projected electrons to a similar extent, by measuring the energy of the electrons using EELS, it is possible to determine which type of scattering caused the deflection.
For example, if an electron with a speed or kinetic energy that is far too high to be achieved through elastic scattering is detected along a certain trajectory, it can be concluded that the electron was deflected due to inelastic scattering. Through such mechanisms, TEM-EELS serves as a magnifying glass for modern scientists and is widely utilized across various fields of research.
To maximize the physical and chemical properties of devices used in modern society, atomic-level experiments are essential. Without TEM-EELS, confirming the atomic arrangement or type of a material using other indirect methods is quite cumbersome and difficult. For example, the semiconductor production process currently commercialized in Korea uses units of 10 nm, and there are only 100 atoms in 10 nm. If even one of these 100 atoms is misaligned, the semiconductor will fail to function properly. Without directly observing the atomic structure, it is impossible to even identify any abnormalities in the semiconductor. This underscores the importance of TEM-EELS as a high-performance magnifying glass.
It is therefore necessary to delve deeper into what scientists of the past believed about atoms. The concept of atoms traces back to ancient Greek philosophers. Democritus believed that matter was composed of indivisible units called “atoms” (ἄτομος). However, his theory was based entirely on philosophical reasoning and was not supported by scientific experiments or evidence. For this reason, atoms remained nothing more than a hypothesis for many years.
In the 19th century, as science gradually became more systematic, atomic theory once again gained attention. John Dalton reintroduced the concept of atoms while studying the chemical reactions of gases to explain the phenomenon of substances reacting in fixed proportions.
His research was an important first step in scientifically supporting the possibility that atoms actually exist, but it was still impossible to prove the existence of atoms through direct observation. Until substantial evidence for atoms was presented, scientists maintained a cautious stance toward atomic theory. At the time, physicists believed that if atoms actually existed, experimental evidence was needed to explain their properties. In this situation, the discovery of Brownian motion provided an important clue to the existence of atoms.
Robert Brown observed the phenomenon of microscopic pollen particles moving irregularly in water. Brownian motion opened up the possibility of explaining this phenomenon as 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.
As such, the scientific establishment of the concept of atoms required extensive discussion and experimentation. In this context, the invention and development of the electron microscope marked a significant turning point in the history of science and played a decisive role in the establishment of atomic theory, which forms the foundation of modern science. Equipment such as TEM-EELS has gone beyond being mere tools, opening up a world invisible to the naked eye and deepening our understanding of the nature of matter.
