A Microscope That Can See Atoms

All matter is made of atoms. An atom is the smallest particle that cannot be further divided by chemical change. You have probably seen diagrams in textbooks that represent crystals with small, round balls packed together in space, or molecules with balls connected to each other. However, very few people have actually seen how atoms are arranged.

Atoms are very small particles, slightly smaller than one-billionth of a meter (1 nm). Since the wavelength of light is about one-two-millionth of a meter (500 nm), this is a world that cannot be seen with an optical microscope.

One of the microscopes that can see this world of atoms is the scanning tunneling microscope (STM). This microscope detects surface irregularities by "tracing" them. A metal tip, sharpened to a single atom at its apex, is brought close to the surface to be observed. When the tip gets as close as about 1 nm, a current flows due to a quantum mechanical effect, even though they are not in contact. This is called the tunneling current. The value of the current depends exponentially on the distance between the tip and the surface. In other words, by reading the current value while moving the tip, the distance between the tip and the sample at that location can be determined, allowing for the observation of fine surface irregularities—that is, the arrangement of atoms. An example of this is shown in Figure 1. It certainly looks like an array of round balls. You can also see areas where atoms are missing (vacancies) and where other substances (atoms or molecules) are adsorbed. Incidentally, since this is not a photograph, the colors in the STM image are not the actual colors; we choose colors that seem appropriate.

Most STMs that allow us to see the world of atoms are large instruments like the one shown in Figure 2. The microscope itself is placed in a vacuum comparable to outer space, at one-trillionth of an atmosphere. If oxygen, nitrogen, water vapor, or other atmospheric components adhere to the surface, they interfere with the observation of individual atoms of the material. Buildings typically vibrate by several tens of micrometers (100,000 to 1,000,000 times the scale of atomic irregularities!). Therefore, the entire vacuum system is placed on a platform called a vibration isolation table, causing it to float gently above the floor. Furthermore, if the atoms on the surface or adsorbed molecules move around due to thermal energy at room temperature, the microscope is cooled with liquid nitrogen or liquid helium. In other words, we observe a frozen surface.

The microscope in the photo is one I built and brought to Yagami when I was a researcher at a national research institute. In fact, I am currently building another new instrument. I draw the blueprints, assemble the parts, do the wiring, and then collect data with the completed microscope. There are so many things I want to see, such as next-generation nano-electronic materials that operate on new principles, catalysts that contribute to energy savings, and new materials for gas storage that might solve environmental problems.