How to make a molecule move
11 May 2007 (Volume 2 Issue 5)
Against the grain
The 38-year-old Kim has taken an unusual career path for a Korean scientist. When he was a student at Seoul National University, the conservative chemistry community held that people aiming for an advanced career should go to the US for their doctorate.
But Kim believed in going to the best group regardless of country. At that time, Kim wanted to study photocatalysts, and the front-runner was a Japanese researcher, Akira Fujishima at the University of Tokyo. In 1994, Kim met Fujishima at a conference in Seoul, and was invited to join his lab.
South Korea is Japan’s nearest country, but until recently historical disputes meant that each country had complex feelings about the other—so people around Kim were against his idea to go to Japan. “But I thought it would be interesting to explore an undeveloped path,” Kim says.
A photocatalyst is a substance that accelerates a chemical reaction when irradiated by light, as in a plant’s photosynthesis. Fujishima was renowned for his 1968 finding that the surface of a titanium oxide electrode acts as a photocatalyst and accelerates the dissolution of water into hydrogen and oxygen. It was thought that the finding would help create clean energy sources, which Kim was highly interested in.
But when Kim joined his lab in 1996, Fujishima didn’t let him take on photocatalyst research. Given his electrochemical background, Kim was sent to one of Fujishima’s other projects, newly launched at the Kanagawa Academy of Science and Technology near Tokyo. “I even didn’t touch a photocatalyst,” Kim says with laugh, adding that he thought tackling a new theme sounded more enjoyable.
Tomokazu Iyoda, a polymer chemist who supervised Kim, remembers he made a fluent speech in Japanese and made people laugh at a welcome party for newcomers. “He’s very friendly, cheerful and good at making jokes,” says Iyoda, now a professor at the Tokyo Institute of Technology.
In 1999, Kim earned his PhD by studying how individual fullerene molecules gather to form nanometer-scaled structure when light is shone on them surface. After that, Kim thought about taking the orthodox path to the US, but on Fujishima’s recommendation joined RIKEN’s Surface Chemistry Laboratory, which was run by Maki Kawai.
The world of the surface chemistry
Kim was once again sent to a different laboratory, and spent his first six months learning how to use the STM. It was a time when the STM was becoming a hot device after researchers in the US achieved a breakthrough by obtaining vibrational spectra for a single molecule absorbed onto a solid surface1.
The chemical bonds that bind atoms into a molecule vibrate constantly provided it is at the right temperature. Depending on the energy of the electrons given to a molecule, its bonds can be disbanded to change the molecule’s properties, and researchers can also stimulate the vibrating electronic state between a molecule and the surface, instead of cutting bonds, and then see how this affects the molecule.
At Kim’s lab, two huge metallic STMs are wrapped in aluminum foil to control the temperature. Inside the machine is an ultra-high vacuum, and the temperature is cooled to a superconducting level. “We create the Universe in the chamber,” Kim says. One molecule is placed on a metal plate inside the chamber. Kim then injects billions of electrons onto the target site in a molecule from a micron-sized tip. The idea of using electrons to induce a chemical reaction was not new, but Kim was the first to control the position of a single molecule and induce excitation of vibrational modes selectively.
Making a molecule hop
In 2002, Kim made a carbon monoxide molecule hop in only one direction on a palladium plate2. Later that year came his greatest achievement so far3 (Fig.2). Using an unsaturated hydrocarbon absorbed onto a palladium plate, he cut the bond between carbon and hydrogen and made the molecule more responsive by meticulously adjusting the energy of electrons to match the frequency of vibration. Other researchers had shown only the dissociation of bonds—Kim was the first to suggest that the process is controllable.
Later, Kim has made an unsaturated hydrocarbon molecule rotate4. Earlier this year, his team also observed how an achiral carbon molecule, adamantine, becomes chiral in the process of self-assembling to form a monolayer5. More recently, they succeeded in the reversible bond rearrangement of a single molecule-metal contact through the combination of catalytic surface chemistry and an STM-tip induced reaction6.
So far, Kim has used molecules and metals that are known for an effective catalytic reaction. Kim is now trying new molecules, and also moving beyond cutting bonds to creating them. Although he admits cutting bonds is much more difficult and still has a long way to go, he is experimenting with combining a nanotube with an organic molecule. Kim is also keen to develop a library for nanotech researchers worldwide about molecules and the energy levels that electrons need for particular chemical reactions.
Kim says RIKEN is the best place for him to work. There is still room to improve the environment, such as support for the family, but the benefits far outweigh the disadvantages, he says. As a senior scientist at RIKEN, Kim often organizes drinks and karaoke with young researchers, takes care of temporary overseas researchers, and promotes friendship between Korea and Japan outside work. “Kim’s presence is important for RIKEN’s internationalization,” says Kawai.