Professor, Graduate School of Nanoscience and Nanotechnology, Waseda University
What can be observed on a surface?
—Unlocking surface physics with computational science—
Tremendous progress has been made in surface physics since the invention of the scanning tunneling microscope (STM) in 1982. Even though atomic images were obtained, how exactly they were taken was unknown. “In order to determine the mechanisms, I developed a method to do theoretical simulations using first principle computations. From the simulation, I found that the tunneling current concentrated on the atom of the probe closest to the sample surface, and this was the key to STM with atomic scale resolution,” said Prof. Tsukada.
Prof. Tsukada is also interested in non-contact atomic force microscopy (ncAFM). Using an ncAFM slight changes in vibrational frequency that occur when a probe vibrating at its resonant frequency scans a sample surface can be measured. He says, “The implementation of theoretical studies led us to be able to determine what is seen in ncAFM images.” Between the probe and the sample surface, there are short-range chemical interaction forces and long-range van der Waals forces. Van der Waals forces work over long ranges, and they are not sensitive to changes in the position of the probe on the atomic scale. On the other hand, chemical forces operate in the range between 0.2 and 0.3 nm, and they are sensitive to changes in the position of the probe on the atomic scale. Thus, ncAFM images show variations in the chemical forces with changes in the position of the probe. The chemical forces are affected not only by the structure of the surface but also by combinations of the constituent elements on the sample surface and the probe, and therefore, it is possible to identify each element. Prof. Tsukada developed a method for theoretically simulating an ncAFM experiment in ultra high vacuum utilizing first principles. However, it can also be used as a tool to observe biomolecules in air or in solution. He thinks that the observation of biomolecules under these conditions is theoretically very interesting and challenging.
Prof. Tsukada has also been trying to determine the electrical conductivity of individual molecules, especially fullerenes and carbon nanotubes with a cage-like surface. “Carbon nanotubes with a cage-like surface are made by inserting five-membered rings or seven-membered rings between six-membered rings. There are no defects in the structure of five-membered rings, but there are phase defects that change the electric wave properties when a wave goes around the ring. It is interesting to determine how the phase defects influence the wave functions,” says Prof. Tsukada. He found that when two metallic carbon nanotubes with different radii are connected with five-membered rings and seven-membered rings, the electric current between these carbon nanotubes depends only on the radius ratios because of the scaling rule for any structures of the connected area.
He also found that, when a magnetic field is applied to the inside of a carbon nanotube torus, a coherent permanent current could be generated. Because of the unique band structure of carbon nanotubes, a slight difference in energy is generated by a magnetic field between a clockwise current and a counterclockwise current in the torus. “To great amusement, the current flows in the direction that increases the applied magnetic field, and therefore, the torus is magnetized even with a low magnetic field. This could be used for several different kinds of magnetic devices,” says Prof. Tsukada. Calculations based on first principles are used to derive the structures and the properties of a system by determining the electronic states within the system using fundamental quantum mechanics without any experimental parameters and by obtaining the interatomic forces. This method is more accurate than theoretical analysis using a model, but it requires a large amount of computational time. Therefore, it is limited to systems which have only a few hundred atoms.
Prof. Tsukada said, “No one tries to solve the mysteries of the universe using individual stars. With a galaxy, you deal with millions of stars as a unit. This concept also applies to materials. A group of basic units, which form a periodic structure, can be considered as one unit. If the unit should be more detailed, it can be simulated by using the hierarchical structure of materials.” Today, various kinds of calculations can be done using ready-made software due to progress in simulation software. He says, “Sometimes, computational science may not seem so important, but I do not want young researchers to think that it is less important.” Theoretical scientists have to figure out what should be computed and what it means. True theoretical scientists can come up with the answers. He adds, “In order to truly understand what is behind phenomenon, a theoretical scientist needs insights into physics.”
