Watching hardworking electrons
20 April 2007 (Volume 2 Issue 4)
New materials, such as fullerene and carbon nanotubes, are attracting considerable attention because of their potential to achieve major breakthroughs in science and technology and industries in all areas, including information and communications, energy, the environment, medicine, machinery, and architecture. The function of a material depends on the state of the electrons contained in the material. However, it has been difficult to clearly observe the electrons in the material. Masaki Takata, Chief Scientist at the Structural Materials Science Laboratory in the SPring-8 Center of the RIKEN Harima Institute, and others, are working on an extensive examination of the electrons in materials by making use of synchrotron X-rays from SPring-8 and their unique analytical approach based on information theory, resulting in a major revolution in the development of new materials.
Visualizing structures from trace amounts of powder
“I was a boy astronomer. I used to photograph Mercury and other celestial bodies using my father’s camera I smuggled out of the house,” says Takata, looking back on the past. “I like observing things. Even now I enjoy photography as a hobby.”
Takata specializes in visualization of the atoms and electrons that form materials. After spending some time using electron microscopy for his research at graduate school, he shifted his research tool to X-rays. Around 1990, under Makoto Sakata of Nagoya University, he began analyzing the structure and electron distribution of atoms that constitute functional materials using trace amounts of their powder. “X-ray crystallography often involves the use of large single crystals as samples to obtain better measurement data,” says Takata. “But most new materials under development are only available in the form of trace amounts of powder.” He adds that determining the structures of materials with powder samples would allow new materials to be developed based on the findings. “We have been engaged in extensive exploration, not only of atomic arrangements, but also of electron distributions in substances using powder samples,” he continues. “The properties of a substance depend on the electron configuration. What I want to know most in the context of developing new materials is the electron distribution.”
To this end, Takata and others use the maximum entropy method (MEM), a technique that has emerged from information theory. “Since MEM is somewhat difficult to understand, only two scientists—Professor Sakata and I, myself—were using it for X-ray crystallography in Japan around 1990,” says Takata. In fact, as he points out, worldwide, no researchers were working on MEM with the exception of France, where there was a still only a small amount of activity using the technique.
So what is MEM? “Using X-rays, we cannot obtain direct observations of magnified views as we can with optical microscopy, which employs visible light,” Takata explains. This is because the sort of lenses used for optical microscopy will not converge scattered light at X-ray wavelengths to produce images. “We use MEM in place of the lens.” When an X-ray beam is directed onto a sample, the X-rays are scattered by the electrons in the subject material. Measured data on the scattered X-rays are converted using a computer to derive images. It should be noted, however, that there is more than one possible candidate for the image derived from the measured data. Which image to select?—this problem is solved by MEM. “Remember science experiments at school. Any experiment is unavoidably accompanied by measurement errors; a plot of the distribution of measured data shows dispersion,” says Takata. “For example, assume we draw a line here [through a set of data points]. There are then a variety of positions where we could draw the line. MEM offers the simplest line that involves no subjective judgment. Additionally, MEM enables us to extend the line even to a region where measured data are unavailable.” However, Takata adds that if the precision of the measured data is poor, only obscure images can be selected, even using MEM. “Hence, we have made efforts to improve the precision of measured data, including the early adoption of imaging-plate technology.”
Elucidating the structure of metallofullerene
In 1995, for the first time in the world, Takata succeeded in visualizing how metal atoms are incorporated into fullerene (Fig. 1). Fullerene, which was discovered in 1985, is a molecule comprising carbon atoms gathered in a cage form like a soccer ball. “The first expectation was that artificially placing metal atoms in the fullerene cage might result in various new properties, including superconductivity,” says Takata. In fact competition was stimulated worldwide to analyze the structure of a synthetic compound of fullerene and metal atoms. However, it took a great deal of time to determine whether the metal atoms adhere to the outside of the cage or are incorporated in the cage. The fine molecular structure of the compound was difficult to analyze because its powder could only be produced in very small amounts.
Figure 1: Examples of metallofullerene containing internalized metal atom, visualized by X-ray structural analysis.
(Joint research with Hisanori Shinohara of Nagoya University)
Why, then, did Takata succeed in the structural analysis of metallofullerene for the first time in the world? “Just before the successful analysis, I conceptualized an analytical method that combined MEM and the Rietveld method,” recounts Takata. He explains that in those days, only the structure of fullerene on its own was known. The Rietveld method derives the true structure by comparing such available information and theoretical models with actual measured data. First, the structure of fullerene alone is compared with the structure estimated by MEM from the measured data. Because the measured data include the contributions of metal atoms, the two structures would be expected to differ. “Then I attempted to locate the discrepancy, and found a distribution of electrons in the fullerene that was inconsistent with the measured data,” he adds. “When applying metal atoms to that portion, the structure of metallofullerene was precisely derived.”
A new material that exhibits property change on gas adsorption
Determining whether the results of a structural analysis are correct requires a level of judgement that is augmented by intuition and sharpened by experience. “The derivation of the true structure of a substance is very beautiful. If the structure obtained is incorrect, it appears to be unnatural and we feel uneasy about its appearance,” says Takata, who is now working on structural analysis making use of X-rays at the world’s highest intensity produced by SPring-8 in combination with his unique analytical approach, including the MEM–Rietveld method.
“We were all astonished and excited at the incredibly beautiful structure,” says Takata, looking back on his first success in structural analysis of polyporous chelate polymer carrying adsorbed oxygen. The polyporous chelate polymer is a new material that was prepared from metal ions and organic molecules by Susumu Kitagawa and others at Kyoto University. This material has regularly arranged small pores of nanometer size (one billionth of a meter), which permits a high degree freedom in the design of the size and shape. As such, the polyporous chelate polymer had been attracting attention as a material capable of adsorbing gases efficiently just by reducing the temperature. “However, it was not known how the gas molecules were adsorbed,” Takata recounts. It was suggested that the gas molecules probably randomly adhered to the inner walls of the pores. Actual structural analysis, however, revealed that the oxygen molecules were floating in series in each pore.” (See upper panel on front cover; oxygen molecules shown in red.) The analytical method was able to indicate the number of electrons belonging to each molecule and atom. “The number of electrons belonging to the oxygen molecule was found to be 16 [the electron number of atomic oxygen],” Takata reveals. “Hence, the electrons did not transfer from the oxygen molecule, but were floating, supported by a very weak force known as the intermolecular interaction.”
The major point to note from this analysis is the finding that magnetism develops with the adsorption of oxygen molecules. Individual oxygen molecules have magnetism. However, because the oxygen atoms in a gas, and hence their magnetic dipoles, are oriented randomly, gaseous oxygen as a whole does not exhibit magnetism. Structural analysis suggests that when oxygen molecules are adsorbed, they are arranged linearly so that magnetism might be developed as a result of uniform orientations of the oxygen dipoles. “Actual measurements detected weak but significant magnetism,” Takata notes. These results strongly stimulated the imagination of scientists who were working on physical properties. The polyporous chelate polymer was found to present a new material phenomenon in that it exhibits property change on gas adsorption. Furthermore, it was found that altering the kind of gas to be adsorbed and the size and shape of the pores produced a broad range of new properties and functions. “For example,” says Takata, “The polyporous chelate polymer exhibits ferroelectricity on adsorbing gaseous carbon monoxide.”
The results of this structural analysis has led to the polyporous chelate polymer becoming a major research theme in nanotechnology, including fullerene and carbon nanotubes. Moreover, the polyporous chelate polymer has immediate potential for industrial applications. “This is because the polyporous chelate polymer can easily be produced chemically, under ordinary conditions, at one atmospheric pressure, and at ambient temperature, in contrast to expensive fullerene and carbon nanotubes, which are still difficult to prepare in large amounts at low cost,” explains Takata.
Visualizing hydrogen using X-rays
It is expected that hydrogen will provide a source of clean energy that does not emit carbon dioxide and other greenhouse gases, but a major problem arises concerning the storage of hydrogen, which is gaseous at normal temperatures. For example, vehicles powered by fuel cells will not become feasible until a way is found to load the hydrogen fuel into the car in a compact way. Although hydrogen-absorbing substances that are capable storing hydrogen in compact volumes at high density are under development, it has been difficult to visualize hydrogen in any material using X-rays. This is caused by hydrogen atoms scattering X-rays weakly, because each atom has only one electron. Takata and others succeeded in visualizing hydrogen absorbed on magnesium (Fig. 2). It was found that hydrogen is bound to magnesium through not only ionic bonds, but also covalent bonds, which are weaker than ionic bonds. At present, new hydrogen-absorbing alloys are being developed by improving the relationship between the metal–hydrogen bonding strength and hydrogen-absorption efficiency.
Figure 2: Hydrogen (H) absorbed in magnesium (Mg)
(Joint research with Toyota Central R&D Labs, Inc.)
Visualizing the behavior of electrons during reactions
“Since its construction in 1997, SPring-8 has been updated to provide enhanced performance, including synchrotron radiation,” says Takata. “However, we remain unable to utilize the radiation to the fullest. Taking my work, for example—an even broader range of phenomena could be visualized by making use of light in the X-ray region.”
Takata and others are striving to visualize reaction processes behind material functions using X-ray pulses from SPring-8 to imitate stroboscopic light from ordinary cameras. One of the materials under investigation is DVD-RAM (DVD random-access memory). When the surface of this type of rewritable optical disc is exposed to laser light, the crystalline phase, in which atoms are regularly arranged, dissolves to form a liquid phase. The liquid phase immediately cools down to form an amorphous phase, in which atoms are arranged irregularly. The crystalline phase can be restored by illuminating the amorphous phase with laser light. Light reflectance differs between the crystalline and amorphous phases. Information is recorded on DVD-RAM by assigning the integer ‘0’ or ‘1’ to differences in the light reflectance. “In certain materials, phase change occurs in the extremely short time of 20 nanoseconds (two hundred millionths of a second),” notes Takata. However, he adds that the reason why the phase change is so fast remained unknown because no one had ever been able to watch the reaction pathway.
In October 2006, Takata and others demonstrated the structural difference between materials showing fast phase change and those showing slow phase change in a joint research program with Noboru Yamada and others at Matsushita Electric Industrial Co., Ltd. (Fig. 3). In materials with a fast phase change, the crystalline and amorphous phases are structurally alike. In materials with a slow phase change, the two phases are not alike. It seems that the fast transition may be thanks to a consistency in the basic structure between the crystalline and amorphous phases.
Figure 3: Phase-change models. MR stands for membered ring.enlarge image
“In our ongoing study, however, we have only succeeded in watching the beginning and end of the phase change. I want to visualize the entire reaction process of the phase change to help develop a new type of DVD-RAM that offers even faster recording speeds,” says Takata. “As I mentioned before, the functions of materials are due to electrons being exchanged in reaction processes. Making use of the advanced X-rays from SPring-8 would enable us to visualize these exchanges in times of the order of picoseconds (one trillionth of a second).
Takata and others are also working to visualize electron spins by using the polarity of the X-rays from SPring-8. Electron spins represent a motion like the rotation of a celestial body, occurring in two directions: either upward or downward. The spin direction of the electrons in a material determines the properties of the material as a whole, such as its magnetism and the nature of the electric currents produced. “Now we are acquiring a firm footing for the development of a methodology for visualizing a three-dimensional distribution of electron spins using SPring-8,” says Takata. “To visualize how the distribution of electrons and the directions of spin change in reaction processes that take place in an extremely short time, hence to visualize all the behavior patterns of electrons that govern physical properties —this is my ultimate goal.”
On the way to this ultimate goal, Takata expects to see the development of a series of new materials that will improve people’s lifestyle and benefit society.