A research group including Professor Shinobu Aoyagi of Nagoya City University, Professor Boris Gorshnov of Moscow Institute of Physics and Technology, Associate Professor Hal Suzuki of Kindai University, and Professor Motohiro Nakano of the Graduate School of Science at the University of Osaka has clarified in detail the motion state of a single Li ion (Fig. 2) trapped in a soccer-ball-shaped carbon molecule called fullerene C₆₀ (Fig. 1) using broadband absorption spectroscopy (Fig. 3) covering wavelengths from terahertz to infrared rays, as well as theoretical calculations. The movement of ions within C₆₀, as revealed in this study, may be applicable to molecular-sized switches and elements. Furthermore, the absorption of electromagnetic waves by ion motion may be applicable to high-resolution imaging, sensors, and encryption technology.

Research Background

Fullerene C₆₀ which is soccer ball-shaped and made up of six- and five-membered carbon rings, is a hollow, cage-like molecule with a diameter of 0.7 nanometers (Fig. 1). Inside the hollow molecule can contain metal atoms, and in many cases these atoms provide electrons to C₆₀ to become metal ions. By controlling and utilizing the state of the active metal ions protected and isolated inside the stable C₆₀, it may be possible to develop new nanodevices such as molecular switches. In this study, the research group focused on Li@ C₆₀-based endofullerene and clarified the low-temperature dynamics of a single active lithium ion trapped in a C₆₀ carbon cage using broadband terahertz and infrared spectroscopy, as well as theoretical calculations. Terahertz waves are electromagnetic waves with wavelengths intermediate between radio waves and infrared rays, while infrared waves are electromagnetic waves with wavelengths intermediate between terahertz waves and visible light, and are suitable for investigating the motion state of atoms and molecules within matter.

Fig. 1 A structure of fullerene C₆₀ carbon molecules cage Credit: Motohiro Nakano

Research Contents

In order to investigate the motion of a single Li ion within the C₆₀ cage, two types of Li@ C₆₀-based endofullerenes with different masses were prepared using lithium isotopes. By combining each with the hexafluorophosphate ion PF₆⁻, two types of crystals with the same molecular arrangement were obtained. Li@ C₆₀-based endofullerenes selectively absorb electromagnetic waves of specific wavelengths (absorption lines), thereby exciting the motion of specific Li ions (Fig. 2). The broadband absorption terahertz and infrared spectroscopy of these two types of crystals were measured and compared, and clear wavelength changes due to differences in the mass of Li ions were observed in several absorption lines. In addition, by measuring the temperature change of the absorption spectrum, the researchers observed changes in the wavelength of the absorption line and increases or decreases in the transmission coefficient due to changes in the temperature of the Li ion's motion state (Fig. 3).

By comparing the isotope effects and temperature changes of the measured absorption spectrum with the results of X-ray crystal structure analysis and theoretical calculations, the research group was able to determine almost all of the atomic motions that caused each of the numerous observed absorption lines. The following has been experimentally proven regarding the movement of a single Li ion within C₆₀:

At temperatures above 100 K (-173 °C), Li ions undergo thermal hopping or quantum delocalized rotational motion on a sphere with a diameter of 0.3 nanometers, rotating along the inner wall of the carbon cage (Fig. 2a). At temperatures below 100 K, this motion is gradually suppressed as the temperature decreases, and Li ions are localized with equal probability at stable positions near the centers of the two six-membered rings stabilized in the crystal. At temperatures below 40 K (−233°C), Li ions localized near the centers of the two six-membered rings vibrate at different frequencies in directions parallel and perpendicular to the plane of the six-membered rings. Furthermore, they simultaneously occupy two stable positions separated by 0.3 nanometers by quantum tunneling (Fig. 2b).

Fig. 2 Temperature fluctuation of the motion of a single Li ion trapped in the C₆₀ cage

a: High temperature above 100 K (-173°C), b: Low temperature below 40 K (-233°C)
Credit: Motohiro Nakano

Fig. 3 Temperature fluctuation of terahertz and infrared absorption spectroscopy of ⁶Li@C₆₀PF₆ crystal

Top: 300-60 K (from 27 to -213°C) (blue line is C₆₀), bottom: 5 K (-268°C)
Credit: Motohiro Nakano

Social Impact of the Research

The motion of the Li ions trapped in C₆₀ observed in this experiment changes not only with temperature but also with the shape of the carbon cage and the arrangement of anions. This means that controlling the motion of ions may be applicable to molecular-sized switches and elements in the future. Furthermore, the motion of these ions is caused by the absorption of electromagnetic waves of a distinctive wavelength, which may be utilized in applications such as high-resolution imaging, sensors, and encryption technology.

Notes

The article, “Terahertz and infrared spectroscopy of the Li@C₆₀PF₆ endofullerene,” was published in Physical Review B at DOI: 10.1103/49zf-ypl3.

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