**9. Conclusions**

The author's group succeeded in inducing the formation of Al nanoparticles by electron irradiation of metastable θ-Al2O3, followed by manipulation of the nanoparticles. A series of phenomena was observed without heating using high-resolution TEM (HRTEM), with an electron beam intensity as low as 1019–20 e/cm<sup>2</sup> s. The typical morphology of the nanoparticles was that of a nanodecahedron surrounded by (111) surfaces with twins. Electron beam irradiation of a group of Al nanoparticles promoted their rotation, revolution, and migration to the irradiation centre, resulting in bonding and embedding into an α-Al2O<sup>3</sup> matrix. The driving force is considered to be the momentum transfer from electrons spiralling across the pole piece of the HRTEM in a strong magnetic field to the Al nanoparticles. When nanoparticles were placed on an amorphous carbon film, onion-like fullerene nucleated and grew beneath them, and finally, a metallofullerene or Al-atom-intercalated structure was formed by electron irradiation.

To develop the manipulation technology for other types of nanoparticles, an electron beam was used to irradiate Cu nanoparticles of 10–50 nm in diameter at an irradiation intensity of 5.5 <sup>×</sup> <sup>10</sup><sup>22</sup> e/cm<sup>2</sup> s, Pt nanoparticles at 1.0–3.3 <sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s, and W nanoparticles derived from WO<sup>3</sup> at 9 <sup>×</sup> <sup>10</sup><sup>20</sup> to 4 <sup>×</sup> <sup>10</sup><sup>23</sup> e/cm<sup>2</sup> s. The behaviour of Cu, Pt, and W nanoparticles under electron irradiation was similar to that of Al, and a nanofilm was finally formed. The CSL boundary structures at the bonded interface of Cu nanoparticles were found to be unstable Σ7 and Σ13b, which are different from the stable Σ3 obtained in Al and Pt with weaker electron beam irradiation.

The possible scientific contribution of electron irradiation is the synthesis of materials in a metastable state through a nonequilibrium reaction in vacuum, as well as the induction of hybridised nano-/mesostructures. It also enables the study of the nature of materials in a pristine and controlled environment, for example, without the formation of an oxide. From the viewpoint of application to devices, nanosized balls, dots, wires, and tubeforming three-dimensional structured circuits may be used as elements of nanodevices, and chemically active points embedded in the substrate for use as a catalyst can be achieved by the manipulation of electron irradiation. With respect to industrial applications, our technologies will contribute to the development of micro- and nanoelectromechanical systems, memories, photonics, battery electrodes, H<sup>2</sup> storage, and more.

**Funding:** This research partly received funding from ERATO, JST.

**Acknowledgments:** The author gratefully acknowledges the research scientists at Tanaka Solid Junction Project, JST, BingShe Xu (now: Shaanxi Univ. Sci. Tech.) and Yoshitaka Tamou (now Mitsubishi Materials), and graduate students at Nagoya Institute of Technology, Toru Kameyama (now: Sony GMO Corp.) and Yoshiki Miki (now Nippon Giant Tire Co., Ltd.).

**Conflicts of Interest:** The author declares no conflict of interest.
