*Review* **Self-Organized Nanostructures Generated on Metal Surfaces under Electron Irradiation**

**Keisuke Niwase**

Department of Physics, Hyogo University of Teacher Education, Kato 673-1494, Hyogo, Japan; niwase@hyogo-u.ac.jp; Tel.: +81-795-44-2210

**Abstract:** Irradiation of high-energy electrons can produce surface vacancies on the exit surface of thin foils by the sputtering of atoms. Although the sputtering randomly occurs in the area irradiated with an intense electron beam of several hundred nanometers in diameter, characteristic topographic features can appear under irradiation. This paper reviews a novel phenomenon on a self-organization of nanogrooves and nanoholes generated on the exit surface of thin metal foils irradiated with high doses of 360–1250 keV electrons. The phenomenon was discovered firstly for gold irradiated at temperatures about 100 K, which shows the formation of grooves and holes with widths between 1 and 2 nm. Irradiation along [001] produces grooves extending along [100] and [010], irradiation along [011] gives grooves along [100], whereas no clear grooves have been observed for [111] irradiations. By contrast, nanoholes, which may reach depths exceeding 20 nm, develop mainly along the beam direction. The formation of the nanostructures depends on the irradiation temperatures, exhibiting an existence of a critical temperature at about 240 K, above which the width significantly increases, and the density decreases. Nanostructures formed for silver, copper, nickel, and iron were also investigated. The self-organized process was discussed in terms of irradiation-induced effects.

**Keywords:** electron irradiation; metal surface; sputtering; groove; hole; self-organization; pattern


**Citation:** Niwase, K. Self-Organized Nanostructures Generated on Metal Surfaces under Electron Irradiation. *Quantum Beam Sci.* **2021**, *5*, 4. https://doi.org/10.3390/qubs5010004

Received: 3 December 2020 Accepted: 14 January 2021 Published: 19 January 2021

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