*3.3. Ni*

A nanostructure, which appeared for a Ni(001) foil irradiated with 400 keV electrons along [001] direction at 105 K to a dose of 7.0 <sup>×</sup> <sup>10</sup><sup>27</sup> electrons m−<sup>2</sup> [22], is shown in Figure 14. The diameter of electron beam for the irradiation was about 200 nm of which the center is near the center of Figure 14a. The micrograph was taken under a kinematic and slightly under-focus condition. One can see that the pattern near the center of the electron beam of photo (c) is rather different from that in the outer area of photo (c). The extension of bright images is not observed along [100] and [010] directions, but they extend slightly along [110] and [110] instead. The pattern near the center of the electron beam is similar to those of Ag and Cu, which are, respectively, shown in Figure 11b,c. The results for Ni mentioned above are different from those of Au, where the pattern is almost the same in

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 11 of 20

the whole area, although the extension of the grooves becomes longer near the beam center, as seen in Figure 2. mentioned above are different from those of Au, where the pattern is almost the same in the whole area, although the extension of the grooves becomes longer near the beam center, as seen in Figure 2.

A nanostructure, which appeared for a Ni(001) foil irradiated with 400 keV electrons along [001] direction at 105 K to a dose of 7.0 × 1027 electrons m−2 [22], is shown in Figure 14. The diameter of electron beam for the irradiation was about 200 nm of which the center is near the center of Figure 14a. The micrograph was taken under a kinematic and slightly under-focus condition. One can see that the pattern near the center of the electron beam of photo (c) is rather different from that in the outer area of photo (c). The extension of bright images is not observed along [100] and [010] directions, but they extend slightly along [110] and [11 0] instead. The pattern near the center of the electron beam is similar to those of Ag and Cu, which are, respectively, shown in Figure 11b,c. The results for Ni

**Figure 14.** (**a**) TEM micrograph of nanostructures generated on the exit surface of Ni(001) foil irradiated with 400 keV electrons along [001] direction at 105 K to a dose of 7.0 × 1027 electrons m−2. (**b**) A magnified view of the outer area of the electron beam. (**c**) A magnified view of the area near the beam center. **Figure 14.** (**a**) TEM micrograph of nanostructures generated on the exit surface of Ni(001) foil irradiated with 400 keV electrons along [001] direction at 105 K to a dose of 7.0 <sup>×</sup> <sup>10</sup><sup>27</sup> electrons m−<sup>2</sup> . (**b**) A magnified view of the outer area of the electron beam. (**c**) A magnified view of the area near the beam center.

Figure 15 shows the Ni(001) specimen, which was tilted to the direction near [011] and irradiated obliquely with 400 keV electrons along [011] direction at 105 K to a dose of 8.4 × 1027 electrons m−2. Bright images of nanogrooves tend to extend along [100], as shown in Figure 15a, similar to the cases of Au, Ag, and Cu (Figure 12). The irradiation was prolonged to a dose of 4.2 × 1028 electrons m−2, then the penetration of nanogrooves from the electron exit surface to the incident surface occurred, as seen in Figure 15b. We can find large holes appear near the center of an electron beam, and several nanowires were generated. The nanowires tend to elongate along [100], reflecting the initial pattern of nanogrooves seen in Figure 15a. Figure 15 shows the Ni(001) specimen, which was tilted to the direction near [011] and irradiated obliquely with 400 keV electrons along [011] direction at 105 K to a dose of 8.4 <sup>×</sup> <sup>10</sup><sup>27</sup> electrons m−<sup>2</sup> . Bright images of nanogrooves tend to extend along [100], as shown in Figure 15a, similar to the cases of Au, Ag, and Cu (Figure 12). The irradiation was prolonged to a dose of 4.2 <sup>×</sup> <sup>10</sup><sup>28</sup> electrons m−<sup>2</sup> , then the penetration of nanogrooves from the electron exit surface to the incident surface occurred, as seen in Figure 15b. We can find large holes appear near the center of an electron beam, and several nanowires were generated. The nanowires tend to elongate along [100], reflecting the initial pattern of nanogrooves seen in Figure 15a.
