*3.4. Iron*

*3.3. Ni* 

A nanostructure formed for a Fe(111) foil irradiated with 400 keV electrons along [111] direction at 300 K to a dose of 4.4 <sup>×</sup> <sup>10</sup><sup>28</sup> electrons m−<sup>2</sup> [22] is shown in Figure 16. The TEM micrograph was taken under a kinematic and slightly under-focus condition. The beam center is near the edge of the specimen. The nanostructure generated by the electron irradiation can be roughly divided into two types; one is high density of nanoholes formed in the outer area of the electron beam, as seen in the magnified view, and the other is nanogrooves, which are observed near the beam center.

The appearance of the two types of structure should be due to the difference in the total dose, as the beam intensity is stronger near the beam center. The irradiation dose described above is an average value. In situ observation has revealed that nanoholes are generated at first and then developed to nanogrooves with increasing dose. The diameter of nanoholes is about 2–4 nm. Formation of high density of nanoholes under electron irradiation has not been observed for Au, Ag, and Cu [21] but has been reported for Si [27].

**Figure 15.** TEM micrographs of Ni(001) specimen tilted to the direction near [011] direction. Irradiation was done with 400 keV electrons along [011] direction at 105 K to doses of (**a**) 8.4 × 1027 em<sup>−</sup><sup>2</sup> and (**b**) 4.2 × 1028 em<sup>−</sup>2. Penetration of the foil can be seen in photo (**b**). **Figure 15.** TEM micrographs of Ni(001) specimen tilted to the direction near [011] direction. Irradiation was done with 400 keV electrons along [011] direction at 105 K to doses of (**a**) 8.4 <sup>×</sup> <sup>10</sup><sup>27</sup> <sup>e</sup> <sup>−</sup>m−<sup>2</sup> and (**b**) 4.2 <sup>×</sup> <sup>10</sup><sup>28</sup> <sup>e</sup> <sup>−</sup>m−<sup>2</sup> . Penetration of the foil can be seen in photo (**b**). specimen suggest the remarkable effect of surface diffusion. Therefore, we guess that thermal evaporation of Fe atoms from the surface becomes remarkable at a temperature above 623 K to form the steps and to change the structure. Moreover, material transport on Fe surface by surface diffusion is not so significant as to change the nanostructure below 823 K.

with larger diameters, while the hillocks decreased in height, as shown in Figure 8. Then, the stepwise annealing was carried out with 50 K/10 min steps from 373 to 823 K in the transmission electron microscope to know the thermal stability of the nanostructure generated **Figure 16.** TEM micrographs of the Fe(111) foil irradiated with 400 keV electrons along [011] direction at 300 K to a dose of 4.4 × 1028 electrons m−2. A magnified view of the nanoholes is given in the inset. **Figure 16.** TEM micrographs of the Fe(111) foil irradiated with 400 keV electrons along [011] direction at 300 K to a dose of 4.4 <sup>×</sup> <sup>10</sup><sup>28</sup> electrons m−<sup>2</sup> . A magnified view of the nanoholes is given in the inset.

for Fe. The changes on annealing at 623 and 823 K are, respectively, shown in Figure 17a,b. The pattern formed on the Fe surface scarcely changed up to 573 K, but a clear change can be observed above 623 K. The steps denoted by arrows are seen to move downwards. In spite of such an apparent change in the specimen surface on annealing at 623 K (Figure 17a), One should note that a nanostructure with several nanometer size appeared at 300 K in the case of Fe. In the case of Au, on the other hand, the nanostructure appeared at 110 K and became unstable on annealing at room temperature. Nanoholes are transformed into voids with larger diameters, while the hillocks decreased in height, as shown in Figure 8. Then, the stepwise annealing was carried out with 50 K/10 min steps from 373 to 823 K in

**Figure 17.** The change in nanostructure of the Fe(111) foil on annealing at (**a**) 623 K and (**b**) 823 K. The nanostructure was generated by the irradiation with 400 keV electrons along the [011] direction at 300 K to a fluence of 4.4 × 1028 electrons m−2. Steps that appeared on annealing are denoted

However, residual O2 gas in the conventional TEM may affect the formation or the behavior of nanostructure similarly to the case of silicon [27]. Nonetheless, the electron

by arrows in photos (**a**,**b**).

the transmission electron microscope to know the thermal stability of the nanostructure generated for Fe.

surface diffusion is not so significant as to change the nanostructure below 823 K.

we can still observe some nanogrooves and nanoholes whose size and pattern are similar to those before annealing (Figure 16). At 823 K, on the other hand, a remarkable change such as disappearance of the nanostructures can be observed (Figure 17b). However, there still remains some nanoholes whose diameters are almost the same as those before annealing. The result is clearly different from that of Au, where nanoholes were transformed into voids with larger diameters, and the surface became smoother. The changes in the Au specimen suggest the remarkable effect of surface diffusion. Therefore, we guess that thermal evaporation of Fe atoms from the surface becomes remarkable at a temperature above 623 K to form the steps and to change the structure. Moreover, material transport on Fe surface by

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 13 of 20

The changes on annealing at 623 and 823 K are, respectively, shown in Figure 17a,b. The pattern formed on the Fe surface scarcely changed up to 573 K, but a clear change can be observed above 623 K. The steps denoted by arrows are seen to move downwards. In spite of such an apparent change in the specimen surface on annealing at 623 K (Figure 17a), we can still observe some nanogrooves and nanoholes whose size and pattern are similar to those before annealing (Figure 16). At 823 K, on the other hand, a remarkable change such as disappearance of the nanostructures can be observed (Figure 17b). However, there still remains some nanoholes whose diameters are almost the same as those before annealing. The result is clearly different from that of Au, where nanoholes were transformed into voids with larger diameters, and the surface became smoother. The changes in the Au specimen suggest the remarkable effect of surface diffusion. Therefore, we guess that thermal evaporation of Fe atoms from the surface becomes remarkable at a temperature above 623 K to form the steps and to change the structure. Moreover, material transport on Fe surface by surface diffusion is not so significant as to change the nanostructure below 823 K. **Figure 16.** TEM micrographs of the Fe(111) foil irradiated with 400 keV electrons along [011] direction at 300 K to a dose of 4.4 × 1028 electrons m−2. A magnified view of the nanoholes is given in the inset.

**Figure 17.** The change in nanostructure of the Fe(111) foil on annealing at (**a**) 623 K and (**b**) 823 K. The nanostructure was generated by the irradiation with 400 keV electrons along the [011] direction at 300 K to a fluence of 4.4 × 1028 electrons m−2. Steps that appeared on annealing are denoted by arrows in photos (**a**,**b**). **Figure 17.** The change in nanostructure of the Fe(111) foil on annealing at (**a**) 623 K and (**b**) 823 K. The nanostructure was generated by the irradiation with 400 keV electrons along the [011] direction at 300 K to a fluence of 4.4 <sup>×</sup> <sup>10</sup><sup>28</sup> electrons m−<sup>2</sup> . Steps that appeared on annealing are denoted by arrows in photos (**a**,**b**).

However, residual O2 gas in the conventional TEM may affect the formation or the behavior of nanostructure similarly to the case of silicon [27]. Nonetheless, the electron However, residual O<sup>2</sup> gas in the conventional TEM may affect the formation or the behavior of nanostructure similarly to the case of silicon [27]. Nonetheless, the electron exit surface of Fe where the pattern developed should be rather clean due to sputtering. Further investigations on this point are desired.
