*4.3. Al2O<sup>3</sup> Nanoparticle Encapsulation*

electron beam irradiation [13].

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 12 of 21

The surface of the Al2O<sup>3</sup> particles was easily covered by an amorphous hydrocarbon layer through a reaction with residual gases in the TEM, where CO, H2O, and H<sup>2</sup> remained even in a highly evacuated atmosphere. When the particles were irradiated by electrons through the outer layer, their inner volume was pulverised into smaller nanoparticles. An example is shown in Figure 20, for the case in which δ-Al2O<sup>3</sup> particles with 200 nm in diameter were transformed into δ- and α-Al2O<sup>3</sup> nanoballs with 2–20 nm in diameter, encapsulated by a hydrocarbon skin. This structure was generated by irradiation with an intensity of 1.6 <sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s, which was as high as that used for nanoparticle preparation and manipulation. Nanoparticle encapsulation technology may be applicable to drug delivery systems in medicine [15]. **Figure 18.** The interface of the -Al2O3 nanowire and nanoball, shown in Figure 17, exhibits a (111)//(101) epitaxial relationship. The nanoball is covered with an amorphous Al-O layer [14].

**Figure 17.** -Al2O3 nanowire and nanoball grown under flashing mode electron beam irradiation (i.e., rapid switching between intensities of 5.5 × 1022 and 5 × 1019 e/cm2sec). They connected and grew toward the irradiation centre from the original -Al2O3 particle surface. The nanowire grew epitaxially maintaining the same plane as the parent -Al2O3 particle. (**a**) -Al2O3 particles before irradiation, with X indicating the irradiation centre. (**b**) -Al2O3 nanowire and nanoball grown after

electrons through the outer layer, their inner volume was pulverised into smaller

**Figure 19.** -Al2O3 nanowire and nanoball grown under flashing mode electron beam irradiation with intensities switching between 5.5 × 1022 and 5 × 1019 e/cm2sec. They connected and grew toward the irradiation centre from the original -Al2O3 particle surface. (**a**) -Al2O3 particles before irradiation, with X indicating the irradiation centre. (**b**) -Al2O3 nanowire and nanoball grown after electron beam irradiation [13]. **Figure 19.** δ-Al2O<sup>3</sup> nanowire and nanoball grown under flashing mode electron beam irradiation with intensities switching between 5.5 <sup>×</sup> <sup>10</sup><sup>22</sup> and 5 <sup>×</sup> <sup>10</sup><sup>19</sup> e/cm<sup>2</sup> s. They connected and grew toward the irradiation centre from the original δ-Al2O<sup>3</sup> particle surface. (**a**) δ-Al2O<sup>3</sup> particles before irradiation, with X indicating the irradiation centre. (**b**) δ-Al2O<sup>3</sup> nanowire and nanoball grown after electron beam irradiation [13]. nanoparticles. An example is shown in Figure 20, for the case in which -Al2O3 particles with 200 nm in diameter were transformed into - and -Al2O3 nanoballs with 2–20 nm in diameter, encapsulated by a hydrocarbon skin. This structure was generated by irradiation with an intensity of 1.6 × 1020 e/cm2sec, which was as high as that used for nanoparticle preparation and manipulation. Nanoparticle encapsulation technology may be applicable to drug delivery systems in medicine [15].

**Figure 20.** - and -Al2O3 nanoball-encapsulated structures obtained from -Al2O3 particles by electron beam irradiation at 1.6 × 1020 e/cm2s for 300 s. An outer amorphous hydrocarbon layer is formed from residual gas contaminants in TEM [15]. **Figure 20.** δ- and α-Al2O<sup>3</sup> nanoball-encapsulated structures obtained from δ-Al2O<sup>3</sup> particles by electron beam irradiation at 1.6<sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s for 300 s. An outer amorphous hydrocarbon layer is formed from residual gas contaminants in TEM [15].

#### **5. Other Nanoparticles 5. Other Nanoparticles**

shown in Figure 22 [16].

#### *5.1. W Nanoparticles and Manipulation 5.1. W Nanoparticles and Manipulation*

In Section 2, a novel method for preparing oxide-free Al nanoparticles from metastable oxides using electron beam irradiation was discussed. This method can be extended to other nanoparticles of easy oxide-forming elements, such as W. W has a In Section 2, a novel method for preparing oxide-free Al nanoparticles from metastable oxides using electron beam irradiation was discussed. This method can be extended to other nanoparticles of easy oxide-forming elements, such as W. W has a heavier specific

Although the fundamental electron optics in the TEM were the same as those used for Al nanoparticles, electrons from the field emission source provided a higher intensity of 1023 e/cm2s than the 1020e/cm2s obtained from the LaB6 filament. Using a Hitachi HF-2000 TEM equipped with a field emission gun with an intensity of 4 × 1023 e/cm2s, Tamou and Tanaka reported the formation of W nanoparticles with an average diameter of 4.3 nm, as shown in Figure 21. The EELS spectra showed no oxygen atoms on the W surface, as

weight of 19.3 g/cm<sup>3</sup> , and W-O has a larger bonding enthalpy such that a higher electron irradiation intensity is required to obtain W nanoparticles from WO3. Although the fundamental electron optics in the TEM were the same as those used for Al nanoparticles, electrons from the field emission source provided a higher intensity of 10<sup>23</sup> e/cm<sup>2</sup> s than the 1020e/cm<sup>2</sup> s obtained from the LaB<sup>6</sup> filament. Using a Hitachi HF-2000 TEM equipped with a field emission gun with an intensity of 4 <sup>×</sup> <sup>10</sup><sup>23</sup> e/cm<sup>2</sup> s, Tamou and Tanaka reported the formation of W nanoparticles with an average diameter of 4.3 nm, as shown in Figure 21. The EELS spectra showed no oxygen atoms on the W surface, as shown in Figure 22 [16]. *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 14 of 21 *Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 14 of 21

**Figure 21.** W nanoparticles deposited by electron beam irradiation of a WO3 particle at 4 × 1023 e/cm2s (6 × 108 A/m2) for a few seconds. The diameter of the W particles ranged between 2 and 6 nm with an average of 4.3 nm [16]. **Figure 21.** W nanoparticles deposited by electron beam irradiation of a WO<sup>3</sup> particle at <sup>4</sup> <sup>×</sup> <sup>10</sup><sup>23</sup> e/cm<sup>2</sup> s (6 <sup>×</sup> <sup>10</sup><sup>8</sup> A/m<sup>2</sup> ) for a few seconds. The diameter of the W particles ranged between 2 and 6 nm with an average of 4.3 nm [16]. **Figure 21.** W nanoparticles deposited by electron beam irradiation of a WO3 particle at 4 × 1023 e/cm2s (6 × 108 A/m2) for a few seconds. The diameter of the W particles ranged between 2 and 6 nm with an average of 4.3 nm [16].

**Figure 22.** EELS spectra before and after electron irradiation of WO3. Electron irradiation induced the disappearance of the O-Kα peak to form pure W nanoparticles [16]. **Figure 22.** EELS spectra before and after electron irradiation of WO3. Electron irradiation induced the disappearance of the O-Kα peak to form pure W nanoparticles [16]. **Figure 22.** EELS spectra before and after electron irradiation of WO<sup>3</sup> . Electron irradiation induced the disappearance of the O-Kα peak to form pure W nanoparticles [16].

#### *5.2. W Migration to Bond and Fullerene Formation 5.2. W Migration to Bond and Fullerene Formation 5.2. W Migration to Bond and Fullerene Formation*

nanoparticles shown in Figures 10–12 [16].

Further electron irradiation of two W nanoparticles, obtained as in Figure 21 at 1.9 × 1021 e/cm2s, which is an irradiation 10 times higher than that used to form Al as shown in Figure 6, induced migration, bonding, and coalescence, as shown in Figure 23 [16]. Graphitic shells also nucleated beneath the W nanoparticles from the amorphous carbon film and grew to onion-like fullerene, which is the same phenomenon observed with Al nanoparticles shown in Figures 10–12 [16]. Further electron irradiation of two W nanoparticles, obtained as in Figure 21 at 1.9 × 1021 e/cm2s, which is an irradiation 10 times higher than that used to form Al as shown in Figure 6, induced migration, bonding, and coalescence, as shown in Figure 23 [16]. Graphitic shells also nucleated beneath the W nanoparticles from the amorphous carbon film and grew to onion-like fullerene, which is the same phenomenon observed with Al Further electron irradiation of two W nanoparticles, obtained as in Figure 21 at 1.9 <sup>×</sup> <sup>10</sup><sup>21</sup> e/cm<sup>2</sup> s, which is an irradiation 10 times higher than that used to form Al as shown in Figure 6, induced migration, bonding, and coalescence, as shown in Figure 23 [16]. Graphitic shells also nucleated beneath the W nanoparticles from the amorphous carbon film and grew to onion-like fullerene, which is the same phenomenon observed with Al nanoparticles shown in Figures 10–12 [16].

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 15 of 21

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 15 of 21

**Figure 23.** Effects of electron beam irradiation of W nanoparticles on an amorphous carbon film at 1.9 × 1021 e/cm2s (300 A/cm2). W nanoparticles migrated together and coalesced, followed by fullerene formation between the W particles and the carbon film. The general features of nanostructure evolution were the same as those observed with irradiation of Al nanoparticles **Figure 23.** Effects of electron beam irradiation of W nanoparticles on an amorphous carbon film at 1.9 <sup>×</sup> <sup>10</sup><sup>21</sup> e/cm<sup>2</sup> s (300 A/cm<sup>2</sup> ). W nanoparticles migrated together and coalesced, followed by fullerene formation between the W particles and the carbon film. The general features of nanostructure evolution were the same as those observed with irradiation of Al nanoparticles shown in Figure 10 [16]. shown in Figure 10 [16]. *5.3. Bonding of Pt and Cu Nanoparticles*  Electron irradiation of a group of Pt and Cu nanoparticles induced bonding to form

#### shown in Figure 10 [16]. *5.3. Bonding of Pt and Cu Nanoparticles* nanofilms, as shown in Figures 24–26 [19,20]. In these cases, nanoparticles were prepared by Ar ion sputtering with a diameter of 10 nm for Pt and 50 nm for Cu. The irradiation

*5.3. Bonding of Pt and Cu Nanoparticles*  Electron irradiation of a group of Pt and Cu nanoparticles induced bonding to form nanofilms, as shown in Figures 24–26 [19,20]. In these cases, nanoparticles were prepared by Ar ion sputtering with a diameter of 10 nm for Pt and 50 nm for Cu. The irradiation Electron irradiation of a group of Pt and Cu nanoparticles induced bonding to form nanofilms, as shown in Figures 24–26. In these cases, nanoparticles were prepared by Ar ion sputtering with a diameter of 10 nm for Pt and 50 nm for Cu. The irradiation intensity for Pt was the same as that for Al, whereas it was 100 times higher for Cu. Bonded Pt/Pt mainly showed three stable Σ3 twin boundaries. The Cu particles migrated to the irradiation centre and bonded, as shown in Figures 26 and 27. The driving force was also the momentum transfer from electrons in the pole piece of the TEM, as shown in Figures 4 and 5 [17–20]. intensity for Pt was the same as that for Al, whereas it was 100 times higher for Cu. Bonded Pt/Pt mainly showed three stable 3 twin boundaries. The Cu particles migrated to the irradiation centre and bonded, as shown in Figures 26 and 27 [20]. The driving force was also the momentum transfer from electrons in the pole piece of the TEM, as shown in Figures 4 and 5 [17–20].

**Figure 24.** Pt nanoparticles bonded by electron irradiation with an intensity of 2.1 × 1020 e/cm2s for 700 s. The bonded Pt/Pt nanoparticles had tilt boundaries of Σ3 and Σ11 [19]. **Figure 24.** Pt nanoparticles bonded by electron irradiation with an intensity of 2.1 <sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s for 700 s. The bonded Pt/Pt nanoparticles had tilt boundaries of Σ3 and Σ11 [19].

700 s. The bonded Pt/Pt nanoparticles had tilt boundaries of Σ3 and Σ11 [19].

**Figure 24.** Pt nanoparticles bonded by electron irradiation with an intensity of 2.1 × 1020 e/cm2s for

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 16 of 21

**Figure 25.** Histogram of tilt boundaries under electron irradiation at three intensities from 1.0 to 3.3 × 1020 e/cm2s for 700 s. Σ3 CSL boundaries were predominant as a low energy structure [19]. **Figure 25.** Histogram of tilt boundaries under electron irradiation at three intensities from 1.0 to 3.3 <sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s for 700 s. Σ3 CSL boundaries were predominant as a low energy structure [19]. **Figure 25.** Histogram of tilt boundaries under electron irradiation at three intensities from 1.0 to 3.3 × 1020 e/cm2s for 700 s. Σ3 CSL boundaries were predominant as a low energy structure [19].

**Figure 26.** Change of the bright-field images and electron diffraction patterns of Cu nanoparticles irradiated with electrons at an intensity of 5.5 × 1022 e/cm2s for 60 s. Cu nanoparticles migrated to the irradiation centre and bonded with each other in the marked irradiation area. The electron diffraction patterns, typical Cu Debye rings, did not change during irradiation. The nanostructures of the bonded interface (i.e., CSL) boundary, were obtained in regions (**a**) and (**b**) after 60 s of irradiation, as shown in Figure 27 [20]. **Figure 26.** Change of the bright-field images and electron diffraction patterns of Cu nanoparticles irradiated with electrons at an intensity of 5.5 × 1022 e/cm2s for 60 s. Cu nanoparticles migrated to the irradiation centre and bonded with each other in the marked irradiation area. The electron diffraction patterns, typical Cu Debye rings, did not change during irradiation. The nanostructures of the bonded interface (i.e., CSL) boundary, were obtained in regions (**a**) and (**b**) after 60 s of **Figure 26.** Change of the bright-field images and electron diffraction patterns of Cu nanoparticles irradiated with electrons at an intensity of 5.5 <sup>×</sup> <sup>10</sup><sup>22</sup> e/cm<sup>2</sup> s for 60 s. Cu nanoparticles migrated to the irradiation centre and bonded with each other in the marked irradiation area. The electron diffraction patterns, typical Cu Debye rings, did not change during irradiation. The nanostructures of the bonded interface (i.e., CSL) boundary, were obtained in regions (**a**) and (**b**) after 60 s of irradiation, as shown in Figure 27 [20]. **Figure 26.** Change of the bright-field images and electron diffraction patterns of Cu nanoparticles irradiated with electrons at an intensity of 5.5 <sup>×</sup> <sup>1022</sup> e/cm2s for 60 s. Cu nanoparticles migrated to the irradiation centre and bonded with each other in the marked irradiation area. The electron diffraction patterns, typical Cu Debye rings, did not change during irradiation. The nanostructures of the bonded interface (i.e., CSL) boundary, were obtained in regions (**a**) and (**b**) after 60 s of irradiation, as shown in Figure 27 [20].

irradiation, as shown in Figure 27 [20].

**Figure 27.** Superposed view of Cu nanoparticle migration and bonding during the irradiation times of 0 s (dotted line) and 30 s (solid line). The circle indicates the electron beam irradiation area of 200 **Figure 27.** Superposed view of Cu nanoparticle migration and bonding during the irradiation times of 0 s (dotted line) and 30 s (solid line). The circle indicates the electron beam irradiation area of 200 nm diameter. Nanoparticles migrated toward the irradiation centre and finally bonded together. Unbonded small nanoparticles seemed to revolve clockwise around the irradiation centre [20]. **Figure 27.** Superposed view of Cu nanoparticle migration and bonding during the irradiation times of 0 s (dotted line) and 30 s (solid line). The circle indicates the electron beam irradiation area of 200 nm diameter. Nanoparticles migrated toward the irradiation centre and finally bonded together. Unbonded small nanoparticles seemed to revolve clockwise around the irradiation centre [20].

**Figure 27.** Superposed view of Cu nanoparticle migration and bonding during the irradiation times

nm diameter. Nanoparticles migrated toward the irradiation centre and finally bonded together. Unbonded small nanoparticles seemed to revolve clockwise around the irradiation centre [20].
