**1. Introduction**

The size range of several tens of nanometres represents a transition region from "top-down" to "bottom-up" processes in nanotechnology. The author has proposed a bottom-up technology utilising active beam irradiation to achieve nanostructures with a size of 3–40 nm. Recent developments in focusing and scanning technologies for active beams, such as electrons or ions, in tabletop apparatuses enable the evolution and control of various types of nanostructures, which can provide desirable hybrid structures, materials, and phases at the desired positions on the nanometre scale.

The source of the electron beam used by the author's group is a transmission electron microscope (TEM) equipped with either an LaB<sup>6</sup> filament or a field emission gun. Although TEM has been widely used as an analysis tool for studying nanostructure and element distributions, we consider a specimen stage of 3 mm in diameter as a reaction field, and focused electrons with an intensity more than 50 times higher than that used under normal observation conditions. The electron irradiation intensity ranged from 5×10<sup>19</sup> to <sup>4</sup> <sup>×</sup> <sup>10</sup><sup>23</sup> e/cm<sup>2</sup> <sup>s</sup> (8 <sup>×</sup> <sup>10</sup>4–6 <sup>×</sup> <sup>10</sup><sup>8</sup> A/m<sup>2</sup> ) in our experiment, and we succeeded in producing oxide-free nanoparticles via electron irradiation of the oxide and facilitated their subsequent manipulation.

In this paper, three topics on the manipulation and control of ceramics and metals by electron beam irradiation are discussed: (1) the preparation of Al nanoparticles and their nanostructure evolution starting from metastable Al2O3, (2) their manipulation by electrons, and (3) the effects of electron irradiation on other nanoparticles, such as Cu, Pt, and W, to prepare nanofilms. This review is based on papers published between 1995 and 2005.

**Citation:** Tanaka, S.-I. Control and Modification of Nanostructured Materials by Electron Beam Irradiation. *Quantum Beam Sci.* **2021**, *5*, 23. https://doi.org/10.3390/ qubs5030023

Academic Editor: Akihiro Iwase

Received: 20 April 2021 Accepted: 12 July 2021 Published: 21 July 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

2005.

*2.1. Overview*

e/cm2s).

#### **2. Electron Excited Reaction Field** ions, based on the proposed concept of an "excited reaction field." One of the

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#### *2.1. Overview* characteristics of such field is that the beams are obtained on the specimen stage of a TEM

**2. Electron Excited Reaction Field**

In the "Exploratory Research for Advanced Technology" (ERATO) "Tanaka Solid Junction Project," JST [1], held in 1993–1998, I commenced an innovative challenge to fabricate nano-/microstructures by irradiation with an energy beam such as electrons and ions, based on the proposed concept of an "excited reaction field." One of the characteristics of such field is that the beams are obtained on the specimen stage of a TEM for electrons, and on the milling/thinning stage for ions. In other words, observation or specimen preparation apparatuses are utilised for their beam source and excited reaction fields. Irradiation with these energy beams has the following merits: first, it facilitates the selection of the site/energy/reaction. Second, it can induce nonequilibrium/catalytic reactions, making it possible to manipulate atomic clusters, synthesise nanomaterials, control the nanostructure and phase, and modify the nanospace. for electrons, and on the milling/thinning stage for ions. In other words, observation or specimen preparation apparatuses are utilised for their beam source and excited reaction fields. Irradiation with these energy beams has the following merits: first, it facilitates the selection of the site/energy/reaction. Second, it can induce nonequilibrium/catalytic reactions, making it possible to manipulate atomic clusters, synthesise nanomaterials, control the nanostructure and phase, and modify the nanospace. *2.2. Effects of Electron Beam Irradiation on -Al2O3* The normal electron beam intensity for observation by a TEM equipped with an

and W, to prepare nanofilms. This review is based on papers published between 1995 and

fabricate nano-/microstructures by irradiation with an energy beam such as electrons and

#### *2.2. Effects of Electron Beam Irradiation on θ-Al2O<sup>3</sup>* LaB6 filament is on the order of 1018 e/cm2s (1600 A/m2) for bright-field imaging and 5×1016

The normal electron beam intensity for observation by a TEM equipped with an LaB<sup>6</sup> filament is on the order of 10<sup>18</sup> e/cm<sup>2</sup> s (1600 A/m<sup>2</sup> ) for bright-field imaging and <sup>5</sup> <sup>×</sup> <sup>10</sup><sup>16</sup> e/cm<sup>2</sup> s s (80 A/m<sup>2</sup> ) for selected area diffraction measured by a fluorescent plate. We increased the electron beam density by increasing the current in the condenser lens to between 10<sup>19</sup> and 10<sup>22</sup> e/cm<sup>2</sup> s to enable electron irradiation of the nanomaterials. e/cm2s s (80 A/m2) for selected area diffraction measured by a fluorescent plate. We increased the electron beam density by increasing the current in the condenser lens to between 1019 and 1022 e/cm2s to enable electron irradiation of the nanomaterials. When an electron beam irradiates metastable -Al2O3 particles at an intensity of 1020

When an electron beam irradiates metastable θ-Al2O<sup>3</sup> particles at an intensity of 10<sup>20</sup> e/cm<sup>2</sup> s in a TEM, successive reactions occur: <sup>1</sup> Al nanoparticle inducement, <sup>2</sup> rotation, revolution, and migration to coalesce/embed, and <sup>3</sup> formation of onion-like fullerenes and Al intercalation, as shown in Figure 1. Furthermore, <sup>4</sup> θ- or δ-Al2O<sup>3</sup> nanostructures are obtained under flashing-mode irradiation (rapid switching between 10<sup>19</sup> and 10<sup>22</sup> e/cm<sup>2</sup> s). e/cm2s in a TEM, successive reactions occur: ① Al nanoparticle inducement, ② rotation, revolution, and migration to coalesce/embed, and ③ formation of onion-like fullerenes and Al intercalation, as shown in Figure 1. Furthermore, ④ - or -Al2O3 nanostructures are obtained under flashing-mode irradiation (rapid switching between 1019 and 1022

**Figure 1**. Evolution of the reactions and nanostructures induced by electron beam irradiation of metastable -Al2O3 in TEM. Numbers ①–④ denote the reaction routes explained [1]. **Figure 1.** Evolution of the reactions and nanostructures induced by electron beam irradiation of metastable θ-Al2O<sup>3</sup> in TEM. Numbers <sup>1</sup> – <sup>4</sup> denote the reaction routes explained [1].

#### **3. Nanostructure Evolution** from -Al2O3 to -Al2O3 upon heating. The starting powder used in this experiment was

**3. Nanostructure Evolution**

*3.1. Al Nanoparticles*

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vaporised metal combustion method (Admatechs Company Limited) [2].

transformation or rearrangement from -Al2O3 to stable -Al2O3 also occurred.

#### *3.1. Al Nanoparticles* obtained by melting Al metal powder, vaporisation, collision of droplets, and quenching

The starting material consists mainly of a metastable θ-Al2O<sup>3</sup> monoclinic structure accompanied by orthorhombic δ-Al2O<sup>3</sup> (Al2O<sup>3</sup> allotrope), and it is different from stable α-Al2O<sup>3</sup> with a trigonal structure. The θ-Al2O<sup>3</sup> powder was synthesised using the vaporised metal combustion method (Admatechs Company Limited) [2]. into the metastable phase [2]. The powder particles had a spherical shape with an average diameter of approximately 10 μm. No stable -Al2O3 structure was observed by X-ray diffraction, and an equilibrium thermodynamic consideration was invalid for metastable -Al2O3, where the binding energy for Al-O was lower than that for -Al2O3. The reaction was expected to proceed via a nonequilibrium route. Electron irradiation


The starting material consists mainly of a metastable -Al2O3 monoclinic structure accompanied by orthorhombic -Al2O3 (Al2O3 allotrope), and it is different from stable -Al2O3 with a trigonal structure. The -Al2O3 powder was synthesised using the

Electron irradiation (2.1 × 1020 e/cm2s for 50 s) of one -Al2O3 particle with a diameter of 100 nm on the 3 mm specimen stage of the TEM led to the formation of Al nanoparticles with a diameter of 2–20 nm and a stable -Al2O3 particle, as shown in Figure 2 [3]. The TEM used was a JEOL JEM-2010 equipped with an LaB6 filament and a specimen chamber under a vacuum of 10−7 Pa, obtained by a direct coupling sputter ion pump. Under these conditions, the electrons cut the Al-O bond in -Al2O3, which was followed by the loss of oxygen to the vacuum and Al atom recombination to form nanoparticles, as schematically shown in Figure 3. In the electron-irradiated area,

Electron irradiation (2.1 <sup>×</sup> <sup>10</sup><sup>20</sup> e/cm<sup>2</sup> s for 50 s) of one θ-Al2O<sup>3</sup> particle with a diameter of 100 nm on the ϕ3 mm specimen stage of the TEM led to the formation of Al nanoparticles with a diameter of 2–20 nm and a stable α-Al2O<sup>3</sup> particle, as shown in Figure 2 [3]. The TEM used was a JEOL JEM-2010 equipped with an LaB<sup>6</sup> filament and a specimen chamber under a vacuum of 10−<sup>7</sup> Pa, obtained by a direct coupling sputter ion pump. Under these conditions, the electrons cut the Al-O bond in θ-Al2O3, which was followed by the loss of oxygen to the vacuum and Al atom recombination to form nanoparticles, as schematically shown in Figure 3. In the electron-irradiated area, transformation or rearrangement from θ-Al2O<sup>3</sup> to stable α-Al2O<sup>3</sup> also occurred. promoted the decomposition of metastable -Al2O3, recombination of Al atoms, loss of a part of oxygen atoms into vacuum, and transformation to stable -Al2O3, as shown in Figure 3, which resembles one stile of the electron-stimulated desorption. Al nanoparticles have a twinned decahedron structure surrounded by {111} surfaces, which has been reported in typical face-centred cubic noble metals, such as Au, Pt, or Ag, whereas no report has been published on Al because of its easily oxidised surface. Decahedra appeared with diameters in the range of 10–20 nm, and further electron irradiation of a set of nanodecahedra enabled their manipulation, as discussed in the next section.

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embed the particles.

3.2.1. Migration and Bonding

*3.2. Manipulation* In the ERATO project, TEM has been used not only as an observation apparatus but θ- and δ-Al2O<sup>3</sup> appear as low-temperature phases in the allotropic transformation from γ-Al2O<sup>3</sup> to α-Al2O<sup>3</sup> upon heating. The starting powder used in this experiment was obtained by melting Al metal powder, vaporisation, collision of droplets, and quenching

to bottom, where the specimen stage is located slightly above the centre. Based on the kinetic features analysed by Horiuchi et al. [4], the momentum transfer from electrons to nanoparticles is the source of their movement. The spiral trajectory of the electrons causes Al nanoparticles on the specimen stage to experience both a tangential force to rotate and revolve and a centripetal force to migrate, bond, and embed. The features around the

**Figure 4.** Model of the interaction between electrons and particles on the specimen stage in TEM. The magnetic field inside a pole piece is 104 Gauss in a 200 keV TEM. (**a**) Electrons follow a spiral trajectory in the magnetic field to transfer forces or momentum such that (**b**) the tangential force results in rotation and revolution and the centripetal force induces migration to gather, bond, and

A set of Al nanodecahedra migrated and bonded to the irradiation centre of the electron beam, as shown in Figure 5. Analysis by superposition before and after irradiation, as shown in Figure 5c, revealed that migration, diameter decrease,

specimen stage and the driving forces are illustrated in Figure 4.

into the metastable phase [2]. The powder particles had a spherical shape with an average diameter of approximately 10 µm. No stable α-Al2O<sup>3</sup> structure was observed by X-ray diffraction, and an equilibrium thermodynamic consideration was invalid for metastable θ-Al2O3, where the binding energy for Al-O was lower than that for α-Al2O3. The reaction was expected to proceed via a nonequilibrium route. Electron irradiation promoted the decomposition of metastable θ-Al2O3, recombination of Al atoms, loss of a part of oxygen atoms into vacuum, and transformation to stable α-Al2O3, as shown in Figure 3, which resembles one stile of the electron-stimulated desorption.

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Al nanoparticles have a twinned decahedron structure surrounded by {111} surfaces, which has been reported in typical face-centred cubic noble metals, such as Au, Pt, or Ag, whereas no report has been published on Al because of its easily oxidised surface. Decahedra appeared with diameters in the range of 10–20 nm, and further electron irradiation of a set of nanodecahedra enabled their manipulation, as discussed in the next section. **Figure 3.** Schematic of Al nanodecahedra formation by electron irradiation in a TEM. Electrons cut the Al-O bond of metastable -Al2O3, which decomposes into Al and O atoms, and finally the Al atoms recombine to form twinned decahedra surrounded by {111} surfaces.
