**1. Introduction**

Generation of nanosized structures is technologically important, and the controlled formation of structures in solids on a nanometer scale is critical to modern technology [1–3]. Such structures may have properties different from those of the bulk materials. Nanoholebased nanomaterials with a size less than the wavelength of an excitation laser beam, for example, are promised for applications such as chemical and biological sensing, membrane biorecognition, unique optical responses under laser excitation, etc.

It should be worthwhile to know the smallest size of nanostructured material. The scanning tunneling microscope has been employed to control the deposition of atoms on or extraction of atoms from the surface on an atomic scale [4–6]. However, there seems to be a minimum width when one tries to produce deep nanoholes or nanogrooves.

High-energy particle irradiation is one of method to generate interesting nanomaterials [7,8]. Aggregation of surface vacancies produced homogeneously by ion sputtering may produce pits that can become rather deep. Electron irradiation, which can induce back sputtering on the surface of thin foil specimens, is also one of the techniques used for nanometer scale etching, lithography and hole formation, and intense convergent electron beams have been utilized, so far. Deep nanoholes have been formed in MgO using a convergent nanosized electron beam, the smallest widths so far achieved being about 1 nm [9,10]. Parallel electron beams, on the other hand, of 500–1000 nm diameter have been shown to produce pits on the exit surface of Au(111) foils by sputtering over the electron energy range of 0.4–1.1 MeV [11].

Self-organization is one method to produce characteristic structures and several selforganization phenomena of defect clusters under high-energy particle irradiations such as

voids, bubbles, and stacking fault tetrahedra have been reported so far [12–16]. Spontaneous well-ordered periodicity can be developed on a broad surface by ion beam sputtering and a numerical model has been proposed as the formation process [17]. taneous well-ordered periodicity can be developed on a broad surface by ion beam sputtering and a numerical model has been proposed as the formation process [17]. The present paper reviews our studies on the evolution of nanosized structures resulting from the sputtering of atoms from the exit surface of thin metal specimens during

Self-organization is one method to produce characteristic structures and several selforganization phenomena of defect clusters under high-energy particle irradiations such as voids, bubbles, and stacking fault tetrahedra have been reported so far [12–16]. Spon-

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 2 of 20

The present paper reviews our studies on the evolution of nanosized structures resulting from the sputtering of atoms from the exit surface of thin metal specimens during homogeneous electron irradiation, focusing on a novel self-organization phenomenon, which can occur on the electron irradiated surface [18–22] and give additional data on the temperature dependence of the formation of nanoholes for gold. homogeneous electron irradiation, focusing on a novel self-organization phenomenon, which can occur on the electron irradiated surface [18–22] and give additional data on the temperature dependence of the formation of nanoholes for gold. **2. Materials and Methods** 

#### **2. Materials and Methods** In the present study, we used the wedge-shaped specimens produced from 99.998%

In the present study, we used the wedge-shaped specimens produced from 99.998% Au, 99.9999% Ag, 99.999% Cu, 99.998% Ni, and 99.997% Fe foils with a thickness of about 100 µm. After eliminating lattice defects by annealing, they were thinned by jetpolishing to make wedge-shaped specimens. The electron irradiations were performed by using two transmission electron microscopes (TEM) of JEOL-ARM 1250 and JEOL-4000FX, both equipped with GATAN liquid-nitrogen cooling stages. Au, 99.9999% Ag, 99.999% Cu, 99.998% Ni, and 99.997% Fe foils with a thickness of about 100 μm. After eliminating lattice defects by annealing, they were thinned by jet-polishing to make wedge-shaped specimens. The electron irradiations were performed by using two transmission electron microscopes (TEM) of JEOL-ARM 1250 and JEOL-4000FX, both equipped with GATAN liquid-nitrogen cooling stages. The irradiations were done along crystallographic directions near the [100], [110], or

The irradiations were done along crystallographic directions near the [100], [110], or [111] direction of grains with surface orientations near {100}, using a beam of about 200–800 nm diameter as illustrated in Figure 1 and a flux density of about 10<sup>24</sup> electrons m−<sup>2</sup> s <sup>−</sup><sup>1</sup> with total fluences ranging from 10<sup>27</sup> to 10<sup>28</sup> electrons m−<sup>2</sup> . The irradiation direction was adjusted by tilting the specimen. Although the surface orientation and the observation directions were not perfectly aligned with the crystallographic orientations in question, such crystallographic indices are used hereafter to show the surface orientation and observation directions. The irradiation time ranged from several 10 s to several 1000 s. A small fraction of incident electrons (10−<sup>6</sup> for 1.0 MeV electrons) can give sufficient energy with target atoms on the electron exit surface to cause sputtering [23]. The electron microscopy observations were made with strongly reduced beam currents to suppress additional sputtering. Detailed experimental and theoretical study on the sputtering yield and angular distribution of sputtered atoms, among others, for gold foils in a high-voltage electron microscope have been given by Cherns et al. [23]. [111] direction of grains with surface orientations near {100}, using a beam of about 200– 800 nm diameter as illustrated in Figure 1 and a flux density of about 1024 electrons m−2 s−<sup>1</sup> with total fluences ranging from 1027 to 1028 electrons m−2. The irradiation direction was adjusted by tilting the specimen. Although the surface orientation and the observation directions were not perfectly aligned with the crystallographic orientations in question, such crystallographic indices are used hereafter to show the surface orientation and observation directions. The irradiation time ranged from several 10 s to several 1000 s. A small fraction of incident electrons (10−6 for 1.0 MeV electrons) can give sufficient energy with target atoms on the electron exit surface to cause sputtering [23]. The electron microscopy observations were made with strongly reduced beam currents to suppress additional sputtering. Detailed experimental and theoretical study on the sputtering yield and angular distribution of sputtered atoms, among others, for gold foils in a high-voltage electron microscope have been given by Cherns et al. [23].

**Figure 1.** A schematic view of a disk specimen of 3 mm diameter. The irradiations were carried **Figure 1.** A schematic view of a disk specimen of 3 mm diameter. The irradiations were carried out at thin parts near a hole with an electron beam of about 200–800 nm diameter.

out at thin parts near a hole with an electron beam of about 200–800 nm diameter.

#### **3. Results 3. Results 3. Results**

*3.1. Gold 3.1. Gold 3.1. Gold* 

3.1.1. Self-Organization of Nanostructure 3.1.1. Self-Organization of Nanostructure 3.1.1. Self-Organization of Nanostructure

Figure 2 shows a typical self-organized pattern observed for an Au(001) foil irradiated with 400 keV electrons along [001] direction at 95 K with an electron beam of about 300 nm in diameter. The micrograph was taken under a kinematic and slightly under-focus condition. Clear bright images extending along [100] and [010] with a width of 1–2 nm can be observed. Stereomicroscopy revealed that these bright lines are grooves formed on the electron exit surface. Figure 2 shows a typical self-organized pattern observed for an Au(001) foil irradiated with 400 keV electrons along [001] direction at 95 K with an electron beam of about 300 nm in diameter. The micrograph was taken under a kinematic and slightly underfocus condition. Clear bright images extending along [100] and [010] with a width of 1–2 nm can be observed. Stereomicroscopy revealed that these bright lines are grooves formed on the electron exit surface. Figure 2 shows a typical self-organized pattern observed for an Au(001) foil irradiated with 400 keV electrons along [001] direction at 95 K with an electron beam of about 300 nm in diameter. The micrograph was taken under a kinematic and slightly underfocus condition. Clear bright images extending along [100] and [010] with a width of 1–2 nm can be observed. Stereomicroscopy revealed that these bright lines are grooves formed on the electron exit surface.

**Figure 2.** (**a**) TEM micrographs of an Au(001) foil irradiated along [001] with 400 keV electrons at 95 K; (**b**) a magnified view of black square area in photo (**a**). **Figure 2.** (**a**) TEM micrographs of an Au(001) foil irradiated along [001] with 400 keV electrons at 95 K; (**b**) a magnified view of black square area in photo (**a**). **Figure 2.** (**a**) TEM micrographs of an Au(001) foil irradiated along [001] with 400 keV electrons at 95 K; (**b**) a magnified view of black square area in photo (**a**).

Figure 3 shows an oblique observation of an Au(001) foil, which was irradiated along [112] with 400 keV electrons at 95 K and observed along [001] by tilting the specimen. Deep nanoholes and hillocks formed on the exit surface of an Au(001) foil can be seen. Aspect ratio on the nanohole is very high in spite of the small diameter of 1–2 nm. Figure 3 shows an oblique observation of an Au(001) foil, which was irradiated along [112] with 400 keV electrons at 95 K and observed along [001] by tilting the specimen. Deep nanoholes and hillocks formed on the exit surface of an Au(001) foil can be seen. Aspect ratio on the nanohole is very high in spite of the small diameter of 1–2 nm. Figure 3 shows an oblique observation of an Au(001) foil, which was irradiated along [112] with 400 keV electrons at 95 K and observed along [001] by tilting the specimen. Deep nanoholes and hillocks formed on the exit surface of an Au(001) foil can be seen. Aspect ratio on the nanohole is very high in spite of the small diameter of 1–2 nm.

3.1.2. Irradiation Directional Dependence on the Structure

Figure 4a–c exhibit TEM images obtained under kinematic and slightly under-focus conditions for a (001)-oriented foil after irradiation with 800 keV electrons at 110 K to a

fluence of 8 <sup>×</sup> <sup>10</sup><sup>26</sup> electrons m−<sup>2</sup> , which were, respectively, irradiated along [001], [011] and [111] directions. Stereomicroscopy indicated that the elongated bright contrasts along [001] and [010] for the [001] irradiation were due to grooves at the electron exit surface of the foil. Different results were obtained after the irradiations along [011] or [111] directions, as seen in Figure 4b,c, respectively. We can find bright contrasts extending along [100] for [011] irradiation, which are due to grooves but cannot find groove formation after [111] irradiation. The white contrasts in Figure 4c are caused by pits. These results suggest strongly that the groove formation is affected by the angle between the beam direction and the surface normal. fluence of 8 × 1026 electrons m−2, which were, respectively, irradiated along [001], [011] and [111] directions. Stereomicroscopy indicated that the elongated bright contrasts along [001] and [010] for the [001] irradiation were due to grooves at the electron exit surface of the foil. Different results were obtained after the irradiations along [011] or [111] directions, as seen in Figure 4b,c, respectively. We can find bright contrasts extending along [100] for [011] irradiation, which are due to grooves but cannot find groove formation after [111] irradiation. The white contrasts in Figure 4c are caused by pits. These results suggest strongly that the groove formation is affected by the angle between the beam direction and the surface normal.

Figure 4a–c exhibit TEM images obtained under kinematic and slightly under-focus conditions for a (001)-oriented foil after irradiation with 800 keV electrons at 110 K to a

*Quantum Beam Sci.* **2021**, *5*, x FOR PEER REVIEW 4 of 20

3.1.2. Irradiation Directional Dependence on the Structure

**Figure 4.** TEM micrographs of an Au(00l) foil irradiated at 110 K with 800 keV electrons to a fluence of about 8 × 106 electrons m−2. The directions of irradiations and observations are (**a**) [001], (**b**) [011], and (**c**) [111]. **Figure 4.** TEM micrographs of an Au(00l) foil irradiated at 110 K with 800 keV electrons to a fluence of about 8 <sup>×</sup> <sup>10</sup><sup>6</sup> electrons m−<sup>2</sup> . The directions of irradiations and observations are (**a**) [001], (**b**) [011], and (**c**) [111].

A detailed analysis has led us to schematic pictures on the nanostructure formed on the exit surface of gold films under electron irradiation as shown in Figure 5 [18]. The sputtered structure mainly consists of anisotropic nanogrooves, deep nanoholes, and hillocks. No clear grooves appear for the [111] irradiation. The nanoholes grow parallel to the irradiation direction both under [001] and [111] irradiations without much change in their widths but are zigzagged under [011] irradiation. Note the large aspect ratio of these nanoholes with extremely small dimension; the width and depth of the smallest nanoholes formed by

the [111] irradiation, for example, are about 1.5 nm and more than 20 nm, respectively. In all three cases, the hillocks develop along the irradiation directions, although not indicated in Figure 5a for clarity. formed by the [111] irradiation, for example, are about 1.5 nm and more than 20 nm, respectively. In all three cases, the hillocks develop along the irradiation directions, although not indicated in Figure 5a for clarity.

A detailed analysis has led us to schematic pictures on the nanostructure formed on the exit surface of gold films under electron irradiation as shown in Figure 5 [18]. The sputtered structure mainly consists of anisotropic nanogrooves, deep nanoholes, and hillocks. No clear grooves appear for the [111] irradiation. The nanoholes grow parallel to the irradiation direction both under [001] and [111] irradiations without much change in their widths but are zigzagged under [011] irradiation. Note the large aspect ratio of these nanoholes with extremely small dimension; the width and depth of the smallest nanoholes

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

**Figure 5.** Schematic views of the sputtered rear surface of Au(001) foils irradiated with 800 keV electrons along (**a**) the [001] direction, (**b**) the [011] direction, and (**c**) the [111] direction. **Figure 5.** Schematic views of the sputtered rear surface of Au(001) foils irradiated with 800 keV electrons along (**a**) the [001] direction, (**b**) the [011] direction, and (**c**) the [111] direction.

Similar results were obtained for the groove and pit formation for 1250 keV electron irradiation of an Au(001) foil at 110 K. However, additionally, the formation of stackingfault tetrahedra were observed, corresponding well to the estimated threshold electron energy of about 1150 keV for the Frenkel pair production in gold at 110 K [24]. Similar results were obtained for the groove and pit formation for 1250 keV electron irradiation of an Au(001) foil at 110 K. However, additionally, the formation of stackingfault tetrahedra were observed, corresponding well to the estimated threshold electron energy of about 1150 keV for the Frenkel pair production in gold at 110 K [24].
