**3. Results and Discussion**

Figure 2a shows a photograph of the sample surface irradiated with Ar<sup>+</sup> ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> using a 5 kV ion gun. The color of the sample changed from reddish brown to dark blue-purple at an irradiation spot. This darkening was visible through a viewing port, and started at the fluence as low as 10<sup>14</sup> cm−<sup>2</sup> . On the sample surface irradiated to a fluence of 10<sup>14</sup> cm−<sup>2</sup> , the color of the edge at the beam spot was not clear as can be seen in Figure 2b, indicating that the border between irradiated and unirradiated areas was not so abrupt. Thus, the uniformity of a beam intensity inside the beam spot could be estimated by the uniformity of color. The minimum fluence to recognize the beam spot with naked eyes will be examined for further discussion of the sensitivity and applicability of the chromatic change for a beam viewer.

**Figure 2.** Photographs of the surface of samples irradiated with 5 keV-Ar<sup>+</sup> ions to fluences of <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> (**a**) and <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>14</sup> cm−<sup>2</sup> (**b**). The photographs were taken after removing the samples from the vacuum chamber.

The mechanism of the observed chromatic change was discussed below, along with RBS, NRA, and XPS data. Figure 3 shows the backscattering spectrum of the Cu oxide layer formed on a Cu plate before irradiation. The chemical composition and thickness of the oxide layer was estimated to be CuO0.4 and 1.9 <sup>×</sup> <sup>10</sup><sup>17</sup> CuO0.4 cm−<sup>2</sup> , respectively, by fitting a simulation spectrum to experimental data, where the program SIMNRA 6.06 [16] was used to obtain the simulation spectrum. The thickness of 1.9 <sup>×</sup> <sup>10</sup><sup>17</sup> CuO0.4 cm−<sup>2</sup> can be converted into 38 nm, much larger than the ion projected range of 5 nm predicted by the SRIM simulation [17], assuming the atomic density to be 5 <sup>×</sup> <sup>10</sup><sup>22</sup> CuO0.4 cm−<sup>3</sup> . The chemical composition CuO0.4 indicates that the oxide layer contains the mixture of metallic copper and copper oxides.

**Figure 3.** RBS spectrum of the sample before irradiation (blue open circles). A simulated spectrum (red solid line) is also shown.

Figure 4 presents the NRA spectra of the sample with and without irradiation. Peaks located around channel number of 260 correspond to protons emitted by the <sup>16</sup>O(d, p)17O reaction. The peak intensities are (2.15 <sup>±</sup> 0.05) <sup>×</sup> <sup>10</sup><sup>3</sup> and (2.01 <sup>±</sup> 0.04) <sup>×</sup> <sup>10</sup><sup>3</sup> counts for the unirradiated and irradiated samples, respectively. The oxygen content in the irradiated sample was determined to be (7.1 <sup>±</sup> 0.2) <sup>×</sup> <sup>10</sup><sup>16</sup> O atoms·cm−<sup>2</sup> by the SiO2/Si standard sample, smaller by 6% than that in the unirradiated sample ((7.6 <sup>±</sup> 0.2) <sup>×</sup> <sup>10</sup><sup>16</sup> <sup>O</sup> atoms·cm–2). Thus, oxygen atoms were found to be released from the CuO0.4 layer by the irradiation with 5 keV-Ar<sup>+</sup> ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> . The SRIM simulation [17] predicts the sputtering yield of O in CuO0.4 to be 4.9 O atoms·ion−<sup>1</sup> , which means that the sputtered O atoms will be approximately 5 <sup>×</sup> <sup>10</sup><sup>15</sup> O atoms·cm−<sup>2</sup> , corresponding to a difference in the oxygen contents between the unirradiated and irradiated samples. The NRA results and the SRIM calculation suggest that the release of O atoms originates in physical sputtering. In the sputtering process of CuO0.4 bombarded with 5 keV-Ar<sup>+</sup> ions, approximately 1.4 <sup>×</sup> <sup>10</sup><sup>15</sup> Cu atoms·cm−<sup>2</sup> as calculated by the SRIM will be lost, leading to the change in chemical composition from CuO0.4 to CuO0.38 in the layer.

**Figure 4.** Nuclear reaction analysis (NRA) spectra of the unirradiated and irradiated areas in the sample irradiated with 5 keV-Ar+ ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> .

Figure 5 shows XPS Cu 2p core level photoemission spectra (PS) of the CuO0.4 layer with and without irradiation. Considering the inelastic mean free path of photoelectrons whose kinetic energy is approximately 300 eV, information about copper valence states can be obtained within 0.8 nm [18] below the surface by the photoemission spectra. As can be seen in Figure 5, a broad shake-up satellite line due to the charge transfer [19,20] appeared in the binding energy (BE) region from 937 to 941 eV of PS for the unirradiated sample and decreased in intensity after irradiation. More quantitatively, the intensity ratio of the satellite to the main line at BE of 932.4 eV was 0.16 and 0.10 for the unirradiated and irradiated samples, respectively. These results indicate that the unirradiated sample included the copper valence state Cu2+ and it was transformed into Cu<sup>+</sup> and/or Cu<sup>0</sup> (Cu+/Cu<sup>0</sup> ) by irradiation. Thus the 5 keV-Ar<sup>+</sup> irradiation reduced Cu2+ to Cu+/Cu<sup>0</sup> . The irradiation-induced reduction observed in the present work could be seen in the change in the shape of the Cu 2p3/2 lines before and after irradiation.

**Figure 5.** XPS Cu 2p core level photoemission spectra of the unirradiated and irradiated areas in the sample irradiated with 5 keV-Ar<sup>+</sup> ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> .

Figure 6a,b depicts Cu 2p3/2 lines for the samples before and after irradiation, respectively. Each spectral line consists of three components denoted by I, II, and III. The lines, after background subtraction by the Shirly method [21], were decomposed by three Voigt functions using a curve fitting procedure. The component I located at BE of 932.4 eV was assigned to be Cu and/or Cu2O. The BE of Cu (932.6 eV) [22–24] is very close to that of Cu2O (932.5 eV) [25–28], therefore the component I cannot be further decomposed by a curve fitting. The components II and III located at BE of 933.6 eV and 934.8 correspond to CuO [28–31] and Cu(OH)<sup>2</sup> [28,32], respectively. The fractions of each component obtained by the curve fitting are summarized in Table 1 for the samples with and without irradiation. The fractions corresponding to copper valence state Cu2+ (CuO and Cu(OH)2) decrease, while the fraction of Cu+/Cu<sup>0</sup> (Cu2O/Cu) increased by irradiation, indicating that the Ar<sup>+</sup> irradiation reduced Cu2+ to Cu+/Cu<sup>0</sup> . This was consistent with the result deduced from the decrease in intensity of shake-up satellite line as described above.

**Figure 6.** Detailed XPS Cu 2p3/2 core level photoemission spectra of the unirradiated (**a**) and irradiated (**b**) areas in the sample irradiated with 5 keV-Ar<sup>+</sup> ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> . Each spectrum was decomposed into three components denoted by I, II, and III.


**Table 1.** Compositions of Cu, Cu2O, CuO and Cu(OH)<sup>2</sup> determined by Cu 2p3/2 photoemission spectra (PS) lines for the unirradiated and irradiated samples.

<sup>1</sup> These values were obtained by the assumption that the composition of the analyzing layer was CuO0.4 before irradiation and the fraction of Cu was unchanged after irradiation.

The concentration ratio of Cu2O to Cu can be indirectly determined by the atomic ratio O/Cu of analyzing layer using the fractions of three components. Of course, the O/Cu can be calculated by the intensity ratio of O 1s to Cu 2p PS lines. However, it is impossible to accurately determine the atomic ratio O/Cu because of the presence of adventitious carbon contamination that includes oxygen atoms in the outermost layer. Therefore, the atomic ratio O/Cu in the analyzing layer was assumed to be 0.4 that was determined by RBS, resulting in the concentration ratio Cu2O/Cu of 8.2/65.7 for the sample without irradiation. These values are presented with parenthesis in Table 1. For the sample with irradiation, the atomic ratio O/Cu in the analyzing layer was definitely different from that in the oxide layer, and thus, the composition CuO0.38 determined by the combination of RBS and NRA could not be used to calculate the fraction of Cu2O/Cu. Therefore, the fraction of metallic copper was assumed to be unchanged after irradiation. In fact, Panzner et al. [33] demonstrated that the oxide CuO was reduced to Cu2O, while the oxide Cu2O was much more stable and no more reduction to Cu occurred under sputtering with 3–5 keV Ar<sup>+</sup> to a low fluence. Then the concentration ratio Cu2O/Cu was calculated to be 28.4/65.7 for the sample with irradiation. These values were also presented inside parenthesis in Table 1.

As described above, XPS analysis revealed that the 5 keV-Ar<sup>+</sup> irradiation reduced Cu2+ to Cu+/Cu<sup>0</sup> in the cuprate. This result is consistent with the previous studies that CuO thin films were transformed into Cu2O by ion irradiation [33,34]. Next, the relationship between the reduction and the chromatic change was discussed briefly.

It is well known that the color of cuprous oxide (Cu2O) powder is red. In addition, Cu2O in the form of both the thin film [35,36] and nanoparticle [37,38] would be reddish considering their optical absorption spectra. On the other hand, the color of the irradiated layer was found to be dark blue-purple, different from the color of pure Cu2O. Fredj and Burleigh [39] showed that the color of a copper oxide layer in which Cu2O is primarily included varied from bare copper color to green depending on its thickness. Thus, they demonstrated the possibility that the Cu2O layer exhibited the color other than red. One possibility for the chromatic change is the presence of Cu2O phase in a metallic Cu phase. Another possibility is the change in a refractive index of the oxide layer accompanied by the change in chemical states of Cu and/or radiation damage by Ar ions. Unfortunately, the dominant effect on the chromatic change is unclear at present.

As mentioned in the Introduction, the beam spot in the sample irradiated with 5 keV-Ar<sup>+</sup> ions using the other machine turned bare copper color due to the removal of an oxide layer by sputtering, different from the present work. The reason for this difference is unknown but may result from the initial thickness and composition of the oxide layer. Further studies with various thicknesses and compositions are needed to clarify the mechanism underlying the irradiation-induced chromatic change in copper oxide layers. In addition, the minimum fluence at which the chromatic change can be recognized is necessary to examine for the applicability of the new type of a beam viewer. Such investigations are now in progress.
