**2. Materials and Methods**

Oxygen-free copper plates of 10 <sup>×</sup> <sup>10</sup> <sup>×</sup> 1 mm<sup>3</sup> (purity 99.99%) were supplied from NILACO, Tokyo, Japan. The Cu plate was put on a laboratory hot plate setting to 473 K in ambient air for 2 min to form the Cu oxide layer. The maximum temperature measured on the surface with chromel-alumel thermocouple was 440 K, somewhat lower than the setting temperature of 473 K, as recorded in Figure 1. The color of heat-treated Cu plate was reddish-brown. The samples were irradiated with 5 keV-Ar<sup>+</sup> ions up to a fluence of <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>15</sup> cm−<sup>2</sup> using a 5 kV-ion gun installed in a vacuum chamber with a base pressure of <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>6</sup> Pa. The ion incidence was normal with respect to the sample surface.

**Figure 1.** Temperature measured on the sample surface with chromel-alumel thermocouple during oxidation.

X-ray photoelectron spectroscopy (XPS) using Mg Kα radiation (*hν* = 1253.6 eV) was performed with a JEOL 9010 X-ray photoelectron spectrometer (JEOL, Tokyo, Japan) to analyze chemical states of Cu before and after the irradiation. The XPS analysis was carried out immediately after the irradiation to avoid the change in chemical states upon the ambient air exposure. Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA) using the <sup>16</sup>O(d, p)17O reaction were conducted for chemical composition analysis with 2 MeV He ions and 0.85 MeV deuterons, respectively, produced from the Van de Graaff accelerator of Hiroshima University. The standard sample for the NRA was a SiO<sup>2</sup> layer formed on a Si wafer (SiO2/Si), which contains 5.8 <sup>×</sup> <sup>10</sup><sup>17</sup> O atoms·cm−<sup>2</sup> determined by RBS.
