2.1.

Sample

The details of the sample information used in this study are described in this section. We used single-layer graphene (8-mm square) transferred onto a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate (15-mm square) with a thickness of $625\mu \mathrm{m}$. The single-layer graphene was formed on a Cu/sapphire substrate through chemical vapor deposition,¹¹ followed by the coating and baking of a polymethyl methacrylate (PMMA) protective film at a temperature of 180°C for 1 min. Thereafter, the following processes were carried out: etching of the Cu/sapphire substrate using ammonium persulfate (0.1 M), pure water rinsing, transferring onto the ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate, and finally removing the PMMA film using acetone. To reduce the PMMA residue, the PMMA removal process was performed at a temperature of 60°C for 30 min. In addition, to prevent erosion from the edges of the graphene sheets and clarify the irradiation area, the sample was irradiated by high-speed AO on an aluminum jig (50 mm square, 6 mm thick) that had a 1-mm-diameter pinhole at the center, as shown in Fig. 1.

Fig. 1

(a) Overview of the aluminum jigs. The top cover has a 1-mm diameter pinhole at the center to avoid the irradiation for edge parts of the graphene sheets and clarify the irradiation area. Graphene sample on (b) a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate on the sample stage and (c) its schematic diagram.

2.2.

High-Speed AO Irradiation Tests

This section summarizes the experimental setup and conditions for high-speed AO irradiation tests.

In this study, we investigated tolerance for high-speed AO in a low-orbit case with an altitude of $\sim 500\mathrm{km}$ corresponding to a relative velocity of $\sim 8\mathrm{km}/\mathrm{s}$ and a kinetic energy of $\sim 5\mathrm{eV}$. To achieve our purposes, a laser-detonation AO beam source at Kobe University¹² was used for the high-speed AO irradiation simulations. The expected AO fluence depends strongly on a variety of factors of launch conditions and space environments, such as the location of instruments, the operational altitude of the spacecraft, and a solar activity. For example, in the case of a previous X-ray satellite, $Hitomi$ (ASTRO-H), the required AO fluence for the thin-film devices consisting of polyimide films was $\sim 5\times {10}^{20}\mathrm{atoms}/{\mathrm{cm}}^2$.² Therefore, an aluminum coating was applied on the polyimide films to impart high tolerance to them to withstand for such a high fluence, as serious damages to the polyimide film following high-speed AO irradiation were reported and expected.^13–15 Considering the fact that high-speed AO tolerance for single-layer graphene has not been investigated in detail even though AO adsorption is known to occur on graphene in not so high-speed AO environments,^16–18 in this study, we evaluated the behaviors of single-layer graphene by irradiating high-speed AO without a metal coating. Thus, high-speed AO was irradiated with fluence values of $2\times {10}^{15}$, $2\times {10}^{16}$, $2\times {10}^{17}$, $2\times {10}^{18}$, and $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$. The fluence value of $2\times {10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2$ corresponds to the minimum value in our experimental setup. The energy distribution during irradiation is estimated by measuring the time of flight (TOF) of ionized high-speed AO using a quadrupolar mass spectrometer located downstream of the high-speed AO beam line. The velocity was converted from the TOF and the averaged velocity was calculated to be $\sim 6\mathrm{km}/\mathrm{s}$. The fluence was determined using the mass loss data of a reference polyimide sample, and the flux was $1.8\times {10}^{15}\mathrm{atoms}/{\mathrm{cm}}^2/\mathrm{shot}$. These tests were conducted under a vacuum level of $\sim {10}^{-6}$ Pa and almost vertical ($\sim {83}^{°}$) AO impact was expected for all of the samples during the irradiation tests.¹⁹ The beamline and inside of the sample chamber are shown in Fig. 2.

Fig. 2

(a) Overview of the high-speed AO irradiation test facility¹² and (b) the single-layer graphene sample and a polyimide reference in the sample chamber.

3.1.

Analysis

To evaluate the status of single-layer graphene before and after high-speed AO irradiation, we used a Raman spectrometer that is usually employed to evaluate the molecular and crystal structures of carbon materials, such as graphite, CNT, and graphene and a scanning electron microscope (SEM) to observe the surface condition. The Raman spectroscopy measurements were performed using a Renishaw instrument with an exposure time of 3 s, laser power of 5%, and wavelength of 532 nm. Because the laser light is blocked near the edge of the jig aperture, the measurements were performed at the 1 mm center aperture, and their averaged value was adopted for certain points as the initial condition before the tests. Mapping measurements were also performed to estimate an averaged value and a (1-sigma) spatial variation before and after the tests under the identical conditions as described above in the central $200\mu \mathrm{m}\times 200\mu \mathrm{m}$ with a pitch of 6 to $10\mu \mathrm{m}$.

We extracted the G and D band intensities through a fitting procedure, individually assuming a single Gaussian function in each Raman spectrum, and finally focused on a D/G ratio, which is an indicator usually used to quantify the degree of disorderliness and structural defects present in carbon materials. The G band ($\sim 1580{\mathrm{cm}}^{-1}$) is the primary mode in graphene caused by a planar molecular motion, whereas the D $\sim 1350{\mathrm{cm}}^{-1}$ band is activated by disorderliness and defects. Thus, the ratio increases with decreasing structural quality of the graphene sample. As supplementary data, the intensity of the 2D mode, which also results from the characteristic mode of graphene, was also obtained as in previous studies on carbon materials. The 2D band intensity is also expected to decrease in a decreasing graphene structure. The SEM observations were performed using a JEOL SEM system with a magnification of $\times 70$ at 2.00 kV.

3.2.

Results

Some of the typical observed Raman spectra for each high-speed AO irradiation fluence are presented shown in Fig. 3. The observed spectra with fluence values of $2\times {10}^{15}$, $2\times {10}^{16}$, and $2\times {10}^{17}\mathrm{atoms}/{\mathrm{cm}}^2$ are very similar to each other, whereas the behavior of the $2\times {10}^{18}\mathrm{atoms}/{\mathrm{cm}}^2$ case is found to be different from the others, and no line features are seen for the $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ case. As for the cases with fluence values of $2\times {10}^{15}$, $2\times {10}^{16}$, and $2\times {10}^{17}\mathrm{atoms}/{\mathrm{cm}}^2$, very weak D band intensity was detected, whereas relatively very strong G band intensity was confirmed. In the $2\times {10}^{18}\mathrm{atoms}/{\mathrm{cm}}^2$ case, the contrast between the D and G bands becomes much smaller, which suggests that a significant change in terms of the D/G ratio occurs in the fluence values from $2\times {10}^{17}$ to $2\times {10}^{18}\mathrm{atoms}/{\mathrm{cm}}^2$. As for the 2D band, emissions were detected except for the $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ case, although the relative intensities of the D and G bands changed drastically. The observed Raman spectra with the best fit models are shown in Fig. 6 of Appendix.

Fig. 3

Raman spectra of representative points for each fluence ($2\times {10}^{15}$ to $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$). The spectral intensity has been corrected for clarity.

Next, we extracted each intensity of each band in each Raman spectrum and found that the D/G ratios before and after the high-speed AO irradiation changed from 0.02 ± 0.01 to 0.02 ± 0.01, 0.03 ± 0.02 to 0.02 ± 0.01, and 0.04 ± 0.03 to 0.05 ± 0.03 for $2\times {10}^{15}$, $2\times {10}^{16}$, and $2\times {10}^{17}\mathrm{atoms}/{\mathrm{cm}}^2$, respectively. Thus, we conclude that there are no significant changes. In contrast, the D/G ratio before and after the high-speed AO irradiation changed from 0.04 ± 0.03 to 0.8 ± 0.4 for $2\times {10}^{18}\mathrm{atoms}/{\mathrm{cm}}^2$ drastically in both the averaged value and 1-sigma range. Furthermore, the D/G ratio could not be measured following the $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ high-speed AO irradiation because no peaks were observed in both the G and D bands, which suggests that very few graphene sheets exist following the $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ irradiation. We confirmed that the behavior of the 2D band is similar to that of the G band. The observed D/G ratios are depicted and summarized in Fig. 4 and Table 1, respectively. Consequently, to prevent the single-layer graphene sheets from erosion, a special treatment such as coating is needed to survive in the LEO for $\gtrsim$ a day.

Fig. 4

D/G ratios before and after the high-speed AO irradiation tests as a function of the high-speed AO fluence. The ratio after the $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ irradiation was not plotted because the G and D bands were not observed.

Table 1

Summary of the D/G ratios for each fluence obtained by Raman spectroscopy before and after the high-speed AO irradiation tests. The ratio is not listed for that fluence because the G and D peaks were not observed after the 2×1019  atoms/cm2 irradiation. The 1-sigma ranges were estimated based on Raman mapping measurements.

Further, as in the case of the Raman spectroscopic results, there is a significant change in the contrast of the SEM images between the irradiated and unirradiated areas following the $2\times {10}^{18}\mathrm{atoms}/{\mathrm{cm}}^2$ irradiation. We could not see the difference in contrast up to the fluence value of $2\times {10}^{17}{\mathrm{atoms}/\mathrm{cm}}^2$; however, a clear contrast could be seen at larger fluence values of $2\times {10}^{18}$ and $2\times {10}^{19}{\mathrm{atoms}/\mathrm{cm}}^2$. In particular, in the case of $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$, the contrast of the irradiated area is consistent with that of the ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate area, suggesting that very few graphene sheets exist. The SEM images following the high-speed AO irradiation with fluence values of $2\times {10}^{17}$, $2\times {10}^{18}$, and $2\times {10}^{19}\mathrm{atoms}/{\mathrm{cm}}^2$ are presented in Fig. 5.

Fig. 5

SEM images after the high-speed AO irradiation with fluence of (a) $2\times {10}^{17}$ (b) $2\times {10}^{18}$, and (c) $2\times {10}^{19}$ ${\mathrm{atoms}/\mathrm{cm}}^2$. The red dotted line corresponds to the boundary between irradiated and unirradiated areas.