**3.2. Growth of carbon nanowalls from methane/hydrogen mixture**

As is obvious, CNWs can also be fabricated employing the CH4/H2 mixture using RI-PECVD. In terms of controlling the wall density (or interspaces between adjacent nanowalls), total pressure and VHF power were changed. In these experiments, the heights of the CNWs were adjusted to 800 ± 50 nm, by varying the growth period. For all growth conditions, the films were uniform and exhibited a similar morphology. The thickness of individual CNWs in the films was approximately 10 nm. To consider what chemical species would affect the determi‐ nation of wall density in these experiments, a plasma diagnosis was carried out using optical emission spectroscopy (OES). By introducing Ar gas into plasma region with a flow rate of 3 sccm, the actinometric measurements were carried out. Here, for determining the relative densities of H atoms and CH radicals, the emission intensity ratios ([CH]/[Ar] and [Hα]/[Ar]) were monitored by detecting the spectral lines associated with Hα 656.1 nm (excitation threshold energy, E = 12.1 eV), CH 431.2 nm (E = 14.6 eV), and Ar 751.4 nm (E = 13.27 eV).

Figure 12 shows top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at total pressures of (a) 1 Pa, (b) 3 Pa, and (c) 5 Pa under a constant VHF power of 300 W. As the total pressure increased, the wall density decreased or interspaces between adjacent nanowalls increased. Figure 12(d) shows the intensities of the CH and Hα emissions relative to Ar as a function of total pressure at a constant VHF power of 300 W. As the total pressure increased, [CH]/[Ar] decreased and [H]/[Ar] increased. Figure 13 shows top view and crosssectional SEM images of CNWs grown on SiO2 substrates at VHF powers of (a) 200 W, (b) 300 W, and (c) 400 W under a constant total pressure of 1 Pa. As the VHF power increased, the wall density increased or interspaces between adjacent nanowalls decreased. As shown in Figure 13(d), [CH]/[Ar] increased and [H]/[Ar] decreased with the increase of VHF power. It was found from the results shown in Figures 12 and 13 that the wall density could be controlled using the total pressure and the VHF power. The above results suggest that H and CH radicals are the important chemical species and the density ratio [CH]/[H] can be useful and simple

system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided

Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700°C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700°C. These results

The selective oxidation techniques of the edges without modification of the graphene planes are essential for the fabrication of novel carbon nanoelectronic devices. A selective etching from the top edges of CNWs using remote oxygen plasma has been demonstrated [38]. CNW film samples were prepared on Si substrates for 30 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38]. The CNW sample was placed on the heater stage 20 cm from the exit of the remote

Figure 14. Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching at 700°C for 5 min. Insets show

Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700°C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700°C. These results

For comparison, we examined reactive ion etching (RIE) reactions using oxygen plasma. The RIE was carried out for 1 min using dual frequency (60 and 2 MHz) CCP system [38]. Figure 15 shows cross-sectional SEM images of CNW samples (a) before and (b) after oxygen RIE at 20°C for 1 min. As a result of the oxygen RIE, the height of the CNWs decreased

Figure 14. Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching at 700°C for 5 min. Insets show

indicate that atomic oxygen would react with the top edges of CNWs preferably without ion irradiations.

drastically, and the top edges of the CNWs were sharpened and spearlike structures were formed.

indicate that atomic oxygen would react with the top edges of CNWs preferably without ion irradiations.

index for controlling the wall density. Obviously, the OES provides information about only radicals of which optical emission transitions are permitted by selection rules. Other important carbon-containing species, including CH3, CH2, and C2H2 should be measured by other diagnostics such as absorption spectroscopy and mass spectrometry for further investigation on the growth mechanism. CH2, and C2H2 should be measured by other diagnostics such as absorption spectroscopy and mass spectrometry for further investigation on the growth mechanism. CH2, and C2H2 should be measured by other diagnostics such as absorption spectroscopy and mass spectrometry for

further investigation on the growth mechanism.

formed with and without O2 gas addition as functions of growth period are shown in Figure 11. In the case of the CNW growth without O2 addition, distinct G-band peak was observed in 2 min, and the *I*G/*I*D ratio increased gradually with increasing growth period, indicating that vertical nanographene formation started after the 2-min growth. In the case of the CNW growth with O2 addition, on the other hand, distinct G-band peak was observed at 3 min, indicating that vertical nanographene formation started after 3 min. Moreover, the *I*G/*I*<sup>D</sup> ratio was higher than that for the CNWs synthesized without O2, revealing that the O2 gas addition is effective

In the process without O2 gas addition, amorphous nanoislands were formed on the Si substrate, and the Si surface was completely covered with these nanoislands in the initial stage, resulting in the formation of a 10-nm-thick amorphous carbon interface layer. Vertical nanographene started to grow at nuclei on the surface of the interface layer. On the other hand, distinct interface layer was not formed in the process with O2 gas addition, and vertical nanographenes were formed on isolated nanoislands. O2 gas addition to C2F6/H2 is effective in suppressing the formation of carbon nanoislands and thereby in controlling CNW nucleation.

As is obvious, CNWs can also be fabricated employing the CH4/H2 mixture using RI-PECVD. In terms of controlling the wall density (or interspaces between adjacent nanowalls), total pressure and VHF power were changed. In these experiments, the heights of the CNWs were adjusted to 800 ± 50 nm, by varying the growth period. For all growth conditions, the films were uniform and exhibited a similar morphology. The thickness of individual CNWs in the films was approximately 10 nm. To consider what chemical species would affect the determi‐ nation of wall density in these experiments, a plasma diagnosis was carried out using optical emission spectroscopy (OES). By introducing Ar gas into plasma region with a flow rate of 3 sccm, the actinometric measurements were carried out. Here, for determining the relative densities of H atoms and CH radicals, the emission intensity ratios ([CH]/[Ar] and [Hα]/[Ar]) were monitored by detecting the spectral lines associated with Hα 656.1 nm (excitation threshold energy, E = 12.1 eV), CH 431.2 nm (E = 14.6 eV), and Ar 751.4 nm (E = 13.27 eV).

Figure 12 shows top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at total pressures of (a) 1 Pa, (b) 3 Pa, and (c) 5 Pa under a constant VHF power of 300 W. As the total pressure increased, the wall density decreased or interspaces between adjacent nanowalls increased. Figure 12(d) shows the intensities of the CH and Hα emissions relative to Ar as a function of total pressure at a constant VHF power of 300 W. As the total pressure increased, [CH]/[Ar] decreased and [H]/[Ar] increased. Figure 13 shows top view and crosssectional SEM images of CNWs grown on SiO2 substrates at VHF powers of (a) 200 W, (b) 300 W, and (c) 400 W under a constant total pressure of 1 Pa. As the VHF power increased, the wall density increased or interspaces between adjacent nanowalls decreased. As shown in Figure 13(d), [CH]/[Ar] increased and [H]/[Ar] decreased with the increase of VHF power. It was found from the results shown in Figures 12 and 13 that the wall density could be controlled using the total pressure and the VHF power. The above results suggest that H and CH radicals are the important chemical species and the density ratio [CH]/[H] can be useful and simple

**3.2. Growth of carbon nanowalls from methane/hydrogen mixture**

for obtaining highly graphitized CNWs.

156 Graphene - New Trends and Developments

Figure 12. Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at total pressures of (a) 1 Pa, (b) 3 Pa, and (c) 5 Pa at constant VHF power of 300 W. (d) [CH]/[Ar] and [H]/[Ar] ratios as a function of total pressure [31]. **Figure 12.** Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at total pressures of (a) 1 Pa, (b) 3 Pa, and (c) 5 Pa at constant VHF power of 300 W. (d) [CH]/[Ar] and [H]/[Ar] ratios as a function of total pressure [31]. Figure 12. Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at total pressures of (a) 1 Pa, (b) 3 Pa, and (c) 5 Pa

at constant VHF power of 300 W. (d) [CH]/[Ar] and [H]/[Ar] ratios as a function of total pressure [31].

using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38]. The CNW sample was placed on the heater stage 20 cm from the exit of the remote Figure 13. Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at VHF powers of (a) 200 W, (b) 300 W, and (c) 400 W at constant total pressure of 1 Pa. (d) [CH]/[Ar] and [H]/[Ar] ratios as a function of VHF power [31]. **Figure 13.** Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at VHF powers of (a) 200 W, (b) 300 W, and (c) 400 W at constant total pressure of 1 Pa. (d) [CH]/[Ar] and [H]/[Ar] ratios as a function of VHF pow‐ er [31].

**3.3. Etching of carbon nanowalls**

ICP.

ICP.

views from the top [38].

views from the top [38].

without ion irradiations.

#### **3.3. Etching of carbon nanowalls** ratios as a function of VHF power [31].

The selective oxidation techniques of the edges without modification of the graphene planes are essential for the fabrication of novel carbon nanoelectronic devices. A selective etching from the top edges of CNWs using remote oxygen plasma has been demonstrated [38]. CNW film samples were prepared on Si substrates for 30 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38]. The CNW sample was placed on the heater stage 20 cm from the exit of the remote ICP. **3.3. Etching of carbon nanowalls**  The selective oxidation techniques of the edges without modification of the graphene planes are essential for the fabrication of novel carbon nanoelectronic devices. A selective etching from the top edges of CNWs using remote oxygen plasma has been demonstrated [38]. CNW film samples were prepared on Si substrates for 30 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38].

of (a) 200 W, (b) 300 W, and (c) 400 W at constant total pressure of 1 Pa. (d) [CH]/[Ar] and [H]/[Ar]

Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700°C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700°C. These results indicate that atomic oxygen would react with the top edges of CNWs preferably without ion irradiations. The CNW sample was placed on the heater stage 20 cm from the exit of the remote ICP. Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700C. These results indicate that atomic oxygen would react with the top edges of CNWs preferably

**Figure 14.** Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching **Figure 14.** Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching at 700°C for 5 min. Insets show views from the top [38].

at 700C for 5 min. Insets show views from the top [38]. For comparison, we examined reactive ion etching (RIE) reactions using oxygen plasma. The RIE was carried out for 1 min using dual frequency (60 and 2 MHz) CCP system [38]. Figure 15 shows cross-sectional SEM images of CNW samples (a) before and (b) after oxygen RIE at 20C for 1 min. As a result of the oxygen RIE, the height of the CNWs For comparison, we examined reactive ion etching (RIE) reactions using oxygen plasma. The RIE was carried out for 1 min using dual frequency (60 and 2 MHz) CCP system [38]. Figure 15 shows cross-sectional SEM images of CNW samples (a) before and (b) after oxygen RIE at 20°C for 1 min. As a result of the oxygen RIE, the height of the CNWs decreased drastically, and the top edges of the CNWs were sharpened and spearlike structures were formed.

Moreover, CNWs were subjected to hydrogen peroxide (H2O2) treatment [39]. It has been reported that H2O2 treatment can induce oxidative functional groups, such as hydroxyl groups on CNT surfaces, and can selectively oxidize disordered parts on the graphene surface [40]. Accordingly, H2O2 treatment has potential for modifying the surfaces of CNWs composed of nanographene domains. CNW film samples were prepared on Si substrates for 45 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film samples were treated with 30% H2O2 solution for 6 and 12 h at 90°C. Then these samples were dried in air at 110°C on a hot plate. Figures 16(a) -16(c) show cross-sectional SEM images of CNWs

**Figure 15.** Cross-sectional SEM images of the CNW films (a) before and (b) after oxygen RIE for 1 min [38].

structures were formed.

**3.3. Etching of carbon nanowalls**

158 Graphene - New Trends and Developments

ratios as a function of VHF power [31].

**3.3. Etching of carbon nanowalls** 

irradiations.

without ion irradiations.

min. Insets show views from the top [38].

at 700C for 5 min. Insets show views from the top [38].

The selective oxidation techniques of the edges without modification of the graphene planes are essential for the fabrication of novel carbon nanoelectronic devices. A selective etching from the top edges of CNWs using remote oxygen plasma has been demonstrated [38]. CNW film samples were prepared on Si substrates for 30 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38]. The CNW sample

The selective oxidation techniques of the edges without modification of the graphene planes are essential for the fabrication of novel carbon nanoelectronic devices. A selective etching from the top edges of CNWs using remote oxygen plasma has been demonstrated [38]. CNW film samples were prepared on Si substrates for 30 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film sample was exposed to oxygen atoms provided using remote ICP source, where two electrically grounded metal meshes were installed at the exit in order to remove irradiation of electrons and ions [38]. The CNW sample was placed on the heater stage 20 cm from the exit of the remote ICP.

**Figure 13.** Top view and cross-sectional SEM images of CNWs grown on SiO2 substrates at VHF powers of (a) 200 W, (b) 300 W, and (c) 400 W at constant total pressure of 1 Pa. (d) [CH]/[Ar] and [H]/[Ar]

Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700°C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700°C. These results indicate that atomic oxygen would react with the top edges of CNWs preferably without ion

Figures 14(a) and 14(b) show SEM images of CNW films before and after atomic oxygen etching, respectively. As a result of the atomic oxygen etching at 700C for 5 min, the height of CNW film was reduced by approximately 160 nm, without change of wall thickness. In contrast, CNWs were not etched in the O2 atmosphere without plasma at 700C. These results indicate that atomic oxygen would react with the top edges of CNWs preferably

**Figure 14.** Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching

**Figure 14.** Cross-sectional SEM images of the CNW films (a) before and (b) after atomic oxygen etching at 700°C for 5

For comparison, we examined reactive ion etching (RIE) reactions using oxygen plasma. The RIE was carried out for 1 min using dual frequency (60 and 2 MHz) CCP system [38]. Figure 15 shows cross-sectional SEM images of CNW samples (a) before and (b) after oxygen RIE at 20°C for 1 min. As a result of the oxygen RIE, the height of the CNWs decreased drastically, and the top edges of the CNWs were sharpened and spearlike structures were formed.

Moreover, CNWs were subjected to hydrogen peroxide (H2O2) treatment [39]. It has been reported that H2O2 treatment can induce oxidative functional groups, such as hydroxyl groups on CNT surfaces, and can selectively oxidize disordered parts on the graphene surface [40]. Accordingly, H2O2 treatment has potential for modifying the surfaces of CNWs composed of nanographene domains. CNW film samples were prepared on Si substrates for 45 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film samples were treated with 30% H2O2 solution for 6 and 12 h at 90°C. Then these samples were dried in air at 110°C on a hot plate. Figures 16(a) -16(c) show cross-sectional SEM images of CNWs

For comparison, we examined reactive ion etching (RIE) reactions using oxygen plasma. The RIE was carried out for 1 min using dual frequency (60 and 2 MHz) CCP system [38]. Figure 15 shows cross-sectional SEM images of CNW samples (a) before and (b) after oxygen RIE at 20C for 1 min. As a result of the oxygen RIE, the height of the CNWs

was placed on the heater stage 20 cm from the exit of the remote ICP.

before and after the H2O2 treatment for 6 and 12 h. The magnified views of CNW sheets before and after the H2O2 treatment were shown in Figures 16(d) -16(f). As a result of H2O2 treatment, characteristic nanometer-scale asperities were formed on the wall surfaces of the CNWs as shown in Figures 16(d) -16(f), while the height of CNWs hardly changed. The size of the dents observed in Figures 16(e) and 16(f) was 20-30 nm. The morphology with nanometer-sized asperities on the surfaces of CNWs was stable after the H2O2 treatment. This result indicates that the radicals in H2O2 solution, such as hydroxyl radicals, react preferentially with the surfaces of CNWs at domain boundaries and induce the characteristic changes in their morphology. **Figure 15.** Cross-sectional SEM images of the CNW films (a) before and (b) after oxygen RIE for 1 min [38]. Moreover, CNWs were subjected to hydrogen peroxide (H2O2) treatment [39]. It has been reported that H2O2 treatment can induce oxidative functional groups, such as hydroxyl groups on CNT surfaces, and can selectively oxidize disordered parts on the graphene surface [40]. Accordingly, H2O2 treatment has potential for modifying the surfaces of CNWs composed of nanographene domains. CNW film samples were prepared on Si substrates for 45 min using the RI-PECVD system employing C2F6/H2 as described in Section 3.1. The CNW film samples were treated with 30% H2O2 solution for 6 and 12 h at 90°C. Then these samples were dried in air at 110°C on a hot plate. Figures 16(a)–16(c) show cross-sectional

SEM images of CNWs before and after the H2O2 treatment for 6 and 12 h. The magnified

**Figure 16.** Cross-sectional SEM images of the CNWs (a) before and after H2O2 treatment for (b) 6 and (c) 12 h. Insets show top views. (d-f) Magnified views of the areas denoted by white squares in the insets [39].

In the case of atomic oxygen etching, CNWs are selectively etched from the top edges with almost no change in wall surface morphology, as shown in Figure 14(b). On the other hand, the H2O2 treatment induces the characteristic changes in their morphology with keeping the size of CNWs constant. The nanometer-scale asperities on the CNW surface increase the surface area, which would be useful as a platform for supporting metal nanoparticles and organopollutant degradation devices [24,41]. It is noted that such asperities could be reduced by O radical exposure after H2O2 treatment, resulting in the reduction of the thickness of CNW sheets. These results, including atomic oxygen etching, oxygen RIE, and H2O2 treatment suggest the possibility of realizing etching and thickness control of walls in CNWs, which should be essential for controlling the electrical properties of graphene materials and realizing their applications to electronic devices.
