**3. Fabrication of carbon nanowalls using Radical Injection Plasma Enhance Chemical Vapor Deposition (RI-PECVD)**

### **3.1. Growth of carbon nanowalls from fluorocarbon/hydrogen mixture**

In the case of PECVD with hydrocarbon/hydrogen system, for example, both CH3 radical and H atoms are thought to play important roles in the formation of several carbon structures. The parallel-plate CCP might be useful to produce plenty of hydrocarbon radicals such as CH3 radicals effectively [33] and also useful for the large-area deposition of the film. However, the CCP itself is not suitable for the growth of diamond and nanodiamond films because of the shortage of H atoms [34]. In contrast, high-density plasmas such as microwave plasma and ICP are suitable for dissociating H2 molecules efficiently. Proof Corrections Form

nucleation of nanographene at the very early growth stage [30]. The growth of CNWs was enhanced occasionally by using metal substrates such as Ni and iron (Fe) [4]. The spacing between adjacent nanowalls and thickness of nanowalls would be affected by the density ratio of CxHy radicals to H atoms [31]. The addition of Ar into the source gas would induce the secondary nucleation at the wall surface, resulting in the formation of highly branched CNWs with high surface to volume ratio as shown in Figure 2(c). On one hand, branching could be suppressed and straight and large-size monolithic carbon nanosheet could be obtained by the addition of oxygen into the source gas [32]. In view of the practical use of CNWs for device applications such as biosensors or electrochemical sensors in the form of micrototal analysis system, postprocesses such as integration techniques, including etching and coating of CNWs and surface functionalization should be established. Figure 5 shows schematic illustration of CNW structures that should be controlled in the nucleation and growth stages and modified by the postprocesses, including etching and surface functionalization. Hereafter, we describe the recent development of CNW fabrication with emphasis on the structure control for realizing carbon nanoplatform working in the area of electrochemical and bio applications.

etching

Functionalization

surface functionalization Page No.

2 Fig. 1

7 Fig. 6

Line

5 25 CNWs, and CNWs and

domain

Growth Etching

**Figure 5.** Schematic illustration of structures of CNWs to be controlled in the nucleation and growth stages and modi‐

**3. Fabrication of carbon nanowalls using Radical Injection Plasma Enhance**

In the case of PECVD with hydrocarbon/hydrogen system, for example, both CH3 radical and H atoms are thought to play important roles in the formation of several carbon structures. The parallel-plate CCP might be useful to produce plenty of hydrocarbon radicals such as CH3

boundary

secondary nucleation

number of graphene layers

fied by the postprocesses, including etching and surface functionalization.

**Chemical Vapor Deposition (RI-PECVD)**

distance

**3.1. Growth of carbon nanowalls from fluorocarbon/hydrogen mixture**

electrical property

In view of the practical applications using CNWs, desirable structures and electrical and chemical properties of CNWs depend on the area of their applications. Therefore, structures, electrical properties, surface chemical properties of CNWs, and related sheet nanostructures should be controlled according to their applications. Although the nucleation mechanism of CNWs is still unclear, ion bombardment on the substrate would have some effect on the nucleation of nanographene at the very early growth stage [30]. The growth of CNWs was enhanced occasionally by using metal substrates such as Ni and iron (Fe) [4]. The spacing between adjacent nanowalls and thickness of nanowalls would be affected by the density ratio of CxHy radicals to H atoms [31]. The addition of Ar into the source gas would induce the secondary nucleation at the wall surface, resulting in the formation of highly branched CNWs with high surface to volume ratio as shown in Figure 2(c). On one hand, branching could be suppressed and straight and large-size monolithic carbon nanosheet could be obtained by the addition of oxygen into the source gas [32]. In view of the practical use of CNWs for device applications such as biosensors or electrochemical sensors in the form of micrototal analysis system, postprocesses such as integration techniques, including etching and coating of CNWs and surface functionalization should be established. Figure 5 shows schematic illustration of CNW structures that should be controlled in the nucleation and growth stages and modified by the postprocesses, including etching and surface functionalization. Hereafter, we describe the recent development of CNW fabrication with emphasis on the structure control for realizing carbon nanoplatform working in the area of

application, while vertical alignment and crystallinity of CNWs can be less crucial.

electrochemical and bio applications.

150 Graphene - New Trends and Developments

radical ion

metal

Nucleation

growth of CNWs. We can expect a wide variety of applications based on their structure or electrical properties. For the electron emitters, CNWs with atomically thin edges, moderate spacing, and excellent height uniformity are required. For the membrane filter using honeycomb structure of CNWs, the spacing between adjacent nanowalls should be controlled. Moreover, CNWs should be detached from the substrate to obtain freestanding membrane, and the CNW membrane should be attached to the different materials. On the other hand, less aligned, dense CNW film with large surface area can be used as gas storage

> Although we have managed to control radical densities in the plasma by changing the mixing ratio of source gases, it is not easy to produce multiple species with different roles effectively at the same time using single plasma. As a solution, hydrocarbon or fluorocarbon gases were excited by a parallel-plate CCP, while the H atom density around the growing surface was actively increased by the injection from the external high-density H2 plasma. This is the idea of radical injection. We have previously developed a radical-injection plasma-enhanced chemical vapor deposition (RI-PECVD) system that has allowed superior control of the properties of CNWs [5,9,10,14,15,32,35,36]. Figure 6 shows a schematic of the RI-PECVD system using very high frequency (VHF: 100MHz) plasma mainly, which was developed with the aim of achieving large-area growth of CNWs with a reasonable growth rate [10,11,14,15,32,35]. The RI-PECVD system used here is composed of a parallel plate VHF CCP and a surface wave-excited microwave (2.45 GHz) H2 plasma (H2 SWP) in tandem structure, as shown in Figure 6 [14,35]. In the case of CNW growth using fluorocarbon source gas, C2F6 was introduced into the VHF CCP region with a flow rate of 50 sccm. H2 with a flow rate of 100 sccm was introduced into the microwave SWP region, and H radicals were injected into the VHF CCP. The total gas pressure ranged from 0.6 to 1.2 torr (80 to 160 Pa). During the CNW growth, the substrate was heated at 600°C. By using this system, the heated substrate was showered with both fluorocarbon (or hydrocarbon) radicals and plenty of H atoms in a controlled manner. 1. Final Chapter Title:Nanoplatform Based on Vertical Nanographene 2. Author details Scientific Title: PhD Full Name:MineoHiramatsu Affiliation:Meijo University Position: Professor No. Delete Replace with 1 17 O2 O<sup>2</sup> 1 18 H2O2 H2O<sup>2</sup> 3 29 3(b)show 3(b)show Nanographite domain Domain boundary

9 20 this layer was composed of this layer composed of **Figure 6.** Schematic of VHF plasma assisted by H2 microwave surface wave plasma [14,35].

10 25 Figures 10(a) -10(d)show Figures 10(a)-10(d)show 14 10 14(b)show 14(b)show 15 5 16(f)was 16(f)was Figure 7(a) shows the H and C atom densities in the VHF plasma of RI-PECVD employing C2F6/H2 mixture, together with the height variation of CNW films. The measurement of atom densities was conducted using the vacuum ultraviolet absorption spectroscopy (VUVAS) system as a function of the total pressure during the CNW growth at microwave and VHF

1

wider.

H2

C2F6 or CH4

powers of 250 and 270 W, and C2F6 and H2 flow rates of 50 and 100 sccm, respectively [35]. As the total pressure increased in the range from 0.1 to 0.6 torr (13.3 to 80 Pa), the H atom density increased drastically from 1012 to 1014 cm−3, while the C atom density was almost constant at 5 × 1012 cm−3, as shown in Figure 7(a). In contrast, the height of the CNW films decreased with an increase in the total pressure. SEM images of CNWs grown at different total pressures are shown in Figures 7(b) -7(d) [35]. CNW film with narrow interspaces was obtained at a low total pressure of 0.1 torr (13.3 Pa), while CNW film with wider interspaces of 30-40 nm was grown at a high pressure of 0.6 torr (80 Pa). As the H/C atom density ratio increased, the growth rate of the CNWs decreased and the average interspaces between the walls became wider. Figure 7(a) shows the H and C atom densities in the VHF plasma of RI-PECVD employing C2F6/H2 mixture, together with the height variation of CNW films. The measurement of atom densities was conducted using the vacuum ultraviolet absorption spectroscopy (VUVAS) system as a function of the total pressure during the CNW growth at microwave and VHF powers of 250 and 270 W, and C2F6 and H2 flow rates of 50 and 100 sccm, respectively [35]. As the total pressure increased in the range from 0.1 to 0.6 torr (13.3 to 80 Pa), the H atom density increased drastically from 1012 to 1014 cm−3, while the C atom density was almost constant at 5 × 1012 cm−3, as shown in Figure 7(a). In contrast, the height of the CNW films decreased with an increase in the total pressure. SEM images of CNWs grown at different total pressures are shown in Figures 7(b)-7(d) [35]. CNW film with narrow interspaces was obtained at a low total pressure of 0.1 torr (13.3 Pa), while CNW film with wider interspaces of 30-40 nm was grown at a high pressure of 0.6 torr (80 Pa). As the H/C atom density ratio increased, the growth rate of the CNWs decreased and the average interspaces between the walls became

Figure 6. Schematic of VHF plasma assisted by H2 microwave surface wave plasma [14,35].

Quartz window

100 MHz (VHF)

substrate

CCP

Surface wave plasma

Microwave

H

To pump

Figure 7. Dummy Text (a) H and C atom densities in VHF plasma employing C2F6/H2 mixtures, together with the height variation of CNW films, as a function of total pressure during the CNW formation at a microwave power of 250 W and a VHF power of 270 W. (b-d) SEM images of CNWs grown at pressures of 0.1, 0.4, and 0.6 torr (13.3, 53.3 and 80 Pa), respectively [35]. **Figure 7.** (a) H and C atom densities in VHF plasma employing C2F6/H2 mixtures, together with the height variation of CNW films, as a function of total pressure during the CNW formation at a microwave power of 250 W and a VHF power of 270 W. (b-d) SEM images of CNWs grown at pressures of 0.1, 0.4, and 0.6 torr (13.3, 53.3 and 80 Pa), respec‐ tively [35].

In addition, O2 and N2 gases were introduced into the VHF-CCP region. The crystallinity of vertically standing CNWs is improved by introducing O2 into the plasma used for CNW growth [32], while N addition is expected to control the electrical properties of the CNWs [35]. Figure 8(a) shows the cross-sectional SEM image of the typical CNW film grown on a Si substrate, and the inset shows the top view SEM image of the same CNW film. CNW growth was conducted on a Si substrate using a C2F6/H2 mixture, resulting in the formation of slightly branching carbon sheets standing almost vertically on the substrate, as shown in Figure 8(a). The thickness of CNW film grown for 30 min was approximately 1 μm. Figure 8(b) shows TEM image of CNWs grown using C2F6/H2, where randomly oriented, small overlapping multilayered graphene domains were observed. The inset shows magnified image of the area enclosed by square, where a bent multilayered graphene structure with a thickness of approximately 9 nm was observed. In contrast, Figure 8(c) In addition, O2 and N2 gases were introduced into the VHF-CCP region. The crystallinity of vertically standing CNWs is improved by introducing O2 into the plasma used for CNW growth [32], while N addition is expected to control the electrical properties of the CNWs [35]. Figure 8(a) shows the cross-sectional SEM image of the typical CNW film grown on a Si substrate, and the inset shows the top view SEM image of the same CNW film. CNW growth was conducted on a Si substrate using a C2F6/H2 mixture, resulting in the formation of slightly branching carbon sheets standing almost vertically on the substrate, as shown in Figure 8(a). The thickness of CNW film grown for 30 min was approximately 1 µm. Figure 8(b) shows TEM image of CNWs grown using C2F6/H2, where randomly oriented, small overlapping multilay‐ ered graphene domains were observed. The inset shows magnified image of the area enclosed by square, where a bent multilayered graphene structure with a thickness of approximately 9 nm was observed. In contrast, Figure 8(c) shows a cross-sectional SEM image of the CNW film grown for 40 min using C2F6/H2 with O2 addition, and the inset shows the top view SEM image

multilayered graphene domains were observed. The inset shows magnified image of the area enclosed by square, where a

In addition, O2 and N2 gases were introduced into the VHF-CCP region. The crystallinity of vertically standing CNWs is improved by introducing O2 into the plasma used for CNW growth [32], while N addition is expected to control the electrical properties of the CNWs [35]. Figure 8(a) shows the cross-sectional SEM image of the typical CNW film grown on a Si substrate, and the inset shows the top view SEM image of the same CNW film. CNW growth was conducted on a Si

Figure 7. Dummy Text (a) H and C atom densities in VHF plasma employing C2F6/H2 mixtures, together with the height variation of CNW films, as a function of total pressure during the CNW formation at a microwave power of 250 W and a VHF power of 270 W. (b-d)

SEM images of CNWs grown at pressures of 0.1, 0.4, and 0.6 torr (13.3, 53.3 and 80 Pa), respectively [35].

while the C atom density was almost constant at 5 × 1012 cm−3, as shown in Figure 7(a). In contrast, the height of the CNW films decreased with an increase in the total pressure. SEM images of CNWs grown at different total pressures are shown in Figures 7(b)-7(d) [35]. CNW film with narrow interspaces was obtained at a low total pressure of 0.1 torr (13.3 Pa), while CNW film with wider interspaces of 30-40 nm was grown at a high pressure of 0.6 torr (80 Pa). As the H/C atom density ratio increased, the growth rate of the CNWs decreased and the average interspaces between the walls became

Figure 7(a) shows the H and C atom densities in the VHF plasma of RI-PECVD employing C2F6/H2 mixture, together with the height variation of CNW films. The measurement of atom densities was conducted using the vacuum ultraviolet absorption spectroscopy (VUVAS) system as a function of the total pressure during the CNW growth at microwave and VHF powers of 250 and 270 W, and C2F6 and H2 flow rates of 50 and 100 sccm, respectively [35]. As the total pressure increased in the range from 0.1 to 0.6 torr (13.3 to 80 Pa), the H atom density increased drastically from 1012 to 1014 cm−3, while the C atom density was almost constant at 5 × 1012 cm−3, as shown in Figure 7(a). In contrast, the height of the CNW films decreased with an increase in the total pressure. SEM images of CNWs grown at different total pressures are shown in Figures 7(b)-7(d) [35]. CNW film with narrow interspaces was obtained at a low total pressure of 0.1 torr (13.3 Pa), while CNW film with wider interspaces of 30-40 nm was grown at a high pressure of 0.6 torr (80 Pa). As the H/C atom of the same CNW film. CNW films grown with O2 had larger plane sheets with wider interspaces than those grown without O2. By the addition of O2 at a flow rate of 5 sccm into source gas mixture, less branching, monolithic graphene sheets were obtained, as shown in Figure 8(c). However, as a result of O2 addition, the growth rate was reduced by approximately 33%. Figure 8(d) shows TEM image of CNWs grown with additional O2, together with magnified image of square area as an inset. Monolithic self-sustaining graphene sheets larger than 200 nm in size were clearly observed in the CNWs grown with O2. A highly orientated, smooth multilayered graphene structure was clearly observed in the inset of Figure 8(d) [32]. bent multilayered graphene structure with a thickness of approximately 9 nm was observed. In contrast, Figure 8(c) shows a cross-sectional SEM image of the CNW film grown for 40 min using C2F6/H2 with O2 addition, and the inset shows the top view SEM image of the same CNW film. CNW films grown with O2 had larger plane sheets with wider interspaces than those grown without O2. By the addition of O2 at a flow rate of 5 sccm into source gas mixture, less branching, monolithic graphene sheets were obtained, as shown in Figure 8(c). However, as a result of O2 addition, the growth rate was reduced by approximately 33%. Figure 8(d) shows TEM image of CNWs grown with additional O2, together with magnified image of square area as an inset. Monolithic self-sustaining graphene sheets larger than 200 nm in size were clearly observed in the CNWs grown with O2. A highly orientated, smooth multilayered graphene structure

was clearly observed in the inset of Figure 8(d) [32].

wider.

powers of 250 and 270 W, and C2F6 and H2 flow rates of 50 and 100 sccm, respectively [35]. As the total pressure increased in the range from 0.1 to 0.6 torr (13.3 to 80 Pa), the H atom density increased drastically from 1012 to 1014 cm−3, while the C atom density was almost constant at 5 × 1012 cm−3, as shown in Figure 7(a). In contrast, the height of the CNW films decreased with an increase in the total pressure. SEM images of CNWs grown at different total pressures are shown in Figures 7(b) -7(d) [35]. CNW film with narrow interspaces was obtained at a low total pressure of 0.1 torr (13.3 Pa), while CNW film with wider interspaces of 30-40 nm was grown at a high pressure of 0.6 torr (80 Pa). As the H/C atom density ratio increased, the growth rate of the CNWs decreased and the average interspaces between the walls became wider.

Figure 6. Schematic of VHF plasma assisted by H2 microwave surface wave plasma [14,35].

Quartz window

100 MHz (VHF)

substrate

SEM images of CNWs grown at pressures of 0.1, 0.4, and 0.6 torr (13.3, 53.3 and 80 Pa), respectively [35].

**Figure 7.** (a) H and C atom densities in VHF plasma employing C2F6/H2 mixtures, together with the height variation of CNW films, as a function of total pressure during the CNW formation at a microwave power of 250 W and a VHF power of 270 W. (b-d) SEM images of CNWs grown at pressures of 0.1, 0.4, and 0.6 torr (13.3, 53.3 and 80 Pa), respec‐

In addition, O2 and N2 gases were introduced into the VHF-CCP region. The crystallinity of vertically standing CNWs is improved by introducing O2 into the plasma used for CNW growth [32], while N addition is expected to control the electrical properties of the CNWs [35]. Figure 8(a) shows the cross-sectional SEM image of the typical CNW film grown on a Si substrate, and the inset shows the top view SEM image of the same CNW film. CNW growth was conducted on a Si substrate using a C2F6/H2 mixture, resulting in the formation of slightly branching carbon sheets standing almost vertically on the substrate, as shown in Figure 8(a). The thickness of CNW film grown for 30 min was approximately 1 µm. Figure 8(b) shows TEM image of CNWs grown using C2F6/H2, where randomly oriented, small overlapping multilay‐ ered graphene domains were observed. The inset shows magnified image of the area enclosed by square, where a bent multilayered graphene structure with a thickness of approximately 9 nm was observed. In contrast, Figure 8(c) shows a cross-sectional SEM image of the CNW film grown for 40 min using C2F6/H2 with O2 addition, and the inset shows the top view SEM image

density ratio increased, the growth rate of the CNWs decreased and the average interspaces between the walls became

CCP

Surface wave plasma

Microwave

H

To pump

H2

C2F6 or CH4

152 Graphene - New Trends and Developments

wider.

tively [35].

Figure 7. Dummy Text (a) H and C atom densities in VHF plasma employing C2F6/H2 mixtures, together with the height variation of CNW films, as a function of total pressure during the CNW formation at a microwave power of 250 W and a VHF power of 270 W. (b-d) In addition, O2 and N2 gases were introduced into the VHF-CCP region. The crystallinity of vertically standing CNWs is Figure 8. Dummy Text (a) Cross-sectional SEM image of CNW film grown on a Si substrate for 30 min using C2F6/H2, together with SEM top view image of identical CNW film as an inset. (b) TEM image of bent CNWs grown using C2F6/H2, together with magnified image of square area as an inset. (c) Cross-sectional SEM image of CNW film grown for 40 min using C2F6/H2 with additional O2 gas, together with SEM top view image of identical CNW film as an inset. (d) TEM image of straight CNWs grown with additional O2, together with **Figure 8.** (a) Cross-sectional SEM image of CNW film grown on a Si substrate for 30 min using C2F6/H2, together with SEM top view image of identical CNW film as an inset. (b) TEM image of bent CNWs grown using C2F6/H2, together with magnified image of square area as an inset. (c) Cross-sectional SEM image of CNW film grown for 40 min using C2F6/H2 with additional O2 gas, together with SEM top view image of identical CNW film as an inset. (d) TEM image of straight CNWs grown with additional O2, together with magnified image of square area as an inset [32].

magnified image of square area as an inset [32].

improved by introducing O2 into the plasma used for CNW growth [32], while N addition is expected to control the electrical properties of the CNWs [35]. Figure 8(a) shows the cross-sectional SEM image of the typical CNW film grown on a Si substrate, and the inset shows the top view SEM image of the same CNW film. CNW growth was conducted on a Si substrate using a C2F6/H2 mixture, resulting in the formation of slightly branching carbon sheets standing almost vertically on the substrate, as shown in Figure 8(a). The thickness of CNW film grown for 30 min was approximately 1 μm. Figure 8(b) shows TEM image of CNWs grown using C2F6/H2, where randomly oriented, small overlapping multilayered graphene domains were observed. The inset shows magnified image of the area enclosed by square, where a bent multilayered graphene structure with a thickness of approximately 9 nm was observed. In contrast, Figure 8(c) Here, the morphology and structure of deposits formed using C2F6/H2 in the early growth stage were investigated in detail. Figures 9(a)-9(d) show tilted SEM images of the deposits formed during the nucleation of CNW growth. At the very early stage of nucleation, as shown in Figure 9(a), nanoislands were formed on the Si substrate in 30 s. The density of nanoislands (number of nanoislands per area) increased with growth period. In 1 min, most of the surface of Si was covered with nanoislands (Figure 9(b)). The thickness of this layer composed of nanoislands was approximately 10 nm. At this moment, some nanoflakes have started to form at the aggregations of nanoislands forming the first layer. Subsequently, randomly oriented nanoflakes were formed on the first layer (Figure 9(c)). In 3 min, these sheet structures grow preferentially in a vertical direction to form vertical CNWs, while the number density of these nanoflakes was less than that observed at 2 min (Figure 9(d)). Figure 9(e) shows a crosssectional TEM image of CNWs grown for 30 min, indicating that the interfacial layer exists between the CNWs and the Si surface. The thickness of the interface layer is approximately 10 nm, which is identical to the thickness of the first layer formed during the nucleation stage. Similar interface layer was also observed in the CNW films grown on Si and SiO2 substrates using inductively coupled plasma (ICP) with CH4/H2/Ar mixtures [13].

**Figure 9.** SEM images of the deposits formed on a Si substrate using a C2F6/H2 system for (a) 30 s, (b) 1 min, (c) 2 min, and (d) 3 min. (e) Cross-sectional TEM image of CNWs and an interface layer synthesized for 30 min [14].

So far, several papers have been published on the observation of CNW growth in the early growth stage and the nucleation mechanism for the formation of vertical layered-graphenes on Si and SiO2 substrates using various CVD methods [5,7,8,13-17,30]. It is common in previous studies that there is an induction period of 1-5 min for the nucleation of vertical nanographene. In addition, there exists an interface layer between the vertical nanographenes and substrate surface. Raman spectra were recorded for the deposits in the initial growth stages. D- and Gbands were not observed in the Raman spectra of nanoislands formed on the substrate for 1 min or less despite the fact that carbon was detected in these samples by X-ray photoelectron spectroscopy (XPS) analysis [14]. The nanoislands and the interface layer underlying twodimensional nanographene are considered to be amorphous carbon. In most cases using several PECVD methods, the interface layer under the CNWs is considered to be an amorphous carbon [5,7,15,17,30,37]. Due to the existence of amorphous carbon interface layer, it is possible to grow CNWs and similar structures on a variety of substrates without catalyst. In contrast, Zhu et al. suggested that graphenes parallel to the substrate surface would grow at first. In their model, at the grain boundaries of these horizontal few-layer graphenes, spreading edge of the top layers of few-layer graphenes would curl upward, resulting in the vertical orientation of these sheets [8].

Figures 10(a)-10(d) show tilted SEM images of the deposits on the Si surface during the nucleation of CNW growth with O2 addition. At 30 s, no deposits were observed on the surface [Figure 10(a)]. In 1 min, nanoislands were formed on the Si substrate [Figure 10(b)]. It took longer time to nucleate nanoislands in the case of the growth with O2 gas addition, compared with the case without O2 shown in Figure 9(a). In 2 min, some small two-dimensional nano‐ flakes have started to grow at isolated nanoislands, while the fractional surface coverage was low, as shown in Figure 10(c). A distinct interface layer was not formed. As shown in Figure 10(d), isolated wall structures had grown in 3 min, while the number density of start-up CNWs was lower than that without O2,

between the CNWs and the Si surface. The thickness of the interface layer is approximately 10 nm, which is identical to the thickness of the first layer formed during the nucleation stage. Similar interface layer was also observed in the CNW films grown on Si and SiO2 substrates

**Figure 9.** SEM images of the deposits formed on a Si substrate using a C2F6/H2 system for (a) 30 s, (b) 1 min, (c) 2 min,

So far, several papers have been published on the observation of CNW growth in the early growth stage and the nucleation mechanism for the formation of vertical layered-graphenes on Si and SiO2 substrates using various CVD methods [5,7,8,13-17,30]. It is common in previous studies that there is an induction period of 1-5 min for the nucleation of vertical nanographene. In addition, there exists an interface layer between the vertical nanographenes and substrate surface. Raman spectra were recorded for the deposits in the initial growth stages. D- and Gbands were not observed in the Raman spectra of nanoislands formed on the substrate for 1 min or less despite the fact that carbon was detected in these samples by X-ray photoelectron spectroscopy (XPS) analysis [14]. The nanoislands and the interface layer underlying twodimensional nanographene are considered to be amorphous carbon. In most cases using several PECVD methods, the interface layer under the CNWs is considered to be an amorphous carbon [5,7,15,17,30,37]. Due to the existence of amorphous carbon interface layer, it is possible to grow CNWs and similar structures on a variety of substrates without catalyst. In contrast, Zhu et al. suggested that graphenes parallel to the substrate surface would grow at first. In their model, at the grain boundaries of these horizontal few-layer graphenes, spreading edge of the top layers of few-layer graphenes would curl upward, resulting in the vertical orientation

Figures 10(a)-10(d) show tilted SEM images of the deposits on the Si surface during the nucleation of CNW growth with O2 addition. At 30 s, no deposits were observed on the surface [Figure 10(a)]. In 1 min, nanoislands were formed on the Si substrate [Figure 10(b)]. It took longer time to nucleate nanoislands in the case of the growth with O2 gas addition, compared with the case without O2 shown in Figure 9(a). In 2 min, some small two-dimensional nano‐

and (d) 3 min. (e) Cross-sectional TEM image of CNWs and an interface layer synthesized for 30 min [14].

of these sheets [8].

using inductively coupled plasma (ICP) with CH4/H2/Ar mixtures [13].

154 Graphene - New Trends and Developments

**Figure 10.** SEM images of the deposits formed on Si substrate using C2F6/H2 with O2 addition for (a) 30 s, (b) 1 min, (c) 2 min, and (d) 3 min [14].

**Figure 11.** Temporal behaviors of *I*G/*I*D ratios of CNWs formed with and without O2 gas addition [14].

Raman spectra were recorded for the deposits formed without and with O2 addition in the initial growth stages. The intensity ratios of the G-band (*I*G) to the D-band (*I*D) of the deposits 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 for obtaining highly graphitized CNWs.

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.
