**2. Brief description of carbon nanowalls**

the large anisotropy between the in-plane and out-of-plane directions. Planar graphene films with respect to the substrate have been synthesized by thermal decomposition of carbonterminated silicon carbide and chemical vapor deposition (CVD) on metals such as nickel (Ni) and copper (Cu) substrates [1-3]. On the other hand, plasma-enhanced CVD (PECVD) is among the early methods to synthesize vertically standing carbon sheet structures [4-17]. These structures are called as carbon nanowalls (CNWs), carbon nanoflakes, and carbon nanosheets. CNWs and related nanocarbon structures consist of nanographene sheets standing vertically on a substrate. Figure 1 shows a schematic illustration of CNWs, where few-layer graphenes composed of nanographite domains form a self-supported network of wall structures. The mazelike architecture of CNWs with large-surface-area graphene planes and a high density of graphene edges would be useful as platform for electrochemical applications as well as tissue

CNWs and related sheet nanostructures have been synthesized using several PECVD techni‐ ques, which are similar to those utilized for growing carbon nanotubes (CNTs) and diamond thin films. For the growth of CNWs, typically, a mixture of methane (CH4) and hydrogen (H2) is employed as source gases. A certain amount of hydrogen (H) atoms are required for growing CNWs. In general, microwave plasma and inductively coupled plasma (ICP) have been used for the growth of CNWs. These are high-density plasmas and are suitable for decomposing H2 molecules efficiently. Or more specifically, radio frequency (rf) capacitively coupled plasma (CCP) with H radical injection and very high frequency (VHF) plasma with H radical injection have been applied to synthesize of CNWs. Pressures are ranging from 1 Pa to atmospheric pressure. Preparation of metal catalysts such as iron (Fe) and cobalt (Co) on the substrate is essential for the growth of CNTs. Unlike the CNT growth, CNWs do not require such catalysts for their nucleation. CNW growth has been conducted on several substrates including Si, SiO2, and Al2O3 without the use of catalysts at substrate temperatures of 500-700°C [5]. In view of the practical use of CNWs for device applications such as biosensors or electrochemical sensors in micrototal analysis system, further investigations should be performed to enable

In this chapter, fabrication techniques of CNWs and possible applications using CNWs as nanoplatform in the area of electrochemistry and tissue engineering are described. In the

engineering such as scaffold for cell culturing [18-25].

control of structures and surface properties of CNWs.

**Figure 1.** Schematic illustration of CNWs.

146 Graphene - New Trends and Developments

CNWs are mazelike architecture consisting of few-layer graphenes standing vertically on a substrate, as was illustrated in Figure 1. The CNW sheet itself is composed of nanodomains of a few tens of nanometers in size. Scanning electron microscopy (SEM) images of CNWs with different morphology are shown in Figures 2(a) -2(d). The morphology of CNWs depends on the synthesis conditions, including pressure, substrate temperature, source gas mixtures, and the type of plasma used for the synthesis. Typical mazelike architecture (Figure 2(a)), isolated vertical nanosheets (Figure 2(b)), and highly branched type (Figure 2(c)) have been fabricated. Moreover, straight and aligned CNWs with regular spacing (Figure 2(d)) was obtained on the substrate set perpendicular to the electrode plate in the case of growth using rf CCP with H radical injection [9].

Figures 3(a) and 3(b) show typical transmission electron microscopy (TEM) images of CNW with a micrometer-high planar nanosheet structure, which was synthesized using electron beam excited plasma-enhanced CVD [12]. Despite the relatively smooth surface, each sheet in CNWs is actually composed of nanographite domains of a few tens of nanometers distin‐ guished by domain boundaries as shown in Figure 3(a). Graphene layers are clearly observed in the high-resolution TEM image of the CNW shown in Figure 3(b). The spacing between neighboring graphene layers was approximately 0.34 nm.

Figure 4 shows a typical Raman spectrum of CNW film formed on Si substrate, which was measured at room temperature using a 514.5-nm line of an argon laser. Typical Raman spectrum for the CNWs has two strong peaks at 1590 cm-1 (G band), indicating the formation of graphitized structure and at 1350 cm-1 (D band) corresponding to the disorder-induced Figures 3(a) and 3(b) show typical transmission electron microscopy (TEM) images of CNW with a micrometer-high planar nanosheet structure, which was synthesized using electron beam excited plasma-enhanced CVD [12]. Despite the relatively smooth surface, each sheet

observed in the high-resolution TEM image of the CNW shown in Figure 3(b). The spacing

**Figure 2.** SEM images of CNWs with different morphologies.

[28,29].

2 nm 50 nm Figure 3. Dummy Text (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b) high-resolution TEM image of CNW showing graphene layers [12]. **Figure 3.** (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b) high-resolution TEM image of CNW showing graphene layers [12].

phonon mode. The peak intensity of D band is comparable or twice as high as that of G band. The sharp and strong D band peak suggests a more nanocrystalline structure, and the presence of graphene edges and small graphite domains. It is noted that the G band peak is accompanied by a shoulder peak at 1620 cm-1. This shoulder peak is often designated as D' band and associated with finite-size graphite crystals and graphene edges [26,27]. The strong and sharp D band peak and D' band peak are prevalent features of CNWs [8,11,13]. The 2D band peak at 2690 cm-1 is used to confirm the presence of few-layer graphene. It originates from a double resonance process that links phonons to the electronic band structure [28,29]. **Figure 3.** (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b) high-resolution TEM image of CNW showing graphene layers [12]. Figure 4 shows a typical Raman spectrum of CNW film formed on Si substrate, which was measured at room temperature using a 514.5-nm line of an argon laser. Typical Raman Figure 4 shows a typical Raman spectrum of CNW film formed on Si substrate, which was measured at room temperature using a 514.5-nm line of an argon laser. Typical Raman spectrum for the CNWs has two strong peaks at 1590 cm-1 (G band), indicating the formation of graphitized structure and at 1350 cm-1 (D band) corresponding to the disorder-induced phonon mode. The peak intensity of D band is comparable or twice as high as that of G band. The sharp and strong D band peak suggests a more nanocrystalline structure, and the presence of graphene edges and small graphite domains. It is noted that the G band peak is accompanied by a shoulder peak at 1620 cm-1. This shoulder peak is often designated as D band and associated with finite-size graphite crystals and graphene edges [26,27]. The strong and sharp D band peak and D band peak are prevalent features of CNWs [8,11,13]. The 2D band peak at 2690 cm-1 is used to confirm the presence of few-layer graphene. It originates from a double resonance process that links phonons to the electronic band structure

Carbon materials such as grassy carbon and conductive doped diamond have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electrocatalytic activity for a variety of redox

> Carbon materials such as grassy carbon and conductive doped diamond have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electrocatalytic activity for a variety of redox reactions. For the electrochemical applications, these carbon-based electrodes are often decorated with catalyst nanoparticles such as platinum (Pt). As was illustrated in Figure 1, CNW film has many graphene edges, and the CNW sheet itself is composed of nanographite domains of a few tens of nanometers in size. These graphene edges and domain boundaries are chemically reactive and are modified easily with several types of surface termination, e.g., C-NH2, C-OH, and C-COOH. Furthermore, Pt nanoparticles were preferably deposited on the defects such as grain boundaries on the surface of graphite [24]. Therefore, the structure of CNWs can be suitable for the platform of the electrochemical and biosensing applications. This kind of vertical-nanographene-based electrochemical platform with the high surface area and electrocatalytic activity offers great promise for creating revolutionary nanostructured electrodes for electrochemical sensing and biosensing, fuel cells and energy-conversion applications.

> The morphology and electrical properties of CNW film depend on the synthesis conditions, including source gases, pressure, process temperature, and the type of plasma used for the 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

Figure 4. Typical Raman spectrum of CNWs.

CNWs for device applications such as biosensors or electrochemical sensors in the form of micrototal analysis system,

band), indicating the formation of graphitized structure and at 1350 cm-1 (D band) corresponding to the disorder-induced phonon mode. The peak intensity of D band is comparable or twice as high as that of G band. The sharp and strong D band peak suggests a more nanocrystalline structure, and the presence of graphene edges and small graphite domains. It is noted that the G band peak is accompanied by a shoulder peak at 1620 cm-1. This shoulder peak is often designated as D band and associated with finite-size graphite crystals and graphene edges [26,27]. The strong and sharp D band peak

Figure 4. Typical Raman spectrum of CNWs. **Figure 4.** Typical Raman spectrum of CNWs.

[28,29].

phonon mode. The peak intensity of D band is comparable or twice as high as that of G band. The sharp and strong D band peak suggests a more nanocrystalline structure, and the presence of graphene edges and small graphite domains. It is noted that the G band peak is accompanied by a shoulder peak at 1620 cm-1. This shoulder peak is often designated as D' band and associated with finite-size graphite crystals and graphene edges [26,27]. The strong and sharp D band peak and D' band peak are prevalent features of CNWs [8,11,13]. The 2D band peak at 2690 cm-1 is used to confirm the presence of few-layer graphene. It originates from a double

**Figure 3.** (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b) high-resolution

(b)

(b)

**Figure 3.** (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b)

image of CNW showing graphene layers [12].

Figure 4 shows a typical Raman spectrum of CNW film formed on Si substrate, which was measured at room temperature using a 514.5-nm line of an argon laser. Typical Raman

Figures 3(a) and 3(b) show typical transmission electron microscopy (TEM) images of CNW with a micrometer-high planar nanosheet structure, which was synthesized using electron beam excited plasma-enhanced CVD [12]. Despite the relatively smooth surface, each sheet in CNWs is actually composed of nanographite domains of a few tens of nanometers distinguished by domain boundaries as shown in Figure 3(a). Graphene layers are clearly observed in the high-resolution TEM image of the CNW shown in Figure 3(b). The spacing

(b)

200 nm

1 µm

(c) (d)

300 nm

500 nm

2 nm

Carbon materials such as grassy carbon and conductive doped diamond have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electrocatalytic activity for a variety of redox reactions. For the electrochemical applications, these carbon-based electrodes are often decorated with catalyst nanoparticles such as platinum (Pt). As was illustrated in Figure 1, CNW film has many graphene edges, and the CNW sheet itself is composed of nanographite domains of a few tens of nanometers in size. These graphene edges and domain boundaries are chemically reactive and are modified easily with several types of surface termination, e.g., C-NH2, C-OH, and C-COOH. Furthermore, Pt nanoparticles were preferably deposited on the defects such as grain boundaries on the surface of graphite [24]. Therefore, the structure of CNWs can be suitable for the platform of the electrochemical and biosensing applications. This kind of vertical-nanographene-based electrochemical platform with the high surface area and electrocatalytic activity offers great promise for creating revolutionary nanostructured electrodes for electrochemical sensing and biosensing, fuel cells and energy-conversion applications.

The morphology and electrical properties of CNW film depend on the synthesis conditions, including source gases, pressure, process temperature, and the type of plasma used for the 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

2 nm

between neighboring graphene layers was approximately 0.34 nm.

(a)

148 Graphene - New Trends and Developments

**Figure 2.** SEM images of CNWs with different morphologies.

**Figure 2.** SEM images of CNWs with different morphologies.

(a)

(a)

50 nm

TEM image of CNW showing graphene layers [12].

[28,29].

50 nm

high-resolution TEM image of CNW showing graphene layers [12].

Carbon materials such as grassy carbon and conductive doped diamond have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electrocatalytic activity for a variety of redox

resonance process that links phonons to the electronic band structure [28,29].

Figure 4. Typical Raman spectrum of CNWs.

reactions. For the electrochemical applications, these carbon-based electrodes are often decorated with catalyst nanoparticles such as platinum (Pt). As was illustrated in Figure 1, CNW film has many graphene edges, and the CNW sheet itself is composed of nanographite domains of a few tens of nanometers in size. These graphene edges and domain boundaries are chemically reactive and are modified easily with several types of surface termination, e.g., C-NH2, C-OH, and C-COOH. Furthermore, Pt nanoparticles were preferably deposited on the defects such as grain boundaries on the surface of graphite [24]. Therefore, the structure of CNWs can be suitable for the platform of the electrochemical and biosensing applications. This kind of vertical-nanographene-based electrochemical platform with the high surface area and electrocatalytic activity offers great promise for creating revolutionary nanostructured electrodes for electrochemical sensing and biosensing, fuel cells and energy-conversion applications. Carbon materials such as grassy carbon and conductive doped diamond have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electrocatalytic activity for a variety of redox reactions. For the electrochemical applications, these carbon-based electrodes are often decorated with catalyst nanoparticles such as platinum (Pt). As was illustrated in Figure 1, CNW film has many graphene edges, and the CNW sheet itself is composed of nanographite domains of a few tens of nanometers in size. These graphene edges and domain boundaries are chemically reactive and are modified easily with several types of surface termination, e.g., C-NH2, C-OH, and C-COOH. Furthermore, Pt nanoparticles were preferably deposited on the defects such as grain boundaries on the surface of graphite [24]. Therefore, the structure of CNWs can be suitable for the platform of the electrochemical and biosensing applications. This kind of vertical-nanographene-based electrochemical platform with the high surface area and electrocatalytic activity offers great promise for creating revolutionary nanostructured electrodes for electrochemical sensing and biosensing, fuel cells and energy-conversion applications. The morphology and electrical properties of CNW film depend on the synthesis conditions, including source gases,

Figure 3. Dummy Text (a) TEM image of CNW grown using electron beam excited plasma-enhanced CVD and (b) high-resolution TEM Figure 4 shows a typical Raman spectrum of CNW film formed on Si substrate, which was measured at room temperature using a 514.5-nm line of an argon laser. Typical Raman spectrum for the CNWs has two strong peaks at 1590 cm-1 (G band), indicating the formation of graphitized structure and at 1350 cm-1 (D band) corresponding to the disorder-induced phonon mode. The peak intensity of D band is comparable or twice as high as that of G band. The sharp and strong D band peak suggests a more nanocrystalline structure, and the presence of graphene edges and small graphite domains. It is noted that the G band peak is accompanied by a shoulder peak at 1620 cm-1. This shoulder peak is often designated as D band and associated with finite-size graphite crystals and graphene edges [26,27]. The strong and sharp D band peak The morphology and electrical properties of CNW film depend on the synthesis conditions, including source gases, pressure, process temperature, and the type of plasma used for the 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 honey‐ comb 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 application, while vertical alignment and crystallinity of CNWs can be less crucial. pressure, process temperature, and the type of plasma used for the 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 application, while vertical alignment and crystallinity of CNWs can be less crucial. 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

and D band peak are prevalent features of CNWs [8,11,13]. The 2D band peak at 2690 cm-1 is used to confirm the presence of few-layer graphene. It originates from a double resonance process that links phonons to the electronic band structure 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 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 electrochemical and bio applications.

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. 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

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

In view of the practical applications using CNWs, desirable structures and electrical and

electrical properties, surface chemical properties of CNWs, and related sheet nanostructures should be controlled according to their applications. Although the nucleation mechanism of

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

**Figure 5.** Schematic illustration of structures of CNWs to be controlled in the nucleation and growth stages and modi‐ fied by the postprocesses, including etching and surface functionalization.
