**1. Introduction**

It is well known that sp2 carbon can lead to various kinds of layered structures. Among these structures, graphene (monolayer and few layers) is an actual two-dimensional material with

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

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

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 control of structures and surface properties of CNWs.

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 beginning, characterizations of CNWs are outlined. Then synthesis method for CNWs using VHF CCP with H radical injection is presented. Radical injection technique was successfully applied to fabricate straight and large-size monolithic carbon nanosheet. The VHF CCP with H radical injection was developed with the aim of achieving large-area growth of CNWs with a reasonable growth rate. The structure of CNWs was controlled by changing the total pressure and VHF power. In addition, the structure of CNWs was modified by O2 plasma etching and hydrogen peroxide (H2O2) treatment.

In the latter half of this chapter, the electrochemical application of CNWs is described. Biosensing with CNWs is a promising application. Dopamine, ascorbic acid, and uric acid are compounds of great biomedical interest, which all are essential biomolecules in our body fluids. CNWs were used as electrode to detect these biomolecules. In addition, CNWs were oxidized by the surface treatment using atmospheric pressure plasma, and proteins such as bovine serum albumin were immobilized on these surface. Electrochemical properties of surface-decorated electrodes were investigated. Moreover, CNWs were used as scaffold for cell culturing. The dependence of the cell-culturing rates and morphologi‐ cal changes of HeLa cells on CNW scaffolds with different densities and wettability were systematically investigated.
