Carbon Nanowalls

Graphite-related materials have long been a subject of interest. Since the report of carbon nanotubes by Iijima [1], the fabrication of carbon nanostructures has been studied intensively. One-dimensional carbon nanostructures, such as carbon nanotubes and carbon nanofibers, have attracted interest for applications such as electrochemical devices, electron field emitters, hydrogen storage materials, and scanning probe microscopy owing to their particular physical, chemical, and mechanical characteristics [2–5].

The first report on the fabrication of two-dimensional carbon nanostructures dates back more than 10 years. Ando et al. [1] found petal-like “carbon roses” during the fabrication of carbon nanotubes in 1997. In 2002, Wu et al. [2] reported the fabrication of two-dimensional carbon nanostructures, “carbon nanowalls,” standing vertically on catalyzed substrates. Both cases were found incidentally, during the fabrication of carbon nanotubes. Shang et al. [3] prepared carbon nanoflake films using hot filament chemical vapor deposition (CVD). To date, carbon nanowalls and similar structures have been grown using various CVD methods such as microwave plasma [2, 4, 5], radio frequency (RF) inductively coupled plasma [6], RF capacitively coupled plasma assisted by H radical injection [7, 8], DC plasma [9], helicon-wave plasma [10], electron beam excited plasma [11], hot-filament CVD [3, 12], and even by sputtering of a glassy carbon target [13, 14].

Carbon nanowalls are graphite nanostructures with edges comprised of stacked planar graphene sheets standing vertically on a substrate. The sheets form a wall structure with thicknesses in the range of a few nanometers to a few tens of nanometers, and with a high aspect ratio.

Thus far, carbon nanowalls have been grown using various chemical vapor deposition (CVD) methods such as, microwave plasma-enhanced CVD, radio frequency (RF) inductively coupled plasma-enhanced CVD, and hot-filament CVD. Among a variety of fabrication methods for carbon nanostructures, plasma-enhanced CVD (PECVD) is becoming one of the most promising techniques due to its feasibility and potentiality for large-area production with reasonable growth rates at relatively low temperatures.

In the case of film formation using plasma-enhanced chemical vapor deposition (CVD), high performance can be achieved by (1) the selective production of specific reactive species crucial for the film growth and nucleation, (2) the efficient transport of important species onto the growing surface, and (3) the control of surface reaction for both nucleation and subsequent growth. In the case of carbon nanostructure fabrication, it is important to elucidate the specific species such as carbon-containing radicals and hydrogen atoms that contribute to the growth and then determine the morphology of the nanostructures. Moreover, on the basis of the knowledge of the species, it is necessary to control the process plasma in order to obtain carbon nanostructures with structure and morphology customized for a specific application. Therefore, measurement of the radicals responsible for the formation of the carbon nanostructures is of great interest for practical applications. This chapter addresses issues on the growth mechanism of carbon nanowalls. Examples of radical density measurements in the plasma are described in the beginning. Then, the growth mechanisms of carbon nanowalls in the steady-state growth and nucleation stage are discussed. Furthermore, carbon nanowalls are synthesized using multi-beam CVD system consisting of ion, fluorocarbon radical, and H radical sources, and the role of ion bombardment for the nucleation of carbon nanowalls is discussed.

Carbon nanowalls have a high density of atomic scale graphitic edges that are potential sites for electron field emission, which might lead to the application in flat panel displays and light sources. So far, a number of publications have reported the field emission properties from carbon nanowalls and related structures [1–17].

Owing to the large surface area (high surface-to-volume ratio) of carbon nanowalls, we can expect a variety of applications using carbon nanowalls such as batteries, capacitors, and gas sensors. To this end, carbon nanowalls are decorated with nanoparticles or films of metals, semiconductors, and insulators, by using several techniques including vacuum evaporation, sputtering, CVD, and plating. Previously, Wu et al. used carbon nanowalls as templates to fabricate large surface-area materials, including Au, Cu, Zn, Ni, CoNiFe, Se, ZnO, TiO₂, SiO _x , SiN _x , and AlO _x [1–3].

Self-organized graphite nanostructures composed of graphene have been studied intensively. Carbon nanowalls and related sheet nanostructures are layered graphenes with open boundaries. The sheets form a self-supported network of wall structures with thicknesses in the range from a few nanometers to a few tens of nanometers, and with a high aspect ratio. The large surface area and sharp edges of carbon nanowalls could prove useful for a number of different applications. Electron field emitters and electrodes for fuel cell would be promising applications. To date, carbon nanowalls and related materials have been grown using various CVD methods. The morphology and structure of carbon nanowalls depend on the source gases, pressure, process temperature as well as the type of plasma used for the growth of carbon nanowalls. Isolated nanosheets, vertically standing nanowalls with a maze-like structure, highly branched type, and a kind of porous film have been fabricated. We can expect a wide variety of applications based on their structure or morphology, as illustrated in Fig. 8.1. Dense, highly branched carbon nanowall films with extremely large surface area are used for gas storage application. For this purpose, vertical alignment and crystallinity of carbon nanowalls can be less crucial, while low-temperature growth with relatively high growth rate may be important for the practical use. In the case of the application for membrane filter, honeycomb structure with controlled spacing is required. For this application, some methods for exfoliation of carbon nanowalls from the substrate to obtain free-standing membrane, or attachment of carbon nanowall membrane to the different materials should be developed. For the electron emitters, carbon nanowalls with atomically thin edges, moderate spacing, and excellent height uniformity are required. For the practical use of carbon nanowalls in the electron emitter application, evaluations of stability and lifetime of emitters are also important.