Introduction

Figure 1.1 Major allotropes of carbon. 1.2 Two-Dimensional CarbonAmong all carbon allotropes, graphene stands out because of its unique lattice structure: a monatomic honeycomb lattice with a perfect 2D dimensionality. The specific lattice structure in combination with the valence electron configuration of carbon atoms gives rise to peculiar electronic band structures, which distinguish graphene from other allotropes. The quasi-particles (or electrons and holes) in graphene behave like massless relativistic particles, or Dirac fermions, with the electrons and holes degenerated at the Dirac points [12−16]. This gives rise to a number of peculiar physical properties that are either not found or superior to those found in other carbon allotropes [17]. Some of the unique physical phenomena that have been observed or explored so far include unconventional integer quantum Hall effect (IQHE) [9, 10], Klein tunneling [18−20], valley polarization [21, 22], universal (non-universal) minimum conductivity [23−26], weak (weak anti-) localization [23, 27−31], ultrahigh mobility [23, 32−34], specular Andreev reflection at the graphene−superconductor interface [35, 36], exceptional thermal conductivity [37, 38], and superior mechanical properties [39]. Since the discovery of single-layer graphene, tremendous progress has been made in the development/redevelopment of vari-ous types of techniques for synthesizing both single-layer graphene

(SLG) and few-layer graphene (FLG) sheets, such as epitaxial growth on both SiC and metallic substrates [40−44], reduction from graphite oxide [45], chemical vapor deposition (CVD) [7, 46−48], and electrical discharge [49]. It is worth noting that most of these techniques are not new and they have been used to grow various types of 2D graphitic materials before the discovery of graphene. Depending on the synthesis techniques and conditions, in addition to pure graph-ene, various secondary forms of graphene can also be formed. These carbon nanostructures are typically multilayer graphene with a varying degree of curvature, defects, and morphology. Although dif-ferent terminologies have been introduced to describe these nanocarbon forms [6], in general, they can all be referred to as two-di-mensional carbon, which is the focus of this book. Just like diamond and graphite, perfect crystalline materials are always desirable, but they are more difficult to produce and thus often too expensive for large-scale applications; on the contrary, partially perfect carbons such as synthetic graphite/diamond, activated carbon, and DLC are more widely used in industry. The same scenario may also happen to graphene, which warrants a book to discuss 2D carbon in a more inclusive manner instead of purely on graphene. 1.3 Scope of This BookThis book is not intended to focus on the fascinating properties of graphene that have already been covered by other books. Instead, after a brief introduction of the band structure and electronic properties of graphene, we focus more on the synthesis and characterization of 2D carbons in general and the associated applications, in particular, in the area of energy storage. Based on this spirit, this book is organized into 11 chapters. Following the introduction, Yihong Wu gives a brief overview of electronic band structure and properties of graphene in Chapter 2. In addition to the description of band structure based on the tight-binding model, several unique electron transport properties of graphene are discussed, including quantum Hall effect, weak (weak anti-) localization, and electrical conductivity and mobility. Chapters 3 and 4 cover the growth of graphene on SiC substrates by Xiaosong Wu and on metallic substrates by Qingkai Yu, respectively. The former discusses the growth mechanism of graphene on both Si-face and C-face of SiC, while the latter deals with the growth of graphene on nickel and copper substrates using CVD. Chapter 5

discusses the growth and electrical transport properties of carbon nanowalls on different types of substrates. Emphasis is placed on how to design and form different types of electrical contacts that allow for the study of electrical transport properties of material structures with an unusual surface morphology. This is then followed by Chapter 6, in which Masaru Tachibana writes about the structural characterization of carbon nanowalls using Raman spectroscopy and transmission electron microscopy, and their potential applications in energy storage such as lithium ion batteries and fuel cells. In Chapter 7, Zexiang Shen discusses the structural properties of 2D carbon based on Raman spectroscopy studies. Chapters 8 and 9 are devoted to the energy storage applications of graphene obtained by the chemical reduction route, which is more cost effective compared with other vapor deposition-based techniques. X. S. Zhao focuses on the applications of 2D carbon in supercapacitor in Chapter 8, followed by Zhaoping Liu dealing with battery applications in Chapter 9. The photonic properties of graphene are discussed by Yoo Won Jong in Chapter 10. In Chapter 11, Hua Zhang discusses another important material derived from graphenegraphene oxideand its potential applications in sensor and memory devices. The current interest in graphene is phenomenal, as evidenced by the large number of publications published in the last few years. Many reviews have been written on graphene, covering various aspects from fundamental physics and electronic properties [16, 23, 50−55] to material synthesis [40−45, 56, 57] and applications [58−62]. Several books with different emphases are already available. It is not possible for any book to cover all the relevant topics on graphene and related nanostructures. By extending the coverage to both flat and vertically aligned graphene sheets, it is hoped that this book can serve as a good reference for research on 2D carbon in general rather than graphene only. References

1. Marder M.P. (2010) Condensed Matter Physics, 2nd ed, JohnWiley, Hoboken, New Jersey. 2. Dresselhaus M.S., Dresselhaus G., Eklund P.C. (1996) Science of

Fullerenes and Carbon Nanotubes, Academic Press, San Diego. 3. Iijima S. (1991) Helical microtubules of graphitic carbon, Nature, 354,

56−58.