Graphene: Fabrication, Characterizations, Properties and Applications

Graphene: Fabrication, Characterizations, Properties and Applications presents a comprehensive review of the current status of graphene, especially focused on synthesis, fundamental properties and future applications, aiming to giving a comprehensive reference for scientists, researchers and graduate students from various sectors. Graphene, a single atomic layer of carbon hexagons, has stimulated a lot of research interest owing to its unique structure and fascinating properties.

The book is devoted to understanding graphene fundamentally yet comprehensively through a wide range of issues in the areas of materials science, chemistry, physics, electronics and biology. The book is an important resource of comprehensive knowledge pertinent to graphene and to related expanding areas. This valuable book will attract scientists, researchers and graduate students in physics and chemistry because it aims at providing all common knowledge of these communities including essential aspects of material synthesis and characterization, fundamental physical properties and detailed chapters focused on the most promising applications.

• Presents a comprehensive and up-to-date review of current research of graphene, especially focused on synthesis, fundamental properties and future applications
• Includes not only fundamental knowledge of graphene materials, but also an overview of special properties for different potential applications of graphene in the fields of solar cells, photodetectors, energy storage, composites, environmental materials and bio-materials
• Emphasizes graphene-based applications that are quickly emerging as potential building blocks for nanotechnological commercial applications

• Language: English
• Publisher: Academic Press
• Release date: Sep 15, 2017
• ISBN: 9780128126523

Book preview

China

Preface

Hongwei Zhu, Zhiping Xu and Dan Xie, Tsinghua University, Beijing, China

Ying Fang, National Center for Nanoscience and Technology, Beijing, China

Graphene, a single atomic layer of carbon hexagons, has stimulated a lot of research interest owing to its unique structure and fascinating properties. This book will cover the current research on graphene, including the preparation methods, characterization techniques, properties, and potential applications, aiming to giving a comprehensive reference for scientists, researchers, and graduate students from various sectors. The first edition of this book was published in 2011 in Chinese. In the past 6 years, the breadth of the research activities involving graphene has spread widely, so it is an appropriate time to compile a book presenting a comprehensive review of the current status of graphene, specially focused on synthesis, fundamental properties, and future applications.

This book is devoted to understanding graphene fundamentally yet comprehensively through a wide range of issues in the areas of materials science, chemistry, physics, electronics, and biology. The importance of this book is that it represents a resource of comprehensive knowledge pertinent to graphene and to the expanding related areas. This book offers the following features:

1. A focus on Materials Science and Engineering;

2. Includes not only fundamental knowledge of graphene materials but also an overview of special properties for different potential applications of graphene in the fields of solar cells, photodetectors, energy storage, composites, environmental materials, and biomaterials;

3. Appeals to the communities working on graphene and graphene-like 2D nanomaterials. Especially useful to the scientists, engineers, and graduate students who are concerned with graphene research.

The book is divided into 10 chapters, which are authored as follows: Z. Zhen and H. W. Zhu wrote Chapter 1, Structure and Properties of Graphene; Q. Chen and H. W. Zhu wrote Chapter 2, Structural Characterizations of Graphene; R. J. Zhang and X. Li wrote Chapter 3, Multidimensional Assemblies of Graphene; Z. P. Xu wrote Chapter 4, Fundamental Properties of Graphene and Chapter 8, Graphene Composites; Y. L. Sun, M. X. Sun, and D. Xie wrote Chapter 5, Graphene Electronic Devices; T. T. Yang, X. L. Zhao, Y. J. He, and H. W. Zhu wrote Chapter 6, Graphene-Based Sensors; X. B. Zang wrote Chapter 7, Graphene-Based Flexible Energy Storage Devices; J. D. Shi and Y. Fang wrote Chapter 9, Biomedical Applications of Graphene; R. J. Zhang and H. W. Zhu wrote Chapter 10, Potential Applications and Perspectives.

We would like to express our gratitude to the publishers for their help in many ways. In this respect we would also like to acknowledge the financial support of the National Natural Science Foundation of China.

Spring 2017

1

Structure and Properties of Graphene

Zhen Zhen and Hongwei Zhu

Abstract

Theoretically, graphene is not a new object. However, before the discovery of graphene, these was always a debate over whether carbon could exist in a two-dimensional (2D) form. In fact, it was commonly recognized that no standalone 2D crystal is stable under certain temperatures in which layers or macromolecules of such material would not be able to grow in a crystalline structure according to theoretic predictions. Therefore, it is quite strange that graphene appears frequently in our daily life but no one ever found it until 2004. More interestingly, unlike many other scientific discoveries, the first observation of graphene is highly dramatic.

Keywords

Carbon; graphene; two-dimensional materials

Chapter Outline

1.1 Carbon Allotropes and Related Materials 1

1.2 Discovery of Graphene 3

1.3 Structure of Graphene 5

1.4 Properties of Graphene 8

References 11

Theoretically, graphene is not a new object as mentioned in the previous chapter. However, before the discovery of graphene, these was always a debate that whether carbon could exists in a two-dimensional (2D) form. In fact, it was commonly recognized that no standalone 2D crystal is stable under certain temperatures in which layers or macromolecules of such material would not be able to grow in crystalline structure according to theoretic predictions. Therefore, it is quite strange that graphene appears frequently in our daily life but no one ever found it until 2004. More interestingly, unlike many other scientific discoveries, the first observation of graphene is highly dramatic.

1.1 Carbon Allotropes and Related Materials

Carbon was one of the first elements known to humans, and is one of the most remarkable of all chemical elements. The use of carbon materials for a multitude of applications derives from the materials’ unique diversity of structures and properties that extend from chemical bonding between carbon atoms to nanostructures, crystallite alignment, and microstructures. Diamond and graphite are both three-dimensional (3D) crystalline forms of the element carbon. Graphite consists purely of sp² hybridized bonds, whereas diamond consists purely of sp³ hybridized bonds. The carbon atoms in diamond are arranged in a lattice, which is a variation of the face-centered cubic (fcc) crystal structure [1]. It has superlative physical qualities, most of which originate from the strong covalent bonding between its atoms (sp³ hybridization). Unlike diamond, graphite in particular is described as consisting of a lamellar (layered, planar) structure [2]. In each layer, the carbon atoms are arranged in a hexagonal lattice with separation of 0.142 nm (sp² hybridization), and the distance between planes (layers) is 0.335 nm. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have very similar physical properties. The lamellar structures have much stronger forces within the lateral planes than between the planes.

Graphitic carbon nanomaterials can be regarded as members of the same group because they consist primarily of sp² carbon atoms arranged in a hexagonal network. This common structure means that they all have some common properties, although they also have significant differences due to their different sizes and shapes. Graphene [3] is the basic structural element of other allotropes [4], including graphite, fullerene (e.g., C60) [5], carbon nanotube (CNT) [6], graphyne [7], and other related materials (e.g., carbon fiber (CF), amorphous carbon (AC), charcoal) [8] (Fig. 1-1). It can also be considered as an indefinitely large aromatic molecule, the limiting case of the family of flat polycyclic aromatic hydrocarbons.

Figure 1-1 Carbon allotropes and related materials. CNT, carbon nanotube.

Fullerenes (also called buckyballs) are molecules of varying sizes composed entirely of carbon that take on the form of hollow spheres or ellipsoids [5]. Fullerenes were the subject of intense research in the 1990s, both because of their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.

A CNT is considered as a cylinder formed by rolling up a graphene sheet with caps which are similar to parts of the fullerenes, and exhibits extraordinary strength and unique electrical properties and is an efficient conductor of heat [9]. Diameters of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs) are typically 0.8–2 nm and 5–20 nm, respectively, although MWNT diameters can exceed 100 nm. Individual CNT walls can be metallic or semiconducting depending on the orientation of the lattice with respect to the tube axis, which is called chirality.

CFs are a long, thin strand of materials about 5–10 μm in diameter, and composed mostly of carbon atoms that are bonded together in microscopic crystals aligned parallel to the long axis of the fiber [10]. The crystal alignment makes the fiber incredibly strong for its size. Several thousand CFs are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is combined with epoxy and wound or molded into shape to form various composite materials. CFs-reinforced composite materials are used in many areas, including spacecraft parts, racing car bodies, golf club shafts, bicycle frames, fishing rods, automobile springs, sailboat masts, and other components where light weight and high strength are needed.

Graphite intercalation compounds (GICs) are complex materials having a formula CXm where the ion Xn+ or Xn− is inserted (intercalated) between the oppositely charged carbon layers [11]. Typically m is much less than 1. These materials are deeply colored solids that exhibit a range of electrical and redox properties of potential applications. Expanded graphite (EG) is made by immersing natural flake graphite in a bath of chromic acid, then concentrated sulfuric acid, which forces the crystal lattice planes apart, thus expanding the graphite.

AC refers to carbon that does not have a crystalline structure. The properties of AC depend on the ratio of sp² to sp³ hybridized bonds present in the material. Materials that are high in sp³ hybridized bonds are referred to as tetrahedral AC (owing to the tetrahedral shape formed by sp³ hybridized bonds), or diamond-like carbon (owing to the similarity of many of its physical properties to those of diamond). AC materials may be stabilized by terminating dangling-π bonds with hydrogen. Activated carbon, also called activated charcoal (usually derived from charcoal), is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions.

Graphyne is a one-atom-thick planar sheet of sp- and sp²-bonded carbon atoms arranged in a crystal lattice [7]. It can be seen as a lattice of benzene rings connected by acetylene bonds. Graphyne has been a theorized allotrope of carbon for a long time and attracted great attention after the discovery of graphene [12,13].

(C=C)n [8].

1.2 Discovery of Graphene

The discovery of graphene took 60 years to go from controversial predications to successful cleavage of graphene from graphite. In 1947, Philip Wallace [14] started to investigate the electronic structure of a single graphite layer in which graphene was not yet defined. Later in 1956, the equation of wave function of such a layer was derived by J. W. McClure [15]. In 1984, G. W. Semenoff [16] proposed a similar equation to this wave function named the Dirac equation. However, many scientists still held doubts on the nature of graphene. In 1960, the electric conductivity of graphene is questioned by Linus Pauling [17], a winner of Nobel Prizes for both Chemistry and Peace. The term graphene is also not used to describe the single layer of graphite until 1987 by S. Mouras [18]. Since then graphene was commonly regarded as a virtual expression of unit structure in graphite and subsequent CNT in calculation and simulation, instead of a realistic form of carbon nanomaterial. The reason for such a point of view can be traced by to 1934 when L. D. Landau and R. E. Peierls [19] suggested that the thermodynamic instability of 2D crystalline material would cause rapid decomposition under room temperature and atmospheric pressure. Moreover, David Mermin and Herbert Wagner [20] present a theory called the Mermin–Wagner theory in 1966, indicating that the surface undulation would destroy the long-range order of 2D crystals. Therefore, although theoretical physicists were familiar with graphene long before its discovery, they did not put too much expectation on it.

Nevertheless, expectations on graphene are quite different for scientists in experimental physics and materials science. Professor Rodney Rouff [21] from University of Texas at Austin used to experiment on obtaining single layer graphene by rubbing graphite on silicon wafer when he was at Washington University. Unfortunately, further examination of the thickness of the outcome materials was neglected. Another scientist, Philip Kim [22] from Columbia University, also obtained layers of graphite by writing with a nano-pencil on a surface. This work gained a minimum of 10 layers of graphene, which is one step away from discovering graphene and could potentially have been awarded with Noble Prizes with some further efforts. However, luck favored two other Russian scientists across the ocean at the University of Manchester.

Andre Geim, a winner of the IgNobel Prize in the year 2000, became a real winner of the Nobel Prize in Physics in 2010 for the discovery of single layer graphene with his former student Konstantin Novoselov. Back in 2004, they published the very first article about the synthesis of graphene in Science, introducing a mechanical exfoliation method to obtain graphene and its Field Effect Transistor (FET) characteristics [3]. Interestingly, the key of this cleavage method is using tape to remove the surface layer of highly oriented pyrolytic graphite (HOPG) which is somehow considerably the simplest method to obtain thin layer of material. In fact, they have tried many advanced techniques with complicate procedures, but they did not get anything. Tape is very effective to clean and to collect graphite waste. The utmost care on experimental details allowed them to find graphene in a huge amount of graphite wastes. As we can see, scientific discovery indeed requires certain luck but only with a great passion for research and a sufficient amount of work.

For now, the mechanical exfoliation method could produce graphene in a size of 1 mm which can be observed with magnifying glass. Small pieces of graphene can also be obtained by rubbing two graphites together. Such a method can be applied to all crystalline materials with a layer structure to obtain single layer pieces [23].

As shown in Fig. 1-2, Novoselov demonstrated the schematic procedure of the mechanical exfoliation of graphene with tape. Graphene flakes with various thicknesses were transferred onto the surface of a silicon wafer, illustrated as those colorful fragments by optical microscope.

Figure 1-2 Schematic procedure of mechanical exfoliation of graphene.

However, if we look back to the early theories and experimental results, the structure of graphene would not be stable compared to other curved structure such as graphite powder, fullerene, and CNT. So, why can graphene be obtained instead of it crumpling into another structure? The answer can be explained by the misunderstanding of Mermin-Wagne theory: (1) a 2D crystal could maintain a stable structure with certain size and conditions instead of forming a new 3D structure; (2) Graphene is not a 100% flat surface since a large amount of fluctuation of about 1 nm amplitude is observed with TEM due to the self-adjustment of length between carbon bonds to adapt thermodynamic fluctuation as shown in Fig. 1-3 [24,25]. The surface roughness gained by the crumples and adsorption might be the key reason to maintaining its stability.

Figure 1-3 Fluctuation of graphene surface [25,26].

Overall, the discovery of graphene is neither a coincidence, nor something that could be done without tremendous effort. The Noble winner Geim himself once confirmed that the early researches of graphene went through many difficulties, in which many of the works on graphene can be traced back to 1970s. During the whole period, the competition of research progress never stopped. In 2005, in the journal Nature, the Geim group and the Kim group published two articles almost simultaneously, confirming the unique electrical properties of single layer graphene [26,27]. Moreover, a researcher from Georgia Institute of Technology, Walt de Heer sent a public letter to the Noble committee named Early development of graphene electronics, suggesting that his work is in fact earlier than Gem’s work. Fig. 1-4 is a diagram quoted from his work on epitaxial growth of graphene on SiC in early 2004 [28]. He also provided several pieces of evidence such as a foundation application of a graphene-related project and a patent about thin film graphite devices. However, just like the controversial discovery of CNTs, the one who first realized the significance of a research outcome should be the one who earns the prize of such a discovery. This was suggested long ago by A.N. Whitehead: Familiar things happen, and mankind does not bother about them. It requires a very unusual mind to undertake the analysis of the obvious.

Figure 1-4 TEM image and diffraction pattern of graphene.

1.3 Structure of Graphene

Graphene, by definition, is an allotrope of carbon in the form of a 2D, atomic-scale, hexagonal lattice in which one atom forms each vertex with sp² hybridization. As shown in Fig. 1-5, the length of the carbon–carbon bond is about 0.142 nm. There are three σ bonds in each lattice with strong connections forming a stable hexagonal structure. The electrical conductivity of graphene is mostly attributed from the π bond located vertically to the lattice plane. Graphene’s stability is due to its tightly packed carbon atoms and a sp² orbital hybridization—a combination of orbitals s, px, and py that constitute the σ-bond. The final pz electron makes up the π-bond. The π-bonds hybridize together to form the π-band and π*-bands. These bands are responsible for most of graphene’s notable electronic properties, via the half-filled band that permits free-moving electrons. Graphene can be considered as a unit structure of graphite, CNT, and fullerene, as well as aromatic molecules with infinite size, such as extremely planar polycyclic aromatic hydrocarbons.

Figure 1-5 Hexagonal lattice of graphene.

In other words, graphene is composed of a closely packed single layer of carbon atoms, forming a 2D honeycomb lattice plane. In single layer graphene, carbon atoms bond with surrounding carbon atoms with sp² hybridization forming a benzene ring in which each atom donates an unpaired electron.

The thickness of graphene is only 0.35 nm which is 1/200,000th the diameter of a human hair. Nevertheless, the structure of graphene is quite stable. The connection between carbon atoms is tough enough to endure external force by a twisting lattice plane to avoid the reconfiguration of atoms. Graphene, with a limited structure, can exist as a nanoribbon such that an energy barrier occurs near the central point due to a lateral charge movement. Such an energy barrier is increased with a decrease in width of the nanoribbon [29]. Thus, by manipulating the width of the graphene nanoribbon, the energy barrier can then be precisely controlled, which is a promising characteristic for potential graphene-based electronic devices. In addition to this, similar to a CNT, the edge of graphene can be classified into zigzag and armchair according to different carbon chains as shown in Fig. 1-6. The variety in edges leads to various conducting behaviors. A graphene nanoribbon with a zigzag edge usually behaves like a metal while a nanoribbon with an armchair edge could conduct electricity like either metal or a semiconductor.

Figure 1-6 Graphene nanoribbon.

1.4 Properties of Graphene

Graphene possesses many outstanding properties in terms of optical transparency, electric conductivity, mechanical strength, and thermal conductivity. Detailed properties will be discussed in Chapter 5, Graphene Electronic Devices, while this chapter will simply cover the main ideas.

Graphene is a super light material with a planar density of 0.77 mg/m². As shown in Fig. 1-5, the unit structure of graphene is a hexagonal carbon ring with an area of 0.052 nm². Such a ring only consists of two carbon atoms since each atom at vertex is shared by three unit rings. Graphene only consists of one atomic layer of carbon atoms which also contributes to its advantages of being superthin and ultralight.

Benefiting from one atomic thickness, graphene has a very high transparency of 97.7% i.e., it only absorbs 2.3% of visible light [30]. As shown in Fig. 1-7, the difference in transparency between a substrate and single layer graphene as well as a single layer and bilayer graphene are both 2.3%. Because of this, the transparency of graphene becomes an effective indication of the numbers of graphene layer, which is confirmed by simulations using noninteracting Dirac fermion theory as shown in Fig. 1-8.

Figure 1-7 Transparency of graphene.

Figure 1-8 Theoretical transparence of graphene [31].

According to the refraction and interference of light, graphene with various layers would exhibit different colors and contrasts which can be used to distinguish the layers of graphene [31]. The theoretical and experimental results both suggest an excellent optical property which can be manipulated by the thickness of graphene. Combined with its high conductivity, graphene becomes a very competitive transparent conductive membrane which could potentially replace many traditional membranes, such as indium tin oxide (ITO), fluorine doped tin oxide (FTO). The use of graphene in this area could solve the problem of fragility, environmental pollution, and limited indium resources. A graphene-based membrane could be applied in dye-sensitized solar cells and LEDs as windows barriers. Moreover, after certain modification such as doping, graphene is not only capable of being an acceptor in optoelectronic devices but could also be used as an electrode in supercapacitors. Furthermore, the adsorption of light of graphene would reach saturation if the intensity of light exceeds a certain critical value. This kind of saturated absorption in graphene occurs at the near-infrared region due to a wide range of adsorption and zero band gap of graphene. This characteristic could be utilized in ultrafast photonics such as fiber lasers.

Another interesting property of graphene is the electron mobility. Graphene is the most conductive material so far at room temperature, with a conductivity of 10⁶ S/m and a sheet resistance of 31 Ω/sq [32]. This is attributed to its ultrahigh mobility of 2×10⁵ cm²/V s [33] which is almost 140 times the mobility in silicon. As mentioned earlier, carbon atoms in graphene are sp² hybridized, donating one extra electron to the π bond. The π electrons, in this case, are totally free to move with little interference at room temperature, yielding a high conductivity. Beside this, graphene is a typical semimetal [34] in which there is a small overlap between its conduction band and valence band. Therefore, the electrons at the top of the valence band could flow into the bottom of the conduction band with lower energy without any heat stimulation. Even at the temperature of absolute zero, a certain concentration of electrons is already in the conduction band while a certain concentration of holes is in the valence band. As shown in Fig. 1-9, the valence band and conduction band of graphene exhibit cone-like structures which intercept at the Dirac point. The electron transport in graphene exhibits the anomalous quantum Hall effect [26,27] and characteristics of relativistic particles [35]. These features are even more promising for graphene nanoribbon with its semiconductive nature. For now, graphene has been used to establish high performance next-generation FETs and many other advanced electronic