24 Introduction, Synthesis and Characterizations of Graphene

Dr. Anchal Srivastava

Contents of this Unit

1. Introduction

2. Crystal Structure of Graphene

3. Synthesis

3.1 Top Down Approach

3.1.1 Mechanical Exfoliation

3.1.2 Chemical Exfoliation

3.2 Bottom Up Approach

3.2.1 Chemical Vapour Deposition (CVD)

3.2.2 Epitaxial Growth

4. Transferring of Graphene

5. Characterizations

5.1 Optical Microscopy

5.2 Raman Microscopy

5.3 Atomic Force Microscopy (AFM)

5.4 Transmission Electron Microscopy (TEM)

6. Summary

Learning Outcomes

• After studying this module, you shall be able to
• Know about some basic information about the novel material graphene
• Learn various synthesis approaches for synthesizing 2D materials such as mechanical exfoliation, chemical exfoliation, CVD etc.
• Get the idea how confirm the presence of mono and few layer graphene from various characterization techniques.

1. INTRODUCTION

Graphene is a one atom thick sheet of carbon atoms densely packed in a honeycomb crystal lattice. It was first time isolated by Andre Geim and Konstantin Novoselov in 2004 and Noble prize of physics in 2010 has been awarded to them for their ground breaking work on graphene. Graphene is a strictly two-dimensional material exhibiting such a high crystal quality that electrons can travel submicrometer distances without scattering. It is hundreds of times stronger than most steels by weight. It has the highest known thermal and electrical conductivity, displaying current densities 106 times that of copper.

2. Crystal Structure of Graphene

A single atomic layer of graphite is called graphene. For graphene, a primitive unit cell can be defined as: the smallest building block from which we can construct the graphene lattice. The primitive unit cell of graphene consists of two atoms due to the hexagonal structure, which we can label as A and B (Fig. 1). Size of the primitive unit cell of graphite depends on how the individual graphene layers stack to form the graphite crystal. Graphite is found in nature with various stacking arrangements, but here we will concentrate on the most common and thermodynamically stable stacking: Bernal (or ABAB) stacking. In Bernal stacking the B atom in the second layer is directly above the A atom in the first layer, and then in the third layer there is an A atom at this location, just as in the first layer.

Lattice vectors can be defined as the vectors joining equivalent points in adjacent unit cells, for mono-layer graphene. Equation 1.1 gives these (two-dimensional) lattice vectors, in terms of the sp2 C–C bond length in graphene a0: The accepted values for the bond length and lattice constant are a0 = 1.42 Å and a = 2.46 Å. We can also (Equation 1.2) define the reciprocal lattice vectors b1 and b2 for the graphene lattice using the relations a1 ⋅ b1 = 2π, a1 ⋅ b2 = 0, etc.:

Fig. 1. (a) Atomic structure of Bernal (ABAB) stacked graphite, (b) Primitive unit cell (shaded) and lattice vectors a1 and a2 of graphene.

3. Synthesis of Graphene

To realize the applications of graphene a consistent, reliable, simple and inexpensive method of growing high-quality, continuous, and uniform, single and few-layer graphene films is a prerequisite. A knowledge of the different synthesis methods is necessary to understand why different graphene sample has the exhibited properties, and to understand which synthesis methods can produce suitable samples for a given experiment or application. The scalability and cost of the different production methods also varies widely.

3.1. Top Down Approach:

As the name suggests, the top-down approach means from top(larger) to bottom(smaller). This approach is similar to making a statue made of stone. As in making of a statue, a bulk or big piece of stone is taken, similarly in top-down approach; a bulk piece of material is taken. Then carving and cutting is done until desired shape is achieved.

3.1.1. Mechanical Exfoliation:

The mechanical exfoliation, or “scotch tape” method, was the method first used to isolate mono-layer, bi-layer and few-layer graphene in 2004. The method exploits the weakness of the van der Waals bonding between the atomic layers of graphite flakes. A crystal of graphite is peeled apart many times with adhesive tape (Fig. 2) until, in some locations, there are flakes of mono-layer, bilayer and few- layer graphene left adhered to the tape. The adhesive tape, with flakes attached, is then pressed against a substrate and peeled away. In some cases the graphene crystals remain attached to the substrate, along with much larger quantities of few-layer graphene and thin graphite crystals. The mono-layer graphene flakes produced by this method occupy only a tiny part of the area of the substrate, and are mixed in with a much larger number of bi-layer and few layer graphene flakes, as well as a lot of thin graphite crystals.

Fig. 2. An illustrative procedure of the Scotch-tape based micromechanical cleavage of HOPG.

However, the extremely small throughput and time-consuming procedure to identify the graphene samples ensures that this techniques completely unsuitable for commercial production of graphene samples. For this, we must look primarily to the liquid-phase exfoliation and chemical vapor deposition techniques described in the next two sections.

3.1.2. Liquid-phase Exfoliation:

The mechanical exfoliation method (shown in Fig. 2) produces extremely small quantities of high-quality mono-layer graphene alongside much larger quantities of few-layer graphene and thin graphite crystals and the extremely small quantity of graphene produced renders it unsuitable for commercial applications. As an alternative, the liquid-phase exfoliation method has been developed. This method involves immersing graphite flakes in a solvent and subjecting the suspension to ultrasound, in order to simply shake the graphite apart into graphene flakes. It can produce far greater quantities of graphene than the mechanical exfoliation process. To begin, the liquid-phase exfoliation process consists of subjecting a suspension of graphite flakes in a solvent to ultrasound. The solvent is chosen such that the energetic cost of exfoliating the graphite flakes into graphene flakes surrounded by solvent molecules is as small as possible. After sonication, a small proportion of the original graphite sample is exfoliated into mono-layer, bi-layer and few-layer graphene samples, while most remains in the form of thicker graphite flakes suspended in the solvent. The thicker flakes can then be removed using centrifugation to leave a suspension of mono-layer, bi-layer and few-layer graphene samples in the solvent. It is worth noting, however, that the graphite sediment removed during the centrifugation process can be subjected to further attempts at exfoliation to increase this yield. In summary, the liquid-phase exfoliation process described here is important due to its scalability to produce large quantities of graphene for commercial use. However, it suffers from the crucial disadvantage that (just as for the mechanical exfoliation process) the mono-layer graphene samples produced are mixed in with a large quantity of few-layer graphene and thin graphite samples. In addition, the sample quality is not as high as that achieved using the mechanical exfoliation process.

Fig. 3. An illustrative procedure of liquid phase exfoliation for obtaining graphene from graphite.

3.2. Bottom Up Approach:

Whilst the mechanical and liquid-phase exfoliation processes are both top-down processes, efforts have been made since the beginning of graphene research to produce graphene in a bottom-up process instead. The aim is to produce large single crystals of graphene on substrates suitable for further study in a process scalable for commercial use. The mechanical and liquid-phase exfoliation processes both fail to achieve this. The first bottom-up process developed for graphene synthesis was epitaxial growth and the second (more widely used process) is chemical vapor deposition.

3.2.1. Epitaxial Growth:

Epitaxial growth (shown in Fig. 4) of graphene is typically performed on the silicon-terminated (0001) face of 6H-SiC by annealing under vacuum or argon at a temperature of 1250°C–2000°C. Annealing at ambient pressure under argon was a later (2008) development which was found to significantly increase the grain size of the graphene flakes. In either case, the graphene is grown from carbon atoms which come from the top layer of the SiC when it decomposes at high temperature. The silicon atoms are lost to the atmosphere via sublimation and the carbon atoms are then free to form graphene. The growth can be made self-limiting as, once the graphene layer is formed, further sublimation of the silicon cannot occur. The crystalline quality achieved by the best epitaxially grown graphene samples is very high However, there are drawbacks to the epitaxial growth technique. In particular the fact that, as is usually the case in epitaxial growth, the graphene layer interacts strongly with the substrate. The orientation of the graphene layer is determined by the orientation of the substrate lattice and the substrate can even cause the appearance of a superlattice in the graphene. The high cost of SiC wafers is an additional barrier to the commercial use of graphene grown by this method. For these reasons, despite the promising early results achieved using the epitaxial growth process, chemical vapour deposition is now the more widely used technique for large-scale graphene growth.

Fig. 4. Schematic for the epitaxial growth of graphene from SiC

3.2.2. Chemical Vapor Deposition (CVD):

The method of graphene growth most likely to prove suitable for commercial applications is chemical vapour deposition (CVD) on a transition metal substrate. A variety of precursor gases and experimental conditions have been utilized, but the most common method is vacuum CVD at approximately 1000°C with methane as the precursor gas and hydrogen also present. The process is catalytic because the transition metal substrate catalysis the decomposition of the precursor gas to produce the carbon which forms the graphene layer. Copper is the most commonly used substrate, followed by nickel, but cobalt, ruthenium, palladium, rhenium, iridium and platinum have also been used

The crystalline quality of the graphene grown by CVD is lower than that of graphene produced by the mechanical exfoliation method. There are two explanations for this. Firstly, when methane is introduced to the reactor the growth process commences simultaneously in various different locations on the substrate. The graphene sample therefore consists of a large number of small crystals instead of one single crystal, as illustrated in Fig. 5. Secondly, the growth temperature is relatively low. Growth typically takes place at a temperature of 1000°C whilst the HOPG and natural graphite used as the starting materials for the mechanical and liquid-phase exfoliation processes have been subjected to temperatures above 2000°C in the factory or in the earth’s interior. Unfortunately, in the case of copper, a higher growth temperature would result in the substrate melting. Despite these limitations the quantum Hall effect has been observed in graphene samples grown

Copper is the most widely used substrate because of the high proportion of the substrate which can be covered in mono-layer graphene (as opposed to bi-layer graphene or few-layer graphene). This is due to the growth of graphene on copper being a self-limiting process. The growth process commences with the copper substrate being annealed at high temperature in hydrogen. When copper is subjected to ambient conditions an oxide layer (CuO, Cu2O) rapidly forms on the surface. When annealing takes place the hydrogen reacts with this copper oxide surface layer, leaving pristine copper exposed to methane molecules when they are introduced to the reaction chamber. Pristine copper does not react with carbon and the solubility of carbon in copper is extremely low (0.001–0.008 wt.%) . Hence, virtually no carbon is absorbed into the copper substrate. The sole source of carbon for growth is carbon atoms resting on the surface of the copper after the copper has acted as a catalyst for the decomposition of the methane molecules. Once a single atomic layer of graphene has grown on the copper surface any more methane molecules that arrive do not have access to the copper to catalyse their decomposition. Hence, even for long growth times, the thickness of the graphene produced is limited to one layer. Graphene samples grown by CVD on copper are over 95% mono-layer. Generally, growth on nickel leads to multilayer graphene but graphene samples 87%–91% mono-layer have been grown on single crystal nickel by utilizing a very rapid cooling rate. The interaction between the copper substrate and the graphene sample after growth is also extremely weak, compared to that between graphene and other transition metals (or silicon carbide for that matter). This is due to the electronic structure of copper. The 3d shell is full, and hence the only bonding possible between graphene and copper is some charge transfer from the graphene π orbitals to the half-full 4s orbital in the copper. The weakness of this interaction is reflected in the behavior of the system upon cooling after growth.

When graphene is grown by CVD on copper, there is also a substantial mismatch in thermal expansion coefficient between the graphene crystal and the substrate – the substrate compresses significantly upon cooling and the graphene crystal does not. However, due to the interaction between graphene and copper being much weaker, the graphene grown by CVD on copper is not left under strain. The Raman peaks of the graphene are in the same position as those of (unstrained) graphene produced on SiO2 using the mechanical exfoliation process. Instead, substantial wrinkles form in the graphene crystal. Finally, it is essential to understand the key role played by the hydrogen gas present during the CVD process. In a typical CVD growth process for graphene the copper substrate in the CVD furnace is first heated up under low hydrogen pressure (~0.1 Torr) to above 1000°C, before methane is introduced to initiate growth of graphene. During the heating stage, the hydrogen is responsible for the removal of the oxide layer from the copper surface as discussed earlier. However, the hydrogen flow is continued during the growth period. During this period the hydrogen plays a different, and every bit as important, role. On the copper surface, the methane gas molecules decompose to free hydrogen and carbon, but the free hydrogen and carbon can also recombine to form methane. So an equilibrium state exists where both reactions are taking place simultaneously (Equation 1.3) due to le Chatelier’s principle)

Fig. 5. Schematic diagram illustrating stages in the growth of graphene by CVD on copper. When growth commences, a number of small crystallites form simultaneously at different locations on thesubstrate. Upon completion of growth (the entire substrate covered in a mono-layer of graphene) defects are present at the boundaries between the crystallites.

4. Transfer of graphene to different substrates:

It is often necessary to obtain a graphene sample on a substrate tailored to a specific experiment or application. For instance, to perform an electron diffraction experiment on graphene, it is necessary to utilize a sample mounted on a substrate with an aperture, and to study the electric field effect in graphene it is necessary to utilize a sample mounted on an insulating substrate on which contacts can easily be deposited. Fortunately, it is relatively straightforward to transfer graphene to different substrates following synthesis/exfoliation. The most common procedure to transfer graphene is to cover the graphene sample in a thin layer of the glass poly(methyl methacrylate) (PMMA). The original substrate is then etched away. There are a number of reagents which can etch away the substrates on which graphene is typically synthesized/exfoliated whilst leaving the graphene layer and PMMA intact, for instance, nitric acid solution can etch away an SiO₂ substrate in this procedure. The PMMA layer (with graphene attached) is then placed on the new substrate, after which the PMMA is removed by dissolving it in acetone. The graphene is then left on the new substrate in the desired location. This procedure (described schematically in Fig. 6) can be performed routinely on graphene samples produced by the mechanical exfoliation method on SiO₂ and by CVD on copper. In chemical vapor deposition (CVD), carbon is supplied in gas form and a metal is used as both catalyst and substrate to grow the graphene layer.

Fig. 6. Schematic illustration of CVD growth of graphene and its transfer process.

5. Characterization:

5.1. Optical Microscopy:

Monolayer graphene becomes visible on SiO2 using an optical microscope. The contrast depends on the thickness of SiO2, the wavelength of light used and the angle of illumination. This feature of graphene is useful for the quick identification of few- to single-layer graphene sheets, and is very important for mechanical exfoliation. Fig. 7 shows the optical contrast of one, two and three layers of exfoliated graphene under different wavelengths of illumination and different thicknesses of SiO₂.

Fig. 7. Optical microscope images of graphene. Multilayer graphene sheet on Si/SiO2 showing optical contrast at different wavelengths and thicknesses.

5.2. Raman:

Raman spectroscopy is an important characterization tool used to probe the phonon spectrum of graphene. Raman spectroscopy of graphene can be used to determine the number of graphene layers and stacking order as well as density of defects and impurities. The three most prominent peaks in the Raman spectrum of graphene and other graphitic materials are the G band at ∼1580 cm−1, the 2D band at ∼2680 cm−1 and the disorder-induced D band at ∼1350 cm−1.

Fig. 8. Layer dependence of graphene Raman spectrum. Raman spectra of N = 1-4 layers of graphene on Si/SiO2 and of bulk graphite.

The G band results from in-plane vibration of sp2 carbon atoms and is the most prominent feature of most graphitic materials. This resonance corresponds to the in-plane optical phonons at the Γ point.

The 2D band arises as a result of a two phonon resonance process, involving phonons near the K point, and is very prominent in graphene as compared to bulk graphite. The D band is induced by defects in the graphene lattice (corresponding to the in-plane optical phonons near the K point), and is not seen in highly ordered graphene layers. The intensity ratio of the G and D band can be used to characterize the number of defects in a graphene sample. The line shape of the 2D peak, as well as its intensity relative to the G peak, can be used to characterize the number of layers of graphene present as illustrated in Fig. 8. Single-layer graphene is characterized by a very sharp, symmetric, Lorentzian 2D peak with an intensity greater than twice the G peak. As the number of layers increases the 2D peak becomes broader, less symmetric and decreases in intensity.

5.3. Atomic Force Microscopy (AFM):

Atomic-force microscopy is a type of scanning probe microscopy with sub-angstrom resolution. It is able to gather data on the mechanical and electrical properties of materials and surfaces by ‘feeling’ or ‘touching’ the surface with a cantilevered mechanical probe controlled by Piezoelectric components. Atomic force microscopy (AFM), in particular, is used extensively since it provides three-dimensional images that enable the measurement of the lateral dimensions of graphene films as well as the thickness, and by extension the number of layers present.

AFM analysis of CVD grown graphene has been shown in Fig. 9 (a, b) it shows that the films are highly continuous and uniform. A large number of wrinkles is also visible on these films. Wrinkle formation occurs because of thermal stresses developed during cooling of the substrate after growth as there is a wide difference in thermal expansion coefficients of Cu and graphene. The thickness profile measurement using AFM shows that the typical film is approximately 3 layers thick. The observed thickness using AFM for single layer graphene is 0.8 nm.

Fig. 9. AFM image of CVD-grown graphene transferred onto a SiO2 substrate. It is clearly evident that the films are uniformand highly continuous.The wrinkles formed on the films during transfer are also clearly visible. The linear thickness analysis also shows that the films are about 1.5 nm thick.

5.4. HR-TEM:

Transmission electron microscopy can be used to image single-layer graphene suspended on a micro fabricated scaffold. TEM image of single-layer graphene displayed long range crystalline order despite the lack of a supporting substrate. Suspended graphene shows considerable surface roughness with out-of-plane deformations of up to 1 nm. The VA-CVD grown graphene films after transferring to lacey carbon-coated grids for TEM investigations (Figures 10, a, b, c, d). From High-resolution TEM examination shows that the films have one, two, three and four layers depending on growth conditions Typically, 1-4 layers are observed depending on the experimental conditions

Fig. 10. TEM images of CVD grown graphene showing (a) one, (b) two, (c) three, and (d) four layers.

6. SUMMARY

• In this module, you study
• The crystal structure of Graphene.
• Various synthesis techniques for graphene growth. Advantages and disadvantage of these techniques.
• Characterization of graphene from different techniques such as Optical microscopy, Raman spectroscopy, Atomic force microscopy, and High resolution transmission electron microscopy.

Suggested Reading

For More Details ( on this topic and other topics discussed in Text Module) See

1. An Introduction to graphene and carbon nanotubes by John Edward Proctor et. al. CRC Press.
2. Applications of graphene and graphene-oxide based nanomaterials by Sekhar Chandra Ray, Elsevier.
3. Graphene for Transparent Conductors Synthesis, Properties and Applications by Qingbin Zheng et. al., Springer.
4. Experimental review of graphene by Daniel R. Cooper et. al.
5. Making graphene visible by P. Blake and E. W. Hill, APL, 91, 063124 (2007).
6. Anchal Srivastava et. Al., Novel liquid precursor based facile synthesis of large-area, continuous, single and few layer graphene films, Chem. Mat. ACS Nano, 2010.