Graphene: An Introduction to the Fundamentals and Industrial Applications

Preface

Science is an ever-continuing quest to understand the intricacies of nature right from atomic scale to vastness of the universe. One of such realm of learning is venturing into materials at a particularly defined size of 1–100 nm—encompassing a science called nano-science and nanotechnology. Graphene is the outcome of research and knowledge based on carbon nanotechnology. Graphene is now at the pinnacle of glorious achievements and has motivated multi-disciplinary research towards developing feasible solutions in various sectors. There have been several advances in the field of graphene-based materials, such as in energy-related applications as fuel cells, super-capacitors and photovoltaic devices. Graphene, by virtue of its unique properties, and graphene composites have also found an important relevance in energy harvesting. Furthermore, applications of graphene in filtering heavy metal ions and other pollutants are also of importance in the current scenario. The recent Nobel Prize–winning research work on graphene has attracted significant attention on account of its exceptional capabilities particularly in the field of electronics.

This book is our humble effort to present the state of the art of graphene research intended for various applications. We have tried to place these developments in scientific, technical, as well as commercial and economic context to assess the likelihood of uptake of these technologies and their relevance to world’s pressing needs of energy, miniaturization, communication, transportation and health.

The scope of this book includes scientific and technological details along with present day industrial approach and needs. This book is intended for new entrants and active researchers in the field of graphene science and technology in industry and academia, medical, government officials responsible for research, innovation, entrepreneur and industrialists venturing into applications of graphene, students and interested lay persons. We assume readers have academic training, but no expertise in graphene-technology.

Madhuri Sharon

Maheshwar Sharon

May 2015

Chapter 1

The History of Graphene

A pencil and a dream can take you anywhere.

Joyce A. Meyers

Prior to excavating the history of graphene, one has to know graphite, which is composed of many layers of graphene stacked together. This stacking makes a three-dimensional structure, the graphite, whereas graphene is a two-dimensional, one-atom-thick material. Evidence of the uses of graphite in Europe has been recorded in pottery decorated with graphite some 6000 years ago. The present concept and clarity about graphite is nearly 500 years old. Graphite ore (Figure 1.1) was found and mined in England in the sixteenth century.

Figure 1.1 Graphite ore. Courtesy: http://en.wikipedia.org/wiki/Graphite.

People used graphite to mark their sheep. However, it was believed that this mineral was lead ore and it was called plumbago. Scheele, in 1779, demonstrated that plumbago is actually carbon, not lead. Because people used it to write marks on their sheep, a German scientist, Verner (1789) named it graphite (a Greek word for writing). With the development of the pencil industry, it has been used as a writing material in a pencil (Figure 1.2) since the eighteenth century.

Figure 1.2 A lead pencil tip made of graphite. Courtesy: http://commons.wikimedia.org/wiki/File:Pencils_hb.jpg.

Because of its layered morphology and weak dispersion forces between adjacent sheets, it was utilized as solid lubricant. Before proceeding further with the history of graphene, it is necessary to define what a graphene is.

The term graphene first appeared in 1987 (Mouras et al. 1987) to describe single sheets of graphite as one of the constituents. The term graphite layers was replaced with graphene by the IUPAC commission. According to the recent definition, graphene is a two-dimensional monolayer of carbon atoms, which is the basic building block of graphitic materials (i.e., fullerene, carbon nano tubes, graphite). Graphene is a two-dimensional material. It consists of a single layer of carbon atoms arranged in a honeycomb-like structure (Figure 1.3B).

Figure 1.3 Schematic diagram of (a) Graphite and (b) Four layers of graphene from graphite.

The carbon-carbon bond length in graphene is about 0.142 nanometers (Figure 1.3B). Its layer height was measured to be just 0.33nm (Figure 1.3A). It is the thinnest material known, and yet is also one of the strongest. Graphene is almost completely transparent. Its structure is so dense that even the smallest atom helium cannot pass through it. It conducts electricity as efficiently as copper and outperforms all other materials as a heat conductor.

In 1859 a British chemist, Benjamin Bordie, prepared a highly lamellar structure by thermally reducing graphite oxide by reacting graphite with potassium chlorate and fuming nitric acid, resulting in the formation of a suspension of graphene oxide crystallite. This graphene oxide was later woven into a paper. An early study the properties of this graphene oxide paper was completed by Kohlschutter and Haenni in 1919. Graphene, a molecule arranged in a single atomic plane, is accepted as a two-dimensional crystal. Earlier it was believed it could not be grown, because thermodynamics had been shown to prevent the formation of two-dimensional crystal in free state by Landau (1930).

Wallace (1947), while trying to study the electronic properties of three-dimensional graphite, came up with the band theory of graphite. According to him,

The structure of the electronic energy bands and Brillouin zones for graphite is developed using the ‘tight binding’ approximation. Graphite is found to be a semi-conductor with zero activation energy, i.e., there are no free electrons at zero temperature, but they are created at higher temperatures by excitation to a band contiguous to the highest one which is normally filled. The electrical conductivity is treated with assumptions about the mean free path. It is found to be about 100 times as great parallel to as across crystal planes. A large and anisotropic diamagnetic susceptibility is predicted for the conduction electrons; this is greatest for fields across the layers. The volume optical absorption is accounted for.

The next milestone work regarding graphene was the publication of the first TEM image of a few layers of graphene by Ruess and Vogt (1948).

Ubbelohde and Lewis (1960) isolated a single-atom plane of graphite and reported surprisingly higher basal-plane conductivity of graphite intercalation compounds as compared to that of the original graphite. They pointed out that graphite consists of layers, which are a network of hexagonal rings of carbon atoms.

Hanns-Peter Boehm and his coworkers isolated and identified single graphene sheets by TEM and XRD in 1961. Their work was published in 1962. Boehm also authored the IUPAC (International Union of Pure and Applied Chemistry) report, formally defining the term graphene in 1994. It is surprising that many reviews and papers have mentioned that graphene was discovered in 2004. The TEM taken by Boehm et al. remained the best observation for over forty years.

These forty years (between 1960 and 2000) exhibited that the research of graphene has grown slowly in multifarious directions, including synthesis. The hope of observing superior electrical properties from thin graphite or graphene layers while obtaining graphene was considered to be a formidable task in both theoretical and experimental aspects. In the graphite intercalation systems, large molecules were inserted between atomic planes, generating isolated graphene layers in a three-dimensional matrix. The subsequent removal of the larger molecules produced a mixture of stacked or scrolled graphene layers without affecting the structure. During this period of research, the cause of the high conductivity of graphite intercalation compounds and the future applications were the main concerns.

There have been attempts to grow graphene using the same approach as the approach generally used for growth of carbon nanotubes, but it allowed the formation of thicker than ≈ 100 layers graphite films (Krishnan et al. 1997).

Hess and Ban (1966) were the first to use a chemical-vapor-deposition (CVD) technique, in which carbon atoms were supplied from a gas phase, to achieve the formation of monolayer graphite or graphene.

However, efforts to epitaxially grow few-layer graphene through the chemical vapor deposition of hydrocarbons on metal substrates (Land et al. 1992 and Nagashima et al. 1993) and on top of other materials (Oshima and Nagashima 1997) as well as by thermal decomposition of SiC have also been successful.

Epitaxial growth of graphene offers probably the only viable route towards electronic applications and, with so much at stake, rapid progress in this direction is expected. The approach that seems promising but has not been attempted yet is the use of the previously demonstrated epitaxy on catalytic surfaces (Land et al. 1992 and Nagashim et al. 1993), such as Ni or Pt, followed by the deposition of an insulating support on top of graphene and chemical removal of the primary metallic substrate.

This epitaxial graphene consists of a single-atom-thick hexagonal lattice of sp² bonded carbon atoms, as in free-standing graphene. However, there is significant charge transfer from the substrate to the epitaxial graphene, and, in some cases, hybridization between the d orbitals of the substrate atoms and π orbitals of graphene, which significantly alters the electronic structure of the epitaxial graphene. The fact that electric current would be carried by effectively massless charge carriers in graphene was pointed out theoretically by Semenoff et al. in 1984.

Properties such as the layered morphology and weak dispersion forces between adjacent sheets have made graphite an ideal material for use as a dry lubricant, along with the similarly structured but more expensive compounds hexagonal boronnitride and molybdenum disulfide. High, in-plane electrical (104 Ω–1 cm–1) and thermal conductivity (3000 W/mK) enable graphite to be used in electrodes and as heating elements for industrial blast furnaces (Bouchard et al. 2001).

The beginning of the twenty-first century saw many important discoveries related to graphene. Enoki et al. in 2003 explained the anisotropy of graphite’s material properties. Bulk graphite was first intercalated by Dresselhaus and Dresselhaus (2002) so that graphene planes became separated by layers of intervening atoms or molecules. This usually resulted in new three-dimensional materials. However, in certain cases, large molecules could be inserted between atomic planes, providing greater separation, such that the resulting compounds could be considered as isolated graphene layers embedded in a three-dimensional matrix.

Shioyama et al. (2001) and Hirata et al. (2004) demonstrated that one can often get rid of intercalating molecules in a chemical reaction to obtain a sludge consisting of restacked and scrolled graphene sheets.

Graphene was patented two years before the Nobel Prize Prize–winning work of Andre Geim and Kostya Novoselov (2004) by a company called Nanotek Instruments (US patent number 7071258, entitled Nano-scaled graphene plates of 2002, owners, Bor Jang and Wen Huang). This patent includes a sketch of carbon nanotubes unrolling to form graphene sheets and multilayer graphene sheets. It is surprising that this visionary patent is not acknowledged by most graphene researchers today; perhaps because most scientific researchers from the academic world never bother to look into the patent literature, whereas industry leaders tend to follow the scientific literature very closely. We hope that the efforts made in recent years to promote the collaboration between industry and academia would promote the sharing of knowledge.

It is worth mentioning here that Dr. Bor Jang (owner of the first graphene patent) did heaps of work on graphene. He has over forty patents related to graphene production and applications, including the first patent for single layer graphene in 2002 and the first patent on graphene-reinforced metal, glass, carbon and ceramic-matrix composites and single layer graphene-reinforced polymer composites. However, Dr. Jang almost never published scientific papers, for which reason he is almost unknown in academia.

2004 was a golden year for graphene research. There have been a number of efforts to make very thin films of graphite by mechanical exfoliation from 1990 to 2004, but nothing thinner than fifty to 100 layers was produced during these years. In 2004, Andre Geim and Kostya Novoselov at Manchester University, UK, managed to extract single-atom-thick crystallites (graphene) from bulk graphite and transfer them onto thin silicon dioxide on a silicon wafer by a famous Scotch Tape Technique. The idea of using Scotch tape for exfoliating graphene was suggested by Oleg Shklyarevskii, who had been using it to polish the graphite rod of pencils. In this micromechanical Scotch tape exfoliation method, graphene is peeled off from graphite using adhesive tape. Initially multiple-layer graphene gets attached to the sticky tape. Then, folding and peeling the tape several times results in the separation of progressively thinner layers and eventually to a single layer of carbon. To detach the tape, acetone is used. Then one last peeling is performed with unused tape by placing a sample of graphite onto sticky tape. By this method, the best quality of graphene is obtained. However, it is difficult to scale up this method. Other methods, like reduction of exfoliated graphene oxide, are used for scaling up, but the quality of graphene produced is poor. Desorption of Si from SiC or growth on metal both gives good quality graphene and is also scalable.

Though graphene was known earlier, it would not be an exaggeration to write that Geim and Novoselov rediscovered graphene in its new incarnation. Apart from receiving the Nobel Prize in physics in 2010, Geim received several awards for his pioneering research on graphene, including (i) the Mott medal for the discovery of a new class of materials—freestanding two-dimensional crystals—in particular graphene in 2007, (ii) the EuroPhysics Prize (together with Novoselov) for discovering and isolating a single free-standing atomic layer of carbon (graphene) and elucidating its remarkable electronic properties in 2008, (iii) Körber Prize for developing the first two-dimensional crystals made of carbon atoms in 2009 and (iv) in 2010 Geim and Novoselov were granted knighthood.

Geim and Novoselov did the Electric field study of graphene. The silicon beneath the SiO2 was used as a back gate electrode to vary the charge density in the graphene layer over a wide range. Their studies revealed that graphene (monolayer) and even its bilayer have simple electronic spectra; both are zero-gap semiconductors (or zero-overlap semimetals) with one type of electron and one type of hole. For three and more layers, the spectra become increasingly complicated: Several charge carriers appear (Novoselov et al. 2004 and Morozov et al. 2005), and the conduction and valence bands start notably overlapping (Novoselov et al. 2004 and Partoens and Peeters 2006); this led to an explosion of research in synthesis, characterization, properties and research into the potential applications of graphene.

Morozove et al. (2005) and Zhang et al. (2005) suggested that because the screening length in graphite is only ≈5Å (that is, less than two layers in thickness), one must differentiate between the surface and the bulk even for films as thin as five layers. The study allows one to distinguish between single-, double- and few- (3 to <10) layer graphene as three different types of two-dimensional crystals of graphene. Thicker structures are to be considered as thin films of graphite.

In 2005 the Anomalous Quantum Hall Effect was detected, showing the massless nature of charge carriers in graphene by Novoselov et al. (2005) and Zhang et al. (2005). They used a micromechanical cleavage technique that led them to make the first observation of the Anomalous Quantum Hall Effect in graphene. They also observed evidence of the theoretically predicted π Berry’s phase of massless Dirac fermions in graphene. In 1984 DiVincenzo and Mele were the first to point out the massless Dirac equation, and it is known that the magnetic field of an electronic Landau level occurs precisely at the Dirac point. This level is responsible for the anomalous integer quantum hall effect. In 2006, the Anamolous Quantum Hall Effect was observed at room temperature by Novoselov et al. in graphene.

Schedin et al. (2007) were the first to show the detection of a single molecule adsorption event. They demonstrated that μm-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene’s surface. The adsorbed molecules change the local carrier concentration in graphene by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

In 2008 Bolotin et al. demonstrated extremely high carrier mobility in suspended graphene. They could achieve mobilities in excess of 200,000 cm² V–1s–1 at electron densities of ~2 × 10¹¹ cm–2 by suspending single-layer graphene. Suspension ~150 nm above a Si/SiO2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks were reduced by a factor of 10 compared to traditional, non-suspended devices. This advancement allowed for accessing the intrinsic transport properties of graphene.

2009 was a year of initiation of many commercialization activities for graphene, for example:

In June—graphene-Info was launched

In October—IBM researchers used graphene to develop ultra-fast photo detectors

In November—two industrial companies took up graphene work:

– Samsung started research in graphene; and

– Fujitsu entered into the production of graphene transistors at low temperatures

In December—Angstron Materials awarded $1.5 million to develop nanographene platelets

In 2010 Andre Geim and Kostya Novoselov were awarded the Nobel Prize in physics for their work on graphene.

Figure 1.4 Nobel Laureates Andre Geim and Kostya Novoselov receiving the Nobel Prize for physics for their work on graphene

Graphene has a higher carrier mobility than silicon, but lacks a band gap, which has kept the on-off ratio of graphene transistors dismally low—usually less than 10 compared to hundreds for silicon. In January 2010, IBM researchers created a tuneable electrical band gap (up to 130meV) for their bi-layer graphene field-effect transistors (FET) that could someday rival complementary metal oxide semiconductors. According to IBM, this was one of the last roadblocks to commercialization of graphene-based technology.

By February of the same year, IBM developed a 100-GHz graphene RF Transistor, on 2" wafers, which operates at room temperature, and is more than two times faster than silicon transistors with the same gate length (40GHz). IBM’s next aim is to increase the speed of the graphene transistor to 1 THz. The graphene RF transistors were created for the Defence Advanced Research Project Agency under its Carbon Electronics for RF Applications (CERA) program. Transistors were fabricated at the wafer scale using epitaxially grown graphene processing techniques that are compatible with those used to fabricate silicon transistors. Later in 2011, IBM developed a 155 GHz graphene transistor and a 10 GHz graphene based IC.

In September of the same year, UCLA researchers, led by biochemist Xiangfeng Duan, developed a 300 GHz graphene transistor using a nanowire as the self-aligned gate.

In April 2010, the European startup graphenea, with $3.8 million investment, was established to produce graphene, and Xolve, with an investment of $2 million, entered into graphene production.

Samsung, which started research and development on graphene in 2009, managed to fabricate a 30 graphene sheet by roll-to-roll process, and later in 2011, produced a 40 graphene sheet. Samsung is the company that holds the largest amount of graphene patents in the world.

Wei Han et al. (2010) achieved tunneling spin injection from Co into single-layer graphene (SLG) using TiO2 seeded MgO barriers. A non-local magneto-resistance (∆RNL) of 130 Ω at room temperature was observed, which is the largest value observed in any material. Investigating ∆RNL vs. SLG conductivity from the transparent to the tunneling contact regimes demonstrates the contrasting behaviors predicted by the drift-diffusion theory of spin transport. Furthermore, tunnel barriers reduce the contact-induced spin relaxation and are therefore important for future investigations of spin relaxation in graphene.

In 2011 Vorbeck Materials entered into making graphene-based ink to develop a Siren alarm (security) tag, the world’s first graphene-based ink product. By 2012 they started shipping it.

Realizing the importance and potential for application of graphene, many government agencies started investing in graphene research in 2011.

Sweden granted $6 million for graphene research.

The UK government decided to invest £50 million in graphene commercial opportunities. In 2012, the UK gave £21.5 million more for graphene research. In 2013 the UK government funded £5 million pounds for graphene membrane research at the University of Manchester. In the beginning of 2013 (January), the University of Cambridge established a new £12 million graphene center, and signed a co-development agreement with Plastic Logic in August 2013. In the same year, ERDF awarded £23 million to Manchester University’s NGI (National Graphene Insist). Moreover, UK launched a $1.5 million project to develop graphene-filled epoxy resin.

Korea allocated $40 million towards graphene commercialization efforts in 2013.

MIT (USA) opened a new center for graphene devices and systems (MIT-CG).

The EU decided to grant €1 billion for graphene research initiatives over 10 years.

2011 saw another applicable output of graphene. Dickerson’s group created a film of graphene oxide that either causes water to bead up and run off or alternatively be spread out in a thin layer. As graphene sheets are transparent, this film can be used in (a) car windshields, where water will shed so quickly that wipers will not be needed, (b) to make ships glide through water very efficiently, (c) to make water-repellent clothes, or (d) self-cleaning glasses, which were developed by Dickerson’s group.

Researchers at the Korea Advanced Institute of Science and Technology (KAIST) used graphene to create bendable batteries. The researchers developed a graphene-based hybrid electrode and produced a flexible lithium rechargeable battery. The cathode material (V2O5) was grown on a graphene sheet using pulsed laser reposition and the anode was lithium-coated graphene. This battery has promising performance compared to non-flexible batteries—higher energy density, power density and a better cycle life. They also believe that this technology can be used not only in batteries but also in solar cells, OLED displays and catalysis.

2012 and 2013 saw much commercial growth and developments in graphene technology, for example:

Bluestone Global Tech started producing 24 by 300 graphene films on copper.

HEAD developed a graphene racket.

Vorbeck and PNNL started marketing graphene-based lithium-ion batteries; and

Cabot launched a graphene-based additive for high density lithium-ion batteries.

UCLA entered into the commercialization of laser-scribed graphene supercapacitors.

Grafoid started to mass produce affordable high-quality graphene materials called MesoGraf with an investment of $3.5 million.

Durham graphene Science raised £1.2 million to commercialize graphene mass production.

IBM developed a graphene-based Terahertz frequency photonic filter and polarizer.

Sony could produce a 100-meter long graphene sheet with a new R2R method.

The National University of Singapore invested $11 million in a graphene production facility.

Haydale announced the availability of HDPl as graphene-based inks.

Lockheed Martin developed a new graphene-based water desalination technology, which they hope to commercialize by 2014–2015.

China launched production of 15" graphene transparent film.

2012 and 2013 witnessed many milestones in research as well.

A breakthrough in the photovoltaic cell was announced by MIT researchers in 2012. They developed a flexible graphene-based solar panel (flexible, light solar cells): Researchers developed a new approach using graphene sheets coated with nanowires.

This new solar (photovoltaic) cell is made from several graphene sheets coated with nanowires. Here, graphene is used as a replacement for ITO in the solar panels. The new electrode material is cheaper and provides several advantages over ITO: Flexibility, low weight, mechanical strength and chemical robustness. A series of polymer coatings to modify the graphene properties, allowing them to bond a layer of zinc oxide nanowires to it, and then an overlay of a material that responds to light waves, i.e., either lead-sulphide quantum dots or a type of polymer called P3HT, is used. Despite these modifications, graphene’s innate properties remain intact. A solution-based process was used to deposit the zinc oxide nanowires on the graphene electrodes, which can be done entirely at temperatures below 175oC. This process is highly scalable. The graphene itself is synthesized through CVD and then coated with the polymer layers.

Improving solar cells demands (a) increased efficiency of their energy conversion, (b) lowering the cost of manufacturing and (c) now Jeffrey Grossman’s team at MIT are opening another avenue for improvement, aiming to produce the thinnest and most lightweight solar panels possible. They have come up (2013) with a graphene/molybdenum solar cells to achieve ultimate power conversion, which is made from a stack of two one-molecule-thick materials: Graphene and molybdenum disulfide. They claimed that these two sheets together are thousands of times thinner than conventional silicon solar cells.

Singh et al. (2011) of Monash University, Australia, published a report suggesting that graphene coating can make copper almost 100 times more resistant to corrosion. Graphene coating on copper (Cu) increased the resistance of the metal to electrochemical degradation by one and a half orders of magnitude. Detailed electrochemical characterization in aggressive chloride environment study showed that the impedance of Cu dramatically increases and the anodic and cathodic current densities of the coated Cu becomes nearly one to two orders of magnitude smaller when coated with graphene. The observations are counterintuitive, as graphite in contact with metal, increases metallic corrosion. The results are expected to bring paradigm changes in the development of anti-corrosion coatings using conformal, ultrathin graphene film.

Another graphene miracle surfaced in 2012 when a team led by Novoselov showed that graphene can repair itself by fixing holes in its structure if it is exposed to loose carbon atoms.

Seon Jeong Kim’s group in 2012 (Min Kyoon Shin et al. 2012) created the world’s toughest fibers by mixing a polymer sheet with reduced graphene oxide and carbon nano tubes during spinning.

Researchers from the Georgia Institute of Technology managed to create a substantial bandgap in graphene nanoribbons by coating bilayer graphene on silicon carbide nanometer-scale steps. This could lead the way towards graphene-based electronics.

Graphenea has produced up to 50% stronger ceramic by adding graphene to ceramic alumina by a simple, fast and scalable method. Moreover, it makes the alumina a hundred million times electronic more conductive to electricity.

Recently, graphene’s negative resistance–enabled bandgap-less transistor has been designed by Guanxiong Liu and his group from the University of California, Riverside, who developed a graphene-based transistor based on negative resistance rather than trying to open up a bandgap. Negative resistance is the counterintuitive phenomenon in which a current entering a material causes the voltage across it to drop. It was previously demonstrated that graphene demonstrates negative resistance in certain circumstances.

IBM researchers are working on various applications of graphene. They have developed a graphene-based infrared detector, driven by intrinsic plasmons, which is much more photo-responsive compared to non-plasmonic graphene detectors. CVD technique was used to grow graphene on copper foil. The copper was etched away and the graphene sheet was transferred to a silicon/silicon-oxide chip. The researchers patterned graphene ribbons (widths of 80 to 200 nm). Graphene is less useful than a semiconductor to detect light in the visible range, but in the infrared range (and also in the terahertz range) graphene is very good, as its high mobility and zero-gap nature gives it fast optoelectronic response and detection. In April 2013, IBM discovered that plasmons lose their energy very slowly