Graphene is a material consisting of a monoatomic layer of carbon atoms (i.e., having a thickness equivalent to the size of a single atom). It has the theoretical strength of diamond and the flexibility of plastic. As suggested by the ending –ene of the name, the atoms are hybridized in the form sp2, and are then arranged to form hexagons with angles of 120°.
An ideal graphene layer consists exclusively of hexagonal cells; pentagonal or heptagonal structures are defects. In particular, in the presence of an isolated pentagonal cell, the planar graphene layer deforms to a conical shape; if, however, there are 12 pentagonal structures, there is a fullerene. Similarly, the presence of an isolated heptagonal cell causes a deformation that transforms the planar structure into a saddle, and the controlled insertion of pentagonal or heptagonal cells allows the realization of very complex structures. In contrast, the presence of single pentagons or heptagons causes surface ripples.
Single-walled carbon nanotubes can be considered graphene cylinders; sometimes at the ends of these nanotubes are hemispherical structures consisting of graphene sheets containing 6 pentagonal structures, which act as a “cap”.
Discovered in 2004 by scientists Andre Gejm and Konstantin Novoselov, who for this reason were awarded the Nobel Prize for Physics 2010, graphene is the thinnest material in the world and is virtually transparent, has the resistance of diamond, the flexibility of plastic and is an excellent thermal conductor. These characteristics make it usable in a wide variety of fields, from electronics (flexible displays) to aeronautics (sensors, batteries), from medicine (prosthetics, tissue engineering) to space exploration (molecular filters, heat exchangers). The good mechanical and electrical properties of graphene allow it to be used as a nano-additive, to be added to plastics or composite materials to make them more resistant or electrically conductive.
The physics of graphene is determined by the nature of the energy spectrum near the maximum of the valence band and the minimum of the conduction band, the so-called “neutral charge points” that correspond to the six vertices of the hexagon of the Brillouin zone. In these points the bands are in contact: graphene is in fact the only semiconductor material with zero energy gap. Generally, we study the properties of the system in two points of the Brillouin zone, called “Dirac points” and indicated with K and K’, since the other four are equivalent to this pair and are connected to the latter by reciprocal lattice vectors. In the vicinity of the neutral charge points, the bands assume a typical conical shape, and the scattering law shows a linear trend with the wave vector k. This has as a direct consequence that the charge carriers obey the Dirac equation, thus mimicking the behavior of relativistic particles. Its charge carriers are therefore regarded as electrons that have lost their rest mass m0 or, equivalently, as neutrinos that have acquired electronic charge, and are therefore called “massless Dirac fermions”. Graphene thus constitutes a bridge between quantum electrodynamics and solid-state physics. The particular band structure of graphene means that on it are found experimentally electrical and optical effects of particular importance.
Mechanical exfoliation of graphite consists in applying a force to the surface of highly oriented graphite crystals in order to detach and unfold the crystalline layers until the single layer is obtained. The first attempts were made as early as 1998, when the interaction of AFM (atomic force microscope) and STM (tunnel effect microscope) analysis tips with the graphite surface was exploited to provide enough energy to overcome the inter-plane attraction forces and lead to the removal and isolation of the crystalline monoatomic layer.
Later, André Geim’s group of researchers developed a very simple method, universally known as the scotch-tape method, which uses simple tape to exfoliate graphite. The technique involves placing the surface of a graphite crystal on the tape, peeling off the tape, and thus peeling off a few layers of material. The tape with the graphite imprint is then folded back on itself and unwound several times. Each time, the deposited flakes split into thinner and thinner layers. At the end of the process, the thin adhered flakes can be easily transferred to an insulating substrate.
Mechanical exfoliation is the simplest and most accessible method for isolating graphene flakes as small as a few square microns, which is useful for basic research into its properties. Unfortunately, this method is not suitable for industrial production.
Liquid phase exfoliation
The method is based on the use of pressure forces generated within a liquid. Graphite powder is mixed with a solvent having the appropriate physical qualities such as viscosity, surface tension, etc. (typically 1-methyl-2-pyrrolidone) or in a mixture of water and surfactant.
The suspension is then subjected to mixing via ultrasonic waves, or high shear force mixer, or ball mill, etc. These processes create both shear and cavitation forces within the liquid that cause the graphite crystals to break down according to the basal plane, reducing them to increasingly thin sheets and, ideally, single sheets of graphene.
The resulting suspension from the process is then purified by ultracentrifugation. This method appears to be one of the most promising from the point of view of scalability, and allows large amounts of excellent graphene to be obtained. In contrast, the flakes turn out to be rather small in lateral size.
Graphene oxide reduction
To date, efforts have been directed primarily toward exfoliation of graphite oxide and subsequent reduction to graphene. Graphite oxide is a material having the same lamellar structure as graphite in which, however, some carbon atoms have bonds with oxygen in the form of hydroxyls (-OH) or carbonyls (C=O) or, more rarely, carboxyls, and in which the distance between graphene layers increases due to the oxygen clutter.
Its strongly hydrophilic nature makes it possible to achieve, using ultrasonic acoustic waves, the intercalation (i.e., reversible inclusion of molecules within other molecules or groups) of water molecules and, consequently, an almost complete exfoliation (~90%) of the graphene oxide material. Graphene is subsequently synthesized by reduction of graphene oxide.
Both chemical reduction methods (by hydrazine N2H4, hydroquinone, sodium boron hydride, or even vitamin C) and thermal or UV methods have been successfully tested, yielding materials with conductivities in the range of 102 S/cm. Chemical synthesis of graphene, via graphene oxide reduction, is a methodology that has the advantage of high yields and ample opportunity to carry out the process on a large scale.
However, the quality of the product from chemical synthesis is rather poor, due to partial reduction of graphene oxide and abundance of defects in the crystal lattice, which makes the product more suitable for applications that do not strictly require qualitative graphene, such as use in polymer composites.
Periodically stacked graphene and its insulating isomorph provide a fascinating structural element in the implementation of highly functional superlattices at the atomic scale, which offers possibilities in the design of nanoelectronic and photonic devices. Various types of superlattices can be obtained by stacking graphene and its related forms. The energy band in layered superlattices turns out to be more sensitive to the barrier width than that in conventional III – V semiconducting superlattices. When more than one atomic layer is added to the barrier in each period, the coupling of the electronic wavefunctions in neighboring potential wells can be significantly reduced, which leads to degeneration of the continuous subbands into quantized energy levels. When the width of the well is varied, the energy levels in the potential wells along the L-M direction behave distinctly from those along the K-H direction.
A superlattice corresponds to a periodic or quasi-periodic arrangement of different materials and can be described by a superlattice period that gives a new translational symmetry to the system, affecting their phonon dispersions and consequently their thermal transport properties. Recently, uniform monolayer structures of graphene-hBN have been successfully synthesized via lithographic schemes coupled with chemical vapor deposition (CVD). Furthermore, graphene-hBN superlattices are ideal model systems for the realization and understanding of coherent (wavelet) and incoherent (particle) phonon thermal transport.
Applications of graphene
Graphene, referred to as the wonder material, has aroused the enthusiasm of researchers who are now actively trying to explore its full potential in different application areas.
One of the first applications successfully investigated is the preparation of polymer nano-composites for which extraordinary improvements in several properties such as electrical conductivity, thermal stability, elastic modulus or tensile strength are observed, following the inclusion of graphene or other graphene-based nano-structures in the polymer matrix.
The use of graphene in electronics is very promising due to the high mobility of charge carriers and low noise, peculiarities that can be well exploited in the fabrication of high-performance field-effect transistors (FETs). In February 2010, a graphene FET fabricated on a 2-inch wafer with a cutoff frequency of 100 GHz was announced; starting from a graphene bilayer, a dual-gate FET with an on/off ratio of about 100 at room temperature and equal to 2,000 at 20 °K was also realized.
The zero bandgap makes graphene lose an essential requirement in the field of digital electronics. One way to overcome this problem is to use graphene nanoribbons (GNRs). GNRs possess bandgaps of sufficient size for applications in digital electronics; the width of the bandgap is related to both the width of the ribbon and the geometry of its edges (zigzag or armchair), however, it is difficult to obtain at the atomic scale the control necessary to fabricate GNRs of precise width and direction. Recently, starting from molecular precursors, however, the possibility of growing GNRs on metallic substrates with atomic precision has been demonstrated.
Graphene is extremely interesting in applications where device operation can be achieved by alternative charge transport mechanisms to classical ones.
The excellent electrical conductivity and high optical transparency of graphene make it an ideal candidate for the realization of transparent and conductive electrodes, with important spin-offs in optoelectronics and photovoltaics.
The combination of its mechanical and electrical properties allows its use in flexible, bendable and stretchable electronics.
The saturable absorption property has relevance for possible applications in the field of lasers and ultrafast photonics. The fabrication of devices that exploit clean energy sources could benefit from the properties of graphene, which, in fact, is already used as an electrode for rechargeable lithium-ion batteries and in ultracapacitors. There are numerous studies on the use of graphene for hydrogen storage in fuel cells.
The possibility of using graphene as a metal replacement in Schottky junctions has also been evaluated. Graphite has already been usefully employed in this application in combination with a wide range of semiconductors, including Si, GaAs, and 4H-SiC; the Schottky barrier formed at the graphite/semiconductor interface is extremely robust and offers many advantages over that determined by traditional metals. Due to the strong bonding between carbon atoms and the relatively small size of this atom, no carbon migration occurs in the semiconductor, thus preserving all the rectifying properties of the barrier. In addition, unlike metal whose Fermi level is fixed, graphite can be doped, which allows the height of the Fermi energy to be modulated and consequently that of the Schottky barrier as well. Finally, because it is not a heavy metal, graphite is not toxic. Replacing graphite with graphene would add to all these advantages also the resolution of the problem that so far has limited the use of Schottky devices in photovoltaics, namely the absorption of incident light by the frontal metal layer: in this case graphene, with its high transparency, allows the incident light to pass almost unchanged, thus opening the way to the use of Schottky devices in photovoltaics.
The exceptionally high ratio of surface area to volume finally led the scientific community to investigate the potential of graphene in the field of gas detection; in fact, the ability to detect the presence of even a single interacting molecule has already been demonstrated in the work of Schedin et al. (Detection of individual gas molecules adsorbed on grapheme, Nat. Mater. 6, 652-652, 2007) After this pioneering paper, subsequent works have verified the high sensitivity of graphene at room temperature to a wide range of analytes; however, this material is not exempt from the chronic problem of solid-state chemical sensors operating at room temperature, i.e., slow analyte desorption, low selectivity, and poor electrical stability under ambient conditions. So far, however, the realization of the chemical sensor based on the single graphene flake is still difficult to achieve due to the complexity of the whole process, starting from the synthesis and/or isolation of graphene to its introduction into the device architecture. To date, several works involving the fabrication of gas sensors use a much more easily manageable material such as reduced graphene oxide as the sensing layer.