Graphene: Synthesis, bio-applications, and properties

Abstract

The properties of graphene, carbon sheets that are only one atom wide, have led researchers and companies to consider its synthesis, properties, and the applications in numerous fields. High-quality graphene is physically powerful, light, nearly transparent, and an exceptional conductor of heat and electricity. Its interactions with other materials and with light and its naturally two-dimensional nature produce unique properties, such as the bipolar transistor effect, ballistic transport of charges, and large quantum oscillations. The following review introduces the many potential applications of graphene.

Keywords::

• bipolar
• conductor
• grapheme
• two-dimensional nature

Introduction

Graphene is a wonder material with many superlatives to its name. It is the thinnest material in the universe and the strongest ever measured. Graphene is a two-dimensional material with layers of carbon atoms that the six pieces of the rings are that together have been (Geim 2009). Graphene, the latest member of carbon allotropes, is truly the basic building block of other important carbon allotropes, including 3D graphite, 1D carbon nanotubes (CNTs), and 0D fullerene (C60) (Figure 1). Therefore, there has long been an interest of many earlier theoretical studies. Previous experimental works related to it are mostly about graphite intercalation compounds and graphite oxide (Dreyer et al. 2010).

Figure 1. Graphene is the basic building block for other carbon allotropes.

Graphene is an exciting material (Geim and Novoselov 2007). It has a large theoretical specific surface area (2630 m²g^{− 1}), high intrinsic mobility (200,000 cm² v^{− 1} s^{− 1}) (Bolotin et al. 2008, Morozov et al. 2008), high Young's modulus (1.0 TPa) (Lee et al. 2008) and thermal conductivity (5000 Wm^{− 1} K^{− 1}) (Balandin et al. 2008), and its optical transmittance (97.7%) and good electrical conductivity merit attention for applications such as for transparent conductive electrodes (Li et al. 2009), among many other potential applications (Zhu et al. 2010).

The most general preparation method of graphene starts from the oxidation and exfoliation of graphite to graphene oxide (GO) (Park and Ruoff 2009, Park et al. 2009). The structure of graphene oxide sheets consists of basal plane of the sheet decorated with hydroxyl and epoxy functional groups, carbonyl groups are present not only as carboxylic acids along the sheet edge but also as organic carbonyl defects within the sheet (Stankovich et al. 2007, Lerf et al. 1998).

Throughout the past half decade, graphene has been studied in the context of many applications, such as polymer composites, energy-related materials, sensors, “paper”-like materials, field-effect transistors (FETs), and biomedical applications, due to its excellent electrical, mechanical, and thermal properties (Geim and Novoselov 2007, Park and Ruoff 2009). Graphene chemistry has the possibility of playing an ever more important role in future developments, such as stoichiometric derivatives that offer a way to control the electronic structure, which is of interest for many applications including electronics. The reduction of electrically insulating graphene oxide, which is exfoliated from graphite oxide (GO), and use of the colloidal suspensions of reduced graphene oxide are among the most promising ways to produce electrically conducting graphene-based platelets on a large scale (Park and Ruoff 2009, Park et al. 2009, Li et al. 2008, Niyogi et al. 2006, Si and Samulski 2008), and therefore its potential in composites (Stankovich et al. 2006, Wang et al. 2010, Liang et al. 2009), paper-like materials, and thin films (Chen et al. 2008, Dikin et al. 2007); as substrates (Lee et al. 2010, Kim et al. 2010); as a coating layer (Han et al. 2010); and as transparent conductive films (Watcharotone et al. 2007, Eda et al. 2008).

High yields of GO monolayers, 0.78 nm in thickness, can be readily prepared by treatment of graphite under strongly acidic and oxidizing conditions (Hummers and Offeman 1958, Patil et al. 2009). The GO single sheets can be readily dispersed in water and mounted onto substrates using spin-, dip-, and spray-coating techniques, and reduced in situ to produce isolated graphene monolayers or transparent 2D films (Watcharotone et al. 2007, Gómez-Navarro et al. 2007, Wang et al. 2008a), with conductivities ranging from as low as 2 S cm^{− 1} (Gómez-Navarro et al. 2007), or 23 S cm^{− 1} (Schniepp et al. 2006), to a value of 550 S cm^–1 for a 10 nm-thick film (Gómez-Navarro et al. 2007), as good as with polycrystalline graphite (1250 S cm^–1) (Patil et al. 2009).

Properties

Graphene, a single layer of carbon atom with a honeycomb structure, has attracted intense attention in the fields of physical, chemical, and biological sciences since its discovery in 2004 (Geim and Novoselov 2007, Sensale-Rodriguez et al. 2011). It has been reported that graphene has very high stiffness and breaking strength. The values measured correspond to a Young's modulus of E = 1.0 TPa and intrinsic strength of 130 GPa, which would suggest that graphene is the strongest material ever measured (Ren et al. 2011). It also reported that graphene has a very high thermal conductivity, measured between 4.84 and 5.30 kW m^{− 1} K^{− 1} (Jeong et al. 2009). The carrier mobility and electron density of graphene have also been measured to extremely high values, 200,000 cm² V^–1 s^–1 and 2 × 1011 cm⁻², respectively.

Optical properties

The high-frequency conductivity for Dirac fermions in graphene has been stated to be a constant, equal to π e²/ 2 h, from the infrared through the visible range of the spectrum (Peres et al. 2006, Gusynin et al. 2007). Inter-band optical transitions in graphene have been probed by infrared spectroscopy, and gate-dependent optical transitions have been reported (Wang et al. 2008b). Due to its unique conical and symmetric band structure, graphene has been proposed for a rich array of novel devices and applications. Its optical properties have also been extensively explored; on the other hand, to date, there are only a few studies of graphene-based THz devices in the literature, including emitters, detectors, and nano-antennas (Otsuji et al. 2010, Kawano and Ishibashi 2010, Rudin 2009, Sun et al. 2009).

Electronic properties

The most explored aspect of graphene physics is its electronic properties. In spite of being recently reviewed (Geim and Novoselov 2007, Neto et al. 2009), this subarea is so important that it necessitates a short update. From the most general perspective, several features make graphene's electronic properties truly unique and different from those of any other known condensed matter system. Essentially discussed is of course graphene's electronic spectrum. Electron waves propagating through the honeycomb lattice completely lose their effective mass, which results in quasi-particles that are described by a Dirac-like equation rather than the Schrödinger equation (Geim and Novoselov 2007, Neto et al. 2009). The latter—so successful for the understanding of quantum properties of other materials—does not work for graphene's charge carriers with zero rest mass. Electron waves in graphene propagate within a layer that is only one atom thick, which makes them accessible and amenable to various scanning probes, as well as sensitive to the proximity of other materials such as high-κ dielectrics, superconductors, and ferromagnetics. This feature offers many enticing possibilities in comparison with the conventional 2D electronic systems (2DES). Graphene exhibits an astonishing electronic quality. Its electrons can cover submicron distance without scattering, even in samples placed on an atomically rough substrate, covered with adsorbates, and at room temperature. Finally, due to the mass-less carriers and little scattering, quantum effects in graphene are robust and can survive even at room temperature (Geim 2009). An application of semi-classical analysis to graphene quantum electron dynamics is demonstrated for one important problem of constructions of semi-classical approximation of Green's function in electronic waveguide inside graphene structure with linear potential and omogeneous magnetic field. The problem can be described by the following 2D Dirac system with magnetic and electrostatic potentials in axial gauge A = 1/2B(−x2, x1, 0) (Neto et al. 2009).

$${\upsilon }_{\text{F}}<-iħ\nabla +\frac{e}{c}A,\overline{\sigma }>\Psi (x)+U(x)\Psi (x)=E\Psi (x),\Psi (x)=\left(\begin{array}{c}u\\ v\end{array}\right),$$

Where x = (x1, x2), u, v are the components of spinor wave function describing electron localization on sites of sublattice A or B of honeycomb graphene structure, e is the electron charge, c is the speed of light, and v_F is the Fermi velocity, the symbol <,> means a scalar product, and σ = (σ₁, σ₂) with Pauli matrices (Geim 2009).

$${\sigma }_1=\left(\begin{array}{cc}0& 1\\ 1& 0\end{array}\right),{\sigma }_2=\left(\begin{array}{cc}0& -i\\ i& 0\end{array}\right).$$

Antibacterial properties

Researchers have made the unexpected finding that graphene-based nanomaterials possess excellent antibacterial properties. So far, biological studies of graphene have been relatively limited. Graphene oxide, graphene oxide, and reduced graphene oxide can effectively inhibit bacterial growth. Chunhai Fan, a professor in the Laboratory of Physical Biology at the Shanghai Institute of Applied Physics, tells Nanowerk, “This is a significant finding as both previous and our own studies have proven that graphene, particularly graphene oxide, is biocompatible and cells can grow well on graphene substrates.”

Mechanical properties

The mechanical properties of monolayer graphene including the Young's modulus and fracture strength have been investigated by numerical simulations such as molecular dynamics (Zhu et al. 2010).

Thermal properties

Since the carrier density of nondoped graphene is relatively low, the electronic contribution to thermal conductivity (Wiedemann–Franz law) is negligible. The thermal conductivity (κ) of graphene is thus dominated by phonon transport, namely diffusive conduction at high temperature and ballistic conduction at sufficiently low temperature (Yu et al. 2005).

Synthesis

Several methods have been proposed to obtain single-layer or few-layer graphene (FLG) at large scale; however, the methods proposed so far are not scalable, produce thick graphite, or highly defective graphene layers, or the cost of graphene production is so high that it becomes prohibitive for mass production (Forbeaux et al. 1998, Viculis et al. 2003, Wu et al. 2004, Gilje et al. 2007).

Chemical vapor deposition

Chemical vapor deposition is a simple, scalable, and cost-efficient method to prepare single- and few-layer graphene films on various substrates; it opens a new route to large-area production of high-quality graphene films for practical applications (Figure 2).

Figure 2. Complete apparatus set-up for chemical vapor deposition of graphene (De Arco et al. 2011).

Synthesis of graphene on Ni supported on Si/SiO₂ wafers facilitated the breakthrough approach for large-scale graphene production; particularly because Ni films provide an excellent geometrical fit of the ordered graphene/graphite phase of carbon to the crystalline metal surface as well as convenient interactions that favor bond formation between carbon atoms at specific conditions (Zhang et al. 2010, Eizenberg and Blakely 1979; Figure 3).

Figure 3. Ni/SiO₂/Si wafers and copper foils are loaded in the CVD graphene system (De Arco et al. 2011).

High temperature-high pressure technique

Graphene has been produced by a high pressure-high temperature (HPHT) growth process from the natural graphitic source material by utilizing the molten Fe–Ni catalysts for dissolution of carbon. The method may lead to a more reliable graphene synthesis and facili- tate its purification and chemical doping (Parvizi et al. 2008).

Chemical oxidation of graphite

The most popular method employed for the synthesis of graphene is chemical oxidation of graphite (Dreyer et al. 2010). This method involves oxidation of graphite to GO using highly oxidizing reagents and subsequently reducing GO to graphene using various reductants (Figure 4). Chemical oxidation of graphite is an established method using concentrated acids (sulfuric acid, nitric acid, and phosphoric acid) and highly oxidizing agents (potassium permanganate and potassium perchlorate) (Ming 2010). Since the 19th century, graphite oxide has been produced by Hummer's method, which typically involves oxidation of graphite in the presence of strong oxidants and acids for extended periods (Su et al. 2009).

$${\text{KMnO}}_4+3{\text{H}}_2{\text{SO}}_4\to {\text{K}}^{+}+{\text{MnO}}_3^{+}+{\text{H}}_3{\text{O}}^{+}+3{\text{HSO}}_4^{-}$$$${\text{MnO}}_3^{+}+{\text{MnO}}_4^{-}\to {\text{Mn}}_2{\text{O}}_7$$

Figure 4. Schematic image of the oxidation and thermal exfoliation process used to derive graphene sheets from graphite (Rafiee et al. 2010).

Formation of dimanganeseheptoxide from KMnO₄ in the presence of strong acid (Koch 1982).

Silicon carbide sublimation

Silicon carbide (6H-SiC) is heated between 1080°C and 1320°C, leading to the growth of graphene. The face of the SiC used for graphene creation, Siterminated or C-terminated, highly influences the thickness, mobility, and carrier density of the graphene (Charrier et al. 2002).

Unfolding CNTs

Graphene ribbons are produced by cutting and unfolding CNTs via chemical etching (KMnO₄-H₂SO₄) or plasma etching. At this scale, the unzipping has been accomplished by harsh acids and the right thermodynamic conditions (Baraton et al. 2011; Figure 5).

Figure 5. Modeling illustration of the SWCNT unzipped into monolayer graphene (Baraton et al. 2011).

Applications

Graphene, with its unique combination of physical and electronic properties, is a very promising material not only for electronics, but also for biosensor and bioelectronic applications (Figure 6).

Figure 6. Various applications of graphene and graphene oxide (Bae et al. 2012).

Transistors

A quantitative understanding of the sensitivity of graphene solution gated field effect transistors (SGFET) capable of operation in aqueous environments is important (Ferrari et al. 2006). The transistor conductance can be significantly altered (10⁶ on–off ratio at RT) by a reversible chemical modification (Echtermeyer et al. 2008).

Biomedical applications

In 2008, from the first study on graphene for biomedical applications, much remarkable work has been carried out to explore the use of graphene for widespread biomedical applications, ranging from drug/gene delivery, biological sensing and imaging, antibacterial materials, to biocompatible scaffold for cell culture (Guo and Dong 2011, Jiang 2011).

Biofunctionalization with DNA

The large 2D aromatic surface of graphene makes it an ideal substrate for adsorption of certain biomolecules. Importantly, ssDNA can be strongly interfaced onto the graphene surface, the tethering process of ssDNA on graphene is especially on thicker sheets (− 8 nm), and on wrinkled rather than flat surfaces of chemically modified graphene. Patterning ssDNA on graphene layers increases the hole density in the graphene layer in the order of 5.61 × 10¹²/cm² (Mollazade et al. 2013).

Graphene-based FRET biosensors

There is increasing attention toward the use of graphene for the development of FRET biosensors. FRET involves the transfer of energy from a donor fluorophore to an acceptor fluorophore and is one of the advanced tools available for measuring nanometer-scale distance and changes, both in vivo and in vitro (Selvin 2000).

Characterization

XRD theory and concept of interlayer distances of graphene

The principle of XRD for materials is based on Bragg's law, and it is expressed by

$$n\lambda =2d(hkl)\sin \theta$$

where λ is the wavelength of the X-ray, θ is the scattering angle, n is an integer representing the order of the diffraction peak, d is the interplane distance of the lattices, and (hkl) are Miller indices. The XRD peak of the (002) facet of GP gives rise to a critical d₀₀₂ value as well as the information required for the lattice size and quality (Figure 7). The GP thickness can be estimated using Sherrer's equation (Ahmadi et al. 2014, Nejati-Koshki et al. 2013, Pourhassan-Moghaddam et al. 2013, Mollazade et al. 2013, Rezaei-Sadabady et al. 2013, Valizadeh et al. 2012, Akbarzadeh et al. 2012a, 2012b, 2012c, 2012d, Akbarzadeh et al. 2013), which is expressed by D₀₀₂ = Kλ/Bcosθ, where D₀₀₂ is the thickness of crystallite (here, GP thickness), K is a constant dependent on the crystallite shape (0.89), λ is the X-ray wavelength, B is the full width at half maximum (FWHM), and θ is the scattering angle (Tang et al. 2012).

Figure 7. XRD spectra of graphite and graphene (Huh 2011).

AFM

AFM studies have shown “flat” 10-nm domains of 0.5 nm roughness; other regions can show bowing and bending, with deviation from a hexagonal structure. Graphene is unusually susceptible to a Fermi-level shift because of chemical doping, which can result from edge chemical functionalization in finite-size pieces, and/or from charge transfer by adsorbed or bound species (Liu et al. 2008; Figure 8).

Figure 8. AFM images of oxidized single-layer (1L) and double-layer (2L) graphene.

Raman spectra

The Raman spectra of graphene includes the G peak located at 1580 cm^{− 1} and 2D peak at 2700 cm^{− 1}, caused by the in-plane optical vibration (degenerate zone center E 2 g mode) and second-order zone boundary phonons, respectively. The D peak, located at 1350 cm^{− 1} due to first-order zone boundary phonons, is absent from defect-free graphene, but exists in defected graphene. Raman spectra could be used to distinguish the “quality” of graphene and to determine the number of layers for n-layer graphene (for n up to 5) by the shape, width, and position of the 2D peak (Ferrari et al. 2006; Figures 8 and 9).

Figure 9. Comparison of Raman spectra at 514 nm for bulk graphite and graphene. They are scaled to have similar height of the 2D peak at 2700 cm^{− 1} (Ferrari et al. 2006).

Authors’ contributions

MK conceived the study and participated in its design and coordination. AA and MM participated in the sequence alignment and drafted the manuscript. EA helped in drafting the manuscript. All authors read and approved the final manuscript.

Acknowledgments

The authors thank the Department of Medical Nanotechnology, Faculty of Advanced Medical Science of Tabriz University, for all the support provided.

Declaration of interest

The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.