Objetivo 1 La socio génesis de la representación

The relatively simple, two-dimensional structure of graphene is the primary reason that graphene monolayers were not successfully isolated until a decade ago. Prior to 2004, graphene was viewed as a theoretical, two dimensional macromolecule, with the assumption that the stability of the carbon lattice was insufficient to maintain the planar configuration of a graphene sheet.33 This understanding was transformed with the release of Geim and Novoselov’s seminal work documenting the first successful fabrication of graphene17, for which they received the Nobel Prize in Physics in 2010. The method continues to be used in preparing pristine graphene for small-scale research, and is known as mechanical exfoliation.

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Mechanical exfoliation is a technique that utilises the intermolecular forces acting on a graphene sheet in order to remove graphene from bulk graphite. In particular, it requires balancing the cohesive forces between carbon layers in graphite and the adhesive forces used to delaminate graphitic layers. Novoselov et al. used common adhesive tape in order to overcome the weak cohesive forces (300 nN/µm2) required to exfoliate layers of graphene from graphite.17 Using a stick-and-peel process, the researchers repeatedly removed graphitic multilayers from a piece of highly ordered pyrolytic graphite (HOPG) attached to photoresist until a graphene monolayer sample was obtained. The graphene flakes were then transferred to a Si / SiO2 substrate by dissolving the photoresist with acetone and floating the graphene onto the substrate. A silicon oxide layer of 300 nm provided suitable contrast to identify the graphene films manually using polarised optical microscopy. In this manner, the mechanical exfoliation of graphene from bulk graphite was achieved, providing a simple, effective method of preparing defect-free graphene samples.

However, the mechanical exfoliation method used to produce graphene presents a number of disadvantages. The mechanical exfoliation of graphene commonly produces in graphitic films of non-uniform thickness and size. Given the irregularity of the particles both in size and quantity, this method could also prove unfavourable when attempting to locate the graphene films on the substrate, by contributing to low yields and a labour intensive process overall. Furthermore, production of graphene using mechanical exfoliation requires the sample to be floated using an organic solvent or transferred to a substrate during the production process. The presence of a solid or volatile transfer medium severely limits practical considerations such as storage, handling and methods of application, and may also be incompatible with further

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manufacturing and processing stages. Therefore, the mechanical exfoliation of graphene is generally unsuitable for implementation in mass production processes.

3.3.1

Current Methods of Graphene Synthesis and Production

Despite the introduction of the mechanical exfoliation method over a decade ago, a solution to preparing defect free graphene in commercial, large-scale quantities at low cost still remains elusive. Consequently there is a great amount of interest in graphene processing techniques within the scientific community, with a wide variety of methods being presented in the literature. The majority of these methods can be divided into approximately a dozen broad categories. Of these methods, three main techniques have the potential to be suited to the fabrication and manipulation of graphene on a commercial scale: chemical vapour deposition (CVD), growth on silicon carbide and liquid phase exfoliation.

3.3.1.1 Chemical Vapour Deposition (CVD)

Chemical vapour deposition is a well-established, “bottom-up” technique used to generate large, individual, high-quality graphene sheets. It involves the decomposition and deposition of hydrocarbon precursors onto transition metal substrates in order to synthesise graphene layers. In one of the early investigations involving this technique, nanometre-sized graphene layers were deposited onto a Pt (1 1 1) substrate through the decomposition of ethylene.34 Graphene has been deposited on a wide range of transition metal substrates including Ni35, Pd36, 37, Ru38, 39, Ir40, and Cu2, 3, 41 by decomposing hydrocarbons such as methane42-44, benzene45, carbon monoxide, ethanol41 and actetylene46 precursors. Although CVD typically produces graphene layers of slightly lesser structural quality in terms of lattice structure that those produced using the mechanical exfoliation method, it is capable of generating large areas of uniformly

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structured, polycrystalline graphene.3 For example, large area graphene sheets up to 30 inches in size have been deposited using CVD onto thin copper substrates.47 These layers can be precision patterned, and thus have great potential in applications requiring large area, high quality, mass produced graphene components such as nano- and microelectronics, transparent conductive layers, sensors and flexible opto-electronic devices.48 However, a number of disadvantages remain with the technique, including difficulties in controlling grain size, ripples in the graphene layers, as well as the number of graphene layers deposited.3 The CVD graphene synthesis process is also generally expensive due to high energy demands compared to other production methods, and can require the removal of the substrate, typically through complex transfer processes. Consequently, graphene produced using CVD is not appropriate for use in all large-scale commercial applications.

3.3.1.2 Growth on Silicon Carbide

The growth of graphene on silicon carbide is another “bottom-up” method of producing graphene with the potential to be used for large-scale fabrication and manipulation. This process centres on the thermal decomposition of a silicon carbide substrate under ultra-high vacuum, and involves the sublimation of silicon atoms from the substrate, followed by reorganisation of the carbon-rich surface to produce graphene layers. The size of the graphene sheets is limited only by the size of the supporting substrate. Meanwhile, the thickness, charge carrier mobility and carrier density of the resulting graphene sheets are largely influenced by the surface properties of the SiC substrate.49 Nevertheless, graphene grown on silicon carbide generally exhibits high quality lattice structures, as evidenced by high charge carrier mobilities that are slightly lower than that of mechanically exfoliated graphene.50 It has also been shown that the growth of graphene layers on silicon carbide provides greater control over the number of graphene

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layers compared to the CVD method.51 However, despite these advantages, significant challenges remain with implementing the technique in large-scale commercial processes. These include the high cost of the SiC substrates, as well as the extreme temperatures necessary to produce the graphene layers. In particular, the decomposition of silicon carbide typically requires temperatures in excess of 1000 °C, which are generally not compatible with the use of silicon carbide as a semiconductor material in high-power electronics. Thus, the ability to grow high quality, single layer graphene on SiC substrates has great potential in niche electronic applications, yet is unsuitable for industrial applications that require large-scale fabrication and manipulation of graphene.

3.3.1.3 Liquid Phase Exfoliation

Liquid phase exfoliation comprises a family of techniques with the ability to generate large-scale quantities of mono- and few layer graphene particles in suspension.52-54 It involves exposing bulk graphite powder to a liquid in which an increase in the total surface area of graphite crystals is favoured. Ultrasonication is generally then applied to encourage exfoliation of the graphitic sheets from the bulk graphite. Exfoliation of graphitic layers can also be initiated by less common methods such as rapid depressurisation in a supercritical fluid55, electrochemical techniques56, high-speed laminar flows57, as well as by spontaneous, self-exfoliation58. Centrifugation is often performed following exfoliation in order to separate the multilayer residues from the mono-, bi and few-layer graphitic sheets. Thus, liquid phase exfoliation can result in the production of stable suspensions enriched with pristine graphene nanosheets.

Graphene suspensions produced through liquid phase exfoliation possess a number of important advantages and limitations. One of the most significant advantages of this technique compared with other graphene production methods is its ability to reliably

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manufacture monolayer graphene in scalable quantities, a feature essential for large- scale industrial implementation. Indeed, bulk quantities of graphene generated using liquid phase exfoliation techniques are currently available on the tonne scale commercially.2 Graphene particles dispersed in the liquid phase also have the advantage of being transferred to substrates conveniently, with liquid phase exfoliated graphene being found to be compatible with a variety of industrial processes including spin coating, spray coating59, drop casting60, and immersion dip coating61. However, liquid phase exfoliation often results in graphitic layers of varying lateral size and thickness, producing layered materials with overall lower quality than those produced using CVD and SiC supported growth. Consequently, this method is generally not suited to the fabrication of high-performance electronic components or semiconductor devices, but rather, is finding use in applications such as conductive inks62, paints, functional coatings63, reinforced composites53 and hierarchically structured bulk materials.