GRAPHENE BASED COATINGS

Graphene is strictly two-dimensional material with exceptionally high crystal and surface quality.Due to the combination of unique electronic, optical and mechanical properties, graphene has already been proposed for a number of potential applications, including different fields in electronics, high-energy physics, anti-corrosion coatings. Graphene is also stronger and stiffer than diamond, and its surface area is the largest known for its weight. Coatings made with graphene—the thinnest, strongest material ever made—have been found conduct heat and electricity as well as copper and help in preventing UV degradation and corrosion. The high electrical conductivity and high optical transparency make graphene a candidate for transparent conducting electrodes commonly found in displays, touchscreens, liquid crystal displays, organic photovoltaic cells, and organic light-emitting diodes. Compared to the brittle indium tin oxide, graphene is preferable for its mechanical strength and flexibility.Driven by the huge expectation for the applications, considerable research efforts are devoted to the growth and synthesis processes. CVD techniques provide a promising route for the scalable growth of large area continuous, transparent, and highly conducting few-layered graphene films. Although only single layer large defect-less crystal should be qualified as Graphene, the synthesized materials are often generically referred to as Graphenes or as Few Layer Graphenes (FLG), a material equally promising for many applications. Research focuses on large graphene crystals growth, aiming both at reducing the number of graphene layers to one and at increasing crystal quality and size. At ENEA laboratories a CVD reactor has been implemented with a fast cooling system for the graphene layer growth. In this process, carbon-bearing gaseous species react at high temperatures in presence of a metal catalyst, which serves both in the decomposition of the carbon species and in the nucleation of the graphene lattice. The mechanism of graphene growth on metals is influenced by several factors, including the carbon solubility limit in the metal, its crystal structure, lattice parameters and thermodynamic parameters such as the temperature and pressure of the system. Large area graphene synthesis using Cu catalyst has received widespread attention since it was first reported in 2009 [1]. Compared to other metal substrates, copper, besides having a very low carbon solubility, when annealed at temperature close to its melting point, experiences grain size growth needed for the achievement of large uniform graphene domains. Graphene films grown by thermal CVD of methane or ethanol on copper substrates, by varying process parameters, have been transferred on appropriate substrates to be characterized as conductive coatings.SEM analysis were performed to assess the morphology characteristics, while Raman spectroscopy was used to study the quality of the films in terms of homogeneity and number of layers. Sheet resistance and conductivity of the graphene films were measured by four-points probe measurements.Graphene films with area of the order of 1 cm2 have been grown and transferred to SiO2/Si substrate by a wet procedure consisting in four steps: i) dissolution of the copper substrate in acid solutions ii) subsequent “scooping” of the floating graphene film with the help of any flat substrate exhibiting large wettability iii) washing into distilled water rinsing solution in order to remove acid bath residues iv) drying. First we present the results obtained on one of the sample grown by methane CVD that exhibited the best figure in term of number of layer.In fig 1 the SEM micrograph of the sample shows a continuous film composed of regions with different brightness. At least four regions with different contrast are evidenced. The number of graphene layers determines the brightness of the different regions: the thinner the film, the brighter the color (due to the lower electron absorption).Micro Raman analysis coupled with optical microscopy allowed to evidence that the graphene film consisted of a mixture of regions more o less thick, as suggested by the different brightness, as shown in fig 2 (a). The Raman spectrum measured on one of the brighter zones (Fig. 2(b)), indicated on the figure by a black arrow, showed a IG/I2D ratio of 0.55 and a relatively narrow 2D linewidth of 42 cm-1, that corresponds to less than 3 layers [2]. Values ranging from 0.5 to 1.9 were found in the other regions of the same sample. The four-points probe measurements demonstrated linear voltage-current (V-I) characteristics for all the tested samples. Figure 3 shows the V-I curves for specimens grown with the same parameter setting in methane or ethanol atmosphere. It results that the sheet resistance is equal to about 900 Ω/□ which corresponds to an effective electrical conductivity of 5.56•105 S/m, assuming an average film thickness of 2 nm.