Near Room-temperature Synthesis of Transfer-free Graphene Films

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Near Room-temperature Synthesis of Transfer-free Graphene Films
Kwak, Jinsung
Kwon, Soon-Yong
Graphene; Nickel; Grain boundary; Carbon diffusion
Issue Date
Graduate School of UNIST
Graphene is a single layer of only carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities and is the basis for understanding of physical or chemical properties of the various carbon-based materials. In graphene lattice, the carbon bonds are sp2 hybridized, where the in-plane σ bond is one of the strongest bonds in materials and the out-of-plane π bond, which contributes to a delocalized network of electrons, is responsible for the electron conduction of graphene and provides the weak interaction between graphene and substrates. In addition, the energy dispersion at K and K’ points in the first Brillouin zone is linear, which closely resembles the Dirac spectrum for massless fermions. With these unique structural characteristics and the band structure, graphene has shown exceptional physical properties, which have attracted enormous research interest in both scientific and engineering fields. One of the most remarkable properties of graphene is that the charge carriers behave as Dirac fermions, which give rise to extraordinary effects such as mobility up to 200,000 cm2V-1s-1, ballistic transport distances of up to a micron at room temperature, half-integer quantum Hall effect. Graphene also possesses the excellent mechanical strength, such as the breaking strength of ~ 42 Nm-1 and the Young’s modulus of 1.0 TPa. Its thermal conductivity is measured with a value of ~ 5,000WmK-1. In addition, graphene is highly transparent, with absorption of ~ 2.3% towards visible light. For applying these outstanding properties of graphene to various fields, the development of various methods has stimulated a vast amount of research in recent years and thus there are four different methods; the mechanical or chemical exfoliation of graphite, sublimation of SiC, and CVD growth on metal substrates. Among these, large-area graphene films are currently best synthesized via the CVD process onto polycrystalline metal surfaces and this method is the most promising method for realization of graphene-based flexible optoelectronic display technology. However, even in the CVD process, there are several problems for direct device applications, such as additional transfer process, introduction of high process temperature and high process costs. Therefore, in this study, we describe a very low-temperature and transfer-free approach to controllably deposit graphene films onto desired substrates, which we refer to as Diffusion-Assisted Synthesis (DAS) method. Our synthesis methodology exploits the properties of a ‘diffusion couple’, wherein a Ni thin film is deposited first on the substrate, and solid carbon (graphite powders) is then deposited on top of the Ni, and allowed to diffuse along the Ni layer to create a thin graphene film at the Ni-substrate interface. First we conducted our DAS process on the hard substrate, such as SiO2 layers, at temperatures below 260 °C. In this case, the as-synthesized graphene films are wrinkle-free and smooth over large areas. Interestingly, we find that the morphologies of regions covered with mono- and bi-layer graphene resemble those of the grains, and the multi-layer graphene ridges, the grain boundaries in the Ni thin films. The electrical properties of graphene layers on SiO2/Si obtained at low-temperature (T ≤ 260 °C) have been evaluated with back-gated graphene-based field-effect transistor (FET) devices and by using transmission line model method. The estimated hole mobility is ~667cm2V-1s-1 at room temperature in ambient conditions and the sheet resistance is found to be ~1,000Ω per square, suggesting that the as-synthesized graphene films are of reasonable quality. We also find that graphene films obtained range from 25 °C to 260 °C have similar structural quality, but the surface coverage of graphene on SiO2 shows a strong dependence on the growth temperature. Furthermore, we have explored the possibility of using our approach to grow graphene in air instead of inert Ar atmospheres. Surprisingly, we find that the surface morphology, areal coverage and Raman structure of the graphene films grown in Ar as well as in air are similar. In addition, we studied the characteristics of the DAS-graphene grown on SiO2/Si substrates at high-temperature growth regime (300 °C ≤ T ≤ 600 °C). In this study, we observe the formation of nanocrystalline graphene layers by precipitation and the morphologies of graphene films are largely independent of process temperature, time and microstructure of poly-Ni films in this process regime. Also we find that the layers contain no graphene ridges at all. From above experimental results and theoretical estimation using Fisher model and DFT calculations, we propose a mechanism for the growth of graphene layers in the DAS process as follows: (1) the resulting C atoms from solid carbon source are transported across the Ni film primarily along the grain boundaries to the Ni-SiO2 interface at low-temperatures and (2) upon reaching the Ni-SiO2 interface, C atoms precipitate out as graphene at the grain boundaries and (3) excess C atoms reaching the graphene ridges, diffuse laterally along the graphene-Ni (111) interface and lead to the growth of graphene over large areas, driven by the strong affinity of C atoms to self-assemble and expand the sp2 lattice. Finally, we demonstrated the applicability of our approach to prepare large-area graphene on the soft material substrates, such as PDMS, PMMA, and glass. To this purpose, we use T ≤ 160 °C and do not anneal the Ni thin films so as to minimize thermal degradation of the substrates. In contrast to graphene on SiO2, the graphene films on plastic and glass substrates are continuous over large areas at all temperatures, possibly due to the decrease in distance between grain boundaries. The as-grown layers on the soft material substrates are nanocrystalline graphene.
Materials Science Engineering
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