CVD GRAPHENE ON LIQUID SUBSTRATES (REPLACEMENTS FOR ITO IN SOLAR AND DISPLAY APPLICATONS)

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Abstract

Graphene, a two-dimensional sheet of sp2 carbon, has been the focus of research communities across the globe due to its remarkable combination of properties. Chemical vapour deposition (CVD) has the potential to produce graphene films on a large-scale, enabling its use in many applications. Typically, graphene is grown on a solid substrate, with the structure of the substrate influencing the resulting graphene. Thus, there has been a move towards using liquid metal substrates in recent years. However, whilst there are many studies reporting on the effect of growth parameters of CVD graphene over solid copper (Cu) and nickel (Ni) substrates, there is little on liquid substrates. This challenge is addressed in this thesis, with a consideration of the material properties of liquid substrates (e.g. carbon solubility) on CVD graphene nucleation and growth. In particular, low melting point substrates were studied with the aim of transferring the graphene directly from the liquid metal without any need to solidify it. Firstly, the graphene films were grown by atmospheric pressure CVD (APCVD) on liquid Cu substrates, which were found to show characteristic hexagonal defects when the Cu was held in a molybdenum (Mo) crucible not present when using a tungsten (W) crucible. These defects were identified as thin precipitates of Mo2C, highlighting the effect of the support substrate on the quality of the grown graphene film. Thermodynamic calculations indicated that under typical CVD conditions, using a CH4/H2 process gas, Mo2C is stable in the presence of C whereas WC is not. A mechanism driven by the vapour phase transport of the volatile MoO3 phase from the metal boat surface was subsequently proposed. Low melting point metal, tin (Sn) and indium (In) were then studied as growth substrates. The effect of hydrogen, hydrocarbon feedstock, growth temperature, reactants' residence time and growth time were also explored. The relationship between the partial pressure of the reactive gases and the resultant graphene was found to differ across the two metals. That is, high quality graphene on liquid Sn was obtained at high partial pressure of the hydrocarbon and long residence time. Conversely, in the case of high quality graphene on liquid In, the opposite was found to occur. The excellent catalytic nature of Cu in graphene growth is well-known. Thus, after optimising the growth conditions for high quality graphene on liquid Sn and In, this study aimed to explore the effect of adding different doses of Cu to form Cu-Sn and Cu-In alloys. Different compositions of low melting alloys, Sn-In have also been explored. It was found that the optimal growth conditions for the alloys matched those of the dominant metals, with sub- optimal growth occurring for the alloys with relatively even concentrations of metals. Finally, this study attempted to find direct transfer routes of graphene from liquid growth substrates, with the aim of avoiding the strain and damage induced in traditional transfer approaches. Various transfer substrates with different surface treatments were tested. A variation in the degree of coverage of the transferred graphene areas was seen on the different target substrates. Consistency of the quality of the directly transferred graphene with the as- grown graphene was observed in all cases, carrying excellent potential for improvements in the creation of support-free and direct graphene transfer.

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