Chemical vapour deposition of graphene: fundamental aspects of synthesis and characterization

Abstract

The chemical vapour deposition (CVD) synthesis of graphene is an unequivocal and the most promising route to produce large-area graphene films suitable for myriad of novel technical applications such as flexible touch screens, smart windows, high-frequency transistors and flexible solar cells. Nonetheless, optimizing CVD growth of large-area graphene has since revealed multiple challenges brought by the need of exploring an extensive experimental parameter space due to the multiple components involved. The control of the graphene coverage, the number of layers, the quality and energy bandgap opening are among the challenges faced by graphene research today. The reasonable way to overcome these challenges is to develop fundamental understanding of the growth processes. This thesis is designed to contribute to the growing body of work in developing large-area graphene films suitable for technical applications by exploring the growth process conditions (i.e. fluid flows, temperature, pressure, cooling and time, among others) in a CVD reactor. For fundamental research, the exfoliated graphene is very useful. A smaller part of this work is devoted to characterization of exfoliated graphene mainly by Raman spectroscopy and imaging. Some findings in exfoliated graphene are useful in optimizing CVD graphene growth. The edge chirality investigation, for example, (Zig-zag or Arm-chair) in exfoliated graphene provides an insight into the domain shape at the initial stage of the CVD graphene growth. The CVD graphene domains assume hexagonal shapes or lobed-shapes consisting of four to six lobes depending on the growth procedure and the parameters used. These shapes at the initial stage suggest the growth mechanism where certain edges grow faster than others during the graphene growth by the CVD process. The strain and doping effects are determined from the Raman modes depending on the position of the G and 2D modes of pristine exfoliated graphene. The large-area monolayer graphene films are obtained at elevated temperatures (~1000 °C) and low methane (CH4) to H2 flow ratios on Cu foil in the CVD reactor. High ratios yield non-uniform, disordered multilayer graphene. The quality of the graphene samples synthesised with low ratios is comparable to that of the exfoliated graphene. For bilayer and the multilayer graphene growth, the underlayer growth concept was successfully investigated and utilised. The kinetics’ theory in the CVD process on low C solubility on Cu foil was experimentally investigated. Theoretically, there are two gas fluxes within the reaction chamber in the CVD process: mass transport through the boundary layer (Jmt) and the surface reaction ( Jsr). The slower of the processes is the growth rate-limiting step. The exclusion of one flux (Jmt) in the atmospheric pressure CVD (AP-CVD) yields results similar to those obtained under low pressure CVD. These results shed more light into the growth mechanisms of large-area graphene by CVD, further explaining why graphene growth under AP-CVD differs from growth under low pressure chemical vapour deposition (LP-CVD).