Large-Area Synthesis of High-Quality Graphene Films for Gas-Barrier Applications

Abstract

Chapter 1 describes general introduction of the thesis. The properties and synthesis methods of graphene are introduced briefly. Chapter 2, improving the properties of graphene has been intensively studied to facilitate the practical applications for electronics. In addition, developing novel approaches to lower the growth temperature and simplify the fabrication steps of graphene devices are demanded. Here, we report a simple method to prepare uniformly and selectively grown monolayer graphene by chemical vapor deposition (CVD) using acetylene (C2H2) at temperature as low as 800 °C, which was evidenced by Raman Spectroscopy, SEM, and TEM. The organic field-effect transistors (OFETs) based on the selectively grown graphene electrodes exhibit enhanced performance compared to photolithography-patterned graphene electrodes. We expect that our method would enable the cost-effective high-performance synthesis and applications of graphene by providing lower synthesis temperature that does not require quartz chambers and simplifying the complicated post-growth pattering processes of graphene electrodes. Chapter 3, two dimensional semiconductor such as MoS2 are an emerging material with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth and control over lattice orientation remain a challenge. Here, we have demonstrated metal organic chemical vapor deposition growth of monolayer MoS2 on sapphire by the use of liquid phase Mo-precursor. The AFM and SEM images revealed that the MoS2 flake is well-defined triangular shape and MoS2 flake has observed step heights of individual layers of 0.7-0.8 nm. Furthermore, Raman and PL spectrum indicate that monolayer MoS2 is less doped and smaller structural disorder. We believe that our versatile MOCVD leads to synthesis high quality monolayer MoS2, allowing its use in future electronic and optoelectronic devices. Chapter 4, preventing reactive gas species such as oxygen or water is important to ensure the stability and durability of organic electronics. Although inorganic materials have been predominantly employed as the protective layers, their poor mechanical property has hindered the practical application to flexible electronics. The densely-packed hexagonal lattice of carbon atoms in graphene does not allow the transmission of small gas molecules. In addition, its outstanding mechanical flexibility and optical transmittance are expected to be useful to overcome the current mechanical limit of the inorganic materials. In this paper, we reported the measurement of the water vapor transmission rate (WVTR) through the 6-layer 10x10 cm2 large-area graphene films synthesized by chemical vapor deposition (CVD). The WVTR was measured to be as low as 10-4 g/m2?day initially, and stabilized at ~0.48 g/m2?day, which corresponds to 7 times reduction in WVTR compared to bare polymer substrates. We also showed that the graphene-passivated organic field-effect transistors (OFETs) exhibited excellent environmental stability as well as a prolonged life time even after 500 bending cycles with strain of 2.3 %. We expect that our results would be a good reference showing the graphenes potential as gas barriers for organic electronics.