Studies on CVD-grown Graphene with Minimal Defects for High Performance Graphene-based Devices

Abstract

Graphene is a promising candidate for electronic devices, due to its exceptional electronic, mechanical and optical properties. Many efforts have been introduced to obtain large-area graphene with high quality for industrial applications. Graphene synthesized using chemical vapor deposition (CVD) is a strong candidate to meet this need. However, line defects, such as grain boundaries and wrinkles, are formed during CVD owing to uncontrollable nucleation and difference between graphene and catalytic substrate. Furthermore, in wide-area applications, the micro-structured and nano-structured graphene must be obtained by an intricate patterning process. Patterning methods composed of several steps such as lithography and etching can create point defects such as oxidized sites on graphene, and degrade its poor quality. This thesis presents a systematic investigation of various line defects (grain boundaries and wrinkles) on graphene and the development of bottom-up method to synthesize patterned graphene with minimal defects for high-quality graphene-based devices. In chapter 2, I develop an easy method to unveil the surface information of CVD-grown graphene by thermal evaporation of gold (Au) adatoms. Development of high-quality graphene-based electronics requires understanding of how line defects such as grain boundaries and wrinkles affect the electronic and mechanical properties of CVD-grown graphene. However, simultaneous visualization of both grain boundaries and wrinkles with detailed structural information has not been reported. Because defects have higher binding energy than that of intact graphene, thermal evaporation of Au adatoms induced selectively deposited gold particles near defect sites. Single lines and gold nanoparticles (Au NPs) formed along GBs, and double lines Au NPs formed along wrinkles. Furthermore, stitched grain boundaries can be distinguished from overlapped grain boundary, and standing wrinkles can be distinguished from folded wrinkles. The width of a standing collapsed wrinkle can be determined. Theoretical calculations revealed that morphology of defects on CVD-grown graphene is caused by distinct binding energies of defects, which affect diffusion of Au NPs. This systematic approach could be further extended to correlate line defects with their effects on electronic and mechanical properties of graphene. In chapter 3, I propose bottom-up method to synthesize patterned graphene with minimal defects from chemically-patterned polymer film as a solid carbon source. Precise graphene patterning is essential for graphene-based electronics. However, a top-down patterning process requires several steps such as lithography and etching, which inevitably cause point defects in the graphene lattice. To avoid these defects during patterning process, a bottom-up method to synthesize patterned graphene was used. The use of polymer as solid carbon source for graphene growth enables direct conversion of the polymer pattern to a graphene pattern. However, physically-patterned polymer film converts indirectly to graphene be deposition of hydrocarbon vapors that form when the polymer decomposes, and results in fully-grown graphene. Therefore, I used selectively-crosslinked polystyrene film as chemically patterned film and obtained bottom-up grown graphene/amorphous carbon (a-C) heterostructure pattern by using CVD. Graphene was successfully grown from neat polystyrene regions, whereas amorphous carbon (a-C) formed on patterned crosslinked polystyrene regions. Because the electrical conductivity of graphene is at least two orders of magnitude higher than that of a-C, the charge transport in graphene/a-C heterostructure mainly occurs through the graphene region. Measurement of the quantum Hall effect in graphene/a-C lateral heterostructures clearly confirms the reliable quality of graphene and the presence of a well-defined graphene/a-C interface. The direct synthesis of patterned graphene from polymer pattern could be further exploited to prepare versatile heterostructures.