2025 Theses Doctoral

Reproducible chemical vapor deposition of high quality graphene

Yan, Xingzhou

Graphene is one of the most important low dimensional materials. Ever since the inception of graphene, attempts to scale up production of graphene have never stopped. Chemical vapor deposition synthesis of graphene on copper is one of the most promising pathways. However, the poor quality of CVD-derived graphene has hindered synthetic graphene for large scale basic science and commercial applications. The lack of reproducibility of CVD graphene research, and the inferior quality of CVD graphene points to potential hidden variables and misunderstanding of the graphene CVD process. In this thesis, it is identified that trace oxygen is a key factor in determining the quality and growth trajectory for graphene grown by low-pressure CVD. By innovative design in the CVD system layout and meticulous control over the substrate quality, it is demonstrated that by eliminating oxygen below µTorr level in the growth chamber, high quality graphene comparable to exfoliated graphene can be obtained. The ultrahigh graphene quality is showcased by a combination of Raman spectroscopy, electrical transport measurement and various scanning probe techniques including scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Moreover, a graphite-gated device encapsulated by hexagonal boron nitride (h-BN) shows well developed fractional quantum hall effect.

Using the oxygen free CVD (OF-CVD) system as a platform, the effect of oxygen is revealed to be inducing etching effect at the µTorr limit. Addition of hydrogen delays the etching effect with reduced graphene growth rate. At high hydrogen concentrations, µTorr-level of oxygen is found to be inducing amorphous carbon contaminations to the graphene surface, at the same time, the quality of the resulting graphene deteriorates with increasing level of oxygen, as characterized by AFM, XPS, Raman spectroscopy and electrical transport measurements.

OF-CVD enables unprecedented tunability of the graphene growth behavior in terms of growth rate and nucleation density. Controlled experiments reveal the individual effect of all experimental parameters such as temperature, and partial pressure of methane and hydrogen are studied. Their effect on the initial growth rate of graphene can be modeled by a compact model based on competitive adsorption of methane and hydrogen onto the copper surface. The mechanism of overall coverage-time evolution is further revealed by phase-field modeling. Collectively, the theoretical insights of the CVD process of graphene pave way for graphene synthesis by design. The underlying mechanism and principles provide insights for understanding and optimizing other 2D materials growth mediated by catalytic substrates.