Junzhu Li
Ph.D. Candidate supervised by Prof. Xixiang Zhang

Event Location: Ibn Sina Building (Bldg. 3), Room 5220
Zoom link: https://kaust.zoom.us/j/97174478278

ABSTRACT:  As a promising advanced two-dimensional (2D) material with remarkable physical-chemical properties, graphene shows considerable potential applications in semiconductor nanodevices and many other fields. Chemical vapor deposition (CVD) has been widely used to synthesize 2D materials, including graphene. To date, many efforts have been made on the CVD growth of higher quality graphene film on arbitrary substrates (e.g., metals and insulators). However, it remains very challenging due to the underlying complex physical mechanisms and numerous influence factors in the CVD growth process. Furthermore, the growth of inch-scale high-quality graphene on insulating substrates is desirable for electronic and optoelectronic applications, but remains challenging, so far, due to the lack of metal catalysis.

In this dissertation, we reveal the existence of a fractal-growth-based mechanism in the CVD synthesis of several 2D materials, to which we build a 2D-DLA model based on an atomic-scale growth mechanism and the traditional fractal theory. The strength of this model is validated by the perfect correlation between theoretically simulated data and experimental results obtained from the CVD growth of graphene, hBN, and TMDs. By applying the 2D-DLA model and carefully tuning the crucial factor of the single-domain net growth rate, we synthesize various fractal-morphology high-quality single-crystal 2D materials, achieving, for the first time, the precise control of the 2D-material CVD growth. Our work lays the theoretical foundation for the precise adjustment of the morphologies and physical properties of 2D materials, which is essential to the use of fractal-shaped nanomaterials for the fabrication of new-generation neural-network nanodevices.

Based on the deeper understanding of the CVD growth mechanism, we have successfully achieved the synthesis of wafer-scale adlayer-free ultra-flat single-crystal monolayer graphene on sapphire substrates. We converted polycrystalline Cu foil placed on Al2O3(0001) into single-crystal Cu(111) film via annealing, and then achieved epitaxial growth of graphene at the interface between Cu(111) and Al2O3(0001) by a multi-cycle plasma-etching-assisted CVD method. We have also successfully proposed a model to formulate the growth mechanism of the single-crystal monolayer graphene in the nanoreactor. This work breaks a bottleneck of synthesizing wafer-scale single-crystal monolayer graphene on insulating substrates and could contribute to next-generation graphene-based nanodevices.