Mechanisms of 2D Materials Growth - Theoretical Studies

二維材料生長機制的理論研究

Student thesis: Doctoral Thesis

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Author(s)

  • Jichen DONG

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date6 Sept 2016

Abstract

Two dimensional (2D) materials have been demonstrated in recent years to hold great potential in various applications including nano-electronics, energy harvesting, sensors, catalysts, protective coatings, etc., due to their excellent physical and chemical properties such as massless Dirac fermions, superb strength, large specific surface area, edge dependent electronic structures, etc. To make the most of these properties, fabrication of large-sized high-quality 2D materials at large scale is a prerequisite. During the last decade, chemical vapor deposition (CVD) has been proved to be one of the most promising methods of achieving this goal. A systematic understanding on the growth mechanisms of 2D materials is thus both fundamentally interesting and practically important. In this dissertation, we present theoretical investigations on the growth mechanisms of 2D materials, focusing on: (i) developing a general approach for predicting the growth and etching behaviors of 2D materials of various crystallographic symmetries; (ii) revealing the mechanisms of graphene growing on polycrystalline substrates and (iii) shedding light on the formation mechanisms of different types of graphene grain boundaries (GBs).

(i) A general approach for predicting the growth and etching behaviors of 2D materials of any crystallographic symmetries is proposed, which is based on simple structure analysis and density functional theory (DFT) calculations. With this method, the growth and etching rate profiles of 2D materials can be estimated. By employing traditional kinetic Wulff construction (KWC) and inversing the KWC, we systematically reveal the growth-etching-regrowth processes of graphene, hexagonal BN (h-BN), FeSe and black phosphorus monolayers. This proposed approach is validated by experimental observations of graphene growth and etching, growth of h-BN monolayer and growth of FeSe monolayer. The shapes of grown 2D materials are found to be only determined by the edges exhibiting locally minimal growth rates, and show no dependence on nuclei's shapes. In comparison, both the initial domain shape and the edges with locally maximal etching rates are critical factors determining the etching of 2D materials.

(ii) The growth behavior of graphene on polycrystalline Cu substrates is systematically investigated. From DFT calculations, it is revealed that graphene GBs are hardly to be formed when graphene grows across substrate GBs, because the adsorption energy increase induced by the change from alignment to misalignment between graphene and substrate is not large enough to compel graphene domains to form GBs to keep aligned with the substrate, and thus graphene GBs are only likely to be formed by coalescence of graphene domains. This finding clarifies that high-quality single crystalline graphene domains can be synthesized on polycrystalline substrates at large scale. Molecular dynamics (MD) simulations of graphene growing across substrate GBs were also carried out to corroborate this finding. Following this result, feasible strategies of growing large-sized single crystalline graphene domains are proposed. Moreover, the origin of the distribution of misorientation angles of graphene GBs is discussed. Conclusions from this study may be also suitable for the growth of other 2D materials.

(iii) The formation mechanisms of covalently bonded grain boundaries (CBGBs) and overlapping grain boundaries (OLGBs) in CVD grown graphene are systematically explored. During the coalescence of two graphene domains with different orientations or a translational mismatch, a linear GB is expected to form. It is broadly acknowledged that such graphene GBs are composed of strings of non-six-membered C rings covalently bonded together. However, recent experiments showed another kind of graphene GBs which are formed by edge overlapping between neighbored graphene domains. These two kinds of graphene GBs have a profound effect on the quality of grown graphene films because they show very different electronic, mechanical and thermal properties. By DFT calculations, we find that both the hydrogen passivation of graphene edges and the misorientation angle between graphene domains are important factors for the formation of overlapping GBs, which is verified by experimental observations. The appearance of these two kinds of graphene GBs is determined by their relative stabilities. This study provides a deep insight into graphene growth and may be helpful for understanding the GB formation of other 2D materials.