铜表面催化石墨烯CVD生长动力学的理论研究

Among the various polycrystalline metal substrates, copper has been one of the most popular catalysts due to its significant advantages in the production of large-area and high-quality graphene films, especially in its better control of the number of graphene layers compared to other metals. However, to date, it is still a great challenge to get a comprehensive understanding of the effects of the growth parameters on elementary steps in the growth dynamics of uniform and high-quality graphene, which include the concentration of the feedstock, the roughness of copper substrate, and the physical properties of copper surface, and so forth. To achieve the high-quality graphene growth with a precise control of layer thickness, it is necessary to gain a deeper insight of graphene CVD nucleation and growth mechanism at the atomic level. In this project, by performing the molecular dynamics simulations in combination of the first-principles calculations, we will provide the atomistic pictures of the nucleation and growth of graphene on copper surface, involving the processes of catalyzed pyrolysis of hydrocarbon, the diffusion and aggregation of active carbon species, and finally nucleation and growth of graphene on copper surface. Here, we will focus on the dynamic interaction between active carbon species and copper surface during the evolution of graphene growth as well as its influence both on the structure of the carbon clusters and the morphology of copper surface. In this study, we will demonstrate the effects of carbon concentration and the degree of roughness as well as the physical properties of the copper surface on the atomic dynamics of graphene nucleation and growth, and then identify the key factors which determine the number of layer and the quality of graphene films. In particular, we will illustrate the surface-catalyzed nucleation and growth mechanism of CVD graphene, which will provide the theoretical rationale and guidance to produce the high-quality and uniform graphene in experiments by selecting the optimal growth parameters.

由于Cu表面CVD生长的石墨烯具备大尺寸高质量以及较好的层数可控性等优点，Cu基底被公认为是石墨烯CVD生长领域最有前景的金属催化剂之一。然而，活性碳浓度、Cu表面的微观形貌结构等参数对石墨烯CVD生长动力学、样品质量和层数控制方面的原子作用机制我们却知之甚少。本项目旨在通过分子动力学模拟和第一性原理计算相结合的方法探索Cu表面碳氢化合物催化热解、活性碳物种在表面扩散迁移、聚集成核到生长成石墨烯的原子动态过程；研究活性碳团簇与金属表面原子的动态交互作用及其对彼此结构和形貌动力学演化的影响；从原子水平上理解活性碳物种浓度、金属表面缺陷结构和不同物理属性如何影响石墨烯成核和生长的模式，进而阐明决定石墨烯生长质量和层数的关键因素。通过揭示石墨烯CVD表面催化生长的微观机制，为实验上寻找最优的石墨烯生长条件提供理论依据和指导。

本项目通过分子动力学模拟，研究了在Cu表面CVD生长石墨烯过程中活性碳团簇与Cu表面原子相互作用的交互过程。我们发现石墨烯在Cu表面生长的基本过程和在Ni与Fe等金属表面类似，历经碳原子或者C2等活性基团在金属表面迁移、形成碳链和Y型支链、聚集成五元环或六元环、形成石墨烯岛、最后生长成大面积石墨烯的动态过程。我们研究了熔融状态Cu表面上碳团簇的生长过程，发现熔融状态下，Cu原子流动性较大，其表面碳原子或者碳链会在Cu原子流动作用下在Cu金属表面迁移，个别碳原子或者碳活性基团会迁入Cu表面的下层区域。形成 “57” 缺陷碳环之后，在Cu原子作用下，“57”缺陷碳环会快速转变为“66”碳环，熔融Cu表面对石墨烯的生长起到了从缺陷碳环到“66”碳环的修复作用，这是在其它金属表面上没有观察到的现象，所以Cu表面比其他金属表面生长的石墨烯质量高很多。我们还研究了不同碳浓度下石墨烯的生长过程，发现在碳浓度相对较低的情况下，生长的石墨烯质量较高。在低碳浓度下，碳原子和碳链有足够的时间迁移，它们可以沿着能垒比较低的路径聚集成核，形成相对较少的缺陷碳环，而且缺陷碳环生成后有足够的时间在Cu原子作用下修复成六元环。在石墨烯生长过程中，碳原子和碳团簇的迁移和聚集破坏了Cu金属表面的原子结构，Cu表面形成了很多点缺陷、线缺陷和面缺陷等缺陷结构，这些点缺陷和线缺陷是石墨烯结构中碳原子或碳团簇的锚点，很多Cu原子在碳原子的作用下迁移到石墨烯表面。我们的理论模拟从原子水平上探索了活性碳物种的浓度、金属表面的物理属性和金属表面原子的流动性等对石墨烯成核和生长的影响因素，从微观尺度探索了石墨烯的生长机制，为实验上寻找最优的石墨烯生长条件提供了理论依据。