化学气相沉积法制备的石墨烯晶畴的氢气刻蚀

利用化学气相沉积法在抛光铜衬底上制备出六角形石墨烯晶畴, 并且在高温条件下对石墨烯晶畴进行氢气刻蚀, 利用光学显微镜和扫描电子显微镜对石墨烯晶畴进行观测, 发现高温条件下石墨烯晶畴表面能够被氢气刻蚀出网络状和线状结构的刻蚀条纹. 通过电子背散射衍射测试证明了刻蚀条纹的形态、密度与铜衬底的晶向有密切关系. 通过对比实验证明了石墨烯表面上的刻蚀条纹是由于石墨烯和铜衬底的热膨胀系数不同, 在降温过程中, 石墨烯表面形成了褶皱, 褶皱在高温氢气气氛下发生氢化反应形成的. 对转移到二氧化硅衬底的石墨烯晶畴进行原子力显微镜测试, 测试结果表明刻蚀条纹的形貌、密度与石墨烯表面褶皱的形貌、密度十分相似. 进一步证明了刻蚀条纹是由于褶皱结构被氢气刻蚀引起的. 实验结果表明, 即使在六角形石墨烯晶畴表面也存在褶皱和点缺陷. 本文提供了一种便捷的方法来观察铜衬底上石墨烯褶皱的分布与形态; 同时, 为进一步提高化学气相沉积法制备石墨烯的质量提供了更多参考.

Hydrogen etching of chemical vapor deposition-grown graphene domains

In this paper we analyze the reason of the etching trenches in chemical vapor deposition (CVD) graphene domain and study the influence factor in the distribution and morphology of wrinkles. Graphene is synthesized on Cu substrate. The Cu substrate is annealed at 1050℃ for 60 min with 1000 sccm Ar and 200 sccm H₂. After annealing, 500 sccm Ar, 20 sccm H₂, and 1 sccm dilute CH₄ (mixed with Ar) are introduced into the CVD system for graphene growth. Hydrogen etchings of graphene are conducted with flows of 500 sccm Ar and 200 sccm H₂ at atmospheric pressure, and etching are performed at 950 and 1050℃. The striated and reticular etching trenches are observed after etching via optical microscope and scanning electron microscope. Every graphene domain is divided into island structures by these etching trenches. However, the edge of graphene domain is not etched and the size of domain is not changed. Electron backscatter diffraction (EBSD) is conducted to analyze the different morphologies of etching trenches. According to the EBSD analysis, the etching trench is closely associated with the Cu crystal orientation. Different Cu planes result in differences in mode, shape, and density of the etching trench. We conduct a verification experiment to judge whether the etching trenches are caused by the gaps between graphene and Cu substrate or by the hydrogenation of wrinkles. The graphene domains grown on Cu substrate with the same growth condition are etched immediately after growth without cooling process. We select graphene which grows across the Cu grain boundary, via optical microscope. A small number of regular hexagons are observed in graphene surface and the region of Cu boundary, but no etching trench is found. As the graphene growing across Cu boundary is the suspending graphene and there is no etching trench, we consider that the gap between graphene and Cu species is not a significant factor of forming etching trench. For comparison, the etching trenches are observed in the graphene domains with cooling process. Thus, the trench formation is bound up with the cooling process after growth, which can lead to the wrinkle formation on the graphene surface, giving rise to a large thermal expansion coefficient difference between the graphene and Cu species. As a major type of structural imperfection, wrinkles can show that enhanced reactivity is due to hydrogenation because of high local curvature. So we consider that the trench formation is caused by the hydrogenation of wrinkles. Then the as-grown graphene domains are transferred to SiO₂ substrate and atomic force microscope (AFM) is employed to measure the surface appearance of graphene. The AMF image shows lots of wrinkles in the graphene surface. The morphology and density of wrinkles are similar to those of the etching trenches extremely. Thus, the AFM testing result provides another evidence to prove that the etching trenches are related to the hydrogenation of wrinkles. From the above we can draw some conclusions. Numerous trenches are observed in the graphene domains after etching, and the trench patterns are closely associated with the Cu crystal orientation. A different Cu crystal orientation leads to variations in mode, shape, and density of the etching trench. We prove that the etching trenches are caused by the hydrogenation on wrinkles formed in the cooling down process instead of the gap between Cu and graphene. This hydrogen etching technology is a convenient way to detect the distribution and morphology of wrinkles. Furthermore, it provides a reference for improving the quality of CVD graphene.