Edge-Epitaxial Growth of Graphene on Cu with a Hydrogen-Free

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Edge-epitaxial Growth of Graphene on Cu with a Hydrogen-free Approach Ruizhe Wu, Yao Ding, Ka Man Yu, Ke Zhou, Zhouyang Zhu, Xuewu Ou, Qicheng Zhang, Minghao Zhuang, Wen-Di Li, Zhiping Xu, Michael S. Altman, and Zhengtang Luo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00147 • Publication Date (Web): 11 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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Edge-epitaxial Growth of Graphene on Cu with a Hydrogenfree Approach Ruizhe Wu1, 2‡, Yao Ding1‡, Ka Man Yu3‡, Ke Zhou4‡, Zhouyang Zhu5, Xuewu Ou1, Qicheng Zhang1, Minghao Zhuang1, Wen-Di Li5, Zhiping Xu4, Michael S. Altman3 and Zhengtang Luo1* 1 Department of Chemical and Biological Engineering, William Mong Institute of Nano Science and Technology and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 2 Wuhan Institute of Marine Electric Propulsion, Wuhan 430064, China 3 Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 4 Applied Mechanics Laboratory, Department of Engineering Mechanics and Center for Nano and Micro Mechanics, Tsinghua University, Beijing, P. R. China 5 Department of Mechanical Engineering, The University of Hong Kong, Pokfulam, Hong Kong ‡

These authors contributed equally to this work. *E-mail: [email protected]

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Abstract We demonstrate a new concept of edge-epitaxial growth that enables the van der Waals (vdWs) epitaxial graphene growth on different Cu facets. This approach simply entails turning off hydrogen during the nucleation stage of the atmospheric pressure chemical vapor deposition (APCVD) process. Fundamentally different from conventional vdWs growth, this new type of epitaxial growth benefits from the strong binding between the graphene edge and a metal step in a hydrogen-absent atmosphere. This interaction fixes the orientation of graphene grains in energetically preferable configurations at the early nucleation stage. Specifically, single-crystal graphene grains grown on Cu (111) have irregular shapes with rough edges and are misaligned slightly with respect to Cu (111) lattice. Graphene grains form with two possible orientations that are rotated by ~30° on the Cu (100) surface, and grains with three preferred orientations are observed on Cu (110). Density function theory calculations further prove the enhanced edge-metal step interaction and thus validate the experimental observations of these specific graphene crystal orientations on Cu (100) and Cu (110). Observation of this new type of edge-epitaxial growth helps to clarify many contradictory graphene growth phenomena that have been observed previously and may provide insights that will facilitate an understanding of the growth of other materials using the chemical vapor deposition method.

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Introduction Since its discovery in 2004,1 graphene has become an appealing two-dimensional (2D) material due to its remarkable mechanical, thermal and electrical properties.2-4 In order to realize its full potential in applications, great efforts have been made to fabricate large area graphene with high quality on transition metal substrates such as Cu foil using the chemical vapor deposition (CVD) method over the past few years.5-9 Since graphene growth on Cu is believed to occur by a surface-mediated growth mechanism,10 Cu surface features, such as roughness,11 defects,12, 13 and facets,14 can have a significant influence on graphene nucleation and growth. Moreover, in recent years the influence of substrate grain boundaries, temperature and interlayer interaction on the growth of two-dimensional material, including graphene and MoS2 etc., has also been reported.15-17

Despite extensive efforts to understand and control graphene growth in the past, the underlying mechanism, i.e. whether the growth on Cu is epitaxial in nature or not, remains elusive because of the contradictory results obtained by various groups. On one hand, it has been demonstrated that the density of graphene nuclei and the shapes and orientations of graphene grains depend strongly on the crystallographic orientation of the Cu facet that growth occurs on, especially in the case of low pressure chemical vapor deposition (LPCVD) growth.14, 18-22 This strong correlation indicates an epitaxial nature of graphene grown on the Cu surface. In contrast, the shapes and orientations of graphene grains, even on the same single crystal Cu facet orientation, can vary 3


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tremendously in different reports.19,

20, 23, 24

To make the situation even more

complicated, single crystal graphene grains were seen to cross the Cu grain boundaries without the change of shape or orientation when grown on polycrystalline Cu foil by the atmospheric pressure chemical vapor pressure (APCVD) method. This behavior implies that the interaction between graphene and Cu substrate is rather weak.7, 25 These inconsistent observations of the relationships between graphene shape/orientation and growth substrate have puzzled the graphene synthesis community and impeded progress towards an understanding of how the graphene/Cu interaction affects the growth process. This hinders efforts to fabricate high-quality graphene that is suitable for future applications.

On the other hand, significant efforts have also been made to understand the roles of hydrogen and oxygen in the graphene growth process over the past few years. It is known that hydrogen serves a dual role as both an activator for methane decomposition and as a graphene etching reagent,26 and that oxygen can passivate the active sites on the Cu substrate. The passivation by oxygen has been exploited to control the graphene nucleation density and to adjust the graphene growth rate.7-9,

27, 28

Although such

importance is acknowledged, the exact roles that hydrogen and oxygen play in the graphene/Cu interaction unfortunately remain elusive.

In this work, we demonstrate a facile “hydrogen-absent” approach to achieve controllable graphene orientation on different single crystal Cu facets of a 4



polycrystalline Cu foil. Growth is controlled in this approach by the manipulation of graphene nucleation during the initial growth stage. The as-grown graphene grains were shown to be strongly correlated with the crystallographic structure of the Cu (111), (100) and (110) substrates. Evidence of this correlation is provided by electron backscatter diffraction (EBSD) and low energy electron microscopy/diffraction (LEEM/LEED), which identified both the graphene orientations and edge types on different Cu facets. We demonstrate that edge-epitaxial growth was achieved by the enhancement of the interaction between graphene edges and Cu steps in the absence of hydrogen flow during the nucleation process. Here, unlike the conventional epitaxial growth where the grown material mimics the orientation of the substrate, the edge-epitaxial growth is defined as that the growth of a crystalline material on another crystalline substrate, i.e. graphene or other two-dimensional materials, whose orientation is determined only by the edge and substrate coupling, such as the edge/step pairing described in this work.29 This growth mechanism is confirmed by results of density function theory (DFT) based calculations. Our findings deepen the understanding of graphene/Cu interaction during the initial nucleation stage and shed light on the control of graphene lattice orientation on various substrates for mass production of high-quality graphene film.

Experimental section Pretreatment of Cu foil Cu foil (99.8%, Alfa Aesar No. 13382) was first treated by acetic acid for 20 min and then rinsed 3 times with distilled deionized water. Then, the Cu foil was blown dry by 5


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N2 gas and inserted into the quartz tube with a quartz substrate as the support. Growth of aligned graphene grains growth under “hydrogen-absent” conditions After a 15 min of purging with 400 sccm Ar gas (Hong Kong Specialty Gases Co., Ltd., 99.999%, oxygen concentration

Edge-Epitaxial Growth of Graphene on Cu with a Hydrogen-Free

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