Chemical vapor deposition (CVD) on metal substrate is considered as the most promising method to grow high-quality graphene. However, due to the higher energy stability of AB stack, the double-layer graphene grown by high temperature CVD is more inclined to form AB stack than twist double-layer graphene. Therefore, it is an important challenge to break the energy advantage of AB stacked graphene and realize interlayer torsion at high temperature.
Recently, Peking University, Academician Liu Zhongfan of Beijing Graphene Research Institute and their collaborators put forward the growth strategy of "ectopic nucleation". The nucleation position of the second layer of graphene is controlled by introducing airflow disturbance in the growth process, so that the lattice orientation of the two layers of graphene is induced by substrates in different regions, and a large proportion of twisted double-layer graphene is obtained (Figure 1).
Figure 1. Growth strategy and growth results of ectopic nucleation method
Generally speaking, the growth of graphene on copper surface follows the "self-limiting" growth model. However, when the hydrogen partial pressure is high, the edge of graphene will change from metal passivation to hydrogen saturation termination, which will weaken the interaction between the edge and metal and hinder the growth of single-layer graphene, so activated carbon species can "drill holes" between the first layer of graphene and copper for the growth of the second layer. The interaction between the second graphene layer and the substrate is stronger than that between graphene layers, which makes interlayer distortion possible. However, the substrate alone is not enough to form the distortion, because the lattice orientation of graphene is determined in the initial stage of nucleation. If two graphene layers nucleate at the same position, the same nucleation environment will make the lattice orientation of the two graphene layers consistent, forming AB stacked graphene.
The researchers found that when the nucleation sites of two graphene layers are different, the probability of interlaminar torsion will increase significantly due to the different micro-environment of the substrate such as steps, kinks, dislocations or particles. In order to realize the controllable nucleation and growth of the second layer of graphene, the researchers adopted the strategy of disturbed growth, that is, changing the partial pressure of hydrogen and methane during CVD growth, and adjusting the final state of graphene edge and the concentration of local carbon species nearby. This method has been verified by 12C/ 13C isotope labeling growth experiment: introducing "disturbance" at 5 min and 10 min respectively, the second layer nucleation time coincides with 5 min and 10C, and the second layer nucleation position coincides with12c//kloc-. At the same time, the undisturbed result is AB stacking double graphene, which proves the effectiveness of this method.
Figure 2. Experimental results of isotope labeling
The researchers also summarized the key parameters of the "perturbation-ectopic nucleation" method. By controlling the ratio of hydrogen source to carbon source in the two-step growth method (Figure 3), tBLG with high torsion ratio (88%) was obtained. High-resolution transmission electron microscopy showed clear moire fringes (Figure 4); Electrical transport measurement shows that it has an ultra-high room temperature carrier mobility (68,000 cm2v1s1) (Figure 5). Angle-resolved photoelectron spectroscopy measurement shows clear linear energy band structure and van Hough singularity. All these prove that the tBLG obtained by this method has super high quality.
Figure 3. Growth parameters of ectopic nucleation method
Figure 4. TEM characterization results
Figure 5. Mobile test results
The author puts forward the strategy of ectopic nucleation. By introducing airflow disturbance to control the nucleation of the second layer of graphene, the lattice orientation of the two layers of graphene is induced by substrates in different regions, thus breaking the limit of the lowest AB stacking energy and realizing the preparation of large-scale twisted double-layer graphene. This method provides a new idea for the preparation of twisted graphene and two-dimensional materials, and lays a material foundation for the emerging torsional electronics research in recent years.
The related research results were published in the journal Nature Communication, titled "Ectopic Nucleation of Twisted Double-layer Graphene with a Wide Range of Twisting Angles". Peking University, academician of Beijing Graphene Research Institute, Professor Peng Hailin, postdoctoral fellow of National University of Singapore, Associate Professor Huang of University of Science and Technology of China as the correspondent of this article, Dr. Sun of Beijing Graphene Research Institute, Dr. Wang Yuehan, Ph.D. student of Manchester University, and Wang of Peking University as the first author. Collaborators include Professor Kostya S. Novoselov from Manchester University, Professor Mark H. Rummeli from Suzhou University, Professor Li Zhenyu from the University of Science and Technology of China and Professor Chen Yulin from Oxford University. The research work of this thesis is supported by School of Chemistry and Molecular Engineering of Peking University, Beijing National Molecular Science Research Center of Ministry of Science and Technology, National Natural Science Foundation of China and Beijing Municipal Science and Technology Commission.
Paper link:
/articles/s 4 1467-02 1-22533- 1