Design growth of large areas of arbitrarily angled double-layer graphene

Recently, Professor Liu Kaihui of Peking University, Professor Wang Zhujun of ShanghaiTech University, and Liu Can, researcher of Chinese Min University, proposed a new growth strategy of “pre-stacked substrate – angle replication single crystal growth”, under the condition of macroscopic control of the rotation angle of the substrate, the strict epitaxial angle reproduction growth behavior of the two-dimensional crystal and the self-propagation effect of the pre-melt plane of the metal substrate were successfully controlled. This strategy provides a new path for accurately manipulating the double-layer stacking structure at macro scale in the field of two-dimensional crystal preparation, which is expected to provide a new low-cost solution for the large-scale preparation of controllable corner multilayer two-dimensional materials.

On September 15, 2022, the research results were published online in Nature Materials under the title of “Designed growth of large bilayer graphene with arbitrary twist angles”.

Liu Can, researcher of Chinese Min University, Li Zehui, distinguished associate researcher of Peking University, Qiao Ruixi, distinguished associate researcher of Nanjing University of Aeronautics and Astronautics, and Wang Qinghe, doctoral student of Peking University, are the co-first authors of the paper. Professor Liu Kaihui of Peking University, Professor Wang Zhujun of ShanghaiTech University and Researcher Liu Can of Chinese min University are co-corresponding authors.

In recent years, due to its extreme atomic layer structure and excellent physical properties such as electricity, optics and mechanics, two-dimensional materials have gradually developed into hot research directions in basic disciplines such as condensed matter physics and materials science, and are expected to produce a series of transformative technology applications in many fields such as microelectronic devices, photonic chips, and information storage. At the same time, the surface atomic arrangement characteristics of two-dimensional materials and their strong interlayer coupling interaction characteristics provide a new degree of freedom for the regulation of the physical state of two-dimensional materials. By regulating the relative angle between layers of two-dimensional materials, the structure of the electronic energy band can be effectively changed and various novel physical phenomena can be generated, such as unconventional superconductivity, spin polarization correlation states, insulator states, molar excitons, magnetic sequence phase transitions, etc. In order to fully explore the novelty brought about by the corner structure and promote the application of corner electronics, it is urgent to develop a strong interlayer coupling angle double-layer two-dimensional material with controllable corner.

Taking graphene as an example, two single-layer graphene can be stacked to form a double-layer corner structure by mechanical peeling and layer-by-layer transfer, but such methods have problems such as harsh transfer conditions, low output efficiency, and interface pollution. Although the direct growth method can obtain a relatively clean interface, the double layer graphene tends to form a thermodynamically stable stacking structure at an angle of 0° or 30° between layers. In recent years, by controlling the defects, steps, structured surfaces and other high-energy site-shaped nuclei in the substrate, the random growth of double-layer graphene at other angle angles can be achieved, however, the relative angle between layers is still uncontrollable. Therefore, the preparation of double-layer graphene with controllable corners is an important problem to be solved in the field of two-dimensional material growth.

In recent years, studies on the growth mechanism of two-dimensional materials have shown that the growth orientation of single crystals is mainly modulated by the lattice, step and other structures on the surface of the substrate. Therefore, by designing the corners of the macroscopic pre-stacked double-layer substrate and epitaxial growth of single-layer single crystals, thereby realizing the duplication of interlayer angles, it is expected to obtain a large-area double-layer two-dimensional material with controllable angle, strong interlayer coupling and clean interface. Based on this, the research team proposed a new strategy of “pre-stacked substrate-angle replication single crystal growth”, which realizes the preparation of centimeter-level bilayer graphene with controlled corners and clean interface. The specific design scheme includes: (1) the single crystal Cu (111) substrate after macroscopic pre-stacking annealing is at a locking angle, so that the rotation angle between the substrates is the target angle of the expected growth of double-layer graphene; (2) Then, using the characteristics of Cu(111) surface symmetry matching and small lattice mismatch, it is ensured that monocrystalline graphene is epitaxially grown on the pre-stacked Cu(111) substrate, and the rotation angle between the substrates is strictly reproduced. Then, the temperature is accurately controlled and the copper foil plane self-spreading effect is used to obtain a double-layer large-area graphene with a specific angle and a uniformly flat van der Waals interface; (3) Finally, in order to peel off the double layer graphene, the isopotential surface etching method is used to apply a parallel electric field in the etching liquid, etch the copper foil on one side at a uniform speed and make the copper ions migrate along the direction of the electric field, which can effectively avoid non-uniform etching. The etching process of the copper surface is monitored by three-electrode electrochemical method, so that the etching time can be precisely controlled to obtain a complete large-area double-layer graphene finished product.

Figure: Schematic diagram of the growth design of a corner double layer graphene; Results of 14° angle bilayer graphene prepared by b-e; Universal preparation of f-i angled bilayer graphene

Using the above original growth strategy, the research team achieved precise control of large-area bilayer graphene angle, providing a novel method for two-dimensional crystalline materials to accurately manipulate atomic stacking structures at macroscopic scales. Morphological and structural characterization techniques, including electron microscopy, angle-resolved photoelectron spectroscopy, spectroscopy, and photocurrent testing, verify the accuracy and uniformity of the designed angles across the atomic scale to the centimeter scale. This method can theoretically be extended to the corner preparation of other two-dimensional crystalline materials, which is expected to provide a low-cost, easy-to-operate feasible technical solution for large-scale corner double-layer two-dimensional material preparation.

The research work has been supported by the National Natural Science Foundation of China, the National Key R&D Program, the Beijing Municipal Natural Science Foundation, the Guangdong Provincial Basic and Applied Basic Research Major Project, the Strategic Pilot Science and Technology Project of the Chinese Academy of Sciences, the Electron Microscopy Laboratory of Peking University, and the Shanghai Synchrotron Radiation Light Source BL07U NanoARPES Line Station. (Source: Science Network)

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