Liu Kaihui's research group of the School of Physics has made progress in the research of precision atom manufacturing of angular double-layer graphene

Professor Liu Kaihui, Institute of Condensed Matter Physics and Materials Physics, School of Physics, Peking University, and the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, proposed a new growth strategy of "pre-stacked substrate-angle replication single crystal growth", under the condition of macroscopic control of the substrate rotation angle, the strict epitaxial angle reproduction growth behavior of two-dimensional crystals and the self-spreading effect of the pre-melted plane of metal substrate were used to successfully manipulate the double-layer graphene stacking angle growth
。 This strategy provides a new path for precise control of double-layer stacking structures at the macroscopic scale in the field of two-dimensional crystal preparation, and 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
".

In recent years, two-dimensional materials have gradually developed into hot research directions in basic disciplines such as condensed matter physics and materials science due to their extreme atomic layer structure and excellent physical, optical, mechanical and other physical properties, 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 state regulation 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-related states, insulating body 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 coupled angle two-dimensional material
with controllable corners.

Taking graphene as an example, two single-layer graphene stacks can be stacked to form a double-layer corner structure by mechanical peeling, layer-by-layer transfer, etc.
, 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, double-layer graphene often tends to form a thermodynamically stable stacking structure
with a 0° or 30° interlayer angle.
In recent years, by controlling the high-energy point nuclei such as defects, steps, and structured surfaces of the substrate, the random growth of double-layer graphene at other angles can be realized, however, the relative corners between layers are 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.

Since 2016, Professor Liu Kaihui, Academician Wang Enge, Academician Yu Dapeng of the School of Physics of Peking University have carried out systematic research on the problems related to the growth of two-dimensional materials, and gradually developed a set of two-dimensional single crystal atom manufacturing general technology, realizing the use of graphene (Science Bulletin 2017, 62, 1074), hexagonal boron nitride (Nature 2019, 570, 91), and transition metal sulfur compounds (Nature).
Large-size two-dimensional single crystal regulated growth represented by Nanotechnology 2022, 17, 33) and the preparation of more than 30 A4 size high-index single crystal copper foil libraries (Nature 2020, 581, 406).

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 structure of lattices and steps on the surface of the substrate
.
Therefore, by macroscopic pre-stacking double-layer substrate design angle and epitaxial growth of single-layer single crystal, so as to achieve inter-layer corner replication, it is expected to obtain a large-area double-layer two-dimensional material
with controllable angle, strong interlayer coupling and clean interface.

Based on the above accumulation, Liu Kaihui's team and collaborators proposed a new strategy of "pre-stacked substrate-angle replication single crystal growth" to achieve centimeter-level bilayer graphene preparation
with controllable corners and clean interfaces 。 The research team (1) pre-stacked the annealed single crystal Cu(111) substrate macroscopic to lock the angle, so that the rotation angle between the substrates is the target angle of the expected growth of double-layer graphene; (2) Subsequently, the characteristics of Cu(111) surface symmetry matching and small lattice mismatch are used to ensure that monocrystalline graphene is grown epitaxially on the pre-stacked Cu(111) substrate, and the rotation angle
between the substrates is strictly reproduced.
Then accurately control the temperature and use the copper foil plane self-spreading effect to obtain a double layer of 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 parallel electric field is applied to the etching fluid by using the equipotential surface etching method, and the copper foil on one side is etched uniformly and the copper ions migrate along the electric field direction, which can effectively avoid non-uniform etching
.
The etching process of copper surface is monitored by three-electrode electrochemical method to accurately control the etching time and obtain a complete large-area double-layer graphene finished product
.

This study uses the above original growth strategy to realize the precise control of the angle of large-area double-layer graphene, and provides a novel method
for precise manipulation of atomic stacking structures at the macroscopic scale for two-dimensional crystal materials.
Topography and structural characterization techniques (including electron microscopy, angle-resolved photoelectron spectroscopy, spectroscopy and photocurrent testing, etc.
) verify the accuracy and uniformity of the designed corners from atomic to centimeter scales
.
This method can theoretically be extended to the corner preparation of other two-dimensional crystal materials, which is expected to provide a low-cost, easy-to-operate feasible technical solution
for the preparation of large-scale angular double-layer two-dimensional materials.

Figure a Corner bilayer graphene growth design schematic diagram; 14° angle bilayer graphene results prepared by b-e; Ubiquitous preparation of f-i corner bilayer graphene

Liu Can (now a researcher at Chinese Minmin University), a Peking University postdoctoral fellow (now a Peking University Distinguished Associate Fellow), Peking University "Boya" postdoctoral fellow Li Zehui (now a Peking University Distinguished Associate Researcher), Peking University "Boyar" postdoctoral fellow Qiao Ruixi (now a Distinguished Associate Researcher of Nanjing University of Aeronautics and Astronautics), and Wang Qinghe, a 2021 doctoral student in the School of Physics, are the first authors of the paper.
Professor Liu Kaihui of Peking University, Professor Zhujun Wang of ShanghaiTech University, and Liu Can, a researcher at Chinese Minsheng University, are co-corresponding authors
.
Other major collaborators include Academician Wang Enge, Professor Wang Xinqiang and Professor Gao Peng of Peking University, Academician Yu Dapeng of Southern University of Science and Technology, Professor Tan Pingheng of the Institute of Semiconductors of Chinese Academy of Sciences, Professor He Jun of Wuhan University, and Professor Liu Zhongkai of ShanghaiTech University
.

The research work has been supported
by the National Natural Science Foundation of China, the National Key Research and Development Program, the Beijing Municipal Natural Science Foundation, the Guangdong Provincial Major Project of Basic and Applied Basic Research, 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.