The two-dimensional material family covers insulators, semiconductors, metals and superconductors, and exhibits many novel physical properties that are different from three-dimensional materials. It has become a research hotspot in the fields of condensed matter physics and mater

two-dimensional material family covers insulators, semiconductors, metals and superconductors, and exhibits many novel physical properties that are different from three-dimensional materials. It has become a research hotspot in the fields of condensed matter physics and materials science in recent years. Preparing high-quality two-dimensional materials, especially ultra-thin materials at the atomic level, is the basis for cutting-edge exploration of two-dimensional materials. In 2004, Professor Geim and Professor Novoselov, winners of the Nobel Prize in Physics, were the first to develop mechanical cleavage technology and obtain single-layer graphene, setting off a research boom in two-dimensional materials. In the past decade, mechanical cleavage technology has been widely used to prepare various high-quality two-dimensional materials. The intrinsic physical properties of many materials such as graphene, MoS2 and the single-layer high-temperature superconducting material Bi2212 are all observed on mechanically cleaved samples. In artificial crystals such as heterojunctions and corner graphene, mechanically cleaved samples also exhibit unique advantages. The mechanically cleaved sample has weak interaction with the substrate, the preparation process is relatively simple, and the sample quality is high. These advantages make this method a great success in the study of two-dimensional materials. However, with the deepening of research, people found that this method also has many shortcomings, especially low preparation efficiency and small sample size, which limits many advanced experimental methods such as scanning tunneling microscopy (STM), infrared-terahertz spectroscopy and angular spectroscopy. Research on two-dimensional materials using resolved photoelectron spectroscopy (ARPES).

Figure 1. Comparison of interlayer binding energy and interaction energy with gold of different layered materials.

In 2015, Dr. Huang Yuanhe and Professor Peter Sutter from Brookhaven National Laboratory (BNL) in the United States collaborated with Gao Hongjun, an academician of the Chinese Academy of Sciences and a researcher at the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics. , uses oxygen plasma A new cleavage method that enhances the interaction between graphene and the substrate (ACS Nano. 9 (11), 10612 (2015)), successfully obtained millimeter-scale single-layer graphene and high-temperature superconducting material Bi2212, which greatly improved The increase in sample size and preparation efficiency makes it possible to study more physical properties of single-layer single crystal graphene and Bi2212. With this improved mechanical cleavage technology, they also successfully prepared graphene bubbles and wrinkles (Physical Review Letters, 120, 186104 (2018); Carbon 156, 24 (2020)), and for the first time reported graphene bubbles. Newton rings with Raman oscillatory behavior were observed in .

Figure 2. Mechanical cleavage obtains a variety of large-area, high-quality, ultra-thin two-dimensional materials. (a) New mechanical cleavage steps; (b-e) Large-area MoS2 obtained by cleavage on different substrates; (f-g) Various large-area 2D materials obtained by cleavage; (h-j) Heterojunction and suspended 2D materials Raman spectrum and fluorescence spectrum.

Recently, The team of Zhou Xingjiang and Gao Hongjun, researchers at the Institute of Physics, Chinese Academy of Sciences, collaborated with Ji Wei, a professor at Renmin University of China, and Peter Sutter, a professor at the University of Nebraska-Lincoln. has made new progress in the field of mechanical cleavage technology. They developed a gold film-assisted universal mechanical cleavage method that can be used to obtain large-scale ultrathin two-dimensional materials. Combining the interaction rules of different elements in the periodic table of elements, the Jiwei team systematically calculated the interaction between 58 layered material systems and gold substrates (Figure 1). Since there is a van der Waals interaction between two-dimensional material layers, and gold can form quasi-covalent bonds with many two-dimensional materials, this interaction is much greater than the van der Waals interaction. Therefore, with the help of gold as a media layer, it can be achieved without affecting the intrinsic properties of the material. Under the premise of physical properties, large-area single-layer samples can be efficiently cleaved. Huang Yuan, associate researcher at the Institute of Physics, and others have successfully achieved large-area cleavage of 40 two-dimensional materials in experiments. The size of a single layer of two-dimensional materials has reached the order of millimeters or more (Figure 2 and Figure 3), and the preparation efficiency is close to 100%. This study shows that the interaction between the outermost elements of layered materials and the substrate is the most critical factor affecting mechanical cleavage. Therefore, for layered materials whose outermost elements contain main groups VA, VIA, and VIIA, a gold film can be used Auxiliary cleavage methods.

Figure 3. Optical photos of various 2D materials obtained by cleavage.

Figure 4. Atomic images, low-energy electron diffraction spots and energy band structure diagrams obtained on a large-area single-layer sample.

More importantly, this cleavage method has good flexibility, and can control in many aspects. First of all, the preparation process does not require a continuous gold film and can efficiently prepare suspended samples, which provides an ideal research system for studying the intrinsic optical properties and transport properties of materials; secondly, this method can realize the conductivity of the substrate. Regulation can selectively change the conductivity and insulation of the substrate according to different experimental requirements. For vacuum characterization methods that require substrate conductivity, such as scanning tunneling microscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES), can directly cleave two-dimensional materials onto the gold film by increasing the thickness of the gold film. To study its atomic structure and energy band structure (Figure 4). In previous research progress, researcher Liu Guodong and Dr. Zhao Wenjuan of Zhou Xingjiang’s team used ARPES to observe a clear energy band structure on mechanically cleaved large-area single-layer MoS2 (Nano Research, 12(12): 3095 (2019) )). For fluorescence spectrum and electrical transport measurements, the thickness of the metal film can be controlled below 3 nm to form an insulating metal island, thereby obtaining a good fluorescence signal and a high switching ratio field effect transistor (Figure 5), which is also internationally recognized. For the first time, high-performance devices were obtained on ultra-thin metal films, breaking people's previous understanding that device processing must be achieved on conventional oxide insulating substrates. In addition, the preparation process of this method avoids contamination and damage caused by additional transfer, and the gold used is only a few nanometers, which greatly saves the consumption of precious metals and provides new ideas for preparing high-quality two-dimensional materials.

Figure 5. By controlling the thickness of the metal film, an insulating metal film can be obtained, which can achieve high switching ratio and superconducting property measurement in the device.

Huang Yuan and others used this technology to cleave large-area single-layer materials such as FeSe, PtTe2 and PdTe2 for the first time in the world, laying a good foundation for the subsequent exploration of the physical properties of some new materials. This cleavage method shows very good universality and can achieve effective cleavage on transparent and flexible substrates, providing new ideas for a variety of optical research and flexible device design.

This research result provides for the first time universal cleavage rules for different layered materials. It plays an important role in exploring more novel physical properties of two-dimensional materials and also provides future large-area wafer-level two-dimensional materials. provides new possibilities for its preparation and application. Relevant results were published in the recent "Nature Communications" magazine (Nature Communications, 11, 2453 (2020)). This work was funded by the Key R&D Program of the Ministry of Science and Technology, the General Project of the National Foundation of China, the Pilot Program of the Chinese Academy of Sciences, the Youth Promotion Association and the Guangdong Songshan Lake Laboratory, as well as the help of teachers and students in the Microprocessing Laboratory and N07 Group of the Institute of Physics. (Source: Institute of Physics)

Paper link

https://www.nature.com/articles/s41467-020-16266-w