Progress in the preparation of large-area two-dimensional materials by universal mechanical cleavage technology

[ Instrument R & D ] Two-dimensional material family covers insulators, semiconductors, metals and superconductors, and exhibits many novel physical properties different from three-dimensional materials. It is a research hotspot in the field of condensed matter physics and materials science in recent years. The preparation of high-quality two-dimensional materials, especially ultra-thin materials at the atomic level, is the basis for the frontier exploration of two-dimensional materials.

In 2004, Professors Geim and Novoselov, winners of the Nobel Prize in Physics, first developed mechanical cleavage technology and obtained a single layer of graphene, setting off a wave of research on two-dimensional materials. In the past ten years, 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 single-layer high-temperature superconducting material Bi2212 are 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, it was found that this method also has many shortcomings, especially the problem of low preparation efficiency and small sample size, which limits many advanced experimental methods such as scanning tunneling microscope (STM), infrared-terahertz spectroscopy and angle Resolved photoelectron spectroscopy (ARPES) study of two-dimensional materials.

In 2015, Dr. Huang Yuanhe and Professor Peter Sutter of Brookhaven National Laboratory (BNL) in the United States collaborated with Gao Hongjun, academician of the Chinese Academy of Sciences and researcher of the Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics, to enhance graphene with oxygen plasma A new cleavage method that interacts with the substrate (ACS Nano. 9 (11), 10612 (2015)), successfully obtained a single-layer graphene and Bi2212, a high-temperature superconducting material on the order of millimeters, greatly improving the sample size and preparation The efficiency makes it possible to study more physical properties of single-layer single-crystal graphene and Bi2212. With this improved mechanical cleavage technique, they also successfully prepared graphene bubbles and folds (Physical Review Letters, 120, 186104 (2018); Carbon 156, 24 (2020)), and were first observed in graphene bubbles Newton's ring with Raman oscillating behavior.

Recently, the team of researchers Zhou Xingjiang and Gao Hongjun of the Institute of Physics of the Chinese Academy of Sciences cooperated with Professor Ji Wei of Renmin University of China and Professor Peter Sutter of Lincoln University of Nebraska to make new progress in the field of mechanical cleavage technology. They developed a universal mechanical cleavage method assisted by gold film, which can be used to obtain large-scale ultra-thin two-dimensional materials. Based on the interaction law of different elements in the periodic table, Jiwei's team systematically calculated the interaction between 58 layered material systems and gold substrates (Figure 1). Since the two-dimensional material layer is Van der Waals interaction, and gold and many two-dimensional materials can form a quasi-covalent bond, this interaction is much larger than the Van der Waals interaction, so with the aid of gold as a media layer, the material's intrinsic physical properties can be affected without affecting Under the premise of efficiently cleaving a large area of ​​single-layer samples. Huang Yuan, associate researcher of the Institute of Physics, has successfully achieved large-scale cleavage of 40 kinds of two-dimensional materials in experiments, and the size of single-layer two-dimensional materials has reached more than millimeters (Figure 2 and Figure 3), and the preparation efficiency is close to 100% . The study shows that the interaction between the outermost element of the layered material and the substrate is the most critical factor affecting mechanical cleavage. Therefore, for the layered material containing the main groups of VA, VIA, and VIIA, the gold film can be used Auxiliary cleavage method.

More importantly, this cleavage method has good flexibility and can be adjusted in many ways. First, the preparation process does not require a continuous gold film, which can efficiently achieve the preparation of suspended samples, which provides an ideal research system for studying the intrinsic optical properties and transport properties of materials; second, this method can realize the adjustment of the conductivity of the substrate For different experimental requirements, the conductivity and insulation of the substrate can be selectively changed. For vacuum characterization methods such as scanning tunneling microscope (STM / STS) and angle-resolved photoelectron spectroscopy (ARPES) that require substrate conductivity, the thickness of the gold film can be increased to directly cleave the two-dimensional material onto the gold film for Study its atomic structure and energy band structure (Figure 4). In the previous research progress, Zhou Guojiang researchers Liu Guodong and Dr. Zhao Wenjuan and others used ARPES to observe a clear energy band structure on a large-area single-layer MoS2 that was mechanically cleaved (Nano Research, 12 (12): 3095 (2019 )). For fluorescence spectroscopy and electrical transport measurement, the thickness of the metal film can be controlled below 3 nm to form an insulated metal island to obtain a good fluorescence signal and a field effect transistor with a high switching ratio (Figure 5), which is also international Obtaining a high-performance device on an ultra-thin metal film for the first time has broken people's previous understanding that device processing must be realized on a conventional oxide insulating substrate. In addition, the preparation process of this method avoids pollution 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 the preparation of high-quality two-dimensional materials.

Huang Yuan and others used the technology to cleave large-area single-layer FeSe, PtTe2 and PdTe2 materials for the first time in the world, which laid a good foundation for the subsequent exploration of the physical properties of some new materials. This cleaving method shows a very good universality, and can be effectively cleaved on a transparent substrate and a flexible substrate, which provides new ideas for various optical research and flexible device design.

This research result for the first time gives a general cleavage law for different layered materials, which is an important impetus for exploring more novel physical properties of two-dimensional materials, and also for the future large-scale wafer-level two-dimensional materials. Preparation and application provide new possibilities. Related results were published in the recent "Nature-Communications" magazine (Nature Communications, 11, 2453 (2020)). This work was supported by the Ministry of Science and Technology's key research and development plan, the fund commission project, the Chinese Academy of Sciences pilot plan and the Youth Promotion Association and the Guangdong Songshan Lake Laboratory, as well as the micromachining laboratory of the Institute of Physics and the teachers and students of the N07 group.