News Source: Advanced Research Institute of Interdisciplinary Science

Editor: Zhu Qianyun

Reviewer: Tang Shuiyuan

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. In recent years, 2D materials have received widespread attention from scientists and industry for their unique properties. The fabrication strategies of 2D materials can be divided into two strategies: "bottom-up" and "top-down". The "bottom-up" preparation strategy is represented by Chemical vapor deposition (CVD) and molecular beam epitaxy (MBE); the "top-down" fabrication strategy is represented by mechanical exfoliation. In the past two decades, CVD methods for the preparation of low-dimensional nanomaterials, especially for the growth of two-dimensional materials, have been extensively studied and many important advances have been made. However, the "top-down" preparation of solvation techniques has only received extensive attention in the last five years, so there are still many important scientific questions to be solved in this research area, which is a new and dynamic direction in 2D materials research.

The preparation of 2D materials using conventional exfoliation techniques produce small-area 2D monolayers, where the yield ratio is low, making it difficult for various high precision test characterization tools and the need for large area samples in device applications. In 2015, Prof. Peter Sutter and Dr. Yuan Huang proposed a method to clean silicon wafers by oxygen plasma and successfully prepared millimeter-scale monolayer graphene as well as high temperature copper-based superconducting material, bismuth strontium calcium copper oxygen (BSCCO). However, the adsorption energy of other2D materials with oxygen-containing surfaces is not high, leading to the unremarkable effect of this method on other materials. In 2020, Prof. Yuan Huang and the team of Academician Hongjun Gao, the team of Researcher Xingjiang Zhou, and the team of Prof. Wei Ji systematically investigated the method of generalized deconstruction of large-area 2D materials by gold surfaces, and successfully exfoliated 40 millimeter-scale 2D materials (Nature Communications, 2020 11:2453). By controlling the thickness of the gold film, it is possible to directly exfoliate 2D materials onto conductive or insulating substrates, providing a flexible option for conducting various characterization tests. However, the contact between the ultra-flat metal surface and 2D materials will produce charge transfer at the interface, resulting in complete quenching of the fluorescence of the two-dimensional semiconductor, which brings a lot of inconvenience to the optical research of 2D materials. At the same time, the cost of large-area exfoliation techniques with gold as the medium is high, which brings challenges to the development of large-scale preparation and application of two-dimensional materials. The possibility of using other metals instead of gold to reduce the cost and realize the large-area exfoliation is a very important issue for both research and applications. To answer this question, Prof. Wei Ji of Renmin University of China and PhD student Jiaqi Dai et al. used first-principles to calculate the interaction between 16 common 2D materials and Ag (111) surfaces, and the results showed that covalent-like quasi-bonding (CLQB) can be formed between the two interfaces. The strong interactions caused by CLQB exceed the interlayer van der Waals interactions, making Au promising as a new medium for generalized large-area 2D material exfoliation techniques with a mechanism similar to Au-assisted exfoliation. Unlike Au, however, Ag does not wet with Ti, causing the Ag film produced by vapor deposition on the Ti surface to form a nanoparticle-like structure. With the help of finite-domain difference simulations and tests of silver film extinction spectra, the researchers found that this rough Ag surface may excite long-range propagation of surface plasmon polaritons, resulting in a dramatic enhancement of the interaction with light in samples obtained by exfoliation on Ag.

Figure 1. Results of first principles calculation of the interlayer interaction of layered materials and the interaction between layered materials and Ag atom interface.

Figure 2. Roughness characterization of Ag films surface and finite-difference time-domain simulation results

In order to test the feasibility of Ag-assisted exfoliation and whether the silver film indicates the existence of surface plasmon polaritons, Prof. Yuan Huang and Prof. Yeliang Wang of Beijing Institute of Technology, together with Prof. Zhenhua Ni and Junpeng Lu of Southeast University, conducted a joint research to exfoliate 12 types of single-crystalline monolayers with millimeter-size on the silver film and verified the universality of this silver film exfoliation technique. By comparing the low wavelength Raman spectra of 1-5 layers of MoS2 on silver and the overhanging regions of the same layers, it was found that the interlayer Raman vibrational modes of the samples on silver were greatly suppressed, which experimentally demonstrated the existence of quasi-covalent bonding-induced pegging effect between the 2D material and the Ag film. Surprisingly, by measuring the photoluminescence of a monolayer of MoS2 and MoSe2 on Ag, the researchers found that both the 2D semiconductors exhibited extremely strong fluorescence on the Ag film, even several times stronger than their overhanging monolayer regions. Researchers further studied the mechanism of this fluorescence enhancement and the experimental results showed that the Purcell effect and the exciton re-excitation effect can be generated in the interfacial structure of 2D material and Ag, thus making the fluorescence of the two-dimensional semiconductor greatly enhanced, and these two effects can only be generated on the surface of the rough Ag films.

Figure 3. Ag-assisted exfoliation procedures and optical photos and fluorescence imaging of exfoliated samples.

Figure 4. PL behavior of four exfoliated TMDC semiconductors on Ag film.

This work achieves the first one-step method of solving large-area 2D materials and integration of equipartite excitonic structures using Ag by combining theoretical and experimental research methods, and the experimental results well verify the theoretical and simulated inferences. In addition, the work indicates that the modulation of the intensity of light-matter interaction can be achieved by controlling the thickness of Ag. Ag-assisted exfoliation technique is not only important for basic research of emergent 2D materials, but also lays the foundation for the future development of large-area preparation of 2D materials and application directions such as optical waveguide integration. On September 14, 2022, the results were published in the top international journal Advanced Science under the title of "One-Step Exfoliation Method for Plasmonic Activation of Large-Area 2D Crystals" (Adv. Sci.2022, 2204247, Impact Factor: 17.521), with Beijing Institute of Technology as the first author, Prof. Yuan Huang, Prof. Yeliang Wang, Prof. Zhenhua Ni from Southeast University, and Prof. Wei Ji from Renmin University of China as co-corresponding authors and Fu Qiang as the first author. This work was supported by the National Natural Science Foundation of China, Ministry of Science and Technology, Beijing Natural Science Foundation, Chongqing Distinguished Youth Fund, and CAS Pioneer B.

Paper link:

https://onlinelibrary.wiley.com/doi/10.1002/advs.202204247

About the author:

Yuan Huang, professor and PhD supervisor of Advanced Research Institute of Interdisciplinary Science, Beijing Institute of Technology. His main research areas are focused on the exfoliation, characterization, device processing and physical property measurement/modulation of 2D materials. He has published more than 80 SCI papers inNature Physics,Nature Communications,Physical Review Letters,ACS Nano, etc., including more than 40 first-author (including co-author) and corresponding-author articles, with more than 4500 citations in total. He presided over the National Key Research and Development Program (Youth Project), the Outstanding Youth Fund and General Project of the NSFC, and the Outstanding Youth Fund project of Chongqing Municipality. He was elected as a member of CASYP in 2019, and was honored as one of the "Top Ten New Scientists in China" by CASA in 2020, and received the Invention Award of China Invention Association in 2021. He was awarded the second prize of Innovation Award of China Invention Association in 2021 (ranked 1st) and the second prize of Achievement Award of China Invention Association in 2022 (ranked 2nd). He is a reviewer ofPhysical Review Letters,Nature Communications,Advanced Functional Materials,ACS Nano,Nano Letters,Journal of Physics, etc. He is the young editorial board member of the journalsPhysics,Chinese Physics Letters,InfoMatandMaterials.