Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO2/Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS2, MoSe2) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions. DOI:10.1002/advs.202204247
Yuan Huang, Yun-Kun Wang, Xin-Yu Huang, Guan-Hua Zhang, Xu Han, Yang Yang, Yunan Gao, Lei Meng, Yushu Wang, Guang-Zhou Geng, Li-Wei Liu, Lin Zhao, Zhi-Hai Cheng, Xin-Feng Liu, Ze-Feng Ren, Hui-Xia Yang, Yufeng Hao, Hong-Jun Gao, Xing-Jiang Zhou, Wei Ji & Ye-Liang Wang
Abstract
Two-dimensional (2D) materials are highly sensitive to substrates, interfaces, and the surrounding environments. Suspended 2D materials are free from substrate-induced effects, thus an ideal approach to study their intrinsic properties. However, it is very challenging to prepare large-area suspended 2D materials with high efficiency. Here we report a universal method, based on pretreatments of densely patterned hole array substrates with either oxygen-plasma or gold film deposition, to prepare large-area suspended mono- and few-layer 2D materials. Multiple structural, optical, and electrical characterization tools were used to fully evaluate the improved performance of various suspended 2D layers. Some of these observations reported in this study are: (1) Observation of a new Raman low frequency mode for the suspended MoS2; (2) Significantly stronger photoluminescence (PL) and second harmonic generation (SHG) signals of suspended WSe2, which enables the study of new optical transition processes; (3) The low energy electron diffraction pattern on suspended MoS2 also exhibits much sharper spots than that on the supported area; and (4) The mobility of suspended graphene device approaches 300 000 cm2 V−1 s−1, which is desirable to explore the intrinsic properties of graphene. This work provides an innovative and efficient route for fabricating suspended 2D materials, and we expect that it can be broadly used for studying intrinsic properties of 2D materials and in applications of hybrid active nanophotonic and electronic devices.
Yuan Huang#, Yu-Hao Pan#, Rong Yang#, Li-Hong Bao, Lei Meng, Hai-Lan Luo, Yong-Qing Cai, Guo-Dong Liu, Wen-Juan Zhao, Zhang Zhou, Liang-Mei Wu, Zhi-Li Zhu, Ming Huang, Li-Wei Liu, Lei Liu, Peng Cheng, Ke-Hui Wu, Shi-Bing Tian, Chang-Zhi Gu, You-Guo Shi, Yan-Feng Guo, Zhi Gang Cheng, Jiang-Ping Hu, Lin Zhao, Guan-Hua Yang, Eli Sutter, Peter Sutter*, Ye-Liang Wang, Wei Ji*, Xing-Jiang Zhou* & Hong-Jun Gao*
Abstract:
Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.
