Abstract
Exfoliation of large-area monolayers is important for fundamental research and technological implementation of transition-metal dichalcogenides. Various techniques have been explored to increase the exfoliation yield, but little is known about the underlying mechanism at the atomic level. Here, we demonstrate gold-assisted mechanical exfoliation of monolayer molybdenum disulfide, up to a centimeter scale. Detailed spectroscopic, microscopic, and first-principles density functional theory analyses reveal that strong van der Waals (vdW) interaction between Au and the topmost MoS2 layer facilitates the exfoliation of monolayers. However, the large-area exfoliation promoted by such strong vdW interaction is only achievable on freshly prepared clean and smooth Au surfaces, while rough surfaces and surfaces exposed to air for more than 15 min result in negligible exfoliation yields. This technique is successfully extended to MoSe2, WS2, WSe2, MoTe2, WTe2, and GaSe. In addition, electrochemical characterization reveals intriguing interactions between monolayer MoS2 and Au. A subnanometer-thick MoS2 monolayer strongly passivates the chemical properties of the underlying Au, and the Au significantly modulates the electronic band structure of the MoS2, turning it from semiconducting to metallic. This could find applications in many areas, including electrochemistry, photovoltaics, and photocatalysis.
Highlights
Monolayer transition-metal dichalcogenides (TMDCs) possess a wide range of extraordinary optoelectronic, chemical, and mechanical properties unattainable in their bulk form.[1−3] Many bulk TMDCs are semiconductors with an indirect band gap, which transitions to a direct band gap when in monolayer form, rendering them excellent materials for optoelectronics.[4−6] A major challenge for large-scale application of TMDCs is the competition between material quality and production scalability
After 15 min, almost no crystals are exfoliated. This suggests that the adhesion forces between MoS2 and Au strongly depend on how long the Au surface has aged in air, which we summarize in the following three stages (Figure 2g−i): Initially (1), the adhesion between Au and the first MoS2 layer is stronger than the interaction between the first MoS2 layer and the rest of MoS2, resulting in the predominant exfoliation of monolayer MoS2
We present a facile gold-assisted mechanical exfoliation of extraordinarily large monolayer MoS2, up to a centimeter-scale, limited mainly by the parent bulk crystal size
Summary
Monolayer transition-metal dichalcogenides (TMDCs) possess a wide range of extraordinary optoelectronic, chemical, and mechanical properties unattainable in their bulk form.[1−3] Many bulk TMDCs are semiconductors with an indirect band gap, which transitions to a direct band gap when in monolayer form, rendering them excellent materials for optoelectronics.[4−6] A major challenge for large-scale application of TMDCs is the competition between material quality and production scalability. Photoluminescence (PL) measurements, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) collectively reveal that the high-yield exfoliation is facilitated by van der Waals (vdW) interaction between the Au surface and the topmost MoS2 layer. This interaction is stronger than the interlayer vdW interactions in bulk MoS2, facilitating the exfoliation of large-area monolayer films. Mo separations and surface roughness, show excellent agreement with the experiments We successfully applied this exfoliation technique to a variety of metal chalcogenides, including MoSe2, WS2, WSe2, MoTe2, WTe2, and GaSe (Supporting Figure S1), and found that the monolayer yield is generally near-unity, limited only by the size of the parent bulk crystal
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
