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      Friction characteristics of mechanically exfoliated and CVD- grown single-layer MoS 2

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          In this work, the friction characteristics of single-layer MoS 2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS 2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS 2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS 2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS 2 at 800  °C and 850  °C were relatively similar to those of mechanically exfoliated MoS 2 whereas the crystalline quality of the CVD-grown single-layer MoS 2 at 900  °C was lower. The CVD-grown single-layer MoS 2 exhibited higher friction than mechanically exfoliated single-layer MoS 2, which might be related to the crystalline imperfections in the CVD-grown MoS 2. In addition, the friction of CVD-grown single-layer MoS 2 increased as the CVD growth temperature increased. In terms of tribological properties, 800  °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS 2 and elucidating the feasibility of single-layer MoS 2 as protective or lubricant layers for micro- and nano-devices.

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          Most cited references 45

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          Stretching and breaking of ultrathin MoS2.

          We report on measurements of the stiffness and breaking strength of monolayer MoS(2), a new semiconducting analogue of graphene. Single and bilayer MoS(2) is exfoliated from bulk and transferred to a substrate containing an array of microfabricated circular holes. The resulting suspended, free-standing membranes are deformed and eventually broken using an atomic force microscope. We find that the in-plane stiffness of monolayer MoS(2) is 180 ± 60 Nm(-1), corresponding to an effective Young's modulus of 270 ± 100 GPa, which is comparable to that of steel. Breaking occurs at an effective strain between 6 and 11% with the average breaking strength of 15 ± 3 Nm(-1) (23 GPa). The strength of strongest monolayer membranes is 11% of its Young's modulus, corresponding to the upper theoretical limit which indicates that the material can be highly crystalline and almost defect-free. Our results show that monolayer MoS(2) could be suitable for a variety of applications such as reinforcing elements in composites and for fabrication of flexible electronic devices.
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            Integrated circuits and logic operations based on single-layer MoS2.

            Logic circuits and the ability to amplify electrical signals form the functional backbone of electronics along with the possibility to integrate multiple elements on the same chip. The miniaturization of electronic circuits is expected to reach fundamental limits in the near future. Two-dimensional materials such as single-layer MoS(2) represent the ultimate limit of miniaturization in the vertical dimension, are interesting as building blocks of low-power nanoelectronic devices, and are suitable for integration due to their planar geometry. Because they are less than 1 nm thin, 2D materials in transistors could also lead to reduced short channel effects and result in fabrication of smaller and more power-efficient transistors. Here, we report on the first integrated circuit based on a two-dimensional semiconductor MoS(2). Our integrated circuits are capable of operating as inverters, converting logical "1" into logical "0", with room-temperature voltage gain higher than 1, making them suitable for incorporation into digital circuits. We also show that electrical circuits composed of single-layer MoS(2) transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced.
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              Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing NO at room temperature.

              Single- and multilayer MoS(2) films are deposited onto Si/SiO(2) using the mechanical exfoliation technique. The films were then used for the fabrication of field-effect transistors (FETs). These FET devices can be used as gas sensors to detect nitrous oxide (NO). Although the single-layer MoS(2) device shows a rapid response after exposure to NO, the current was found to be unstable. The two-, three-, and four-layer MoS(2) devices show both stable and sensitive responses to NO down to a concentration of 0.8 ppm. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

                Author and article information

                Tsinghua Science and Technology
                Tsinghua University Press (Xueyuan Building, Tsinghua University, Beijing 100084, China )
                05 December 2018
                : 06
                : 04
                : 395-406 (pp. )
                [ 1 ] School of Mechanical Engineering, University of Ulsan, Ulsan 44610, Republic of Korea
                [ 2 ] Department of Physics and Energy Harvest Storage Research Center, University of Ulsan, Ulsan 44610, Republic of Korea
                Author notes
                * Corresponding author: Koo-Hyun CHUNG, E-mail: khchung@

                Dinh Le Cao KY. He received his M.S. degree in mechanical engineering in 2010 from University of Technology, Ho Chi Minh City, Vietnam. He is currently pursuing his PhD degree in the Tribology and Surface Engineering Laboratory at University of Ulsan, Republic of Korea. His research interests include fundamental understanding of friction characteristics of nanomaterials from experiments and molecular dynamics simulation.

                Bien-Cuong TRAN KHAC. He received his M.S. degree in mechanical engineering in 2014 from University of Ulsan, Republic of Korea. He is currently pursuing his PhD degree in the Tribology and Surface Engineering Laboratory at the same university. His research interests include tribology and surface damage characteristics of atomically thin (2D) materials.

                Chinh Tam LE. He received his B.S. degree in material science in 2012 from Ho Chi Minh University of Natural Science in Vietnam. After then, he joined the Semiconductor Device Research Laboratory from 2013 and is currently the PhD student in University of Ulsan, Republic of Korea. His interest research is synthesis and optical characterization of monolayer transition metal dichalcogenides MX2.

                Yong Soo KIM. He received his M.S. and PhD degrees in physics from Seoul National University, Republic of Korea in 1993 and 1998, respectively. After then, he was a senior and principle researcher at R&D division, SK-Hynix Inc. He joined the Department of Physics at University of Ulsan, South Korea from 2008. His current position is associate professor, chair of physics department and director of human resource center for novel materials research experts (BK21+ program). His research interest includes organic- inorganic hybrid solar cell, 2-D layer materials relating optoelectronic device, especially transitional metal dichalcogenide growth and its optical characteristics.

                Koo-Hyun CHUNG. He received his M.S. and PhD degrees in mechanical engineering from Yonsei University, Republic of Korea, in 1997 and 2005, respectively. His current position is an associate professor at the School of Mechanical Engineering, University of Ulsan, South Korea. His research areas cover tribology, micro/nano tribology, adhesion, surface engineering, molecular dynamics simulation, as well as themes relating to material science.


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