Full paperDirectional dependent piezoelectric effect in CVD grown monolayer MoS2 for flexible piezoelectric nanogenerators
Graphical abstract
We report directional dependent piezoelectric effects in chemical vapor deposition grown monolayer MoS2 for flexible piezoelectric nanogenerators. The output power obtained from the nanogenerator with the armchair direction of MoS2 is about two times higher than that from the nanogenerator with the zigzag direction of MoS2 under the same strain.
Introduction
Considerable scientific efforts are being expanded towards realizing electronic components for transparent flexible self-powered electronic switches, skins, sensors, etc. Experimental studies on the physical properties of two-dimensional (2D) materials have grown exponentially since 2D materials offer unique advantages for use in such next-generation devices [1], [2], [3], [4], [5]. Various semiconducting 2D materials have been studied, including transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten diselenide (WSe2), which are likely to bring breakthroughs in future electronic and optoelectronic devices [6], [7], [8], [9], [10], [11]. The physical properties of 2D MoS2 nanosheets have been actively explored particularly as a result of their possible integration in both nano/micro-electromechanical devices and energy harvesting devices [4], [7]. Monolayer MoS2 has a direct band gap and high mobility [6], [11] and has been used to successfully fabricate field-effect transistors [8], [11], [12], [13], [14] so it has emerged as an interesting complement to graphene in various semiconducting applications.
In previous theoretical studies, most of 2D monolayer materials may exhibit piezoelectric properties, unlike its bulk parent crystal [15], [16], [17]. Remarkably, the calculation of the piezoelectric coefficient for monolayer MoS2 according to density-functional theory revealed that the monolayer structure exhibits a stronger piezoelectric coupling than the bulk wurtzite structured materials [15]. Nevertheless, experimental evidence of the piezoelectricity of 2D monolayer MoS2 has not yet been sufficiently provided although very recently few studies on the experimental observation of intrinsic piezoelectric properties of MoS2 reported that the piezoelectricity from MoS2 only exists when there are an odd number of layers in the 2D crystal [5], [8]. Here we report directional dependent piezoelectric effects in chemical vapor deposition (CVD) grown monolayer MoS2 using lateral piezoresponse force microscopy (PFM) [18], [19] measurements. In addition, it was found that the piezoelectric power output from piezoelectric nanogenerators (NGs) fabricated with monolayer MoS2 is strongly dependent on the MoS2 atomic orientation along either armchair or zigzag direction, which further confirms that the magnitude of the piezoelectric polarization in monolayer MoS2 significantly depends on the atomic orientation axis of MoS2.
Section snippets
Synthesis of monolayer MoS2 by CVD
Triangular-shaped monolayer MoS2 flakes has been synthesized in the conventional atmospheric pressure CVD method [20], [21]. In detail, 5 mg of MoO3 (Sigma-Aldrich, 203815) and 200 mg of solid S (Sigma-Aldrich, 344621) were loaded into the tube reactor which are at 210 °C (zone 1) and 780 °C (zone 2). As a synthesized template, SiO2 (300 nm)/Si wafer was faced-down above MoO3 crucible and total growth time is 15 min including ramping up period for both zone 1 and 2. After cooling step, the triangular
Results and discussion
2D layered MoS2 is expected to exhibit piezoelectric effects due to the non-centro-symmetric arrangement of the Mo and S atoms (Fig. 1a). Piezoelectricity in 2D MoS2 structures arises from the development of asymmetrical electrical dipoles that are induced when the material is subjected to an external stress [22]. An optical microscope image in Fig. 1b shows that the present work is based on CVD-grown triangular-shaped single-crystalline monolayer MoS2 flakes [23], [24], [25]. Geometric
Conclusions
In conclusion, the unique directional dependent piezoelectric effect of the CVD-grown triangular-shaped single-crystalline monolayer MoS2 flake was qualitatively studied by using lateral PFM. We successfully perform an experiment where the piezoelectric coefficient, d11, was measured, showing the anisotropic piezoresponse in the single-crystalline monolayer MoS2. It was found that the d11 of MoS2 in the armchair direction is 3.78 pm/V, while the d11 of MoS2 in the zigzag direction is 1.38 pm/V,
Acknowledgments
S. K. Kim and R. Bhatia contributed equally to this work. This work was financially supported by the Center for Advanced Soft-Electronics as the Global Frontier Project (2013M3A6A5073177) and Institute for Basic Science Program (IBS-R011-D1) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning.
Sung Kyun Kim is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU). His research interests include atomic force microscopy studies of piezoelectric, triboelectric and ferroelectric materials and characterization of 2D materials.
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Sung Kyun Kim is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU). His research interests include atomic force microscopy studies of piezoelectric, triboelectric and ferroelectric materials and characterization of 2D materials.
Dr. Ravi Bhatia worked as a Postdoctoral Fellow with Prof. Sang-Woo Kim at Sungkyunkwan University (SKKU), South Korea. He earned his doctorate degree from Department of Physics, Indian institute of Science (IISc), Bangalore in 2012. During Ph.D. tenure, he worked on the low temperature charge transport and magnetic properties of iron-filled multiwall carbon nanotube (MWCNT), and MWCNT based composite systems. His current research interests are focused on growth and applications of two dimensional materials.
Tae-Ho Kim is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU). His research interests include synthesis 2D materials such as graphene, h-BN and 2D materials based piezoelectric nanogenerators.
Daehee Seol is a Ph.D. student under the supervision of Prof. Yunseok Kim at School of Advanced Materials Science & Engineering, Sungkyunkwan university (SKKU). His research interests include scanning probe microscopy based studies of electromechanical, ferroelectric, ionic phenomena at the nanoscale.
Jung Ho Kim is a Ph.D. student under the supervision of Prof. Young Hee Lee at Center for Integrated Nanostructure Physics (CINAP) of Institute of Basic Science (IBS) and Department of Energy Science, Sungkyunkwan University (SKKU). His research interests include fabrication and characterization of electrical devices using two-dimensional materials such as graphene, h-BN, and transition metal dichalcogenides.
Hyun Kim is a Ph.D. student under the supervision of Prof. Young Hee Lee at Center for Integrated Nanostructure Physics (CINAP) of Institute of Basic Science (IBS) and Department of Energy Science, Sungkyunkwan University (SKKU). His research interests include synthesis of two dimensional materials such as transition metal dichalcogenides and black phosphorus.
Wanchul Seung is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU). His research interests are fabrications and characterizations of piezoelectric and triboelectric nanogenerator energy harvesting and their applications in self-powered devices.
Dr. Yunseok Kim was born in Korea, 1979. He received the M.S. and the Ph.D. degrees in Materials Science and Engineering from Korea Advanced Institute of Science and Technology (KAIST), Korea, in, respectively, 2004 and 2007. From 2008 to 2010, he was awarded the Humboldt research fellowship from the Alexander von Humboldt foundation which allowed him to work as a postdoctoral researcher at Max Planck Institute of Microstructure Physics, Germany. Then, from 2011 to 2012, he worked as a postdoctoral researcher at Oak Ridge National Laboratory, USA. Since 2012, Yunseok Kim is Assistant Professor at the School of Materials Science and Engineering, Sungkyunkwan University (SKKU), Korea. His research interests include scanning probe microscopy studies of electromechanical, ferroelectric, transport and ionic phenomena at the nanoscale.
Dr. Young Hee Lee is a director of the Center for Integrated Nanostructure Physics, Institute for Basic Science. He is also a Professor in the Department of Energy Science and Department of Physics at Sungkyunkwan University, Korea. He received his B.Sc. degree in physics from Chonbuk National University, Korea, and his Ph.D. degree in physics from Kent State University, USA. His research interests include exploration of unprecedented physical and chemical properties of 2D layered materials and carbon-based materials and their applications to electronic devices and energy storage.
Dr. Sang-Woo Kim is a Professor in School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). He received his Ph.D. from Kyoto University in Department of Electronic Science and Engineering in 2004. After working as a postdoctoral researcher at Kyoto University and University of Cambridge, he spent 4 years as an assistant professor at Kumoh National Institute of Technology. He joined the School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT) at SKKU in 2009. His recent research interests include piezoelectric/triboelectric/pyroelectric nanogenerators, hybrid energy harvesting/storage devices, flexible sensors, etc. Now he is an Associate Editor of Nano Energy and an Executive Board Member of Advanced Electronic Materials.
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These authors contributed equally to this work.