Laser-directed synthesis of strain-induced crumpled MoS2 structure for enhanced triboelectrification toward haptic sensors
Graphical abstract
Introduction
As human interface technologies continue to develop, an increasing number of electronic devices are capable of being implanted in or attached to the human body. For this reason, small-scale energy devices are regarded as essential to pursuit fourth-generation industrial innovations for an Internet of Things (IoT) electronics system consisting of portable and mobile electronics [1,2]. Because mechanical energy sources are omnipresent in the surrounding environment, a wide range of piezoelectric and triboelectric energy harvesting technologies have been studied that convert biomechanical movement and environmental vibration into electric energy that can be utilized by self-powered electronic devices or sensors [[3], [4], [5], [6]]. In particular, triboelectric nanogenerator (TENG) devices have attracted extensive attention both in academic research and the industrial sector, since they are capable of generating high electrical output as well as being manufactured by low-cost and easy processes [3,7,8].
The mechanism of the TENG system is based on the coupling effect between triboelectrification resulting from surface frictions and electrostatic induction through equivalent circuits [9,10]. Numerous materials for obtaining triboelectric effects from frictional surfaces have been investigated in previous studies, including polymers, ceramics, metals, and even some liquids [11]. Unfortunately, however, in spite of some progress in the development of TENG technologies, the field of self-powered triboelectric devices remains well short of its target: atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) nanomaterials (e.g., MoS2, WSe2, etc.), regarded as the most representative future electronic materials for the flexible, transparent, and/or nanoscale devices of tomorrow [12,13]. While a few studies have been conducted of 2D TMDC nanomaterials in triboelectric devices, they used conventional methods such as exfoliation processes or chemical vapor deposition (CVD) methods [[14], [15], [16], [17]]. These approaches have critical drawbacks, including restrictions in synthesis area, time, and/or cost problems. For example, the active surface area in triboelectric devices is important for the density of current output generated. More significantly, these conventional methods for fabricating 2D TMDC nanomaterials have been unable to demonstrate reliable morphological controllability, even though surface morphology and proper roughness are highly crucial in triboelectric effects and related TENG performance [[18], [19], [20]]. This limitation remains a critical hurdle to overcome in the investigation of triboelectric phenomena in 2D TMDC nanomaterials and related self-powered device applications. The well-known transfer technique after exfoliation or CVD synthesis is ineffective for morphological modifications, particularly for subsequent device fabrications with patterning [12,21,22]. Additional processes using 2D TMDC nanomaterials in device patterning have shown similar limitations with regard to achieving fast and cost-effective device fabrications.
To investigate the triboelectrification principles of 2D TMDC nanomaterials, we present here a direct single-step laser processing method for morphologically controlled synthesis of atomically thin 2D MoS2 layers, using a the photonic thermolysis mechanism. This directly patternable laser synthesis method tunes the surface topography of 2D MoS2 layers in non-vacuum atmosphere, without any additional treatment or modification. The single-step morphological tunability results from interfacial cavities caused by the laser-assisted separation of the underlying SiO2 thin film layer on the Si wafer. Both flat and crumpled MoS2 layers synthesized by laser-directed thermolysis are thoroughly investigated using various material characterizations to analyze triboelectric phenomena on MoS2 layers, as well as TENG device properties more generally. The flat MoS2 TENG device produces an energy harvesting output of 17 V, 0.85 μA, and 1.6 μW, whereas the crumpled MoS2 TENG generates 25 V, 1.2 μA, and 2.25 μW. On the basis of these results, it is evident that the crumpled MoS2 TENG generates ~40% more power than the flat one. Throughout the comprehensive material characterizations, the main determining factors in the performance enhancement are not only increasing the surface roughness but also changing the work function of the laser-assisted crumpled 2D MoS2 structure [23]. Finally, we transfer the patterned crumpled-MoS2 layers onto a flexible plastic substrate to show a practical application. The crumpled MoS2-based self-powered haptic sensor array detects well the movement of a stylus pen. By means of the noteworthy laser synthesis that it uses, this approach overcomes the limitations of previous 2D TMDC nanomaterials in the field of triboelectric energy harvesting and self-powered devices.
Section snippets
Development of laser-directed 2D MoS2 synthesis
Fig. 1a schematically illustrates the synthesis procedure for generally the flat MoS2 as well as for the crumpled MoS2 structure. Our photonic synthesis approach is based on the thermolysis of spin-coated (NH4)2MoS4 precursor film to form atomically thin 2D MoS2 layers, using a laser-directed annealing method on a thermally-oxidized silicon wafer (Si wafer with 300 nm-thick SiO2 thin film). As shown in Fig. S1, the (NH4)2MoS4 precursor (sky-blue colored region) demonstrates strong sensitivity
Conclusion
We have demonstrated a laser-directed synthesis method for achieving rapid, wafer-scale, and patternable 2D TMDC nanomaterial fabrication without using a vacuum. The MoS2-surface morphology and strain can also be easily controlled by the laser irradiation with the synthesis simultaneously. The thermal-interfacial interaction was extensively studied by a variety of characterizations, as well as electronic properties. In particular, it was determined that modulating the work function of MoS2 by
Synthesis method for laser-directed flat and crumpled MoS2
Several pieces of highly p-doped SiO2/Si (thermally oxidated SiO2 thin film, 300 nm in thickness) were sonicated for 10 min under ethanol and deionized water (DI water), in each case. To enhance hydrophilicity of the SiO2 surface, O2 plasma treatment was implemented at 100 W for 100 s, which provides good adhesive properties between the precursor solution and the SiO2/Si wafer. The MoS2 precursor, ammonium tetrathiomolybdate ((NH4)2MoS4, Sigma-Aldrich), was dissolved in the co-solvent of N,N
CRediT authorship contribution statement
Seoungwoong Park: Investigation, Data curation, Formal analysis, Writing - original draft. Jiseul Park: Investigation, Data curation, Formal analysis, Writing - original draft. Yeon-gyu Kim: Investigation, Data curation. Sukang Bae: Validation. Tae-Wook Kim: Validation. Kwi-Il Park: Data curation, Validation. Byung Hee Hong: Supervision, Validation. Chang Kyu Jeong: Conceptualization, Funding acquisition, Investigation, Methodology, Supervision, Validation, Writing - original draft, Writing -
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that might have appeared to influence the research reported in this paper.
Acknowledgements
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT (NRF-2019R1C1C1002571). This article was also assisted by the Jeonbuk National University Writing Center and this work was financially supported by the Korea Institute of Science and Technology Institutional Program. This work is supported by the Technology Innovation Program (20011317) funded by the Ministry of Trade Industry & Energy.
Seoungwoong Park received the B.S. degree in School of polymer Science and Engineering from the Chonnam National University, Gwangju, Korea, in 2016, the M.S. degree in polymer engineering from Chonnam National University, Gwangju, Korea, in 2018. He is currently working toward the Ph.D. degree in the department of chemistry, Seoul National University (SNU), Seoul, Korea. His current research interests include laser pyrolysis of 2D materials and evaluation their electrical properties for
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Seoungwoong Park received the B.S. degree in School of polymer Science and Engineering from the Chonnam National University, Gwangju, Korea, in 2016, the M.S. degree in polymer engineering from Chonnam National University, Gwangju, Korea, in 2018. He is currently working toward the Ph.D. degree in the department of chemistry, Seoul National University (SNU), Seoul, Korea. His current research interests include laser pyrolysis of 2D materials and evaluation their electrical properties for fabricating practical wearable electronics.
Jiseul Park received her B.S. degree in Major of Electronic Materials Engineering, Division of Advanced Materials Engineering, from Jeonbuk National University (JBNU) in 2019. She is studying for M.S. degree in Materials Engineering from JBNU. She is working as a graduate researcher at Biological and Environmental electronics laboratory. Her research topic focus on wearable electronics of laser synthesized 2D materials.
Chang Kyu Jeong is currently a professor in Jeonbuk National University, Korea. He was born in Seoul and received his B.S. degree in Materials Science and Engineering from Hanyang University. He holds his M.S. and Ph.D. degrees from the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST). After working on a postdoctoral research fellow in Institute for NanoCentury (KINC), he was employed as a postdoctoral scholar in Pennsylvania State University. His current research topics are biological and environmental electronic devices for energy and sensor applications using ferroelectric, piezoelectric, and dielectric materials.
Seoung-Ki Lee received the Ph.D. degrees in School of Advanced Materials Science & Engineering from the Sungkyunkwan University, Republic of Korea in 2014, respectively. From 2014 to 2015, he was a post-doctoral researcher in School of Electrical and Electronic Engineering with Yonsei University. After his second post-doctoral researcher in Smalley-Curl Institute and the NanoCarbon Center at Rice University from 2015 to 2016, he is currently a senior researcher with the Functional Composite Materials Research Center, Korea Institute of Science and Technology since 2016. His current research interests include synthesis of low dimensional materials, flexible electronics, and wearable sensor system.
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These authors contributed equally to this work.