Elsevier

Nano Energy

Volume 81, March 2021, 105610
Nano Energy

Piezoionic-powered graphene strain sensor based on solid polymer electrolyte

https://doi.org/10.1016/j.nanoen.2020.105610Get rights and content

Highlights

  • A piezoionic doping tunes the Dirac point voltage of graphene FET (S-GFET) for realizing a piezoionic-powered strain sensor.

  • S-GFET-based strain sensor exhibits stable output signals against continuous strains and distinguishes strain directions.

  • S-GFET device is also suitable for touch sensing via coupling the triboelectrification.

  • The transparent and flexible sensing device can potentially be utilized in rehabilitation processes and elderly care.

Abstract

Doping to graphene is essential for developing graphene-based electronic devices and circuits, while traditional doping methods to graphene are still challenging due to unstable doping characteristics and unavoidable damage to the graphene structure. Here, we demonstrate piezoionic-powered strain and touch sensors using mechanically doped graphene with solid polymer electrolyte (SPE). Due to the piezoionic effect in SPE, the Dirac point voltage of an SPE-coated graphene field-effect transistor (S-GFET) is shifted left/right upon compressive/tensile strain. This mechanical strain tuned piezoionic doping to graphene that enables to obtain different Dirac point voltage of S-GFET for a piezoionic-powered strain sensor. When the S-GFET acted as a strain sensor, the S-GFET exhibited stable output signals against to continuous strain and was able to distinguish between tension and compression without any additional components. The strain sensor mounted on a hand very effectively responded to the hand joint movement. Additionally, it was found that the device is also suitable for touch sensing due to the coupling of triboelectrification and the electronic transport in the S-GFET.

Graphical Abstract

Piezoionic-powered strain sensor was introduced by mechanically doped graphene with solid polymer electrolyte (SPE). The Dirac point voltage of SPE-coated graphene field-effect transistor (S-GFET) enable left/right-shift upon compressive/tensile strain. The S-GFET strain sensor exhibited stable output signals against to continuous strain and strong durability. The S-GFET sensor demonstrates joint motion monitoring for the rehabilitation process.

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Introduction

Graphene, a monolayer of carbon atoms arranged in a honeycomb lattice, has attracted tremendous attentions due to its excellent electronic characteristics and high transparency [1], [2], [3], [4]. In particular, graphene undergoes an ambipolar electric field effect where both holes and electrons can be induced in it depending upon the polarity of a gate bias [5], [6]. For developing graphene-based electronic devices and circuits, doping to graphene is essential, which can tune the Fermi level and carrier concentration of graphene [7]. However, traditional doping methods such as chemical doping and electrostatic doping would inevitably damage the graphene structure or consume extra energy [8], [9], [10], [11], [12], [13]. Piezoelectric and triboelectric potentials have been proposed to dope graphene in field-effect transistors (FETs) and realize low-power consuming sensing devices [14], [15]. Nevertheless, these devices generally require to combine with additional piezo/triboelectric nanogenerators [16], [17], [18], [19], which undoubtedly increase the complexity and manufacturing difficulty of the devices.

Solid polymer electrolyte (SPE), comprised of ionic salt and polymer matrix, typically has a very higher capacitance (1–10 μF/cm2) than conventional dielectric materials such as Al2O3, Ta2O5, TiO2 etc. [20], [21]. Thus, SPE can be an excellent candidate as a gate dielectric for FETs [22], [23], [24]. Under the gate electric field, nanometer-thick electric double layers (EDLs) are formed at the SPE/semiconductor interface and, as a consequence, such an electrolyte-gating allows low operational gate voltages of within ±3 V for FETs [25], [26], [27], [28]. Remarkably, SPEs undergo a piezoionic effect where a non-uniform deformation of the SPEs induces an internal Donnan potential due to the ionic concentration gradient and an inhomogeneous movement of cations and anions [29]. The piezoionic effect can potentially transform the mechanical strain into an electric signal. Like piezoelectric and triboelectric potential-gated FETs [14], [15], [30], [31], the piezoionic potential can also be utilized as a gate bias to realize the self-gated FET.

Here, we demonstrate a piezoionic-powered strain sensor using mechanically doped graphene with SPE. When strain is applied, the piezoionic potential modulates the carrier concentration in the graphene channel. As a consequence, the Dirac point voltage (VDirac) of an SPE-coated graphene FET (S-GFET) undergoes a shift (a positive shift of ~450 mV due to a tensile strain of ~0.20% and a negative shift of ~300 mV due to a compressive strain of ~0.16%). This new doping technique has ability to tune pristine graphene to n-type doping state or p-type doping state. It was found that the S-GFET device effectively responds to the applied strain (from as low as ~0.045% to ~0.231%) with a gauge factor of approximately −16 and fully recovers at the strain removal. It has shown excellent stability with a long operation over 2000 times cycles. Besides, with an opposite current change, the strain sensor can distinguish between the compressive and tensile strains which has in fact been the major limitation with the capacitive and resistive strain sensors [32], [33], [34], [35], [36]. Additionally, the device is multifunctional as it can also be operated as a touch sensor where the triboelectric potential, due to contact electrification between SPE and the external object, modulates the current transport in the S-GFET. The flexible, transparent S-GFET sensor has been demonstrated to very accurately detect joint motion of hand with potentially strong prospects for use in a long-term rehabilitation process.

Section snippets

Material synthesis

Graphene was synthesized via a low-pressure chemical vapor deposition (CVD) method [37]. The copper foil was undergoes a H2 treatment process (gas flow rate is 20 sccm) and then with a CH4 gas flow rate of ~5 sccm for 18 min at 1000 ℃ to grow graphene. The graphene was taken out after the chamber cools down to the room temperature. In order to prepare 0.5M H3PO4-PVA (phosphoric acid-polyvinyl alcohol) SPE, H3PO4 powder and PVA powder were mixed in the deionized water and then the mixture was

Results and discussion

The S-GFET is schematically described in Fig. 1a. The device design is composed of a CVD-grown graphene channel (with a width of 500 µm and a length of 700 µm) with Cr (5 nm)/Au (50 nm) based drain and source electrodes on a flexible PEN substrate. The cross-sectional view of the device is shown in Fig. S1, Supporting Information. The graphene channel is covered with 0.5M H3PO4-PVA SPE at the top. Raman spectra with G-to-2D peaks intensity ratio of about 0.5 indicates the CVD graphene is single

Conclusions

In summary, we have demonstrated a flexible piezoionic-powered strain and touch sensors using mechanically doped graphene with SPE. Due to the piezoionic effect in SPE, the Dirac point voltage of the S-GFET is shifted left (or right) upon the compressive (or tensile) strain. It was found that the mechanical strain can effectively tune piezoionic doping (p-type or n-type doping) to graphene for realizing a piezoionic-powered strain sensor with S-GFET. The S-GFET strain sensor could provide the

CRediT authorship contribution statement

De-Sheng Liu, Hanjun Ryu, Zhiming Wang and Sang-Woo Kim designed and conceptualized the project. De-Sheng Liu, Hanjun Ryu and Jae-Hwan Jung carried out the fabrication and characterization of devices. Usman Khan performed calculations. De-Sheng Liu, Hanjun Ryu, Usman Khan, Cuo Wu, Jiang Wu, Zhiming Wang and Sang-Woo Kim analyzed the data. Sang-Woo Kim supervised the project. All authors contributed to the writing of the paper.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

D.-S.L. and H.R. contributed equally to this work. This work was financially supported by Nano Material Technology Development Program (2020M3H4A1A03084600) through the National Research Foundation of Korea (NRF), the ICT Creative Consilience Program (IITP-2020-0-01821) through the IITP (Institute for Information & communications Technology Planning & Evaluation) funded by the MSIT (Ministry of Science and ICT) of Korea, and the Korea Basic Science Institute (KBSI), National Research Facilities

De-Sheng Liu is currently a Ph.D. candidate at the Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China (UESTC) under the supervision of Prof. Zhiming Wang. His research interests focuses on 2D electronic and optoelectronic devices.

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    De-Sheng Liu is currently a Ph.D. candidate at the Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China (UESTC) under the supervision of Prof. Zhiming Wang. His research interests focuses on 2D electronic and optoelectronic devices.

    Dr. Hanjun Ryu received his Ph.D. degree under the supervision of Prof. Sang-Woo Kim at the School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Korea, in 2019. His research interests are fabrication and characterization of pyroelectric, piezoelectric, and triboelectric nanogenerators for energy harvesting, and portable self-powered devices.

    Dr. Usman Khan obtained PhD in Sensorial and Learning Systems Engineering from University of Rome Tor Vergata in May, 2014. Thereafter, until December, 2018, he served as a postdoc researcher at initially Tor Vergata and then at SKKU. Since December, 2018, he is an Assistant Professor at School of Electrical Engineering and Computer Science, National University of Science and Technology, Islamabad, Pakistan. His research interests include 2D materials, micro-nano systems and nanoelectronics.

    Cuo Wu obtained his B.S. degree from School of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China (UESTC) in 2016. He is currently a Ph.D. candidate at the Institute of Fundamental and Frontier Sciences, UESTC. His current research interests focus on manipulation of surface plasmon polaritons propagation, 2D optoelectronic devices, nanofabrication technology, and numerical simulation.

    Jae-Hwan Jung is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering in Sungkyunkwan University (SKKU). His research interests are 2D material synthesis and biotech applications.

    Dr. Jiang Wu received his PhD degree in Electrical Engineering from the University of Arkansas-Fayetteville in 2011. He was with UESTC as an Associate Professor and then Professor at UESTC from 2011 to 2015. He joined the Photonics group at University College London as a Research Associate in 2012. From 2015, he was a Lecturer at UCL. Since 2019, he has been a full professor at UESTC. His research interests include compound semiconductors and optoelectronics. He is a Fellow of Higher Education Academy, IEEE Senior Member, and Chinese Society for Optical Engineering Senior Member.

    Dr. Zhiming Wang received his Ph.D. degree in condensed matter physics from the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing, China, in 1998. He is currently a professor of the National 1000-Talent Program, working in the UESTC. His research interests include the optoelectronic properties of low-dimensional semiconductor nano-structures and corresponding applications in photovoltaic devices.

    Dr. Sang-Woo Kim is an SKKU Distinguished Professor (SKKU Fellow) at Sungkyunkwan University (SKKU). He received a Ph.D. in Electronic Science and Engineering from Kyoto University in 2004. His recent research interest is focused on piezoelectric/triboelectric nanogenerators, self-powered sensors and body-implantable devices, and 2D materials. Prof. Kim has published over 250 research papers (h-index of 68). Prof. Kim served as Chairman of the 4th NGPT conference at SKKU in 2018. Now he is a Director of the BK21 FOUR SKKU MSE Program, and is currently serving as an Associate Editor of Nano Energy and an Executive Board Member of Advanced Electronic Materials.

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