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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 36312−36322
Ultrasensitive and Stretchable Strain Sensors Based on Mazelike Vertical Graphene Network Shuying Wu,† Shuhua Peng,† Zhao Jun Han,‡ Hongwei Zhu,§ and Chun H. Wang*,† †
School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia CSIRO Manufacturing, P.O. Box 218, 36 Bradfield Road, Lindfield, NSW 2070, Australia § State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China ACS Appl. Mater. Interfaces 2018.10:36312-36322. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/28/18. For personal use only.
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S Supporting Information *
ABSTRACT: Here, we report a new type of strain sensors consisting of vertical graphene nanosheets (VGNs) with mazelike network, sandwiched between poly(dimethylsiloxane) (PDMS) substrates. The new sensors outperform most graphene thin-film-based sensors reported previously and show an outstanding combination of high stretchability of ∼120%, excellent linearity over the entire detection range, and high sensitivity with a gauge factor of ∼32.6. The sensitivity can be tuned by controlling the thickness of VGNs, with sensors consisting of thicker VGNs showing higher sensitivity but slightly lower stretchability (the maximum gauge factor is ∼88.4 with a maximum detection strain of ∼55%). Detailed microscopic examinations reveal that the ultrahigh sensitivity stems from the formation of microcracks initiated in the buffer layer. These microcracks are bridged by strings of graphene/PDMS, enabling the conductive network to continue to function up to a strain level significantly higher than that of previously reported graphene thin-filmbased sensors. Furthermore, the present sensors have been found to be insensitive to temperatures and various liquids, including water and 0.1 mol L−1 sodium chloride solution (similar to the sweat on human skin). Demonstrations are presented to highlight the new sensors’ potential as wearable devices for human motion detection and pressure distribution measurement. KEYWORDS: vertical graphene nanosheets, piezoresistivity, strain sensor, microcracking, human motion detection
1. INTRODUCTION Wearable sensors that can perform a myriad of physical and physiological measurements are currently attracting considerable interests due to their broad applications including human motion detection, soft robotics, electronic skin, and personalized health monitoring.1 Stretchability and high sensitivity are the key characteristics of advanced wearable sensors. To achieve high stretchability and sensitivity, polymer composites that consist of flexible and stretchable polymers and highly electrically conductive nanomaterials have been developed. Examples of the conductive nanofillers include carbon nanotubes,2 carbon nanofibers,3 graphene,4 nanowires,5 and their hybrids,6 whereas the commonly used flexible and stretchable substrates include poly(dimethylsiloxane) (PDMS), 3,5−8 Ecoflex, 9,10 stretchable yarns, 11 polyurethane,12,13 etc. Moreover, multidimensional stretchable strain sensors based on silver nanowires have recently been developed, which demonstrate the capability of sensing complex multiaxial strains.14,15 However, a major challenge remains in simultaneously attaining high sensitivity and a broad sensing range.16 It has been demonstrated that proper structural engineering can © 2018 American Chemical Society
improve the stretchability and sensitivity of the sensors. Taking graphene as an example, extensive efforts have been devoted to improving the performance of graphene-based strain sensors by structural engineering.16−18 Graphene has been identified as a promising sensing material due to its remarkable structural and electrical properties.16−23 However, its stable structure often leads to little band gap opening under strain and thereby low intrinsic piezoresistive sensitivity (with a gauge factor of about 1.9).24 To improve the sensitivity, Yang et al.16 developed sensors based on graphene woven fabric with ultrahigh sensitivity (a gauge factor of 500 for strains below 2%). This macrowoven fabric structure induced a large interfacial resistance between the interlaced ribbons and the formation of oriented zigzag cracks, leading to greatly improved sensitivity as compared to that of sensors consisting of a planar network structure of patterned graphene. Despite their high sensitivity, sensors based on graphene thin films show low stretchability, i.e., typically
