Research Article www.acsami.org
Structural Engineering for High Sensitivity, Ultrathin Pressure Sensors Based on Wrinkled Graphene and Anodic Aluminum Oxide Membrane Wenjun Chen,†,‡ Xuchun Gui,*,†,‡ Binghao Liang,†,‡ Rongliang Yang,†,‡ Yongjia Zheng,†,‡ Chengchun Zhao,†,‡ Xinming Li,∥ Hai Zhu,†,§ and Zikang Tang*,†,‡,⊥ †
State Key Lab of Optoelectronic Materials and Technologies, ‡School of Electronics and Information Technology, and §School of Physics, Sun Yat-sen University, Guangzhou, 510275, P. R. China ∥ Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China ⊥ Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau 999078, China S Supporting Information *
ABSTRACT: Nature-motivated pressure sensors have been greatly important components integrated into flexible electronics and applied in artificial intelligence. Here, we report a high sensitivity, ultrathin, and transparent pressure sensor based on wrinkled graphene prepared by a facile liquid-phase shrink method. Two pieces of wrinkled graphene are face to face assembled into a pressure sensor, in which a porous anodic aluminum oxide (AAO) membrane with the thickness of only 200 nm was used to insulate the two layers of graphene. The pressure sensor exhibits ultrahigh operating sensitivity (6.92 kPa−1), resulting from the insulation in its inactive state and conduction under compression. Formation of current pathways is attributed to the contact of graphene wrinkles through the pores of AAO membrane. In addition, the pressure sensor is also an on/off and energy saving device, due to the complete isolation between the two graphene layers when the sensor is not subjected to any pressure. We believe that our high-performance pressure sensor is an ideal candidate for integration in flexible electronics, but also paves the way for other 2D materials to be involved in the fabrication of pressure sensors. KEYWORDS: graphene, pressure sensor, wrinkled structures, anodic aluminum oxide, flexible electronics
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INTRODUCTION
It is found that assembling the conductive materials with tip structures, such as wrinkles,9 pyramids,33,34 pillars,35,36 and hemispheres,37,38 could further improve the sensitivity and response speed of the pressure sensors. It is known that external compression can change the distance between two electrodes of capacitance-based pressure sensors or conductive paths of piezoresistive sensors, which triggers corresponding electrical responses. In comparison with the film without rough structures, a relatively small pressure is able to increase the contact interface of microstructures and current pathways of tip-structure based pressure sensors, which imposes positive effects on the enhancement of responsivity.39 Experimentally, for instance, graphene was grown on patterned copper foil and transferred in molding polydimethylsiloxane (PDMS) to obtain piezoresistive sensors with hemispheric structures in order to improve the sensitivity.37 Further, a pressure sensor constructed by microstructured reduced graphene oxide has an extremely widened operating pressure range from 0 to 50 kPa.40
Mechanical sensors are of great importance in many application fields.1−8 Particularly, the accomplishment of the pressure sensor with high performance facilitates its integration in advanced wearable electronics, which have attracted much attention and are intended to be widely used.9−14 Pressure sensors based on suitable materials with rational structures are expected to show outstanding properties and the potential to guide application in flexible electronics.15,16 For convenient integration and combination in wearable devices, high-performance pressure sensors with ultralow thickness are needed. Recently, high conductivity gold,17−19 silver,20−22 and metal oxide nanowires23,24 have been used for the fabrication of pressure sensors, but the sensitivity of these sensors needs to be further improved. Two-dimensional (2D) materials, especially graphene, due to its outstanding electrical conductivity, could be properly used as the base element to fabricate high sensitivity mechanical sensors. Although a few research articles have reported graphene films to be fabricated into transparent pressure sensors,25−27 three-dimensional porous macroscopic graphene structures with different configurations28−32 are still the major forms of graphene used in pressure sensors. © XXXX American Chemical Society
Received: April 20, 2017 Accepted: June 28, 2017 Published: June 28, 2017 A
DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of fabrication of pressure sensors. and soaked in a mixed solution of H3PO4 (6 wt %) and H2CrO4 (1.8 wt %) at 60 °C water bath for 2 h, the samples were electrolyzed in 0.3 M H2C2O4 solution at 6−8 °C with the voltage of 45 V for 110, 130, and 150 s to obtain AAO membranes with the thickness of 200, 250, and 300 nm, respectively. Subsequently, as-prepared AAO membranes were soaked in an H3PO4 solution (5%) at 35 °C water bath for 10 min, cleaned in DI water, and dried in an oven at 60 °C. Next, a layer of PMMA was coated onto the AAO membranes and the remaining Al foils were etched away in a mixture solution of HCl and CuSO4. Finally, the poly(methyl methacrylate) (PMMA)/AAO membranes were soaked in an H3PO4 solution (3 wt %) at 35 °C water bath for 18 min to widen some pores, then cleaned in DI water, and dried in an oven at 60 °C. Structural Characterization and Performance Measurement of Pressure Sensors. SEM images were recorded using a Hitachi S4800 with operating voltage of 3 kV. TEM images were taken by a JEOL JEM-2010HR operating at the accelerating voltage of 300 kV. Optical microscopy images were captured using a Zeiss Axio CSM 700. AFM images and corresponding 3D views were characterized by a Veeco Edge. Raman spectra were measured by HORIBA JY HR800 with 633 nm excitation laser. Transmittance spectra were measured using a UV−vis spectrometer (Maya2000 Pro, Ocean Optics, Dunedin, FL). A motion controller (Zolix SC300-3A, 1.25 μm resolution) and a force gauge (Mark 10) were employed to offer different forces on the pressure sensors. The data of electricity tests were collected using a Keithley 2400 operated at the voltage of 1 V.
Accordingly, the high sensitivity and wide working range of pressure sensors take advantage of the graphene crumples, the controllable manufacture of which further tunes the properties of these devices. Here, we propose a resistive pressure sensor based on two layers of wrinkled graphene (WG) with nanoscale wrinkles. WG was self-assembled on the surface of the aqueous solution and transferred onto arbitrary target substrates. The introduction of a layer of porous anodic aluminum oxide (AAO) membrane with a thickness of just 200 nm insulates two layers of conductive WG, which prevents excessive energy consumption from nonworking conditions. The isolation in the “off” state and conduction in the working state greatly promote the sensing sensitivity of the resistive pressure sensor. The sensitivity of the sensors is 6.92 and 0.14 kPa−1 at low and high operating pressure range, respectively. Additionally, this highperformance pressure sensor is composed of three layers of 2D materials. Therefore, its thickness is limited to hundreds of nanometers and the optical transmittance is as high as 68.6%, which ensure its suitability for the integration of flexible electronics.
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EXPERIMENTAL METHODS
Preparation of WG. Graphene was first grown by the method of chemical vapor deposition (CVD) on copper foil, as reported in our previous work.41,42 Typically, The Cu substrate in the tubular furnace was heated to 1010 °C in 40 min and annealed for 20 min under the atmosphere of Ar (200 sccm) and H2 (65 sccm). After introducing CH4 (30 sccm) to synthesize graphene for 5 min, the copper foil was pulled out from the reaction zone quickly to cool down under the protection of Ar (200 sccm) and H2 (65 sccm). Subsequently, the asprepared sample was put onto the surface of 0.5 M FeCl3 solution that the Cu substrate was etched away after approximately 40 min. The floated graphene was transferred onto the surface of deionized water (DI) water for several times in order to eliminate the remaining impurities. Then the clean graphene was transferred onto the surface of ethanol/DI water solution with the concentration of 1.2:1 vol % to obtain WG. A PDMS substrate was previously processed in air plasma for 1 min to increase its hydrophilicity. Finally, the WG was “fished out” by 1-mm-thick PDMS substrate, then dried in an oven at 55 °C. Preparation of AAO Membranes. To start with, high-purity aluminum foils (99.999%) with the thickness of 0.3 mm were degreased in acetone for 10 min and soaked in NaOH (5 wt %) for 20 min. Then the clean Al foils were electrochemically polished in a mixed solution of HClO4 and ethanol (1:4 vol %) with the constant current of 1.2 A for 4 min and dried in nitrogen. Two electrolysis processes were used to prepare AAO. The first is as follows: the polished Al foils were electrolyzed in 0.3 M H2C2O4 solution at 2−6 °C with the voltage of 45 V for 8 h. After being cleaned in DI water
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RESULTS AND DISCUSSION The facile fabrication procedures of WG/AAO/WG resistive pressure sensor are illustrated in Figure 1. First, high-quality graphene was synthesized by CVD. A liquid-phase shrink method was used to assemble WG, as clarified by our group previously.41−43 The clean pristine graphene (PG) was transferred onto the surface of ethanol/DI water solution with the concentration of 1.2:1 vol % and WG was selfassembled with the root-mean-square of 25 nm. Then the PMMA/AAO membrane was placed onto and covered most parts of WG on PDMS. In order to obtain porous AAO/WG on PDMS, the sample was soaked in pure acetone for 15 min to remove the PMMA layer. Subsequently, a silver wire with the diameter of 60 μm was fixed on the WG by spreading silver paint as the electrode. Finally, another layer of WG on PDMS with the same silver electrode was stacked on the AAO/WG on PDMS sample to obtain a resistive pressure sensor with a sandwich-like structure after packaging, which has the potential to be a universal model for the architecture of tactile sensors based on other materials. The size of the sensors was controlled at approximately 10 × 10 mm2. B
DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Characterization of pressure sensors. (a) SEM image and (b) 3D view of WG. (c) SEM image of AAO membrane. Inset is 30°-tilted-view SEM image of AAO membrane with the thickness of 200 nm. (d) SEM image of WG on AAO membrane. (e) Optical microscopy image of AAO membrane with the thickness of 250 nm on WG; the AAO boundary is indicated by a red dotted line. (f) Optical transmittance spectra of 200-nmthick AAO membrane on WG; inset is the photograph of the sample.
Figure 3. Schematics and current responses of pressure sensors. (a) Structural and (b) operating schematics of the pressure sensor. (c) Current responses of different pressure sensors under various pressures. Inset is the photograph of packaged pressure sensor with silver electrodes. (d) Cyclic current responses of pressure sensors based on 200-nm-thick AAO membrane and WG under various pressures. The cyclic current response of the pressure sensor under 300 Pa was multiplied by 100. (e) Stability measurement of pressure sensors based on 200-nm-thick AAO membrane and WG. (f) Cyclic current responses of pressure sensors based on AAO membrane with different thickness and WG.
C
DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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found that current responses scale linearly with the contact area (Figure S6), which revealed the formation of WG current pathways through the AAO pores but not cracks or defects. After releasing the mechanical load, connecting graphene wrinkles are separated and the pressure sensor returns to its originally insulated state resulting from restoration of the PDMS. Operation performance of three different resistive pressure sensors based on WG/200-nm-thick AAO, WG/250nm-thick AAO, and PG/200-nm-thick AAO were studied, respectively. The photograph in the inset of Figure 3c shows an assembled pressure sensor. It is also noted in Figure 3c that the WG/200-nm-thick AAO based sensor has the lowest turn-on compression of 300 Pa among these devices, while the threshold value of the pressure sensor based on WG/250nm-thick AAO is up to 1.3 kPa. The working limit of both sensors is fulfilled for real tactile sensing, especially human touch with pressure from hundreds of pascals to 10 kPa.1,25 The other one fabricated by PG/200-nm-thick AAO is still not working even under a high pressure of 4.5 kPa. Hence, wrinkles on graphene and shallower holes of the AAO membrane are profitable for the achievement of a pressure sensor performing under lower compression because of easier crossover of the conductive graphene layers. However, it is worth mentioning that an AAO membrane with a thickness of less than 200 nm or the hole diameter larger than 90 nm is much more brittle, which degrades the performance of pressure based sensors. In addition, the current response of the WG/200-nm-thick AAO based sensor rises significantly at the low working range of 300 Pa to 1.5 kPa but slows down with the pressure increasing from 1.5 to 4.5 kPa. The contact points of two WG electrodes grow dramatically at the low pressure, but the growing velocity falls down at the high scope until reaching a plateau. Linear I−V curves of the sensor based on WG/200-nm-thick AAO in the “off” states (under the pressure of 100 Pa) and some working states (under the pressure of 500, 1000, and 2000 Pa, respectively) prove that the WG electrodes make a good ohmic contact, as shown in Figure S7. In order to quantitatively evaluate the operating performance of a sensor, resistive sensitivity S is essentially and commonly employed and expressed by5,13,36,37,45
The PG and WG prepared in liquid phase were characterized by Raman spectra and TEM to indicate the high quality (Figure S1). SEM image (Figure 2a) and 3D view of the AFM image (Figure 2b) show that the graphene wrinkles on WG are evenly distributed, and the height of the wrinkles is tens to a hundred of nanometers, which is obviously different from low-height folds induced randomly during growth and transfer procedures on PG (Figure S1b). Optical microscopy image (Figure S1c) and morphology analysis (Figure S1d) of WG further demonstrate the homogeneity of wrinkled structures in a large area (larger than 200 × 180 μm2 and 30 × 30 μm2, respectively). Insulated porous AAO membranes were prepared with the same aperture and different pore depth, which can be conveniently controlled by growth time. The SEM image in Figure 2c shows that the diameter of most of AAO holes is in the range of 80 to 90 nm, which was further clarified by size distribution of the hole diameter (Figure S2). The AAO membranes have rare cracks over a large area (12 × 8.5 μm2), as shown in Figure S3. The significant improvement of stretchability of WG induced by wrinkled structures enables its good combination with AAO membrane as well as perfection without severe damage, as shown in Figure 2d and Figure S4a. However, cracks emerged when the PG sample is transferred on an AAO membrane (Figure S4b). Figure 2e and Figure S5 show that graphene wrinkles still can be observed, though a layer of AAO membrane with the thickness of 250 or 300 nm was transferred onto the WG. On the contrary, it is difficult to see random graphene folds with low height on PG through a 250-nm-thick AAO membrane (Figure S5c,d). The AAO/WG membrane maintains high optical transmittance of 68.6% at the wavelength of 550 nm, as clarified by the transmittance spectra and photograph in Figure 2f. In addition, the WG sample has lower sheet resistance than that of PG, which is reported by our previous work.41 Although the thickness of WG is higher than that of PG because of the graphene wrinkles, graphene wrinkles create extra conduction pathways due to interlayer tunneling, which decreases the vertical resistance.44 Thus, it is reasonable to think that the combination of WG and AAO membrane is promising for the manufacture of the ultrathin pressure sensor with high sensitivity. The structure schematic of piezoresistive based sensor is illustrated in Figure 3a. The insulated AAO membrane with homogeneous holes as a blocking layer can avoid conduction between the top and bottom layers of WG in the “off” state of the pressure sensor, which follows the concept of energy conversion and enhances the operating sensitivity. Furthermore, graphene with wrinkled tips is the ideal material to be applied in this regime, according to the working schematics of the pressure sensor (Figure 3b). The operation of the sensor takes advantage of the deformation of PDMS and contact of wrinkled structures through the pores of the AAO membrane under compression. When pressure is applied to the sensor, high wrinkles of the two WG electrodes contact throughout the AAO pores and form some current pathways. With the increasing pressure, the contact area of high wrinkles increases and more wrinkles with low height connect, which leads to the larger current response. Dense contact points and excellent electrical conductivity of WG could trigger great current responses and sensitive sensation. We have tested one sample with different pressure area. The same pressure of 3 kPa was applied to the sensor based on WG/200-nm-thick AAO with the pressure area of 1.0, 3.1, and 4.0 mm2, respectively. It is
S = δ(ΔR /R 0)/δP
(1)
where the ΔR is the resistance change of sensor under pressure, R0 is the initial resistance of sensor, and P is the applied pressure. The equation can be transformed to S = δ(ΔI /I0)/δP = (δ ΔI /δP)/I0
(2)
where ΔI is the current change of the sensor under pressure, and I0 is the initial current of the sensor, which is taken as the response current under the pressure of 500 Pa. In order to avoid overestimating the sensitivity, the response current under 300 Pa was not taken into account for calculation due to the near-zero current (0.16 μA). Additionally, depending on operating scopes, the current−pressure curve is divided into two parts and linearly fitted respectively (Figure S8a) for assessment of the sensor performance, which is similar to that reported by previous work.5,13,37,45,46 According to the calculation results, the pressure sensor based on WG/200nm-thick AAO exhibits outstanding sensitivity as 6.92 kPa−1 at the working pressure from 300 Pa to 1.5 kPa. High resistive sensitivity (0.14 kPa−1) is maintained under the high pressure from 1.5 to 4.5 kPa, which is comparable with other sensors introduced by previous work.28,45,47 Figure 3d exhibits D
DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Responses to objects applied on pressure sensors. (a) The blue LED is off without an object and (b) on with a mung bean applied on the pressure sensor based on 200-nm-thick AAO membrane and WG. (c) Current responses of a mung bean applied on the pressure sensor based on 200-nm-thick AAO membrane and WG. (d) Current response of a mung bean, a soybean, and a weight applied to the pressure sensor based on 200nm-thick AAO membrane and WG, respectively.
consecutive measurements of the pressure sensor under a wide range of different compressions of 300, 1000, 2000, and 4000 Pa, respectively. The cyclic current response of the pressure sensor under 300 Pa has been multiplied by 100 to make it easier to read on the scale. Higher compression turns the device on with higher current response, and the sensor is restored to its initial isolated state after removing the applied pressure. Relative examinations for its counterparts have been performed and consistent results were obtained, as shown in Figure S8b,c. Our sensors also have excellent cyclability. After the 10 000 cycles test, the current responses of the sensor remain stable (Figure 3e). SEM imaging in Figure S10 further demonstrates that the morphology of the WG electrode maintains its original state after 10 000 cycles compression, which enables the cyclic stability. In addition, series of current response of pressure sensors based on different combinations of WG and AAO under the same applied pressure of 2 kPa are shown in Figure 3f. The WG/300-nm-thick AAO based sensor does not show an on/off effect, because the pressure of 2 kPa is not able to make two WG electrodes contact through the AAO pores. Finally, it is found that these pressure sensors can still normally and stably operate for mechanical detection although it is inferred to be conductive by external preloaded compression. Cyclic current responses in Figure S8d-f confirm the reliability. Thus, the collaboration of WG and AAO membrane can be available for the architecture of pressure based sensors with high sensitivity and stability. Simultaneously, this sandwich-like structure is expected to involve other 2D materials in the application of pressure sensing with tunable properties. Some light objects were used for approving the detection ability of the pressure sensors in reality. In Figure 4a, a pressure sensor is connected with a turn-off blue LED and two dry batteries in series. A small mung bean (0.23 g, 300−400 Pa) put on the pressure sensor leads to the connection of two layers of WG and establishment of conductive paths that turns the LED on, as shown in Figure 4b, and the light went out when the
bean was removed. Corresponding current outputs (Figure 4c) demonstrate the switch effect and repeatability of the pressure sensor in this example. As photographs showed in Figure S10, heavier things such as a soybean (0.80 g, 500−600 Pa) or a weight (1.00g, 730 Pa) applied on the pressure sensor are competent to achieve a brighter LED because of the generation of higher switched-on current. In Figure 4d, the performance of the pressure sensor in successive detection of a mung bean, a soybean, and a weight further demonstrates the 1 order higher turned-on current generated by a soybean or a weight than that by a mung bean. The sensors in this work have high sensitivity and cyclability, and also have the ability to detect the motion, pulse waves of human beings, or a tiny pressure. As a demonstration, two sensors based on WG/200-nm-thick AAO were pasted on the left and right keys of a mouse (Figure S11). In comparison of pressure on a traditional mechanical mouse, a gentle touch leads to current responses of our pressure sensor. In addition, it can identify double clicks with the interval of about 0.3 s. These instances ensure that the pressure sensor based on the combination of graphene with wrinkled structures and AAO membrane can operate as a sensitive switch in practice.
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CONCLUSIONS We have introduced a resistive pressure sensor based on a WG/ AAO/WG sandwich-like structure. Three layers of 2D materials enable its low thickness of hundreds of nanometers and high optical transmittance (68.6%). What is more important, featuring a new operation mechanism, the introduction of the isolated AAO membrane as a blocking layer to separate two conductive layers of WG not only avoids additional consumption of energy in the “off” state of the pressure sensor, but also significantly improves its sensitivity to a high level of 6.92 kPa−1 at working scope of low pressure with high durability. It is also seen that the pressure sensor serves as a sensitive electrical switch in practical applications. ConseE
DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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quently, this high-performance pressure sensor is promising for integration in flexible electronic devices to fulfill tactile sensing, and the operating properties are expected to be tuned by replacement of other 2D materials. We believe that more 2D materials will be engaged in the application of pressure sensing relying on this model.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05515. Characterization of PG, WG, and AAO membrane; SEM and optical microscopy imaging; practical detection for objects; application demonstration of the pressure sensor in mouse keys (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xuchun Gui: 0000-0001-7430-3643 Xinming Li: 0000-0002-7844-8417 Hai Zhu: 0000-0003-2874-5509 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Guangdong Natural Science Foundation (Grant No. 2016A030313346 and 2014A030306022), Guangdong Youth Top-notch Talent Support Program (No. 2015TQ01C201), the Fundamental Research Funds for the Central Universities (16lgzd02), and Funding from University of Macau to Z. K. Tang (SRG201600002-FST).
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REFERENCES
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DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.7b05515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
