Flexible Piezoelectric-Induced Pressure Sensors for Static Measurements Based on Nanowires/Graphene Heterostructures Zefeng Chen,†,⊥ Zhao Wang,‡,⊥ Xinming Li,*,† Yuxuan Lin,§ Ningqi Luo,† Mingzhu Long,† Ni Zhao,† and Jian-Bin Xu*,† ACS Nano 2017.11:4507-4513. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/28/19. For personal use only.
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Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, People’s Republic of China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences, Hubei University, 368 Youyi Road, Wuhan 430062, People’s Republic of China § Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡
S Supporting Information *
ABSTRACT: The piezoelectric effect is widely applied in pressure sensors for the detection of dynamic signals. However, these piezoelectric-induced pressure sensors have challenges in measuring static signals that are based on the transient flow of electrons in an external load as driven by the piezopotential arisen from dynamic stress. Here, we present a pressure sensor with nanowires/graphene heterostructures for static measurements based on the synergistic mechanisms between strain-induced polarization charges in piezoelectric nanowires and the caused change of carrier scattering in graphene. Compared to the conventional piezoelectric nanowire or graphene pressure sensors, this sensor is capable of measuring static pressures with a sensitivity of up to 9.4 × 10−3 kPa−1 and a fast response time down to 5−7 ms. This demonstration of pressure sensors shows great potential in the applications of electronic skin and wearable devices. KEYWORDS: graphene, piezoelectric nanowire, pressure sensor, static measurements, flexible device
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Therefore, the major challenge for piezoelectric-driven pressure sensors is to achieve a stable static signal measurement with low-cost and easy fabrication. Graphene, a classical two-dimensional material, has attracted great attention due to its distinctive properties.15−20 Recently, graphene has been demonstrated to be a promising material for pressure sensors, in which the sensing behavior results from the instant contact of isolated graphene sheets or islands.21−30 The piezoresistive effect in intrinsic graphene under strain is limited by the lattice distortion with a threshold deformation of over 20%, which would lead to variations of the electronic band structure and conduction properties.31 Generally, the sensitivity of intrinsic graphene synthesized by chemical vapor deposition (CVD) is measured at an order of 10−5 kPa−1.32,33 In order to
earable and lightweight pressure sensors are essential electronic components in touch displays, electronic skin, and wearable health monitors, which require fast response and high sensitivity in the low-pressure regime.1−9 To achieve these requirements, many technologies based on microfabrication processing have been developed.6,7 However, these pressure sensors need decent microstructure design; thus the fabrication processes are rather complicated and expensive. On the other hand, piezoelectric materials, which are capable of converting mechanical energy into electrical signals, have also exhibited great potential in building self-powered pressure sensors due to the strain-dependent voltage/current.10−14 The output of the generator is sensitive to the pressure change, and the response time is theoretically on the order of microseconds.14 However, piezoelectric pressure sensors can only be used for measuring dynamic pressures because the output voltage generated by the piezoelectric materials is an impulsive signal and can be detected only when the movement is in the transition between on and off modes. © 2017 American Chemical Society
Received: November 30, 2016 Accepted: April 5, 2017 Published: April 5, 2017 4507
DOI: 10.1021/acsnano.6b08027 ACS Nano 2017, 11, 4507−4513
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Figure 1. (a) Fabrication process of a flexible pressure sensor composed of PbTiO3 nanowires (PTNWs) and graphene. (b) Raman spectrum of graphene. (c) SEM image of PTNWs.
Figure 2. (a) Current change of the PTNWs/G pressure sensor during the press and release process. (b) Current response under the pressure cycles. (c) Rise/fall current response of the PTNWs/G pressure sensor.
RESULTS AND DISCUSSION Materials Preparation and Pressure Sensor Fabrication. PbTiO3 nanowires (PTNWs), possessing an excellent piezoelectric property, are applied to make a flexible pressure sensor with graphene in this work. Figure 1a schematically illustrates the key processes for the fabrication of the flexible pressure sensor. A layer of poly(pyromellitic dianhydride-co4,4′-oxidianiline) (PI) with a thickness of 2.3 μm is used as a flexible substrate for the device, which is placed on silicon. Then a CVD-grown graphene film is transferred onto the PI substrate, followed by evaporation of the gold electrodes. After removing the poly(methyl methacrylate) (PMMA) on graphene, a PTNW solution, which is synthesized by the hydrothermal process, is spread on the surface of graphene. To avoid the peeling-off of the nanowires from the substrate, the device is encapsulated by polydimethylsiloxane (PDMS). Finally, the sample is lifted off from silicon, and a flexible PbTiO3 nanowires/graphene (PTNWs/G) device is obtained. Details of material synthesis and device fabrication are shown in the Materials and Methods. A Raman spectrum of graphene shows an extremely small D peak (∼1350 cm−1), which indicates a low density of defects or disordered carbons in graphene (Figure 1b). Moreover, the intensity of the 2D peak (∼2680 cm−1) is at least twice as large as the intensity of the G peak (∼1580 cm−1), which means the global area of the sample solely consists of single-layer graphene.35 The distributive diameter of the PTNWs shown
improve the sensitivity of graphene to realize the detection of small pressures, a piezopotential-powered pressure sensor based on ion gel gate graphene transistors is demonstrated.34 However, the device still needs complex microfabrication and precise poly(vinylidenefluoride-co-trifluoroethylene) alignment techniques to enhance the coupling efficiency. In this work, we present a pressure sensor for static measurements based on nanowires/graphene heterostructures. A working mechanism is reported where the strain-induced polaron in piezoelectric nanowires can work as charged impurities and thus affect the carrier mobility of graphene. We systematically investigated the performance of this pressure sensor under cyclic strains. Significantly, these devices exhibit a sensing behavior for static measurements, while conventional piezoelectric nanowire devices detect only dynamic pressures. Thanks to the fast response of piezoelectric properties of the nanowires, our devices show a response time of 5−7 ms, which is more sensitive than other piezoresistive sensors.8,9,30 Due to the high carrier mobility of graphene, this device has a pressure sensitivity of up to 9.4 × 10−3 kPa−1, higher than the sensitivity of an intrinsic CVD-grown graphene pressure sensor.32,33 These nanowires/graphene heterostructures are inherently flexible, which makes this device suitable for wearable human health sensors. 4508
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Figure 3. (a) Pressure response of a pure PTNW-based pressure sensor under a pressure pulse. (b) Pressure response of a graphene-based pressure sensor under a pressure pulse. (c) Pressure response of a PTNWs/G transistor under a pressure pulse. The inset is the dynamic process of the pressure pulse.
in Figure 1c is about 500 nm, with the lengths reaching up to 10 μm, which is favorable for bending under pressure. Besides, the PTNWs can be confirmed to possess a tetragonal perovskite structure according to the X-ray diffraction patterns shown in Figure S1a.36 Furthermore, a high-resolution transmission electron microscope image of PTNWs exhibits the lattice constants to be a = 3.94 Å and c = 4.2 Å (Figure S1(b)), which indicates the good crystallinity of the PTNWs. The polarization of the tetragonal PbTiO3 lattice is along the [001] orientation, which is the axial direction of the nanowires. The applied pressure on the top surface of the device will lead to the bending or compression motion of the nanowires, which could be a stretch of the nanowire along the axial direction, and result in a piezoelectric potential along this orientation. Electrical Analysis of a PTNWs/G Pressure Sensor upon Cyclic Static Measurements. This PTNWs/G pressure sensor shows special performance for static measurements. The current of the PTNWs/G sensor is recorded with the change of pressure under a bias voltage of 1 V. Absolute variation of electronic current (ΔI = IP − I0, where I0 and IP denote the current without and with applied pressure, respectively) is plotted as a function of applied pressure, as shown in Figure 2a. The pressure sensitivity R is defined as the ratio of the current changes (R = δ(ΔI/I0)/δP, where P denotes the applied pressure), and this device shows a sensitivity of 9.4 × 10−3 kPa−1 with a good linear response from 0 to 1400 Pa (Figure 2a), which is higher than the sensitivity by an order of 10−5 kPa−1 of an intrinsic CVD-grown graphene pressure sensor (S2 in the Supporting Information).32,33 Particularly, the stretching−releasing process of this PTNWs/G sensor showed reversible behavior, and the hysteresis may be attributed to the elastic deformation and viscoelastic effects of the device. The responses of the PTNWs/G sensor for static pressure with loading and unloading cycles with an applied voltage of 1 V are measured and plotted in Figure 2b. When the force is applied, the current response decreases, which is retainable. When the force is withdrawn, the current response retrieves its initial value. The force exerted by the actuator and the current response of the device show a very good synchronization. It is worth mentioning that the device also shows a low hysteresis, which indicates its fast response. The response time, defined as the time required for the response current to rise/fall from 10% to 90%, can be deduced from Figure 2c. It is clearly seen that
both the rising and falling times are about 5−7 ms, which is faster than those of previous piezoresistor sensors.8,9,30 Performance Comparisons of PTNWs, Graphene, and PTNWs/G Pressure Sensors upon Static Measurements. To confirm this effect of static measurements, different devices, including PTNWs, graphene, and PTNWs/G pressure sensors, have been made for comparison (Figure 3). The pressure response of a pure PTNW pressure sensor under a pressure pulse of 0.04 N is characterized, as shown in Figure 3a. When the device is under pressure, a positive voltage pulse signal appears instantly. However, no response is recorded when the pressure is retained. Afterward, a negative current pulse is generated by the sensor instantly when the pressure is released. This dynamic process of the voltage reflects the dynamic process of the free carriers in the external circuit. In detail, once the force is loaded, polarization charges of PTNWs will be formed at the surface, as well as a polarization potential, so the voltage rises quickly; then the free carriers with opposite polarity are attracted to the PTNWs’ surface to balance the potential, so the voltage decays with time until the potential is equilibrated and the voltage is zero. Once the force is released, the polarization charges disappear, leaving the free carriers, so the voltage quickly falls to a negative value; then the free carriers will move back until an equilibrium state, so the voltage rises to zero again. The band diagram is shown in Supporting Information S3. This phenomenon has been discussed in many papers about nanogenerators.10−13 The short-circuit current of the PTNW pressure sensor is also measured (S4 in the Supporting Information). Particularly, the current response of the PTNW pressure sensor is much smaller than that of the PTNWs/G sensor, on the order of nA, because of the high resistivity of the PTNWs. Such switching behavior of the PTNW pressure sensor is due to the transient flow of electrons during the equilibrium reconstruction driven by the piezopotential arising from dynamic stress,37 which is similar to the process of open-circuit voltage. In detail, when the PTNW is stretched, the electrons in the external circuit should flow in order to compensate the potential at the surface induced by the tensile strain, which will generate the first output signal. When the electrons and the piezoelectric field reach equilibrium, there is no more current flowing. Therefore, the PTNW pressure sensor cannot be used for static measurements because the 4509
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Figure 4. (a) Transfer curves of the PTNWs/G transistor under different pressures. The inset shows the locally enlarged area of the transfer curves. (b) Current variation of the PTNWs/G transistor under different pressures. (c) Dirac point voltage and mobility of graphene derived from the transfer curves. The inset shows the residual carrier concentration under different loading force. (d) Pressure response of PTNWs/G transistor under a pressure pulse. (e and f) Three-dimensional band diagram of PTNWs/G without and with loading force, respectively, corresponding the current response in (d).
Supporting Information). Transfer curves of the pressure transistor with a source−drain voltage (Vds) of 0.3 V under loading forces varying from 0 to 5 N are shown in Figure 4a. The response of the PTNWs/G pressure transistor can be seen in Video S1. The current in graphene decreases as the exerted force increases and the resistance increases. In order to understand such behavior, we extracted the carrier concentration and mobility of the graphene from the transfer curves by fitting the following equations (S7 in the Supporting Information):38,39
pressure-induced response is a transient process and only exists at the moment of switching on/off. For the graphene pressure sensor, there appears no current change under a pressure of 0.4 N with an applied voltage of 1 V (Figure 3b), which agrees with the previous reports.33 The piezoresistive effect in graphene is related to the lattice distortion, and a threshold deformation of 20% is required. Under a vertical weak pressure, there should not be any lattice distortion in graphene, so the carriers can be seen as straightforward transport in graphene, resulting in no current change (S5 in the Supporting Information). For the PTNWs/G pressure sensor, when a force of 0.4 N is loaded with a voltage of 1 V applied, the current of the PTNWs/G pressure sensor decreases quickly and exhibits a negative response current ΔI. This current is retained when the pressure is applied continuously (Figure 3c). A corresponding video is shown in the Supporting Information. Obviously, the pressure response of the PTNWs/G pressure sensor for static measurements is different from the PTNW one. The switching behavior of the PTNW sensor is based on the transient flow of electrons in the external load as driven by the piezopotential arising from dynamic stress,37 which can be detected only at the moment of polarization charge generation or disappearance. It is suitable for detecting the dynamic strain other than steady ones. However, in the PTNWs/G pressure sensor, polarization charges on PTNWs are used to increase the scattering sources in graphene and reduce the conductivity of graphene; thus static pressure can be detected. Mechanism of the PTNWs/G Pressure Sensor and the Role of Graphene. To investigate the mechanism of this pressure sensor, we characterized the properties of graphene in a PTNWs/G pressure transistor under pressure. The PTNWs/ G pressure transistor is fabricated, which is composed of a traditional graphene field effect transistor (G-FET) and PTNWs spread on the active channel of graphene (S6 in the
R tot = R contact + R channel = R contact +
n=
n02 + ng2 =
L /W neμ
n02 + (Cox(Vg − VDirac)/e)2
ng = Cox(Vg − VDirac)/e
(i) (ii) (iii)
where Rtot = Vds/Ids represents the total resistance of the device composed of contact resistance Rcontact and channel resistance Rchannel, e is the elementary charge, n0 is the residual carrier concentration, representing the density of carriers at the Dirac point, and ng is the carrier density originating from the backgate bias. VDirac represents the doping level of graphene. Cox = 11.5 nF/cm2 is the areal capacitance of the 300 nm thick SiO2 dielectric layer. Figure 4b shows the change of current (ΔI) as gate voltage under different loading force. The value of the response current increases as the loading force increases. Figure 4c shows the Dirac point voltage VDirac, carrier mobility, and residual carrier concentration under different loading force. VDirac does not change under different pressures, which means that no charge is transferred into graphene under the loading force. However, as the loading force increases, the carrier mobility μ decreases from 1040 cm2 V−1 s−1 to 970 cm2 V−1 s−1, and its falling rate decreases as well. It is also observed that the residual carrier concentration, n0, which is an indication of the 4510
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Figure 5. (a) Photograph of the measurement for wrist pulses. (b) 20 s real-time record of wrist pulses. The gray region is one periodic wrist pulse, which is enlarged in (c).
shown in Figure 4c. In general, the dominant carrier scattering mechanism is the Coulomb scattering by randomly charged impurities,41,42 which results in the carrier-density inhomogeneities in monolayer graphene. Another kind of scattering mechanism, resonant scattering due to defects, cracks, or boundaries in the graphene, may also influence the mobility.43 However, if the pressure response results from the defects or cracks induced by the strain, the pressure response would be irreversible, which is clearly opposite the experiment results as shown in Figures 2 and 4d. Application of PTNWs/G Pressure Sensors. On the basis of the above working mechanism, the PTNWs/G pressure sensor can be used to monitor the radial artery of a human. The pressure sensor is attached to a human wrist, above the radial artery (Figure 5a). The recording setup is the same as that for the response time tests. A 20 s real-time record of wrist pulse with an applied voltage of 3 V is shown in Figure 5b, which reveals a current response of around 470 μA. After removing the influencing factor of applied voltage, the estimated pressure of the human pulse of 7 kPa can be obtained compared with the previous data in Figure 2. It is clearly seen that the pulse frequency is 63 beats/min with a repeatable pulse shape. Generally, the pulse pressure shape contains information on blood pressure from the left ventricle contracting and the reflective wave from the lower body.44−47 From the enlargement of a periodic wrist pulse as shown in Figure 5c, it is clearly manifested that there exist three peaks following the pulse waveform shape (P1, P2, and P3), from which a few parameters for arterial stiffness diagnosis can be deduced. For example, the radial augmentation index, rAI = P2/P1, can be estimated; namely, rAI ≈ 0.91. The time interval between the first and second peaks, ΔTDVP = t2 − t1, is calculated to be ΔTDVP = 125 ms.
existence of localized charge puddles, or charge variations, increases with the applied force. In order to confirm this phenomenon, more PTNWs/G transistors are fabricated (S8 in the Supporting Information). All the results show that the voltage of the neutral point remains at almost the same voltage and the current decreases when the applied pressure is loaded. These results are different from those of other graphene-based devices, such as a photogating graphene photodetector,40 in which photoinduced carriers (free carriers) will inject into graphene and shift the voltage of the neutral point. In this PTNWs/G pressure transistor, strain-induced polarization charges are bound carriers; it cannot directly increase the carrier concentration of graphene. Also because of the barrier height between graphene and PTNWs, the carriers in graphene cannot get into the PTNWs (further discussion is present in Supporting Information S9). Although, the pressure-induced voltage generated by these polarization charges may induce a variation of the effective gate electric field, the pressure-induced voltage is too low (on the order of 10 mV, while the voltage of the neutral point is 50 V) to change the carrier concentration of graphene. As a result, the voltage of the neutral point remains unchanged under the applied voltage. The role of graphene in the PTNWs/G pressure sensor is further demonstrated by combining a piezoelectric model of PTNWs and a carrier scattering model in graphene. When the PTNWs are under pressure, the induced polarization potential (both positive and negative) will appear on the surfaces of the nanowires. These polarization charges in PTNWs can increase the scatterings of carriers in graphene, and then the carrier mobility of graphene will be decreased. Figure 4d shows the pressure response of a PTNWs/G transistor under a pressure pulse. In detail, without pressure, the carriers of graphene can move in a straight manner (Figure 4e) in the graphene channel. With pressure, the polarization charges form at the surface of the PTNWs, resulting in band bending. Because of the random distribution of nanowires, the polarization potential on the surface of PTNWs is random. Therefore, the carriers of graphene will be scattered by these random potentials and the mobility of graphene will be reduced, resulting in a decrease in current (Figure 4f). With increasing the pressure, the density of polarization charges will increase accordingly. As a result, the scattering magnitude is enhanced and the carrier mobility of graphene decreases consequently. To further confirm our assumption, we estimated the density of variations due to the piezoelectric polarizations of the randomly distributed nanowires, based on the average density of nanowires and approximate charges induced on each polarized nanowire (see S10 in the Supporting Information). The charge density variation induced by the applied pressure was estimated to be 0.98 × 1010 cm−2 N−1, which is in good agreement with the slope of the residual carrier density (1.16 × 1010 cm−2 N−1) as
CONCLUSIONS In summary, we demonstrate a piezoelectric-induced pressure sensor for static measurements based on nanowire/graphene heterostructures. Here, the strain-induced polarization charges in piezoelectric nanowires can function as charged impurities in graphene and affect its carrier mobility, thus enhancing the sensitivity of this graphene-based pressure sensor. Based on this working mechanism, this designed sensor has a featured sensitivity of up to 9.4 × 10−3 kPa−1, higher than the sensitivity of an intrinsic CVD-grown graphene pressure sensor, which has been applied to monitor the radial artery of a human. This study provides a concept of device design with an easy fabrication method for wearable and implantable electronic applications. 4511
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MATERIALS AND METHODS
Foundation of China for the support (51402060, 11504099 and 61229401).
Fabrication of PbTiO3 Nanowires. The PbTiO3 nanowires were synthesized by a hydrothermal process method.36 The details are the following: First, lead acetic hydrate, Pb(CH3COO)2·3H2O, and tetrabutyl titanate, Ti(OC4H9)4, of 3.41 g were dissolved in DI water and ethanol, respectively. Then, the Ti(OC4H9)4 solution was dropped into the Pb(CH3COO2) solution. The pH value of the mixed solution was controlled by NaOH and remained at 13. The final mixture was put into a hydrothermal reactor to perform the hydrothermal treatment at 200 °C for 48 h. After cooling, the obtained products were washed with DI water and ethanol. Fabrication of the Flexible PTNWs/Graphene Pressure Sensor. (i) For the flexible PI substrate on silicon, the 2.3-μm-thick PI film was made by spin-coating (500 rpm for 5 s and 3000 rpm for 30 s) a poly(pyromellitic dianhydride-co-4,4′-oxydianiline) and amic acid solution (Sigma-Aldrich) on a Si wafer, which was then cured at 150 °C for 5 min and 250 °C for another 1 h in ambient. (ii) Graphene was transferred onto the PI substrate. (iii) Cr/Au interdigitated electrodes were deposited with a metallic shadow mask. (iv) The PbTiO3 nanowires were coated on graphene by the drop-casting method. The PbTiO3 nanowire solution was dropped onto the surface of graphene and dried for a while. After that, PDMS was encapsulated on the device. Fabrication of the PTNWs/Graphene Pressure Transistor. CVD-grown graphene was transferred by a wet process to a SiO2/Si substrate (a silicon oxide at a thickness of 300 nm on top of the p-type heavy doping silicon substrate). By utilizing a shadow mask to define the electrodes, Cr/Au (5 nm/80 nm) films were deposited via a thermal evaporation deposition technique. The width between the two electrodes was W = 30 μm, and the length of the graphene channel was L = 250 μm. After the graphene FET fabrication was finished, a drop of PbTiO3 ethanol solution was coated on the top of the FET channel and baked at 60 °C to release ethanol. Afterward, the device was encapsulated by PDMS, leaving the two electrodes for wire bonding.
REFERENCES (1) Chortos, A.; Liu, J.; Bao, Z. Pursuing Prosthetic Electronic Skin. Nat. Mater. 2016, 15, 937−950. (2) Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D.-H.; Brooks, G. A.; Davis, R. W.; Javey, A. Fully Integrated Wearable Sensor Arrays for Multiplexed in situ Perspiration Analysis. Nature 2016, 529, 509−514. (3) Kang, S.; Murphy, R. K. J.; Hwang, S.; Lee, S. M.; Harburg, D. V.; Krueger, N. A.; Shin, J.; Gamble, P.; Cheng, H.; Yu, S.; Liu, Z.; McCall, J. G.; Stephen, M.; Ying, H.; Kim, J.; Park, G.; Chad Webb, R.; Lee, C. H.; Chung, S.; Wie, D. S.; Gujar, A. D.; Vemulapalli, B.; Kim, A. H.; Lee, K.; Cheng, J.; Huang, Y.; Lee, S. H.; Braun, P. V.; Ray, W. Z.; Rogers, J. A. Bioresorbable Silicon Electronic Sensors for the Brain. Nature 2016, 530, 71−76. (4) Luo, N.; Dai, W.; Li, C.; Zhou, Z.; Lu, L.; Poon, C.; Chen, S.; Zhang, Y.; Zhao, N. Flexible Piezoresistive Sensor Patch Enabling Ultralow Power Cuffless Blood Pressure Measurement. Adv. Funct. Mater. 2016, 26, 1178−1187. (5) Yang, T.; Li, X.; Jiang, X.; Lin, S.; Lao, J.; Shi, J.; Zhen, Z.; Li, Z.; Zhu, H. Structural Engineering of Gold Thin Films with Channel Cracks for Ultrasensitive Strain Sensing. Mater. Horiz. 2016, 3, 248− 255. (6) Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A. Nanowire Active-Matrix Circuitry for LowVoltage Macroscale Artificial Skin. Nat. Mater. 2010, 9, 821−826. (7) Schwartz, G.; Tee, B. C.-K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z. Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859. (8) Ramuz, M.; Tee, B. C-K.; Tok, J. B.-H.; Bao, Z. Transparent, Optical, Pressure-Sensitive Artificial Skin for Large-Area Stretchable Electronics. Adv. Mater. 2012, 24, 3223−3227. (9) Choong, C.-L.; Shim, M.-B.; Lee, B.-S.; Jeon, S.; Ko, D.-S.; Kang, T.-H.; Bae, J.; Lee, S. H.; Byun, K.-E.; Im, J.; Jeong, Y. J.; Park, C. E.; Park, J.-J.; Chung, U.-I. Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array. Adv. Mater. 2014, 26, 3451−3458. (10) Wang, Z. L. From Nanogenerators to PiezotronicsA DecadeLong Study of ZnO Nanostructures. MRS Bull. 2012, 37, 814−827. (11) Wu, W.; Wen, X.; Wang, Z. L. Taxel-Addressable Matrix of Vertical-Nanowire Piezotronic Transistors for Active and Adaptive Tactile Imaging. Science 2013, 340, 952−957. (12) Wang, Z.; Zhang, Y.; Yang, S.; Hu, Y.; Wang, S.; Gu, H.; Wang, Y.; Chan, H. L. W.; Wang, J. (K,Na)NbO3 Nanofiber-based SelfPowered Sensors for Accurate Detection of Dynamic Strain. ACS Appl. Mater. Interfaces 2015, 7, 4921−4927. (13) Hu, Y.; Xu, C.; Zhang, Y.; Lin, L.; Snyder, R. L.; Wang, Z. L. A Nanogenerator for Energy Harvesting from a Rotating Tire and its Application as a Self-Powered Pressure/Speed Sensor. Adv. Mater. 2011, 23, 4068−4071. (14) Zhou, J.; Fei, P.; Gao, Y.; Gu, Y.; Liu, J.; Bao, G.; Wang, Z. L. Mechanical−Electrical Triggers and Sensors Using Piezoelectric Micowires/Nanowires. Nano Lett. 2008, 8, 2725−2730. (15) Li, X.; Lv, Z.; Zhu, H. Carbon/Silicon Heterojunction Solar Cells: State of the Art and Prospects. Adv. Mater. 2015, 27, 6549− 6574. (16) Chen, Z.; Li, X.; Wang, J.; Tao, L.; Long, M.; Liang, S.; Ang, L.; Shu, C.; Tsang, H.; Xu, J. Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity. ACS Nano 2017, 11, 430−437. (17) Li, X.; Zhao, T.; Wang, K.; Yang, Y.; Wei, J.; Kang, F.; Wu, D.; Zhu, H. Directly Drawing Self-Assembled, Porous, and Monolithic Graphene Fiber from Chemical Vapor Deposition Grown Graphene Film and Its Electrochemical Properties. Langmuir 2011, 27, 12164− 12171.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b08027. Video S1: Pressure response of the PTNWs/G pressure sensor for static measurements (AVI) Figures S1−S10: Materials and characterization, additional experimental details (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Zefeng Chen: 0000-0002-0689-8443 Xinming Li: 0000-0002-7844-8417 Yuxuan Lin: 0000-0003-0638-2620 Author Contributions ⊥
Z. Chen and Z. Wang contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The work is partly supported by the Research Grants Council of Hong Kong, particularly via Grant Nos. AoE/P-03/08, N_CUHK405/12, T23-407/13-N, AoE/P-02/12, 14207515, 14204616, and CUHK Group Research Scheme. X.M.L., Z.W. and J.B.X. would like to thank the National Natural Science 4512
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ACS Nano
Hydrothermal Method at Low Temperature. Appl. Phys. Lett. 2006, 88, 193120. (37) Wang, Z. L.; Zhu, G.; Yang, Y.; Wang, S. H.; Pan, C. F. Progress in Nanogenerators for Portable Electronics. Mater. Today 2012, 15, 532−543. (38) Liao, L.; Bai, J.; Qu, Y.; Lin, Y. C.; Li, Y.; Huang, Y.; Duan, X. High-κ Oxide Nanoribbons as Gate Dielectrics for High Mobility TopGated Graphene Transistors. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6711−6715. (39) Zhang, Y.; Zhang, L.; Kim, P.; Ge, M.; Li, Z.; Zhou, C. Vapor Trapping Growth of Single-Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic Properties. Nano Lett. 2012, 12, 2810− 2816. (40) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Hybrid Graphene−Quantum Dot Phototransistors with Ultrahigh Gain. Nat. Nanotechnol. 2012, 7, 363−368. (41) Chen, K.; Wan, X.; Liu, D.; Kang, Z.; Xie, W.; Chen, J.; Miao, Q.; Xu, J. Quantitative Determination of Scattering Mechanism in Large-Area Graphene on Conventional and SAM-Functionalized Substrates at Room Temperature. Nanoscale 2013, 5, 5784−5793. (42) Giannazzo, F.; Sonde, S.; Nigro, R. L.; Rimini, E.; Raineri, V. Mapping the Density of Scattering Centers Limiting the Electron Mean Free Path in Graphene. Nano Lett. 2011, 11, 4612−4618. (43) Zhang, Y. B.; Brar, V. W.; Girit, C.; Zettl, A.; Crommie, M. F. Origin of Spatial Charge Inhomogeneity in Graphene. Nat. Phys. 2009, 5, 722−726. (44) Nichols, W. W. Clinical Measurement of Arterial Stiffness Obtained from Noninvasive Pressure Waveforms. Am. J. Hypertens. 2005, 18, 3S−10S. (45) Pauca, A. L.; O’Rourke, M. F.; Kon, N. D. Prospective Evaluation of a Method for Estimating Ascending Aortic Pressure From the Radial Artery Pressure Waveform. Hypertension 2001, 38, 932−937. (46) Zhang, Y.; Zhang, F.; Huang, D.; Gao, X.; Di, C.; Zhu, D. Flexible Suspended Gate Organic Thin-Film Transistors for UltraSensitive Pressure Detection. Nat. Commun. 2015, 6, 6269. (47) Dagdeviren, C.; Su, Y.; Joe, P.; Yona, R.; Liu, Y.; Kim, Y.; Huang, Y.; Damadoran, A.; Xia, J.; Martin, L.; Huang, Y.; Rogers, J. Conformable Amplified Lead Zirconate Titanate Sensors with Enhanced Piezoelectric Response for Cutaneous Pressure Monitoring. Nat. Commun. 2014, 5, 4496.
(18) Li, L.; Li, X.; Du, M.; Guo, Y.; Li, Y.; Li, H.; Yang, Y.; Alam, F.; Lin, C.; Fang, Y. Solid-Phase Coalescence of Electrochemically Exfoliated Graphene Flakes into a Continuous Film on Copper. Chem. Mater. 2016, 28, 3360−3366. (19) Li, X.; Zhu, H.; Wang, K.; Cao, A.; Wei, J.; Li, C.; Jia, Y.; Li, Z.; Li, X.; Wu, D. Graphene-On-Silicon Schottky Junction Solar Cells. Adv. Mater. 2010, 22, 2743−2748. (20) Li, X.; Zhu, H. The Graphene-Semiconductor Schottky Junction. Phys. Today 2016, 69 (9), 46−51. (21) Jang, H.; Park, Y. J.; Chen, X.; Das, T.; Kim, M. S.; Ahn, J. H. Graphene-Based Flexible and Stretchable Electronics. Adv. Mater. 2016, 28, 4184−4202. (22) Shi, J.; Li, X.; Cheng, H.; Liu, Z.; Zhao, L.; Yang, T.; Dai, Z.; Cheng, Z.; Shi, E.; Yang, L.; Zhang, Z.; Cao, A.; Zhu, H.; Fang, Y. Graphene Reinforced Carbon Nanotube Networks for Wearable Strain Sensors. Adv. Funct. Mater. 2016, 26, 2078−2084. (23) Yan, C.; Wang, J.; Kang, W.; Cui, M.; Wang, X.; Foo, C. Y.; Chee, K. J.; Lee, P. S. Highly Stretchable Piezoresistive Graphene− Nanocellulose Nanopaper for Strain Sensors. Adv. Mater. 2014, 26, 2022−2027. (24) Zhao, J.; He, C.; Yang, R.; Shi, Z.; Cheng, M.; Yang, W.; Xie, G.; Wang, D.; Shi, D.; Zhang, G. Ultra-Sensitive Strain Sensors Based on Piezoresistive Nanographene Films. Appl. Phys. Lett. 2012, 101, 063112. (25) Yang, T.; Wang, W.; Zhang, H.; Li, X.; Shi, J.; He, Y.; Zheng, Q.; Li, Z.; Zhu, H. Tactile Sensing System Based on Arrays of Graphene Woven Microfabrics: Electromechanical Behavior and Electronic Skin Application. ACS Nano 2015, 9, 10867−10875. (26) Li, X.; Yang, T.; Yang, Y.; Zhu, J.; Li, L.; Alam, F.; Li, X.; Wang, K.; Cheng, H.; Lin, C.; Fang, Y.; Zhu, H. Large-Area Ultrathin Graphene Films by Single-Step Marangoni Self-Assembly for Highly Sensitive Strain Sensing Application. Adv. Funct. Mater. 2016, 26, 1322−1329. (27) Haniff, M. A. S. M.; Hafiz, S. M.; Wahid, K. A. A.; Endut, Z.; Lee, H. W.; Bien, D. C. S.; Azid, I. A.; Abdullah, M. Z.; Huang, N. M.; Rahman, S. A. Piezoresistive Effects in Controllable Defective HFTCVD Graphene-Based Flexible Pressure Sensor. Sci. Rep. 2015, 5, 14751. (28) Yang, T.; Wang, Y.; Li, X.; Zhang, Y.; Li, X.; Wang, K.; Wu, D.; Li, H.; Zhu, H. High Sensitivity and Wide Dynamic Range Torsion Sensors Based on Graphene Woven Structure. Nanoscale 2014, 6, 13053−13059. (29) Zhu, B.; Niu, Z.; Wang, H.; Leow, W. R.; Wang, H.; Li, Y.; Zheng, L.; Wei, J.; Huo, F.; Chen, X. Microstructured Graphene Arrays for Highly Sensitive Flexible Tactile Sensors. Small 2014, 10, 3625−3631. (30) Yao, H. B.; Ge, J.; Wang, C. F.; Wang, X.; Hu, W.; Zheng, Z. J.; Ni, Y.; Yu, S. H. A Flexible and Highly Pressure-Sensitive Graphene− Polyurethane Sponge Based on Fractured Microstructure Design. Adv. Mater. 2013, 25, 6692−6698. (31) Pereira, V. M.; Neto, A. H. C. Tight-Binding Approach to Uniaxial Strain in Graphene. Phys. Rev. B 2009, 80, 045401. (32) Zhu, S. E.; Ghatkesar, M. K.; Zhang, C.; Janssen, G. C. A. M. Graphene Based Piezoresistive Pressure Sensor. Appl. Phys. Lett. 2013, 102, 161904. (33) Smith, A. D.; Niklaus, F.; Paussa, A.; Vaziri, S.; Fischer, A. C.; Sterner, M.; Forsberg, F.; Delin, A.; Esseni, D.; Palestri, P.; Ö stling, M.; Lemme, M. C. Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes. Nano Lett. 2013, 13, 3237−3242. (34) Sun, Q.; Seung, W.; Kim, B. J.; Seo, S.; Kim, S.-W.; Cho, J. H. Active Matrix Electronic Skin Strain Sensor Based on PiezopotentialPowered Graphene Transistors. Adv. Mater. 2015, 27, 3411−3417. (35) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. (36) Hu, Y.; Gu, H.; Sun, X.; You, J.; Wang, J. Photoluminescence and Raman Scattering Studies on PbTiO3 Nanowires Fabricated by 4513
DOI: 10.1021/acsnano.6b08027 ACS Nano 2017, 11, 4507−4513
