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Highly sensitive and flexible strain sensors based on vertical zinc oxide nanowire arrays
Sensors and Actuators A 205 (2014) 164–169
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Highly sensitive and flexible strain sensors based on vertical zinc oxide nanowire arrays Wengui Zhang, Ren Zhu, Vu Nguyen, Rusen Yang ∗ Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA
a r t i c l e
i n f o
Article history: Received 11 July 2013 Received in revised form 1 October 2013 Accepted 2 November 2013 Available online 9 November 2013 Keywords: ZnO Nanowire array Strain sensor Gauge factor Piezotronic effect
a b s t r a c t In this paper, a highly sensitive strain sensor with vertically aligned zinc oxide (ZnO) nanowire arrays on polyethylene terephthalate (PET) film was reported. The device fabrication includes conventional photolithography, metallization, and ZnO nanowire growth through a hydrothermal method. I–V characteristics of the device were highly nonlinear due to the Schottky contact between the nanowire and the gold (Au) electrode. The conductivity of the device is significantly tuned by the change of ZnO/Au Schottky barrier that reflects the strain-induced piezoelectric potential. A gauge factor up to 1813 was obtained from this strain senor, which is higher than the previously reported device based on a lateral ZnO microwire. Theoretical analysis of the piezotronic effect shows that the working nanowire with the largest conductivity change dominates the performance of the device. The non-working nanowire has limited adverse effect on the performance, which explains the robust performance of this novel strain sensor. The stability and fast response of the sensor were also investigated. The sensitive and robust strain sensor is expected to find applications in civil, medical, and other fields. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Flexible or stretchable strain sensors based on nanowire, nanotube or polymer composite have been widely studied in the last decade for potential applications in portable and wearable personal devices [1–24]. Many nanomaterials like carbon nanotube (CNT) [1–12], graphene [13–18] and ZnO nanostructure [19–24] have been investigated for the design of strain sensors. Compared to conventional rigid strain sensors using metal or silicon, flexible strain sensors based on nanomaterials exhibit high strain tolerance, ultra-fast response, high sensitivity, and low power consumption. Nanowires with piezoelectric and semiconducting properties are particularly suitable for this purpose due to the recently discovered piezotronic effect, in which the piezoelectric potential from a strained wire can dramatically tune the current flow through the nanowire by the change of Schottky barrier [23–27]. Flexible piezotronic strain sensor has been demonstrated based on individual ZnO microwire, with a gauge factor of 1250 [22]. The nanowire-based sensor is expected to provide even better performance due to the much higher surface-volume ratio and significant spatial confinement. In this paper, a novel strain sensor based on vertical ZnO nanowires grown on PET substrate has been designed and
∗ Corresponding author. Tel.: +1 6126264318. E-mail address: [email protected] (R. Yang). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.11.004
fabricated, and the improved sensitivity, flexibility, robustness as well as fast response have been examined. ZnO is an environment friendly material with outstanding piezoelectric and semiconducting properties. ZnO nanowires can be achieved in high yield with physical or chemical approaches [28–30], which facilitates future scale up and massive production. It has been revealed that the I–V characteristics of the strain sensor are dominated by Schottky contact at ZnO/Au interfaces and are thus highly nonlinear. The Schottky barrier height is tuned by piezoelectric potential from stressed ZnO nanowires when the strain sensor is deformed. The strain sensor is extremely sensitive to the local strain and a gauge factor up to 1813 was observed. This gauge factor is higher than that of previous reported strain sensors with single ZnO microwire in lateral configuration [22]. In the Section 2 of this paper, the fabrication procedure of the device and the measurement system are introduced. In Section 3, the performance of the strain sensor is tested. Finally, the performance as well as the piezotronic working principle of the device is discussed. 2. Fabrication procedure and measurement setup The fabrication procedure of the strain sensor and the measurement setup are shown in Fig. 1. A piece of PET substrate with a length of ∼3 cm, a width of ∼1 cm, and a thickness of ∼200 m was rinsed with acetone, isopropyl alcohol, and deionized water and then blow-dried with nitrogen. The dry and clean PET substrate was placed in a RF/DC sputtering system (AJA-Sputter, 200 W), in
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Fig. 1. Schematic fabrication process of the strain sensor and measuring system. (a) Deposit Cr bonding layer and ZnO seedlayer on PET substrate. (b) Open the window through photolithography. (c) Grow ZnO nanowire with hydrothermal solution. (d) Spin-coat PDMS protection layer for ZnO nanowire and dry etch to exposure nanowire tips. (e) Deposit Au as top electrode. (f) Optical image of the final device. (g) Sketch of measuring system that includes sample holder and linear motor with moving rod.
which a bonding layer of Cr (∼40 nm) and a seed layer of ZnO (∼200 nm) were deposited in sequence, as shown in Fig. 1(a). Shipley S1813 photoresist was spin-coated (∼2 m, 3000 rpm for 30 s) on the substrate and a pattern of 2 × 2 windows (50 m × 50 m each) was opened in Fig. 1(b) through photolithography for the growth of ZnO nanowire arrays in Fig. 1(c). In order to obtain uniform, long and high quality of ZnO nanowires, the growth solution of 16 mmol/L hexamethylenetetramine and 16 mmol/L zinc chloride was mixed with 4% volume concentration of ammonia (30% w/w NH3 ). The substrate was floated upside down on the surface of the solution. After the growth at 95 ◦ C for 14 h, ZnO nanowire arrays were obtained in the 2 × 2 windows. A thin layer of polydimethylsiloxane (PDMS, ∼20 m, 3000 rpm for 2 min) was spin-coated on the surface of the PET substrate and cured at 80 ◦ C for 2 h, which served as a protection layer for the nanowire arrays. Top part of the PDMS layer was then etched away in a reactive-ion etching system (STS Dry Etcher, with O2 :CF4 = 1:3 for 15 min), which exposed the tip of nanowires in Fig. 1(d) for electrode connection in Fig. 1(e). A thin layer of Au (∼120 nm) was deposited by a RF/DC sputtering system to serve as the top electrode and form ZnO/Au Schottky barrier. Finally, the strain sensor was packaged by spin-coating a thin layer of poly(methyl methacrylate) (∼2 m, NANOTM 950, MicroChem) on the surface. The final device was compact and highly flexible, as shown in Fig. 1(f). The setup in Fig. 1(g) was used to test the performance of the strain sensor. One end of the strain sensor was fixed tightly on a sample holder while the other end was attached to a moving rod driven by a programmable linear motor. During the measurement, the linear motor was programmed to push/pull the lateral rod with specified distance, acceleration, maximum speed and deceleration, so that the strain sensor was bent back and forth in a well-controlled manner. A sinusoidal bias voltage was applied across the nanowires with a functional generator (Keithley 3390) and the current was monitored with a current amplifier (Keithley
428). The measurement was carried out in atmosphere at room temperature. The strain sensor was located in a home-built Faraday cage so that the environmental influence and electromagnetic noises were excluded. The performance of strain sensor largely depends on the quality of ZnO nanowire arrays. Fig. 2 shows typical ZnO nanowire arrays from hydrothermal growth. Fig. 2(a) confirmed that ZnO nanowire arrays grew uniformly within designed pattern with a length of ∼10 m and a diameter of ∼100 nm. The PDMS layer was spincoated on the substrate and then partially etched to expose the tip of the ZnO nanowire for the electrode deposition, as shown in Fig. 2(b). After etching, ZnO nanowires in Fig. 2(b) were intact and surrounded by PDMS. The distorted and very bright area in Fig. 2(b) is due to the common charging effect in the scanning electron microscopy. 3. Experimental result The typical I–V characteristics of the strain sensor and its response to the strain are shown in Fig. 3. The nonlinear and rectifying behavior in Fig. 3(a) is due to the Schottky contact formed between the ZnO nanowire and the Au electrode. Fig. 3(a) also indicates that the I–V characteristic is dramatically changed when the devices is stressed. Because the dimension of the nanowire is much smaller than PET and PDMS is much thinner (15 m) and more compliant (360–870 kPa) than the PET (200 m and 2–2.7 GPa), PET dominates the elastic behavior of the device. The longitudinal normal strain at the surface of the sensor was estimated from equation
ε=
3aD x (1 − ) l l2
(1)
Here, a is the half thickness of the PET substrate (∼100 m), D is the maximum bending deflection of the device, l is the length from
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Fig. 3. Electromechanical behaviors of the strain sensor. (a) Significant change of nonlinear I–V curve in response to different strain. (b) Gauge factor at 1.5 V bias. (c) Linear dependence of logrithmic current on the strain at different bias of 2 V, 1.5 V, and 1 V. Fig. 2. SEM image of vertical ZnO nanowires from hydrothermal growth. (a) Top view of the as-grown nanowire. Inset: cross-section view of the nanowire arrays. (b) Top view of the nanowire after PDMS protection coating and dry etching to expose nanowire tips.
fixed end to free end, and x is the average distance from the fixed end. Because of the Poisson effect, the normal strain along the surface of the PET induced the strain along the axial direction of the nanowire that grew perpendicular to the surface, and caused dramatic current change. The conductivity of the compressed nanowire was significantly increased, especially when a forward bias was applied. For example, at the bias of +2 V, the current in strain-free nanowires was about 0.02 A. As the normal strain ε reached 0.4%, the current increased to 0.08 A. When the strain sensor was further pushed forward until the strain reached 0.8%, the current reached 0.26 A. The performance of the strain sen
I/I0 ε ,
sor can be characterized with the strain gauge factor, GF =
where I0 is the strain-free current of the device at a fixed forward bias, and ε and I are the strain and the corresponding current change at the same bias. With a forward bias of 1.5 V, the gauge factor of the strain sensor is plotted in Fig. 3(b) and is as high as 1813 at a normal strain ε = 0.6%. In Fig. 3(c), the logarithmic current is plot against the strain at different biases. A linear behavior is revealed and the slope is roughly constant for different biases, and the mechanism is discussed in next section. The stability and response of the sensor is examined in Fig. 4. The “Stress” state and “Release” state in Fig. 4 correspond to the state when the device was mechanically bent and the state when the device returned to its straight and strain-free condition, respectively. It is worthwhile pointing out that, the current always reached the similar value at “Stress” state and went back to its original current level at “Release” state when the substrate was
cyclically bent under different frequencies, which demonstrates the stability of the strain sensor. The instant response to 2.5 Hz excitation indicates a quick switch of the strain sensor. The fast response of the strain sensor with ZnO nanowires was further studied when the device was deformed with different strain rates. The current of the strain sensor was under real time monitor as the linear motor pushed the free end of the substrate at different speeds from 0.001 m/s to 1 m/s to the strain ε = 0.8%. The observed current fluctuation at the beginning and end of the deformation was ascribed to the instability of the acceleration and the deceleration of the linear motor. However, even for the fast speed like v = 1 m/s, the strain sensor responded very well to the mechanical deformation with a response time below 100 ms.
4. Discussion and explanations The high sensitivity of the strain sensor in this study can be ascribed to the piezotronic effect in the piezoelectric and semiconducting nanowires [25,26]. To explain the piezotronic effect, the sensor can be simplified as a one dimensional metalsemiconductor-metal structure. Schottky contact forms at one terminal between the top gold electrode and the tip of nanowires, while Ohmic contact at the other terminal is realized with Cr electrode connected to the continuous ZnO seed layer. The I–V characteristic of this metal-semiconductor-metal structure under forward bias, V > 3kT/q ∼ 77mV, is given by [26]
∗∗ 2
I = SA T exp
−
qB0 kT
exp
q( + V ) E kT
(2)
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Fig. 4. Switching between “Stress” (ε = 0.8%) and “Release” states of the strain sensor at fixed bias of +5 V when it is cyclically bent with at (a) 0.125 Hz, (b) 0.25 Hz, (c) 1 Hz, and (d) 2.5 Hz.
while I–V characteristic in the reverse direction with 3kT/q∼77mV is
q B0
I = SA∗∗ T 2 exp −
where =
2qND
kT
exp
V + Vbi −
q
q/4 kT
kT q
V >
(3)
(Fig. 5(c)) from Eq. (2). As a result, the strain gauge factor, GF, of the strain sensor can be expressed as
I/I0 exp(− qkTB (ε) ) − 1 = GF = ε ε
(5)
, S is the contact area, A** is
the effective Richardson constant, T is the temperature, q is the electron charge, B0 is the Schottky barrier height at zero electric field, Vbi is the built-in potential at the barrier, ND is the donor impurity density, is the permittivity of ZnO and E is the lowering of Schottky barrier under an external electric field. When ZnO nanowire is compressed, piezoelectric potential is generated along its length as shown in Fig. 5(a). This potential decreases the Schottky barrier (Fig. 5(b)) in Eq. (2) in the form of [25]
B =
2 qpiezo Wpiezo
2εs
(4)
where Wpiezo is the width of the polarization layer on the interface, εs is the permittivity of ZnO, and piezo is the density of polarization charges (in units of electron charge) that is proportional to the strain ε along nanowire c-axis. Consequently, the Schottky barrier change, B , is proportional to strain ε. B < 0 when the nanowire is compressed because of the strain-induced negative charge on the ZnO/Au interface and B > 0 when nanowire is stretched due to the strain-induced positive charge. Because is proportional to the strain, it is clear from Eqs. (2) and (4) that the logarithmic current is proportional to the strain and the slope is independent of the applied bias, which agrees well with our observation in Fig. 3(c). The change of the Schottky barrier due to the piezoelectric potential, B , modulates the current through the nanowire
Fig. 5. Explanation of piezotronic working principle. (a) “Original” and “Compressed” states of ZnO nanowire. (b) Corresponding energy band diagram at ZnO/Au interface on two states. (c) Correpsonding I–V characteristics on two states.
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The exponential dependence of the current on the Schottky barrier explains the high gauge factor observed in the strain sensor. Considering the current of nanowires under stress is much higher than the stress-free current and the nanowires are very uniform, we assume that the stress-free conductivity are the same for all nanowires. The stress-induced current change is different for different nanowires because the Schottky barrier and thus the current are very sensitive to the local strain that can vary slightly from one nanowire to another. Therefore, the gauge factor is estimated as
I i exp(− qi (ε) ) − N Ii0 kT GF = = εN ε
(6)
Here, N represents the number of nanowires, i (ε) is the Schottky barrier change in the i-th nanowire. Because of the variation in the nanowire growth and the device fabrication, all nanowires had not the same response to the stress. When the device in Fig. 1(d) was bent to the left such that the nanowire was experiencing compressive strain along the length, the current in some nanowires was significantly increased due to the Schottky barrier drop, while other nanowires might not work well and did not change much. Exponential term in Eq. (6) indicates that the nanowires with dramatic current increase dominated the performance, which agreed with the high sensitivity we observed in Fig. 3. In comparison, when the device was bent to the right such that the nanowire was elongated, the current of the most responsive nanowires drops significantly, but the irresponsive nanowires maintaining at a relatively higher current level dominated the device’s current. Therefore, the total change of current in Eq. (6) is insignificant, resulting in very low sensitivity in this case. Quality of Schottky barrier significantly influences the performance of the strain sensor. In this study, Au was selected to form Schottky contact with ZnO, which has a higher work function than the silver used in the previously reported single-nanowire strain sensor [22,31]. Oxygen plasma clean for the tip of ZnO nanowire during dry etching process improved the quality of Schottky contact [32]. Size of the nanowire array determines the number of nanowires. Compared to the exponential term in Eq. (6), the number of nanowires N is a minor factor and can usually be neglected. However, when there are more nanowires in a larger designed pattern, the variance of nanowires make it difficult to achieve Schottky contact on every nanowire. For this reason, the size of the ZnO nanowire array as well as growth process are controlled for improved Schottky contact and such optimization is expected to further enhance the device’s performance. 5. Conclusion In summary, we have developed a novel strain sensor based on vertical ZnO nanowire arrays grown on a flexible PET substrate from a hydrothermal process. The strain sensor is extremely sensitive and a strain gauge factor up to 1813 has been demonstrated. The high sensitivity is ascribed to the high quality ZnO nanowire arrays and the piezotronic effect. In addition, the stability and fast response of the device have been observed. The excellent performances of the strain sensor promote further applications for delicate and sensitive force, strain and stress measurement in many areas, such as tissue/cell examination, smart skin design, and structural health monitoring. Acknowledgements The authors are truly grateful for the financial support from the Department of Mechanical Engineering and the College of
Science and Engineering of the University of Minnesota. The authors would like to thank the National Science Foundation for its financial support (ECCS-1150147). Parts of this work were carried out in Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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Ren Zhu is currently a PhD candidate in the department of mechanical engineering, University of Minnesota. He received his bachelor’s degree in mechanical engineering and automation at Shanghai Jiao Tong University, China. During his undergraduate study, he was awarded with National Scholarship and Academic Excellence Scholarship. His research interests are piezoelectric nanogenerators, a novel technology for energy harvesting
Biographies
Rusen Yang received his PhD degree in Materials Science and Engineering from Georgia Institute of Technology in 2007, where he continued as Post-Doctoral Associate. He joined Mechanical Engineering at the University of Minnesota-Twin Cities as an assistant professor in 2010. He has discovered novel nanostructures, such as ZnO, SnO2 , Zn3 P2 , and investigated their application potentials. His most recent work on energy harvester based on piezoelectric nanomaterials made significant contribution in the field of renewable energy.
Wengui Zhang received his Master degree in Plasma Physics from the University of Science and Technology of China (USTC), China, in 2010 and Bachelor degree in Space Physics from the University of Science and Technology of China (USTC), China, in 2004. He is currently a PhD candidate in University of Minnesota. His research interests are design and fabrication of novel piezotronic micro/nano-scale devices, including mechanical sensors and gas/chemical sensors.
Vu Nguyen received his B.S. degree in Mechanical Engineering from Worcester Polytechnic Institute, Worcester, Massachusetts in 2012. He is currently pursuing Ph.D. degree at the University of Minnesota, Minneapolis, Minnesota. His research interests are energy harvesting and self-power systems at micro/nano scale.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Highly sensitive and flexible strain sensors based on vertical zinc oxide nanowire arrays Wengui Zhang, Ren Zhu, Vu Nguyen, Rusen Yang ∗ Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA
a r t i c l e
i n f o
Article history: Received 11 July 2013 Received in revised form 1 October 2013 Accepted 2 November 2013 Available online 9 November 2013 Keywords: ZnO Nanowire array Strain sensor Gauge factor Piezotronic effect
a b s t r a c t In this paper, a highly sensitive strain sensor with vertically aligned zinc oxide (ZnO) nanowire arrays on polyethylene terephthalate (PET) film was reported. The device fabrication includes conventional photolithography, metallization, and ZnO nanowire growth through a hydrothermal method. I–V characteristics of the device were highly nonlinear due to the Schottky contact between the nanowire and the gold (Au) electrode. The conductivity of the device is significantly tuned by the change of ZnO/Au Schottky barrier that reflects the strain-induced piezoelectric potential. A gauge factor up to 1813 was obtained from this strain senor, which is higher than the previously reported device based on a lateral ZnO microwire. Theoretical analysis of the piezotronic effect shows that the working nanowire with the largest conductivity change dominates the performance of the device. The non-working nanowire has limited adverse effect on the performance, which explains the robust performance of this novel strain sensor. The stability and fast response of the sensor were also investigated. The sensitive and robust strain sensor is expected to find applications in civil, medical, and other fields. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Flexible or stretchable strain sensors based on nanowire, nanotube or polymer composite have been widely studied in the last decade for potential applications in portable and wearable personal devices [1–24]. Many nanomaterials like carbon nanotube (CNT) [1–12], graphene [13–18] and ZnO nanostructure [19–24] have been investigated for the design of strain sensors. Compared to conventional rigid strain sensors using metal or silicon, flexible strain sensors based on nanomaterials exhibit high strain tolerance, ultra-fast response, high sensitivity, and low power consumption. Nanowires with piezoelectric and semiconducting properties are particularly suitable for this purpose due to the recently discovered piezotronic effect, in which the piezoelectric potential from a strained wire can dramatically tune the current flow through the nanowire by the change of Schottky barrier [23–27]. Flexible piezotronic strain sensor has been demonstrated based on individual ZnO microwire, with a gauge factor of 1250 [22]. The nanowire-based sensor is expected to provide even better performance due to the much higher surface-volume ratio and significant spatial confinement. In this paper, a novel strain sensor based on vertical ZnO nanowires grown on PET substrate has been designed and
∗ Corresponding author. Tel.: +1 6126264318. E-mail address: [email protected] (R. Yang). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.11.004
fabricated, and the improved sensitivity, flexibility, robustness as well as fast response have been examined. ZnO is an environment friendly material with outstanding piezoelectric and semiconducting properties. ZnO nanowires can be achieved in high yield with physical or chemical approaches [28–30], which facilitates future scale up and massive production. It has been revealed that the I–V characteristics of the strain sensor are dominated by Schottky contact at ZnO/Au interfaces and are thus highly nonlinear. The Schottky barrier height is tuned by piezoelectric potential from stressed ZnO nanowires when the strain sensor is deformed. The strain sensor is extremely sensitive to the local strain and a gauge factor up to 1813 was observed. This gauge factor is higher than that of previous reported strain sensors with single ZnO microwire in lateral configuration [22]. In the Section 2 of this paper, the fabrication procedure of the device and the measurement system are introduced. In Section 3, the performance of the strain sensor is tested. Finally, the performance as well as the piezotronic working principle of the device is discussed. 2. Fabrication procedure and measurement setup The fabrication procedure of the strain sensor and the measurement setup are shown in Fig. 1. A piece of PET substrate with a length of ∼3 cm, a width of ∼1 cm, and a thickness of ∼200 m was rinsed with acetone, isopropyl alcohol, and deionized water and then blow-dried with nitrogen. The dry and clean PET substrate was placed in a RF/DC sputtering system (AJA-Sputter, 200 W), in
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Fig. 1. Schematic fabrication process of the strain sensor and measuring system. (a) Deposit Cr bonding layer and ZnO seedlayer on PET substrate. (b) Open the window through photolithography. (c) Grow ZnO nanowire with hydrothermal solution. (d) Spin-coat PDMS protection layer for ZnO nanowire and dry etch to exposure nanowire tips. (e) Deposit Au as top electrode. (f) Optical image of the final device. (g) Sketch of measuring system that includes sample holder and linear motor with moving rod.
which a bonding layer of Cr (∼40 nm) and a seed layer of ZnO (∼200 nm) were deposited in sequence, as shown in Fig. 1(a). Shipley S1813 photoresist was spin-coated (∼2 m, 3000 rpm for 30 s) on the substrate and a pattern of 2 × 2 windows (50 m × 50 m each) was opened in Fig. 1(b) through photolithography for the growth of ZnO nanowire arrays in Fig. 1(c). In order to obtain uniform, long and high quality of ZnO nanowires, the growth solution of 16 mmol/L hexamethylenetetramine and 16 mmol/L zinc chloride was mixed with 4% volume concentration of ammonia (30% w/w NH3 ). The substrate was floated upside down on the surface of the solution. After the growth at 95 ◦ C for 14 h, ZnO nanowire arrays were obtained in the 2 × 2 windows. A thin layer of polydimethylsiloxane (PDMS, ∼20 m, 3000 rpm for 2 min) was spin-coated on the surface of the PET substrate and cured at 80 ◦ C for 2 h, which served as a protection layer for the nanowire arrays. Top part of the PDMS layer was then etched away in a reactive-ion etching system (STS Dry Etcher, with O2 :CF4 = 1:3 for 15 min), which exposed the tip of nanowires in Fig. 1(d) for electrode connection in Fig. 1(e). A thin layer of Au (∼120 nm) was deposited by a RF/DC sputtering system to serve as the top electrode and form ZnO/Au Schottky barrier. Finally, the strain sensor was packaged by spin-coating a thin layer of poly(methyl methacrylate) (∼2 m, NANOTM 950, MicroChem) on the surface. The final device was compact and highly flexible, as shown in Fig. 1(f). The setup in Fig. 1(g) was used to test the performance of the strain sensor. One end of the strain sensor was fixed tightly on a sample holder while the other end was attached to a moving rod driven by a programmable linear motor. During the measurement, the linear motor was programmed to push/pull the lateral rod with specified distance, acceleration, maximum speed and deceleration, so that the strain sensor was bent back and forth in a well-controlled manner. A sinusoidal bias voltage was applied across the nanowires with a functional generator (Keithley 3390) and the current was monitored with a current amplifier (Keithley
428). The measurement was carried out in atmosphere at room temperature. The strain sensor was located in a home-built Faraday cage so that the environmental influence and electromagnetic noises were excluded. The performance of strain sensor largely depends on the quality of ZnO nanowire arrays. Fig. 2 shows typical ZnO nanowire arrays from hydrothermal growth. Fig. 2(a) confirmed that ZnO nanowire arrays grew uniformly within designed pattern with a length of ∼10 m and a diameter of ∼100 nm. The PDMS layer was spincoated on the substrate and then partially etched to expose the tip of the ZnO nanowire for the electrode deposition, as shown in Fig. 2(b). After etching, ZnO nanowires in Fig. 2(b) were intact and surrounded by PDMS. The distorted and very bright area in Fig. 2(b) is due to the common charging effect in the scanning electron microscopy. 3. Experimental result The typical I–V characteristics of the strain sensor and its response to the strain are shown in Fig. 3. The nonlinear and rectifying behavior in Fig. 3(a) is due to the Schottky contact formed between the ZnO nanowire and the Au electrode. Fig. 3(a) also indicates that the I–V characteristic is dramatically changed when the devices is stressed. Because the dimension of the nanowire is much smaller than PET and PDMS is much thinner (15 m) and more compliant (360–870 kPa) than the PET (200 m and 2–2.7 GPa), PET dominates the elastic behavior of the device. The longitudinal normal strain at the surface of the sensor was estimated from equation
ε=
3aD x (1 − ) l l2
(1)
Here, a is the half thickness of the PET substrate (∼100 m), D is the maximum bending deflection of the device, l is the length from
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Fig. 3. Electromechanical behaviors of the strain sensor. (a) Significant change of nonlinear I–V curve in response to different strain. (b) Gauge factor at 1.5 V bias. (c) Linear dependence of logrithmic current on the strain at different bias of 2 V, 1.5 V, and 1 V. Fig. 2. SEM image of vertical ZnO nanowires from hydrothermal growth. (a) Top view of the as-grown nanowire. Inset: cross-section view of the nanowire arrays. (b) Top view of the nanowire after PDMS protection coating and dry etching to expose nanowire tips.
fixed end to free end, and x is the average distance from the fixed end. Because of the Poisson effect, the normal strain along the surface of the PET induced the strain along the axial direction of the nanowire that grew perpendicular to the surface, and caused dramatic current change. The conductivity of the compressed nanowire was significantly increased, especially when a forward bias was applied. For example, at the bias of +2 V, the current in strain-free nanowires was about 0.02 A. As the normal strain ε reached 0.4%, the current increased to 0.08 A. When the strain sensor was further pushed forward until the strain reached 0.8%, the current reached 0.26 A. The performance of the strain sen
I/I0 ε ,
sor can be characterized with the strain gauge factor, GF =
where I0 is the strain-free current of the device at a fixed forward bias, and ε and I are the strain and the corresponding current change at the same bias. With a forward bias of 1.5 V, the gauge factor of the strain sensor is plotted in Fig. 3(b) and is as high as 1813 at a normal strain ε = 0.6%. In Fig. 3(c), the logarithmic current is plot against the strain at different biases. A linear behavior is revealed and the slope is roughly constant for different biases, and the mechanism is discussed in next section. The stability and response of the sensor is examined in Fig. 4. The “Stress” state and “Release” state in Fig. 4 correspond to the state when the device was mechanically bent and the state when the device returned to its straight and strain-free condition, respectively. It is worthwhile pointing out that, the current always reached the similar value at “Stress” state and went back to its original current level at “Release” state when the substrate was
cyclically bent under different frequencies, which demonstrates the stability of the strain sensor. The instant response to 2.5 Hz excitation indicates a quick switch of the strain sensor. The fast response of the strain sensor with ZnO nanowires was further studied when the device was deformed with different strain rates. The current of the strain sensor was under real time monitor as the linear motor pushed the free end of the substrate at different speeds from 0.001 m/s to 1 m/s to the strain ε = 0.8%. The observed current fluctuation at the beginning and end of the deformation was ascribed to the instability of the acceleration and the deceleration of the linear motor. However, even for the fast speed like v = 1 m/s, the strain sensor responded very well to the mechanical deformation with a response time below 100 ms.
4. Discussion and explanations The high sensitivity of the strain sensor in this study can be ascribed to the piezotronic effect in the piezoelectric and semiconducting nanowires [25,26]. To explain the piezotronic effect, the sensor can be simplified as a one dimensional metalsemiconductor-metal structure. Schottky contact forms at one terminal between the top gold electrode and the tip of nanowires, while Ohmic contact at the other terminal is realized with Cr electrode connected to the continuous ZnO seed layer. The I–V characteristic of this metal-semiconductor-metal structure under forward bias, V > 3kT/q ∼ 77mV, is given by [26]
∗∗ 2
I = SA T exp
−
qB0 kT
exp
q( + V ) E kT
(2)
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Fig. 4. Switching between “Stress” (ε = 0.8%) and “Release” states of the strain sensor at fixed bias of +5 V when it is cyclically bent with at (a) 0.125 Hz, (b) 0.25 Hz, (c) 1 Hz, and (d) 2.5 Hz.
while I–V characteristic in the reverse direction with 3kT/q∼77mV is
q B0
I = SA∗∗ T 2 exp −
where =
2qND
kT
exp
V + Vbi −
q
q/4 kT
kT q
V >
(3)
(Fig. 5(c)) from Eq. (2). As a result, the strain gauge factor, GF, of the strain sensor can be expressed as
I/I0 exp(− qkTB (ε) ) − 1 = GF = ε ε
(5)
, S is the contact area, A** is
the effective Richardson constant, T is the temperature, q is the electron charge, B0 is the Schottky barrier height at zero electric field, Vbi is the built-in potential at the barrier, ND is the donor impurity density, is the permittivity of ZnO and E is the lowering of Schottky barrier under an external electric field. When ZnO nanowire is compressed, piezoelectric potential is generated along its length as shown in Fig. 5(a). This potential decreases the Schottky barrier (Fig. 5(b)) in Eq. (2) in the form of [25]
B =
2 qpiezo Wpiezo
2εs
(4)
where Wpiezo is the width of the polarization layer on the interface, εs is the permittivity of ZnO, and piezo is the density of polarization charges (in units of electron charge) that is proportional to the strain ε along nanowire c-axis. Consequently, the Schottky barrier change, B , is proportional to strain ε. B < 0 when the nanowire is compressed because of the strain-induced negative charge on the ZnO/Au interface and B > 0 when nanowire is stretched due to the strain-induced positive charge. Because is proportional to the strain, it is clear from Eqs. (2) and (4) that the logarithmic current is proportional to the strain and the slope is independent of the applied bias, which agrees well with our observation in Fig. 3(c). The change of the Schottky barrier due to the piezoelectric potential, B , modulates the current through the nanowire
Fig. 5. Explanation of piezotronic working principle. (a) “Original” and “Compressed” states of ZnO nanowire. (b) Corresponding energy band diagram at ZnO/Au interface on two states. (c) Correpsonding I–V characteristics on two states.
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The exponential dependence of the current on the Schottky barrier explains the high gauge factor observed in the strain sensor. Considering the current of nanowires under stress is much higher than the stress-free current and the nanowires are very uniform, we assume that the stress-free conductivity are the same for all nanowires. The stress-induced current change is different for different nanowires because the Schottky barrier and thus the current are very sensitive to the local strain that can vary slightly from one nanowire to another. Therefore, the gauge factor is estimated as
I i exp(− qi (ε) ) − N Ii0 kT GF = = εN ε
(6)
Here, N represents the number of nanowires, i (ε) is the Schottky barrier change in the i-th nanowire. Because of the variation in the nanowire growth and the device fabrication, all nanowires had not the same response to the stress. When the device in Fig. 1(d) was bent to the left such that the nanowire was experiencing compressive strain along the length, the current in some nanowires was significantly increased due to the Schottky barrier drop, while other nanowires might not work well and did not change much. Exponential term in Eq. (6) indicates that the nanowires with dramatic current increase dominated the performance, which agreed with the high sensitivity we observed in Fig. 3. In comparison, when the device was bent to the right such that the nanowire was elongated, the current of the most responsive nanowires drops significantly, but the irresponsive nanowires maintaining at a relatively higher current level dominated the device’s current. Therefore, the total change of current in Eq. (6) is insignificant, resulting in very low sensitivity in this case. Quality of Schottky barrier significantly influences the performance of the strain sensor. In this study, Au was selected to form Schottky contact with ZnO, which has a higher work function than the silver used in the previously reported single-nanowire strain sensor [22,31]. Oxygen plasma clean for the tip of ZnO nanowire during dry etching process improved the quality of Schottky contact [32]. Size of the nanowire array determines the number of nanowires. Compared to the exponential term in Eq. (6), the number of nanowires N is a minor factor and can usually be neglected. However, when there are more nanowires in a larger designed pattern, the variance of nanowires make it difficult to achieve Schottky contact on every nanowire. For this reason, the size of the ZnO nanowire array as well as growth process are controlled for improved Schottky contact and such optimization is expected to further enhance the device’s performance. 5. Conclusion In summary, we have developed a novel strain sensor based on vertical ZnO nanowire arrays grown on a flexible PET substrate from a hydrothermal process. The strain sensor is extremely sensitive and a strain gauge factor up to 1813 has been demonstrated. The high sensitivity is ascribed to the high quality ZnO nanowire arrays and the piezotronic effect. In addition, the stability and fast response of the device have been observed. The excellent performances of the strain sensor promote further applications for delicate and sensitive force, strain and stress measurement in many areas, such as tissue/cell examination, smart skin design, and structural health monitoring. Acknowledgements The authors are truly grateful for the financial support from the Department of Mechanical Engineering and the College of
Science and Engineering of the University of Minnesota. The authors would like to thank the National Science Foundation for its financial support (ECCS-1150147). Parts of this work were carried out in Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
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Ren Zhu is currently a PhD candidate in the department of mechanical engineering, University of Minnesota. He received his bachelor’s degree in mechanical engineering and automation at Shanghai Jiao Tong University, China. During his undergraduate study, he was awarded with National Scholarship and Academic Excellence Scholarship. His research interests are piezoelectric nanogenerators, a novel technology for energy harvesting
Biographies
Rusen Yang received his PhD degree in Materials Science and Engineering from Georgia Institute of Technology in 2007, where he continued as Post-Doctoral Associate. He joined Mechanical Engineering at the University of Minnesota-Twin Cities as an assistant professor in 2010. He has discovered novel nanostructures, such as ZnO, SnO2 , Zn3 P2 , and investigated their application potentials. His most recent work on energy harvester based on piezoelectric nanomaterials made significant contribution in the field of renewable energy.
Wengui Zhang received his Master degree in Plasma Physics from the University of Science and Technology of China (USTC), China, in 2010 and Bachelor degree in Space Physics from the University of Science and Technology of China (USTC), China, in 2004. He is currently a PhD candidate in University of Minnesota. His research interests are design and fabrication of novel piezotronic micro/nano-scale devices, including mechanical sensors and gas/chemical sensors.
Vu Nguyen received his B.S. degree in Mechanical Engineering from Worcester Polytechnic Institute, Worcester, Massachusetts in 2012. He is currently pursuing Ph.D. degree at the University of Minnesota, Minneapolis, Minnesota. His research interests are energy harvesting and self-power systems at micro/nano scale.