J Nanopart Res (2014) 16:2714 DOI 10.1007/s11051-014-2714-6
RESEARCH PAPER
ZnO nanoparticles embedded in polyethylene-glycol (PEG) matrix as sensitive strain gauge elements Jun Tang • Hao Guo • Ping An • Meng Chen D. Tsoukalas • Yunbo Shi • Jun Liu • Chenyang Xue • Wendong Zhang
•
Received: 17 May 2014 / Accepted: 21 October 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract In this work, we investigate the strainsensing properties of nanocomposite material made from ZnO nanoparticles (NPs) embedded in polyethylene-glycol (PEG) by measuring three different electrical parameters: piezo-resistance, piezo-capacitance, and piezo-impedance. To understand the conduction mechanisms involved in such a material and optimize its performance, a systematic experimental study of DC and AC conductivity was performed using various concentrations of ZnO NPs in PEG. The size range of the NPs is 10–400 nm in diameter. Our results show that conductivity is reduced as the concentration of ZnO NPs increases that reveals the role of ion trapping by ZnO NPs. We then discuss possible mechanisms of high strain sensitivity observed which is attributed to stress influencing surface defects of
J. Tang H. Guo P. An M. Chen D. Tsoukalas Y. Shi J. Liu C. Xue W. Zhang Key Laboratory of Instrumentation Science and Dynamic Measurement, North University of China, Ministry of Education, Taiyuan 030051, Shanxi, China J. Tang H. Guo P. An M. Chen Y. Shi J. Liu (&) C. Xue W. Zhang Science and Technology on Electronic Test and Measurement Laboratory, North University of China, Taiyuan 030051, Shanxi, China e-mail:
[email protected] D. Tsoukalas Department of Applied Physics, National Technical University of Athens, 15780 Zografou, Greece
ZnO NPs as characterized by Raman measurements. Finally, application of the nanocomposite films on low-cost accelerometers is tested and results demonstrated high sensitivity of piezo-resistance and of piezo-capacitance in the range of 0–1 g. Keywords ZnO nanoparticles Gage factor Piezoresistance Piezo-capacitance Piezo-impedance Accelerometers Thin film
Introduction As key components of micro-electromechanical systems (MEMS) sensors, highly sensitive strain gage elements have been widely investigated (Farcau et al. 2011a, b). Different methods were developed including optical strain gage, piezo-resistive and piezoelectric to name a few (Hsu et al. 1999; Gullapalli et al. 2010). To further increase the performance of the sensing elements, much effort has recently been devoted to the application of nanomaterials as embedded strain gage elements in MEMS sensors (Yamada et al. 2011; Herrmann et al. 2007; Siffalovic et al. 2010). Various nanomaterials and their composites were used, which demonstrated quite promising results. Carbon nanotube film strain sensors have been shown to provide multi-directional and multipoint strainsensing ability. These have also been proven to be an
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excellent material in developing human friendly devices (f.e human motion detectors), which can act as part of human skin and clothing (Yamada et al. 2011). Polymers are also used as ultrasensitive piezoresistive materials. They exhibit quite high sensitivity and have the potential to be also applied as flexible substrates (Laukhina et al. 2009; Latessa et al. 2009). Au nanoparticle films and Au nanoparticle/polymer composites are other type of highly strain sensitive materials investigated which operate on electron tunneling transport between metallic nanoparticles (NPs) (Vossmeyer et al. 2008; Warren et al. 2012; Tang et al. 2011). More particularly, Au or Pt nanoparticle films have already been proven to exhibit a gage factor (GF) two orders of magnitude higher than conventional metal foils (Herrmann et al. 2007; Tanner et al. 2012) due to the exponential dependence of conductivity to the inter-particle distance that is modulated by the applied strain. On the other hand resistance modulation with strain has been also observed in experiments using individual ZnO nanowires (NWs) strained with an AFM tip (Han et al. 2009) that leads to similar high values of GF as above. In another study, Liu et al. using dense assemblies of ZnO NWs report a further increase of GF by six orders of magnitude (Liu et al. 2011). This giant increase is attributed to the influence of strain on the contact area of the individual NWs among them allowing for current flow modulation through the nanomaterial. Interestingly in the same paper a much lower GF is reported for a NW assembly with a different architecture that makes the contact area effect negligible. In between are GF values of ZnO nanorods in a polymer matrix reported recently (Chen et al. 2013). It is worth also reporting that Wang et al. converted nanoscale mechanical energy into electrical energy by means of piezo-electric zinc oxide NW arrays to design the NW-based piezo-electric power generator (Wang and Song 2006; Yu et al. 2013). From the above mentioned literature reports, it can be concluded that phenomena related either to stress effect on the ZnO nanostructure core itself or/and on its interaction with the neighboring nanostructures of the assembly influence the resistance value. It becomes then clear that further understanding on strain effects on conduction mechanisms of ZnO nanomaterials still needs to be developed while at the same time nanomaterial-based strain gage technology has yet to find commercial applications.
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In this paper, we have developed a nanocomposite strain gage made from polyethylene-glycol (PEG)/ ZnO NPs composite and we have investigated not only the resistance, but also the capacitance and impedance responses of the device to applied stress. Our data reveal a different role of ZnO NPs within the polymer that has not been previously reported. We observe that ZnO NPs are able to capture ions which are responsible for pristine conductivity in PEG thus to reduce the conductivity. Application of strain increases conductivity of the nanocomposite either by influencing NPs’ reaction with free ions or by increasing electron flow through the NPs’ assembly. By changing the nanoparticle density and nanoparticle size, various strain-sensing elements with different electrical performance were fabricated and the effect of strain is discussed in terms of physical phenomena involved in order to explain the high GF observed during our measurements. All the strain-sensing elements were fabricated onto a simple beam with mass accelerometer structure in order to demonstrate their application in low-cost, high-sensitivity MEMS sensors. From our study, we have found that the nanomaterials they do induce a high resistance change but at the same time they have a remarkable effect on the piezo-capacitance and piezo-impedance values with strain which has potential applications as additional parameters to monitor strain in MEMS sensor applications.
Experimental ZnO NPs with diameters of 10, 40, and 400 nm were purchased from Hangzhou Wanjing New Material Co., Ltd. And the purity is 99.98 %. The particle size can be calculated by the software SmileView produced by JEOS Ltd. PEG 400 was used as the dispersion and emulsification solution. PEG is an uncharged, hydrophilic, non-toxic, linear polymer which is available in a number of molecular weights. It is non-immunogenic and has a very low order of toxicity. It has been used as solvents for the preparation of polymeric NPs (Liu et al. 2005; Ali and Lamperecht 2013). In our experiment, the purity of PEG is [ 99.0 % and the average molecular weight is 400. All the experiments were performed in a cleanroom area with constant temperature of 20 °C and constant relative humidity at 60 %. First, the PEG 400 was dissolved into deionized water with a
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Fig. 1 a Optical images of the test system; the scanning electron microscope (SEM) image of ZnO NPs’ size b 40 nm; c 400 nm
concentration of 1 %. Next, the nanoparticle powders were dispersed into the prepared PEG solution and the concentrations were controlled by their weight. To improve the uniformity of the ZnO nanoparticle distribution in the PEG solution, a sonic bath was used (Loh and Chang 2011). A simple beam-mass mechanical structure was used, as shown in Fig. 1a, which was fabricated from commercially available printed circuit board (PCB). Gold electrodes 100 lm apart were fabricated on the end of the beam, where the ZnO NPs were deposited as the strain-sensing element. The colloids were then drop-casted on the electrodes using transfer pipette which maintains a constant amount of 2 ll and dried in the air. ZnO nanoparticle films, placed within the electrodes, were strained using a micrometer. The schematic diagram of the setup used to strain the samples is provided in Fig. 2a. The ends of the substrate were free to move as the sample was strained by the micrometer. The electrical responses were measured by Agilent 4156C for DC measurements and Agilent 4284A for AC measurements.
Results and discussion DC piezo-resistance response characterization Measurements without strain The ZnO nanoparticle density was tuned by the ZnO concentrations in the PEG solution. Different initial nanoparticle concentrations were thus prepared: 10, 40, and 70 % and various nanoparticle sizes were also selected: 10, 40, and 400 nm, as shown in Fig. 1b, c. We remark that the resistance of the pristine PEG film (without ZnO NPs) was measured to be 50 MX. Extracted conductivity value seems in agreement with conductivity values reported in the literature (Hampton Research Corp. 2012) for PEG and it is due to ionic conductivity possibly from H?, (H3O)?, or (OH)-. From the test results of unstrained samples, it can be concluded (Fig. 2b) that by increasing the initial colloid concentrations of the 400 nm ZnO NPs, the initial resistance of pristine sample (50 MX) was increased to 0.6 GX at a ZnO NPs’ concentration of 10 % and to 18.5 GX when the ZnO NPs’ concentration was at 70 %.
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Fig. 2 Piezo-resistance responses as a function of the ZnO nanoparticle films with different strain, nanoparticle densities and particles sizes. a The schematic diagram of the setup for the electrical characterizations. b From different colloid concentrations with sizes of 400 nm; c from colloid concentration of 70 % with different sizes; d relative resistance change DR/R versus strain (absolute values for both variables were used); e the g factor of piezo-resistance
The resistance is related to the lateral conductivity (r) by the relationship: 1 w RX ¼ ð1Þ r hL Given the dimensions of our gold electrodes (electrode spacing 100 lm, electrode width 1 mm and gold thickness 20 lm) we estimate from Eq. (1) that the corresponding conductivity values of the above measured resistances would be 0.5 lS/cm (pristine), 0.04 lS/cm (10 %), and 1.35 nS/cm (70 %). From measurements of unstrained samples it is clear that as we
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increase the NPs’ concentration in PEG solution the resistance is increased more than two orders of magnitude. Since the conductivity in pristine PEG solution is mainly of ionic nature we conclude that as we increase the NPs’ concentration we are able to reduce the free ions (particularly protons and hydrogen containing ions which are in equilibrium with water concentration that is trapped in PEG sites) either by trapping them at ZnO NPs’ surface or by reducing their mobility or both. We believe that ZnO NPs can be responsible for both effects since they can either trap ions reducing their number and thus conductivity or/and they can directly trap water
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molecules. In the latter case the mobility of the transported ions through acid based reactions within the continuous water network formed around PEG is reduced due to the disruption of paths of the water network from the adsorption of water molecules by ZnO nps. From the literature it is known that ZnO can easily trap hydrogen or hydrogen containing ions at its surface oxygen vacancy (Xue et al. 2014) and consequently increase the film resistance through the mechanisms discussed above. We remark that our findings cannot be explained assuming that conduction through electron tunneling from nanoparticle to nanoparticle within the ZnO NPs’ assembly is the prevailing current transport mechanism. First if electron transport through ZnO NPs prevailed, we should have observed a decrease of resistance as we increase NPs’ concentration in unstrained samples particularly for NPs’ concentrations above 15 % taken it as the percolation limit of 3-D NPs’ packing (Scher and Zallen 1970). On the contrary we observe a substantial increase of the resistance as the ZnO NPs’ concentration is increased. Second because ZnO is a semiconductor and electron transport through ZnO NPs’ assemblies result in very low conductivity values in vacuum which slightly increases in humid air at 90 % RH (Ghosh et al. 2011). This is explained on one hand by the limited available number of conduction electrons in ZnO due to large bandgap of the material and on the other hand by existing energy barriers to electron movement present in the form of depletion layers near each nanoparticle surface which can be modified by surface reactions influencing conductivity (Ghosh et al. 2011). We mention that during our experiments humidity was kept constant at 60 % RH. This condition could only result in conductivity increase as we increase NPs’ concentration according to the above reference; it is consequently in disagreement with our experimental findings. It is thus likely that measured conductivity decrease by increasing NPs’ concentration is mainly limited by decrease of ion concentration and/or mobility in PEG following the mechanisms proposed in this section while electron transport through ZnO NPs’ contribution to conductivity remains comparatively negligible. Measurements under strain After applying compressive strain, we found that the resistance value has decreased and piezo-resistance
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sensitivity is higher when the initial resistance of the sample was higher. A similar phenomenon was observed when the ZnO nanoparticle sizes were tuned, as shown in Fig. 2c. With the same ZnO colloid concentration, smaller NPs lead to larger initial resistance values, and higher piezo-resistance sensitivity. No effect of resistance change with the application of external strain was observed in pristine samples. The piezo-resistive GF G can be calculated using the following equation (Chen et al. 2013; Liu et al. 2005, 2011): G¼
DR ; Re
ð2Þ
where e is the strain, DR is the change in base resistance R with strain. Examining Fig. 2d, we find that the resistive response of the sample (10 nm, 10 %), used as an example here, to applied strain is closely described by Eq. (2). From the calculation results, with different NPs density and sizes, the max of GF value has reached a value of 434 (shown in Fig. 2e) which is higher than that of the traditional semiconductor materials (50–200). In order to support the above arguments we have performed Raman measurements as a function of strain to investigate if ZnO NPs are under stress in the PEG film when strain is applied and we show the obtained results in Fig. 3b, c. The NPs were packed by the polymer PEG (shown in Fig. 1c). When there is a strain on the ZnO NPs’ film, the ZnO NPs were compressed by the PEG polymer matrix. In our experiment, the Raman peak of ZnO was shifted from 437.52 to 437.75/cm as a function of stress on the sample (400 nm NPs’ diameter, 40 % concentration) as shown in Fig. 3b. The peak exhibits a ‘‘blue shift’’ and it allows calculating the value of stress on the NPs according to the following Eq. (3) (Decremps et al. 2002): r ¼ 227Dx;
ð3Þ
where, the r is stress, and the Dx is Raman shift. The stress on the ZnO NPs was increased to 51 MPa as it can be concluded using Eq. (3) for maximum strain. Figure 3c shows the calculated dependence of stress versus strain following the above procedure. Stress applied to ZnO has been reported to influence material properties like surface energy barrier that can
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Fig. 3 a The schematic diagram of the setup used for the Raman spectrum; b Raman spectrum of the samples as a function of stress; c stress versus strain dependence using Eq. (5)
be modulated with strain according to the previous reports due to change of material band structure and piezo-electric effects (Zhou et al. 2008; Chung et al. 1991). This strain induced effect influences in our experiments the ability of ZnO NPs to capture ions within PEG solution. A second possibility to explain resistance decrease with strain could be attributed to an increase of electron conduction in ZnO NPs’ assembly due to barrier lowering from NP to NP as a result of strain. The fact that high concentrations ZnO solution presents the highest strain sensitivity favors an increased electron transport under strain within the dense NPs’ network. Whatever the exact conduction mechanism prevails, this phenomenon can then be used to sense strain. Piezo-capacitance and piezo-impedance response characterization Piezo-capacitance measurements The piezo-capacitance response was measured as shown in Fig. 4. With increasing nanoparticle density (for higher colloid concentrations), the initial
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capacitance and relative capacitance variations were increased. From the measurement results, high sensitivity factors of the piezo-capacitance (DC/Ce) were also discerned that changed with different ZnO nanoparticle densities and sizes. As it is shown in Fig. 4b, for the 400 nm ZnO NPs, the sensitivity factor of the piezo-capacitance was slightly increased from its value using 10 % colloid concentration to the one measured when using 70 % NPs’ concentration. By changing the nanoparticle size, keeping the same colloid concentration of 70 %, the piezo-capacitance was increased when using 40 nm ZnO NPs compared to 400 nm NPs. The capacitance change of the nanoparticle film with strain is influenced by the strain’s effect on polarization (Zhou et al. 2008). The sensing mechanism for the piezo-capacitive response can thus be mainly attributed to the stress on the nanoparticle films, which resulted in variations of the permittivity by Dem of samples (Kummer et al. 2004; Wen et al. 2000): DC ¼ Dh
oC ðh; em Þ oh
ð4Þ
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Fig. 4 Piezo-capacitance responses as a function of the ZnO nanoparticle films with different strain, nanoparticle densities and particles sizes. a From different colloid concentrations with sizes of 400 nm; b from colloid concentration of 70 % with different sizes; c the sensitivity factor of piezocapacitance
em ¼
P þ1 e0 E
ð5Þ
3Bee 2 2 C11 C12 P¼ þ 2C44 C2 T33 ðl3 m3 n3 Þ ; C
ð6Þ where P is the intensity of piezo-electric polarization, E is the average of the electric field in the medium, e0 is the vacuum dielectric constant, e is the strain, and l3, m3, n3 are the three direction cosines. It might be also considered that triboelectricity was produced by the strain or bending process at the interface between the ZnO and PEG phases. The rubbing between them increase the surface/ interface charges which resulted in changing capacitances. Piezo-impedance response characterizations From the measurement results, shown in Fig. 5a, it can be concluded that with increasing external stress the real part of the impedance decreased. Meanwhile, by reducing the nanoparticle density, for the 400 nm ZnO NPs, the resistance value of the ZnO NPs’ films was
reduced from 1.65 kX at 70 % concentration to 1.05 kX at 10 % concentration which values demonstrate a convergence of AC resistance if particularly compared with corresponding unstrained DC resistance values of these samples which were different an order of magnitude. We believe that this reduction of the resistance values is an indication that at high frequency resistance losses are also determined by electron current transport through ZnO NPs’ assembly and not only by ion transport. AC current reduces also the resistance at metal contacts which are expected to be present in the case of electronic transport mechanism through NPs’ assemblies during DC measurements. As it is noted in (Hill and Jonscher 1979) while only certain electron hops (from NP to NP) could lead to the continuing percolation giving DC conductivity, all short-range hops contribute to the AC conductivity resulting in the observed increase. From the real part response to the external stress, we calculate the GF by Eq. (2) as to 94 (shown in Fig. 5c), lower from DC measurements. From the imaginary part response to the external stress, the GF was calculated by a similar equation to Eq. (2) as to 37 (shown in Fig. 6c) attributed to the influence of strain on polarization as in purely capacitive measurements.
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Fig. 6 Piezo-impedance (imaginary part) responses as a function of the ZnO nanoparticle films with different strain, nanoparticle densities and particles sizes a From different colloid concentrations with sizes of 400 nm; b from colloid concentration of 70 % with different sizes; c the sensitivity factor of piezoimpedance
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Fig. 7 Applications of the ZnO nanoparticle films for the acceleration sensing. a The schematic diagram of the setup for the electrical characterizations; b the resistive responses; c the
capacitive responses; d the real part of the impedance responses; e the imaginary part of the impedance responses
Accelerometers application test
Conclusions
We have subsequently applied a highly sensitive strain gage element as the embedded sensing element of a simple beam-mass structure based accelerometer. As it is shown in Fig. 7a, by changing the direction of the mass to the gravity, we can simulate the external acceleration from 0 to 1 g. From the test results (Fig. 7), it can be concluded that by increasing the nanoparticle density from 10 to 70 %, the sensitivity of the resistance responses is much higher. Also, for accelerations smaller than 0.7 g, the sensitivity is much smaller than the sensitivity of accelerations larger than 0.7 g. For capacitance response to external acceleration, similar stress response was observed. Quite a large capacitance response (be up to 0.484 Pf/g) was achieved within the measurement range of 0–1 g. Meanwhile, the linearity was also good, reaching 99.549 %. The impedance response, which includes the same response to stress induced variations, has further confirmed our conclusions regarding resistance and capacitance responses (Fig. 7e).
In conclusion, nanoparticle films for highly sensitive strain gage applications were studied systematically in this paper. Piezo-resistance, piezo-capacitance, and piezo-impedance were tested using a nanocomposite material with different ZnO nanoparticle densities and sizes in PEG. From the test results, high sensitivity of piezo-resistance was measured which can be about three times the bulk semiconductor piezoresistors. Two different phenomena were also observed in the ZnO nanoparticle films. The first was the high resistance changes observed after application of external stress. The second is the stress effect on the semiconductor nanoparticle core confirmed with Raman spectroscopy. This effect can explain the high strain sensitivity of the nanomaterial observed. Finally, this nanocomposite PEG/ZnO NPs-based strain gage element was embedded in low-cost accelerometers, where sensitivity of the resistance, capacitance, and impedance responses was measured in the range of 0–1 g. Currently, to improve the reproducibility, cost, and long-term stability of the devices we are further investigating various parameters related
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with nanoparticle film deposition conditions, which could be important for the optimization and application of nanoparticle-based nanocomposites in applications of sensitive MEMS sensors. Acknowledgments We acknowledge the financial support from the Natural Science Foundation of China (91123016, 51225504, 61171056, and 51105345), National Basic Research Program of China (2012CB723404), program for the top young academic leaders of higher learning institutions of Shanxi and one hundred persons project of Shanxi. One of us (DT) wants to thank Dr. P. Argitis of NCSR Demokritos for fruitful discussions on PEG conductivity.
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