High performance flexible sensor based on inorganic nanomaterials

Sensors and Actuators B 176 (2013) 522–533

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Review

High performance flexible sensor based on inorganic nanomaterials Bin Hu a , Wen Chen b , Jun Zhou a,∗ a

Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology (HUST), Wuhan 430074, China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China b

a r t i c l e

i n f o

Article history: Received 1 August 2012 Received in revised form 7 September 2012 Accepted 12 September 2012 Available online 20 September 2012 Keywords: Inorganic flexible sensor Carbon Oxide Nanostructure

a b s t r a c t In recent years, the rapid development of flexible electronic devices indicates their attractive perspective in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring. While sensors, as the important components in such multifunctional devices, also required to be flexible and robust for integration. In addition, with the emergency of smart sensor networks, low cost, low energy consumption and easy-fabrication sensors with various functions are demanded urgently. Compared with the flexible organic electronic sensors, inorganic nanomaterials based sensors with long life-time and the high carrier mobility have been attracting the interest of researchers, and the tremendous progress has been made for developing the flexible, high-performance inorganic materials based sensors. In this article, we review the recent advancements of some important inorganic materials in various sensing applications, including carbon material and some transition metal oxides. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two types of flexible sensor based on inorganic nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Carbon-based flexible sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Oxide flexible sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biographies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction As the important component of multifunctional integrated electrical devices, sensors act the role of signal-input for responding various external stimulations. In recent years, with the emergency of smart sensor network that consisted of spatially distributed autonomous sensors, the fabrication of high performance sensors have been a hot issue, which can be utilized for physical or environmental conditions monitoring, such as temperature, sound, vibration, pressure, and motion [1–4]. For establishing these huge networks, great amount of sensors with different responsive func-

∗ Corresponding author. Tel.: +86 27 87793105; fax: +86 27 87792225. E-mail address: [email protected] (J. Zhou). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.09.036

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tions are required. Thus, exploring an efficient way to fabricate low power consumption and highly stable sensors is of large interest. The tremendous progress in the research of inorganic active nanomaterials brings the opportunities for this purpose by considering their capable of scaling up with low cost and energy consumption [5–9]. It has been demonstrated that the robust and high quality inorganic nanostructures could be utilized in high-performance flexible electronics and exhibited comparable performance metrics with traditional organic flexible devices; in this context, the study on inorganic flexible sensors provides an effective approach for overcoming the drawbacks in the organic-based sensors such as relatively poor life-time and the low carrier mobility. In this article, we summarize the recent advancements toward the carbon materials including nanotubes, nanoparticles and graphene, and some transition metal oxide on flexible substrates for various sensing applications.

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2. Two types of flexible sensor based on inorganic nanomaterials 2.1. Carbon-based flexible sensor Carbon-based materials offer a number of exciting possibilities for both new science and applications, and a unique and fascinating aspect of carbon is the variety of forms. From C60 , carbon nanotube (CNT) to graphene [10–12], over the past several decades, large numbers of academic and industrial groups have explored the use of carbon in diverse application possibilities. CNT, especially the single-walled carbon nanotube (SWNT) is a well-known form of carbon material, which can be visualized as graphene sheets rolled-up to certain directions designated by pairs of integers, forming a cylindrical shell of single carbon atomic layer with the diameter of a few nanometers and length up to hundreds micrometers [13–16]. Their chirality and diameter determine the semiconducting, metallic or semimetallic properties, which can be utilized in active channels of transistor devices by virtue of their high mobilities (up to 10,000 cm2 V s−1 at room temperature) [17], or as conductors for advanced electrical interconnects, due to their low resistivities [18–20], high current-carrying capacities (up to 109 A cm−2 ) [21], and high thermal conductivities (up to 3500 W m−1 K−1 ) [22]. The most important feature of SWNT for flexible device applications is the extraordinarily stiff and strong mechanical reinforcement effects [23,24], with the Young’s moduli in the range of 1–2 TPa and the fracture stresses as high as 50 GPa for bundles [25], which yield a density-normalized strength 50 times larger than that of steel wires, and also resist failure under repeated bending [26,27]. Moreover, considering the high weight-normalized surface areas up to 1600 m2 g−1 [28], the properties of SWNT can be very sensitive to adsorbed species. Thereby, combining abovementioned excellent mechanical properties and electrical features, SWNT could be an ideal candidate for various high performance flexible sensors. The electromechanical performance has been widely studied for exploring the electrical properties of SWNT under the influence of tensile stretching, and the large resistance change and piezoresistive gauge factors was observed under axial strains (600–1000 for semiconducting or quasimetallic SWNT and 210 for metallic SWNT) [29,30]. Many works use the polymer to enhance the interfacial bonding between the nanotubes, which can improve the strain transfer, repeatability and linearity of the strain sensor [31,32]. Recent work reported a new stretchable strain sensor using an aligned SWNT thin films fixed on a stretchable polymer substrate. The nanotube films fracture into gaps and islands, and bundles bridging the gaps when stretched as shown in Fig. 1a–c demonstrates the fracture is reversible with extreme level of stretchability. This mechanism allows the films to act as strain sensors and is capable of measuring strains up to 280% (50 times more than conventional metal strain sensors), with high durability (10,000 cycles at 150% strain), fast response (14 ms delay) and low creep (3.0% at 100% strain). The human-friendly devices could be incorporated into clothing or attached directly to the body as seen in Fig. 1d, such as stockings, bandages and gloves for detecting different types of human motion, including movement, typing, breathing and speech [33]. SWNT based gas sensor is capable of detecting the limitation of part per billion (ppb) level, which extends the applications for low vapor pressure analytes monitoring such as chemical warfare agents and explosives. This is due to the SWNT gas sensors respond to the surface coverage of analytes (P/P0 , where P is the partial pressure and P0 is equilibrium pressure, respectively) in place of the concentration (P) response in conventional gas sensors, for which insufficient concentrations loaded on the active materials cannot

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be detected [34–37]. These chemiresistor sensor as the most studied sensor involves electrical contacts at two ends of a SWNT, as the experiment and calculation shown [38,39], if the gas molecules adsorb onto the SWNTs, especially at defect points, the number of mobile charge carriers transferred between adsorbed molecules and valence band of SWNT surface can be changed apparently, resulting in the notable change of the device resistance [40]. SWNT network film on the flexible plastic substrate (polyethylene terephthalate, PET) was demonstrated as an easy-fabrication sensor that is very sensitive (lower to 25 parts per million (ppm)) to the chemical warfare agent simulants for the nerve agents Sarin (diisopropyl methylphosphonate, DIMP) and Soman (dimethyl methylphosphonate, DMMP). Due to the high strong electron-donating properties of DIMP and DMMP [41], large and reproducible resistance changes (75–150%) were observed upon exposure to these organic vapors, as shown in Fig. 2a and b. Unlike the individual SWNT sensor, the interbundle sites of SWNT bundles played an important role for vapor sorption. Sensor function was robust even when the SWNT/PET sensor was bent all the way to a crease, there was only an 8% loss in % R/R for DIMP and almost no change for DMMP, as shown in Fig. 2c. More importantly, the response was robust and exhibited excellent selectivity even in the presence of large equilibrium concentrations of interferent vapors such as hexane, xylene and water that commonly found in battlespace environments, suggesting that both DIMP and DMMP vapors were capable of selectively displacing other vapors from the walls of the SWNTs (Fig. 2d). Moreover, the line patterning method can be used to rapidly screen stimulants at high concentrations for developing more complicated sensor systems [42]. Besides the organic vapor, SWNT is also sensitive to some toxic or dangerous inorganic gases [43–45]. Pd nanoparticles decorated SWNT on the thin plastic substrates have been used as a high-performance hydrogen sensors with excellent mechanical flexibility as seen in Fig. 3a and b. The typical sensors exhibited sensitivity of ∼75% for 0.05% hydrogen in air and response time of ∼3 s for 1% hydrogen at room temperature, the performance of the asfabricated flexible devices kept essentially unchanged even when they were bent to curving profile and after 2000 cycles operation of bending/relaxing (Fig. 3c) [46]. As a developing technology for flexible electronic device fabrication, printing with the advantage of cost-effective is an effective approach for large-area integration of electronics and sensors on nonconventional substrates, such as plastic or paper [47–51], which have been demonstrated and attracted large interest recently [52–61]. Printing the SWNT bundles on cellulosics (like paper and cloth) can detect aggressive oxidizing vapors such as nitrogen dioxide and chlorine at ppb level at room temperature without the aid of a vapor concentrator. As shown in Fig. 4a, inkjet-printed films of CNTs on paper are significantly robust, and exhibited low sensor-tosensor variation, spontaneous signal recovery, negligible baseline drift, and the ability to bend the sensors to a crease without loss of sensor performance. From Fig. 4b, we can see the sensors can be viewed as being selective to a class of highly oxidizing detection of highly toxic chemical warfare agents [62]. SWNTs can also serve as ultrasensitive transducers in biosensors since the diameters and carrier densities of SWNTs are comparable to the sizes and surface-charge densities of bio-macromolecules [63–65]. The hydrophobic interactions, ␲–␲ stacking interactions can help to bind the biomolecules, such as DNAs and proteins to the surfaces of SWNTs nonspecifically [6,66]. Generally, there are two mechanisms were accepted for biomolecules to influence the electronic properties of SWNTs, one is the electrostatic gating or doping of SWNTs, and another is the modulation of the Schottky barrier between SWNTs and contact electrodes [67,68]. Thus, single-strand DNAs bound to SWNTs can serve as probes for their complementary strands, to distinguish the mutant and wild-type alleles, and

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Fig. 1. (a) Scanning electron microscope (SEM) image of the suspended bundles. (b) Paper model of the unstrained and strained states. (c) Photograph of the SWCNT-film strain sensor under strain. (d) Photograph of the sensor adhered to the data glove. Reprinted from Ref. [33] with permission from Nature Publishing Group.

Fig. 2. SWNT/PET sensor response to stimulants at saturated vapor pressure: (a) DMMP at 1620 ppm; (b) DIMP at 299 ppm. Insets: injection volume at 10 ppm. (c) Sensor response of flat and creased (bending angle of about 5◦ ) SWNT/PET device when exposed to stimulant vapors at saturated vapors concentrations. Inset: optical image of sensor when bent to a crease. (d) Sensor response of SWNT/PET device to common organic solvent vapors and DIMP and DMMP under saturated vapor conditions. Reprinted from Ref. [42] with permission from Institute of Physics Publishing.

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Fig. 3. (a) Optical image of a Pd nanoparticles decorated SWNT flexible sensor. (b) Atomic force microscope (AFM) image of Pd nanoparticles electrodeposited on the surfaces of SWNTs. (c) Bending and fatigue tests over a hydrogen sensor when it was exposed to 500 ppm hydrogen in air. The inset presents the optical image of the sensor with curved geometry mounted on a probe holder. Reprinted from Ref. [46] with permission from American Institute of Physics.

offer advantages of smaller footprint and simpler implementation than chemicapacitors and optical-readout sensors [36]. For pure CNT networks, the sensing properties are based on the electron-donating/-accepting analytes due to their semiconducting properties [69,70]. While the surface manipulation such as covalent functionalization or coating with various materials such as metals [71], enzymes[6] or polymers[72] could extend the CNT sensor applications widely. Recent work showed that the CNTs can be assembled on the oxide surface guided by methyl-terminated molecular patterns (Fig. 5a), and functionalized CNTs with glutamate oxidase as a selective biosensors can detect two forms of glutamate substances (l-glutamic acid, a neurotransmitting material shown in Fig. 5c, and monosodium glutamate, a food additive in Fig. 5d) [73]. By these ways, the SWNTs based flexible sensors can function as labels for efficient label-free detection. Fig. 6 demonstrated that electrochemical coating of pH sensitive polyaniline on the thin CNT networks for transparent pH sensor. Compared to the commercial pH sensors constructed around a liquid-filled glass membrane that limits their applications due to size and rigidity, the solid-state pH sensors based on flexible substrates and CNT networks have been a good approach to overcome this disadvantage, and optical transparent is another important feature for integration with next-generation electronic devices

[74–76]. The as-prepared sensor was sensitive to pH allowing the range from pH 1 to 13 as shown in Fig. 6a, and exhibited the excellent performance of linearity, selectivity, stability and fast response [77]. Moreover, as the color-changing material, polyaniline offers a variety of oxidation states and the variety of colors which the polymer can appear, thus the transparent CNT/polyaniline substrate can be used as optical pH sensor as well, in which the color changes considerably with the pH shown in Fig. 6b [78]. Carbon nanoparticles (CNPs) derived from candle soot is another common form of carbon, which have drawn increasing attention owing to their attractive applications in bioimaging and optoelectronic devices [79,80], and can also be a coating layer for self-clean surface [81–84]. As shown in Fig. 7a, a highly flexible, sensitive infrared nanosensor based on carbon nanoparticles can be fabricated via a simple and low-cost flame synthesis process and dry transfer method. The infrared response of the device studied by using an Nd:YAG laser (1064 nm) showed a sharp photoresponse to a pulsed infrared laser with a rise time of 68 ms and strong current change of 16.9% under the power density of 7.8 mW/mm2 . As the rough surfaces and low-surface-energy materials, the carbon nanoparticles possess superhydrophobic property with a contact angle larger than 150◦ and a sliding angle of about 4◦ , which enable the sensor to clean itself [85].

Fig. 4. (a) Inkjet-printed CNT/paper. (b) Selectivity plot for an inkjet-printed CNT/PET film, showing an increase in resistance for common organic vapors and a decrease in resistance for NO2 and Cl2 . Reprinted from Ref. [62] with permission from American Chemical Society.

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Fig. 5. (a) SWNT-based flexible devices. (b) SEM image of line-shape patterns comprised of adsorbed SWNTs and octadecyltrichlorosilane (OTS) passivation regions. (c) Detection of 5 nM l-glutamate using the flexible biosensor with a source-drain bias of 0.01 V and a source-gate bias of 0 V. (d) Detection of 200 ␮M MSG using the flexible biosensor with a source-drain bias of 0.1 V and a source-gate bias of 0 V. Reprinted from Ref. [73] with permission from Institute of Physics Publishing.

Recent discovery of graphene has stimulated considerable research activities on the optical, electronic, and mechanical properties of this single-atom-thickness, 2D hexagonal honeycomb structure carbon material [86]. The study showed that the graphene can be a clinical utility of biomarker to discriminate health and disease with the capability of measuring extremely low concentration proteins, and due to the graphene material properties in nature, it can overcome the hurdles of the previous detection methods such as expensive and complex to realize. It has been demonstrated that the flexible cancer sensor based on layer-by-layer (LBL) selfassembled graphene offers a number of advantages over the current protein detection techniques, the self-assembly process using PET as the substrate make the device easy fabricating and very flexible

(Fig. 8a). According to the conductance change shown in Fig. 8b, the labeled-free graphene sensors are capable of detecting very low concentrations of prostate specific antigen (PSA) down to 4 fg/ml (0.11 fM) and 0.4 pg/ml (11 fM), respectively, which are three orders of magnitude lower than CNT sensors under the same conditions [87]. In addition, paper-like materials have become a another good carrier for loading the graphene nanosheets for flexible electrode and sensor applications, the resultant freestanding graphene paper has shown a unique collection of characteristics, such as mechanical robustness, excellent electrical conductivity, outperforming many other carbon substrates. However, it is known that mechanically derived graphene suffers from the problems in scalable and large-area preparation,

Fig. 6. (a) The time dependence of the open circuit potential in various buffer solutions of the transparent and flexible CNT/polyaniline pH sensors. (b) Optical pH response of CNT/polyaniline: both the absorption and the color change with pH. It could be used as a sensor just by the color changes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Reprinted from Ref. [78] with permission from John Wiley and Sons.

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Fig. 7. (a) SEM images of CNPs grown on ceramic strips, inset: optical image of the CNP based infrared sensors, revealing the uniformity and flexibility of the CNPs after being transferred onto polydimethylsiloxane (PDMS). (b) Current change under the on/off incident pulsed infrared light. Reprinted from Ref. [85] with permission from American Chemical Society.

Fig. 8. (a) Schematic illustration of LbL self-assembled graphene nanocomposite before immunization. (b) Shift in normalized conductance vs PSA concentration for labelfree graphene sensors. Upper left inset: optical image of LbL self-assembled graphene cancer sensor on a flexible PET substrate. Lower right inset: conductance vs time data recorded after alternate delivery of the different concentrations of PSA. Reprinted from Ref. [87] with permission from American Institute of Physics.

Fig. 9. (a) Schematic diagram of an SGGT integrated in a microfluidic chip. (b) Photograph of the flexible microchip at incurve bending statue. (c) Time-dependent channel current of an SGGT characterized in alternate static and flowing KCl solution with different velocities. Reprinted from Ref. [88] with permission from American Chemical Society.

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Fig. 10. (a) Schematic diagram of UV sensor. (b) I–t curves of UV response when UV light is on and off circularly. The inset shows I–V curves under UV illumination (above) and I–V curves without UV illumination (below). Reprinted from Ref. [107] with permission from American Chemical Society.

and chemical exfoliated graphene oxide (GO) with controllable surface defect density can be adopted as an alternative choice for chemical and biological sensing application [88,89], which has been successfully used for detecting a wide range of gas molecules recently [90–93]. For example, the Pt nanoparticles modified reduced graphene oxide paper (rGOP)/MnO2 composite can be a flexible sensor for nonenzymatic detection of H2 O2 , which enables its application in H2 O2 secretion monitoring by live cells with high sensitivity and selectivity [94]. While GO sheets on the PET surface gave high performance in the detection of both NH3 and rogor (lower to 7.6 ppb). Moreover, the content of oxygen functional groups on GO sheets can be controlled by using powertunable laser, which contributed not only the conductivity but also tunable interaction between water molecules and oxygen functional groups on the graphene oxide sheets, thus can be applied in humidity sensor [95]. Few-layer graphene can be integrated in microfluidic systems as a solution-gated graphene field effect transistors (SGGT) shown in Fig. 9a and b. The transfer characteristics of the SGGT with a gate electrode shifted with the change of the ionic concentration of KCl solution in the microchannel, and can be utilized as a flow velocity sensors to measure the streaming potentials in microfluidic channels (Fig. 9c), and the shift of the negative gate potential with pH shows a supra-Nernstian response of 99 meV/pH [96]. These works point to the potential application of graphene in ultrafast and ultralow noise chemical or biological sensors [97]. 2.2. Oxide flexible sensor Metal oxide sensors, like SnO2 , TiO2 , WO3 , ZnO, Fe2 O3 ,VO2 and In2 O3 , have been investigated for several decades, and significant advances have been made especially in chemical sensing, due to their better thermal and environmental stability compared with organic materials as well as better response reversibility of electrical conductivity, which varies with the composition of the surrounding chemical atmosphere [98–103]. It has been proved that reduction of crystallite size went along with a significant increase in sensing performances, and which promotes numerous researches in nanoscale oxide. The devices based on the nanomaterials can operate at extremely low power consumption and can be easily integrated with nanoelectronics. In addition, for building the smart sensor networks, technological and industrial innovations are finding a way to incorporate different sensors into our daily life, thus assembling the oxide based sensor devices on common substrates such as plastic, textile and paper become an important issue to the demands of light handheld, portable consumer devices.

Regenerated cellulose membrane is an ideal material because of its advantages in terms of biodegradable, biocompatible, flexible characteristics as well as low price, such as glucose oxidase immobilized cellulose–tin oxide hybrid nanocomposite as a disposable glucose biosensor. As-prepared sensors display linear response in the range of 0.5–12 mM with correlation coefficient of 0.96, which is able to meet the clinical region for glucose concentration detection [104]. Smart clothing/textiles are becoming very popular in the past decade for strengthening the adaptability of flexible devices, and there have been reported the flexible fiber/oxide structures can be used for energy harvesting from environment or human body [105,106]. Recently, well aligned ZnO nanowire arrays grown on Kevlar fiber was designed as a fiber-based ultraviolet (UV) sensor (Fig. 10a), which displays a fast response, high on/off current ratio (5400) and good repeatability as shown in Fig. 10b, even bending into “U” shape, it still exhibited stable performance and can detect UV illumination quantificationally [107]. Thin and flexible metal foil can be another substrate as well as the electrode simultaneously for sensor fabrication, vertically aligned ZnO nanorod channel on metal foil with graphene-based top conductive electrode was fabricated as new type of flexible ethanol gas sensors, hybrid architectures accommodated the flexural deformation without mechanical or electrical failure for cycling bending. Gas sensing performance demonstrated the ppm level detection of ethanol gas vapor [108]. The rapid growth of research in the field of micro- and nanoelectro-mechanical system (MEMS and NEMS) requires the ultra fast, high-sensitivity and low-power consumption strainsensitive devices. Strain response utilizing the piezoelectric effect in an individual ZnO fine-wires is a simple, reliable, and cost-effective technique. It has been demonstrated the electromechanical sensor of ZnO fine-wires that placed on the flexible polystyrene exhibited extremely high gauge factor of 1250, which is 25% higher than the best gauge factor demonstrated for CNTs. The Schottky barrier height at the interface of silver electrodes and ZnO wire was highly sensitive to strain and can be modulated by external strain as illustrated in Fig. 11a, which plays a crucial role in determining the electrical transport property (Fig. 11b) [109–112]. The I–V curves shift upward with tension strain and downward with compressive strain is attributed to the combination of strain induced band structure change and piezoelectric effect [113]. This coupling effect of piezopotential together with the presence of Schottky contacts as the fundamental physics have a few important nanotechnologies, such as strain/force/pressuretriggered/controlled electronic devices, sensors, and logic gates

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Fig. 11. (a) Energy band diagram illustrates the asymmetric Schottky barrier heights at the source and drain contacts of a piezoelectric fine-wires. (b) I–V characteristics of the device under different strains. Inset is the dependence of ln I (in unit of ampere) on the applied strain. Reprinted from Ref. [113] with permission from American Chemical Society.

[114–119], and the piezoelectric ZnO nanostructures can also be embedded in a stable matrix of paper for both static and dynamic mechanical strain monitoring [120]. Furthermore, the limitation for strain measurement can be improved dramatically through a novel strain sensor prototype based on a ZnO nanowire/polystyrene nanofiber hybrid structure (Fig. 12a), which can detect strains up to 50% with high durability, fast response, high gauge factors, and could be driven by solar cells and has potential applications as an outdoor sensor system (Fig. 12b) [121]. Compared with conventional strain sensors (lower than 5%), the demonstrated high strain

detection sensor provide a way for wearable applications, such as the human motion capturing as we can seen in Fig. 12c. Phase transition in VO2 nanobeam is another response type which can also be utilized for high performance flexible strain sensor and electromechanical switch fabrication. Compared to bulk or film, the single-crystal nature of the nanobeam can withstand a much higher uniaxial strain without plastic deformation or fracture. Preloaded tensile strain can induce the formation of localized insulting M2 phase, which could transform to the relatively low resistance state M1 phase when subjected to external compressive

Fig. 12. (a) SEM image of ZnO nanowire/polystyrene nanofiber hybrid structure. Inset: high-resolution SEM image of the ZnO nanowires. (b) Current response of the strain sensor device at different static strain state under a fixed bias of 10 V. (c) The curent–time response curve of the strain sensor device that was fixed on an index finger at four different bending and release finger motions. The upper insets labeled as I, II, III, and IV demonstrate the four finger motion states. Reprinted from Ref. [121] with permission from John Wiley and Sons.

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Fig. 13. (a) Schematic of the phase transition of M1 and M2 with tensile and compressive strain. (b) The I–V curve under different tensile and compressive strains. (c) I–V characteristic of VO2 nanobeam under different axial compressive strain, which shows that the threshold voltage and current decreased gradually with increasing the external compression. (d) Bias voltage of 9.5 V applied on the device can trigger the Mott transition and kept the nanobeam in a low resistance status, which can be switched to a high resistance status by stretching the substrate. Reprinted from Refs. [122,125] with permission from John Wiley and Sons.

strain. On the contrary, the tensile strain could induce the formation of M2 phase in the nanobeam. Thus, phase fraction is well tuned (Fig. 13a) and the resistance of the nanobeam can be controlled. As seen in Fig. 13b, the high gauge factor with short response time showed the potential application of the VO2 nanobeam for quantifying tiny strain [122]. In addition to strain response basing on the M1 –M2 insulating phase transition, the Mott transition of VO2 is very sensitive to the tiny strain as well, and by coupling self-heating and external strain (Fig. 13c), recent study offers a new way to tune the metal–insulator transition in a single domain [123,124]. Fig. 13d shows that the metallic and insulating phases at two sides of threshold heating voltage can be easily switched by stretching or compressing the substrate, and the sensor exhibited great controllability with very efficient and quick switching [125]. These demonstrated features can enable strategies for the integration of a VO2 nanobeam in advanced and complex functional units such as logic gates for micro/nano-systems action control. With the exception of ZnO and VO2 , other oxide nanomaterials also can be used in bendable or stretchable functional devices by utilizing their unique properties, like the flexible humidity sensor based on TiO2 nanoparticles–organic composite materials [126] and Ga2 O3 /SnO2 core/shell structure [127], while the nanolayer thickness of SnO2 fabricated by layer-by-layer self-assembly technique have been reported for the detection of ethylene gas recently [128]. 3. Conclusions Flexible electronic devices based on the inorganic nanomaterials are the emerging landscape in recent years following the flexible organic electronic devices. In this article, we review flexible sensor application of various carbon forms and metal oxide nanostructures

with major emphases on the types of device structure and issues for realizing practical sensors. These advanced sensing techniques open opportunities to design and control various high performance sensors that have certain functions for monitoring physical or environmental conditions. In addition, the flexible, lightweight, low cost and energy consumption, easy fabrication enable them to built huge smart sensing networks or integrate with portable and wearable devices.

Acknowledgments This work was supported by the NSFC (No: 51072152, 61204001), the Fundamental Research Funds for the Central Universities of China (2012NQ106), National Basic Research Program of China (2012CB619302), a Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201035), the Program for New Century Excellent Talents in University (NCET-10-0397).

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Biographies Bin Hu received his Ph.D. in materials science at Wuhan University of Technology in 2011. From 2009 to 2011, he was a visiting student in Georgia Institute of Technology during 2009–2011. He joined in Wuhan National Laboratory for Optoelectronics (WNOL) from 2012 as an associate professor, and his main research interest is the flexible sensors for integrated self-powered nano- and microsystems.

Wen Chen received his Ph.D. in materials science (1998) from Wuhan University of Technology and was promoted to professor at this university in 1999. Prof. Chen’s research interests include the synthesis, discovery, characterization, and understanding of fundamental physical properties of oxide nanostructures, as well as applications of nanomaterials in energy sciences and electronics.

Jun Zhou received his B.S. degree (2001) and his Ph.D. in material physics and chemistry (2007) from Sun Yat-Sen University, and worked in Georgia Institute of Technology as a research scientist during 2007–2009. He joined in Wuhan National Laboratory for Optoelectronics as a professor (WNOL) from 2009. His research interest is flexible self-powered nano- and microseneor systems.