Piezotronic Nanowire “Taxel” Gives Active/Adaptive Sense of Touch
Authors: Wenzhuo Wu, Xiaonan Wen and Zhong Lin Wang
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, USA.
Emulation of human senses via electronic means has long been a grand challenge in research of artificial intelligence as well as prosthetics, and is of pivotal importance for developing intelligently accessible and natural interfaces between human/environment and machine. Unlike other senses (seeing, hearing, smelling and tasting), capability of skin for touch sensing remains stubbornly difficult to be mimicked, which necessitates the development of large-scale pressure sensor arrays with high spatial-resolution, high-sensitivity and fast response.
Implementations of pressure sensor arrays have been previously reported using assembled nanomaterials or microstructured rubber layers based on the change in capacitance or resistance upon pressure, which have been applied to map strain distribution in a matrix format with resolution in the millimeter-scale [1,2]. In those demonstrations, electronic components like traditional field-effect-transistors (FETs) act as read-out elements for detecting pressure-induced property change in the pressure-sensitive media. Intensive efforts have been devoted to minimize the effect of substrate strain on performance of these electronic components while preserving the deformability of the substrate. This scheme of pressure sensing, whereas, not only requires complicated system integration of heterogeneous components but also lacks proficiency in directly interfacing electronics with mechanical actions in an active way that mechanical straining can be utilized to generate new electronic control/feedback. Therefore, new approaches have to be developed for better mimicking or even competing with the tactile sensing capabilities of human skins.
A direct control over the operation of electronic devices by mechanical action could be advantageous for implementing tactile sensing and realizing direct interfacing between machines and human/ambient. Piezoelectric materials are ideal candidates for this purpose, which produce electrical potential upon variations of applied pressure/stress due to the linear electromechanical coupling/interaction between mechanical and electrical state in materials lacking inversion symmetry. The most well-known piezoelectric material is perovskite-structured Pb(ZrxTi1-x)O3 (PZT), which has been widely used for electromechanical sensing, actuating and energy harvesting. PZT, however, is electrically-insulating and hence less useful for building electronic devices. In addition, the extremely brittle nature of ceramic PZT films and incorporation of lead impose issues such as reliability, durability and safety for long term sustainable operations and hinder its applications in areas such as biomedical devices.
Figure 1: [click on image to view higher resolution] Piezopotential in Wurtzite crystal. (a) Atomic model of the Wurtzite-structured ZnO. (b) Distribution of piezopotential along a ZnO NW under axial strain calculated by numerical methods. The growth direction of the NW is along c-axis. The color gradient represents the distribution of piezopotential in which red indicates positive piezopotential and blue indicates negative piezopotential.
Due to the polarization of ions in a crystal that has non-center symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress  (Figure 1). For piezoelectric semiconductor materials such as ZnO, GaN, InN and CdS, the effect of piezopotential on the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. One of the important effects of piezopotential on p-n junction and metal-semiconductor contact is to tune/control the charge transport across the interface, which is the basis of piezotronic effect (Figure 2) [3,4]. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is called piezotronic device. Our group initiated the research in piezotronics back to 2007 . Since then we have investigated the piezotronic effect for realizing novel applications such as strain-gated piezotronic logic nanodevices  and piezotronic strain memory device . In contrast to conventional CMOS units, the strain-gated electronics is driven by mechanical agitation. A brief comparison between piezotronic transistor and field effect transistor (FET) is shown in Figure 3.
Figure 2: [click on image to view higher resolution] Schematic of energy diagram illustrating the effect of piezopotential on modulating the metal-semiconductor contact and p-n junctions. (a) With compressive strain applied, the negative piezoelectric polarization ionic charges induced near the interface (symbols with “-”) increases the local Schottky barrier height (SBH). (b) With tensile strain applied, the positive piezoelectric polarization ionic charges induced near the interface (symbols with “+”) decreases the local SBH. (c) and (d) With strain applied, the piezoelectric polarization ionic charges are induced near the junction interface.
Figure 3: [click on image to view higher resolution] Comparison between traditional FET and Strain-gated piezotronic transistor (SGT).
Designing, fabricating and integrating arrays of nanodevices into a functional system is the key for transferring nano-scale science into applicable nanotechnology. In our latest work , we report the first and by far the largest 3D array integration of vertical NW piezotronic transistors circuitry (92 x 92 taxels with 234 taxels per inch) as active/adaptive taxel-addressable pressure-sensor matrix for tactile imaging. Here “taxel” is short for tactile pixel, which is the unit in our array device. We combined the top-down microfabrication processes for fabricating device structure with the bottom-up synthesis of ZnO NWs at low-temperature (85oC).
The taxel area density of SGVPT (strain-gated vertical piezotronic transistor) array is 8464/cm2, which is higher than the number of tactile sensors in recent reports (~ 6-27/cm2) [1,2,9] and mechanoreceptors embedded in the human fingertip skins (~ 240/cm2) . The fabricated sensors are capable of mapping spatial profiles of small pressure changes (< 10 kPa). The spatial resolution with taxel dimension (20 x 20 μm2) and pitch size (100 μm) as well as the tactile sensitivity (2.1 μs∙kPa-1) have been demonstrated, enabling a 15-to-25-fold increase in number of taxels and 300-to-1000-fold increase in taxel area density compared to recent reports.
Each taxel in the array device is made of a strain-gated vertical piezotronic transistor (SGVPT) based on vertically-aligned n-ZnO nanowires (NWs) (Figure 4a, d). SGVPT operates based on modulation of local contact characteristics and charge carrier transport by strain-induced ionic polarization charges at the interface of metal-semiconductor contact, which is the fundamental of piezotronics. Using the piezoelectric polarization charges created at metal-semiconductor interface under strain to gate/modulate transport process of local charge carriers, piezotronic effect has been applied to design independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied on the devices into local electronic controlling signals (Figure 4b-c). Figure 4c shows the equivalent circuit to present the operation scheme of SGVPT array.
Figure 4: [click on image to view higher resolution] (a) Comparison between three-terminal voltage-gated NW FET (left) and two-terminal strain-gated vertical piezotronic transistor (right). (b) Schematic illustration of a 3D SGVPT array with taxel density of 92 × 92 and scheme for spatial profile imaging of local stress. (c) Equivalent circuit diagram of the 3D SGVPT array. (d) SEM image of SGVPT array taken after etching-back the SU 8 layer and exposing top portions (~ 20 μm) of the ZnO NWs. Inset, 30o-tilt view of the exposed ZnO NWs for single taxel. (e) Optical image of the transparent 3D SGVPT array on flexible substrate. (f) Current responses for single taxel under different pressures, presenting the gate modulation effect of applied pressure on the electrical characteristics of SGVPT.
The elimination of wrap gate offers a new approach for 3D structuring. The local contact profile and carrier transport across the Schottky barriers, formed between ZnO NW and metal electrodes, can be effectively controlled by the polarization-charge-induced potential. Electrical characteristics of the two-terminal SGVPT are therefore directly modulated by external mechanical actions induced strain, which essentially functions as a gate signal for controlling carrier transport in SGVPT. The modulation effect of applied pressure is shown from the plot of current variations against pressure changes (Figure 4f). Cross-bar electrodes have been configured for multiplexed data acquisition and the spatial profiles of applied stress can be recorded and imaged. The output signal is current response so that it is easy to integrate SGVPT array with back-end interface circuits for fast data processing, recording and transmission.
Three unique capabilities of SGVPT array device, which are not readily available in previous reports, have been demonstrated in this work:
1. The capability of using SGVPT array for multi-dimensional signature recording, which not only records the calligraphy or signature patterns, when people write, but also registers the corresponding pressure/force applied at each location/pixel by the person and the speed or writing. This augmented capability can essentially provide means for realizing personal signature recognition with unique identity and enhanced security.
2. The shape-adaptive sensing capability of SGVPT array, which provides means for detecting the shape change of device in situ in real-time and feeding-back the sensed changes in shape for calibration of other functionalities as well as corresponding control/response performed by the system. The real time detection of shape changes caused by stretching or twisting is a desirable feature for sensors embedded in an artificial tissue or prosthetic device. This unique capability enables the potential integration of SGVPT device for applications in artificial/prosthetic skin in smart biomedical treatments and intelligent robotics.
3. Additionally, the SGVPT devices can also function as self-powered active tactile sensors by converting mechanical stimulations into electrical signals utilizing the piezopotential without applied bias, which emulates the physiological operations of mechanoreceptors in biological entities, such as human hair follicles and hair cells in the cochlea.
Table 1: Comparison between piezoresistive effect and piezotronic effect.
Our approach is based on a completely different mechanism from the traditional designs based on piezoresistive effect. A comparison between piezotronic effect and commonly recognized piezoresistive effect is listed in Table 1. This technology eliminates the wrap-gate electrode for fabricating 3D vertical NW based transistor, taking advantage of piezotronic effect. The structural transformation from three-terminal configuration into two-terminal configuration significantly simplifies the layout design and circuitry fabrication while maintains effective control over individual devices. The approach presented has the potential to be integrated with silicon-based CMOS technology for achieving augmented functionalities in smart systems in the era of “More Than Moore”, such as artificial skin, personal electronics and potential integration with compliant energy harvesting modules for self-powered flexible functional systems . The scalability of this platform in integrating in-place synthesized single-crystalline NWs in controllable manners together with its demonstrated compatibility with state-of-the-art microfabrication techniques enables future large-scale implementation of wurtzite NWs for practical applications in human-electronics interfacing, smart skin and micro/nano-electromechanical systems.
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