Mapping Strain/Pressure with ZnO Nanowire Arrays By Piezotronic and Piezo-Phototronic Effect
Authors: Pan Caofeng , Bao Rongrong 
Published: February 2020

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. In this talk, we present a novel design of ZnO nanowire arrays, which can be used to directly record the strain distribution by piezotronic and piezo-phototronic effect.
First, we have reported large-array three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as active taxel-addressable pressure/force-sensor matrix for tactile imaging with a high resolution of 100 μm1. 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. The device matrix has been demonstrated for achieving shape-adaptive high-resolution tactile imaging and self-powered, multi-dimensional active sensing. However, the signal of the piezotronic transistors array is the change of the resistance of each NWs, which can only be measured in a series way. That means such piezotronic transistors array can only mapping a static strain.
Different with the electrical signal, the optical signal can be measured in a parallel way. And in our previous work, we have demonstrated how the piezo-phototronic effect can be effectively utilized to enhance the emission intensity of an n-ZnO/p-GaN NW LED. The emission light intensity and injection current at a fixed applied voltage has been enhanced by a factor of 17 and 4 after applying a 0.093% compressive strain, respectively. Here, we extend the single NW device to NW LEDs array, for pressure/force sensor arrays for mapping strain with a resolution as high as 2.7 μm. Such sensors are capable of recording spatial profiles of pressure distribution, and the tactile pixel area density of our device array is 6250000/cm2, which is much higher than the number of tactile sensors in recent reports (~ 6-27/cm2) and mechanoreceptors embedded in the human fingertip skins (~ 240/cm2).
When the device is under pressure, the images unambiguously show that the change in LED intensity occurred apparently at the pixels that were being compressed by the molded pattern, while those were off the molded characters showed almost no change in LED intensity. Instead of using the cross-bar electrodes for sequential data output, the pressure image is read out in parallel for all of the pixels at a response and recovery time-resolution of 90 ms. This may be a major step toward digital imaging of mechanical signals by optical means, with potential applications in touch pad technology, personalized signatures, bio-imaging and optical MEMS.
Furthermore, our recent studies achieve such piezo-phototronic effect induced strain mapping in a flexible n-ZnO NWs/p-polymer LEDs array system composed of PEDOT:PSS and patterned ZnO NWs with a spatial resolution of 7 μm for mapping of spatial pressure distributions. The emission intensity of the LED array sensor matrix is dominated by locally applied strains as indicated by piezo-phototronic effect. Therefore, spatial pressure distributions are immediately obtained by parallel-reading the illumination intensities of LED arrays based on electroluminescence working mechanism. A wide range of pressure measurements from 40 MPa to 100 MPa was achieved through controlling the growth conditions of ZnO nanowire array.

Lastly, the piezo-phototronic effect was achieved on Si wafer based on a n-ZnO nanofilm/p-Si micropillar heterostructure (ZSH) LEDs array. White light emissions at room temperature, featuring with emission peaks in both visible and near-infrared regions were obtained. By applying a strain onto the top of the ZSH LEDs, the light emission intensity of ZSH LEDs array was enhanced as well by 120% under -0.05% compressive strains. A pressure map can be created by reading out in parallel the change of the electroluminescent intensities from all the pixels in the near future. This research not only introduce a novel approach to fabricate Si-based light-emitting components with high performances, but also may be a great step toward digital imaging of mechanical signals using optical means, having potential applications in artificial skin, touch pad technology, personalized signatures, bio-imaging and optical MEMS, and even and smart skin.