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Piezotronics: A new field of strain-engineered functional semiconductor devices Figure 3. (a) I–V characteristics of a Ag–ZnO–Ag system under zero, compressive, and tensile strains. (b) (Adapted with permission from Ref. 7. Copyright 2008, American Chemical Society.) (Adapted with permission from Ref. 8. Copyright 2008, American Chemical Society.) Strain-sensing performance of the Ag–ZnO piezotronic device. New transistor and sensor devices— Memory switches and artificial skin The most direct application of the Ppz–band structure relationship presented in Fig. 2 is its modulation of charge transport through the piezoelectric material, that is, a piezotronic transistor. In a piezotronic transistor, the piezoelectric potential induced by strain replaces the conventional gate voltage. This configuration requires only two electrical terminals (electrodes) and is much simpler than regular electrically gated transistors. This is a great advantage for system miniaturization and 3D integration, for example, incorporating vertical NW arrays that can be individually addressed and controlled. The first piezotronic transistor based on a single crystal ZnO microwire (MW) was demonstrated in 2006.5 Through in-situ bending and I–V characterization in a scanning electron microscope chamber, a monotonic reduction of source-drain current was observed when the deflection generated a piezopotential along the MW’s side wall. This work marked the first discovery of the piezotronic effect and quickly led to further research that involved more comprehensive characterization and obtained deeper understanding of the piezotronic phenomenon. For example, using a ZnO NW, Zhou et al. demonstrated a strain-induced I–V characteristic change in a Ag– ZnO–Ag system consisting of back-to-back Schottky barriers (Fig. 3(a)).7 This system can be represented by the MS model shown in Fig. 2(c), where both electrodes share the same magnitude but opposite sign of potential change at the interface. This produces asymmetrical I–V curves, which makes it possible to use the M1–S–M2 structure as a memory switch. Intuitively, strain sensors are a direct application of the strain-regulated conductivity change. Different from the piezoresistivity, which is a bulk property and typically follows a linear relationship with strain, the piezotronic effect controls the interface barrier height, and, thus, the current change follows an exponential relationship with strain (Fig 3(b)). As a result, a piezotronic strain sensor offers a much enhanced gauge factor (the ratio between current change and strain amplitude). The highest reported gauge factor from a piezotronic ZnO NW was about 1250.8 This value significantly exceeds the gauge factors of commercial semiconductor strain sensors (~100–200) and the highest gauge factor reported for carbon nanotubes (~1000). Because of their high sensitivity and simple configuration, NW piezotronic strain sensors represent an ideal solution for artificial skin and human–electronic interfaces. Most recently, Wu et al. developed a large-area flexible piezotronic sensor sheet using individually addressed vertical ZnO NW-bundle arrays.9 When subjected to external force or pressure, Ppz was generated at the ZnO–metal contact interface and modulated the barrier height. Thus, the sensitivity was improved by a factor of at least 30 compared with resistive devices. The sensor array provided shape-adaptive force–pressure imaging with a very high resolution of 8,464 pixel/cm2, which is more than an order of magnitude higher than mechanoreceptors in the skin of human fingertips (~240 /cm2). This transparent and flexible force–pressure sensor sheet is able to mimic the sense of human skin and offers a novel platform for interfacing human body and electronics. Similar to regular transistors, piezotronic transistors also can be used for logic circuits, where the on–off states are switched by strain-induced Ppz. Wu et al. demonstrated that multiple ZnO NW–Ag Schottky junctions that were integrated and operated by straining could perform universal logic operations, including NAND, NOR, and XOR.10 Such mechanically operated logic units offer a new function component for advanced nanoelectromechanical systems. Working with light— Piezophototronics and piezotronicenhanced photovoltaics Piezophototronics involves modulation of optoelectronic phenomena by engineering the band structure using the piezotronic effect. The basic principle also follows the diagram shown in Fig. 2, where the amplitude of band shifting at a heterojunction is controlled by strain to manipulate charge recombination (for light-emitting devices) or separation (for photovoltaic devices) at the junction. Yang et al. reported a dramatic improvement in the emission intensity of an n-ZnO MW/p-GaN-based LED 20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 6


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