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Piezotronics: A new field of strain-engineered functional semiconductor devices Figure 5. (a) The tailoring of the quantum dot solar cells (QDSC) band diagram when a positive Ppz appears at the ZnO–PbS interface. (b) Schematic illustration of corresponding change of the depletion regions in ZnO–PbS QD assembly. (c) J–V characteristics of a ZnO–PbS QDSC when the cell was subjected to various strains. (d) Plot of QDSC efficiency (red circles) and Jsc (blue squares) as a function of strain. matically the difference between these electrochemical potentials, is a new means of modulating the material’s electrochemical activity via its strain state. Recently, we demonstrated a piezocatalysis process in a strained ferroelectric Pb(Mg1/3Nb2/3)O3·32PbTiO3 (PMNPT) beam in a deionized water system. We observed that hydrogen evolution from the water depended strongly on the material’s piezoelectric potential.16 The experiments measured hydrogen gas evolution as a function of time during mechanical oscillation of the PMN-PT cantilever in deionized water at select frequencies (Fig. 6(a)). The hydrogen concentration increased at a rate of ~0.22 ppb/s at 10 Hz oscillation. At 20 Hz oscillation, the hydrogen-gas concentration increased at a rate of ~0.68 ppb/s, demonstrating that more strain cycles could result in a higher hydrogen output per unit time. The electrical-to-chemical energy conversion efficiency (piezocatalytic efficiency) was estimated by comparing the total surface charge generated on the strained piezoelectric material to the amount of hydrogen-gas produced. The efficiency per oscillation was less than 0.7% even under the favorable condition of high piezoelectric potential. However, it could be improved to 2%–2.4% given sufficient time for piezocatalyzed electrochemical reactions to proceed. The water reduction–oxidation system serves as a good example to illustrate the fundamental principle of piezocatalysis. Figure 6(b) demonstrates a means by which piezopotential is sufficient to create a favorable energetic landscape (Adapted from Ref. 13.) for generating Faradic currents on opposing gold electrode surfaces, promoting the reduction of protons in solution (evolving hydrogen-gas) and the oxidation of water. In the limit where the piezoelectric material is a perfect dielectric, the appearance of piezopotential induces a linear shift of the Fermi level. Accordingly, the electron energy levels of both electrodes shift by an equal and opposite amount and the difference is the observed piezoelectric voltage output (solid red lines in Fig. 6(b)). This electronic perturbation induced by the mechanical deformation modifies the electrons’ energy in the gold electrodes and moves it away from equilibrium. The electrochemical potential differences between the electrode and solution are a driving force for electron transfer across the electrode–solution interface and thus induce electrochemical reactions. This process is similar to that which occurs in an electrolysis system, where an applied bias disrupts the Fermi-level equilibrium across the interface resulting in a driving force for electrochemical reactions. Therefore, when the potential on the negative electrode exceeds the proton reduction potential (right Au electrode in Fig. 6(b)), electrons of sufficient energy transfer from the electrode to protons on or near the surface, producing hydrogen. Similarly, when unoccupied electron energy levels of the electrode are made sufficiently positive in potential so as to exist below the water oxidation potential (left Au electrode in Fig. 6(b)), electrons transfer from water molecules to the electrode, producing oxygen. Such piezoelectric-potential-driven electrochemical reactions create Faradic currents in the electrolyte and deplete piezoelectric-induced surface charge. Therefore, the piezoelectric potential drops accordingly, and eventually the reactions cease when the electron energy levels are no longer energetically favorable for net charge transfer (dashed red lines in Fig. 6(b)). In addition to piezocatalyzed water splitting, numerous recent studies have confirmed the broader correlation between electrochemical activity and Ppz. For example, a study conducted using ferroelectric PVDF demonstrated that in-situ piezopotential can influence lithium-ion battery charging behavior.17 Also, electrochemical deposition can be selectively activated by the ferroelectric domain polarization.18 Thus, the novel coupling effect between Ppz and electrochemical processes emboldens a new and promising strategy for mechanically tailoring interface energetics and chemistry. Conclusion Piezotronics is an exciting new interdisciplinary field bridging between piezoelectrics and semiconductors. Promising proof-of-principle devices and systems are revolutionizing our understanding and practice of strain-regulated semiconductor functions. So far, the piezotronic effect has been used to • Create local potential wells for enhanced LED quantum efficiency; • Improve performance in GaN– InGaN quantum well lasers; • Form electromechanical memory diodes; • Increase open-circuit voltage and photocurrent extraction in PV and PEC devices; and • Activate or facilitate electrochemical reactions. Considering that Ppz depends directly 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 6


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