Non-linear electromechanical effects in ZnO nanowires for enhanced energy conversion – LATINO

Piezoelectric (Piezo) semiconductor (SC) nanowires (NWs) have been researched as the building blocks for various energy transducing applications including mechanical energy harvesting, sensors and piezotronic devices, exploiting both the SC and Piezo properties to control the electric transport through the NW. When dealing with the conversion of mechanical energy into electrical energy, three phenomena have to be considered: (i) Classical piezoelectricity is the electromechanical interaction between mechanical and electrical parameters in crystalline materials with no inversion symmetry, i.e. the application of mechanical strain results in the generation of electric charge and vice versa. (ii) Non-linear Piezo, which occur mostly in nanomaterials where terms of higher orders play a major role. (iii) Finally, flexoelectricity which is a universal effect allowed by any symmetry in all materials and couples polarization to strain gradients. The SC nature of these NWs is also essential for transducer applications affecting radically their performance The coupling of mechanical and electrical states by linear and non-linear Piezo effects, flexoelectricity as well as SC effects and, thus the conversion of one form of energy into the other, has a great potential for energy conversion applications from mechanical inputs eventually leading to new mechanical transducers like sensors or energy harvesters powering autonomous and wireless systems. LATINO aims at providing a fundamental understanding of the conversion of mechanical inputs into electricity by the non-linear Piezo and flexoelectric effect in ZnO NWs. A non-linear increase in the effective Piezo coefficient was suggested with decreasing NW diameter making them promising candidates for future mechanical energy transducers. Theoretical predictions have shown that within the nanometer regime, flexoelectric effects may play a significant role and enhance the electromechanical coupling by a factor of 4 to 5. It has further been demonstrated that nanostructures benefit from increased yield strength which may reach the ultimate limit of the corresponding material. Much larger critical strains may then be reached and eventually converted into electrical energy. Furthermore, several studies show the importance of the SC aspect of these NWs for transducer applications. LATINO brings together experts on nano-mechanics, electro-mechanical effects and SC physics, growth of SC ZnO NWs, and developers of novel sensors and energy harvesters. Novel research will be conducted employing unique and complementary experimental in situ techniques: (i) The NW SC properties will be characterized by scanning microwave impedance microscopy (SMIM) coupled, for the first time, to mechanically controlled deformations. (ii) The nano-mechanical behavior of individual NWs will be studied by three-point bending and uniaxial tensile tests in combination with in situ synchrotron X-ray diffraction. By varying the amount of deformation, linear and non-linear terms are accessible as well as flexoelectric effects. Si NWs (non Piezo) will also be studied allowing the decorrelation of the different effects. (iii) The Piezo properties will be investigated on nanocomposites based on NWs embedded in a polymer matrix with contacting electrodes on both sides of the device. The Piezo induced strain generated under bias will be measured in situ by synchrotron X-ray diffraction. Complementary in situ Piezo measurements on individual NWs will be conducted by high-resolution transmission electron microscopy using a novel Fusion biasing sample holder. The experiments will be accompanied by finite element method simulations considering all the experimental inputs, linear and non-linear electro-mechanical effects, mechanics as well as SC physics. The comprehensive experimentally validated model developed by LATINO will thus allow the design of new and improved energy transducers based on SC Piezo NWs.