The aspect ratio of barium titanate (BaTiO3) nanowires is demonstrated to be successfully controlled by adjusting the temperature of the hydrothermal growth from 150 to 240 °C, corresponding to aspect ratios from 9.3 to 45.8, respectively. Polyvinylidene fluoride (PVDF) nanocomposites are formed from the various aspect ratio nanowires and the relationship between the dielectric constant of the nanocomposite and the aspect ratio of the fillers is quantified. It was found that the dielectric constant of the nanocomposite increases with the aspect ratio of the nanowires. Nanocomposites with 30 vol % BaTiO3 nanowires and an aspect ratio of 45.8 can reach a dielectric constant of 44.3, which is 30.7% higher than samples with an aspect ratio of 9.3 and 352% larger than the polymer matrix. These results demonstrate that using high-aspect-ratio nanowires is an effective way to control and improve the dielectric performance of nanocomposites for future capacitor applications.

Nano-electromechanical systems (NEMS) developed using piezoelectric nanowires (NWs) have gained immense interest in energy harvesting applications as they are able to convert several different forms of mechanical energy sources into electric power and thereby function as reliable power sources for ultra-low power wireless electronics. In this work, a piezoelectric NEMS vibrational energy harvester is fabricated through the development of a synthesis process for vertically aligned barium titanate (BaTiO 3 ) nanowire (NW) arrays directly on a conductive substrate. These poled ferroelectric NW arrays are characterized through direct vibration excitation and demonstrated to provide efficient harvesting of mechanical vibrational energy producing an average power density of $6.27 mW cm À3 from 1g acceleration. In order to substantiate the superior energy harvesting performance of the newly developed BaTiO 3 NW arrays, a direct comparison is made with conventional ZnO NW arrays. Here, we clearly report that the ferroelectric BaTiO 3 NW NEMS energy harvester has $16 times greater power density than the ZnO NW NEMS energy harvester from the same acceleration input.

Barium titanate (BaTiO3) nanowires have gained considerable research interest due to their lead-free composition and strong energy conversion efficiency. However, most research has focused on free-standing BaTiO3 nanowires, which are hard to apply for sensing and energy harvesting. Here, a novel method for the growth of vertically aligned BaTiO3 nanowire arrays on a conductive substrate is developed, and their electromechanical coupling behavior is directly evaluated to yield the strain coupling coefficient. The preparation of vertically aligned BaTiO3 nanowire arrays is based on a two-step hydrothermal reaction by first growing oriented rutile TiO2 nanowire arrays and then converting them to BaTiO3 while simultaneously retaining their morphology. A refined piezoelectric force microscopy (PFM) testing method is applied to demonstrate the piezoelectric behavior of BaTiO3 nanowires in the longitude direction. The piezoelectric response (d33 = 43 ± 2 pm/V) of the BaTiO3 nanowires is measured to demonstrate their potential application in sensors, energy harvesting, and micro-electromechanical systems.

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Lead zirconium titanate (PZT) nanowires are synthesized using a scalable two-step hydrothermal reaction. The piezo-electric coupling coefficient of the PZT NWs shows the highest value for PZT nano-wires in the literature (80 ± 5 pm/V). A PZT-NW-based nanocomposite is fabri-cated to demonstrate an energy-harvesting application with an open-circuit voltage up to 7 V and a power density up to 2.4 μW/cm(3) .

From the perspective of bond relaxation and bond vibration, we have formulated the Raman phonon relaxation of graphene, under the stimuli of the number-of-layers, the uni-axial strain, the pressure, and the temperature, in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of the Raman shifts, which are due to the number-of-layers reduction, of the G-peak shift and arises from the vibration of a pair of atoms, while the D- and the 2D-peak shifts involve the z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C(3v) bond geometry to the C(2v) uni-axial bond elongation; (iii) the thermal softening of the phonons originates from bond expansion and weakening; and (iv) the pressure stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift as well as the length, energy, force constant, Debye temperature, compressibility and elastic modulus of the C-C bond in graphene, which is of instrumental importance in the understanding of the unusual behavior of graphene.

It has long been puzzling regarding the trends and physical origins of the size-effect on the elasticity of ZnO nanostructures. An extension of the atomic “coordination-radius” correlation premise of Pauling and Goldschmidt to energy domain has enabled us to clarify that the elastic modulus is intrinsically proportional to the sum of bond energy per unit volume and that the size-induced elastic stiffening arises from (i) the broken-bond-induced local strain and skin-depth energy pinning and (ii) the tunable fraction of bonds between the undercoordinated atoms, and therefore, the elastic modulus of ZnO nanostructures should increase with the inverse of feature size.

Lead-free piezoelectric nanowires (NWs) show strong potential in sensing and energy harvesting applications due to their flexibility and ability to convert mechanical energy to electric energy. Currently, most lead-free piezoelectric NWs are produced through low yield synthesis methods and result in low electromechanical coupling, which limit their efficiency as energy harvesters. In order to alleviate these issues, a scalable method is developed to synthesize perovskite type 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) NWs with high piezoelectric coupling coefficient. The piezoelectric coupling coefficient of the BZT-BCT NWs is measured by a refined piezoresponse force microscopy (PFM) testing method and shows the highest reported coupling coefficient for lead-free piezoelectric nanowires of 90 ± 5 pm V(-1). Flexible nanocomposites utilizing dispersed BZT-BCT NWs are fabricated to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 μW cm(-3). The high electromechanical coupling coefficient and high power density demonstrated with these lead-free NWs produced via a scalable synthesis method shows the potential for high performance NW-based devices.