level, which led to no more electrical output in the external
circuit (Figure 3(c)). When the compression was released as
shown in Figure 3(d), the piezopotential was vanished, which
would break the equilibrium state. Therefore, the energy
difference would be induced on the opposite direction and
lead to an impulsive negative electrical output signal.
Because of the complex and expensive fabrication and
characterization and integration process of piezoelectric
nanogenerators, the accurate modeling and performance
simulation about the electromechanical conversion behavior
of the nanowires were important before the experimental
works. For example, Falconi and coauthors have conducted
finite element method (FEM) simulation works to estimate
the output potential and stored electrostatic energy of a
ZnO nanowire under various deformation configurations
with different positions of the contacts [10, 11]. In order to
simplify the FEM modeling process, the nonlinear behavior
of Schottky contact, uncertain load in the external circuit,
nonzero electrical conductivity of piezoelectricmaterials, and
parasitic effectswere neglected.The simulationwas simplified
as to qualitatively compare the static piezoelectric potential
generated by nanowire with different structure.Their simulation
result suggested that the electrode contact configuration
would have great impact on the piezoelectric potential of a
bending nanowire. Because the highest strain was distributed
at the bottom regain of the nanowires, the highest piezoelectric
potential difference between the electrodes can be
obtained when the electrodes were set up on both sides at the
bottom of the nanowire. However, it is difficult to obtain this
kind of nanoscaled electrodes in practical experiments.They
also compared the performance of bending nanowires with
the vertically compressed nanowires and found that the latter
one exhibited higher energy conversion efficiency than the
bending nanowires with the same electrode configurations.
According to their results, the vertical compression model
with relatively higher electrical output and simplified structures
is more suitable for the practical applications than the
bending models.However, the influence of free charge in the
nanowires to the electromechanical conversion efficiencywas
not considered in this work, which has been reported to be a
key issue for the piezoelectric nanogenerators.