Recently, the polarization screening effects induced by
the free charge carriers have been regarded as an important
issue for optimizing the electromechanical conversion efficiency
of the piezoelectric nanowires. However, it was very
difficult to precisely control the concentration of dopant in
piezoelectric nanowires in practical experiments. Therefore,
the computer analysis through FEM can be used to predict
this effect. For example, Gao and Wang have investigated
the behavior of free charge carriers in a bent piezoelectric
semiconductive nanowire under thermodynamic equilibrium
conditions [25]. They found that the positive side of
the bent nanowire would be partially screened by the free
charge carriers because the conduction band electrons would
be accumulated at the positive side under the piezoelectric
potential. Therefore, the potential in positive side is much
lower than the negative side. Moreover, Falconi et al. have
investigated the screening effect in a vertically compressed
nanowire and found that the piezoelectric potential would
be almost completely screened by the free charge carriers at
a higher level of donor concentrations. They also confirmed
that the length of the vertically compressed nanowire did not
significantly affect the maximum value of the piezoelectric
potential, while the relative dielectric constant surrounding
the nanowire would significantly influence the output voltage
[26].
3.1.2. Fabrication and Performance of Vertical Integrated
Nanogenerators. After finding that the AFM tip-induced
lateral bending of nanowire (ZnO, GaN, CdS, InN, and ZnS)
can generate impulsive voltage output [27–31], another kind
of nanogenerator with similar mechanism was carried out.
In this device, the AFM tips were replaced by a zigzag top
electrode to bend the nanowires [32]. Qin et al. have also
reported a similar nanogenerator with two fibers coated by
the ZnO nanowire arrays [21]. As shown in Figure 4, they
entangle the fibers and brush the nanowires with each other
to make the ZnO nanowires bending. This nanogenerator
can generate voltage with approximately 1mV in amplitude.
Although these early-stage nanogenerators have attracted a
lot of attention, the complicated fabrication process as well as