the low electrical output and poor stability makes them not
suitable for practical application.
According to the modeling discussed above, the vertical
compress of nanowires could generate higher output voltage
than the top-bottom contacted bending nanowires. Xu et al.
have fabricated a vertical nanowire array integrated nanogenerator
(VING) [33]. As shown in Figure 5, the device was
fabricated by packaging the vertical-aligned ZnO nanowire
array with polymethylmethacrylate (PMMA) and connected
the nanowire with top-bottom flat electrodes, which can
generate several tens ofmillivolts in amplitude under external
pressures. The fabrication process of the VING is much
simpler than the bending type nanogenerators. Moreover,
the packaging of PMMA can largely increase the robustness
of the devices. Due to the easy synthesis process of ZnO
nanowire arrays, ZnO VING has become the most popular
design among the reported nanogenerators. Several improvements
have beencarriedout by researchers. For example, Kim
et al. have demonstrated a ZnO VING based on cellulose
paper substrates, which could increase the thermal stability
of the VINGs [34]. Choi and coworkers have reported a ZnO
VING based on transparent and flexible graphene electrodes
[35]. The nanogenerator can be fully rolled for the energy
harvesting and converting process.The output current of this
device is up to 2-3mA/cm2 under nonrolling and rolling
state. Besides the conventional VINGs, some novel designs
on the architecture of VINGs have been demonstrated by
researchers. For example, Hu et al. have fabricated a ZnO
VING which is a free-standing cantilever beam made of
a five-layer structure [36]. By depositing the ZnO seed/Cr
layers on the top and bottom surfaces of the polyester (PS)
substrate, the densely packed ZnO nanowire texture films
were synthesized and then covered by PMMA as blocking
layer. Finally, the top electrodes were deposited on top of
both PMMA layers, and the whole system was packaged by
PDMS.The layered VINGs can generate open-circuit voltage
up to 10V and short-circuit current up to 0.6