application is the piezoelectric ignition/sparkers such as
the cigarette lighters. Moreover, the piezoelectric materials
have been employed for energy harvesting applications. For
example, the energy from human movements and vehicle
movements in public places can be harvested and converted
into electricity for lighting the lamps. Recently, themicroscale
energy harvesters were developed for harvesting the smallscale
mechanical energies by using the piezoelectric nanomaterials,
which is called “piezoelectric nanogenerators.”The
nanogenerators can be used for charging the batteries or
directly driving some low-power microdevices. The recent
progress of piezoelectric nanogenerators will be reviewed in
the next section.
3. Piezoelectric Nanowires for Energy
Harvesting Applications
The first research work about the piezoelectric nanogenerators
was reported by Wang and Song in 2006. After that,
numerous research works have been conducted about the
working mechanism, structural modeling and design, and
performance optimization of the piezoelectric nanogenerators.
Until now, several kinds of flexible nanogenerators
have been developed, which could be used for harvesting
that varies mechanical energies from the environment
or human bodies. The output electrical energy has been
increased from several millivolts to several hundred volts,
which is enough for driving a light-emission diode (LED),
liquid crystal display (LCD), and wireless data transmitting
device. In this section, the author briefly reviewed the
working mechanism, modeling/simulations, and the experimental
progress of piezoelectric nanogenerators according
to the structure of the nanogenerators including the verticalaligned
nanowire arrays, the lateral-aligned nanowire networks,
and the nanowire-based nanocomposites.
3.1. Vertical-Aligned Nanowire Arrays
3.1.1. Working Mechanism and Structural Modeling. Two differentworkingmodels
can be used for describing theworking
process of the nanogenerators based on vertical-aligned
nanowire arrays including the lateral bending and vertical
compression. Due to their different electrical andmechanical
configurations, the working mechanism is different in certain
degrees, but with one consistent basis: the coupling of the
semiconductor behavior and the piezoelectric property of the
piezoelectric nanowire.
Figure 2 illustrated the working mechanism of the nanogenerator
based on a bending nanowire induced by an AFM
tip [20]. As shown, a Schottky barrier was built up between
the nanowire and the AFM tip due to the difference of
working function and electron affinity. The system was in
an equilibrium state and no voltage output was generated
when the nanowire is not bent by the AFM tip. Once the
nanowire was bent by the scanning AFM tip, the asymmetric
piezoelectric potential would be generated due to the stretch
and compression of the inner and outer side of the nanowire.
The piezoelectric potential in the nanowire changed the