Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 146

d n 3 r 4 n g | 2

Figure 7.1 (a) Schematic diagram of the setup of the near-field electrospinning

(NFES) process where the probe-to-collector distance, h, is reduced to
mm range to utilize the stable polymer jet region for better control-
lability of the deposition locations. The polymer is attached to the top of
the tungsten electrode in a manner analogous to that of a dip-pen. (b) An
optical image photo showing the NFES with the dip-pen approach in
operation. Reprinted with permission from ref. 26. Copyright (2006)
American Chemical Society. (c) An optical image photo showing the
continuous NFES in operation by using a syringe to supply the polymer
solution continuously.
Reprinted with permission from ref. 42. Copyright 2008, American
Institute of Physics.

using a syringe instead of a probe as shown in Figure 7.1(c) such that
polymer solution can be supplied continuously to deposit continuous
nanofibers. These innovative modifications make continuous NFES to
maintain the continuous characteristics of conventional FFES with superior
controllability of the deposition locations. These and other similar ap-
proaches on NFES 43-45 can be the foundations to construct nanofiber
nanogenerators previously unachievable by FFES such as parallel arrays of
nanofibers for nanogenerators. 23
The piezoelectricity of PVDF comes from its b-phase crystalline structure.
For PVDF thin films, a post process including both electric poling and
mechanical stretching is required to promote the formation of the b-phase
PVDF. For electrospun PVDF nanofibers, the electrospinning process can
provide both high electrical field and mechanical stretching in situ without a
post-poling process. As a result, most literature reports, as summarized in
Table 1, have shown piezoelectricity or high contents of a b-phase structure
in electrospun PVDF nanofibers. For example, the near-field electrospinning
process has an average electrical field about one order of magnitude larger
than the far-field electrospinning process and this could enhance piezo-
electricity for nanogenerator applications. 22 Other nanofibers in Table 7.1
are from the far-field electrospinning process and only one has poor dipole
alignments by using the second harmonic generation (SHG) method to de-
tect the polarity of the nanofiber. 46 As can be seen, all other reports of the

.