Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 151

d n 3 r 4 n g | 2

Figure 7.3 (a) AFM image of electrospun triglycine sulfate (TGS) nanocrystal embed-

ded poly(ethylene) oxide (PEO) nanofibers showing their morphology by
the far-field electorspinning process. (b) Vertical mode PFM image of the
same sample area with results corresponding to the effective longitudial
piezoelectric coecient, d 33eff , of the nanofiber. (c) Lateral mode PFM
image of the same sample corresponding to the effective shear piezo-
electric coecient, d 15eff , of the nanofiber.
Reprinted with permission from ref. 68. Copyright 2010, American
Institute of Physics.

Figure 7.3(a). For the vertical, out-of-plane PFM measurements in
Figure 7.3(b), the image contrast is roughly proportional to the effective
longitudinal piezoelectric coecient, d 33eff . The response in the lateral mode
PFM is shown in Figure 7.3(c) that reflects the effective shear piezoelectric
coecient, d 15eff . The image shows a random distribution in both phase and
amplitude of the response, which is attributed to the unsystematic orien-
tation of crystals in the nanofiber. Similar experiments were conducted by
Baji et al. for electrospun PVDF nanofibers. 65
PFM is also used for characterizing PZT fibers. For example, Wang et al.
determined the polarization domains of electrospun PZT fibers. 69 The do-
mains of the original fibers were randomly oriented, up, down or parallel to
the substrate on which the fiber was deposited. A post-poling is therefore
necessary to implement these PZT fibers in a nanogenerator.

.

7.3.4 SHG and Raman Spectroscopy
The polarity of a piezoelectric material can also be characterized by SHG as
demonstrated by Isakov et al. who analyzed a 532 nm second harmonic
spectrum from a Nd : YAG laser on b-glycine and g-glycine nanofibers. 61 The
nonlinear optical responses were attributed to the embedded nanocrystals
with their polar axis oriented in parallel with the fiber mat plane. The same
technology has been used to measure the piezoelectric property of other
nanofibers such as electrospun PVDF and a-helical poly(a-amino acid)
fibers. 46 Raman spectroscopy can be used to detect various low frequency
modes in a system. It is used to characterize material properties such as
crystallographic orientation. In PZT fibers, the analysis tool has been applied
to detect the formation of PZT during the annealing process. 63