Increasing the electrical conductivity of poly„vinylidene fluoride… by KrF
excimer laser irradiation
Yaling Ji and Yijian Jiang
a͒
National Center for Laser Technology, Beijing University of Technology, Beijing 100022,
People’s Republic of China
͑Received 20 July 2006; accepted 10 October 2006; published online 27 November 2006͒
This letter describes the increase in electrical conductivity of poly͑vinylidene fluoride͒ induced by
excimer laser irradiation with =248 nm. The electrical conductivity was found to increase from
10
−13
to 10
−4
⍀
−1
cm
−1
. As a result, experiments produced a transition in the property of the
material from an insulator to a conductor. Optimal conditions for laser irradiation were determined
in terms of the laser energy density, the repetition frequency, and the total number of laser shots.
This letter also discusses the micromechanics of increasing conductivity in the light of Raman
spectroscopy and x-ray photoelectron spectroscopy. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2390632͔
Poly͑vinylidene fluoride͒͑PVDF͒ is known to have ex-
tremely high chemical stability and electrical resistivity. It is
also widely used as an organic piezoelectric and ferroelectric
material. If PVDF could be endowed with the properties of a
semiconductor or conductor, it could be used in multifunc-
tional and integrated devices, and the range of its applica-
tions could be further developed. The potential of high-
energy laser beams can be exploited for the modification and
patterning of a material. The interaction between lasers and
polymers induced by photochemical and optothermal reac-
tions has recently attracted great interest among scientists.
Various changes in properties, such as electrical
conductivity
1,2
͑the typical polymer is polyimide͒, adhesive
bonding,
3–5
and hydrophilicity,
6–9
can be induced when high-
energy photons from an excimer laser act on a polymer.
However, the change in the electrical conductivity of PVDF
induced by excimer laser irradiation has never been reported
until now.
In our experiments, the piezoelectric polymer PVDF was
irradiated by a KrF excimer laser with a wavelength of
248 nm, and its electrical conductivity was increased by nine
orders of magnitude. The experimental conditions and oper-
ating procedures were very simple, and the results were re-
markable. If electrical conductivity could be increased fur-
ther, it would be possible to set the conducting and insulating
regions of a sample of PVDF with a high spatial resolution.
Such laser wiring technology could have a significant impact
on the production of semiconductor based integrated circuits.
The thickness of the PVDF sample we used in the ex-
periment was 500
m. It was mounted in the middle of an
electrically controlled rotating plate. It was rotated at a speed
of 40° /s. The frequency of the KrF laser was set to 4 Hz.
The laser energy density ͑E͒ irradiating the sample was con-
trolled by changing the laser output energy and its spot size.
The experiment was performed at room temperature in am-
bient air conditions. The electrical conductivity of the sample
after laser irradiation was measured by a four-probe method.
The sample’s initial electrical conductivity was measured to
be 10
−13
⍀
−1
cm
−1
.
Figures 1͑a͒ and 1͑b͒ show the logarithm of the electri-
cal conductivity of PVDF as a function of laser shots with
laser energy densities of 190 and 240 mJ/cm
2
, respectively.
In Fig. 1͑a͒ the variation in the samples’ electrical conduc-
tivity occurred after 30 shots. The electrical conductivity
maintained a stable order of magnitude as the number of
laser shots was increased, and the maximum value was ap-
proximately 1.0ϫ 10
−4
⍀
−1
cm
−1
. However, after 570 laser
shots the surface of the sample was seriously damaged, and
as a result, the electrical conductivity of PVDF sharply de-
clined to the state of insulation and never regained conduct-
ibility again. Figures 1͑a͒ and 1͑b͒ show that the maximum
number of laser shots inducing the conductibility of a sample
͑n
max
͒ goes down as the laser energy density increases
͑E=190 mJ/ cm
2
, n
max
=570; E=240 mJ/ cm
2
, n
max
=300͒.
The laser-induced conductivity exhibits an energy density
threshold of about 140 mJ/ cm
2
. Below this value no
matter how many shots were applied, the laser could not
induce any electric conductivity in the PVDF sample. If
EϾ290 mJ/ cm
2
the electric conductivity of PVDF could not
be induced either as a result of serious damage to the surface.
In general, both excessive energy density and excessive
number of laser shots failed to induce conductibility in
PVDF.
In order to confirm why PVDF could have the properties
of an electric conductor after laser irradiation, we analyzed
the surface of the sample using Raman spectroscopy and
x-ray photoelectron spectroscopy ͑XPS͒.
Figure 2 shows the sample’s Raman spectra in the region
between 100 and 3500 cm
−1
before and after the excimer
laser irradiation. The peak at 1433 cm
−1
is characteristic of
the CH stretching vibration mode, and two bands ͑1351 and
1590 cm
−1
͒ appear at each side of the peak ͑1433 cm
−1
͒ after
laser irradiation. According to Ref. 10 a graphite single crys-
tal induces a single Raman band at 1600 cm
−1
, while poly-
crystalline graphite gives rise to an additional band at
1350 cm
−1
. The graphite results in the formation of conduct-
ing layers.
11
The molecular formula for PVDF is ͑CH
2
CF
2
͒
n
.
Gas was generated during the process of laser irradiation,
and it was able to change the color of blue litmus paper. We
infer that the acidic gas was HF. Therefore, after analyzing
the Raman spectra, the molecular formula of PVDF, and the
a͒
Electronic mail: yjjiang@bjut.edu.cn
APPLIED PHYSICS LETTERS 89, 221103 ͑2006͒
0003-6951/2006/89͑22͒/221103/3/$23.00 © 2006 American Institute of Physics89, 221103-1