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Physica B 406 (2011) 1919–1925
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Physica B
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A study on optical absorption and constants of doped poly(ethylene oxide)
R.S. Al-Faleh, A.M. Zihlif n
Physics Department, The University of Jordan, Amman, Jordan
a r t i c l e i n f o
abstract
Article history:
Received 12 October 2010
Received in revised form
25 January 2011
Accepted 26 January 2011
Available online 24 February 2011
Thin films of polymer electrolyte based on poly(ethylene oxide) doped with sodium iodide (NaI) were
prepared using the solution cast method. The films obtained have average thickness of 70 mm and
different NaI concentrations. Absorption and reflectance spectra of UV-radiation were studied in the
wavelength range 300–800 nm. The optical results were analyzed in terms of absorption formula for
non-crystalline materials.
The optical energy gap and the basic optical constants, refractive index, and dielectric constants of
the prepared films have been investigated and showed a clear dependence on the NaI concentration.
The interpreted absorption mechanism is a direct electron transition.
The observed optical energy gap for neat poly(ethylene oxide) is about 2.6 eV, and decreases to a
value 2.36 eV for the film of 15 wt% NaI content. It was found that the calculated refractive index and
the dielectric constants of the polymer electrolyte thin films increase with NaI content. Models were
used to describe the dependences of the dielectric constant on the NaI concentration, and the refractive
index on the incident photon energy.
& 2011 Published by Elsevier B.V.
Keywords:
Polymer electrolyte
Thin films
Absorption
Energy gap
Direct transition
Dielectric constant
Refractive index
1. Introduction
Polymer composites and polymer electrolytes are the most
widely used materials in the past few decades. Understanding
their properties is extremely important not only for scientific
knowledge but also for modern and advanced technological
applications. Most polymeric materials are poor conductors of
electricity because of the unavailability of large number of free
electrons or ions to participate in the conduction process, so a
great attention has been focused on enhancing their electrical
conduction and improving their properties. In order to promote
optical, thermal, and electrical properties to polymeric systems,
many different materials have been developed as conducting
polymers electrolytes and polymer composites [1–6].
One important aspect of these characteristics is the transition
from insulating to conductive behavior by adding some conducting or electrolytic substances. Many studies have shown that the
physical behavior of polymer/electrolytes depends on the filler
type or structure and the method of composite preparation.
Understanding the chemical and physical nature of the composites structure helps greatly to evaluate and interpret the conduction mechanism taking place in the composites bulk. Space
charge limited current can be suggested to explain the charge
transfer in polymers in addition to ionic conduction dominated by
the amorphous phase in the polymer matrix [7–10].
Solid polymer electrolytes (SPE) are promising materials for
electrochemical device applications. Nowadays they are successfully used in high energy density rechargeable batteries, full cells
integrated optical devices, and electrochromic displays [5,6].
Sodium iodide is a white, crystalline salt with good properties
and high luminescence efficiency. It is used in radiation detection,
treatment of iodine deficiency, nuclear medicine, and in many
other applications, such as X-ray detectors with high spectrometric quality.
The physical characterization of solid films of polymer electrolyte can be carried out using a variety of analytical techniques
such as impedance, X-ray diffraction, and UV-spectroscopy. Polymer electrolytes with low NaI concentration were prepared for
the purpose of percolation detection or anomalous behavior in
the electrical conductivity as we have recently reported in
PEO/I2 polymer electrolyte with very low I2 content less than
0.5 wt% [14]. In the present study, thin films of PEO doped with
sodium iodide were prepared with different concentrations: 0%,
1%, 2%, 4%, 6%, 8%, 10%, and 15% by weight. Optical and dielectric
constants of those films will be determined under different
UV-radiation wavelengths and sodium iodide concentrations.
2. Experimental work
2.1. Composites films preparation
n
Corresponding author.
E-mail address: [email protected] (A.M. Zihlif).
0921-4526/$ - see front matter & 2011 Published by Elsevier B.V.
doi:10.1016/j.physb.2011.01.076
Poly(ethylene oxide) with average molecular weight of
300,000 g/mol was used to prepare electrolyte films by casting
1920
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
from solution. Poly(ethylene oxide) powder and solid sodium
iodine were mixed together and dissolved in methanol as a
suitable solvent. The solution was then stirred continuously by
a rotary magnet at room temperature for a few hours. The stirring
process lasted until the mixture reached a homogeneous viscous
molten state. The mixture was immediately cast to thin films on a
glass plate, and the methanol was allowed to evaporate completely at room temperature and under atmospheric pressure for a
few days. The composite films were dried in an oven at 40 1C for
2 days. The films obtained have thicknesses of about 70 mm and
sodium iodide concentrations of 0%, 1%, 2%, 4%, 6%, 8%, 10%, and
15% by weight.
2.2. Optical measurements
where k is the extinction coefficient, which is related to the
absorption coefficient and wavelength by
k¼
al
ð4Þ
4p
The dielectric constant (e0 ) and dielectric loss (e00 ) are calculated from the relations [13]:
e0 ¼ n2 k2 , e00 ¼ 2nk
One of the most direct and simplest methods for probing the
band structure of materials is studying their absorption spectra.
In the absorption process, a photon of known energy excites an
electron from a lower energy state to a higher one. By analyzing
the radiation spectra, we can understand the transition mechanism. The fundamental absorption manifests itself by a rapid rise in
absorption, known as the absorption edge. This can be used to
determine the optical energy gap and the type of transition [13,15]. Absorption is expressed in terms of the coefficient
a(o), which is defined as the relative decrease rate in light
intensity. Using a UV-spectrophotometer, the optical absorbance
and reflectance spectra of the PEO/NaI composites were collected
at room temperature in the wavelength range 300–800 nm.
3. Results and discussion
The absorption coefficient a(o) was obtained from the absorbance A. After correction for reflection, a(o) can be calculated
using the Beer Lambert’s formula [12,13]
I ¼ I0 expðaxÞ
ð1Þ
Hence
2:303
I0
2:303
log
AðoÞ
¼
x
x
I
ð2Þ
ð5Þ
Solids absorb an amount of the incident light of intensity I0,
and consequently optical transitions start when the energy of
photon absorbed is higher than or equal to the forbidden energy
gap. If the required energy is almost equal to the difference
between the lowest level of conduction band and the highest level
of valence band, electrons will be transferred from the valence
band to conduction band. At high absorption coefficient levels,
where a(o)4104 cm 1, the absorption coefficient a for noncrystalline materials can be related to the energy of incident
photon energy (_o) according to the formula:
aðoÞ:o ¼ Bð:oEopt Þr
ð6Þ
where B is a constant, Eopt is the optical energy gap, and the
exponent r is an index determined by the type of electronic
transition causing the optical absorption and can take values 1/2,
3/2 for direct and 2, 3 for indirect transitions [15–17].
Figs. 1 and 2 illustrate the absorption and reflectance spectra
of PEO composites with NaI content of 1, 2, 4, 6, 8, 10, and 15 wt%.
It is clearly shown that absorption decreases rapidly with increasing wavelength up to 400 nm. Fig. 3 shows graphs of the product
of absorption coefficient (a) and photon energy (a _o) versus
photon energy (_o) at room temperature. The drawn straight
lines obtained with r ¼1/2 indicate that the electron transition is
direct in k-space. Extrapolation of the linear portion of these
curves gives the value of optical energy gap (Eopt).
8.0
7.5
pure PEO
1% NaI
2% NaI
4% NaI
6% NaI
8% NaI
10% NaI
15% NaI
7.0
6.5
6.0
5.5
Absorption
aðwÞ ¼
where x is the sample thickness; I0 and I are the incident and
transmitted intensities, respectively [12–15]. The refractive index
can be obtained from
8"
9
#1=2
<
4R
R þ1=
2
n¼
k
ð3Þ
: ðR1Þ2
R1 ;
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
300
400
500
600
Wavelength (nm)
Fig. 1. Absorption spectra for PEO/NaI composites.
700
800
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
1921
70
Pure PEO
1% NaI
2% NaI
4% NaI
6% NaI
8% NaI
10% NaI
15% NaI
65
60
Reflectance%
55
50
45
40
35
30
25
20
15
10
5
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Reflectance spectra for PEO/NaI composites.
12
Pure PEO
1% NaI
2% NaI
4% NaI
6% NaI
8% NaI
10% NaI
15% NaI
11
10
(αhω)2 (eV/cm)2 ∗ 106
9
8
7
6
5
4
3
2
1
0
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
hω (eV)
Fig. 3. (a_o)2 versus photon energy for PEO/NaI composites.
Table 1 includes determined values of (Eopt), which decrease
with increasing NaI content from 2.6 eV for pure PEO to 2.36 eV
for 15 wt% composite. At lower absorption coefficient level, in the
range of 1–104 cm 1, a(o) is described by the Urbach formula [16]
aðoÞ ¼ a0 expð:o=DEÞ
ð7Þ
where a0 is a constant and DE is the energy gap tail interpreted as
the width of the tails of localized states in the forbidden band
gap [13,18]. Fig. 4 presents the Urbach plots for the composite
films. The extrapolated (DE) values listed in Table 1 were
determined from the slope reciprocal of the linear part of each
curve and Eq. (7) as
lnðaÞ ¼ :=DE þlnða0 Þ
ð8Þ
Table 1
Optical energy results for (NaI) composite films.
Composite
Eopt (eV)
DE (eV)
Eg + DE (eV)
Pure PEO
1 wt% NaI
2 wt% NaI
4 wt% NaI
6 wt% NaI
8 wt% NaI
10 wt% NaI
15 wt% NaI
2.60
2.56
2.54
2.50
2.48
2.46
2.44
2.36
0.34
0.36
0.37
0.40
0.41
0.42
0.43
0.50
4.94
2.92
2.91
2.90
2.89
2.88
2.87
2.86
The exponential dependence of a(o) on photon energy (_o)
indicates that the absorption processes taking place in the studied
films obey the Urbach rule. These energy tails become smaller as
1922
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
7.00
Pure PEO
1% NaI
2% NaI
4% NaI
6% NaI
8% NaI
10% NaI
15% NaI
6.95
6.90
6.85
6.80
6.75
ln(α)
6.70
6.65
6.60
6.55
6.50
6.45
6.40
6.35
6.30
6.25
6.20
6.15
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
hω (eV)
Refractive index
Fig. 4. Urbach plot of ln(a) versus photon energy for PEO/NaI composites.
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Pure PEO
1wt.% NaI
2wt.% NaI
4wt.% NaI
6wt.% NaI
8wt.% NaI
10wt.% NaI
15wt.% NaI
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. Refractive index versus UV-radiation wavelength.
the concentration of NaI decreases, which is consistent with the
variation of the optical energy gap (Eopt). In general, the sum
Eopt + DE represents the mobility energy gap; Table 1 includes
values of the mobility gap for different PEO electrolyte composites. The increments of the energy tail widths can be explained
by the fact that increasing the NaI content may lead to the
creation of ionic complexes, disorder, and imperfections in the
structure of the composites, a case that may increase localized
states within the forbidden energy gap. The energy tails have the
smallest values due to neatness of the PEO structure and scanty
impurities, which lead to decreased localized states within the
forbidden band gap. This usually contributes to the increase in the
optical energy gap [13,18].
Fig. 5 shows the variation of the refractive index of the
prepared PEO/NaI composites with UV-wavelength, calculated
from Eqs. (3) and (4), which relate the refractive index (n) with
the absorption coefficient (a) and the extinction coefficient (k).
The figure shows that the refractive index decreases rapidly at
lower wavelengths ( o500 nm). The dielectric constant (e0 ) and
00
dielectric loss (e ) of the PEO/NaI composites were calculated
from Eq. (6). Figs. 6 and 7 show the variations of e0 and e00 as a
function of radiation wavelength. Both constants decrease with
increasing wavelength. Fig. 8 shows the effect of NaI content on
the refractive index and the dielectric constant, where both of
them increase with increasing concentration of the electrolyte NaI
in the PEO matrix. The refractive index values ranged from 1.46
for pure PEO to 3.8 for 15 wt% NaI composite, and the dielectric
constant (e0 ) ranged from 4.5 to 12 measured at 660 nm
wavelength.
The observed increase in the dielectric constant, refractive
index, and electrical conductivity due to incorporation of concentration of NaI salt particles in the PEO matrix was reported
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
1923
45
Pure PEO
1wt.% NaI
2wt.% NaI
4wt.% NaI
6wt.% NaI
8wt.% NaI
10wt.% NaI
15wt.% NaI
40
Dielectric Constant (ε′)
35
30
25
20
15
10
5
300
400
500
600
700
800
Wavelength (nm)
Fig. 6. Dielectric constant (e0 ) versus wavelength.
0.07
Pure PEO
1wt.% NaI
2wt.% NaI
4wt.% NaI
6wt.% NaI
8wt.% NaI
10wt.% NaI
15wt.% NaI
Dielectric Loss (ε″)
0.06
0.05
0.04
0.03
0.02
0.01
0.00
300
400
500
600
700
800
Wavelength (nm)
Fig. 7. Dielectric loss (e00 ) versus wavelength.
4.0
Refractive index
3.5
3.0
2.5
2.0
1.5
-2
0
2
4
6
8
10
12
NaI filler concentrations (wt.%)
Fig. 8. Dependence of refractive index on NaI concentrations.
14
16
1924
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
Dielectric Constant (ε′ ) at (λ= 660 nm)
14
Experimental Data
13
EMT Model (Rao et al.)
12
11
10
9
8
7
6
5
4
-2
0
2
4
6
8
10
12
14
16
Volume fraction of NaI (%)
Fig. 9. Fit of EMT model: dielectric constant as a function of NaI concentration.
0.15
pure PEO
1 wt%. NaI
2 wt%. NaI
4 wt%. NaI
6 wt%. NaI
8 wt%. NaI
10 wt%. NaI
15 wt%. NaI
0.14
0.13
0.12
(1/n2-1)
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
8.0
8.2
8.4
8.6
8.8
9.0
9.2
9.4
9.6
9.8
(hν)2 (eV)2
Fig. 10. Plots of (n2–1) 1 versus (hn)2 of the composites.
Table 2
Excitation and dispersion energies for composites.
Composite
E0 (eV)
Ed (eV)
Pure PEO
1 wt% NaI
2 wt% NaI
4 wt% NaI
6 wt% NaI
8 wt% NaI
10 wt% NaI
15 wt% NaI
4.03
3.77
3.71
3.69
3.68
3.65
3.64
3.63
8.7
10.05
11.42
13.37
15.59
17.89
18.67
38.21
in many researches [7,10,11]. Liu et al. [19] fitted a modeling
relation between the dielectric constant and filler content
based on the effective-medium theory (EMT), which relates the
inter-phase between particles, polymer matrix, and composite
morphology. The EMT model is expressed in the form of Rao
equation [5,8] as
e ¼ e1 ½1 þv2 ðe2 e1 Þ=ðe1 þ nð1v2 Þðe2 e1 Þ
ð9Þ
where e, e1, and e2 are the dielectric of composite, PEO matrix, and
discrete fillers, respectively, v1 and v2 are the volume fraction of
continuous matrix and discrete fillers, and n is a factor related to
the morphology of filled particles. Fig. 9 shows the fit of the EMT
model of Eq. (9) to our PEO/NaI composites prepared via the
casting method. The model fit predicts properly the increase of
the dielectric constant (e0 ) of PEO/NaI composites with increasing
filler concentrations, especially at lower NaI filler concentrations.
The refractive index, n, of the PEO/NaI thin films is found to
decrease with increase in the wavelength of incident photon, and
tends to be constant at high wavelengths as shown in Fig. 5.
R.S. Al-Faleh, A.M. Zihlif / Physica B 406 (2011) 1919–1925
The dispersion of refractive index below the inter-band absorption edge according to the Wemple–DiDomenico single oscillator
model [20–23] is given as
n2 ¼ 1þ
E0 Ed
E20 ðhvÞ2
ð10Þ
where E0 is the average excitation energy for electronic transitions, Ed is the dispersion energy, which is a measure of the
strength of inter-band optical transitions, h is Planck’s constant,
n is the frequency, and hn is the photon energy. E0 and Ed values
were calculated from the slope and intercept (E0/Ed) on the
vertical axis of plot of 1/(n2–1) versus (hn)2 (Fig. 10). The values
of E0 and Ed for the PEO/NaI composites are given in Table 2.
Although Ed values increase with increasing NaI concentration,
E0 values tend to decrease.
4. Conclusion
In this work, polymer electrolyte thin films of PEO/NaI have
been prepared by the casting method. The films contain NaI
electrolyte component as a filler at different concentrations of
0%, 1%, 2%, 4%, 6%, 8%, 10%, and 15% by weight. Optical quantities
such as absorption coefficient, optical energy gap, energy tails,
refractive index, and dielectric constants were determined from
analysis of the absorbance and reflectance of UV-radiation spectra. From the optical results obtained it was found that the
electron transition is direct in k-space and the optical energy
gap decreases with increasing sodium iodide concentration. The
observed dielectric constant and the refractive index increase
with NaI content. The fit of the EMT model on the variation of the
1925
dielectric constant with the electrolyte content is, in general,
acceptable.
Acknowledgment
The authors thank the Chemistry Department of Jordan
University for providing the sodium iodide powder.
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