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The refractive index dispersion of Poly (2,2’-oxybis(methylene)bis(4-(hydroxyl(4hydroxymethyl)naphthalene-1-yl)(phenyl)methyl)naphthalene-l-ol)( PHMNP )Thin film
Ali Qassim Abdullah* a , Alaa Yassin AL-Ahmad b and Hadi Z.M. Al-Sawaad c
a
Department of Physics, College of Science, Basrah University .
b
Department of Physics, College of Education, Basrah University .
c
Department of Chemistry , College of Science, Basrah University .
Basrah – Iraq
*
corresponding authors.
E-mail address: [email protected]
E-mail address: [email protected]
Abstract:
In the present work the refractive index spectra of Poly(2,2’-oxybis(methylene)bis(4-(hydroxyl(4hydroxymethyl)naphthalene-1-yl)(phenyl)methyl)naphthalene-l-ol) (PHMNP) thin film have been
studied at room temperature. The transmission and reflectance spectra , at normal incidence of
PHMNP thin film were obtained in the range (200  900 nm) . The refractive index dispersion
parameters such as oscillator energy Eo, dispersion energy Ed ,long wavelength refractive index n
,oscillator length strength So calculated to be about 4.18 eV,11.95eV,1.96,10.024x1013 m-2
respectively. The optical moments M-1 and M-3 , nonlinear optical susceptibility (  (3) ) and nonlinear
refraction index (n2 (0)) were found to be 2.85(eV)-2 ,0.1633(eV)-2 , 4.45x10-13 esu and 8.62x10-12 esu
respectively. The low reflectance and low refractive index of PHMNP thin film in the UV-Visible
region make the materials a prominent one for antireflection coating in solar thermal devices.
Keywords: The refractive index dispersion ,Optical constants, nonlinear optical susceptibility.
1.Introduction:
1
Polymer has received great attention due to its environmental stability, ease of preparation ,and
its optical and electrical properties . Much work has been done on the molecular design ,synthesis ,and
assembly of structures with desired properties[1].Phenols resin are polymers used in a wide variety of
application in different areas as the construction , electronic .. etc. [2].Some common uses are
adhesives ,coating , molding , laminate or plastic manufacturing [3]. Phenolic resins are obtained by
the condensation of phenol and an aldehyde , commonly formaldehyde , in a batch reactor , in acid or
basic media , followed by vacuum distillation to give phenolic products [4] . Information about the
spectral dependence of optical parameters such as refractive index , dielectric constant , reflectivity and
absorption coefficients are essential in the characterization of materials that are used in the fabrication
of optoelectronic devices[5-8].In the present paper , we present the results of the transmission and
reflection measurements performed on the polymer in order to derive the refractive index . The
refractive index dispersion data were analyzed using the Wemple-DiDomenico single-effective –
oscillator model .As the determination of optical constant is expected to expand the available physical
information .
2.Experimental:
Thin
film
of
Poly(2,2’-oxybis(methylene)bis(4-(hydroxyl(4-hydroxymethyl)naphthalene-1yl)(phenyl)methyl)naphthalene-l-ol) (PHMNP )was prepared at room temperature using cast method
on a glass 1.4 cm  2 cm size . In order to determine optical constants of the film as a function to
wavelength the film thickness was kept around 1 m . A measurement of the spectral transmittance (T )
and reflectance (R) were recorded using CE-7200 PC dual beam spectrophotometer in the wavelength
range (200  900 nm) . The Chemical Structure of Poly(2,2’-oxybis(methylene)bis(4-(hydroxyl(4hydroxymethyl)naphthalene-1-yl)(phenyl)methyl)naphthalene-l-ol)( PHMNP) is shown in Figure(1).
Figure 1: the structure of naphthol-formaldehyde resin.
3. Results and discussion
2
The spectral distribution of transmittance (T) and reflection (R) was studied for PHMNP
are illustrated in figures (2) and (3) respectively.
100
T[%]
80
60
40
20
0
200
300
400
500
600
700
800
900
wavelength(nm)
Figure:(2) The spectral transmission (T)as a function of wavelength for PHMNP thin film.
20
18
16
14
R[%]
12
10
8
6
4
2
0
200
300
400
500
600
700
800
900
wavelength(nm)
Figure: (3) Reflectance (R) a function of wavelength for PHMNP thin film.
It could be noted from transmittance and reflectance distributions of the PHMNP thin film, figures (2)
and (3), that at longer wavelengths (λ > 550 nm) the film become transparent. The inequality (R+T) <
3
1 at shorter wavelengths (λ < 550 nm) due to existence of absorption (absorbing region). The refraction
index (n) value provide the optical properties of the film and it is related by the fallowing equation [9].
1 R
n(
)
1 R
(1)
The extinction coefficient (k) can be calculated using the equation :
k

4
(2)
Figures(4)and (5) show the variation of refraction index (n) and extinction coefficient (k) as a function
of wavelength.
2.6
2.4
2.2
2.0
n
1.8
1.6
1.4
1.2
1.0
200
300
400
500
600
700
800
900
wavelength(nm)
Figure: (4) Refraction index as a function of wavelength for PHMNP thin film.
0.16
0.14
0.12
0.10
k
0.08
4
Figure: (5) Extinction coefficient as a function of wavelength for PHMNP thin film.
From Figures (4) and (5) we can see that the values of refraction index(n) and extinction coefficient
(k) are decreasing with the increasing of wavelength. In the region (200-350nm) it is clear that the
values of (n) and (k) sharply decreases with increasing wavelength. At wavelength values >350nm the
values of (n) and (k) are smoothly decreases. Wemple and DiDomenico have analyzed refractive
index data below the inter-band absorption edge using the single effective oscillator equation[10,11].
(n 2  1) 
E d E
( E2  E 2 )
(3)
E is the single oscillator energy and Ed is the dispersion energy . Values of the parameters ( E , Ed )
can be evaluated by plotting of (n 2  1) 1 versus (hv)2 and fitting it to a straight line shown in figure
(6).
5
Figure(6): Variation of (n 2  1) versus (hv) 2 for PHMNP thin film .
The statically refraction index n is evaluated from eq. (3), n 2  1  ( Ed / E ) .The value of Ed and
E calculated from the slope of plotting (n 2  1) 1 verses (hv) 2 in figure (10), from the fitting we
found that values are (11.95) and (4.18) eV respectively. The values of static dielectric constant,
 s  n 2 (0) , and static refractive are also calculated using eq.(3), The value of n(0) is found to be and
1.96 . The moments of optical dispersion spectra M-1 and M-3 ,can be evaluated using the relationships[
10]:
E2 
M 1
M 3
(4)
Ed2 
M 31
M 3
(5)
The values of dispersion parameters and the optical moments of the films are presented in table 1 .
The relation between optical real part of dielectric constant  1 and the square of wavelength  2 is
given by [12]
1  n 2  k 2    
e2
N 2

2 2
4 c   m *
(6)
6
e2
N
A
2 2
4 c   m *
(7)
Where   is infinite high frequency dielectric constant, e is the electronic charge,   the permittivity
of free space ( 8.854  10 -12 F / m ) ,c is the velocity of light, and N / m * is the ratio of carrier
concentration to the effective mass . The high frequency dielectric constant   can be obtained from
plotting n 2 as a function of  2 as shown in figure (7) , it is observed is linear at large wavelengths.
Extrapolating the linear part of this dependence to zero wavelength give value of  () and from the
slope the values of N / m * were calculated according to the Eq.(6) . The obtained values of   and
N / m * are given in the table (1).
Figure (7): Plot of n 2 as a function of wavelength for PHMNP thin film.
The average inter-band oscillator wavelength  can be calculated by the following [13]
7
n2  1

 1  (  )2
2
n 1

(8)
n is the refractive index at infinite wavelength  , the plotting (n 2  1) 1 verses 2 shows linear part
was below the absorption edge as shown in figure(8) .
Figure(8): Plot of (n 2  1) 1 against 2 for PHMNP thin film.
The intersection with (n2  1)1 axis is (n2  1) 1 and hence, n2 at
 equal to   . The average
oscillator strength is given by ,
s 
n2  1
(9)
o 2
The optical conductivity is a measure of frequency response of material when irradiated with light it is
determined from the following relation [14] ,
 opt 
nc
4
(10)
8
Where c is light velocity. The electrical conductivity can be estimated by using the following
relation .
e 
2 opt
(11)

The high magnitude of optical conductivity (1014 sec-1) confirms the presence of very high photo
response of the film. The increased of optical conductivity at high photon energies is due to the high
absorbance of PHMNP thin film and may be due to electron excited by photon energy. The optical and
electrical conductivity have inverse image by increasing photon energy and optical conductivity higher
14
3.5x10
14
3.0x10
14
opt (sec)-1
4.0x10
2.5x10
14
2.0x10
14
1.5x10
14
1.0x10
14
5.0x10
13
e (Ohm.cmsec)-1
than electrical as shown in Figure (9).
6x10
5
5x10
5
4x10
5
3x10
5
2x10
5
1x10
5
0
0.0
1
2
3
4
h (eV)
5
6
7
1
2
3
4
5
6
h (eV)
Figure: (9) Plot (a) optical conductivity opt and (b) electrical conductivitye as a function of
wavelength for PHMNP thin film.
9
7
According to Frumer, the Miller rule is very convenient for visible and near infrared
frequencies. It which relates the third- order of nonlinear polorizability (  (3) ) parameter, and the linear
optical susceptibility  (1) through the following equation [15.16]:
 (3)  A(  (1) ) 4
 A[ Eo E d / 4 ( Eo2  (h ) 2 ]4
(12)
 A /( 4 ) 4 (n 2  1) 4
6.0x10
-12
5.0x10
-12
4.0x10
-12
3.0x10
-12
2.0x10
-12
1.0x10
-12

 esu
Where A is a constant, A  1.7  10 10 . The covalency and iconicity of chemical bonds strongly
influence the magnitude of the non linearity. The values of nonlinear refraction index (n2 ) are
calculated from the semi-empirical relation [16].
(n 2  1) 4
(13)
n2 (esu )  2.6 10 13
n
0.0
200
300
400
500
600
700
800
900
h(eV)
Figure (10): Plot nonlinear optical susceptibility (  (3) ) as a function of wavelength for PHMNP
thin film.
10
n2(esu)
8.0x10
-11
7.0x10
-11
6.0x10
-11
5.0x10
-11
4.0x10
-11
3.0x10
-11
2.0x10
-11
1.0x10
-11
0.0
200
300
400
500
600
700
800
900
h(eV)
Figure (11): Plot nonlinear refraction index (n2 ) as a function of wavelength for PHMNP thin
film.
The varies of nonlinear optical susceptibility (  (3) ) and nonlinear refraction index (n2 ) as a
function of wavelength are shows in Fig.(10) and Fig(11) respectively. It is clear from figures (10)
and (11) that the values of third-order nonlinear susceptibility  (3) and the nonlinear refraction index
(n2 ) are decreasing when the wavelength is increasing.
Table(1). Optical constants of PHMNP thin film
Quantity
Value
Quantity
Value
E (eV )
4.18
N / m (m 3 .kg) 1
23.55×1030
Ed (eV )
11.95
 (nm)
167.33
M 1 (eV ) 2
2.85
S  (m 2 )
10.204×1013
M 3 (eV ) 2
0.1633
 3 (0)(esu )
4.45×10-13
n
1.96
n2 (0)(esu)
8.64×10-12

3.85
4. Conclusions
Optical transmission spectrum is used to calculate the optical, electric and dielectric properties
(i.e. the refractive index , extinction coefficient , optical and electrical conductivity ), for PHMNP
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thin film. The optical conductivity  Opt. were increased with increasing photon energy. The PHMNP
thin film is exhibited more transmittance at high wavelength. The high transmission, , low reflectance
and low refractive index of PHMNP thin film in the UV-Visible region make the materials a prominent
one for antireflection coating in solar thermal devices. The high extinction coefficient value (10 -1 ) and
electric conductivity (105 (Ω cm)-1) show the Semi-insulating behavior of the material. The high
magnitude of optical conductivity (1014 s-1) confirms the presence of very high photo response of the
material.
Acknowledgements: we are thankful to our colleagues in physical department that do all the essential
analyzes for achieving the research.
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Refrees
1-Dr.Noori Jarh " [email protected]
:07801231345
University of Basrah/Iraq. Mobile phone
2-Prof.Dr.Salah hashim [email protected] . Basrah University/Iraq Mobile phone
:07801000106
Qeensland
3- Dr.Muna M.Abbas” [email protected] University of Bagdad / Iraq . Mobile Phone
:07901364642
4- prof.Dr.Salah Shaker:009647801000106, email:[email protected].
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