Macromolecular Symposia
Theoretical Study of Surface Plasmon Resonance in
P3HT:PCBM/Cu nano Film
Journal: Macromolecular Symposia
Manuscript ID masy.202000170.R1
Wiley - Manuscript type: Full Paper
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Date Submitted by the
16-Apr-2020
Author:
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Complete List of Authors: Das, Malyaj; Medi-Caps University, Physics
Kushwah, Kamal Kumar ; Jabalpur Engineering College, Department of
Applied Physics
Sharma, Divakar; Medi-Caps University, Physics Department
Keywords: optics, nanoparticles, interface
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Theoretical Study of Surface Plasmon Resonance in
P3HT:PCBM/Cu nano Film
Malyaj Das,*1 Kamal Kumar Kushwaha, 2 Divakar Sharma 1
1Department
of Physics, Medi-Caps University, Indore, India 453331
Fax: (+91) 7314259501, E mail: malyaj08@gmail.com
2Department of Applied Physics, Jabalpur Engineering College, Jabalpur,
India 482011
Summary: The present paper discusses thoretical study about the surface
plasmon resonance (SPR) properties of P3HT (Poly-3 hexylthiophene-2, 5-
Fo
diyl), PCBM (6,6- PhenylC61 butyric acid methyl ester): Cu nano film with the
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variation of thickness of Cu nano film. The present theoretical studies show
that the variation of thickness of the Cu nano film gives better result as
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compared to gold and silver nano films. The SPR Q-factor infers that copper
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would be good candidates to replace gold as potential plasmonic materials.
Keywords: Optics; nanoparticles; interfaces.
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Macromolecular Symposia
1. Introduction
Recently, surface plasmon resonance (SPR) is becoming emergent phenomenon as it has been
widely used in solar cells, biomedical sensing devices and sub wavelength optical devices and
components.
[1,2]
With the help of this technique, we can also detect a change in refractive
index of the medium signifying the existence or non existence of target molecule which is
useful in biosensors. Many research groups have studied and demonstrated solar cell based
CdS and ZnS thin films. [3,4] Presently, P3HT (Poly-3 hexylthiophene-2, 5-diyl), PCBM (6,6PhenylC61 butyric acid methyl ester) is widely used because their morphology was relatively
easy to optimize. Meanwhile, flexibility, large area, light-weight, and the ability to self-repair
are all attractions for research as compared to in organic solar cell. [5,6,7] Also, in previous
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Macromolecular Symposia
studies,
[8,9]
it has been observed that the P3HT:PCBM/Ag and P3HT:PCBM/Au nano film
give better results. But, SPR sensor preparation from these metals is costly. Cu is also
exhibiting SPR character.[10] So, Cu can be use as a cost effective metal in place of Ag and Au.
Therefore, the present problem was undertaken with a view a) to get theoretical study of SPR
effect on P3HT-PCBM/Cu film using Kretschmann-Raether configuration b) to determine the
SPR Q-factor of Ag, Au and Cu. From the observations, it has been noticed that the
reflectance becomes zero at 10 nm thickness of P3HT-PCBM film and 45 nm thickness of
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copper nano film. We have found that he SPR Q-factor of Cu is more than gold and smaller
than Ag. The enhanced Q-factor of Cu suggests it can be used in place of gold for plasmonic
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materials. To the best of our knowledge, there is no report for theoretical SPR study with
P3HT: PCBM/Cu nano film.
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2. Theoretical Model
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2.1 Surface Plasmon Resonance Reflectance Equations
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One of the ways for excitation of SPP waves is Kretschmann-Raether configuration. [11,12] Its
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design involves a totally reflecting prism with a metal film on its reflecting surface. Usually
such a structure is formally considered as a three-layer structure involving prism glass (p),
metal (m) and sensing layer (s) which is described in Figure 1.
Figure 1: Kretschmann configuration
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Page 3 of 12
For determination of reflection coefficient (r), in Kretschmann configuration, we need to use
3-layer media equation (n1, n2 and n3 are refractive index of each layer) from equation of
dielectric medium [13]
r
r1 e  i   r2 e  i 
1  r1 r2 e  i 
Here, r1 and r2 are Fresnels’ reflectivity coefficients r1 
n1  n2
n1  n2
(1)
at the first and r2 
second- interface and δ is the phase factor which is defined as  
2

n d,
n 2  n3
n 2  n3
here λ is the wave
length of incident light and n is the refractive index and d is the thickness of film.
2
Substitute the expression of wave number k   n , in the formula of δ converted into   k d
r1  r2 e 2 i k d
1  r1 r2 e 2 i k d
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The Equation 1 modified now,
r
(2)
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Similarly, Equation (2) applied to the three medias j (p, m, s) which is defined as Glass prism,
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(p), Metallic film (m), Sensing layer (s), relating p-polarization can be calculated as follows[14,
15]:
rpm  rms e 2 i km d
rpms 
(3)
rR
1  rpm rms e 2 i km d
In this Equation (3), the amplitude reflectance for prism-metal and metal-sensing layer
interfaces are given by
rpm 
n p  nm
n p  nm
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rms 
nm  n s
nm  n s
(4)
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Macromolecular Symposia
The refractive index of prism, metal and sensing layer are np, nm and ns and they are related
with kp, km, ks and kz wave number (extinction coefficients ) of prism, metal, sensing and air
(k   km  p )
(k   k s  m )
rpm  p m
rms  m s
layer by
(5)
(k m  s  k s  m )
(k p  m  km  p )
2
kp   p
km   m
ks   s
kz 
c2
2
c2
2
(6)
(7)
 k z2
 k z2
(8)
n p sin 
(9)
c

c
 k z2
2
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Macromolecular Symposia
The term np is the refractive index of the glass prism and the incident angle is given as θ. The term
2 c

ω corresponds to the angular frequency  
and c represents the velocity of light. Also, εp,
εm, εs and εo are the dielectric constant respective medium prism, metal, sensing and air layer.
  1  i  2
The dielectic constant ε is given as
(10)
Here, ε1 and ε2 are real and imaginary part whuch is defined as [14]
1  n 2  k 2
(11)
 2  2 n  k
(12)
Here, n is refractive index and k is the excitation coefficient. The reflectivity (R) of medium
is given as
Fo
R  rpms
2

rpm  rms e 2 i km d
2
1  rpm .rms e 2 i km d
(13)
At resonance condition, for the excitation of surface plasmon waves by the prism is
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equivalence of the tangential component of incident wave vector with real part of surface
plasmon wave vector [16,17]
2
n p sin  ATR  Re ( k sp )

2
 2     m  s 
n p sin  ATR    

     m   s 
(14)
(15)
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or
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Here, θATR is the SPR angle where a dip is observed in the intensity of light reflected
internally from the film, ksp is surface plasmon wave vector and εm, εs are dielectric constant
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Page 4 of 12
of metal and sensing layer, respectively. Using Equation 13 and 15, we can determine the
reflectivity of our sample P3HT: PCBM/Cu nano film.
2.2 Surface Plasmon Resonance(SPR) Quality factor
SPR Quality factor (Q - factor) is one of the important parameter to compare performamce
of plasmomic material when used in different applications. The plasmonic materials such as
gold and silver have high quality factor. Futher to investigate the SPR performamce, we
need to determine the SPR quality factor. The SPR quality factor is given by following
formula [18]:
Q 
 12
2
(16)
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Here, ε1 and ε2 are real and imaginary part of dielectric constant (Equation 11 and 12).
Using, the Equation 16, we have calculated SPR Q-factor of Ag, Au and Cu.
3. Results and Discussion:
Theoretical models depend on various experimental parameters such as thickness of metal
film and sensing layer, laser wavelength and refractive index of the dielectric material on
either side of the metal film. We performed SPR studies in the visible region because Cu
shows the plasmoic behaviour in visible regsion. We used He-Ne Laser of wavelength 632.8
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nm which lies in visible region and mostly reported in differnt SPR studies. [19] In the present
work for 3-layer model, we took SF10 prism as a prism material (p), Cu as a metallic film (m)
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and P3HT:PCBM as a sensing layer (s) which is illustrated in Figure 2.
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Macromolecular Symposia
Figure 2. Description of 3-layer model
The various physical parameters required for the calculation are enlisted in Table 1 at
wavelength 0.6328 μm of He-Ne Laser.
Table 1 Parameters for Prism, Copper and P3HT: PCBM
S.No
Material
Refractive Index (n)
Extinction Coefficient (k)
1.
Prism (SF10) [20]
1.5151
4.7557 x10-8
1.
Copper [20]
0.270002
2.
P3HT:PCBM [21]
2.0409
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3.4081
0.0074660
Macromolecular Symposia
Using the Equation 9 and 11, we have plotted the variation of reflectance P3HT: PCBM/Cu
nano film with the thickness of Cu and P3HT: PCBM. We have chosen the optimized
thickness of Cu and P3HT:PCBM nano film after plotting differnt grpah of reflectance vs
angle from our parameters. Here, we have plotted those graphs which giving SPR reflectance
zero. Further, description prsented in the form of differnt curves of P3HT: PCBM/Cu nano
film with thickness of copper (Figure 3,4 and 5).
3.1 SPR curve on the variation of thickness of Cu film thickness
The dielectric layer, P3HT:PCBM played important role in SPR study of P3HT:PCBM/Cu
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nano film. When, we fix the thickness of P3HT:PCBM about 30 nm and vary the thickness of
Cu nano film, the significance SPR reflectance curve obtained. For the thickness
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P3HT:PCBM film > 30 nm, no significant curve obtained, so in our present work those curves
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not presented.
a) When the thickness of P3HT: PCBM film is 30 nm.
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We have plotted the variation of P3HT:PCBM/Cu nano film with the variation of thickness
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of Cu nano film vary between 10 nm to 40 nm and shown in Figure 3 and with constant the
thickness of P3HT:PCBM at 10 nm.
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10 nm
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Reflectance ( in a.u.)
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Page 6 of 12
60
20 nm
40
30 nm
20
o
40 nm
SPR Angle 55
0
30
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60
70
80
0
Angle ( in )
Figure 3. Variation of reflectance of P3HT:PCBM/Cu with thickness of Cu nano film.
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We find that from Figure 3 that a broad curve is obtained for 10 nm film thickness and
narrower curve for 40 nm film thickness of Cu nano film. The narrow peak observed for
P3HT:PCBM/Cu nano film at SPR angle 550 for Cu nano film thickness.
From Figure 3, it is clear that for P3HT: PCBM thickness of 30 nm, reflectance of P3HT:
PCBM/Cu nano film becomes zero but curve is not significantly narrow. So, further we
decrease the thickness 30 nm to 10 nm film thicknesses.
b) When the thickness of P3HT: PCBM film is 10 nm.
We have plotted the variation of P3HT:PCBM/Cu nano film with the variation of thickness
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of Cu nano film vary between 10 nm to 40 nm and shown in Figure 4 and with constant the
thickness of P3HT:PCBM at 10 nm.
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100
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90
10 nm
70
60
20 nm
50
30 nm
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40 nm SPR Angle 36.6
10
0
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Reflectance ( in a.u.)
80
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Macromolecular Symposia
60
0
Angle ( in )
70
80
Figure 4. Variation of reflectance of P3HT: PCBM/Cu with thickness of Cu nano film.
The variation of reflectance of Figure 3 and 4 suggest that when we keep the thickness of
P3HT:PCBM film 10 nm, the reflectance curve better than 30 nm thickness. This studies
suggest that the 10 nm thickness may be possess SPR effect at this thickness for
P3HT:PCBM/Cu nano film. Further, we fix the thickness of P3HT:PCBM film 10 nm for our
calculations.
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Macromolecular Symposia
b) When the thickness of P3HT: PCBM film is 10 nm and thickness of Cu film 40 to 55
nm.
We have plotted the variation of P3HT:PCBM/Cu nano film with the variation of thickness
of Cu nano film vary between 40 nm to 55 nm and shown in Figure 5 with constant the
thickness of P3HT:PCBM at 10 nm. From Figure 5, we found that both 40 nm and 45 nm,
thickness very close to zero reflectance but at 45 nm sharper than 40 nm curve. And also, it
this studies established that thickness above 45 nm not significant, as reflectance was not zero.
100
Fo
90
70
50 nm
60
45 nm
50
40
40 nm
55 nm
30
20
10
0
35
40
45
50
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30
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Refletance ( in a.u.)
80
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Angle ( in
0
)
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Figure 5 Variation of reflectance of P3HT: PCBM/Cu with thickness of Cu nano film.
b) When the thickness of P3HT: PCBM film is 10 nm and thickness of Cu film 45 nm.
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Refletance ( in a.u.)
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60
Thickness of Cu film 45 nm
40
20
0
30
40
50
60
70
80
0
Angle in ( )
Figure 6. Variation of reflectance of P3HT: PCBM/Cu with thickness of Cu nano film.
From Figure 3 to Figure 6, we varied the thickness of P3HT: PCBM and Cu nano film.
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Page 9 of 12
The effect of thickness of P3HT: PCBM and Cu nano film on P3HT: PCBM/Cu nano films
are given in Table 2.
Table 2: Effect of thickness of P3HT: PCBM and Cu nano film on P3HT: PCBM/Cu.
S. No.
Thickness of P3HT-PCBM
Thickness of Cu
Remark
1.
30 nm
10 nm to 40 nm
Figure 3
2.
10 nm
10 nm to 40 nm
Figure 4
3.
10 nm
40 nm to 55 nm
Figure 5
4
10 nm
45 nm
Figure 6
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From Table 2, it can be noticed that a thickness of both dielectric and metal (P3HT: PCBM
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and Cu) nano film plays an important role in SPR properties of P3HT: PCBM/Cu nano film.
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The thickness affects the methodology of design of SPR device. Our theoretical results show
that in P3HT: PCBM/Cu nano film reflectance zero at thickness of P3HT: PCBM of 10 nm
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and thickness of Cu 45 nm. The SPR angle calculated from X-axis for this composite is
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36.2ο.
To, the best of our knowledge, there is no any report of SPR study in P3HT-PCBM/Cu film.
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Macromolecular Symposia
So it is difficult to compare the results with current literature on similar work. However, we
compared the obtained results with previously reported data [8,9] on the SPR effect on Ag and
Au metal on P3HT-PCBM dielectric film, the calculated values are enlisted in Table 3.
Table 3: Calculated values of thickness and SPR angle for Cu, Ag and Au nano film.
S. No.
Nano film
Thickness
SPR Angle
1.
P3HT-PCBM/Cu
10 nm/ 45 nm
36.2ο
2.
P3HT-PCBM/ Ag [8]
23 nm/51 nm
37ο
3.
P3HT-PCBM/ Au [9]
15 nm/48 nm
38ο
From Table 3, it is clear that the value of SPR angle of Cu is smaller than the Ag and Au.
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We compared our results with not similar but related article on Cu nano particles. O. A.
Yeshchenko [22] reported the temperature dependence SPR study on Cu/SiO2 composite films
in nano regime. The experimental results on the temprature dependence of SPR energy and
curve in Cu nano particles 17-59 nm thickness embedded in Silica matrix in the temperature
range 293–460 K. Some researchers
[23]
reported that addition of Cu in TiO2 matrix can
enhance absorption towards visible spectrum and can reduce the charge carrier recombination
due to Localized Surface Plasmon Resonance (LSPR). M. Mahanti et al claim that an
enhancement in the UV emission has been occurred at Cu–ZnO interface which is well
Fo
supported by the Plasmon dispersion relation. After, comparison, the above literature it came
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out that the Cu nano film 17 nm to 59 nm showing the SPR property. Also, we got the idea
that addition of Cu nanoparticle enhances the UV absorption in TiO2 [24] The above articles
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suggest that Cu can be use as an efficient SPR active metal..
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3.2 SPR Q-factor curve on the variation of Ag, Au and Cu film:
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In order to have information, about the order of SPR Q-factor in Ag,Au an Cu, we have
thoretically calculayed Q-factor using Equation 16 in the range of 0.19 μm to 1.5 μm
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wavlength of irrdiated light source. The values of n and k are taken from reference for the
differnt values of wavelength[20]. Dependence of SPR Quality factor for Ag, Au and Cu are
shown in Figure 7.
2000
Ag
Au
Cu
1500
SPR Quality factor
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Page 10 of 12
1000
500
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
wavlength ( in  m)
Figure 7. Dependence of SPR Quality factor for Ag, Au and Cu.
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Page 11 of 12
From, Figure 7, it is clear that the order of Q factor for Ag is more than the Au and Cu
sample. One important picture came out from the Figure 7 that Cu shows more Q-factor than
Au in the range of 0.4 to 1.1 μm of wavelength of light. The 0.4 to 1.1 μm of wavelength of
light covered the complete visible and some part of near IR. So, it is clear that the Q factor of
Cu is better than Au. (Blue line copper and red line Au). The above plot also suggests that the
Cu can be used as a metal layer for potential SPR application.
4. Conclusion
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The present theoretical studies show that variation of thickness of the Cu nano film gives
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better result as compared to gold and silver nano films (the SPR angle for Cu is smaller than
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the Ag and Au). Also, the Q factor of Cu is more than the Au. These two merits of Cu
suggest that considering the cost of silver and gold, copper would be good candidates to
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replace silver and gold as plasmonic materials.
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