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Applied Surface Science
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haracterization of poly (3-methyl thiophene) thin films prepared by modifiedhemical bath deposition
andip V. Kamata, J.B. Yadavc, Vijaya Purib, R.K. Puria,∗, O.S. Jooc
Vacuum Techniques & Thin Film Lab., USIC, Shivaji University, Kolhapur, IndiaDepartment of Physics, Shivaji University, Kolhapur, 416004, IndiaClean Energy Research Centre, Korea Institute of Science and Technology, Seoul, Republic of Korea
r t i c l e i n f o
rticle history:eceived 21 April 2011eceived in revised form 20 August 2011ccepted 21 August 2011vailable online 26 August 2011
a b s t r a c t
Poly (3-methyl thiophene) thin films were prepared by chemical bath deposition technique on glasssubstrate; the prepared thin films were characterized for structural, morphological and optical properties.The variation in the oxidant concentration has an influence on the properties of the P3MeT thin films.The increase in the oxidant concentration leads to increase in the thickness of the film. The bindingenergy increases due to increase in oxidation concentration. The P3MeT thin films show smooth surface
eywords:olymerhin filmshemical synthesisPS
morphology with increase in oxidant concentration whereas the contact angle of the thin film decreaseswith increase in oxidant concentration. The optical absorbance of these thin films was found to increasewith decrease in the optical band gap due to increase in oxidant concentration.
Polythiophene and its derivatives are now widely recognized foraving significant potential for the development of many devices
ike Field Effect Transistors, LEDs, Sensors, diodes, photovoltaic,hotoconductive devices and optical modulator devices, sensorpplications [1–7], due to their extraordinary physical and chem-cal properties. A distinguishing feature is that the conductivityf these polymers can be varied from conducting to insulatingtate with proper doping. The main drawback of these polymerss their low solubility, which hampers their use at commercialevel [2]. The presence of inert sulphur atom having high oxi-ation potential makes the preparation of polythiophene moreifficult [7]. This problem can be overcome by using alkyl substi-uted thiophenes. Alkyl substituted polythiophenes are soluble in
ost organic solvents, besides alkyl substituents strongly influ-nces the polythiophenes properties. It has been reported thatlkyl substituted polythiophenes show good environmental stabil-ty [8]. Poly (3-methylthiophene) belongs to polythiophene familyaving good chemical and environmental stability. Several meth-
ds are available for the polymerization of polythiophene, amonghich the simple chemical oxidative polymerization of poly (3-
methylthiophene) (P3MeT) with iron chloride and chloroform giveshigh yield [9].
Most of the reports available on the Poly (3-methyl thiophene)thin films are those using electrochemical polymerization tech-niques and physical methods. Among the various other methodsavailable for thin film deposition, chemical bath deposition (CBD)method is most suitable for the integration in large-scale fabri-cation process. The main advantages of the CBD method are itslow cost, sophisticated instruments not required, low processingtemperature and non-polluting by products [10].
In the present article the optical properties of poly (3-methylthiophene) thin films prepared by chemical bath deposition arereported. The conventional CBD technique was modified and thedeposition was carried out under inert atmosphere. The effectof variation of oxidant concentration on the properties of poly(3-methyl thiophene) thin films like surface morphology, opticalabsorbance, transmittance and refractive index is also reported.
2. Experimental details
2.1. Substrate cleaning
Micro slides of glass with dimensions 2.5 cm × 7.5 cm were usedas substrate. The cleaning procedure has been reported elsewhere[11]. Initially, the substrate were washed with deionized water,boiled in chromic acid for 30 min and washed with detergent, rinsed
n acetone and finally ultrasonically cleaned with deionized waterefore the deposition of thin film.
.2. Polythiophene thin film deposition
(3-methyl thiophene) (AR grade Merck) was purified by distil-ation before prior use. Iron chlorides FeCl3, Chloroform (CH3Cl),
ethanol (CH3OH) were used for the preparation of the polythio-hene thin films. An inert atmosphere is required for the depositionf poly (3-methyl thiophene) thin films so that the conventionalBD method was modified as shown in Fig. 1. The oxidant solu-ion was prepared by dissolving 0.1 M of FeCl3 in chloroform andhe glass slides were immersed in this beaker under constant stir-ing and kept in the glass chamber consisting of three ports ashown in Fig. 1. Nitrogen was introduced from first port and theecond port for outlet of nitrogen, the monomer solution of 0.1 Moncentration of (3-methyl thiophene) in chloroform was addedrop wise in the oxidant solution by a funnel from the third port ofhe glass chamber. The ratio of oxidant to the monomer was kept:1. During the precipitation, heterogeneous reaction occurred andhe deposition of polythiophene took place on the glass substrate.he substrates coated with polythiophene thin films were removedfter a time interval of 1 h from the bath, washed with methanol fol-owed by chloroform and acetone over and over again to removeesidual oxidant and unreacted monomers, dried in air and pre-erved in a vacuum dessicator. The experiment was repeated byhanging oxidation concentration. The oxidant concentration wasaried as 0.1 M, 0.15 M and 0.2 M. For the oxidant concentrationore than 0.2 M it was found that the deposition was nonuniform,
on stoichiometric with precipitate on the substrate.
.3. Characterization techniques
The thickness of these thin films was measured with surface
rofiler. FTIR spectra of poly (3-methyl thiophene) powder andhin films were recorded by using a PerkinElmer’s Spectrum Spec-rometer in the range of 400–4000 cm−1. For FTIR, these filmsere scratched from glass substrate and mixed with KBr. The
echnique for poly (3-methyl thiophene) thin films.
surface morphology and was studied by using the scanning elec-tron microscopy (SEM-HITACHIS-4100 model) at voltage 10 KV thesurface of these thin films coated with carbon to avoid chargingof the surface). XPS spectra were analyzed at excitation energyof 1486.6 eV and a scan step of 0.1 eV, using X-ray photoelec-tron spectroscopy (XPSESCA, PHI-5800 model), carbon correctionsbeing done before characterization. The optical absorption in thewavelength range 350–900 nm of the thin films of poly (3-methylthiophene) was measured by UV–vis spectrophotometer (U-2800,Hitachi, Japan). The water contact angle was measured with Sessiledrop method. Refractive indices of these thin films were calculatedanalytically [12].
3. Result and discussion
3.1. Reaction and growth mechanism
The CBD is based on the formation of solid phase from a solution,which involves the two steps of nucleation and particle growth.In nucleation, the heterogeneous reaction at the substrate sur-face takes place when clusters of the molecules formed undergorapid decomposition and particles combine to grow up to a certainthickness of the film [13]. The film thickness is found to dependupon deposition time, molar concentration of reactants, tempera-ture of bath and speed of rotation. In the present investigation, allparameters were kept fixed and oxidant concentration in the bathwas varied. It is interesting to note that film thickness increaseswith oxidant concentration this can be understood as follows:as soon as monomer is added to the oxidant solution, the freeFe3+ions initiates the polymerization of poly (3-methyl thiophene)[14] by attacking on the weak bonds of monomer (� bond) (Fig. 2).When the glass substrates are immersed into the chemical baththe ions get adsorbed on the substrate due to the forces of attrac-tion between free ions in the solution and surface of the substrate.
These forces may be cohesive forces or Vander Waals forces orchemical attractive forces. Increase in the concentration of FeCl3results into more number of free Fe3+ ions resulting in increase inthe adsorption between glass and ions in the solution, which helps
Fig. 5. XPS spectra of chemical bath deposited poly (3-methyl thiophene)
o increase the film thickness. The possible growth mechanism ofhe poly (3-methyl thiophene) thin film from a bath is as shown inig. 3
.2. FTIR spectroscopy
The FTIR spectra of chemically deposited poly (3-methyl thio-hene) thin films are shown in Fig. 4. The absorption peakorresponds to 3426 cm−1, 3434 cm−1and 3408 cm−1 in all the
3MeT thin films represents the O–H vibrations due to humid-ty in KBr [15]. The peak at 2936 cm−1 is shifted to some extentt 2922 cm−1 and 2916 cm−1 with increase in the concentrationf oxidant which represents the asymmetric stretching modes in
lms at different oxidant concentration (a) 0.1 M (b) 0.15 M and (c) 0.2 M.
the methyl group [16]. The band at 1670 cm−1 is shifted towardslower region at 1635 cm−1 and 1628 cm−1 for the oxidant concen-tration of 0.15 M and 0.2 M belongs to C C stretching modes ofvibrations in the thiophene ring [16]. While the band at 1488 cm−1
and 1424 cm−1at oxidant concentration 0.1 M (Fig. 4(a)) and 0.15 M(Fig. 4(b)) is ascribable to the stretching vibrations of 2,5 disubsti-tuted thiophene ring [17] is absent in other thin film. The peaksin between 1350 cm−1 and 1397 cm−1 are assigned to deforma-tion vibrations of methyl groups [18]. The bands at 1285 cm−1 and
1297 cm−1 assigned to C–H in plane bending vibrations [19]. Thebands 1159 cm−1 is shifted to lower side to 1124 cm−1 rest of thethin films representing the C–H ring breathing [19]. The bands at850 cm−1, 816 cm−1 and 809 cm−1 assigned to C–H out of plane
Fig. 6. SEM images of poly (3-methyl thiophene) thin films at
ending [16] in the ( C–H) thiophene ring [16,20]. While the peakst around 683–669 cm−1 represent the C–S–C stretching vibrationsn the thiophene ring [21].
.3. XPS studies
XPS spectra of chemically deposited poly (3-methyl thiophene)hin films are as shown in Fig. 5. The survey scan illustrates theresence of distinct bands at binding energies corresponding to1s, C1s, S2s, S2p and O2s core levels [22]. The binding energyight be chemically shifted according to state of chemical bond
f the element. The signal of standard carbon is known to be84 eV [23]. At oxidant concentration of 0.1 M (Fig. 5a) the pres-nce of C–S band corresponding to 284.9 eV [24] indicates theuccessful deposition of poly (3-methyl thiophene) (Fig. 5a), the2s, S2s, S2p3/2 and S2p1/2 peaks were observed at 24 eV, 228.7 eV,64 eV (FWHM 0.92 eV) and 165.1 eV (FWHM 0.24 eV) respectively.he energy difference of S2p3/2 and S2p1/2 peaks is 1.1 eV is closeo expected value 1.17 eV [25] these observed values have goodgreement with previous result obtained for P3MeT thin films byehara et al. [26]. However the increase in the oxidant concen-
ration leads to shift to the higher binding energy i.e. the C1ss shifted to 285.1 eV (FWHM 0.92 eV), at oxidant concentration.15 M (Fig. 5b). The S2p3/2 peak shifted to 164.7 eV (FWHM 0.92 eV)nd S2p1/2 is at 165.4 eV (FWHM 0.27 eV). At oxidant concentra-ion 0.2 M (Fig. 5c) the carbon peak shifts towards higher binding
nergy to 285.7 eV, simultaneous shifts was observed in S2p peaks.e. S2p3/2 is at 164.9 eV (FWHM 0.79 eV) and S2p1/2 at 166 eVFWHM 0.25 eV). This shift towards the higher energy indicateshat the change in oxidant concentration is able to affect the S
ent oxidant concentrations (a) 0.1 M (b) 0.15 M and (c) 0.2 M.
atoms which is similar to change in dopant concentration can affectthe S atoms [23]. Also the intensity of C1s changes with respect toincrease in the oxidant concentration; this might be due to dif-ference in photoelectric cross sections caused by changes in thedensity of states [27]. This might be due to disorder phenomenonobserved is C1s peaks like steric effects, cross-linking or chain ter-minators which is responsible for change in the carbon bonding[27].
3.4. Surface morphology
Fig. 6 shows the surface morphology of chemical bath depositedP3MeT thin films. The increase in the oxidant concentration has apropensity to increase the film thickness. Initially at 0.1 M oxidantconcentration P3MeT thin films (thickness ∼370 nm) shows gran-ular porous surface morphology with very small grains (Fig. 6a).The thickness of the film is found to increase (∼435 nm) at0.15 M oxidant concentration showing granular surface morphol-ogy with increased grain size simultaneously reduced porosity(shown by circles in Fig. 6b), which might be one of the causesfor the reduction of the roughness of the film. As the oxi-dation concentration increases to 0.2 M the P3MeT thin film(thickness ∼514 nm) shows smoother surface morphology withrounded and larger grains and indistinguishable grain bound-aries with more reduction in the pores (Fig. 6c). The surface
morphology was found more uniform with increased grain sizeas the thickness of the film increased, similar results wereobserved in case of vacuum evaporated aluminium thin films[28].
˛0 = absorption coefficient, ‘h�’ is the photon energy and ‘n’ is aconstant. The value of n depends on the probability of transition; ittakes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden,
Fig. 7. Contact angle images of poly (3-methyl thiophene) thin film
.5. Wettability test
Wettability of the thin film is one of the important proper-ies and can be affected by various factors. Among those, surfacenergy and surface roughness are the leading factors for the wet-ability. When the surface energy is lowered, the hydrophobicity isnhanced [29]. Also the surface roughness strongly influences theettability of the thin film [30]. The contact angle images of P3MeT
hin films for different oxidant concentrations (0.1 M, 0.15 M and.2 M) are as shown in Fig. 7a, b and c respectively. It is clear fromhe figure that the water contact angle of the thin film decreasesrom 93◦ to 83◦ with increase in the oxidant concentration mak-ng the film hydrophilic in nature. This might be due to increasen the thickness of the film which helps to reduce the defects andhe roughness of the film which can be seen more clearly from SEMmages.
.6. Optical properties
The optical absorbance and transmittance of the P3MeT thinlms is as shown in Fig. 8. It gives �max = 498 nm representing �–�*
0
0.05
0.1
0.15
0.2
0.25
900800700600500400300Wavelength (nm)
Abs
orba
nce
bca
ig. 8. Absorbance spectra of poly (3-methyl thiophene) thin films with differentxidant concentration (a) 0.1 M (b) 0.15 M and (c) 0.2 M.
ifferent oxidant concentrations (a) 0.1 M (b) 0.15 M and (c) 0.2 M.
electronics transitions in thiophene ring. The P3MeT thin filmsshow an increase in optical absorbance with simultaneous decreasein transmission due to increase in the thickness (i.e. concentrationof dopant). This might be due to the increase in grain size [31].
3.7. Optical band gap
Fig. 9 gives the graph of (˛th�)2 as a function of h�. From theabsorption data, the band gap energy was calculated using formula
= [˛o (h� − Eg)n]/(h�)
where ‘Eg’ is the separation between bottom of the conductionband and top of the valence band, = absorption of thin film and
indirect allowed and indirect forbidden transition respectively.
Band Gap
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
2.32.11.91.71.51.31.10.90.70.5Photon energy (eV)
(hνα
t)2(e
V/c
m)2
cba
Fig. 9. Optical band gap of poly (3-methyl thiophene) thin films at different oxidantconcentrations (a) 0.1 M (b) 0.15 M and (c) 0.2 M.
Table 1Optical band gap and refractive index of the P3MeT thin films for various oxidant concentrations.
Sr. No. Oxidant concentration (mol) Thickness of the film (nm) Band gap (eV) Refractive index (n)
01 0.1 370 2.01 1.71 ± 0.01
to
3
(m
n
w–
trtmplcmdsofi
4
doTftwi
A
a
[
[
[
[
[
[
[
[[
[
[[
[
[
[
[
[
[[
[(2000) 754–5760.
02 0.15 435
03 0.2 514
The observed values of band gap are tabulated in Table 1. Fromhe table it is seen that there is a decrease in the optical band gapf the P3MeT thin films due to increase in the thickness of the film.
.8. Refractive index
The refractive indices (n) of the chemical bath deposited poly3-methyl thiophene) thin films were calculated by using analytical
ethod using following formula [12].
=[
ns2Tf + ns
(1 +
√Rf
)2
Tf + ns
(1 −
√Rf
)2
]1/2
here ns – refractive index of the substrate, Tf – transmittance, Rf Reflectance.
The values of refractive indices of the poly (3-methyl thiophene)hin films are given in Table 1. The refractive index of the mate-ial depends on the free volume packing density, polarizability andhe difference between the experimental optical wavelength and
inimum absorption wavelength [32]. Higher density and largerolarizability of the material increases the refractive index, for the
arger thickness material the light travels slowly through it, whichauses enhance in the refractive index. Generally aromatic poly-ers possess higher refractive index than the aliphatic polymers
ue to its better packing density and higher polarizability. From theurface morphology it can be seen that as the concentration of thexidant increases the grain size increases which may be responsibleor scattering losses, which may be one of the reason for increasen refractive index.
. Conclusion
Poly (3-methyl thiophene) thin films were successfullyeposited by chemical bath deposition method; the variation ofxidant concentration strongly affects the properties of these films.hese thin films show change in the structure of the film as seenrom XPS studies, with change in oxidation concentration. Thehickness, absorbance, and refractive index of the films increasehereas the transmittance and band gap decreases due to increase
n oxidant concentration.
cknowledgements
The author S.V. Kamat is thankful to Council of Scientificnd Industrial Research (CSIR) for the award of Senior Research
[
[[
1.97 1.73 ± 0.011.95 1.77 ± 0.01
Fellowship (SRF). One of the authors Vijaya Puri gratefully acknowl-edges the award of Research Scientist from University GrantCommission (UGC), India.
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