1. Introduction

Due to their exclusive optical properties, a wide variety of electrically conducting polymers have been studied. Research on conducting polymers has peaked in the past two decades with a broad range of applications such as light emitting diodes (LEDs)^[1], electrochromic displays^{[2, 3]}, and other optoelectronic applications due to their unique permutation of optical, electrical and magnetic properties. Along with the conducting polymers, polypyrrole (Ppy) is for the most part extensively studied, because the monomer (pyrrole) is easily oxidized, water soluble, and commercially available. Hence, Ppy presents several advantages including environmental stability, good redox properties, and the ability to give high electrical conductivities and sensitive physicochemical properties^[4], and applications such as chemical and biological sensors^[5], nanoscale actuators^[6], drug delivery^[7], electronic chemical sensors^{[8, 9]}, supercapacitors^[10], and thermal transducers of NIR radiation^[11]. The intrinsic properties of Ppy are highly dependent on synthesis conditions, i.e. electropolymerization conditions such as deposition mode, deposition time, and current density. Furthermore, there is a requirement to treat the surface of Ppy thin films with the help of an external source, in order to get the improved properties of Ppy thin films such as tuning of optical band gap and surface modification. Also, Ppy was scrutinized in recent years by several different spectroscopic techniques with the aim to understand the chemical and electronic structure of this polymer. High energy electron irradiation on the surface of electrodeposited Ppy thin films is useful for tuning the optical properties in the visible region. Recently, Hong $et$ $al$. reported fine characteristics tailoring of organic and inorganic nanowires using focused electron-beam irradiation. They examined the changes in the structural, optical, and electrical properties of compartments of single poly (3-methylthiophene) P3MT and TiO$_{2}$ NWs treated with a focused E-beam under different conditions^[12].

In this study, we focus on the effect of, high energy (2 MeV) electron irradiation on the surface of Ppy thin films for the optical properties.

2. Experimental

2.1 Materials

All reagents were of analytical grade. Pyrrole (Spectrochem, Mumbai, India 99%) was distilled under reduced pressure prior to use and kept refrigerated until use. Acetone and H$_{2}$SO$_{4}$ (Loba Chemie) were used as received. Solutions were prepared in double distilled water.

2.2 Substrate cleaning

Initially, the substrates were polished with zero grade emery paper and then washed with distilled detergent and rinsed multiple times with distilled water. The these mechanically cleaned substrates were treated with detergent in an ultrasonic cleaner for 10 min with repeated washing with distilled water and detergent and finally air dried substrates were treated with acetone.

2.3 Electrochemical synthesis of Ppy

The Ppy cauliflowers were synthesized at room temperature by electrodeposition. The room temperature deposition avoids the oxidation and corrosion of metallic substrates. The electrochemical polymerization of pyrrole results in the formation of a cauliflower-like structure on the substrate. Preparative parameters such as deposition potential, deposition time and concentration of the precursor were optimized. In the typical synthesis, (0.1 M) pyrrole and (0.5 M) sulfuric acid are used as the monomer and electrolyte respectively. Ppy thin films were prepared on stainless steel ITO substrates by using the potentiostatic electrodeposition technique at a deposition potential of –0.8 V versus SCE. An electrodeposition study of the Ppy thin films was made using a three-electrode configuration. The Ppy thin film was deposited on the anode (stainless steel and ITO) and a pure graphite plate was used as the cathode, a saturated calomel electrode (SCE) was used as the reference electrode. A thin black colored Ppy thin film was obtained on the substrate.

2.4 High energy electron irradiation normal to the Ppy surface

The electron beam irradiations of the thin film samples were carried out using an industrial electron accelerator (model ILU-6, Budker Institute of Nuclear Physics, Russia) at the Bhabha Atomic Research Centre (BARC), Mumbai, India. The dose rate was 5 kGy/pass and the energy of the electron pulse was 2 MeV. The details of the set up are described elsewhere^[13]. The samples were irradiated with 10, 30 and 50 kGy doses. The temperature rise in the thin film samples during the irradiation process is not expected to be very high. Therefore, the melt down condition of the samples is excluded.

2.5 Polymer characterization

The structural studies of the pristine and electron irradiated Ppy thin films were made with the help of FTIR spectroscopy. The Fourier transform infrared spectrum (FT-IR) was recorded between 4000 and 450 cm$^{-1}$ at a spectral resolution of 2 cm$^{-1}$ on a Perkin–Elmer 1710 spectrophotometer to identify the bonds of Ppy. The surface morphological study of pristine and electron irradiated Ppy thin films was made with the help of scanning electron microscopy (SEM) using JEOL model, JSM-6360 (LA). UV–vis absorption and transmission spectra were recorded at room temperature and near to normal incidence using a 119 SYSTRONICS UV–vis spectrophotometer. The optical reflectance was recorded using a Steller Net Inc USA reflectometer having a UV–vis light source with CCD detector.

3. Results and discussion

The electron irradiated organic polymers were studied with infrared absorption (FT-IR) spectroscopy. Figures 1(a) and 1(b) show the FT-IR spectra of pristine and 50 kGy dose electron irradiated Ppy thin films. The spectra show that all the observed principal bands are the feature bands of the pristine sample. Also, upon 50 kGy electron irradiation the same main bands are observed, i.e. previous nature of Ppy is not destroyed. The conducting Ppy shows a characteristic ``tail of the electronic absorption band" in the range 4000–2000 cm^-1[14]. The broad band centered at 3600 cm$^{-1}$ corresponds to the N–H stretching vibrations^[15], the absorption band observed at 3257 cm$^{-1}$ may correspond to an aromatic C–H band^[16], and the band observed at 2140 cm$^{-1}$ is maybe due to the doped ions. The isolated or conjugated carbonyl groups may be present in the polymers which are assigned at the 1989 cm$^{-1}$ and 1779 cm$^{-1}$. The C=C band is present at the 1615 cm^-1[16], the stretching and bending vibrations of the C=C band are associated with the 1503 cm$^{-1}$ and 1424 cm$^{-1}$ peaks, respectively^{[14, 16]}. The less intense peak at 1385 cm$^{-1}$ may be attributed to the N–C stretching vibrations^[15]. The band centered at 1293 cm$^{-1}$ may correspond to deformation vibrations^[14] or C–C stretching vibrations^[15]. The absorption peak observed at 1181 cm$^{-1}$ is due to the stretching vibrations of C–N bonds^[15].

Figures 1(c) and 1(d) show the morphological characteristics of the pristine and 50 kGy electron irradiated Ppy thin films. SEM images shows that the substrates were well covered with spherical shaped particles. At high magnification the cauliflower-like structure was observed (not shown here). Upon electron irradiation, the structure of the particle is same but decrease in particle size. Generally, Ppys are low melting point compounds and during electron irradiation there is a possibility of heat generation at the surface and this will produce the splitting of particles and due to tendency, the particles again reunite to form the original shape. During this period, the agglomerated or un-agglomerated particle creates shallow defects. Some authors have reported a lack of apparent morphological change upon ion irradiation at 3 keV, 195 nA/mm$^{2}$ for a total ion dose of 6.8 $\mu$C/mm^2[17].

The UV–vis spectrum of the pristine and electron irradiated Ppy thin films are shown in Fig. 2. The spectrum shows that absorbance values increase with an increase in dose of electron irradiation. This is may be due to an increase in number of particles and this is consistent with the morphological study of electron irradiated Ppy thin films^[18]. The absorption peaks observed at 300 to 450 nm are due to $\pi$–$\pi$* transitions^[18]. The absorption peak shifts towards higher wavelengths with electron dose. The pristine Ppy thin film shows the absorption peak at 366 nm. Also there is another absorption band region above 600 nm and this is associated with the transition of electrons from the valence band to another bipolaron band of Ppy and which confirms the conducting state of polymer^[18].

An optical band gap energy study of electrodeposited pristine and electron irradiated polyporrole thin films is illustrated in Fig. 3. The optical band gap of thin films are calculated from the Tauc's relation as^[20] $\alpha$ $=$ $A(h$$\nu$ – $E_{\rm g})$$^{n}$ / $h \nu$, where $n=$ 1/2 for a direct band gap, $h \nu$ is photon energy, $E_{\rm g}$ is the band gap in eV, $\alpha$ is the absorbance coefficient and $A$ is a constant. The pristine Ppy shows an optical band gap of 2.19 eV. After electron irradiation, the band gap decreases up to 1.97 eV. The number of carbon atoms per cluster increases with high energy (2 MeV) electron doses, which are shown in Table 1. The decrease in band gap and increase in number of carbon atoms per cluster is possibly due to the creation of defects and traps in the gap of higher occupied molecular orbits (HOMO) and lower unoccupied molecular orbits (LUMO). This is coupled with the above optical absorption study and morphological characteristic results.

Figure 4 shows the variation of transmittance, reflectance and refractive index profile of pristine and electron irradiated Ppy thin films in the 300 nm to 800 nm range. In Fig. 4(a), the pristine Ppy shows a higher value of transmittance than the electron irradiated Ppy thin films. Also, in case of reflectance (shown in Fig. 4(b, d)), the 50 kGy electron irradiated Ppy shows a lower value of reflectance than the pristine Ppy thin films. The refractive index profile (Fig. 4(c)) shows the different values of maximum refractive index, with the electron doses at the different wavelengths. The value of refractive index ($n$) was calculated from the relation $n= (1+R+R^{1/2}) / (1-R)$, where $R$ is the normal reflectance of Ppy thin films^[21]. We got the maximum value of refractive index 1.26 for 10 kGy electron irradiated Ppy thin films at 506 nm, shown in Fig. 4(c, b) and Table 1. Figure 4(c) shows that the refractive index not only depends on the wavelength but also on the electron dose. Upon high energy electron irradiation, decreases in transmission, reflectance, and refractive index ($n$) may be attributed to the interaction of electrons with the sigma and pi bonded electrons in the polymer backbone. Upon electron irradiation the Ppy thin film becomes less transparent than the 50 kGy electron irradiated one.

Figure 5 shows the extinction coefficient, optical conductivity, and refractive index dispersion relation with the wavelength. The spectral dependence of the refractive index can be evaluated by using the single oscillator model proposed by Wemple and Di Domenico viz; $n^{2}$ $=$ 1 $+$ [$E_{\rm o}$$E_{\rm d}$ / ($E_{\rm o}^{2}$–$E^{2})$], where $E_0$ is the average excitation energy known as the oscillator energy, $E_{\rm d}$ is the dispersion energy called the oscillator strength, and $E$ is the incident photon energy^[22]. The value of extinction coefficient ($k$) is calculated from the relation as $k =$ $\alpha \lambda$/4$\pi$, where $\alpha$ is the coefficient of absorption, $\lambda$ is the wavelength^[23]. The optical conductivity of electrodeposited and high energy electron irradiated Ppy thin films were sketched using the relation as $\sigma$ $=$ $\alpha nc/4 \pi$, where $c$ is the velocity of light in vacuum. The pristine Ppy sample shows a more diffuse value of extinction coefficient and optical conductivity than that of the electron irradiated Ppy thin films. The optical conductivity and extinction coefficient not only depends on the wavelengths but also on the modified surfaces.

In summary, the electrodeposited Ppy thin films were successfully irradiated with high energy electron with different doses. The surface morphology and optical properties of Ppy thin films were tuned by high energy (2 MeV) electrons. So, surface modified and band gap energy tuned Ppy electrodes will play an important role in the field of electro-chromic or smart windows, with the help of high energy electron irradiation. The structure of Ppy thin film is not destroyed upon electron irradiation. The pristine Ppy films show an optical band gap of 2.19 eV, while the 50 kGy electron irradiated Ppy thin films shows 1.97 eV. The extinction coefficient, refractive index and optical conductivity were tuned with the help of electron irradiation.

Acknowledgements: The authors are very grateful to the Department of Science and Technology, New Delhi, for financial support under the DST-PURSE scheme at Shivaji University, Kolhapur. Also, we are grateful to the group members of ILU-6 of RTDD, BARC, Mumbai for the irradiation experiments.