Swift heavy ion irradiation induced modifications in the optical band gap and Urbach’s tail in polyaniline nanofibers

Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806

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Swift heavy ion irradiation induced modifications in the optical band gap and Urbach’s tail in polyaniline nanofibers Somik Banerjee, A. Kumar ⇑ Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, Assam, India

a r t i c l e

i n f o

Article history: Received 17 June 2011 Received in revised form 1 September 2011 Available online 10 September 2011 Keywords: Polyaniline nanofibers Swift heavy ions Optical band gap Urbach’s tail Quantum confinement

a b s t r a c t Optical band gap and Urbach tail width of HCl and CSA doped polyaniline (PAni) nanofibers and the ion beam induced modifications in the band gap and Urbach’s tail of the samples have been studied employing UV–Vis absorption spectroscopy. All the major bands appearing in the FTIR spectra exhibit a decrease in intensity and broadening in their band widths upon interaction with the highly energetic ion beams. This suggests that SHI irradiation induces chain-scissioning events in the PAni nanofibers. An interesting result that comes out from the FTIR analysis is a transition from the benzenoid to quinoid states in the PAni chains, which reveals that there is a decrease in the degree of conjugation in the polymer upon irradiation. Optical absorption studies indicate three direct allowed transitions at 2.64, 3.61 and 4.08 eV for HCl doped PAni nanofibers and at 2.62, 3.49 and 4.02 eV for the CSA doped PAni nanofibers. The optical band gap is found to increase with increasing ion fluence which may be attributed to the reduction in the fiber diameters upon irradiation, which is corroborated by TEM analysis. Increase in the optical band gap also points out to a decrease in the conjugation length due to the larger torsion angles between the adjacent phenyl rings of the polymer with respect to the plane of the nitrogen atoms, which is also supported by FTIR results. The Urbach tail width decreases with increasing ion fluence indicating that structural disorders are annealed out of the PAni nanofibers which is also observed from the plots of (ahm)2 against photon energy (hm) for HCl doped PAni nanofibers. The quantum confinement effect is confirmed by fact that a band gap exhibits a linear dependence on the inverse of the square of the radius of the PAni nanofibers. Infact, the increase in the optical band gap may be a combined effect of the decrease in the Urbach band width and the quantum confinement effect. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Research in the field of conducting polymers in recent years have been focussed on the development of conducting polymer nanostructures, which combine the advantages of organic conductors with low dimensionality and also show better processibility. Several types of conducting polymer nanostructures such as nanofibers, nanotubes, nanoparticles, nanowires and nanoribbons have been synthesized by several techniques including microemulsion, soft and hard template methods, interfacial polymerization, electrospinning, etc. [1–7]. These nanostructures have applications in sensors, memory devices, microelectronics, etc. [8–10] and in majority of the cases show better efficiency than their bulk counterpart. Among the family of p-conjugated polymers, polyaniline (PAni) has attracted much attention because of its properties such as special doping mechanism, good thermal and environmental stability and high conductivity. Conductivity of PAni can be modified

⇑ Corresponding author. E-mail address: [email protected] (A. Kumar). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.09.004

through the variation of either the number of protons, the number of electrons or both [11]. Among the PAni nanostructures, PAni nanofibers have attracted a lot of attention because of their easy synthesis, high processibility, their unique applications in mesoscopic physics and fabrication of nanoscale devices. 1D nanostructures are believed to be the ideal systems for investigation of the dependence of electrical, thermal, optical and mechanical properties on quantum confinement and dimensionality [12]. Ion irradiation of polymers can be used to induce irreversible modifications of their macromolecular structure, in a controlled way, leading to remarkable changes in their chemical, optical, electrical, mechanical, morphological and thermal properties [13,14]. Modern technologies demand polymers with specific properties that can be achieved by using ion beams. The effects of ion–polymer interactions are more pronounced and quite different when they are subjected to swift heavy ions (SHI) irradiation. Irradiation of polymers with swift heavy ions leads to the change of initial structure by cross-linking, chain-scission and emission of atoms, molecules and molecule fragments [15,16]. The effect of high energy heavy ion irradiation on the chemical and optical properties of polymers has been investigated by several groups [17–20]. The

S. Banerjee, A. Kumar / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806

electrical properties of irradiated polymer films have also been studied extensively. It has been observed by Hussain et al. [21,22] that conducting polymers thin films show improvement in conductivity and increase in the degree of crystallinity upon 160 MeV Ni12+ ion irradiations of fluences between 5  1010 and 3  1012 ions cm2. The increase in dc conductivity has been attributed to the creation of defects, which produce new charge carriers (polaron) whereas the enhanced degree of crystallinity is due to production of close packed regions with aligned chains by chain folding or cross-linking. Enhancement in electrochemical stability of the polymers has also been observed. However, the crystallinity of most polymers decreases upon irradiation. A decrease in crystallinity of PAni upon irradiation with 70 MeV carbon ions of fluence as high as 4.5  1013 ions cm2 has also been reported [23,24]. Ramola et al. have reported that above deposition energy in the range 50–70 keV/lm, the crystallinity of the polymer films increases whereas at deposition energy higher than 200 KeV/lm, the polymer gets destructed and as such the degree of crystallinity decreases [25,26]. A change in the crystallinity of polyethylene terephthalate and polyvinylidene fluoride (PVDF) polymers upon irradiation by 180 MeV Ag14+ ions in the fluence range 1  1010– 6.6  1012 ions cm2 have also been reported [27,28] and has been correlated to the crystallinity changes to the ‘secondary radiationinduced crystallization process’. This process has also been observed in c-rays irradiated polypropylene by Mateev and Karageorgiev [29]. Recently the authors have investigated the SHI irradiation effects on PAni nanofibers. It has been observed that upon SHI irradiation PAni nanofibers are fragmented and amorphized [30]. Transmission electron microscopy reveals that the average diameter decreases from 29.35 to 9.45 nm for the HCl doped PAni nanofibers and from 50 to 11.38 nm for the CSA doped PAni nanofibers upon SHI irradiation [30]. The typical average lengths of the HCl and CSA doped PAni nanofibers varies from about 800 nm in the pristine form to about 100 nm upon irradiation with 90 MeV O7+ ions at fluence of 1  1012 ions cm2. Micro-Raman studies of the irradiated PAni nanofibers indicate a benzenoid to quinoid transformation upon exposure to SHI irradiation [31]. In this paper, we investigate the optical properties of the PAni nanofibers and the fluence dependent variations in the optical band-gap and the Urbach energy upon irradiation by 90 MeV O7+ ions with fluence varying from 3  1010 to 1  1012 ions cm2. 2. Experimental details Aniline (p.a Merck) was distilled under reduced pressure prior to use. All other chemicals were analytical grade reagents. Polyaniline nanofibers doped with 1 M hydrochloric acid (HCl) and 1 M camphor sulfonic acid (CSA) synthesized using interfacial polymerization technique [6] were purified by washing with HPLC grade methanol, filtered several times and dispersed uniformly in a 2% PVA solution for casting thin films (50 lm) on 1 cm2 glass slides for irradiation purpose. The samples were designated as sample S1 and S2, respectively. 90 MeV O7+ ions with mean projected range much larger than the thickness of the films were used to irradiate the PAni nanofibers at normal beam incidence at different fluences of 3  1010, 3  1011 and 1  1012 ions cm2 using the 15UD Pelletron accelerator at the Inter University Accelerator Centre (IUAC), New Delhi under very high vacuum (106 Torr). The fluence was controlled by controlling the time up to which the samples were irradiated. The beam current was fixed at 0.5 particle nano ampere (pnA). FTIR spectra of the samples were acquired using a Perkin Elmer spectrum 100 FTIR spectrophotometer. UV–Visible spectroscopy was recorded employing a Shimadzu 2450 UV–Visible spectrophotometer by

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dispersing equal amount of the samples in milli-Q water at room temperature. 3. Results and discussion 3.1. FTIR spectroscopy Fig. 1(a and b) shows the FTIR spectra of the pristine and irradiated PAni nanofibers doped with HCl and CSA, respectively. The vibrational bands at 1670 and 3300 cm1 occur due to the N–H stretching vibration of the benzenoid units of the emeraldine form of PAni. The peaks at 1400 and 1460 cm1 are assigned to the C@C stretching vibrations of the benzenoid and quinoid units of PAni, respectively. The band at 1199 cm1 is ascribed to the C–N bending vibration mode. The band at 1122 cm1 has been ascribed to the C–H in plane deformation while the peaks at 600 and 660 cm1 are assigned to the C–H out of plane deformations. The variations in the band widths and band shifts have been studied quantitatively by deconvoluting the FTIR spectra of the pristine and 90 MeV O7+ ion beam irradiated PAni nanofibers using Lorentzian oscillation curves corresponding to the IR-active modes of PAni nanofibers, in the range of 500–4000 cm1. Each FTIR spectra for the pristine and irradiated samples have been normalized with respect to the maximum value prior to deconvolution using Lorentzian oscillation curves. The detailed position, intensity and band widths of the deconvoluted peaks are listed in Table 1. Although there is a negligible change in the peak positions, the intensity and band widths show remarkable variations with the increase in the irradiation fluence. Fig. 2(a–d) shows the deconvolution of the sub-peaks representing C@C stretching of the benzenoid and the quinoid resonant structures for the pristine and irradiated samples in the FTIR spectra in the range 1360– 1520 cm1 using Lorentzian oscillation curves. A comparison of the deconvoluted sub-peaks representing the C@C stretching of the benzenoid and the quinoid ring for the pristine and irradiated samples has been shown in Fig. 3. The intensity of the peak at 1400 cm1 due to the C@C stretching of the benzenoid ring for the HCl doped PAni nanofibers (S1) irradiated at the fluence of 1  1012 ions cm2 decreases as compared to that for the pristine, while in case of the CSA doped PAni nanofibers (S2) the intensity of the peak decreases almost to three times than that of the pristine. The FWHMs for the HCl doped and CSA doped PAni nanofibers increases by 2.69 and 1.85 cm1, respectively with increasing ion fluence. On the other hand, the peak at 1460 cm1 due to the C@C stretching of the quinoid ring intensifies for both the HCl and CSA doped PAni nanofibers upon SHI irradiation as observed from the Table 1. This suggests that there is a partial transformation from the benzenoid to the quinoid structure in the PAni chains upon SHI irradiation. The fact that there is a partial transition from the benzenoid to the quinoid structure in the PAni chains is further corroborated by the decrease in the intensity of the peaks corresponding to the N–H stretching at 1670 and 3300 cm1 as can be observed from the Table 1. The intensity of the N–H stretching peak at 1670 cm1 decreases from 23.9 to 9.3 for the HCl doped PAni nanofiber (sample S1) whereas from 23.8 to 6.0 for the CSA doped PAni nanofibers (S2). A similar decrement in the intensity is also observed for the N–H stretching peak centered at 3300 cm1. The band is also found to broaden with increasing ion fluence and the band width increases by 214.83 cm1 for the HCl doped PAni nanofibers and by 175.95 cm1 for the CSA doped PAni nanofibers. The authors have already conducted a detailed study of the effect of 90 MeV O7+ ion irradiation on the crystalline nature of PAni nanofibers using X-ray diffraction and lR spectroscopy, which also show a decrease in the overall degree of crystallinity and the local range of order (domain length) of PAni nanofiber upon SHI irradiation. The decrease in the degree of crystallinity is also accompanied by the

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S. Banerjee, A. Kumar / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806

Fig. 1. FTIR spectra of pristine and irradiated (a) HCl doped and (b) CSA doped PAni nanofibers.

fragmentation of PAni nanofibers which increases with increasing irradiation fluence. The authors propose that PAni nanofibers are amorphized and fragmented within the core of the tracks. As the ion fluence increases, the tracks overlap and fragmentation increases leading to a reduction in the size of the PAni nanofibers which has been clearly observed from TEM experiments performed by the same authors and is reported elsewhere [30]. The decrease in the intensity of almost all the prominent peaks in the FTIR spectra indicates that the interaction of the highly energetic ion beams with the PAni nanofibers induces chain scissions that increases with increasing ion fluence leading to a reduction in the crystalline nature and induces fragmentation of the PAni nanofibers upon ion beam treatment. The chain scissioning events lead to the emission of hydrogen atoms and molecular fragments from the aromatic rings of the polyaniline nanofibers backbone resulting in the transformation of benzenoid into the quinoid structure. At lower irradiation fluence the transformation is partial and increases with the increase in fluence and beyond a critical fluence complete transformation from the benzenoid to the quinoid structure is expected. However, PAni nanofibers being highly radiation sensitive burn out before complete transformation of the benzenoid to quinoid structure if the fluence is increased above 1012 ions cm2 .

on k = 800 nm. The peak at 440 nm is ascribed to the transitions between the p and polaron bands, and the second peak centered at 800 nm is due to the polaron–p⁄ band transitions. It is observed that with the increase in irradiation fluence, the peak at 290 nm is blue-shifted, which indicates that the size of the PAni nanofibers decreases with the increase in irradiation fluence. The authors have earlier reported that irradiation of PAni nanofibers with 90 MeV O7+ ions leads to the fragmentation of the nanofibers clearly observed from TEM experiments [30]. The optical absorption coefficient (a) has been determined from the absorption spectra using Eq. (1). After correction for reflection, the absorption coefficient (a) has been calculated from the absorbance (A), using the relation:

3.2. Optical band gap

where A is the absorbance and d is the thickness of the quartz cuvette used for the UV–Vis experiments. The optical band gap may be evaluated for the values of the absorption coefficient using the following relation:

Fig. 4 (a and b) shows the absorption spectra of the unirradiated and irradiated PAni nanofibers doped with HCl and CSA, respectively plotted in the wavelength range 200–900 nm. The unirradiated sample shows three prominent peaks which are the signature of PAni in the emeraldine salt form. In case of conducting polymers, there are always two distinct defect bands within the band gap, arising from a destabilization of the highest occupied band (HOMO), which leads to the lower defect band and a stabilization of the lowest unoccupied band (LUMO) leading to the highest defect band [32]. Stafström et al. have suggested that unlike other conducting polymers, in case of PAni the doubly charged spinless bipolarons become unstable on a polyemeraldine chain resulting in the formation of two polarons, which separates to yield a polaron lattice. The fact that the Pauli susceptibility increases linearly with the degree of protonation in PAni confirms the existence of a polaron lattice in PAni. Thus, in PAni, instead of two bands, a single broad polaron band appears deep in the gap, which is also supported by band structure calculations [33]. The absorption peak occurring at k = 290 nm can be attributed to the p–p⁄ band transition. There are two visible region bands, one at around k = 440 nm and the other, a broad peak centered

I ¼ I0 expðaxÞ

ð1Þ

The Eq. (1) may be written as

a¼

    2:303 I 2:303 ¼ log A; x I0 x

ð2Þ

where x is the thickness of the sample;

a ¼ 2:303

a¼

X

  A d

ai ¼

X Ai ðhm  Egi Þmi ; hm i

ð3Þ

ð4Þ

where the value of Egi and mi correspond to the energy and the nature of the particular optical transition with absorption coefficient ai. For allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, the value of mi corresponds to 1/2, 2, 3/2 and 3, respectively [34]. In an allowed direct transition the electron is simply transferred vertically from the top of the valence band to the bottom of the conduction band, without a change in momentum (wave vector). On the other hand, in materials having an indirect band gap, the bottom of the conduction band does not correspond to zero crystal momentum and a transition from the valence to the conduction band must always be associated with a phonon of the right magnitude of crystal momentum. In the present work, since we are dealing with nanostructures viz., PAni nanofibers, we have attempted to determine the value of mi without pre-assuming the nature of the optical transition in the PAni nanofibers.

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S. Banerjee, A. Kumar / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806 Table 1 Position, intensity and FWHMs of the prominent FTIR peaks of pristine and 90 MeV O7+ ion beam irradiated PAni nanofibers doped with HCl and CSA. Peak (cm1)

600

1122 1400

1460

1670 3350 a b

Designation of the peaks

C–H out of plane deformation C–H in plane deformation C@C stretching vibration of benzenoid ring C@C stretching vibration of quinoid ring Due to N–H stretching vibration of polyaniline

Samples

Lorentzian deconvolution of the FTIR peaks for pristine and irradiated samples at different fluences Position

Intensity

FWHM

Pristine

3  1010

3  1011

3  1012

Pristine

3  1010

3e1011

1  1012

Pristine

3  1010

3  1011

1  1012

S1 S2b

603.37 603.13

602.93 605.77

603.43 603.05

602.93 601.79

23.8 16.7

13.5 15.0

12.7 12.0

10.2 5.0

30.22 27.13

33.93 30.04

34.54 31.75

36.71 40.68

S1 S2 S1 S2

1122.41 1121.61 1401.03 1400.73

1121.00 1119.20 1401.08 1400.93

1119.80 1118.62 1401.39 1400.95

1122.41 1119.68 1401.93 1401.35

49.4 34.5 19.2 13.1

31.5 31.5 12.1 12.3

29.9 27.5 10.1 9.3

26.3 12.9 9.7 4.2

99.57 89.80 24.51 24.37

102.39 98.72 24.95 24.52

106.59 105.87 25.51 25.40

109.29 108.11 27.20 26.22

S1 S2

1459.90 1459.30

1459.28 1458.83

1460.61 1458.21

1458.92 1457.94

3.9 1.7

4.9 5.2

7.2 5.4

7.8 7.8

67.87 101.13

48.44 64.40

45.03 54.80

44.97 44.42

S1 S2 S1 S2

1669.00 1669.17 3277.90 3287.32

1666.96 1669.00 3296.43 3286.34

1669.60 1668.40 3286.42 3271.83

1668.43 1669.70 3279.89 3273.51

23.9 23.8 55.1 35.1

18.4 15.4 36.7 25.3

15.1 14.9 25.1 24.6

9.3 6.0 22.3 9.0

27.95 24.9 731.15 703.74

28.66 27.2 911.17 714.07

34.93 28.4 940.92 718.21

40.25 29.7 945.98 879.69

a

S1 stands for the HCl doped PAni nanofibers. S2 stands for the CSA doped PAni nanofibers.

Fig. 2. Lorentzian deconvolution of the FTIR spectra within 1360–1520 cm1 for (a) pristine HCl doped PAni nanofibers and those irradiated at a fluence of (b) 3  1010, (c) 3  1011 and (d) 1  1012 ions/cm2.

Now, the Eq. (4) can be written as

d½ln ðahmÞ m ¼ dðhmÞ ðhm  EÞ

ð5Þ

for a particular value of mi and Egi (say mi = m and Egi = E). The plot of d[ln (ahm)]/d(hm) vs. hm will show a discontinuity at a particular value hm = E where a possible optical transition might have occured corresponding to a particular band-gap E = Eg1. Fig. 5 shows the plot

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Fig. 3. Comparision of the deconvoluted sub peaks representing the C@C stretching vibration of the benzenoid and quinoid rings of PAni nanofibers.

of d[ln (ahm)]/d(hm) vs. hm for the pristine HCl and CSA doped PAni nanofibers. It is observed that there are three discontinuities corresponding to three possible optical transitions (at E = Eg1, E = Eg2 and E = Eg3) in both the pristine HCl and CSA doped PAni nanofibers. In order to get the m values corresponding to the three optical transitions at 2.64, 3.61 and 4.08 eV for HCl doped PAni nanofibers and 2.62, 3.49 and 4.02 eV for the CSA doped PAni nanofibers, we plot ln (ahm) as a function of ln (hm  E), where E = Eg1, Eg2 and Eg3 for the three types of optical transitions as shown in Fig. 6(a–c). The slopes of the plots have been determined by performing a linear fitting of the experimental data. It has been observed that for all the three types of optical transitions at E = Eg1, E = Eg2 and E = Eg3 for the HCl and CSA doped PAni nanofibers the slope is quite near to 0.5. This confirms that the three types of optical transitions in both the HCl and CSA doped PAni nanofibers are of allowed direct nature. The direct optical band gap of the pristine HCl and CSA doped PAni nanofiber samples have also been determined by plotting (ahm)2 against the photon energy (hm). The relation between the optical absorption coefficient (a) for a direct transition and the photon energy (hm) was given by Fahrenbruch and Bube [35]:

ahm ¼ Aðhm  Eg Þ1=2

ð6Þ

Fig. 5. Plots of d[ln (ahm)]/d(hm) vs. hm for the pristine HCl and CSA doped PAni nanofibers.

where A is a constant, h is Planck’s constant, m is the frequency of the radiation and Eg is the optical energy gap for direct transition. Fig. 7 shows the plot of (ahm)2 vs. photon energy (hm) for the pristine HCl and CSA doped PAni nanofibers. The value of the optical energy gap Eg is determined from the intersection of the extrapolated line with the photon energy axis (at a = 0). The direct optical band gap of the pristine sample is found to be 4.23 and 4.00 eV for the PAni nanofibers doped with HCl and CSA, respectively, which is similar to the values (Eg3) obtained from the d[ln (ahm)]/d(hm) vs. hm plots for the same samples. Extrapolating two straight line portions of the plots to a = 0, one can get two more activation energies viz., 2.31 and 2.98 eV for the PAni nanofibers doped with HCl and 2.41 and 3.31 eV, corresponding to the doping induced polaron defect levels present within the band gap in the samples, which also correspond to the energies (Eg1 and Eg2) of the other two discontinuities observed in Fig. 5. The direct optical band gap for the irradiated samples have been determined directly from the plot of (ahm)2 vs. photon energy (hm), since the optical transitions in PAni nanofibers have been found to be of the allowed direct nature. Fig. 8 (a and b) shows (ahm)2 vs. hm plots for the HCl and CSA doped PAni nanofibers, respectively, irradiated at a fluence of 3  1010, 3  1011 and 1  1012 ions cm2. The optical band gaps (Eg) determined from these plots have been tabulated in Table 2. It is observed that the optical band gap in-

Fig. 4. UV–Visible spectra of pristine and irradiated PAni nanofibers doped with (a) HCl and (b) CSA.

S. Banerjee, A. Kumar / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806

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Fig. 6. (a) Linear fit of ln (ahm) vs. ln (hm  E) where E = Eg1, (b) Linear fit of ln (ahm) vs. ln (hm  E) where E = Eg2 and (c) Linear fit of ln (ahm) vs. ln (hm-E) where E = Eg3 for both the HCl and CSA doped PAni nanofibers.

Fig. 7. (ahv)2 vs. hm plots for the pristine PAni nanofibers doped with HCl and CSA.

creases with increasing irradiation fluence. This increase in the direct optical band gap values can be attributed to two factors: (a) the fragmentation of the PAni nanofibers upon SHI irradiation [30], and (b) a decrease in the extent of conjugation which suggests that the adjacent phenyl rings of the polymer have larger torsion angles with respect to the plane of the nitrogen atoms [36]. This result is corroborated by the FTIR results, which show that there is a benzenoid to quinoid transition in the PAni nanofibers upon SHI

irradiation that is an evidence of the decrease in the conjugation length of PAni nanofibers. It has also been observed from the (ahm)2 vs. (hm) plots of the irradiated PAni nanofibers doped with HCl (Fig. 8a), that the defect level observed at 2.31 eV for the pristine material cannot be detected for the irradiated samples. This suggests that the defect must have annealed out upon SHI irradiation. SHI irradiation can anneal out defects as well as create defects within the forbidden energy gap of materials depending upon the nature of the material irradiated and the ion beam parameters such as fluence, energy and charge state of the ion. The defect level at 2.98 eV observed in the (ahm)2 vs. (hm) plot for the pristine PAni nanofiber is found to shift to about 3.38 eV and remains almost similar for the samples irradiated at different fluences. However, both the defect levels in the CSA doped PAni nanofiber samples still continue to exist at almost the same positions (2.41 and 3.31 eV) even after irradiation. In fact, the defect levels in CSA doped PAni nanofibers become more prominent upon SHI irradiation, which is an indication of an increase in the density of states in the defect levels. The reason as to why the defect level at 2.41 eV still persists after SHI irradiation in the CSA doped PAni nanofibers is not clear. One might assume that the parameters of the ion beam being exactly same for both the samples, the observed phenomenon as discussed above has to do with the property of the dopant CSA, which is quite different from HCl since CSA has a long side chain and is much bulkier than HCl.

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Fig. 8. (ahv)2 vs. hm plots for the irradiated PAni nanofibers (a) doped with HCl and (b) doped with CSA.

Table 2 Optical band gap (Eopt/Eg3), defect levels (Eg1 and Eg2) and Urbach tail width (eV) of PAni nanofibers. Fluence

Radius of the PAni nanofibers (nm) a

Pristine 3  1010 3  1011 1  1012 a b #

b

Eg2 (eV)#

Eopt (Eg3) (eV) a

S1

S2

S1

14.7 ± 0.2 9.9 ± 0.1 6.0 ± 0.3 4.7 ± 0.2

25.0 ± 0.1 20.6 ± 0.1 10.0 ± 0.2 5.7 ± 0.3

4.2 ± 0.2 4.5 ± 0.3 5.3 ± 0.2 5.6 ± 0.2

b

a

Eg1 (eV)#

Urbach energy (eV)

b

S1a

S2b

S1a

S2b

2.3 – – –

2.4 2.4 2.5 2.4

3.0 ± 0.2 2.0 ± 0.1 1.8 ± 0.1 1.0 ± 0.1

2.2 ± 0.1 1.8 ± 0.1 1.3 ± 0.1 0.7 ± 0.2

S2

S1

S2

3.8 ± 0.2 4.4 ± 0.2 4.5 ± 0.1 4.8 ± 0.3

3.0 3.4 3.4 3.4

3.3 3.5 3.3 3.3

S1 stands for PAni nanofibers doped with HCl. S2 stands for PAni nanofibers doped with CSA. The values of Eg2 and Eg1 may have errors up to 5%.

Fig. 9. Plot of ln a vs. hm for the pristine and irradiated PAni nanofibers (a) doped with HCl and (b) doped with CSA.

S. Banerjee, A. Kumar / Nuclear Instruments and Methods in Physics Research B 269 (2011) 2798–2806

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3.3. Urbach’s tail In case of non-crystalline materials there can be dense localized energy states, near the valence (HOMO) and conduction (LUMO) band edges, known as ‘‘Urbach energy bands’’. The absorption coefficient below the fundamental absorption edge for the non-crystalline materials show an exponential dependence on the photon energy (hv) which follows the Urbach formula [37]:

aðmÞ ¼ a0 exp

  hm Eu

ð7Þ

where a0 is a constant, Eu is an energy which is interpreted as the width of the tail of localized states in the forbidden band gap, m is the frequency of radiation and h is Planck’s constant. This region is attributed to the electronic transition between a localized band tail and an extended band. The origin of Eu is considered as thermal vibrations in the lattice [38]. The above relation has been first proposed by Urbach [37] to describe the absorption edge in alkali halide crystals. This relation has also been found to be applicable for amorphous materials. Fig. 9(a and b) shows the logarithm of the absorption coefficient a plotted as a function of the photon energy (hm) for the pristine and irradiated HCl and CSA doped PAni nanofibers. The values of the Urbach energies (Eu) for the pristine and irradiated PAni nanofibers doped with HCl and CSA have been calculated by taking the reciprocal of the slopes of the linear portion of these curves and are listed in Table 2. According to Cody’s model the Urbach tail width (e) can be written as:

eðT; XÞ ¼

  KBh KBh 1 K B hX þ þ 2r0 2r0 r0 expðh=TÞ  1

ð8Þ

where KB is the Boltzmann constant, X is a dimensionless parameter called structural disorder parameter and h is related to the Debye temperature (hD) by hD  4h/3. The second term in the Eq. (8) represents the contribution of electron–phonon and exciton–phonon interactions and the third term originates due to the mean square deviation of the atoms from a perfectly ordered lattice due to structural disorder. In the present work, since all the optical absorption spectra have been recorded at room temperature, the decrease in the Urbach tail width in PAni nanofibers upon SHI irradiation is supposed to be only due to the decrease in structural disorder. The optical band gap calculated using UV–Vis spectroscopy is actually less than the actual band gap of the material. It is due to the band tailing that we are only able to measure the apparent band gap and not the actual one. It is observed that the Urbach’s tail width decreases with increasing ion fluence, which indicates that the localized states due to disorder are annealed out upon SHI irradiation. This leads to the further separation of the valence and the conduction band edges and hence the band gap of the material increases. Similar type of annealing out of disorders and subsequent increase in the band gap has been reported earlier [39]. However, the increase in band gap may be a combined effect of the decrease in the Urbach tail width and the quantum confinement effect, which has been discussed in the following section. 3.4. Quantum confinement effect Another probable reason for the increase in the band gap upon SHI irradiation is the reduction in the fiber diameter upon SHI irradiation, which reveals the existence of quantum confinement effect. Table 2 shows that the absorption edge energy increases as the diameter of the nanofiber decreases, which proves the quantum size effect. According to the theoretical treatment for this small fiber size, the electron and hole wave functions are individ-

Fig. 10. Bandgap vs. the inverse of the square of the radius of the pristine and irradiated nanofibers illustrating the quantum confinement effect in PAni nanofibers.

ually confined. The gap energy, E, is inversely proportional to the square of the particle radius, R, as follows [40]:

E ¼ Eg þ

hp 2lR2

ð9Þ

where Eg is the bulk band gap of PAni nanofibers,  h ¼ h=2p, h is Planck’s constant, and l is the reduced mass of electrons and holes (0.08 m0). Though the Eq. (9) is valid for spherical particles, the equation has also been used for one dimensional nanostructures such as Si nanowires [41,42] and Ge/Si core/shell nanowire heterostructures [43] using a cylindrical confinement potential and effective mass approximation. Fig. 10 shows the absorption edge energy as a function of the inverse of the square of the average radii of the PAni nanofibers. A linear relationship confirms the quantum size effect in PAni nanofibers leading to the enhancement of the optical band gap upon SHI irradiation. The corresponding bulk band-gap energy obtained by interpolating the straight line at 1/R2 = 0 is found to be 3.89 eV for the HCl doped PAni nanofibers whereas for the CSA doped is 3.96 eV. 4. Conclusions We have studied the optical band gap and the Urbach tail width of HCl and CSA doped PAni nanofibers and also investigated the effect of SHI irradiation on the band gap and Urbach’s tail. It has been observed that PAni nanofibers exhibit three allowed direct transitions at 2.64, 3.61 and 4.08 eV for HCl doped and at 2.62, 3.49 and 4.02 eV for the CSA doped PAni nanofibers. The slight variations in the energies of the allowed optical transitions has been attributed to different fiber diameters in case of the two types of PAni nanofibers (HCl doped  30 nm and CSA doped  50 nm). The optical band gap determined from the (ahm)2 against photon energy (hm) is consistent with the energy discontinuities observed in the plot of d[ln (ahm)]/d(hm) vs. hm, where the possible optical transition might take place. Upon irradiation an increase in the optical band gap is observed which has been attributed to the fragmentation of the PAni nanofibers upon SHI irradiation and also a decrease in the extent of conjugation which suggests that the adjacent phenyl rings of the polymer have larger torsion angles with respect to the plane of the nitrogen atoms, which is also corroborated by the FTIR results which indicate a benzenoid to quinoid transition and is an indication of reduction in p-conjugation. The structural disorders have been found to anneal out as a result of

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SHI irradiation as confirmed by the decreasing value of Urbach tail width with increasing ion fluence, which enhances the band gap. Acknowledgment The authors would like to acknowledge Dr. D.K. Avasthi for his valuable suggestions during the experiments. The pelletron staff of Inter University Accelerator Centre (IUAC), New Delhi is highly acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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