Nonlinear optical properties of conducting polyaniline and polyaniline–Ag composite thin films

Chemical Physics Letters 477 (2009) 164–168

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Nonlinear optical properties of conducting polyaniline and polyaniline–Ag composite thin films Ali Sezer a, Ullas Gurudas b,*, Brian Collins b, Andrew Mckinlay b, Daniel M. Bubb b a b

Department of Chemistry and Physics, California University of Pennsylvania, California, PA 15419, United States Department of Physics, Rutgers-Camden, Camden, NJ 08102, United States

a r t i c l e

i n f o

Article history: Received 6 April 2009 In final form 23 June 2009 Available online 26 June 2009

a b s t r a c t The intensity dependent refractive indices of polyaniline and polyaniline–silver nanocomposites are measured. While the material are not good candidates for all-optical switching, they do exhibit reversible processes of saturable and reverse saturable absorption at the same wavelength, a trait that makes them suitable materials for optical pulse compression and limiting. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Polyaniline (PANI) is a p-conjugated conducting polymer with desirable electrical, optical, and electrochemical properties [1]. It can be prepared in different oxidation states with high stability, and the photo-physical properties can easily be fine-tuned by simple organic pathways. PANI may also be prepared as a nanocomposite, for example, with silver [2]. Previous research on PANI has noted that the nonlinear optical response of PANI is highly sensitive to the oxidation state [3] and doping level [4,5]. Other nonlinear optical (NLO) chromophores have shown the ability to be tuned as the result of slight alterations to their chemical and physical structure [6], so it stands to reason that PANI may be optimized in a similar fashion. The motivation behind this current work is to investigate the nonlinear optical properties of PANI in the conducting form, both as a nanocomposite with silver and in the native state. PANI has been extensively investigated to determine its suitability for optical switching applications [3–5]. Transmission or nonlinear based optical applications require materials with a large nonlinear refractive index, minimal one- or multi-photon absorption losses, and a fast nonlinear refractive index and response time [3]. There has not been widespread agreement in the literature about the nonlinear optical properties of PANI. Using the z-scan technique [7], Petrov et al. found that a nanocomposites of PANI (in different oxidation states) and polyvinyl alcohol at 532 nm had suitable nonlinear properties for photonics applications [4]. Another study probed the nonlinear optical properties of the emeraldine salt form of polyaniline [5] in the visible and near-IR and found n2 values an order of magnitude less than Petrov et al.’s study, leading to the conclusion that PANI is not promising for

* Corresponding author. E-mail addresses: [email protected] (A. Sezer), [email protected] (U. Gurudas), [email protected] (D.M. Bubb). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.06.070

optical applications due to a large bleaching signal. Finally, Maciel et al. studied the response of thin films and solutions of PANI in the fully reduced and partially oxidized form [3]. They found that PANI in the leucoemeraldine base form was most suitable for optical applications. Once again, however, there were differences between their reported results and earlier work, particularly with respect to the magnitude of the nonlinearity. Despite these disagreements, there are two salient points to be taken from these studies. First, the nonlinear optical properties of PANI are sensitive to the oxidation state and doping level. Second, PANI seems unsuited for optical switching applications, particularly in the visible, due to its large linear absorption. In light of these previous works, we have measured the nonlinear optical properties of both PANI and PANI–Ag using the z-scan technique. In particular, we have observed both saturable (SA) and reverse saturable absorption (RSA) in these films while using a 532 nm picosecond laser. The simultaneous appearance of SA and RSA at the same wavelength suggests that these films might be used for optical pulse compression and limiting. 2. Results and discussion 2.1. Experimental details The emeraldine salt form of PANI was synthesized in our laboratory by a standard procedure [8]. The procedure and materials are described in detail elsewhere [9]. Thin films were prepared by matrix-assisted pulsed laser deposition using an ESC Derma 20 Er:YAG laser system (110 mJ/pulse, 350 ls, 10 Hz, 2937 nm). They were characterized by Infrared, Raman, and UV–Vis spectroscopy. More details about this film deposition method may be found here [9,10]. PANI and PANI–Ag films were deposited on 1 cm2 sapphire substrates from frozen suspensions in methanol. To prepare the PANI–Ag composites, either 0.5 or 1 wt.% silver nanoparticles (Sigma–Aldrich, <100 nm, 99.5% metals basis) was added to the

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solution and co-deposited with the PANI. The experimental details of the z-scan remain the same as previously reported [11,12]. 2.2. Characterization The UV–Vis spectra of the films were in accordance with previous reports of PANI [9], Ag nanoparticles [12,13], and PANI–Ag [14] and is shown in Fig. 1. We will note that the characteristic absorption band at 450 nm in PANI is broadened due to the presence of the Ag nanoparticles in the composite film. This occurs because the surface plasmon resonance (SPR) band in Ag near 400 nm overlaps this spectral region. The strong absorption in the UV region below 300 nm corresponds to the p—p band gap transitions associated with the p-electrons extended molecular orbitals on the backbone of the polymer [3]. Maciel et al. [3] observed this band around 330 nm in their studies on the PANI in the emeraldine base form and leucoemeraldine base form. The upward arrow in the figure corresponds to the excitation wavelength for the z-scan measurements. At this wavelength the excitation is quasi resonant with the absorption of the PANI and PANI–Ag film that appears around 450 nm. The FTIR spectra are also consistent with our previous observations [9]. In addition to the transmission spectroscopies discussed above, Raman spectra were taken of the films. In the PANI films, the characteristic Raman spectrum is observed, with all of the expected bands between 400 and 1700 cm1 present [15]. However, when the films are doped with Ag, there is a large increase of the signal consistent with surface enhanced Raman (SERS) [16]. These results are displayed in Fig. 2. We take this as unambiguous evidence that a true composite of PANI–Ag is formed. The AFM characterization of the films was performed with a Digital Instruments Nanoscope III in tapping mode and is shown in Fig. 3. The films were found to consist of overlapping nanoparticles. The thickness of the films determined using the associated software was found to be about 140 nm and 130 nm for PANI and PANI–Ag, respectively.

Fig. 2. SERS spectra of PANI recorded from the Ag–PANI composite film. B – Benzenoid ring; Q – quinoid ring.

2.3. z-Scan results In Fig. 4, the open aperture z-scan data is displayed for the PANI–Ag film. Very similar results are obtained for the PANI film. At low laser excitation levels, the data is characteristic of saturable

Fig. 1. Absorption spectra of Ag, PANI and Ag–PANI films used for the present study. The upward arrow corresponds to the excitation wavelength for the z-scan measurement.

absorption (SA). As the laser irradiance is increased, the nonlinear behavior changes from SA to reverse saturable absorption (RSA). Such effects were observed before in longitudinal SPR bands in Au nanoparticles [17], copper-doped silicate glasses [18], Ag nanoparticles [12], and a linear polymer in dimethyl formamide [19]. The reversible transformation from SA to RSA suggests threshold behavior and that as the laser intensity is increased additional nonlinear processes come to dominate the response of the material. One mechanism capable of producing such an effect is transient free carrier absorption [12,20]. The films also exhibit optical limiting. After an initial rise, as the laser irradiance is increased over 5.3 GW/cm2 in the PANI film and 3.8 GW/cm2 in the PANI–Ag film, the transmission steadily falls. We found the threshold of limiting at 19.2 GW/cm2 for the PANI film and 20.8 GW/cm2 for the PANI–Ag film, respectively. The threshold of limiting is defined as the incident irradiance for which the transmittance falls to ½ of the initial value [17]. This is a noteworthy result, as the simultaneous presence of limiting and reversible SA/RSA suggests that these materials have possible application for optical pulse compression [21]. Materials that exhibit RSA can also be used in applications that require optical limiting, for example optical sensors that need to be protected from intense laser pulses. The nonlinear absorption coefficient, b and hence Imvð3Þ , for both PANI and PANI–Ag is found by fitting the open aperture z-scan data to the theory presented by Bahae et al. [7]. We found values of b ¼ ð1:99  0:2Þ  107 m=W and Imvð3Þ ¼ ð7:09  0:04Þ

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Fig. 3. AFM pictures of the films used for the present study. (a) PANI and (b) Ag–PANI.

108 esu for the PANI film of thickness 140 nm and b ¼ ð2:74 0:1Þ  107 m=W and Imvð3Þ ¼ ð9:81 0:1Þ  108 esu for the PANI–Ag film of thickness 130 nm. In the presence of SA, the intensity dependent absorption coefficient aðIÞ is given by [22]

aðIÞ ¼

a0 1 þ ðIIs Þ

ð1Þ

where a0 is the low intensity absorption coefficient and Is is the saturation intensity. If excitation intensity I is less than Is , SA can be considered as a third-order process and in such cases a0 =Is is equivalent to b [23]. The duration of the laser pulse can also play a major role in the nature of SA/RSA and the magnitude of b. It has been reported in the case of metal oxides that b and hence the vð3Þ values increase on increasing the laser pulse duration (femto ? pico ? nanosecond) [24]. Longer pulses can make competing nonlinear absorption processes such as two-photon induced free carrier absorption dominant, leading to a larger loss than two-photon absorption alone. Not accounting for such effects results in overestimation of b, sometimes by orders of magnitude [25]. The duration of the laser pulse used for our z-scan measurements is 25 ps, so the dif-

ference in our measured values with that reported earlier [3–5] may be due to the difference in the experimental conditions. Samoc et al. [22] reported different ways to analyze the z-scan signal in a conjugated polymer, poly(indenofluorene), when the excitation wavelength is in resonance with strong one-photon absorption. They reported the dependence of the magnitude of the degenerate four-wave mixing (DFWM) signal on the light intensity I is quite different from the characteristic cubic dependence for nonresonant third-order nonlinearity. The observed power dependence of the DFWM signal was found to be proportional to I1:5 . Under such conditions it is unreasonable to use the description of the nonlinearity in terms of the nonlinear refractive index c because the effective value of c is intensity dependent. In our z-scan studies, however, the excitation wavelength at 532 nm is in quasi resonant with the absorption of PANI and PANI–Ag that appears around 450 nm. Moreover, we cannot rule out the p—p transition due to two-photon absorption with 532 nm. Under such circumstances it is reasonable to measure the nonlinear refractive index of our samples. The closed aperture z-scan data was used to find the nonlinear index, c, and Revð3Þ . The laser pulse irradiance was fixed at 2.3 GW/ cm2 for these measurements. There are significant differences between the PANI and PANI–Ag films’ closed aperture z-scan data. For PANI, the closed and open z-scan data appear very similar, but the closed aperture data has a maximum transmittance at z  2 mm instead of z ¼ 0 like the open aperture scan. Here, the Imvð3Þ response dominates the Revð3Þ component in the z-scan transmittance curve. Similar results were obtained in our earlier studies on Ag nanodots [12] and Ag nanocrystal–glass composites [26]. In order to extract the nonlinear refractive index, we used the procedure described in Chapple et al.’s review paper [27]. Briefly, the closed aperture data is divided by the open aperture data using the following equation:

T refr ðzÞ ¼

Fig. 4. Open aperture z-scan data of Ag–PANI at different laser intensities.

T a ðzÞ ½T na ðz=1:25Þp

ð2Þ

Here, p = S + 0.67(1  S)2, with S as the aperture linear transmittance which is 0.4 in our case. Ta and Tna are the transmittance with the aperture and no aperture, respectively. The result is shown in Fig. 5 along with the result for the PANI–Ag film. Using the formulas given in Ref. [7], we obtain the following values: c ¼ þð1:15 0:4Þ  1014 m2 =W and Re vð3Þ ¼ þð1:0  0:2Þ 108 esu for the nonlinear refractive index and Re v(3), respectively, for the PANI film of thickness 140 nm.

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Fig. 5. Closed aperture z-scan data of (a) PANI derived from the open and closed aperture data using Eq. (2) and (b) Ag–PANI. The solid line is the theoretical fit using the equation given in [7].

Interestingly, PANI–Ag shows a negative refractive nonlinearity. The closed aperture z-scan shows a dispersive character and yields the following values: c ¼ ð0:57  0:03Þ  1014 m2 =W and Re vð3Þ ¼ ð0:48  0:2Þ  108 esu for the PANI–Ag film of thickness 130 nm. The different nature of the closed aperture z-scan transmittance curve and the sign reversal of Re vð3Þ for PANI–Ag relative to PANI is very interesting; one possible explanation for this phenomena is that the SERS process plays a role in the vð3Þ response through cross phase modulation [28,29]. Another reason for such an effect can be due to the effective dielectric constant of the composite mixture of thin films given by Maxwell–Garnett model [30,31]. The correct application of the Maxwell–Garnett model requires that (i) the Lorentz-local field correction applies, (ii) the induced polarization of one particle from its neighbors is instantaneous and (iii) the distance between the particles are so large that they act as independent scatterers [32]. Considering these conditions and the strong SERS signal obtained from PANI–Ag, we assume that the former is more probable. This is of course speculative, and will be investigated further. As the presence of two-photon absorption can limit the efficiency of any high v(3) material [33], the films do not show promise for all-optical switching applications. Upon computation of the Stegeman figures of merit [34], we find that they fall far short. The condition for all-optical switching may be expressed in compact and elegant form as [33]:

8p

Imvð3Þ <1 Re vð3Þ

ð3Þ

The values for our films are 184 and 515 for PANI and PANI–Ag, respectively.

3. Conclusion From our observations of the nonlinear optical properties of PANI and PANI–Ag, we conclude that the material is not suitable for all optical applications. The presence of the SPR in the PANI– Ag composite did not improve the required conditions for optical switching. However, we did observe a sign reversal of the nonlinear index in PANI–Ag and intensity dependent reversible SA and RSA in both films. The interesting property of SA and RSA at the same wavelength commends the material as a candidate for optical pulse compression and protecting sensor from high power laser pulses.

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