Nonlinear Optical Properties of Poly(3,4-ethylenedioxythiophene

Japanese Journal of Applied Physics 48 (2009) 04C123

REGULAR PAPER

Nonlinear Optical Properties of Poly(3,4-ethylenedioxythiophene) Synthesized by Electropolymerization Shengchun Mao, Youlong Xu, Jianhua Niu, Xin Wang, and Minqiang Wang School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China Received September 22, 2008; accepted November 25, 2008; published online April 20, 2009 Poly(3,4-ethylenedioxythiophene) (PEDOT) films were synthesized on indium-doped tin oxide (ITO) electrodes by electropolymerization in propylene carbonate (PC) electrolyte. The morphology of the PEDOT films was characterized by field emission scanning electron microscopy and atomic force microscope. The third-order nonlinear optical properties of PEDOT films were investigated by Z -scan technique at 532 nm with 7 ns pulses from a Nd:YAG laser. The result shows that the third-order nonlinear refractive index of the PEDOT film is as high as 1013 m2 W1 . At the same time, the properties of PEDOT oligomer dissolved in PC solution were investigated. The corresponding nonlinear refractive index in PC electrolyte exhibited up to 1017 m2 W1 . The perfect stability and ultra-high third order nonlinear refractive indexes of PEDOT films on ITO and PEDOT in PC solution indicate that PEDOT synthesized by electropolymerized have great potential application for nonlinear optical devices. # 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.04C123

1.

Introduction

Conjugated polymers with strong delocalization of electrons in the polymeric backbone are considered as promising candidates due to their wide-ranging applications, such as supercapacitors, light-emitting diodes, solar cells, and nonlinear optical (NLO) devices.1–5) Conjugated polymers are usually chemically synthesized and dissolved in organic solvents or fabricated into films on various substrates via spin-coating to study their NLO properties.6,7) However, such chemical polymerizations involve many processes and the films obtained are inhomogeneous. In contrast, the films integrated by electrochemical polymerization are more compact and possesses higher conductivity.8) The process is simple and controllable. Among the numerous nonlinear organic polymers, thiophene based polymers are currently under intensive investigation for nonlinear optics because of their large third-order response, high chemical stability, and readiness of functionalization.5,9) In this study, poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesized by the galvanostatic electropolymerization method. The microstructure and third-order optical nonlinearities properties of PEDOT films on indium-doped tin oxide (ITO) glass and PEDOT in PC solution were investigated. 2.

Experimental Methods

EDOT monomer (Baytron, 99%), was distilled. For the electrochemical polymerization, propylene carbonate (PC; 99.5%) was used as electrolytes in this experiment, toluenep-sulfonic acid (TOSH) as surfactant and supporting salt, ITO as working electrode and tantalum sheet as counter electrode were used. ITO and tantalum sheet were cleaned used the method as described by Hass.10) 0.19 g TOSH was added into 20 ml PC solution and stirred for 5 min for better dissolution. Next, 0.2 ml EDOT monomer was dripped into the solution by burette. The PEDOT thin films were deposited directly on the ITO electrodes (ITO glass, 10 10 1 mm3 ) by galvanostatic electropolymerization. At the same time, some PEDOT oligomer near the positive plate could dissolve in the PC solution and formed PEDOT solution. By applying currents

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of different magnitudes, different PEDOT films and PEDOT solution samples were fabricated. All the experiments were carried out at room temperature. The surface morphology of the PEDOT films was observed by field emission scanning electron microscopy (FE-SEM; JSM-6700F) and atomic force microscope (AFM). The morphology of PEDOT dispersed on PC electrolyte was measured by high-resolution transmission electron microscope (HR-TEM; JEM-3010). The optical transmittance of PEDOT films on the ITO electrode and PEDOT solution were investigated using JASCO-570 ultraviolet/visible (UV/ vis) spectrophotometer. The nonlinear optical properties of PEDOT films and PEDOT solution were characterized using a modified Z-scan technique.11,12) A Q-switched Nd:YAG laser frequency-doubled at 532 nm and characterized by pulse duration of 7 ns was employed as the light source. The light beam was a TEM00 -mode optical Gaussian spatial profile, and it was focused by the lens with focal length of 160 mm. The radius at beam waist, !0 was 10 mm. The transmitted beam energy, the reference beam energy, and their ratio were measured by an energy ratio meter simultaneously. The linear transmittance of the pulse energy passing the aperture to the total pulse energy was measured to be 0.10 (S ¼ 0:10) without sample loaded. The measurement system was using CS2 as standard calibration. To avoid cumulative thermal effects, data were collected in single shot mode. The Z-scan experimental setup is shown in Fig. 1. 3.

Results and Discussion

3.1 PEDOT films on ITO glass electrode The PEDOT films on ITO substrate are transparent and their color is light blue. Figure 2 shows the optical transmittance of the PEDOT film (synthesized with 0.5 mA/cm2 current density for 5 min) on ITO glass substrate. The sample shows good transparency from 350 to 800 nm. The transmittance is about 71% at 532 nm. Figures 3(a) and 3(b) show the FE-SEM and AFM images on the surface of the sample. It could be observed that the film is compact and smooth. Film roughness represented by root-mean-square value in 10 10 mm2 is only 9.4 nm. The thickness of PEDOT film on ITO substrate was determined to be about 200 nm by -stepper profiler. The Z-scan experimental curve of PEDOT film in PC is shown in Fig. 4, in which the incident energy is 3 mJ. The

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Jpn. J. Appl. Phys. 48 (2009) 04C123 Beam Attenuator Splitter

Lens

S. Mao et al.

Aperture

Sample

Nd: YAG Laser -Z

Aperture Polarimeter

+Z

D2 stepper

D1

Computer Rm6600A

Fig. 1. Schematic illustration of Z -scan experimental setup. Fig. 4. Measured Z -scan of PEDOT film using 7 ns pulses at ¼ 532 nm, indicating a large negative nonlinearity (self-defocusing). Solid line is the best theoretical fitting obtained with eq. (1).

Transmittance(%)

100

(b) 80

(a)

60 40 20

0 200 300 400 500 600 700 800 900

Wavelength(nm) Fig. 2. Optical transmittance property curves: (a) the PEDOT film synthesized at the current density of 0.5 mA/cm2 for 5 min; (b) ITO glass substrate.

effect is a third-order response rather than thermal effect. We made Z-scan measurements at several laser intensities and found that the relation of n=I0 versus the peak input irradiance I0 displayed a horizontal straight line. This indicates that the nonlinear effect is a third-order response. According to the work reported by Chapple et al.13) and Yin et al.,14) optical nonlinear refractive and absorptive parameters can be simultaneously estimated from the closed aperture. With the thin-sample approximation, the normalized transmittance (T) is given as TðzÞ ¼ 1 þ

4x 2ðx2 þ 3Þ ; ð1Þ ðx2 þ 9Þðx2 þ 1Þ ðx2 þ 9Þðx2 þ 1Þ

where x ¼ z=z0 , z is the position of the sample, while z0 is the Rayleigh range of the lens; ¼ kI0 Leff is the phase shifts due to refractive nonlinearity; ¼ I0 Leff =2 is the phase shifts due to absorptive nonlinearity. k ¼ 2= is the wave number, I0 is the intensity of the incident laser beam, Leff ¼ ð1 e0 L Þ=0 is the effective length of the sample, (m2 /W) is nonlinear refractive index, and is the absorption coefficient. The value of and can be obtained through the best theoretical fit from the Z-scan curve as shown in Fig. 4. Then and can be calculated. The real and imaginary part of the third-order nonlinear susceptibility can be deduced by the following equations:

(a)

µm /div 0.0094

Re ð3Þ ¼ 2n20 c"0 ; Im

0 5.0 µm /div

5.0 µm /div (b)

Fig. 3. (Color online) The surface morphology of the PEDOT film electropolymerization on ITO at 1 mA/cm2 for 5 min: (a) FE-SEM image; (b) AFM image.

solid line shows the best theoretical fitting curve to the data. The signal profile with a peak-to-valley indicates a negative (self-defocusing) nonlinear refractive index. Since ITO substrate does not contribute to NLO properties, the result was from the PEDOT film. The laser-induced damage threshold of PEDOT films on ITO is around 6 mJ. Thus the nonlinear

ð3Þ

¼

n20 c"0 =2:

ð2Þ ð3Þ

The calculated value of nonlinear refractive index, (m2 / W), is 9:25 1013 m2 W1 . The real and imaginary part of third-order optical susceptibility [Re ð3Þ (SI), Im ð3Þ (SI)] are 9:63 1015 m2 V2 and 4:01 1017 m2 V2 , reð3Þ spectively. Then, the third-order optical susceptibility SI isffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 9:63 1015 m2 V2 using ð3Þ ¼ ðRe ð3Þ Þ2 þ ðIm ð3Þ Þ2 . Converting into the electro static unit (esu), the nonlinear refractive index, n2 , is 3:09 106 esu, and the third-order susceptibility, ð3Þ (esu) is 6:90 107 esu. Compared to some representative organic polymer nonlinear materials and ferroelectric materials such as PLT30 films15) and SBT films,16) PEDOT films show much higher nonlinear refractive index and the third-order susceptibility. 3.2 PEDOT solution The color of PEDOT solution is light blue and its color

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Transmittance(%)

120

(b)

100 80

(a)

60 40 20 0 200 300 400 500 600 700 800 900

Wavelength(nm) Fig. 5. Optical transmittance property curves: (a) the PEDOT solution in PC at current density of 0.5 mA/cm2 for 5 min; (b) PC pure solution.

Fig. 7. TEM morphology of PEDOT in PC solution.

electropolymerization are light blue, compact and smooth and its third-order nonlinear refractive index has shown to be the value as large as 1013 m2 W1 . On the other hand, PEDOT in PC solution is rodlike as observed from the TEM morphology and its third-order nonlinear refractive index is found to be as large as 1017 m2 W1 . Both PEDOT film and solution exhibit perfect stability and ultra-high third order nonlinear refractive indexes, indicating that the PEDOT as promising candidate for optical apparatus. Acknowledgements Fig. 6. Measured Z -scan of PEDOT solution in PC using 7 ns pulses at ¼ 532 nm, indicating a high negative nonlinearity (self-defocusing). Solid line is the best theoretical fitting obtained with eq. (1).

becomes gradually deeper as polymerization time increases. Figure 5 shows the optical transmittance of PEDOT solution with 5 min electropolymerization (current density: 0.5 mA/ cm2 ). The sample is tested in 1 mm quartz cuvette and the transmittance is tested about 74% at 532 nm. The Z-scan experimental curve of PEDOT solution in PC is shown in Fig. 6, in which the incident energy is 10 mJ. Solid line shows the best theoretical fitting curve to the data. It is also a negative (self-defocusing) nonlinear refractive index. Since the quartz cuvette substrate has no NLO property at 532 nm, the high NLO properties is attributed to the PEDOT solution. The measurement and calculation of NLO property of PEDOT films were the same as mentioned in §3.1. The nonlinear refractive index, n2 is 2:40 1010 esu, and the third-order susceptibility, ð3Þ (esu), is 5:35 1011 esu. The results are more than one order of magnitude higher than that of other reported polymer solutions.5–7) To study the reasons for this high optical nonlinearity, we further inspected our sample by TEM morphology. From Fig. 7, it can be seen that PEDOT is rodlike grown under our process condition. Due to electropolymerization, the monomer grows up directly. This directional growth could increase the conductivity of the polymer, which accounts for the enhancement in the optical nonlinearity. 4.

Conclusions

PEDOT films on ITO and PEDOT in PC solution were obtained by electropolymerization. Both of them have good transparency as measured by UV/vis spectrophotometer. Their third-order optical nonlinearities were studied using Z-scan technique. The PEDOT films on ITO synthesized by

The authors acknowledge the financial supported by the National Nature Science Foundation of China (Grant No. 50473033) and the Specialized Research Fund for Doctoral Program of Higher Education of China (Grant No. 20040698016).

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