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Nonlinear optical and optical limiting properties of graphene oxide–Fe3O4 hybrid material

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 J. Opt. 13 075202 (http://iopscience.iop.org/2040-8986/13/7/075202) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING J. Opt. 13 (2011) 075202 (7pp)

JOURNAL OF OPTICS doi:10.1088/2040-8978/13/7/075202

Nonlinear optical and optical limiting properties of graphene oxide–Fe3O4 hybrid material
Xiao-Liang Zhang1,2 , Xin Zhao1 , Zhi-Bo Liu1 , Shuo Shi1 , Wen-Yuan Zhou2 , Jian-Guo Tian2 , Yan-Fei Xu3 and Yong-Sheng Chen3
1 The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School, Nankai University, Tianjin 300457, People’s Republic of China 2 School of Physics, Nankai University, Tianjin 300071, People’s Republic of China 3 Key Laboratory of Functional Polymer Materials and Center for Nanoscale Science and Technology, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China

E-mail: rainingstar@nankai.edu.cn (Z-B Liu) and jjtian@nankai.edu.cn

Received 19 December 2010, accepted for publication 13 April 2011 Published 12 May 2011 Online at stacks.iop.org/JOpt/13/075202 Abstract The nonlinear optical (NLO) and optical limiting properties of a graphene oxide hybrid material coordinated with Fe3 O4 nanoparticles (GO–Fe3 O4 ) were studied by using the Z-scan technique at 532 nm in the nanosecond and picosecond regimes. Results show that GO–Fe3 O4 exhibits enhanced NLO and optical limiting properties in comparison with the pristine GO in the nanosecond regime. Compared with fullerene (C60 ) in toluene at different concentrations, GO–Fe3 O4 exhibits a weaker optical limiting effect than C60 at high concentration, but shows a stronger optical limiting effect than C60 at low concentration in the high input ?uence region.
Keywords: nonlinear optics, graphene oxide, hybrid materials, Z-scan

(Some ?gures in this article are in colour only in the electronic version)

1. Introduction
The development of laser science and technology has motivated a lot of interest in designing optical limiters. A practical optical limiter can attenuate an optical beam strongly for high intensity or ?uence, while exhibiting high transmittance for low intensity or ?uence. Materials with large NLO properties can be promising candidates for optical limiting and they have attracted considerable interest in studying the NLO properties of new materials. Up to now, numerous materials, including phthalocyanines [1, 2], porphyrins [3, 4], fullerenes [5, 6], carbon nanotubes (CNTs) [7–10], inorganic nanoparticles [11, 12], graphene, and graphene oxide (GO) [13–15], have been reported to have NLO properties and optical limiting effects. Several NLO mechanisms, particularly multiphoton absorption, reverse saturable absorption (RSA),
2040-8978/11/075202+07$33.00

nonlinear scattering, and nonlinear refraction have been found to dominate different kinds of NLO materials [16]. In the past few decades, great efforts have been made to promote the NLO properties by modifying the structures of the NLO materials. Recently, carbon-based hybrid materials decorated with organic dye [17–21] or semiconductor nanoparticles [22, 23] have been shown to exhibit enhanced NLO properties due to the combination of multiple NLO mechanisms and the proposed photoinduced electron or energy transfer in the hybrid materials, which provides a good approach to obtaining materials with high values for their NLO properties. Among various graphene-base hybrid materials, GO decorated with magnetic Fe3 O4 nanoparticles has attracted attention due to its potential application in the fabrication of functional polymer composites, sensors, waste water
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? 2011 IOP Publishing Ltd Printed in the UK & the USA

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Figure 1. Scheme of the synthesis of the GO–Fe3 O4 hybrid material.

treatment, and drug delivery [24–26]. What is more, Fe3 O4 nanomaterials have shown strong excited state absorption and nonlinear scattering [27, 28]. Hence, it is expected that GO decorated with magnetic Fe3 O4 nanoparticles would have high values for its NLO properties owing to a combination of nonlinear mechanisms of GO and Fe3 O4 nanoparticles. However, there are few detailed reports on the NLO properties of this kind of hybrid material [29]. In this paper, we study the nonlinear absorption, nonlinear refraction, nonlinear scattering properties, and optical limiting effect of the GO–Fe3 O4 hybrid material. The results show that GO–Fe3 O4 exhibits enhanced nonlinear refraction, nonlinear scattering, and optical limiting effect compared with the pristine GO. To evaluate the NLO properties and optical limiting effect of GO–Fe3 O4 , we also compared it with the benchmark optical limiting material of fullerene C60 in toluene.

2. Experimental section
GO was prepared from puri?ed natural graphite according to a modi?ed hummer method [30, 31]. The oxygen-containing groups in GO make it strongly hydrophilic and water soluble. The statistical analysis using atomic force microscopy shows that the size of GO sheets is mainly distributed between 200 and 500 nm. The synthesis of GO–Fe3 O4 hybrid was prepared by chemical deposition of iron ions using water soluble GO as carrier, and Fe3 O4 is bound onto the GO surface by the coordination interaction between the –COOH and Fe3 O4 [24, 29] (as shown in ?gure 1). The formation of this hybrid was veri?ed by Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and x-ray powder diffraction [29]. The size of Fe3 O4 nanoparticles is 2–4 nm with a narrow size distribution, and some Fe3 O4 aggregation is also observed [29]. The Z-scan experiments were conducted with a linearly polarized 5 ns and 35 ps pulsed laser at 532 nm generated from a frequency doubled Q -switched Nd:YAG laser (Continuum Surelite-II) and a mode-locked Nd:YAG laser (Continuum model PY61), respectively. The optical limiting experiments 2

were only conducted with a 5 ns pulsed laser. The pulsed laser was set at a repetition rate of 10 Hz for Z-scan and single pulse mode for optical limiting experiments. The spatial pro?le of the pulsed beam was of nearly Gaussian distribution after spatial ?ltering. The pulsed beam was split into two parts: the re?ected part was used as reference, and the transmitted part was focused onto samples by using a 25 cm focal length lens. The re?ected and transmitted pulses energies were measured simultaneously by using two energy detectors (PE9-SH-ROHS Ophir). In the Z-scan experiments, samples were moved along the propagation direction of the focused beam. In the optical limiting measurements, the samples were placed at the focus where the focused spot radius was about 23 μm (1/e2 ); an aperture with a diameter of 8 mm was placed between the detector and the sample where all the transmitted energy could just go through it when the sample was far away from the focus. The 8 mm aperture was used to fully take advantage of negative nonlinear refraction (self-defocusing) and nonlinear scattering. All the energy through the aperture was focused into the detector by a lens. In the nonlinear scattering measurements, a small area lens was placed at an angle of 22? with respected to the Z axis to collect the scattered signals. C60 toluene solution was employed as a reference, GO and GO–Fe3 O4 were in water and all the samples were contained in 5 mm thick quartz cells, no nonlinear response or damage from the quartz cell was observed in our experiments.

3. Results and discussion
Figure 2(a) gives the UV–visible absorption spectra of GO and GO–Fe3 O4 in water with the same concentration of 0.033 mg ml?1 . GO shows a broad absorption continuously decreasing from 220 to 800 nm. Compared with GO, GO– Fe3 O4 shows a similar broad absorption, but it exhibits weak absorption at the short wavelength region and stronger absorption at the longer wavelength region. The difference can be attributed to two factors, one is the partial removal of the epoxide and the hydroxyl groups on GO, which were deoxygenated during Fe3 O4 nanoparticle deposition by

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Figure 2. (a) Absorption spectra of GO and GO–Fe3 O4 in water with the same concentration of 0.033 mg ml?1 . (b) The plot of absorption value at 400 nm versus concentration for GO and GO–Fe3 O4 in water. Solid lines are linear ?ts.

Figure 3. Open-aperture Z-scan curves of GO (a) and GO–Fe3 O4 (b) with the same concentration of 0.375 mg ml?1 at different on-axis peak intensity with nanosecond pulses. Solid lines are theoretical ?ts.

treating with aqueous NaOH solution [29, 32], the other is the absorption of Fe3 O4 nanoparticles. UV–visible absorption spectra of GO and GO–Fe3 O4 with various concentrations were also measured. To avoid the strong absorption in the UV region beyond the ability of the spectrophotometer, the concentration was controlled not to be higher than 0.08 mg ml?1 for the absorption spectra measurements. The absorbance values at 400 nm were plotted against concentration and are shown in ?gure 2(b). The observed absorption is linearly dependent on concentrations (Lambert– Beer’s law) and similar results are also obtained at other wavelengths, which indicates that both GO and GO–Fe3 O4 are dispersed homogeneously in water. Since GO–Fe3 O4 was prepared by chemical deposition of Fe3+ and Fe2+ ions using GO as carriers and no pristine Fe3 O4 nanoparticles were synthesized [29], Fe3 O4 nanoparticles were not measured in our experiments. The nonlinear optical properties of the samples were measured by the Z-scan technique [33] at 532 nm in nanosecond and picosecond regimes. Figure 3 shows the openaperture Z-scan curves of GO, GO–Fe3 O4 in water with the same concentration of 0.375 mg ml?1 at different on-axis peak intensity. As shown in ?gure 3, at low intensity, GO shows a symmetrical transmittance peak at the focus (z = 0), indicating that the saturable absorption (SA) is dominant. As intensity increases, a valley inside the peak appears at the focus and becomes deeper gradually. This implies that the RSA-like 3

behavior occurs similarly to the results in [13]. Unlike GO, the open-aperture Z-scan curves of GO–Fe3 O4 exhibit only a valley at the focus, and the valley becomes increasingly deeper. Compared with the transition from SA to RSA-like behavior of GO as input intensity increases, GO–Fe3 O4 keeps the strong RSA-like behavior. This change can be attributed to the coordination of GO with Fe3 O4 nanoparticles. To evaluate the NLO properties of GO and GO–Fe3 O4 quantitatively, we ?t the experimental data by solving the propagation equation of the electric ?eld envelope E : 1 ? ?E r r ?r ?r

? 2ik α(I ) =

2k 2 ?E ? ikα E + ?z n0

nE = 0

(1) (2) (3)

α0
1+
I IS

+ βeff I

n = n 2eff I

where a modi?ed nonlinear absorption coef?cient α(I ) is used to combine the SA and two-photon absorption (TPA) coef?cients [34, 35], α0 is the linear absorption coef?cient, I is the laser radiation intensity, IS is saturable intensity, n 0 is linear refraction index, n 2eff is the effective nonlinear refraction coef?cient, βeff is the effective TPA coef?cient and k is the wavevector. α0 is 3.39 cm?1 and 3.99 cm?1 for GO and GO–Fe3 O4 at 532 nm with the same concentration of

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Figure 4. The values of the effective TPA coef?cient βeff as a function of on-axis peak intensity for GO and GO–Fe3 O4 with the same concentration of 0.375 mg ml?1 in the case of nanosecond pulses.

0.375 mg ml?1 , respectively. For GO, IS = 1.2 × 108 W cm?2 is obtained, while IS is near to in?nity for GO–Fe3 O4 . To illustrate the difference in mechanism of NLO in GO and GO–Fe3 O4 , we give βeff as a function of input intensities for the two samples, As shown in ?gure 4, the effective TPA coef?cient βeff is nearly a constant of 7 cm GW?1 at different input intensities for GO, indicating the dominant TPA mechanism. However, the βeff increases with input intensity for GO–Fe3 O4 , which indicates that besides TPA from the GO moiety, nonlinear scattering may also play an important role, since strong nonlinear scattering signals were observed for GO–Fe3 O4 , while no nonlinear scattering signals were observed for GO during the Z-scan measurements. (GO has no nonlinear scattering, while GO–Fe3 O4 has strong nonlinear scattering at low intensity or ?uence, as shown in ?gure 7.) Figure 5 shows the open-aperture and closed-aperture Zscan results of GO and GO–Fe3 O4 with the same intensity and concentration. For GO, the obvious peak–valley feature of the closed-aperture Z-scan curves indicates the strong negative nonlinear refraction, while the peak of the curve is seriously suppressed for GO–Fe3 O4 , suggesting that stronger nonlinear absorption/nonlinear scattering exists. By theoretical ?tting, the effective TPA coef?cients βeff and effective nonlinear refraction coef?cients n 2eff were obtained as 7.8 cm GW?1 , 9.74 × 10?14 cm2 W?1 for GO and 26 cm GW?1 , 2.83 × 10?13 cm2 W?1 for GO–Fe3 O4 , respectively. So both the effective TPA and nonlinear refraction were enhanced in GO–Fe3 O4 compared with the pristine GO. Since the beam waist radius at focus is about 23 μm, the build-up time of the thermally induced optical nonlinearities is about 16 ns. Compared with the pulse-width of 5 ns, the thermally induced optical nonlinearities are highly transient [36]. So the observed negative nonlinear refraction should be attributed to the transient thermally induced optical nonlinearities and the intrinsic nonlinear refraction of the samples. Three factors may contribute to the enhancement of the NLO properties of GO–Fe3 O4 . Firstly, the Fe3 O4 nanoparticles in GO–Fe3 O4 should have high values of their 4

Figure 5. Open-aperture and closed-aperture Z-scan curves of GO and GO–Fe3 O4 with the same concentration of 0.375 mg ml?1 at an on-axis peak intensity of 0.57 GW cm?2 with nanosecond pulses. Solid lines are theoretical ?ts.

NLO properties [27, 28]. Secondly, during the synthesis of GO–Fe3 O4 , partial reduction of GO will increase the thermal conductivity and enhance the NLO properties of GO–Fe3 O4 . In GO, epoxide and hydroxyl functional groups mostly are on the basal plane, while carboxyl groups are located at the sheet edges [37, 38]. During the synthesis of GO–Fe3 O4 , Fe3 O4 nanoparticles mainly deposited on the edge of the GO sheet coordinated with carboxyl groups [29], while the epoxide and the hydroxyl groups on GO were partially removed by NaOH [29, 32], which increases the conjugation network of the nanostructure. The resulting integrated structure will transfer crystal lattice vibrations more rapidly, and thus the thermal conductivity of GO–Fe3 O4 increases. This will lead to enhancement of nonlinear scattering and nonlinear refraction due to thermal effects. Thirdly, Fe3 O4 nanoparticles deposited on GO may increase the size of the scattering center over that of the pristine GO, resulting in an enhanced nonlinear scattering. Figure 6 gives the open-aperture Z-scan results of GO and GO–Fe3 O4 at 532 nm with 35 ps pulse. Different from the case of a nanosecond pulse, GO–Fe3 O4 shows weaker NLO properties than GO. Since nonlinear scattering is usually inef?cient under a picosecond pulse [39], the observed weaker NLO properties of GO–Fe3 O4 may be attributed to the nonlinear absorption mechanics (TPA and/or RSA).

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Figure 6. Open-aperture Z-scan curves of GO and GO–Fe3 O4 with the same concentration of 0.375 mg ml?1 with picosecond pulses.

Summarizing the results demonstrated above, we can see that the hybrid material GO–Fe3 O4 exhibits strong nonlinear absorption, nonlinear refraction, and observed nonlinear scattering properties, which may make it a good optical limiting material under a nanosecond pulse. Fullerene (C60 ) has been reported to have a strong optical limiting effect and is usually used as a reference material. The NLO properties of C60 come from the well known RSA mechanism, i.e. their ?rst singlet and triplet states have larger absorption cross sections than the ground state, while some research groups reported that nonlinear scattering and nonlinear refraction also play important roles in C60 for optical limiting [40, 41]. To

evaluate the NLO properties and optical limiting effect of GO– Fe3 O4 , we measured the optical limiting effect of GO–Fe3 O4 , compared with C60 and the pristine GO with the same linear transmittance of 49% and 87%, respectively. The high and low linear transmittance was obtained by adjusting the mass concentration of the samples. As shown in ?gures 7(a) and (c), with the linear transmittance of 49%, GO–Fe3 O4 exhibits enhanced optical limiting effect, compared with GO, but it is weaker than C60 . For example, at the input ?uence of 20 J cm?2 , the output ?uences are 1.32 J cm?2 , 3.30 J cm?2 , and 0.56 J cm?2 , and the optical limiting thresholds (de?ned as the input ?uences at which the transmittance falls to 50% of the normalized linear transmittance) are 2.82 J cm?2 , 10.19 J cm?2 , and 0.41 J cm?2 , for GO–Fe3 O4 , GO, and C60 , respectively. The lowest output ?uence and optical limiting threshold of C60 indicate that C60 exhibits the best optical limiting effect at high concentration. As shown in ?gures 7(b) and (d), with the linear transmittance of 87%, C60 shows the lowest output ?uence and normalized transmittance for input ?uence lower than 2.33 J cm?2 , but it shows a higher output ?uence and normalized transmittance than GO–Fe3 O4 for input ?uence higher than 2.33 J cm?2 . At an input ?uence of 20 J cm?2 , the output ?uences are 2.81 J cm?2 , 5.06 J cm?2 , and 3.33 J cm?2 , the optical limiting thresholds are 3.70 J cm?2 , 10.38 J cm?2 , and 8.58 J cm?2 , for GO–Fe3 O4 , GO, and C60 , respectively. This indicates that GO–Fe3 O4 shows the best optical limiting effect at low concentration in the high input ?uence region. In our experiments, nonlinear scattering signals were also measured for the samples. From ?gures 7(c) and (d), we

Figure 7. The optical limiting of GO–Fe3 O4 , GO and C60 with the same linear transmittance of 49% and 87% with nanosecond pulses. (a) and (b) show output ?uence versus input ?uence. (c) and (d) show nonlinear transmittance and scattered signals’ spectra versus input ?uence.

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can see that the scattered intensity increase along with the decrease of normalized transmittance for the three samples at high input ?uence indicate that nonlinear scattering exists and is responsible for the optical limiting at high input ?uence. However, we noticed that the onset of the growth of nonlinear scattering is higher than that of the decrease of normalized transmittance, which is much more pronounced for C60 , indicating the existence of other nonlinear mechanisms, such as nonlinear absorption and/or nonlinear refraction. For the linear transmittance of 49% as shown in ?gure 7(c), we can see that GO shows the weakest scattered intensity and the weakest optical limiting effect; GO–Fe3 O4 shows a stronger scattered intensity but weaker optical limiting effect than C60 . For the linear transmittance of 87% as shown in ?gure 7(d), C60 exhibits signi?cant scattered signals and leads to the near constant output ?uence for input ?uence higher than 10 J cm?2 , but GO–Fe3 O4 shows a stronger scattered intensity and lower output ?uence than C60 for input ?uence higher than 2.33 J cm?2 . So GO–Fe3 O4 exhibits better optical limiting performance than C60 at low concentration in the high input ?uence region due to the strong nonlinear scattering properties combined with negative nonlinear refraction and TPA. A practical optical limiter requires high linear transmittance, large broadband NLO properties, and fast response time. Considering the strong scattering properties, even at low input ?uence and low concentration, the strong negative nonlinear refraction, and the obvious nonlinear absorption under picosecond pulses for GO–Fe3 O4 , it is expected that the hybrid material GO–Fe3 O4 may be a good candidate for optical limiter.

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4. Conclusions
The NLO properties and optical limiting of GO and GO–Fe3 O4 were studied. Results show that GO–Fe3 O4 exhibits different NLO properties and enhanced optical limiting effect compared with GO. The enhanced nonlinear optical behaviors may arise from enhanced nonlinear scattering combined with TPA. GO– Fe3 O4 exhibits larger NLO properties and stronger optical limiting effect than the benchmark optical limiting material of C60 at low concentration in the high input ?uence region, and smaller NLO properties and weaker optical limiting effect than C60 at high concentration. This can be attributed to the different NLO mechanisms between GO–Fe3 O4 and C60 . Since GO–Fe3 O4 exhibits strong nonlinear optical properties and nonlinear scattering signals even at low concentration or high linear transmittance, we expect that GO–Fe3 O4 will be an excellent candidate for broadband optical limiters.

Acknowledgments
This work is supported by NSFC (10974103), Chinese National Key Basic Research Special Fund (2011CB922003), the Program for New Century Excellent Talents in University (NCET-09-0484), the Natural Science Foundation of Tianjin (09JCYBJC04300), and the Key Project of the Chinese Ministry of Education (109039). 6

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