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Nov 16, 2012 - Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India. ⊥. Department of Material Science and Engin...
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Optical Power Limiting in Fluorinated Graphene Oxide: An Insight into the Nonlinear Optical Properties Panit Chantharasupawong,† Reji Philip,‡,# Narayanan T. Narayanan,§ Parambath M. Sudeep,∥ Akshay Mathkar,§ Pulickel M. Ajayan,§ and Jayan Thomas*,†,‡,⊥ †

College of Optics and Photonics, CREOL, University of Central Florida, Orlando, Florida 32816, United States NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States § Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, United States ∥ Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India ⊥ Department of Material Science and Engineering, University of Central Florida, Orlando, Florida 32816, United States ‡

S Supporting Information *

ABSTRACT: Fluorination of carbon nanomaterials has many advantages due to the unique nature of the carbon−fluorine (C−F) bond. In this work, we report the optical power limiting properties of fluorinated graphene oxide (F−GO) using the optical z-scan technique. In addition, we used the photoacoustic technique to gain insight into the nonlinear processes involved in the optical limiting of samples. The photoacoustic technique enabled us to confirm that optical limiting observed in F−GO at low fluence arises from nonlinear absorption, while that at higher fluence is due to nonlinear scattering. Moreover, we found that F−GO has high nonlinear absorption and nonlinear scattering and its optical limiting threshold is about an order of magnitude better than that of graphene oxide (GO).

1. INTRODUCTION

Fluorination of carbon nanomaterials has many advantages due to the unique nature of the carbon−fluorine (C−F) bond. For instance, the C−F bond demonstrates excellent oxidative and thermal stability.18 Due to the high electronegativity of fluorine atoms, C−F bonds have high polarity and low surface free energy. Partially fluorinated GO (F−GO) can even be paramagnetic due to the presence of localized F-bonds. F−GO will be an attractive material in many applications such as super amphiphobic surfaces, multimodality imaging, and photonic devices.19 Optical limiting is a nonlinear optical process in which the transmittance of a material decreases as the input light intensity increases.20 Optical limiters can be used to fabricate devices for protecting eyes and sensors from powerful laser beams. In this paper, we report the optical limiting performance of F−GO with 5 ns laser pulses at a wavelength of 532 nm. It is found that F−GO has large nonlinear absorption and nonlinear scattering which are beneficial for optical limiting applications. When compared to GO, F−GO exhibits an order of magnitude improvement in the optical limiting performance.

Graphene oxide was first discovered in 1859 by Benjamin Brodie through the exfoliation of graphite oxide.1 GO is an electrically hybrid material between the conducting π-states of sp2 carbon sites, which contribute to the bandgap formation of the material, and the σ-states of sp3 carbon sites. The GO bandgap can be tuned by adjusting the ratio of sp2 carbon atoms to sp3 carbon atoms via a chemical reduction process. Chemical reduction can transform GO from an insulator to a semiconductor and also to a metal-like state, in the form of graphene. While graphene possesses excellent electrical properties, mechanical flexibility, optical transparency, thermal conductivity, and a low thermal expansion coefficient,2−6 its precursor GO has interesting properties of its own. For instance, unlike graphene, GO possesses several oxygen containing hydroxyl, epoxide, diol, ketone, and carboxyl functional moieties. These functional groups allow GO to interact with a wide array of materials, both organic and inorganic, which results in the high processability of GO. Due to their versatility, functionalized GOs have become promising candidates for various applications such as drug delivery,7,8 magnetic resonance imaging (MRI),9 memory devices,10 supercapacitors,11,12 and optoelectronic devices.13,14 Similarly, the optical limiting property of GO is found to be greatly enhanced with organic as well as inorganic decorations.15−17 © 2012 American Chemical Society

Received: September 28, 2012 Revised: November 14, 2012 Published: November 16, 2012 25955

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Figure 1. (a) XRD pattern of GO, F−GO, and HF−GO and (b) schematic representation of F−GO.

Figure 2. (a) UV−vis absorption spectrum of F−GO and (b) HF−GO. The high absorbance in the short wavelength region indicates the possibility of RSA upon optical irradiation at 532 nm.

Figure 3. (a) The optical open-aperture z-scan configuration with a photodiode kept at an angle to the incident beam for measuring scattered light. (b) Open aperture z-scan curves. Solid curves are numerical fits to the data.

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Figure 4. (a) Variation of sample output fluence with input fluence, calculated from the z-scan data for different samples. (b) Variation of sample transmission with input fluence. The dotted line corresponds to 50% linear transmission.

2. EXPERIMENTAL SECTION 2.1. Preparations and Characterizations of Fluorinated Graphene Oxides. All graphene oxide (GO) based samples were synthesized using an improved synthesis route reported elsewhere.21 In the case of pure GO, graphite powder (Bay carbon, Inc. SP-1 grade 325 mesh) is used as a raw material, while, for the synthesis of fluorinated graphene oxide, fluorinated graphite polymer (Alpha Aesar) is used as a starting raw material. Detailed structural information showing the planar honeycomb lattice structures, including C13 NMR results, are discussed in our previous report.2,19 Figure 1a shows the XRD of GO, F−GO, and highly fluorinated graphene oxide (HF−GO). The XRD pattern indicates an increased lattice spacing for GO (∼6 Å) compared to pristine graphite powder (∼3.3 Å). This observation suggests the exfoliation of graphite. F−GO is hydrophilic similar to GO, whereas highly fluorinated GO (HF−GO) is relatively hydrophobic.19 A schematic representation of F−GO is given in Figure 1b. The increase in hydrophobicity of HF−GO can be attributed to the low surface energy of C−F bonds. Both F−GO and HF−GO have well-defined absorption peaks (Figure 2). The absorption spectrum of FGO is almost identical to GO, showing a peak at 225 nm22 which corresponds to the π → π* transition. The weak shoulder at ∼300 nm is due to the n → π* transition of the carbonyl bonds.22 HF−GO shows no such shoulder at 300 nm, while showing a slightly less intense peak at 220 nm. 2.2. Measurements of Optical Limiting Properties. We measured optical limiting in GO/water, F−GO/water, and HF−GO/NMP dispersions using the open aperture z-scan technique.23,24 A schematic of the experimental setup is shown in Figure 3a. All samples were prepared to have a linear transmission of 50% at an excitation wavelength of 532 nm. A Q-switched, frequency-doubled Nd:YAG laser (Minilite I, Continuum) was used to generate 5 ns (fwhm) pulses at this wavelength. The laser output was spatially filtered to obtain a neat Gaussian beam profile, and then focused using a 200 mm focal length plano-convex lens. The beam radius at the focus (w0) was measured to be 30 ± 2 μm. The sample was taken in a

1 mm path length cuvette and translated along the axis of the laser beam (z-axis) by a linear translation stage (Newport, ILS150PP). By fixing the input laser pulse energy (Ein) at a suitable value and translating the sample along the laser beam near the focal region, the incident laser fluence on the sample (Fin(z)) was varied. Maximum fluence is attained at the beam focus (z = 0), and the fluence reduces as a Lorentzian function away from the focal point (i.e., for z > 0 and z < 0). The transmitted energy for different sample positions (z) was measured using a pyroelectric energy probe (LaserProbe, RjP735). By mounting a photodiode near the sample at an angle to the beam axis, linear and/or nonlinear light scattering was also measured. The normalized transmission (Tnorm) was then calculated by dividing the measured transmission with the linear transmission of the sample. The obtained z-scan curves for an input energy of 30 μJ are shown in Figure 3b. All samples show an optical limiting (OL) property, since the transmission decreases with increase in input fluence. The z-scan data can be used to plot the variation of sample transmission with input laser fluence, because, for a spatially Gaussian beam, the light fluence Fin(z) at any position z can be calculated from the corresponding beam radius ω(z) and the input laser pulse energy Ein. The position-dependent fluence can be calculated from the expression Fin(z) =

4 ln 2 E in π 3/2w(z)2

(1)

where the beam radius is given by ⎛ z ⎞2 w(z) = w(0) 1 + ⎜ ⎟ ⎝ z0 ⎠

(2)

The plot of sample transmission versus input fluence is shown in Figure 4a, and that of output fluence versus input fluence is shown in Figure 4b. Results reveal that the optical limiting efficiencies of all fluorinated graphene samples are significantly higher than that of GO in water. The optical limiting threshold (input fluence at 25957

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which transmission decreases to 50% of the linear transmission due to the nonlinearity) of F−GO/water and H−FGO/NMP are at 0.8 and 1.5 J/cm2, respectively. The optical limiting thresholds of fluorinated GO samples are better than that of benchmark materials like C60 in toluene and carbon black in water.25 When comparing with the threshold values of other well-known optical limiters such as single-wall carbon nanotubes in ethanol (∼1 J/cm2 with 42% linear transmission)26 and multiwall carbon nanotubes in water (∼0.9 J/cm2 with 50% linear transmission),27 our F−GO/water dispersion has a lower limiting threshold. The limiting threshold of GO could not be measured due to its relatively lower limiting efficiency. According to Xiao-Liang et al.,28 the limiting threshold of GO/water (49% linear transmission) is at 10.2 J/cm2, for ns pulses at 532 nm, which is an order of magnitude higher than the thresholds of fluorinated GO samples. The optical limiting properties of F−GO in N-methyl-2-pyrrolidinone (NMP) were also studied to find out the solvent contribution to the optical limiting performances when comparing F−GO/water to HF− GO/NMP dispersions (see the Supporting Information). It was found that, at 50% linear transmission, F−GO in NMP also had better optical limiting efficiency than HF−GO in NMP. The measured open-aperture z-scan curve can be fitted numerically to the transmission equation for a third-order nonlinear process, given by T (z ) =

1 π q0(z , 0)

−∞

∫+∞

Figure 5. Variation of scattering signal amplitudes with input fluence of GO and fluorinated GO samples. Scattering signals of pure solvents are also given.

example, optical limiting of F−GO in water has an onset of optical limiting at about 0.05 J/cm2, whereas its nonlinear scattering appears only later, at around 0.14 J/cm2. This observation suggests that the optical limiting action of fluorinated graphene oxide in the ns excitation regime is not exclusively due to nonlinear scattering. In fact, it is possible that nonlinear absorption could also be enhanced in the fluorinated graphene oxide. In order to verify this assumption, we employed a photoacoustic measurement technique. Photoacoustics (PA) is a well-known sensitive method for measuring low level absorptions in materials.35−39 In pulsed PA, transient heating by a laser pulse generates an acoustic wave in the material, which is detected by an acoustic transducer. The magnitude of the acoustic signal is proportional to the absorbed energy.39 Thus, PA measurement can give insight into how much light energy is being absorbed by a given system under irradiation. While the optical z-scan signal is affected by both NLA and NLS, the PA signal is affected only by linear absorption and NLA. In our PA measurements, an in-house fabricated PA cell was used for capturing the absorption-induced PA signals from the samples (Figure 6). The design of this cell is similar to that reported by Yelleswarapu and Kothapalli.40 The PA cell is made from brass and has a diameter of 8 cm and height of 5 cm. An ultrasonic transducer (Olympus NDT, model A315-SU) and glass windows are fixed on the circular cell wall as shown in the figure. The cell has a brass lid with a slotted Teflon cap in the center, through which the cuvette containing the sample is inserted. During the experiment, the PA cell was filled with water in order to achieve good acoustic coupling between the sample and the transducer. A laser beam was focused on the sample. The beam radius incident upon the sample was 80 μm. The peak amplitude of the PA signal from the transducer was measured using an oscilloscope with varied input optical energy. In the thermoelastic regime, the amplitude of the PA signal originating from a large laser radius Rf, observed at a distance r, is given by41

2

ln[1 + q0(z , 0)e−t ] dt

(3)

where T(z) is the normalized energy transmittance, with q0(z, t) = βeffI0(t)Leff/(1 + (z2/z02)), with βeff being the third-order nonlinear absorption coefficient and I0 the peak on-axis intensity. βeff calculated from the numerical fits are 1.40, 0.7, and 0.35 nm/W for F−GO/water, HF−GO/NMP, and GO/ water, respectively. In materials, optical limiting behavior arises from nonlinear absorption and/or nonlinear scattering. Processes such as twophoton (or multiphoton) absorption and excited state absorption (also known as reverse saturable absorption, RSA) belong to the class of nonlinear absorption (NLA). Nonlinear scattering (NLS) in dispersions/solutions in the context of optical limiting refers to the scattering of photons from refractive index variations, microbubbles, and microplasma, which are caused by laser-induced heating of the medium. In absorbing media like carbon based materials, optical limiting of ns laser pulses is caused mostly by RSA and/or NLS. For example, carbon particle suspensions (CS) show strong optical limiting due to NLS caused by thermally induced microplasma,29 whereas fullerenes (C60) exhibit robust limiting due to large RSA.30−32 In addition, multiwalled and single walled carbon nanotubes (CNTs) exhibit broadband optical limiting due to NLS,27,33 while GO exhibits limiting due to RSA.34 By measuring the scattered light from our samples, it was found that all fluorinated graphene oxide samples exhibited strong nonlinear scattering (Figure 5), whereas there is no nonlinear scattering present in GO. This finding suggests that there is a significant enhancement in nonlinear scattering due to the presence of C−F bonds in the fluorinated samples. This enhancement, as a result, contributes to better optical limiting properties. Comparing the z-scan and scattering data, it can be seen that, for fluorinated graphene oxide samples, the onsets of the optical limiting start earlier than the onsets of nonlinear scattering. For

SL = 25958

γν 2αE πCpR f 3/2r1/2

(4)

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Figure 6. Schematic of the PA cell used for PA measurements.

where γ is the thermal expansion coefficient, v is the acoustic velocity, Cp is the specific heat at constant pressure, E is the input energy, and α is the absorption coefficient. At higher input energies (E′) when the medium exhibits third-order optical nonlinearity, the absorption coefficient can be written as

α = α0 + β0I

(5)

where β0 is the nonlinear absorption coefficient and I is the intensity. Substituting eq 5 into eq 4 gives the nonlinear PA signal as S NL =

γν 2(α0 + β0I )E′ πCpR f 3/2r1/2

(6)

Dividing SNL by the linear signal SL gives the normalized PA signal Snorm =

α0 + β0I E′ S NL = · α0 SL E

(7)

which is a measure of the nonlinear optical absorption of the medium. By measuring Snorm for various input energies and fitting the data to eq 7, the nonlinear absorption coefficient β0 can be determined. The normalized PA signal plotted against input energy is shown in Figure 7. According to the plot, the PA signal is the highest in F−GO, while GO has the lowest PA signal and HF− GO assumes an intermediate value. This observation renders the fact that, besides NLS, NLA also plays a significant role in the optical limiting of fluorinated GO samples. The fitted β0 values are 9 × 10−10 m/W for F−GO, 4.5 × 10−10 m/W for HFGO, and 2.04 × 10−10 m/W for GO. The PA results confirm that fluorinated graphene oxide is a stronger nonlinear absorber than graphene oxide.

Figure 7. PA measurements of GO/water, F−GO/water, and HF− GO/NMP dispersions. Higher PA signals for F−GOs compared to GO infer that, besides NLS, NLA also plays a significant role in the optical limiting of fluorinated GO samples. Solid curves are numerical fits to eq 7.

and thus reduces its optical limiting efficiency. As-prepared GO exhibited inferior nonlinear absorption as compared with partially reduced GO due to its lower number of localized sp2 domains. In the case of fluorinated graphene oxide, the interaction of fluorine atoms with the graphene oxide layers is accomplished by covalent attachment of fluorine atoms to the layers. This interaction is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atoms from the trigonal (sp2) to tetrahedral (sp3) hybridization due to the formation of the additional σ bond between carbon and fluorine atom. The large difference between the local band gaps of the sp3 and sp2 sites creates band edge fluctuations, with the sp3 sites acting as tunnel barriers between the π states of sp2 clusters.43 These tunnels create strongly localized isolated sp2 domains which act as defects in the electronic band. Therefore,

3. DISCUSSION Recently, X. F. Jiang et al. reported significantly enhanced nonlinear absorption of GO upon partial reduction.42 This enhancement in nonlinear absorption was attributed to localized sp2 domains. Upon further reduction of GO, larger sp2 domains are formed. The interconnectivity of the sp2 domains results in increased nonradiative recombination rates 25959

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MURI: “Synthesis and Characterization of 3-D Carbon Nanotube Solid Networks” award no.: FA9550-12-1-0035.

similar to partially reduced GO, it is possible that larger nonlinear absorption in the fluorinated samples is due to an increase in the number of localized sp2 domains which are created by the sp3 attachments of F atoms. However, XPS analyses revealed that the ratios of the number of sp2 carbons to sp3 carbons in our GO and F−GO are similar (see the Supporting Information), suggesting that there are similar numbers of sp2 domains in both samples. This finding suggests that the presence of sp3 defects created by a highly electronegative atom such as fluorine promotes larger nonlinear absorption and better optical limiting than the sp3 sites formed with other functional groups in GO. On the other hand, the ratio of sp2 to sp3 carbons of HF−GO is much lower than both of FGO and GO (Supporting Information, Figure S2). The lower nonlinear absorption of HF−GO when compared to F− GO can be attributed to the decrease in the number of localized sp2 domains. This is analogous to the case of inferior nonlinear absorption of as-prepared GO compared to partially reduced GO reported previously.42 Furthermore, the fact that HF−GO is a stronger nonlinear absorber than GO, even though it has a lower number of sp2 to sp3 carbons, confirms the significance of fluorine sp3 sites in the enhancement of nonlinear absorption and thus optical limiting.

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4. CONCLUSIONS In summary, we have investigated the optical limiting properties of F−GO dispersions in water. It is found that the optical limiting efficiency of F−GO is significantly greater than that of GO. Nonlinear scattering measurements reveal that scattering is higher in F−GO when compared to GO, while photoacoustic measurements indicate that nonlinear absorption is also superior in F−GO. Limiting observed in F−GO at low fluence arises from NLA, while NLS has a contribution at higher fluences. Nonlinear scattering occurs at an optical fluence which is about 4 times higher than that required for the onset of limiting. The limiting threshold of F−GO is an order of magnitude higher than that of GO. These observations suggest that F−GO dispersions are potential candidates for optical limiting applications.

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ASSOCIATED CONTENT

S Supporting Information *

Optical limiting data of F−GO in NMP and C-1s XPS spectra of GO, F−GO, and HF−GO. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Mailing address: Nanoscience Technology Center, 12424 Research Parkway, Suite 480, Orlando, FL 32826, USA. E-mail: [email protected]. Present Address

# Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560080, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.T. acknowledges University of Central Florida start-up funding for the completion of this work. N.T.N. acknowledges funding sponsorship from the U.S. Department of Defense: U.S. Air Force Office of Scientific Research for the Project 25960

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