J. Phys. Chem. C 2010, 114, 12517–12523
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Nonlinear Optical Transmission of Nanographene and Its Composites Boshan Zhao,† Baobao Cao,‡ Weilie Zhou,‡ Dan Li,§ and Wei Zhao†,* Department of Chemistry, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204, AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana 70148, and Department of Materials Engineering, Monash UniVersity, Melbourne, Vic 3800, Australia ReceiVed: May 15, 2010; ReVised Manuscript ReceiVed: June 19, 2010
Low-cost broad-band optical limiters based on nanographene have been studied, finding optical-limiting properties superior to those of current standards, carbon fullerenes (C60) solutions and carbon black suspensions. Further examination indicates that the presence of π conjugation improves the optical-limiting responses. Superior limiting performance is retained regardless of solvent viscosity and polarity, a unique feature of graphene not observed in C60 and carbon black. Graphene suspensions in organic solvents can work under 10 Hz laser pulses without losing excellent limiting performance. More significantly, outstanding limiting properties are also preserved in a gel matrix. These nanographene-based optical limiters can thus work in solutions and solid matrixes for devices used for protecting human eyes and optical sensors from high-power lasers. I. Introduction Following the demonstration of the first functioning laser fifty years ago,1 laser-limiting behaviors based on nonlinear optical (NLO) processes were investigated.2,3 As lasers become more powerful,4 the need for protection becomes greater. Accidental discharges or their use as a weapon makes protection from them an increasing necessity. Damage to human eyes and optical sensors can be reduced by the use of new optical-limiting materials and devices with a high linear transmission up to a predetermined input intensity, above which the NLO properties of the materials or devices limit the transmission of light.5-8 In past decades, various materials, including organic dyes, carbon black suspensions, organometallics, fullerenes, semiconductors, liquid crystals, and nanostructures, were studied as optical limiters.5,7-11 However, for practical applications, no single material or limiting mechanism can meet the stringent application requirements.6 For example, C60 solutions are benchmark standards for optical limiters at 532 nm. However, they suffer from a low damage threshold and are not broad-band optical limiters. Carbon black (CB) suspensions are benchmark standards for broad-band optical limiters. However, they do not work well for short pulses, such as picosecond pulses. They exhibit a turnover behavior at a 10 Hz repetition rate in some solvents with relatively high viscosities10,11 and lose stability over time due to carbon particle aggregation. Finding a suitable solid matrix is desirable for practical device applications. In this work, we demonstrated that nanographene, a single-atomic layer carbon nanostructure, is an excellent optical limiter, outperforming the current standards, C60 and CB, a three-dimensional (3D) nanographite material.12 Graphene has recently emerged as a next-generation material. A flat monolayer of carbon atoms packed tightly into a 2D honeycomb lattice, graphene’s structure has been thought impossible to exist in the free state.13 It was discovered by a group of scientists using a simple scotch tape peeling approach * To whom correspondence should be addressed. E-mail: wxzhao@ ualr.edu. Tel: (501) 569-8823. † University of Arkansas at Little Rock. ‡ University of New Orleans. § Monash University.
from single-crystal graphite.13 Graphene is the basic building block for graphitic materials of all other dimensionalities, including 0D fullerenes, 1D nanotubes, and 3D graphite. By varying its size, 2D graphene can be also turned into 0D dots and 1D nanoribbons. Within the 2D classification, graphene can be specifically designated as single-, double-, and few- (3-10) layered, arranged in three different types of 2D crystals. These structures may possess significantly different electronic and optical properties. For example, 2D single-layer graphene is a zero-gap semiconductor with one type of electron and one type of hole,13 whereas quasi-2D graphene ribbons, products of shrinking one of the dimensions of 2D graphene to 99%), 30 wt % hydrogen chloroaurate in dilute HCl solution (HAuCl4, 99.99%), tris(hydroxymethyl)-aminomethane (Tris, >99.9%), orthoxylene (98%), acetonitrile (ACN, 99.9%), tetrahydrofuran (THF, 99.9%), N,N-dimethylformamide (DMF, 99.9+%), and carbon disulfide (CS2, 99.9+%), were purchased from SigmaAldrich. Polyvinyl alcohol (PVA, 99-100% hydrolyzed, molecular weight ) 93 000) was from Eastman Kodak. Borax (sodium borate, tetra crystals) was from Fischer Scientific. The buffer solution was Tris buffer (10 mM, pH 8.0). B. Preparation of Graphene, Carbon Black, and AuGraphene Aqueous Suspensions and C60 Solutions. The synthesis and structural characterization of GO and chemically reduced GS have been reported in ref 25. Because carboxylic acid groups are unlikely to be reduced by hydrazine under the given reaction conditions, the GS contain carboxylic acid groups, as revealed by IR absorption spectroscopy.25 The graphene sheets have dimensions of about 200 by 300 nm with a height of ∼0.9 nm, as shown in the atomic force microscopy (AFM) image in Figure 1A. The AFM image was taken in tapping mode using Veeco SPM Dimension 3100. The height of ∼0.9 nm measured by AFM indicates single-layer graphene.14An aqueous suspension of GS was found to be unstable under shaking and particularly when mixed with other solutions. Therefore, a modified surface functionalization approach using DNA was adopted to stabilize it.26 A dsDNA solution was prepared by dissolving 10 mg of dsDNA in 5 mL of Tris buffer. About 1 mL of the GS suspension (∼0.2 mg GS/mL) was added into the solution. The mixture was sonicated (Sonics model VCX 130 PB, 20 kHz, output power ) ∼8 W) in a hot bath, the temperature reaching ∼80 °C, which allowed the double-
Zhao et al. stranded DNA to unzip and coat on graphene, stabilizing it in solution.26 To compare size effects, three GS samples with sheet sizes GS1 > GS2 > GS3 were also prepared by a simple centrifugation process. The GO aqueous suspension was stable over a long period of time. It was diluted with distilled water to the appropriate concentration. The CB suspension was prepared by dissolving water-soluble CB (∼0.1 mg) in 5 mL of distilled water or 1 wt % SDS aqueous solution with mild sonication. A saturated C60 solution was made by dissolving excess amounts in 1 mL of orthoxylene, which showed the same limiting performance as in toluene.5 Lastly, a suspension (∼1 mL) of a Au-graphene (Au-GS) composite was prepared by reducing HAuCl4 (5 µL in 300 µL of Tris buffer) with the DNA-graphene suspension (∼0.1 mL) at pH 9.5, adjusted with 0.5 M NaOH, then adjusted back to pH 8 in Tris buffer using 0.1 M HCl. The pH was monitored using an Orion model 420 pH meter with a Fisher AccupHast Microprobe electrode. The size of the Au particles is about 150-200 nm, composed of a few smaller gold nanoparticles, as revealed by transmission electron microscopy (TEM) (Figure 1B). The TEM measurement was performed using a JEOL TEM 2010. Next, all samples of suspensions or solutions were hosted in 1 mm quartz cells, measuring their optical absorption spectra in a Varian Cary 5000 UV-vis-NIR spectrophotometer. The samples’ concentrations were adjusted to optimal transmittance, ∼70% transmittance at 532 nm (path length ) 1 mm). After concentration adjustment, the samples were transferred to 1 mm quartz cuvettes for opticallimiting measurements. For mechanism studies, GS and GO suspensions in 1 cm quartz cuvettes with an ∼50% transmittance at 532 nm were also prepared. C. Graphene Suspensions in Organic Solvents and PVA Gel. To test the effects of different solvents on limiting, GS were dispersed in ACN, THF, DMF, and a mixture of DMF and CS2 (v/v ) 1:1). No stable GS suspension was made in pure CS2. The starting GS suspension (0.2 mL) was centrifuged at 16 000g in a VWR Galaxy 16 Microcentrifuge for 10 min. The top water layer was carefully decanted, leaving the watersaturated GS precipitate. One milliliter of an above-mentioned solvent was added into the GS precipitate, which was then sonicated in an ultrasonic bath (Branson model 1510R-MT, 42 kHz, output power ) 70 W) ∼3 to 5 min to disperse the GS. About 8 wt % of water remained in the organic suspensions, as determined by IR absorption spectroscopy. GS suspensions in the organic solvents were stable over a few days, long enough to take optical measurements. The absorption spectra of the suspensions were taken using 1 mm quartz cells; each solution’s concentration was adjusted to give an ∼50% linear transmittance at 532 nm (path length ) 10 mm). Preparation of GS in PVA gel was performed by mixing a dsDNA-GS suspension in a calculated volume with a 4 wt % borax aqueous solution; this mixture was added to a 4 wt % PVA aqueous solution in a 1:9 volume ratio of the borax solution to the PVA solution. The resulting gel was homogenized in a hot water bath of ∼70 °C and then allowed to sit overnight at room temperature to allow air bubbles to leave the gel. The resulting GS-PVA gel has a linear transmittance of ∼50% at 532 nm with a path length of 10 mm. The above samples were transferred into 1 cm quartz cuvettes for optical-limiting measurements. D. Optical-Limiting Measurements. The laser used in the limiting experiments was a Continuum Powerlite Precision II Series model 8000 Injection Seeded Nd:YAG nanosecond pulsed laser with output λ ) 1064.2 nm and an 8 ns pulse duration. The laser was used to pump two LaserVision optical parametric oscillators and optical parametric amplifier systems
Nonlinear Optical Transmission of Nanographene
Figure 2. Absorption spectra of suspensions of GS, CB, and Au-GS composite and of C60 solution in othoxylene. The inset is the GO absorption spectrum.
to generate two adjustable laser beams covering 532 nm to 5 µm. Here, two laser beams with wavelengths of 532 and 730 nm were used to examine the optical-limiting properties of the samples. To further assess the broad-band limiting properties of GS, two near-IR laser beams of 800 and 1300 nm, the latter wavelength used in a powerful laser system,4 were also used in this work. A measurement setup similar to that in ref 27 was used. Briefly, the 532 nm laser beam was passed through a CaF2 lens with a focus length of 50 cm and focused onto the samples. The focus diameter was ∼100 µm. The intensity of the laser before and after passing the sample was recorded with a pyroelectric detector (J8LP, Coherent-Molectron, Inc.) that was connected to a computer with a National Instruments data processing board and a LabView program. The transmittance T was then measured as a function of laser incident energy or incident fluence. The input energy was varied using a series of optical filters with different optical densities. Unless otherwise stated, the limiting data were taken for the first laser shot. The data gathering steps were repeated with the red laser beam of 730 nm and the two near-IR laser beams of 800 and 1300 nm and a focus size of ∼230 µm. For each sample, T was measured a minimum of three times at each input power. For all data sets, the average standard deviation was less than 10%. The graphene data were analyzed by plotting output fluence as a function of input fluence and compared with data of CB, C60, and Au-GS in order to evaluate its effectiveness. The limiting mechanisms were also evaluated by fitting selected data with a TPA equation.28 The normalized nonlinear transmittance of graphene in organic solvents and PVA gel was plotted as a function of input energy. The limiting threshold, defined as the input power at 50% of the linear transmittance, was determined from the plot of each sample. III. Results and Discussion A. Linear Absorption and Optical-Limiting Responses of Graphene in Aqueous Suspensions. Figure 1 shows the aqueous suspensions of GS, CB, and Au-GS and C60 solution in orthoxylene hosted in 1 mm quartz cuvettes. Figure 1A shows the AFM image of the graphene sample with dimensions of about 200 by 300 nm and a height of 0.9 nm, typical for singlelayer graphene measured by AFM.14 Each gold nanoparticle in the Au-graphene composite was approximately 150 nm in size, as illustrated in the TEM image in Figure 1B. They appear to be aggregates of several smaller gold nanoparticles. The absorption spectra in Figure 2 reveal properties inherent to each material. The concentration of all samples has been adjusted so they have nearly the same absorbance at 532 nm.
J. Phys. Chem. C, Vol. 114, No. 29, 2010 12519 C60 has strong absorption near the UV range and relatively weak absorption in the visible range. Suspensions of GS, CB, and Au-GS exhibit broad-band absorption, ranging from ultraviolet (UV) to near-IR. The GO suspension has a strong absorption peak near 250 nm, with absorption tailing off in the near-infrared due to disrupted π conjugation. The optical-limiting responses of these samples are shown in Figure 3 under single shots. At 532 nm with a 3 ns pulse duration, all of the samples exhibited limiting responses to a certain extent (Figure 3a). A line that represented 63% linear transmittance (TL), (7% loss due to reflection by quartz cuvette surfaces; in total, 70% transmittance) was not exceeded by any sample. C60 and GS had similar limiting properties, with CB ranking third. Au-GS exhibited the least limiting of the four. The optical-limiting response of these samples at 730 nm is shown in Figure 3b. All samples except for C60 exhibited limiting. C60 absorbed little of the incoming light, consistently having transmittances of over 90% in an almost perfectly linear relationship, which could be related to its lack of absorption at red and longer wavelengths (Figure 2). In contrast to C60, GS, CB, and Au-GS continued maintained optical-limiting behaviors due to their broad-band absorption. Among them, GS demonstrated limiting the best, followed by CB and Au-GS. It should be noted that Au-GS did exhibit good limiting performance. It appeared that the laser broke down the Au nanoparticles due to laser photothermal reshaping,29 enhancing absorption effects at 532 nm (Figure S1 in the Supporting Information). At 730 nm, the performance was comparable to that of CB. From here, it appears that graphene composites have prospects for further exploration. The size effects of GS samples were also examined. The optical-limiting responses of the samples are shown in Figure 4. At 532 nm, all GS samples are superior to the CB suspension and the GO suspension. GS1 sample has a limiting response comparable to that of C60. At 730 nm, GS1 and GS2 samples outperform the CB suspension. The performance of GS3 is between CB and GO, whereas GO is a weak optical limiter, close to C60. The results suggest that size-related nonlinear scattering may play an important role in the observed GS limiting behaviors.24,30 B. Limiting Performance and Limiting Mechanisms of Graphene and Graphene Oxide. Extended π conjugation and defects may play an active role in optical-limiting behaviors. The limiting response of GS is better than that of GO, as shown in Figure 5; examination of the scales of each graph shows GS limiting almost twice as well. This difference may be related to the degree of π conjugation in the samples. As shown in Figure 6, GS has extended π conjugation, whereas in GO, the π conjugation is disrupted due to defects, such as carboxylic acid and phenolic hydroxyl groups.25 The results suggest that extended π conjugation enhances limiting performance. Understanding the limiting mechanisms will provide insight into designing better limiters. Two mechanisms for optical limiting that are most interesting are TPA and reverse saturable absorption (RSA).31 TPA materials widely studied are semiconductors, such as GaAs, with a band-gap energy greater than the photon energy of the light source, giving the desirable feature of high transmittance for low incident light. At high intensities, processes associated with TPA, including free carrier generation and absorption and defocusing caused by an induced change in refractive index, limit the throughput of the limiter.8,32 TPA limiters include bulk semiconductor materials as well as semiconductor nanoparticles.31
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Figure 3. Optical-limiting responses of aqueous suspensions of GS, Au-GS, and CB and of C60 solution in orthoxylene at 532 (a) and 730 nm (b). For the samples at 730 nm, TL ) 73% for GS and CB, 59% for Au-GS, and 90% for C60. For comparison among the samples, the TL is normalized to 1 in plotting (b).
Figure 4. Optical-limiting responses of aqueous suspensions of GS1-3, GO, and CB and of C60 solution in orthoxylene at 532 (a) and 730 nm (b). TL at 532 nm is 63% for samples GS1-3 and CB and 56% for GO and C60. For the same samples at 730 nm, TL ) 67% for GS1-3, 68% for CB, 73% for GO, and 81% for C60. For comparison, the TL is normalized to 1 for plotting both panels.
Figure 5. Limiting comparison of aqueous suspensions of GS and GO for (532 nm at top, 730 nm at bottom, path length ) 1 cm). Red line: fitting with eq 1.
In RSA, the excited state has greater absorption than the ground state, thus giving rise to limiting effects. For RSA, there must be a certain amount of linear absorption to keep the sample in an excited state.5,31 RSA limiting materials include fullerenes, such as C60, and some organic dyes, such as metalloporphyrins. Other mechanisms include nonlinear refraction and nonlinear scattering. Nonlinear refraction may induce defocusing, whereas in nonlinear scattering, laser bursts superheat the sample, causing avalanche ionization and
formation of microplasmas, which expand to the surrounding liquid and strongly scatter the incident laser light33 as well as forming bubbles that contribute to the scattering effect. CB suspensions exhibit this type of limiting, in which the main components are graphite nanoparticles.12 In considering limiting mechanisms (TPA in this case), eq 1 is for TPA for a laser beam with a Gaussian spatial and temporal profile in conjunction with linear absorption28
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Figure 6. Structural diagrams of GO and GS.
Io )
Iie-RL
(1)
1 + (1 - e-RL)βIi /2√2R
where R is the linear absorption coefficient, β is the TPA coefficient, L is the path length, and Ii and Io are the input power and the out power, respectively. The sample surface reflection is neglected in the equation. The equation was used to fit the data sets in Figure 5 for GS and GO. It seems that the data of GS cannot be fitted by the TPA equation because it should be a semimetal. However, there are a few reasons to try; perfect GS has a cone-shaped π band structure near the high-symmetry Dirac point.15 Even with a zero band gap, graphene electrons in the valence band of the lower cone can absorb a wide range of wavelengths of light and are excited into empty states in the conduction band of the upper cone, accounting for its broad optical absorption.15 The TPA process may be possible for GS based on its unique band structure. Second, GS with defects may become semiconducting,25 thus suitable for the TPA process. Third, the fitting may result in an effective TPA coefficient because the data may appear to fit the pure TPA equation, but the actually undergoing mechanisms may involve more complicated mechanisms, such as nonlinear scattering and nonlinear refraction.30 The fitted TPA coefficients are listed in Table 1, which provide a relative quantity for evaluating the limiting performance between GS and GO. It is difficult to compare those values with those reported by other groups because experimental conditions, including the concentration of these samples, could differ widely, causing variations in the TPA coefficients. The obtained TPA coefficients based on eq 1 are about 2 times larger than those obtained by the TPA equation with an incident light beam of a uniform transverse intensity distribution,6,7,31 in agreement with the theoretical calculation.7 The TPA coefficients are largest for GS in both green and red wavelengths, about 2 times larger than GO at 532 nm. From the fitting of data, it also appears that GO has a better fit with the TPA equation. This may indicate that GO’s TPA effects contribute more significantly to its limiting. GS may also have more complicated mechanisms involved, possibly nonlinear scattering and nonlinear refraction;10,11 further investigation would be required to explicate the detailed mechanisms. C. Optical-Limiting Responses of Graphene in Organic Suspensions. The optical-limiting responses of GS in organic solvents at 532 nm are superior to that of GS in aqueous solution, as shown in Figure 7. The viscosity and polarity of solvents with their respective limiting thresholds are listed in Table 2. Graphene in THF performs the best with the lowest TABLE 1: TPA Absorption Coefficient β (cm/GW) at Different Laser Wavelengths samples
532 nm
730 nm
GO GS
2.8 ( 0.1 7.1 ( 0.3
0.38 ( 0.02 2.4 ( 0.1
Figure 7. Optical-limiting responses of various GS suspensions in aqueous and organic solvents at 532 nm. The inset plot shows opticallimiting responses of a GS suspension in THF at 800 and 1300 nm.
TABLE 2: Solvent Viscosities, Dielectric Constants, and Experimental Limiting Thresholds for GS Suspensions
environment
viscositya (mPa · s)
water DMF THF ACN CS2 DMF-CS2 (v/v ) 1:1) PVA gel
0.890 0.794 0.456 0.369 0.352 0.415b 528d
polarity (dielectric constanta)
limiting threshold (mJ/J cm-2)
80.1 38.25 7.52 36.64 2.63 20.4c
0.25/3.2 0.15/1.9 0.097/1.2 0.11/1.4 0.15/1.9 0.20/2.5
a From ref 34. b Calculated value based on the Refutas equation in ref 35. c Calculated volume-fraction-weighted value assuming an ideal mixture based on ref 36. d From ref 37.
limiting threshold, 1.2 J/cm2, outperforming those reported in ref 24 where solvents N,N-dimethylacetamide (DMA), N-methyl-2-pyrrolidone, and γ-butyrolactone were used. The best limiting threshold (defined there as 85% of the linear transmittance) was about 2 J/cm2 in solvent DMA.24 It is known that the optical-limiting behaviors are related to the viscosity and the polarity of the host solvent.11 Here, the viscosities of the solvents used follow the order water > DMF > THF > DMF-CS2 > ACN > CS2; their polarities (dielectric constant) follow water > DMF > ACN > DMF-CS2> THF > CS2. Clearly, the lower-viscosity solvents, such as THF and ACN, performed better than higher-viscosity solvents, such as water. However, there is one exception here. DMF has a similar viscosity to that of water, but the former has a better limiting response than the later. This difference might be related to the difference in their polarities. One of the unique optical features of GS is its broad linear optical absorption extending from UV to IR.15 It is expected that its limiting behaviors will also be observed in other wavelengths, as a broad-band limiter. To further study that, the optical-limiting responses of a GS suspension in THF were measured, as shown in the inset of Figure 7, using 800 and 1300 nm laser beams. Compared with a wavelength of 532 nm where a limiting threshold of 1.2 J/cm2 was determined (Table 2), its limiting strength decreases at longer wavelengths, with a
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Figure 8. Comparison of optical-limiting responses of GS in a polymer gel matrix PVA with an aqueous GS suspension at 532 nm.
limiting threshold of 3.4 J/cm2 at 800 nm and 13 J/cm2 at 1300 nm, consistent with previous observations in other carbon-based broad-band limiting materials24 and semiconducting nanoparticles.31 D. Optical-Limiting Responses of Graphene in a Polymer Gel Matrix. The optical-limiting behaviors of GS in a polymer gel PVA were also investigated in order to explore the possible device applications. The gel matrix provides several advantages over liquids or solids. First, it contains over 90 wt % of water, but possesses a clear, glass-transparent, solid-like structure, making the dopant stable in the matrix over a long period of time without precipitation, while also maintaining an opticallimiting environment similar to liquids. Second, the laser damage threshold of the gel may be greater than that of a solid polymer matrix, approaching that of the liquid suspension. Third, the gel matrix can become a fluid at temperatures of 60-80 °C, allowing for reshaping the gel or rehomogenizing the dopant, important features for recoverable and reusable devices. As shown in Figure 8, with a linear transmittance of 50% at 532 nm with a 1 cm path length, GS in PVA gel has the same optical-limiting performance as the GS aqueous suspension, one of the best optical-limiting samples in this study. This result is significant, the first demonstration of an excellent opticallimiting performance of graphene preserved in a polymer gel matrix, indicating great promise for solid optical-limiting devices.
Zhao et al. E. Optical-Limiting Responses of Graphene under 10 Hz Pulses. As shown in Figure 9, GS samples in organic solvents also show one more notable limiting property; they work as optical limiters for 10 Hz pulses, a more challenging condition for CB.10,11 In Figure 9, the first dozens of shots near zero transmittance indicate the baseline by blocking the laser beam off the sample. A jump-in transmittance occurs when the laser beam is on a sample. Limiting takes place in the organic solvents fairly well under continuing 10 Hz pulse shots at 532 nm, with a constant nonlinear transmittance, which is always lower than the linear transmittance. This performance was also observed at 800 and 1300 nm, as shown in Figure S2 in the Supporting Information for GS in THF. For GS in water and PVA gel, the limiting goes well for the first a few shots; bleaching and subsequent loss of limiting occur under continuing shots. Further measurements of GO indicate that this material has a good limiting performance without the bleaching that occurs in aqueous GS suspensions. Even more interestingly, a similar limiting performance of GO is retained in the gel matrix as well (Figure S3 in the Supporting Information). As estimated from the TPA coefficients in Table 1, GO is about 2 times weaker than GS at 532 nm. Therefore, a compromise may be made for a better limiting device by using a composite composed of GO and GS. In addition, no turnover behavior was observed for GS and GO even under high input fluences, a behavior significantly different from CB suspensions.11 These interesting results suggest that GS and GO perform differently from CB with better optical-limiting responses. IV. Conclusions In summary, graphene was superior as a broad-band optical limiter to the C60 solution and the carbon black suspension, the current optical-limiting standards. Further examination indicated that the presence of π conjugation improves the optical-limiting responses. A superior limiting performance was retained regardless of solvent viscosity and polarity. Graphene suspensions in organic solvents can work under 10 Hz laser pulses without losing excellent limiting performance. More significantly, the outstanding limiting properties are also preserved in a polymer gel matrix, suggesting
Figure 9. Nonlinear transmittance change as a function of the number of laser pulses (532 nm, 0.78 mJ, 10 Hz) for GS in different solvents and in PVA gel.
Nonlinear Optical Transmission of Nanographene possible solid device applications. Further work may be extended to understand these unique results and reveal the detailed mechanisms. The potential of nanographene could be further evaluated through modification of graphene nanosheet and nanoribbon dimensions to adjust limiting behaviors, chemical functionalization in the 2D network, and edges through the introduction of various chemical groups and hybridization of materials to get stronger broad-band limiting. Acknowledgment. We thank Mr. Marc B. Mu¨ller for taking the AFM image of graphene. Supporting Information Available: Figure S1, absorption spectra of a Au-GS suspension before and after laser irradiation; Figure S2, optical-limiting responses of GR in THF at 800 and 1300 nm; and Figure S3, optical-limiting responses of GO in aqueous solution and PVA gel. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Maiman, T. H. Nature 1960, 187, 493–494. (2) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S. P. S.; Whinnery, J. R. Bull. Am. Phys. Soc. 1964, 9, 501. (3) Leite, R. C. C.; Moore, R. S.; Whinnery, J. R. Appl. Phys. Lett. 1964, 5, 141–143. (4) Hecht, J. Laser Focus World 2010, 40, 36–41. (5) Sun, Y.-P.; Riggs, J. E. Int. ReV. Phys. Chem. 1999, 18, 43–90. (6) Wood, G. L.; Mott, A. G.; Sharp, E. J. Proc. SPIE 1992, 1692, 2–14. (7) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Chem. ReV. 2008, 108, 1245–1330. (8) Tutt, L.; Boggess, T. F. Prog. Quantum Electron. 1993, 17, 299– 338. (9) Zhang, L.; Allen, S. D.; Woelfle, C.; Zhang, F. J. Phys. Chem. C 2009, 113, 13979–13984. (10) Mansour, K.; Soileau, M. J.; Van Stryland, E. W. J. Opt. Soc. Am. B 1992, 9, 1100–1109. (11) Hernandez, F. E.; Shensky, W., III; Cohanoschi, I.; Hagan, D. J.; Van Stryland, E. W. Appl. Opt. 2002, 41, 1103–1107. (12) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (13) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191.
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