Control of Optical Limiting of Carbon Nanotube Dispersions by Changing Solvent Parameters

Control of Optical Limiting of Carbon Nanotube Dispersions byChanging Solvent Parameters

Jun Wang,*,† Daniel Fruchtl,† Zhenyu Sun,‡ Jonathan N. Coleman,† and Werner J. Blau†

School of Physics and the Centre for Research on AdaptiVe Nanostructures and NanodeVices (CRANN),Trinity College Dublin, Dublin 2, Ireland, and Beijing National Laboratory for Molecular Sciences, Institute ofChemistry, Chinese Academy of Sciences, Beijing, 100190, P.R. China

ReceiVed: December 11, 2009; ReVised Manuscript ReceiVed: February 17, 2010

Nonlinear optical and optical limiting properties of a range of single-walled carbon nanotube dispersionsprepared in N-methyl-2-pyrrolidinone (NMP) were studied using the open aperture Z-scan technique at 532nm. As the appropriate thermodynamic properties of the solvents are much more important than the bundlesize of nanotubes for improving the optical limiting performance, the solvent parameters were controlled byeither changing the temperature of the dispersions or blending a secondary solvent. While the optical limitingperformance can be varied freely by increasing or decreasing the temperature from room temperature to100 °C, the reduction of temperature below the freezing point of NMP and then down as far as -80 °C haslittle influence on the limiting performance. As a result of adding a small amount of organic solvent into theNMP dispersions, the nonlinear optical responses were enhanced significantly due to the reduction of surfacetension and other parameters. By contrast, the addition of water leads to a decrease in the optical limitingresponse. Nanotube dispersions in water/surfactant exhibit a similar limiting performance to the nanotubes inNMP. Our results reveal that the optical limiting performance of the nanotube dispersions can be engineeredby adjusting the solvent properties. Because the carbon nanotube dispersions are typical of the thermallyinduced light scattering dominated optical limiting materials, we believe the conclusions fit not only thenanotubes but also other nanomaterials with the similar limiting mechanism.

1. Introduction

As a consequence of the rapid development of nanotechnol-ogy over the past few decades, a variety of nanomaterials havebeen shown to possess remarkable nonlinear optical (NLO)properties.1-4 The most representative products are carbon-basednanomaterials: from 3-D graphite (carbon black) to 0-Dfullerenes, to 1-D carbon nanotubes (CNTs), and then to 2-Dgraphenes, discovered most recently. The 1-D structure of CNTsgives the material outstanding mechanical, thermal, electricaland optical properties, that is, high Young’s modulus, tensilestrength, thermal conductivity, carrier mobility, and third-ordernonlinear polarizability.5-7 In particular, the unique NLOproperties of CNTs motivates the design and fabrication ofnanophotonic devices, for example, optical switches,8 mode-lockers,9 and optical limiters.10

As an effective protection device for optical detectors, sensors,and the human eye from intense laser beams, optical limitershave huge potential for military and day to day applications. Asuccessful optical limiter should strongly attenuate intense andpotentially dangerous laser beams, while exhibiting high-transmittance for low-intensity ambient light. Following on fromoptical limiting studies on carbon black suspensions andfullerenes, CNTs have been intensively investigated as an opticallimiting material over the past decade.2-4,10-12 It is highlyadvantageous that the nanotubes combine the advantages of theother two allotropes; carbon black suspensions have broadbandoptical limiting and the fullerenes act as a favorable counterpart

for functional materials. The 1-D nanostructure of CNTs actsas a favorable host for functional materials, for example, metalnanoparticles,13 organic molecules,14-19 and polymers,20-22 al-lowing for the formation of versatile optical limiting composites.In opposition to materials such as phthalocyanines,23-26

porphyrins,27-29 and fullerene,30-32 which display limiting effectsin specific visible spectral regions due to reverse saturableabsorption, CNT dispersions exhibit prominent optical limitingbehavior over a broad wavelength range from the visible to thenear-infrared, resulting from effective scattering by laser-inducedmicrobubbles and microplasmas.

The thermally induced light scattering is a common phenom-enon for nanomaterials for optical limiting.3 The scatteringprocess can disperse the highly intense beam into a larger spatialdimension and hence reduce the intensity of the direct incidentbeam. According to Mie scattering theory, the nanoscaleparticles alone cannot scatter a light beam effectively. Theeffective scattering arises from the formation of scatteringcenters with a size of the order of the wavelength of the incidentlaser beam. In general, the scattering centers, initiating fromnanostructures, consist of microbubbles and microplasmas. InCNT dispersions, the microbubbles originate from the thermalenergy transfer from the nanotubes to the solvent and themicroplasmas are induced by the ionization of nanotubes byintense laser pulses.33,34 Because the scattering efficiency islargely dependent on the formation dynamics of the scatteringcenters, the optical limiting performance can be influenced bythe structure of nanotubes and the thermodynamic propertiesof the solvent used to disperse the nanotubes. CNTs with largerbundles have bigger initial sizes of scattering centers and moreeffective energy transfer to solvents, resulting in a largerscattering cross-section and better limiting efficiency. On the

* Corresponding author. E-mail: [email protected]; [email protected].

† Trinity College Dublin.‡ Chinese Academy of Sciences.

J. Phys. Chem. C 2010, 114, 6148–61566148

10.1021/jp9117248 2010 American Chemical SocietyPublished on Web 03/10/2010

other hand, the heat-induced microbubbles or plasmas growfaster in a solvent with lower surface tension and boiling point,so they can reach the critical size for scattering in a shortertime; consequently, a more effective limiting can be achieved.

Very recently, we verified experimentally that the appropriatethermodynamic properties of the solvents are much moreimportant than the bundle size of nanotubes for improving theoptical limiting performance and that the surface tension playsthe most essential role in comparison with the other parameterssuch as boiling point and viscosity.35 While keeping thenanotube dispersions stable against sedimentation, the thermo-dynamic parameters and, hence, the optical limiting perfor-mances were improved by increasing the temperature of thedispersions or by blending suitable secondary organic solvents.In this work, we report that the NLO response of nanotubedispersions can be enhanced or suppressed freely by adjustingthe temperature. The influence of adding different solvents,especially water, on the optical limiting is also investigatedintensively.

2. Experimental Section

N-Methyl-2-pyrrolidinone (NMP) is a good dispersant forCNTs due to the large interaction energies between nanotubesand solvent molecules.36,37 The surface energy of NMP matchesvery well with that of graphite (70-80 mJ m-2), resulting in aminimal energy cost in overcoming the van der Waals forcesbetween two nanotubes, hence, the effective exfoliation toindividual nanotubes or small bundles. High quality NMPdispersions were employed as the primary optical limitingmaterials in this work. We used a similar preparation procedureto prepare nanotube dispersions as those described in ourprevious works.34,36,37 Single-walled nanotubes (HiPCO) fromCarbon Nanotechnologies were used without further purification.Dispersions of nanotubes were prepared in NMP at an initialconcentration of 0.1 mg/mL. For the most part, initial dispersionswere sonicated for 2 min using a high-power ultrasonic tipprocessor, model GEX600 (120 W, 60 kHz). This was followedby 4 h in a low-power ultrasonic bath and then further sonicationfor 1 min. All dispersions were subsequently centrifuged at 5500rpm for 90 min to remove any large aggregates. The concentra-tions of nanotubes after centrifugation were deduced from theabsorbance ratios of dispersions before and after centrifugation.The final dispersions were then consecutively diluted to producedispersions with a fixed concentration of 0.01 mg/mL. Thedetailed study of the nature and quality of these nanotubedispersions has been reported previously.36 The prepared 0.01mg/mL dispersions were consecutively blended with a rangeof solvents, for example, acetone, tetrahydrofuran, toluene,chloroform, N,N-dimethylacetamide (DMA), water, and so on,to modify their thermodynamic properties. The detailed ther-modynamic parameters of the solvents are given in the Sup-porting Information (Table S1). Even after the addition of thesolvents up to 20 vol %, the dispersions remained free fromsedimentation and further aggregation for a period of weeks.While the pure NMP dispersions were heated up to 100 °C bya temperature controller system, another batch of NMP disper-sions was frozen in a freezer at different temperatures from -16to -22 °C then to -80 °C. The nanotube dispersions were quitestable for a few hours when the temperature was raised up to100 °C. No visible aggregations were observed for dispersionsat temperatures above -22 °C when the samples remained atthese temperatures for up to 24 h. However, large amounts ofsedimentation were observed in the dispersions frozen at-80 °C after a period of 10 h. NLO measurements were carried

out on the frozen dispersions after the samples were allowed torecover to room temperature (RT).

The Z-scan is a well-known technique to characterize theNLO properties of materials, including nonlinear absorption,scattering, or refraction.38 A standard open-aperture Z-scansystem was used to measure the nonlinear coefficients, as wellas the optical limiting properties. This measures the totaltransmittance through the sample as a function of incident laserintensity, while the sample is gradually moved through the focusof a lens (along the z-axis). Effective extinction (absorption and/or scattering) coefficients are calculated by fitting theory reportedpreviously.23,38 The normalized transmittance as a function ofposition z, Tnorm(z), is given by

where q0(z) ) q00/[1 + (z/z0)2]; z0 is the diffraction length ofthe beam; q00 ) �effI0Leff; �eff is the effective intensity-dependentnonlinear extinction (NLE) coefficient; and I0 is the intensityof the light at focus. Leff is known as the effective length of thesample defined in terms of the linear absorption coefficient, R0,and the true optical path length through the sample, L, Leff )[1 - exp(-R0L)]/R0.

All experiments were performed using a 6 ns-pulsed Q-switched Nd:YAG laser. The laser was operated at the secondharmonic, 532 nm, with a pulse repetition rate of 10 Hz. Thebeam was spatially filtered to remove higher-order modes andtightly focused using a 9 cm focal length lens. The waist radiusfor all experiments was calculated as ∼20-23 µm. The incidentlaser energy was altered from 0.03 to 0.3 mJ per pulse. Thecorresponding on-focus beam intensity was varied from 0.2 to2.0 GW/cm2. Simultaneously, a focusing lens setup was placedat ∼35° to the direction of the incident beam to monitor thescattered light from dispersions. For the sake of setting thetemperature controller system, 10 mm quartz cuvettes were usedfor all experiments. The stability and bundling process of thedispersions at different temperatures were studied using a staticlight scattering experiment, focusing lens setup at 45° to thedirect incident beam. In addition, an atomic force microscope(AFM, Multimode Nanoscope IIIA) was used to measure thebundle size distribution of nanotubes in NMP. The AFMsamples were prepared by casting few drops of nanotubedispersions on precleaned Si substrates, followed by a hightemperature vacuum treatment to get rid of the residual NMP.

3. Results and Discussion

Figure 1 depicts open-aperture Z-scan and correspondingscattering results for the nanotube dispersions at differentincident intensities and at different temperatures. All open-aperture Z-scan measurements performed in this work showeda clear reduction in the transmission followed by an increasein the scattering about the focus of the lens. This is typical oflight scattering induced optical limiting of the incident light.Because the solvent parameters of NMP, for example, surfacetension and viscosity, are sensitive to the temperature and addedsolvents, the NLO and optical limiting performances of thenanotube dispersions, as shown in Figure 1, can thus beengineered very well by controlling these two factors.

3.1. Temperature Effect. Of the two methods of controllingthe optical limiting responses, the temperature method is quiteinstant and flexible. To identify the feasibility, we designed threeheating schemes (as given by the dash-dotted heating curves in

Tnorm(z) )log[1 + q0(z)]

q0(z)(1)

Optical Limiting of Carbon Nanotube Dispersions J. Phys. Chem. C, Vol. 114, No. 13, 2010 6149

Figure 2): (1) heating the dispersions by increasing the tem-perature from RT (∼20 °C) to higher constant values, such as,40, 60, 80, or 100 °C, and then decreasing back to RT,respectively; (2) continuously increasing the temperature fromRT to 100 °C, and then decreasing back to RT at 20 degreeincrements; (3) switching the temperature between RT and100 °C. As shown in Figure 2, the NLE coefficient �eff wasimproved significantly by increasing the temperature up to100 °C and it was precisely adjusted to a certain level byincreasing or decreasing the temperature. Most importantly, theoptical limiting response can be programmed by setting theheating profile in advance.

In our experiment, strong light scattering was seen when thedispersions passed through the focus of incident beam. Wecollected a fraction of scattered light using a convex lens at∼35° horizontally to the direct incident beam. Figure 3a showsthe nonlinear transmission and scattering as functions of incidentenergy density for the NMP dispersions at different tempera-tures. It is clearly seen that the scattered intensities increasesignificantly along with the decrease of transmission and theonset of the growth of scattered signals is synchronous withthe onset of the decrease of transmission for all dispersions,indicating that nonlinear light scattering is responsible for theoptical limiting. When the temperature was switched between

20 and 100 °C, the transmission trend and scattering signal werevaried reversibly. We employ the value of energy density, whichcorresponds to the normalized transmittance of 85%, to evaluatethe optical limiting response of dispersions. The linear transmit-tance is set to unity for all dispersions. For simplicity, we namethe value as the so-called “optical limiting threshold”. Figure3b shows the limiting threshold and scattered intensity asfunctions of the temperature of the dispersions. Because thesurface tension of NMP decreases gradually as the temperature,39

Figure 1. Open-aperture Z-scan results with normalized transmissionand scattered signal for the NMP dispersions at different temperatures(a) and for the NMP/acetone and NMP/DMA dispersions at differenton-focus intensities (b).

Figure 2. Nonlinear extinction coefficient �eff as a function of heatingprofile (dash-dotted curves). The concentration of all dispersions is 0.01mg/mL.

6150 J. Phys. Chem. C, Vol. 114, No. 13, 2010 Wang et al.

it is evident that a smaller surface tension results in a lowerlimiting threshold and more significant scattering intensity. Inaddition, the bundle size of the nanotubes in NMP was measuredfrom AFM images by reading out the height of the bundle above

the Si surface because of the influence of tip convolution onlateral measurements at this scale. About 200-300 nanotubeswere counted from each dispersion which was used at the sametime for the Z-scan measurements. Figure 3c gives the histo-grams of bundle diameters of the dispersions at differenttemperatures. In contrast to the root-mean-square (rms) diam-eters, 3.79 ( 1.47 nm at RT, the diameters were increased to6.52 ( 2.17 nm after the heating treatment at 100 °C. It is clearlyseen that the large bundles did not hasten optical limitingperformances much, as the limiting responses were degradedwhen the dispersions were cooled down to RT, as shown inFigure 3a,b. Thus, for the optical limiting of the well-dispersednanotubes, the contribution of solvent parameters is much largerthan that of bundle size.

The bundling of CNTs in liquid dispersions is a spontaneousand irreversible process. Because the bundled nanotubes are farmore stable than the isolated, the bundles cannot be separatedeasily without the injection of external force, say, ultrasonica-tion.40 At a higher temperature level the bundles are more readilyformed as the free energy barriers between adjacent nanotubes,which are proportional to the surface tension of NMP, aredecreased as well.40 A static light scattering experiment, i.e.focusing lens setup at 45° to the direct incident beam, was usedto study the stability and bundling dynamics of the dispersionsat different temperatures. To avoid any possible microbubblesformed, low energy and repetition rate (1 Hz) laser pulses wereemployed. Figure 4 presents the scattered intensities as afunction of heating time. The decrease of the scattered signalresults from the formation of big bundles and subsequentsedimentation. The NMP dispersions can remain stable againstsedimentation over 140 h at 40 °C. In contrast, the lifetime ofthe dispersions was shortened evidently at 80 and 100 °C, whichwould be an obstacle when it comes to real-life applications asthe performance of a capable optical limiter is expected toremain stable with changing environmental temperature.

The adaptability of CNTs for use in optical limiting isrevealed in part from the Z-scan measurements for nanotubedispersions frozen at different temperatures. Figure 5a showsthe NLE coefficient for the 0.01 mg/mL dispersions frozen at arange of temperatures. Due to the using of different batches ofnanotubes from Carbon Nanotechnologies, the �eff in Figure 5ais slightly larger than that in Figure 2. We did not observe an

Figure 3. (a) Nonlinear transmission and scattering of the NMPdispersions at the temperature switched between 20 and 100 °C. (b)Optical limiting threshold and scattered intensity as functions of thetemperature. The open symbols refer to the dispersions cooled downto RT from higher temperatures. (c) Histograms of bundle diametersfor the nanotube dispersions at 20 and 100 °C. Inset: rms diameter ofthe nanotubes as a function of the temperature. The concentration ofall dispersions is 0.01 mg/mL.

Figure 4. Scattered light intensity as a function of time for the NMPdispersions at different temperatures. The concentration of all disper-sions is 0.005 mg/mL.


apparent difference for the dispersions at frozen temperatureshigher than -22 °C and at RT. This implies that the opticallimiting properties of the nanotube dispersions do not changesignificantly after treating in the frozen environment. A largeamount of sedimentation was observed in the dispersions frozenat -80 °C for 10 h. These dispersions exhibited a larger �eff

compared with the other dispersions in which no sedimentationwas observed. The better NLE effect is mainly due to a largerbundle size in sedimented dispersions. The sedimentation isprobably due to the interaction between nanotubes and NMPmolecules being destroyed at such a low temperature. Thebundle diameter increases due to nanotube aggregation after-80 °C, while the total number density of nanotube bundlesdecreases compared with the dispersions frozen at highertemperatures. This is due to the fact that the dispersions at-80 °C have the same mass concentration as the otherdispersions. Provided that the properties of the NMP remainunaltered after -80 °C, the enhanced NLE effect for thedispersions at -80 °C indicates that, at the same massconcentration, the bundle diameter has more contribution thanthe bundle density to the optical limiting of CNT dispersions.Vivien et al. compared the optical limiting properties ofnanotube suspensions in water and chloroform.41,42 Due to themore effective coalescence of gaseous bubbles in water than in

chloroform, a small quantity of well-defined large bubbles wasobserved in water suspensions from the shadowgraphic pictures,and a large quantity of ill-defined small bubbles was observedin chloroform suspensions. However, the chloroform suspen-sions exhibit a better optical limiting response than the watersuspensions. Vivien’s results seem inconsistent with our inter-pretation. It should not be forgotten, however, that chloroformhas a lower surface tension (27.5 dyn/cm at 20 °C) and lowerboiling point (61.2 °C) than water, which results in part in alarger volume fraction of the gaseous phase in chloroformsuspensions. Moreover, although the linear transmittance wasthe same for suspensions of water and chloroform, the bundlesize distribution was not reported.41,42 In our experiment, wefocused on the effect of the variation of bundle diameter onoptical limiting by fixing all the other conditions.

As shown in Figure 1b, it is noted that the minimumtransmission (Tmin) in the Z-scan data decreases gradually withincreasing on-focus energy density (J cm-2). The inset of Figure5a shows the minimum transmission as a function of the incidentenergy density for the dispersions at RT and -80 °C. Thebundles in the dispersion at -80 °C possess the larger initialsize of bubble and the more effective energy transfer to thesurrounding liquid, resulting in a lower Tmin and better opticallimiting. We cannot tell the distinct difference in the decreasingtrend of Tmin for dispersions frozen at temperatures higher than-22 °C. Figure 5b, in which the normalized transmission wasplotted as a function of input energy density (J/cm2), shows theoptical limiting behavior of three frozen dispersions. The opticallimiting performance of the sedimented dispersion at -80 °Cslightly exceeds that of the dispersions without sedimentation.The limiting thresholds of all dispersions are illustrated in theinset of Figure 5b. The dispersion at -80 °C exhibits the lowestlimiting threshold (∼1.95 J/cm2) and all other dispersions havea similar limiting threshold (∼2.15 J/cm2).

3.2. Solvent Effect. To study the correlation of the opticallimiting performance and the thermodynamic properties ofnanotube dispersions, we measured the NLO properties of theNMP dispersions blended with a range of solvents with aconstant ratio of 4:1 in volume. Considering the physical processof optical limiting in the nanotube dispersions, we chose fourcorresponding parameters that could influence the NLO re-sponses, namely, surface tension, boiling point, viscosity, andrefractive index. The physical parameters of the binary solventmixtures can be deduced theoretically by various models.43-46

For the purpose of simplification, we adopt an ideal equationto predict the parameters given by43-46

where δbin,1,2 and x1,2 are the physical parameters and the molefraction of the binary mixtures, components 1 and 2, respec-tively. It should be pointed out that, whereas the estimated valuescertainly deviate from the real values, it should not affect toomuch the subsequent qualitative conclusions. Figures 6 and 7show the NLE coefficient, limiting threshold and scatteredintensity as functions of surface tension, boiling point, viscosity,and refractive index of the binary solvent mixtures. Overall, itis evidently seen that the optical limiting performances, that is,�eff, limiting threshold and scattering were enhanced when theseparameters were decreased, except in the case of refractiveindex. While the three thermodynamic parameters have anintuitive positive effect on optical limiting, the effect of surfacetension is more pronounced in comparison with the other two.The scattering cross-section of the microbubbles or microplas-

Figure 5. (a) Variation of the NLE coefficient for nanotube dispersionsat different frozen temperatures. Inset: the minimal transmission as afunction of on-focus energy density for various dispersions. (b) Opticallimiting for various frozen dispersions. Inset: plot of threshold energydensity against the temperature. The concentration of all dispersions is0.01 mg/mL. The dashed lines are intended as a visual guide.

δbin ) δ1x1 + δ2x2 (2)


mas in the dispersions is determined not only by the radius ofthe bubbles but also by the ratio of refractive indices insideand outside of the scattered centers.47 However, as shown inFigures 6d and 7d, the refractive index has less of an effect onthe NLO responses of various dispersions except the NMP/waterdispersions.

We present a qualitative analysis to show the credibilityof this conclusion. When a light beam is incident on anindividual particle, the total extinguished power Pext isproportional to the incident intensity Iinc, Pext ) (σabs +σsca)Iinc, where σabs and σsca are the absorption and scatteringcross sections, respectively.48 When the theoretical predictionof Belousova et al. for the absorption and scattering crosssections of carbon particle suspensions under high intensitylaser illuminations was used,49 it can be shown that, as thesize of the vapor bubbles increases, the scattering cross-section increases significantly, while the absorption cross-

section decreases until it is negligible, effectively limitingthe incident power. Although that model was based on quasi-spherical carbon particles, the prediction of the Mie theoryis appreciable qualitatively for CNTs. The Mie theory wasapplied by Vivien et al. to model the optical extinctionscattering profile in CNT suspensions.50 The extinctionthrough the absorption of nanotubes is ignored and theextinguished power is attributed completely to scattering, Pext

) σscaIinc. Because the scattering cross-section σsca is stronglydependent on the size of the vapor bubbles, the expansionprocess of bubbles in the duration of a pulse largelydetermines the scattering efficiency of the nanotube disper-sions to incident laser beam. The dynamics of the expansionor collapse process of a spherical bubble is described by theRayleigh-Plesset equation.51,52 Assuming equilibrium condi-tions, the equation becomes a form of Laplace’s equation

Figure 6. Nonlinear extinction coefficient �eff of the nanotube dispersions as a function of surface tension, boiling point, viscosity, and refractiveindex of the binary solvent mixtures.

Figure 7. Optical limiting threshold and scattered intensity as functions of surface tension, boiling point, viscosity, and refractive index of thebinary solvent mixtures.


where pB is the pressure inside of the bubble, p∞ is thepressure far from the bubble, r is the radius of the bubble,and γ is the surface tension. Supposing that the thermalenergy exchange and mass transport between the gas bubbleand the surrounding solvent are ignored in an instantaneousprocess at the end of a laser pulse, the pressure in the bubblepB can be estimated via the ideal gas law

where rB is the radius of the bubble at the end of a laserpulse, n is the number of moles of gas, R is the universalgas constant, and T is the absolute temperature in the bubble.Equating 3 and 4, the relationship between the surface tensionγ and the bubble size rB is found to be

By substituting realistic values of n and T for the bubbles innanotube dispersions into eq 5, we found that a lower surfacetension results in a larger bubble size, hence, producing moreeffective scattering and optical limiting. In contrast, the lowerthe boiling point of the solvent, the shorter the time neededand the lesser the incident energy needed to initiate a bubble, aprocess that rarely has an effect on the growth dynamics of agas bubble.

It is noticed in Figures 6 and 7 that the addition of waterinto the NMP dispersions resulted in a serious reduction of theoptical limiting performance. To define the negative effect ofwater, we conducted the Z-scan measurements for a range ofwater diluted NMP dispersions. Figure 8a presents the NLEcoefficients for the dispersions diluted by water and NMP,respectively. For the NMP/water dispersions, �eff decreasesgradually with the addition of water. At 0.2-2.0% water dilutionfraction, �eff is 1.38 cm/GW. This decreases gradually andapproaches 0.7 cm/GW as the dilution fraction approaches 20%.In contrast, �eff of the pure NMP dispersions remains ap-proximately constant at 1.4 cm/GW. For pure NMP dispersions,slightly decreasing the nanotube concentration from 0.01 to0.008 mg/mL does not affect the NLE coefficient significantly.The AFM measurements reveal that the rms diameter of theNMP/water dispersions increases monotonically with the ad-dition of water, whereas the diameter of the NMP dispersions(∼2.6 nm) remains roughly constant. The diameter of the pureNMP dispersions, however, is always smaller than that of theNMP/water dispersions.53 The difference in diameter for the twosets of dispersions is ∼0.9 nm as the dilution fraction reaches5% and approaches ∼1.6 nm at 20% dilution fraction. For lowerdilution fraction dispersions (0.2-5%), the NLE coefficientsof the NMP/water dispersions are close to those of the NMPdispersions. This indicates that the variation of the bundle sizes(∼0.9 nm) as well as the slight variation of the thermodynamicproperties of solvents does not affect the NLO responses of thenanotube dispersions. We noticed that a decrease in nanotubeconcentration does not exhibit apparent negative influence on�eff of the pure NMP dispersions. Also, the larger bundlediameter should have a positive effect on the NLO response of

the NMP/water dispersions. Thus, we conclude that the trendof decreasing �eff for the NMP/water dispersions is mainlyattributed to the change of the thermodynamic properties ofsolvents. Water has a high surface tension (72 dyn/cm at 25°C) and low boiling point (100 °C) compared with NMP (202°C boiling point and 41 dyn/cm surface tension at 25 °C). Incomparison with pure NMP, the mixtures of NMP and waterhave higher surface tension and viscosity but lower boilingpoint.54,55 The addition of a small amount of water (10-20%)results in a distinct decrease of �eff, implying that the surfacetension has a larger contribution to the NLO response. Therefore,it must have a larger contribution to the optical limiting responseof nanotube dispersions than the boiling point.

The minimal transmission Tmin plotted as a function of theon-focus energy density is depicted in the inset of Figure 8afor the dispersions diluted by water, where each data point onthe plot represents an independent open-aperture Z-scan of thedispersion in question. It can be clearly seen that Tmin increaseswith increasing dilution fraction, implying a reduced opticallimiting performance of water-incorporated nanotube disper-sions. Figure 8b illustrates the comparison of optical limitingperformances for the various NMP/water dispersions. In agree-ment with the NLE results in Figure 8a, the optical limiting

pB - p∞ ) 2γr

(3)

pB(43

πrB3 ) ) nRT (4)

2γ ) 3nRT

4πrB2- p∞rB (5)

Figure 8. (a) Nonlinear extinction coefficient as a function of thedegree of dilution using water and NMP, respectively. Inset: the minimaltransmission as a function of on-focus energy density for variousdispersions diluted by water. (b) Optical limiting for various dispersionsdiluted by water. Inset: plot of threshold energy density against thedegree of dilution using water and NMP. The dashed lines are intendedas a visual guide.


performance of the NMP/water dispersions decreases withincreasing fraction of dilution. The inset of Figure 8b showsthe threshold energy density versus the dilution fraction ofdispersions. It can be noted that the threshold of the pure NMPdispersions remains approximately constant (∼2.07 J/cm2) andlies in a lower level when compared with that of the NMP/water dispersions. In contrast, the threshold of NMP/waterdispersions increases gradually with increasing dilution fraction(approaching ∼4.41 J/cm2 as the dilution fraction approaches20%). As discussed above, the formation process of bubblesdetermines the scattering efficiency and, hence, the performanceof optical limiting. The higher surface tension of NMP/waterdispersions suppresses the expansion rate of bubbles. Moreincident energy is needed to make the bubbles grow to a criticalsize for effectively scattering light.

In addition, a surfactant-stabilized CNT dispersion wasprepared in water. The concentration of the surfactant, sodiumdodecylbenzene sulfonate (SDBS), was 10 mg/mL. Figure 9shows the optical limiting and nonlinear scattering of thedispersions in NMP and water/SDBS at the same level of lineartransmission (∼70%). Because the surfactant largely reducedthe surface tension of water, the appropriate surface tension ofthe water/SDBS results in an effective debundling of nanotubes.As a result of the similar surface tension, the water/SDBSdispersions exhibit slightly better optical limiting and largerscattered signal than the NMP dispersions.

4. Conclusion

While the reduction of nanotube dispersion temperaturesdown to as low as -80 °C has little influence on the opticallimiting performance, the limiting responses can be varied freelyby programming the heating profile from RT to 100 °C. TheNLO investigation of nanotubes in dispersants with differentNMP/solvent mixtures gives us an insight into how thermody-namic properties affect the optical limiting performance ofnanotube dispersions. The surface tension of the solvent playsa more important role than the viscosity or boiling point andthe appropriate solvent properties contribute to the nonlinearscattering dominated optical limiting phenomenon more thanthe bundle size. Whereas the addition of water leads to adecrease in the optical limiting response of the dispersions, the

nanotube dispersions in water/surfactant exhibit similar limitingperformance to the nanotubes in NMP. It should be mentionedthat the nanotube dispersions in NMP are stable with the additionof another solvent or with changing environmental temperatureand at the same time exhibit an enhanced optical limitingperformance. These findings are of relevance not only to carbonnanotube dispersions but also to other nonlinear scatteringdominated optical limiting materials.3

Acknowledgment. This work was supported by the ScienceFoundation Ireland (SFI) under Grant No. 08/CE/I1432. J.W.thanks the SFI for his postdoctoral research fellowship andacknowledges Mr. Niall McEvoy for his helpful discussion inthis work.

Supporting Information Available: Thermodynamic pa-rameters of the solvents used to blend with the carbon nanotubedispersions in NMP. This material is available free of chargevia the Internet at http://pubs.acs.org.

References and Notes

(1) Sun, Y. P.; Riggs, J. E. Int. ReV. Phys. Chem. 1999, 18, 43.(2) Chen, Y.; Lin, Y.; Liu, Y.; Doyle, J.; He, N.; Zhuang, X. D.; Bai,

J. R.; Blau, W. J. J. Nanosci. Nanotechnol. 2007, 7, 1268.(3) Wang, J.; Blau, W. J. J. Opt. A: Pure Appl. Opt. 2009, 11, 024001.(4) Wang, J.; Chen, Y.; Blau, W. J. J. Mater. Chem. 2009, 19, 7425.(5) Ajayan, P. M. Chem. ReV. 1999, 99, 1787.(6) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002,

297, 787.(7) Blau, W. J.; Wang, J. Nat. Nanotechnol. 2008, 3, 705.(8) Chen, Y. C.; Raravikar, N. R.; Schadler, L. S.; Ajayan, P. M.; Zhao,

Y. P.; Lu, T. M.; Wang, G. C.; Zhang, X. C. Appl. Phys. Lett. 2002, 81,975.

(9) Set, S. Y.; Yaguchi, H.; Tanaka, Y.; Jablonski, M. IEEE J. Sel.Top. Quantum Electron. 2004, 10, 137.

(10) Sun, X.; Yu, R. Q.; Xu, G. Q.; Hor, T. S. A.; Ji, W. Appl. Phys.Lett. 1998, 73, 3632.

(11) Chen, P.; Wu, X.; Sun, X.; Lin, J.; Ji, W.; Tan, K. L. Phys. ReV.Lett. 1999, 82, 2548.

(12) Vivien, L.; Lancon, P.; Riehl, D.; Hache, F.; Anglaret, E. Carbon2002, 40, 1789.

(13) Chin, K. C.; Gohel, A.; Elim, H. I.; Chen, W. Z.; Ji, W.; Chong,G. L.; Sow, C. H.; Wee, A. T. S. J. Mater. Res. 2006, 21, 2758.

(14) Izard, N.; Menard, C.; Riehl, D.; Doris, E.; Mioskowski, C.;Anglaret, E. Chem. Phys. Lett. 2004, 391, 124.

(15) Webster, S.; Reyes-Reyes, M.; Pedron, X.; Lopez-Sandoval, R.;Terrones, M.; Carroll, D. L. AdV. Mater. 2005, 17, 1239.

(16) Ni Mhuircheartaigh, E. M.; Giordani, S.; Blau, W. J. J. Phys. Chem.B 2006, 110, 23136.

(17) Wang, J.; Blau, W. J. Chem. Phys. Lett. 2008, 465, 265.(18) He, N.; Chen, Y.; Bai, J.; Wang, J.; Blau, W. J.; Zhu, J. J. Phys.

Chem. C 2009, 113, 13029.(19) Liu, Z. B.; Tian, J. G.; Guo, Z.; Ren, D. M.; Du, F.; Zheng, J. Y.;

Chen, Y. S. AdV. Mater. 2008, 20, 511.(20) O’Flaherty, S. A.; Murphy, R.; Hold, S. V.; Cadek, M.; Coleman,

J. N.; Blau, W. J. J. Phys. Chem. B 2003, 107, 958.(21) O’Flaherty, S. M.; Hold, S. V.; Brennan, M. E.; Cadek, M.; Drury,

A.; Coleman, J. N.; Blau, W. J. J. Opt. Soc. Am. B 2003, 20, 49.(22) Jin, Z. X.; Sun, X.; Xu, G. Q.; Goh, S. H.; Ji, W. Chem. Phys.

Lett. 2000, 318, 505.(23) O’Flaherty, S. M.; Hold, S. V.; Cook, M. J.; Torres, T.; Chen, Y.;

Hanack, M.; Blau, W. J. AdV. Mater. 2003, 15, 19.(24) Doyle, J. J.; Wang, J.; O’Flaherty, S. M.; Chen, Y.; Slodek, A.;

Hegarty, T.; Carpenter, L., II.; Wohrle, D.; Hanack, M.; Blau, W. J. J. Opt.A: Pure Appl. Opt. 2008, 10, 075101.

(25) Chen, Y.; Hanack, M.; Araki, Y.; Ito, O. Chem. Soc. ReV. 2005,34, 517.

(26) Chen, Y.; Gao, L.; Feng, M.; Gu, L. L.; He, N.; Wang, J.; Araki,Y.; Blau, W. J.; Ito, O. Mini-ReV. Org. Chem. 2009, 6, 55.

(27) Blau, W.; Byrne, H.; Dennis, W. M.; Kelly, J. M. Opt. Commun.1985, 56, 25.

(28) Senge, M. O.; Fazekas, M.; Notaras, E. G. A.; Blau, W. J.;Zawadzka, M.; Locos, O. B.; Mhuircheartaigh, E. M. N. AdV. Mater. 2007,19, 2737.

(29) Zawadzka, M.; Wang, J.; Blau, W. J.; Senge, M. O. Chem. Phys.Lett. 2009, 477, 330.

Figure 9. Normalized transmission and scattered signal as functionsof the incident pulse energy density for the nanotube dispersions inNMP and water/SDBS. Inset: the minimal transmission as a functionof on-focus energy density for various dispersions.


(30) Blau, W. J.; Byrne, H. J.; Cardin, D. J.; Dennis, T. J.; Hare, J. P.;Kroto, H. W.; Taylor, R.; Walton, D. R. M. Phys. ReV. Lett. 1991, 67,1423.

(31) Tutt, L. W.; Kost, A. Nature 1992, 356, 225.(32) Doyle, J. J.; Ballesteros, B.; de la Torre, G.; McGovern, D. A.;

Kelly, J. M.; Torres, T.; Blau, W. J. Chem. Phys. Lett. 2006, 428, 307.(33) Wang, J.; Blau, W. J. J. Phys. Chem. C 2008, 112, 2298.(34) Wang, J.; Blau, W. J. Appl. Phys. B 2008, 91, 521.(35) Wang, J.; Fruchtl, D.; Blau, W. J. Opt. Commun. 2010, 283, 464.(36) Giordani, S.; Bergin, S. D.; Nicolosi, V.; Lebedkin, S.; Kappes,

M. M.; Blau, W. J.; Coleman, J. N. J. Phys. Chem. B 2006, 110, 15708.(37) Bergin, S. D.; Nicolosi, V.; Streich, P. V.; Giordani, S.; Sun, Z. Y.;

Windle, A. H.; Ryan, P.; Niraj, N. P.; Wang, Z. T.; Carpenter, L.; Blau,W. J.; Boland, J.; Hamilton, J. P.; Coleman, J. N. AdV. Mater. 2008, 20,1876.

(38) Sheikbahae, M.; Said, A. A.; Wei, T. H.; Hagan, D. J.; Vanstryland,E. W. IEEE J. Quantum Electron. 1990, 26, 760.

(39) Kahl, H.; Wadewitz, T.; Winkelmann, J. J. Chem. Eng. Data 2003,48, 580.

(40) Mac Kernan, D.; Blau, W. J. EPL 2008, 83, 66009.(41) Vivien, L.; Riehl, D.; Hache, F.; Anglaret, E. Phys. B (Amsterdam,

Neth.) 2002, 323, 233.(42) Vivien, L.; Moreau, J.; Riehl, D.; Alloncle, P. A.; Autric, M.; Hache,

F.; Anglaret, E. J. Opt. Soc. Am. B 2002, 19, 2665.

(43) Carreira, J.; Mendonca, A.; Saramago, B. D. V.; Soares, V. A. M.J. Colloid Interface Sci. 1997, 185, 68.

(44) Yang, C. S.; Xu, W.; Ma, P. S. J. Chem. Eng. Data 2004, 49, 1794.(45) Labutin, V. A.; Labutina, A. V. Theor. Found. Chem. Eng. 2001,

35, 520.(46) Sharma, S.; Patel, P. B.; Patel, R. S.; Vora, J. J. E-J. Chem. 2007,

4, 343.(47) He, G. S.; Qin, H. Y.; Zheng, Q. D. J. Appl. Phys. 2009, 105,

023110.(48) Jones, A. R. Prog. Energy Combust. Sci. 1999, 25, 1.(49) Belousova, I. M.; Mironova, N. G.; Yur’ev, M. S. Opt. Spectrosc.

2003, 94, 86.(50) Vivien, L.; Riehl, D.; Delouis, J. F.; Delaire, J. A.; Hache, F.;

Anglaret, E. J. Opt. Soc. Am. B 2002, 19, 208.(51) Brennen, C. E. CaVitation and Bubble Dynamics; Oxford University

Press: New York, 1995.(52) Wang, J.; Hernandez, Y.; Lotya, M.; Coleman, J. N.; Blau, W. J.

AdV. Mater. 2009, 21, 2430.(53) Sun, Z.; O’Connor, I.; Bergin, S. D.; Coleman, J. N. J. Phys. Chem.

C 2009, 113, 1260.(54) Kahl, H.; Wadewitz, T.; Winkelmann, J. J. Chem. Eng. Data 2003,

48, 1500.(55) Henni, A.; Hromek, J. J.; Tontiwachwuthikul, P.; Chakma, A.

J. Chem. Eng. Data 2004, 49, 231.

JP9117248


Documents

Control of Optical Limiting of Carbon Nanotube Dispersions by Changing Solvent Parameters