J. Phys. Chem. C 2009, 113, 13979–13984
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Influence of Polymer Structures on Optical Power Limiting Performance of Single-Walled Carbon Nanotubes Liangmin Zhang* and Susan D. Allen Arkansas Center for Laser Applications and Science, Department of Chemistry and Physics, Arkansas State UniVersity, State UniVersity, Arkansas 72467
Caroline Woelfle and Fajian Zhang Department of Materials Science and Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061 ReceiVed: February 26, 2009; ReVised Manuscript ReceiVed: June 19, 2009
Purified single-walled carbon nanotubes are dispersed in two different types of polymer matrices. In these stable dispersed systems, the interaction between the polymer matrix and the single-walled carbon nanotubes, optical power limiting effect, and two-photon absorption are investigated. Transmission electron micrographs show the rigid polymer backbone interacts with the single-walled carbon nanotubes and results in the formation of nanoribbons and nanobundles in the dispersions. Optical measurements show that the nanoribbons and nanobundles significantly improve the optical power limiting performance through increased nonlinear optical absorption. Detailed comparison of optical power limiting, two-photon absorption, and two-photon excited fluorescence emission is presented. Our experimental results show that the different polymer backbones have a considerable effect on the nonlinear optical properties of carbon nanotubes. 1. Introduction Since the discovery of carbon nanotubes in 1991,1 there has been considerable interest in using carbon nanotubes to fabricate photonic and electronic devices.2-5 However, carbon nanotubes easily form naturally packed and entangled crystalline ropes due to strong van der Waals attractions. This aggregation phenomenon hinders their applicability and diminishes the specific photonic and electronic properties of each nanotube. Therefore, the use of carbon nanotubes in a practical application requires incorporating the nanotubes into a matrix and allowing for the fabrication of films, coatings, and suspensions. Substantial efforts have been dedicated to obtaining well-separated and functionalized carbon nanotubes.4-7 The existing alternate techniques used to functionalize carbon nanotubes can be separated into two different approaches: covalent8-10 and noncovalent functionalizations.7,11,12 The nonconvalent technique involves the physical adsorption of molecules along the sidewalls of carbon nanotubes, and while providing new functionalities on the surface of carbon nantubes and tailoring their properties, the modification do not destroy the surface of the nanotubes and do not alter their intrinsic electronic nature. Thus, finding functional conjugated polymers that enable one to noncovalently tailor carbon nanotubes’ properties is considerably urgent for practical devices. Previously it has been shown that nanotubes can be held in suspensions by using poly(m-phenylenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene),13 poly(9,9di-n-octylfluorenyl-2,7′-diyl),14 and octadecyamine.14,15 Some polymers with flexible and rigid aromatic backbones have been used to wrap around the carbon nanotubes.16,17 Chen et al. find that rigid polymers, poly(aryleneethynylene)s (PPEs), can enhance the solubility of single-walled carbon nanotubes * To whom correspondence should be addressed. E-mail: lzhang@ astate.edu.
(SWNTs) by about 20-fold due to π-π interactions.7 Because of the π-π interaction between the SWNTs and a rigid conjugated polymer backbone, rigid conjugated polymers can result in more stable dispersions of the nantubes versus those with flexible backbones. The strong π-π interaction between SWNTs and a rigid conjugated polymer matrix is attributed to the fact that the atomic arrangement of carbon atoms in an aromatic group is similar to their arrangement on the surfaces of SWNTs.18,19 Therefore, it is significantly important to investigate the influence of the interaction between a conjugated polymer and the nanotubes on the resulting nonlinear optical properties. The nonconvalent polymer-functionalized SWNTs have emerged as a new type of material for optical power limiting applications.13,20,21 Optical power limiters are of significant importance for the protection of human eyes, optical elements, or optical sensors from intense laser pulses. To work effectively, they must rely on nonlinear responses such as nonlinear absorption22 or nonlinear scattering13 to dissipate the incident light as a function of its intensity. Mansour et al. found that raw carbon-black suspensions showed optical power limiting effect23 and Sun et al. reported that the dominant nonlinearity leading to optical limiting was thermally induced nonlinear scattering.24 Some researchers concluded that nonlinear scattering due to thermally induced solvent-bubble formation and sublimation of carbon nanotubes was the main mechanism leading to optical limiting in SWNT suspensions.25-28 Riggs et al.20,21 performed optical-limiting experiments on suspended and solubilized full-length and shortened SWNTs and multiwalled carbon nanotubes (MWNTs). They found that the dominant optical-limiting mechanism was nonlinear scattering for the suspended systems and nonlinear absorption for the solubilized systems. Thus there remains some debate as to whether the
10.1021/jp901791n CCC: $40.75 2009 American Chemical Society Published on Web 07/09/2009
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Zhang et al. where the imaginary part is related to the 2PA coefficient β through
χ(3) I )
Figure 1. Molecular structures of the three polymers used in this work. PPE and PFE monomers are connected by rigid triple bonds while the PFO polymer monomer is linked by single flexible bonds.
observed nonlinear effects are due to either scattering or absorption or both mechanisms. In this work, two types of three different conjugated polymers are used to functionalize purified SWNTs in chloroform. The polymers used are the commercially available poly(9,9-di-nhexylfluorenyl-2,7-diyl) (PFO) that possesses a flexible backbone, and poly(2,5-dioctylphenylene-1,4-ethynylene) (PPE) and poly(9,9-dioctylfluorenyl-2,7-yleneethynylene) (PFE) that have rigid backbones. A systematic investigation of optical power limiting effect and two-photon absorption mechanism has been performed on the first two composite systems. Because PFE has a similar backbone structure to that of PPE, the PFE-SWNT system is employed to verify the effect of the rigid backbone on the optical power limiting performance. Figure 1 shows the structures of these three polymers. We have found that the resulting optical limiting performance significantly depends on the structure (rigid or flexible) of the polymers although it is difficult to develop a theory to quantitatively predict the optical power limiting based on the polymer and SWNT’s structures. 2. Theory When a laser beam propagates in a medium, considering twophoton absorption13,29 the beam intensity change along the propagation direction (for example, z axis) can be written as
∂I ) -(RI + βI2) ∂z
(1)
where R is the linear absorption coefficient and β is the twophoton absorption (2PA) coefficient, which is related to the imaginary part of the third-order susceptibility χI(3). If β ) 0, that means that only linear absorption (single-photon absorption) exists in the medium. If two-photon absorption coexists in the medium (β * 0), with increasing input power, the transmission power should increase slowly then linearity. This is the so-called power limiting effect. Considering 2PA, the third-order nonlinear susceptibility is now considered to be a complex quantity:
χ(3) ) χR(3) + iχ(3) I
(2)
n2ε0cλβ 2π
(3)
where n is the linear refractive index, ε0 is the permittivity of free space, c is the speed of light, and λ is the wavelength of the incident light. The open-aperture and closed-aperture Z-scan techniques30,31 can be used to determine the nonlinear absorption coefficient and the nonlinear refractive index of nonlinear optical materials, respectively. Large refractive nonlinearities in materials are commonly associated with a resonant transition, which may be of single or multiphoton nature. The nonlinear absorption in such materials arising from either direct multiphoton absorption, saturation of the single photon absorption, or dynamic freecarrier absorption have strong effects on the measurements of nonlinear refraction when using the Z-scan technique. Under thin sample approximation, the open-aperture Z-scan is sensitive to nonlinear absorption while the closed-aperture Z-scan is sensitive to nonlinear refraction. In this paper, we concentrate on nonlinear optical absorption measurements using the openaperture Z-scan technique. For this technique, the normalized transmittance as a function of z, TNorm(z), is given by13,30,31
TNorm(z) )
ln[1 + q0(z)] q0(z)
(4)
q00 1 + (z/z0)
(5)
where q0(z) is expressed by
q0(z) )
z0 is the diffraction length of the beam, and q00 ) βI0L, where I0 is the intensity of the laser beam at focus and L is known as the effective length of the sample. If one can obtain TNorm(z) experimentally, using eqs 3-5, one can calculate the 2PA coefficient β and χI(3). 3. Materials Raw SWNTs produced by chemical vapor deposition (CVD) were purchased from Optics Innovations, Inc. The SWNTs were purified according to a method developed by Chiang et al.32 Raw SWNTs were heated in air in a tube furnace for 18 h at 225 °C. The oxidized SWNTs were sonicated in concentrated hydrochloric acid for 15 min. The resulting dispersion was centrifuged at 2500 rpm for 20 min, and the acidic supernatant was removed. The remaining SWNTs were filtered and rinsed several times with deionized water until the pH of the filtrate became neutral to eliminate any remaining traces of acid. The black powder was further oxidized at 325 °C for 1.5 h, and the same centrifugation, filtration, and rinsing steps as previously described were applied. Three polymers were purchased from Sigma-Aldrich. Solutions of SWNTs and polymers were prepared by mixing SWNTs with a 10 mL solution of polymer in chloroform. The mixture was sonicated for 30 min and left in a refrigerator undisturbed for several days to check the stability. Then, the performance of optical power limiting and two-photon absorption were measured.
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Figure 2. Schematic diagram of the experimental arrangement for optical power limiting and open-aperture Z-scan measurements. BS, beamsplitter; D, detector; Ls: lenses; OI, optical isolator.
Figure 4. A comparison of optical power limiting performance of PFO/ SWNT-2, PPE/SWNT-2, and PFE/SWNT-2 dispersions. Each dispersion is made by solubilizing 5 mg of polymer and 0.25 mg of SWNTs into 10 mL of chloroform.
Figure 3. A comparison of optical power limiting performance of PFO/ SWNT-1, PPE/SWNT-1, and PFE/SWNT-1 dispersions. Each dispersion is made by solubilizing 5 mg of polymer and 0.5 mg of SWNTs into 10 mL of chloroform.
4. Experiment and Discussion The optical power limiting experiments and open-aperture Z-scan measurement were performed by using a Coherent Verdi10 pumped Mira-900 Ti:Sapphire laser operating at 800 nm, with a 75.8 MHz pulse repetition rate and a 200 fs pulse width. All samples were measured in a quartz cuvette with a 2-mm path length and the surface reflection losses were not corrected. The experimental setup for optical power limiting measurements is shown in Figure 2. It consists of the laser system, an optical isolator to keep the laser cavity stable, a half-wave plate and a polarizer to control the polarization of the laser beam, a focus lens, a stage, and a power meter to measure the input power and the transmittance after the sample cell. Figure 3 shows optical power limiting performances of three dispersions. For comparison purposes, the transmission for each of the dispersions is normalized by using its highest value. The laser beam is focused by a lens and the diameter of the beam is about 300 µm before the sample. With increasing input energy, one can see that the PPE/SWNT-1 dispersion (O) exhibits optimal optical power limiting performance. While the PFO/SWNT-1 (0) also shows an optical power limiting effect, the performance is obviously worse than that of PPE/SWNT-1. This indicates that the interaction of between the SWNTs and PPE can significantly increase the optical power limiting performance. Chen et al.7 find that SWNTs can interact with polymers with rigid backbonds (like PPE) via π-stacking to have high solubility and form nanoribbons and nanoropes in CHCl3. In contrast, polymers with flexible backbones (like PFO) could easily wrap around SWNTs and reduce the solubility of SWNTs. To provide further evidence that the π-stacking can enhance optical power limiting performance, we use another rigid backbone polymer PFE to make a dispersion and its normalized transmission is also shown in Figure 3 (∆). One can see that the PFE/SWNT-1 also exhibit a stronger optical power limiting effect than the PFO/SWNT-1. Similar measurements are then performed in the polymer/SWNT dispersions with lower concentration of SWNTs and the result obtained is shown as Figure 4. Again, PPE/SWNT-1 has a stronger optical power
Figure 5. Performance of three polymer solutions (PFO, PPE, and PFE) without SWNTs. The solutions were made by solubilizing 5 mg of polymer into 10 mL of chloroform.
limiting behavior, especially at high incident energies. To confirm the contribution of the optical power limiting effect from polymers, we then measure the solutions without SWNTs. The result is included in Figure 5. Except for the slight increase of transmission at the very low energy level, the straight line behavior indicates that without inclusion of SWNTs, the polymer solutions have no considerable optical power limiting effect. Therefore, the optical power limiting effect arises from the interaction between the polymers and SWNTs. To analyze the interaction between the polymers and SWNTs in suspensions, a Philips 420 T Transmission Electron Micrograph (TEM) system was used to visualize the SWNTs dispersed with PFO and PPE polymers. The TEM samples were prepared by drying a drop of the polymer/SWNT solutions on a Lacey Carbon TEM grid (200 mesh), followed by rinsing with pure chloroform to remove excess polymer. This method enables us to observe the dispersed SWNTs. Figures 6 and 7 show the TEM measurements of SWNTs in the PFO/SWNT-1 and PPE/ SWNT-1 suspensions, respectively. Compared with the modified SWNTs in these two figures, one can see that wider nanobundles and nanoribbons of SWNTs are established through a more effective interaction between PPE and SWNTs in PPE/SWNT-1 dispersion while the SWNTs in PFO were modified more slightly as shown in Figure 6. The difference in the nature of the interaction can be explained by analyzing the structures of these two polymers.14,33 The PFO polymer consists of an aromatic, rigid repeat unit connected through flexible linkages. This type of structure can provide the flexibility needed for encapsulation of SWNTs. This is in contrast to PPE where the monomer is connected by rigid triple bonds. It has been shown previously that the PFO polymer tends to map onto the structure
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Figure 8. Fluorescence spectra of PFO and PFO/SWNT-2. Figure 6. TEM image of SWNTs in the PFO/SWNT-1 dispersion.
Figure 9. Flurescence spectra of PPE and PPE/SWNT-2. Compared with Figure 8, one can see that the fluorescent intensity is much weaker from PPE/SWNT-2. This is due to the energy transfer to SWNTs from the excited PPE molecules through the π-π interaction.
Figure 7. TEM image of SWNTs in the PPE/SWNT-1 dispersion. Note that wider nanoribbons and nanobundles of SWNTs can be seen.
of nanotubes.14,7 On the basis of the previous work and the TEM measurements, it can be reasoned to infer that the PFO backbone binds efficiently to the SWNTs. The PPE polymer monomer is connected by rigid triple bonds and cannot wrap around SWNTs efficiently; some research work shows that the polymers with a similar backbone and function groups of PPE and PFE can stably stack on the surface of SWNTs through π-π interaction.7,33,35 This interaction is much stronger than wrapping. The effective attraction between rigid conjugated polymers like PPE and SWNTs arises from the atomic arrangement of carbon atoms in an aromatic group being similar to their arrangement on the surface of an SWNT. The interactions of the polymers with SWNTs can also be verified by fluorescence intensity. This fluorescence measurement is carried out by replacing the detector and power meter in Figure 2 with a collection lens coupled by a fiber to direct the emitting light into a Hitachi-F-4500 spectrophotometer with a wavelength resolution of 1 nm. Without SWNTs, PFO/ chloroform and PPE/chloroform solutions emit strong fluorescence under excitation of 800 nm due to two-photon absorption as shown on the upper curves in Figures 8 and 9. The lower curves in Figures 8 and 9 show the fluorescence spectra of PFO/SWNT-2 and PPE/SWNT-2 dispersions. It can be seen that on both lower curves, the fluorescence
intensity is decreased by the presence of SWNTs in the dispersions, which is attributed to the energy transfer from the polymers to the SWNTs.7,33,35 The fluorescence is quenched by the energy transfer from the excited state of the polymer molecules to SWNTs through the interaction. The fluorescence peak is significantly lower in the PPE/ SWNT-2 dispersion. The fluorescence measurement reaffirms that the π-π interaction between the PPE matrix and SWNTs is stronger than polymer wrapping, which occurs between the PFO matrix and SWNTs. These measurements indicate that one can introduce neutral and ionic functional groups onto the carbon nanotube surface to tailor their properties. The open-aperture Z-scan technique can be used to extract two-photon absorption coefficient β of two suspensions and make a quantitative comparison. In the setup shown in Figure 2, we use a micrometer drive stage (with a resolution of 10 µm) to adjust the position of the sample cell. At different positions, the transmitted power through the cell can be obtained. For PFO/SWNT-2 and PPE/SWNT-2, the measured normalized transmissions as a function of z are shown in Figures 10 and 11 (symbols), respectively. z ) 0 indicates the center of the cell is placed right at the focal plane. Using the theory outlined in section 2, one can fit the curves theoretically to extract the two-photon absorption coefficient β and the imaginary part χI(3). The solid curves in Figures 10 and 11 represent the fitted results. To obtain more reliable results, for each suspension, we conduct the open-aperture
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J. Phys. Chem. C, Vol. 113, No. 31, 2009 13983 5. Conclusions
Figure 10. Normalized typical open-aperture measurement data for PFO/SWNT-2. z ) 0 corresponds to the sample cell being right at the focal point. The average laser power is 501 mW, which equals a peak intensity I0 ) 662 MW/cm2 at the focal plane. The theoretical fit gives χI(3) ) 2.28 × 10-7 (esu).
Figure 11. Normalized typical open-aperture measurement data for PFO/SWNT-2. z ) 0 corresponds to the sample cell being right at the focal point. The average laser power is 300 mW, which equals a peak intensity I0 ) 396 MW/cm2 at the focal plane. The curve fit gives χI(3) ) 8.47 × 10-7 (esu).
Figure 12. χ(3) I of PPE/SWNT-2 (upper) and PFO/SWNT-2 as functions of the focal intensity I0. One can see that the coefficient does not change significantly with different incident energies.
Z-scan measurement at five different powers. The calculated result for χI(3) is shown in Figure 12. One can see that the coefficient does not change significantly with different incident energy powers. This is consistent with theory. Overall, the coefficient of PPE/SWNT-2 is about three times larger than that of PFO polymer wrapped SWNTs. This indicates that the π-π interaction between the PPE polymer and SWNTs is more effective in generating the nonlinear absorption coefficient than wrapping.
Three conjugated polymers, PFO, PPE, and PFE, with different backbone structures, have been used to disperse purified SWNTs in chloroform. With changes in the structure, different optical power limiting performance has been observed. PPE/SWNT dispersions exhibit a significantly high optical power limiting effect. TEM measurements show that SWNTs in the PPE dispersions form some wider nanoribbons and nanobundles, which can significantly improve the nonlinear absorption and scattering. The nanoribbons and nanobundles arise from the π-π interaction between the molecules of PPE and SWNTs in the dispersions. The existence of the π-π interaction is attributed to the similar molecular structures between the aromatic groups of the rigid backbone of the PPE polymer and the hexagonal carbon atom network existing on the surface of SWNTs. The PFO matrixbased composites also exhibit an optical power limiting effect and two-photon absorption. Because the PFO backbone can only bind or wrap on the SWNTs, the TEM image shows that the wide nanoribbons and nanobundles cannot be formed in the PFO/SWNTs dispersions. Therefore, the interaction between the PFO molecules and SWNTs cannot improve the optical power limiting and nonlinear absorption as effectively as that between the PPE molecules and SWNTs. There are potential host polymers that remain to be tested to find which kind of polymer shows the best optical power limiting performance and strongest nonlinear absorption. The result infers that polymer matrices can tailor nonlinear optical properties of carbon nanotubes through the interaction between a polymer matrix and carbon nanotubes. Acknowledgment. One of the authors (L.Z.) acknowledges R. Lee Williams and R. O. Claus at NanoSonic, Inc. for their assistance regarding the composite preparation and analysis. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Chang, M. C. Y.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2004, 126, 15392. (3) Hu, C.; Zhang, Y.; Bao, G.; Zhang, Y.; Liu, M.; Wang, Z. L. J. Phys. Chem. B 2005, 109, 20072. (4) Shim, M.; Kam, N. W. S.; Chen, R. J.; Li, Y.; Dai, H. Nano Lett. 2002, 2, 285. (5) Song, C.; Pehrsson, P. E.; Zhao, W. J. Mater. Res. 2006, 21, 2817. (6) Star, A.; Gabriel, J. C. P.; Bradley, K.; GrUner, G. Nano Lett. 2003, 3, 459. (7) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2004, 124, 9034. (8) Bahr, J. L.; Yang, J. S.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2002, 123, 6536. (9) Chen, X. H.; Chen, C. S.; Chen, Q.; Cheng, F. Q.; Zhang, G.; Chen, Z. Z. Mater. Lett. 2002, 57, 734. (10) Qin, S.; Qin, D.; Ford, W. T.; Herrera, J. E.; Resasco, D. E.; Bachilo, S. M.; Weisman, R. B. Macromolecules 2004, 37, 3965. (11) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. Nano Lett. 2003, 3, 269. (12) Parades, J. I.; Burghard, M. Langmuir 2004, 20, 5149. (13) 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. (14) O’Flaherty, S. M.; Murphy, R.; Hold, S. V.; Cadek, M.; Coleman, J. N.; Blau, W. J. J. Phys. Chem. B 2003, 107, 958. (15) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Yao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95. (16) O’Connell, M.; Boul, P.; Huffman, L. M.; Wang, C.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (17) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (18) Katz, E. J. Electroanal. Chem. 1994, 357, 157. (19) Ehrlich, J. E.; Ananthavel, S. P.; Barlow, S.; Mansour, K.; Mohanalingam, K.; Marder, S. R.; Perry, J. W.; Rumi, M.; Thayumanavan, S. Nonlinear Opt. 2001, 27, 121.
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