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Stretching Effect of Linearly Polarized Ar+ Laser Single-Beam on Azo Polymer Colloidal Spheres Yaobang Li, Yaning He, Xiaolan Tong, and Xiaogong Wang* Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua UniVersity, Beijing, 100084, PRC ReceiVed October 26, 2005. In Final Form: December 19, 2005 This work shows that a linearly polarized Ar+ laser single-beam irradiation can cause stretching deformation of azo polymer colloidal spheres along the polarization direction of the laser beam. An epoxy-based polymer, containing 4-amino-4′-carboxyazobenzene at each repeat unit, was used to construct the colloidal spheres. The colloidal spheres were prepared by gradual hydrophobic aggregation of the polymeric chains in a THF/H2O dispersion medium, which was induced by a steady increase in the water content. When the obtained colloidal spheres were exposed to the spatially filtered and collimated Ar+ laser beam (488 nm, 150 mW/cm2), the colloids were stretched along the polarization direction of the laser beam. In the testing period (20 min), the colloids were deformed continuously as the irradiation time increased. When 2D close-packed arrays of the colloidal spheres were irradiated by the polarized laser singlebeam, the colloidal spheres were all uniformly stretched along the polarization direction of the laser beam. On the contrary, when the arrays were irradiated by the interfering p-polarized laser beams, only the colloidal spheres in the bright regions of the interference pattern were significantly deformed.
Introduction It is now well-known that light can exert forces through different mechanisms on various substances ranging from microscopic particles to macroscopic objects.1-5 Laser radiation forces have been widely explored to optically accelerate, slow, stably trap, and manipulate micrometer-sized dielectric particles and atoms.1 This has led to a diversity of scientific and practical applications, especially in areas where small particles play a key role.2 On the other hand, photoinduced deformations have been achieved for polymers containing aromatic azo chromophores (azo polymers for short).3-5 The deformations are caused by the trans-cis photoisomerization of the azobenzene units and the consequent structure variation at different levels. Thin films of cross-linked liquid-crystal azo polymers can be significantly contracted or bent by light because of the variation of the orientation order and related conformational change of the polymer backbones.3 When azo polymer thin films are exposed to an interference pattern of coherent Ar+ laser beams, surface-relief-gratings (SRGs) can be formed on the film surfaces.4,5 SRGs are stable at a temperature below the glass transition temperatures (Tg’s) of the polymers and can be removed by optical erasure or by heating samples to a temperature above Tg’s.4 Although several models have been proposed to describe the SRG formation, the forming mechanism has not been fully understood until the present time.5 Colloidal particles, which have at least one dimension within the nanometer-micrometer range, have been widely applied in many industrial products.6 Recently, monodispersed colloidal * Corresponding author. E-mail:
[email protected]. (1) (a) Ashkin, A. Phys. ReV. Lett. 1970, 24, 156. (b) Ashkin, A. Science 1980, 210, 1081. (c) Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288. (2) (a) Burns, M. M.; Fournier, J. M.; Golovchenko, J. A. Science 1990, 249, 749. (b) Mio, C.; Marr, D. W. M. Langmuir 1999, 15, 8565. (3) (a) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warmer, M. Phys. ReV. Lett. 2001, 87, 015501. (b) Li, M. H.; Keller, P.; Li, B.; Wang, X. G.; Brunet, M. AdV. Mater. 2003, 15, 569. (c) Yu, Y. L.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (4) (a) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. (b) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166. (5) (a) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (b) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139.
spheres have been extensively explored as building blocks to construct two-dimensional (2D) and three-dimensional (3D) ordered arrays.7 On the basis of those periodic 2D and 3D arrays and the structures further developed by using the arrays as masks or templates, a variety of novel materials with long-range mesoscopic order or mesoporous constructions have been prepared.7,8 More recently, the effort to produce nonspherical colloids from spherical colloids, such as by mechanically stretching colloidal spheres embedded in polymeric matrixes, starts to arouse significant interest due to the potential applications in photonic band gap (PBG) crystals.9 Although laser radiation forces have been widely explored to optically move, trap, and manipulate colloidal particles,1,2 using light as a tool to modify colloid shape is still lacking in scientific literature. Recently, we demonstrated that for the colloidal spheres formed from an amphiphilic azo polymer, interfering p-polarized Ar+ laser beams can cause significant shape deformation.10 The twobeam interference nature of the experiment implies that the deformation could be caused by the same optical effect that results in the SRG formation and can be explained on the basis of similar theoretical models. For the SRG formation, the space variation of the light intensity or polarization direction is required to induce the surface deformation.4,5 However, it is unclear whether those conditions are necessary for causing shape deformation of the colloids. In current work, we were somewhat surprised to observe that a linearly polarized Ar+ laser single-beam with uniform intensity can also cause the shape deformation for the same type of the colloidal spheres. This work shows for the first time that the polarized uniform Ar+ laser beam at normal incidence can stretch (6) Hiemenz, P. C. Principles of Colloid and Surface Chemistry (2nd ed.); Marcel Dekker: New York, 1986. (7) (a) Li, Z. Y.; Wang, J.; Gu, B. Y. Phys. ReV. B 1998, 58, 3721. (b) Li, Z. Y.; Wang, J.; Gu, B. Y. J. Phys. Soc. Jpn. 1998, 67, 3288. (c) Lu, Y.; Xiong, H.; Jiang, X.; Xia, Y.; Prentiss, M.; Whiteside, G. M. J. Am. Chem. Soc. 2003, 125, 12724. (8) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. H. Nature 1997, 386, 143. (9) (a) Lu, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2001, 13, 271. (b) Lu, Y.; Yin, Y.; Li, Z. Y.; Xia, Y. Langmuir 2002, 18, 7722. (10) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. J. Am. Chem. Soc. 2005, 127, 2402.
10.1021/la052884b CCC: $33.50 © 2006 American Chemical Society Published on Web 01/21/2006
Ar+ Laser Stretching of Azo Polymer Colloidal Spheres
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the “isolated” azo polymer colloids along the polarization direction of the laser beam. In contrast to the nonuniform deformation caused by the interfering laser beams, the polarized laser singlebeam can uniformly stretch every colloidal sphere in their 2D close-packed arrays. Experimental Section Materials. Analytical pure tetrahydrofuran (THF) from a commercial source was refluxed with cuprous chloride and distilled for dehydration before use. Deionized water (resistivity > 18 MΩ) was obtained from a Millipore water purification system and used for the experiments described below. The chemical structure of the azo polymer (BP-AZ-CA) used to prepare the colloidal spheres is given as
which contains pseudo-stilbene-type azo chromophores11 and can form SRGs upon laser light irradiation.12 The number-average molecular weight of the polymer was estimated to be 41 000 with the polydispersity index of 2.2 obtained by the GPC measurement. The preparation and characterization details of BP-AZ-CA can be seen in our previous paper.12 Sample Preparation. The colloidal spheres were prepared by self-assembly of the polymer chains in selective solvent through hydrophobic aggregation.13 In the process, BP-AZ-CA was first dissolved in tetrahydrofuran (THF) with an initial concentration of 0.4 mg/mL, and then a suitable amount of Milli-Q water was added dropwise into the THF solution. When the water content reached a critical value (50%, vol %), uniform colloidal spheres were obtained. Then a large amount of water was added to quench the structures formed, and the suspension was dialyzed against water for 72 h to remove THF. When the initial concentration of BP-AZ-CA in THF changed from 0.1 to 1.0 mg/mL, the average size of the spheres could be adjusted in a range from 160 to 310 nm. The average size of the spheres used in this study was 212 nm with the polydispersity index of 0.03. Silicon wafers as substrates were treated with a solution containing 30% hydrogen peroxide and 70% sulfuric acid overnight and then washed thoroughly with Milli-Q water. The samples containing “isolated” colloids on the surfaces were prepared by dropping the water suspension of the colloidal spheres (2 mg/mL) on the clean silicon wafers and casting under gravity. The colloidal sphere arrays were prepared by the vertical deposition method.14 The substrates were immersed vertically into beakers containing the sphere suspensions (about 0.2 mg/mL). When water evaporated slowly (about 0.05 mL/h) in a 40 °C oven, the colloidal spheres were driven by the capillary force to assemble gradually into 2D arrays on the substrates as the liquid surface moved down. After the preparation, the samples were dried in a 30 °C vacuum oven for 12 h. Laser Irradiation Setup. A linearly polarized beam from an Ar+ laser at 488 nm was used as the light source. The spatially filtered laser beam was expanded and collimated. The intensity of the laser beam was about 150 mW/cm2. For the single-beam irradiation experiment, the linearly polarized laser beam was incident perpendicularly to the wafer surfaces containing the colloids. To carry out the experiment with interfering laser beams, a p-polarized Ar+ laser beam was split by a mirror and the reflected half-beam (11) Rau, H. Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. II, Chapter 4. (12) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43, 7325. (13) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (14) (a) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (b) Norri, D. J.; Arlinghaus, E. G.; Meng, L. L.; Heiny, R.; Scriven, L. E. AdV. Mater. 2004, 16, 1393. (c) Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760.
Figure 1. (A) TEM image of the azo-polymer colloidal spheres with an average diameter of 212 nm. (B) SEM image of the colloidal spheres after exposure to a linearly polarized Ar+ laser beam for 10 min. (C) TEM image of the deformed colloids after irradiation with the laser beam for 15 min and release from the surface of the silicon wafer by sonication. coincided with the other half on the sample surface. This experimental setup was similar to those reported for SRG fabrication.4 All experiments were carried out at room temperature under an ambient condition. Characterization. TEM observation was carried out on a JEOL JSM-1200EX transmission electron microscope with the accelerating voltage of 120 kV. SEM measurement was performed on a field emission microscope (JEOL JSM-6301F) with the accelerating voltage of 5 kV. All the samples prepared for SEM studies were coated with thin layers of carbon (∼5 nm in thickness). The atom force microscope (AFM) images were obtained by using a Nanoscope-IIIa scanning probe microscope in the tapping mode. The average size and the polydispersity index of the colloidal spheres were measured with a Marvern Zetasizer 3000 dynamic light scattering instrument equipped with a multi-τ digital time correlation and a 632-nm solid-state laser light source. The scattering angle used for the measurement was 90°, and the temperature of the suspensions was controlled at 25 °C.
Results and Discussion The colloidal spheres were prepared by hydrophobic aggregation of the polymer chains, induced by gradual addition of the selective solvent (H2O) into a THF solution of BP-AZ-CA.10,13 Under conditions given in Experimental Section, uniform colloidal spheres were obtained. The spheres were characterized by using transmission electron microscopy (TEM) and dynamic light scattering (DLS). Figure 1A shows a typical TEM image of the colloidal spheres. The average size of the colloidal spheres was 212 nm with the polydispersity index of 0.03 obtained from the DLS measurement. The samples for the optical-stretching experiments were prepared by casting a water suspension of the colloidal spheres (2 mg/mL) on clean silicon wafers, which left the “isolated” colloids on the surfaces. The colloidal spheres were exposed to the spatially filtered and collimated laser beam that was incident perpendicularly to the wafer surface. Figure 1B gives a typical scanning electron microscope (SEM) image of the “isolated” colloidal spheres after being irradiated by the linearly polarized Ar+ laser beam (488 nm, 150 mW/cm2) for 10 min. The elongated direction of the colloids is perpendicular to the casting direction and parallel to the polarization direction of the laser beam. Figure 1C shows the TEM image of the deformed colloids, which were irradiated by the laser beam for 15 min and then released from the silicon wafer into water by sonication. The picture confirms that the spheres are deformed to ellipsoids after the irradiation. Figure 2 shows SEM images of the colloidal particles observed before irradiation and after irradiation for different periods of time. In the testing period (20 min), the colloids are stretched
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Figure 2. SEM images of colloidal spheres before irradiation (A); and after irradiation for different periods of time; (B) 5 min, l/d ) 1.28; (C) 10 min, l/d ) 1.65; (D) 15 min, l/d ) 1.98; and (E), 20 min, l/d ) 2.35.
Figure 3. Relationship between the average axial ratios (l/d) of the colloidal particles and the irradiation time.
Figure 4. SEM image of the colloids, after being irradiated by the laser single-beam for 30 min.
continuously along the polarization direction as the irradiation time increases. The average axial ratio (l/d) of the colloids (estimated statistically from SEM images of 100 colloidal particles) is used to indicate the deformation degree of the colloids. Figure 3 gives the relationship between the average axial ratios and the irradiation time, which indicates that the average axial ratios increase almost linearly with the irradiation time. Figure 4 shows the morphology of the colloids irradiated for an even longer time (30 min). It can be seen that the “isolated” colloids are further stretched in the polarization direction. Some colloids with ill-defined shapes can be seen in the figure and are formed
Li et al.
Figure 5. Typical SEM image of the colloids, after being irradiated by two laser beams with orthogonally polarized directions each for 10 min.
Figure 6. Typical SEM image of 2D close-packed arrays of the colloidal spheres obtained by the vertical deposition method. Insert: a FFT plot obtained from an AFM image of a 10 × 10 µm2 size sample.
by merging two or three deformed colloids in the elongated direction. After the colloids had been stretched in one direction, the colloids could be stretched in the other directions (such as that orthogonal to the first one) by the polarized laser single-beam irradiation. A typical SEM image of the colloids, which were obtained after being irradiated by two laser beams with orthogonally polarized directions each for 10 min, is shown in Figure 5. It can be seen that the colloids are stretched in two orthogonal directions. It further confirms that the deformation of the colloids is a stretch along the polarization direction of the laser single-beam. To compare the irradiation effects of the polarized laser singlebeam and the interfering p-polarized laser beams, 2D arrays of the colloidal spheres were prepared by the vertical deposition method.14 By this method, a substrate is immersed vertically in a suspension containing colloidal spheres. By evaporating the dispersion medium or lifting the substrate out of the suspension at constant speed, the surface of the liquid moves down and the colloids deposit onto the substrate during the decline of the surface. The attractive capillary force at the meniscus can cause the colloidal spheres to organize into close-packed 2D arrays. In this work, silicon wafers were immersed vertically in the suspension of the colloidal spheres (about 0.2 mg/mL), and the 2D arrays were obtained by evaporating the dispersion medium at a constant
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Figure 7. SEM images of the 2D colloidal arrays after being irradiated by (A) the linearly polarized laser beam for 15 min; (B) the p-polarized interfering laser beams for 20 min.
rate (about 0.05 mL/h). Figure 6 shows the SEM image of the 2D arrays, and the inset gives a 2D Fast Fourier Transform (FFT) plot obtained from an AFM image of a 10 × 10 µm2 size sample. A hexagonally close-packed structure is a predominant alignment pattern in the sample. Figure 7 shows the SEM images of the colloidal arrays after being exposed to the polarized laser single-beam for 15 min (Figure 7A) and the interfering p-polarized laser beams for 20 min (Figure 7B). After being irradiated by the polarized laser single-beam, the colloidal spheres are uniformly stretched along the polarization direction of the laser beam. The deformation degree of the colloids in the array is less than that of the “isolated” colloids, because of the restraint of the neighboring colloids. The deformation degree of colloids can as well be controlled by the irradiation time. The irradiation experiments showed that the assembly direction of the arrays did not influence the stretching effect. In contrast to the single-beam irradiation, after being irradiated with the interfering laser beams, the colloids in some regions are almost unchanged but in other regions are significantly deformed. Comparing with the SRG inscription study of the polymer,12,15 it can be concluded that the spheres in the dark regions of the interference pattern remain unchanged while the spheres in the bright regions are deformed significantly. It is well-known that various radiation force effects can be caused by the optical-field gradient force of laser beams, which is attributed to a time average of the Lorentz force over a cycle of the electromagnetic fields.16 The SRG formation induced by interfering laser beams has been attributed to the polymer chain migration caused by the gradient force, where the force is proportional to the gradient of the light intensity and along the polarization direction.15,16 The colloid deformation observed in the two-beam interference experiment could also be attributed to the gradient force of the laser irradiation. However, for the (15) (a) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Lee, T. S.; Tripathy, S. K. Appl. Phys. Lett. 1998, 72, 2096. (b) Bian, S.; Williams, J. M.; Kim, D. Y.; Li, L.; Balasubramanian, S.; Kumar, J.; Tripathy, S. K. J. Appl. Phys. 1999, 86, 4498.
single-beam irradiation, the light intensity of the spatially filtered and collimated laser beam (1.5 cm in diameter) should be uniform in submicrometer size. One possible explanation for this somewhat unexpected observation is that the optical-field gradient force could be caused by the lenslike property of the colloidal particles, which resulted in the nonuniform light intensity in the colloids. The microlens effect of polymeric colloids has been reported by Xia et al. for polystyrene colloids.17 Although the mechanism for the colloidal sphere deformation is still not understood at the current stage, the single-beam method can show some obvious technical advantages over the interfering two-beam irradiation scheme. Nonspherical colloids and their arrays can be more feasibly prepared by the single-beam irradiation. Artificial defects in colloidal arrays can be selectively fabricated by this method. Those applications could be important in areas such as fabrication of PBG material with complete band gaps and optical crystals with special light-manipulation properties.7-9 In summary, photoinduced shape deformation has been observed for the colloidal spheres formed from an azo polymer after being irradiated by a linearly polarized Ar+ laser singlebeam (488 nm, 150 mW/cm2). The spatially filtered and collimated laser beam can stretch the colloids along the polarization direction of the laser beam. In the testing period (20 min), the colloids are deformed continuously as the irradiation time increases. When irradiated by the polarized laser single-beam, the colloidal spheres in 2D close-packed arrays are uniformly stretched along the polarization direction of the laser beam. On the other hand, the interfering p-polarized laser beams only deform the colloidal spheres in the bright regions of the interference pattern. Acknowledgment. Financial support from NSFC under Projects 50533040 and 20374033 is gratefully acknowledged LA052884B (16) Smith, P. W.; Ashkin, A.; Tomlinson, W. J. Opt. Lett. 1981, 6, 284. (17) Lu, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2001, 13, 34.
