Fabrication of Photoprocessible Azo Polymer Microwires through a

prepare both the soft lithographic masters and the microwires. The masters were ... the masters. With use of the stamps and a solution of BP-AZ-CA as ...
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Langmuir 2006, 22, 10233-10237

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Fabrication of Photoprocessible Azo Polymer Microwires through a Soft Lithographic Approach Bin Liu, Yaning He, and Xiaogong Wang* Laboratory for AdVanced Materials, Department of Chemical Engineering, Tsinghua UniVersity, Beijing, 100084, P. R. China ReceiVed July 5, 2006. In Final Form: September 9, 2006 In this work, a soft lithographic approach has been developed to fabricate free-standing azo polymer microwires with unique photoprocessible characteristics. In the process, an epoxy-based azo polymer (BP-AZ-CA) was used to prepare both the soft lithographic masters and the microwires. The masters were prepared by photofabricating surface relief gratings on BP-AZ-CA thin films. Then the elastomeric stamps were prepared by replica molding of poly(dimethylsiloxane) prepolymer against the masters. With use of the stamps and a solution of BP-AZ-CA as “ink”, the microwires were prepared by contact printing and wet etching. The microwires possessed a uniform sub-micrometerscale transverse dimension and macroscopic longitudinal dimension. Those characteristic sizes depended on the adjustable features of the masters and stamps used in the process. The transverse dimension of the microwires could be altered after exposure to a linearly polarized Ar+ laser single beam with the polarization direction perpendicular to the longitudinal axes of the microwires. Upon irradiation of interfering p-polarized Ar+ laser beams, regular surface relief structures could be inscribed on the microwires along the longitudinal direction, which coincided with both the polarization direction of the laser beams and the grating vector direction of the interference pattern. The microwires with photoprocessible properties are potentially usable as sub-micrometer-scale materials in future miniaturized components and devices. The approach reported in this work can be further extended to the fabrication of nano-/ microwires from other polymeric materials.

Introduction Nano-/microwires have aroused tremendous research interest in recent years for their potential applications in electronic, optoelectronic, electrochemical, and electromechanical devices.1 Those 1D nano-/microstructures have been fabricated through different methods by using a variety of materials such as metals,2 metal oxides,3 and semiconductors4 and polymers.5 Because of their unique properties, polymeric nano-/microwires are attracting more and more attention.6-12 Nano-/microwires of conductive polymers (CPs) have been fabricated by methods such as template synthesis,6 photolithography,7 self-assembly,8 direct electrochemical synthesis,9 dip-pen nanolithography,10 microcontact printing (µCP),11 and micromolding in capillaries (MIMIC).12 * Corresponding author. E-mail: [email protected]. (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (2) Yan, X.-M.; Kwon, S.; Contreras, A. M.; Bokor, J.; Somorjai, G. A. Nano Lett. 2005, 5, 745. (3) Kolmakov, A.; Moskovits, M. Annu. ReV. Mater. Res. 2004, 34, 151. (4) See, for example: (a) Yin, Y. D.; Gates, B.; Xia, Y. N. AdV. Mater. 2000, 12, 1426. (b) Sun, Y. G.; Khang, D.-Y.; Hua, F.; Hurley, K.; Nuzzo, R. G.; Rogers, J. A. AdV. Funct. Mater. 2005, 15, 30. (c) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83. (d) Liang, Y. Q.; Zhen, C. G.; Zou, D. C.; Xu, D. S. J. Am. Chem. Soc. 2004, 126, 16338. (e) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (5) Aleshin, A. N. AdV. Mater. 2006, 18, 17. (6) See, for example: (a) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (b) Jerome, C.; Jerome, R. Angew. Chem., Int. Ed. 1998, 37, 2488. (c) Tchepournaya, I.; Vasilieva, S.; Logvinov, S.; Timonov, A.; Amadelli, R.; Bartak, D. Langmuir 2003, 19, 9005. (7) Jager, E. W. H.; Smela, E.; Inganas, O. Science 2000, 290, 1540. (8) (a) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684. (b) Qiu, H. J.; Zhai, J.; Li, S. H.; Jiang, L.; Wan, M. X. AdV. Funct. Mater. 2003, 13, 925. (9) Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. H. Angew. Chem. 2002, 114, 3817. (10) See, for example: (a) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522. (b) Noy, A.; Miller, A. E.; Klare, J. E.; Weeks, B. L.; Woods, B. W.; Deyoreo, J. J. Nano Lett. 2002, 2, 109. (11) Yu, J. F.; Holdcroft, S. Chem. Commun. 2001, 14, 1274. (12) Zhang, F. L.; Nyberg, T.; Inganas, O. Nano Lett. 2002, 2, 1373.

Those 1D nano-/microstructures of conjugated polymers can exhibit unique electrical transport and optical excitation properties, which are promising as elements in nanoelectronic devices, sensors, and other miniaturized components or devices.5-12 More recently, exceptional photothermal effect of polyaniline nanowires has been discovered and explored for material processing applications.13 In general, the shape and size of polymeric nano-/ microwires, which are determined by the techniques and conditions used to prepare them, cannot be further adjusted after the preparation. Therefore, it is still a challenging problem to endue polymeric nano-/microwires with processing flexibility and to fabricate them through a feasible approach. Azo polymers (polymers containing aromatic azo chromophores) have been well-documented for their capability to be processed in sub-micrometer-scale upon light irradiation.14 Reversible surface relief gratings (SRGs) can be photofabricated on azo polymer films by interfering Ar+ laser beam irradiation.15 Upon the light irradiation, SRGs are formed at a temperature well below the glass transition temperatures (Tg’s) of the polymers. SRGs are stable below Tg’s of the polymers and can be removed by heating samples to a temperature above their Tg’s or erased optically even below Tg’s. The SRG formation has been attributed to an internal pressure gradient caused by an isomerizationdriven free volume expansion in the bulk,16 the dipolar interaction of the azo chromophores with the optically induced electric field gradient,17 translational wormlike diffusion caused by the photoisomerization of the azobenzene chromophores,18 and a mean-field force caused by the molecule alignment.19 More (13) Huang, J. X.; Kaner, R. B. Nat. Mater. 2004, 3, 783. (14) (a) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (b) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (15) (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. (16) (a) Barrett, C. J.; Rochon, P.; Natansohn, A. J. Phys. Chem. 1996, 100, 8836. (b) Barratt, C. J.; Rochon, P.; Natansohn, A. J. Chem. Phys. 1998, 109, 1505.

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recently, it has been reported that colloidal azo polymer spheres can be stretched by irradiation of the polarized Ar+ laser single beam or interfering beams.20 It can be expected that nano-/ microwires made of azo polymers can exhibit photoprocessible properties. However, to our knowledge, no such result has been reported in the literature. In this paper, we present a method to fabricate photoprocessible free-standing azo polymer microwires through a soft lithographic approach. The microwires, made of an epoxy-based azo polymer (BP-AZ-CA), possess a uniform sub-micrometer-scale transverse dimension and macroscopic longitudinal dimension, which can be further adjusted by exploiting the changeable features of the masters and stamps used in the process. The microwires can be processed by Ar+ laser (488 nm) irradiation after the preparation. Irradiation of the polarized Ar+ laser single beam or interfering beams can alter the transverse dimension of the microwires or inscribe regular surface relief structures on the microwires. Experimental Section Materials. The chemical structure of the epoxy-based azo polymer (BP-AZ-CA) is given as

which contains the pseudo-stilbene-type azo chromophore at each repeating unit. The BP-AZ-CA sample was synthesized in the authors’ laboratory and possessed the number-average molecular weight of 41000 with the polydispersity index of 2.2. The synthesis and characterization of the polymer have been reported elsewhere.21 The PDMS elastomer kit was bought from Dow Corning (Sylgard 184). Tetrahydrofuran (THF), dimethylformamide (DMF), and hydrofluoric acid (HF) were all commercially purchased and used without further purification. Milli-Q water (resistivity > 18 MΩ‚ cm) was supplied by a Millipore water purification system. Characterization. The atomic force microscopy (AFM) images were obtained by using a scanning probe microscope (Nanoscope IIIa, Digital Instrument) in the tapping mode. The scanning electron microscopy (SEM) images were obtained by applying an Hitachi S-4500 electron microscope operated at an accelerating voltage of 15 kV. The samples prepared for SEM studies were coated with a thin layer of gold (∼10 nm in thickness) before the measurement. Transmission electron microscopy (TEM) images were recorded on a JEM-1200EX (JEOL) microscope at an accelerating voltage of 100 kV. The TEM samples were prepared by transferring the floated microwires (obtained after HF etching) onto the copper grids. Master Fabrication. A solution of BP-AZ-CA in DMF (10%, wt %) was prepared and spin-coated onto glass slides to obtain polymer thin films (about 1 µm in thickness). The fabrication of SRGs is similar to the method reported in the literature.15,21 A linearly polarized Ar+ laser beam (488 nm, 100 mW/cm2) was used as the light source. The p-polarized laser beam was obtained through spatial filtering, expanding, and collimating. Then the beam was split into two halves. One-half of the beam was incident on the films directly and the other half of the beam was reflected onto the films from a (17) (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. (18) Lefin, P.; Fiorini, C.; Nunzi, J. M. Pure Appl. Opt. 1998, 7, 71. (19) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S.; Hilvsted, S. Phys. ReV. Lett. 1998, 80, 89. (20) (a) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. J. Am. Chem. Soc. 2005, 127, 2402. (b) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. Langmuir 2006, 22, 2288. (21) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43, 7325.

Liu et al. mirror. In a typical condition, the films were exposed to the interference pattern for 30 min at room temperature. The films inscribed with the SRGs were used as masters for PDMS replication. PDMS Stamp Preparation. The elastomeric stamps were prepared by replica molding. Poly(dimethylsiloxane) (PDMS) prepolymer was obtained by mixing the elastomer base and curing agent (Sylgard 184, Dow Corning) at a proper ratio (10:1, w/w). The prepolymer was cast against the masters and then cured in a 60 °C oven for 4 h. After curing, the PDMS stamps were peeled off and used for the microwire preparation. Microwire Fabrication. The stamps were “inked” with a THF solution of BP-AZ-CA (5 mg/mL), and the excessive liquid on the surfaces was removed by careful wiping. The PDMS stamps were printed on clean glass slides, where the inked surfaces were pressed conformally against the substrates for a few seconds (such as 10 s). After that, the stamps were peeled off and the printed patterns were left on the substrates. The samples were kept in a 40 °C vacuum oven for 2 days. In the etching step, the glass slides with the patterned surfaces upward were put in a plastic Petri dish. Deionized water was added into the dish until the liquid level reached the upper surfaces of the glass slides. Then hydrofluoric acid (HF) was added dropwise near the slides. Etching from the lateral sides (along the grating vector) showed a better result, which could produce wellseparated microwires. While the substrates were slowly etched, the microwires gradually formed and released from the substrates. The floated microwires were washed with deionized water and then transferred to target substrates (glass slides, silicon wafers, or copper grids). Photoprocessing. The apparatus and experimental setup for photoprocessing the microwires were similar to those described above for the SRG fabrication. The Ar+ laser beam (488 nm) was spatially filtered, expanded, and collimated to possess the intensity of 100 mW/cm2. For the single-beam irradiation experiment, the linearly polarized laser beam was incident perpendicularly to the wafer surfaces containing the microwires. The polarization direction was adjusted to be approximately orthogonal to the longitudinal axes of the microwires. To carry out the experiments with interfering laser beams, the microwires were irradiated by the p-polarized interfering Ar+ laser beams, where both the polarization direction and the grating vector of the interference pattern were approximately parallel to the longitudinal direction of the microwires. The grating spatial period was adjusted by changing the intersection angle of the interfering beams.15,16 Under typical conditions, the samples were exposed to the light irradiation for 30-50 min.

Results and Discussion The fabricating procedure of the microwires is illustrated in Figure 1. First, a master is prepared by photoinscribing SRGs on a BP-AZ-CA thin film and then an elastomeric stamp is prepared by replica molding of PDMS prepolymer against the master. The SRGs on the azo polymer film is complementarily transferred to the stamp surface. Second, the PDMS stamp is “inked” with a solution of BP-AZ-CA and pressed against a glass slide for a few seconds (step (a) in Figure 1). After the stamp is peeled off, a patterned negative relief structure is left on the glass slide (step (b) in Figure 1). Third, hydrofluoric acid (HF) is used to etch the glass substrate surface. During the process, the relief bands on the glass slide are transformed to microwires, which are floated off the substrate on to the water surface (step (c) in Figure 1). After washing, the free-standing microwires are transferred to a clean substrate and dried to remove water. The preparation and characterization results are given and discussed as follows. The masters were prepared from the epoxy-based azo polymer BP-AZ-CA (Mn ) 41000, PDI ) 2.2). For SRG preparation, the spin-coated BP-AZ-CA thin films were exposed to an interference pattern of p-polarized Ar+ laser beams (488 nm, 100 mW/cm2) for a proper time period. The grating depth and spatial period

Photoprocessible Azo Polymer Microwire Fabrication

Figure 1. Schematic representation of the preparation of photoprocessible free-standing microwires: (a) contact printing with an “inked” PDMS stamp, (b) formation of the patterned surface relief structure after printing, (c) HF etching, (d) irradiation with linearly polarized Ar+ laser single beam, and (e) irradiation with interfering p-polarized Ar+ laser beams.

can be adjusted by altering the irradiation time and intersection angle of the interfering beams. In this study, the masters with sinusoidal surface relief structures were prepared by irradiation with the interfering Ar+ laser beams for 30 min at room temperature. The grating spatial period and trench depth were 790 and 230 nm estimated from AFM measurement. The surface pattern was stable below the glass transition temperature of BPAZ-CA (Tg ) 160 °C). The PDMS stamps for contact printing were prepared by casting a mixture of the elastomer base and curing agent (10:1, w:w) against the masters and then cured in a 60 °C oven for 4 h. A THF solution of BP-AZ-CA (5 mg/mL) was used as the ink. The PDMS stamps were inked by touching the polymer solution for about 1 s, which left a thin liquid layer adsorbed on the patterned surfaces. Although the ink concentration was low, the solvent of the inked thin liquid layer vaporized very quickly and a highly concentrated and viscous thin film formed on the SRG surfaces.22 When the inked surfaces of the stamps were pressed against clean glass slides for a few seconds (such as 10 s), the BP-AZCA material was transferred to the slide surfaces and shaped by the trenches on the stamps. After the PDMS stamps were peeled off, the samples were kept in a 40 °C vacuum oven for 2 days before subsequent experiments Figure 2 gives AFM images of some typical examples of the masters, the PDMS replicas, and the printed surface relief structures. The master (Figure 2a) and the PDMS stamp (Figure 2b) show a very similar surface pattern, which have regular sinusoidal surface relief with the grating period and trench depth of 790 and 230 nm for the master and 800 and 170 nm for the stamp. The printed pattern also exhibits the SRG characteristics (Figure 2c). The grating period of the printed pattern is 790 nm, which is equal to the period of the master and 10 nm less than that of the stamp. The trench depth is 165 nm, which is 5 nm (22) Liu, B.; Wang, M. Q.; He, Y. N.; Wang X. G. Langmuir 2006, 22, 7405.

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Figure 2. AFM images of surface structures: (a) BP-AZ-CA master, (b) PDMS stamp, and (c) the printed surface relief structure of BPAZ-CA on a glass slide.

shallower than that of the PDMS stamp. The result indicates that, through the replica molding and contact printing, the sinusoidal relief structures of BP-AZ-CA can be fabricated on the glass substrates. The printed surface structures could be directly transformed into microwires by carefully etching the substrates. In the process, the glass slides were put in a plastic Petri dish with the printed surfaces upward. A suitable amount of deionized water was added into the dish until the liquid level reached the upper surfaces of the glass slides. Then HF was added dropwise into the water in the dish. Etching from the lateral side of the printed bands was found to be preferable, which could produce well-separated microwires. While HF diffused into the interface between the printed bands and the glass substrate, the substrate surface was etched and the polymeric microwires were gradually peeled off and floated on to the water surface. Due to the surface tension, the polymeric microwires isolated from each other and formed uniform free-standing microwires with some flexibility, which could be transferred to other surfaces. The soft lithographic procedure mentioned above is necessary to prepare the microwires. Although SRGs can be directly inscribed on the polymer film surface, the trench bottom cannot reach the slide surface even with irradiation for an extremely long time.14,15,21 Therefore, directly etching the SRGs-inscribed polymer film cannot produce microwires. Figure 3 gives some typical SEM and AFM images of the azo polymer microwires. The longitudinal dimension of the microwires is in the macroscopic scale (at least 1 mm) as shown in the SEM picture (Figure 3a). From the enlarged SEM image (Figure 3b), the microwires show a transverse dimension of about 480 nm. AFM observation indicates that the microwires maintain some sinusoidal characteristics in the transverse cross section (Figure 3c,d). The horizontal dimension (HD) and vertical dimension (VD) of the microwires in the cross section are 580 and 130 nm for Figure 3c and 570 and 130 nm for Figure 3d, respectively. These microwires could be transferred to copper grids, TEM observation also confirmed the morphology of the microwires (Figure S1(a) and S1(b), in the Supporting Information). The dimensions of the microwires can be further adjusted by using the masters and stamps with different grating spatial

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Figure 3. SEM and AFM images of the epoxy-based azo polymer microwires: (a) SEM image of the microwires; (b) enlarged SEM image of the microwires, 480 nm in the width; (c) AFM image of the microwires, HD ) 580 nm and VD ) 130 nm in the transverse cross section; (d) enlarged AFM image of the microwires, HD ) 570 nm and VD ) 130 nm in the transverse cross section.

periods and depths, which can be obtained by changing the light irradiation condition and exploiting the elastomeric property of PDMS. One interesting property of the free-standing microwires is their photoprocessibility due to the photoresponsive nature of BP-AZ-CA. The transverse dimension of the microwires can be altered by irradiation of a linearly polarized Ar+ laser single beam (step (d) in Figure 1). The microwires can show alternately varied modulations along the longitudinal direction after exposure to the interfering patterns of p-polarized Ar+ laser beams (step (e) in Figure 1). In the single-beam experiment, the microwires were irradiated by a linearly polarized Ar+ laser beam (488 nm, 100 mW/cm2) incident vertically upon the sample. The polarization direction of the laser beam was orthogonal to the longitudinal axes of the microwires. The microwires used for each comparison experiment were obtained from the same batch and possessed uniform transverse sizes. AFM measurement showed a size deviation less than (5%, which was estimated from randomly selected 40 different locations of the microwires. Before and after the light irradiation, the transverse dimension of the samples, which were statically selected in almost the same region, was detected by AFM. Figure 4 gives some typical AFM images obtained before irradiation and after irradiation for 30 min. Before the light irradiation, the horizontal dimensions (HDs) are 580 nm (Figure 4a) and 570 nm (Figure 4c). After the light irradiation, the dimensions are changed to 620 nm (Figure 4b) and 610 nm (Figure 4d). Correspondingly, the vertical dimensions (VDs) of the cross section vary from 130 nm (Figure 4a) and 135 nm (Figure 4c) to 110 nm (Figure 4b) and 115 nm (Figure 4d) after the light irradiation. The section analysis on the figures indicates that the transverse shape of the microwires is altered after the light irradiation. Increasing the light irradiation time, the transverse dimension of the microwires can be further altered. Figure S2 (in the Supporting Information) gives the AFM images before and after irradiation with the same light source for 100 min. From the defects on the microwires, it can be seen that the

Figure 4. AFM images of the microwires before and after irradiation with polarized Ar+ laser single beam (488 nm, 100 mW/cm2) for 30 min. (a) Before the irradiation, HD ) 580 nm, VD ) 130 nm. (b) After the irradiation, HD ) 620 nm and VD ) 110 nm, respectively. (c) Before the irradiation, HD ) 570 nm and VD ) 135 nm. (d) After the irradiation, HD ) 610 nm and VD ) 115 nm.

observations were carried out on the exact same location. This result can further confirm the transverse size alteration given in Figure 4. In the experiment using the interfering laser beams, the interference pattern of Ar+ laser beams was produced by a similar setup used to inscribe SRGs. Both polarization direction of the laser beam and the grating vector direction of the interference pattern coincided with the longitudinal direction of the microwires. The grating period of the interference pattern was adjusted by changing the intersection angle between the interfering beams. After exposure to interfering p-polarized Ar+ laser beams for a period of time, more complicated shape deformation can be induced on the photoresponsive microwires. Figure 5 and Figure S3 (in the Supporting Information) give the AFM images of the microwires after the light irradiation, which indicates that surface relief structures are formed on the microwires. Figure 5a and 5b show 2D- and 3D-view AFM images of the sample, which were obtained after the light irradiation for 50 min. The images show that the distance between two humps (the spatial period) is 360 nm and the height of the hump is 100 nm. Upon exposure to the interference patterns with different grating periods, similar surface modifications can be produced on the microwires. Figure 5c and 5d show the 2D- and 3D-view AFM images of the microwires with the modulation periods of 770 nm and the height of 60 nm, which was obtained after the light irradiation for 50 min. Figure 5e and 5f show the AFM images of the microwires with the modulation periods of 910 nm and the height of 35 nm, which was obtained after the light irradiation for 30 min. SEM observation further confirms the structure formed on the microwires (Figure 6). SEM picture shows regularly alternating bright and dark regions along the axial direction of the microwires, which correspond to the topographic contrast caused by the humps and valleys on the

Photoprocessible Azo Polymer Microwire Fabrication

Figure 5. AFM images of the microwires after being irradiated by p-polarized interfering Ar+ laser beams (488 nm, 100 mW/cm2). (a) 2D-view image, the modulation period and height are 360 and 100 nm, the irradiation time is 50 min; (b) 3D-view image of picture (a); (c) 2D-view image, the modulation period and height are 770 and 60 nm, the irradiation time is 50 min; (d) 3D-view image of picture (c); (e) 2D-view image, the modulation period and height are 910 and 35 nm, the irradiation time is 30 min; (f) 3D-view image of picture (e).

microwires. SEM observation also indicates that the surface modulation with different spatial periods can be inscribed on the surface. For the same samples as those given in Figure 5, the distances between SEM bright regions (the humps) are 360 nm (Figure 6a), 780 nm (Figure 6b), and 910 nm (Figure 6c), respectively. Comparing with the SRG study reported before,17 it is believed that the valleys in AFM images and the dark regions in the SEM images correspond to the bright region of the interference pattern of the laser beams. The structure formation induced by the light irradiation can also be seen from TEM observation (Figure S1(c) and S1(d), in the Supporting Information). Like the SRG formation, the mechanism for the photoinduced deformation is not fully understood at the current stage. The above results indicate that the light irradiation can cause two effects. First, the single laser beam irradiation can cause transverse shape deformation (Figure 4). Although the deformation degree is much smaller than those observed for colloidal spheres,20b which could be attributed to the differences in the inner structure and geometric shape, the photoinduced effect is also believed to be a stretching deformation along the polarization direction of the laser beam. Second, the irradiation with interfering laser beams can cause the mass migration from bright regions to dark regions of the interference pattern (Figures 5 and 6), which is similar to the effect causing SRG formation on the azo polymer films.14,15,21 In both cases, the repeated trans-cis-trans isomer-

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Figure 6. SEM images of the microwires after being irradiated by p-polarized interfering Ar+ laser beams (488 nm, 100 mW/cm2). (a) The modulation period is 360 nm, the irradiation time is 50 min; (b) the modulation period is 780 nm, the irradiation time is 50 min; (c) the modulation period is 910 nm, the irradiation time is 30 min.

ization cycles of the azo chromophores play a key role in softening the materials through the free volume expansion.14(b),16

Conclusion In summary, a facile method to prepare azo polymer microwires through soft lithography and wet etching has been developed in this work. The free-standing BP-AZ-CA microwires prepared by this method possess a uniform sub-micrometer-scale transverse dimension and macroscopic longitudinal dimension. Due to the photoresponsive nature, the flexible microwires can be photoprocessed after the fabrication. Different size and shape deformation can be induced by the irradiation of the polarized Ar+ laser single beam and interfering beams. Acknowledgment. Financial support from NSFC under Projects 50533040 and 20374033 is gratefully acknowledged. Supporting Information Available: More results obtained from the structure characterization. This material is available free of charge via the Internet at http://pubs.acs.org. LA0619426