Duplication of Photoinduced Azo Polymer Surface-Relief Gratings

Using the azo polymer films as masters, stamps of poly(dimethylsiloxane) (PDMS) were .... The soluble MWNTs were prepared by the method as reported in...
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Langmuir 2006, 22, 7405-7410

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Duplication of Photoinduced Azo Polymer Surface-Relief Gratings through a Soft Lithographic Approach Bin Liu, Mingqing Wang, Yaning He, and Xiaogong Wang* Department of Chemical Engineering, Laboratory for AdVanced Materials, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed April 29, 2006 In this work, a soft lithographic approach has been developed to duplicate photoinduced surface-relief-gratings (SRGs) of azo polymer films to generate the surface pattern replicas composed of different materials on various substrates. For this purpose, thin films of an epoxy-based azo polymer (BP-AZ-CA) were prepared by spin-coating, and SRGs with different structures were inscribed by exposing the films to interference patterns of Ar+ laser beams at modest intensity (150 mW/cm2). Using the azo polymer films as masters, stamps of poly(dimethylsiloxane) (PDMS) were prepared by replica molding. The PDMS stamps were then used to transfer the solutions of poly(3-hexylthiophene) (P3HT), multiwalled carbon nanotube (MWNT), and BP-AZ-CA to different substrates by contact printing. Through this process, surface pattern replicas made of the functional materials were obtained. The pattern formation and quality depended on the factors such as the solution concentration, contacting time in the printing process, and printing pressure. Under the proper conditions, the printed patterns showed the same grating periods as the masters and the same relief depths as the stamps (replicas of the masters). This approach, showing some attractive characteristics such as the easiness of master preparation and the versatility of soft fabrication processes, can be applied to the fabrications of optical functional surfaces, sensors, and photonic devices.

Introduction Photoinduced reversible surface-relief-gratings (SRGs) have been well documented as a unique and fascinating property of azobenzene-containing polymers (azo polymer for short).1-5 Upon exposure to an interference pattern of Ar+ laser beams at modest intensity, large surface modulations can be produced on azo polymer films.1,2 The photofabricated surface structures are stable below the glass transition temperatures (Tgs) of the polymers and can be removed by optical erasure or by heating samples to a temperature above their Tgs.6-8 It is easy to fabricate various surface patterns by selecting suitable photofabrication conditions.9,10 This one-step approach can precisely control the modulation depths (by adjusting irradiation energy), eliminate the contact and wet processing, and avoid the etching or developing steps in photolithographic methods. Complex surface relief structures can be prepared by methods such as sequentially superimposed inscriptions with beams having different wavelengths or polarization directions.4,9,10 Azo polymers with the SRG formation ability and their potential applications have been extensively explored in recent years.3,4 On the other hand, this surface patterning method also faces some obvious limitations, for instance, which can only be used for azo polymers or related * Corresponding author. E-mail: [email protected]. (1) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136138. (2) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166-1168. (3) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817-1846. (4) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139-4176. (5) He, Y.; Wang, X.; Zhou, Q. Polymer 2002, 43, 7325-7333. (6) Barrett, C.; Natansohn, A.; Rochon, P. J. Phys. Chem. 1996, 100, 88368842. (7) Ho, M. S.; Barrett, C.; Paterson, J.; Esteghamatian, M.; Natansohn, A.; Rochon, P. Macromolecules 1996, 29, 4613-4618. (8) Jiang, X. L.; Li, L.; Kumar, J.; Kim, D. Y.; Tripathy, S. K. Appl. Phys. Lett. 1998, 72, 2502-2504. (9) Tripathy, S. K.; Kim, D. Y.; Li, L.; Kumar, J. Chemtech 1998, 28, 34-40. (10) Viswanathan, N. K.; Kim, D. Y.; Bian, S.; Williams, J.; Liu, W.; Li, L.; Samuelson, L.; Kumar, J.; Tripathy, S. K. J. Mater. Chem. 1999, 9, 1941-1955.

materials. Therefore, it cannot be directly extended as a general processing way for other functional materials. Soft lithography as a new microfabrication technique has attracted great attention in recent years.11-19 This innovative technique uses patterned elastomers as stamps, molds, and masks to fabricate a variety of microstructures. Those soft fabrication tools can be prepared by casting a liquid prepolymer of the elastomers against a master that has a patterned relief structure on its surface. Available relief structures such as diffraction gratings or TEM grids have been used as the replica molding masters.11 More generally, masters have been prepared by micromachining and various microlithographic techniques.20,21 Among them, photolithography is one of the most widely used methods for fabricating masters with complex structures. Masters with feature sizes greater than or equal to 20 µm have been prepared by a rapid photomask formation scheme based on computer programming and ink-jet printing.22 Because of its deformable ability, low interfacial free energy, and optical transparency, poly(dimethylsiloxane) (PDMS) has been widely used as the elastomeric material to duplicate a master through (11) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550575. (12) Zhao, X.; Xia, Y.; Whitesides, G. M. J. Mater. Chem. 1997, 7, 10691074. (13) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1171-1196. (14) Quist, A. P.; Pavlovic, E.; Oscarsson, S. Anal. Bioanal. Chem. 2005, 381, 591-600. (15) Xia, Y.; Kim, E.; Zhao, X. M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347-349. (16) Bowden, N.; Huck, W. T. S.; Paul, K. E.; Whitesides, G. M. Appl. Phys. Lett. 1999, 75, 2557-2559. (17) Quake, S. R.; Scherer, A. Science 2000, 290, 1536-1540. (18) Wilbur, J. L.; Jackman, R. J.; Whitesides, G. M.; Cheung, E. L.; Lee, L. K.; Prentiss, M. G. Chem. Mater. 1996, 8, 1380-1385. (19) Rogers, J. A.; Jackman, R. J.; Schueller, O. J. A.; Whitesides, G. M. Appl. Opt. 1996, 35, 6641-6647. (20) Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672-2682. (21) Wilbur, J. L.; Kumar, A.; Kim, E.; Whitesides, G. M. AdV. Mater. 1994, 6, 600-604. (22) Qin, D.; Xia, Y.; Whitesides, G. M. AdV. Mater. 1996, 8, 917-919.

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replica molding.16-19 On the basis of those elastomeric stamps and molds, a variety of high-quality surface patterns and structures with resolution down to submicron-scale have been prepared by soft lithographic methods such as replica molding (REM), microcontact printing (µCP), microtransfer molding (µTM), micromolding in capillaries (MIMIC), and solvent-assisted micromolding (SAMIM).11 Those microfabrication methods have been used for preparing optically functional surfaces of transparent polymers,23 patterns of regioregular polythiophenes,24 conductive nanowires and nanodots of poly(3,4-ethylenedioxythiophene) (PEDOT),25 patterned light-emitting diodes of PEDOT,26 and patterned structures of single-walled carbon nanotubes.27 New approaches with good controllability and simplicity are still being actively explored.28, 29 Due to the topographic features, the surface-patterned azo polymer films can be directly used as a soft lithographic master to cast elastomeric stamps or molds. As a master, it can show advantages such as high resolution, reversible patterning ability, and ease of fabrication. Through soft lithographic processes, the surface patterns of azo polymers can be feasibly duplicated to fabricate the surface pattern replicas made of different functional materials on various substrates. This approach, which combines the advantages of the surface patterning technique based on photoresponsive azo polymers and soft lithography, can supply a new method to fabricate optical functional surfaces, sensors, and photonic devices among others. However, to our knowledge, such investigations have not yet been reported in the literature. In this work, thin films of an epoxy-based azo polymer (BPAZ-CA) inscribed with SRGs were used as replica molding masters. By casting the liquid prepolymer of PDMS against the masters, PDMS stamps were prepared and then used to transfer the solutions of poly(3-hexylthiophene) (P3HT), multiwalled carbon nanotube (MWNT), and BP-AZ-CA to substrates through contact printing. Through this process, replicas of the surface relief structures composed of the materials were obtained. The printed replicas showed the same grating periods as the masters and the same relief depths as the stamps. Experimental Section Materials. The chemical structure of the epoxy-based azo polymer (BP-AZ-CA) is given as

which contains pseudo-stilbene-type azo chromophores at each repeating unit. The number-average molecular weight of the polymer was estimated to be 41 000 with the polydispersity index of 2.2 obtained by the gel permeation chromatography (GPC) measurement. The preparation and characterization details of BP-AZ-CA can be seen in our previous paper.5 The PDMS prepolymer was purchased from Dow Corning (Sylgard 184). Other chemicals and solvents, (23) Grzybowski, B. A.; Qin, D.; Whitesides, G. M. Appl. Opt. 1999, 38, 2997-3002. (24) Zhai, L.; Laird, D. W.; McCullough, R. D. Langmuir 2003, 19, 64926497. (25) Zhang, F.; Nyberg, T.; Inganas, O. Nano Lett. 2002, 2, 1373-1377. (26) Granlund, T.; Nyberg, T.; Roman, L. S.; Svensson, M.; Inganas, O. AdV. Mater. 2000, 12, 269-273. (27) Meitl, M. A.; Zhou, Y.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Nano Lett. 2004, 4, 1643-1647. (28) Park, J.; Hammond, P. T. AdV. Mater. 2004, 16, 520-525. (29) McLellan, J. M.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10830-10831.

Liu et al. such as N,N-dimethylformamide (DMF), tetrahydrofuran (THF), and toluene, were commercially purchased and used without further purification. The glass slides, silicon wafers (GRITEK, crystallographic orientation (100), n-type), and indium tin oxide (ITO) coated glass slides were ultrasonically treated in water for 10 min, rinsed with acetone and blown dry with air before use. Characterization. 1H NMR spectra were obtained on a JEOL JNM-ECA600 NMR spectrometer. The molecular weights and their distributions of the polymers were determined by gel permeation chromatography (GPC) utilizing a Waters model 515 pump and a model 2410 differential refractometer with 3 styragel columns HT2, HT3, and HT4 connected in a serial fashion. THF was used as the eluent at a flow rate of 1.0 mL/min. Polystyrene standards with dispersity of 1.08-1.12 obtained from Waters were employed to calibrate the instrument. The patterned features on the substrates and the printed film thickness were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM measurements were performed on a field emission microscope (Hitachi S-4500) with the accelerating voltage of 15 kV. All of the samples prepared for SEM studies were observed without spurting treatment. The AFM images were obtained by using a NanoscopeIIIa Scanning Probe Microscope in the tapping mode. The grating depths were measured as the vertical distance from the peak to the valley (i.e., the trench depth) in patterned relief structures. The average film thickness was obtained by gashing the printed films and measuring the vertical distance from substrate surface to the middlepoint of the trench depth. UV-vis spectra were recorded on an Agilent-8453 spectrophotometer. Synthesis of 3-Hexylthiophene. 1-Bromohexane (18.1 g) was dissolved in 20 mL of anhydrous ether, and then 2.4 g of magnesium (in 20 mL anhydrous ether) was added dropwise into the solution. After the magnesium was dissolved, 60 mg of NiDPPPCl2 was added as the catalyst. 3-Bromothiophene (16.3 g, dissolved in 20 mL anhydrous ether) was added into the mixture. The reaction was carried out at 0 °C for 18 h. Then the solution was poured into a mixture of hydrochloric acid and ice. After phase separation, the brown supernatant liquid was collected and dried with anhydrous MgSO4 for 12 h and filtered. The filtrate was purified twice by reduced pressure distillation. The product (85-87 °C, at 7 mmHg) was collected. 1H NMR (CDCl3): δ 7.22 (s, 1H), 6.91 (m, 2H), 2.61(t, 2H), 1.61 (t, 2H), 1.25-1.35 (m, 8H), 0.89 (t, 3H). Synthesis of Poly(3-hexylthiophene) (P3HT). The polymer was prepared by the chemical oxidative polymerization as those reported previously.30 FeCl3 (6.5 g) was dispersed in 50 mL chloroform under sonication and then added dropwise into a solution of 3-hexylthiophene (1.6 g, in 50 mL chloroform). After polymerization at 0 °C for 12 h, an excessive amount of methanol was added to the reaction mixture. The precipitate was collected by filtration and treated with a 30% (vol %) solution of ammonia in ethanol for 1 h to remove the surplus FeCl3. Finally, the product was rinsed with a large amount of ethanol and dried under reduced pressure at room temperature for 48 h. The number average molecular weight of the polymer was estimated to be 17 870 with the polydispersity index of 4.2 obtained by the GPC measurement. 1H NMR (CDCl3): δ 6.98, 2.79, 2.56, 1.69, 1.34, 0.90. Preparation of Soluble Carbon Nanotubes. Multiwalled carbon nanotubes (MWNTs), prepared by the catalytic chemical vapor deposition (CCVD) method in a fluidized-bed reactor,31,32 were supplied by courtesy of Beijing Key Laboratory of Green Reaction Engineering and Technology in the authors’ department. The soluble MWNTs were prepared by the method as reported in the literature.33-35 MWNTs (1 g) were ultrasonically dispersed in a (30) Liu, Y.; Oshime, K.; Yamauchi, T.; Shimomura, M.; Miyauchi, S. J. Appl. Polym. Sci. 2000, 77, 3069-3076. (31) Wang, Y.; Wei, F.; Luo, G.; Yu, H.; Gu, G. Chem. Phys. Lett. 2002, 364, 568-572. (32) Yu, H.; Zhang, Q.; Wei, F.; Qian, W.; Luo, G. Carbon 2003, 41, 28552863. (33) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834-840. (34) Wang, Y.; Wu, J.; Wei, F. Carbon 2003, 41, 2939-2948.

Duplication of Surface-Relief Gratings mixture of 30 mL of H2SO4 (98%) and 10 mL of HNO3 for 12 h, then washed with an excessive amount of water, and dried in a vacuum oven at 80 °C for 24 h. The acid-treated MWNTs were added into a solution of dodecylamine (1.7 mmol) in 10 mL of DMF, and then a solution of dicyclohexylcarbodiimide (DCCI) in THF (10 mL) was added dropwise into the reaction system. The solution was stirred at room temperature for 72 h under a nitrogen atmosphere. The product was collected by filtration with an 800 nm-pore PA filter and washed with DMF, THF, and methanol each for four times, then dried at room temperature under vacuum for 48 h. Master Fabrication. A suitable amount of BP-AZ-CA was dissolved in DMF to obtain a solution with a concentration about 0.1 g/mL. The solution was filtered through a 0.45 µm membrane. The films were prepared by spin-coating the solution onto clean glass slides. By adjusting the spinning rate, the thickness of the films was controlled to be about 1.0 µm. After being dried in a 70 °C vacuum oven for 48 h, the films were stored in a desiccator for further applications. The experimental setup for SRG fabrication was similar to those reported before.1,2,5 A linearly polarized Ar+ laser beam (488 nm, 150 mW/cm2) was used as the light source. SRGs were optically inscribed on the polymer films with p-polarized interfering laser beams, which were obtained by splitting the Ar+ laser beam with a mirror and letting the reflected half beam to coincide with the other half on the film surface. The diffraction efficiency of the gratings, which was probed by measuring the first-order diffracted beam intensity of an unpolarized low power He-Ne laser beam (633 nm) in transmission mode, was used to monitor the modulation depth in the process. Preparation of Stamps. The elastomeric stamps were prepared by replica molding. The PDMS prepolymer was prepared by mixing the elastomer base and curing agent (Sylgard 184, Dow Corning) in a proper ratio (10:1, wt/wt). Cylindrical molds were prepared by fixing plastic tubes (with a diameter about 1.3 cm) on the azo polymer films that had been inscribed with SRGs as mentioned above. The prepolymer was poured into the molds and cured in a 60 °C oven for 4 h. The PDMS stamps were then separated from the molds and used for contact printing. Contact Printing. P3HT was dissolved in toluene to obtain a solution with the concentration of 13 mg/mL. BP-AZ-CA was dissolved in THF to obtain a solution with a proper concentration (10 mg/mL for a typical condition). Soluble MWNTs were dissolved in methylene dichloride with the concentration of 8 mg/mL. These solutions were used as “inks” for contact printing. The PDMS stamps were dipped into the solutions (“inks”) for about 1 s and carefully blotted with a piece of filter paper. Then, the stamps were pressed on the substrates in a conformal contact manner with a moderate pressure (such as 50 kPa) for a few seconds. The achieved patterns on the substrates were dried in a 25 °C vacuum oven for 12 h.

Results and Discussion The soft lithographic approach for duplicating the photoinduced surface patterns of azo polymer films is illustrated in Figure 1. The PDMS stamp is prepared through replica molding (step a). The elastomeric stamp is “inked” by a proper solution of the functional material (step b). In this step, when the featured surface of the PDMS stamp touches the liquid surface, the “ink” solution is immediately filled in the grooves of the stamp and also adsorbed on the stamp surface. (In order to see clearly, the vertical dimension of the adsorbed layer is un-proportionally enlarged.) As the solvent of the “inked” liquid thin layer vaporizes very quickly, a highly concentrated and viscous liquid film is left on the stamp surface (step c). Then, the PDMS stamp is conformally printed onto the substrate for a few seconds, and a thin surfacepatterned layer of the materials is formed on the substrate after the solvent evaporation (step d). In the process, the volatile solvent (35) Feng, W.; Zhou, F.; Wang, X.; Bai, X. D.; Liang, J.; Yoshino, K. Jpn. J. Appl. Phys. 2003, 42, 5726-5730.

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Figure 1. Schematic representation of the soft lithographic procedure: (a) PDMS stamp is fabricated through replica molding. (b) PDMS stamp is “inked” with a solution of the functional materials. (c) A highly concentrated and viscous “ink” layer is left on the stamp after the solvent evaporation. (d) The elastomeric stamp is printed on the substrate for a few seconds. (e) After peeling off PDMS stamp, patterned layer of functional material is formed on the substrate.

Figure 2. Photographic images of cylindrical PDMS stamps with cast surface-relief-gratings.

plays an important role for the pattern formation. The processing details and factors controlling the pattern formation will be given and discussed in the following parts. Replica Molding Against SRGs. The replica molding masters were prepared by producing surface-relief-gratings (SRGs) on azo polymer films through the photofabrication method. The BP-AZ-CA thin films with smooth surfaces were obtained by spin-coating. SRGs were optically inscribed on the polymer films through irradiation with p-polarized interfering Ar+ laser beams (488 nm, 150 mW/cm2). The grating depths were controlled by the irradiation time. The profile spacing S of the gratings (grating period) was adjusted according to the following equation

S ) λ/(2 sin θ)

(1)

where λ is the wavelength of the writing beam and θ is the angle between the beam propagation axis and the mirror plane. The experimental details and theories for SRG fabrication can be seen in the references.3-7 The transparent stamps with replicated SRGs on the surfaces were obtained by casting PDMS prepolymer against the masters. The liquid prepolymer of PDMS was prepared by mixing the elastomer base and curing agent (Sylgard 184, Dow Corning) in a proper ratio (10:1, wt/wt). The cast molds containing the prepolymer were cured in a 60 °C oven for 4 h. Figures 2 and 3 show the photographic images of the stamps and AFM images of the masters and stamps. Brilliant and iridescent colors, resulting from the diffraction of the surface gratings, can be observed

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Figure 4. Some typical AFM images of patterned structures on the substrates prepared by the contact printing; (a) P3HT pattern on silicon wafer, (b) P3HT pattern on ITO glass, (c) BP-AZ-CA pattern on glass slide. (d) MWNT pattern on glass slide.

Figure 3. AFM images of the azo polymer masters and the PDMS replicas, (a) master (period ) 1860 nm, depth ) 140 nm, (b) replica (period ) 1850 nm, depth ) 120 nm); (c) master (period ) 790 nm, depth ) 230 nm), (d) replica (period ) 800 nm, depth ) 170 nm; (e) master inscribed with two sets of gratings orthogonal to each other (modulation depth ) 210 nm), (f) replica of the master given in (e) having a modulation depth of 150 nm.

from the photographic images. For a typical master (Figure 3a), the grating period and depth of the master were measured to be 1860 and 140 nm. For the PDMS replica (Figure 3b), these two parameters were 1850 and 120 nm. The replica, which possesses almost the same period but somewhat shallower trenches than those of the master, replicates the topologically inversed structures of the master with the high fidelity. This method is feasible to replicate the SRGs with different characteristic sizes. Morphology investigation of the master and replica revealed that perfect surface features could be molded down to hundreds of nanometers. Figure 3c shows a typical AFM image of the SRGs on the azo polymer film with the grating period and depth of 790 and 230 nm. The AFM image of the PDMS replica is given in Figure 3d, which shows the grating period and depth of 800 and 170 nm, respectively. More complicated surface structures can be replicated by the same method. Figure 3e shows the AFM image of the structure produced by recording two sets of the gratings orthogonal to each other on the same location of the azo polymer film. This type of structure can also be replicated by the PDMS stamp with the high fidelity (Figure 3f). The modulation depths estimated from the AFM measurements are 210 nm for the SRGs and 150 nm for the replica. In all of the cases, the periods of the relief features of the replicas are almost the same as those of the masters. When the SRGs have small grating periods and deep trenches, the depths of the surface modulation on the replicas are obviously smaller

than those of the masters. It has been pointed out that the surface energy of PDMS might not be low enough for duplicating tiny features with a lower submicron size or a large ratio of depth to width.36 To test whether the replicating could cause damages to the masters, the surface morphologies of the masters were investigated before and after the replication. Both the optical microscope and AFM observations confirmed that there were no obvious damages to the masters even after 30 times of replica molding. In addition to the feasibility for the pattern design, high resolution and preparation easiness, this durable property is another advantage of the SRG-inscribed azo polymer masters for replica molding applications. Patterning through Contact Printing. The PDMS stamps were used to rapidly produce the surface patterns of P3HT, MWNT, and BP-AZ-CA on various substrates through contact printing. In the processes, the materials were dissolved or dispersed in proper media and the solutions or suspension were used as “inks”. The stamps were dipped smoothly on the liquid surfaces of the “inks”, which result in a thin layer of liquid adsorbed on the patterned surfaces. The superfluous liquid was carefully removed by absorbing with a piece of filter paper. Although the “ink” concentration was low, the solvent of the “inked” thin liquid layer vaporized very quickly and a highly concentrated and viscous liquid film formed on the SRGs. The stamps were pressed on the substrates before the solvents had been completely volatilized. The stamps were kept in conformal touch with substrates for a proper period of time (see the discussions in the following section) and then peeled off. In the printing step, a suitable pressure (such as 50 kPa) was required. After the patterns formed, the samples were dried in a 25 °C vacuum oven for 12 h before further measurements. Figure 4 shows typical AFM images of the patterns produced by this method. It can be seen that relief structures of those materials can be successfully printed onto the surfaces. Figure 4a shows the patterned structure of P3HT on a silicon wafer, prepared by using the PDMS stamp with surface structure shown (36) Rolland, J. P.; Hagberg, E. C.; Denison, G. M.; Carter, K. R.; De Simone, J. M. Angew. Chem., Int. Ed. 2004, 43, 5796-5799.

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Figure 6. Relationship between ink concentration and the grating depth; the contacting time and pressure in the printing process were 3 min and 50 kPa.

Figure 5. Some typical SEM images of patterned structures on silicon wafers prepared by contact printing: (a) P3HT, (b) BPAZ-CA, (c) MWNT, and (d) an enlarged image of (c).

in Figure 3b. Figure 4b shows the patterned structure of P3HT on ITO glass, obtained by using the PDMS stamp with surface structure shown in Figure 3f. Both were prepared by using a toluene solution of P3HT as the “ink”. The other two patterns in Figure 4c,d were prepared by using the PDMS replica with relief structure shown in Figure 3d. The BP-AZ-CA pattern on glass slide is shown in Figure 4c and the pattern of the soluble MWNT on glass slide is shown in Figure 4d, where THF and methylene dichloride were used as the solvent or dispersing medium for the “inks”, respectively. Figure 5 shows the SEM images of the patterns on silicon wafers. All of the patterns in Figure 5 were prepared by using the PDMS stamp with relief structure shown in Figure 3d. The patterns in Figure 5a,b were made of P3HT and BP-AZ-CA, respectively. The pattern in Figure 5c was made of the soluble MWNT, and Figure 5d shows its enlarged image. The results show that for these materials the required patterns can be formed through the contact printing process on the substrates such as silicon wafer, glass slide, and ITO glass. For those substrates, the printed patterns show almost the same features and quality, where the effect of substrates can almost be ignored. During the printing process, the relief structures on the PDMS stamps are transferred negatively (having convex and concave structures reversed). In Figure 4a, the grating period and depth are 1840 and 120 nm. The modulation depth in Figure 4b is 145 nm. The grating period and depth are 780 and 170 nm in Figure 4c, and 780 and 165 nm in Figure 4d. In the four SEM images (Figure 5a-d), the grating periods are 785, 775, 785, and 780 nm, respectively. The printed patterns show the same grating periods as the masters and the same relief depth as the stamps (replicas of the masters). From above results and optical microscope observations, it can be further concluded that the transferred structures can be treated as a patterned thin layer of the materials on the substrates. Factors Influencing the Pattern Formation and Quality. The quality of the transferred pattern depends on the properties of “inks”. Solutions of P3HT in toluene and BP-AZ-CA in THF can produce good quality patterns through the contact printing (see Figures 4a-c and 5a,b), as indicated by the clear edges of the patterns and the smooth surfaces. The good quality is attributed to the good solubility of the polymers in the solvents and film-

forming ability of the solutions. The patterns formed by MWNT show rough edges (Figures 4d and 5c,d) because of its poor film-forming ability compared to polymers. The “isolated” carbon nanotubes can be seen in the enlarged SEM image (Figure 5d). Besides those factors, the wettability of the solvents to PDMS is also important, by which the “inks” can be adsorbed on the stamp surfaces and further transferred to the substrates. A proper volatility of the solvents is also necessary, which allows the “inks” to dry quickly and leaves the polymers on substrates after printing. On the basis of these requirements, toluene, THF, chloroform, and methylene dichloride are suitable candidates for contact printing of P3HT, BP-AZ-CA, and MWNT. For the proper “inks”, the pattern formation and quality also depend on the factors such as the solution concentration, contacting time in the printing process, and printing pressure. The results, obtained from contact printing on silicon wafer with the THF solution of BP-AZ-CA as “ink”, are given and discussed as follows. In the process, the PDMS stamp with the grating period and depth of 790 and 155 nm was used. Figure 6 shows the relationship between the grating depth obtained from the AFM measurement and the “ink” (BP-AZ-CA in THF) concentration. It can be seen that, when the concentration is lower than 5 mg/mL, the grating depth increases with the concentration. However, for the concentration in range from 5 to 50 mg/mL, the grating depth no longer changes with the concentration. Figure 7a shows the influence of the “ink” concentration on the average film thickness, which measures the vertical distance from the substrate surface to the middle-point of the trench depth. It indicates that the average film thickness increases almost linear with the concentration increase in the concentration range. Comparing with the result given in Figure 6, it can be seen that when the “ink” concentration is higher than about 2.5 mg/mL there is a thin BP-AZ-CA layer lies between the trench bottom and substrate surface. For the concentration of 5 and 30 mg/mL, the “buffer” layer thickness can be estimated to be about 20 and 287 nm. Figure 7b gives the relationship between the maximum absorbance (Amax, λmax ) 430 nm) and the “ink” concentration. The maximum absorbance also increases linearly with the concentration in the range. Figure 7c gives a plot of Amax versus the average film thickness. The linear relationship indicates that the density of the film almost keeps constant when the concentration increases in the range. Figure 8 gives the influences of the contacting time in the printing process (Figure 8a) and printing pressure (Figure 8b) on the grating depths. The result shows that except for the extremely short contacting time (