Direct Fabrication of Micro/Nano-Patterned Surfaces by Vertical-Directional Photofluidization of Azobenzene Materials Jaeho Choi,† Wonhee Cho,† Yeon Sik Jung,‡ Hong Suk Kang,*,§ and Hee-Tak Kim*,†,∥ Departments of †Chemical and Biomolecular Engineering, ‡Materials Science and Engineering, and ∥KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea § Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *
ABSTRACT: Anisotropic movement of azobenzene materials (i.e., azobenzene molecules incorporated in polymer, glass, or supramolecules) has provided significant opportunities for the fabrication of micro/nanoarchitectures. The examples include circular holes, line gaps, ellipsoidal holes, and nanofunnels. However, all of the previous studies have only focused on the lateral directional movement for the structural shaping of azobenzene materials. Herein, we propose structural shaping based on a vertical directional movement of azobenzene materials. To do this, light with oblique incidence, containing normal direction light polarization, was illuminated onto azobenzene materials film contact with patterned elastomeric molds (i.e., PDMS) so that the resulting vertical directional movement of azobenzene materials fills in the cavities of the molds and results in pattern formation. As a result, a range of patterns with sizes of features from micro- to sub-100 nm scale was successfully fabricated in a large area (few cm2), and the structural height was deterministically controlled by simply adjusting irradiation time. In addition to the notable capability of fabricating the single-scale structures, the technique provides a facile way to fabricate complex hierarchical multiscale structures, ensuring its versatility and wide applicability to various applications. As a selected exemplary application of the multiscale structures, a superhydrophobic surface has been successfully demonstrated. KEYWORDS: micro/nano patterning, azobenzene materials, athermal photofluidization, imprint lithography, multiscale structures features.11 Although low-temperature processes using solvent or UV-curable materials are available, a drawback of all these methods is material shrinkage, either by solvent evaporation, cooling, or during curing steps.12 The azobenzene molecule-containing materials (referred to as azo-materials), in which azobenzene molecules are incorporated into host materials, such as polymer, glass, and dendrimer, have attracted considerable attention for their distinct ability to produce micro/nanopatterned surfaces.13−22 The structural shaping ability stems from its peculiar optical behavior, known as athermal directional photofluidization: Under illumination, azo-materials become fluidized at room temperature and directionally flow parallel to the light polarization.23−26 Such polarization-dependent fluidization allows one to deterministically control the mass migration of azo-materials via light polarization manipulation, thereby providing remarkable flexibility in the control of structural
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icro- and nanolithography techniques are in high demand for a wide range of applications in the fields of electronics, optics, surface sciences, and biosciences.1,2 For decades, photolithography has been the mainstay of micro/nanolithography techniques; however, with the ever-increasing demand for smaller features, photolithography has approached its technological and economical limits.3−5 Alternatively, some of the oldest yet conceptually simplest forms of lithography based on imprinting, embossing, or molding have been intensively explored for their potential to serve as the basis for cost-effective micro/nanofabrication techniques. These examples include nanoimprint lithography (NIL),6 capillary force lithography (CFL),7 solvent-assisted soft-stamp-based NIL,8,9 and ultraviolet NIL.10 In these methods, the softening (or fluidizing) of molding materials (e.g., polymer) is crucial in pattern formation, in the way that the softened material fills up the cavities of a patterned mold (e.g., quartz, polydimethylsiloxane (PDMS), or perfluoropolyether) either by capillarity or pressure-assisted relaxation. For softening, applying heat is generally exploited, but the thermal process causes distortion of the imprinted structures or mold © 2017 American Chemical Society
Received: September 2, 2016 Accepted: January 12, 2017 Published: January 12, 2017 1320
DOI: 10.1021/acsnano.6b05934 ACS Nano 2017, 11, 1320−1327
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Figure 1. Structural shaping based on a vertical-directional photofluidization of PDO 3. (a) Schematic illustration of the structural shaping process: slantwise irradiation with circularly polarized light on the stacks of a line-patterned PDMS mold and PDO 3 film. Light polarization in the normal direction (i.e., normal E-component) serves as a driving force to flow the PDO 3 upward along with the sidewall of the contact PDMS mold. (b) Cross-sectional SEM image of vertically migrated PDO 3 after slantwise irradiation with an incidence angle of 15°. (c) The degree of vertical migration (i.e., structure height) with a varying incidence angle. The measured structure heights of PDO 3 (red dots) are well fitted to the black line (i.e., sin 2θ). (d) Influence of the fluence on the structure height. The black line is a Gaussian fit to the measured structure heights (red dots) and corresponds to the profile of the used laser beam. (e) The plot of the structure heights (red dots) with different irradiation times; the corresponding exponential fit is shown as a solid black line.
features such as sizes and shapes.27−32 Examples include the following: inscription of two orthogonal gratings;33,34 twodimensional (2D) directional photofluidic reconfiguration of a micron-scaled, simple line shape into an ellipsoidal hole with nanoscale;35−37 and the round pores in the breath figure array into rectangular, rhombic, and parallelogram-shaped pores38 and spherical colloids into ellipsoidal shaped colloids.39,40 In addition, when the irradiation stops, everything becomes “frozen” again, and hence, the reconfigured structural fidelity of azo-materials is quite stable. However, most of the previous studies have only focused on the lateral directional photofluidization for the structural shaping of azo-materials; structural shaping based on vertical-directional photofluidization has not yet been fully developed. Recently, Probst et al. reported a structural shaping method based on the vertical movement of azo-material.12 However, the structural shaping is driven by the capillarity of its photofluidic motion of azomaterial, which only allows structural height as low as 100 nm, thereby hindering its general applicability to a variety of applications as well as to the architecture of complex structures. In the present work, we propose a structural shaping based on the vertical movement of azo-material driven by directional photofluidization. The structural shaping relies on cavity filling of elastomeric molds by forced vertical migration (i.e., by directional photofluidization) of azo-material; hence, we refer to it as directional photofluidization imprint lithography (DPIL). In DPIL, light with oblique incidence, containing normal direction light polarization, is illuminated onto azomaterial film in contact with elastomeric molds for vertical migration, followed by normal irradiation for a structural
leveling. By the two-step light exposure process, a range of patterns with sizes of features from the micro- to sub-100 nm scale, with structural heights up to a few micron, is successfully fabricated in a large area (ca. cm2 scale). Furthermore, owing to its solvent-free, reaction-free, and heat-free nature, the material shrinkage issue, which is unavoidable with thermal, solvent, and UV-curable assisted molding techniques, can be eliminated. In addition to the notable capability of fabricating single-scale structures, the technique provides a facile way to design complex hierarchical multiscale structures, ensuring its versatility and broad applicability to various applications. As a selected exemplary application of the multiscale structures, the superhydrophobic surface is successfully demonstrated.
RESULTS/DISCUSSIONS Directional photofluidization of azo-materials, a key feature for DPIL, originates from photoinduced molecular motions. When light with an absorption wavelength (450−600 nm, herein 532 nm) is illuminated onto the azo-materials, the photochromic azobenzene molecules grafted to the main chain undergo a repetitive trans−cis−trans photoisomerization reaction until the long axis of the azobenzene molecules is entirely aligned in a direction perpendicular to the incident light polarization.13,14 Such photoinduced molecular motions make the azo-materials behave as a fluid even below the glass transition temperature (Tg) or melting temperature (Tm), and, most notably, the direction of their fluidity strongly depends on the light polarization; the fluidized azo-materials flow in the direction parallel to the light polarization.23−26 The features of directional photofluidization clearly appear in the reconfiguration of azo1321
DOI: 10.1021/acsnano.6b05934 ACS Nano 2017, 11, 1320−1327
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Figure 2. (a) Schematic diagram of DPIL: (i) making conformal contact between PDMS mold and PDO 3 film; (ii) slantwise irradiation for pattern evolution; (iii) normal irradiation for a structural leveling; and (iv) removing PDMS mold. SEM images of resulting line pattern after (b) slantwise irradiation and (c) normal irradiation.
direction polarization component (θ = 0°), did not evoke notable vertical migration, as can be seen in Figure S4a, Supporting Information. The degree of vertical migration should be varied with θ. With increasing θ, the normal light polarization component increases, whereas light intensity per area is reduced. As a result of the trade-off relationship between the normal light polarization component and light intensity per area, the light intensity of the normal polarization component (Inor) depends on θ, as given in eq 1:
material square posts into specific shapes according to the light polarization. Polydisperse orange 3 (PDO 3), an epoxy-based azobenzene molecule containing amorphous polymer, was employed in this work as a proof-of-concept; however, the principle is not restricted to any particular polymer, and similar results were also obtained using noncovalently functionalized azo-material systems from our earlier studies.37 The structural characterization for the PDO 3 is given in Figure S1, Supporting Information. The PDO 3 posts, developed by the soft-imprinting method, were elongated in the direction of the linearly polarized laser electric field and were reconfigured into an isotropic dome by the irradiation of a circularly polarized light (i.e., the linear electric vector of circularly polarized light being continuously rotated along the wavevector results in radial diffusion, Figure S2, Supporting Information). Notably, the distinct rectangular post became a rounded dome under light irradiation, owing to the tendency to minimize surface tension, which is substantial evidence of PDO 3 fluidic behavior by light irradiation at room temperature. Such directional photofluidic behavior elucidates that the migration of PDO 3 originates solely from photodriven molecular motions, not by a photothermal effect (i.e., athermal photofluidization). The filling of the PDO 3 in the cavities of PDMS mold is driven by the directional photofluidic behavior; light polarization in the normal direction (i.e., in z-axis, Figure 1a) serves as a driving force to flow the PDO 3 upward along with the sidewall of the contact PDMS mold. To demonstrate the concept of the vertical migration by the normal light polarization component, a circularly polarized laser beam with a maximum intensity of 4.2 W/cm2 with a beam diameter of 1.1 mm was irradiated on the stacks of a line-patterned PDMS and a 5 μm-thick PDO 3 film for 3 h at various incidence angles, as shown in Figure 1a (see the detailed optical setup described in Figure S3, Supporting Information). The width, spacing, and height of the line array of the PDMS mold (hard-PDMS/softPDMS composite molds are used in this study for replication fidelity) are 0.7 μm, 50 μm, and 3 μm, respectively. Unlike perpendicular irradiation, slantwise irradiation possesses a normal light polarization component (z-axis direction in Figure 1a). Indeed, a slantwise irradiation with an incidence angle (θ) of 15° induced mass-migration along the sidewall of PDMS (Figure 1b), while perpendicular irradiation, having no z-
Inor = Isin 2θ
(1)
where I is light intensity per area. As theoretically predicted, the plot of vertical displacement as a function of θ from 0° to 75° exhibits a maximum displacement at 45°, as can be seen in Figure 1c, supporting the vertical migration of the PDO 3 driven by the normal light polarization component. The essential role of normal light polarization can be further evidenced by slantwise irradiation with s-polarized light, whose the electric vector is always perpendicular to the z-direction irrespective to θ. As shown in Figure S4b, Supporting Information, the slantwise irradiation with s-polarized light hardly induces vertical migration. In addition, we compared the pattern height versus light irradiation time for circularly polarized and s-polarized light irradiation under a relatively low light intensity of 100 mW/cm2 and at an incident angle of 45° (see Figure S5, Supporting Information). As a result, we found that only the circularly polarized light induced the upward movement; the s-polarized light was not capable of evolving the upward movement even with a long irradiation time. It further verifies that the upward movement of PDO 3 is dominated by the directional photofluidization rather than other side effects (e.g., heat expansion). The degree of vertical migration should also be varied with fluence (i.e., the product of light intensity and irradiation time). In the experiment, light with an incident angle of 45° was used, and low fluence (maximum light intensity of 4.2 W/cm2 and 7 min light irradiation time) was chosen to visualize the Gaussian intensity profile of the laser. As can be seen in Figure 1d, the maximum structure heights measured at different positions in the exposed area can be fitted with a Gaussian function of laser profile, which indicates that the filling rate is proportional to the 1322
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Figure 3. SEM images of PDO 3 patterns formed by DPIL: (a) Micron pillar structure; submicron sized (b) pillar and (c) line structure; and (d) sub-100 nm line structure. The inset in (b) provides optical image of the pattern fabricated over a large area (1.2 cm2) with high uniformity. The logo is used with permission from KAIST.
area. The following second irradiation at θ of 0° was designed for a structural leveling. The conversion of PDO 3 from solid to quasi-liquid state by the second irradiation dissipates any stresses exerted on the PDMS, resulting in the elastic recovery of the PDMS mold and structural leveling, as demonstrated in Figure 2c. Owing to the absence of a normal light polarization component in the second irradiation, further vertical drift of the PDO 3 did not occur. By using DPIL, various structures in the micron- to sub-100 nm scale were successfully fabricated in a large area with high fidelity. In addition to the micron-sized line pattern (Figure 2c), a micron-sized pillar pattern was also fabricated (Figure 3a), which indicates that the light irradiation can vertically drift few micron-wide azo-material as high as a few microns (ca. 2 μm), which is not achievable with the previous method utilizing capillarity-based photofluidization imprint lithography.12 In the submicron scale, such structural shaping is possible, as typically shown for pillar and line patterns (Figure 3b,c). Moreover, adjusting the exposure time for the first irradiation allows us to deterministically control the structural height, providing a convenience in structural variation, for example, 5 and 10 min slantwise irradiation followed by 5 min perpendicular irradiation induced sub-μm pillar pattern with structural heights of 200 and 400 nm, respectively (Figure S6, Supporting Information). The pattern formation is readily possible in a large area (1.2 cm2, Figure 3b inset), and the structural uniformity achieved over a large area is evidenced by the wellordered SEM moire images (Figure S7, Supporting Information). In addition to the various micron-sized patterns, DPIL can produce nanosized patterns (i.e., sub-100 nm, Figure 3d), which ensures its superior structural shaping capability for a
applied fluence. It is attributed that the viscosity of athermally photofluidized PDO 3 is inversely proportional to the incident light intensity, and the mass transport of PDO 3 is determined as a balance of traction force caused by light irradiation and the friction force defined by the rheological properties (i.e., viscosity).41 In addition, the degree of vertical migration is found to show an exponential dependence on exposure time according to eq 2: ⎡ ⎛ t ⎞⎤ h(t ) = h0⎢1 − exp⎜ − ⎟⎥ ⎝ τ ⎠⎦ ⎣
(2)
where t is the exposure time, h0 the maximum structure height, and τ the build-up time constant (Figure 1e). The value for τ is around 33.3 min irrespective of the incidence angle. Exploiting the light-driven upward drift of the azo-material, a structural shaping process (i.e., DPIL) was developed. The overall process features two successive irradiations at different incident angle, as illustrated in Figure 2a. For the pattern formation, a PDMS mold with feature sizes ranging from a few microns to sub-100 nm is placed on PDO 3 film coated on a substrate. A circularly polarized light is then irradiated on the stack of the PDMS mold and a PDO 3 film at θ of 45°, which induces the vertical migration of the PDO 3 into the mold cavities. As confirmed from the SEM image of the PDO 3 pattern after the first irradiation (Figure 2b), a micron-sized line pattern evolves during the slantwise irradiation. However, the bottom of the channel was slightly tilted, as seen in Figure 2b. This tilting can be attributed to the driving force for PDO 3 to move perpendicularly to the irradiation direction and the resulting elastic deformation of the PDMS mold in the land 1323
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Figure 4. Multiscale hierarchical structures formed by DPIL. (a) Schematic illustration of the fabrication process: The two-step light exposure process is performed on the stack of the PDMS mold and prestructured PDO 3 film. Cross-sectional SEM images of (b) a prestructured box pattern (40 μm width, 80 μm spacing, and 40 μm height) and (c) dual-scale structure with a submicron pillar pattern (500 nm diameter, 500 nm spacing, and 600 nm height) on top. (d) The triple-scale structure with a sub-100 nm line pattern on top and a micron-pillar pattern (2 μm diameter, 2 μm spacing, and 1.6 μm height) on the second level. Superhydrophobic property of (e) the single-scale and (f) dual-scale structure; static contact angle before and after mechanical vibration and advancing and receding contact angles are measured. (g) Demonstration of the self-cleaning effect at a slightly tilted angle (
