Light-Induced Surface Patterning of Silica - ACS Nano (ACS

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Light-Induced Surface Patterning of Silica Hong Suk Kang, Seungwoo Lee, Jaeho Choi, Hongkyung Lee, Jung-Ki Park, and Hee-Tak Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b03946 • Publication Date (Web): 21 Sep 2015 Downloaded from http://pubs.acs.org on September 24, 2015

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Light-Induced Surface Patterning of Silica Hong Suk Kang,† Seungwoo Lee,‡ Jaeho Choi,∥Hongkyung Lee,∥ Jung-Ki Park, ∥,†* and HeeTak Kim∥*

† Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701 (Korea) ‡ SKKU Advanced Institute of Nanotechnology (SAINT) & School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, 440-746 (Korea) ‖ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701 (Korea)

Corresponding Authors *E-mail (Prof. Jung-Ki Park): [email protected] *E-mail (Prof. Hee-Tak Kim): [email protected]

KEYWORDS: Azobenzene materials, Silica precursor, Photo-fluidization, Pyrolytic conversion, Micro/nano silica patterning

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ABSTRACT Manipulating the size and shape of silica precursor patterns using simple far-field light irradiation and transforming such reconfigured structures into inorganic silica patterns by pyrolytic conversion are demonstrated. The key concept of our work is the use of an azobenzene incorporated silica precursor (herein, we refer to this material as azo-silane composite) as ink in a micro-molding process. The moving direction of azo-silane composite is parallel to light polarization direction; in addition, the amount of azo-silane composite movement can be precisely determined by controlling light irradiation time. By exploiting this peculiar phenomenon, azosilane composite patterns produced using the micro-molding technique are arbitrarily manipulated to obtain various structural features including high-resolution size or sophisticated shape. The photo-reconfigured patterns formed with azo-silane composites are then converted into pure silica patterns through pyrolytic conversion. The pyrolytic converted silica patterns are uniformly formed over large area, ensuring crack-free formation and providing high structural fidelity. Therefore, this optical manipulation technique, in conjunction with the pyrolytic conversion process, opens a promising route to the design of silica patterns with finely tuned structural features in terms of size and shape. This platform for designing silica structures has significant value in various nanotechnology fields including micro/nanofluidic channel for lab-on-a-chip devices, transparent superhydrophobic surfaces, and optoelectronic devices.


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Tailor-made micro/nano silica patterns are very attractive in a number of miniaturized system fields

such

as

electronics/photonics,1-5

chemical/biotechnology,6

surface

science,7-11

microelectromechanical systems (MEMS),12 and micro/nano fluidics.13 This is mainly because such silica patterns possess inherent rigid properties4-5,7-13 (e.g., high mechanical strength, thermal resistance, and chemical endurance) and possible functional properties1-3,6,10-15 (e.g., high transmittance ranging from visible to NIR, low electrical conductance, biochemical compatibility, low autofluorescence, and potential to form photonic crystals). For example, periodically roughened silica patterns have been used as highly robust superhydrophobic and anti-reflective surfaces; these surfaces were found to have good durability/longevity performance under prolonged exposure to chemical or mechanical stress.7-11 Also, well-ordered silica line patterns have been used as micro/nano fluidic channels in DNA analysis lab-on-a-chip devices, taking advantage of inherent silica properties including low autofluorescence, high stability, and compatibility with numerous bio/chemical materials.6,13 In addition, silica patterned arrays have been employed as templates for high-temperature plasma-enhanced chemical vapor deposition (CVD), taking advantage of the thermal resistance property of silica. This process enabled the realization of periodically or randomly patterned poly-silicon thin film solar cells; these cells exhibit total internal reflectance inside the absorber layer and showed an increase of the light path length, thereby increasing the absorption of sunlight.4-5 Conventionally, the photolithographic process constitutes a major approach toward the fabrication of tailor-made silica patterns.6,10,12 However, such patterns require relatively complex steps (e.g., clean-rooms and etching processes with long lag times) and are difficult to fabricate into nano-sized architectures due to optical diffraction limits. Alternatively, micro-molding techniques such as soft-lithography (SL) and nanoimprinting lithography (NIL), which utilize


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pryolytic conversion of silica precursors (i.e., carbosilane molecules) to pure silica structures, have been widely exploited to create silica patterns.2,5,7,9,13,16-19 Such micro-molding techniques are particularly interesting because they enable the rapid fabrication of well-ordered silica patterns without need for clean rooms or etching processes. However, current micro-molding methods, while promising, have several critical shortcomings. First, the dimensions of the skeletal structures are heavily dependent on the size and shape of the master template; therefore, a new master template must be fabricated for every single tuning of the pattern size and shape. This lack of controllability of the pattern size and shape impedes the design of optimal structures for practical device applications. Second, it is still difficult to design high-resolution structural features (on scales of a few nanometers) because master patterns for micro-molding templates are generally fabricated by conventional photolithography, using which process it is hard to create nano-scale patterns due to resolution limits. For these reasons, to structure silica materials, there is great need in this field for the development of a novel approach that can provide highly-adjustable structural features in terms of size and shape. In the present work, we propose a new platform for designing silica patterned arrays by simple far-field light irradiation and subsequent pyrolytic conversion process. This new process enables the control of geometries different from those of the original master patterns, allowing the direct generation of arbitrarily-shaped silica structural features on various scales ranging from a few microns to tens of nanometers. The key to the success of our strategy is the use of photoreconfigurable azo-silane composite, in which the azobenzene moiety is linked to the silica precursor as a micro-molding material. 20-21 Under polarized light irradiation, azo-silane composite shows anisotropic fluidic movement in parallel to the polarization direction of incident light; the degree of movement can be precisely controlled by varying the light irradiation time. Making use


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of these peculiar phenomenon, the size and shape of the azo-silane composite pattern produced by micro-molding techniques can be precisely and deterministically tailored into desired forms using simple far-field light irradition.22-27 The photo-reconfigured patterns formed using azo-silane composite are then converted into pure silica patterns through calcination in O2 atmosphere. The azo-silane composite patterns are uniformly converted into silica phase over large area, ensuring that there is crack-free formation and offering high structural fidelity (in this paper, the structural fidelity is used to indicate how much of the shape of the original silica precursor remains after pyrolytic conversion). Herein, in order to prove our concept, we started with micron-sized line, hole, and pillar shaped azo-silane composite patterns and transformed them into various shaped and sized silica patterns where geometries are different from those of the original master pattern (i.e., line, hole, and pillar silica patterns ranging from the micron-scale to below 30 nm; ellipsoidal pillar- and hole-shaped pattern). The conversion chemistry was characterized by energy-dispersive X-ray spectroscopy (EDX) and fourier-transform infrared spectroscopy (FT-IR); the obtained silica structures were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM), and optical image. In the end, the robustness (i.e., the thermal and mechanical stability) of the obtained silica patterns was evaluated. The resulting silica patterns were capable of withstanding high temperatures (up to 1000 °C) in air and providing good mechanical strength (Young’s modulus of 33.7 GPa and hardness of 2.63 GPa).

RESULTS AND DISCUSSION As a proof of the principle and versatility of our method, we designed and performed the experiments as shown in Scheme 1. The process can be categorized into three main steps: i)


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preparation of a pristine patterned array of azo-silane composite materials (line-, hole-, and pillarshaped arrays) by solvent-assisted micro-molding process; ii) light-induced structural manipulation of the pristine patterned azo-silane composite arrays: the pristine features are reconfigured simply by controlling the polarization direction and exposure time of light; and iii) pyrolytic conversion of the photo-reconfigured azo-silane pattern into a robust silica pattern. The azo-silane composites used as photo-reconfigurable molding ink were synthesized by connecting azobenzene molecules (2-(4-dimethylaminophenylazo)benzoic acid in Na salt form) and carbosilane molecules ((3-aminopropyl)triethoxysilane (APTES)) through ionic bonding as described in Scheme 2 and in the Experimental Section. The ionic bonding approach

20-21

offers

two advantages over the previous covalent bonding approaches:21 i) room temperature synthesis without any side reactions and ii) possible to precisely control the azobenzene content in the azosilane composite owing to a nearly complete conversion (96.7 % conversion). In the methanol solution of the azobenzene and carbosilane, a stoichiometric amount of HCl was added to protonate the amino groups of APTES, thereby creating an ionic bond between the positively charged amino group and the negatively charged carboxylate group and releasing NaCl.20-21 Care should be taken not to exceed the HCl stoichiometry, because excess HCl converts azobenzene to a nonphotoreactive form.20 Next, azo-silane composite pre-patterned arrays of line, hole, and pillar were prepared from the azo-silane composite in N-methyl-2-pyrrolidone (NMP) by using a solvent-assisted micromolding process. The detail solvent-assisted micro-molding process is described in Experimental Section. With mild heat treatment (45 °C for 3h) during the molding processes, the mold ink was solidified via NMP evaporation and the resulting azo-silane frame was further consolidated by the hydrolysis of the APTES moieties and subsequent condensation reaction (Si-OH + Si-OH → Si-


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O-Si). These reactions led to a cross-linking of the carbosilane matrix, as illustrated in Figure S1; this cross-linking was confirmed by the appearance of Si-O-Si peak and the increase of this peak with heat treatment time in the FT-IR spectra (Figure S2). The process of cross-linking enhances the mechanical integrity of the azo-silane composite but reduces the moving speed upon lightirradiation due to an increase of the molecular weight.29 In consideration of these trade-off properties, a heat treatment condition (45 °C for 3 h) that allows apparent solidification and practically acceptable moving speed (about 60 nm/min) under light irradiation (intensity of 20 mW/cm2 and wavelength of 532 nm) was selected. Scanning electron microscope (SEM) images of the resulting pre-patterned arrays of azo-silane composites are shown in Figure 1. The spacing, line width, and height of the line arrays (Figure 1a) are 4.1 μm, 2.5 μm, and 2.8 μm, respectively; the spacing, diameter, and depth of the hole arrays (Figure 1b) are 1.3 μm, 2.7 μm, and 2 μm, respectively; and spacing, diameter, and height of pillar arrays (Figure 1c) are 1.3 μm, 2.7 μm, and 2 μm, respectively. Structural uniformity achieved over a large area was evidenced by the wellordered SEM moiré images (see Figure S3). When a polarization-controlled beam with appropriate wavelength ranging from 400 nm to 550 nm (Figure S4) is exposed onto the azo-silane composite surface, the azobenzene moieties undergo trans-cis-trans photoisomerization until the axis is oriented perpendicular to the polarization direction.23,30 This peculiar photo-reconfiguration of azobenzene moieties allows azobenzene incorporated materials to fluidize even further below melting temperature (Tm) by light irradiation. More importantly, the direction of the light-induced fluidization of azo-silane composites is parallel to the irradiation polarization direction,22-27 contrary to the direction in thermal induced isotropic fluidization;28 and the degree of azo-silane composites movement is precisely controlled by varying the light irradiation time. As a consequence, the shape and size


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of azo-silane composite pattern can be easily controlled by varying the light polarization and light irradiation time, respectively. Many previous reports have suggested possible mechanisms for this anisotropic movement of azobenzene incorporated materials (including azobenzene incorporated polymers,31-41 dendrimers,42 and molecular glasses 43); however, none of the mechanisms proposed so far can fully explain all phenomena responsible for the movement of azobenzene containing materials; thus, understanding of this mechanism is still evolving. As a first example of the light-induced structural manipulation, size-tunings for various patterns were demonstrated. These included control of the gap between two lines (Figure 2a), the hole diameter (Figure 2b), and the pillar diameter (Figure 2c). The employed polarizations were spolarization for the line arrays, and circular-polarization for the hole and pillar arrays. The wavelength and intensity of incident light were 532 nm and 20 mW/cm2, respectively. The detailed experimental setup for the light irradiation process is presented in Experimental Section and the Supporting Information (Figure S5). As shown in Figure 2a, the spacing between two lines was indeed uniformly reduced with increasing irradiation time; the line spacing was reduced from 4.1 μm to 2 μm upon 20 min irradiation. Interestingly, upon longer irradiation (50 min), the spacing was scaled down to sub-30 nm (27 nm); this resolution is comparable to those of direct writing methods such as e-beam lithography. The same idea can be used to control the diameter of the holes and pillars. Under right-handed circularly polarized light irradiation (i.e., the linear electric field vector rotates clockwise) to the hole and pillar pattern arrays, isotropic in-plane movement of the azo-silane composite occurred owing to equal polarizations in all in-plane directions. As shown in Figure 2b, the initial hole diameter of 2.7 μm was decreased to 1.5 μm with 20 min light irradiation; a longer irradiation of 50 min resulted in a pattern of 29 nm diameter holes. In the case of the pillar patterned array (Figure 2c), the right-handed circularly polarized light irradiation


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enlarged the diameter of the pillar from 2.7 to 3.3 μm upon 20 min light irradiation (bottom left panel in Figure 2c) and to 3.7 μm upon 50 min of light irradiation (bottom right panel in Figure 2c). In our experiment, we found that the limited structural feature which we allow to successfully handle in uniform and scalable manner is around 20 nm. Such scaling limit is caused from fluidic motion of azo-silane composite under light irradiation: in sub-20 nm regime, the spatially separated two fluidic surfaces are readily merged due to Van Der Waals interaction, with reducing surface energy. The rate of movement of azo-silane composite was found to be gradually increased with the increase of azobenzene molecular contents. This arises because the number of azobenzene molecules that can potentially participate in isomerization and the resultant photofluidization is gradually increased during light irradiation. Theses aspects of movement depending on contents of azobenzene molecules are presented in Figure S6a-b. We also found that the light intensity significantly influences azo-silane composite movement as shown in Figure S6c. In the high light intensity regime (100 mW/cm2), the moving speed of azo-silane composite increases because a large number of azobenzene molecules are participated in isomerization cycle. However, the maximum distance of azo-silane composite movement decreases because the temperature is raised due to the high light intensity, thereby accelerating cross-linking of the silane component. This cross-linking of host silane matrix eventually causes an immobilization of azo-silane composite molecules, thereby decreasing maximum moving distance of azo-silane composite. Conversely, in the low light intensity regime (10 mW/cm2), the moving speed of azo-silane composite pattern decreases, whereas the maximum moving distance of the pattern increases. After the light-induced structural manipulation, we transformed the photo-reconfigured azosilane patterns to silica patterns using a pyrolytic conversion process. The transformation of the


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chemical structure in the pyrolytic conversion process is presented in Figure 3a. Prior to the calcination process, the azo-silane composites were slowly heated to 200 °C and held for 1 h to increase the cross-linking degree of the carbosilane matrix via hydrolysis and self-condensation reaction (Figure S1); this was done as a stabilization step to avoid pattern collapse or crack formation during the subsequent pyrolytic conversion process.19 After the structural stabilization, the furnace temperature was gradually increased to 500 °C and kept at that temperature for 1 h in order to fully remove the organic moieties from the azo-silane composites. This elimination of the organic moieties should lead to large volume shrinkage, with a loss of the original in-plane geometry.9,44-45 For the pristine line-patterned array of the azo-silane composite, significant volume shrinkage was found both in the lateral and vertical directions (see Figure S7) at different rates; this random shrinkage can also be found in the previous examples.5, 44-45 Interestingly, in the case of the photo-reconfigured pattern, the calcination step resulted in volume shrinkage only in the vertical direction, as shown in Figures 3b and 3c. This peculiar behavior derives from the photo-fluidic effects of the azo-silane composite pattern: under light exposure, the pattern becomes softened and even fluidic, like ice melting, and consequently turns into a round arch-shaped pattern, as shown in Figure 3b. Owing to adhesion between the photo-reconfigured structure and the substrate, volume shrinkage takes place exclusively in the vertical direction, thus preserving the in-plane pattern dimensions even after the calcination (see Figure 3c).45 The transformation of the surface profiles after pyrolytic conversion was further evaluated using atomic force microscopy (AFM), with results displayed in Figures 3d-f. The height of the structural features was reduced from 560 nm to 220 nm (see Figure 3f); meanwhile, the pattern width did not change much, clearly demonstrating the preservation of the in-plane structure.


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Figures 4a-c provide schematic illustrations and SEM images of the line- (Figure 4a), hole(Figure 4b), and pillar-shaped (Figure 4c) silica patterns obtained after the pyrolytic conversion process. As can be seen in these images, the transformation to pure inorganic phase is apparently free from any structural deformations or defects. It should be stressed that the sizes of the patterns (the gaps between two lines (Figure 4a), the hole diameter (Figure 4b), and the pillar diameter (Figure 4c)) were nearly unchanged even after pyrolytic conversion from the structures before the pyrolytic conversion process (Figure 3). More surprisingly, structural features miniaturized to 30 nm (see the inset in Figure 4a and Figure 4b) were retained after pyrolytic conversion without losing any structural regularity. This high-resolution of silica patterns on a few nano-scale is very difficult to achieve using previous lithographic techniques.46-48 As shown in Figure 4c, the photoreconfigured pillars were also converted to silica pillars, preserving their diameters. Figures 4d-f display comparisons of the pattern dimensions before and after pyrolytic conversion for the line patterns (Figure 4d), hole patterns (Figure 4e), and pillar patterns (Figure 4f) prepared with varying irradiation times. As expected, the adjustment of the light irradiation time affords a deterministic control of the azo-silane composite pattern, and the manipulated patterns can be converted into silica patterns while maintaining their in-plane dimensions. We believe that the error bars indicate inherent variations in the light irradiation experiments such as vibration, temperature fluctuation, and noise in the photons.25 The slower dimensional changes with irradiation time indicate that the moving speed of the azo-silane composite was gradually reduced as the irradiation time increased. This reduction probably originated from the decreased number of azobenzene molecules that can possibly participate in isomerization.25 However, the deceleration with longer irradiation can be highly profitable in that the structure in the nano-scale region can be more precisely manipulated.


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In nanolithography, the ability to tune pattern shapes is of practical significance for numerous applications. In addition to size tuning, the other interesting feature of light-induced structural manipulation is its capacity for shape tuning: a silica pattern whose shape is different from that of a molding stamp is also achievable. The key to the success of the shape tuning process is the anisotropic reconfiguration of the azo-silane composite via control of the light-polarization direction with respect to the geometry of the azo-silane patterns.23-27 To verify the process of shape tuning, we irradiated the hole- and pillar-shaped patterns with light of different polarization directions and transformed the photo-reconfigured patterns into corresponding silica patterns, as shown in Figure 5. The irradiation of linearly polarized light (s-polarized light) on the circular hole patterned array of the azo-silane composite (Figure 1b) resulted in a transformation to an ellipsoidal-pattern (see Figure S8a); the subsequent pyrolytic conversion process generated an ellipsoidal silica pattern, dictating the pattern shape (Figures 5a). The orientation of the ellipsoidal pattern was tuned by changing the polarization direction (Figures 5b-c), also indicating that these structural transformations originate from photo-alignment of azobenzene materials rather than from photo-thermal softening, which would result in isotropic shape-change. The circular pillar array of the azo-silane composite can be transformed into an ellipsoidal silica pillar array through photo-reconfiguration with linearly polarized light (Figure S8b) and subsequent pyrolysis (Figures 5d-f). The pillars were elongated in the polarization direction (Figures 5e and 5f). The light irradiation time for the configurations shown in Figure 5 was 30 min. Taking all of these observations into consideration, this process of directional photo-reconfiguration can control not only the dimensions of silica patterns but also their shapes in a simple and parallel manner. The conversion chemistry from the azo-silane composite to silica was investigated using optical images (color alternation of specimens, Figure 6a-c), FT-IR measurement (Figure 6d-e), and EDX


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analysis (Figure 6f). The optical images in Figures 6a-c were taken after three important steps. After the molding process of the line pattern array, a reddish color, which is the original color of the azo-silane composites, was uniformly formed (Figure 6a). After the light-induced structural manipulation, an iridescent greenish color was observed at the photo-reconfigured area (marked by the orange arrow in Figure 6b). The generation of a shimmering green color was presumably caused by light diffraction from the photo-reconfigured line patterned array (see the insets in Figure 6b). After the pyrolytic conversion process (500 °C for 1 h), the sample was transformed into an iridescent white colored pattern (Figure 6c). The generation of iridescent white color in spite of the transparent nature of the silica stems from light diffraction by the periodic line pattern (see low magnification optical image in Figure S9). This color transition from reddish to iridescent white color is significant evidence of the conversion chemistry of the azo-silane composite to silica. For compositional analysis of the conversion chemistry, we monitored the variation of the FT-IR spectra for the azo-silane composite with increasing temperature from 45 to 500 °C and compared results with those for fused-silica; results are shown in Figures 6d-e. The FT-IR spectra were found to feature 6 characteristic bands: A (2700-3000 cm-1), B (1630-1700 cm-1), C (1350-1450 cm-1), D (1130-1200 cm-1), E (1030-1090 cm-1), and F (930-960 cm-1). The assignments of the observed FT-IR peaks, based on information from previous papers15,49 are summarized in Figure 6e. It was clearly found that the intensity of the organic compound peaks (-CH3 stretch (A), C=O stretch (B), and -CH2 deformation (C)) gradually diminished as the temperature increased and eventually disappeared above 400 °C. In contrast, the inorganic silica peak (Si-O-Si long chain peak (E)) gradually increased with temperature. The spectra taken after heating at 500 °C is nearly identical to that of fused-silica, proving the nearly complete conversion from azo-silane composite to pure silica. The peak at 1630-1700 cm-1 (C=O stretch (B)), which was generated from azobenzene


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molecules, was found to be significantly diminished at 200 °C, indicating the decomposition of the azobenzene molecules. This is consistent with previous observations of the restricted movement of azo-silane composite above 200 °C.20 The intensity of the peaks in the range of 11301200 cm-1 (O–Si–O short chain with terminal bonds of –CH3 or -OH (D) from low degree of condensation reaction of APTES) gradually decreased with temperature, while the intensity of the absorption peak at 1030-1090 cm-1 (O–Si–O long chain vibration (E)) increased. These peak variations indicate an increased degree of cross-linking. In addition, the intensity of the 930-960 cm-1 peak (Si-OH stretch peak (F)), which is generated from intermediate compounds during the condensation reaction of APTES (Figure S1), gradually deceased with temperature and disappeared at around 200 °C. This result suggests that the condensation reaction of APTES accelerated with increasing temperature and eventually terminated below 200 °C. The surface chemical compositions of the as-prepared azo-silane composite and the calcinated silica pattern determined from EDX spectroscopy are shown in Figure 6f. In order to rule out the substrate contribution from Si or O to EDX mapping, we transferred these films to an Al substrate. After pyrolysis, the C and N signals from the organic moieties were substantially reduced to below 2 atomic %. Moreover, the O/Si atomic ratio after pyrolysis (1.998) was close to that of pure silica (2). Therefore, the EDX results support the nearly complete conversion (96.7 % conversion) of the azo-silane composite to silica. To assess the thermal stability of the resulting silica patterns, thermo-gravimetric analysis (TGA) was conducted. As shown in Figure 7a, during a scan to 1000 oC at a constant heating rate of 10 ºC /min in air, the resulting silica pattern (500 °C in 1h) exhibited only 2.8 % weight loss, which can be attributed to the loss of residual carbon and nitrogen (see Figure 6f). The mechanical properties of the azo-silane composite films pyrolyzed at various temperatures were measured


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using the nano-indentation method (Figure 7b). For each sample, testing was carried out at 9 sites and the averaged values were used as the Young’s modulus and hardness. The Young’s modulus and hardness of the as-prepared azo-silane composite were around 1 GPa and 0.14 GPa, respectively. These values were found to monotonically increase up to around 18.2 GPa and 1.14 GPa after annealing at 300 °C; these values finally reached 33.7 GPa and 2.63 GPa, respectively, after the annealing at 500 °C. The resulting mechanical robustness is close to that found in the relevant study related to pyrolytic converted silica (previously reported Young’s modulus and hardness were 31.62 GPa and 2.31 GPa, respectively) 50 and far above that of traditional organic materials (In general, organic materials have Young’s modulus values below 10 GPa and hardness values below 0.2 GPa).51-53 These results clearly indicate that the silica pattern maintains the intrinsic mechanical robustness of silica. The excellent stability of the resulting silica structures will enable their use in various devices that are subjected to harsh environmental conditions demanding tolerance of high temperatures, corrosion, and friction.

CONCLUSIONS We have demonstrated a new approach to precisely manipulate the size and shape of silica patterns through simple far-field light irradiation. Azo-silane composite, in a photo-reconfigurable mold, was successfully synthesized by attaching a photo-active azobenzene moiety to carbosilane via ionic bonding. Owing to anisotropic movement of the azo-silane composite along the polarization direction of incident light, the azo-silane composite patterns can be arbitrarily manipulated according to the light polarization. The photo-reconfigured azo-silane patterns were converted to pure silica patterns through the pyrolytic conversion process. The resulting silica


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patterns showed excellent retention of the in-plane geometry of the azo-silane patterns due to anisotropic volume shrinking during the pyrolytic conversion. We experimentally demonstrated variously scaled (line, hole, and pillar patterns ranging from micro to sub-30 nm) and arbitraryshaped (orientation-controlled ellipsoidal hole and pillar patterns) silica patterns in a simple, parallel, and scalable manner. The resulting silica patterns have excellent thermal stability up to 1000 °C and mechanical robustness (high Young’s modulus and hardness of 33.7 GPa and 2.63 GPa, respectively) comparable to those of previous pyrolytic converted silica. This unprecedented manipulation capability of silica’s structural feature is able to provide significant advantages over previous methods (simple micro-molding techniques) in terms of actual device application. For example, the very narrow silica gap, which is essential for DNA analysis yet difficult to be achieved from previous methods (micro-molding techniques or photolithography), can be easily obtained. In addition, highly efficient light-trapping silica structure for solar cell can be readily designed by our method. Our method is also applicable to the manipulation of 3D silica structures such as 3D photonic crystals. Based on its simplicity and efficacy in size and shape tuning, the proposed

method

represents

important

progress

toward

more

advanced

silica

micro/nanolithographic tools

EXPERIMENTAL SECTION Preparation of azo-silane composite molding ink The azo-silane composite solution was prepared by the method developed by Kulikovska et al. 2021

First, 0. 62 g of 2-(4-dimethylaminophenylazo)benzoic acid in Na salt (2.12 mmol, Sigma-

Aldrich) was dissolved in 4 mL of methanol. After this, (3-aminopropyl)triethoxysilane (APTES,


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0.66 mL, 2.83 mmol, Sigma-Aldrich) was added dropwise to the solution. After the solution was stirred for 1 h under dry condition (humidity ~10 %, at 22 ˚C), 0.18 mL of concentrated HCl (37 %, Aldrich) was added to the solution and mixture was left to settle for 1h. This mixing procedure was performed inside the desiccator containing dried porous silica gel particles (Silica gel blue, Junsei) for high dryness. NaCl precipitated with excess acetone and was removed from the solution by vacuum filtration. The filtered solution was kept in a vacuum chamber in order to remove the methanol. The desiccated azo-silane composite was dissolved in N-methyl-2-pyrrolidone (NMP) solvent to a level of 10 wt% for use as molding ink.

Preparation of polydimethylsiloxane (PDMS) mold We first fabricated line-, hole-, and pillar-shaped silicon patterns by conventional photolithography. After surface treatment with tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane, a liquid prepolymer and an initiator (Sylgard 184, Dow Corning, Midland, MI) were mixed at a ratio of 10:1 (by weight) and subsequently degassed using a vacuum stage. The PDMS solution was then carefully poured onto the obtained master pattern to prevent the generation of bubbles. Finally, the PDMS solution was cured at 80 °C in an oven for 1 h. After being fully cured, the PDMS mold was peeled off from the master pattern.

Solvent-assisted micro-molding process for the preparation of azo-silane composite pattern We carefully dropped the precursor ink (i.e., azo-silane composite is dissolved in NMP solvent by 10 wt%) on the freshly cleaned substrate (i.e., glass wafer). Then, a PDMS mold with an area of


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2-4 cm2 was placed face down on the surface. A pressure of roughly 25 psi (ca. 1.7 atm) was applied to the stack of the PDMS mold and substrate for squeeze-out of the solution from the PDMS/substrate contact areas for minimization of residue after the molding process. Simultaneously, a solution state azo-silane composite spontaneously penetrated the voids of the patterned PDMS mold. When the solvent (i.e., NMP) was completely evaporated (45 °C for 3 h), a replicated azo-silane composite pattern was uniformly generated as presented in Figure 1.

Optical setup The detailed light irradiation setup used for tailoring the pattern structure is presented in Figure S5. The intensity and wavelength of the laser light source (Opto Engine LLC, 3W) were 20 mW/cm2 and 532 nm, respectively. The intensity of the beam was precisely controlled by using a neutral density (ND) filter and the polarization of irradiating beam was controlled by adjusting the director of the wave plate. Using a set of a spatial filter and lens, the 1 mm diameter of the initial Gaussian profile intensity beam was converted into a 2 cm diameter uniform profile intensity plane beam; the final beam size was further controlled using an iris. The light irradiation time was precisely controlled using an electronic shutter.

Heat treatment The photo-reconfigured azo-silane composite pattern was heat treated in an electric resistance furnace (home-built equipment) under O2 atmosphere. The procedure of heat treatment for pyrolytic conversion is as follows: (1) the sample was slowly heated to 200 °C and held for 1 h


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(this process enables the increase of cross-linking degree of the carbosilane matrix via hydrolysis and condensation); (2) the furnace temperature was gradually increased to 500 °C (constant ramp rate of 2 °C/min) and kept at that temperature for 1 h (this process permits the elimination of any remaining organic components, densification of the film, and conversion to inorganic silica phase).

Characterization All of the structures developed in this study were characterized by scanning electron microscopy (SEM, FEI, Sirion) with field emission, atomic force microscopy (AFM, PSIA XEI-100 systems), an FT-IR Microscope (Bruker Optiks, IFS66V/S & HYPERION 3000), thermos-gravimetric analysis (TGA, TG 209 F3, Netzsch), a nano indentation system (MTS, Nano Indenter XP), and a digital camera (Canon, EOS 100D). The AFM measurement was operated in non-contact mode under ambient conditions. The SEM samples were prepared by coating with 10 Å of platinum and samples were examined at 5-10 keV. Both plane and cross-sectioned views of the samples were examined using SEM.

ASSOCIATED CONTENT Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org

Conflict of Interest


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The authors declare no competing financial interest

AUTHOR INFORMATION Corresponding Author *E-mail (Prof. Jung-Ki Park): [email protected] *E-mail (Prof. Hee-Tak Kim): [email protected]

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (code# NRF-2012R1A1A2042558). This work was also supported by KAIST own project (code# N01150446 and code# N01150005).

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Scheme 1. Schematic diagram of the structural manipulation of silica arrays in various patterns (line-, hole-, and pillar-pattern). i) Pattern transfer on azo-silane composite by micro-molding process; ii) light-induced structural manipulation of the azo-silane patterns; iii) pyrolytic conversion of the azo-silane patterns to pure silica patterns. 177x121mm (300 x 300 DPI)


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Scheme 2. Synthetic route for the azo-silane composite. Gold-colored bar indicates azobenzene molecule (ionized 2-(4-dimethylaminophenylazo)benzoic acid). 81x24mm (300 x 300 DPI)


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Figure 1. SEM images of the pristine azo-silane composite patterns. (a) Line arrays; (b) circular hole arrays; (c) circular pillar arrays. The insets in (a-c) provide high magnification SEM images. 178x46mm (300 x 300 DPI)


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Figure 2. Schematic illustration and resulting SEM images of light-induced size manipulation of azo-silane composite patterns. Control of (a) gap width between two lines, (b) hole diameter, and (c) pillar diameter with varying light irradiation time. Insets in (a-b): close-up views. 178x55mm (300 x 300 DPI)


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Figure 3. Pyrolytic conversion of azo-silane composite. (a) Transformation of the chemical structure of azosilane composite in pyrolytic conversion. Tilted-view (ca. 75°) SEM images of the photo-reconfigured line patterned arrays (10 min light irradiation, s-polarization light) (b) before and (c) after the pyrolytic conversion. The red dotted box in (b-c) is the silhouette of the original structure before pyrolytic conversion. AFM images of the photo-reconfigured line pattern (d) before and (e) after the pyrolytic conversion; and (f) the surface profiles. 176x189mm (300 x 300 DPI)


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Figure 4. Schematic illustration and resulting SEM images of pyrolytic conversion of photo-reconfigured azosilane composite pattern. (a) Line-, (b) hole-, and (c) pillar-shaped silica patterns with different light irradiation times (light irradiation time of 20 min (bottom left panel) and 50 min (bottom right panel)). Insets in (a-b): close-up views. The plots of the size variation with different light irradiation times for both before and after pyrolytic conversion: (d) gap between the lines, (e) hole diameter, and (f) pillar diameter. 178x103mm (300 x 300 DPI)


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Figure 5. Schematic illustration and resulting SEM images of shape manipulation of silica patterns. (a) Overall scheme for fabricating ellipsoidal silica hole pattern from circular azo-silane composite hole pattern: i) anisotropic movement of azo-silane composite in a direction parallel to the light polarization, allowing transformation of the round hole pattern to an ellipsoidal hole pattern; ii) this photo-reconfigured structure is transformed into a silica pattern by pyrolytic conversion. (b-c) SEM images of resulting ellipsoidal silica hole patterns. The structural orientation of the ellipsoidal hole pattern is controlled by the polarization direction of light: (b) vertical direction and (c) lateral direction. (d) Overall scheme for fabricating ellipsoidal silica pillar pattern from circular azo-silane composite pillar pattern; the same working principle is applied as above. (e-f) SEM images of resulting ellipsoidal silica pillar patterns. 178x107mm (300 x 300 DPI)


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Figure 6. Evidence of transformation into pure silica pattern analyzed by optical images, FT-IR, and EDX. Optical images taken after (a) molding process (line pattern fabrication); (b) light-induced structural manipulation (tailoring of gap between two lines); (c) pyrolytic conversion (500 °C for 1 h). Insets in (a-c): SEM images of etch figures. Color of the specimens was altered from reddish to iridescent white color. (d) FT-IR spectra of azo-silane composite structure under different heating conditions. (e) Observed IR peaks and their assignment. (f) Elemental analysis of azo-silane composite before and after heat treatment (500 °C for 1 h). 178x188mm (300 x 300 DPI)


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Figure 7. Thermal and mechanical properties of silica structure obtained from the pyrolytic conversion. (a) TGA of the pyrolytic converted silica structure (500 °C for 1 h in oxygen atmosphere). (b) Young’s moduli (upper panel) and hardness (bottom panel) of the spin-coated azo-silane composite films with different pyrolysis temperatures. The modulus and hardness were determined from nano-indentation experiment. 176x68mm (300 x 300 DPI)


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Light-Induced Surface Patterning of Silica - ACS Nano (ACS

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