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Jul 17, 2013 - Fabrication of Three-Dimensional Nanostructured Titania Materials by Prism Holographic Lithography and the Sol−Gel Reaction. Sung-Gyu...
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Fabrication of Three-Dimensional Nanostructured Titania Materials by Prism Holographic Lithography and the Sol−Gel Reaction Sung-Gyu Park,† Tae Yoon Jeon,‡ and Seung-Man Yang*,‡ †

Advanced Functional Thin Films Department, Korea Institute of Materials Science (KIMS), Changwon, Gyeongnam 641-831, Korea National Creative Research Initiative Center for Integrated Optofluidic Systems; Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Korea

‡

ABSTRACT: We present a simple, easy method for fabricating high-quality titania inverted replicas of 3D holographically featured structures. A combination of single-prism holographic lithography and sol−gel chemistry was used to prepare 3D titania inverse structures with flat and completely open surfaces without the use of additional postprocessing steps, such as reactive ion etching, ion-beam milling, and/or polishing steps. A hydrophobic, stable liquid titania precursor facilitated the complete infiltration of the precursor into the hydrophobic 3D SU-8 polymer template, which produced very uniform high-quality titania inverse structures. Although the degree of film shrinkage during the calcination process was large (∼34%), the optical strength of the 3D titania inverse photonic crystals doubled because of the high-refractive-index contrast. Compared to titania inverse opal structures, the filling fraction (∼27%) of titania materials has been doubled. This is the first work to fabricate titania inverse photonic crystals with a high filling fraction by utilizing prism holographic lithography and the sol−gel chemistry reaction of a stable titania precursor. The X-ray diffraction patterns indicated the presence of a crystalline anatase or rutile phase depending on the calcination temperature.

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INTRODUCTION Titanium dioxide or titania (TiO2) is a key semiconductor material used in practical applications in the fields of photovoltaics,1−3 photocatalysis,4−6 hydrogen storage,7,8 and photonic biosensors.9−12 Recently, extensive research efforts have been applied toward improving the infiltration of highrefractive-index (RI) crystalline titania into a 3D porous template.13−18 The major challenge has been the introduction of crystalline morphology into the 3D structures while maintaining the highly ordered structural integrity of the materials. Although dry processes, such as atomic layer deposition (ALD)19 and chemical vapor deposition (CVD),20 provide conformal homogeneous deposition into the interconnected pores of a 3D template, only partial backfilling of the high-RI materials is possible because of pinch-off problems,21 resulting in core−shell morphologies or pattern collapse during the removal of the polymer template. Solution-based methods, such as electrodeposition16,22,23 and sol−gel chemistry reactions,2,3,18,24 are attractive for their simplicity, costeffectiveness, and ease of fabrication. Importantly, these methods allow the complete filling of the precursor solution of high-RI materials into the 3D interconnected porous template, yielding 3D photonic crystals (PC) with complete photonic band gaps (PBG) after the removal of the polymer template.23 Here, we present a simple and easy method for fabricating a high-quality 3D inverse TiO2 PC by prism holographic lithography (PHL) and sol−gel chemistry. A 3D polymer © XXXX American Chemical Society

template with face-centered cubic (fcc) symmetry was defined by PHL, and titania inverse structures were formed via a sol− gel chemistry reaction, followed by a calcination process. Bartl et al. developed a hydrophobic and stable titania precursor for preparing planar TiO2 inverse opal structures.18 More specifically, they mixed the unstable titanium alkoxide with a fluorinated organic solvent and hydrochloric acid to make a highly air- and moisture-stable TiO2 precursor by slowing down the condensation kinetics. The high stability and hydrophobicity of the TiO2 precursor enabled the complete infiltration into the hydrophobic 3D SU-8 polymer template and led to very uniform, high-quality titania inverse structures. After the calcination process, which removed the polymer template and induced the formation of the crystalline-phase titania materials, the reflection peak of the 3D inverse titania structures was found to be blue-shifted, from 1.99 μm for the SU-8 polymer template to 1.44 μm for the inverse titania structures. The blue shift in the PBG was attributed to the large volume shrinkage of the 3D titania inverse structures during the calcination process. Although the film shrank by 34% along the [111] direction, the optical strength of the 3D titania inverse PC increased from 29% for the SU-8 template to 59%. Bragg’s law was used to calculate a titania filling fraction of 27%, assuming that the RI of titania was 2.3. The crystalline phase of Received: March 22, 2013 Revised: July 16, 2013

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Scheme 1. Schematic Illustration of the Titania Sol−Gel Method for Backfilling Titania Sol−Gel into the 3D SU-8 Polymer Templatea

a (a) Three-dimensional SU-8 polymer template. (b) Infiltrated polymer/titania sol−gel composite. (c) Three-dimensional titania inverse structures after calcination.

Figure 1. Scanning electron microscopy (SEM) images of an SU-8 polymer template. (a) Top view of a holographically featured 3D polymer template. Cross-sectional views of (b) 2-layer, (c) 6-layer, (d) 11-layer, and (e) 13-layer polymer templates. (f) Pore size distribution of 13-layer structures along the [111] direction. The x axis corresponds to the pore position shown in e. acid (12 M, Aldrich) while stirring vigorously. After 20 min, the viscosity of the solution was adjusted by adding 2 mL of ethanol and 2 mL of ethylene glycol to the titania precursor. The formation of an overcoat on the 3D template was prevented by affixing a coverslip to the top of the 3D polymer template, and the titania sol−gel precursor was allowed to infiltrate the polymer template via capillary action between the substrate and the coverslip.29 Twenty microliters of precursor solution was applied to the edge of the coverslip. In this way, the precursor solution completely filled the interconnected pores of the 3D template. The infiltrated polymer template/titania composite was then allowed to dry overnight. After drying, the top coverslip was removed. The sample was then calcined in air by slowly increasing the temperature (1 °C/min) to the calcination temperatures (450, 650, and 850 °C) and then maintaining this temperature for 0.5 h. Threedimensional anatase titania PC was obtained without pattern collapse. After the calcination step, the titania inverse structures were rinsed with acetone, IPA, and DI water. Characterization. The morphologies of the 3D nanostructures fabricated by PHL were investigated by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The film thickness and interlayer distance along the [111] direction were measured from the cross-sectional SEM images. The reflection spectra of the photonic crystals were characterized by FT-IR spectrometry (Bruker IFS 66 V/ S) combined with IR microscopy imaging (Hyperion 3000, Nikon). A 20× objective with a numerical aperture of 0.45 was used for measurement. A high-reflectance broadband mirror (R > 0.99) was used as a reference. X-ray diffraction (XRD) was performed on a Rigaku diffractometer operated in reflection mode with Cu Kα radiation (40 kV, 45 mA) and with a diffracted -beam monochromator.

the titania inverse structures was examined by conducting X-ray diffraction studies. The XRD diffraction pattern of the samples calcined at 450 and 650 °C coincides with that of the anatase phase. The XRD pattern of the sample calcined at 850 °C indicates the presence of titania in the rutile phase.

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EXPERIMENTAL SECTION

Prism Holographic Lithography. The SU-8 photoresist (PR) was prepared by mixing the SU-8 resin (150 wt % to solvent) and a cationic photoacid generator (PAG, triarylsulfonium hexafluorophosphate, 1.3 wt % to resin) in solvent (γ-butyrolactone (GBL)). The optical setup and detailed experimental procedures can be found in previous reports.25−28 Briefly, a 1−10-μm-thick PR film was obtained by spin-casting the solution onto a glass substrate, followed by soft baking at 95 °C to evaporate the solvent. Next, a laser beam (HeCd laser, 325 nm, 50 mW, Kimmon, beam diameter 1 mm) was passed through a beam expander and directed perpendicularly to a top-cut fused silica prism with a refractive index of 1.48 at 325 nm. A postexposure bake was performed at low temperatures (55 °C) to relieve the thermal stress that built up during the development process.26 A 3D SU-8 polymer template was fabricated after developing the sample with propylene glycol methyl ether acetate (PGMEA) and rinsing with isopropanol (IPA). Fabrication of Three-Dimensional Titania Photonic Crystals through Sol−Gel Chemistry. The titania precursor solution was prepared according to the procedure described in the literature.18 Two milliliters of titania ethoxide (Aldrich) was added to a mixture of 1.6 mL of trifluoroacetic acid (99%, Aldrich) and 0.4 mL of hydrochloric B

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Figure 2. Scanning electron microscopy (SEM) images of the titania inverse structures. (a) Top view of 3D titania inverse structures. (b) Magnified image of panel a. (c) A 45° tilted view of 6-layer titania inverse structures. (d) Cross-sectional view of 11-layer titania inverse structures. The scanning range was 20−60° (2θ) with a step size of 0.02° and a scan speed of 1°/min.

result of UV laser absorption of PAG in the SU-8 PR. Attenuation of the laser fluence by absorption in the PR limits the maximum number of layers that can be made dimensionally homogeneous by HL. It is clear that as the 3D film thickness increases, the bottom layers become more porous (the red dotted region of Figure 1e) because of the low cross-linking density caused by light absorption in the top region. The attenuation can be reduced by offsetting a smaller PAG concentration with a longer laser exposure time. However, the long exposure time often provides the mechanical instability of the optical components, which leads to nonuniform interference pattern. Normally, an SU-8 film containing 1.0−1.5% PAG exhibits uniform and homogeneous 3D structures. But even in this PAG concentration range the maximum thickness of an SU-8 film is about 10 μm.19,23,25−28 Therefore, in the case of HL, the maximum number of TiO2 layers strongly depends on the number of SU-8 templates, not the TiO2 sol−gel reaction. Because the sol−gel method is a wet chemical process, complete infiltration of TiO2 precursor solution into the 3D polymer template can be achieved regardless of the polymer template thickness. Commonly, titanium alkoxides are directly used as titania precursors to infiltrate the sacrificial opal structures.13−17 However, titanium alkoxides are typically air- and moisturesensitive, making it difficult to control the degree of infiltration in an open environment.18 Various strategies have been employed to overcome this limitation, such as diluting the precursor solution in solvent to lower the viscosity and improve the wetting properties of the sol. However, multiple infiltrations are generally required to prevent pattern collapse.17,31 In contrast to using unstable titanium alkoxide solutions directly as precursors for the infiltration process, a hydrophobic, stable titania precursor was prepared by mixing titanium alkoxide solutions with fluorinated organic and strong inorganic acids. Solubilizing titanium alkoxides in organic/inorganic acid mixtures results in a highly air- and moisture-stable titania precursor by slowing down the condensation kinetics. The high stability and hydrophobicity of the titania precursor enabled the complete infiltration into the hydrophobic 3D SU-8 polymer

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RESULTS AND DISCUSSION The process by which 3D titania inverse structures were fabricated is illustrated in Scheme 1. First, an SU-8 polymer template with fcc symmetry was obtained by PHL. The PHLfeatured SU-8 templates were then infiltrated with the titania sol−gel precursor (Scheme 1b). The interconnected pores of the polymer template were filled with the titania sol−gel precursor through capillary action. The formation of an overcoat on the 3D template was prevented by affixing a coverslip to the top of the 3D polymer template.18,29 The high stability and hydrophobicity of the TiO2 precursor enabled complete infiltration into the hydrophobic 3D SU-8 polymer template and led to the formation of uniform, high-quality titania inverse structures. The infiltrated polymer template/ titania composite was then allowed to dry overnight. After drying, the top coverslip was removed. A calcination process was used to decompose the cross-linked SU-8 materials and convert the amorphous titania into a crystalline titania phase (Scheme 1c). Figure 1 shows the scanning electron microscopy (SEM) images of the holographically defined 3D polymer template. A highly uniform 3D polymer template was formed by PHL, as shown in Figure 1a. The 3D SU-8 structures displayed a network of interconnected pores arranged into an fcc lattice with a lattice constant of 650 nm. The thickness of SU-8 PR determines the number of layers of 3D polymer structures. Figure 1b−d shows cross-sectional views of 2-layer, 6-layer, and 11-layer polymer templates. We demonstrated that hexagonally arranged plasmonic gold dot arrays were fabricated by using 2layer polymer structures.30 Figure 1e shows a cross-sectional view of a 13-layer polymer template. The 3D structures had nonuniform pores generated through the depth of the polymer film. The pore size along the [111] direction (white dotted arrow in Figure 1e) was gradually increased from 290 nm at the top layer to 570 nm at the bottom layer. This was attributed to reduced interference intensity in the bottom region mainly as a C

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Figure 3. (a) Optical reflectance spectra of the 11-layer titania inverse photonic crystals infiltrated with various solvents. (b) Linear trend of the peak shift as a function of the refractive index.

shrinkage along the [111] direction was observed, the optical strength of the 3D titania inverse PC increased to 59% from 29% for the SU-8 template (Table 1). As the infilling medium

template and led to very uniform, high-quality titania inverse structures. Three-dimensional titania inverse structures with a flat, completely open surface were clearly obtained (Figure 2a,b). It should be noted that the sol−gel infiltration method may produce fully open top surfaces without the use of additional postprocessing steps, such as reactive ion etching, ion-beam milling, and/or polishing steps. Inverse opal structures prepared by the sol−gel chemistry and calcination process commonly have microcrystallites, as confirmed by TEM and BET methods.32 The formation of less-dense titania materials by the sol−gel method is mostly due to the small particle size of the crystallites. In our experiments, we can see many microcrystallites with small particle size (Figure 2b). However, dry processes produce denser high RI materials than does the sol−gel reaction.19,20 Our method produced the complete infiltration of the titania sol−gel precursor, regardless of the template thickness (Figure 2c,d). During the calcination step, the sol−gel film was densified. A large degree of volume shrinkage was therefore expected. The degree of film shrinkage of the 3D titania film was quantified by comparing the lattice constant in the (111) plane and interlayer spacing in the [111] direction. The interlayer spacing in the [111] direction was reduced to 490 nm from 750 nm for the polymer template, corresponding to a shrinkage of 34.4%, whereas the lattice constant in the (111) plane remained constant at 650 nm. The 3D titania inverse structures, however, did not collapse during the high volume shrinkage process. A slow temperature ramp is important for densifying the titania gel before removal of the polymer template to prevent film collapse. To this end, we selected a slow temperature ramp rate of 1 °C/min, to 450 °C, for the calcination step. In this way, we obtained high-quality titania inverse structures. Figure 3 shows the optical properties of an 11-layer TiO2 PC filled with materials that displayed different refractive indices: air = 1, H2O = 1.33, ethanol = 1.361, 1:1 mixture of H2O and ethanol = 1.345, and IPA = 1.377. The SU-8 polymer template yielded an incident light reflection peak at 1.99 μm,25 whereas the incident light reflection peak for the TiO2 inverse structures was blue-shifted to 1.44 μm in the [111] direction. Once the polymer template had been inverted to the high-RI materials, the effective RI increased, resulting in a red shift of the reflectance peak in the normal direction;23,33 however, in our experiments, the interlayer distance in the titania PC along the [111] direction was reduced by 34.4%, which resulted in a blue shift in the reflectance peak. Although a large degree of volume

Table 1. Comparison of the Optical Properties, Volume Shrinkage, and Filling Fraction of the 3D Photonic Crystals photonic crystals

reflectance peak position (μm)

interlayer spacing along the [111] direction (nm)

reflectance (%)

filling fraction of skeleton materials (%)

SU-8/air TiO2/air

1.99 1.44

750 490

29 59

44 27

was changed from air to the higher-RI liquids, the reflection spectra were modulated and red-shifted (Figure 3b). The filling fraction of TiO2 in the structures could be calculated using Bragg’s equations. For d = 490 nm (as measured from the SEM images) and n = 2.3 for TiO2, the filling fraction of titania (ffTiO2) was calculated to be 27%. Considering that the filling fraction of the polymer template was 44%, ffTiO2 was expected to be 56% under complete filling of the titania materials (Table 1). This was attributed to the formation of less-dense titania materials by the sol−gel method. Similar results were obtained from the titania inverse opals. Although the theoretical ffTiO2 value should have been 26%, considering the opal structures, the experimental ffTiO2 value was only 12%.18 The mechanical properties of 3D inverse structures are highly dependent on the filling fraction of skeleton materials (in our experiments, titania materials). In the case of inverse opal structures, the maximum filling fraction of skeleton materials would be 26%, considering the close-packing of opal structures. However, in our experiments, we achieved 27% ffTiO2, which is similar to that (ffTiO2 = 30%) of titania inverse structures prepared from ALD.19 Complete filling of the polymer template with titania materials can be achieved by the electrodeposition method.16 The optical properties of 3D inverse structures are highly dependent on the structural uniformity and surface roughness. Commonly, the optical strength of the titania inverse PCs was increased by the large RI contrast. In our experiments, the optical strength of the 3D titania inverse PC increased to 59% from 29% for the SU-8 template. The peak reflectance of 3D titania inverse PC prepared by HL and ALD was 76%.19 And titania inverse structures prepared by HL and electrodeposition D

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had a peak reflectance of 76% due to high ffTiO2 and a smooth top surface.16 Titania inverse opal structures prepared by a sol− gel reaction had a peak reflectance of 70%.18 As for the maximum number of layers for achieving high-quality photonic crystals, 10-layer Cu2O inverse structures are sufficient for achieving 100% reflectance.34 Because refractive indices of crystalline Cu2O and TiO2 in the anatase phase are similar, 11layer TiO2 structures are sufficient for high-quality PCs. A high degree of structural order and a smooth surface are crucial factors in achieving high-quality PCs. Our TiO2 inverse structures had a relatively rough surface and less-dense titania materials. Alkoxide precursors generally produce an amorphous oxide, which requires thermal treatment to produce the crystalline phase.14−18 Titania has several crystalline phases, including anatase, rutile, brookite, and their mixtures.14 The anatase and brookite crystalline phases, which are stable at low temperature, transform into rutile when the sample is calcined at high temperature.35 The calcination of titania gel at 450 and 650 °C yields an anatase phase with a refractive index of 2.3−2.5. XRD techniques were used to characterize the bulk titania film formed on the glass substrate. The XRD patterns exhibited strong diffraction peaks at 25° from the (101) crystal plane and 48° from the (200) crystal plane and weak peaks from the (103), (004), (112), (105), and (211) crystal planes,14 indicating the presence of TiO2 in the anatase phase (Figure 4). No reflections of the rutile and brookite phases were

high-quality 3D titania inverse photonic crystals were successfully fabricated. The reflectance spectra and SEM images provided evidence for anisotropic film shrinkage during the calcination processes. The filling fraction of titania materials has been doubled, compared to that of inverse opal structures. Also, compared to the dry deposition process, our sol−gel infiltration method produced fully open top surfaces without the use of additional postprocessing steps, such as reactive ion etching, ion-beam milling, and/or polishing steps. This is the first trial to fabricate titania inverse PCs with a high filling fraction (∼27%) by utilizing prism holographic lithography and the sol−gel chemistry reaction of a stable titania precursor. Although a large degree of film shrinkage (∼34%) occurred during the calcination step, the optical strength of the 3D titania inverse photonic crystals doubled as a result of the high refractive index contrast.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-350-3962. Fax: +82-42-350-5962. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Science and Technology for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems”. This research was also supported by R&D Program grants from the Korea Institute of Materials Science (KIMS).

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REFERENCES

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Figure 4. XRD diffraction pattern of the crystalline titania film after the calcination process. The XRD diffraction pattern of the samples calcined at 450 and 650 °C coincides with that of the anatase phase. The XRD pattern of the sample calcined at 850 °C indicates the presence of titania in the rutile phase.

observed at 450 and 650 °C. The XRD patterns of the samples calcined at 850 °C exhibited strong diffraction peaks at 27° from the (110) crystal plane, 36° from the (101) crystal plane, and 55° from the (211) crystal plane and weak peaks from the (200), (111), (210), and (220) crystal planes, indicating the presence of the rutile phase (Figure 4).36

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CONCLUSIONS We have presented a simple, cheap, and easy method for fabricating high-quality titania inverted replicas of 3D holographically featured structures. Through a combination of single-prism holographic lithography and sol−gel chemistry, E

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