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Apr 4, 2017 - NPs dispersed in 20% sol−gel binder are patterned using thermal- or UV-assisted imprinting with a PDMS stamp. The composition and ...
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Direct Patterning of Robust One-Dimensional, Two-Dimensional, and Three-Dimensional Crystalline Metal Oxide Nanostructures Using Imprint Lithography and Nanoparticle Dispersion Inks Rohit Kothari, Michael R. Beaulieu, Nicholas R. Hendricks, Shengkai Li, and James J. Watkins* Center for Hierarchical Manufacturing, Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: Dimensionally stable one-dimensional (1-D), two-dimensional (2-D), and three-dimensional (3-D) high aspect ratio crystalline metal oxide nanostructures are fabricated using soft nanoimprint lithography with inks comprised of nanoparticle (NP) dispersions in solvent or in sol−gel precursors for the metal oxide. Crystalline TiO2 and indium tin oxide (ITO) NP dispersions in solvent are imprinted using a solvent permeable patterned poly(dimethylsiloxane) (PDMS) stamp to yield robust crystalline nanostructures that are dimensionally stable to calcination (less than 8% linear shrinkage in imprinted feature heights upon heat treatment at 500 °C). Inks comprised of 80% crystalline NPs dispersed in 20% sol−gel binder are patterned using thermal- or UV-assisted imprinting with a PDMS stamp. The composition and physical properties of the dimensionally stable imprinted metal oxides (TiO2 and ITO) can be altered by varying the composition of the ink. Rapid printing of high aspect ratio nanostructures and sub-100 nm features are easily realized. Residual layer free, direct imprinting of isolated features is achieved by using an ink with the appropriate surface energy to ensure dewetting at the stamp−substrate interface. The technique is extended to create 3-D mesh nanostructures by deploying a simple layer-by-layer imprint strategy. TiO2 3-D mesh nanostructures are robust and mechanically stable to calcination at temperatures of 1000 °C, which results in an anatase to rutile transition. The direct fabrication of high quality dimensionally stable metal oxide nanostructures opens the door to solution based and roll-to-roll processing of robust and efficient inorganic electronic, optical, and energy generation and storage devices. atterned metal oxide and semiconductor films are ubiquitous in functional device architectures. Typical fabrication pathways are subtractive by nature and contain multiple steps, including deposition of a planar film by chemical or physical deposition methods such sputtering or chemical vapor deposition, application of a photoresist, patterning of the resist using optical lithography, development of the resist, etching the metal oxide, and stripping of the residual resist. The fabrication sequence is usually carried out in a clean room facility and employs batch processes, some of which operate at elevated temperatures and reduced pressures. Consequently, patterned inorganic film fabrication is often cost, equipment, materials, and energy intensive and is not amendable to continuous as opposed to batch fabrication. The development of additive or direct patterning techniques for metal oxide and semiconductor films, particularly using approaches that are scalable for continuous production and operate at ambient pressure and modest temperatures, would be enabling for the low cost production of fully functional printed electronics or for devices that require large active areas. Among the most significant challenges are fabrication rate and integration density. Many printing approaches yield structures with minimum features sizes of tens or hundreds of

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© 2017 American Chemical Society

micrometers, while acceptable performance thresholds often require features in the deep submicrometer range. For example, features below 250 nm are required for the manipulation of visible light while features below 1 μm are required to approach electronic device integration densities achieved by the semiconductor industry in the 1980s. Advances toward direct writing and other approaches to directly patterned inorganic device layers have been realized over the past decade. Lewis and co-workers used direct ink writing (DIW), a serial writing approach, to produce patterned structures via coagulation of polyelectrolyte,1 colloidal,2 or sol− gel precursor inks3 printed via extrusion through micrometerscale nozzles. For example, amorphous metal oxide structures could be produced using sol−gel inks in arbitrary 1-D and 3-D structures with lateral dimensions on the order of 2 and 4 μm, respectively. For TiO2 fabrication, calcination of the printed inks to yield anatase resulted in 80% volume shrinkage and minimum feature sizes on the order of 0.25 and 0.5 μm for the 1-D and 3-D structures.3 Micrometer-scale inorganic structures Received: December 20, 2016 Revised: April 3, 2017 Published: April 4, 2017 3908

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

Article

Chemistry of Materials can also be produced by first creating a sacrificial organic template structure by DIW using polyelectrolyte inks followed by coating the structure using CVD or another deposition method and removing the template by calcination. Such an approach has been used for the fabrication of silicon woodpile structures, for example.4 Electrohydrodynamic-inkjet printing technology is one of DIW methods that is capable of printing high resolution structures using micro-/nanodroplets of functional inks and has potential for commercialization.5 While an effective additive approach, challenges for DIW include rate limitations due to serial writing and direct printing of deep submicrometer features without relying on volumetric shrinkage of precursor structures. Nanoimprint lithography (NIL) and soft lithography offer the potential for large area, rapid, and scalable patterned film fabrication. Thermal imprint lithography (embossing) and UVassisted NIL (light-induced polymerization of a liquid resin in contact with a master) have been scaled for roll-to-roll production and can produce a patterned polymer film with critical dimensions as small as 50 nm at feet-per-minute rates.6 NIL in combination with thermal/UV induced polymerization of oligomers containing Si, Ti, or Zr atoms in order to improve refractive index has been used to imprint optical waveguides and 2-D photonic structures.7,8 Recently, UV-NIL has been employed for the patterning of optical gratings9 nano-tomicroscale 2-D photonic patterns,10 and 3-D photonic crystals11 using a hybrid polymer/nanoparticle (NP) resists system of tunable refractive index and metal oxide NP loadings of up to 90 wt %. Thermal or UV-NIL patterning of metal oxide films is typically pursued by imprinting sol−gel precursor containing solutions followed by calcination to drive formation of the metal oxide structure, remove organics, and in some cases crystallize the nascent amorphous metal oxide.12−14 The transition from sol to amorphous gel phase in the soft mold features typically leads to a height reduction of the imprinted features of 40−60%.14,15 High temperature calcination to obtain a crystalline phase is accompanied by additional reductions in dimensions (60−80%), which causes further dimensional instability and cracking.12,13 Alterative approaches have been explored. Choi et al. fabricated a hierarchical TiO2 structure using a two-step hybrid approach by combining sol− gel-based soft NIL to form conical microscale patterns that serve as a template followed by hydrothermal growth of TiO2 nanorods from the microscale patterns.16 As an alternative to thermal or UV-assisted NIL, one variation of soft imprint lithography employs solvent-permeable stamps to remove solvent from a particulate ink system, leaving behind solids. Such an approach has been used to pattern lines of metal NPs, including gold that can be subsequently annealed to produce conductive features.14,17 However, elevated temperatures required postimprinting to impart conductivity and to remove organic ligands bound to the gold NPs to impart stability to the ink, which leads to significant shrinkage and feature distortion. For example, the removal of organics from a NP ink stabilized with 18 wt % ligand17 leads to 80% volumetric shrinkage based on component densities. There are few reported attempts to prepare patterned metal oxide films directly using metal oxide NP dispersions as inks. In one example, Yang et al. fabricated hexagonally patterned indium tin oxide (ITO) dots using ITO NP dispersions, but the approach involves high imprint pressures (0.5 MPa) and long imprint times (1 h).18 One disadvantage of the use of a soft stamp

material (Sylgard-184 PDMS) combined with high imprint pressure is significant compression of the stamp during imprinting which in turn leads to dimensional instability during the pattern transfer process. Continuous 1-D, 2-D, or 3-D structures using this approach were not reported. Here we report the direct fabrication of patterned 1-D, 2-D, and 3-D dimensionally stable crystalline metal oxide nanostructures using a solution-based soft NIL technique. A solvent permeable patterned poly(dimethylsiloxane) (PDMS) stamp is used to imprint an ink containing crystalline NPs in an organic solvent. The composition and physical properties of imprinted metal oxides can be altered by doping crystalline NP based inks with UV/thermally curable metal oxide sol−gel precursors. Use of the curable sols in the ink imparts fluidity and enables imprinting at low solvent concentration in the ink. The use of NPs dispersed in sols as inks eases the requirement of solvent transport into the stamp during patterning, simplifying the process and providing advantages in speed and scalability. The as-imprinted nanostructures are highly crystalline with low residual organics and therefore undergo little shrinkage (less than 5% in some cases) upon thermal annealing. High aspect ratio nanostructures and sub-100 nm features are easily realized. Residual layer free direct imprinting (without etching) was achieved by choosing the ink with the appropriate surface energy and optimum spin coating conditions to ensure dewetting at stamp-substrate interface. The technique was further extended to create 3-D nanostructures by deploying a layer-by-layer imprint strategy. The method is scalable and can produce large area device quality nanostructures in a rapid fashion at a low cost. Here nanopatterns of ITO, a transparent conductive oxide, and TiO2, a transparent high refractive index material, are illustrated for their potential use in solar cell electrodes and photonic devices. Recent progress is the design and synthesis of myriad NPs with well controlled size below 15 nm, size distribution, and surface chemistry by solvothermal and hydrothermal techniques provides a palette of starting materials for broad ranges of applications.

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RESULTS AND DISCUSSION We have imprinted metal oxide nanostructures using inks that are comprised of stable suspensions of crystalline NPs in polar organic solvents and mixtures of crystalline NPs with metal oxide sol−gel precursors. A solvent permeable PDMS stamp replicated from a hard master mold is used for imprinting. NP inks were obtained by replacing isopropyl alcohol (IPA) or water in commercially available pH-stabilized NP dispersions with low vapor pressure polar organic solvents, for example, Nmethyl-2-pyrrolidone (NMP) or an NMP−methanol solvent mixture. Dispersions of NPs functionalized with significant quantities of covalently bound organic ligands were avoided as removal of the ligands can lead to large volume contraction and feature deformation. A schematic of solvent-assisted soft nanoimprinting of NPs for patterned metal oxide films is shown in Figure 1. After spin-coating the inks, a PDMS stamp is placed on the film. Good air permeability of the PDMS stamp and capillary forces promotes conformal contact without formation of air inclusions at the interface, resulting in uniform filling of the stamp features with the ink. The ink dries as the solvent permeates through PDMS, creating rigidified NP based nanostructures as guided by the stamp features. Nanostructures obtained after removal of the stamp are stable as the strong electrostatic interaction between the NPs upon removal of solvent prevents collapse. The thickness of spin coated films is 3909

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

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Chemistry of Materials

Information. Figure 3a,b shows the cross-section SEM images at two different length scales of ITO grating structures imprinted with a square inch-sized composite−PDMS stamp made using the silicon master mold shown in Figure S1. Line width (LW) at the base of the lines, line pitch (LP), and line height (LH) of printed grating structure are ∼130 nm, ∼450 nm, and ∼380 nm, respectively, whereas the LW, LP, and LH of the silicon master from which the patterned composite− PDMS stamp is made are ∼210 nm, ∼450 nm, and ∼480 nm, respectively. The reduction in the printed feature dimensions is due to the volumetric shrinkage accompanied by removal of solvent through PDMS during imprinting. The imprinted features are uniform in size and shape throughout the pattern. Once the imprinting is achieved, these crystalline NP-based features maintain structural integrity upon heating at elevated temperatures, which is often required during calcination and annealing to improve physical properties, for example, conductivity in the case of ITO. Figure 3c shows the crosssection view of the grating structure (LW, ∼130 nm; P, ∼450 nm; LH, ∼365 nm) after heating at 500 °C for 1 h, providing a feature height shrinkage of ∼4%. X-ray diffraction (XRD) patterns for planar ITO NP-based thin film before and after heat treatment at 500 °C indicate Bragg reflections corresponding to cubic In2O3 (space group Ia3̅) crystal structure. The average grain sizes before and after heat treatment as determined by the line broadening of primary (222) reflection using the Scherrer equation were 14.6 and 16.2 nm, respectively, suggesting that shrinkage upon heating is mostly due to the removal of residual organic material in the films. Figure 3d,e shows via structures with hole radii (HR) ∼ 125 nm, hole pitch (HP) ∼ 340 nm, and hole depth (HD) ∼ 355 nm imprinted in a square array pattern. The imprinted structure therefore has minimum hole−wall thicknesses of ∼90 nm, suggesting that sub-100 nm features can also be imprinted using this strategy. Figure 3e,f shows top views at two different magnifications of patterns after heating at 500 °C for 1 h. The HR, HP, and HD of the 500 °C heated sample were ∼125 nm, ∼340 nm, and ∼340 nm, respectively, suggesting a low linear shrinkage of less than 5%. The surface areas of these high aspect ratio line and hole patterns is ∼2.7-fold and ∼4.3-fold that of their projected areas, respectively. Such high surface area transparent conductive oxide nanopatterns are useful for many applications, for example, as solar cell electrodes for improved device efficiency.20,21 The residual ITO layer at the base of the patterns is desirable for such an application and can be

Figure 1. Schematic of solvent-assisted soft nanoimprint lithography for patterning NP-based nanostructured films: (a) ink comprised of a NP dispersion in an organic solvent is applied onto a silicon wafer by spin coating. (b) A patterned PDMS stamp is placed on the ink; solvent permeates through the stamp, creating a rigidified NP-based nanostructure as guided by the features in the stamp. (c) A patterned NP-based film is obtained upon removal of the stamp.

influenced by ambient humidity with higher humidity leading to thinner films. Spin speed and ink composition can be adjusted in order to tune residual-film thickness beneath the imprinted structures. Inks comprised of ITO NPs dispersed in NMP are obtained by replacing isopropyl alcohol (IPA) with NMP in commercially available ITO dispersion in IPA. The choice of solvent is driven by three factors, the solvent should have low vapor pressure to allow a broad time window for spin-coating and imprinting, it should not promote excessive swelling of PDMS stamp, and it must have similar polarity as of the solvent present in the initial dispersion in order to ensure dispersion stability after the solvent exchange step. Figure 2a shows the size distribution of the NPs and NP aggregates in suspensions before and after exchanging solvent, as obtained by dynamic light scattering (DLS). The average aggregate sizes before and after exchanging solvent from IPA to NMP were 49 and 54 nm, respectively, suggesting that there is negligible additional agglomeration of NPs after exchanging solvent. Figure 2b shows the optical image of the ITO NP suspension in NMP obtained after solvent exchange step, which is stable for over a year. Figure 2c shows transmission electron microscopy (TEM) image of ITO NPs with the majority of the particles below 30 nm in size. Figure 3 shows SEM images of ITO nanostructures imprinted using crystalline ITO NP-based ink and a patterned bilayer composite−PDMS stamp. The composite−PDMS stamp consists of a thin layer of patterned h-PDMS backed by a thick layer of Sylgard-184 PDMS.19 The pattern in the hPDMS layer of the composite−PDMS stamp is replicated using the silicon master mold shown in Figure S1 in the Supporting

Figure 2. (a) Particle size distribution of 5 wt % ITO dispersion in IPA (before solvent exchange) and 5 wt % ITO dispersion in NMP (after solvent exchange), as obtained by dynamic light scattering. (b) Digital photograph of ITO dispersion in NMP. (c) TEM image of ITO NPs. 3910

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

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Chemistry of Materials

Figure 3. SEM images of ITO nanostructuresbefore and after calcination at 500 °C for 1 himprinted using ITO NPs dispersed in NMP. (a, b) Cross section view of ITO grating pattern (line width at the base of the line (LW), ∼130 nm; line pitch (LP), ∼450 nm; line height (LH), ∼380 nm) imprinted with a 2.54 × 2.54 cm2 composite−PDMS stamp made using the silicon master mold shown in Figure S1 in the Supporting Information. (c) Line pattern after heating at 500 °C for 1 h (LW, ∼130 nm; LP, ∼450 nm; LH, ∼365 nm). (d) Top SEM image of ITO via features imprinted in square array pattern (hole radius (HR), ∼125 nm; hole pitch (HP), ∼340 nm; hole depth (HD), ∼355 nm) imprinted from ITO NPs dispersed in NMP using a 10 × 10 mm2 composite−PDMS stamp. (e, f) Top view of features after heating at 500 °C for 1 h (HR, ∼125 nm; HP, ∼340 nm; HD, ∼340 nm). Inset in (f) shows cross section view of features after heating at 500 °C. Angle of tilt in (a)−(c) is less than 4° and in (f) is 8°. Imprinting for all samples was performed at room temperature, and imprinting time of several minutes was observed before stamp was removed.

Imprinting into the inks comprising high loading of crystalline ITO NPs in an ITO sol prepared using metal salt precursors also offers good dimensional stability and rapid processability. The ink is prepared by mixing ITO NP suspensions in NMP and ITO sols (indium nitrate and tin(IV) acetate solution in NMP) in the desired proportion, i.e., to provide “x” wt % ITO solid content from NP and “100 − x” wt % from sol upon calcination at 500 °C. The compositions are denoted by “x NP/100−x sol ITO-ink”, and the corresponding films are likewise denoted by “x NP/100−x sol ITO”. Figure 5

eliminated if required by control of the substrate surface energies and solvent wetting. The dynamics of pattern generation depend on the rate of solvent removal via diffusion into the PDMS stamp, which can result in prolonged imprint times at room temperature. The diffusion rate can be improved by imprinting at moderately high temperatures. To assess the speed and reproducibility for creating the patterned nanostructures, we imprinted 24 consecutive 2.54 cm × 2.54 cm array patterns of ITO gratings using a single PDMS stamp (replicated from a silicon master with LW, ∼475 nm; LP, ∼950 nm; LH, ∼475 nm) at 120 °C. The ink was spin coated on Si wafers for 2 min, and the stamp was placed on the substrate. The samples were then heated at 120 °C for 15 s, and the stamp was removed to obtain the patterned nanostructure. Figure 4 shows a digital photograph of the 24 samples; the line height for all samples as measured by atomic force microscopy (AFM) varied between 350 ± 12 nm (Supporting Information Figure S2). Such short imprinting times are crucial for high volume manufacturing of nanostructures.

Figure 5. (a) ITO gratings imprinted in 30 s at 150 °C using an ink prepared by mixing ITO NP suspension and ITO sol in a proportion to provide 80 wt % ITO content from NPs suspension and 20 wt % from sol. The height of the imprinted features is ∼385 nm. Imprints were performed with a 2.54 × 2.54 cm2 composite−PDMS stamp made using a silicon master mold shown in Figure S1 in the Supporting Information. (b) Grating pattern upon calcination at 500 °C for 1 h. The structure height is reduced to ∼355 nm (approximately 8% shrinkage).

shows ITO gratings imprinted using 80 NP/20 sol ITO-ink. The composite−PDMS stamp used for imprinting was made using the silicon master mold shown in Figure S1 in the Supporting Information. Unlike NP films coated from solvent only, films coated from NP/sol dispersions could be imprinted even several hours after spin coating. To achieve rapid patterning the composite−PDMS stamp was placed and the substrate was heated at 150 °C for 30 s to convert the imprinted viscous NP/sol dispersion into a rigidified structure. Figure 5a shows gratings imprinted at 150 °C with ∼385 nm tall lines. The pattern was then calcined at 500 °C for 1 h to

Figure 4. Digital photograph of 24 ITO grating line pattern samples of size 2.54 × 2.54 cm2 imprinted at 120 °C with 15 s imprint time per sample. Color variation between the imprinted samples in the photograph arises due to different interference colors produced by the grating patterns at different viewing angles. 3911

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

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Chemistry of Materials

NP/5 sol ITO, 100 NP ITO, and 100 sol ITO films with similar thicknesses (180−230 nm) were 2000 Ω/□, 9000 Ω/□, and 200−250 Ω/□, respectively. The sheet resistances of our films are also superior or comparable to that reported by other researchers using solution processable NP22,23 and sol−gel based techniques.24−26 The tremendous improvement in conductivity by adding sol to provide just 20 wt % ITO content from sol can be attributed to reduced porosity of the films, improved interfacial contact, and potentially superior quality of ITO from sols compared to the ITO NPs. The porosity in NP-based films arises in part from the interstitial voids between assembled NPs upon removal of the solvent, which can strongly vary with the size distribution of the NPs in the film. To compare porosity in the thin films, we measured refractive index (RI) using spectroscopic ellipsometry. RI fitting was performed by minimizing mean-square error (MSE) using a general oscillator layer model for ITO. Figure 6b shows RI measured between 450 and 800 nm for 500 °C calcined 100 NP ITO, 80 NP/20 sol ITO, and 100 sol ITO thin films on silicon substrates. The thicknesses of the films, RIs at 633 nm, and mean square error (MSE) values are shown in Table 2. The small MSE values

remove residual organic material and convert the nascent amorphous binder into a crystalline phase, which resulted in line height reduction to 355 nm (∼8% linear shrinkage). The shrinkage in feature height at each step during the fabrication process using 80 NP/20 sol ITO-ink and also using NP-based ITO inks without sol−gel precursors described earlier are summarized in Table 1. Figure 6c shows XRD data for the Table 1. Line Height (LH) and Shrinkage in Feature Height during Imprinting and Heat Treatment Steps for Fabrication of ITO Gratings Using Ink Composed of Different Proportions of ITO NPs and ITO Sol−Gel Precursora

100 NP ITO 80 NP/20 sol ITO

LH of master mold

LH/shrinkage-I (for as-imprinted sample)

LH/shrinkage-HT (for 500 °C heat treated sample)

480 nm 480 nm

380 nm/21% 385 nm/20%

365 nm/4% 355 nm/8%

a

Shrinkage observed in the LH of as-imprinted samples with respect to the LH of master mold is represented as “shrinkage-I”, and shrinkage observed in the LH of heat treated samples with respect to the LH of as-imprinted samples is represented as “shrinkage-HT”.

calcined planar 100 sol ITO and 80 NP/20 sol ITO thin films indicating a crystalline morphology with average grain sizes 7.6 and 16.2 nm, respectively. Inks with lower ITO sol content, for example, 90 NP/10 sol ITO-ink, resulted in rigid films after spin coating for several minutes. Such inks therefore required solvent-assisted imprinting, akin to patterning complete NPbased films as described earlier. The relative proportions of crystalline NPs and sol in the ink determines dimensional stability upon calcination. Figure 6a shows reductions in film thickness measured using a mechanical profilometer of room temperature dried planar films upon calcination at 500 °C for 1 h. Films with greater than 80 wt % ITO content from NPs yielded thickness reductions of less than 20%. Films fabricated using NPs and sol also showed lower sheet resistance compared to the films fabricated using only NPs. To measure sheet resistance, planar films were calcined for 1 h at 500 °C in air and annealed for 3 h at 300 °C in a stream of nitrogen to increase the concentration of oxygen vacancies. The sheet resistances of 200 nm thick 80 NP/20 sol ITO films were 400−450 Ω/□. The sheet resistances of a 95

Table 2. Fitted Thickness, MSE, and RI of Planar ITO Films on Silicon Substrate Calcined at 500 °C for 1 h 100 NP ITO 80 NP/20 sol ITO 100 sol ITO

thickness (nm)

MSE

RI at 633 nm

114 166 74

9.7 11 12

1.528 1.558 1.564

(less than 10) signifies good fitting of RI data. Taking the RI of dense ITO as 1.858 at 633 nm,27,28 porosities as calculated using the Lorentz−Lorenz effective medium approximation model29 are approximately 31.5% for 100 NP ITO, 28.3% for 80/20 ITO, and 27.6% for 100 sol ITO films. Planar ITO films made by NP-based ink and NP/sol-based inks showed excellent optical transparency of more than 90% in the visible range (Figure S3a in the Supporting Information), which also suggests that there is no precipitation of large aggregates during film formation by these methods. To further extend solvent assisted NP-based imprinting to other metal oxides, we demonstrate imprinting of TiO2

Figure 6. (a) Shrinkage in planar ITO thin films composed of different proportions of ITO NPs and ITO sol−gel precursor (based on solid content contributed by the two components upon calcination at 500 °C). Initial thickness is measured after drying the spin coated planar film at room temperature for 72 h. Percentage loss in thickness is determined after calcination at 500 °C for 1 h. (b) Refractive index of planar ITO thin films calcined at 500 °C. (c) XRD diffraction of planar ITO thin films with Bragg reflections corresponding to the cubic In2O3 crystal structure. 3912

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

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Chemistry of Materials

Figure 7. Particle size distribution of 5 wt % TiO2 dispersions in water (before solvent exchange) and in the NMP−methanol mixture (after solvent exchange), as obtained by dynamic light scattering. (b) Digital photograph of TiO2 dispersion in the NMP−methanol mixture. (c) TEM image of TiO2 NPs.

Figure 8. Cross-section SEM images of patterned TiO2 gratings imprinted with inks having different weight ratios of TiO2 NPs and UV curable TPA sol. (a−c) As-imprinted gratings: (a) 100 NP (LH, 385 nm), (b) 80 NP/20 sol (LH, 380 nm), and (c) 100 sol (LH, 275 nm) imprinted with a 2.54 × 2.54 cm2 composite−PDMS stamp made using the silicon master mold shown in Figure S1 in the Supporting Information and cured under UV. (d−f) Gratings upon calcination at 500 °C for 3 h: (d) 100 NP (LH, 360 nm), (e) 80 NP/20 sol (LH, 340 nm), and (f) 100 sol (LH, 140 nm). (g) 100 NP grating pattern (LH, 320 nm) upon calcination at 750 °C for 3 h.

nanostructures. TiO2 NP dispersions for inks were obtained by replacing the majority of the water in commercially available acid-stabilized aqueous TiO2 (anatase) NP dispersions with an NMP−methanol solvent mixture. The particle size distribution in suspensions, as shown in Figure 7a, of TiO2 aggregates before (8.7 nm) and after (7.9 nm) solvent change from water to NMP−methanol mixture remained almost the same. Figure 7b shows the optical image of the NP suspension in the NMP− methanol mixture, which is stable for over a year with negligible precipitation. Figure 7c shows the TEM image of drop-casted TiO2 NPs with the majority of particles below 8 nm. Titania nanostructures were imprinted with inks comprised of different proportions of TiO2 NP dispersions (in NMP− methanol mixtures) and a UV-curable TiO2 sol, titanium diisopropoxide bis(acetylacetonate) (TPA), in IPA. TPA is one of the most common TiO2 precursors as acetylacetonate (AcAc) chelates Ti and forms a stable metal complex. AcAc or more generally β-diketonate ligands have an optical absorption band in the UV range from the π − π* transition. This photoexcitation of AcAc ligands gives rise to the dissociation of the chelate bonds of the metal complex, increasing the reactivity of the metal precursor to form an amorphous TiO2 network.30

Imprinting was performed with a composite−PDMS stamp made using the master mold shown in Figure S1, Supporting Information. Samples were subjected to a 10 J cm−2 energy dose of long wavelength UV light after placing the stamp for imprinting in order to cure the TPA sol. Figure 8 shows the cross sectional SEM of imprinted features composed of 100% TiO2 NP (100 NP TiO2), 80/20 (by weight) TiO2 NP/TPA (80 NP/20 sol TiO2), and 100% TPA (100 sol TiO2) with feature height of 385, 380, and 270 nm, respectively, which upon calcination at 500 °C for 3 h reduced to 360 nm (∼6% shrinkage), 340 nm (∼11% shrinkage), and 140 nm (∼48% shrinkage), respectively. The shrinkage in the feature height at each step during the fabrication process for 100 NP TiO2, 80 NP/20 sol TiO2, and 100 sol TiO2 is shown in Table 3. Figure 9b shows XRD of planar films before and after being calcined at 500 °C for 3 h, having an anatase−TiO2 reflection with mean crystallite size growing form 3.8 to 13.5 nm. The dimensional instability associated with enormous shrinkage observed upon calcination to obtain a crystalline phase in the conventionally used sol−gel based soft NIL technique leads to distorted features (Figure 8f); in contrast to this, low shrinkage upon calcination in features majorly comprising crystalline NPs imprinted using the solvent-assisted soft NIL technique 3913

DOI: 10.1021/acs.chemmater.6b05398 Chem. Mater. 2017, 29, 3908−3918

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Chemistry of Materials Table 3. Line Height (LH) and Shrinkage in Feature Height during Imprinting and Heat Treatment Steps for Fabrication of TiO2 Gratings Using Ink Composed of Different Proportions of TiO2 NPs and UV Curable TPA Sola

100 NP TiO2 80 NP/20 sol TiO2 100 sol TiO2

LH of master mold

LH/shrinkage-I (for as-imprinted sample)

LH/shrinkage-HT (for 500 °C heat treated sample)

480 nm 480 nm 480 nm

385 nm/20% 380 nm/21% 270 nm/44%

360 nm/6% 340 nm/11% 140 nm/48%

Table 4. Fitted Thickness, MSE, and RI of Planar TiO2 Films on Silicon Substrate Calcined at 500 °C for 3 h 100 NP TiO2 80 NP/20 sol TiO2 100 sol TiO2

thickness (nm)

MSE

RI at 633 nm

86 103 70

7 17 9

1.960 1.917 2.056

This potentially could be due to the alteration in surface charge on TiO2 NPs in the presence of TPA sol, leading to changes in coagulation dynamics during drying of the film. We believe that the RI in the NP based films can be further improved by choosing a dispersion with broader particle size distribution. Many applications require free-standing features without a residual layer. By taking advantage of low surface energy of the stamp, ink solutions with low viscosity and appropriate advancing and receding contact angles on the stamp material can fill the recesses without leaving any residue on the surface.32 Such a discontinuous wetting approach has been used to fill various elastomeric stamps with metal oxide sols, followed by imprinting on the substrate to achieve a residual-layer free pattern.12,14 The rate of filling of the ink and the swelling of the stamp encountered due to an additional thick layer of solvent applied between stamp and substrate in order to pull out depressed solidified ink from stamp recesses onto the substrate can limit the imprinting speed in this method.14 In addition to discontinuous wetting of the stamp, residuallayer free direct imprinting can be facilitated by application of modest pressure to squeeze out low viscosity inks from between the stamp/substrate interface during the imprinting process. Imprinting of gold features with minimal residual layers has been optimized using a low viscosity and low surface tension gold NP suspension in α-terpineol at a 5 psi imprint pressure.17 This technique has also recently been optimized to imprint residual-free patterns of small organic molecules, which are then polymerized by thermal initiation.33,34 Here we take a similar approach to demonstrate residual-layer free imprinting of titania lines. The ink is comprised of an ∼3 wt % TiO2 NP dispersion in a propanediol−methanol solvent mixture, obtained by diluting 20 wt % TiO2 NP stock dispersion in propanediol by methanol. The ink was spin-coated (at 3000 rpm) onto a silicon wafer in a 70−80% relative humidity

a

Shrinkage observed in the LH of as-imprinted samples with respect to the LH of master mold is represented as “shrinkage-I”, and shrinkage observed in the LH of heat treated samples with respect to the LH of as-imprinted samples is represented as “shrinkage-HT”.

demonstrated here rendered dimensionally stable nanostructures (Figure 8d,e). Figure 8g shows the 100 NP TiO2 sample upon heating at 750 °C for 3 h, which resulted in line height reduction to ∼320 nm (∼17% shrinkage). XRD on 750 °C treated 100 NP TiO2 film confirmed a transition from anataseto rutile-TiO2 phase and grain growth to 29.8 nm (Figure 9b). Note that these structures remain mechanically stable despite significant crystal phase transition and grain coarsening. Planar films of 100 NP TiO2 and 80 NP/20 sol TiO2 showed transmittance greater than 90% in the visible regime (Figure S3b in the Supporting Information), suggesting that NPs do not precipitate to form large agglomerates upon evaporation of the solvent during film formation. TiO2 due to its high RI and transparency is interesting for applications in photonic devices to manipulate visible light. Figure 9a shows measured RI of anatase-TiO2 planar films calcined for 3 h at 500 °C. The Cauchy model was used to fit the RI data, which is suitable for TiO2 because of its transparency and low absorbance in the visible spectrum. The RIs of films containing NPs are lower than those of films made using only TPA sol. The thicknesses of the films, MSE values, and RIs at 633 nm are shown in Table 4. Taking the RI of dense TiO2 as 2.49 at 633 nm,31 porosities as calculated using Lorentz−Lorenz effective medium approximation model29 are 23.3% in 100 NP TiO2 film, 25.6% in 80 NP/20 sol TiO2 film, and 18.5% for 100 sol TiO2 film. Unlike in the case of ITO, the RI of the TiO2 NP films did not improve by addition of the sol.

Figure 9. Characterization of planar TiO2 thin films made with inks having different weight ratios of TiO2 NPs and UV curable TPA sol. (a) Refractive Index of 100 NP, 80 NP/20 sol, and 100 sol films calcined at 500 °C for 3 h. (b) XRD patterns of 100 NP, 80 NP/20 sol, and 100 sol films calcined at different temperatures for 3 h. 3914

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second step the structure is planarized with a UV-cross-linkable low viscosity organic thiolene−acrylate prepolymer (NOA60, Norland Products Inc.) by spin-coating from a 20 wt % NOA 60 solution in propylene glycol monomethyl ether acetate (PGMEA) solvent followed 10 wt % NOA60 solution in PGMEA, with UV curing after each spin coating step. A low power oxygen plasma treatment before coating each planarization layer is found to improve of the quality of the planarization layer. The low viscosity of NOA60 allows effective planarization (grating height reduces to 2−5 nm as measured by AFM) while leaving a thin residual layer (100−150 nm) atop of the first grating structure. The imprinting and planarization process is then simply repeated for the desired number of layers. Finally, the organic planarization layer is removed by calcination. Figure 12a,b shows a bilayer and a tetralayer with ∼240 nm grating

environment for about 2 min after which a PDMS stamp (LW, ∼475 nm; LP, ∼950 nm; LH, ∼480 nm) was placed and light pressure applied. The substrate was then heated to 120 °C for 15 s, and the stamp was removed to obtain a residual-layer free line pattern (Figure 10) with ∼300 nm line height. In this case,

Figure 10. (a) Top view and (b) tilted view of a residual layer-free TiO2 grating pattern imprinted over 2.54 × 2.54 cm2 area.

high relative humidity was found to be important to obtain high quality residual-layer free imprints. High humidity leads to thinner NP-based films suggesting that the interaction of ink with substrate is weaker at higher humidity, which will promote dewetting of the ink at the stamp/substrate interface during the imprinting process. Continuous 3-D nanostructures composed of high loadings of ZnO NPs in UV-curable resins has been fabricated using soft UV-NIL to pattern grating layers on prepatterned sacrificial poly(vinyl alcohol) (PVA) substrate followed by transfer stacking and dissolving of the PVA substrate in water.35 Such high RI photonic crystal patterns on LED showed enhanced light extraction from LED. 3-D photonic crystals of high RI TiO2 NPs/polymer composites were fabricated using soft UVNIL to pattern gratings on a sacrificial planar PVA substrate, followed by dissolving the substrate and stacking the patterned grating layers to form the 3-D photonic crystals.11 The ability to print NP-based features with minimal residual layers enables the fabrication of completely inorganic continuous 3-D nanostructures using a simple print− planarize−print approach (Figure 11). The first grating layer

Figure 12. TiO2 3-D nanostructures fabricated using the print− planarize−print approach on a 2.54 × 2.54 cm2 Si coupon. (a) Bilayer and (b) tetralayer after removal of organic planarizing layer by calcination. (c) Bilayer heated to 850 °C for 1 h. (d) Tetralayer heated at 1000 °C for 1 h.

width at the top of the grating line following calcination to remove the planarization layer. Figure 12c shows the bilayer after annealing at 850 °C for 1 h with grating width reduced to ∼200 nm. The structure remains stable despite conversion of TiO2 from anatase to rutile and significant coarsening of the NP crystallites, as indicated by the sharp primary rutile peak in XRD (Figure S4, Supporting Information). Figure 12d shows the tetralayer after calcination at 1000 °C for 1 h with XRD (Figure S4, Supporting Information) suggesting highly crystalline rutile morphology with sharp higher order peaks. A slight sagging of the structure can be observed due to softening of TiO2 at such high temperature. Continuous 3-D inorganic nanostructures using direct imprinting methods to stack residual-free patterned layers have been demonstrated in the past by transfer stacking a patterned precursor layer onto a prepatterned substrate36 and by transfer imprinting ink filled in soft mold recesses onto a prepatterned substrate.14 The drawback of the transfer stacking method is that it used the conventional thermal NIL technique for patterning, which involved a hard master mold to imprint into palladium organic ink at high processing pressure (50 bar).36 The excessive pressure used during imprinting and transferring caused cracking of stacked grating lines in some cases. The imprinted line pattern also suffered a massive line height reduction of 62% upon calcination due to the high organic content of the ink. Moreover, instead of appearing as a 3-D structure, the bilayer structure resembled a 2-D structure

Figure 11. Schematics of fabrication of a 3-D metal oxide nanostructure by a print−planarize−print approach followed by calcination to remove the organic planarizing layer.

on a silicon wafer is imprinted using an ∼3 wt % TiO2 NP dispersion in a propanediol−methanol solvent mixture as ink and a PDMS stamp (LW, ∼475 nm; LP, ∼950 nm; LH, ∼480 nm) in 60−80% relative humidity environment. Imprinting time is 10 s at 120 °C. The height of the imprinted line can be tuned between 250 and 300 nm by simply varying spin speed (2000−3000 rpm) or the concentration of the ink. In the 3915

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dimethylsiloxane (hydride terminated, Gelest Corp.) was dropwise added and stirred for 1 min. The h-PDMS mixture was then immediately spin-coated on a patterned silicon master at 1000 rpm for 40 s, followed by heating at 60 °C for 2 min to cure h-PDMS. An ∼3 mm-thick layer of Sylgard 184 prepolymer mixture (made as described above) was poured onto the h-PDMS layer and cured at 70 °C for 5 h. The cured bilayer composite−PDMS was then peeled away from the master mold. Preparation of NP Dispersions for Inks. Solvent exchanged ITO NP dispersions in NMP were obtained by adding NMP to a commercially available ITO dispersion in IPA, followed by evaporation of IPA. Equal weights of an ITO (∼30 wt %, 20−30 nm average particle size, In2O3:SnO2 = 90:10 by weight) NP dispersion in IPA (Nanostructured & Amorphous Materials, Inc.) and NMP (N-methyl2-pyrrolidone 99%, Sigma-Aldrich Co. Ltd.) are mixed and dried under a stream of air to remove IPA. The solid content of ITO in the dispersion obtained after solvent exchange, as determined by heat treating the NP dispersion at 500 °C for 1 h, was found to be 25.9 wt %. To obtain the TiO2 NP dispersion NMP−methanol solvent mixture, acid stabilized titanium oxide (anatase, 15 wt %, 5−30 nm average particle size) the NP dispersion in water (Nanostructured & Amorphous Materials, Inc.) was mixed with NMP and methanol in a 2:1:1 weight ratio and dried under a stream of air to remove the majority of solvents (up to 50% solvent remaining in the obtained mixture). The resulting dried slurry was redispersed in a 1:1 NMP− methanol solvent mixture using vortex-mixing and sonication to obtain a 15 wt % TiO2 NPs dispersion, which can be further diluted with methanol in order to adjust the thickness of the spin coated films. Solvent exchanging with only NMP caused agglomeration. Addition of methanol upon removal of water improves the dissociation of the acid sufficiently to avoid destabilization of the suspension. Moreover, the NMP−methanol solvent mixture provided high quality spin coated films, whereas the use of the NMP−water mixture caused striation in the films. To obtain the ∼3 wt % of TiO2 NP dispersion in the propanediol− methanol solvent mixture, one part (by volume) of titanium oxide (anatase, around 20 wt %, 15 nm average particle size) NPs dispersion in 1,2-propanediol (US Research Nanomaterials, Inc.) was diluted with 5 parts of methanol (ACS grade, Fisher Scientific). All NP dispersions were subject to ultrasonication for a few minutes before using as an ink. Preparation of NP/Sol Inks. To prepare NP/sol ITO-ink, an ITO NP dispersion in NMP was mixed with ITO sol in NMP. The ITO sol in NMP was prepared by dissolving 0.235 g of tin(IV) acetate (Sn(CH3CO2)4, Sigma-Aldrich Co. Ltd.) in 6.75 g of NMP at 80 °C, followed by solubilizing 3.02 g of indium(III) nitrate hydrate (In(NO3)3·xH2O (x = 4.5, as determined by weight loss upon heating in vacuum at 105 °C for 24 h), Sigma-Aldrich Co. Ltd.). This composition of metal slats is chosen to create ITO with In2O3:SnO2 = 90:10 (by weight) upon calcining the sol at 500 °C. To prepare 80 NP/20 sol TiO2 ink, a 15 wt % TiO2 NP dispersion in NMP−methanol was mixed with titanium diisopropoxide bis(acetylacetonate) (TPA, [(CH3)2CHO]2Ti(C5H7O2)2, 75 wt % in isopropanol, Sigma-Aldrich Co. Ltd.) in proportions to provide 80:20 (by weight) NPs to TPA in the ink. Characterization. Aggregate size distributions in NP dispersions diluted to 5 wt % solids were measured by dynamic light scattering (Malvern Zetasizer 3000 HSa instrument). Transmission electron microscopy was performed in bright field imaging mode using a JEOL 2000 FX TEM operated at an accelerating voltage of 200 kV. The silicon master was gold sputtered using a sputter coater (CR 108) for 60 s prior to imaging. Scanning electron microscopy was performed on a field emission scanning electron microscope (Magellan 400). Thickness measurements of planar films were performed with a surface profilometer (Veeco Dektak 150). Sheet resistance of planar ITO films (on SiO2 (300 nm)/Si wafer) were measured using a fourpoint probe technique (Keithley 2400 instrument) with evaporated gold electrodes for contacts. Transmittance measurements of planar films on quartz substrate were performed on an UV/vis/NIR

because of collapse of the soft metal organic ink pattern during the heat treatment step. Bilayers generated by directly transfer imprinting silver NP ink filled in soft mold recesses onto a prepatterned substrate also suffered from structural instability where the top layer collapsed and touched the substrate, probably due to sagging of the soft mold features during the imprinting step.14 In contrast to this, our multilayer structures do not collapse upon heating up to 850 °C (Figure 12a−c) because of the rigidity and robustness of as-imprinted grating layers. Moreover, our approach enables the fabrication of many layers.

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CONCLUSION We have shown that a variety of high aspect ratio crystalline inorganic oxide nanostructures can be directly fabricated by practicing solvent/thermal/UV-assisted soft nanoimprint lithography on inks containing metal oxide nanoparticles. Our nanoparticle based soft imprinting technique is capable of producing large area nanopatterns in a rapid fashion. The low shrinkage at elevated temperatures observed in crystalline nanoparticle based nanopatterns (less than 8% linear shrinkage in the imprinted feature height upon heat treatment at 500 °C) is a substantial development over previously explored sol−gel based methods that suffer from significant shrinkage when the structures are first condensed to form the metal oxide network and then heated to obtain a crystalline morphology. The porosity in the nanoparticle based features is comparable to that obtained by sol−gel based methods. The technique is also extended to fabricate residual-layer free nanopatterns and 3-D nanopatterns by using a print−planarize−print approach.

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METHODS

Fabrication of PDMS Stamps. Patterned PDMS stamps were made by casting PDMS against silicon master molds. The silicon masters were cleaned in concentrated sulfuric acid and followed by treating for 15 min in an oxygen plasma cleaner. The silicon masters were then placed in a degassed glass jar at 80 °C, to which 1 vol % of heptadecafluoro-1,1,2,2-tetrahydrodecyl)dimethylchlorosilane (Gelest Corp.) is introduced for a 12 h period. In this process a self-assembled monolayer of a long chain fluorinated alkane is created on silicon master molds which reduces their surface energy and facilitates easy removal of cured PDMS stamps casted against them in the next step. Sylgard 184 silicone elastomer kits (Ellsworth Adhesive) were used to make PDMS stamps from the silicon masters with low aspect-ratio line patterns (LW, ∼425 nm; LP, ∼950 nm; LH, ∼480 nm). A 10:1 (by weight) ratio of base to curing agent was thoroughly mixed, and an ∼3 mm thick layer was poured on the silicon master molds. Vacuum was pulled (for 5−10 min) to remove air inclusions and dissolved gases. The PDMS replica was then cured at 70 °C for 5 h and peeled away from the master mold to obtain the patterned stamp form. Bilayer composite−PDMS structures were used to make stamps from hard silicon master molds bearing high aspect-ratio and densely packed features. The bilayer stamp consists of a patterned thin layer of h-PDMS backed with a thick soft Sylgard 184 PDMS layer and was made using a method described elsewhere.19 An h-PDMS layer (see Methods) is essential as Sylgard 184 PDMS has insufficient mechanical strength to prevent deformation of high aspect ratio densely packed features. In a typical process to make h-PDMS, 1.7 g of (7.0−8.0% vinylmethylsiloxane)−dimethylsiloxane copolymer (trimethylsiloxyl terminated, VDT-731, Gelest Corp.) was mixed with one drop of 2,4,6,8-tetramethyltetravinylcyclotetrasuloxane (Fluka-87927, SigmaAldrich Co. Ltd.) and 9 μL of platinum−divinyltetramethyldisiloxane complex in xylene (SIP-6831, Gelest). The mixing was done in a 25 mL glass vial for 30 s using a magnetic stirrer. The mixture was then immediately degassed on a Schlenk line by pulling vacuum for 1 min. To the degassed mixture, 0.5 mL of (25−30% methylhydrosiloxane)− 3916

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Chemistry of Materials spectrometer (PerkinElmer Lambda 1050) using an integrating sphere accessory. X-ray diffraction (XRD) experiments were performed using an X-ray diffractometer (Philips PANalytical X’Pert) equipped with a Cu Kα X-ray (0.1542 nm) source operating at 45 kV and 40 mA. The average crystallite size by XRD was measured using the Scherrer equation. Spectroscopic ellipsometry measurements for determination of the refractive index (RI) were performed using a variable-angle spectroscopic ellipsometer (RC2-DI from J. W. Woollam Co., Inc.). Ellipsometric data was collected for 55°, 60°, 65°, and 70° angles of incidence. RI for ITO was determined by fitting ellipsometric data in the 450−1500 nm spectral range with a bilayer model consisting of a general oscillator layer on the silicon substrate. The fitting was performed using the instrument’s VASE-software (complete EASE version 2.30 software, J.A. Woollam Co., Inc.). RI for TiO2 was determined by fitting ellipsometric data in the 380−1500 nm spectral range with a bilayer model consisting of a Cauchy layer on the silicon substrate. Porosity was calculated using Lorentz−Lorenz effective medium approximation model as29

neff 2 − 1 2

neff + 2

= (1 − f )

(5) Park, J.-U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. k.; Lee, C. Y.; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A. High-resolution electrohydrodynamic jet printing. Nat. Mater. 2007, 6, 782−789. (6) John, J.; Tang, Y.; Rothstein, J. P.; Watkins, J. J.; Carter, K. R. Large-area, continuous roll-to-roll nanoimprinting with PFPE composite molds. Nanotechnology 2013, 24, 505307. (7) Schueller, O. J. A.; Whitesides, G. M.; Rogers, J. A.; Meier, M.; Dodabalapur, A. Fabrication of photonic crystal lasers by nanomolding of solgel glasses. Appl. Opt. 1999, 38, 5799−5802. (8) Kim, W.-S.; Yoon, K. B.; Bae, B.-S. Nanopatterning of photonic crystals with a photocurable silica-titania organic-inorganic hybrid material by a UV-based nanoimprint technique. J. Mater. Chem. 2005, 15, 4535−4539. (9) Jo, H.-B.; Byeon, K.-J.; Lee, H.; Kwon, M.-H.; Choi, K.-W. Fabrication of ZnO nano-structures using UV nanoimprint lithography of a ZnO nano-particle dispersion resin. J. Mater. Chem. 2012, 22, 20742−20746. (10) Choi, J.-H.; Jo, H.-B.; Choi, H.-J.; Lee, H. Fabrication of TiO2 nano-to-microscale structures using UV nanoimprint lithography. Nanotechnology 2013, 24, 195301. (11) Beaulieu, M. R.; Hendricks, N. R.; Watkins, J. J. Large-Area Printing of Optical Gratings and 3D Photonic Crystals Using SolutionProcessable Nanoparticle/Polymer Composites. ACS Photonics 2014, 1, 799−805. (12) Hampton, M. J.; Williams, S. S.; Zhou, Z.; Nunes, J.; Ko, D.-H.; Templeton, J. L.; Samulski, E. T.; DeSimone, J. M. The patterning of sub-500 nm inorganic oxide structures. Adv. Mater. 2008, 20, 2667− 2673. (13) Ganesan, R.; Dumond, J.; Saifullah, M. S. M.; Lim, S. H.; Hussain, H.; Low, H. Y. Direct Patterning of TiO2 Using Step-andFlash Imprint Lithography. ACS Nano 2012, 6, 1494−1502. (14) Hwang, J. K.; Cho, S.; Dang, J. M.; Kwak, E. B.; Song, K.; Moon, J.; Sung, M. M. Direct nanoprinting by liquid-bridge-mediated nanotransfer moulding. Nat. Nanotechnol. 2010, 5, 742−748. (15) Li, M.; Tan, H.; Chen, L.; Wang, J.; Chou, S. Y. Large area direct nanoimprinting of SiO2 andTiO2 gel gratings for optical applications. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2003, 21, 660− 663. (16) Choi, H.-J.; Shin, J.-H.; Choo, S.; Kim, J.; Lee, H. A tunable method for nonwetting surfaces based on nanoimprint lithography and hydrothermal growth. J. Mater. Chem. A 2013, 1, 8417−8424. (17) Ko, S. H.; Park, I.; Pan, H.; Grigoropoulos, C. P.; Pisano, A. P.; Luscombe, C. K.; Frechet, J. M. J. Direct nanoimprinting of metal nanoparticles for nanoscale electronics fabrication. Nano Lett. 2007, 7, 1869−1877. (18) Yang, K.-Y.; Yoon, K.-M.; Lim, S.; Lee, H. Direct indium tin oxide patterning using thermal nanoimprint lithography for highly efficient optoelectronic devices. J. Vac. Sci. Technol. B 2009, 27, 2786− 2789. (19) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Improved pattern transfer in soft lithography using composite stamps. Langmuir 2002, 18, 5314−5320. (20) Wang, F.; Subbaiyan, N. K.; Wang, Q.; Rochford, C.; Xu, G.; Lu, R.; Elliot, A.; D’ Souza, F.; Hui, R.; Wu, J. Development of nanopatterned fluorine-doped tin oxide electrodes for dye-sensitized solar cells with improved light trapping. ACS Appl. Mater. Interfaces 2012, 4, 1565−1572. (21) Manca, M.; Beke, S.; De Marco, L.; Pareo, P.; Qualtieri, A.; Cannavale, A.; Brandi, F.; Gigli, G. 3D Photoelectrode for Dye Solar Cells Realized by Laser Micromachining of Photosensitive Glass. J. Phys. Chem. C 2014, 118, 17100−17107. (22) Cheong, D.-S.; Yun, D. H.; Kim, D. H.; Han, K. R. Indium tin oxide (ITO) coatings fabricated using nanoparticle slurry and sol. Han'guk Seramik Hakhoechi 2011, 48, 516−519. (23) Cho, Y.-S.; Kim, H.-M.; Hong, J.-J.; Yi, G.-R.; Jang, S. H.; Yang, S.-M. Dispersion stabilization of conductive transparent oxide nanoparticles. Colloids Surf., A 2009, 336, 88−98.

nc 2 − 1 nc 2 + 2

where neff is the measured RI of the film, f is the volume fraction of the pores (percentage porosity= 100 × f), and nc is the RI of dense material in the absence of porosity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05398. Cross section SEM of silicon master mold with grating pattern, AFM phase image and line height measurement of one of the 24 ITO line-space patterned coupons, transmittance of ITO and TiO2 thin film on quartz, and XRD showing rutile reflections of TiO2 multilayer logpile patterns after annealing at 850 and 1000 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.J.W.). ORCID

Rohit Kothari: 0000-0002-0256-1632 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the National Science Foundation under Grants CMMI-1025020 (Center for Hierarchical Manufacturing) and CMMI-1258336. R.K. would like to acknowledge assistance with the background XPS characterization experiment from Irene Howell.

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REFERENCES

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