Nonlithographic Fabrication of Nanostructured Micropatterns via

Feb 6, 2014 - Yang Ou†, Liang-Wei Zhu†, Wen-Da Xiao†, Hao-Cheng Yang†, Qing-Jun Jiang‡, Xia Li‡, Jian-Guo Lu‡, Ling-Shu Wan†*, and Zhi...
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Nonlithographic Fabrication of Nanostructured Micropatterns via Breath Figures and Solution Growth Yang Ou,† Liang-Wei Zhu,† Wen-Da Xiao,† Hao-Cheng Yang,† Qing-Jun Jiang,‡ Xia Li,‡ Jian-Guo Lu,‡ Ling-Shu Wan,†,* and Zhi-Kang Xu† †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China ‡ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: Micropatterning techniques independent of high-cost facilities are highly appreciated in bioanalysis and optoelectronics. Here we report a novel nonlithographic method based on self-assembled honeycomb films with through pores for micropatterning of zinc oxide nanowires (ZnO NWs). The ordered films were prepared via the breath figure method and used as templates for the solution growth of ZnO NWs. The resultant ZnO NW micropatterns were characterized by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, and photoluminescence spectrometry. Room-temperature photoluminescence spectra indicate that the micropatterned ZnO NWs show greatly enhanced near-band-edge emission and have potential as high-efficiency blue or near-UV light emitters. This facile and versatile approach is further demonstrated by templating biomimetic hydroxyapatite and silver nanoparticles on polydopamine-coated substrates. This work provides an alternative route to fabricating micropatterned functional surfaces at low cost and high efficiency.

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INTRODUCTION Micropatterning is of critical importance for various applications such as biochips and photovoltaic devices.1,2 Until now, micropatterning techniques, including photolithography, electron/ion beam lithography, directed self-assembly of block copolymers, imprinting and soft lithography, have been widely investigated and tremendous progress has been made over recent decades.1−4 A simple and convenient patterning process independent of high-cost facilities would be highly expected for microstructures where pattern periodicity is prior to shape intricacy. As an alternative technique, the breath figure (BF) method provides a simple, robust and efficient bottom-up strategy to fabricate hexagonally ordered two-dimensional films with a pore size in the range of micrometer and submicrometer.5−8 Conceptually, micropatterning based on the BF method can be the hexagonal film itself, which arises from the self-assembly of water droplets in the cooling process of solvent evaporation.9−12 The BF process can also form hierarchical structures with site-specific hydrophilic groups or nanoparticles induced by the interaction between functional items and the condensed water molecules.13−15 Using BF films as micropatterning templates has also received great attention.16−20 However, such BF films with nonthrough-pore make it mandatory to introduce extra templating or pattern transfer procedures, e.g., poly(dimethylsiloxane)-based soft lithography,16 pyrolyzation of UV cross-linked polymer/functional precursor hybrid films,17,18,20 or combination of sputtercoating and reactive ion etching.19 Therefore, it is still © 2014 American Chemical Society

challenging to develop a facile, versatile, and one-step BF templating procedure for microfabrication of advanced functional materials. In this work, we demonstrate a versatile, straightforward, and nonlithographic method to fabricate micropatterns of nanostructures. BF films with through-pore structures were prepared from a block copolymer at an air/ice interface and applied as templates for micropatterning on silicon substrates. By replicating the ordering of the templates, ZnO NW micropatterns were obtained via confined solution growth, showing a significant intensity enhancement of near-band-edge exciton emission. The robustness of the proposed method was further confirmed by templating the growth of hydroxyapatite and silver nanoparticles. The key to the success of this straightforward micropatterning method is a modest thermal treatment of the BF films on the substrates, which is ease of control and hence quite reproducible. This technique directly uses ordered through-pore BF films as the patterning templates, opening a novel alternative route to the fabrication of micropatterned surfaces.

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EXPERIMENTAL METHODS Materials. The synthesis of the block copolymer, polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) Received: October 9, 2013 Revised: February 6, 2014 Published: February 6, 2014 4403

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(PS247-b-PDMAEMA14, Mn = 27 900 g mol−1, Mw/Mn = 1.24) by atom transfer radical polymerization was described elsewhere.21 Silicon wafer was purchased from Tianjin Semiconductor Material Factory (China). Before use, the Si wafer was cleaned by piranha solution (2:1 mixture of 98% H2SO4 and 30% H2O2) for 15 min, rinsed by deionized water, and blow dried by nitrogen gas. The 3-hydroxytyramine hydrochloride (dopamine) was purchased from Sigma-Aldrich and used as received. Water used in all experiments was deionized and ultrafiltrated to 18.2 MΩ with an ELGA LabWater system. All other reagents were acquired from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification. Preparation of Through-Pore Breath Figure Films. Breath figure (BF) films with through-pore structures were prepared according to our reported procedure.22 Typically, a solution of PS247-b-PDMAEMA14 in carbon disulfide (1 mg mL−1, 50 μL) was cast onto the surface of ice under a humid airflow at room temperature. The relative humidity of the airflow was maintained around 80% by bubbling through distilled water, and the flow rate was controlled via a needle valve and measured by a flow meter. After complete evaporation of the solvent and water (about 1 min), a thin opaque film formed on the ice/water surface and could be easily transferred onto Si wafer or other substrates for further experiments. Confined Hydrothermal Growth of ZnO NWs. First, a cleaned Si wafer was coated with a 50 nm thick ZnO seed layer by magnetron plasma sputter, which is an optimal thickness for the continuous growth of ZnO NWs.23 A piece of as-prepared through-pore BF film was then transferred onto the coated wafer and dried at room temperature. To further enhance the interfacial adhesion between the film and wafer, the wafer with a BF film was pressed between two pieces of microscope slides by a Hoffmann clamp, and kept in an oven at 75 °C in vacuum for 1 h. Before hydrothermal growth of ZnO NWs, the substrate was immersed in ethanol for 10 s to completely replace the air captured in the pores and fully wet the substrate. Typically, an equimolar aqueous solution (10 mL) of zinc nitride (Zn(NO3)2·6H2O, 5 mM) and hexamethylenetetramine (HMTA, 5 mM) was used as the nutrient solution. It is noteworthy that the Si substrate was put face-down floating on the nutrient solution surface by the solution surface tension, thereby protecting any unexpected ZnO precipitates from physical deposition onto the substrate surface, which would inhibit the growth of desired ZnO nanostructures or possibly initiate secondary growth.24 The whole system was kept at 70 °C for 24 h. After the reaction vessel was cooled down to room temperature, the substrate was taken out of the nutrient solution and rinsed in deionized water and isopropyl alcohol for 5 min sequentially to protect the vertical NWs from sweeping down by the surface tension of evaporated liquid droplets.24 Finally, the substrate was dried in air at room temperature, and the upper surface of the BF film was carefully removed by an adhesive tape or solvent such as toluene if necessary. Preparation of ZnO NW Film. The preparation process of ZnO NW film includes the hydrothermal growth of ZnO NWs in the absence of the ordered through-pore template. An Si wafer coated with ZnO seed layer was immersed in ethanol for 10 s in advance, and an equimolar aqueous solution (10 mL) of Zn(NO3)2·6H2O (5 mM) and HMTA (5 mM) was used as the nutrient solution. The Si substrate was put face-down floating on the nutrient solution surface by virtue of the solution surface

tension, and the whole system was kept at 70 °C for 24 h. After the reaction vessel was cooled down to room temperature, the substrate was taken out of the nutrient solution and rinsed in deionized water and isopropyl alcohol for 5 min sequentially. Finally, the substrate was dried in air at room temperature. Fabrication of Hydroxyapatite Micropatterns. Briefly, a cleaned Si substrate was immersed in an aqueous solution of dopamine (2 mg mL−1 in 50 mM Tris buffer, pH = 8.5) for 12 h at room temperature. Then the polydopamine-coated substrate was rinsed in acetone for 5 min, washed with deionized water, and finally dried by nitrogen gas. The transfer and heat pressing treatment of the ordered through-pore film onto the substrate were similar to that of ZnO NW micropatterns. Subsequently, the Si substrate was wetted in ethanol for 10 s and placed into a modified simulated body fluid (mSBF) and incubated at room temperature for 5 d. The composition of mSBF was as follows: NaCl, 141.0 mM; KCl, 4.0 mM; MgSO4, 0.5 mM; MgCl2, 1.0 mM; NaHCO3, 4.2 mM; CaCl2, 2.5 mM; and KH2PO4, 1.0 mM. Finally, the substrate was rinsed with deionized water and dried with nitrogen gas. Formation of Silver Nanoparticle Micropatterns. The precoating of polydopamine, transfer, and heat pressing treatment of through-pore film templates onto a Si substrate were mentioned above. Then the substrate was wetted in ethanol for 10 s and immersed in an aqueous solution of AgNO3 (50 mM) for 24 h under dark. The Ag-coated substrate was rinsed with deionized water and dried at room temperature. Characterization. The surface morphology of the samples was observed using a field emission scanning electron microscope (FE-SEM, 3 kV, S-4800, Hitachi) with energy dispersive X-ray spectrometry (EDX, 20 kV). Samples were precoated with Platinum through an ion sputter prior to imaging, except for the silver nanoparticle arrays. The crystal structures of ZnO NWs were investigated by X-ray diffraction (XRD, X′Pert PRO, PANalytical) and high-resolution transmission electron microscopy (HRTEM, 200 kV, Tecnai G2 F20, FEI). Photoluminescence (PL) characteristics of ZnO NWs were measured by a photoluminescence spectrometer (FLS920, Edinburgh Instruments) with an He−Cd laser (25 mW, 325 nm) and Xe lamp (450 W, 325 nm) as the excitation source, respectively. The average excitation power was 50 W/ cm2 for the focused He−Cd laser and 0.5 W/cm2 for the Xe lamp.

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RESULTS AND DISCUSSION The fabrication process of hexagonal ZnO NW arrays on a Si substrate is schematically illustrated in Figure 1. First, a block copolymer, PS-b-PDMAEMA, was dissolved in carbon disulfide, and 50 μL of the as-prepared solution (1 mg mL−1) was cast at an air/ice interface under a humid airflow (ca. 80% relative humidity). During this process, condensed water droplets self-assemble into hexagonal arrays due to the thermocapillary force and Marangoni convection, and penetrate the polymer solution film reasoned by the excess of the surface tension differential pressure to the critical rupture pressure across the pores.22 After complete evaporation of solvent and water, a hexagonally ordered BF film with through-pore structures is formed and floated on the ice/water surface. The film can be easily transferred onto various substrates such as Si wafer, gold foil, plastic film, and even some nonplanar or 3-dimensional supports, which may introduce an attractive circumvention of the limitations of cracking or fracture of the films directly coated on nonplanar surfaces and hence expand 4404

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heat pressing process,29 a modest and reproducible thermal treatment was performed by heating the BF films on Si substrate at 75 °C, a temperature slightly below the glass transition temperature of PS-b-PDMAEMA, at which the interfacial adhesion is increased while keeping the ordering of the BF film. The through-pore BF templates were then used for fabricating ZnO NW micropatterns by low-temperature hydrothermal growth.30 The vertical growth of ZnO NWs on a Si wafer was assisted by a 50-nm thick layer of polycrystalline ZnO seeds deposited via magnetron plasma sputter. Meanwhile, prior to the hydrothermal growth, the Si substrate with a BF template was quickly prewetted with ethanol to completely replace the air captured in the film pores to allow full penetration of nutrient solution to the substrate surface, as BF films are generally at the Cassie state where wetting by water is resisted.31 Vertically aligned ZnO NW arrays were synthesized in a nutrient solution containing Zn(NO3)2 (5 mM) and HMTA (5 mM) at 70 °C for 24 h. Due to the small diameter and large aspect ratio of the as-grown NWs, the substrate needs a careful drying when the hydrothermal growth is complete. Or else, the surface tension of the droplet may sweep the vertical NWs down onto the substrate.24 In this work, the NWs on substrate were rinsed in deionized water first, and then isopropyl alcohol was used to replace the residual water on the surface of NWs, which has a much smaller surface tension (21.7 mN/m at 20 °C) than water (72.8 mN/m at 20 °C). Therefore, the surface tension of the evaporated droplets was significantly reduced to protect the NWs from falling off the substrate. The micropatterned ZnO NWs were imaged by SEM (Figure 2c,d). The interfacial adhesion between the film and wafer after thermal treatment is strong enough to prohibit the nutrient solution from diffusing into the interfacial areas of the template and substrate, providing a large number of patterned hexagonal cells (Supporting Information, SI, Figure S1, ) for the growth of ZnO NWs. Moreover, the length of grown NWs is below the thickness of the through-pore templates, allowing the same lateral dimension defined by the holes.24 Consequently, the growth of ZnO NWs is confined in the pores of the BF template, resulting in uniformly patterned ZnO NW arrays in a large scale of at least 1.5 × 1.5 cm2 (Figure 2d inset). A further removal of the upper surface of the honeycomb template simply using an adhesive tape or solvent (e.g., toluene) gives a micropatterned ZnO NW surface replicating the hexagonal pore structures on Si substrate (Figure 2e,f). Multiple NWs with the diameter ranging from 85 to 180 nm grew out of one single spot (Figure 2f, inset), attributed to a considerably larger size of the patterned openings compared to the scale of ZnO seed grains.24 The polydispersity of the ZnO NW diameters is probably ascribed to the coalescence effect of the NWs in close proximity.32,33 Additionally, the morphology of ZnO NWs can be tuned by varying nutrient solution concentration and growth time (SI Figures S2 and S3). Multiple ZnO NWs are inclined to merge together to form thicker NWs when increasing the nutrient solution concentration from 5 mM to 100 mM. The growth is initiated homogeneously within the pores, and the density of vertically aligned NWs increases as the reaction time is elongated from 4 to 24 h. The hexagonal contours of ZnO NW arrays are very consistent with and dependent on the BF templating film. Therefore, the scale of ZnO NW arrays can be facilely modulated in the range of 2−8 μm by tuning the period of BF

Figure 1. Schematics of the fabrication process of ZnO NW arrays using an ordered through-pore film as the template. (a) Si substrate, (b) ZnO seed layer coating, (c) transferred porous template, (d) confined growth of ZnO NWs, and (e) micropatterns after removing the template.

the scope of micropatterning based on BF films.25−27 The SEM images of a typical through-pore BF film on a piece of nanofiber mesh28 are shown in Figure 2a,b. Nanofibers underlying the BF

Figure 2. Top-down SEM images of (a, b) a through-pore BF film on a piece of nanofiber mesh. The inset in (b) shows a digital image of the film on a polyethylene terephthalate (PET) support (scale bar: 5 mm). (c, d) ZnO NW arrays templated by a 4-μm-diameter through-pore film. The inset in (d) is a digital image of ZnO NW micropatterns on Si substrate (scale bar: 5 mm). (e, f) ZnO NW arrays after removing the template. The inset in (f) shows a higher magnification of ZnO NWs (scale bar: 500 nm).

film can be clearly seen through the pores. The film displays uniform bright iridescent colors when viewed with a reflected light (Figure 2b, inset), indicative of a relatively large area and perfectly ordered polymeric bubble arrays. It is important to increase the interfacial adhesion between the BF template and the substrate for patterning fidelity. Otherwise, irregular growth of ZnO NWs or even destruction of the template takes place (data not shown). Considering that the adhesion of a polymeric film can be greatly improved by a 4405

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Figure 3. Top view SEM images of ZnO NW arrays templated by through-pore films with tunable pore size prepared at different airflow speeds of (a) 4 L/min (∼2 μm), (b) 2 L/min (∼6 μm), and (c) 1 L/min (∼8 μm).

perpendicularly to the growth direction are clearly visible, and the interplane spacing (0.26 nm) indicates the growth along the ZnO [0001] direction. The associated selected area electron diffraction (SAED) pattern (Figure 4e) further confirms a single crystalline growth along the wire axis. The crystal structure and vertical alignment of the ZnO NWs are revealed by XRD pattern and rocking curve measurements (SI Figure S4). XRD θ-2θ scan of the ZnO NW arrays on the Si substrate reveals only the ZnO [0002] plane, and the θ-rocking curve of the peak at 34.57° exhibits a full width at half-maximum value of 0.10° (SI Figure S4 inset), indicating almost perfect vertical growth on the basal plane of Si substrate. Room-temperature photoluminescence (PL) spectra of ZnO NW arrays, ZnO NW film, and ZnO seed layer were measured using a 325 nm He−Cd laser as the excitation source (Figure 5). The ZnO seed layer exhibits a weak band-edge UV emission

arrays via, for example, an appropriate dynamic control over the airflow speed in the film formation process (Figure 3). Meanwhile, it has been proven that stretching or compressing of a polymer mesh permits BF pore geometries beyond hexagonal, e.g., rectangle, square, or triangle.34 Moreover, BF films formed on an inclined substrate may produce rhombohedral or distorted-square arrangement of holes,5,35 instead of hexagonal symmetry, thus making it possible to provide a feasible and interesting route to the generation of micropatterns with variable fashions. The chemical composition of the micropatterned ZnO NWs was characterized using EDX, as indicated in Figure 4a. The

Figure 5. Room-temperature PL spectra of (a) ZnO NW arrays (templated by a 4-μm-diameter through-pore film), (b) ZnO NW film, and (c) ZnO seed layer.

at 380 nm and two defect-related green peaks at 404 and 458 nm (SI Figure S5), which are commonly observed emissions of defects in ZnO nanostructures.36,37 After solution growth, the micropatterned ZnO NW arrays exhibit a relatively much stronger band-edge UV emission at 375 nm and a weaker broad defect-derived green-yellow peak at 553 nm, whereas ZnO NW film shows a prominent green-yellow emission around 568 nm and an obviously weaker UV peak at 378 nm. In addition, the PL spectrum of ZnO NW arrays was measured under the Xe lamp excitation with an intensity 2 orders of magnitude weaker than the He−Cd laser at room temperature (SI Figure S6). Both the intensities of band-edge UV emission and defectderived peak are attenuated when using the Xe lamp as excitation source. However, the proportion of the above peaks remains similar to that obtained from He−Cd laser excitation, which proves that the defect levels of ZnO NWs are not saturated in our characterization. Therefore, it reveals a higher

Figure 4. (a) EDX spectrum of micropatterned ZnO NWs. (b) Distribution of zinc element (points measured by EDX mapping) on the surface, the inset shows the corresponding SEM image of the examined area. (c−e) TEM images of a single ZnO NW, (c) lowmagnification image, (d) high-magnification image, and (e) corresponding SAED pattern.

NWs are only composed of Zn and O, and signals of C and Pt originate from the templating film residue and ion sputter process prior to the observation, respectively. Mapping of the zinc element displays a well patterned distribution in accordance with hexagonal shapes of the ordered arrays on the Si substrate (Figure 4b). An individual ZnO NW was investigated using HRTEM, as shown in Figure 4c−e. The vertically grown NWs have a typical length of ∼370 nm indicated from Figure 4c. ZnO crystal planes aligned 4406

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the arrays were assisted by polydopamine because of the facile adhesion property and postmodification of polydopamine coating.41,42 Micropatterns of biomimetic hydroxyapatite and silver nanoparticles may find applications in selective osteoblastic cell adhesion43,44 and surface enhancement of Raman scattering,45 respectively. Through-pore BF templates on the Si substrate remain intact after the wet chemical growth of hydroxyapatite and silver nanoparticles for up to 15 days. Due to the excellent stability and durability of the template toward the nutrient solution, this nonlithographic method may provide an alternative and versatile approach for micropatterning in water phase.

optical excitement quality of ZnO NW arrays compared to unpatterned ZnO NW film. The slight blue-shift and significant intensity enhancement in the peak position of the near-bandedge exciton emission probably arise from the charge transfer of electron−hole recombination38,39 and a decrease in defect density due to the formation of micropatterned ZnO NW arrays.40 Furthermore, optical properties of ZnO NW arrays were assessed by low-temperature PL measurements (Figure 6). It is clearly seen that the intensity of the UV band-edge

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CONCLUSIONS A versatile and straightforward nonlithographic method has been developed for the fabrication of micropatterns of ZnO NWs, biomimetic hydroxyapatite, and silver nanoparticles. Selfassembled breath figure films with through-pores can be applied as templates for the micropatterning process in water. This novel micropatterning technique is easy to operate, independent of high-cost facilities, environmentally benign without using a heavy dose of organic solvent, and time saving for the production of micropatterns with an area of up to several square centimeters, which is useful as a novel alternative route to the fabrication of micropatterned functional materials. It has also been demonstrated that micropatterns of ZnO NWs show significant intensity enhancement of near-band-edge exciton emission, which show potential for high-efficiency blue or nearUV light emitters. The micropatterns may also be used in tissue engineering, biochips, and surface enhancement Raman scattering substrates.

Figure 6. PL spectra as a function of temperature from 30 to 298 K for ZnO NW arrays templated by a 4-μm-diameter through-pore film using He−Cd laser as excitation source.

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luminescence significantly increases with the decrease of temperature, while the visible defect-derived peaks are suppressed and finally diminished as lowering temperature. The micropatterned ZnO NW arrays will be rather useful in applications of high-efficiency blue/near-UV light emitters for instance.30 This straightforward technique can also be used for micropatterning other items such as biomolecules and particles. We prepared hydroxyapatite and silver nanoparticles hexagonal arrays on Si substrate to demonstrate the versatility of this nonlithographic micropatterning technique (Figure 7). Both of

ASSOCIATED CONTENT

S Supporting Information *

Tilt view SEM images of ZnO NW arrays, SEM images of ZnO NW micropatterns prepared at various conditions, XRD spectra, and PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87953763; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 51173161 and 21374100). The authors thank Mr. C. Shang at Zhejiang University for assistance in shooting the digital photographs.

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Figure 7. SEM images of hydroxyapatite (a, b) and silver nanoparticles (c, d) hexagonal arrays, templated by a 6-μm-diameter through-pore film. 4407

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