Supporting Information

Transcription

1 Supporting Information Fabrication of Free-Standing, Self-aligned, High-Aspect-Ratio Synthetic Ommatidia Brian M. Jun, Francesca Serra, Yu Xia, Hong Suk Kang, and Shu Yang* Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, USA * S-1

2 Methods Fabrication of microlens arrays (Figure S1). First, we performed replica molding of a square array of PDMS porous membrane (diameter of 10 µm, height of 10 µm) to photoresist SU-8 (SU , Microchem) at 95 o C. After peeling the SU-8 micropillar array from PDMS mold, the sample underwent thermal reflow at 55 C for 25 s, using a method similar to the literature 1-2, to form hemispherical lenses with a radius (r) of ~ 7.5 µm and height (h) of ~ 8 µm. The microlens array was then exposed to UV light (97435 Oriel Flood Exposure Source, Newport) at an intensity of 54 mw/cm 2 until fully cured. The pattern was replicated to PDMS with Sylgard 184 (Dow Corning) and curing agent in 10:1 weight ratio twice and cured at 65 o C for 1 h each time to get a positive microlens template, each of which was treated with (tridecafluoro-1,1,2,2- tetrahydrooctyl) dimethylchlorosilane (Gelest), referred as fluorosilane, to facilitate the separation the replica from the mold. The positive microlens template was then placed on a glass covered with a thin layer of PDMS, followed by heating at 65 C for 30 min and 160 C for 10 min to completely crosslink the PDMS layer sandwiched between the substrate and the template. The positive microlens template was peeled off, leaving behind a negative microlens template on glass for fabrication of the waveguides. Fabrication of waveguides (Figure 1). The glass slide with the negative PDMS microlens array is treated with UV ozonolysis (Jelight UVO Cleaner, Model 144AX) for 1 h and immediately put in a solution consisting of poly(vinyl alcohol) (PVA, Sigma Aldrich), oxalic acid (Sigma Aldrich), and tap water in a 1:10:500 mass ratio. After drying the microlens array, SU-8 was spin coated (Cee 200X, Brewer Science) on top of it at a ramp rate of 500 rpm and spinning speed at 4000 rpm for 30 s. The spin coated SU-8 film was first soft baked at 55 C for 2 h to minimize bubble formation on the surfaced treated template, followed by baking at 95 C for 15 min to S-2

3 completely remove the solvent, cyclopentanone. To fabricate longer waveguides while avoiding the formation of unwanted bubbles in preparation of thicker films, the samples were spin coated with SU-8 solution multiple times. First, SU-8 solution was spin coated at 4000 rpm. After drying, a new layer of SU-8 solution was spin coated on top at a speed of 2300 or 4000 rpm for 30 s, followed by soft-baking at 95 C for 15 min. The entire setup was then exposed to 365 nm UV light from the glass substrate side such that the light went through the microlens array first before reaching the SU-8 photoresist. The dosages were 35 mj cm -2 for sample spin coated only once at 4000 rpm (45 µm thick, sample A), 72 mj cm -2 for sample spin coated at 4000 rpm twice (70 µm thick, sample B), and 84.5 mj cm -2 for sample spin coated at 4000 rpm/s first then at 2300 rpm (85 µm, sample C). The SU-8 was post-exposure baked at 95 C for 5 min, developed in propylene glycol monomethyl ether acetate (PGMEA) (Sigma Aldrich) for 30 min, and then exposed to 365 nm UV light at a dosage of 35 mj cm -2 for 30 min to fully cure the waveguides while submerged in PGMEA. Then the sample was rinsed with isopropanol (IPA, Fisher Scientific) and left in a 1% wt PVA solution in water for 1 h. After briefly rinsing with ethanol (Fisher Scientific), the sample was submerged in a 1.5 cm-thick mixture of diethyl ether, PDMS and curing agent. Diethyl ether was evaporated slowly at increased temperature (room temperature, 6h; 35 C, 1 h; 40 C, 1h). The remaining PDMS is then cured at 55 C for 1 h. Additional PDMS with curing agent was poured over the sample and cured at 65 C for 1h. The PDMS layer was then peeled off from the glass with the waveguide. While diethyl ether is a good solvent of PDMS, no swelling of the PDMS layer was observed, possibly due to the surface coating of PVA on SU-8. Characterization. Transmission of light through the synthetic ommatidia was tested under an optical microscope (Olympus BX61) by focusing on the tip of the waveguide and comparing it to S-3

4 the focal point when only microlenses were present. Further characterization of the light-guiding behavior was performed on an Axio-Zeiss optical microscope. Scanning Electron Microscopy (SEM) images for the synthetic ommatidia were obtained from JEOL 7500F by sputtering the samples with 4 nm iridium and imaged at a voltage of 1.00 kv. The microlenses were characterized by an environment SEM (Quanta 600 FEG ESEM), so PDMS sample could be imaged with surface uncoated, with a voltage of 10 kv and a pressure of 0.9 torr. To characterize waveguiding properties, Verdi V6 dioded-pumped laser (Coherent Inc.) with the wavelength of 532 nm was used to illuminate the waveguides from the lens side (incidence angle of 5 o and light intensity of 1 mw/cm 2 ). a PDMS mold Mold b SU-8 Reflow 55 C, 25 s h r 10 μm Figure S1. Fabrication of the SU-8 microlens array. (a) Schematics of the fabrication process. (b) Cross-sectional SEM image of the SU-8 microlens array. Hansen solubility parameter Using the Hansen solubility parameter, the relative energy difference (RED) is calculated as RED = Ra Ro = 4(δ *+ δ *- ) - + (δ 0+ δ 0- ) - + (δ 1+ δ 1- ) - Ro S-4

5 where R a is the distance between Hansen parameters in Hansen space, and R 0 is the interaction radius of the solute. δ *, δ 0, and δ 1 are dispersion, polar, and hydrogen-bonding contributions to solubility parameter, respectively. RED < 1.0 means high affinity between the polymer and the solvent, RED > 1.0 means poor solubility of the polymer in a solvent, and RED = 1.0 means a q condition where the polymer is partially dissolved in a solvent Table S1. Values for δ *, δ 0, δ 1, and R o for the materials used in experiments*. Materials δ * δ 0 δ 1 R o EPON PDMS N/A PVA (BT =20 min) PVA (BT =1 h) PVA (BT =4 h) diethyl ether N/A * The value of Epon 1001 (Shell Chemical Corp.), a bisphenol-a epoxy resin, is used as an approximation of SU All R 0 values were obtained from Hansen 5 except that of EPON 1001 was obtained from Ford et al. 3, and that of PDMS was obtained from Uragami et al. 6 For PVA, the data was taken from literature 7 by measuring the breakthrough time (BT), which is a function of solvent permeability of the polymer. Table S2. Calculated RED values. Pair of materials RED EPON 1001-diethyl ether 1.3 PVA (B t =20 min)-diethyl ether 1.16 PVA (B t =1 h)-diethyl ether 1.52 PVA (B t =4 h)-diethyl ether 1.45 EPON 1001-PDMS 1.42 S-5

6 In calculation of RED values, R o is considered the interaction radius of the polymers. In the case of EPON PDMS pair, since there is no data of R o for PDMS, the RED was calculated using R o of EPON From Table S2, it is clear that EPON and PVA do not have good affinity to diethyl ether. Reference: 1. Peng, C.; Liang, X.; Fu, Z.; Chou, S. Y., High Fidelity Fabrication of Microlens Arrays by Nanoimprint Using Conformal Mold Duplication and Low-Pressure Liquid Material Curing. J. Vac. Sci. Technol. B 2007, 25, Zhang, Y.; Lin, C.-T.; Yang, S., Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU-8. Small 2010, 6, Ford, J.; Marder, S. R.; Yang, S., Growing "Nanofruit" Textures on Photo-Crosslinked SU-8 Surfaces through Layer-by-Layer Grafting of Hyperbranched Poly(Ethyleneimine). Chem. Mat. 2009, 21, Kim, H.-N.; Kang, J.-H.; Jin, W.-M.; Moon, J. H., Surface Modification of 2D/3D SU-8 Patterns with a Swelling-deswelling Method. Soft Matter 2011, 7, Hansen, C. M., Hansen Solubility Parameters: A User's Handbook. CRC Press: Boca Raton, FL, Uragami, T.; Sumida, I.; Miyata, T.; Shiraiwa, T.; Tamura, H.; Yajima, T., Pervaporation Characteristics in Removal of Benzene from Water through Polystyrene- Poly(Dimethylsiloxane) IPN Membranes Mater. Sci. Appl. 2011, 2, Hansen, C. M.; Hansen, K. M., Performance of Protective Clothing: Second Symposium. In ASTM International, Mansdort, S. Z.; Sager, R.; Neilsen, A. P., Eds. 1988, pp 197. S-6