SUPPLEMENTARY INFORMATION Structural colour printing using a magnetically tunable and lithographically fixable photonic crystal Hyoki Kim 1, Jianping Ge 2, Junhoi Kim 1, Sung-eun Choi 1, Hosuk Lee 1, Howon Lee 1, Wook Park 1, Yadong Yin 2, Sunghoon Kwon 1 1 School of Electrical Engineering and Computer Science, Seoul National University San 56-1, Shillim 9-dong, Gwanak-ku, Seoul 151-744, SOUTH KOREA 2 Department of Chemistry, University of California Riverside, CA 92521, USA Section S1: Material & Experimental Setup M-Ink preparation M-Ink is three phase mixture composed of superparamagnetic CNCs, solvation liquid and photocurable resin. Superparamagnetic CNCs were synthesized based on a high-temperature hydrolysis reaction followed by a modified Stöber process 1. Synthesized superparamagnetic CNCs are initially dispersed in ethyl alcohol. This CNCs solution was collected by magnetic separation, and re-dispersed in photocurable resin without complete desiccation of ethanol. Remnant ethyl alcohol adsorbed on the surface of CNCs is used as a solvation liquid. Mixture of CNCs, solvation layer and photocurable resin was vortexed for 5min. This material preparation process is illustrated in Figure S1. If ethyl alcohol is fully desiccated on performing solvent exchange from ethanol to photocurable resin, solvation layer cannot be formed on the surface of CNCs so that CNCs are aggregate each other 2. Thus CNCs do not show its unique diffraction nature photonics www.nature.com/naturephotonics 1
supplementary information property under external magnetic field. We used poly(ethylene glycol) diacrylate (PEG-DA, Sigma-Aldrich, M n =258) with 15 wt% of photoinitiator (2,2-dimethoxy-2-phenylacetophenone, Sigma-Aldrich) as the photocurable resin. Figure S1. Process of material preparation. M-Ink is 3-phase system: superparamagnetic CNCs, photocurable resin, solvation liquid. Synthesized superparamagnetic CNCs are magnetically separated from the ethyl alcohol, and dispersed in photocurable resin without full dessication of remnant ethanol which plays a part of solvation liquid. Printing substrate Structural colour printing is performed on two layered substrate: elastic PEG film, glass slide (Figure S2). Elastic PEG layer on the glass slide was made by deposition of poly(ethylene glycol) diacrylate (PEG-DA, Sigma-Aldrigh, M n =575) with 15 wt% of photoinitiator (2, 2-dimethoxy-2- phenylacetophenone) on the glass slide, and photopolymerization of the prepolymer with UV light for 5 sec. M-Ink is deposited on this two layered substrate, and successive colour tuning and 2 nature photonics www.nature.com/naturephotonics
supplementary information fixing is performed as illustrated in Figure 1 in main manuscript. This substrate preparation process is illustrated in Figure S2. PEG layer prevents aggregation of CNCs on the bare glass surface. Figure S2. Substrate preparation. Two layered substrate is prepared before structural colour printing. Experimental setup: Magnetic actuation, Maskless lithography NdFeB permanent magnet was used to generate magnetic field which was attached to controlling stage at the maskless lithography system. Magnetic intensity profile was measured with gaussmeter (455 DSP Gaussmeter, Lakeshore). Typical range of magnetic field intensity for colour tuning is from 100 Gauss to 800 Gauss. Photopolymerization setup was based on maskless lithography using DMD spatial light modulator 3. Optical microscope (IX71, Olympus), UV source (200W, mercury-xenon lamp, Hamamatsu) and digital mirror device (DMD, Texas Instrument) was aligned for photopolymerization. Exposure pattern of UV light was controlled by digital micromirror array (DMD, Texas Instrument) with self-designed computer program which synchronize magnetic field actuation, pattern of DMD and UV exposure (Figure S2). DMD based maskless lithography enables instant immobilization of magnetically self-assembled photonic nanostructure and high resolution patterning of colours from the structure. nature photonics www.nature.com/naturephotonics 3
supplementary information Figure S3. Experimental setup. UV light reflects from the spatial light modulator (DMD) whose pattern is dynamically controllable. Patterned UV light is scaled down when it passes through objective lens. (a) Measured magnetic field profile. (b) Loaded mask pattern to spatial light modulator. (c) Optical micrograph of patterned structural colour corresponded to mask pattern. Optical characterization Optical micrographs were acquired by true-colour charge coupled device (CCD) camera (DP71, Olympus) which is directly aligned to the inverted microscope (IX71, Olympus). Spectrum data 4 nature photonics www.nature.com/naturephotonics
supplementary information was acquired by spectrometer (Acton, Princeton Instrument) which is connected to the inverted microscope (Eclipse Ti, Nikon). Built-in field stop shutter in the spectrometer was used for isolating optical signal from background noise and other neighboring structures. Figure 3-(d), Figure 3(h), and Figure 4-(a), (b), (f), (h) were obtained with the commercially available digital camera (IXUS 870 IS, Canon). nature photonics www.nature.com/naturephotonics 5
supplementary information Section S2: Characterization of UV dose dependent structural variations UV dose dependent spectra shift We investigated short time photopolymerization characteristics using this novel material and special instrumentation as an experimental window on the discovery of nanoscale nature of polymerization kinetics at the optical regime. We observed spectra blue shift as increasing dose of UV light exposed to fix chain-like photonic nanostructure. This implies that more UV energy induces denser polymer network, which pull each of the superparamagnetic CNCs so that interparticle distance decreases, as expected with Bragg theory, m λ = 2nd sinθ, where m stands for order of scattering, λ for diffraction wavelength, n for refractive index of medium, d for interparticle distance, and θ for angle between incident light and axis of chain-like ordering of superparamagnetic CNCs. Also, spectra variation saturates as increasing UV dose, which implies that the crosslinked polymer network is fully cured (Figure S4). To quantify shrinkage of polymer network, we measured variation of spectra of two different coloured structures, each of which is produced under different magnetic field intensity (284 Gauss, 446 Gauss). Interparticle distance of chain-like photonic nanostructure fabricated under stronger magnetic field intensity is smaller than that of the structure fabricated under weaker magnetic field. In our case, measured spectra shift to the shorter wavelength is Δλ 1 ~ 19.9nm for the structure generated under 284 Gauss, and spectra shift of the structure generated under 446 Gauss is Δλ 2 ~ 17.7nm. Approximate calculation shows that shrinkage of interparticle distances are Δd 1 ~ 6.6nm for the structure fabricated under 284 Gauss, Δd 2 ~ 5.9nm for the structure fabricated under 446 Gauss (Figure S4). 6 nature photonics www.nature.com/naturephotonics
supplementary information For perfectly aligned CNCs in a given volume with interparticle distance with d, illuminated by incident light parallel to the axis of chain-like photonic nanostructure, and taken account of first order Bragg diffraction ( 2 nδd = Δλ ), then volumetric shrinkage of polymer structure can be estimated as follows: Δ V = V 3 3 3 3 1 V2 V1 ( V1 NΔd) = V1 ( V1 NΔλ / 2n), where N stands for number of CNCs in a chain, n for refractive index of medium, Δd for shrinkage of interparticle distance given by Δ d = d d ( 3 V 3 V )/ N, V 1 for volume of polymer before 1 2 1 2 shrinkage, V 2 for volume of polymer after shrinkage. Preservation of photonic nanostructure in polymer network Polymer network which is not fully cured usually shrinks when it is dried. Thus, desiccation of prepolymer liquid leads to distortion of chain-like photonic nanostructure which results in quenching the diffracted light. However, we observed that strong UV exposure solidifies the polymer matrix denser and retains chainlike ordered CNCs structure in the polymer network when fully dried. First, to verify preservation of photonic nanostructure, we generated two identical coloured structures under same magnetic field intensity with same UV energy. Then we removed remnant M-Ink, and immersed the two coloured structures in prepolymer solution, PEG-DA, Mw: 258, as illustrated in Figure S5-(a). Secondly, additional UV light was exposed to the one (structure on the right side of Figure S5 (a)) for 500ms, and additional UV exposure was not applied to the other structure (left). In this step, spectra blue shift at the particle with additional UV exposure was occurred because of the shrinkage of polymer network, which is described in Figure S4. Finally, we removed prepolymer solution with ethanol and fully dried in air. While colour from the structure without additional UV quenched, colour from structure with nature photonics www.nature.com/naturephotonics 7
supplementary information additional UV remained, which implies that additional UV exposure makes polymer network denser and photonic nanostructure in a fully cured polymer network does not suffer from distortion of its structure when dried, and therefore retains colour. Figure S4. UV dose dependent spectra shift. (a) Schematic illustration: Increase of UV dose further densifies the polymer network, shrinks the interparticle separation, and leads to blue shift of the spectra. (b) Spectral data of coloured structure produced under 284 Gauss as increasing UV exposure. (c) Spectral data of coloured structure produced under 446 Gauss as increasing UV exposure. (d) Plot of peak wavelength as increasing UV. (e) Plot of peak intensity as increasing UV. 8 nature photonics www.nature.com/naturephotonics
supplementary information Figure S5. Preservation of photonic nanostructure in polymer matrix. (a) Two identical structures produced under same magnetic field intensity with same UV exposure. (b) Additional UV expose to right side of the structural colour. No additional UV was applied to the left one. Spectra blue shift was occurred, or greener, due to decrease of interparticle distance resulted from the densification of polymer network. (c) Two samples were dried after removal of prepolymer PEG-DA with ethanol. While structural colour with additional UV exposure retains colour when fully dried, structural colour without additional UV exposure quenches since the shrinkage of polymer network which pulls each of CNCs to aggregate. nature photonics www.nature.com/naturephotonics 9
supplementary information Section S3: Transmission micrograph of demonstrated structural colour pattern Figure S6. Reflection micrograph and related transmission micrograph. Image reconstructed from structural colour shows unique transmission/reflection characteristic, quite different from chemical dye or pigment, which does verify the formation of structural colour. 10 nature photonics www.nature.com/naturephotonics
supplementary information Section S4: Additional spectral data of spatial colour mixing As same analogy with the spatial colour mixing technique in conventional dye/pigment printing, reflected wavelength can be modulated by distribution of various structural colour dots whose size is smaller than human eye s resolution. Spectra of various colour dot arrays demonstrated in this work can be seen in Figure S7. Figure S7. Spectral data of different colour dot arrays. Green lines stands for spectra of colour dot located at (1,1) of 4ⅹ4 dot array. Orange lines stands for spectra of colour dot at (1,2), gray lines for mathematical addition of green and orange, blue lines for spectra of area including colour dots, (1,1), and (1,2). Insets are micrographs of selected dot arrays at Figure 3-(f) in main manuscript. nature photonics www.nature.com/naturephotonics 11
supplementary information Section S5: Fabrication of flexible ultrathin film photonic crystal As illustrated in Figure S8, key idea to fabricate mechanically flexible photonic crystal film is peeling off elastic membrane where patterned photonic crystal structures are immobilized. For the elastic membrane, we used PEG layered glass slide as a printing substrate (Figure S2 in Section S1). M-Ink is deposited on the elastic membrane and produce artificial structural color by sequential color tuning and fixing process. Insufficiently cured polymer network usually shrinks when it is dried, which results in distortion of chain-like photonic nanostructure, thus quenches the diffracted light (Figure S5). Also, complete curing by long time UV exposure cannot guarantee the fidelity of high resolution pattern due to free-radical diffusion 4. We overcome this trade-off by two step curing process as demonstrated in Figure S8. First we produce high-resolution feature with instantaneous immobilization (Figure S8 (a)-(c)) and washout remnant M-ink with photocurable prepolymer (PEG-DA, Mw: 258). During washing out, prepolymer molecules diffuse into the pre-cured network. Then, we expose UV for complete curing the pre-cured feature so that polymer network fully densifies, which preserves periodic arrangement of photonic nanostructure when desiccated. Thus it stably retains colour from the structures (Figure S8 (d)-(f)). Since chain-like CNCs photonic nanostructures are immobilized in the polymer network on the elastic PEG layer, where structurally coloured features are covalently bonded with the PEG layer, we can peel off the features from the glass slide (Figure S8 (g)). Mechanical property of PEG can be found in the previous publication 5. 12 nature photonics www.nature.com/naturephotonics
supplementary information Figure S8. Schematic illustration of flexible photonic crystal thin film for artificial structural colour. (a) Deposition of M-Ink whose color is magnetically tunable and lithographically fixable on the PEG layer. (b)-(c) Artificial structural color patterning using sequential colour tuning and fixing process. (d)-(f) Prevention of photonic nanostructure when dried by additional strong UV exposure. (g)-(i) Peel off the photonic crystal film from the glass slide, then transfer to arbitrary flexible substrate. nature photonics www.nature.com/naturephotonics 13
supplementary information Section S6: Spectra variation: angular relationship between observer, angle of incident light, and axis of chain A chain-like ordered CNCs photonic nanostructure is a colouration unit. Due to the structural origin, spectra variation occurs along with angular relationship of various parameters: position of observer, angle of incident of light, and axis of chain (Figure S9). We investigated the angular dependency of spectra variation. First, if the magnetic field is applied on the skew to the substrate during the colour fixing process of M-Ink, then the particle chain is also skewed with respect to the substrate (Figure S10 (a)). The variation of the angle of the particle chain changes observed colour as shown in Figure S10 (b). In addition to the tilt angle of the chain, the angle between incident light and axis of chain also determines colour seen by observer (Figure S10 (c)). We illuminated light to the sample with increasing incident angle, angle between the axis of chain and incident light, and observed the spectra blue shift (Figure S10 (d)). All of these spectra shift occur along with various angular relations that can be explained by simple physical model, modified Bragg model. Optical path difference is given by nδ d = n( Δd + Δd2) = nd(cosθ + cos( θ + θ )), where n is refractive index of medium, d is 1 t t i total path difference, d is interparticle distance between CNCs, θ t is angle between axis of observer and axis of tilted chain, and θ i is angle between axis of observer and incident light. Corresponding spectra peak is be given by mλ = nd(cosθ + cos( θ + θ )), where m stands for order of diffraction, λ for spectra peak. 2 t t i 14 nature photonics www.nature.com/naturephotonics
supplementary information Figure S9. Experimental setup and schematic illustration of chain-like scatterer and angular relationship. (a) Experimental setup for measurement of spectra shift by anglular relation. (b) Spectra variation occurs with regards to various parameters: position of observer, incident light, and tilt of chains. nature photonics www.nature.com/naturephotonics 15
supplementary information Figure S10. Spectra variation with respect to the angular relationships. (a) Schematic illustration of skewed chain structure and spectra shift. (b) Optical micrograph of structural colour features with gradual skew of chain. Diffracted colour shifts to the shorter wavelength with gradual tilt of external magnetic field when fixing the colour. (c) Schematic illustration of spectra variation with angle of incidence. (d) Incident light dependent colour shift from fabricated structural colour film. 16 nature photonics www.nature.com/naturephotonics
supplementary information Section S7: Mechanical flexibility and related spectra variation Figure S11. Spectra variation with increasing curvature. (a) Increase of curvature of the photonic crystal film and spectra blue shift (optical image). Inset shows the cross section of the film. (b) When curvature increases, angular relationship between chain-like scatterer and incident light changes thus result in spectra variation. (c) Measured diffraction peak values of the film (Top: peak wavelength vs viewing angle, Down: peak intensity vs viewing angle). nature photonics www.nature.com/naturephotonics 17
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