Structural colour printing using a magnetically tunable ......Structural colour printing using a magnetically tunable and lithographically fixable photonic crystal ... UV source (200W,
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property under external magnetic field. We used poly(ethylene glycol) diacrylate (PEG-DA,
Sigma-Aldrich, Mn=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, Mn=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
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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
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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.
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.
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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.
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.
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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 diffusion4. 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
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.
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.
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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.
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).