structural colour printing using a magnetically tunable ......structural colour printing using a...
TRANSCRIPT
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
nature photonics | www.nature.com/naturephotonics 1
1
SUPPLEMENTARY INFORMATION
Structural colour printing using a magnetically tunable and lithographically
fixable photonic crystal
Hyoki Kim1, Jianping Ge2, Junhoi Kim1, Sung-eun Choi1, Hosuk Lee1, Howon Lee1, Wook Park1,
Yadong Yin2, Sunghoon Kwon1
1School of Electrical Engineering and Computer Science, Seoul National University
San 56-1, Shillim 9-dong, Gwanak-ku, Seoul 151- 744, SOUTH KOREA
2Department 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 process1. 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 other2. Thus CNCs do not show its unique diffraction
© 2009 Macmillan Publishers Limited. All rights reserved.
2 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
2
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
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 3
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
3
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 modulator3. 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.
© 2009 Macmillan Publishers Limited. All rights reserved.
4 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
4
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
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 5
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
5
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).
© 2009 Macmillan Publishers Limited. All rights reserved.
6 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
6
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, θλ sin2ndm = , 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 Δλ1 ~ 19.9nm for
the structure generated under 284 Gauss, and spectra shift of the structure generated under 446
Gauss is Δλ2 ~ 17.7nm. Approximate calculation shows that shrinkage of interparticle distances
are Δd1 ~ 6.6nm for the structure fabricated under 284 Gauss, Δd2 ~ 5.9nm for the structure
fabricated under 446 Gauss (Figure S4).
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 7
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
7
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 ( λΔ=Δdn2 ), then volumetric shrinkage of polymer structure can be
estimated as follows: 3311
331121 )2/()( nNVVdNVVVVV λΔ−−=Δ−−≈−=Δ , where N
stands for number of CNCs in a chain, n for refractive index of medium, Δd for shrinkage of
interparticle distance given by NVVddd /)( 32
3121 −≈−=Δ , V1 for volume of polymer before
shrinkage, V2 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
© 2009 Macmillan Publishers Limited. All rights reserved.
8 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
8
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 9
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
9
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
10 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
10
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 11
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
11
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
12 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
12
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
be found in the previous publication5.
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 13
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
13
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
14 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
14
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
))cos((cos)( 21 ittndddndn θθθ ++=Δ+Δ=Δ , where n is refractive index of medium, ∆d is
total path difference, d is interparticle distance between CNCs, θt is angle between axis of
observer and axis of tilted chain, and θi is angle between axis of observer and incident light.
Corresponding spectra peak is be given by ))cos((cos2 ittndm θθθλ ++= , where m stands for
order of diffraction, λ for spectra peak.
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 15
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
15
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
16 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
16
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.
© 2009 Macmillan Publishers Limited. All rights reserved.
nature photonics | www.nature.com/naturephotonics 17
SUPPLEMENTARY INFORMATIONdoi: 10.1038/nphoton.2009.141
17
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).
© 2009 Macmillan Publishers Limited. All rights reserved.
18 nature photonics | www.nature.com/naturephotonics
SUPPLEMENTARY INFORMATION doi: 10.1038/nphoton.2009.141
18
REFERENCE 1. Ge, J., Yin, Y. Magnetically tunable colloidal photonic structures in alkanol solutions. Adv.
Mater. 20, 3485-3491 (2008).
2. Raghavan, S. R., Walls, H. J. & Khan, S. A. Rheology of silica dispersions in organic liquids:
new evidence for solvation forces dictated by hydrogen bonding. Langmuir 16, 7920-7930
(2000).
3. Chung, S. E. et al. Optofluidic maskless lithography system for real-time synthesis of
photopolymerized microstructures in microfluidic channels. Appl. Phys. Lett. 91, 041106 (2007).
4. Panda, P. et al. Stop-flow lithography to generate cell-laden microgel particles. Lab Chip 8,
1056-1061 (2008).
5. Kim, P., Suh, K. Y. Rigiflex, Spontaneously wettable polymeric mold for forming reversibly
bonded nanocapillaries. Langmuir 23, 4549-4553 (2007)
© 2009 Macmillan Publishers Limited. All rights reserved.