Supporting Information

Transcription

1 Supporting Information Chameleon-Inspired Mechanochromic Photonic Films Composed of Nonclose-Packed Colloidal Arrays Gun Ho Lee, Tae Min Choi, Bomi Kim, Sang Hoon Han, Jung Min Lee, and Shin-Hyun Kim *, Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea The 4th R&D Institute, Agency for Defense Development, Daejeon 34060, Republic of Korea * To whom correspondence should be addressed. kim.sh@kaist.ac.kr

2 Supplementary Figures Figure S1. Fabrication procedures for the mechanochromic photonic films. The photocurable dispersion of silica particles in poly(ethylene glycol) phenyl ether acrylate (PEGPEA) is infiltrated into a gap between two parallel glasses separated by 50 µm with spacers. The dispersion is irradiated by ultraviolet (UV) to photopolymerize PEGPEA. The resulting film is released from glasses, which is then subjected to reactive ion etching (RIE) with SF 6 gas to reduce surface tension; both sides of the film are hydrophobized. Reflectance (%) Transmittance (%) Wavelength (nm) Figure S2. The optical property of photonic films. Reflectance and transmittance spectra of photonic films composed of silica particles with diameter, d, of 209 nm in volume fraction, ϕ, of 0.33.

3 A volume fraction of spheres in nonclose-packed fcc lattice, ϕ, can be calculated by considering a unit cell which contains 4 spheres:, (1) where d is the diameter of spheres and a is lattice constant. The center-to-center distance between two nearest neighbors, d cc, is same to / 2 from the geometric consideration on (111) plane. Therefore, the volume fraction can be written with d cc :, (2) where 2t is a surface-to-surface separation between two nearest neighbors. For nonclosepacked fcc lattice composed of silica particles with a diameter of d = 150 nm and thickness of solvation layer of t = 36.5 nm, ϕ is calculated as from eq 2, which is assigned as threshold volume fraction for crystallization, ϕ th. Assuming that the thickness of solvation layer remains unchanged with particle diameter, ϕ th can be expressed as a function of the diameter:, [3] where d has a unit of nm. The values of ϕ th are for d = 148 nm, for d = 166 nm, for d = 175 nm, for d = 195 nm and for d = 216 nm. Figure S3. Threshold volume fraction for crystallization. (a) The unit cell of nonclose-packed fcc lattice. (b) A cartoon showing two nearest neighbors separated by solvation layers.

4 Figure S4. Influence of particle diameter on the color of the photonic film. (a, b) A set of optical microscope (OM) images (a) and reflectance spectra (b) of photonic films prepared from photocurable dispersions of silica particles with d = 148, 166, 175, 195, and 216 nm at ϕ = (c) Diameter dependence of λ max. The black line indicates Bragg s equation for staked (111) planes of nonclose-packed fcc structures. Figure S5. The elastic modulus of the photonic film. (a) Stress-strain curves of silica-free poly(pegpea) film (black) and composite photonic film (red). (b) Elastic moduli of silica-free poly(pegpea) film and composite photonic film. The values are obtained from the average slope of stress-strain curves.

5 Figure S6. Surface modification of photonic films. (a) Optical microscope (OM) images of pristine (left) and SF 6 -treated (right) photonic films taken in reflection mode. (b) Scanning electron microscope (SEM) image showing the surface of the SF 6 -treated film. (c) Reflectance spectra of pristine (red) and SF 6 -treated photonic films (black). There is the negligible effect of the surface treatment on color and spectrum. (d) Image showing a contact angle (CA) of water drop on pristine film (CA = 67.7⁰). (e) Image showing CA of water drop on SF 6 - treated film, where CA is measured after 0, 1, and 7 days. The value of CA decreases as the effect of surface treatment slowly fades out.

6 Figure S7. Reversibility and thermal stability. (a, b) Reflectance spectra (a) and OM images (b) of the photonic films at stress-free initial state, stretched to ε = 0.4, stored at ε = 0.4 for 3 days at 25⁰, and recovered to ε = 0. (c, d) The same set for the film stretched at 70⁰. (e, f) The same set for the film stretched at -20⁰. At all three temperatures, the photonic films fully recover original color and reflectance spectrum when they are released from extensional stress. b

7 Figure S8. Long-term stability of the photonic films. (a, b) Reflectance spectra (a) and OM images (b) of the photonic films after 0 day and 180 days. No change in spectra and color indicates that the photocurable dispersion and photonic film contain negligibly small amount of volatile solvent. Figure S9. Colloidal arrangement in the photonic film. (a, b) Lattice model and SEM image showing the cross-section of photonic films at ε = 0% (a) and ε = 41% (b). The regular hexagonal arrangement and elongated hexagonal arrangement are marked in SEM images of (a) and (b), respectively.

8 Figure S10. Estimation of refractive-index contrast from lattice models. (a) Lattice models showing deformation of fcc structure for the extensional strains of ε = 0, 21.0, 41.2, and 60.8% from the leftmost. The images in the top row show four (111) planes stacked, where slices containing (111) plane are labeled red and intermediate slices are labeled blue. The images in the middle and bottom rows show the slice containing (111) plane and the intermediate slice, respectively. Three small images correspond to the top, middle, and bottom cuts of each slice. (b) Effective refractive indices as a function of extensional strain for the slice containing (111) plane (red squares) and the intermediate slice. Because the volume fraction of silica particles in the slice containing (111) plane decreases, whereas that in the slice containing intermediate plane increases, the index contrast decreases as the strain increases. The refractive indices are calculated by Maxwell-Garnett average.

9 Figure S11. In-situ visualization of strain around the elbow. (a, b) Photonic film attached to the skin: The film on folded elbow shows red (a) and that on unfolded elbow shows sky blue (b), where the film is attached to the skin when the elbow is folded. Supplementary Movies Movie S1: Reflection-color change (first half) and transmission-color change (second half) of a photonic film with extensional strain. Movie S2: A sticky photonic film without surface modification (first half) and non-adhesive photonic film whose surfaces are fluorinated by reactive ion etching (second half) with SF 6. Movie S3: Reflection-color change of a photonic film along with extensional strain and its recovery under observation with an optical microscope (first half) and reflectance-spectrum change and its recovery (second half). Movie S4: Development of dual-color patterns by compression of a photonic film with a macroscopic stamp (K pattern; first half) and a microscopic stamp (line pattern; second half). Movie S5: Photonic film on a skin around an elbow, where the film shows reversible color change with folding and unfolding the elbow.