Developing Non-Iridescent Structural Color on Flexible Substrates

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Applications of Polymer, Composite, and Coating Materials

Developing Non-Iridescent Structural Color on Flexible Substrates with High Bending Resistance Mario Echeverri, Anvay Patil, Ming Xiao, Weiyao Li, Matthew D. Shawkey, and Ali Dhinojwala ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04560 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Materials & Interfaces

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Developing Non-Iridescent Structural Color on

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Flexible Substrates with High Bending Resistance

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Mario Echeverri

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Dhinojwala

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†

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§

8 9

‡

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¬

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*

†,¬

, Anvay Patil

†,¬

, Ming Xiao

†,§

†

‡

, Weiyao Li , Matthew D. Shawkey and Ali

*,†

Department of Polymer Science, The University of Akron, Akron, OH, 44325

John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138 Evolution and Optics of Nanostructures Group, Biology Department, University of Ghent, 9000, Ghent, Belgium These authors contributed equally to this work.

Correspondence to: [email protected]

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Keywords: polydopamine, electrophoretic deposition, structural colors, flexible coatings, directassembly

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ABSTRACT

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Nanostructured materials producing structural colors have a great potential in replacing toxic

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metals or organic pigments. Electrophoretic deposition (EPD) is a promising method for

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producing these materials in a large scale, but it requires improvements in brightness, saturation,

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and mechanical stability. Herein we use EPD assembly to co-deposit silica (SiO 2) particles with

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precursors of synthetic melanin, polydopamine (PDA), to produce mechanically robust, wide-

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angle structurally colored coatings. We use spectrophotometry to show that PDA precursors

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enhance saturation of structural colors and nanoscratch testing to demonstrate that they stabilize

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particles within the EPD coatings. Stabilization by PDA precursors allows us to coat flexible

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substrates that can sustain bending stresses, opening an avenue for electro-printing on flexible

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materials.

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INTRODUCTION

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Structural colors produced by nanostructures offer an intriguing approach to color production.

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Unlike organic dyes and pigments,1,2 structural colors are not

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sunlight.3–5 Moreover, they have many applications such as decorations, 6 smart windows,7

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humidity-responsive systems,8 photonic inks9 or pigments,10 and non-toxic colors for

oxidized upon exposure to

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cosmetics.11

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The last two decades have seen an accelerated effort to self-assemble colloidal particles into

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structural colors through many methods such as evaporation-based self assembly10,12–14 or spray

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painting.15–18 Electrophoretic deposition (EPD) is a versatile technique with a high potential for a

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rapid, low-cost, energy-efficient coatings technology. By applying a DC voltage, charged

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particles migrate to the oppositely charged electrode, creating a microns-thick layer of deposited

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particles. By controlling the size of the particles, a pallette of colors can be produced. In addition,

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EPD can also be applied to curved surfaces.6 The faster depostion rate of particles in EPD

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enables to create semi-ordered colloidal arrays,6,19 typically difficult to achieve due to strong

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tendency of monodispersed micron-sized particles to crystallize to photonic opals. 15,20 Initial

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EPD experiments using silica (SiO2) or polystyrene particles21 produced washed-out structural

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colors due to incoherent scattering. Saturation was improved by adding absorbing materials such

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as carbon black.6 Although this strategy produced robust colors on conductive substrate, the

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cohesion of these particles is weak and they tend to quickly flake off of surfaces. Recently, a


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third component (a polycationic polymer) in addition to an absorbing material was added to

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improve the mechanical stability of electrophoretic deposited (EPDed) coatings, 19 but this

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process adds an additional step in their fabrication process.

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Melanin, a ubiquitous natural pigment, offers a robust method to produce saturated colors

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because it can absorb incoherently scattered light,22,23 provide UV protection24,25 and bind

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materials.26–28 Here we report a one-pot strategy of co-depositing SiO 2 particles in conjunction

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with precursors of polydopamine (pre-PDA) (a synthetic mimic of melanin) to fabricate self-

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assembled coatings with improved mechanical strength to withstand bending stresses. We have

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extended this approach to coat on conductive indium tin oxide (ITO)-coated plastic sheets using

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a mixture of in-situ synthesized pre-PDA and SiO2 particles. This methodology allows us to use

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a simple masking approach to develop patterns. Enhanced cohesive and adhesive strength

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enables these coated flexible sheets to be bent without delamination, a critical property for

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coloring flexible materials.

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RESULTS AND DISCUSSION

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Preparation and optical properties of EPDed coatings

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Figure 1A illustrates the EPD process we have used to deposit semi-ordered arrays of negatively

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charged silica particles on the anode using electric field. The optical micrographs and

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photographs of EPDed coatings from SiO2 particles only (SiO2-only) with their corresponding

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reflectance curves for each SiO2 particle size are shown in Figure 1B. The SiO2-only EPDs show

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different structural colors by varying the size of the SiO2 particles – blue (220 nm), green (260

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nm) and red (300 nm). These colors are whitish and pale due to the strong contribution of the

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incoherently scattered light across the entire visible spectrum.


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Inspired by the fact melanin can increase saturation of structural colors, 23 we incorporated

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synthetic melanin (pre-PDA) into the SiO2 suspension for EPD. Polydopamine (PDA)

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polymerization and the final morphology is sensitive to the monomer concentration, pH,

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proportion of ethanol:water mixture, and time.29 By manipulating the above parameters, we

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reached an intermediate stage between an oligomer and a particle . The deposited films from a

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mixture of in-situ synthesized pre-PDA and SiO2 particles (SiO2-prePDA), show vivid structural

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colors to naked eyes (Figure 1C). Their reflectance peak values are lower than SiO2-only films

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due to the strong absorption of incoherently scattered light across the complete visible spectrum

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by the pre-PDA. A comparison plot of reflectance spectra with respect to dopamine monomer

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concentration has been shown in Figure S1A. It is observed that as the dopamine monomer

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concentration is increased, there is a continuous drop in the intensity and the peak reflectance

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values show a red-shift indicating that the coatings are getting darker (Figure S1B). Considering

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the length-scale of the SiO2 particles employed and the thickness of the EPDed coatings formed,

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we hypothesize that the structural color of these EPDs is caused by multiple scattering 11 between

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the SiO2 particles with additional absorption by pre-PDA.

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To evaluate the angle-dependence of EPD films, we performed angle-resolved specular

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reflectance measurements, where the incident angle and the detection angle were simultaneously

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varied from 5° to 35° (with respect to the surface normal) in steps of 10° (Figure 2A). In the

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case of SiO2-only EPDs, we observed shifts in the peak reflectance values with the incident angle

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(Figure 2B), indicating some level of iridescence. However, angle-independent reflectance in

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SiO2-prePDA EPDs (Figure 2C), indicates that they are non-iridescent. The position of the

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spectral peak shifts by less than 10 nm and is independent of the viewing angle for the SiO 2-

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prePDA coating, while for the SiO2-only system it shifts by around 50 nm (Figures 2D and S2).


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Morphology of the EPDs

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Figures 3A and 3B show SEM images of SiO2-only and SiO2-prePDA EPDed films,

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respectively. In contrast to SiO2-only system, the EPD of pre-PDA (along with the nanoparticles)

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impedes the periodicity of the arrangement, inducing strong randomization in the packing of the

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particles, rather than forming small and dispersed crystalline domains as in the former EPDed

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system. Periodic arrangement (long-range order) of SiO2 nanoparticles leads to a bright

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reflectance with angle-dependency due to the Bragg diffraction.30 A random, densely packed

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photonic structure exhibits short-range order and less long-range periodicity, and is characterized

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by circular or ring-shaped 2D fast Fourier transform (FFT) patterns, as shown in the insets of

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Figure 3A (distinct rings with bright spots indicative of semi-ordered colloidal array) and

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Figure 3B (broad halo indicative of a more amorphous system). The effect of dopamine

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monomer concentration on the morphology of EPDed coatings can be seen in Figure S3. With

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increasing concentration, there is a tendency to produce more disordered structures, exhibiting a

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transition from semi-ordering to more disordering. The hallmark of amorphous-like colloidal

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arrays is that they produce non-iridescent structural colors unlike crystalline-like packing

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observed for monodisperse nanoparticles. 31,32

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We analyzed the cross section of EPDed coatings using SEM to characterize the thickness of

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films (Figure 3C). While it is generally necessary to sandwich a sample between an epoxy layer

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and the substrate to protect it from mechanical damage during cross-sectioning, 6 here we

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obtained a clean cut by simply breaking the ITO/glass for SiO2-prePDA system. The stability of

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such EPDs is perceived from the cross-sectional SEM images; the breaking of ITO/glass

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preserved the structure. The SiO2-prePDA coatings on ITO/glass with three different sizes of

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particles: 220, 260 and 300 nm, resulted in 8 ± 1 µm, 12 ± 5 µm and 16 ± 6 µm thick films,


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respectively (Figure S4). All the three films were deposited using the same conditions. The

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statistical study using a Welch’s ANOVA revealed that the cross-section thickness varied

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significantly across the three treatment groups (F2,11.5 = 12.307, P = 0.001). The cross-section

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thickness of the 220 nm SiO2 treatment was significantly lower than that of the 300 nm SiO2

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treatment (Games-Howell, P = 0.002). The 260 nm SiO 2 treatment’s cross-section thickness,

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however, was not significantly different from that of either the 220 nm SiO 2 treatment or the 300

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nm SiO2 treatment (Games-Howell; 260 nm vs. 220 nm: P = 0.103 and 260 nm vs. 300 nm: P =

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0.289). Although not dramatic, it has been observed that particle size affects the thickness of the

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EPD layer for an applied voltage and operation time,33 as can be observed in this case.

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Mechanical characterization of the EPDs

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Figure 4A shows a schematic demonstrating the nanoscratch test (top) along with the profile

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exhibited by the normal force (middle) and by the lateral displacement (bottom) as a function of

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time. When the diamond tip penetrates the layer of EPDs, the force (normal force) rapidly

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increases to a set value, followed by the lateral movement of the tip to push the particles (i.e. a

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scratch), giving the value of the lateral force. The red line indicates when the scratch is

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performed. A scratch is generated when the lateral force is high enough to break the bonding

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between particles and push particles away. The nanoscratch test, one of the most extensively

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used techniques to test the cohesive and adhesive properties of coatings and thin films, 34 is often

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used to quantify the mechanical properties of the EPDed coatings and their mechanical

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robustness. The mean lateral force curves for the three sizes of SiO2 particles in SiO2-only and

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SiO2-prePDA EPDs are shown in Figure S5.

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Figure 4B shows that the maximum lateral forces increase when pre-PDA is incorporated during

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the EPDs: an increase of 20 μN when applied with a normal force of 500 μN (10% increase) and


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an increment of 88 μN (20% increase) for a normal force of 1000 μN. The increase in the

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maximum lateral force for SiO2-prePDA EPDs indicates stronger binding between silica

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particles. Furthermore, the particles exert resistance to the motion of the tip, recorded as the

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friction coefficient of both (SiO2-only and SiO2-prePDA) EPDs, as shown in Figure 4C for a

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normal force of 500 μN and Figure 4D for 1000 μN. These plots clearly indicate the increased

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resistance to the tip motion offered by the incorporation of pre-PDA in the EPDed system. To be

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precise, the mean friction coefficient for SiO2-prePDA coatings are higher than their

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counterparts. Even though some of these numbers appear to be a modest increase for SiO 2-

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prePDA films, the integrity of these coatings is dramatically different during the bending test.

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The nanoscratch results for the other two SiO2 sizes (220 and 300 nm) can be found in the

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Supporting Information (SI) (Figure S6).

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In addition to enhancing the saturation of the structural colors of the EPDs, pre-PDA also binds

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the particles to the substrate and maintains the structural integrity of the EPDed coating.

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Extensive research has attempted to uncover the forces that dictate self- and directed-assembly

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processes,35,36 however, little is known about how to improve the structural integrity of the

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resultant assembled structures, especially EPDed SiO 2 nanoparticle-based coatings. The addition

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of small amounts of dopamine within the system is a step towards solving this problem.

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Although it is difficult to quantify the state of pre-PDA formed during the in-situ reaction, it

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appears to bind to SiO2 particles due to its proclivity for the SiO2 surface. PDA coats SiO2

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substrates37 and nanoparticles38 owing to the dehydration reaction between the silanol groups on

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the SiO2 surface and the catecholic moieties of PDA, resulting in a strong chemical bond.

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Flexible and patternable structural color coatings

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Structural colors are typically fabricated on rigid glass substrates coated with ITO via EPD.

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However, there is a demand of flexible coatings for flexible displays due to the low weight,

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breaking resistance, and adaptability. Here, we can electrophoretically deposit structurally

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colored coating onto flexible ITO/poly (ethylene terephthalate) (PET) substrates. Figure 5A

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shows that pre-PDA improves the structural integrity of the deposition, offering the capacity to

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strongly bend the coating to large extents without damaging it. We bent EPDed films around a

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series of cylinders of different diameters (0.5, 0.4, and 0.25 in.). The SiO 2-only EPDs sloughed

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off its particles at even the largest cylinder size, demonstrating poor substrate – particle and

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particle – particle interactions. In case of SiO2-prePDA coatings, we hypothesize that the pre-

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PDA forms strong bonds with the substrate and among the particles and thereby helps to

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dissipate the bending stresses enhancing both cohesive and adhesive strength. Moreover, the

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dopamine monomer concentration also affects the mechanical properties of EPDed coatings as

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shown in Figure S7. An optimum monomer concentration is essential to achieve desired bending

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resistance without compromising on the optical properties (i.e. brilliance of structural colors –

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Figure S1). The effective adhesion of pre-PDA to many substrates is attributed to its strong

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molecular adhesion through the catechol group.39 Similar behavior occurs regardless of particle

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size (Figure S8).

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Finally, we illustrate the concept of using masking approach to create patterns on PET substrates.

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We used a fluorinated monomer – 1H,1H,2H-perfluoro-1-dodecene (perfluoro) to plasma coat

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defined regions of the substrate to form a hydrophobic pattern. 40 We hypothesize that due to

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weak adhesion between the particles and the fluorinated polymer, the particles are washed away

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during the drying process. This results in the production of the patterns as observed in Figure


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5B. Hence, we have developed a facile way of patterning structural color coatings via the EPD

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process using a cold plasma treatment of the conductive, flexible substrates. These experiments

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illustrate the advantages of using these simple approaches to fabricate large-scale patterns.

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CONCLUSIONS

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We have developed a facile method to enhance the saturation of structural colors and also

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improve the mechanical integrity of particle coatings using EPD. We have EPDed SiO 2

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nanoparticles along with pre-PDA. The use of pre-PDA not only enhances the color saturation of

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the films due to absorption of incoherently scattered light, but also leads to random packing of

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the SiO2 particles to produce angle-independent (non-iridescent) structural colors. In addition,

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we demonstrated that pre-PDA increases mechanical property of the films using nanoscratch

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measurements and fabrication of bending-resistant flexible coatings. While designing such

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systems, it is essential to take into account the effect of dopamine monomer concentration for

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obtaining optimal optical and mechanical properties. This system can be scaled and provides a

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good opportunity to the flexible displays and electronics industry to employ flexible structural

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color coatings for aesthetics, long-lasting colors and potential substitutes to toxic, metal- or

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organic-based pigments. Furthermore, the cold plasma approach provides an easy way to pattern

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flexible PET substrates.

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EXPERIMENTAL METHODS

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Preparation of EPDed coatings and flexible structural colors. EPD of SiO2-only and SiO2-

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prePDA on ITO/glass and ITO/PET has been adapted from previous literature. 6,19 Both

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ITO/glass and ITO/PET were cleaned in ethanol via sonication for 2 h and then blow dried with

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nitrogen gas. The suspension of silica nanoparticles (20 mg/mL) was prepared in 4:1

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ethanol:water mixture, in addition to 70 μL NH4OH solution. For the preparation of SiO2-


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prePDA mixture, the same formulation was used except we added 0.3 mg/(1 mL of 4:1

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ethanol:water mixture) of dopamine monomer, which self-polymerizes to polydopamine chains

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during the 20 h oxidative reaction at room temperature. 29 To study the effect of dopamine

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monomer concentration on the final EPDs, reactions were also carried out using 0.1 and 0.6

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mg/(1 mL of 4:1 ethanol:water mixture) of dopamine monomer. The suspension underwent

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successive color changes from white to yellow and finally to milky brown. The silica

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nanoparticles (three sizes – 220, 260 and 300 nm) were purchased from Superior Silica LLC.

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(SUPSILTM PREMIUM) and ITO/glass was purchased from Nanocs Inc. with a resistance 100

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/sq. To observe the formation of pre-PDA, the oxidative polymerization of dopamine

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monomer was carried out under the same reaction conditions without the silica particles and

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monitored over time via in-situ IR absorption measurements for 20 h (Figure S9). In an attempt

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to comment on the form of the material, the pre-PDA thus formed at the end of 20 h was

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evaluated using light scattering measurements, i.e. dynamic and static (Figure S10). The details

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for the characterization of pre-PDA can be found in SI.

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The directed (electric field-driven) assemblies were carried out on ITO/glass pieces of

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approximately 13 mm length (l) and 7 mm width (w) and for ITO/PET, the size of the pieces

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were 15 mm l x 10 mm w. For both, SiO2-only and SiO2-prePDA EPDs, the voltage was kept

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constant at 10 V for the assembly run of 5 mins. The current dropped during the process from

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750 mA to 100 mA. The equipment used was a DC-regulated power supply CSII2005S.

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Transmission FTIR spectra (KBr protocol) of SiO2-prePDA and SiO2- only (control) system was

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performed to detect pre-PDA signatures in the coating (methodology can be found in SI). The

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two spectra (Figure S11) were almost identical to each other denoting that the amount of pre-


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PDA incorporated into the coating during EPD process is too small to be detected by IR in

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comparison to SiO2.

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Patterning via cold plasma treatment. Patterns were created on ITO/PET sheets using stencils

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by placing the sheets under each pattern with the ITO-side facing the deposition front. The

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exposed part was coated with perfluoro using radio frequency – plasma-enhanced chemical

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vapor deposition (rf-PECVD) via a procedure developed before.41 Perfluoro or 1H,1H,2H-

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perfluoro-1-dodecene (C10F21–CH=CH2) (97% pure) was purchased from Matrix Scientific and

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is an exemplar of the class of long-chain perfluoro alkyl monomers. The surface obtained by

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plasma depositing perfluoro monomers has been reported to be superhydrophobic and

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oleophobic, having a low surface energy value of 7.5 mN m-1.40 The plasma deposition was

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performed in an inductively-coupled, cylindrical vacuum chamber wherein, the coupling was

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achieved through an impedance-matching network that connects the coil wound around the

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chamber to the rf-generator operating at 13.56 MHz. The deposition chamber was connected to

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the roughing pump via a pressure gauge and a liquid nitrogen cold trap. The glass tube

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containing the perfluoro monomer was connected to the inlet of the chamber and the control

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valve was used to adjust and monitor the vapor pressure inside the chamber. For this particular

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monomer, the plasma deposition was carried out for 10 mins at a vapor pressure of 140 – 160

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mTorr and the chamber was operated in a continuous mode at an input power of 35 W. Prior to

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deposition, a cleaning cycle was done for an hour using air plasma at a vapor pressure of 180 –

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200 mTorr and an input power of 50 W. We had used clean ITO/PET sheets and silicon wafers

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as reference substrates to evaluate the plasma coating by contact angle measurements.

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Following the patterning process via the cold plasma treatment, SiO2-prePDA EPDs were carried

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out on these patterned ITO/PET substrates at a constant voltage of 10 V and for a period of 5


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mins. The same suspensions were employed as described earlier. After the EPD, the substrates

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were immediately taken out and laid onto a levelled surface to allow gradual evaporation of

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ethanol, thereby giving the desired patterns after complete drying.

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Characterization of EPDed coatings. Optical images of the EPDs were taken using an

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Olympus BX51 microscope (Olympus Corp.). We used a white standard of high density Teflon

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tape (TaegaTech) for measuring the white reference. Percentage of reflectance was measured

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using an Avaspec spectrometer with a xenon white source of beam size ~ 5 mm (Avantes Inc.,

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Broomfield, USA) attached to a goniometer. Before any spectral measurements were collected,

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two calibration steps, using a white and black reference were performed for ensuring all

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measurements were done using the same protocol. The detector and the incident light beam were

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positioned at 5° (from the surface normal) and increased in steps of 10° till 45° in order to carry

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out specular, angle-resolved reflectance measurements. These measurements were taken on the

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coatings EPDed on ITO/glass substrate.

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The top and cross-sectional view of the nanostructures resulting from the directed assembly were

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characterized using a field-emission scanning electron microscope (SEM) (JEOL-7401, JEOL

16

Ltd.). To measure the thickness of the EPDed coatings, we cut the ITO/glass underneath the

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deposition using a diamond tip, and then break along the defect line with hands. Later, the pieces

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were vertically aligned along the cut edge for SEM imaging. No sputter coating was necessary

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for the aforementioned samples. We used the ImageJ software (http://imagej.nih.gov/ij/) to

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measure the thickness of the coating from the SEM images. The error bars mentioned in the

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thickness values are representative of the standard deviation, resulting from the analyses of eight

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cross-sectional SEM images of each type of coating (25 measurements performed on each SEM

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image). A Welch’s analysis of variance (ANOVA) was used to compare mean cross-section


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thicknesses across the three SiO2 treatments (220nm, 260nm, and 300nm). This nonparametric

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analysis was used in place of a parametric ANOVA because the variances between the groups

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were heterogeneous (Hartley’s Fmax test; P > 0.05) and this violates one of the assumptions of

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parametric statistical analyses. The residuals of the data were normally distributed (Shapiro-Wilk

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test; P > 0.05). In the case of a significant whole model, a Games-Howell multiple comparisons

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test, which does not assume homogeneous variance between the treatment groups, was used to

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determine which groups were significantly different.

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Nanoscratch test. The nanoscratch data were collected for the EPDed coatings on ITO/glass

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using Hysitron TI Premier (Bruker Corp.). A diamond conical tip was used in order to have a

10

constant scratch, to eliminate the edge effects from the Berkovich tip during scratching. The

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normal force profile used was a constant load scratch and it is described in Figure 4A. It

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involved a fast, linear increase in the normal force, stable step at the maximum normal force, and

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finally, a rapid and linear decrease in the normal force. In this experiment, we had used two

14

normal forces to penetrate the sample at different extents: 500 μN and 1000 μN. The tip

15

simultaneously moves across the sample surface exerting a lateral force, providing a measure of

16

the force required by the tip to overcome the resistance of the particles (friction) during lateral

17

movement, resulting in a compressive stress built-up in front of the moving tip. 42 Once the tip

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penetrated the coating, the scratch was carried out for 17 seconds, traveling 10 μm through the

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EPD. For the sake of reproducibility, four scratches were run on the same sample at different

20

regions using one normal force each time.

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Bending test. The EPDed coatings of SiO2-only and SiO2-prePDA on ITO/PET were tested by

22

utilizing different diameter (0.5, 0.4 and 0.25 in.) steel cylinders, to evaluate the bending

23

capacity of the EPDs. The flexible material was placed on the cylinder and then applied with


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force to conform to the shape of the cylinder, held steady for 30 seconds and later, released and

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passed onto the next cylinders.

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FIGURES

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Figure 1. (A) Schematic of the substrate (ITO/glass) coated by SiO2 nanoparticles and pre-PDA using EPD technique; the voltage and time were kept constant in these experiments, being 10 V and 5 min respectively. (B) Rightmost panel: Photographs of coatings prepared via EPD with only SiO2 nanoparticles of 220 (top), 260 (middle) and 300 nm (bottom) diameter. Middle panel: Optical micrographs of depositions made with only SiO 2 nanoparticles; top: blue (220 nm), middle: green (260 nm) and, bottom: red (300 nm). Left: Reflectance spectra of SiO2 depositions with each colored curve representing the color obtained from the EPD of SiO 2 nanoparticles of a particular size (blue: 220 nm, green: 260 nm, red: 300 nm). (C) Rightmost panel: Photographs of EPDs of SiO2 nanoparticles (size 220 (top), 260 (middle) and 300 nm (bottom) diameter) with incorporated pre-PDA. Middle panel: Optical micrographs of depositions with the combination of SiO2-prePDA; top: navy blue (220 nm), middle: olive green (260 nm) and, bottom: deep red (300 nm). Left: Reflectance spectra of SiO2-prePDA depositions with each colored curve


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representing the color obtained from the EPD of SiO2-prePDA combination with a particular size of SiO2 nanoparticles employed (navy blue: 220 nm, olive green: 260 nm, deep red: 300 nm). Here the concentration of dopamine in each case is kept constant ~ 0.3 mg/ (1 mL of 4:1 ethanol:water mixture). Scale bars of optical micrographs, 200 µm.

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Figure 2. (A) Schematic of the setup for the specular, angle-dependent reflectance measurements with the angle ϴ (between the incident light and the surface normal) varied from 5° to 35° in steps of 10°. (B) Angle-dependent reflectance spectra for the coating of SiO2 nanoparticles (260 nm diameter) on ITO/glass at incident angles from 5° to 35° (with respect to the surface normal). (C) Angle-dependent reflectance spectra for the coating of SiO 2 (260 nm)-prePDA on ITO/glass at incident angles from 5° to 35° (with respect to the surface normal). In both plots B and C, the reflectance spectra have been off-set along y-axis for clarity. (D) Plot of spectral peak positions at varying angles of incidence with respect to the surface normal for both SiO 2-only (filled black squares) and SiO2-prePDA (filled red circles) EPDs.

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Page 16 of 25

Figure 3. EPDs containing SiO2 nanoparticles of size 260 nm diameter, with or without prePDA. (A) SEM image of SiO2-only EPD (Top View), (B) SEM image of SiO2–prePDA EPD (Top View) and (C) SEM cross-sectional image of the EPD of SiO2–prePDA. The insets of panels (A) and (B) are the 2D Fourier power spectra. Scale bars of all the images, 1 µm.

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Figure 4. Quantitative analysis of the forces for an EPD of SiO2 nanoparticles (260 nm diameter) with or without the incorporation of pre-PDA. (A) Top: A schematic demonstrating the nanoscratch test. Middle: The profile employed for the nanoscratch test using a normal force of 1000 μN. Bottom: The profile corresponds to the lateral displacement of the tip during the scratching test. (B) A compiled histogram of maximum lateral force against normal force of 500 μN and 1000 μN for both SiO2-only (black bar) and SiO2-prePDA (red bar) EPDs. The error bars are the standard deviations calculated from 4 measurements for each sample and force. Mean friction coefficient plotted against time at a normal force of 500 μN (C) and 1000 μN (D) after averaging over four runs for both SiO2-only (black curve) and SiO2-prePDA (red curve) EPDs.

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Figure 5. (A) Left: Schematic of cylinders of different diameters used for the bending test. Right: Demonstration of bending test for both SiO2-only and SiO2-prePDA EPDs, each containing 260 nm SiO2 nanoparticles. The SiO2-only EPD fails instantly on bending slightly (top) while the SiO2-prePDA EPD is mechanically robust and invariant to bending (middle and bottom). (B) Demonstration of the patterned flexible structural color coatings via EPD process. A cold plasma treatment was used for selectively patterning the ITO/PET substrates. The letters “U” and “A” had been plasma treated. The green and blue colors were obtained by employing 260 nm and 220 nm SiO2 nanoparticles, respectively. The structural color coatings were formed by the SiO2-prePDA EPD. The panels C and D show that the patterns were not destroyed or smudged even after bending the substrates, exhibiting the robustness of these EPDs.

20

ASSOCIATED CONTENT

21

Supporting Information is available free of charge on the ACS Publications website or from the

22

author. A description of the nanoscratch results for the other two SiO 2 sizes (220 and 300 nm),

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angle-dependent reflectance spectra of the two types of EPDs (SiO 2-only and SiO2-prePDA)

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along with the plot for the spectral peak shifts with respect to the incident angles, cross-section

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SEM images of SiO2–prePDA EPDed coatings, and the bending test results for both SiO2-only

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and SiO2-prePDA EPDs, containing 220 nm and 300 nm SiO2 nanoparticles are all included in

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SI. In addition, details about characterization of pre-PDA via IR absorption and light scattering

28

study, IR absorbance spectra of SiO2 and SiO2-prePDA via transmission FTIR geometry, and


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Page 18 of 25

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figures supporting the effect of dopamine monomer concentration on the optical, mechanical and

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morphological properties of SiO2-prePDA coatings can be found here.

3

AUTHOR INFORMATION

4

Corresponding Author

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Prof. A. Dhinojwala Department of Polymer Science, The University of Akron, Akron, OH, 44325, USA E-mail: [email protected]

8

Author Contributions

9

All authors contributed towards designing the experiments. M.E. ¬, A.P.¬, and A.D. conducted the

10

experiments. The analysis of the data and the writing of the manuscript was possible through

11

contributions of all authors. All authors have given approval to the final version of the

12

manuscript. ¬ These authors contributed equally to this work.

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Funding Sources

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This project was supported by the Air Force Office of Scientific Research under the Multi-

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University Research Initiative (MURI) grant (FA9550-18-1-0142).

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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The authors would like to thank Edward Laughlin for assisting in the design and fabrication of

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the EPD set-up. We acknowledge the National Polymer Innovation Center (NPIC) (U Akron)

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and Zhorro Nikolov for giving us access and assisting in the nanoscratch measurements. We

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would like to extend special thanks to Kent Displays Inc for providing the ITO/PET sheets. We

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would like to thank Austin Garner for his assistance in statistical analysis. In addition, we also


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Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

recognize the assistance from Cheng Liu and Steven S. C. Chuang for in-situ FTIR

2

characterizations. We would also like to extend our gratitude to Saranshu Singla, Siddhesh Dalvi,

3

Zhuang Xu, Liliana D’Alba and Asritha Nallapaneni for insightful discussion and comments on

4

the manuscript. We acknowledge financial support from the Air Force Office of Scientific

5

Research under the Multi-University Research Initiative (MURI) grant (FA9550-18-1-0142).

6

ABBREVIATIONS

7

EPD, electrophoretic deposition; DC, direct current; UV, ultra-violet; PDA, polydopamine; pre-

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PDA, precursors of polydopamine; ITO; indium tin oxide; SEM, scanning electron microscopy;

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FFT, fast Fourier transformation; PET, poly (ethylene terephthalate); ATR, attenuated total

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reflectance; FTIR, Fourier-transform infrared spectroscopy; DLS, dynamic light scattering; SLS,

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static light scattering

12

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TABLE OF CONTENTS GRAPHIC


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Developing Non-Iridescent Structural Color on Flexible Substrates

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