Developing Non-Iridescent Structural Color on ... - ACS Publications

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 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

ACS Applied Materials & Interfaces

1

Developing Non-Iridescent Structural Color on

2

Flexible Substrates with High Bending Resistance

3

Mario Echeverri

4

Dhinojwala

5

†

6 7

§

8 9

‡

10

¬

11

*

†,¬

, 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]

12 13 14

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

15 16

ABSTRACT

17

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

18

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

19

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

20

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

21

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

22

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

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 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

Page 2 of 25

1

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

2

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

3

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

4

materials.

5

INTRODUCTION

6

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

7

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

8

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

9

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

oxidized upon exposure to

10

cosmetics.11

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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


2

Page 3 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

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

2

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

3

process adds an additional step in their fabrication process.

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

coloring flexible materials.

14

RESULTS AND DISCUSSION

15

Preparation and optical properties of EPDed coatings

16

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

17

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

18

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

19

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

20

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

21

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

22

incoherently scattered light across the entire visible spectrum.


3


Page 4 of 25

1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

the SiO2 particles with additional absorption by pre-PDA.

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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


4

Page 5 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

Morphology of the EPDs

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

observed for monodisperse nanoparticles. 31,32

17

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

18

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

19

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

20

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

21

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

22

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

23

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


5


Page 6 of 25

1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

Mechanical characterization of the EPDs

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

SiO2-prePDA EPDs are shown in Figure S5.

22

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

23

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


6

Page 7 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

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

Supporting Information (SI) (Figure S6).

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22 23


7


Page 8 of 25

1

Flexible and patternable structural color coatings

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

size (Figure S8).

19

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

20

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

21

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

22

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

23

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


8

Page 9 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

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

2

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

3

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

4

CONCLUSIONS

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

flexible PET substrates.

18

EXPERIMENTAL METHODS

19

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

20

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

21

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

22

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

23

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


9


Page 10 of 25

1

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

2

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

3

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

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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


10

Page 11 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

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

2

comparison to SiO2.

3

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

4

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

5

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

6

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

7

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

8

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

9

plasma depositing perfluoro monomers has been reported to be superhydrophobic and

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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


11


Page 12 of 25

1

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

2

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

3

ethanol, thereby giving the desired patterns after complete drying.

4

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

5

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

6

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

7

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

8

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

9

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

10

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

11

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

12

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

13

coatings EPDed on ITO/glass substrate.

14

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

15

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

17

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

18

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

19

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

20

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

21

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

22

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

23

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


12

Page 13 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

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

2

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

3

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

4

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

5

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

6

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

7

determine which groups were significantly different.

8

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

9

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

11

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

12

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

13

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

18

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

19

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

20

regions using one normal force each time.

21

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


13


Page 14 of 25

1

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

2

passed onto the next cylinders.

3

FIGURES

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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


14

Page 15 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 2 3 4


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.

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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.

25 26 27 28


15


1 2 3 4

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.

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

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.

25 26 27 28


16

Page 17 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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

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),

23

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

24

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

25

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

26

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

27

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


17


Page 18 of 25

1

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

2

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

3

AUTHOR INFORMATION

4

Corresponding Author

5 6 7

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.

13

Funding Sources

14

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

15

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

16

Notes

17

The authors declare no competing financial interest.

18

ACKNOWLEDGMENT

19

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

20

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

21

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

22

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

23

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


18

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-

8

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

9

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

10

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

11

static light scattering

12

REFERENCES

13

(1)

14 15

14862-14889. (2)

16 17

(3)

Franck, J.; Livingston, R. Remarks on the Fluorescence, Phosphorescence and Photochemistry of Dyestuffs. J. Chem. Phys. 1941, 9, 184–190.

(4)

20 21

Booth, C. Evolutionary Significance of Ontogenetic Color Change in Animals. Biol. J. Linn. Soc. 1990, 40, 125–163.

18 19

Sun, J.; Bhushan, B.; Tong, J. Structural Coloration in Nature. RSC Adv. 2013, 3 (35),

Fukuda, M.; Kodama, K.; Yamamoto, H.; Mito, K. Evaluation of New Organic Pigments as Laser-Active Media for a Solid-State Dye Laser. Dye. Pigment. 2004, 63, 115–125.

(5)

Potyrailo, R. A.; Ghiradella, H.; Vertiatchikh, A.; Dovidenko, K.; Cournoyer, J. R.; Olson,


19


Page 20 of 25

1

E. Morpho Butterfly Wing Scales Demonstrate Highly Selective Vapour Response. Nat.

2

Photonics 2007, 1, 123–128.

3

(6)

Katagiri, K.; Tanaka, Y.; Uemura, K.; Inumaru, K.; Seki, T.; Takeoka, Y. Structural Color

4

Coating Films Composed of an Amorphous Array of Colloidal Particles via

5

Electrophoretic Deposition. NPG Asia Mater. 2017, 9, e355.

6

(7)

Ge, D.; Lee, E.; Yang, L.; Cho, Y.; Li, M.; Gianola, D. S.; Yang, S. A Robust Smart

7

Window: Reversibly Switching from High Transparency to Angle-Independent Structural

8

Color Display. Adv. Mater. 2015, 27, 2489–2495.

9

(8)

Xiao, M.; Li, Y.; Zhao, J.; Wang, Z.; Gao, M.; Gianneschi, N. C.; Dhinojwala, A.;

10

Shawkey, M. D. Stimuli-Responsive Structurally Colored Films from Bioinspired

11

Synthetic Melanin Nanoparticles. Chem. Mater. 2016, 28, 5516–5521.

12

(9)

Park, J. G.; Kim, S. H.; Magkiriadou, S.; Choi, T. M.; Kim, Y. S.; Manoharan, V. N. Full-

13

Spectrum Photonic Pigments with Non-Iridescent Structural Colors through Colloidal

14

Assembly. Angew. Chemie - Int. Ed. 2014, 53, 2899–2903.

15

(10)

Xiao, M.; Li, Y.; Allen, M. C.; Deheyn, D. D.; Yue, X.; Zhao, J.; Gianneschi, N. C.;

16

Shawkey, M. D.; Dhinojwala, A. Bio-Inspired Structural Colors Produced via Self-

17

Assembly of Synthetic Melanin Nanoparticles. ACS Nano 2015, 9, 5454–5460.

18

(11)

19 20 21

Kinoshita, S.; Yoshioka, S.; Miyazaki, J. Physics of Structural Colors. Reports Prog. Phys. 2008, 71, 076401.

(12)

Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Single-Crystal Colloidal Multilayers of Controlled Thickness. Chem. Mater. 1999, 11, 2132–2140.


20

Page 21 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


(13)

Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, O.; Marti, J.;

2

Ralchenko, V. G. Carbon Structures of 3D Periodicity at Optical Wavelengths. Science.

3

1998, 282, 897–901.

4

(14)

Hatton, B.; Mishchenko, L.; Davis, S.; Sandhage, K. H.; Aizenberg, J. Assembly of Large-

5

Area, Highly Ordered, Crack-Free Inverse Opal Films. Proc. Natl. Acad. Sci. 2010, 107,

6

10354–10359.

7

(15)

Takeoka, Y.; Yoshioka, S.; Takano, A.; Arai, S.; Nueangnoraj, K.; Nishihara, H.;

8

Teshima, M.; Ohtsuka, Y.; Seki, T. Production of Colored Pigments with Amorphous

9

Arrays of Black and White Colloidal Particles. Angew. Chemie - Int. Ed. 2013, 52, 7261–

10 11

7265. (16)

Ge, D.; Yang, L.; Wu, G.; Yang, S. Angle-Independent Colours from Spray Coated Quasi-

12

Amorphous Arrays of Nanoparticles: Combination of Constructive Interference and

13

Rayleigh Scattering. J. Mater. Chem. C 2014, 2, 4395–4400.

14

(17)

15 16

Ge, D.; Yang, L.; Wu, G.; Yang, S. Spray Coating of Superhydrophobic and AngleIndependent Coloured Films. Chem. Commun. 2014, 50, 2469–2472.

(18)

Kohri, M.; Nannichi, Y.; Taniguchi, T.; Kishikawa, K. Biomimetic Non-Iridescent

17

Structural Color Materials from Polydopamine Black Particles That Mimic Melanin

18

Granules. J. Mater. Chem. C 2015, 3, 720–724.

19

(19)

Katagiri, K.; Uemura, K.; Uesugi, R.; Inumaru, K.; Seki, T.; Takeoka, Y. Structurally

20

Colored Coating Films with Tunable Iridescence Fabricated via Cathodic Electrophoretic

21

Deposition of Silica Particles. RSC Adv. 2018, 8, 10776–10784.


21


1

(20)

2 3

Page 22 of 25

García, P. D.; Sapienza, R.; Blanco, Á.; López, C. Photonic Glass: A Novel Random Material for Light. Adv. Mater. 2007, 19, 2597–2602.

(21)

Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Ostrander, J. W.; Ragoisha, G. A.

4

Electrophoretic Deposition of Latex-Based 3D Colloidal Photonic Crystals: A Technique

5

for Rapid Production of High-Quality Opals. Chem. Mater. 2000, 12, 2721–2726.

6

(22)

Xiao, M.; Hu, Z.; Wang, Z.; Li, Y.; Tormo, A. D.; Le Thomas, N.; Wang, B.; Gianneschi,

7

N. C.; Shawkey, M. D.; Dhinojwala, A. Bioinspired Bright Noniridescent Photonic

8

Melanin Supraballs. Sci. Adv. 2017, e1701151.

9

(23)

Shawkey, M. D.; Hill, G. E. Significance of a Basal Melanin Layer to Production of Non-

10

Iridescent Structural Plumage Color: Evidence from an Amelanotic Steller’s Jay

11

(Cyanocitta Stelleri). J. Exp. Biol. 2006, 209, 1245–1250.

12

(24)

Kapp, F. G.; Perlin, J. R.; Hagedorn, E. J.; Gansner, J. M.; Schwarz, D. E.; O’Connell, L.

13

A.; Johnson, N. S.; Amemiya, C.; Fisher, D. E.; Wölfle, U.; et al. Protection from UV

14

Light Is an Evolutionarily Conserved Feature of the Haematopoietic Niche. Nature 2018,

15

558, 445–448.

16

(25)

Cheng, D.; He, M.; Ran, J.; Cai, G.; Wu, J.; Wang, X. In Situ Reduction of TiO2

17

Nanoparticles on Cotton Fabrics through Polydopamine Templates for Photocatalysis and

18

UV Protection. Cellulose 2018, 25, 1413–1424.

19

(26)

20 21

Wu, T. F.; Hong, J. D. Dopamine-Melanin Nanofilms for Biomimetic Structural Coloration. Biomacromolecules 2015, 16, 660–666.

(27)

Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-Molecule Mechanics of Mussel


22

Page 23 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 2

Adhesion. Proc. Natl. Acad. Sci. 2006, 103, 12999–13003. (28)

Ran, J.; He, M.; Li, W.; Cheng, D.; Wang, X. Growing ZnO Nanoparticles on

3

Polydopamine-Templated Cotton Fabrics for Durable Antimicrobial Activity and UV

4

Protection. Polymers. 2018, 10, 495.

5

(29)

6 7

Jiang, X.; Wang, Y.; Li, M. Selecting Water-Alcohol Mixed Solvent for Synthesis of Polydopamine Nano-Spheres Using Solubility Parameter. Sci. Rep. 2014, 4, 6070.

(30)

Lopez-Garcia, M.; Masters, N.; O’Brien, H. E.; Lennon, J.; Atkinson, G.; Cryan, M. J.;

8

Oulton, R.; Whitney, H. M. Light Induced Dynamic Structural Color by Intracellular 3D

9

Photonic Crystals in Brown Algae. Sci. Adv. 2018, 4, eaan8917.

10

(31)

11 12

Shi, L.; Zhang, Y.; Dong, B.; Zhan, T.; Liu, X.; Zi, J. Amorphous Photonic Crystals with Only Short-Range Order. Adv. Mater. 2013, 25, 5314–5320.

(32)

Liu, X. Y.; Diao, Y. Y. Modeling Biomineralization and Structural Color Biomimetics by

13

Controlled Colloidal Assembly. In Bioinspiration: From Nano to Micro Scales; Liu, X.

14

Y., Ed.; Springer US: New York, 2012; pp 221-274.

15

(33)

Mori, Y.; Nobuzane, Y.; Nishimura, K.; Yamada, K.; Tsuchiya, K. Thin Film Structure of

16

Titania Nanoparticles Prepared by Electrophoretic Deposition. Chem. Eng. Trans. 2017,

17

57, 1507–1512.

18

(34)

19 20 21

Tomastik, J.; Ctvrtlik, R. Nanoscratch Test — A Tool for Evaluation of Cohesive and Adhesive Properties of Thin Films and Coatings. EPJ Web Conf. 2013, 48, 27.

(35)

Turki, D.; Fatah, N. Description of Consolidation Forces on Nanometric Powders. Brazilian J. Chem. Eng. 2010, 27, 555–562.


23


1

(36)

Page 24 of 25

Min, Y.; Akbulut, M.; Kristiansen, K.; Golan, Y.; Israelachvili, J. The Role of

2

Interparticle and External Forces in Nanoparticle Assembly. Nat. Mater. 2008, 7, 527–

3

538.

4

(37)

Liu, J.; Li, J.; Yu, B.; Ma, B.; Zhu, Y.; Song, X.; Cao, X.; Yang, W.; Zhou, F. Tribological

5

Properties of Self-Assembled Monolayers of Catecholic Imidazolium and the Spin-Coated

6

Films of Ionic Liquids. Langmuir 2011, 27 (18), 11324–11331.

7

(38)

8 9

General Route to Superhydrophobic Particles. J. Am. Chem. Soc. 2012, 134, 9879–9881. (39)

10 11

Zhang, L.; Wu, J.; Wang, Y.; Long, Y.; Zhao, N.; Xu, J. Combination of Bioinspiration: A

Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448, 338–341.

(40)

Laguardia, L.; Ricci, D.; Vassallo, E.; Cremona, A.; Mesto, E.; Grezzi, F.; Dellera, F.

12

Deposition of Super-Hydrophobic and Oleophobic Fluorocarbon Films in Radio

13

Frequency Glow Discharges. Macromol. Symp. 2007, 247, 295–302.

14

(41)

Badge, I.; Bhawalkar, S. P.; Jia, L.; Dhinojwala, A. Tuning Surface Wettability Using

15

Single Layered and Hierarchically Ordered Arrays of Spherical Colloidal Particles. Soft

16

Matter 2013, 9, 3032–3040.

17 18

(42)

Kareer, A. Nano-Scratch Hardness and the Lateral Size Effect (LSE). Ph.D. Dissertation, University of Leicester, Leicester, UK, March 2015.

19 20 21


24

Page 25 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


TABLE OF CONTENTS GRAPHIC


25

Developing Non-Iridescent Structural Color on ... - ACS Publications

Recommend Documents