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Dec 16, 2016 - Livermore, California 94550, United States. Corresponding Author: [email protected]. Abstract: Amorphous photonic structures exhibit interesting ...
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Tunable amorphous photonic materials with pigmentary colloidal nanostructures

T. Y. Han, J. Han, E. Lee, M. A. Worsley, J. R. I. Lee, M. Bagge-Hansen, A. J. Pascall, J. D. Kuntz

December 16, 2016

Advanced Optical Materials

Disclaimer This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.

Tunable amorphous photonic materials with pigmentary colloidal nanostructures Jinkyu Han1, Elaine Lee2, Jessica K. Dudoff1, Michael Bagge-Hansen1, Jonathan R. I. Lee1, Andrew J. Pascall2, Joshua D. Kuntz1, Trevor M. Willey1, Marcus A. Worsley1, T. Yong-Jin Han1* 1. Physics and Life Science Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States 2. Engineering Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States Corresponding Author: [email protected] Abstract: Amorphous photonic structures exhibit interesting optical properties such as non-iridescent angle-independent structural colors and isotropic photonic band gaps. Here, we demonstrate colloidal assemblies of engineered amorphous photonic materials, using pigmentary α-Fe2O3/SiO2 core/shell nanoparticles, exhibiting non- iridescent and tunable colors. The observed colors result from combination of colloidal particle arrangements, giving arises to the structural colors, along with the inherent pigmentary color of the αFe2O3/SiO2 nanoparticles. Colloidal particle assemblies of α-Fe2O3/SiO2 core/shell nanoparticles, and therefore the resulting colors, can be manipulated by shell thickness, particle concentration and external electrical stimuli. α-Fe2O3/SiO2 nanoparticles based amorphous photonic structures exhibit short-range order on a length scale comparable to optical wavelengths and are weakly correlated to each other, as confirmed by ultra-smallangle X-ray scattering measurements. Dynamic tunability of α-Fe2O3/SiO2 nanomaterials in the visible wavelengths is demonstrated using electrophoretic deposition process with a noticeable difference between transmitted and reflected colors.

Amorphous

structures

have

been

investigated

both

theoretically

and

experimentally due to their unique properties as compared with perfectly ordered crystalline structures[1]. Prototypical amorphous photonic materials can be created with nanostructured assemblies, including colloidal nanoparticles with relatively low polydispersity (5% < δ < 10%)[2] to exhibit unique angle-independent structural coloration[3].

Angle-independent colors arise from isotropic nature of nanoparticle

assemblies with only limited short-range order, whereas, conventional photonic crystals are generated by monodispersed colloidal particles that are highly ordered possessing periodic structures[4, 5]. Amorphous structures with short-range order producing striking structural coloration are easily found in various living organisms such as birds, insects, sea animals, and even in plants[6]. Color tunability and transparency is also an interesting aspect in an organism’s structural/pigmentary color scheme which can be used for signaling, camouflage, or mating purposes in different environments[7]. While some groups have demonstrated tunable structural color by fabricating artificially responsive photonic crystals[8,

9, 10]

,

typically using white body colored polystyrene and silica particles, few have integrated inherent pigmentary color with structural color to demonstrate tunable behavior in amorphous photonic materials despite the benefit of enhanced color contrast. Recently, in order to improve the structural color contrast, many groups have synthesized or utilized black body colored materials such as Fe3O4, carbon black, and polydopamine black particles

[8, 11, 12]

. Although the resulting colors from assemblies of these materials quite

striking, the colors they produce are somewhat limited. One of the reason why black bodied materials can’t achieve wide color spectrum is because black pigmentary

materials do not have color mixing effects whereas truly pigmentary materials, such as Fe2O3, can have a significant color mixing effects when combined with structural colors. Examples of combining pigmentary color materials with structural coloration schemes are limited. This may be due to difficulties of fabricating pigmentary materials with optimal particle size, polydispersity and specific refractive index that can be manipulated to control their assembly[4]. In addressing these challenges, we have specifically chosen to use α-Fe2O3/SiO2 core/shell nanoparticles, synthesized with optimized size distributions, to provide pigmentary color that can be systematically assembled to also provide structural colors. The highly faceted single crystal α-Fe2O3 core enhances the color contrast with pigment-induced absorption, while SiO2 shell coating improves the suspension properties and controls the inter-core distance, which contributes to the structural colors.

Here, we demonstrate the assembly and tuning of α-Fe2O3/SiO2

core/shell nanoparticle arrays to generate a tunable structural color with distinct reflected and transmitted color behaviors. The use of α-Fe2O3/SiO2 core/shell nanoparticles with moderate polydispersity (δ ~ 7%), as confirmed by synchrotron-based ultra-small-angle X-ray scattering (USAXS), along with variation in the shell thickness and particle concentration provide multiple pathways to tune the color spectrum of the assemblies. The color tunability observed by varying the concentrations can also be emulated by modulating the electric field applied to a diluted suspension of particles inside an electrophoretic deposition (EPD) cell. Figure 1 A and B show the SEM and TEM images of single crystalline α-Fe2O3 nanocubes, respectively. Based on SEM and TEM image analysis, the average diameter of the synthesized particles is 42.9  3.1 nm (poly-dispersity,  = 7.2%). Fe2O3

nanocubes were uniformly coated with various silica shell thickness to improve the stability of the particle suspensions and to control the inter-particle electrostatic interactions between the particles (Figure 1C-F, Figure S1). It is noteworthy that the use of PVP ligands on the surface of Fe2O3 nanocubes plays an important role in formation of the silica coating on the Fe2O3 nanocubes, as they act as anchoring sites for silica deposition[10]. The surface charge on Fe2O3/SiO2 is negative due to the ionization of the surface hydroxyl groups of the SiO2 shell. This negative surface charge on the particles is confirmed by the observed zeta potential of -25.2 mV. The thickness of the silica shell (tSiO2) can be controlled from 16.5  3.2 nm to 42.0  3.8 nm with reaction time and concentration of TEOS precursor added during reaction. (See Table S1 and Figure 1C-F). The as-prepared suspensions of Fe2O3/SiO2 (dFe2O3 =42.9  3.1 nm, tSiO2 = 42.0  3.8 nm) colloids are solvent exchanged to propylene carbonate, upon which they selfassemble and generate structural ordering due to electrostatic repulsion between particles. Propylene carbonate was chosen as a solvent because it exhibits excellent electrochemical stability and low vapor pressure[13]. The resulting ordered structures provide colors in both the visible and the UV spectrum regime that are tunable by controlling the particle concentrations (the minimum particle concentration to observe structural color for the α-Fe2O3/SiO2 system was 5 wt %).

The suspensions of

Fe2O3/SiO2 particles in propylene carbonate below 5 wt% concentration display the typical orange-red pigmentary colors of the Fe2O3 core, however, as the particle concentration increases, the suspensions exhibit yellowish-green reflective photonic color (11-16 wt%) and gradually evolve to purple and deep purple color at 21- 35 wt%, indicating that the reflected color includes not only pigmentary but also structural colors

(Figure 2A).

Figure 2B shows the reflective spectra of Fe2O3/SiO2 in propylene

carbonate with different particle concentrations. The spectra confirm that the observed reflected colors are produced by a combination of the structural color from Fe2O3/SiO2 nanoparticle arrangement, observed in the range of 350 – 550 nm, and pigmentary color of Fe2O3, observed in the range of 550 – 800 nm. The pigmentary color of Fe2O3 is attributed to the transition of ground and first excited states of ferric iron (d5 states) in the Fe2O3 crystal structure[14]. The reflectivity spectrum of bare Fe2O3 nanocubes is characterized by a local reflectivity maximum near 730 nm, a shoulder centered near 610 nm assigned to 6A1  4T2, and low reflectivity near 510 and 460 nm assigned to 6A1  (4E, 4A1) transition, which is consistent with the reflectivity of pure hematite α-Fe2O3[14, 15]

, indicating that as-made Fe2O3 nanocubes have a single-phase hematite structure

(Figure S2). The structural photonic color produced by the self-assembled nanoparticle arrays blue-shifts as the particle concentration increases, as shown in Figure 2B and Table 1. This is mainly due to shorter inter-particle distances between the Fe2O3/SiO2 nanoparticles at higher nanoparticle concentrations. Nanoparticles at the same particle concentration but with thinner silica shell (tSiO2), i.e. shorter core-to-core distance, also exhibit a blue shift in the reflectance peak from structural color, as shown in Figure 2C, 2D and Table 1. We should note that the reduction of effective refractive index due to increasing volume fraction of SiO2 is negligible since the difference of refractive index between SiO2 (n ~ 1.45)[16] and propylene carbonate (n ~ 1.42)[17] is small and significantly smaller compared to the differences in the refractive index between Fe2O3 (n ~ 2.8)[18] and propylene carbonate. In addition, the full width half maximum (FWHM) of

the reflectance bandwidths are gradually reduced at higher nanoparticle concentration, with FWHMs of ~82 nm at 14 wt%, ~57 nm at 21 wt%, and ~45 nm at 35 wt%. Indeed, the bandwidth of reflectance in a photonic crystal is strongly correlated to the ordering and crystallinity of the particle arrangement[5, 11] and thus the structure of Fe2O3/SiO2 particle arrays in suspension is likely more ordered as the particle concentration increases in the range of 5-35 wt%. Interestingly, the reflection intensity appears to be affected by the periodicity of nanoparticle assemblies. The reflection intensity increases with increasing particle concentration up to ~ 21 wt% and then decreases with increasing particle concentration in 21 – 35 wt% as shown in Figure 2B. We postulate that at lower concentration (i.e. 10-21 wt%), the incident light is more likely to interact with particle arrays in suspension resulting in enhancement of reflection intensity as the periodicity and order of the structure (particle concentration) increases. The cause of reduction of reflection intensity above ~21 wt% remains unclear. However, we hypothesize that the reduction is attributed to the enhancement of transmittance in particle structures, when particle assemblies become nearly crystalline-like. In general, the transmittance increases with increasing order of the particle assemblies thereby reducing scattering events (i.e. reflection)[19]. Thus, at higher concentartion (>21 wt%), the enhancement of transmission is likely more dominant as compared with that of reflection, thereby reducing overall reflection intensity. A decrease in peak intensity at ~600 nm (6A1  4T2 transition in Fe2O3) relative to that at 730 nm (local reflectance), I600/I730, is also observed with increasing particle concentration (lowest at 35 wt%), suggesting that the particle concentrations and arrangement affects not only the structural photonic color but also pigmentary color. At this point, the cause of the reduction of I600/I730 at higher particle

concentrations remains undetermined. We propose that the absorption of incident light in Fe2O3 to produce pigmentary color is decreased by the reflection from the structural color, thereby reducing the reflectance of the pigments (I600). This proposal is viable since the I600/I730 is inversely proportional to the spectral overlap between the absorption spectra of Fe2O3 and reflectance spectra from structural color of Fe2O3/SiO2 as shown in Figure S3. High fidelity examination of the structural ordering and inter-particle distances of Fe2O3/SiO2 suspensions with different particle concentrations was performed with ultrasmall-angle X-ray scattering (USAXS) measurements. USAXS provides a measurement of the structural correlation in nanostructured ensembles, which can then be used to understand the interactions of visible light with the nanostructural assembly. Figure 3A displays the X-ray scattering intensity of Fe2O3/SiO2 nanostructures with 14 and 21 wt% of Fe2O3 in propylene carbonate as a function of q, the scattering wavevector. Both curves show Porod behavior, where I(q) scales to q-2.9 for q > 0.01 A-1 and the porod slope (2.9) is consistent with a typical value of cubes[20]. The existence of several minima for q > 0.1 A-1 indicates a moderate polydispersity which is also confirmed via curve fitting using the IGOR Pro 6.37/IRENA software[21]. Based on modeling of the USAXS data, the Fe2O3 nanoparticle mean diameter is 42.4 nm and poly-dispersity (δ) is 7 %, with a shell thickness of 39.5 nm, which is within experimental error of the values determined from TEM (dcore, TEM = 42.9 nm (δ = 7%), tSiO2, TEM = 42.0 ± 3.8 nm). In order to examine the inter-particle interactions in our Fe2O3/SiO2 nanoparticle system with different particle concentration in more detail, we extracted the structure factor S(q) by dividing I(q) with the form factor representing the specific size and shape of the scatters. Figure 3B exhibits the structure factor S(q) at (0.001 < q < 0.01 A−1),

which are relevant for visible structural color production[22]. Both S(q) profiles show an obvious peak at qpk = 0.00344 A-1 and 0.00410 A-1 for 14 and 21 wt%, respectively. Using the peak position qpk, the inter-particle distance (spatial correlation length, s) can be calculated by using: s = 2π/qpk

(1)

where s = 182.65 nm (qpk = 0.00344 A-1) for 14 wt% and s = 153.2 nm (qpk = 0.00410 A-1) for 21 wt%. The range of both s values indicates that the presence of a dominant length scale of structural periodicity is on the order of visible wavelength and as anticipated, the inter-particle distance is shorter at higher particle concentration. It is somewhat surprising that a relatively small difference of inter-particle distance between 14 wt % and 21 wt % (~ 30 nm) gives rise to a significant blue-shift of color from green to purple as shown in Figure 2A. We should note that both structures possess a second peak at higher q = 0.0067 and 0.0074 A-1 for 14 and 21 wt%, respectively, due to the structural correlation with primary 1st peak. However, the correlation between 1st and 2nd peak is most likely weak in both structures since the bandwidth of the second peak is broad and less distinctive than is typically observed for a highly ordered structure[23], suggesting that both structures are far from perfectly ordered, i.e. amorphous structures. In order to further demonstrate the crystallinity of the structures, we examined the width (FWHM, Δq) of the primary USAXS peak to quantify the extent of spatial periodicity, ξ (ξ = 2π/Δq). ξ is 539.4 nm for 14 wt% and 636.6 nm for 21 wt%, which is consistent with the decreased bandwidth of the optical reflectance spectrum at 21 wt% (FWHM ~ 57 nm) as compared with the spectrum at 14 wt% (FWHM ~ 82 nm) in suspension. For both cases, ξ is only a few times the dominant length-scale of spatial

correlations (s) (i.e. ξ = 2.95s for 14 wt% and ξ = 4.15s for 21 wt%), implying that both structures possess short-range order and are amorphous. Schematics of the nanoparticle arrangement for 14 and 21 wt% Fe2O3/SiO2, based upon the structural information derived from the USAXS, are shown in Figure 3C and D, respectively. Moreover, we can indirectly estimate the effective refractive index (neff) of Fe2O3/SiO2 in propylene carbonate with different concentrations by correlating reflection measurements with USXAS structural data using equation (2). =

2

(2)

, where λmax is the wavelength of maximum reflection and 2π/q = the average interscatterer or nearest- neighbor spacing, and neff is effective refractive index of nanostruture31. Calculated values for neff at 14 wt% is 1.452 and is 1.485 at 21 wt% assuming the nSiO2 is comparable to that of propylene carbonate (n ~ 1.42). It is not surprising that neff increases with increasing Fe2O3 concentration due to increased volume fraction of Fe2O3 (n ~ 2.8) in propylene carbonate. The obtained structural and optical values for both 14 and 21 wt% Fe2O3/SiO2 suspensions are summarized in Table 2. To demonstrate the dynamic tunability of Fe2O3/SiO2 suspensions, EPD experiments were performed using a custom cell. EPD cells were fabricated using transparent top and bottom ITO glass electrodes separated by 25 μm kapton tape spacer. Interestingly, as cells were assembled, we observed a clear difference between transmitted and reflected colors from EPD cells (Figure 4), giving rise to behavior comparable with the Lycurgus cup effect[24]. The difference between the transmitted and reflected colors in the cell is attributed to the difference between the pigmentary color (intrinsic color) of Fe2O3/SiO2 and the structural color from Fe2O3/SiO2 nanoparticle

arrangement. With 25 μm spacer in place, the beam path length of a EPD cell is relatively short compared to the thicker 5 mm quartz cuvette, where the transmittance of the Fe2O3/SiO2 suspensions increases while the absorption of incident light in the suspensions decreases according to the Beer-Lambert law[25]. The decreased absorption at thinner cell spacing minimizes the observed reflected pigmentary color effect. Therefore, the structural color is dominant at the thinner spacing, and results in an obvious color difference between transmitted (white background) and reflected color (black background, LLNL logo) as shown in Figure 4 inset. The spectra in Figure 4 shows transmitted color of Fe2O3/SiO2 suspension in an EPD cell with 25 μm spacer at max ~ 600 nm, whereas the reflective colors can be tuned depending on the particle concentration (from 518 nm to 433 nm as particle concentration increases from 14 wt % to 25 wt%). This observation confirms that the reflected color is primarily from structural coloration and the transmitted color is attributed to the pigmentary coloration. It is noteworthy that the bandwidth of the structural reflection is not highly sensitive to the particle concentration at 25 μm spacing (FWHM ~ 62 ± 2.5 nm) implying that the crystallinity of these nanostructures is comparable and not strongly affected by the particle concentration. The combined effect of particle concentration and thickness of cell spacing on the crystallinity of nanostructures remains under investigation. The schematics of fabricated EPD cell in the presence of applied voltage (ON state) and in the absence of applied voltage (OFF state) are shown in Figure 5A. Due to their negative surface charge, Fe2O3/SiO2 nanoparticles can be concentrated on the positive electrode under an external electric field, resulting in structural color changes. The resulting nanoparticle arrangements and associated color arise from the balance

between the electrostatic repulsion between the particles and the assembly of colloidal particles at the electrode in the presence of an electric field. The reflected color of the Fe2O3/SiO2 structures (14 wt% of Fe2O3 in propylene carbonate) is changed from green (OFF state) to blue as the applied voltage is increased to 4 V (ON state), while the transmitted color remains orange (Figure 5B, Movie S1). The color change is fully reversible and the response time corresponding to applied voltage is almost instantaneous, as shown in Movie S2 and S3. The tunability of the particle assembly and the resulting color were tested by varying the applied voltage. The reflected color from the EPD cell containing Fe2O3/SiO2 suspensions (~14 wt% Fe2O3) is bluish green (λmax ~ 495 nm) in the range of 1-3V and is significantly blue-shifts when applied voltage increases beyond 3.0 V. A continued increase in the applied voltage to 3.5 V results in the color changing to blue (λmax ~ 450 nm) and a further increase to 4 V results in a deep blue (λmax ~ 425 nm) (Figure 5C). The reflected color range is also tunable by controlling the particle concentration and tSiO2 (Figure 5D, Movie S3 and 5E). As discussed earlier, the path length of light plays an important role in determining the reflected color by controlling the relative intensity of structural and pigmentary colors; therefore, the reflected color can also be tuned by the cell thickness. For example, the reflected colors observed in the absence and presence of applied voltage under same electric field with a 500 μm spacing is obviously different from that of a 25 μm spacing due to enhanced relative intensity of pigmentary color and possibly different response to electric stimuli (Figure S4A and B). The photonic color with a 500 μm spacing changed from yellow to pink and then to deep red as the applied voltage increases (Figure S4C). This demonstrate the versatility of the EPD cell device which can be used to generate the full visible color

spectrum via changing the spacer thickness, particle concentration, silica shell thickness, tSiO2, and applied voltage. Amorphous states of α-Fe2O3/SiO2 core/shell nanoparticles colloidal assembly exhibit short range order, as confirmed by USAXS data, and no preferential orientation thereby generating angle independent structural color. Figure 6 shows the reflective spectra of Fe2O3/SiO2 nanostructures (14 wt% Fe2O3) in an EPD cell with varying viewing angles. The spectral shifts less than 10 nm depending on the viewing angle in the range of 0-60°, indicating that the exhibited structural color is nearly non-iridescent and insensitive to the detection angle (Figure S5). Nevertheless, the color generated by constructive interference in these amorphous photonic assemblies depends highly on the incident light source and the angle between the directions of illumination and detection. To determine the exact role of specular and diffused reflection in these structures, sophisticated angle-dependent optical experiments will likely be needed in future studies. In summary, engineered Fe2O3/SiO2 amorphous photonic materials were successfully fabricated. The structures of concentrated Fe2O3/SiO2 suspensions exhibit short-range order and the inter-particle distances that decrease at higher particle concentrations.

The short range order and inter-particle distances were successfully

measured by USAXS with inter-particle distances of ~180 nm and ~150 nm for 14 wt% and 21 wt% solutions, respectively. Interestingly, the reflected and transmitted photonic color is distinguishable due to the difference between the inherent pigmentary color of Fe2O3/SiO2 nanoparticles and the structural color arising from their arrangements. The reflected photonic color is effectively tunable with different particle concentration and shell thickness (tSiO2) and dynamically tunable in response to electric stimuli. These novel

amorphous photonic materials composed of pigmentary colloidal particles can not only imitate interesting colored living organisms in nature but also can lead to novel noniridescent electronic paper displays and colored-reflective photonic displays with excellent color tunability at low operating voltages and fast response time.

Experimental Synthesis of α-Fe2O3 nanocubes. α-Fe2O3 nanocubes were prepared adopting a previously reported hydrothermal approach[26]. Briefly, 0.9 g of poly (vinyl pyrrolidone) (PVP, Sigma-Aldrich, Mw = 55,000) was added to 20 mL of anhydrous N,N-dimethylformamide (Sigma-Aldrich, 99.8%) and stirred for 30 min to completely dissolve the reagents. Thereafter, 0.05 mmol of Fe(NO3)3.9H2O (Sigma-Aldrich, >99.95%) was added in the solution, vigorously stirred for 15 min, and transferred into a Teflon-lined stainless-steel autoclave (with a capacity of 30 mL). The autoclave was subsequently heated to 180°C for 30 h and removed to convectively cool to room temperature. As-prepared samples were later isolated from the mother liquor by centrifugation at 10000 rpm for 20 min and washed with the mixture of water and ethanol (EtOH) (vwater: vEtOH = 1:1) for 3 times and stored in EtOH for future use. Synthesis of α-Fe2O3 /SiO2 core/shell nanoparticles α-Fe2O3/SiO2 core/shell colloids were prepared through a modified Stöber method[27]. Typically, 20 mg of α-Fe2O3 nanocubes was added to 20 mL EtOH with 1 ml ammonium hydroxide (J.T.Baker, 28%) and 3 mL Mill-Q water and stirred vigorously for 10 min. 0.04 ml Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99.999%) was added to the

solution every 20 min for the desired time (e.g.1 to 2 h) so the total volume of TEOS was 0.08 -0.32 mL and stirred for another 0 - 22 h at room temperature. The obtained core– shell particles were washed with EtOH through repetitive centrifugation and redispersion. The nanoparticles were finally dispersed in propylene carbonate at greater than 5 wt% of Fe2O3/SiO2, which is the critical concentration for exhibiting reflective structural color. Fabrication of an EPD cell and patterned Ti-Pt deposited electrode The Fe2O3/SiO2 colloids were then injected between two ITO electrodes (1 inch × 1 inch) separated by a 25 μm spacer of kapton tape to investigate the electrophoretic response of structural color from Fe2O3/SiO2 colloids in propylene carbonate. The fabrication of Ti (t = 20 nm) and Pt (t = 200 nm) deposited electrodes were prepared by the patterned sputter deposition using conventional lift-off lithography techniques. Characterization SEM and TEM images were obtained at 3 kV and 30 kV on a JEOL JSM-7401F instrument, respectively. Specimens for all microscopy experiments were prepared by dispersing the as-prepared product in ethanol, sonicating for 2 min to ensure an adequate dispersion of the nanostructures, and dipping one drop of the solution onto a Si wafer for SEM and a 300 mesh Cu grid, coated with a lacey carbon film for TEM. USAXS measurements were performed on core-shell particle suspensions sealed within a cell that contains an aperture for x-ray transmission with a path length of 1mm. The cell is composed of a polymer spacer containing a central aperture that is sealed on the front and back surfaces by 1mm thick glass slides. USAXS experiments were conducted using an absolute intensity-calibrated instrument at beamline 09-ID-C at the Advanced Photon Source (Argonne Naitonal Laboratory)

[28]

. An X-ray energy of 21 keV and a beam size

of 0.3 mm × 0.4 mm were used. Data analysis was based on fitting the scattering curve to an appropriate model by a least-squares method using the IRENA software[21]. The zeta potential of the suspended particles was measured with a ZetaSizer Nano ZS90 (Malvern) The optical properties of Fe2O3/SiO2 colloids in propylene carbonate were measured using a Perkin-Elmer Lambda 950 spectrophotometer equipped with an integrating sphere in the static state to examine the effect of concentration and silica shell thickness on the structural coloration. An Ocean Optics HR2000+ with a balanced tungsten source (400 – 900 nm) was used to investigate the effect of applied voltage on the coloration in an EPD cell and angle dependency of the structural color.

Acknowledgement: Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. We acknowledge assistance from Jan Ilavsky and Ross Andrews of the APS, Argonne National Laboratory. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. LLNL-JRNL-715397

Figure 1. Representative (A) SEM and (B) TEM images of Fe2O3 nanocubes (dFe2O3 = 42.9  3.1 nm) and bright field SEM images of Fe2 O3/SiO2 core/shell nanoparticles with silica coating reaction times of (C) 1 h (tSiO2 =16.5  3.2 nm), (D) 2 h (tSiO2 =26.8  3.2 nm), and dark field SEM images with (E) 4 h (tSiO2 =34.3  3.2 nm), and (F) 24 h (tSiO2 = 42.0  3.8 nm) silica coating reaction time. Scale bars = 100 nm.

Figure 2. (A) Photographs of Fe2O3/SiO2 (tSiO2 ~ 42 nm) colloids in propylene carbonate with different Fe2O3 concentration (5-35 wt%). (B) Normalized reflection spectra of Fe2O3/SiO2 (tSiO2 ~ 42 nm) suspensions with different particle concentrations (10- 35 wt%) and (C) 18 wt% Fe2O3 solution with different shell thickness (tSiO2). (D) Photographs of Fe2O3/SiO2 (tSiO2 ~ 42 nm) colloids in propylene carbonate with different tSiO2.

Figure 3. (A) Raw and fitted USAXS data of Fe2O3/SiO2 nanoparticles at 14 and 21 wt% of Fe2O3 in propylene carbonate as function of scattering vector, q, on a log-log plot. The curve fitting was carried out by using IRENA software. (B) The structure factor S(q) of Fe2O3/SiO2 suspensions with 14 and 21 wt% Fe2O3 particle concentration obtained from the raw data of Figure 3(A). Schematics of Fe2O3/SiO2 arrangement of (C) 14 wt% and (D) 21 wt% of Fe2O3 concentration in propylene carbonate based on the spatial correlation length (s) and the range of spatial order (ξ).

Figure 4. The reflectance (solid lines) and normalized transmittance (dotted line) spectra of Fe2O3/SiO2 suspension in an EPD cell with 25 μm spacing with varying Fe2O3 concentrations. The insets show the photographs of Fe2O3/SiO2 suspension in an EPD cell with 25 μm spacing with different Fe2O3 concentration (i.e. 14 and 21 wt%) under diffusive illumination with no applied voltage. Black carbon tape (LLNL logo) with a white paper was put on the backside of the cell in order to distinguish the reflected and transmitted color more clearly.

Figure 5. (A) The schematic of an EPD cell in the absence (OFF state) and presence (ON state) of applied voltage. (B) Photograph of Fe2O3/SiO2 (~14 wt% Fe2O3) suspensions in an EPD cell with black and white (triangle) background put on the backside of the cell under 0V and 4V. (C) Normalized reflection spectra of the Fe2O3/SiO2 suspension (~14

wt% Fe2O3) in an EPD cell at various applied voltages. (D) Photograph of Fe2O3/SiO2 suspension in an EPD cell at various Fe2O3 concentration (6, 14, and 18 wt%) and applied voltage (0-5V) and (E) effects of various silica shell thickness (tSiO2 , at 27, 34, and 42 nm) and applied voltage (0-5V) .

Figure 6. Reflection spectra of Fe2O3/SiO2 suspension (~14 wt% Fe2O3) in an EPD cell at various view angles (5–60°).

Table 1. The peak of reflective spectra related to structural coloration depending on various Fe2O3/SiO2 particle concentration and tSiO2 Concentration of Fe2O3 in PC

35 wt%

25 wt% 21 wt%

tSiO2 (nm)

42.0  3.8

42.0  3.8

42.0  3.8

26.8  3.2

34.3  3.2

42.0  3.8

λmax related to structural coloration

371 nm

433 nm 455 nm

416 nm

440 nm

472 nm 518 nm 532 nm

18 wt%

14 wt% 10 wt% 42.0  3.8

42.0  3.8

Table 2. Structural and optical characteristic data of Fe2O3/SiO2 core/shell nanoparticles in propylene carbonate (PC). Core weight % in PC

Inter-particle distance (s = 2π/q)

Coherent length (ξ = 2π/Δq)

λmax from reflection spectra

Effective refractive index (neff)

14 wt%

182.65 nm

539.35 nm = 2.95 s

518 nm

1.459

21 wt%

153.24 nm

636.60 nm = 4.15 s

455 nm

1.485

dcore (nm)

tSiO2 (nm)

42.9 ± 3.1

42.0 ± 3.0

Volume fraction of core(φ) 3.6% 6.1%

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A novel amorphous photonic structures using pigmentary α-Fe2O3/SiO2 core/shell nanoparticles are succesfully fabricated. The resulting non-iridicent brilliant colors are in combination of pigmenary and structural colororation and manipulated by shell thickness, particle concentration and external electrical stimuli using electrophoretic deposition process. In the process, fully reversible and instantaneous color change as well as noticeable difference between transmitted and reflected colors is observed. Keyword: nanoparticle assemblies, amorphous materials, photonic materials, structural colors, electrophoretic deposition Jinkyu Han1, Elaine Lee2, Jessica K. Dudoff1, Michael Bagge-Hansen1, Jonathan R. I. Lee1, Andrew J. Pascall2, Joshua D. Kuntz1, Trevor M. Willey1, Marcus A. Worsley1, T. Yong-Jin Han1* Reversible

OFF

ON Instantaneous Transmitted (pigmentary) color

OFF

Reflected (pigmentary) color

ON

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016.

Supporting Information

Tunable amorphous photonic materials with pigmentary colloidal nanostructures

Jinkyu Han1, Elaine Lee2, Jessica K. Dudoff2, Michael Bagge-Hansen1, Jonathan R. I. Lee1, Andrew J. Pascal2, Joshua D. Kuntz1, Trevor M. Willey1, Marcus A Worsley1, T. Yong-Jin Han1 1. Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States 2. Engineering Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States

Figure S1. Representative lower magnification TEM image (dark field) of Fe2O3/SiO2 core/shell nanoparticles reacted for 24 h (dFe2O3 =42.9  3.1 nm, tSiO2 = 42.0  3.8 nm)

Figure S2. Normalized reflection spectra of bare Fe2O3 nanocubes

Figure S3. Absorption spectra of bare Fe2O3 (dotted line) and reflectance spectra of Fe2O3/SiO2 in propylene carbonate with different concentration of Fe2O3.

Figure S4. (A) Reflective spectra and B) photographs of Fe 2O3/SiO2 suspensions (tSiO2 = 27 nm and 14 wt% Fe2O3) in an EPD cell with 25 and 500 μm spacing under same electric field and (C) applied voltage.

Figure S5. Photograph of Fe2O3/SiO2 suspensions (tSiO2 = 42 nm and 14 wt% Fe2O3) in an EPD cell with 25 μm spacing at various viewing angles (5- 60°).

Table S1. The reaction conditions for the deposition of SiO2 on Fe2O3 and corresponding tSiO2. Note that the volume of EtOH (20 ml), amount of water (3 ml), NH4OH (1 ml) and bare Fe2O3 particles (20 mg) are fixed at all conditions.

Amount of TEOS (ml)

Reaction time (h)

tSiO2 (nm)

Image

0.08

1

16.5  3.2

Figure 1C

0.16

2

26.8  3.2

Figure 1D

0.32

4

34.3  3.2

Figure 1E

0.32

24

42.0  3.8

Figure 1F

Supporting movies files are included as Production Data file.