Electro-optical device with tunable transparency using colloidal core

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Electro-optical device with tunable transparency using colloidal core/shell nanoparticles Jinkyu Han, Megan C Freyman, Eyal Feigenbaum, and Thomas Yong-Jin Han ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01337 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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Electro-optical device with tunable transparency using colloidal core/shell nanoparticles Jinkyu Han1*, Megan C. Freyman1, E. Feigenbaum2, T. Yong-Jin Han1* 1. Physics and Life Science Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States 2. National Ignition Facility, Lawrence Livermore National Laboratory, Livermore, California 94550, United States Keywords: core/shell nanoparticles, particle assemblies, transparency tunability, suspended particle device, light matter interaction. Corresponding Authors: [email protected] (J.H), [email protected] (T.Y.H) Abstract: A suspended particle device (SPD) adapted for controlling the transmission of electromagnetic radiation has become an area of considerable focus for smart window technology, due to their desirable properties such as instant and precise light control and cost-effectiveness. Here, we demonstrate a SPD with tunable transparency in the visible regime using colloidal assemblies of nanoparticles. The observed transparency using ZnS/SiO2 core/shell colloidal nanoparticles is dynamically tunable in response to external electric field with increased transparency when applied voltage increases. The observed transparency change is attributed to structural ordering of nanoparticle assemblies and thereby modify the photonic band structures, as confirmed by the finite- difference timedomain simulations of Maxwell’s equations. The transparency of the device can also be manipulated by changing the particle size and the device thickness. In addition to transparency, structural colorations and their dynamic tunability are demonstrated using α-Fe2O3/SiO2 core/shell nanomaterials, resulting from the combination of inherent optical properties of α-Fe2O3/SiO2 nanomaterials and coloration due to their tunable structural particle assemblies in response to electric stimuli.

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Motivated by the potential for significant energy savings from reduced cooling and heating loads, smart window technologies have significant implications for future design and outfitting of commercial and residential buildings1,2. In particular, smart window based on suspended particle device (SPD) has become an area of considerable focus, due to their unique properties such as instant and precise light control, long lifetime, and excellent stability as compared with electrochromic (EC) devices, which tune their optical properties via electrochemical redox reactions under an applied voltage3-6. Most techniques for fabricating SPD use one- dimensional needle-like or rod shaped dichroic materials of polyhalide crystals (e.g. iodoquinine sulfate, herapathite) as electrically responsive materials that align in the presence of applied electric field, enabling the light to pass through1,5. However, the current technology requires a relatively high AC voltage (24 – 100V) to control light transmission and the choice of the responsive materials is limited due to the difficulty of fabricating one-dimensional shaped materials with dichroic properties7. Spherical nanoparticles with controlled size and narrow size distributions have been previously used as building blocks of colloidal photonic crystal, resulting in interesting optical characteristics such as photonic band-gaps, and frequency of stop-and pass-bands when the assembled structures strongly interact with electromagnetic waves8,9. For example, assembled nanoparticle structures can generate brilliant structural color since the incident light with certain frequency is reflected rather than transmitted through the stop-band of photonic crystals8,10. Furthermore, the resulting colors can be manipulated by controlling the inter-particle distance in response to external stimuli such as temperature, humidity, electric or magnetic field, and mechanical stretching and thus a


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variety of photonic and optical applications such as reflective color display, optical waveguide and lasers, have been demonstrated based on the controlled colloidal particle assemblies10-16. Despite of these efforts, there have not been any extensive efforts to correlate nanoparticle arrangements with corresponding transparency and to fabricate or demonstrate transparency tunable device in response to external field by controlling colloidal nanoparticle arrangements. Therefore, in this work, we aim to perform dynamically tunable transparency under relatively small (1-5 V) external electrical voltage by tuning the particle arrangements using easily manufactured spherical colloidal nanoparticles as responsive materials. Furthermore, in addition to transparency tunability, the device can reflect visible colors of light that are tunable in response to electric field using optical properties of these spherical nanoparticle assemblies. We have purposely chosen ZnS/SiO2 and α-Fe2O3/SiO2 cores/shell nanoparticles as electrically responsive materials. ZnS nanoparticle is a white body colored material resulting in pale and weak structural color from the particle arrangements due to strong incoherent light scattering (i.e. white color) and thereby enable us to clearly observe the transparency tunability (i.e. from opaque to clear) in SPD. In contrast, α-Fe2O3 nanoparticle was chosen as the other core material to demonstrate not only the transparency tunability but also the structural color effects. SiO2 shell coating on these core materials generates the surface charge on the core/shell particles thereby improving the suspension properties and is able to respond to electric field17. The use of colloidal nanoparticles provides multiple pathways to tune the transparency as well as color spectrum of the particle assemblies including variation in the core/shell composition, the particle size, and device thickness. In addition, we have


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performed finite-difference time-domain (FDTD) with full-wave numerical simulations to probe and support the transparency tunability observed by varying the particle arrangement. We believe that our approach to tune the transparency without loss in performance as well as controlling light reflection through the combination of inherent optical properties of responsive materials and their particle assemblies with relatively low energy requirement can open up different avenues for novel smart window technology. ZnS nanoparticle clusters with various particle size were successfully synthesized through homogeneous nucleation from zinc nitrate and thioacetamide (TAA) precursors (Figure 1A-C and Figure S1). Figure 1 A-C show the SEM images of ZnS nanoparticle clusters synthesized with different particle sizes and based on SEM image analysis, the average diameters of these particles are 110.3 ± 5.7 nm (poly-dispersity, δ = 5.2 %), 72.6 ± 4.2 nm (δ = 5.8 %), and 62.1 ± 3.1 nm (δ = 5.0%), respectively. Nanoparticle clusters are formed when small nanoparticle (~ 5 nm) aggregates and the particle size of clusters is readily controlled by manipulating the ratio of amount of zinc nitrate and thioacetamide18,19. In addition, ZnS nanoparticle clusters were uniformly coated with silica shell (tSiO2 ~ 21.4 ± 1.8 nm) through a modified Stöber method in the presence of PVP ligands on the ZnS surface, which act as anchoring sites for silica deposition20,21 (Figure 1D, Figure S1). The surface charge on ZnS/SiO2 is negative due to the presence of the surface hydroxyl groups of the SiO2 shell20. The as-prepared suspensions of ZnS/SiO2 colloids are solvent exchanged to propylene carbonate (~ 10 wt% ZnS/SiO2 in solvent). Propylene carbonate was chosen as a solvent because it exhibits excellent electrochemical stability and low vapor pressure22.


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To demonstrate the dynamic transparency tunability of ZnS/SiO2 suspensions, we fabricated a SPD cell using transparent ITO glass electrodes separated by Kapton tape as shown in Figure 2A. Due to their negative surface charge, ZnS/SiO2 nanoparticles can migrate and concentrate on the positive electrode under an external electric field, resulting in changes to structural parameters during particle assembly (i.e. inter-particle distance and the order of particle assemblies). The schematics of fabricated SPD cell in the presence of applied voltage (ON state) and in the absence of applied voltage (OFF state) are shown in Figure 2A. During ON state, the resulting nanoparticle arrangements arise from the balance between the electrostatic repulsion between the particles and the electrostatic attraction between surface charged particles and the external electric field. The tunability of the particle assembly and the resulting transparency were tested by varying the applied field. The transparency of SPD cell composed of ZnS/SiO2 nanostructures gradually increases as the applied field increases (Figure 2B, Movie S1) and the cell changes from opaque (OFF state) to mostly clear at an applied field of 2000 V/cm (ON state, i.e. 5 V at 25 µm spacing and 10 V at 50 µm spacing, Figure 2B). For example, during ON state, the transmission of the device at 50 µm spacing with dZnS/tSiO2 of 73/21 nm is increased from ~6 to ~48%, ~25 to ~71 %, ~47 to ~78%, measured at 450, 550, 650 nm, respectively (Figure 2E). Note that the characters (i.e. Lawrence Livermore National Laboratory) with a white paper were put on the backside of the cell to qualitatively estimate the transparency of the SPD. Furthermore, the opacity of the SPD cell is significantly affected by spacing thickness of the cell. It is obvious that the opacity at 50 µm spacing of SPD is higher compared to the 25 µm spacing at OFF state despite of similar transparency of the cells at 25 and 50 µm at ON state and therefore the


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transparency effect is more pronounced at 50 µm spacing of SPD (Figure 2B). Note that although the structural color can be generated from nanoparticle assemblies with weakly ordered structures, the intensity of the structural color is not noticeable since the structural color is likely to be overwhelmed by significant incoherent scattering from white body color of ZnS. Moreover, we observed that the particle size of colloidal nanoparticles is also critical to the transparency tunability. The opacity of the device increases with increasing particles size of ZnS/SiO2 nanoparticles (Figure 2B-2D). Specifically, the device with too small of particles (i.e. dZnS/tSiO2 ~ 62/21 nm) is already somewhat transparent even at OFF state (Figure 2C) and SPD with too large of particle (i.e. dZnS/tSiO2 ~ 110/21 nm) is still hazy even at ON state (Figure 2D), suggesting a range of “sweet spot” particle sizes for given wavelength ranges. According to Mie scattering theory23 and Beer-Lambert law24, the light scattering by the particles is significantly influenced by the particle size and light path length through the material and thus it is reasonable that the transmission decreases with increasing the particle size and the thickness of the device. Note that the silica shell thickness can also affect the optical scattering of core/shell particle assemblies and we previously showed that the stop-band position from the particle assemblies is red-shifted as the shell thickness increases at the same size of the core particles due to increased inter-particle distance of core particles25. However, since the refractive index difference between ZnS core particles (nZnS = 2.37) and dispersing media, propylene carbonate (PC) (npc = 1.42), is significantly larger than the difference that between SiO2 shells (nSiO2 = 1.45) and PC, the optical scattering is significantly affected by volume fraction of the core materials as compared with that of the SiO2 shell materials.


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The transmission spectra of the SPD cell (Figure 2E) confirm that the transmission is clearly enhanced at smaller particle size and dramatically increases at ON state as compared with OFF state. For example, the transmission at 550 nm is ~ 6% for the device with dZnS/tSiO2 of 110/21 nm, ~25% at that with dZnS/tSiO2 of 73/21 nm, and ~37% at that with dZnS/tSiO2 of 62/21 nm at OFF state. During the ON state, the transmission of the device increases from 6% to 19% for dZnS/tSiO2 of 110/21 nm, from 25% to 70%, and 37% to 71 % for dZnS/tSiO2 of 73/21 nm and dZnS/tSiO2 of 62/21 nm, respectively, measured at 550 nm. We propose that the dynamic transparency tunability in response to electric field is attributed to the change of structural ordering of particle assemblies. Since the particles are concentrated on the positive electrode under the applied field, the nanoparticles are not aggregated in a random manner but likely to form more ordered structures due to the presence of electrostatic repulsion between the particles thereby enhancing the transparency of the SPD cell. This hypothesis can be supported by our previous ultra small angle x-ray scattering (USAXS) results25, which showed that coherent length of particle assemblies is longer at higher particle concentration in suspension media. Although the structural color from the light reflection through the stop-band of particle assemblies is suppressed by incoherent scattering from ZnS/SiO2 nanoparticles, the light reflection arising from the stop-band characteristics of the particle assemblies can be obtained by manipulating the device fabrication and the particle size of ZnS/SiO2. We modified the device using a thinner spacer (10 µm), smaller particle size, and a glassy carbon as a backside electrode to reduce the scattering of the particles and to absorb incoherent scattering from white body color of ZnS. Figure 3 shows the reflection spectra


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of the modified SPD with smaller ZnS/SiO2 nanoparticle assemblies (dZnS/tSiO2 of 62/21 nm) at ON and OFF states. Although the incoherent scattering is not completely removed, it is obvious that the stop band position is blue-shifted as the applied field increases due to smaller inter-particle distance in the structures and the peak positions of reflection spectra are located in the near UV - UV range, which is near 393 nm and 320 nm at OFF and ON state, respectively. Since the stop-band position at ON state is in UV range, the reflected color under the incident light in the visible range is not noticeable in the presence of an applied field as shown in the inset pictures in Figure 3. We previously demonstrated that the structures of our colloidal core/shell nanoparticles exhibit shortrange order and are weakly correlated to each other (i.e. amorphous structures), as confirmed by ultra-small-angle X-ray scattering measurements in our previous report25. We proved that the resulting amorphous particle structures show non-iridescent structural colors and isotropic photonic band gaps, indicating that the photonic band characteristics of our particle structures are independent of viewing angle, which is consistent with other reports of engineered amorphous photonic crystals26,27. In order to further confirm the transparency tunability by varying the particle assemblies, we performed numerical analysis, based on FDTD calculations of Maxwell’s equations. Figure 4A displays the simulation schematics: the structure is excited by plane wave with perfectly matched layers surrounding the simulation domain to avoid reflections at the boundaries. The transmission power is normalized by the source excitation power per wavelength. Figure 4B shows the schematics of the particle arrangement and individual particle positions at various randomness of the particle


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positions (rA) at a given inter-particle distance and diameter of core/shell particles (D). As expected, the particle assemblies are more disordered at higher rA. Figure 4 C and D show the simulated transmission spectra with different rA at A = 1.1D and 1.5D, respectively. Note that D = 100 ± 6 nm (Dcore = 60 ± 6 nm, tshell = 20 nm) and refractive index of core (ncore = 2.367) and shell (nshell = 1.45) are the parameters we chose for the simulation to represent ZnS/SiO2 core/shell nanoparticles. All spectra show photonic band structures (i.e. stop- and pass-band) and the stop-band position is blueshifted as inter-particle distance (A) decreases and is 413 nm and 325 nm at A = 1.5D and 1.1D, respectively, which is consistent with experimental observation (Figure 3) and follows the Bragg-Snell’s law. We suggest that A at OFF state in experimental conditions appears to be slightly less than 1.5D since the stop-band position from simulation data is somewhat red-shifted (i.e. 413 nm, Figure 4D) as compared with that from experimental observation (i.e. 393 nm). Interestingly, the transmission through the pass-band of particle assemblies noticeably decreases with increasing rA as shown in Figure 4C-E. The transmission of particle assemblies can be determined by both the band structures from particle assemblies and particle scattering. The complete band structure is formed when the particles assemblies are perfectly ordered possessing periodic structures28, which refers to the rA = 0 in our case. On the contrary, at the limit of complete disorder, the band structure no longer exists and the transmission is solely affected by the scattering events (i.e. more scattering at short wavelength and larger particle size according to Rayleigh/Mie scattering theory23,29). In this respect, as rA increases, the transmission is likely to be dominated by particle scattering, resulting in the collapse of the photonic band structures, especially the pass-bands, thereby decreasing the transmission.


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Specifically, the transmission at 370 and 570 nm and A = 1.5D (Figure 4D) decreases from 79% to 21% for 370 nm and 79% to 55 % for 570 nm as rA increases from 0 to 0.5D, indicating the structural ordering of the particle assemblies plays critical roles in affecting the transparency. In order to more clearly compare the simulated transmission spectra with experimental observation, the simulated transmission spectra of ZnS/SiO2 structures at smaller A and rA (i.e. A = 1.1D, rA = 0) and larger A and rA (i.e. A = 1.5D, rA = 0.5D), assumed to be similar conditions of experimental observation at ON and OFF state, respectively, are shown in Figure 3E. The overall trends of the transmission spectra features from the simulation and the experimental observation (i.e. Figure 2E) at ON and OFF state are comparable. In addition, the averaged transmission at A = 1.1D and rA = 0 and at A = 1.5D and rA = 0.5D is ~ 72 %, ~40 % in the range of 350-700 nm which is somewhat comparable to the averaged transmission of ON/OFF ratio from experimental observation (i.e. ~58% at ON state and ~26% at OFF state) although the transmission values from the simulation is slightly higher than that from the experimental observation due to different experimental and simulation conditions (i.e. device thickness). Thus, with the support of simulation data, it is reasonable to postulate that increased transparency at larger electric field can be viewed collectively as a consequence of enhanced structural ordering of particle assemblies and thereby improved photonic band structures. We postulate that this novel approach to dynamically tune the transparency in response to electric stimuli can lead not only to design new types of SPD but also to improve the functionality of SPD by utilizing various inherent optical properties of colloidal nanoparticles. In this regard, we chose Fe2O3/SiO2 nanoparticles to demonstrate transparency as well as coloration tunable devices in response to electric field.


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We successfully fabricated Fe2O3 nanoparticle clusters with different particle size in the range of ~ 40 – 95 nm (Figure S2, and Figure 5A) prepared by microwave assisted hydrothermal method and these particles are coated with silica with modified Stöber methods21 (Figure S3, and Figure 5B). SPD cell with Fe2O3/SiO2 core/shell nanoparticles with dFe2O3 = 93.4 ± 9.8 nm and tSiO2 = 24.3 ± 3.8 nm (Figure 5A and 5B) in propylene carbonate (~15 wt% Fe2O3/SiO2) shows largest transparency difference between ON and OFF state (Figure 5C, S3, and Movie S2). Specifically, the opacity at OFF state increases with increasing particle size (Figure S3), making it difficult to observe the letters on the white paper that was put on the backside of SPD cell with dFe2O3/tSiO2 of 93/24 nm (Figure 5C). Nonetheless, the opacity decreases with increasing applied voltage and the letters in the backside of SPD cell become observable at 5V (Figure 5C). Based on the transmission spectra of SPD cell at OFF (0 V) and ON (5 V) state, the transmission at ON state (~64 % at 625 nm) is dramatically higher than that at OFF state (11% at 625 nm) as shown in Figure 5D. In addition, based on FDTD simulations using the structural parameters of Fe2O3/SiO2 particle assemblies (Figure S4), the transmission of particle assemblies significantly increases with decreasing rA, indicating higher transparency can be obtained at more ordered particle assemblies, which is consistent with the simulation and experimental results of ZnS/SiO2 particle assemblies and the experimental observation of tunable transparency of Fe2O3/SiO2 structures under the applied field. Furthermore, the opacity of the device at OFF state as well as the transparency difference between ON and OFF state decrease as the particle concentration in propylene carbonate increases from 15 wt% to 20 wt% (Figure S5). As confirmed with FDTD simulation and our previous USAXS results, these observed effects are attributed to increased structural


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ordering due to reduced rA at higher applied voltage or particle concentration. Note that no significant transmission in the range of 350 to 550 nm is observed mainly due to inherent absorption properties of Fe2O3 nanoparticles (see gray line in Figure 5D), which is one of indications that the characteristics of SPD can be tunable by inherent optical properties of responsive materials. Interestingly, beyond transparency effects, observed coloration from light reflections through the particle arrangements is also dynamically tunable under applied voltage as shown in Figure 5C. Similar to our previous report25, the reflected colors are generated by a combination of inherent pigmentary color of Fe2O3, observed in the range of 550 – 700 nm and the structural color from Fe2O3/SiO2 nanoparticle arrangement (Figure 5E). The reflected color of the device is yellowish orange (λmax ~ 620 nm) at OFF state (0 V) and is mainly due to inherent pigmentary color. As applied voltage increases, the structural color contribution to reflected coloration is observable in the range of 300 to 550 nm and blue-shifted from λpeak ~ 478 nm and 398 nm at 3 V to λmax ~ 330 nm at 5 V (Figure 4E) due to smaller inter-particle distance. The reflected color, which is a combination of structural and pigmentary colors, changes to orange-yellow (0-1 V), reddish purple (3 V) and deep-reddish purple (5 V) as shown in Figure 5C. We observed previously that the pigment color is also tunable under the applied field since the intensity ratio (i.e. I610/I730) near 610 nm, assigned to 6A1 → 4T2 transition of Fe2O3 and 730 nm, characterized by a local reflectivity maximum, decreases with decreasing inter-particle distance. The cause of the reduction of I610/I730 at shorter inter-particle distance still remains unclear but we proposed that the absorption of incident light in Fe2O3 to produce pigmentary color is decreased by the reflection from the structural color, thereby


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reducing the reflection of the pigments (I610) when the reflection spectra from particle assemblies match well with the absorption spectra of Fe2O3. This proposal is viable since the I610/I730 is inversely proportional to the amount of spectral overlap between the absorption spectra of Fe2O3 and reflectance spectra from structural color of Fe2O3/SiO225. Based on the transparency and coloration tunabilities using controlled particle assemblies and optical inherent properties of core/shell nanoparticles, it is reasonable to propose that novel and various light matter interactive devices can be generated by using optical nanomaterials such as quantum dots, lanthanide activated metal oxide, and metal nanoparticles. Future studies will explore these types of novel electro-optical device and investigate the optical interactions between the nanoparticle assemblies and inherent optical properties of the nanoparticles. As shown in Movie S1 and S2, our SPD instantaneously responds to applied field. The responding time of the device is typically in the range of tens milliseconds30, which is similar to existing SPD devices31,32 but much faster than commercial electrochromic (EC) windows (hundreds seconds33). In addition, we demonstrated that the reflected color as well as the transmission of the device can be precisely controlled by the strength of applied field, the particle composition, the particle size, the thickness of spacing of the device whereas current SPD and EC device have mainly focused on the transparency tunability. Moreover, the light control in our SPD can be performed with relatively low energy requirement (up to 5 V), which is comparable to EC device (0 – 5 V)33 but significantly less than existing SPD system (24 – 110 V)31,32. Furthermore, we investigate the operational stability of our SPD (Figure S6) and the variations of the spectra at ON-OFF cycle are not noticeable up to 200 cycles, indicating our SPD shows


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excellent stability. However, in terms of ON-OFF ratio of the transparency tunability, our SPD needs to be further improved as compared with existing SPD. The average transmission at ON state in existing SPD is typically 50 – 60 % in the visible range31,32, which is similar to our SPD (~58 %), but the average of the light blockage at OFF state in current system is around 5% in the visible range31,32, which is significantly lower than our SPD (i.e. ~26%), suggesting that the current existing SPD device shows more efficient ON-OFF transparency tunability. We believe the performance of transparency tunability of our SPD can be improved by further improvements in processing and the development of device fabrications and the particle compositions and dispersing media, which is beyond the scope of this work. In summary, we successfully fabricated novel transparency tunable device in response to electric stimuli using colloidal spherical core/shell nanoparticles. We synthesized relatively monodispersed ZnS/SiO2 (δ = 5%) as electrically responsive materials and optimized the particle size to maximize the transparency tunability. The transparency of the device increases with decreasing the particle size and device thickness and more importantly, increasing the electric field due to improved photonic band structures from enhanced structural ordering of nanoparticle assemblies, which is confirmed by FDTD simulations. Interestingly, when using pigmentary Fe2O3/SiO2 nanoparticles with optimal particle size, the coloration accompanied with the transparency is dynamically and effectively tunable in response to electric stimuli. Furthermore, the transmission of our SPD at ON and OFF state is not affected by a number of ON-OFF cycles (up to 200 cycles), suggesting the operational stability of the device is excellent and comparable to current existing SPD. This demonstration of the


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transparency as well as coloration tunability of the SPD not only improve the versatility and the functionality of smart window device but also provide a pathway toward the rational design of new types of multi-functional light matter interactive devices.

Experimental Synthesis of ZnS nanoparticle clusters. ZnS colloidal nanoparticle clusters were prepared by homogeneous precipitation from acidic zinc nitrate aqueous solutions using the thermal decomposition of thioacetamide (TAA) as a source of sulfide ions18,19. Poly (vinyl pyrrolidone) (PVP) acted as both as a structure-directing agent and a stabilizer for the growing ZnS. In a typical synthesis of 72.6 ± 4.2 nm ZnS nanoparticle clusters, 10 g of PVP (Sigma-Aldrich, Mw = 55,000) was dissolved in 150 ml of Mill-Q deionized (DI) water and heated to 80°C (solution (1)). In parallel, 0.01 mol of Zn(NO3)2.9H2O (J.T. Baker, 99%) was dissolved in 50 ml DI water and the solution was heated to 80°C (solution (2)). Then, 0.2 mol of TAA (Sigma-Aldrich, 99%) and 0.2 ml of HNO3 (Sigma-Aldrich, 70%) were added to solution (1) and vigorously stirred for 20 min. Subsequently, the solution (2) was quickly added into solution (1) and uniformly stirred (~800 rpm) for 1.5 h at 80°C. The particle size can be varied from ~60 to ~400 nm by manipulating the ratio of amount of zinc nitrate and TAA and increases with increasing the molar ratio of zinc nitrate to TAA (i.e. dZnS ~ 60 nm at [Zn]/[TAA] = 0.025, and dZnS ~ 400 nm at [Zn]/[TAA] = 0.1). Synthesis of α-Fe2O3 nanoparticle clusters. α-Fe2O3 nanoparticle clusters were prepared modifying a previously reported hydrothermal approach34. Briefly, 1.8 g of PVP was added to 40 mL of anhydrous N,N-


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dimethylformamide (Sigma-Aldrich, 99.8%) and stirred for 30 min to completely dissolve the reagents. Thereafter, 0.1 mmol of Fe(NO3)3.9H2O (Sigma-Aldrich, >99.95%) was added in the solution, vigorously stirred for 15 min, and then 5–80 µl HNO3 (SigmaAldrich, 70%) was added and stirred for another 15 mins. The particle size increases with increasing the amount of HNO3 (i.e. dFe2O3 ~ 42 nm at 5 µl of HNO3 and dFe2O3 ~ 95 nm at 80 µl of HNO3). Subsequently, the solution was transferred into a sealed and specialized glass vessel with a capacity of 80 mL and then placed in a microwave furnace (CEM Corp., model SP-X with frequency of 2.45 GHz and variable power up to 300 W) and heated to 180°C for 3 h. 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 ZnS/SiO2 and α-Fe2O3 /SiO2 core/shell nanoparticles α-Fe2O3/SiO2 and ZnS/SiO2 core/shell colloids were prepared through a modified Stöber method21 assisted with microwave heating. Typically, 40 mg of α-Fe2O3 or 20 mg ZnS nanoparticle clusters was added to 35 mL EtOH with 2 ml ammonium hydroxide (J.T. Baker, 28%) and 6 mL Mill-Q water and stirred vigorously for 20 min. Then, 1 ml Tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99.999%) was added to the solution and subsequently placed in a microwave furnace (CEM Corp., model SP-X with frequency of 2.45 GHz and variable power up to 300 W) and heated to 50°C for 20 min. The obtained core–shell particles were washed with EtOH through repetitive centrifugation and redispersion. The nanoparticles were finally dispersed in propylene carbonate at 10 wt% of ZnS/SiO2 or 15-20 wt% of Fe2O3/SiO2. Fabrication of a SPD cell


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The colloidal nanoparticles were then injected between two ITO electrodes (1 inch × 1 inch) separated by a 25 or 50 µm spacer of Kapton tape to investigate the electrophoretic response of transparency as well as structural color from colloidal nanoparticles in propylene carbonate. In order to address the photonic band gap of ZnS/SiO2 nanoparticle assemblies, the colloidal particles were injected between a ITO electrode (i.e. front side) and a glassy carbon electrode (i.e. back side) separated by a 10 µm spacer of double-side tape. FDTD simulation The FDTD simulations were performed with a commercial package (Lumerical inc.). We define the parameters for the simulation, where D = diameter of core/shell particles, A = inter-particle distance defined as the distance from the center to center of adjacent core/shell particles (in units D), rA = randomness of the position of the particles (in units of D, uniform random distribution in [-rA,rA]), and rcore and rshell = the poly-dispersity of core and shell size (i.e. 0 – 100%, with 0% signifying complete particle uniformity), respectively. Note that rA is chosen in the range from 0 to gap distance between the particles (i.e. (A-1)D) to avoid overlap between particles. D = 100 nm (Dcore = 60 nm, tshell = 20 nm), rcore = 10% and rshell = 0% and D = 160 nm (Dcore = 100 nm, tshell = 30 nm), rcore = 0% and rshell = 0% are the parameters we chose for the simulation to represent ZnS/SiO2 and Fe2O3/SiO2 core/shell nanoparticles, respectively.

In this work, the

refractive indices (n) of the ZnS, Fe2O3, and silica were assumed to be 2.367, 2.8, and 1.45, respectively. Characterization


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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. The reflection properties of the device with Fe2O3/SiO2 and ZnS/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 applied voltage on the coloration. An Ocean Optics HR2000+ with a balanced tungsten source (400 – 900 nm) was used to investigate the effect of applied voltage on the transparency in a SPD cell.

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. Portions of this work was supported by the Laboratory Directed Research and Development (LDRD) project 16-ERD-019.


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References (1) Baetens, R.; Jelle, B. P.; Gustavsen, A. Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: A state-of-the-art review. Solar Energy Materials and Solar Cells 2010, 94, 87-105. (2) Kamalisarvestani, M.; Saidur, R.; Mekhilef, S.; Javadi, F. Performance, materials and coating technologies of thermochromic thin films on smart windows. Renewable and Sustainable Energy Reviews 2013, 26, 353-364. (3) Azens, A.; Granqvist, C. Electrochromic smart windows: energy efficiency and device aspects. Journal of Solid State Electrochemistry 2003, 7, 64-68. (4) Ghosh, A.; Norton, B. Durability of switching behaviour after outdoor exposure for a suspended particle device switchable glazing. Solar Energy Materials and Solar Cells 2017, 163, 178-184. (5) Chakrapani, S.; Slovak, S. M.; Saxe, R. L.; Fanning, B.: SPD films and light valves comprising same. Google Patents, 2002. (6) Vergaz, R.; Pena, J.; Barrios, D.; Pérez, I.; Torres, J. Electrooptical behaviour and control of a suspended particle device. Opto-Electronics Review 2007, 15, 154-158. (7) Wang, Q.: Nano smart glass system. Google Patents, 2013. (8) Kim, S. H.; Lee, S. Y.; Yang, S. M.; Yi, G. R. Self-assembled colloidal structures for photonics. NPG Asia Materials 2011, 3, 25-33. (9) Yoshino, K.; Shimoda, Y.; Kawagishi, Y.; Nakayama, K.; Ozaki, M. Temperature tuning of the stop band in transmission spectra of liquid-crystal infiltrated synthetic opal as tunable photonic crystal. Applied Physics Letters 1999, 75, 932-934. (10) Ge, J.; Hu, Y.; Yin, Y. Highly tunable superparamagnetic colloidal photonic crystals. Angewandte Chemie 2007, 119, 7572-7575. (11) Lee, K.; Asher, S. A. Photonic crystal chemical sensors: pH and ionic strength. Journal of the American Chemical Society 2000, 122, 9534-9537. (12) Ballato, J.; James, A. A ceramic photonic crystal temperature sensor. Journal of the American Ceramic Society 1999, 82, 2273-2275. (13) Yin, S. N.; Wang, C. F.; Liu, S. S.; Chen, S. Facile fabrication of tunable colloidal photonic crystal hydrogel supraballs toward a colorimetric humidity sensor. Journal of Materials Chemistry C 2013, 1, 4685-4690. (14) Chen, S.; Roh, K.; Lee, J.; Chong, W. K.; Lu, Y.; Mathews, N.; Sum, T. C.; Nurmikko, A. A photonic crystal laser from solution based organo-lead iodide perovskite thin films. ACS nano 2016, 10, 3959-3967. (15) Toyotama, A.; Yamanaka, J.; Yonese, M.; Sawada, T.; Uchida, F. Thermally driven unidirectional crystallization of charged colloidal silica. Journal of the American Chemical Society 2007, 129, 3044-3045. (16) Park, G. C.; Xue, W.; Taghizadeh, A.; Semenova, E.; Yvind, K.; Mørk, J.; Chung, I. S. Hybrid vertical‐cavity laser with lateral emission into a silicon waveguide. Laser & Photonics Reviews 2015, 9.


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(17) Branda, F.; Silvestri, B.; Costantini, A.; Luciani, G. Effect of exposure to growth media on size and surface charge of silica based Stöber nanoparticles: a DLS and ζ-potential study. Journal of Sol-Gel Science and Technology 2015, 73, 54-61. (18) Hosein, I. D.; Liddell, C. M. Homogeneous, core− shell, and hollow-shell ZnS colloid-based photonic crystals. Langmuir 2007, 23, 2892-2897. (19) Velikov, K. P.; van Blaaderen, A. Synthesis and characterization of monodisperse core− shell colloidal spheres of zinc sulﬁde and silica. Langmuir 2001, 17, 4779-4786. (20) Han, M. G.; Shin, C. G.; Jeon, S. J.; Shim, H.; Heo, C. J.; Jin, H.; Kim, J. W.; Lee, S. Full Color Tunable Photonic Crystal from Crystalline Colloidal Arrays with an Engineered Photonic Stop-Band. Advanced Materials 2012, 24, 6438-6444. (21) Stöber, W.; Fink, A.; Bohn, E. Controlled growth of monodisperse silica spheres in the micron size range. Journal of colloid and interface science 1968, 26, 62-69. (22) Nasirzadeh, K.; Neueder, R.; Kunz, W. Vapor Pressures of Propylene Carbonate and N,N-Dimethylacetamide. Journal of Chemical & Engineering Data 2005, 50, 26-28. (23) Mie, G. Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Annalen der Physik 1908, 330, 377-445. (24) Calloway, D. Beer-lambert law. J. Chem. Educ 1997, 74, 744. (25) Han, J.; Lee, E.; Dudoff, J. K.; Bagge‐Hansen, M.; Lee, J. R.; Pascall, A. J.; Kuntz, J. D.; Willey, T. M.; Worsley, M. A.; Han, T. Y. J. Tunable Amorphous Photonic Materials with Pigmentary Colloidal Nanostructures. Advanced Optical Materials 2017, 5. (26) Takeoka, Y.; Honda, M.; Seki, T.; Ishii, M.; Nakamura, H. Structural Colored Liquid Membrane without Angle Dependence. ACS Applied Materials & Interfaces 2009, 1, 982-986. (27) Saranathan, V.; Forster, J. D.; Noh, H.; Liew, S.-F.; Mochrie, S. G. J.; Cao, H.; Dufresne, E. R.; Prum, R. O. Structure and optical function of amorphous photonic nanostructures from avian feather barbs: a comparative small angle X-ray scattering (SAXS) analysis of 230 bird species. Journal of the Royal Society Interface 2012, 9, 25632580. (28) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N.: Photonic Crystals: Molding the Flow of Light; Princeton University Press, 1995. (29) Clays, K.; Persoons, A. Hyper-Rayleigh scattering in solution. Physical review letters 1991, 66, 2980. (30) Lee, I.; Kim, D.; Kal, J.; Baek, H.; Kwak, D.; Go, D.; Kim, E.; Kang, C.; Chung, J.; Jang, Y.; Ji, S.; Joo, J.; Kang, Y. Quasi-Amorphous Colloidal Structures for Electrically Tunable Full-Color Photonic Pixels with Angle-Independency. Advanced Materials 2010, 22, 4973-4977. (31) Ghosh, A.; Norton, B.; Duffy, A. Measured overall heat transfer coefficient of a suspended particle device switchable glazing. Applied Energy 2015, 159, 362-369. (32) Ghosh, A.; Norton, B.; Duffy, A. Effect of sky conditions on light transmission through a suspended particle device switchable glazing. Solar Energy Materials and Solar Cells 2017, 160, 134-140.


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(33) Piccolo, A.; Simone, F. Performance requirements for electrochromic smart window. Journal of Building Engineering 2015, 3, 94-103. (34) Zheng, Y.; Cheng, Y.; Wang, Y.; Bao, F.; Zhou, L.; Wei, X.; Zhang, Y.; Zheng, Q. Quasicubic α-Fe2O3 Nanoparticles with Excellent Catalytic Performance. The Journal of Physical Chemistry B 2006, 110, 3093-3097.


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Figure 1. Representative SEM images of ZnS nanoparticle clusters with different particle size of (A) dZnS = 110.3 ± 5.7 nm, (B) dZnS = 72.6 ± 4.2 nm, and (C) dZnS = 62.1 ± 3.1 nm and (D) TEM images of ZnS/SiO2 core/shell nanoparticles (dZnS = 72.6 ± 4.2 nm, tSiO2 = 21.4 ± 1.8 nm). Insets in Figure (B) and (D) represent the corresponding SEM and TEM images obtained at higher magnification. Scale bars in every image are 200 nm.


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Figure 2. (A) The schematic of a SPD cell in the absence (OFF state) and presence (ON state) of applied voltage. (B) Photograph of ZnS (dZnS = 72.6 ± 4.2 nm)/SiO2 suspensions in a SPD cell under different applied voltage at 25 and 50 µm spacing. (C) Photograph of ZnS/SiO2 suspensions with (C) smaller (dZnS = 62.1 ± 3.1 nm) and (D) larger (dZnS = 110.3 ± 5.7 nm) core size in a SPD cell (50 µm spacing) at OFF and ON state. Note that the characters (i.e. Lawrence Livermore National Laboratory) with a white paper was put on the backside of the cell to qualitatively estimate the transparency of the SPD and tSiO2 of all samples is comparable to 20-25 nm. (E) Transmission spectra of the ZnS/SiO2 suspension with various particle size in a SPD cell at ON and OFF state.


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Figure 3. Reflection spectra of the ZnS/SiO2 (dZnS = 62.1 ± 3.1 nm, tSiO2 =21.4 ± 1.8 nm) suspension in a modified SPD cell (i.e. with at 10 µm spacing and a glassy carbon as a backside electrode) at 0 and 5 V. The insets show the photograph in a modified SPD cell under 0 V and 5 V under the light in the visible range illumination.


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Figure 4. (A) FDTD simulation settings: simulations boundaries in orange, structure spheres in cyan, and power monitors in yellow. The arrow in magenta indicates the excitation source direction. (B) The schematic of the particle arrangement at A = 1.5D with rA = 0, 0.25D, and 0.5D. Simulated transmission spectra of ZnS/SiO2 particle structures with (D = 100 nm (Dcore = 60 nm, tshell = 20 nm), rcore = 10% and rshell = 0%) at A = (C) 1.1D, (D) 1.5D with different rA. We define the parameters, where D = diameter of core/shell particles, A = inter-particle distance, rA = randomness of the position of the particles, and rcore and rshell = the poly-dispersity of core and shell size, respectively. (E) Simulated transmission spectra of the ZnS/SiO2 particle structures at A = 1.1D with rA = 0, and A=1.5D with rA = 0.5D, which are assumed to be similar conditions in experimental observation.


ACS Photonics 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

Figure 5. Representative (A) SEM and (B) TEM images of Fe2O3 nanoparticle clusters (dFe2O3 = 93.4 ± 9.8 nm) and Fe2O3/SiO2 core/shell nanoparticles (dFe2O3 = 93.4 ± 9.8 nm and tSiO2 = 24.3 ± 3.8 nm), respectively. Scale bars in all images are 200 nm. (C) Photograph of Fe2O3/SiO2 suspensions (~15 wt%) in a SPD cell under different applied voltage. The characters (i.e. Lawrence Livermore National Laboratory) with a white paper put on the backside of the cell. (D) Transmission spectra of the Fe2O3/SiO2 suspensions in a SPD cell at 0 and 5 V and absorption spectra of Fe2O3 nanoparticle clusters. (E) Reflection spectra of Fe2O3/SiO2 suspension in a SPD cell at 0, 3 and 5V.


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A table of content


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TOC figure 254x190mm (128 x 128 DPI)


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Figure 1. Representative SEM images of ZnS nanoparticle clusters with different particle size of (A) dZnS = 110.3 ± 5.7 nm, (B) dZnS = 72.6 ± 4.2 nm, and (C) dZnS = 62.1 ± 3.1 nm and (D) TEM images of ZnS/SiO2 core/shell nanoparticles (dZnS = 72.6 ± 4.2 nm, tSiO2 = 21.4 ± 1.8 nm). Insets in Figure (B) and (D) represent the corresponding SEM and TEM images obtained at higher magnification. Scale bars in every image are 200 nm. 338x190mm (96 x 96 DPI)


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Figure 2. (A) The schematic of a SPD cell in the absence (OFF state) and presence (ON state) of applied voltage. (B) Photograph of ZnS (dZnS = 72.6 ± 4.2 nm)/SiO2 suspensions in a SPD cell under different applied voltage at 25 and 50 µm spacing. (C) Photograph of ZnS/SiO2 suspensions with (C) smaller (dZnS = 62.1 ± 3.1 nm) and (D) larger (dZnS = 110.3 ± 5.7 nm) core size in a SPD cell (50 µm spacing) at OFF and ON state. Note that the characters (i.e. Lawrence Livermore National Laboratory) with a white paper was put on the backside of the cell to qualitatively estimate the transparency of the SPD and tSiO2 of all samples is comparable to 20-25 nm. (E) Transmission spectra of the ZnS/SiO2 suspension with various particle size in a SPD cell at ON and OFF state. 338x190mm (96 x 96 DPI)


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Figure 3. Reflection spectra of the ZnS/SiO2 (dZnS = 62.1 3.1 nm, tSiO2 =21.4 1.8 nm) suspension in a modified SPD cell (i.e. with at 10 m spacing and a glassy carbon as a backside electrode) at 0 and 5 V. The insets show the photograph in a modified SPD cell under 0 V and 5 V under the light in the visible range illumination. 338x190mm (96 x 96 DPI)


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Figure 4. (A) FDTD simulation settings: simulations boundaries in orange, structure spheres in cyan, and power monitors in yellow. The arrow in magenta indicates the excitation source direction. (B) The schematic of the particle arrangement at A = 1.5D with rA = 0, 0.25D, and 0.5D. Simulated transmission spectra of ZnS/SiO2 particle structures with (D = 100 nm (Dcore = 60 nm, tshell = 20 nm), rcore = 10% and rshell = 0%) at A = (C) 1.1D, (D) 1.5D with different rA. We define the parameters, where D = diameter of core/shell particles, A = inter-particle distance, rA = randomness of the position of the particles, and rcore and rshell = the poly-dispersity of core and shell size, respectively. (E) Simulated transmission spectra of the ZnS/SiO2 particle structures at A = 1.1D with rA = 0, and A=1.5D with rA = 0.5D, which are assumed to be similar conditions in experimental observation.

338x190mm (96 x 96 DPI)


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Figure 5. Representative (A) SEM and (B) TEM images of Fe2O3 nanoparticle clusters (dFe2O3 = 93.4 9.8 nm) and Fe2O3/SiO2 core/shell nanoparticles (dFe2O3 = 93.4 9.8 nm and tSiO2 = 24.3 3.8 nm), respectively. Scale bars in all images are 200 nm. (C) Photograph of Fe2O3/SiO2 suspensions (~15 wt%) in a SPD cell under different applied voltage. The characters (i.e. Lawrence Livermore National Laboratory) with a white paper put on the backside of the cell. (D) Transmission spectra of the Fe2O3/SiO2 suspensions in a SPD cell at 0 and 5 V and absorption spectra of Fe2O3 nanoparticle clusters. (E) Reflection spectra of Fe2O3/SiO2 suspension in a SPD cell at 0, 3 and 5V. 338x190mm (96 x 96 DPI)


Electro-optical device with tunable transparency using colloidal core

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