Iridescence in nematics: Photonic liquid crystals ofnanoplates in absence of long-range periodicityMinxiang Zenga, Daniel Kinga, Dali Huangb, Changwoo Doc, Ling Wanga,d, Mingfeng Chena,e, Shijun Leia,Pengcheng Line, Ying Chene, and Zhengdong Chenga,b,1

aArtie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843; bDepartment of Materials Science andEngineering, Texas A&M University, College Station, TX 77843; cNeutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831; dSchoolof Materials Science and Engineering, Tianjin University, 300350 Tianjin, China; and eGuangdong Provincial Key Laboratory on Functional Soft CondensedMatter, School of Materials and Energy, Guangdong University of Technology, 510006 Guangzhou, China

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved July 23, 2019 (received for review May 7, 2019)

Photonic materials with positionally ordered structure can interactstrongly with light to produce brilliant structural colors. Here, wefound that the nonperiodic nematic liquid crystals of nanoplatescan also display structural color with only significant orientationalorder. Owing to the loose stacking of the nematic nanodiscs, suchcolloidal dispersion is able to reflect a broad-spectrum wavelength,of which the reflection color can be further enhanced by addingcarbon nanoparticles to reduce background scattering. Upon theaddition of electrolytes, such vivid colors of nematic dispersion canbe fine-tuned via electrostatic forces. Furthermore, we took advan-tage of the fluidity of the nematic structure to create a variety ofcolorful arts. It was expected that the concept of implanting nematicfeatures in photonic structure of lyotropic nanoparticles may openopportunities for developing advanced photonic materials for dis-play, sensing, and art applications.

self-assembly | nematic colloids | photonic liquid crystals | neutronscattering | 2D nanomaterials

Followed by the rise of graphene, significant advances havebeen witnessed in the field of 2D nanomaterials (1). This has

led to particular research interest in understanding the collectivebehaviors of 2D nanomaterials in suspension (2). Like otheranisotropic particles (e.g., rods), 2D nanomaterials can formliquid-crystal (LC) phases when dispersed in a solvent (3).Driven by entropic interaction, the self-assembly of 2D nanodiscscan produce a fascinating variety of LC structures ranging fromorientationally ordered nematic (N) to positionally orderedstructures, including lamellar (L) and columnar (C) phases withpositional order in 1 and 2 dimensions, respectively (4). Thesesuperstructures with controlled ordering have emerged as prom-ising paradigms addressing challenges in energy storage (5), con-trolled drug delivery (6, 7), and self-healing materials (8). Forexample, by controlling liquid-crystalline alignment of 2D titaniumcarbide (MXene), researchers have demonstrated thickness-independent capacitance of vertically aligned nanosheet-basedfilm (5). Additionally, the integration of LC ordering of 2Dnanomaterials and additive manufacturing techniques (e.g., 3Dprinting), may enable advanced assembly technology for fabri-cating flexible and wearable electronics (9, 10). Therefore, de-veloping approaches of understanding and manipulating themesoscopic ordering of nanodiscs will significantly forwardpractical application of 2D nanomaterials.Among various types of ordered nanoarchitectures, photonic

liquid crystals (PLCs) of 2D nanomaterials have attracted recentattention owing to the ability of dynamically interacting with thelight of interest to achieve brilliant reflection colors (11). Gen-erally, the color of PLCs originates from the periodic structureswith long-range positional ordering, such as lamellar or helicalarrangement (12–17). In particular, lamellar structure is one ofthe most studied designs for fabricating photonic materials of 2Dbuilding blocks due to the structural similarity to natural nacreand pearls (18). For example, several lamellar 2D monolayers,

including graphene oxide and titanium oxide nanosheets (19, 20),have been developed into colorful PLCs. By contrast, formingvisible color by nematic structure is notoriously difficult due to thelack of long-range periodicity in most nematic phases. Most the-ories of lyotropic nematic phase have focused on its orientationallyordered features (21, 22), while the possibility of structural col-oration has yet to elicit much attention. To date, the only colorfulnematic materials were found to be chiral nematic (N*) phasewith the prerequisite of a chiral structure (23), which limits thechoices of nanoscale building blocks. In addition, the colorfulchiral phase has been mainly found in small-molecule thermo-tropic LC systems, while it remains debated if colorful N* phase ofnanosheets is feasible (23). In consequence of smaller order pa-rameter (i.e., less crystalline), the nematic phase may allow higherflexibility and larger directional diffusivity (24, 25), and thus ispromising for dynamic chemical/biological sensing, which wouldbe complementary to existing photonic materials with prolongedresponse to stimuli (26, 27). In addition, as lamellar structurerequires high monodispersity in nanoplate thickness, toxic chem-icals or strong oxidizers are often necessary to prepare monolayersduring exfoliation (19, 20, 28), while the less-ordered nematicstructure may circumvent such requirement (29). Despite theseadvantages, however, developing a photonic structure based onachiral nematic phase remains a formidable task, as the nematic

Significance

The striking colors of organisms, like butterflies, have sparkedtremendous research interest in developing artificial photoniccrystals with extraordinary optical properties. In most cases,the color of photonic crystals originates from the periodic mi-crostructure that manipulates light through optical interfer-ence. Here, we report a photonic structure that does not relyon long-range periodic arrangements. Instead, such structuralcolor comes from the nematic liquid crystals of nanodiscs asopposed to the conventional achiral nematic phases thatare colorless under white light. This finding challenges thestereotypical design of photonic liquid crystals based mainlyon periodically layered or helical structures. We expect thatthe concept of nematic photonic nanoparticles may openresearch opportunities for developing advanced photonic materials.

Author contributions: M.Z., L.W., M.C., and Z.C. designed research; M.Z., D.K., D.H., C.D.,and M.C. performed research; M.Z., C.D., S.L., P.L., Y.C., and Z.C. analyzed data; and M.Z.and L.W. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Published under the PNAS license.1To whom correspondence may be addressed. Email: zcheng@tamu.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1906511116/-/DCSupplemental.

Published online August 23, 2019.

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platelets dispersion must be developed in a way that can selectivelyreflect light in absence of long-range periodicity.Here, we report a photonic crystalline dispersion based on

unexfoliated 2D materials (zirconium hydrogenphosphate, ZrHP)without involving any strong long-range positional ordering. In-stead of developing a lamellar structure by monolayers, we tookadvantage of the local correlation of nematic nanodiscs withcontrolled size and polydispersity, resulting in reflecting a broadreflection wavelength with vivid color. The ZrHP platelets withlow polydispersity were prepared by a method involving gravity-driven size segregation followed by centrifugation of selectedfractions (Fig. 1A). Under gravity, the size segregation occurs aslarger particles tend to sediment faster due to Stokes’ law, wherethe terminal sedimentation velocity increases as the square ofparticle size (R2) for spherical particles (30). As shown in Fig. 1B,we loaded the nanodisc suspension into a cylindrical glass vial,allowing gravitational fractionation for 20 h, after which the de-sired fractions of suspension were collected and centrifuged toyield the final product. A much longer sedimentation time wasalso performed on ZrHP samples, where we observed an irides-cent layer at the top of the sediments. The size segregation wasconfirmed using scanning electron microscopy (SEM). As shownin Fig. 1C, the pristine ZrHP platelets showed a polydispersemorphology, whereas a much-narrowed distribution of plateletsize was seen for fractionated ZrHP (Fig. 1D). Quantitatively, thefractionation process caused a significant reduction in plateletpolydispersity (σ) from 0.58 to 0.13, while a similar decrease indiameter was also observed (SI Appendix, Fig. S1). Measured byatomic force microscopy, the average thickness of ZrHP plateletswas 46.6 nm of ZrHP (SI Appendix, Figs. S2 and S3). Remarkably,we observed that an iridescent structure started to form when thefractionation experiment proceeded at high volume fraction (ϕ >15%, Fig. 1 D, Inset). These suspended colloidal nanoplatesstrongly diffract visible light and render the solution vivid andmonochromic colors. Such diffraction of visible light can originatefrom the positional order of lamellar structure or local correlationof nematic nanodiscs (Fig. 1E). Despite increasing advances incolorful lamellar structure (19, 20), direct evidence of photonicstructure based on local correlation of nematic nanodiscs has notbeen experimentally proven yet.

To investigate the color behavior of the ZrHP platelets, wesuspended platelets at various concentrations in water. To re-duce the background scattering effect and enhance the colorcontrast, we added a small amount of carbon black (0.1 wt %)into the ZrHP dispersions. As shown in Fig. 2A, a variety ofstructural colors (including red, green, and blue) were seen uponincreasing the platelet volume fraction. By changing [ZrHP]from 29.7 to 14.8%, the structural color of ZrHP can besmoothly modulated from the blue (497 nm) to green (559 nm)and to red (644 nm) in a wide spectral range (Fig. 2B). It wasfound that the UV-vis reflection peaks throughout the system arequite broad, signaling the presence of short-range order of ne-matic phase since scattering pattern of nematic phase is typicallymuch wider than that of lamellar phase (Fig. 2 B, Inset) (4). Theuniform and single color throughout the system suggests theformation of an ordered structure that follows Bragg–Snellequation λmax = 2(d/m)(n2 − sin2θ)1/2, where d is the averagelattice spacing, m is the order of the Bragg reflection, θ is theincidence angle of light, and n is the average refractive index ofthe suspension (19, 31). Fig. 2C shows the peak wavelength(λmax) of ZrHP dispersion as a function of (1 − ϕ)/ϕ; a linearcorrelation was observed, which is comparable with small-molecule photonic system (32). The interlayer spacing (d) wasalso estimated by Bragg’s equation (Fig. 2C), showing a similarlylinearly proportional relation to (1 − ϕ)/ϕ. To calculate waterlayer thickness, we applied a modified Bragg’s equation for atwo-component system (SI Appendix, Fig. S4). It was found thatthe water layer thickness was in the range of 130 to 200 nm; suchbulky interparticle distance (102 nm) is much larger than theintermolecular distance of thermotropic LC (∼1 nm) (33, 34).Despite that the colors can be tuned by particle concentration,we found the color is not highly sensitive to surface anchoring.Upon shearing with different types of substrates, the PLC showsno significant change in color intensity or wavelength (additionaldiscussion on ZrHP color under shearing/disturbance can befound in SI Appendix, Figs. S5 and S6).Based on UV-vis analysis, we expected the structural color of

ZrHP origins from the long-range positional ordering, e.g.,smectic or columnar phases. To understand the self-assembly ofnanodiscs, we used field-emission SEM to analyze the photonic

Fig. 1. The gravity-driven fractionation of ZrHP platelets. (A) Schematic illustration of fractionation process under gravity. (B) Digital camera images of ZrHPsuspension: (A) t = 0 min; (B) t = 4 min; (C) t = 5 h; (D) t = 9 h; (E) t = 20 h; (F) t = 100 h. (C and D) SEM images of (C) pristine ZrHP and (D) fractionated ZrHP. (D,Inset) The dispersions of fractionated ZrHP in water. The concentration of ZrHP in dispersion increases from left to right, showing a blue shift in structuralcolor. (E) Schematic illustration of proposed color mechanisms of lamellar and nematic colors. The color of lamellar structure comes from the multilayerinterference, while the local alignment of nematic nanoplates may also produce structural color under suitable conditions.

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structure in microscale. As the electrostatic repulsion betweenZrHP platelets would disappear with the evaporation of water,the microstructure of PLC dispersion would completely lose itsinitial alignment. To prevent the collapse of ZrHP nanodiscs, weintroduced a freeze-drying process to preserve the structuralfeatures of ZrHP dispersion followed by SEM analysis of cross-

section samples. As shown in Fig. 3A, no long-range orderedlamellar structure was observed in the samples. Instead, the SEMimage revealed a rather twisting and bending structure showingelongation with periodic lines in some local regions. Such alignmentof platelets was in direct contrast to the positional-ordered lamellarstructure with long-range periodicity (e.g., smectic structure) (4).

Fig. 2. The structural colors of platelet suspensions. (A) Digital camera images of colorful ZrHP drops with different concentrations: (A) ϕ = 0.15; (B) ϕ = 0.23;(C) ϕ = 0.28; (D) ϕ = 0.32; (E) ϕ = 0.39; (F) ϕ = 0.39 (unfractionated ZrHP). (B) UV-vis spectra of platelet suspensions showing a wide range of colors. The insetproposes different scattering patterns from nematic (N) and lamellar (L) phases. (C) The maximum reflection wavelength (black) and calculated averaged spacing (blue) of platelets with different volume fraction.

Fig. 3. Structure characterization of platelet suspensions. (A) Field-emission SEM images of freeze-dried platelet samples. (B) SANS measurements of plateletsuspensions. (C) Theoretical prediction of phase diagram of discotic particles. (D) Polarized light microscopy of an as-prepared platelet suspension showingflow-induced birefringence. A nematic defect can be observed after equilibrating overnight (Inset). (E) UV-vis reflection spectra of platelet suspensionsshowing the structural colors tuned by NaCl (long dash) and NaH2PO4 (short dash). (F) Reflection spectra of platelet suspensions showing the structural colorstuned by charged polymer. (Inset) The structural color blue-shifting with CNC concentration. (Scale bars in A, Inset and D, Inset are 5 and 10 μm, respectively.)

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Furthermore, one of the most conclusive tools to determine col-loidal structure is the small-angle scattering techniques (35). Inparticular, the small-angle neutron scattering (SANS) was pre-ferred for our unexfoliated nanodiscs, as SANS has a higher pen-etration of bulk unexfoliated samples than small-angle X-rayscattering. As shown in Fig. 3B, the formation of high positionalordering was also disapproved as there is no observable signal oflong-range positional ordering within the q range investigated de-spite varying of the vol % of ZrHP from 0.1 to 31.4%. This suggeststhat there is no visible smectic layer ordering with a long correla-tion length in the ZrHP dispersion. At the highest concentration of31.4%, a small peak around q = ∼0.9 Å−1 was observed, whose

length scale�

2πq = ∼7Å

�reasonably agrees with the well-known

interlayer distance of ZrHP (36). Since the contrast for this inter-layer structure is not strong enough, similar peaks were not ob-served in the lower concentrations. For positional-ordered lamellarstructure, a series of sharp scattering peaks were expected with a qratio of 1:2:3. To rule out the possible positional ordering in alarger length scale, we also performed extended q-range SANS. Asshown in SI Appendix, Fig. S7, no observable peak of significantpositional ordering was found. This was also confirmed by 2DSANS measurements (SI Appendix, Fig. S8).To identify the phase details of ZrHP, we next investigated the

LC feature of nanodisc dispersion by polarized optical micros-copy (POM). As shown in Fig. 3D, the POM image of ZrHPsuspension revealed a strong anisotropic texture upon loadingsamples due to the shear-induced birefringence. After equili-brating the sample overnight, a characteristic nematic texturewas seen (Fig. 3 D, Inset) (37, 38). No focal conical defect or oilystreak defect, typical defects for smectic or cholesteric phase, wasobserved. The POM results were well consistent with SANS re-sults where no significant positional ordering was found. In fact,theoretical works have made extensive effort on predicting LCphase formation for a specific shape and volume fraction ofnanoplates. Based on Onsager’s second virial theory, Wensinkand Lekkerkerker studied the hard colloidal platelet phase dia-gram for different aspect ratios using the Parson–Lee decouplingapproximation for multiple-body interactions (39). The phasediagram predicts liquid-crystalline phase formation: isotropic,nematic, and columnar phases. As the volume fraction increases,an I-N-C phase transition would be expected (Fig. 3C). In ourstudy, the aspect ratio was estimated to be 0.031 and the volumefraction of nanodisc suspension ranged from 0.15 to 0.40, whichfalls into the nematic region in the phase diagram and confirmsour experimental observation.The structural color that we observed in nematic phase is sort

of counterintuitive considering that nematic mesogens possesssolely long-range orientational order and lack periodically posi-tional order. Here we attributed the structural color to the strongstacking of the nematic nanodisc that forms local order. Such ne-matic phase, often called cybotactic or columnar nematic, consistsof platelets that tend to stack over short distances, forming locallyaligned structure (4). Although the detailed reason remains un-clear, we hypothesize that the large plate size and low thicknesspolydispersity might improve interlayer interactions, producing astable smectic-like layer ordering with a short correlation length inthe N phase. Such locally aligned structure of neighboring flakesforms an average stacking distance that is at the same length scaleof visible-light wavelength (∼102 nm, Fig. 2C). In contrast to ZrHP-based LC, the spacing of small-molecule nematic LCs is severalorders of magnitude smaller than the wavelength of visible light,and thus these LCs usually do not show structural color. The dis-tinctive optical behavior of nematic ZrHP dispersion highlights thedifference of discotic particle-based LCs from small-molecule LCs.The cybotactic nematic structure was confirmed by SEM images:The platelets situated far apart are not correlated in long range,while it is evident that some neighboring flakes are locally stacked.

In addition, the UV-vis reflection peaks throughout the system areconsiderably broad, confirming a marked positional short-rangeorder of nematic phase. As ZrHP are charged plates (zeta po-tential = −31.0 ± 8.5 mV), we expected that a change of the Debyelength induced by ionic concentration may offer a versatile ap-proach to manipulate the self-assembling color. To quantify it, weadjusted the ionic strength of nanodisc suspension by varying saltconcentration with different anions including Cl− and H2PO4

−. Asshown in Fig. 3E, the structural color of PLC suspension blue-shifted sharply with increasing NaCl concentration from zero to0.2 mM and then to 2 mM; this indicated that strong electrolyteNaCl induced a Debye screening of charged platelets, and thusdecreased average d spacing. The addition of NaH2PO4 showed asimilar trend of color shift, although the change of color was lesssignificant. In addition to the small-molecule electrolyte, we alsostudied the effect of polymer electrolyte on the structural color ofZrHP, in which we added the cellulose nanocrystals (CNC) with thecharged sulfate groups (-OSO3-). Upon increasing the CNC con-tent, the peak wavelength decreased continuously (Fig. 3F), allow-ing for PLCs with optically tunable red, green, and blue color, asdemonstrated in Fig. 3F (Inset). It is worth noting that CNC canform chiral nematic alignment, and even achieve various structuralcolors at high concentration (>50 wt %) owing to its periodical pitchat high concentration (40). However, as the added CNC concen-tration is relatively low throughout our system (<2 wt %), we canignore the intrinsic color of CNC in the CNC/ZrHP mixture (41).As the discotic nematic PLC is not composed of monolayers,

we expected an improved thermal stability in our PLCs. It wasreported that conventional heating/drying processes of monolayer-based LC material can cause irreversible structural change toPLCs of 2D monolayers (31, 42). First, we evaluated the thermalstability of unexfoliated ZrHP dispersion by multiple heating–cooling cycles. In each heating–cooling cycle, the sample was heldfirst at 20 °C for 10 s, then heated to 80 °C at a rate of 1 °C/s,followed by being held at 80 °C for 10 s, and finally cooled to 20 °Cat a controlled rate of 1 °C/s. Despite multiple heating–coolingcycles, the color of PLCs remained almost unchanged after 200and 400 cycles, as shown in Fig. 4A. This indicates the exceptionalcolor stability of the nematic dispersion over this temperaturerange. For comparison, monolayer-based PLCs were prepared andevaluated under heating–cooling cycles, showing an obviouschange in color intensity (SI Appendix, Fig. S9). In addition, wefound the ZrHP showed an even higher thermal stability. Afterthermally treating ZrHP powder at 200 °C for 100 h, the disper-sion of these particles (Fig. 4 B, Right) demonstrated the almostidentical color feature in comparison with the as-prepared one(Fig. 4 B, Left). It is worth mentioning that the ZrHP dispersionbecomes blue-shifting at 80 °C in comparison with its structuralcolor at room temperature (20 °C), as shown in SI Appendix, Fig.S10. This can be explained by the fact that heating process pro-motes the dissociation of ion pairs on the ZrHP surface to en-hance the ionic strength, leading to a decrease in Debye lengthand a blue-shifting structural color (19).The low elasticity and high flexibility of PLC enabled the

possibility of developing colorful “inks” and “papers” using thesame material. As different structural colors can be seen underdifferent ZrHP concentration, a photonic palette including thepigments with red, yellow, green, and blue colors was obtained bysimply adding the proper amount of water into concentrated ZrHPsuspensions (Fig. 5A). These photonic inks can be manipulatedwith different shapes and angles, allowing direct ink writing on aglass substrate (Fig. 5B, Lower). Meanwhile, owing to the stimuli-responsive nature of photonic inks, a small amount of dopant CNCcan be used to control on-demand the color of writing (Fig. 5B,Upper). To obtain a photonic paint, we employed a strategy in-volving pretreated glass as the art template (see details in SI Ap-pendix, Fig. S11). A selected pattern was first taped on a glasssubstrate before a commercially available hydrophobic coating was

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applied. After removing the templates, the photonic ink can bereadily applied on the glass, forming a photonic “wave” patternwith good resolution (Fig. 5C). In addition to direct ink writing onglass, a “water marbling” can be obtained by applying shearingforce on the surface of photonic ink. Such idea is “borrowed” fromancient Japanese art, i.e., so-called suminagashi, in which a shearforce was skillfully applied to the surface of the ink. Without ap-plying any shear force, the PLCs automatically aligned at the air/water interface, forming a “blank state” (Fig. 5D). By applying ashear force to stir photonic ink (ϕ = 0.32) using a plastic stick, a

heart-shape suminagashi with blue color was obtained. In addition,a cyan suminagashi of “flower” can also be made by changing theplatelet content to ϕ = 0.30, demonstrating water-based photonicarts with different structural colors.In summary, we demonstrated that nonperiodic nematic LCs

of nanoplates can show iridescent color. Despite the nematicmesogens possessing solely long-range orientational order, thelarge platelet size and improved polydispersity favored interlayerinteractions, yielding locally aligned structure in the length scale ofvisible wavelength. This finding reshaped the stereotypical design

Fig. 4. Thermal stability of ZrHP platelets and their suspension. (A) The thermal stability of ZrHP dispersion. A colorful droplet after 200 and 400 heating–cooling cycles showed the same blue color without observable change. The PLC droplets were sealed in centrifuge tubes during the thermal cycling to preventpossible H2O evaporation. (B) The thermal stability of ZrHP particles over 100 h. Upon dispersion in water, the ZrHP particles showed the almost same colorbefore (Left) and after thermal treatment (Right) at 200 °C for 100 h.

Fig. 5. Photonic patterning and coffee art by platelet suspension. (A) A color palette obtained by ZrHP with different volume fractions. (B) Iridescent“TAMU” and “123” made by ZrHP suspension on glass substrate. (C) Photonic pattern before (Left) and after adding ZrHP suspension (Right) on glasssubstrate with hydrophobic coating (Scale bar, 1 cm.) (D) The dispersion of ZrHP in water showing various drawings in comparison with coffee art.

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of PLCs based mainly on periodically layered or helical structures.Furthermore, we demonstrated that such structural colors arehighly tunable and can be readily assembled into different pho-tonic patterns. Due to the high stability of unexfoliated nematicflakes, we showed the structural color was stable after 400 heat-ing–cooling cycles. The research offers a facile, straightforward,and highly flexible approach of assembling 2D nanomaterials intonematic photonic liquid, which is expected to find applications inchemical sensors, optical display, and biological indictors.

Materials and MethodsMaterials. Crude ZrHP powder was obtained from Sunshine Factory Co., Ltd.Sodium chloride (98%) was purchased from Avantor Performance Materials.Sodium dihydrogen phosphate (99%) was obtained from Sigma-Aldrich.Carbon black (99.9%)was purchased fromAlfa Aesar. CNCwas obtained fromUMaine Process Development Center.

Fractionation of ZrHP. First, 5 g of pristine ZrHP powder was dispersed in 20mLdeionized H2O. Then, the ZrHP dispersion was vortexed for 1 min, followedby sonication for 30 s, allowing the ZrHP platelets to be fully homogenizedin aqueous suspension. Next, the resulting white dispersion was transferredinto a cylindrical glass vial before the fractionation occurred. After 20 h, theresulting suspension was separated into different fractions to obtain frac-tionated platelets. Of the liquid, 20–80% fractions were collected andcentrifuged, whereas the other fractions were discarded. Finally, theobtained ZrHP was dried at 65 °C in an oven before usage. To enhance thestructural color, 0.1 wt % of carbon black particles was added.

Photonic Arts. For the photonic ink experiments, we prepared differentconcentrations of ZrHP in water as paints, where a small amount of carbonblack was added to enhance color contrast. The photonic arts were achievedby simply dropping ZrHP paint on a pretreated glass substrate on whichtemplate tapes and hydrophobic coating were applied.

SANS Measurement. SANS measurements were conducted at the SpallationNeutron Source at the Oak Ridge National Laboratory using extendedq-range SANS diffractometer (beam line 6) (43, 44). To cover the q range(0.003 Å−1 < q < 1 Å−1), three configurations were used at sample-to-dectector distances of 1.3, 2.5, and 4 m, with wavelength bands definedby minimum wavelength of 2.5, 2.5, and 10 Å. Samples were loaded to 1-mmquartz cells. The empty cell scattering was subtracted from all data. Mea-sured data have been corrected for detector sensitivity, dark currentnoise, angle-dependent transmission, and time-of-flight corrections usingMantidPlot software package (45).

Characterization. The reflection colors of platelets were measured by UV-visspectra (Hitachi U-4100 UV-Vis-NIR spectrophotometer). A field-emission SEM(JEOL JSM-7500F) was used to obtain SEM images of samples. The thermalcycling test was performed using a Px2 thermal cycler (Thermo Electron Cor-poration). To prevent possible H2O evaporation, PLC samples were loaded in0.2-mL plastic tubes which were then sealed for thermal cycling analysis.

ACKNOWLEDGMENTS. The authors would like to acknowledge financialsupport from Texas A&M Water Seed Grant (TEES-163024). This research usedresources at the Spallation Neutron Source, a Department of Energy Office ofScience User Facility operated by the Oak Ridge National Laboratory.

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