Reversible Nanoparticle Cubic Lattices in Blue Phase Liquid Crystals Mohamed Amine Gharbi,†,‡ Sabine Manet,†,‡ Julien Lhermitte,‡ Sarah Brown,†,‡ Jonathan Milette,† Violeta Toader,† Mark Sutton,‡ and Linda Reven*,† †
Centre for Self-Assembled Chemical Structures (CSACS/CRMAA), Department of Chemistry, McGill University, Montreal, Québec H3A0B8, Canada ‡ Department of Physics, McGill University, Montreal, Québec H3A2T8, Canada S Supporting Information *
ABSTRACT: Blue phases (BPs), a distinct class of liquid crystals (LCs) with 3D periodic ordering of double twist cylinders involving orthogonal helical director twists, have been theoretically studied as potential templates for tunable colloidal crystals. Here, we report the spontaneous formation of thermally reversible, cubic crystal nanoparticle (NP) assemblies in BPs. Gold NPs, functionalized to be highly miscible in cyanobiphenyl-based LCs, were dispersed in BP mixtures and characterized by polarized optical microscopy and synchrotron small-angle X-ray scattering (SAXS). The NPs assemble by selectively migrating to periodic strong trapping sites in the BP disclination lines. The NP lattice, remarkably robust given the small particle size (4.5 nm diameter), is commensurate with that of the BP matrix. At the BP I to BP II phase transition, the NP lattice reversibly switches between two different cubic structures. The simultaneous presence of two different symmetries in a single material presents an interesting opportunity to develop novel dynamic optical materials. KEYWORDS: blue phase, liquid crystal, nanoparticles, colloidal crystal
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The assembly of micron-size particles in liquid crystalline solvents, known as LC colloids, has been studied over the last two decades.8 LC colloidal microstructures (i.e., chains and cellular networks), not observed in isotropic solvents, form due to strong anisotropic, long-range interparticle forces generated by the distortion of the LC director field at the microparticle surfaces. By varying the anchoring strength at the particle surfaces to influence the orientation of the LC molecules, the generated LC defects and their interactions can be controlled, allowing establishment of the rules for making specific microstructures. As the particle size approaches the molecular scale, the long-range particle−particle interactions due to the inclusion-induced LC topological defects diminish. Entropic effects and decreased local order around the NPs become important and can lead to changes in the mesophase order and phase transition temperatures. Small NPs tend to aggregate, often uncontrollably, to reduce the volume of disordered LC.9 Whereas LC defects produced by microparticles and their interactions are easily studied by optical microscopy, only large
he blue phases (BPs) I and II of chiral liquid crystals were initially academic curiosities due to their 3D cubic crystalline orientational order, unique among liquid crystals (LCs), combined with very narrow temperature ranges. Later on, another blue phase, BP III (or fog phase), was discovered, which has the same symmetry as the isotropic phase. Stabilization of blue phases through the addition of polymers or nanoparticles and the synthesis of new mesogens is now driving efforts to develop ultrafast displays and photonic band gap materials1,2 from these fundamentally fascinating crystalline liquids.3 Recent theoretical studies4−6 predict that blue phases should be effective hosts to template a variety of dynamic micro- and nanoparticle 3D assemblies. However, such structures have yet to be experimentally realized. Here, we report the first 3D crystalline nanoparticle structure templated by a LC host. Furthermore, unlike previous 3D nematic colloidal crystals, constructed by positioning individual microparticles with optical tweezers,7 this nanocolloidal crystallization is spontaneous, reversible, and occurs throughout the blue phase matrix volume. The effect of variation of the temperature, pitch of the cholesteric phase, and particle concentration on the gold NP assembly are reported. © XXXX American Chemical Society
Received: November 23, 2015 Accepted: February 22, 2016
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Figure S5), the addition of the NPs does not significantly alter the BP lattice similar to other studies of gold NPs dispersed in BPs.19−21 In some cases, an increase in the pitch upon addition of the nanoparticles was reported.19 SAXS data were taken at 0, ∼1, 2.5, and 10 wt % Au NPs in the 50:50 6OCB/CB15 mixtures and at 0.9 wt % Au NPs for 45:55 and 55:45 6OCB/CB15 mixtures. The scattering profiles for each phase are quite different but have same features for the same phase. Representative SAXS data for 2.5 and 10 wt % Au NPs (0.13 and 0.5 Au vol %, respectively) dispersed in a 50:50 6OCB/CB15 mixture as a function of temperature is presented in Figures 1 and 2. Comparison with a pure 6OCB/CB15
NPs (>20 nm) can be observed by dark-field microscopy, and the behavior of smaller NPs remains difficult to predict.9 Despite this knowledge gap, our group and others have successfully used nematic,10−12 cholesteric,13,14 and smectic15−17 LCs to organize NPs into regular micron-scale structures. Both permanent and reversible structures have been formed via LC phase transitions and/or periodic defect structures, exploiting the tendency of NPs to locate at high energy LC defect sites and interfaces. As discussed in a recent review,9 the dimensions of the LC templated NP patterns are controlled by varying the particle concentration, film thickness, boundary conditions, and other experimental parameters.
RESULTS AND DISCUSSION Although NPs have been used to stabilize blue phases,18−24 ordered NP assemblies in BPs have not been previously reported. Unlike other LC phases where periodic defect structures are induced by specific boundary conditions, blue phases spontaneously form ordered 3D defect structures. Blue phase molecules display helical ordering in two orthogonal directions, visualized as double twist cylinders. The cylinders, packed in a cubic crystalline arrangement, are threaded by a lattice of disclination lines. The lattice constant, on the order of several hundred nanometers, is determined by the pitch that can be varied by the amount of chiral dopant. The blue phase mixture, 4′-hexyloxy-4-cyanobiphenyl (6OCB) and 4-cyano-4′(2-methylbutyl)-biphenyl (CB15), the chiral dopant in our case,25 was chosen because it has been well studied and is compatible with functionalized gold NPs previously optimized to be miscible in cyanobiphenyl based LCs.26 The gold NPs, with an average diameter of 4.7 nm, are functionalized with a mixed ligand shell of 4′-(12-mercaptoalkyloxy)biphenyl-4carbonitrile (CBO(CH2)12SH) and hexanethiol (CH3(CH2)5SH) in a 1:1 ratio.26 6OCB/CB15 ratios of 45:55, 50:50, and 55:45 were examined to vary the pitch. The particle concentration was 0.9 or 1.0 wt % gold NPs as the mixture ratio was varied. For the 50:50 6OCB/CB15 mixture, higher particle concentrations up to 50 wt % gold NPs were examined. The pure 50:50 6OCB/CB15 mixture displays blue phases I and II within a narrow temperature range of 31.7 to 32.6 °C (ΔT ∼ 0.9 °C).25 The addition of the functionalized gold NPs lowers the transition temperatures by a few degrees and increases the BP temperature range to a maximum of ∼2.5 °C, similar to previously studied Au NP-BP dispersions.19−21 Gold NP additives have been reported to stabilize BP I in favor of BP II, but equal stabilization of both phases has also been observed. The 50:50 and 45:55 mixtures with 0.9 wt % Au NPs both showed BP II at the isotropic-BP transition. In the 55:45 mixture with 0.9 wt % Au NPs, BP I was observed to immediately appear upon cooling while BP II only appeared upon heating, whereas the undoped mixture showed both phases under the same conditions. Polarized optical microscopy (POM) shows that the gold NPs are well dispersed (no visible aggregates) in both the isotropic and blue phases up to concentrations of ∼25 wt % gold NPs, although aggregates smaller than the microscope resolution (10 μm. The sharp peaks (inset of Figure 2c) reappear upon heating back up to the blue phases and then disappear again in the isotropic phase, indicating a reversible process. Upon increasing the NP concentration to 10 wt % Au NPs, the intensities and widths of the Bragg peaks increase, and the scattering profile becomes more powder-like with well-defined rings (Figure 2). The sharpest peaks approach the narrowest widths given by the finite size of the X-ray beam (20 × 20 μm2). The broadest peak still gives a crystal size around 1 μm. Even the width of the Bragg peaks are features of a well-defined lattice and not sensitive to aggregates. The Bragg peaks of the NP lattices in both BP I and BP II can be indexed by a simple cubic indexing based on a lattice constant indicative of the body centered cubic (BCC) lattice of BP I of the 6OCB/CB15 mixtures. The complete description of B
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shows that these NP pseudolattice parameters, aNP, range between 370 and 420 nm, similar to those of the blue phases.
Figure 3. Pseudolattice constant of the cubic structures of the Au NPs in the LC for different concentrations of 6OCB/CB15 mixture as a function of Ttransition − T. Ttransition is the isotropic-BPII temperature transition, and T is the temperature of the system. For the 50:50 6OCB/CB15 mixture, Ttransition = 32.9 °C. For the 45:55 6OCB/CB15 mixture, Ttransition = 25.6 °C. For the 55:45 6OCB/ CB15 mixture, Ttransition = 36.7 °C. For the 50:50 mixture (blue), the filled squares represent the BP II phase and empty ones represent the BP I phase. For the 45:55 mixture (red filled circles), only the BP II phase exists. For the 55:45 mixture (magenta), a coexistence between BP I and BP II was observed. All Bragg peaks disappear in the cholesteric phase.
Given these dimensions and assuming homogeneous dispersions, there are ∼4 NPs per unit cell for the BCC BP I and/or 1/2 NP for the simple cubic (SC) BP II for 0.9 wt % Au NP samples. This estimate is an upper limit given that there may still be a contribution from randomly dispersed NPs. On the basis of the analysis of the scattering intensities, the randomly spaced NPs are also located within the BP crystals, rather than only at the interfaces or between the BP platelets (lattice planes of BP crystallites) and seem to sit on unoccupied lattice sites. Since the blue phase crystallites are not completely randomly oriented, it is not possible to accurately estimate the relative intensities of the Bragg reflections and diffuse scattering. One can see from the figures that both blue phase structures have significant disorder scattering as well as the powder rings. This disorder scattering increases with the volume fraction of NPs. The diffuse scattering also increases at the BP II to BP I phase transition, suggesting a higher concentration of gold NPs within the BP crystallites (as opposed to concentrating in the grain boundaries, i.e., between the BP platelets). In Figure 3, the variations of the lattice constants with temperature, particle concentration, and chiral dopant are presented. Upon cooling from the isotropic phase, the lattice constant of the pure 50:50 6OCB/CB15 mixture (no gold NPs) in BP II, aBPII, first increases from ∼340 to 380 nm, and then in BP I, aBPI decreases from 480 to 400 nm as the temperature is lowered (see ref 25). For the same mixture with 1 wt % gold NPs in BP I, the gold NP pseudolattice constant,
Figure 2. Scattering intensity and their corresponding isotropically averaged intensities of the Au NPs/LC mixture in the isotropic phase (a), the BPII (b), the BPI (c), and the cholesteric phase (d) for 10 wt % Au NPs in 50:50 6OCB/CB15. Insets of b, d, f, and h show the polarized optical microscopy images and the schematics of LC molecule alignments in each mesophase. The platelet sizes of the BP in BP I and BP II vary between ∼5 and 30 μm. The inset of c shows a sharp peak for 0.9 wt % Au NPs in 50:50 6OCB/CB15 mixture.
this indexing is much more complex and for the sake of continuity will be discussed in more detail further along this article. Analysis of the SAXS data, summarized in Figure 3, C
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Table 1. Table Showing the Good Agreement between the Calculated Wave Vectors qcal and the Measured Ones qexp of the Bragg Peaks in both BP I and BP II for 0.9 wt % Au NP Samples in a 50:50 6OCB/CB15 Mixture BP II at T = 32.5 °C, aNP = 376.228 nm qexp (10−3) Å−1 qcal (10−3) Å−1
BP I at T = 32 °C, aNP = 396.298 nm
(h,k,l) = (2,2,0)
(h,k,l) = (2,2,2)
(h,k,l) = (2,2,1)
(h,k,l) = (2,2,2)
(h,k,l) = (4,2,1)
4.73882 4.72361
5.78220 5.78521
4.76477 4.75641
5.45966 5.49223
7.23682 7.26554
aNP, instead increases slightly from ∼370 to 390 nm as the temperature is lowered. With less chiral dopant, the NP pseudolattice constant increases from ∼375−400 nm in the 45:55 6OCB/CB15 mixture to ∼410−420 nm in the 55:45 the 6OCB/CB15 mixture, which shows a BP I appearing at the phase transition by POM. These values fall within the relative size ranges of BP I and BP II lattice constants mentioned above, where aBPI > aBPII. Whereas in the isotropic phase the NPs diffusion is too fast to be measured by X-ray photon correlation spectroscopy (XPCS), (see Figure S9) no significant mobility of the ordered NPs was detected on a time scale of 10 min. The XPCS measurements (see Supporting Information) on the diffuse scattering in isotropic and BP show fast dynamics on the experimental time scale, 0.25 s, which decreases with temperature and dramatically increases in the cholesteric phase where we measure a compressed exponential line shape with a time constant scaling as 1/q. When left to anneal at a fixed temperature, the Bragg peak intensities increase and sharpen, indicating that the crystal structure becomes better defined. After a few minutes, the Bragg peak intensities remain constant, indicating the system has equilibrated, and if the NPs are not static, then at least as many NPs would have to enter as leave the trapping sites in order to keep the peak intensities constant. The occupancy of the trapping sites is constant, but the increase in scattering with NPs concentration suggests that the diffuse scattering arises from partially occupied sites. The time evolution seen within the diffuse scattering by XPCS indicates that the NPs may be hopping from site to site. According to the rules for the Miller indices (h, k, l) of allowed reflections for cubic crystals, all values are allowed for simple cubic crystals, the sums of the indices h + k + l are even for BCC, and the indices (h, k, and l) are all even or all odd for face centered cubic (FCC) lattices. Given the dimensions of the X-ray beam (20 × 20 μm2) and the 40 μm sized platelets (faces of the crystallites) of the nonaligned, polycrystalline BP, we can expect multiple reflections for each spot probed along the capillary tube. We used the indexing for cubic crystals with the lattice constant compatible with the BP I that has BCC symmetry. At the isotropic-BP II transition of 50:50 6OCB/ CB15 mixture with 0.9 wt % Au NP, (220) and (222) reflections appear. The (222) peak is present throughout the entire BP temperature ranges in all the mixtures. At 32.3 °C, corresponding to the BP II to BP I transition of the 50:50 mixture, the intensity of the (220) peak diminishes and appears to switch to a new (221) reflection whose intensity increases with cooling (Figures 1 and 2). Additional Bragg peaks appear including (421) reflections. Upon increasing the particle concentration to 10 wt % Au NPs, the same Miller indices, extracted from well-defined rings, are observed in the same temperature sequence and yield similar NP lattice constants. Table 1 shows an example of measuring the wave vectors of Bragg peaks and their indexing in blue phase and how these values agree with the calculated ones using the following relationship:
qcal = (2π /a) h2 + k 2 + l 2
(1)
In the 55:45 6OCB/CB15 mixture with 0.9 wt % Au NP, where only BP I is observed by POM, the (221) reflection appears immediately at the isotropic-BP I transition. Conversely, in the 45:55 6OCB/CB15 dispersion, which should favor the formation of BP II due to a larger quantity of chiral dopant, the (221) peak does not appear, and only a few other even/odd (h,k,l) combinations are observed. We therefore identify BP II with the appearance of the (220) peak and BP I with the (221) peak. The changes in the Miller indices with temperature indicate that the gold NPs occupy different sites in the BP I and BP II phases. We note that besides the amount of chiral dopant, the temperature ranges of BP I and BP II are highly sensitive to the cooling/heating rates. In addition to a relatively wide isotropicBP II biphasic region,25 the coexistence of BP I and BP II was sometimes observed by POM for the 50:50 6OCB/CB15 mixtures if the samples were not sufficiently equilibrated. Likewise, the 55:45 6OCB/CB15 samples showed the coexistence of BP I and BP II at the phase transition by POM. Such coexistence, possibly enhanced by a temperature gradient along the capillary, was also observed for the gold NP lattices at the BP I-BP II transition of the 50:50 mixture by the simultaneous presence of the (220) and (221) Bragg peaks. The mechanism of the thermally reversible switch of the NP cubic lattice is of interest as it could give insight into the BP II to BP I transition itself. There is virtually no theoretical work or understanding of the phase-change kinetics apart from the work of Henrich et al.28 who did simulations of the BP nucleation from the isotropic and cholesteric phases. This study proposes the formation of a metastable amorphous defect network with the reemergence of the stable BP via a second nucleation. More relevant to the results here are the theoretical studies of colloidal crystallization in BPs for photonic applications predicting that the most stable structures are different from those of the BP matrixes. Specifically, FCC colloidal crystals should form in body-centered cubic BP I and BCC crystals in simple cubic BP II.4,6 One dominant colloidal crystal structure in BP I should prevail, whereas the small energy differences for the BP II templated colloidal crystals indicate that polydomains are likely.4 Another recent study of colloid-BP composites predicts a wide variety of templated structures. For weak anchoring, colloidal crystals, gels, and helical colloidal ropes were found, whereas strong anchoring inhibits BP templating.5 BP templating of colloidal crystals of micron or submicron-size particles has not been experimentally realized so far. A recent study shows that the addition of large colloids (>0.5 μm) leads to localized melting of the BP.29 As mentioned previously, the Bragg peaks of the NP lattices formed in BP I and II were indexed using cubic lattice constants compatible with the BP I BCC lattice. We note that the SC BP II lattice itself can be indexed using the BP I lattice constant, (hkl)BPII = 2 (hkl)BPI (the peaks (220) and (222) are in reality (110) and (111) peaks, respectively). The absence of certain D
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obvious subsets or combinations of these lattices do not account for the observed X-ray scattering of the NPs in BP I. The persistence of the (222) reflection indicates that at the BP I to BP II phase transition, the NPs likely remain trapped at these sites, the midpoints of the disclination lines. The other NPs that were trapped in the intersections of the BP II disclination lines, which disappear as these sites are not present in BP I, must populate other sites along the defect line lattice in a complex yet highly periodic fashion in order to give rise to the new Bragg peaks at (221) and (421). We emphasize that the selective trapping sites of the BP lattices are remarkably robust given that the same Bragg reflections for each phase appear independent of NP concentration or whether the sample is cooled or heated.
lower order reflections, such as (110) or (002), indicates that the strong trapping sites responsible for the NP lattice are a subset or a combination of cubic structures leading to extinction of these normally allowed peaks. A recent synchrotron SAXS study of polymer stabilized BP I, where the polymer decorates the lattice of disclination lines, accounted for the absence of certain allowed BCC cubic peaks by calculating the structure factors for a lattice of rods.30 A similar calculation, assuming that the Au NPs fill the disclination lines, does not account for the observed X-ray scattering and rather leads to complete cancellation due to the symmetry because fully filling in the disclination lines preserves the body-centered symmetry. Instead, we can account for the (220) and (222) peaks of the BP II dispersion if the NPs are located at the centers of the tetrahedral arms of the disclination lines and at the midpoints of these defect lines (see Figure 4).
CONCLUSIONS Existing theoretical studies of nanoparticle dispersions in BPs have only concerned BP stabilization and have not examined the possibility of forming ordered structures. Besides the small particle size, the particular materials that influence the particle− particle and particle−LC interactions are critical factors that remain to be explored. Despite the small particle size, the periodic arrays of trapping sites responsible for the formation of the hypothetical BP microcolloidal crystals4−6 are also able to capture the NPs without greatly disordering the BP structure as predicted for larger particles.5 Similar to the BP micron-size colloidal crystals, the symmetries of our nanocolloidal crystals are different from those of the BP matrix.4 The possible existence of different symmetries of the two components, i.e., the particle lattice and the BP orientational order, in a single material has been recently theoretically explored as a means to create tunable photonic crystals.6 Future work will concern BP templating of larger nanoparticles to see if this results in different cubic structures. The application of electric fields to alter the BP structure to allow electrical control over the nanocolloidal crystal dimensions and/or structure is possible.30 Colloidal crystals of other nanomaterials with desirable optical, magnetic, or electric properties, such as quantum dots or semiconductor nanorods, may be possible as long as these materials can be suitably functionalized to prevent bulk phase separation. Finally, depending on the competition for defect sites, the addition of UV-curable monomers followed by photopolymerization could both serve to extend the BP temperature range and freeze in the 3D cubic crystalline nanostructures.
Figure 4. Model showing the possible arrangement of the Au NPs in BPII into a tetrahedral structure. Eight unit cells are shown along with the specific occupied Wycoff sites. The (0,0,0) sits at the intersections of the tetrahedral arms and the (1/4,1/4,1/4) at the midpoints of the tetrahedral arms.
These correspond to two and four sites in the SC BP II structure. Such a configuration would be equivalent to a combination of the BP colloidal crystal structures in (Figure 2D) of ref 4, namely, the 4PC, with colloids at the middle of the disclination lines, and the BCC colloidal crystal where the particles sit at the intersections. The NPs sites located at the centers of the tetrahedral arms of the disclination lines (BCC) give a (220) peak in our indexing but not the (222), whereas NPs sites situated at the midpoints of the disclination lines (4PC) give a (222) but not the (220). Given that this leads to 6 sites per BP II unit cell, there must be many unoccupied sites for 1 wt % NP. This most likely explains the disorder scattering observed between the powder peaks. Given that micron sized particles may strongly distort the disclination lattice4 and even lead to melting of the BP,29 the BCC colloidal crystal is predicted to be the energetically favorable structure in a BP II matrix.4 However, the trapping sites of the 4PC structure, that is, the midpoints of the BP II disclination lines, may be favorable for smaller particles, and NPs thus populate the sites of both the BCC and 4PC colloidal crystals. For BP I, the proposed SC, BCC, or FCC decorations of the BP disclination lattice4 can be ruled out by the observed (221), (222), and (421) reflections. However, the simplest and most
METHODS Sample Preparation. 4′-Hexyloxy-4-cyanobiphenyl (6OCB) and 4-cyano-4′-(2-methylbutyl)-biphenyl (CB15) (Kingston Chemicals Limited) were used to prepare the BP mixtures. Before any studies, the mixtures with different ratios (6OCB/CB15 ratios of 45:55, 50:50, and 55:45) were maintained at 50 °C for 1 h to mix the components. Functionalized Au NPs are then added in the BP mixture in a solution in dichloromethane (Sigma-Aldrich). The solvent was then evaporated under a slow flow of inert gas at 50 °C overnight. Finally, the samples were vacuum-dried at the same temperature for 1 h to eliminate any residual solvent. Small Angle X-ray Scattering. For the synchrotron small-angle X-ray scattering (SAXS) experiments, capillary tubes were filled with the BP-Au NP dispersions. The X-ray data were collected after heating the sample to the isotropic phase to erase any thermal history and then slowly cooled. Measurements were taken at several areas along the capillary to minimize any radiation damage. For each temperature, the sample was allowed to equilibrate for at least 10 min. The E
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measurements were performed in transmission geometry using a 7.35keV X-ray beam. The beam size was 20 × 20 μm2, and the incoming flux was 3.7 × 109 photons/s. The diffracted X-rays were measured in the far field (4 m) with an EIGER 1MP counting detector (by Dectris, https://www.dectris.com). The q range probed spanned from 0.002 to 0.017 Å−1, and the resolution was 2.75 × 105 Å5 per pixel.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07379. Additional SAXS data, optical microscopy, reflectance measurements, XPCS data, calculation of the X-ray structure factors, and Figures S1−S9 and Tables S1−S2 (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by Le Fonds de recherche du Quebec - Nature et technologies (FQRNT) and Natural Sciences and Research Council (NSERC). The use of the Advanced Photon Source was supported by the U.S. DOE Contract No. DEAC02-06CH11357. We thank the staff at the Advanced Photon Source: Dr. Suresh Narayanan, Dr. Eric Dufresne, and Dr. Alec Sandy for their hard work and support. REFERENCES (1) Kikuchi, H. Liquid Crystalline Blue Phases. In Liquid Crystalline Functional Assemblies and Their Supramolecular Structures; Springer: Berlin, Germany, 2008; pp 99−117. (2) Yoshizawa, A. Material Design for Blue Phase Liquid Crystals and Their Electro-Optical Effects. RSC Adv. 2013, 3, 25475−25497. (3) Wright, D. C.; Mermin, N. D. Crystalline Liquids: The Blue Phases. Rev. Mod. Phys. 1989, 61, 385−432. (4) Ravnik, M.; Alexander, G. P.; Yeomans, J. M.; Zumer, S. ThreeDimensional Colloidal Crystals in Liquid Crystalline Blue Phases. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 5188−5192. (5) Stratford, K.; Henrich, O.; Lintuvuori, J. S.; Cates, M. E.; Marenduzzo, D. Self-Assembly of Colloid-Cholesteric Composites Provides a Possible Route to Switchable Optical Materials. Nat. Commun. 2014, 5, article no. 3954.10.1038/ncomms4954 (6) Stimulak, M.; Ravnik, M. Tunable Photonic Crystals with Partial Bandgaps from Blue Phase Colloidal Crystals and Dielectric-Doped Blue Phases. Soft Matter 2014, 10, 6339−6346. (7) Nych, A.; Ognysta, U.; Skarabot, M.; Ravnik, M.; Zumer, S.; Musevic, I. Assembly and Control of 3D Nematic Dipolar Colloidal Crystals. Nat. Commun. 2013, 4, article no. 1489.10.1038/ ncomms2486 (8) Musevic, I.; Skarabot, M. Self-Assembly of Nematic Colloids. Soft Matter 2008, 4, 195−199. (9) Blanc, C.; Coursault, D.; Lacaze, E. Ordering Nano-and Microparticles Assemblies with Liquid Crystals. Liq. Cryst. Rev. 2013, 1, 83−109. (10) Milette, J.; Cowling, S. J.; Toader, V.; Lavigne, C.; Saez, I. M.; Lennox, R. B.; Goodby, J. W.; Reven, L. Reversible Long- Range Network Formation of Gold Nanoparticle - Nematic Liquid Crystal Composites. Soft Matter 2012, 8, 173−179. (11) Milette, J.; Toader, V.; Soulé, E. R.; Lennox, R. B.; Rey, A. D.; Reven, L. A Molecular and Thermodynamic View of the Assembly of F
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