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We investigate the dispersibility of spherical gold nanoparticles in three phases of a chiral liquid crystal – blue phase I, blue phase II and the cholesteric phase – by UV-visible spectroscopy and optical microscopy. UV-visible spectroscopy revealed that a gradual red-shift and broadening of local surface plasmon resonance occur in the blue I and cholesteric phases. Moreover, optical microscopy revealed a clear difference in the aggregation behavior of nanoparticles depending on the phase, with uniform textures being observed in the blue II phase, and agglomerates forming in blue I and cholesteric phases. The difference in the dispersibility of nanoparticles is discussed in terms of the structure of each liquid crystal phase.
© 2013 Optical Society of America
1. Introduction
Nanoparticle(NP)-doped liquid crystals (LCs) are attracting growing interest owing to the novel and improved properties that emerge in these nano-systems. To date, various materials, from metals to fullerenes have been introduced in LCs, and various effects on the properties of LCs have been reported [1, 2]. Another exciting possiblility of NP-doped LCs is their application in the field of metamaterials [3, 4]: since LC molecules can easily be reoriented by an external field, the dielectric constant in the vicinity of the particle can be modulated, leading to tunable optical properties. Changing the orientation of nanomaterials themselves through cooperative motion may also lead to giant changes in the optical properties.
Among the various LC phases, blue phases (BPs) are attracting interest as nano-templates to realize tunable photonic crystals and metamaterials [5–7]. BPs possess spontaneous three-dimensional order with a periodicity of a few hundred nanometers, and are observed in chiral nematic LCs with strong helical twisting powers. We schematically illustrate the formation and structure of BPLCs in Fig. 1. When chiral molecules are doped in a rod-like LC, twisting of the nematic director is induced, which at low helical twisting powers results in a uniaxial helicoidal structure: the resultant structure is referred to as the cholesteric liquid crystal and has been investigated for use in displays and tunable lasers (Fig. 1(a)) [8–10]. In systems with strong helical twisting powers such that the helical pitch falls to below 500 nm, the LC director twists in 360°, forming a doubly twisting local structure. The double-twist cylinders self-assemble to fill three-dimensional space, resulting in two kinds of cubic lattices, one with body-centered symmetry and another with simple cubic symmetry, referred to as BP I and BP II, respectively (Fig. 1(b)).
Fig. 1 Schematic illustration of (a) chiraity-induced twisting of the director, (b) director distribution in BPs I and II, and (c) close-up of an intersection of three double-twist cylinders with left-handed helicity. The LC director can connect continuously in the intersection with right-handed stacking while it cannot in the intersection with left-handed stacking (emphasized by red coloring).
The complex three-dimensional structure of BPs I and II requires that they contain a periodic network of disclination lines at locations where the LC director cannot be defined uniquely [11, 12]. As shown in Fig. 1(c), a disclination line is formed when three double twist cylinders stack with the same handedness as the helical sense of the cylinder itself. The disclination lines possess high free energies and as a result, the thermal stability of BPs is limited to a few K near the isotropic clearing point. However, recent theoretical investigations have revealed that these disclination lines are what make BPs attractive as nano-templates. Three-dimensional trapping and alignment of nanoparticles (NPs) have been predicted to occur through elastic interactions in the director field [7]. Moreover, the NPs trapped in the disclination lines have been shown to stabilize the BP, by removing the effective volume of the high energy-costing disclination lines.
We have previously demonstrated that BPs doped with gold NPs (AuNPs) indeed have a temperature range that is wider than the non-doped material [13–15]. Other groups have also reported the stabilization of BPs by the addition of spherical or rod-like NPs [16,17]. However, while there are numerous studies devoted to NP-induced stabilization of BPs, experimental investigation of the dispersibility of NPs, and/or their optical properties is largely missing. Understanding how BPs behave as solvents for NPs, and how the optical properties of NPs appear in BPs is important for future applications as tunable photonic crystals and metamaterials. Especially, comparing the dispersion behavior of NPs in the two BPs is interesting in that it can provide insight into how the structure affects the macroscopic properties. From an applications perspective, metamaterials with better properties will likely be realized using the phase with better NP dispersibility. In this study, we study the behavior of AuNPs (d∼8.2 nm) dispersed in the two BPs by UV-visible spectroscopy and optical microscopy. The cholesteric phase, which has a uniaxial helical structure, is also investigated. We perform careful experiments to rule out the effect of temperature on the solubility of NPs, and show that there is a distinct difference in the behavior of NPs depending on the phase. BP II was found to have a higher ability to disperse AuNPs than BP I and the cholesteric phase, which we attribute to the difference in the density of disclination lines.
2. Experimental procedure
We prepared three chiral nematic LC samples in this study; two AuNP-doped samples with different chiral dopant concentrations, and a reference sample without AuNPs. The AuNP-doped samples were prepared by first preparing a AuNP-doped nematic LC, and mixing it into a chiral nematic LC that is known to exhibit the BP. The AuNP-doped nematic LC was prepared by the following procedure [14]. 300 mg of 4-cyano-4′-pentylbiphenyl (5CB, Merck) was placed in a cylindrical glass container and placed in a sputter deposition apparatus (Ulvac Kiko: VPS-20), and deposited with gold using a DC plasma generated by applying voltage of 1 kV between electrodes, for 2 hours (4 min. of sputtering repeated 30 times) at a reduced pressure of ∼20 Pa. The size of the AuNPs were measured using a transmission electrion microscope (TEM; Hitachi, H-7650), by taking a small amount (3 μL) of the sample on a carbon coated copper grid in the isotropic phase, and rinsing the sample in acetone. Figure 2 shows a typical TEM image of the fabricated particles along with statistical data: the mean diameter was d̄∼ 8.2 nm, with a log-normal distribution (σ ∼ 2.1 nm). A plausible model for the formation of AuNPs is that the atoms sputtered into the liquid crystal gradually coalesce into nano-sized particles as they drift through the liquid crystal, with the host LC adsorbed on the particles serving as the capping layer to promote dispersion. As shown in the inset of Fig. 2(a), the particles do not aggregate but are separated by ∼2 nm, which approximately matches the length of the 5CB molecule. The TEM data suggests that the 5CB molecules are adsorbed with the long molecular axis aligned normal to the particle surface. The concentration of the AuNPs was determined from wavelength dispersive X-ray fluorescence spectroscopy (XRF; Rigaku, Supermini) by making a standard curve from colloidal gold with a known concentration (BBInternational, EM. GC5). The concentration was found to be 3.3 mg/cm3.
The BPLC mixture was prepared by doping a chiral dopant (CD; 2,5-bis-[4 -(hexyloxy)-phenyl-4-carbonyl]-1,4;3,6-dianhydride-D-sorbitol, synthesized) into a 1:1 by weight mixture of nematic LCs 5CB and JC-1041XX (JNC). The AuNP-doped 5CB was added to the mixture at a weight ratio of 9 wt%, and the concentration of the CD was varied to either 6 wt% or 8 wt%. The reference sample without AuNPs was prepared by using a non-doped 5CB instead of the AuNP-doped 5CB, and mixing the CD at a concentration of 8 wt%. From previous studies, the mixture used here is known to exhibit either BP I only or BPs I and II, depending on the amount of CD in the sample [15,18]: the concentration was chosen so that the 6 wt% sample would only show BP I and the 8 wt% sample would show both BPs I and II. The phase sequence of the samples as determined from polarized optical microscopy (POM) and microscopic polarized reflection spectroscopy are summarized in Table 1. The temperature dependence of the Bragg reflection wavelength is shown in Appendix A. Consistent with previous studies, both BPs I and II were found to be stabilized by the addition of AuNPs. [13, 15]
Table 1. Phase Sequence of the Samples Used in Study
The samples were infiltrated by capillary action into glass sandwich cells with an approximate cell-gap of 100 μm. The behavior of the AuNPs in the samples were evaluated by UV-visible spectroscopy and optical microscopy. The UV-visible spectra of the samples were measured using a commercial spectrophotometer (Shimadzu, UV-3150) while cooling the sample at a rate of 0.1 °C/min., on a home-made hotstage. Measurement was performed over a circular spot with an approximate diameter of 3 mm. Optical microscopy was performed on an upright microscope (Nikon, Eclipse E-600 POL) equipped with a 10x objective lens and a commercial hotstage (Linkam, 10013 with controller LTS-350).
3. Results and discussion
Figure 3 shows the extinction spectra of the sample with 8 wt% CD, obtained at different temperatures: the temperatures correspond to the isotropic, BP II, BP I and cholesteric phases in decreasing order. Due to Bragg reflection, high extinction coefficients were observed at ∼470 nm for BPI and ∼420 nm for BPII, while in the cholesteric phase, a significantly larger extinction was observed over a broad wavelength range due to light scattering by the focal conic texture (note that the cells were not treated with alignment layers in this study) [19]. In the AuNP-doped sample, another peak in the extinction coefficient appeared at ∼550 nm, attributed to the local surface plasmon resonance (LSPR) of the AuNPs. Figure 3(c) shows the peak wavelength and the full width at half-maximum (FWHM) of the LSPR, obtained by fitting the spectrum to a Gaussian function. The LSPR was similar in the isotropic phase and BP II, but showed a small red-shift in BP I, which shifted further in the cholesteric phase (see dotted line in Fig. 3(c) drawn as a guide to the eye). Also, the FWHM became broader in the cholesteric phase. The results here indicate that there is a change in the distribution of the NPs in the sample upon phase transition. Since the LSPR is known to red-shift when the particles become larger or form clusters, one can suspect that some aggregation has occurred in BP I and the cholesteric phase [20]. However, it is difficult to compare the dispersion behavior based on the UV-visible measurements alone because measurements are performed over a wide area (∼3 mm) and the spectrum is averaged even if there are inhomogeneities due to the formation of agglomerates. To gain a fuller understanding of the behavior of the NPs, the samples were inspected visually on a microscope both between crossed polarizers and with the analyzer removed.
Fig. 3 Extinction spectra acquired at various temperatures, for the (a) reference (non-doped) and (b) AuNP-doped sample. (c) FWHM and peak wavelength of the LSPR peak observed in the AuNP-doped sample. The dotted line has been drawn as a guide to show the red-shift of the LSPR peak in BP I and cholesteric phases.
Figure 4 shows the transmitted microscopic images of the reference and AuNP-doped sample with 8 wt% CD, acquired as the sample was cooled at a rate of 0.1 °C/min. In Appendix B, we show the transmitted and reflected polarized microscopic images of the reference and the AuNP-doped samples. The two samples show similar textures in both transmitted and reflected polarized optical microscopy; namely, platelet textures corresponding to different lattice orientations are observed in the BPs, and a typical focal conic texture is observed in the cholesteric phase. However, when the analyzer is removed, different textures are observed as shown in Fig. 4. The AuNP-doped sample has a darker color, possibly because of light extinction due to LSPR. Moreover, in contrast to the reference sample which shows a small change in texture even upon phase transition (except in the cholesteric phase, which shows strong scattering), purple fringes, or the formation of particles were observed in the AuNP-doped sample. The texture changed depending on the phase, with purple fringes appearing in BP II only, micrometer-sized agglomerates forming in BP I, and the agglomerates clustering into larger particles in the cholesteric phase (see insets of figure).
Fig. 4 Optical microscope images of the (a)reference and (b)AuNP-doped samples acquired at various temperatures. See Fig. 10 in Appendix B for polarized transmission and reflection images of each sample.
Since the purple color was not observed in the reference sample, the color can be attributed to the LSPR of the AuNPs. A careful observation of Fig. 4 further reveals that in BP II, the purple region exists between the BP platelets, thereby suggesting that the fringe is due to the difference in the dispersibility of AuNPs between the isotropic phase and BP II. To investigate the behavior of AuNPs in the BP II platelets, the sample was observed as the platelets were grown by keeping the temperature at 40.5 °C. Figure 5 shows the optical texture of the sample acquired between crossed polarizers and with the analyzer removed. As can be seen from the figure, only small platelets are observed in the isotropic background when the sample first reaches 40.5 °C; however, maintaining the sample at the same temperature causes the platelets to grow, eventually covering the entire field of view. As the platelets grew, the purple fringe was seen to gather in a smaller area (where the sample is isotropic) and grow deeper in color; however, when the platelets grew large enough to cover the whole sample, the fringe disappeared and produced a uniform texture. The results therefore indicate that while the AuNPs tend at first to remain in the isotropic region, BP II is capable of holding the particles without causing aggregation.
Fig. 5 Optical microscope images of the AuNP-doped sample with 8 wt% CD acquired while maintaining the sample temperature at 40.5 °C. (a) Transmission through crossed polarizers, and (b) transmission with analyzer removed.
A similar experiment was performed in BP I to examine its ability to hold AuNPs. However, completely different results were obtained as shown in Fig. 6: as soon as the sample turned into BP I, the purple fringes started to coalesce, and within several minutes, turned into round agglomerates with an approximate size of 5 μm. The formation of agglomerates is qualitatively consistent with the red-shift observed in the LSPR of the sample. The result seems to suggest that there is a distinct difference in the dispersibility of AuNPs in BP I and II; however, one may also argue that the phenomenon is merely a consequence of having different temperatures, since in general, the solubility of impurities in LCs is greater at elevated temperatures. To rule out the effect of temperature, we observed the sample with 6 wt% CD content, which shows the BP I directly below the isotropic phase, at temperatures higher than the temperature of BP II in the sample with 8 wt% CD. Figure 7 shows the optical microscope images of the sample acquired at various temperatures. Agglomerates were found to form as soon as the whole field of view was filled by the BP I platelets, thereby providing direct evidence that the ability to hold AuNPs is fundamentally different between the two BPs.
Fig. 6 Optical microscope images of the AuNP-doped sample with 8 wt% CD acquired in BP I. (a) Transmission through crossed polarizers, and (b) transmission with analyzer removed.
Fig. 7 Optical microscope images of the AuNP-doped sample with 6 wt% CD acquired at various temperatures. (a) Transmission through crossed polarizers, and (b) transmission with analyzer removed.
In Fig. 8, we show the optical texture of the sample in the cholesteric phase, and the redispersion of NPs observed in the isotropic phase. On entering the cholesteric phase, the agglomerates which were distributed rather randomly in BP I (see Figs. 6 and 7) were found to gather into cluster-like objects. The agglomerates varied in size and shape, and also appeared to have a darker color than they have had in BP I. The red-shift and broadening of the LSPR observed in the cholesteric phase may be a consequence of forming such agglomerations. Upon heating the sample back into the isotropic phase, the agglomerates gradually dissolved back into the LC, regaining their original purple color. As can be seen in Fig. 8(d), a uniform texture was obtained after heating the sample for approximately 9 minutes (2 min. at 42 °C, 7 min. at 70 °C).
Fig. 8 Redisperion of the aggregated AuNPs in the isotropic phase. Image in the (a) cholesteric phase at 35 °C, (b) isotropic phase at 42.0 °C, (c) after 2 minutes at 42.0 °C, and (d) 7 minutes after heating the sample to 70.0 °C (after taking image (c))
The reversibility achieved between the dissolved and aggregated states shows the high miscibility of the AuNPs with the host LC material. One reason for such high miscilibity could be that the 5CB molecules are adsorbed on the particle surface, as suggested by the TEM image (see inset of Fig. 2) and the substantial amount of 5CB molecules in the host BPLC (close to 50 %). However, since the solubility of NPs in LCs is greatly affected by the combination of the capping agent and the host LC, it should be kept in mind that the results may vary if AuNPs with a different capping layer are used [21]. Nevertheless, we wish to emphasize that for the particular AuNPs used in this study and the BP material used, BP II was found to have the highest ability to hold the NPs without causing aggregation, followed by BP I, and then the cholesteric phase. The results of both UV-visible spectroscopy and optical microscopy are in agreement with each other. Since we have ruled out the effect of temperature, the difference should be a genuine consequence of having different structures.
One possible explanation for the difference in the dispersibility is that the density of the disclination lines is different in each phase. While disclination-mediated trapping of NPs in BPs has not been confirmed experimentally, disclination lines and other local disorders in LC alignment have been found to act as spatial traps for polymers and colloidal particles in other LC phases such as the nematic phase [22–24]. Therefore, it is plausible to believe that the phase in which the disclination lines take up more volume should have a higher ability to hold the NPs. Among the three phases studied, both BPs I and II possess a lattice of disclination lines with a spatial periodicity of a few 100 nms, while the cholesteric phase does not. Furthermore, a rough estimate assuming a cylindrical disclination core with diameter of 10 nm [11] distributed in BPs I and II as shown in Fig. 1(b), and lattice constants of 200 nm for BP I and 140 nm for BP II (estimated from the Bragg reflection wavelengths in each phase) indicates that the volume the disclination lines occupy in BP II is more than six-fold the volume in BP I. While the exact ratio between the two phases may vary depending on the actual size, or the coherence length of the disclination core, this description yields results that are qualitatively consistent with the behavior observed experimentally.
4. Conclusion
Plasmonic AuNPs with average size of 8.2 nm were fabricated in 5CB by sputtering, and their dispersion behavior in three chiral LC phases – BP I, II and the cholesteric phase – was investigated by UV-visible spectroscopy and optical microscopy. A distinct difference in the dispersion behavior was found depending on the phase, with the ability to hold AuNPs without aggregation being highest in the BP II, followed by BP I and then the cholesteric phase. We believe that the difference is attributed to the difference in the structure of each phase, in particular the density of disclination lines. Since the optical properties of the particles is not affected by aggregation, our findings clearly suggest that BP II is more suited for use as a nano-template to realize LC-based photonic crystals and metamaterials.
Appendix A: Bragg reflection wavelength of prepared samples
Figure 9 shows the temperature dependence of the Bragg reflection peak wavelengths for the 3 samples prepared in this study. Measurement was performed on a POM, using an objective lens with 10× magnification and a fiber-coupled multichannel spectrometer with a core diameter of 1 mm (Hamamatsu, PMA-11). The blue and pink background colors indicate the temperature range of BPs I and II.
Fig. 9 Temperature dependence of the Bragg reflection peak wavelengths for the 3 samples prepared in this study.
Appendix B: Polarized transmission and reflection microscope images of sample with 8 wt% CD
Polarized transmission and reflection microscope images of the samples with 8 wt% CD are shown in Fig. 10. The two samples show similar optical textures regardless of the presence of AuNPs.
Fig. 10 Polarized microscope images of the reference and AuNP-doped samples acquired at various temperatures. (a) Transmission through crossed polarizers, and (b) reflection through crossed polarizers of the reference sample. (c) Transmission through crossed polarizers and (d) reflection through crossed polarizers of the AuNP-doped sample.
Acknowledgments
This study was supported by JSPS KAKENHI Grant Numbers 23562210 and 24656015, MEXT KAKENHI Grant Number 23107519, JSPS Asian CORE Program and the JST PRESTO Program. The authors thank Prof. H. Kikuchi of Kyushu University, Japan and JNC Corporation for providing the liquid crystal materials, Prof. S. Kuwabata and Dr. T. Tsuda of Osaka University for help in performing TEM observations of the sample, and Dr. Y. Inoue and Mr. S. Yabu for experimental assistance.
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