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Self-Organized, One-Dimensional Periodic Structures in a Gold Nanoparticles-Doped Nematic Liquid Crystal Composite Piotr Lesiak, Karolina Bednarska, Wiktor Lewandowski, Micha# Wójcik, Sylwia Polakiewicz, Maciej Bagi#ski, Tomasz Osuch, Konrad Markowski, Kamil Orzechowski, Micha# Makowski, Jan Bolek, and Tomasz R. Woli#ski ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b03302 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019
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Self-Organized, One-Dimensional Periodic Structures in a Gold Nanoparticles-Doped Nematic Liquid Crystal Composite Piotr Lesiak†*, Karolina Bednarska†, Wiktor Lewandowski‡, Michał Wójcik‡, Sylwia Polakiewicz‡, Maciej Bagiński‡, Tomasz Osuch₴, Konrad Markowski₴, Kamil Orzechowski†, Michał Makowski†, Jan Bolek†, Tomasz R. Woliński† †Faculty
of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, POLAND
‡Faculty
of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warszawa, POLAND
₴Faculty of Electronics and Information Technology, Institute of Electronic Systems, Warsaw University of
Technology, Nowowiejska 15/19, 00-665 Warszawa, POLAND *corresponding author
[email protected] Abstract Composite structures exhibiting a periodic arrangement of building blocks can be found in natural systems at different length scales. Recreating such systems in artificial composites using the principles of self-assembly has been a great challenge, especially for 1D microscale systems. Here, we present a purposely designed composite material consisting of gold nanoparticles and a nematic liquid crystal matrix that has the ability to self-create a periodic structure in the form of a one-dimensional photonic lattice through a phase separation process occurring in a confined space. Our strategy is based on the use of a thermo-switchable medium that reversibly and quickly responds to both heating and cooling. We find that the period of the structure is strongly related to the size of the confining space. We believe that our findings will allow us to not only better understand the phase separation process in multicomponent soft/colloid mixtures with useful optical properties but also improve our understanding of the precise assembly of advanced materials into one-dimensional periodic systems, with prospective applications in future photonic technologies. ACS Paragon Plus Environment
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Keywords: nematic liquid crystal, nanoparticles, self-assembly, self-organization, phase separation, liquid crystal composite, periodic structure
Nature can develop and create biological systems with a significant level of order through a complex but lowenergy bottom-up approach. Nature’s principles have been successfully applied to build 2D and 3D periodic structures using polymers,1 organic-inorganic hybrid materials,2 photonic crystals3 or doped liquid crystals (LC).4,5 However, spontaneous formation and structural/morphological control of 1D-periodic materials resembling the order of those structures observed in spider silk,6 bamboo7 or cactus spine8 have just begun,9 and the preparation of these materials is cumbersome. Different types of structures can be created through the self-organization of nanoparticles, such as chains,10 lattices,11 or even three-dimensional crystals12 or other complex structures.13-16 Self-assembly can be achieved using several methods. One of these methods is known as templating, in which different host objects may be used to template an assembly of nanoparticles into a predefined shape.17 Another method is self-assembly at different types of interfaces using various techniques.18 A separate group of methods are assisted technologies that use external electric19 or magnetic fields.20 Structures made of composites are of particular interest for metamaterials and photonics applications21,22 as an interesting alternative to one-dimensional layered structures due to the broad range of LC tuning possibilities. An enormous advantage of such structures is the ability to extensively modify their periodicity depending on the geometry of the confining space. In recent years, the creation of chiral structures in capillaries filled with liquid crystals has been shown,23-25 and it was demonstrated that doping nanoparticles into liquid crystals enables some level of control over the self-assembly process of such structures by different means.26 However, the authors of previous papers23-26 focused only on boundary conditions, and they did not obtain any visibly periodic systems. In this paper, we present an approach to self-induce a one-dimensional photonic lattice only through the mechanism of phase separation. This self-assembly process is completely reversible and can be controlled by modifying the diameter of the confining space. Because the proposed LC-based structures are nonsolid (in contrast to a 1D PC, i.e., thin film filters), it is possible to dynamically adjust the periodicity of
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this type of structure. This phenomenon in turn may allow the design and fabrication of spectrally adjustable photonics devices with a broad tuning range. Results/Discussion We prepared gold (Au) nanoparticles-doped LC mixtures based on two host nematics: 4-cyano-4'pentylbiphenyl, commonly referred to as 5CB and 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The materials have well-described properties, exhibit a room-temperature LC phase and are often used as LC host materials for various nano- and microinclusions. The gold nanoparticles (2.5 ± 0.4 nm diameter for the Au core) were covered with the pro-mesogenic ligand N,N–dioctyl–4–[(4’–(10mercaptodecyloxy)biphenyl–4–ylo)xymethyl]benzamide (2NC8) (Fig. 1). The choice of Au as the core material was dictated by surface high stability and well-known chemistry of gold nanoparticles.
Figure 1. An idealized model of nanoparticles incorporation into a liquid crystal matrix resulting from the pro-mesogenic ligand and host molecules interaction. Sizes of nanoparticles and organic molecules are not pictured to scale. It is important to note that several features of Au@L1 favor interactions with the LC hosts. First, the small size of the nanoparticle core makes the Au@L1 particles comparable in size with the LC matrix molecules and ensures high surface curvature, which is thought to favor LC host molecule infiltration into the organic shell of NPs.27 Second, the rod-like and aromatic structure of L1 ligands enables efficient steric and π-π interactions with the host molecules. Finally, the organic shell of Au@L1 is flexible enough to adopt different ACS Paragon Plus Environment
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shapes, as previously confirmed in their neat state.27,28 All these features translate to tactoidal deformation of the organic shell of nanoparticles, which means that ligands act as spatial stabilizers of such an arrangement that is forced by the anchoring conditions on the surface of the sample. Our NP-LC composites were in a well dispersed state, as confirmed by SAXS measurements. The analysis of the infinite volume issue suggests that as the concentration of nanoparticles increases, the nematic-isotropic phase transition temperature (TN-I) increases.29,30 We have verified this trend experimentally (Fig. 2), and our results are in line with previous works.31-33 35.8 35.7 35.6 35.5 TNI [0C]
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35.4 35.3 35.2 35.1 35 34.9 0
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Figure 2. Nematic-isotropic transition temperature changes with different concentrations of nanoparticles in the investigated LC composites. The mechanism of phase separation is analogous to formation of spherical capsules and other geometries from nanoparticles by a particle-sorting process described previously.34-36 In our experiments nanoparticles are homogenously dispersed in the nematic phase. When the temperature increases, isotropic domains begin to nucleate (Fig, 3a) and grow (Fig. 3b). Nanoparticles are expelled from these expanding nonordered regions and are sorted into nematic domains. The isotropic domains subsequently merge (Fig. 3c), and the nematic domains (which contain most of the particles) shrink. Stopping the heating process (before the whole volume of sample is melted) and then cooling the sample restores the nematic phase in the whole volume; however, nanoparticles are no longer uniformly dispersed in the sample. Fig. 4 shows that the shrinking of the isotropic domains (Fig. 4a) does not change the location of the nanoparticles that have been pushed out of the isotropic phase. Consequently, the disappearance of the isotropic phase (Fig. 4b) reveals a bright area characterized by ACS Paragon Plus Environment
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a low nanoparticles concentration in the nematic phase. After cooling the sample to the initial temperature (Fig. 4c), it can be seen that the sample has a visible division into bright and dark areas which correspond to higher and lower concentrations of nanoparticles, respectively. Only within the bright areas the nematic phase has transitioned into the isotropic phase. If the temperature of the whole volume of the liquid crystal increases high enough to reach the nematic-toisotropic phase transition, then the brighter and darker areas are still visible (Fig.5a). However, when the LC material is cooled to the isotropic-to-nematic phase transition temperature, nematic domains begin to nucleate and grow initially in the areas of high gold nanoparticles concentration (Fig. 5b) and then expand to the remaining areas of the sample (Fig. 5c).
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Fig. 3. Optical polarized microscopy images of the Au@L1/5CB sample in the initial stages of heating.
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Fig. 4. Optical polarized microscopy images of the Au@L1/5CB sample in the process of cooling from partially isotropic-phase-transitioned state.
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Fig. 5. Optical polarized microscopy images of Au@L1/5CB sample in the process of cooling from fully isotropic-phase-transitioned state. The above described process leads to macroscopic separation of the nematic and isotropic phases, during which the nanoparticles seem to be pushed out of the isotropic phase and into the nematic phase, creating a difference in densities of nanoparticles in LC. This phenomenon in turn results in a growing difference in both phases around TN-I. The surface tension creates a complex pattern in such cases. This mechanism describing the characteristics of such mixtures was the subject of intense studies by Cahn and Hilliard in 195837 and by Weinan and Palffy-Muhoray in 199738 and is still being developed. In 2013, Milette et al.32 provided a thermodynamic model which showed that self-organization is driven by a subtle balance of nanoparticle entropic forces, excluding volume, interface-mediated mesogen and nanoparticle molecular interactions, as well as couplings between conserved and non-conserved order parameters. It should be noted that structures formed through phase separation of LC systems doped with nanoparticles in planar cells were already described in the literature.31-33 Here, we show that it is possible to create a periodic structure if the mixture is placed in capillaries with diameters ranging from several to several dozens of micrometers. During the infiltration process, the anchoring conditions on the glass/nematic border induce a planar alignment of the liquid crystal molecules (schematically depicted in Fig. 7a). The previously described process of phase separation in liquid crystal cells is analogous to that in capillaries in the initial stages. A domain may appear and start to grow at any position in the entire volume of the mixture due to nanoparticle density fluctuations. However, the internal diameter of the capillary stops the spherical growth of isotropic domains; therefore, further Brownian motion of domains is restrained by the friction induced by the viscosity of the liquid crystal on the glass/nematic border. As a result, the initial position of the domain does not affect the resulting structure. The surface of the domain ACS Paragon Plus Environment
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is then perpendicular to the long axis of the capillary and takes the form of a convex meniscus, as observed in Fig. 6 (in the isotropic phase). Such a form seems to come from the forces anchoring liquid crystal molecules on the glass-nematic border, which are stronger than the van der Waals forces that keep the molecules in the nematic phase.
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Figure 6. Polarizing optical microscope images of domains growing at random positions in Au@L1/6CHBT. The measurements were performed in a crossed-polarizer setup; thus, the nematic phase is visible as light areas, whereas the isotropic phase appears as dark areas. Formation of a structure through the growth of the isotropic phase can be observed (arrows indicate the positions of the polarizer and analyzer). More domains appear and grow between the already existing domains in areas where the concentration of nanoparticles is the lowest (Fig. 6, 30 [s] row). Thus, the nematic phase becomes divided into sections that are decreasing in size (schematically depicted in Fig. 7b) until the isotropic domains are separated by regions with such a high concentration of nanoparticles that further isotropic domain creation appears to be impossible without a significant rise in temperature. The concentration of nanoparticles is the highest at the borders ACS Paragon Plus Environment
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around the nematic phase because the nanoparticles are pushed out from the isotropic phase. The dynamics of
c)
this phenomenon seems to prevent nanoparticles from diffusing deeper into the nematic phase. We believe that such a local increase in the density of nanoparticles results in a local increase in the phase transition temperature at the border between the nematic and isotropic phases. This increase in turn would prevent subsequent isotropic phase domains from appearing in areas with a high concentration of nanoparticles. In the areas further from the nematic phase borders, the concentration of nanoparticles decreases, so the probability for isotropic domains to appear is greater. This process involves the creation of areas around a single domain in which other domains cannot appear. By analogy to atom interactions in a crystal lattice, the increase of the concentration of nanoparticles around an isotropic domain acts as a potential barrier that counteracts the appearance of another isotropic domain in the vicinity of the already existing domain. However, the appearance of another isotropic domain is possible in an area with a lower nanoparticle concentration. This mechanism is only responsible for regulating the thickness of phases and thus contributes to a one-dimensional and alternating arrangement of nematic and isotropic phases in the capillary. The structure that forms during the above described process is schematically shown in the isotropic phase in Fig. 7c, while an experimentally obtained spatial distribution of nanoparticles can be observed in Fig. 8. Slight variations in the periodic arrangement of formed structures may appear due to the random simultaneous creation of two domains close to each other, so a separation area rich in nanoparticles cannot form in time. The potential barrier is then too low, and the domains collapse into each other. A larger isotropic domain results in a larger barrier around it. However, defects in such a photonic structure only contribute to local changes in the period of the structure and do not affect the macroscopic period. It is worth noting here that the diffusion of nanoparticles to a dispersed state is not an immediate process.39 By observing the diffusion in an LC cell, we find that it takes approximately a day for the nanoparticles to fully disperse.
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Figure 7. Initial distribution of nanoparticles (dots) in the nematic phase of a liquid crystal host (a), distribution after heating to the temperature range around the nematic/isotropic phase transition (the formation of the periodic structure) (b), and distribution after heating to the isotropic phase (c).
Figure 8. Nanoparticle concentration observed after a structure formed in the 60 μm capillary with Au@L1/6CHBT, presented as the absorbance of the formed structure: image showing the phase change observed after digital holographic reconstruction of a coherent 633 nm wavelength front passing through the ACS Paragon Plus Environment
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sample (inset a), and photo taken with parallel polarizers of the isotropic phase just after the structure formed (inset b). In the case of cooling we have noticed the nucleation of the nematic phase in areas that are characterized with a higher concentration of nanoparticles, which was confirmed by the observation of the capillary with parallel polarizers (Fig. 8b). The obtained results allow suspecting that gold nanoparticles act as nuclei of the nematic phase. In the areas with a low nanoparticles concentration the nematic-to-isotropic transition takes place at a lower temperature, near the phase transition of pure 5CB. The situation described above is presented in Fig. 9, in which the speed of temperature changes was 0.1oC/min.
35,7°C
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Figure 9. Polarizing optical microscope images of nematic domains growing at fixed positions in a high concentration of Au@L1/5CB. The measurements were performed in a crossed-polarizer setup; thus, the nematic phase is visible as light dots, whereas the isotropic phase appears as dark areas. The formation of a structure can be observed through the growth of the nematic phase (arrows indicate the positions of the polarizer and analyzer).
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We observed the formation of a periodic structure for a set concentration of Au@L1 independent of the liquid crystal host (5CB or 6CHBT). In all cases, dependence of the period on the internal diameter of the capillary was observed. To determine the period of the formed structures, we created an algorithm that calculates the distances between neighboring domains in a picture taken on the microscope. The obtained dependence of the period on the capillary diameter d is similar for diameters in the 3-20 μm range, leading to the conclusion that doubling the capillary diameter results in a two-fold increase in the period of the structure (Fig. 10a, points). Additionally, we prepared a tapered capillary to investigate whether a continuous change in the capillary diameter continuously changes the period of the formed structure (Fig. 10c). We used the part of the tapered capillary with the diameter continuously changing from 6.8 to 13.6 µm to perform a preliminary experiment. Our result confirms that a continuous change of the capillary diameter results in a continuous change of the structure period. As it was observed in the previous experiment, domains in a tapered capillary appear and start to grow at any position in the entire volume of the mixture due to nanoparticle density fluctuations. However, the internal diameter of the tapered capillary stops the spherical growth of isotropic domains at different times in different parts of the tapered capillary – faster for a smaller diameter. Finally, the period of the obtained structure is changed in a continuous way from 18.7 µm to 35.8 µm. These results are in good agreement with those obtained for capillaries with fixed diameters (Fig. 10b). b)
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Figure 10. Nematic domain structure period in capillaries (dots), with the inset showing the obtained ACS Paragon Plus Environment
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structures (a), domain structure period for fixed diameter capillaries (dots) and the tapered capillary (triangles), with the line representing the averaged dependence of the period on the taper diameter (b), and structure formed in the tapered capillary filled with Au@L1/5CB (c).
Conclusions In our work, we have shown that it is possible to obtain one-dimensional periodic structures spontaneously formed by spatial confinement of a nematic liquid crystal doped with gold nanoparticles. The mechanism of this formation can be understood within the frame of a 60-year-old theory,37 which allows us to understand how the initial fluctuations of the nanoparticle concentration within the LC host allow for a phase transition into discrete domains, thus creating a self-organized photonic structure. We have shown that both fixed and variable (so-called chirp) structures can be obtained when tapered capillaries are used. In particular, wavelength tuning, spectral width adjustment, and the introduction of a narrow transmission band within the photonic band gap of self-organized periodic structures are possible. In this way, optical devices, such as lowpower-consuming tunable filters or reflectors, light shutters or electrically controllable intensity modulators may be realized. Moreover, these modifications are reversible, which makes it possible to create optical memory devices.40 A related area of applications for the proposed optical element is the development of dynamically tunable diffractive optical elements. We envision that our strategy can be readily extended to systems composed of other LC media and a broad range of NP compositions, sizes and shapes. We believe that our findings will allow us to not only better understand the phase separation process in multicomponent soft/colloid mixtures with useful optical properties but also improve the understanding of the precise assembly of advanced materials into 1D periodic systems. Methods/Experimental Au@L1 nanoparticle synthesis. An aqueous solution of HAuCl4 (90 ml, 88 mM) was mixed with a solution of methyltrioctylammonium chloride in toluene (240 ml, 42 mM). The two-phase mixture was vigorously stirred. Consequently, the phases were separated, and dodecanethiol (0.84 mmol) was then added to the organic phase and stirred for 20 min. Then, a freshly prepared aqueous solution of sodium borohydride ACS Paragon Plus Environment
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(25 ml, 0.4 M) was quickly added. After further stirring for 2 h, the organic phase was separated, washed with water (2 x 100 ml), and precipitated with 200 ml of ethanol. The precipitate was washed twice with ethanol (100 ml) and dissolved in toluene. The obtained Au NPs were used as the starting material for the preparation of hybrid NPs, denoted Au@L1. To a solution of NPs (10 mg in 2 ml of toluene), L1 ligand solution was added (10 mg in 2 ml of toluene). The reaction proceeded at room temperature for 36 h. Then, NPs were precipitated with 15 ml of ethanol and centrifuged (8000 rpm, 10 min). The supernatant containing unbound thiol ligands was discarded. The precipitate was dissolved in 2 ml of toluene, and 2 ml of ethanol was added. Then, the particles were centrifuged (8000 rpm, 10 min), and the process was repeated until no traces of free mesogenic ligand remained, as determined by thin-layer chromatography. Doping NPs into the LC host. For each sample, a proper amount of the toluene NP solution was added to a centrifuge tube with a given amount of LC (5CB or 6CHBT), and then, the mixture was thoroughly mixed by sonication at room temperature for 2 min. The volume of the NP solution was varied for different samples but was always above 100 μl. Then, the open vessel was kept at 60°C for 12 h to remove toluene. Next, the tube was transferred to a sonication bath and sonicated at 80°C for 2 min. Upon removal, it was vigorously shaken while cooling down to room temperature. The last step was removal of aggregates by centrifugation at 5000 rpm for 5 min. The model of nanoparticle incorporation into a liquid crystal is presented in Fig. 1. To observe the formation of periodic structures in capillaries, we used a Nikon Eclipse polarized optical microscope with parallel and crossed polarizers and a Linkam THMS600 microscope stage with an accuracy of 0.1°C. The microscope was connected to a computer so that the obtained images and movie clips could be saved. SAXS measurements. From a macroscopic point of view, in Au@L1/5CB and Au@L1/6CHBT composites, nanoparticles are well dispersed in the matrix, as determined by small angle X-ray scattering measurements (SAXS). SAXS experiments were performed with the Bruker Nanostar system (CuKα radiation, parallel beam formed by cross-coupled Goebel mirrors and a 3-pinhole collimation system, area detector VANTEC 2000) at room temperature (Fig. 11). Samples were prepared in thin-walled glass capillaries (for SAXS measurements for the determination of nanoparticle sizes and measurements of spatially
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confined LC/NP composite materials), as thin films on a Kapton tape substrate (for SAXRD measurements of Au in the neat state) . The exemplary diffractogram of nanoparticles-doped liquid crystal compounds collected at 80°C (Fig. 11) revealed only a monotonic decrease of scattered intensity with an increasing scattering angle, which is characteristic of an unstructured dispersion of nanoparticles. The experimental results were fitted with scattering of spheres with the diameter 2.6 +/- 0.4 nm. These results evidence that in at the isotropic phases Au nanoparticles are well separated within the LC matrix. The same results were achieved at 30°C, indicating that although NPs are expelled to the boundary region of the LC phase, the NPs are not structured.
Figure 11. SAXS measurements of nanoparticles in 5CB attesting the good dispersion of nanoparticles in the LC matrix (no visible peak). Infiltrating capillaries with Au/LC mixtures. In our study, we filled silica capillaries with internal diameters ranging from 3 to 60 μm with Au@L1/5CB and Au@L1/6CHBT wt% nanoparticles through capillary action. Taper preparation. A section of a capillary was thermally tapered using horizontal pulling and stretching techniques. As a result, a symmetrical tapered capillary with linearly reduced inner and outer diameters and a 6 µm waist diameter was obtained. Acknowledgments. This research is supported by NCN under grant no. UMO-2015/19/B/ST7/03650. The authors are thankful to P. Palffy-Muhoray from the Liquid Crystal Institute, Kent State University for discussions and comments. WL and MB work was funded by the REINFORCE project (Agreement No. First ACS Paragon Plus Environment
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TEAM2016–2/15) carried out within the First Team program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. Part of this work was also supported by National Science Centre Poland grant no. UMO 2014/15/D/ST5/02570 (MW).
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