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Abstract
Blue phase (BP) liquid crystals are materials with unique self-assembling properties. They can be regarded as 3D photonic crystals as they organize in 3D cubic structures with sub-micrometer range periodicity and display selective optical bandgaps. Yet, the obtained BP crystals are usually polycrystalline or micrometer-sized monocrystals. Producing large BP monocrystals has proven to be a challenging and time-consuming endeavor, due to BP crystal growth being notoriously slow and the complex requirements for achieving a reasonable size and monocrystalline structure. In this work we successfully obtained large BP monocrystals (single lattice orientation) by fast self-assembly. Our fabrication process, which is about 100× faster than in previous reported research, uses relatively simple techniques, therefore demonstrating a considerable improvement towards the manufacturing of 3D photonic crystals.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Blue phase (BP) liquid crystals are materials with unique self-assembling properties that make them promising candidates for novel photonic applications. BPs self-organize spontaneously in 3D structures with sub-micrometer range periodicity; as a result, they can be considered 3D photonic crystals [1,2].
The structure of BPs [2] is fairly complex, molecules arrange in helices that assemble in double-twisted cylinders which in turn organize in cubic crystals. It is important to realize that the unit-cell sizes range in the 100’s nm range, thus qualifying these structures for generation of unique optical phenomena such as photonic gaps and spectral filters. BPs appear in nematic liquid crystals (LC) having strong helical twisting power (also known as cholesteric liquid crystals, CLC), which can show different crystalline phases, the most relevant being Blue phase I (BPI) and Blue phase II (BPII). These phases usually exist in narrow temperature ranges between the isotropic (liquid) and CLC phases –i.e., a cooling sequence could be isotropic → BPII → BPI → CLC.
Stabilizing these materials in a single crystal structure has not been possible until very recently, though [3]. At present, BPs can be obtained as almost perfect crystals by using different technologies, such as different orienting layers, microstructure stabilization, continuous applied voltage or photopatterning [4–7]. Nevertheless, in most cases, fabrication methods lead to either limited-sized BP crystals (micrometer range), polycrystalline BP (platelets) or non-stabilized BP crystals.
Notwithstanding the preparation technology, a distinctive common feature of all methods described in literature is the BP growth rate. Indeed, self-assembly and growth of BP crystals is a very slow process –previous works quantify BP crystal growth rate to be around 5µm/min [8]– therefore the temperature rate for the crystals to yield a reasonable size is consequently low. Conventionally, the temperature rate to obtain BP layers has been set to 0.1-0.5°C/min, regardless whether the BP is obtained by heating up from the CLC phase or by cooling down from isotropic state [3–9].
In this work we report large size BP photonic crystals organized in a single lattice orientation. The crystal homogeneity and organization of the 3D cubic structure was assessed by optical methods and by transmission electron microscopy (TEM). Besides this remarkable result, the most relevant issues of this work are speed –our fast self-assembly procedure is about 100× faster than previous reported works– and stability: the material remains as a BP in a temperature range over 70K. BP macrocrystals are 3D photonic crystals, featuring sub-micrometer-sized cubic periodic structures and selective optical bandgap. The procedure described herein is relatively simple and fast; therefore, it paves the way to the preparation of perfect macrosized 3D photonic crystals for multiple applications.
2. Results
Figure 1 shows two BP textures observed by polarizing optical microscopy (POM) in reflection mode. A mixture that displays a BP in a certain temperature range (see Methods) was filled into sandwiched glass cells organized in two groups. Glass surfaces of the first group did not undergo any surface conditioning treatment, while glass surfaces of the second group were treated with an alignment layer. Cells of the first group show a polycrystalline structure (Fig. 1, top) which corresponds to the classic BP platelet structure, where BP crystallites are overall scattered about the cells surface. Each platelet is a group of BP cubes positioned in a particular orientation.
Fig. 1. Blue phase crystal textures observed by POM and corresponding Kossel patterns. BP platelet structure is obtained when the glass cell has no surface conditioning (top). A uniform single BP crystal is obtained for surface treated glass cells (bottom). In this case, the easy axis is parallel to the polarizer direction. Field of view is ≈14 mm2
Platelet colors are given by Bragg reflections from specific lattice planes, each color corresponding to a different cubic lattice orientation. It had been suggested in previous works that BPs need not alignment layers, thus the cell manufacturing process could be simplified. However, platelet structure increases scattering and worsens the optical performance [3]. Figure 1 (bottom) shows a homogeneous BP structure, as shown in cells with alignment layers. These BP structures are uniform along the whole cell and show a single distinctive reflection color, thus suggesting that all BP cubes are organized and oriented in a monocrystalline 3D structure.
The BP single crystal homogeneity was assessed by generating the Kossel patterns of different regions of each cell. Kossel patterns are carried out with visible light, but are formally equivalent to X-ray diffraction patterns, with a scale factor derived from the sub-micrometer size of the BP cubic structures. A well-oriented BP liquid crystal is a periodic structure, thus a converging monochromatic light incident on the BP crystal will fulfill the Bragg condition, producing a diffraction pattern. Distinctive Kossel patterns are obtained for each BP phase –either BPI or BPII– and lattice orientation. The order parameter of the BP crystal is revealed by the presence of a single pattern or a mix of them [10]. Figure 1 (right) shows the Kossel patterns obtained for polycrystalline and monocrystalline BPs respectively. It can be clearly seen that the first is a mix of several patterns corresponding to mixed BP lattice orientations, while the second corresponds to a single BP orientation –in this case a BPI phase with crystallographic orientation (200).
BP crystals were stabilized by polymerization (see Methods). Figure 2(a) shows the POM texture of a stabilized BP crystal in a glass cell and its Kossel pattern, corresponding to BPI (110). The pattern remains sharp and does not rotate or become dimmer in different cell areas, suggesting that there exist a single BPI in a unique lattice orientation and a fixed azimuthal angle –in this case, the crystal azimuthal angle is 45°±0.5° with respect to the rubbing direction of the alignment layer. The BP reflection wavelength and lattice orientation differs in this case from the one shown in Fig. 1. The resulting BP crystal depends on a number of factors, one of them being the liquid crystal mixture composition. In particular, the chiral dopant concentration seems to influence the resulting lattice orientation, which is the subject of our current research.
Fig. 2. Blue Phase crystal uniformity assessment: a) BP crystal texture, corresponding Kossel pattern and stabilized BP in a glass cell (field of view: ≈14 mm2). b) Side Bragg reflection produced by the BP monocrystal when the cell is side-illuminated (cell active area = 1 cm2). c) Side Bragg reflection in a BP with a defect and Kossel pattern.
Crystal uniformity was assessed by observing side illumination Bragg reflections. Since BPs are periodic cubic structures, they show Bragg reflection in three dimensions (the selective reflection of cholesteric liquid crystals is produced only in one dimension) [11,12]. A bright side Bragg reflection is generated when illuminating a stabilized BP crystal from the side of the glass cell. The glass cell was kept standing in vertical position, while the incoming light illuminates the BP crystal from above generating a bright Bragg reflection that is observed through the sample glass. Figure 2(b) shows that the resulting reflection, following the Bragg condition, shifts to shorter wavelengths when the viewing angle increases. The photos were then taken at different viewing angles of the camera with respect to the sample, and there is a single reflection color for each viewing angle.
By observing the side Bragg reflection, the topology of the BP layer becomes clearly visible. The homogeneity of the BP crystal can be analyzed in search for defects, because crystal defects result in variations in lattice orientation about the area and reflected light changes. Additionally, disorganized BP crystals have a tendency to show scattering and haze when side-illuminated. In our BPs, the crystal shows virtually perfect homogeneity about the whole glass cell (2 cm2), with no spots, dim areas or haze. The discernable faint lines inside the cell are caused by the ITO pixel edge, which seems not to have any impact on the BP crystal uniformity. In Fig. 2(c) a defect on a different BP crystal sample can be spotted on the lower right side of the glass cell, where the reflected light becomes dimmer. Indeed, this area of the BP crystal showed less uniformity when checked by POM which also revealed a blurred Kossel pattern (Fig. 2(c) inset).
POM, Kossel pattern and side Bragg reflection tools provide useful information about the BP crystal uniformity in the XY plane and, to some degree, about the homogeneity of the bulk (Z axis). Defects and inhomogeneities in BP crystals are manifested by POM and side Bragg reflection as changes in color, texture, dim reflecting areas and appearance of haze. However, small defects or changes in lattice orientation occurring in the bulk might not be noticeable by optical methods, especially for thick crystal samples (d>20 µm). Dim or blurred Kossel patterns are a clear indicator of non- monocrystalline samples, because the resulting diffracted pattern is an average of the combined changes and orientation variations in the (hkl) lattice planes [10,13]. However, local crystal misorientations, translated into slightly dimmer Kossel patterns, would be difficult to measure or even estimate. Consequently, it is concluded that a full verification of our crystals would require direct observation of the crystal bulk. This was accomplished by Transmission Electron Microscopy (TEM).
Observing BPs by TEM imposes some technical difficulties, because the BP crystals are created inside thin glass cell sandwiches having at most some 10’s µm thickness. The crystals need to be extracted from the glass cell in order to be sliced and observed. Stabilized BP crystals are still mainly composed by a liquid crystalline material. The polymer used for BP stabilization acts as a scaffold, supporting the cubic structure by getting inserted into the BP disclination lines [14]. When a BP glass cell is disassembled, this polymer scaffold breaks, the liquid crystal pours out from it and the BP structure disappears.
Subsequently, it was decided to prepare a specific set of BP mixtures for TEM observations. These mixtures had increasingly higher concentrations of monomer compounds, so that the main building blocks of the BP cubic structure would be replaced by a rigid polymer, and the BP crystal could be extracted from the cell without collapsing the structure. The polymer employed in the fabrication process comes from a monomer that displays a mesogenic phase itself. Therefore, it was possible to obtain blue phases with increasing concentrations of this monomer in the same kind of mixture, albeit inducing a reduction in the temperature ranges where BP phases appear (see Methods).
Uniform BP crystals, equivalent to those shown in Fig. 2, were obtained with high monomer concentrations. The most stable BPs were obtained when the main monomer concentration was higher than 80%. The remaining nematic mixture improves the temperature range where the BP appears as well as the 3D structure. These BP crystals were identified by their Kossel patterns as BPI (110) and the layer homogeneity was analyzed by side Bragg reflection.
After polymer stabilization, the glass cells were disassembled, and the BP film was extracted. Consecutive cross-sections of the BP crystal films were performed with an ultramicrotome. The BP films thickness was d ≈ 80 nm. Figure 3 shows consecutive cross section TEM images of a BP crystal. The film edge corresponds to where the substrate surface should be, and the structures are reflections from the BP cubic distribution. Since the film thickness is smaller than the BP lattice size, the film contains less than one unit cell in height. Previous research showed that the LC textures observed by TEM appear bright where the LC director is perpendicular to the film surface and dark when parallel [15–18]. The LC director is, on average, parallel to the BP double twist cylinder axes, therefore, bright areas correspond to double twist cylinders that are positioned perpendicular to the film surface [15]. Taking this into account, the BP organization can be estimated, and the lattice size can be measured. The micrographs show a sharp organized periodic BP structure assembled in the [110] direction. A crystal lattice size a = 280 nm was measured. The estimated lattice size derived from the Bragg reflected wavelength is a = 285 nm, [4]
that is calculated from λ = 644 nm. Here n, the effective refractive index, is estimated n = 1.60, and the corresponding Miller indices for the specific lattice orientation (hkl) = (110). The TEM observation provides a direct verification of the BP crystal structure in the bulk. Defects in the bulk that would not be measurable using optical instruments are visible by TEM. For instance, some BP films showed small defects or positional shift in the cube arrangement (Fig. 3 bottom left micrograph). In general, though, all samples showed uniform monocrystalline structures, and all consecutive TEM cross-sections displayed the same BP lattice orientation.Fig. 3. Consecutive cross-sections of a BP crystal observed by TEM. The organized crystalline structure corresponds to BPI in (110) lattice orientation, the sample edge would be at the glass cell surface. Scale bars: 2 µm and 1 µm for the micrographs on the left and right side respectively.
According to previous studies on BP crystal growth, the BPI crystal growth rate was quantified to be around 2-5 µm/min, which is in fact significantly smaller (by a factor of 100) than the growth rate measured for the BPII formation [8,9]. Speeding up the BPI crystal growth by increasing the temperature rate provokes the nucleation of a large number of crystallites or platelets, preventing the formation of a single crystal.
Interestingly, we found that the BP crystal assembly for all of our samples occurred much faster than anticipated. Figure 4 shows the formation of the BP crystal when cooling from the isotropic state at a temperature rate of ≈ 20°C /min. The images show the edge of the glass cell as seen by POM – the adhesive gasket corner is visible. Decreasing the temperature, at 6 seconds (Fig. 4(b)), a bright texture appears in the isotropic from the edge of the cell, which is identified as BPII (110). At 10 seconds (Fig. 4(c)), a red texture corresponding to BPI (110) appears from the BPII in the edge of the cell, growing towards the center – since there is a slight temperature gradient near the cell borders. The BPI phase continues growing, the texture becomes bright red and expands until it covers the whole cell surface at 23 seconds (Fig. 4(d), (e)) with a uniform homogeneous structure. The BP is then polymerized into stable crystals depicted in Fig. 2.
Fig. 4. Fast crystal growth of monocrystalline BPI from isotropic phase: a) Isotropic state, b) Starting point of BPII, c) Starting point of BPI on the edge, d-e) BPI growth in the rest of the cell, f) Non-uniform BPI obtained by heating from the CLC phase and Kossel pattern (inset).
It is worth mentioning that, during the thermal process, the BPI can only be kept as a monocrystal as long as no cholesteric phase appears at all. When the BPI is formed from the CLC phase by heating, randomly oriented BP crystals appear. The BP structure is still BPI but the azimuthal angle of the lattice size is not fixed, thus the resulting BP is polycrystalline. Figure 4(f) shows such BPI structure (red domains) obtained from CLC (blue), the Kossel pattern corresponds to a mix of patterns of BPI (110) oriented at different angles.
A BP crystal formation without the presence of alignment layers grows into platelets. Because each platelet needs to form around an independent nucleation point, increasing the temperature rate hinders the platelet growth and increases the number of crystallites. Our results suggest that a surface-assisted crystal growth is apparently inducing a speedy formation of BP into remarkably uniform structures.
Yet, the observed results need a more complete understanding and are still under study. A difference between published manufacturing procedures and the procedures described here arise from conditioning layers. Conventional polyimide alignment layers have been commonly employed as liquid crystal alignment layers. Instead, we have used polyamide layers: Nylon 11, Nylon 6 and Nylon 6-6. Previously, polyamides have been used almost exclusively for ferro- and antiferroelectric smectic liquid crystals (FLC, AFLC), as these layers produce excellent alignment and electrooptical behavior due to their weak anchoring energy conditions [19,20]. It has been demonstrated that an increased in surface anchoring energy tends to increase the devices working voltages and the grayscale dynamic range is deteriorated. Certainly, surface anchoring is essential for FLCs when the cell thickness is in the range of the surface coupling, and [21,22]. In contrast, conventional polyimides produce strong anchoring layers, where the average anchoring energies, W, are in the order of W≈10−4 J/m2, whereas alignment layers like Nylon, photoalignment layers or layers with soft rubbing conditions, produce weak anchoring that is one or two orders of magnitude smaller [23,24].
According to previous studies, the anchoring energy at the interface could be the driving force to promote a specific crystallographic orientation [25,26]. The same mechanisms would be applicable in this case, the free energy at the interface can dominate in some lattice orientations while penalizing others, which would aid in the speedy formation of a BP crystal in a specific single crystal lattice orientation
This anchoring energy change at the interface has been used as well in BP mixtures when combined with weak anchoring interface inducing monomers, thus reducing driving voltage of polymer stabilized BPs [27]. Another strategy involves using electrostriction effect by a repetitive applied AC field to induce metastable lattice structures in BPI [28].
A number of commercial polyimide layers with strong surface anchoring were examined as alignment for blue phases to compare to Nylon layers, like polyimides SE-130, RN-1199, SE-1211, SE-7492, among others. None of these alignment layers produced monocrystalline BP oriented layers, in most cases, disorganized platelets or semi organized crystallites appeared. It may be inferred that, at least within our manufacturing protocol, the use of polyamides plays a decisive role in the swift generation of macroscopic monocrystal samples. Additionally, in order to avoid strong anchoring conditions, several polyimide layers treated with very soft rubbing conditions were tested as well. The resulting BP layers showed slightly better homogeneity, some samples produced small monocrystalline areas, although the layers still had abundant defects and crystal misorientations.
Previous research demonstrated that BPs crystals organized in unidirectionally well-orienting layers were responsible for obtaining improved electrooptical responses compared to non-uniform BPs [3]. Several BP crystals manufactured by fast-assembly as shown in Fig. 2, were prepared into ITO covered glass cells for electrooptical characterization. The devices were measured at room temperature after the polymer stabilization process and all measurements were performed for two orthogonal polarizations in reflection mode.
Figure 5 shows the dynamic response and switching time measurement a BP monocrystal stabilized in a 5 µm cell measured in reflection mode. Using a 1kHz sin wave modulated by a 0.5Hz square wave response times were measured as the switching between 90% and 10% reflection. The average switching ON and OFF times were ton = 43.9 µs, toff = 4.8 ms (5µm), and the average threshold and saturation voltages were Vth = 16V and Vsat = 66V respectively.
Fig. 5. Dynamic response in reflection mode of a BP monocrystal stabilized in a 5 µm glass cell – switching times ON (left) and OFF (right).
All measurements obtained in the electrooptical characterization were essentially the same for the two orthogonal polarizations in all cases. Variations of 4% in reflection occurred for static response at most, thus the devices demonstrate polarization independency of BP crystals.
3. Methods
In-house prepared nematic mixture whose major compositions are three fluorinated biphenyl cyclohexyls (JC-1041xx), 4-cyano-4'-pentylbiphenyl (55.0:45.0), and chiral dopant ISO(6OBA)2. Standard BP crystals were prepared with this LC mixture and monomers RMM-34C (Merck), EHA (1:1). Electrooptical measurements were performed on BP crystals prepared from this mixture. BPs with high monomer concentration were formulated with the previous compounds. The most stable polymeric BP crystals resulted from mixtures where the monomer composition was higher than 80%. The BPs observed by TEM contained LC:RMM34C:EHA = (9.5:86.0:4.5). The mixtures were injected into glass cells consisting of two ITO coated ultraflat glass substrates –conditioned with polyamide layers (Nylon 11), thermal cycle and rubbed in antiparallel configuration– assembled into a sandwich cell of 5 or 15 µm thick. Several polyamides were tested: Nylon 11, Nylon 6, as well as Nylon 6-6, produced monocrystalline and oriented BP crystals.
The BP crystals were prepared and analyzed in an Instec HCS402 hot stage platform while cooling down from isotropic state to cholesteric liquid crystal. All BP phases were checked in an Olympus BX51 polarizing microscope in reflection mode with a 5×/0.15 objective. Kossel patterns were obtained in conoscopic configuration with a 60×/0.70 objective (550 nm or 450 nm light source). After fast thermal treatment, BP monocrystals were stabilized with a 365 nm light source (Hamamatsu Lightningcure LC8) at 0.5 mW/cm2 for 15 minutes, or 2 minutes in case of the polymeric BPs. Reflection spectra were measured with Ocean Optics Flame-T spectrometer. Transmission of the BP crystals in the glass cells were measured T=85%, (measured outside the photonic bandgap, @550 nm).
The polymeric BP crystals were analyzed by TEM in a JEOL JEM 1400 microscope. Sections of the films were cut dried and embedded in epoxy resin. Consecutive cross sections of the films were cut with an ultramicrotome (d≈70-90 nm) and deposited over a grid (100µm). The samples were observed by TEM, and the resulting images were analyzed with an acquisition software system for TEM.
For electrooptical characterization, the BP crystals were prepared into ITO coated glass cells, sealed and wired. The devices were addressed using waveform function generator HP 33120A, high voltage linear amplifier FLC F20ADI, and the obtained reflection signals were analyzed and recorded with Tektronix TDS 2014 digital oscilloscope and Tektronix OpenChoice acquisition software.
4. Conclusions
In this work we manufactured large 3D Blue Phase monocrystals by fast self-assembly – the BP crystal assembly is about 100× faster than previously reported – and analyzed the resulting crystals by TEM, confirming the monocrystalline structure. Our findings show that once a crystal seed is formed in the edge of the cell, the monocrystal growth rapidly progresses through the cell volume, because surface assist layers promote a thermodynamic preference for an ordered structure. The BP crystals electrooptical characterization shows considerably short response times and polarization independency. Our method demonstrates a significant progress towards manufacturing 3D photonic crystals, which is typically a complex task, as the fabrication requirements for achieving large sized crystals and uniform structures are particularly high. Integrating tunable BP monocrystals in structured devices could, as well, produce different optical functions, making a notable improvement in photonic crystal structures for devices in specific photonic applications. Such materials will provide novel or alternative solutions in Photonics and Nanotechnology to build new components; for instance microlasers, optical sensors, intelligent displays, and security technologies.
Funding
Narodowe Centrum Nauki (UMO-2016/23/P/ST7/04261).
Disclosures
The authors declare no conflicts of interest.
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