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Large-scale single-crystal blue phase through holography lithography

Large-scale single-crystal blue phase through holography lithography Research Article Large-scale single-crystal blue phase through holography lithography Xiaowan Xu, Jiawei Wang, Yanjun Liu , and Dan Luo* Southern University of Science and Technology, Department of Electrical & Electronic Engineering, Shenzhen, China Abstract. The blue phase, which emerges between cholesteric and isotropic phases within a three- dimensional periodical superstructure, is of great significance in display and photonic applications. The crystalline orientation plays an important role in the macroscopic performance of the blue phase, where the single crystal shows higher uniformity over the polydomain and monodomain, resulting in higher Bragg re- flection intensity, lower hysteresis, and lower driving voltage. However, currently reported methods of forming a single-crystal blue phase based on thermal controlling or e-beam lithography are quite time-consuming or expensive for large-scale fabrication, especially in the centimeter range, thus hindering the broad practical applications of single-crystal blue-phase-based photonic devices. Herein, a strategy to fabricate a large scale single crystalline blue-phase domain using holography lithography is proposed. Defect-free single-crystal domains both in blue phase I and blue phase II with a desired orientation of over 1 cm are fabricated based on a nanopatterned grating with periodic homeotropic and degenerate parallel anchoring, with colors from red and green to blue. This holography lithography-assisted strategy for fabrication of a large-scale single- crystal blue phase provides a time-saving and low-cost method for a defect-free single crystalline structure, leading to broad applications in liquid crystal displays, laser devices, adaptive optics elements, and electro- optical devices. Keywords: blue phase; single crystal; holography lithography; liquid crystals. Received Oct. 12, 2022; revised manuscript received Nov. 26, 2022; accepted for publication Dec. 31, 2022; published online Feb. 11, 2023. © The Authors. Published by SPIE and CLP under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.APN.2.2.026004] On the other hand, the crystalline orientation plays an impor- 1 Introduction tant role in the macroscopic performance of the blue phase, The blue phase (BP), existing between the cholesteric phase where the single crystal shows higher uniformity over the poly- and isotropic phase (ISO) of liquid crystals within a three- domain and monodomain, resulting in higher Bragg reflection dimensional periodical superstructure, possesses three forms of intensity, lower hysteresis, and lower driving voltage. Usually, blue phase I (BP I), blue phase II (BP II), and blue phase III the BP fragments form inside the volume simultaneously as a 1,2 (BP III). BP III possesses a noncrystalline symmetry with phase transition occurs, leading to polydomain BP structures, disordered structure, and BP I and BP II exhibit body-centered which consist of many platelets of different orientations. The cubic structure and simple cubic structure, respectively, with a mismatches in polydomain BPs increase the scattering in the 3–6 double-twisted cylinder nanostructure. The narrowed temper- volume and affect the reflectivity of the Bragg reflection and ature range and nonuniformity of the crystalline structure are 25–29 the optical-electrical properties of BPs. Recent studies indi- two important issues that hinder the practical applications of 30–32 cate that several methods, such as electric fields, thermal blue phase liquid crystal (BPLC). On one hand, the temperature 33 34–36 control, and surface treatments can be applied to form 7,8 range of the BP has been extended by polymerization or nano- monodomain BPs, which consist of many single-crystal plate- 9,10 particle doping, leading to useful applications such as dis- lets with one lattice plane orientated in the same direction. The 11–14 15–17 18–20 21–24 plays, lasers, sensors, and photonic devices. structures of monodomain BPs significantly increase the reflec- tivity of the Bragg reflection. However, many defects still *Address all correspondence to Dan Luo, luod@sustech.edu.cn exist between the separated platelets with a grain boundary. Advanced Photonics Nexus 026004-1 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Therefore, to further improve the optical performance of BPs, while varying the pitch of the nanopattern grating from 230 and it is highly desirable to develop methods of fabricating ideal, 190 nm to 170 nm, along with a high reflectance of 44% and single-crystal BPs. 45% to 42% and a narrow bandwidth of 19.0 and 17.0 nm to Several methods have been proposed to generate a single 14.6 nm, respectively. This holography lithography-assisted crystal blue phase. Chen et al. reported the fabrication of an strategy for fabrication of a large-scale single-crystal BP extraordinarily large single-crystal BP I by controlling the self- provides a time-saving and low-cost method for a defect-free assembly processes of BPLC. The gradient-temperature tech- single-crystalline structure that significantly improves the prop- nique enables growth of a single crystalline BP with the size of erty of the BP, including an increase in the intensity of the Bragg 1cm . However, long periods of thermal control over several reflection, leading to broad applications such as liquid crystal days were required, which leads to low efficiency in fabrication, displays, laser devices, adaptive optics elements, and electro- and the crystal orientation of the BP could not be controlled due optical devices. to the similar treatment of the substrate compared to traditional BPs. de Pablo et al. proposed a method to form a single crys- 2 Results and Discussion talline BP II through a nanopatterned surface with binary In our experiments, both BP I [with body-center cubic lattice 38–42 homeotropic/planar anchoring through e-beam lithography. structure, Fig. 1(a)] and BP II (with simple cubic lattice struc- This method is capable of generating a single-crystal BP over a ture, Fig. 1(b)] are investigated. The wavelength of reflected pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi macroscopic region (at a scale of mm ) with designed crystallo- 2 2 2 light from the BP is determined by λ ¼ 2n a∕ h þ k þ l , eff graphic plane orientation based on the simulation of free energy where λ is the wavelength of reflective light, n is the refractive eff from Landau–de Gennes theory (see Supplementary Material). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 index (n ¼ ð2n þ n Þ∕3; n and n are the ordinary and However, the fabrication is highly dependent on the nanopattern eff o e o e extraordinary refractive indices of liquid crystal materials), a is fabricated by e-beam lithography, which is time-consuming and the lattice constant of crystal, and h; k; l are the Miller indices. expensive, thus hindering the practical application of single- The BP crystallographic planes are denoted by (hkl), and crystal BP photonic devices. Therefore, a time-saving and low- BP represents a BP with orientation of the (hkl) plane cost method, rather than e-beam lithography, to fabricate the ðhklÞ nanopattern at a larger scale for large-scale single-crystal BPs parallel to the surface. is highly desirable. The anchoring pattern of the substrate is designed based on In this paper, a strategy to fabricate large scale single crys- the morphology of the orientating lattice plane on the surface. talline BP liquid crystals is demonstrated through holography Figure 1(c) shows the morphology of BP I plane at the ð110Þ lithography. Defect-free single-crystal domains both in BP I and boundary of crystal lattice with l ¼ 0 (where l means the dis- c c BP II with a desired orientation of over 1cm are fabricated placement from the boundary of the unit lattice). The region of based on a nanopatterned grating with periodic homeotropic blue ellipses represents the cross section of a double-twist and degenerate parallel anchoring. A large-scale single-crystal cylinder on the substrate surface, where the liquid crystal (LC) BP with reflection colors from red and green to blue are realized molecules orient from 45 deg to 90 deg (the angles between the Fig. 1 (a) BP I with body-center cubic lattice structure; (b) BP II with simple cubic lattice structure; (c) morphology of BP I plane; (d) morphology of BP II plane; (e) reduced morphology of ð110Þ ð100Þ BP I plane; (f) reduced morphology of BP II plane; (g) simplified grating of BP I plane; ð110Þ ð100Þ ð110Þ (h) simplified grating of BP II plane. ð100Þ Advanced Photonics Nexus 026004-2 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography substrate surface and the director of the LC molecule). This re- The morphologies under other cutting conditions are shown in gion will be treated to be a homotropic surface anchored in a Fig. S1 in the Supplementary Material (l ¼ 0 to l ¼ d ) and c c BPI simplified alignment model. The rest of the region represents Fig. S2 in the Supplementary Material (l ¼ 0 to l ¼ d ), c c BPII the defect structure in the crystal lattice, which can be seen respectively. However, the fabrication process of the duty ratio as isotropic materials. The pitch along two orthogonal directions (0.7:1.3) is much more difficult. Therefore, we try to use a gra- pffiffiffi ting with a duty ratio of 1:1 to replace the duty ratio of 0.7:1.3, is represented by 2 a , and a , where a is the lattice BPI BPI BPI which shows good and acceptable results in the following ex- constant of BP I, leading to an effective period of grating formed pffiffiffiffiffiffiffiffi periments. Figures 1(g) and 1(h) demonstrate the simplified gra- by the region of blue ellipses to be d ¼ 2∕3a . This re- BPI BPI ting deduced from Figs. 1(e) and 1(f), respectively. To reduce gion will be treated as a degenerate parallel surface anchored in the difficulty of fabrication, the duty ratio of grating is chosen a simplified alignment model. Figure 1(d) shows the morphol- to be 1:1 in both cases. This simplification process proved to ogy of BP II , which is similar to BP I. The region of the blue ð100Þ be effective and efficient, leading to a single crystal forming in rectangle with an oblique line represents the long-axis cutting both the BP I and BP II planes. ð110Þ ð100Þ section of the double-twist cylinder, where the LC molecules In our experiment, two-beam holography lithography was orient from 0 deg to 45 deg (the angles between the substrate applied to generate the grating on a large scale. Figure 2(a) de- surface and the director of LC molecule). Both the blue rectan- picts the schematic illustration of the fabrication process, where gle and the rest of the region will be treated as a degenerate a linearly polarized laser beam (the polarization direction is per- parallel surface anchored in a simplified alignment model. pendicular to the plane of the optical table) with a wavelength of The effective period of grating formed by the region of blue 325 nm splits into two beams by passing through a beam splitter, circles and the rest is d ¼ a . BPII BPII and there is interference at the surface of substrate coated with The single crystalline BPs are mediated by the strain induced homeotropic alignment material (OTE) and photoresist. For by the corresponding patterned anchoring conditions with the two-beam holography lithography, the obtained interference lower free energy. The grating pattern consists of homeotropic light field is a grating. The pitch of the formed grating is deter- anchoring and degenerate parallel anchoring, resulting from mined by the formula of 2d sin θ ¼ λ, where d is the pitch of the boundary molecular distribution, which successfully induced grating, λ is the wavelength of the laser, and the interference 38,39 the formation of single crystalline BPs. angle θ is half the angle between two interference beams. Figures 1(e) and 1(f) demonstrate the reduced morphology The parameters of BP samples (S1, S2, and S3) used in our of BP I and BP II planes at the boundary of the crystal ð110Þ ð100Þ experiment are shown in Table 1. The data indicate the process lattice with l ¼ 0, where the blue region represents the homeo- c of phase transitions and the corresponding information between tropic surface anchoring and the rest, the yellow region, repre- substrates and lattice parameters; for example, Iso-BP I means sents degenerate parallel surface anchoring. The reduced this sample is started from an isotropic phase and stabilized at morphology of surface anchoring can be further simplified to BP I through cooling. Once the pitch is fixed, the corresponding a grating in both cases, and the duty ratio is defined as d ∶d , 1 2 interference angle, lattice constant, and chiral dopant concentra- where d and d are the corresponding widths of the region with 1 2 tion can be calculated. In our experiments, three pitches of gra- the blue part and the region without the blue part, respectively. ting were chosen to be 230, 190, and 170 nm, and the calculated The duty ratios are around 0.7:1.3 (where d ¼ d þ d )and BPI 1 2 interference angle θ should be 45.0 deg, 58.8 deg, and 72.9 deg, 1:1 (where d ¼ d þ d ) in Figs. 1(e) and 1(f), respectively. BPII 1 2 respectively. The corresponding lattice constant a is calculated Fig. 2 Fabrication process of patterned substrate with periodic OTE grating for large-scale single- crystal BP generation. (a) Two-beam holography lithography based on the substrate with OTE and photoresist; (b) developing; (c) O plasma etching; (d) stripping; (e) periodic orientation of liquid crystal molecules induced by grating patterned substrate. Advanced Photonics Nexus 026004-3 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Table 1 Parameters of BP samples. Transition Pitch Interference Lattice Chiral Dopant Phase Temperature d Angle θ Constant Concentration Sample Transition (°C) (nm) (deg) a (nm) c (wt.%) S1 Iso-BP I 89.2 230 45.0 281.7 3.0 S2 BP II-BP I 85.8 190 58.8 232.7 3.6 S3 Iso-BP II 88.5 170 72.9 170.0 3.4 to be 281.7 and 232.7 nm for BP I (according to d ¼ alignment-induced single crystal. Polydomain BPs [Figs. 3(a), BPI pffiffiffiffiffiffiffiffi 3(d), and 3(g)] contain plenty of single crystals with different 2∕3a ), and 170 nm for BP II (according to d ¼ a ), BPI BPII BPII orientations and a disordered mosaic image with several colors. which require the concentration of chiral dopant of R5011 −1 Monodomain BPs [Figs. 3(b), 3(e), and 3(h)] contain pieces of (HTP ¼ 126 μm ) to be 3.0, 3.6, and 3.4 wt.%, respectively. a single crystal with a uniform orientation but still with defects The pattern of the grating received by the positive photoresist and inconspicuous boundaries between the pieces. In contrast, where the bright region in the light field forming the grating will the patterned grating alignment successfully induces the single be developed and removed, while the dark region in the light crystal [Figs. 3(c), 3(f), and 3(i)] in all three cases of S1, S2, and field forming the grating will be retained for the next developing S3, where the pitch of grating is 230, 190, and 170 nm with step [Fig. 2(b)]. After developing, the O plasma etching was corresponding reflection colors of red, green, and blue. The applied [Fig. 2(c)], where the OTE covered with photoresist single crystal forms a uniform crystalline structure in a large (yellow) was kept from etching by O plasma and while the domain of macroscopic size, and the obtained single crystal OTE part uncovered with photoresist was etched. After stripping has no defect in the volume. of the photoresist, a grating consisting of the region with OTE Figures 4(a)–4(c) demonstrate the images of fabricated three (which would lead to perpendicular alignment of the LC) and gratings observed under scanning electron microscopy (SEM). the region without OTE (which would lead to degenerate paral- The experimentally measured pitch is 231.5, 190.5, and lel alignment of the LC) on substrate were fabricated, as shown 170.9 nm, respectively, which is highly consistent with the in Fig. 2(d). Figure 2(e) shows the orientation of liquid crystal designed pitch of d ¼ 230 nm, d ¼ 190 nm, and molecules induced by the surface anchoring of the patterned BPIðS1Þ BPIðS2Þ d ¼ 170 nm. The POM images of S1, S2, and S3 (cor- substrate. The patterned homeotropic and degenerate parallel BPIIðS3Þ anchoring of the substrate induces periodic orientation of liquid responding to grating patterned in Figs. 4(a)–4(c), respectively) crystal molecules on the patterned substrate and leads to uni- with corresponding Kossel diagrams (as inset figures) for the form formation of BPs with a cubic structure. The cell with lattice planes of BP I ,BP I , and BP II are shown ð110Þ ð110Þ ð100Þ a grating pattern was made up of a bare glass and a glass with in Figs. 4(d)–4(f), respectively. According to the principle of grating-patterned OTE, and 5 μm spacers were used to define lattice diffraction, the Kossel diagrams of BP I have six ð110Þ the cell thickness. lines in Fig. 4(d) and four lines in Fig. 4(e), which derives from The phase transitions among isotropic phase, BP I, and BP II diffractive limitation. The lattice constant of sample S1 is larger can be categorized as isotropic phase BP I, BP II–BP I, and iso- than S2 within diffractive limitation; therefore, more Kossel tropic phase BP II during the temperature decrease process. lines can be seen. The Kossel diagrams with distinct Kossel lines Figures 3(a)–3(c) show the polarization optical microscopy indicate a uniform orientation. The Bragg reflection spectra of (POM) images of sample S1 in BP I (cooling from isotropic S1, S2, and S3 are shown in Figs. 4(g)–4(i), respectively. The phase at 89.5°C to BP I at 85.7°C with a rate of 0.1°C/min) single-crystal BPs of samples S1, S2, and S3 possess high re- at the homeotropic, parallel, or grating-patterned alignment of flectance of 44%, 45%, and 42% (the theory limitation is 50% the substrate, leading to polydomain, monodomain, and a single due to the circularly polarized light of reflection) at central crystal, respectively. Figures 3(d)–3(f) show the POM images wavelengths of 648, 540, and 512 nm, and narrow bandwidths of sample S2 in BP I (cooling from isotropic phase at 88.8°C of 19.0, 17.0, and 14.6 nm, respectively. According to the rela- to BP II at 86.3°C and then to BP I at 84.9°C with rate of tionship of the grating period and the lattice constant for BP I pffiffiffiffiffiffiffiffi 0.1°C/min) at homeotropic, parallel, or grating-patterned align- and BP II (d ¼ 2∕3a , and d ¼ a ), the calculated BPI BPI BPII BPII ment of the substrate, leading to polydomain, monodomain, and lattice constants of induced single-crystal BPs are a ¼ BPIðS1Þ a single crystal, respectively. Figures 3(g)–3(i) show the POM 283.6nm, a ¼ 233.3nm, and a ¼ 170.9nm, BPIðS2Þ BPIIðS2Þ images of sample S3 in BP II (cooling from isotropic phase at respectively. Compared to the measured lattice constants 89°C to BP II at 86.8°C with rate of 0.1°C/min) at homeotropic, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 parallel, or grating-patterned alignment of the substrate, leading (λ ¼ 2n a∕ h þ k þ l ) of 289.3, 241.3, and 161.6 nm, eff to polydomain, monodomain, and a single crystal, respectively. the deviation values between the calculated and measured values The changes of the clear point and temperature range of BPs of lattice constants Δa are 5.7, 8.0, and 9.3 nm, respectively, with these three surface treatments are negligible. The parallel which are quite small compared to those previously reported anchoring derives from rubbing on polymide (PI) materials with in Ref. 38. The parameters including pitch, calculated and mea- one preferred azimuth in the horizontal direction, as shown in sured lattices, and the deviation values for samples S1–S3 are Figs. 3(b), 3(e), and 3(h). The uniformity of images from the shown in Table 2. homeotropic and parallel alignment-induced polydomain and The reflection spectra and POM textures of BPs are valid monodomain BP are apparently worse than that from the grating proof to differentiate polydomain, monodomain, and single- Advanced Photonics Nexus 026004-4 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Fig. 3 POM images of S1, S2, and S3 at different alignment conditions. S1 in BP I (a) at homeo- tropic alignment with polydomain, (b) at parallel alignment with monodomain, and (c) at grating alignment (d ¼ 230 nm) with single crystal. S2 in BP I (d) at homeotropic alignment with polydo- main, (e) at parallel alignment with monodomain, and (f) at grating alignment (d ¼ 190 nm) with single crystal. S3 in BP II (g) at homeotropic alignment with polydomain, (h) at parallel alignment with monodomain, and (i) at grating alignment (d ¼ 170 nm) with single crystal. Scale bar: 100 μm. crystal BPs, as shown in Fig. S3 in the Supplementary Material. fabricated based on a nanopatterned grating with periodic The polydomain BPs are easy to differentiate. The spectra of homeotropic and degenerate parallel anchoring. The grating monodomain and single-crystal BPs are quite similar, which pattern consisting of homeotropic anchoring and degenerate makes them difficult to distinguish. Monodomain BPs are parallel anchoring resulting from boundary molecular distribu- formed with pieces of single crystals with the same orientatio- tion successfully induced the formation of a single crystalline nand show similar Bragg structure colors. The defects and mis- blue phase. A single-crystal BP in BP I ,BP I , and ð110Þ ð110Þ matches of angles between pieces of crystalline structures exist BP II planes with reflection color from red and green to blue ð100Þ in monodomain BPs, while single-crystal BPs exhibit perfect are realized while varying the pitch of the nanopattern grating morphology in the POM texture. from 230 and 190 nm to 170 nm, with a high reflectance of Figures 5(a)–5(c) show the photo images of fabricated single- 44% and 45% to 42% at the central wavelengths of 648 and 540 crystal BPs S1, S2, and S3 with a large scale of over 1cm in to 512 nm, and narrow bandwidths of 19.0 and 17.0 nm to red, green, and blue, respectively. In principle, the fabrication 14.6 nm, respectively. This holography lithography-assisted size can be further increased by using a larger diameter of laser strategy for fabrication of a large-scale single-crystal BP pro- beam with higher laser power. The large scale of single-crystal vides a time-saving and low-cost method for formation of a BPs based on holography lithography induced grating pattern defect-free single crystalline structure, leading to broad applica- of the substrate enables defect-free crystalline structures, lead- tions in liquid crystal displays, laser devices, adaptive optics ing to potential applications in displays, lasers, and electro- elements, and electro-optical devices. optical devices. 4 Experimental Section 3 Conclusion 4.1 Materials We demonstrated a strategy to fabricate a large-scale single crys- talline BP through holography lithography. Defect-free single Liquid crystal host HTG135200-100 (n ¼ 1.717, n ¼ 1.513 of e o 2 −1 crystals both in BP I and BP II with size over 1cm are 589 nm at 20°C) and chiral dopant R5011 (HTP ¼ 126 μm ) Advanced Photonics Nexus 026004-5 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Fig. 4 Surface morphology of gratings, Kossel diagram, and reflection spectra of single crystals. SEM images of gratings were used to align of BP samples (a) S1, (b) S2, and (c) S3. The scale bar is 500 nm. (d) POM image of single crystalline BP I formed by S1. Inset: Kossel diagram of BP I ; (e) POM image of single crystalline BP I formed by S2. Inset: Kossel diagram of BP I ; ð110Þ ð110Þ (f) POM image of single crystalline BP II formed by S3. Inset: Kossel diagram of BP II . The ð100Þ reflection spectra of (g) S1, (h) S2, and (i) S3. Table 2 Parameters of BP fabricated in our experiment. Calculated Measured Pitch lattice lattice Deviation d constant constant value Sample (nm) a (nm) a (nm) Δa (nm) S1 231.5 283.6 289.3 5.7 S2 190.5 233.3 241.3 8.0 Fig. 5 Photo images of (a) S1 in BP I, (b) S2 in BP I, and (c) S3 in S3 170.9 170.9 161.6 9.3 BP II. Scale bar: 2 mm. were purchased from Hecheng Display. Homeotropic alignment 4.2 Fabrication Process materials octadecyltriethoxysilane [OTE, n-C H SiðOCH CH Þ ] 18 37 2 3 3 were purchased from Sigma Aldrich and used without further puri- Liquid crystal mixtures preparation: The nematic liquid crystal fication. Parallel alignment material polyimide (PI) was purchased HTG135200-100 and chiral dopant R5011 were mixed together, from Beijing Bomi Company. Photoresist mr-P 1202LIL, devel- stirred at the isotropic phase, and cooled down to the BP. oper mr-D 374 and stripper mr-Rem 660 (based on N-methylpyr- Substrates preparation: First, the OTE film was developed on rolidon (NMP)) were purchased from Micro Resist Technology. the substrates. The substrate glass was cleaned by sonication in Advanced Photonics Nexus 026004-6 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography 2. W. Hu et al., “Ultrastable liquid crystalline blue phase from acetone and ethyl alcohol. The glass was immersed in the mix- molecular synergistic self-assembly,” Nat. Commun. 12, 1440 ture of concentrated H SO and 30% aqueous H O (2:1 v/v) 2 4 2 2 (2021). for 30 min. The samples were immersed in the mixture of 3. S. Meiboom, M. Sammon, and W. F. Brinkman, “Lattice of dis- 240 mg OTE and 70 ml toluene with 0.25 mL n-BuNH as cata- clinations: the structure of the blue phases of cholesteric liquid lyst for 90 min. The samples should be vertically inserted into crystals,” Phys. Rev. A 27(1), 438–454 (1983). the solution and should not touch each other. To remove excess 4. S. S. Gandhi and L.-C. Chien, “Unraveling the mystery of the silane aggregates in the solution, the samples were stirred in a blue fog: structure, properties, and applications of amorphous blue toluene bath and rinsed with deionized water several times. phase III,” Adv. Mater. 29(47), 1704296 (2017). Then they were blown dry with nitrogen and allowed to cure for 5. H. M. Jin et al., “Soft crystal martensites: an in situ resonant soft x-ray scattering study of a liquid crystal martensitic transforma- 2 h at 150°C in a vacuum oven. After development with OTE, tion,” Sci. Adv. 6(13), 5986 (2020). the substrates were spin-coated with photoresist mr-P 1202LIL 6. J. Liu et al., “Diffusionless transformation of soft cubic superstruc- (4000 r, 30 s) and baked at 100°C for 60 s, and then exposed ture from amorphous to simple cubic and body-centered cubic under two-beam interference for 30 s to get the structure of the phases,” Nat. Commun. 12, 3477 (2021). grating. A 325 nm He–Cd laser was used as the interference 7. H. Kikuchi et al., “Polymer-stabilized liquid crystal blue phases,” light source. The substrate was developed in developer mr-D Nat. Mater. 1(1), 64–68 (2002). 374 for 25 s. The resulting grating pattern was transformed onto 8. D. M. Xu et al., “Electro-optic response of polymer-stabilized blue the OTE film layer by exposing the sample to oxygen plasma phase liquid crystals,” Appl. Phys. Lett. 105(1), 011119 (2014). (50 W, 60 s), followed by stripping the photoresist in stripper 9. H. Yoshida et al., “Nanoparticle-stabilized cholesteric blue phases,” Appl. Phys. Express 2(12), 121501 (2009). mr-Rem 660 for 1 min. 10. M. Ravnik et al., “Three-dimensional colloidal crystals in liquid The cell with the grating pattern was made up of a bare glass crystalline blue phases,” Proc. Natl. Acad. Sci. U. S. A. 108(13), and a glass with grating-patterned OTE, and 5 μm spacers were 5188–5192 (2011). used to define the cell thickness. A liquid crystal mixture was 11. F. Castles et al., “Stretchable liquid-crystal blue-phase gels,” Nat. injected into the cell through capillary action. The cell was Mater. 13(8), 817–821 (2014). heated until the mixtures were at the isotropic phase, and then 12. Y.-S. Zhang et al., “Stretchable freestanding films of 3D nano- slowly cooled down by 0.1°C per minute until the single crys- crystalline blue phase elastomer and their tunable applications,” talline BP was fully formed in the cell. Adv. Opt. Mater. 9(1), 2001427 (2020). The cell with homeotropic alignment was made up of a bare 13. F. 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Chanishvili, “Liquid crystal blue phases Optical characterization was performed by using the cross- interconversions based real-time thermal imaging device,” Opt. polarized and reflective mode of the microscope (ECLIPSE Ci Express 27(9), 13526–13531 (2019). POL, Nikon) with a 10× objective. Samples were heated up and 17. J. Liu et al., “Single-, dual-, triple-, and quadruple-wavelength cooled down by using a temperature controller hot stage (TS62, surface-emitting lasing in blue-phase liquid crystal,” Adv. Mater. 34(9), 2108330 (2022). INSTEC). Visible spectra of BPs were obtained by spectrometer 18. V. Sridurai et al., “Electrically tunable soft photonic gel formed (USB2000+, Ocean Optics). The He–Cd laser (IK3151R-E, by blue phase liquid crystal for switchable color-reflecting mirror,” KIMMON) was used to generate two-beam interference. The ACS Appl. Mater. Interfaces 9(45), 39569–39575 (2017). laser power was 10 mW, and the beam diameter of laser beam 19. Y. Yang et al., “Structural and optical response of polymer- was 2 cm. stabilized blue phase liquid crystal films to volatile organic compounds,” ACS Appl. Mater. Interfaces 12(37), 42099–42108 Acknowledgments (2020). 20. Y. Yang et al., “Bioinspired color-changing photonic polymer This work was supported by the National Natural Science coatings based on three-dimensional blue phase liquid crystal Foundation of China (NSFC) (Grant Nos. 61875081 and networks,” ACS Appl. Mater. Interfaces 13(34), 41102–41111 62175098) and the Guangdong Basic and Applied Basic (2021). Research Foundation (Grant No. 2021B1515020097). The 21. F. Castles et al., “Blue-phase templated fabrication of three- authors declare no conflict of interest. dimensional nanostructures for photonic applications,” Nat. Mater. 11(7), 599–603 (2012). 22. J. Yang et al., “Photonic shape memory polymer based on liquid Code, Data, and Materials Availability crystalline blue phase films,” ACS Appl. Mater. Interfaces 11(49), 46124–46131 (2019). The data supporting the findings of this study are available from 23. W. Hu et al., “Humidity-responsive blue phase liquid-crystalline the corresponding author upon reasonable request. film with reconfigurable and tailored visual signals,” Adv. Funct. Mater. 30(43), 2004610 (2020). References 24. K. R. Schlafmann and T. J. White, “Retention and deformation of 1. D. C. Wright and N. D. Mermin, “Crystalline liquids: the blue the blue phases in liquid crystalline elastomers,” Nat. Commun. phases,” Rev. Mod. Phys. 61(2), 385–432 (1983). 12, 4916 (2021). Advanced Photonics Nexus 026004-7 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography 25. R. Manda et al., “Polymer-stabilized monodomain blue phase 40. X. Li et al., “Mesoscale martensitic transformation in single diffraction grating,” Adv. Mater. Interfaces 7(9), 1901923 (2020). crystals of topological defects,” Proc. Natl. Acad. Sci. U. S. A. 26. E. Oton et al., “Orientation control of ideal blue phase photonic 114(38), 10011–10016 (2017). crystals,” Sci. Rep. 10(1), 10148 (2020). 41. X. Li et al., “Sculpted grain boundaries in soft crystals,” Sci. Adv. 27. E. Oton et al., “Monodomain blue phase liquid crystal layers for 5(11), 9112 (2019). phase modulation,” Sci. Rep. 7, 44575 (2017). 42. X. Li et al., “Nucleation and growth of blue phase liquid crystals 28. K. Kim et al., “A well-aligned simple cubic blue phase for a liquid on chemically-patterned surfaces: a surface anchoring assisted crystal laser,” J. Mater. Chem. C 3(21), 5383–5388 (2015). blue phase correlation length,” Mol. Syst. Des. Eng. 6(7), 534–544 29. P. Nayek et al., “Effect of the grain size on hysteresis of liquid- (2021). crystalline Blue Phase I,” J. Soc. Inf. Disp. 20(6), 318–325 (2012). 30. Y. Chen and S.-T. Wu, “Electric field-induced monodomain blue Xiaowan Xu received her PhD from the Department of Electrical and phase liquid crystals,” Appl. Phys. Lett. 102(17), 171110 (2013). Electronic Engineering, Southern University of Science and Technology. 31. M. Chen et al., “Electrically assisting crystal growth of blue phase She currently works at Shenyang Ligong University. Her research liquid crystals,” Opt. Mater. Express 4(5), 953–959 (2014). interests include blue phase liquid crystal devices, single crystalline blue 32. J. Yan et al., “A full-color reflective display using polymer- phases, and nanofabrication. stabilized blue phase liquid crystal,” Appl. Phys. Lett. 102(8), 081102 (2013). Jiawei Wang is currently a PhD candidate in the Department of Electrical 33. H.-S. Chen et al., “Hysteresis-free polymer-stabilized blue phase and Electronic Engineering, Southern University of Science and Tech- liquid crystals using thermal recycles,” Opt. Mater. Express 2(8), nology. His research interests include optical metasurfaces and their 1149–1155 (2012). dynamic modulation, liquid crystal photonic devices, and nanofabrication. 34. Z. G. Zheng et al., “Light-patterned crystallographic direction of a self-organized 3D soft photonic crystal,” Adv. Mater. 29(42), Yanjun Liu received his PhD in photonics from Nanyang Technological 1703165 (2017). University, Singapore, in 2007. He is currently an associate professor 35. S. Liu, I. Nys, and K. Neyts, “Two-step photoalignment with in the Department of Electrical and Electronic Engineering, Southern high resolution for the alignment of blue phase liquid crystal,” University of Science and Technology, China. His research group mainly Adv. Opt. Mater. 10(17), 2200711 (2022). focuses on liquid crystal photonics, active plasmonics, and metamaterials/ 36. J. Xiong et al., “Holo-imprinting polarization optics with a reflective metasurfaces. He has authored/co-authored more than 200 peer-reviewed liquid crystal hologram template,” Light Sci. Appl. 11, 54 (2022). journal publications, 10 granted patents, 5 book chapters, and 30 invited 37. C. W. Chen et al., “Large three-dimensional photonic crystals based talks in his research fields. on monocrystalline liquid crystal blue phases,” Nat. Commun. 8, 727 (2017). Dan Luo received his PhD in photonics from Nanyang Technological 38. J. A. Martinez-Gonzalez et al., “Directed self-assembly of liquid University, Singapore, in 2012. He is currently an associate professor crystalline blue-phases into ideal single-crystals,” Nat. Commun. in the Department of Electrical and Electronic Engineering, Southern 8, 15854 (2017). University of Science and Technology. His research interests include 39. X. Li et al., “Perfection in nucleation and growth of blue-phase liquid crystal photonic devices, liquid crystal optical sensor, and liquid single crystals: small free-energy required to self-assemble at crystal elastomer actuator. He has published more than 140 peer- specific lattice orientation,” ACS Appl. Mater. Interfaces 11(9), reviewed journal publications with citation of more than 2600 times 9487–9495 (2019). and H-index of 27. Advanced Photonics Nexus 026004-8 Mar∕Apr 2023 Vol. 2(2) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Photonics Nexus SPIE

Large-scale single-crystal blue phase through holography lithography

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Abstract

Research Article Large-scale single-crystal blue phase through holography lithography Xiaowan Xu, Jiawei Wang, Yanjun Liu , and Dan Luo* Southern University of Science and Technology, Department of Electrical & Electronic Engineering, Shenzhen, China Abstract. The blue phase, which emerges between cholesteric and isotropic phases within a three- dimensional periodical superstructure, is of great significance in display and photonic applications. The crystalline orientation plays an important role in the macroscopic performance of the blue phase, where the single crystal shows higher uniformity over the polydomain and monodomain, resulting in higher Bragg re- flection intensity, lower hysteresis, and lower driving voltage. However, currently reported methods of forming a single-crystal blue phase based on thermal controlling or e-beam lithography are quite time-consuming or expensive for large-scale fabrication, especially in the centimeter range, thus hindering the broad practical applications of single-crystal blue-phase-based photonic devices. Herein, a strategy to fabricate a large scale single crystalline blue-phase domain using holography lithography is proposed. Defect-free single-crystal domains both in blue phase I and blue phase II with a desired orientation of over 1 cm are fabricated based on a nanopatterned grating with periodic homeotropic and degenerate parallel anchoring, with colors from red and green to blue. This holography lithography-assisted strategy for fabrication of a large-scale single- crystal blue phase provides a time-saving and low-cost method for a defect-free single crystalline structure, leading to broad applications in liquid crystal displays, laser devices, adaptive optics elements, and electro- optical devices. Keywords: blue phase; single crystal; holography lithography; liquid crystals. Received Oct. 12, 2022; revised manuscript received Nov. 26, 2022; accepted for publication Dec. 31, 2022; published online Feb. 11, 2023. © The Authors. Published by SPIE and CLP under a Creative Commons Attribution 4.0 International License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.APN.2.2.026004] On the other hand, the crystalline orientation plays an impor- 1 Introduction tant role in the macroscopic performance of the blue phase, The blue phase (BP), existing between the cholesteric phase where the single crystal shows higher uniformity over the poly- and isotropic phase (ISO) of liquid crystals within a three- domain and monodomain, resulting in higher Bragg reflection dimensional periodical superstructure, possesses three forms of intensity, lower hysteresis, and lower driving voltage. Usually, blue phase I (BP I), blue phase II (BP II), and blue phase III the BP fragments form inside the volume simultaneously as a 1,2 (BP III). BP III possesses a noncrystalline symmetry with phase transition occurs, leading to polydomain BP structures, disordered structure, and BP I and BP II exhibit body-centered which consist of many platelets of different orientations. The cubic structure and simple cubic structure, respectively, with a mismatches in polydomain BPs increase the scattering in the 3–6 double-twisted cylinder nanostructure. The narrowed temper- volume and affect the reflectivity of the Bragg reflection and ature range and nonuniformity of the crystalline structure are 25–29 the optical-electrical properties of BPs. Recent studies indi- two important issues that hinder the practical applications of 30–32 cate that several methods, such as electric fields, thermal blue phase liquid crystal (BPLC). On one hand, the temperature 33 34–36 control, and surface treatments can be applied to form 7,8 range of the BP has been extended by polymerization or nano- monodomain BPs, which consist of many single-crystal plate- 9,10 particle doping, leading to useful applications such as dis- lets with one lattice plane orientated in the same direction. The 11–14 15–17 18–20 21–24 plays, lasers, sensors, and photonic devices. structures of monodomain BPs significantly increase the reflec- tivity of the Bragg reflection. However, many defects still *Address all correspondence to Dan Luo, luod@sustech.edu.cn exist between the separated platelets with a grain boundary. Advanced Photonics Nexus 026004-1 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Therefore, to further improve the optical performance of BPs, while varying the pitch of the nanopattern grating from 230 and it is highly desirable to develop methods of fabricating ideal, 190 nm to 170 nm, along with a high reflectance of 44% and single-crystal BPs. 45% to 42% and a narrow bandwidth of 19.0 and 17.0 nm to Several methods have been proposed to generate a single 14.6 nm, respectively. This holography lithography-assisted crystal blue phase. Chen et al. reported the fabrication of an strategy for fabrication of a large-scale single-crystal BP extraordinarily large single-crystal BP I by controlling the self- provides a time-saving and low-cost method for a defect-free assembly processes of BPLC. The gradient-temperature tech- single-crystalline structure that significantly improves the prop- nique enables growth of a single crystalline BP with the size of erty of the BP, including an increase in the intensity of the Bragg 1cm . However, long periods of thermal control over several reflection, leading to broad applications such as liquid crystal days were required, which leads to low efficiency in fabrication, displays, laser devices, adaptive optics elements, and electro- and the crystal orientation of the BP could not be controlled due optical devices. to the similar treatment of the substrate compared to traditional BPs. de Pablo et al. proposed a method to form a single crys- 2 Results and Discussion talline BP II through a nanopatterned surface with binary In our experiments, both BP I [with body-center cubic lattice 38–42 homeotropic/planar anchoring through e-beam lithography. structure, Fig. 1(a)] and BP II (with simple cubic lattice struc- This method is capable of generating a single-crystal BP over a ture, Fig. 1(b)] are investigated. The wavelength of reflected pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi macroscopic region (at a scale of mm ) with designed crystallo- 2 2 2 light from the BP is determined by λ ¼ 2n a∕ h þ k þ l , eff graphic plane orientation based on the simulation of free energy where λ is the wavelength of reflective light, n is the refractive eff from Landau–de Gennes theory (see Supplementary Material). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 index (n ¼ ð2n þ n Þ∕3; n and n are the ordinary and However, the fabrication is highly dependent on the nanopattern eff o e o e extraordinary refractive indices of liquid crystal materials), a is fabricated by e-beam lithography, which is time-consuming and the lattice constant of crystal, and h; k; l are the Miller indices. expensive, thus hindering the practical application of single- The BP crystallographic planes are denoted by (hkl), and crystal BP photonic devices. Therefore, a time-saving and low- BP represents a BP with orientation of the (hkl) plane cost method, rather than e-beam lithography, to fabricate the ðhklÞ nanopattern at a larger scale for large-scale single-crystal BPs parallel to the surface. is highly desirable. The anchoring pattern of the substrate is designed based on In this paper, a strategy to fabricate large scale single crys- the morphology of the orientating lattice plane on the surface. talline BP liquid crystals is demonstrated through holography Figure 1(c) shows the morphology of BP I plane at the ð110Þ lithography. Defect-free single-crystal domains both in BP I and boundary of crystal lattice with l ¼ 0 (where l means the dis- c c BP II with a desired orientation of over 1cm are fabricated placement from the boundary of the unit lattice). The region of based on a nanopatterned grating with periodic homeotropic blue ellipses represents the cross section of a double-twist and degenerate parallel anchoring. A large-scale single-crystal cylinder on the substrate surface, where the liquid crystal (LC) BP with reflection colors from red and green to blue are realized molecules orient from 45 deg to 90 deg (the angles between the Fig. 1 (a) BP I with body-center cubic lattice structure; (b) BP II with simple cubic lattice structure; (c) morphology of BP I plane; (d) morphology of BP II plane; (e) reduced morphology of ð110Þ ð100Þ BP I plane; (f) reduced morphology of BP II plane; (g) simplified grating of BP I plane; ð110Þ ð100Þ ð110Þ (h) simplified grating of BP II plane. ð100Þ Advanced Photonics Nexus 026004-2 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography substrate surface and the director of the LC molecule). This re- The morphologies under other cutting conditions are shown in gion will be treated to be a homotropic surface anchored in a Fig. S1 in the Supplementary Material (l ¼ 0 to l ¼ d ) and c c BPI simplified alignment model. The rest of the region represents Fig. S2 in the Supplementary Material (l ¼ 0 to l ¼ d ), c c BPII the defect structure in the crystal lattice, which can be seen respectively. However, the fabrication process of the duty ratio as isotropic materials. The pitch along two orthogonal directions (0.7:1.3) is much more difficult. Therefore, we try to use a gra- pffiffiffi ting with a duty ratio of 1:1 to replace the duty ratio of 0.7:1.3, is represented by 2 a , and a , where a is the lattice BPI BPI BPI which shows good and acceptable results in the following ex- constant of BP I, leading to an effective period of grating formed pffiffiffiffiffiffiffiffi periments. Figures 1(g) and 1(h) demonstrate the simplified gra- by the region of blue ellipses to be d ¼ 2∕3a . This re- BPI BPI ting deduced from Figs. 1(e) and 1(f), respectively. To reduce gion will be treated as a degenerate parallel surface anchored in the difficulty of fabrication, the duty ratio of grating is chosen a simplified alignment model. Figure 1(d) shows the morphol- to be 1:1 in both cases. This simplification process proved to ogy of BP II , which is similar to BP I. The region of the blue ð100Þ be effective and efficient, leading to a single crystal forming in rectangle with an oblique line represents the long-axis cutting both the BP I and BP II planes. ð110Þ ð100Þ section of the double-twist cylinder, where the LC molecules In our experiment, two-beam holography lithography was orient from 0 deg to 45 deg (the angles between the substrate applied to generate the grating on a large scale. Figure 2(a) de- surface and the director of LC molecule). Both the blue rectan- picts the schematic illustration of the fabrication process, where gle and the rest of the region will be treated as a degenerate a linearly polarized laser beam (the polarization direction is per- parallel surface anchored in a simplified alignment model. pendicular to the plane of the optical table) with a wavelength of The effective period of grating formed by the region of blue 325 nm splits into two beams by passing through a beam splitter, circles and the rest is d ¼ a . BPII BPII and there is interference at the surface of substrate coated with The single crystalline BPs are mediated by the strain induced homeotropic alignment material (OTE) and photoresist. For by the corresponding patterned anchoring conditions with the two-beam holography lithography, the obtained interference lower free energy. The grating pattern consists of homeotropic light field is a grating. The pitch of the formed grating is deter- anchoring and degenerate parallel anchoring, resulting from mined by the formula of 2d sin θ ¼ λ, where d is the pitch of the boundary molecular distribution, which successfully induced grating, λ is the wavelength of the laser, and the interference 38,39 the formation of single crystalline BPs. angle θ is half the angle between two interference beams. Figures 1(e) and 1(f) demonstrate the reduced morphology The parameters of BP samples (S1, S2, and S3) used in our of BP I and BP II planes at the boundary of the crystal ð110Þ ð100Þ experiment are shown in Table 1. The data indicate the process lattice with l ¼ 0, where the blue region represents the homeo- c of phase transitions and the corresponding information between tropic surface anchoring and the rest, the yellow region, repre- substrates and lattice parameters; for example, Iso-BP I means sents degenerate parallel surface anchoring. The reduced this sample is started from an isotropic phase and stabilized at morphology of surface anchoring can be further simplified to BP I through cooling. Once the pitch is fixed, the corresponding a grating in both cases, and the duty ratio is defined as d ∶d , 1 2 interference angle, lattice constant, and chiral dopant concentra- where d and d are the corresponding widths of the region with 1 2 tion can be calculated. In our experiments, three pitches of gra- the blue part and the region without the blue part, respectively. ting were chosen to be 230, 190, and 170 nm, and the calculated The duty ratios are around 0.7:1.3 (where d ¼ d þ d )and BPI 1 2 interference angle θ should be 45.0 deg, 58.8 deg, and 72.9 deg, 1:1 (where d ¼ d þ d ) in Figs. 1(e) and 1(f), respectively. BPII 1 2 respectively. The corresponding lattice constant a is calculated Fig. 2 Fabrication process of patterned substrate with periodic OTE grating for large-scale single- crystal BP generation. (a) Two-beam holography lithography based on the substrate with OTE and photoresist; (b) developing; (c) O plasma etching; (d) stripping; (e) periodic orientation of liquid crystal molecules induced by grating patterned substrate. Advanced Photonics Nexus 026004-3 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Table 1 Parameters of BP samples. Transition Pitch Interference Lattice Chiral Dopant Phase Temperature d Angle θ Constant Concentration Sample Transition (°C) (nm) (deg) a (nm) c (wt.%) S1 Iso-BP I 89.2 230 45.0 281.7 3.0 S2 BP II-BP I 85.8 190 58.8 232.7 3.6 S3 Iso-BP II 88.5 170 72.9 170.0 3.4 to be 281.7 and 232.7 nm for BP I (according to d ¼ alignment-induced single crystal. Polydomain BPs [Figs. 3(a), BPI pffiffiffiffiffiffiffiffi 3(d), and 3(g)] contain plenty of single crystals with different 2∕3a ), and 170 nm for BP II (according to d ¼ a ), BPI BPII BPII orientations and a disordered mosaic image with several colors. which require the concentration of chiral dopant of R5011 −1 Monodomain BPs [Figs. 3(b), 3(e), and 3(h)] contain pieces of (HTP ¼ 126 μm ) to be 3.0, 3.6, and 3.4 wt.%, respectively. a single crystal with a uniform orientation but still with defects The pattern of the grating received by the positive photoresist and inconspicuous boundaries between the pieces. In contrast, where the bright region in the light field forming the grating will the patterned grating alignment successfully induces the single be developed and removed, while the dark region in the light crystal [Figs. 3(c), 3(f), and 3(i)] in all three cases of S1, S2, and field forming the grating will be retained for the next developing S3, where the pitch of grating is 230, 190, and 170 nm with step [Fig. 2(b)]. After developing, the O plasma etching was corresponding reflection colors of red, green, and blue. The applied [Fig. 2(c)], where the OTE covered with photoresist single crystal forms a uniform crystalline structure in a large (yellow) was kept from etching by O plasma and while the domain of macroscopic size, and the obtained single crystal OTE part uncovered with photoresist was etched. After stripping has no defect in the volume. of the photoresist, a grating consisting of the region with OTE Figures 4(a)–4(c) demonstrate the images of fabricated three (which would lead to perpendicular alignment of the LC) and gratings observed under scanning electron microscopy (SEM). the region without OTE (which would lead to degenerate paral- The experimentally measured pitch is 231.5, 190.5, and lel alignment of the LC) on substrate were fabricated, as shown 170.9 nm, respectively, which is highly consistent with the in Fig. 2(d). Figure 2(e) shows the orientation of liquid crystal designed pitch of d ¼ 230 nm, d ¼ 190 nm, and molecules induced by the surface anchoring of the patterned BPIðS1Þ BPIðS2Þ d ¼ 170 nm. The POM images of S1, S2, and S3 (cor- substrate. The patterned homeotropic and degenerate parallel BPIIðS3Þ anchoring of the substrate induces periodic orientation of liquid responding to grating patterned in Figs. 4(a)–4(c), respectively) crystal molecules on the patterned substrate and leads to uni- with corresponding Kossel diagrams (as inset figures) for the form formation of BPs with a cubic structure. The cell with lattice planes of BP I ,BP I , and BP II are shown ð110Þ ð110Þ ð100Þ a grating pattern was made up of a bare glass and a glass with in Figs. 4(d)–4(f), respectively. According to the principle of grating-patterned OTE, and 5 μm spacers were used to define lattice diffraction, the Kossel diagrams of BP I have six ð110Þ the cell thickness. lines in Fig. 4(d) and four lines in Fig. 4(e), which derives from The phase transitions among isotropic phase, BP I, and BP II diffractive limitation. The lattice constant of sample S1 is larger can be categorized as isotropic phase BP I, BP II–BP I, and iso- than S2 within diffractive limitation; therefore, more Kossel tropic phase BP II during the temperature decrease process. lines can be seen. The Kossel diagrams with distinct Kossel lines Figures 3(a)–3(c) show the polarization optical microscopy indicate a uniform orientation. The Bragg reflection spectra of (POM) images of sample S1 in BP I (cooling from isotropic S1, S2, and S3 are shown in Figs. 4(g)–4(i), respectively. The phase at 89.5°C to BP I at 85.7°C with a rate of 0.1°C/min) single-crystal BPs of samples S1, S2, and S3 possess high re- at the homeotropic, parallel, or grating-patterned alignment of flectance of 44%, 45%, and 42% (the theory limitation is 50% the substrate, leading to polydomain, monodomain, and a single due to the circularly polarized light of reflection) at central crystal, respectively. Figures 3(d)–3(f) show the POM images wavelengths of 648, 540, and 512 nm, and narrow bandwidths of sample S2 in BP I (cooling from isotropic phase at 88.8°C of 19.0, 17.0, and 14.6 nm, respectively. According to the rela- to BP II at 86.3°C and then to BP I at 84.9°C with rate of tionship of the grating period and the lattice constant for BP I pffiffiffiffiffiffiffiffi 0.1°C/min) at homeotropic, parallel, or grating-patterned align- and BP II (d ¼ 2∕3a , and d ¼ a ), the calculated BPI BPI BPII BPII ment of the substrate, leading to polydomain, monodomain, and lattice constants of induced single-crystal BPs are a ¼ BPIðS1Þ a single crystal, respectively. Figures 3(g)–3(i) show the POM 283.6nm, a ¼ 233.3nm, and a ¼ 170.9nm, BPIðS2Þ BPIIðS2Þ images of sample S3 in BP II (cooling from isotropic phase at respectively. Compared to the measured lattice constants 89°C to BP II at 86.8°C with rate of 0.1°C/min) at homeotropic, pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 parallel, or grating-patterned alignment of the substrate, leading (λ ¼ 2n a∕ h þ k þ l ) of 289.3, 241.3, and 161.6 nm, eff to polydomain, monodomain, and a single crystal, respectively. the deviation values between the calculated and measured values The changes of the clear point and temperature range of BPs of lattice constants Δa are 5.7, 8.0, and 9.3 nm, respectively, with these three surface treatments are negligible. The parallel which are quite small compared to those previously reported anchoring derives from rubbing on polymide (PI) materials with in Ref. 38. The parameters including pitch, calculated and mea- one preferred azimuth in the horizontal direction, as shown in sured lattices, and the deviation values for samples S1–S3 are Figs. 3(b), 3(e), and 3(h). The uniformity of images from the shown in Table 2. homeotropic and parallel alignment-induced polydomain and The reflection spectra and POM textures of BPs are valid monodomain BP are apparently worse than that from the grating proof to differentiate polydomain, monodomain, and single- Advanced Photonics Nexus 026004-4 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Fig. 3 POM images of S1, S2, and S3 at different alignment conditions. S1 in BP I (a) at homeo- tropic alignment with polydomain, (b) at parallel alignment with monodomain, and (c) at grating alignment (d ¼ 230 nm) with single crystal. S2 in BP I (d) at homeotropic alignment with polydo- main, (e) at parallel alignment with monodomain, and (f) at grating alignment (d ¼ 190 nm) with single crystal. S3 in BP II (g) at homeotropic alignment with polydomain, (h) at parallel alignment with monodomain, and (i) at grating alignment (d ¼ 170 nm) with single crystal. Scale bar: 100 μm. crystal BPs, as shown in Fig. S3 in the Supplementary Material. fabricated based on a nanopatterned grating with periodic The polydomain BPs are easy to differentiate. The spectra of homeotropic and degenerate parallel anchoring. The grating monodomain and single-crystal BPs are quite similar, which pattern consisting of homeotropic anchoring and degenerate makes them difficult to distinguish. Monodomain BPs are parallel anchoring resulting from boundary molecular distribu- formed with pieces of single crystals with the same orientatio- tion successfully induced the formation of a single crystalline nand show similar Bragg structure colors. The defects and mis- blue phase. A single-crystal BP in BP I ,BP I , and ð110Þ ð110Þ matches of angles between pieces of crystalline structures exist BP II planes with reflection color from red and green to blue ð100Þ in monodomain BPs, while single-crystal BPs exhibit perfect are realized while varying the pitch of the nanopattern grating morphology in the POM texture. from 230 and 190 nm to 170 nm, with a high reflectance of Figures 5(a)–5(c) show the photo images of fabricated single- 44% and 45% to 42% at the central wavelengths of 648 and 540 crystal BPs S1, S2, and S3 with a large scale of over 1cm in to 512 nm, and narrow bandwidths of 19.0 and 17.0 nm to red, green, and blue, respectively. In principle, the fabrication 14.6 nm, respectively. This holography lithography-assisted size can be further increased by using a larger diameter of laser strategy for fabrication of a large-scale single-crystal BP pro- beam with higher laser power. The large scale of single-crystal vides a time-saving and low-cost method for formation of a BPs based on holography lithography induced grating pattern defect-free single crystalline structure, leading to broad applica- of the substrate enables defect-free crystalline structures, lead- tions in liquid crystal displays, laser devices, adaptive optics ing to potential applications in displays, lasers, and electro- elements, and electro-optical devices. optical devices. 4 Experimental Section 3 Conclusion 4.1 Materials We demonstrated a strategy to fabricate a large-scale single crys- talline BP through holography lithography. Defect-free single Liquid crystal host HTG135200-100 (n ¼ 1.717, n ¼ 1.513 of e o 2 −1 crystals both in BP I and BP II with size over 1cm are 589 nm at 20°C) and chiral dopant R5011 (HTP ¼ 126 μm ) Advanced Photonics Nexus 026004-5 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography Fig. 4 Surface morphology of gratings, Kossel diagram, and reflection spectra of single crystals. SEM images of gratings were used to align of BP samples (a) S1, (b) S2, and (c) S3. The scale bar is 500 nm. (d) POM image of single crystalline BP I formed by S1. Inset: Kossel diagram of BP I ; (e) POM image of single crystalline BP I formed by S2. Inset: Kossel diagram of BP I ; ð110Þ ð110Þ (f) POM image of single crystalline BP II formed by S3. Inset: Kossel diagram of BP II . The ð100Þ reflection spectra of (g) S1, (h) S2, and (i) S3. Table 2 Parameters of BP fabricated in our experiment. Calculated Measured Pitch lattice lattice Deviation d constant constant value Sample (nm) a (nm) a (nm) Δa (nm) S1 231.5 283.6 289.3 5.7 S2 190.5 233.3 241.3 8.0 Fig. 5 Photo images of (a) S1 in BP I, (b) S2 in BP I, and (c) S3 in S3 170.9 170.9 161.6 9.3 BP II. Scale bar: 2 mm. were purchased from Hecheng Display. Homeotropic alignment 4.2 Fabrication Process materials octadecyltriethoxysilane [OTE, n-C H SiðOCH CH Þ ] 18 37 2 3 3 were purchased from Sigma Aldrich and used without further puri- Liquid crystal mixtures preparation: The nematic liquid crystal fication. Parallel alignment material polyimide (PI) was purchased HTG135200-100 and chiral dopant R5011 were mixed together, from Beijing Bomi Company. Photoresist mr-P 1202LIL, devel- stirred at the isotropic phase, and cooled down to the BP. oper mr-D 374 and stripper mr-Rem 660 (based on N-methylpyr- Substrates preparation: First, the OTE film was developed on rolidon (NMP)) were purchased from Micro Resist Technology. the substrates. The substrate glass was cleaned by sonication in Advanced Photonics Nexus 026004-6 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography 2. W. Hu et al., “Ultrastable liquid crystalline blue phase from acetone and ethyl alcohol. The glass was immersed in the mix- molecular synergistic self-assembly,” Nat. Commun. 12, 1440 ture of concentrated H SO and 30% aqueous H O (2:1 v/v) 2 4 2 2 (2021). for 30 min. The samples were immersed in the mixture of 3. S. Meiboom, M. Sammon, and W. F. Brinkman, “Lattice of dis- 240 mg OTE and 70 ml toluene with 0.25 mL n-BuNH as cata- clinations: the structure of the blue phases of cholesteric liquid lyst for 90 min. The samples should be vertically inserted into crystals,” Phys. Rev. A 27(1), 438–454 (1983). the solution and should not touch each other. To remove excess 4. S. S. Gandhi and L.-C. Chien, “Unraveling the mystery of the silane aggregates in the solution, the samples were stirred in a blue fog: structure, properties, and applications of amorphous blue toluene bath and rinsed with deionized water several times. phase III,” Adv. Mater. 29(47), 1704296 (2017). Then they were blown dry with nitrogen and allowed to cure for 5. H. M. Jin et al., “Soft crystal martensites: an in situ resonant soft x-ray scattering study of a liquid crystal martensitic transforma- 2 h at 150°C in a vacuum oven. After development with OTE, tion,” Sci. Adv. 6(13), 5986 (2020). the substrates were spin-coated with photoresist mr-P 1202LIL 6. J. Liu et al., “Diffusionless transformation of soft cubic superstruc- (4000 r, 30 s) and baked at 100°C for 60 s, and then exposed ture from amorphous to simple cubic and body-centered cubic under two-beam interference for 30 s to get the structure of the phases,” Nat. Commun. 12, 3477 (2021). grating. A 325 nm He–Cd laser was used as the interference 7. H. Kikuchi et al., “Polymer-stabilized liquid crystal blue phases,” light source. The substrate was developed in developer mr-D Nat. Mater. 1(1), 64–68 (2002). 374 for 25 s. The resulting grating pattern was transformed onto 8. D. M. Xu et al., “Electro-optic response of polymer-stabilized blue the OTE film layer by exposing the sample to oxygen plasma phase liquid crystals,” Appl. Phys. Lett. 105(1), 011119 (2014). (50 W, 60 s), followed by stripping the photoresist in stripper 9. H. Yoshida et al., “Nanoparticle-stabilized cholesteric blue phases,” Appl. Phys. Express 2(12), 121501 (2009). mr-Rem 660 for 1 min. 10. M. 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Meng et al., “High-resolution erasable “live” patterns based on controllable ink diffusion on the 3D blue-phase liquid crystal glass and a glass coated with OTE, which was an intermediate networks,” Adv. Funct. Mater. 32(15), 2110985 (2022). product in substrate preparation before photolithography. 14. Y. Huang et al., “Optimized blue-phase liquid crystal for field The cell with parallel alignment was made up of a bare glass sequential-color displays,” Opt. Mater. Express 7(2), 641–650 and a glass coated with PI. The PI-coated glass was rubbed with (2017). fabric beforehand to induce parallel alignment. 15. M. Wang et al., “Bias-polarity dependent bidirectional modulation of photonic bandgap in a nanoengineered 3D blue phase polymer scaffold for tunable laser application,” Adv. Opt. Mater. 6(16), 4.3 Measurements 1800409 (2018). 16. G. Petriashvili and A. 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Yang et al., “Structural and optical response of polymer- was 2 cm. stabilized blue phase liquid crystal films to volatile organic compounds,” ACS Appl. Mater. Interfaces 12(37), 42099–42108 Acknowledgments (2020). 20. Y. Yang et al., “Bioinspired color-changing photonic polymer This work was supported by the National Natural Science coatings based on three-dimensional blue phase liquid crystal Foundation of China (NSFC) (Grant Nos. 61875081 and networks,” ACS Appl. Mater. Interfaces 13(34), 41102–41111 62175098) and the Guangdong Basic and Applied Basic (2021). Research Foundation (Grant No. 2021B1515020097). The 21. F. Castles et al., “Blue-phase templated fabrication of three- authors declare no conflict of interest. dimensional nanostructures for photonic applications,” Nat. Mater. 11(7), 599–603 (2012). 22. J. Yang et al., “Photonic shape memory polymer based on liquid Code, Data, and Materials Availability crystalline blue phase films,” ACS Appl. Mater. Interfaces 11(49), 46124–46131 (2019). The data supporting the findings of this study are available from 23. W. Hu et al., “Humidity-responsive blue phase liquid-crystalline the corresponding author upon reasonable request. film with reconfigurable and tailored visual signals,” Adv. Funct. Mater. 30(43), 2004610 (2020). References 24. K. R. Schlafmann and T. J. White, “Retention and deformation of 1. D. C. Wright and N. D. Mermin, “Crystalline liquids: the blue the blue phases in liquid crystalline elastomers,” Nat. Commun. phases,” Rev. Mod. Phys. 61(2), 385–432 (1983). 12, 4916 (2021). Advanced Photonics Nexus 026004-7 Mar∕Apr 2023 Vol. 2(2) Xu et al.: Large-scale single-crystal blue phase through holography lithography 25. R. Manda et al., “Polymer-stabilized monodomain blue phase 40. X. Li et al., “Mesoscale martensitic transformation in single diffraction grating,” Adv. Mater. Interfaces 7(9), 1901923 (2020). crystals of topological defects,” Proc. Natl. Acad. Sci. U. S. A. 26. 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Lett. 102(17), 171110 (2013). Electronic Engineering, Southern University of Science and Technology. 31. M. Chen et al., “Electrically assisting crystal growth of blue phase She currently works at Shenyang Ligong University. Her research liquid crystals,” Opt. Mater. Express 4(5), 953–959 (2014). interests include blue phase liquid crystal devices, single crystalline blue 32. J. Yan et al., “A full-color reflective display using polymer- phases, and nanofabrication. stabilized blue phase liquid crystal,” Appl. Phys. Lett. 102(8), 081102 (2013). Jiawei Wang is currently a PhD candidate in the Department of Electrical 33. H.-S. Chen et al., “Hysteresis-free polymer-stabilized blue phase and Electronic Engineering, Southern University of Science and Tech- liquid crystals using thermal recycles,” Opt. Mater. Express 2(8), nology. His research interests include optical metasurfaces and their 1149–1155 (2012). dynamic modulation, liquid crystal photonic devices, and nanofabrication. 34. Z. G. Zheng et al., “Light-patterned crystallographic direction of a self-organized 3D soft photonic crystal,” Adv. Mater. 29(42), Yanjun Liu received his PhD in photonics from Nanyang Technological 1703165 (2017). University, Singapore, in 2007. He is currently an associate professor 35. S. Liu, I. Nys, and K. Neyts, “Two-step photoalignment with in the Department of Electrical and Electronic Engineering, Southern high resolution for the alignment of blue phase liquid crystal,” University of Science and Technology, China. His research group mainly Adv. Opt. Mater. 10(17), 2200711 (2022). focuses on liquid crystal photonics, active plasmonics, and metamaterials/ 36. J. Xiong et al., “Holo-imprinting polarization optics with a reflective metasurfaces. He has authored/co-authored more than 200 peer-reviewed liquid crystal hologram template,” Light Sci. Appl. 11, 54 (2022). journal publications, 10 granted patents, 5 book chapters, and 30 invited 37. C. W. Chen et al., “Large three-dimensional photonic crystals based talks in his research fields. on monocrystalline liquid crystal blue phases,” Nat. Commun. 8, 727 (2017). Dan Luo received his PhD in photonics from Nanyang Technological 38. J. A. Martinez-Gonzalez et al., “Directed self-assembly of liquid University, Singapore, in 2012. He is currently an associate professor crystalline blue-phases into ideal single-crystals,” Nat. Commun. in the Department of Electrical and Electronic Engineering, Southern 8, 15854 (2017). University of Science and Technology. His research interests include 39. X. Li et al., “Perfection in nucleation and growth of blue-phase liquid crystal photonic devices, liquid crystal optical sensor, and liquid single crystals: small free-energy required to self-assemble at crystal elastomer actuator. He has published more than 140 peer- specific lattice orientation,” ACS Appl. Mater. Interfaces 11(9), reviewed journal publications with citation of more than 2600 times 9487–9495 (2019). and H-index of 27. Advanced Photonics Nexus 026004-8 Mar∕Apr 2023 Vol. 2(2)

Journal

Advanced Photonics NexusSPIE

Published: Mar 1, 2023

Keywords: blue phase; single crystal; holography lithography; liquid crystals

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