High Photoinduced Ordering and Controllable Photostability of

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High Photoinduced Ordering and Controllable Photo-Stability of Hydrophilic Azobenzene Material Based on Relative Humidity Yue Shi, Chenxiang Zhao, Jacob Yeuk-Lung Ho, Feng Song, Vladimir G. Chigrinov, Dan Luo, Hoi Sing Kwok, and Xiao Wei Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00039 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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High Photoinduced Ordering and Controllable Photo-Stability of Hydrophilic Azobenzene Material Based on Relative Humidity Yue Shi, †,‡,§ Chenxiang Zhao, § Jacob Yeuk-Lung Ho, § Feng Song, ‡ Vladimir G. Chigrinov, § Dan Luo,*, † Hoi Sing Kwok,*, § Xiao Wei Sun† †

Department of Electrical and Electronic Engineering, Southern University of Science and

Technology, No. 1088, Xueyuan Rd., Xili, Nanshan District, Shenzhen, Guangdong 518055, P. R. China ‡

School of Physics, Nankai University, Tianjin 300071, P. R. China

§

State Key Laboratory on Advanced Displays and Optoelectronics Technologies, Department of

Electronic and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China. KEYWORDS: azobenzene; photoalignment; photo-stability; relative humidity; J-aggregation

ABSTRACT

Azobenzene materials provide an effective way for liquid crystal (LC) alignment besides traditional rubbing technology. A strong relationship between relative humidity (RH) and the

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photoalignment quality of hydrophilic azobenzene dye brilliant yellow (BY) has been investigated. Good photoalignment quality can only be ensured at about 40% RH or below. On the other hand, the photo-stability of the alignment layer is also influenced dramatically by RH. The rewritability can be guaranteed at extremely low RH (≤ 10%). It is gradually lost with increasing RH, and the alignment layer becomes photo-stable against further light exposure when at 40% RH or above. Therefore, the BY photoalignment layer can be tuned from rewritable to photo-stable by simply adjusting RH, and thus multi-step photo-patterned alignments can be obtained and reserved based on this method. Similar properties are also expected for other hydrophilic azobenzene photoalignment materials, where the specific RH values may vary correspondingly. The reason is due to the strong intermolecular interaction and J-aggregate formation of BY molecules with water insertion. Moreover, the lyotropic LC formed by Jaggregated BY molecules in aqueous solution is reported here.

TEXT INTRODUCTION The photoalignment technology, as a noncontact alignment approach for LC, could avoid contaminations and static charges compared to traditional rubbing method. Photo-dimmerization cinnamate materials,1 photo-decomposition polyimides,2 photo-isomerization and reorientation azobenzene materials3-5 have been proven as good photoalignment candidates. In particular, the photoalignment of azobenzene material is basically a reversible process.6,7 When exposed to linearly polarized light, the azobenzene molecules are aligned perpendicularly to the polarization direction of excitation light, and the molecules can be reoriented by polarization direction


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change. Based on this property, the azobenzene alignment layer can be patterned to multidomains by multi-step exposure due to its rewritability, which is important for the applications on modern LC display and various photonic devices.8,9 On the other hand, however, the rewritability of azobenzene materials prevents their further applications since photo-stability is required against degradation. Therefore, several approaches have been explored to improve the photo-stability of azobenzene material, such as adding crosslinking fragments to the molecules,10 or incorporating crosslinking matertials with the azobenzene materials.11-16 The main purpose is to maintain the azobenzene rewritability during photo-patterning process, and stabilize the patterned alignment layer afterwards. Various azobenzene photoalignment materials have been explored,3-5,17-19 among which the azobenzene sulfonic dyes SD1 and BY have been used widely due to their easy fabrication process and good alignment quality.9,19-21 Our previous studies have shown that the photoalignment quality of SD1 was affected greatly by RH, where the insertion of water into SD1 molecules resulted in phase transition from amorphous solid to semicrystal.22 And the studies of BY material by other groups showed the similar properties.23,24 In this study, we explored the RH influence on BY material in detail, where the photo-induced phase retardation, order parameter and the anchoring energy for LC are measured, giving a recommended RH working window as fabrication guidance for BY material. On the other hand, however, only onestep photoalignment process of BY layers was found in reported literatures,19-21 which is contradictory to the traditional azobenzene materials with photo-rewritability. Herein, we find that the photo-stability of the BY film also depends greatly on RH. At extremely low RH (≤ 10%), the BY layers are rewritable and can be photo-patterned to multi-domains by multi-step exposure method, while the rewritability is gradually lost with increasing RH. Our exploration


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shows that the BY layers can be tuned from rewritable to photo-stable by adjusting RH from low to high value, probably due to strong intermolecular interaction and J-mesoaggregate formation of the BY molecules with water insertion. Thus, a simple method for improving photo-stability of the hydrophilic azobenzene photoalignment material is proposed. A comparison of the RH influence on BY and SD1 materials are discussed based on the experimental results and their molecular structures, which may be referenced as molecular guidelines for the hydrophilic azobenzene material design. Moreover, the hydrogen-bond-assisted J-aggregation of BY molecules is evident by X-ray diffraction (XRD) and absorption spectrum investigations, and the lyotropic LC formed by J-aggregated BY molecules in aqueous solution is reported. EXPERIMENTAL SECTION Sample preparation. The BY powder (Sigma-Aldrich, Figure 1a) is stored in an Argon-filled glovebox (RH < 1%) as received to avoid water contact. A 0.5wt% BY solution in N,Ndimethylformamide (DMF; Sigma-Aldrich, 99%, water content ≤ 0.5%) is spin-coated on glass substrate at 1500 rpm for 40 s. The coated substrate is then soft-baked at 90 °C for 20 min for solvent evaporation, giving a 6 nm-thick homogeneous solid film (measured by Ellipsometer). The BY film-coated substrate is then put in a sealed chamber and taken out of the glovebox. A RH generator (InstruQuest Inc.) is connected to the sealed chamber afterwards to achieve a desired RH inside. Before light exposure, the BY film is kept in the chamber with desired RH for 10 min, which is long enough to reach equilibrium since no difference is observed by comparing the result with the one kept at that RH for 30 min. For the recovered films from high to low RH, the BY layer were stored at different RHs and then back to low RH at room temperature for 10 min before LPUV exposure.


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Photoalignment Property Investigation. Upon exposed to linearly polarized ultraviolet (LPUV) or blue light, the randomly distributed azobenzene molecules tend to align perpendicularly to the light polarization.7 The small in situ phase retardation change of the thin film is measured by a 632.8 nm He-Ne probing laser, which is beyond the absorption range of the BY material, with two crossed Glan-Thompson polarizers and a photoelastic modulator and sample in between.22 The writing dynamics of the BY film under LPUV and blue light has the same orientation trend but different writing speeds, which are proportional to the absorption of BY film at different wavelengths. Therefore, 365 nm LPUV light is used as excitation source here, and the results are applicable to other wavelengths. The alignment quality could be checked by dichroic ratio, order parameter S, and azimuthal anchoring energy of the BY layer. After 2.5 J/cm2 LPUV light exposure, the film absorption spectra are measured (Ocean Optics) parallelly (∥ ) and perpendicularly ( ) to the molecule orientation direction. The dichroic ratio of alignment layer is ∥ ⁄ , and the order parameter is = (∥ − )⁄(∥ + 2 ) , calculated at the absorption peak. For measurement, twist nematic (TN) LC cells are assembled with 5CB (Dainippon Ink and Chemicals Inc.) filled in between two orthogonally photoaligned BY substrates in a customized glove box with corresponding RH environment inside. The actual twist angle of LC is measured, and the azimuthal anchoring energy can be calculated as = 2 ⁄( 2) , where is the deviation angle compared to the easy axis of the photoaligned film. Rewritability Investigation. To investigate the rewritability of BY material, the in situ phase retardation is measured under LPUV exposure at different RHs, where the polarization direction changes 90° alternatively when the phase retardation reaches the same value (opposite sign) in the other direction. In addition, binary grating cells are made based on two-step exposure


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method, which is commonly adopted for binary LC grating fabrication using rewritable azobenzene photoalignment layer. At different RHs, the BY films are first exposed with uniform LPUV. And then the same substrate is turned 90° and exposed again for the same dosage through a 1D amplitude mask with 100 µm period, tending to reorient the exposed areas to be orthogonal to the original orientation of mask shadow regions. Afterwards, the photo-patterned substrate is assembled with a rubbed polyimide (PI) substrate (optically passive) to form a 5 µm-thick cell. The rubbing direction is parallel to the orientation direction of BY molecules after the 1st exposure. Thereafter, LC 5CB is injected into the cell by Capillary effect at the corresponding RH in the customized glove box. If the BY film is rewritable, an alternate 1D planar alignment (PA) and TN domains should be observed in the LC cell, and the typical 1D binary grating diffraction pattern should be observed with a 543 nm He–Ne laser incidence. On the other hand, the rewritability is also checked for the BY films that was exposed to LPUV at low RH and then humidification-enhanced afterwards to higher RH value, which was then re-exposed to LPUV light with orthogonal polarization. Aggregation Characterization. The crystalline ordering is studied using XRD on a Rigaku Smartlab 9KW with Cu Kα radiation (1.54 Å). Thick BY films (110 nm) on Si substrate, obtained by spin coating a 10 wt% BY DMF solution, is used for XRD analysis. The BY films are exposed to different RH conditions, including the open air with non-saturated RH environment and saturated RH environment with increasing exposure time. The BY film absorption spectra at different RH condition are measured using UV/VIS spectrometer (Perkin Elmer) in the sealed chamber. RESULTS AND DISCUSSION


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Photolignment at Different Relative Humidity. A strong dependence of the in situ phase retardation change ∆nd on RH is observed for the BY photoalignment film with LPUV exposure, as shown in Figure 1b. The BY molecules can be photoaligned below 50% RH, and the resulting phase retardation is saturated at high value when the RH is at 45% or below, enabling high surface anisotropy. When the RH is at or above 50%, the photoalignment ability becomes poor and it is eventually lost at high RH. The initial orientation speed, which is the phase retardation rate at time approaching to 0, is shown in Figure 2a. The orientation speed at extremely low RH (≤ 10%) is slower than that at 20%-40%, which is probably due to the limited free volume around azobenzene molecules.25,26 However, higher exposure energy gives much better ordering, especially at low RH range, as indicated by the dichroic ratio and order parameter of the photoaligned film with longer exposure time (Figures 2b-c). In addition, the azimuthal anchoring coefficient after 2.5 J/cm2 LPUV exposure is on the range of 10-5-10-4 J/m2 under proper RH range (Figure 2d), which is strong enough for various applications.27 The micrograph of the TN LC cells under polarized optical microscope (POM) shows good uniform alignment at 40% RH or below (Figure 2e-f). If further increasing RH, the anchoring energy gets smaller and the alignment quality becomes worse, where non-uniform alignment starts to show up. For example, the LC cell assembled at 60% RH shows obvious partial alignment, while the uniform alignment is totally lost at 80% RH (Figures 2g-h). Linear writing property is expected for azobenzene photoalignment process, where the film alignment quality only depends on exposure energy. In that case, the alignment ordering with high power illumination is the same as the one with lower power but prolonged exposure time. For BY alignment layers, this linear process can be guaranteed at 40% RH or below, where the normalized phase retardation vs. exposure energy overlaps for different LPUV powers (Figures


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3a-c). When the RH reaches 50%, the BY layer cannot be photoaligned under weak light exposure (Figure 3d), and the linearity is not valid anymore. This is due to the intrinsic semicrystalline property of BY material with water insertion.22-24 When the RH is at 40% or below, weak intermolecular interaction exists. The 365 nm LPUV light is stronger than this weak interaction, and thus could align BY molecules properly. When the RH reaches 50% or above, more water molecules are bounded to the hydrophilic functionalities of azobenzene molecules, and the hydrogen-bond-assisted intermolecular interaction becomes stronger. Therefore, reduced photoalignment ability is observed and the breakage of their linkage depends on the LPUV power, giving a non-linear process. Therefore, the photoalignment quality of BY films depends strongly on RH. The RH range for BY alignment layers is below 50%. Especially when the RH is at 40% or below, good photoalignment quality can be guaranteed, which is recommended as the working window for BY photoalignment application. When the RH is above 50%, the photoalignment quality is poor and should be avoided. Compared to SD1 material, whose RH working window is recommended as 50%−70%,22 less water insertion in BY material will destroy its photoalignment ability. This is probably due to less hydrophilic terminal groups of BY molecule. Therefore, different RH working ranges exist for different hydrophilic azobenzene photoalignment materials, which should be chosen and designed properly according to the local situation for their applications. Photo-Stability at Different Relative Humidity. Under LPUV or blue light irradiation with changing polarization directions, reorientation is expected for azobenzene materials, where the absorption oscillators could be realigned to different directions.6.7 This is important for applications based on multi-domain alignment. For example, multi-domain LC alignment can improve the display viewing angle,8 and various LC photonic devices are fabricated by multi-


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step exposure method.9 However, in the literatures using BY as photoalignment material, almost all the works were done by one-step exposure method and no reports were found based on its rewritability.19-21 Herein, we explore the rewritability of BY as a function of RH. The in situ phase retardation measurement under LPUV exposure with alternatively 90° changed polarization direction is shown in Figure 4a. The BY films show good rewritabilty at 10% RH or below. The reorientation speed decreases with increasing RH. When the RH reaches 20%, the rewritability becomes very poor, where complete reorientation needs prolonged exposure time. At 30% RH, the BY film shows very slow ordering change by rewriting, which is too slow to consider as rewritable. When the RH is above 40%, the BY film is not rewritable anymore, where light exposure with orthogonal polarization doesn’t cause reorientation of the BY molecules, and an unusual slow order increasing until saturation is even observed along previous direction. To further explore the rewritability of BY films, we prepare the binary LC grating using BYcoated substrates based on two-step exposure method at different RHs. If the BY film is rewritable, an alternate 1D planar alignment (PA) and TN domains should be observed in the LC cell, as illustrated in the schematic diagram in Figure 4b. Figures 4c-d present the optical micrographs of the LC cells prepared at different RHs under POM. The cell made at 5% RH shows alternate PA and TN domains, and obvious 1D grating diffraction is observed from the cell. The cell prepared at 20% RH shows very poor TN area, and only week diffraction is observed. However, the cell prepared at 40% RH shows only uniform PA throughout the cell and there is no grating diffraction. Therefore, the BY film is rewritable and can be patterned to several domains by multi-step exposure method if the RH is low enough (≤ 10%). However, this extremely low RH can hardly be reached in the open air, which is probably the reason why


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people only use BY as one-step photoalignment layer. Therefore, a glovebox or a sealed chamber with ≤ 10% RH inside is recommended for rewriting and multi-step exposure purpose of BY photoalignment material. Relative Humidity Change. The humidification-enhanced ordering after photoalignment was observed in the hydrophilic azobenzene materials due to stronger intermolecular interaction with more water insertion,22-24 where the prealigned molecules provide a torque to neighbors toward the alignment direction.22 The same humidification-enhanced ordering after light exposure is also expected for the BY films. As shown in Figures 5a-b, both the phase retardation and order parameter show great enhancement after light excitation with increased RH. The anzimuthal anchoring energy also increases dramatically (Figure 5c). Photo-stable BY films is also expected after the humidification-enhancement. The in situ phase retardation measurement shows that the BY films cannot be rewritten when the RH increases to 40% or above (Figure 5a). An unusual ordering incensement is even observed below 70% RH with the 2nd excitation. If the RH increases to 80% after photoalignment, the enhanced ordering decreases to a saturated value with the 2nd excitation, but still keeps the original alignment direction with a much higher ordering compared to the one before RH increment. The photostability of the BY film after humidification-enhancement is also tested by a TN LC cell, which is assembled with a BY film and a rubbed PI substrate. The BY film was uniformly photoaligned at 5% RH, and then the RH increased to 70% for humidification-enhancement before cell assembling. The assembled LC cell is then exposed to the orthogonally LP 410 nm light with a 2D square lattice mask (10 mm period), no significant difference is observed with up to 33 J/cm2 exposure compared to the original cell (Figures 5d-e). Therefore, the BY alignment layer is rewritable and can be photo-patterned to multi-domains at extremely low RH (≤ 10%), and then


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the alignment ordering can be enhanced and stabilized after the RH increasing to above 40%. On the other hand, the electrical properties of the BY photoaligned LC (MDA-01-4679 from Merck) cell show no significant change after RH change. For example, for the LC cell made at 5% RH, the voltage holding ratio (VHR) is measured to be 97.5% at room temperature and 87.8% at 60º C, and the residual DC (RDC) voltage is 58 mV; For the BY layers photoaligned at 5% RH and then humidification-enhanced to 50% RH afterwards, the LC cell VHR is 98.4% at room temperature and 88.3% at 60ºC, and the RDC voltage is 54.5 mV. Proper water added to the BY layer does not worsen the electrical properties of the photoaligned LC cell, probably since that the absorbed water are bounded to the azobenzene molecules. Therefore, the BY azobenzene photoalignment layer can be tuned from rewritable to photo-stable by RH adjustment, and thus various photo-patterned alignments can be reserved and stabilized. In this way, a chamber or glove box is recommended for environment control, and additional cell sealing is recommended after cell fabrication for device storage against environment change. Although the humidification control method using a chamber or glove box is not suitable for large device fabrication in industry, it is suitable for research demo for photoaligned optical devices that require optical stability afterwards, and it may be possible for small optical element fabrication. On the other hand, incorporation of BY molecules with crosslinking groups or reactive mesogens may further improve its electrical properties and stability against light and moisture.11-16,28 In contrast to humidification-enhancement, there is only a very small ordering decrease of the prealigned BY film after lowering RH (Figure 6a). However, the recovered BY films become photoalignable again if the original RH is no more than 60% (Figures 6b). The in situ phase retardation measurement shows that the recovered BY films have different orientation speeds, indicating that the BY films are only partially recovered. On the other hand, the BY films


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become rewritable again if the RH is recovered from no more than 60% to below 10%. An example is given in Figures 6c that the film recovered from 40% to 5% RH shows good rewritability. Therefore, the water molecules are loosely bounded to BY materials at or below 60% RH, and can be partially lost by lowering RH. However, after being exposed to 65% RH or above, the BY layer loses photoalignment ability and cannot be recovered anymore by lowering RH at room temperature, indicating strong bonding between the BY and water molecules. Aggregation Behavior. The crystalline property of the hydrophilic azobenzene materials with water insertion have been reported.22-24 Here, we use XRD to investigate the BY films at different conditions (Figure 7a). At non-saturated RH environment, no diffraction signal is observed. However, diffraction peaks starts to show up after exposure to saturated RH environment. The two peaks at 2θ = 19.0º (d = 4.67 Å) and 2θ = 9.5º (d = 9.29 Å) are observed. The 4.67 Å d-spacing is the thickness of azobenezene units, indicating the repeat stacking of the BY molecules,29 and the 9.29 Å d-spacing corresponds to two stacking thickness. These two peaks have similar diffraction intensity and profile, indicating half-and-half overlapping of the molecules that produces the brickwork structure.30 It is a typical structure of J-aggregation,31-34 where the hydrogen bonds between the middle sodium sulfonate and terminal hydroxyl groups of the disazo molecules make the brickwork structure favorable. This hydrogen-bond-assisted aggregates of BY molecules grows bigger and the ordering becomes better with more water molecule insertion, indicated by the two higher and sharper XRD peaks. Since no other diffraction peaks are observed from the BY films from the θ/2θ XRD scan, the BY molecule stacking on substrate is the preferential orientation growth of the film, as indicated in Figure 7b. If without water, on the other hand, the aggregation is hindered by intermolecular repulsive electrostatic force of the BY hydrophilic groups, and therefore amorphous state is preferred.


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To double check the aggregation behavior of BY molecules, the absorption spectra are measured (Figure 7c). Bathochromical shift is observed in the BY films with increasing RH, and an absorption shoulder shows up at longer wavelength (~ 500 nm) when the RH is extremely high or saturated. These are the typical phenomena for J-aggregation of dye molecules with slipped arrangements,31-34 consistent with the XRD results. On the other hand, there is no optical anisotropy observed under POM without saturated water (Figure 7d), indicating that the BY aggregates without condensed water are mesoscopic.35 At or below 60% RH, the BY molecules are loosely packed, where the absorption spectra show continuous bathochromical shift with more water. When reaching 65% RH or above, the BY molecules are strongly packed into mesoaggregates, where the absorption spectrum has a sudden bathochromical shift and the shoulder at longer wavelength becomes more obvious. In that case, the intermolecular interaction is strong and thus the photoalignment ability is lost, and it cannot be recovered by lowering RH. If there is condensed water, the mesoaggregates are concluded to form macroaggregates,35 and the BY film becomes optically anisotropic where crystalline texture is observed (Figure 7e). The J-aggregation is also evident by the absorption spectra of BY aqueous solution. When BY concentration increases to ~ 2 wt% in water, the absorption shoulder increases at longer wavelength (Figure 7f). Moreover, a typical lyotropic LC texture is observed at 2 wt% BY aqueous solution, as shown in Figure 7g, which is the first-time observation of lyotropic LC formed by BY material to our best knowledge. Conclusions The BY photoalignment quality and stability have been demonstrated to depend strongly on RH. The BY layer is photoalignable below 50% RH, where good photoalignment quality can be guaranteed at 40% RH or below, which is recommended as the RH working window of BY


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photoalignment layer. When the RH is above 40%, the photoalignment ability is gradually lost due to stronger intermolecular interaction with water molecule insertion. Humidificationenhanced photoalignment ordering after light excitation is also observed. On the other hand, we demonstrate that the photo-stability of BY alignment layer also depends on RH. At extremely low RH (≤ 10%), the BY film is rewritable, while the rewritability is gradually lost with increasing RH. The BY photoalignment layer becomes photo-stable when at 40% RH or above. The stabilization against light exposure has also been proven for the humidification-enhanced BY film after prealignment. Therefore, the BY photoalignment layer can be tuned from rewritable to photo-stable by adjusting the RH from low to high value, and thus various multistep photo-patterned alignments can be obtained and reserved. This property is also expected for other hydrophilic azobenzene photoalignment materials, where the specific RH ranges may vary correspondingly. Brickwork-type J-mesoaggregates are probably formed due to hydrogen-bondassisted self-assembly of BY molecules with water molecule insertion, especially when the RH reaches 65% or above, where the BY layer cannot be photoaligned anymore. If exposed to saturated RH with more water molecules insertion, the mesoaggregates are concluded to form macroaggregates which shows macroscopic anisotropy.


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FIGURES

Figure 1. (a) Chemical structure of BY molecule. (b) In situ phase retardation change measurement of the BY photoalignment films with LPUV exposure under different RHs. The LPUV power is 10 mW/cm2.


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Figure 2. Ordering comparison of the BY films induced by 2.5 J/cm2 365 nm LPUV at different RHs. (a) Orientation speed. (b) Dichroic ratio. (c) Order parameter. (d) Azimuthal anchoring coefficient. The comparisons of dichroic ratio and order parameter exposed to 2.5 J/cm2 (red circles) and 10 J/cm2 dosages (blue squares ) are also provided. (e-h) POM LC textures of TN cells prepared under different RHs: (e) 20%, (f) 40%, (g) 60%, and (h) 80%. The scale bar is 100 µm.


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Figure 3. Linearity comparison of normalized BY phase retardation change induced by LPUV of different power at different RHs: (a) 5%, (b) 30%, (c) 40%, and (d) 50%. Inset: In situ phase retardation change of BY film at 50% RH, where the photoalignment quality is poor under weak light exposure.


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Figure 4. Rewritability of BY photoliangment layer under different RHs. (a) In situ phase retardation measurement with 90° alternatively changed LPUV light exposure. (b) Schematic structure of the 1D PA-TN LC grating cell assembled by rubbed PI and BY layer after 2-step exposure. (c-e) Micrographs of LC gratings under POM, prepared under different RHs: (c) 5%, (d) 20%, and (e) 40%. The grating period is 100 µm. (f-h) Diffraction patterns of the LC cells correspondingly.


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Figure 5. Humidification-enhancement and stablization of BY film after light exposure. (a) In situ phase retardation measurement. The BY films are first exposed by s-polarized LPUV light at 5% RH for 4 min. Then the RH increases to different values for about 16 min, and then the films are re-exposed to p-polarized LPUV light with the same intensity. (b) Ordering enhancement of the BY films with increasing RH after light exposure. (c) Anchoring energy enhancement of the BY films when the RH changes from 5% to 70% after light exposure. (d) A TN LC cell assembled by humidification-enhanced BY film (5% to 70% RH) and rubbed PI. (e) The same TN cell exposed to 33 J/cm2 orthogonally polarized 410 nm light with a 2D square lattice mask (10 mm period). No difference is observed compared to (d).


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Figure 6. The effect of RH decrease to BY photoalignment layer. (a) In situ phase retardation measurement of BY film illustrated by LPUV light at 30% RH, and then the RH decreases to 5% after light off. (b) In situ phase retardation measurement of recovered BY films from different RH to 20% for LPUV exposure. (c) In situ phase retardation measurement of BY films recovered from 40% to 5% RH with alternatively 90° changed polarization direction.


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Figure 7. (a) XRD signal of 110 nm-thick BY films from θ/2θ XRD scan. Black line: BY film in the open air (~ 70 % RH); Green to magenta lines: BY films exposed to saturated RH with increasing time. (b) The layered brickwork structure proposed for BY molecules on glass substrate. (c) Absorption spectra of BY films under different RH environments. (d-e) Micrographs of 110 nm-thick BY films under POM at different RHs: (d) 70% and (e) > 100%. The scale bar is 100 µm. (f) Absorption spectra of BY aqueous solution with increasing concentrations. (g) Lyotropic LC texture of a 2 wt% BY aqueous solution in a 30 nm-thick glass cell under POM. The scale bar is 200 µm.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].


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* E-mail: [email protected]. Funding Sources This work is supported by Innovation and Technology Fund ITC-PSKL12EG02; Natural National Science Foundation of China (NSFC) 61405088 and 11674183; Shenzhen Science and Technology Innovation Council JCYJ20150601155130435 and KQTD2015071710313656; National Key Research and Development Program of China administrated by the Ministry of Science and Technology of China 2016YFB0401702, and Foshan Innovation Project 2014IT100072. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge Material Characterization and Preparation Center of South University of Science and Technology and Materials Characterization & Preparation Facility of the Hong Kong University of Science and Technology.

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Figure 2 1627x1458mm (96 x 96 DPI)


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Figure 4 1170x895mm (96 x 96 DPI)


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High Photoinduced Ordering and Controllable Photostability of

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