Novel Photoalignment Method Based on Low ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 9032−9037

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Novel Photoalignment Method Based on Low-Molecular-Weight Azobenzene Dyes and Its Application for High-Dichroic-Ratio Polarizers Su Pan,* Jacob Y. Ho, Vladimir G. Chigrinov, and Hoi Sing Kwok State Key Lab on Advanced Displays and Optoelectronics, Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

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S Supporting Information *

ABSTRACT: Photoalignment is a simple technique for the manipulation of molecular orientations, which has been widely used in liquid crystal displays. Here, we propose a novel photoalignment method, in which an azobenzene dye thin film is deposited by thermal evaporation and in situ exposed to linearly polarized light simultaneously. We obtain polarizers with a dichroic ratio of up to 62, which is the highest value ever realized by a photoalignment method. Moreover, the polarizing thin film has a thickness of just 200 nm, compatible with flexible substrates, making it perfect for use as a dichroic polarizer in ultrathin and flexible displays.

KEYWORDS: photoalignment, azobenzene dye, polarizer, dichroic ratio, order parameter support in the device.5 Therefore, a novel technique capable of producing an ultrathin polarizer without this shrinkage issue is highly anticipated for ultrathin and flexible displays. A thin polarizer with a thickness less than 1 μm and based on lyotropic liquid crystalline (LLC) molecules has been reported.6 However, the dichroic ratio of this LLC polarizer is smaller than 25, which cannot satisfy the requirements of high-performance displays. Additionally, a high-contrast polarizer with a thickness of 5 μm and based on the guest−host effect has been demonstrated by Peeters et al.,2 but the film is not sufficiently thin and the fabrication process is complicated. The photoalignment method is a simple way to manipulate the orientation of molecules in a noncontact way by exposure to linearly polarized light.7 Photoalignment is advantageous because it can avoid mechanical damage and electrostatic charges, which are inevitably generated by the rubbing technique. Moreover, photoalignment offers great convenience to realize complex orientations.8−11 Therefore, it has wide applications in liquid crystal alignment,7,12−14 optical actuators,10 polarized light emission,15,16 and polarizers.17−19 Azobenzene dye molecules are common materials applied for dichroic polarizers by photoalignment.18,20,21 The dichroic ratios previously achieved by photoalignment are around 20,17,18,22 except for the value of 50 obtained by photosensitive ionic self-assembly complexes.19 Further improvement to

1. INTRODUCTION Dichroic polarizers are indispensable elements in flat panel displays, which are ubiquitous in modern life. Conventional polarizing thin films are based on mechanically stretching polyvinyl alcohol (PVA) doped with iodine; such polarizers are of low cost and have a high dichroic ratio (N > 60).1 The dichroic ratio (N) is the key parameter to evaluate the optical performance of a dichroic polarizer, which is defined as N = A⊥/A∥, where A⊥ and A∥ are the absorbances perpendicular and parallel to the transmission axis of the polarizer, respectively. For high-performance liquid crystal displays (LCDs), polarizers with a dichroic ratio of more than 50 are required.2 Besides the optical performance, the thickness of the elements in the displays is also a crucial selection criterion. With the rapid development of ultrathin displays, the demand for ultrathin polarizers is urgent, as the thickness of conventional polarizers is around 25 μm, which is not satisfactory for such displays. Polarizers with a thickness of 5 μm have been developed by the stretching method recently,3 but further reduction of the thickness is very challenging because ultrathin PVA films are fragile to handle during the stretching process. Besides the reduction of the thickness and weight, ultrathin polarizers have an overwhelming advantage in flexible LCDs because they can be placed inside the liquid crystal cells to eliminate the light leakage problem resulting from the birefringence of the flexible substrates.4 Furthermore, there is a serious concern with using conventional polarizers in thin and flexible displays. The shrinkage force generated in the stretching process will cause panel bending and display distortion when there is little rigid © 2018 American Chemical Society

Received: January 3, 2018 Accepted: February 14, 2018 Published: February 14, 2018 9032

DOI: 10.1021/acsami.8b00104 ACS Appl. Mater. Interfaces 2018, 10, 9032−9037

Research Article

ACS Applied Materials & Interfaces

chemical structure of the low-molecular-weight azobenzene dye 4,4′-([1,1′-biphenyl]-4,4′-diylbis(diazene-2,1-diyl))bis(N,N-dibutylaniline) (AD-1) used in this study, which is synthesized and additionally purified by preparative column chromatography and recrystallization,23,24 is shown in Figure 1b. The AD1 material is melted at 144 °C and shows good thermal stability until 320 °C. (thermogravimetric analysis data are shown in Figure S1.) The molecular orbitals of AD-1 are highly anisotropic, which is critical to obtain high dichroism. The ultraviolet−visible (UV−vis) absorption spectrum of AD-1 in toluene solution is shown in Figure 1c. The absorption peaks are located at 321 and 464 nm. During the deposition of the AD-1 thin film, the vacuum in the chamber is kept as low as 10−6 Torr. The deposition rate is controlled by the heating temperature of AD-1 powders. The absorption spectrum of a 200 nm thick deposited AD-1 film (without alignment) is shown in Figure 2a. The absorbance band in the visible region is broadened, and the peak is slightly blue-shifted to 439 nm, which is due to the molecular aggregation and π−π stacking.23,25 Besides, the optical density of the 200 nm thick thin film is quite large, which is beneficial to realize an ultrathin polarizer. The surface morphology of the thin film measured by atomic force microscope is shown in Figure 2b. The root mean square of the roughness in a 10 μm × 10 μm area is 2.327 nm, which indicates a smooth surface of the AD-1 thin film. To align the AD-1 molecules, the thin film is illuminated by linearly polarized light. The absorption of the molecules is proportional to cos2 θ, where θ is the angle between the absorption oscillator of the AD-1 molecules and the polarization of the pump light. The molecules located at the smaller θ are more able to be excited and rotate via iterative isomerization and diffusion. Finally, the molecules will reach a steady anisotropic angular distribution, and an orientation perpendicular to the polarization is preferred.7 In the in situ photoalignment method, the irradiation starts and stops synchronously with the thermal evaporation process of the AD-1 thin film. A linearly polarized He−Cd laser (442 nm) is expanded and guided to the glass substrate in a vacuum chamber through an optical window. The light is uniformly distributed around a 15 mm × 15 mm area, and its intensity is fixed at 5 mW cm−2. The exposure dosage is varied by changing the exposure time, and the exposure time is the same as the deposition time, which is changed according to the deposition rate for thin films of a given thickness. In our experiments, the 200 nm thickness of the AD-1 film is a result of balancing the transmission and contrast ratio of the polarizer. We first compare the polarizers produced by the in situ method and the conventional photoalignment method, in which all the conditions are kept the same as those for the in situ method, except that the irradiation starts after the thin film has been completely deposited. The polarizing optical microscopy pictures of the AD-1 thin films obtained by the two methods are demonstrated in Figure 3. With the conventional method, the AD-1 molecules are randomly oriented and show birefringence in domains under cross polarizers, which is shown in Figure 3a. The domains disappear after the photoalignment (Figure 3b), which indicates the homogenous alignment direction of the AD-1 molecules. The microscopy image of the AD-1 polarizer produced by the in situ photoalignment is shown in Figure 3c, and there are no domains because the AD-1 is photoaligned during deposition. Moreover, the crystalline properties of the AD-1 powders and

achieve a dichroic ratio for high-performance LCDs (N > 50) has never been realized by using the existing photoalignment methods before. In this paper, we introduce a novel in situ photoalignment technique and demonstrate an ultrathin high-dichroic-ratio polarizer with a dichroic ratio of 62. This high-dichroic-ratio polarizer can fulfill the requirement of the high-performance displays. Moreover, the polarizer is 200 nm thick, which can reduce the thickness of the displays and also make the polarizer suitable to be placed inside the liquid crystal cells to improve the optical performance. Additionally, the in situ photoalignment method eliminates the shrinkage problem, and the polarizing thin films produced are highly compatible with flexible displays.

2. RESULTS AND DISCUSSION In the in situ method, we deposit and simultaneously irradiate an azobenzene dye thin film with linearly polarized light, as shown in Figure 1a. The thermal evaporation technique is intensively employed in thin film deposition because of its low cost, minimal damage to the surface of functional materials, and low tendency to unintentional heat to the substrate. Lowmolecular-weight molecules are suitable for thermal evaporation because their melting temperature is relatively low. The

Figure 1. (a) Schematic diagram of the in situ photoalignment technique. The linearly polarized light is guided into the vacuum chamber with a small oblique angle to the substrate. The AD-1 source is put at the bottom of the chamber, right below the glass substrate. The deposition rate is controlled by the heating temperature. The irradiation of the AD-1 molecules starts and stops synchronously with the deposition process. (b) The chemical structure of the AD-1 molecule. (c) The UV−vis absorption spectrum of AD-1 in toluene solution. The absorption peaks are located at 321 and 464 nm. 9033


Research Article


Figure 2. (a) Absorption spectrum of this AD-1 thin film deposited by vacuum evaporation. Two absorption peaks are located at 322 and 439 nm. (b) Surface topography of the evaporated AD-1 thin film with a thickness of 200 nm. The root mean square of the roughness in a 10 μm × 10 μm area is 2.327 nm.

Figure 3. Polarizing optical microscopy images of the AD-1 thin films (200 nm) produced by both conventional photoalignment and in situ photoalignment techniques. (a) Vacuum-deposited AD-1 thin films. (b) AD-1 thin films by conventional photoalignment. (c) AD-1 thin films by in situ photoalignment. The scale bars are 100 μm.

Figure 4. (a) XRD of AD-1 powders. The three main peaks appear at 2θ = 8.1°, 20.1°, and 24.6°, which indicates the strong crystalline property of the AD-1 powders. (b) XRD results of the AD-1 thin films by both conventional photoalignment and in situ photoalignment methods.

films are investigated by the X-ray diffraction (XRD) technique. The AD-1 powders show several obvious diffraction peaks, as exhibited in Figure 4a, which indicates that the AD-1 molecules in powders are packed with strong crystalline properties. The XRD results of the AD-1 thin films produced by both the conventional and in situ photoalignment methods show a sharp peak and a broad amorphous peak with several small peaks, as shown in Figure 4b. The small peaks of the AD-1 thin film by in situ photoalignment is more obvious than those by conventional photoalignment, which may indicate a stronger semicrystalline property of the AD-1 polarizer by the in situ method and favorability to achieve a higher order parameter. The order parameter (S) is a parameter describing the degree of the order of the molecules, and it can be calculated by the dichroic ratio (S = (N − 1)/(N + 2)) under the assumption that the transition dipole moment is parallel to the molecular long axis.26 Polarized UV−vis absorption spectra parallel and perpendicular to the polarization of the pump light are measured. The

calculated dichroic ratios (at 450 nm) of the polarizers produced by both methods are demonstrated in Figure 5. The dichroic ratio achieved by the in situ method is much larger than that produced by the conventional method under the same exposure energy. During the photoalignment process, the free volume surrounding the excited molecules is significant for the molecular reorientation. It is generally known that a minimum local free volume is essential for the isomerization of azobenzene dyes.27,28 What is more, it has been reported that the order parameter of azobenzene dyes after photoalignment is improved with more free volume.29 For the conventional photoalignment method, the AD-1 thin films are deposited first and the molecular aggregation is completed before the photoalignment, so there is not much free volume for the molecular movement, which results in a low dichroic ratio. However, in the in situ method, the azobenzene molecules are evaporated and irradiated when the molecular packing is not very strong because the surrounding molecules have not settled down and the upper molecules have not yet been evaporated. 9034


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Figure 5. Comparison of dichroic ratios (at 450 nm) achieved by the in situ and conventional photoalignment methods. The substrate temperature is room temperature (23 °C). The light intensity of the linearly polarized He−Cd laser (442 nm) is fixed at 5 mW cm−2, and the film thickness is 200 nm. In the conventional photoalignment method, the irradiation starts after the thin film has been completely deposited, and the other conditions are kept the same as that in the in situ method. The maximum dichroic ratios realized by the in situ and conventional photoalignment methods are 62 and 15 at a dosage of 33 J cm−2, respectively.

Therefore, there are large free volumes for the freshly evaporated AD-1 molecules, and they can easily rotate with small steric hindrance when they are excited by the pump light and are finally oriented with a high order parameter. The dichroic ratios achieved by both methods grow as the exposure energy increases. The maximum dichroic ratios realized by the in situ and conventional photoalignment methods are 62 and 15 at a dosage of 33 J cm−2, respectively. It should be noted that for the in situ photoalignment method, the dichroic ratio decreases to 35 when the exposure energy increases to 50 J cm−2. This is because there are more free volumes during the photo-reorientation in this method, which may result in out-of-plane alignment under a very high excitation energy, reducing the dichroic ratio. The value of 62 is the highest dichroic ratio achieved by a photoalignment method so far and satisfies the requirement for highperformance LCDs. The absorption spectra parallel and perpendicular to the polarization of the pump light are exhibited in Figure 6a, and the wavelength-dependent dichroic ratio is shown in Figure 6b. The photographs of the produced AD-1 polarizer on a polarized lighting device are shown in Figure 6c, and its video is provided in the Supporting Information (Video S1). Moreover, the energy consumed at the maximum dichroic ratio is just 33 J cm−2. This is much lower than that for the photosensitive ionic self-assembly complexes method, in which a dosage of 850 J cm−2 is required to obtain a dichroic ratio of 50.19 Consequently, this in situ technique is the most efficient and economical photoalignment method. Next, we investigate the effect of the substrate temperature, which is an influential factor in the photoisomerization and diffusion processes.30,31 We change the substrate temperature during the in situ photoalignment and preserve the other conditions of the process in which the dichroic ratio of 62 is achieved. The dependence of the dichroic ratio on the substrate temperature is demonstrated in Figure 7. It is found that the dichroic ratio decreases rapidly to 20 when the substrate temperature increases to 50 °C. The increase of the temperature will accelerate the thermal diffusion toward isotropy.32 Besides, the trans isomers are more stable at an elevated temperature, which decelerates the molecular reorientation by trans−cis−trans isomerization.33 Therefore, a high

Figure 6. (a) The measured absorbance parallel and perpendicular to the polarization of the pump laser after in situ photoalignment at a dosage of 33 J cm−2. (b) The calculated dichroic ratio. The peak is located around 450 nm. (c) Photographs of the produced AD-1 polarizer on a polarized lighting device. In the left-hand picture, the polarization axis of the AD-1 polarizer is placed parallel to the polarization of the light box. The sample that is rotated by 90° is shown in the right-hand picture. The size of the sample is 20 mm × 20 mm. An area of around 15 mm × 15 mm in the center is exposed to the linearly polarized He−Cd laser (442 nm), whereas the surrounding area corresponds to the unexposed AD-1. The blank spaces at two corners are the sites at which we fixed the glass substrate, so there are no AD-1 molecules.

Figure 7. The influence of the substrate temperature on dichroic ratios (at 450 nm) in the in situ photoalignment method at a dosage of 33 J cm−2. The optimal substrate temperature is room temperature (23 °C).

temperature is not favorable in the photoalignment process. However, the dichroic ratio is not improved further under lower temperatures and is reduced to 35 at 5 °C because the rotational mobility of the molecules decreases when the temperature decreases.34 What is more, it is reported that no reorientation is observed when the temperature is below a threshold. Therefore, the room temperature (23 °C) is optimal to obtain the highest dichroic ratio by balancing the photoreorientation process and thermal diffusion. This in situ photoalignment is advantageous to integrate with flexible display manufacturing because no extra heating is necessary and the flexible substrates’ tolerance to heat is of no concern. 9035



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Finally, the compatibility of this in situ photoalignment technique with flexible substrates is investigated. We bond a piece of 150 μm thick polyethylene terephthalate (PET) film on a glass substrate to ensure a flat surface during the evaporation because curved substrates can cause nonuniformity of the incident angle and light distribution. After the in situ photoalignment process, a flexible polarizer with a dichroic ratio around 50 at 450 nm is obtained under the previous optimal conditions. The optical performance is presented in Figure S2, and the video of the optical performance on a polarized lighting device is shown in the Supporting Information (Video S2). The small drop in the dichroic ratio may be due to the change of surface tension and roughness. The thickness of the polarizing thin film is just 200 nm, which enables the film to be inserted inside the liquid crystal cells to avoid optical performance deterioration resulting from the birefringence of flexible substrates. Photoalignment is a simple and low-cost way to produce dichroic polarizers, which is advantageous for flexible displays because the polarizers that are produced are ultrathin, which can be integrated inside the liquid crystal cells and are free from the shrinkage issue. The main hindrance to the practical application of dichroic polarizers produced by photoalignment is the insufficient dichroic ratio. In this work, a polarizer with a dichroic ratio of more than 60 is, for the first time, achieved by the photoalignment method. By in situ photoaligning the dye molecules simultaneously with the thin film deposition process, the molecules rotate under unfastened circumstances and are free to orient themselves with a high order parameter. They can therefore show a much higher dichroic ratio than that in the conventional photoalignment processes. This in situ photoalignment technique paves a new way to realize highperformance polarizing thin films that can rival commercial PVA/iodine polarizers. Moreover, this technique achieves molecules with an order parameter as large as 0.95 and may be applied in polarized emission to double the optical efficiency of displays. Finally, the photoalignment based on azobenzene dyes is a reversible process, and additional passivation is necessary to stabilize the molecules after the alignment.35,36 This means that we can reorient the direction of the molecules by additional exposure, and the method possesses particular advantages in realizing patterned alignments, which show great potential in photonic applications.8

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b00104. Experimental details including material characterization, fabrication process, atomic force microscopy, and spectroscope measurement; thermogravimetric analysis results of the AD-1 molecules; the measured absorbance and dichroic ratio of 200 nm AD-1 polarizing thin film on the 150 μm thick PET substrate after the in situ photoalignment; and optical performance of the commercial PVA/iodine polarizer (PDF) AD-1 polarizers on a polarized lighting device (AVI) Optical performance of the polarizer on a polarized lighting device (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Su Pan: 0000-0002-0639-9468 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by the State Key Laboratory on Advanced Displays and Optoelectronics Technologies of The Hong Kong University of Science and Technology. The characterizations were performed in the Materials Characterization and Preparation Facility of The Hong Kong University of Science and Technology. The authors acknowledge Dr. Vashchenko V. V. and Krivoshey A. I. for the preparation of AD-1 according to HKUST-STCU project #P641a.

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REFERENCES

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3. CONCLUSIONS In conclusion, we have proposed an in situ photoalignment method and successfully demonstrated an ultrathin highdichroic-ratio polarizer. Compared with conventional photoalignment methods, this novel technique has realized oriented dye molecules with a large order parameter, which show a very high dichroic ratio, up to 62. Moreover, the energy consumption is low, which is more economical for practical manufacturing. Furthermore, the thickness of the produced polarizer is only 200 nm, and the fabrication process is highly compatible with flexible substrates, which is urgently demanded for ultrathin and flexible displays. We believe that this polarizer has overwhelming advantages for next-generation displays, and the superior photoalignment technique has promising applications in the manipulation of molecular orientation. 9036


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