aState Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
bResearch Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, China
cBOE Technology Group Co., Ltd., Beijing 100176, China
Transparent photoresists with a high refractive index (RI) and high transmittance in visible wavelengths have promising functionalities in optical fields. This work reports a kind of tunable optical material composed of titanium dioxide nanoparticles embedded in acrylic resin with a high RI for ultraviolet (UV)-imprint lithography. The hybrid film exhibits a tunable RI of up to 1.67 (589 nm) after being cured by UV light, while maintaining both a high transparency of over 98% in the visible light range and a low haze of less than 0.05%. The precision machining of optical microstructures can be imprinted easily and efficiently using the hybrid resin, which acts as a light guide plate (LGP) to guide the light from the side to the top in order to conserve the energy of the display device. These preliminary studies based on both laboratory and commercial experiments pave the way for exploiting the unparalleled optical properties of nanocomposite resins and promoting their industrial application.
The liquid crystal display (LCD) is the most popular flat-panel display technology, with widespread applications [1], [2], [3]. Light guide plates (LGPs) are currently key supporting materials for LCD modules and product production [4], [5], [6]. With the aim of fabricating displays with high resolution, brightness, and contrast, a key materials challenge is the integrated innovation of material components and device structures at the nanometer to micrometer scale to achieve controllable photon propagation and intensity distribution. In the general sense, this challenge involves processing and manufacturing various basic materials into nanostructures at the wavelength or subwavelength scales to achieve the flexible regulation of light transmission characteristics [7], [8], [9], [10]. Optically clear adhesive (OCA) resin is a kind of special adhesive material used to bond transparent optical components such as lenses; it has high visible light transmittance, good bonding strength, and small curing shrinkage, and can be cured at room temperature or higher [11], [12], [13], [14]. A variety of technologies have been developed for the preparation of microstructures and nanostructures based on the use of different OCA polymers, including laser engraving [14], [15], thermal curing [16], [17], and injection molding [18]. Among them, ultraviolet (UV)-imprint technology has the advantages of high efficiency, environmental friendliness, and high reproduction accuracy [19], [20]. Its rapid structural prototyping of various polymers makes the UV-curing lithography technology unique in the preparation of micro-nano optical and optoelectronic devices [21]. This technology can be used in many fields, including light-emitting diodes (LEDs), displays [22], metalenses [23], spectral detection [24], [25], [26], electro spraying [27], and virtual and augmented realities [28], thereby playing a key role in the commercialization of nanotechnology [29], [30], [31].
As a key factor in the imprinting process, the photoresist restricts the integrity, resolution, and aspect ratio of the pattern. Compared with thermal imprinting, UV imprinting can be performed at room temperature, which reduces the time needed for heating to transform the photoresist form, greatly improves the imprinting efficiency, and avoids deformation due to inappropriate temperature [19], [20]. The refractive index (RI) is the basic property of optical materials. It is generally accepted that components made of optical resin with a relatively high RI are smaller, lighter, and have wider applicability than those made of ordinary optical materials for the same focal length requirements. Along with other works, we have demonstrated that the addition of ZrO2 nanoparticles to aliphatic epoxy resin [32], silicone [33], polyvinyl alcohol [34], and polyurethane acrylate [35] improves the light refractive performance of the polymers as encapsulation materials in LED devices for energy conservation and lighting. However, as far as we are aware, few studies have reported on transparent photoresists composed of inorganic nanoparticles embedded in an acrylic resin with enhanced RI values for UV-nanoimprint lithography.
In this work, with the aim of fabricating a UV-curable resin with a high RI, nanocomposites composed of titanium dioxide (TiO2) nanoparticles embedded in acrylic resin were synthesized via a short reaction route under mild reaction conditions. The TiO2 nanoparticles were used as filler materials to enhance the RI value due to their intrinsic characteristics of a high RI, low absorption coefficient in the visible range, and easy processability [36], [37], [38]. The sol-gel method was applied to synthesize TiO2 nanoparticles coated with a silane coupling agent, which were then embedded in an acrylic resin. The micromorphology and optical properties of the nanocomposites, including their RI values, transparency, and haze in the visible wavelength range, were systematically studied. A prototype component of an LGP with precision-machined optical microstructures was demonstrated through both laboratory and commercial experiments using the hybrid resin.
2. Material and methods
2.1. Materials
Tetraisopropyl titanate (TIPT, > 97%), acetone, acetylacetone, acrylic acid, aniline, benzyl alcohol, 1-butanol, chloroform, cyclohexane, dimethyl sulfoxide (DMSO), ethanol, ethyl acetate, ethylene glycol, hexane, methanol, methyl ethyl ketone (MEK), n-methyl-2-pyrrolidone (NMP), propylene glycol monomethyl ether acetate (PGMEA), tetrahydrofuran (THF), toluene, and p-xylene were purchased from Shanghai Macklin Biochemical Co., Ltd. (China). Nitric acid (HNO3, 65%-68%) was obtained from Beijing Chemical Works (China), while 3-methacryloyltrimethoxypropylsilane (KH570, > 98%) was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (China). Commercial basic UV-curing resin epoxy acrylate (EA) and photoinitiator 1173 were purchased from Ryoji Chem’s branch office in Germany. Deionized (DI) water was purified using a Hitech Smart-S30 reverse osmosis system (Shanghai Hitech Instruments Co., Ltd., China). Polycarbonate (PC) molds for optical films with precision-designed microstructures were obtained from BOE Technology Group Co., Ltd. (China). All chemicals were used without further purification.
2.2. Synthesis of the TiO2 nanodispersion
A TiO2 nanodispersion in methanol was synthesized using the modified sol-gel method [39]. In brief, 1.44 mL of DI water and 50.6 mL of methanol were mixed under vigorous stirring in a three-necked flask, and 0.32 mL of aqueous HNO3 solution was added to adjust the pH of the solution to around 1. Then, 10.33 g of TIPT was dropped into the solution with a constant pressure funnel at the speed of 1 mL∙min−1. After that, a certain amount of KH570 (0.90, 0.45, or 0.30 g) was dropped into the flask, corresponding to mole ratios of Ti to Si of 10:1, 20:1, or 30:1, respectively. These three samples were named TS10, TS20, and TS30. Finally, the reacted solutions were collected after 1 h at room temperature.
2.3. Preparation of the nanocomposite materials
The EA resin and photoinitiator 1173 were mixed and dropped into NMP to form a homogeneous solution. The solution was then dropped into the TiO2 nanodispersion under vigorous stirring. The TiO2 content accounted for 5, 10, 20, or 30 wt% of the resin, respectively, while the amount of photoinitiator 1173 was 5 wt% of the nanocomposite material. Finally, the mixture was heated to 50 °C in a rotary evaporator to remove the solvents. To test the optical properties of the hybrid films, the composite materials were coated onto a glass slide or spin coated onto a silicon wafer. For the spin coating of the samples, silicon wafers were cleaned with acetone and absolute ethanol in an ultrasonic cleaner for 10 min, respectively. The program parameters of the spin-coating processes were 1200 r∙min−1 for 6 s, followed by 8000 r∙min−1 for 20 s. For the fabrication of hybrid films, 100 μL of hybrid resin was coated onto a glass slide and put into a vacuum oven. A piston vacuum pump was used to achieve a vacuum of 0.1 MPa; then, nitrogen (N2) was sent into the oven until normal pressure was reached. This operation was repeated three times to ensure that the N2 in the oven was sufficient. Next, a UV lamp positioned 20 cm away from the resin was turned on to cure the resin; the irradiance of the UV lamp was 14 mW∙cm−2 at 365 nm, and the curing time was 10 min. A sample of pure resin underwent the same processes for further characterization.
2.4. Preparation of UV-curing patterns
The composite material was coated onto a PC substrate by means of blade coating to form a thin film, while a transparent PC mold was placed on the top. After UV irradiation in a N2 atmosphere, the mold was peeled off, leaving the pattern imprinted onto the composite film. This process was performed at ambient pressure and room temperature.
2.5. Characterization
Transmission electron microscope (TEM) images were taken by an HT7700 TEM (Hitachi High-Technologies Corporation, Japan) operating in bright field mode with an accelerating voltage of 120 kV. Selected area electron diffraction (SAED) images were taken using a JEM-2100F (JEOL Ltd., Japan). Surface morphologies were observed via a JSM-6360LV scanning electron microscope (SEM; JEOL Ltd.). The size distribution of the dispersions was obtained by means of Malvern Nano ZS90 (Malvern Panalytical, USA) dynamic light scattering (DLS). The Fourier-transform infrared spectroscopy (FTIR) spectra of solid samples were obtained using a PerkinElmer Spectrum GX FTIR spectroscopy system (PerkinElmer, USA). Thermogravimetric analysis (TGA) measurements were performed on a NEXTA STA200 simultaneous thermal analyzer (Hitachi High-Technologies Corporation) with a heating rate of 10 °C∙min−1 from room temperature to 800 °C in an air atmosphere. The X-ray diffraction (XRD) patterns of the samples were measured by an XRD-6000 diffractometer (Shimadzu Inc., Japan). The RI of the films was measured by an ellipsometer (UVSEL, HORIBA, France) from a wavelength of 400-900 nm. Atomic force microscope (AFM) images were obtained using an AFM DMFASTSCAN2-SYS (Bruker, Germany).
3. Results and discussion
The reaction process and the surface modification of the TiO2 particles are shown in Fig. 1(a). Methanol was used as a solvent for the reaction and water was used as a reactant for hydrolysis. Nitric acid acted as a hydrolysis inhibitor to prevent excessive hydrolysis of the TIPT. The chemical reactions involved in the process are shown in Eqs. (1), (2). Typically, the hydrolysis of TIPT and the polycondensation of Ti(OH)4 occur simultaneously to form TiO2 nanoparticles in the acid solution. The KH570 is adsorbed onto the surface of nanometer-sized TiO2 particles via covalent cross-linking. The TIPT used in the raw material was assumed to be fully (100%) hydrolyzed within 1 h, as the reaction conditions were more than adequate. Also, since there was no post-processing, such as separation, after the nanoparticle synthesis, it was considered that the KH570 was also completely transferred to the dispersion.
According to the DLS analyses of the different samples shown in Fig. 1(b), after surface modification with KH570, the particle size of the TiO2 in the aqueous solution was significantly reduced compared with that of the pure TiO2, indicating that the grafting of the silane coupling agent increased the dispersion of the particles. The average particle size of TS20 was (10 ± 4) nm, with a range from 6 to 14 nm. Further enhanced amounts of KH570 had no significant benefit for the dispersion of TiO2. Therefore, 20:1 was identified as the optimized ratio, and subsequent experiments for the synthesis of TiO2 nanodispersions were performed using a 20:1 mole ratio of titanium (Ti) and silicon (Si). The FTIR spectra of the pure TiO2 and the KH570-modified TiO2 are provided in Fig. 1(c). After modification with KH570, new bands appeared at 1716 and 2821 cm-1 for the stretching of the C=O and -CH3 groups in KH570, respectively. For the TiO2-KH570 sample, stretching of the Ti-O-Si band appeared at 930-950 cm-1, indicating the bonding of the KH570 on the TiO2 surface.
Both the AFM images in Figs. 2(a) and (b) and the TEM images in Fig. 2(c) show that the size of the TiO2 particles is around 6 nm. It should be noted that the TEM images indicate the actual size of the sample in the dried state, whereas the size measured via DLS is the hydrodynamic diameter in the hydrated state, which has a larger hydrodynamic volume due to the solvent effect. Fig. 2(c) shows a TEM image and an SAED image of the TiO2 nanoparticles, revealing the amorphous crystalline state of the particles. The XRD spectra of freshly prepared TiO2 powder and of samples stored for 90 and 180 days in Fig. 2(d) demonstrate no significant diffraction peaks, indicating the formation of stable amorphous TiO2 nanostructures.
The dispersibility of nanoparticles in the solvent is critical in the preparation of polymer-based nanocomposites using solution blending. The Hansen solubility parameters (HSPs) are used to predict the dispersion of nanomaterials in solvents and polymer substrates [40], [41]. In this work, we developed the use of the HSPs to evaluate the dispersion of TiO2-KH570 and the EA resin. In brief, the total solubility parameter δ is divided into δD (dispersion), δP (polarity), and δH (hydrogen bonding):
To determine whether solvents are “good” or “bad” for the desired purpose, the relative energy difference (RED) value is calculated; if a solvent is “good” (RED ≤ 1), then the polymer or nanoparticles will disperse in the solvent. The smaller the RED value, the better the dispersion.
Fig. 3(a) illustrates how to judge whether the TS20 nanoparticle or the resin can be dispersed in a solvent or not. If it cannot be dispersed in the solvent (RED > 1), the laser will be scattered. Otherwise, the Tyndall effect will be obvious, and the light path will be very clear (RED ≤ 1). Fig. 3(b) shows the Hanson dispersion sphere of the TS20 nanoparticles, demonstrating that the TS20 nanoparticles can be dispersed in a variety of solvents, that their polarity is not strong, and that the coordinates of (δD, δP, δH) are (17.06, 6.39, 10.55), while the radius is 10.2. Similarly, Fig. 3(c) shows the Hanson dispersion sphere of the resin, demonstrating that the coordinates of (δD, δP, δH) are (16.40, 6.76, 12.24), and the radius is 12.0. All the data can be fitted and calculated, and the fit = 1.0. Table 1 shows the values of the RED, where a value of less than 1 indicates dispersion. It can be seen that some areas of the two spheres overlap (Fig. 3(d)), so the blending solvent can be selected in these areas. NMP has low volatility, is colorless, and has a transparent appearance; it is a commonly used solvent in the electronics industry, so it was chosen as a blending solvent in this case.
Fig. 4(a) shows the preparation process for the nanocomposite film material. The nanodispersion was blended with the EA resin in NMP to form a homogeneous solution; next, the solvent was removed by rotary evaporating. Finally, the nanocomposite consisting of TiO2 and EA resin was coated onto a glass sheet to form a thin film. It can be seen from the digital photo in Fig. 4(b) that preparing the nanoparticles via the sol-gel method and mixing them with the EA resin allowed the nanoparticles to be uniformly dispersed in the composite material, exhibiting high transparency. The AFM images in Figs. 4(c) and (d) show that the hybrid film prepared through this method has an outstanding flatness and a roughness of only 0.196 nm. The relationship between the light transmittance (T) of nanocomposites with randomly distributed spherical nanoparticles can be described by Eq. (4) [42], [43], [44].
where r is the radius of the sphere; Vp is the volume fraction of the nanoparticles; np and nm are the RI of the particle and the resin matrix, respectively; I and I0 are the intensities of the transmitted and incident light, respectively; λ is the wavelength of the light; and x is the optical path length. It is only when the nanocomposite contains nanoparticles with a size much smaller than the wavelength of light (for spherical particles 2r < λ/10) that light scattering can be significantly reduced, in which case the principle of an RI mismatch can be ignored. In this method, the TiO2 nanoparticles are ultra-small and uniformly dispersed, so the nanocomposite material exhibits high transparency.
To analyze the doping amount of TiO2 in the hybrid resin, TGA tests were performed; the results are shown in Fig. 5(a). As can be seen from the curves, the pure EA resin (RJ361) is almost decomposed at 600 °C, and the pure KH570 loses all weight at around 210 °C. The weight loss of the hybrid resin decreases from 100 to 89, 79, and 72 wt% at 600 °C as the TiO2 content in the resin increases from 0 to 10, 20, and 30 wt%. As the TiO2 content increases, the curve shifts to the high-temperature area, indicating that the addition of TiO2 improves the thermal stability of the resin. The RI values of hybrid films with different TiO2 solid contents were measured in the range of 400-800 nm; the results are presented in Fig. 5(b) and show that the RI of the hybrid film increases as the content of TiO2 increases. The RI reaches 1.67 at 589 nm when the TiO2 content is 30 wt% in the hybrid film; however, the RI of the pure EA resin is only 1.53. Furthermore, the dependence of the RI on the volume fraction of TiO2 was calculated using the Lorentz-Lorenz effective medium expansion theory, expressed as follows:
where n is the RI of the nanocomposites; and ϕp and ϕm represents the volume fractions of the nanoparticle and the resin, respectively. For hybrid composite films with nanoparticle weight ratios of 0, 10, 20, and 30 wt%, the volume fractions were 0, 5.3, 11.1, and 15.9 vol%, respectively. The RI value of the nanocomposites is related to the volume fraction of the nanoparticle; the fitting formula is y = 0.8605xp + 1.5306.
Fig. 5(c) shows the transmittance spectra of nanocomposite films with different concentrations of TiO2 embedded in EA resin. The films with a TiO2 content ranging from 10 to 30 wt% maintained high transparency in the range of 400-800 nm, and the transmittance was greater than 98% at the wavelength of 589 nm. The haze of the EA resin with the glass slide was 0% as the blank sample; as shown in Fig. 5(d), the hazes of the other composite resins were all less than 0.05%, which is very beneficial for the backlight performance in a display. To examine their usage under extreme conditions, the hybrid films were heated at 60 °C for 24 h. As shown in Fig. 5(e), the transmittance was maintained at over 98%; moreover, no cracks formed on the films. This result means that the material can adapt to a wide temperature range. After that, the hardness of the nanocomposites was tested by the pencil hardness test. The nanocomposites were coated on the glasses to form uniform films. Mitsubishi pencils that meet international standards were selected for testing for the hardness of 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, and 9H, a total of 17, from soft to hard. The hardness of the original EA resin was less than 6B; however, as the TiO2 content increased, the film hardness improved under the same UV curing time, as shown in Fig. 5(f). On the premise of maintaining the visible light transmittance, utilizing the effect of ultra-small nanoparticles on the enhancement of UV light scattering and the difference in RI between the nanoparticles and the organic polymer substrate can induce multiple reflections at interphase interfaces in an interfacial composite. As a result, the uniformity of the UV light distribution at the microscale is improved, and the time required to reach the required hardness can be shortened.
The internal structure of an LCD devices contains two main parts: the display and the backlight module. The backlight module mainly consists of an LGP, an LED light source, and other functional film materials. An LGP with microstructures can be made using the proposed hybrid resin; the UV-curing progress is shown in Fig. 6(a). As shown in Fig. 6(b), the stripe patterns are completely imprinted, and the stripe spacing is 20 μm; also, as shown in the cross-sectional view (Fig. 6(c)), the base side of the triangle is about 7.5 μm and the height is 2.5 μm. The mapping images in Fig. 6(d) show that the embossed composite elements are evenly distributed, indicating that the distribution of titanium dioxide in the resin is relatively uniform. The microscopic morphology can be observed with higher magnification in Figs. 6(e) and (f). The nanoparticles are distributed in the nanocomposite resin without aggregation, and the titanium (Ti) element is also uniformly distributed.
Finally, this structure was transferred onto a PC plastic plate as a simple LGP, and an LED light strip was placed on the bottom side. A schematic diagram of the optical path is provided in Figs. 7(a) and (b). When the LED light source incidents from the side of the LGP, reflection, refraction, scattering, and other optical phenomena of the light occur in an internal transmission process to change the trajectory of the light, converting the side point light source into an area light source.
where n1 and n2 are the RI of medium 1 and medium 2, respectively; θ1 is the incidence angle; θ2 is the refraction angle; and θc is the critical angle. When the material of the LGP is selected as PC (the RI is 1.59), according to the law of refraction (Eqs. (6), (7)), total reflection will occur when the incident angle is less than 38.9°, causing the light to be reflected to the LGP and be unable to exit from the light-emitting surface. When high-RI polymer microstructures are added to the surface of the LGP, the main purpose is to correct the exit angle of the light, concentrate the exiting light, and improve the utilization rate of light energy. The software LightTools (9.1.0 version) was used to simulate the irradiance distribution of the light output from the upper surface of the LGP. In this work, we simulated a 14 mm × 14 mm LGP; five identical LED lights were incident parallel from the side, which was similar to the actual situation. The specific dimensions of the microstructure were as follows: The base edge of the triangle was 7.5 μm, the height was 2.5 μm, and the stripe spacing was 20 μm. A plane receiver was set at the same height above the LGPs. It can be seen from the simulation results that the irradiance distribution of the LGP with microstructures (Fig. 7(c)) is more uniform and the irradiance is larger, indicating that the microstructure effectively guides the light from the side to the top. However, the LGP without microstructures (Fig. 7(d)) has less irradiance above. Two LGPs were placed at the same time above a background paper with written text in a dark environment; LED lights were then lit. A schematic diagram of the experimental procedure is provided in Fig. 7(e). It is clear from the digital images in Fig. 7(f) that the LGP without microstructures cannot guide the light from the side to the top, so the patterns below are blurry. Due to the exist of the microstructures on the PC substrate, the light is guided to the top, so patterns far from the LED light source can still be seen very clearly.
4. Conclusions
Optical functional nanomaterials are revolutionary materials with applications ranging from individual mobile communication to television displays. A key global scientific and technological challenge is how to bring newly developed nanotechnology and nanocomposites with excellent performance out of the laboratory and transform them into new device products that can enter millions of households. The challenge ahead involves the ingenious fusion of scientific thinking with engineering thinking in accelerating the translation of new materials and technologies in industrial applications. In this work, we report a type of transparent photoresist composed of TiO2 nanoparticles embedded in acrylic resin for UV-imprint lithography. This hybrid film exhibits a tunable RI of up to 1.67 (589 nm) after being cured by UV light, while maintaining both a high transparency of over 98% in the visible light range and a low haze of less than 0.05%. An organic-inorganic composite method was demonstrated to be an advanced way to precisely adjust the RI of the material. Finally, the nanocomposites were used to fabricate microstructures on an LGP. Light-curable resins with an ultrahigh RI of 2.0 (589 nm) were also prepared via a change of the material of the basic polymer and in the amounts of TiO2 nanoparticles. The hybrid resin can be used for the precision machining of optical microstructures easily and efficiently, and can be applied in 10.5 inch (1 inch = 2.54 cm) flat-panel display devices. The knowledge obtained from both laboratory and commercial experiments on photon regulation at various nanoscale and microscale interfaces, along with the application of advanced nanocomposites to empower optical devices, is expected to assist in the promotion of human health via high-precision medical, lighting, and new display products applications.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (22288102 and 22278027).
Compliance with ethics guidelines
Yinglu Liu, Dan Wang, Changlin Liu, Qianqian Hao, Jian Li, Jie-Xin Wang, Xiuyun Chen, Peng Zhong, Xibin Shao, and Jian-Feng Chen declare that they have no conflict of interest or financial conflicts to disclose.
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