Figure 2 illustrates the X-ray photoelectron spectroscopy (XPS) data of the brush-coated GO-ZnO composite films [29–31]. The spectra confirmed the presence of carbon, zinc, and oxygen based on the bonding energy regions. The Zn 2p peak was observed in the range of 1020–1045 eV. The O 1s peak appeared around 530 eV, and the C 1s peak was observed near 280 eV. The Zn 2p spectrum was analyzed using a Gaussian filter and showed Zn 2p1/2 and Zn 2p3/2 spin-orbit doublets with peaks at 1021.8 and 1044.5 eV, respectively. The area ratio between the two spin-orbit peaks was approximately 1:2, and these peaks gradually decreased with increasing GO ratios. The O 1s spectrum consisted of two components: oxygen-metal bonds centered at 530 eV and oxygen vacancies at 532 eV. In the pure ZnO, the proportion of oxygen-metal bonds was higher than oxygen vacancies; however, doping ZnO with 15% GO resulted in decreased oxygen-metal bonds and increased oxygen vacancies. The C-C/C = C bonds were centered at 284 eV, C-O bonds were centered at 286.1 eV, and C = O bonds were centered at 288.0 eV. The intensity of C-C/C = C bonding increased with increasing doping concentration of GO, indicating the presence of aromatic bonding in GO. In addition, for the reliability of the XPS analysis data, the survey data is shown in Fig. 3. The scan range is 0-1400 eV, and all elements of the GO-ZnO thin film can be checked at a glance.
Figure 4 shows the AFM images of the GO-ZnO composite thin films based on GO doping concentrations [32]. For the 15% doped GO-ZnO, the surface roughness (rq) is 70.25 nm; this value is much larger than the 2–10 nm roughness typically seen for rubbed PI films. Therefore, the surface roughness obtained from brush coating is sufficient for orienting the LCs without rubbing. Additionally, the surface tension from brush coating revealed the presence of surface alignment structures. During brush coating, the GO-ZnO mixture in the solution creates a retraction force, which leads to formation of the surface alignment structures during solidification. A significant surface alignment structure was observed for high GO content and 15% doping compared to a low doping rate. This indicates that the retraction force during brush coating is particularly effective in the presence of GO.
Figure 5 shows the results of the XRD and TEM analyses to confirm the crystallinity of the GO-ZnO mixture. XRD patterns were analyzed for ZnO and 15% doped GO-ZnO. No diffraction peaks were observed in the XRD patterns under any doping conditions, indicating that the GO-ZnO composites were in the amorphous state. This amorphous phase was also observed through TEM analysis. The selected-area electron diffraction (SAED) pattern of GO exhibited a clear spot pattern, indicating long-range order and crystallinity in one direction. However, an intermediate distribution in a ring pattern was observed, indicating randomly oriented crystal structures. At 15% doping of GO-ZnO, a ring-like diffraction pattern was not observed, suggesting lack of crystallinity. The corresponding high-resolution TEM (HRTEM) image in the inset of Fig. 5c also confirms the absence of a crystalline structure while indicating the presence of randomly oriented crystals without long-range order.
Figure 6 shows the optical transmittances of the GO-ZnO films at doping ratios of 0%, 5%, and 15% over a wavelength range of 200 − 850 nm; the average optical transmittances in the visible region (380 − 780 nm) were 87.5%, 85.4%, and 87.1% at doping ratios of 0%, 5%, and 15%, respectively. In LCDs, rubbed PI films are conventionally used as the alignment layer and show similar transparency values (80 − 85%) in the visible range. Hence, from this perspective, the proposed GO-ZnO films can be considered as good alternative candidates for LC alignment layers.
Figure 7 shows the POM analysis results confirming the LC orientation state and thermal stability. The LC cell is placed between crossed polarizers and analyzed using an optical microscope. A black image represents the case where the LC molecules are aligned well in the same direction as the surface orientation, resulting in no light scattering. On the other hand, when light scattering occurs between the crossed polarizers under certain conditions, it indicates that the LC molecules are not aligned in a single direction, causing polarized light from the bottom to scatter owing to the optical refractive index anisotropy of the LC molecules. When the LC cell, which exhibits a well-aligned black image, is rotated by 45° between the crossed polarizers, the bottom-scattered light is homogeneously aligned by the LC molecules and appears as bright light. From this observation, it can be inferred that the LC molecules on the GO-ZnO film obtained through brush coating exhibit uniform and homogeneous alignment regardless of the doping level of GO. From the thermal stability evaluation, it was found that the ZnO film mixed with GO loses its LC orientation property at 90°, while the ZnO film without GO loses its orientation property at 120°; this indicates that the thermal stability of the GO-ZnO film is not as good as that of ZnO. A pretilt angle of approximately 0.03° was obtained with a low error ratio; this demonstrates the homogeneous LC alignment characteristics of the GO-ZnO film regardless of doping ratio. The pretilt angles obtained were 0.034°, 0.004°, and 0.001° for doping ratios of 0%, 5%, and 15%, respectively. Brush-coated GO-ZnO not only exhibits surface alignment but also horizontally fixes the LC molecules through a combination of ZnO's hydrophilic properties and the pi-bonds between GO, ZnO, and LC molecules, which have aromatic rings. Therefore, it can be observed that the pretilt angles decrease further as the doping amount of GO with low pretilt angle increases. The surface wetting properties of GO-ZnO, as shown in Fig. 8, show contact angles of 69.041°, 64.028°, and 63.828° at 0%, 5%, and 15% doping ratios of GO, respectively, indicating increased hydrophilic properties with increasing doping ratios of GO.
Figure 9 illustrates the homogeneous orientations of the LC molecules on the brush-coated GO-ZnO films. As confirmed by the crystal structure analysis, the atomic crystallinity of GO-ZnO was amorphous. Therefore, the LC molecules are not aligned on the surface based on the crystal structure. However, surface anisotropy was observed through surface shape analysis. First, the LC molecules near the surface were rearranged by the aligned surface protrusions; then, they were mutually attracted to the neighboring LC molecules through van der Waals forces, resulting in the alignment of all LC molecules. As a result, the LC alignment in the GO-ZnO film is mainly attributed to the surface tension generated during brush coating rather than the crystal structure.
Figure 10 shows the residual DC and V-C characteristics. The residual DC characteristics are used to demonstrate the image sticking effects of LCDs. Image sticking refers to a phenomenon where a previous image persists and affects the subsequent image owing to the residual charges. Residual DC is defined as the voltage difference between the rise and fall directions at half the value of the maximum capacitance when the DC voltage is swept from − 10 V to + 10 V. From the positive and negative cycles, residual DC values of 0.811 V and 0.833 V were obtained, respectively. These values are similar to the 0.7–0.85 V range of values observed for the conventional rubbed polyimide (PI) films. The V-C characteristics show that the capacitance changes because of dielectric anisotropy when applying a voltage and switching the LCs. The switching threshold voltage can be determined from the graph, where the capacitance value changes at 1.7 V.