Growth of ZnO Nanorods and Their Applications for Liquid Crystal

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Growth of ZnO Nanorods and its Applications for Liquid Crystal Devices Mu-Zhe Chen, Sheng-Hsiung Yang, and Shie-Chang Jeng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00202 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Nano Materials

Growth of ZnO Nanorods and its Applications for Liquid Crystal Devices Mu-Zhe Chen,† Sheng-Hsiung Yang‡ and Shie-Chang Jeng*, § †

Institute of Photonics System, ‡Institute of Lighting and Energy Photonics, and § Institute of Imaging and Biomedical Photonics, College of Photonics, National Chiao Tung University, Tainan 711, Taiwan ABSTRACT Different morphologies of ZnO nanorods were synthesized by using the two-step hydrothermal method and they were applied for liquid crystal (LC) alignment films. The morphology of ZnO nanorods, including length, diameter and density, were controlled by the growth time from 16 to 26 min. The morphology and the chemical feature both contributed to the surface wettability of ZnO nanorods. The LC pretilt angle on ZnO nanorods was successfully controlled from homeotropic (~89.0°) to homogenous (~ 1.7°) alignments as the contact angle of water on ZnO nanorods was changed from 93° to 28° with the gowth time. The analysis of X-ray diffraction and X-ray photoelectron spectroscopy showed that the crystallite size of ZnO nanorods was increased with the growth time, while the oxygen vacancy was decreased. The ZnO nanorods grown on ITO glass substrate also improve the transmission of LCDs due to the antireflection effect.

Keywords: ZnO nanorods, liquid crystal alignment, liquid crystal devices, pretilt angle, surface wettability

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1. Introduction The alignment of LC molecules is the most crucial technique for the fabrication of liquid crystal device (LCDs), so many works have been conducted to find the alternative methods for replacing the buffed polyimide (PI) films adopted in LC panel industry.1,2 The electro-optical (EO) properties of LCDs are influenced by the features of LC molecules anchoring on the alignment films, such as the polar, azimuthal angles and anchoring energy, and they are all related to the properties of LC materials and the solid alignment films. Therefore, the studies of LCs orientation on the thin alignment films have been widely conducted due to the importance in the academic research and photonic applications.1,2 The long-range orientational order of LCs can be also influenced by dispersed nanomaterials. Recently, the LC materials doped with nanoparticles have also been applied to change the textures, phase transition temperature, electrical, electrooptical and optical properties of LCDs,3-7 such as low threshold voltage,3,5 decreased relaxation frequency3 and memory effects.7 The variety of alignment techniques can be classified into different processes and/or materials, such as mechanically buffed PI films, obliquely evaporated silicon oxide (SiOx) thin films, and photo alignment.1,2 Those methods determine the conformation and molecular orientation of LCs anchored on the alignment films. It was found that the mechanism of LCs anchoring on alignment films depends on the chemical properties and the geometrical structure of the surface. It can be characterized by the pretilt angle and anchoring energy in polar and azimuthal orientations. The anchoring energy is a measure of how strong the anchoring of the LC molecules is to the alignment films. Previous studies have shown that the short-range interactions and long-range van der Waals interactions between the alignment film surface and the LC molecules provide the anchoring energy that determines the LC alignment.1,2,8,9 The short-range interfacial 2


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interactions include steric interaction, charge–charge interaction, charge–dipole interaction, dipole–dipole interaction, hydrogen bonding and chemical bonding.1,2,8,9 Moreover, the patterned alignment films via the spatially-controlled anchoring and pretilt angles can be applied for different operation modes of LC displays10,11 and diffractive optical elements.12 Unfortunately, the conventional PI material for alignment films adopted in LCD industry can only generate near zero (homogeneous) and 90 degrees (homeotropic) in pretilt angles, and their alignment properties are difficult to control by chemical synthesis. The organic PI is also sensitive to UV light and temperature; therefore, inorganic alignment materials for LCDs are promising since they can be operated in severe conditions, such as high temperature and light intensity, where a robust material is needed.13 Several methods have also been studied for controlling the pretilt angles of LCs via the change of surface wettability of the alignment films.14-22 We have applied ZnO nanostructured films for LC alignment and we have successfully controlled the LC pretilt angle on ZnO nanostructured films via annealing20,21 and UV exposure recently.22 Both of the annealing and UV exposure processes produce the controllable defect properties of ZnO nanostructured films, therefore the surface wettability is modified and it can generate the tunable LC pretilt angle aligned on ZnO nanostructured films. The advantages of oxide semiconductors, such as InGaZnO, ZnO and In2O, in their superior electrical, mechanical and optical properties have attracted many interests recently and generated many applications, including sensors, thin film transistors (TFT) and energy

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generators.23-29 InGaZnO has been intensively developed and used for TFT arrays in LCD industry due to the high mobility.23 Recently, ZnO nanostructures have also been applied for self-powered energy-harvesting nanosystems and piezoelectric sensor devices.24-27 The control of the length of ZnO nanorods can also serve as antireflection films. ZnO nanorods have exhibited broadband and omnidirectional antireflection characteristics for incident light, and they can greatly improve the performance of photonic devices.30-32 A variety of growth methods has been developed and produced several different morphologies accompanying with miscellaneous material properties.2527

In this work, we applied the various morphologies of ZnO nanorods as the alternative LC alignment films for LC pretilt control rather than using the complicated post-treatment in our previous works.20-22 Therefore, the facility cost is expected to be reduced for industry applications. Furthermore, the high temperature annealing treatment, that is, 170 ~ 300 °C in our previous work, may not be suitable for flexible LCDs using plastics substrates with the glass transition temperature less than the annealing temperature. The morphologies, including the length, diameter and density, of ZnO nanorods prepared by the two-step hydrothermal method were modified by the growth time. The ZnO nanorods also serve as the antireflection films to improve the transmittance of LCDs. The scanning electron microscope (SEM) were applied for the 4


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measurement of morphologies and the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were adopted for studying the chemical features of ZnO nanorods. It was found that the LC pretilt angle on ZnO nanorods was changed by different growth time, which produces different morphology and surface wettability. 2. EXPERIMENTAL METHODS 2.1 Materials In this research, the ZnO nanorods were prepared via the solution-based two-step hydrothermal method similar to our previous study.20-22 The schematic illustration of fabrication process of the ZnO nanorods is shown in Figure 1. A ZnO seed layers were first formed on the ITO glass substrates, followed by the nanorods growth in the solution state. For the first step, a mixture of zinc acetate dihydrate (0.22 g, 1 mmol) and isopropanol (10 mL) heated to 120 °C with stirring. 2-(Dimethylamino)ethanol (0.089 g, 1 mmol) was slowly added to the as-prepared mixture. The mixture was continuously stirred at 120 °C for 2 hr. The formed solution was deposited on ITO glass substrate, stood still for 30 sec and followed by spin-coating process at 2000 rpm/10 sec. The asprepared film was then annealed at 200 °C in air for 1 hr to form a ZnO seed layer. In the second step, the ZnO seed layer was immersed in a precursor solution. To prepare the growth solution, a mixture of ammonium chloride (1.07 g, 20 mmol) and zinc sulfate heptahydrate (0.144 g, 0.5 mmol) in de-ionized water (50 ml) was prepared. The pH of the mixture solution was monitored by a pH meter and carefully adjusted to 10.5 by slowly adding 2M NaOH aqueous solution. The bath solution was heated to 95 °C in an oven, and the substrates with ZnO seed layers were immersed in the growth solution (ZnO seed layer downward) for growing ZnO nanorods. The growth time were set to 16, 19, 22, 23, 24, 25 and 26 min for obtaining different lengths of nanorod arrays. The 5



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Figure 1. The schematic illustration of fabrication process of the ZnO nanorod arrays on the ITO glass substrate. solution was not changed during the growth process. The substrates were then taken out, sequentially rinsed with de-ionized water and annealed in air at 170 oC for 1 hr to obtain the as-prepared ZnO nanorods. 2.2 Sample Preparation For being the LC alignment films, the surface of the as-prepared ZnO nanorods with different growth times was mechanically buffed once by a desktop rubbing machine. The EO properties of LC cells using ZnO nanorods alignment films were determined by using the antiparallel LC cells with mylar spacers (cell gap ~ 5.5 μm). The positive dielectric anisotropy nematic LC material (E7, Δε = 14.1, ne = 1.74, no = 1.52, Tc= 61 °C, Daily Polymer Corp.) was filled into the cell at a temperature ~70 °C for reaching the uniform alignment. The components and mass composition of E7 LC are summarized in Figure S1 (in Supporting Information). It is noted that the clear temperature of E7 LC does not change with the ZnO nanorods alignment films having different morphologies. 2.3 Characterization The morphology of ZnO nanorods with different growth time was characterized by SEM (Hitachi SU8000). The diameter, density and length were analysed by the software ImageJ, where the image threshold was set by auto adjustment and the minimum area

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and circularity were set at 100 nm2 and 0.6, respectively. The transmission spectra of ZnO nanorods grown on ITO glass substrates was measured by the UV-visible spectrometer (Princeton Instruments, Acton SP2150). The structure and crystallinity of ZnO nanorods were studied by XRD (D/MAX-2500, Rigaku) with the Cu-Kα radiation at λ=1.54Å. The chemical properties of ZnO nanorods were characterized by XPS (PHI 5000 Versa-Probe, PHI). The measurement of CA of water of buffed-ZnO nanorods using a contact angle measurement system (CAM-100, Creating Nano Technologies) was applied to evaluate the surface wettability. The modified crystal rotation method was applied to characterize the pretilt angles of LC cells.33 An interferometer (Agilent 5530) was adopted to measure the rotation angle dependent phase retardation for the LC cells.34 The EO properties of the LC cells were examined by observing the the polarizing optical microscope (POM, Olympus BX51) photographs and the measurements of voltage dependent phase retardation. 3. RESULTS AND DISCUSSION Figure 2 shows the SEM images of the ZnO nanorods with different growth time. A seed layer of ZnO was first fabricated by the sol-gel method on the ITO glass substrate. The length of ZnO nanorods was controlled via growth time at the second step of the hydrothermal process. At the early stage of the second-step hydrothermal growth, the supersaturation of the solution is the highest, and the ZnO nanograins on which nanorods grow are generated with the help of ZnO seed layer. Previous studies have shown that the orientation of ZnO nanorods is determined by the nucleation and growth of the first few layers of zinc and oxygen atoms, and the texturing of the ZnO seed layer seems to be an intrinsic thermodynamic feature of the growth of these nuclei.35 As illustrated in Figure 2, most ZnO nanorods grew vertically on the substrate. Figure S2 (in Supporting

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Figure 2. The SEM images of ZnO nanorods with various growth time of (a) 16, (b) 19, (c) 22, (d) 23, (e) 24, (f) 25 and (g) 26 min.

Information) illustrates the top-view SEM images of ZnO nanorods after mechanical rubbing process; the topographic grooves are clearly seen as shown in Figure S2, which are essential for obtaining certain LC alignment in azimuthal direction. Figure 3 shows the average length and diameter of the ZnO nanorods with the growth time. The average length of the ZnO nanorods increases from 55 to 198 nm as the growth time is from 16 to 26 min. Moreover, the growth rate keeps almost constant at the first 24 min, and it slightly increases when the growth time is beyond 24 min. The average diameter of the ZnO nanorods shows an increase from 31 to 48 nm with the growth time. A fast growth rate is observed at the first 19 min and a nearly constant growth rate is found after 19 min. Besides, the average density of the ZnO nanorods (the number of nanorods per μm 2

) decreases from 2494 to 920 μm－2 with the growth time as shown in Figure 4, and it

does not show much difference when the growth time is longer than 19 min. The growth 8


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conditions, such as concentration of precursor, growth time, pH and temperature, can produce great influence on the morphology of ZnO nanorods.36,37 The results shown in Figures 3 and 4 imply that the early stage of hydrothermal process favors the growth in the radial direction, i.e. the high concentration of precursors benefits the formation of ZnO nanorods with larger diameter. 36,37

Figure 3. The average length and diameter of the ZnO nanorods vs the growth time.

Figure 4. The average density of the ZnO nanorods vs the growth time.

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Figure 5 illustrates the XRD results of the as-prepared ZnO nanorods with three different growth time (16, 22 and 26 min). The diffraction peaks are identified by matching with the standard ZnO diffraction data (JCPDS file no.36-1451). The results in Figure 5 show three peaks around 31.7°, 34.4°, and 36.4°, revealing the presence of polycrystalline wurtzite structure of ZnO nanorods. The intensity for 16 min growth time is barely seen for the same experimental conditions. Based on our XRD data, the average ZnO crystallite sizes D are determined according to the Scherrer’s relation, D=0.94λ/(βcosθ),38 where λ=1.54 Å depicts the X-ray wavelength, β represents the full width at half-maximum (FWHM) of the XRD signal, and θ is the diffraction angle. The FWHM values of ZnO nanorods at (002) plane for the growth time of 22 and 26 min are estimated to be 0.50° and 0.44°, respectively. The calculated crystallite size is thus varied from 17.3 to 19.7 nm with increasing the growth time, indication of improved crystallinity of ZnO nanorods. The calculated crystal size of (002) plane has no relation to the diameter shown in Figure 3. However, the average crystallite size of (002) plane should be a good indication for the crystallinity of ZnO nanorods.

Figure 5. XRD patterns of ZnO nanorods with various growth time. 10


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It is known that the chemical features of ZnO nanorods are related to the defect and the crystal structure of surface. Therefore, the chemical properties of ZnO nanorods are changed with the growth time according to the XRD results shown in Figure 5. Three types of defects are mainly found in ZnO materials, including zinc vacancy, zinc interstitial and oxygen vacancy.39 The technique of XPS is generally utilized to distinguish the stoichiometry and defect nature of ZnO nanorods. Figure S3 (in Supporting Information) illustrates the obtained XPS spectra of ZnO nanorods with three different growth times. The binding energies of these photoelectron peaks for zinc and oxygen atoms are detected and assigned to the Zn 2p, 3s, 3p, 3d, and O 1s signals.40 Furthermore, the O 1s signal was carefully characterized to give information about oxygen defect properties of ZnO nanorods as shown in Figure 6.41 Three Gaussian-like curves located at ~530.0, ~531.0 and ~532.0 eV were applied for fitting the O 1s peak. The peak of O 1s located at 530.0 eV indicates the amount of oxygen atoms in a fully oxidized stoichiometric surrounding. The peak at 531.0 eV means those O2－ ions in the oxygen-deficient environment, i.e. oxygen vacancies. Besides, the peak at 532.0 eV correlates with those loosely bound oxygen atoms on the surface. The evolution of the peaks at 531.0 and 532.0 eV is related to the surface wettability. Once these oxygen vacancies and loosely bound oxygen are produced, water molecules in environment are easily attracted to these defect sites, leading to the enhancement of surface wettability. According to the results shown in Table S1 (in Supporting Information), the relative intensity ratio of the oxygen to total O 1s at 530.0 eV increases from 28% to 69%. The increase of ZnO stoichiometry with the growth time agrees with the XRD results in Figure 5. On the contrary, the defects of oxygen contributed from 531.0 and 532.0 eV decreases from 72% to 31% as the growth time increases from 16 to 26 min as depicted in Table S1. It indicates that the surface wettability should decrease with increasing the 11



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Figure 6. O 1s high resolution XPS spectra of ZnO nanorods with different growth time of (a) 16, (b) 22 and (c) 26 min.

growth time according to the amount of oxygen defects. According to the results of SEM, XRD and XPS, the morphology and chemical features of ZnO nanorods are changed with the growth time. Those changes could influence the ZnO nanorods wettability, which will be shown later. The results of contact angle (CA) of distilled water and LC pretilt angle on the ZnO nanorods with different growth time are illustrated in Figure 7. The decrease of CA of water from 93° to 28° with the growth time from 14 to 26 min is observed. It indicates that the growth time can modify the surface properties and increase the ZnO nanorods wettability by increasing the growth time. The wettability of a film can be influenced by 12


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the morphology and the chemical feature of the surface. From the results of Figures 3 and 4, the length of ZnO nanorods increases with the growth time and the diameter and density of ZnO nanorods mainly show dependence on the growth time for the first 16 and 19 min. The results of XRD in Figure 5 and XPS in Figure 6 both show that the chemical features of ZnO nanorods are changed, while the surface wettability should decrease with the growth time. Therefore, the tendency of surface wettability of ZnO nanorods toward the growth time is mainly determined by the morphology especially the ZnO nanorods length. It is noted that LC pretilt angle depends on the surface wettability (CA of water) shown in Figure 7. And, the surface wettability is related to the morphology (length, diameter, and density in Figure 2) and surface chemistry (defects properties in Figures 5 and 6). Therefore, the tendency of CA of water, pretilt angle and length of ZnO nanorods with growth time is different for each other. In contrast to our previous works for LC molecules aligned on annealed and UV-treated ZnO nanostructured films,21-22 the surface wettability is mainly changed due to the change of the chemical defect properties of ZnO nanostructured films via the annealing and UV exposure processes.

Figure 7. CA of water and LC pretilt angle on ZnO nanorods vs the growth time. 13



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Figure 7 shows that the pretilt angle decreases from 89.0° to 1.7° with increasing the growth time. Both the CA of water and the pretilt angle decrease as the growth time increases according to Figures 7. The results confirm that the LC pretilt angle on ZnO nanorods has positive correlation with the surface wettability. The semi-empirical Friedel-Creagh-Kmetz (FCK) rule has been proved for the dependence between the LC pretilt angle and the wettability of an alignment film.14-22 The rule says that LCs tend to align paralleled to an alignment film when its CA of water is relatively small and the LCs inter-molecular interaction is less than the interface interaction, and vice versa. The transmittance of an alignment film is an important feature for LCDs applications. The PI films adopted in LCDs industry have the problem of yellowish coloration due to the extent of diimide fragment conjugation.42 The UV–vis transmission spectra of the conventional PI film (~ 80 nm coated on ITO glass substrate), ITO glass substrate and ZnO nanorods grown on ITO glass substrate (ZnO nanorods/ITO) for three different growth time were characterized and compared as illustrated in Figure 8. The ZnO nanorods/ITO exhibits a high transmittance around 90% in the visible region due to the antireflection property produced by ZnO nanorods. A very wide reflection suppression in the range of 400 to 1200 nm by using ZnO nanowire arrays has been demonstrated for the solar cells.30-32

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Figure 8. UV–vis transmission spectra of the PI film, ITO substrate and ZnO nanorods/ITO for different growth time.

Figure 9 shows the POM photographs of LC test cells (Voff) using ZnO nanorods alignment films with different growth time. The LC cells with the same cell gap and different pretilt angles will exhibit different colors due to the different corresponding phase retardations. Although the topographic grooves after mechanical rubbing process shown in Figure S2 are essential for obtaining certain LC alignment in azimuthal direction, the line and dot defects shown in Figures 9 (b) and (c) are often generated by the laboratory-grade rubbing machine limited to its mechanical stability and roller uniformity. This issue can be overcome by the industry-grade rubbing machine adopted in current LCD industry.

Figure 9. The photographs of LC cells using ZnO nanorods alignment films observed by POM for different growth time of (a) 16, (b) 22 and (c) 26 min, where there is no driving voltage.

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Figure 10 shows the phase retardations of LC cells using ZnO nanorods alignment films under various applied voltages for three different growth time. When the driving voltage is less than the threshold voltage, the phase retardation P of an LC cell equals P=

neeff =

2π

λ

(neeff −no )d

(1)

ne ⋅ no ne2

sin θ p + n o2 cos 2 θ p 2

,

where θp is the pretilt angle, no and ne are the ordinary and extraordinary refractive indices of the LC material, respectively. According to the Equation (1), an LC cell with the lower pretilt angle has the higher P while the driving voltage is less than the threshold value. Figure 10 illustrates that the P increases with the growth time, indicating that the LC pretilt angle decreases as the growth time increases. The results are the same as the results shown in Figures 7 and 9.

Figure 10. Phase retardation-voltage characteristics of LC cells using ZnO nanorods alignment films for three different growth time. 4. CONCLUSION We have reported an alternative approach to change the LC pretilt angle on ZnO nanorods prepared by the two-step hydrothermal process. This study confirms that the growth time can mediate the surface morphology and chemical property of ZnO nanorods, producing 16


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influence on the surface wettability. The influences of morphology and chemical features are different upon the wettability of ZnO nanorods. The relationship between the wettability of ZnO nanorods and the LC pretilt follows the empirical FCK rule. Finally, we demonstrate the LC pretilt angle is controlled from 89.0° to 1.7° as the CA of water changes from 93° to 28°.

ASSOCIATED CONTENT Supporting Information available: [Components and mass composition of E7 nematic liquid crystal, SEM images (top view) of ZnO nanorods after rubbing process with various growth time, XPS survey spectra of ZnO nanorods with three different growth time, the relative intensity ratio of the oxygen at 530.0, 531.0 and 532.0 eV to the total O 1s signal in ZnO nanorods with different growth time.]

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors appreciate the support by the Ministry of Science and Technology of Taiwan under contract: MOST 106-2112-M-009-003, MOST 103-2112-M-009-013-MY3, and MOST 106-2221-E-009-135.

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Growth of ZnO Nanorods and Their Applications for Liquid Crystal

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