UV-treated ZnO films for liquid crystal alignment

UV-treated ZnO

films for liquid crystal alignment

The surface wettability of ZnOfilms prepared by a sol–gel process is altered from hydrophobicity to hydrophilicity via ultraviolet (UV) light irradiation. The results indicate that hydrophilicity of the ZnO films is enhanced along with the increase of UV irradiation energy. The measurements of X-ray photoelectron spectroscopy and photoluminescence show that the amount of oxygen vacancies in ZnO films is increased with the UV irradiation. The results of UV-vis absorption spectroscopy and X-ray diffraction also show that the crystalline quality of the ZnO films is slightly changed after UV treatment. The increase in oxygen vacancies means that water molecules can easily coordinate into the oxygen vacancy sites, leading to the increase of surface wettability. It is observed that liquid crystal (LC) molecules can be aligned on the UV-treated ZnOfilms, and the orientation of LCs on the UV-treated ZnO films can be tuned from homeotropic to homogeneous alignment by controlling the surface wettability of ZnOfilms. Our results show that the pretilt angle of LCs on ZnOfilms depends on their surface wettability, and it can be successfully adjusted over a wide range from 89.5
to 0.5
as the contact angle of water on ZnO films changes from 97
_{to 60
.}

1.

Introduction

The electro-optical properties of LCDs strongly depend on the alignmentlms for orientating LC molecules. Conventionally, the mechanically buffed organic polyimide (PI) alignment lms are applied to anchor the LC molecules with a specic orien-tation and pretilt angle, which is the angle between the director of the LC molecules and the alignmentlms. It is a mature technology in the LCD industry to use PIlms for aligning LCs nearly parallel and perpendicular to the substrates using homogeneous PI and homeotropic PI, respectively.

The technique for controlling pretilt angle is required for developing LC devices with different electro-optical properties, such as different LC displays modes1,2 _{and diffractive optical}

elements using inhomogeneous anchoring and pretilt angle.3 As a result, many methods have been developed to control the pretilt angle of LCs, such as polymer stabilization,4,5mixture of different materials,6–8 _{and treatment of alignment lms.}9–12

Control of the pretilt angle of LCs has also been widely studied by adjusting the surface wettability of the alignment lms.5,8,10,12–15_{It is observed that a hydrophobic surface favors}

the homeotropic alignment and a hydrophilic surface favors the homogeneous alignment. Therefore, we can reasonably suppose that alm with controllable surface wettability can be applied for tuning the pretilt angle of the LC molecules.

Study of new alignment technologies is very important for academic and practical applications. Applications of inorganic ZnOlms for aligning LCs are very promising for LCD systems operated in severe conditions, where a highly durable material is needed.16 Surface wettability of ZnO nanostructured lms modied by photo irradiation have been reported.17,18 _The

wettability of ZnO lms can be changed from hydrophobic surfaces to hydrophilic surfaces via ultraviolet (UV) irradiation. Besides, the ion beam (IB) bombardment on ZnOlms as an alternative alignment approach has been reported by Seo's group.9,11However, only a limited range of pretilt angle control (few degrees) was obtained.9,11 Recently, we found that the pretilt angle of LCs aligned on ZnO nanoparticle arrays can be controlled by annealing.12 _{However, the method cannot be}

applied to fabricate LCDs requiring patterned alignmentlms. The technique of photo alignment, irradiation of photon through a photo mask, can enable an area of alignmentlms to be divided into several domains with different alignment,19

which has found many applications, such as wide viewing angle LCDs,20patterned retarders21and LC lens.10

In this work, we applied UV treatment on ZnOlms prepared by sol–gel process. The characteristics of UV-treated ZnO lms were investigated by means of scanning electron microscope (SEM), atomic force microscopy (AFM), water contact angle (WCA) measurement, photoluminescence (PL) spectra, X-ray photoelectron spectroscopy (XPS), UV-vis absorption spectra, and X-ray diffraction (XRD). The UV-treated ZnO lms were further applied as LC alignmentlms. The controllable pretilt angle of ZnOlms can be achieved by using UV irradiation with different exposure times, which produce different surface wettability. The results show that the pretilt angle of LCs on ZnO

a_{Institute of Photonics System, National Chiao Tung University, Tainan 711, Taiwan} b_{Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan}

711, Taiwan

c_{Institute of Imaging and Biomedical Photonics, National Chiao Tung University,}

Tainan 711, Taiwan. E-mail: [email protected]; Tel: +886 63032121 Cite this: RSC Adv., 2016, 6, 52095

Received 22nd March 2016 Accepted 23rd May 2016 DOI: 10.1039/c6ra07454e www.rsc.org/advances

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lms can be tuned over a wide range from 89.5
_{to 0.5
as the}

WCA on ZnOlms changes from 97
to 60
.

2.

Experimental methods

2.1 Materials

ZnO thin lms were synthesized by the sol–gel method. A solution of zinc acetate dihydrate (0.22 g, 1 mmol) in 10 mL of isopropanol was stirred vigorously at 120
C for 10 min. 2-(Dimethylamino)ethanol (0.089 g, 1 mmol) was slowly added to the above solution and stirred at the same temperature for an additional 2 h to form a homogeneous sol–gel solution. The solution was then spin-cast into a thinlm on the ITO glass substrate at room temperature. The as-prepared thinlm was annealed at 200
C in air for 90 min to form the ZnO thinlm, followed by UV exposure with different irradiation energy. The surface of the UV-treated ZnO alignment lm was then sub-jected to rubbing treatment using a nylon cloth in such a way that the ZnO alignmentlm was rubbed once in each direction. The inuence of UV treatment on ZnO lms was conducted by irradiating the samples for the certain exposure times of 2, 4, 6, 8, 10, 12, and 14 min using an UV light (EXECURE 4000, HOYA) with a constant output intensity of 1.0 mW cm2measured by an UV radiometer (UVX-36, UVP).

2.2 Sample preparation

To determine optical properties of LC molecules on UV-treated ZnO alignmentlms, antiparallel LC cells were assembled with the 5.5mm Mylar lms as the spacers. The LC cells were then lled with negative dielectric anisotropic LC molecules (D3 ¼ 2.8, 3t ¼ 6.1, ne ¼ 1.579, no ¼ 1.483, Tc ¼ 77
C) at

a temperature100
C for obtaining a uniform alignment. It is worth to note that the UV-treated ZnO lms also show the similar alignment properties for using the positive dielectric anisotropic LC molecules, such as E7.

2.3 Characterization

The morphology of the ZnO lms was examined using SEM (SU8000, Hitachi) and AFM (Innova, Bruker). The PL measure-ment was conducted at room temperature by using a He–Cd laser for excitation (IK3301R-G, Kimmon). The PL and UV-vis absorption spectra were measured by a spectrometer (SP2150, Acton) and recorded by a photomultiplier tube (PD471, Princeton Instruments). Here, the 2.5 mmol sol–gel solution was applied to obtain a thick ZnOlm for increasing PL signal. Electron spectra of ZnOlms for chemical analysis were carried out by an XPS system (PHI 5000 Versa-Probe, PHI). The struc-ture and crystallinity of ZnO was carried out by an XRD system (D/MAX-2500, Rigaku) using Cu-K_aradiation source (l ¼ 1.54 ˚A). Here, the 5.0 mmol sol–gel solution was applied to obtain a thick ZnOlm for increasing XRD signal. The wettability of UV-treated ZnOlms was evaluated by measuring the WCA on their surfaces using a contact angle analyzer (CAM-100, Creating Nano Technologies). The pretilt angles of LC cells were measured at room temperature by the modied crystal rotation method,22where a laser interferometer (5530, Agilent)

was used to determine the phase retardation.23 The optical properties of the LC cells were also evaluated by means of the polarizing optical microscope (POM).

3.

Results and discussion

The nanostructure and the morphology of the UV-treated ZnO lms observed by SEM and AFM are shown in Fig. 1 and 2, respectively, where the sample without UV treatment is compared with the longest UV exposure time (14 min). The thickness and grain size of the ZnOlms is around 50 and 58 nm, respectively as shown in Fig. 1. The roughness of the ZnO nanostructures is determined as2 nm by AFM as shown in Fig. 2. No signicant difference in morphology is observed for ZnOlms before and aer UV irradiation.

The results of WCA on the UV-treated ZnO thinlms with different UV exposure time are shown in Fig. 3. It shows that the WCA decreases from 97
to 60
as the UV exposure time increases from 0 to 14 min. The results indicate that UV irra-diation can mediate and increase the surface wettability of the ZnO lms by increasing UV exposure time, while the ZnO surface is changed from hydrophobic to hydrophilic property. It is known that the wettability of a solid surface is determined by both the chemical property and the morphology of the surface. The morphology of ZnOlms does not show any dependence on UV treatment as shown in Fig. 1 and 2; therefore, the further discussion that follows will center about the inuence of UV irradiation on chemical properties of ZnOlms.

The chemical composition of ZnOlms relates to the defect chemistry and the surface crystal structure. It is reported ZnO has a rich and complicated defect property.24 Defects of ZnO lms are intrinsic properties and highly depend on the synthetic process.24–28 Although the defect properties of ZnO

Fig. 1 SEM images of ZnO thinfilms (a) before and (b) after UV irra-diation with an exposure time of 14 min.

materials have been intensive studied for more than 40 years, ZnO still has a controversy.24–28 _{The main defects in ZnO}

materials are zinc interstitial, oxygen vacancy and zinc vacancy. XPS was applied to evaluate the stoichiometry and defect property of UV-treated ZnOlms as shown in Fig. 4. It illustrates

the XPS survey spectra of treated-ZnOlms before and aer UV irradiation with an exposure time of 14 min. Clear photoelec-tron peaks from zinc and oxygen atoms in the binding energy region between 0 and 1200 eV are observed, such as Zn 2p, 3s, 3p, 3d, and O 1s signals.29_{Besides, the O 1s signal can be further}

analyzed to provide additional information about different bonding types around oxygen atoms as shown in Fig. 5.17,30_This

O 1s signal is composed of two components that correspond to different bonding environments of oxygen atoms existing in the UV-treated ZnO lms.11,17,30 _{Firstly, the main binding}

compo-nent located at 530.1 eV corresponds to O2ions in the ZnO lattice (Zn–O bonds) with hexagonal wurtzite structure. The amount of ZnO is expected to increase as the relative intensity of this signal is raised. Secondly, the shoulder component located around 531.5 eV is attributed to those O2ions in the neigh-boring vacant oxygen sites, i.e. oxygen vacancies. The evolution of this shoulder signal reveals the change of oxygen vacancies in the ZnO lms. According to the previous statement, the O 1s signal is resolved into two Gaussian-like curves centered at 530.1 and 531.5 eV as shown in Fig. 5. Aer UV exposure for 14 min, the relative intensity ratio of the main binding component from Zn–O bonds at 530.1 eV to total O 1s is decreased from 53.7% to 42.5%, indicating a decrease in the concentration of Zn–O bounds. Furthermore, it is noted that the shoulder component grows stronger from Fig. 5(a) and (b). The relative intensity ratio of the oxygen vacancies at 531.5 eV to total O 1s is increased from 46.3% to 57.5% aer UV treatment. It is re-ported that UV irradiation generates electrons and holes in the ZnO lattice.17 Some of the holes react with lattice oxygen,

Fig. 2 AFM images of the ZnOfilm (a) before and (b) after the UV irradiation with an exposure time of 14 min.

Fig. 3 WCA of UV-treated ZnOfilms as a function of UV exposure time.

Fig. 4 XPS survey spectra of ZnOfilms obtained before and after UV irradiation with an exposure time of 14 min.

Fig. 5 High resolution O 1s XPS of ZnOfilms prepared (a) before and (b) after UV irradiation with an exposure time of 14 min.

leading to the formation of freely leaving oxygen molecules and defective sites on the surface of ZnO, so-called oxygen vacancies. Once these oxygen vacancies are formed, water molecules in air are consequently coordinated to those vacant sites and resulted in the increase of surface wettability, as shown previously in Fig. 3. Therefore, ZnOlms provide photosensitive surfaces to be switched from hydrophobicity to hydrophilicity by UV irradiation.

The defect properties of UV-treated ZnOlms with different exposure times are also investigated by PL measurement as shown in Fig. 6, where three exposure times are plotted for comparison. The PL spectra of UV-treated ZnO lms are featured by a strong ultraviolet (UV) emission near 380 nm from the free exciton recombination and a broad defect-related emission in the visible region.24–28 Specic energies of emis-sion within the visible band have been assigned as a variety of defects in ZnO with different energies.24_{The increased intensity}

of visible emissions with UV exposure time presented in Fig. 6 indicates that the concentration of point defect is increased.

The optical absorption spectra and band gap energy of ZnO lms before and aer UV irradiation with an exposure time of 14 min are also investigated as shown in Fig. 7 and 8, respec-tively. The optical absorption property is related to the funda-mental energy gap, which depends on the properties of

materials, such as the crystal structure31 _{and carrier}

concen-tration.32 _{For a semiconductor with a direct band gap, the}

energy band gap energy Egcan be estimated by the Tauc model:

(ahn)2_{¼ A(hn  E}

g),33wherea is the absorption coefficient, A is

a constant, hn is the photon energy of the incident light. The Eg

is determined by extrapolating the tangential line to the photon energy axis in the plot of (ahn)2_{versus hn as shown in Fig. 8. The}

Egis then obtained as 3.30 eV and 3.32 eV for ZnOlms before

and aer UV treatment, respectively. The slight change of Eg

may indicate that the features of crystal structure and point defect of ZnOlms are slightly changed with UV treatment.

The XRD results of as-prepared ZnO lms on the glass substrate with different UV exposure times are shown in Fig. 9. The XRD patterns are identied by using the standard powder diffraction data of ZnO, referred at JCPDS le no. 36-1451. The XRD patterns show three main peaks at around 31.7
, 34.3
, and 36.3
. These peaks represent the existence of polycrystalline wurtzite structure of ZnOlms. According to the XRD results, the average crystallite sizes D can be derived from the Scherrer's relation34_{as D¼ 0.94l/b cos q, where l ¼ 1.54 ˚A is the}

wave-length of the Cu-Ka radiation, b is the full width at half-maximum (FWHM) of the diffraction peak, and q is the diffraction angle. The FWHM for ZnO lms is determined as 1.04
and 0.94
for the (002) plane of ZnOlms before and aer

Fig. 6 PL of UV-treated ZnOfilms with three different exposure times.

Fig. 7 Absorbance spectra of ZnOfilms (a) before and (b) after UV irradiation with an exposure time of 14 min.

Fig. 8 Optical band gap energy of ZnOfilms (a) before and (b) after UV irradiation with an exposure time of 14 min.

Fig. 9 XRD of ZnO films before and after UV irradiation with an exposure time of 14 min.

UV irradiation with an exposure time of 14 min according to Fig. 9. The calculated crystallite size of ZnO thin lms is changed from 9.7 to 10.3 nm aer UV exposure. The slightly increased grain size in ZnOlms indicates that the crystalline quality is slightly improved with UV treatment.

Our XPS results in Fig. 5 are consistent with our PL results in Fig. 6, in which they both show that defects (oxygen vacancies) of ZnO lms increase with UV exposure time. However, the geometrical structure of ZnO lms still remains unchanged aer UV exposure as shown in Fig. 1 and 2. It is well known that the surface chemistry and the surface roughness play an important role in the wetting properties of a solid surface, and the chemical composition of ZnO lms relates to the defect chemistry and the surface crystal structure.35 _{Our studies}

conrm that the UV irradiation on the ZnO lms brings about the generation of oxygen vacancies, which produces inuence on the chemical composition and contributes to the formation of highly hydrophilic surfaces. The results of band gap energy in Fig. 8 and XRD in Fig. 9 also show that the crystallinity of ZnO lms is slightly changed aer UV treatment. The chemical properties of UV-treated ZnO lms produce inuence on the pretilt angle of LCs, which will be shown later.

The pretilt angle of LCs and WCA on ZnOlms for different UV exposure times are both shown in Fig. 10. The pretilt angle

decreases from 89.5 to 0.5
as the CA of water decreases from 97
to 60
. Both the pretilt angle and the WCA decrease as the UV exposure time increases. The results conrm that the pretilt angle of LCs on the UV-treated ZnOlm strongly depends on its surface wettability, and the pretilt angle of the ZnOlm can be easily controlled by exposure time of UV irradiation. The rela-tionship between the surface wettability and pretilt angle of an alignment lm has been widely investigated.5,8,10,12–15,36 _An

alignmentlm with more polar surfaces gives a lower pretilt angle due to the increased interaction between LC molecules and the alignmentlm.13,14,36

The POM photographs of antiparallel LC cells with UV-treated ZnO alignmentlms with different exposure times are shown in Fig. 11. The transmission of LC cells changes from dark state (0 min, pretilt angle89.5
) to bright state (14 min, pretilt angle0.5
). The LC cells with different color represent the different pretilt angles corresponding to the different phase retardations. These results agree with the results shown in Fig. 10.

The stability of alignment materials under the high temperature and photo illumination environments are impor-tant for LCDs' applications. Recently, photostability of organic PI and inorganic silicon-dioxide (SiO2) alignment lms were

studied and compared by C.-H. Wen et al.16_{Their results}

indi-cated that SiO2alignmentlms are superior than conventional

PIlms under severe conditions. In order to study the reliability of UV-treated ZnO alignment lms unambiguously, the contamination produced by the rubbing process should be avoided.37 We are developing several non-rubbing methods37 and the stability of the as-prepared ZnO lms under severe conditions will be evaluated in the near future.

4.

Conclusions

In conclusion, we have applied UV treatment on ZnO lms prepared by sol–gel process. The UV-treated ZnO lms were further applied as LC alignmentlms. The controllable pretilt angle of LCs on ZnO lms can be obtained by different UV exposure times. The results of XPS and PL have shown that the chemical compositions of ZnOlms have been changed with

Fig. 11 POM photographs of antiparallel LC cells with UV-treated ZnO alignmentfilms with different UV exposure times: (a) 0, (b) 2, (c) 4, (d) 6, (e) 8, (f) 10, (g) 12, and (h) 14 min.

Fig. 10 Pretilt angle of LCs and WCA on ZnOfilm as a function of UV exposure time.

the UV irradiation, which produce inuence on the surface wettability. The results of band gap energy and XRD measure-ments also show that the crystallinity of ZnOlms is slightly changed aer UV treatment. Our results show that the pretilt angle of LCs on ZnOlms depends on their surface wettability, and it can be successfully adjusted over a wide range from 89.5
to 0.5
as the WCA on ZnOlms changes from 97
to 60
. Our proposed method of making inorganic ZnO alignment lms with tunable pretilt angles is very simple and easy for scale-up production in the current LCD industry.

Acknowledgements

We thank the anonymous reviewers for their constructive comments. The authors thank the Ministry of Science and Technology of Taiwan fornancially supporting this research under contract MOST 103-2112-M-009-013-MY3, and NSC102-2113-M-009-007.

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