Zinc oxide nanolevel surface transformation for liquid crystal orientation by ion bombardment

Byeong Yun Oh, Won Kyu Lee, Young Hwan Kim, Dae Shik Seo

Research output: Contribution to journalArticlepeer-review

22 Citations (Scopus)

Abstract

This paper introduces the characteristics of the zinc oxide (ZnO) inorganic film deposited by radio-frequency magnetron sputtering as an alternative alignment layer for liquid crystal display (LCD) applications. The crystalline structure related to the texture formation of ZnO (10 1- 3) was observed with a tilt angle of approximately 28.1° to the ZnO (0001) plane, leading to a smooth surface and high-density structure. Ion beam (IB) bombardment at various incident angles was used to induce liquid crystal (LC) alignment and cause the measured pretilt angle on ZnO films to assume a triangular contour. The orientation order of liquid crystal molecules was due to the van der Waals force for the vertical alignment of LCs with selective breaking of O-Zn bonds by IB bombardment. The contact angle contour as a function of the IB incident angle resembled the behavior of the pretilt angle. The pretilt angle is controllable by adjusting the surface features on ZnO films with IB bombardment. The electro-optic characteristics of vertically aligned (VA)-LCD based on ZnO film were comparable to those of VA-LCD based on polyimide, showing good potential of ZnO film as a LC alignment layer.

Original languageEnglish
Article number054506
JournalJournal of Applied Physics
Volume105
Issue number5
DOIs
Publication statusPublished - 2009

Bibliographical note

Funding Information:
This research was supported by the Ministry of Knowledge Economy (MKE), Korea under the Information Technology Research Center (ITRC) Support program supervised by the Institute of Information Technology Advancement (IITA) (Grant Nos. IITA-2008-C1090-0801-0018). The authors would like to thank Gun Hee Kim and Byung Du Ahn providing valuable discussions. Table I. Calculated surface energies of various IB-bombarded ZnO surfaces as a function of the incident angle. Incident angle(deg) Dispersive energy ( mJ   m − 2 ) Polar energy ( mJ   m − 2 ) Surface energy ( mJ   m − 2 ) 15 9.95 38.10 48.05 30 10.10 36.36 46.46 45 18.37 25.64 44.01 60 14.41 32.02 46.43 75 13.93 33.47 47.40 FIG. 1. Cross-sectional FESEM photograph of ZnO films deposited on (a) bare and (b) ITO-coasted glass substrates with an apparent viewing angle of 85°. The inset is a larger-scale top view FESEM photograph of the ZnO film at a 60° viewing angle. FIG. 2. (a) XRD pattern of ITO film and ZnO film deposited on an ITO-coated glass substrate at a temperature of 400   ° C . The inset is for ZnO film deposited on a bare glass substrate under the same conditions. (b) Schematic structure of the ZnO ( 10 1 ¯ 3 ) plane. (c) Growth direction of the ZnO [ 10 1 ¯ 3 ] with a tilt angle of approximately 28.1° to the ZnO [0001] direction. FIG. 3. Tilt angles on the IB-bombarded ZnO film as a function of the incident angle for an incident energy of 1.8 keV and an exposure time of 2 min. The inset shows the pretilt angle calculated from the tilt angle. FIG. 4. Optical transmittance spectra of (a) ITO/glass, (b) as-deposited ZnO/ITO/glass, and (c) IB-bombarded ZnO/ITO/glass substrates. FIG. 5. XPS spectra of O 1 s core-level of as-deposited and IB-bombarded ZnO films for an incident angle of 45°, an incident energy of 1.8 keV, and an exposure time of 2 min. FIG. 6. Contact angles on the IB-bombarded ZnO film as a function of the incident angle for an incident energy of 1.8 keV and an exposure time of 2 min. FIG. 7. (a) rms roughness values of the IB-bombarded ZnO film as a function of the incident angle for an incident energy of 1.8 keV and an exposure time of 2 min, and (b) the surface morphology of the as-deposited and IB-bombarded ZnO films for an incident angle of 45° under the same conditions. FIG. 8. Optical photomicrographs of the LC on the IB-bombarded ZnO films for an incident angle of 45°, an incident energy of 1.8 keV, and an exposure time of 2 min: (a) off-state with no applied voltage and (b) on-state with applied voltage of 10 V; the blue spots are due to the spacers. “A” indicates analyzer and “P” indicates polarizer. FIG. 9. Response time characteristics for the VA-LCD using the ZnO film bombarded by IB at an incident angle of 45°, an incident energy of 1.8 keV, and an exposure time of 2 min. Results for the rubbed PI layer are shown for comparison.

All Science Journal Classification (ASJC) codes

  • Physics and Astronomy(all)

Fingerprint

Dive into the research topics of 'Zinc oxide nanolevel surface transformation for liquid crystal orientation by ion bombardment'. Together they form a unique fingerprint.

Cite this