Browsing by Author "Wu, LH"

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Effects of O-2 plasma treatment on the electric and dielectric characteristics of Ba0.7Sr0.3TiO3 thin films
(2000-04-01)
Leu, CC
;
Chan, SH
;
Chen, HY
;
Horng, RH
;
Wuu, DS
The effects of the O-2 plasma treatment on the electric and dielectric characteristics of Ba0.7Sr0.3TiO3 (BST) thin films were investigated. As a result of the exposure of the as-deposited or the annealed BST films to the O-2 plasma, the leakage current density of the BST films can be improved. Typically, the leakage current density can decrease by three orders of magnitude as compared that of the non-plasma treated sample at an applied voltage of 1.5 V. It is found that the plasma treatment changes the surface morphology. The capacitance of the BST films was reduced by 10%similar to 30%. The improvement of the leakage current density and the reduction of a dielectric constant for the plasma treated samples could be attributed to the reduction of carbon contaminations of BST thin films. The 10 year life time of the time-dependent dielectric breakdown (TDDB) studies indicates that all the samples have a life time of over 10 years of operation at a voltage bias of 1 V. (C) 2000 Elsevier Science Ltd. Al rights reserved.
Obliquely tilted discotic phase compensation films
(2000-08-15)
Wu, LH
;
Luo, SJ
;
Hsu, CS
;
Wu, ST
;
應用化學系
A three-step method was developed to fabricate the obliquely tilted discotic negative birefringence phase compensation films. Model fitting reveals that the tilt angle of the discotic directors is about 46 degrees from surface normal. The total phase retardation value of the film can be controlled by the discotic layer thickness. These asymmetric phase retardation films are useful for widening the viewing angle of a liquid crystal display device.
Photo-alignment of liquid crystals using a crosslinked discotic film
(2001-02-01)
Wu, LH
;
Lee, WC
;
Hsu, CS
;
Wu, ST
;
應用化學系
A new photo-alignment layer using a crosslinked discotic compound was demonstrated. This discotic compound exhibits excellent solubility in common organic solvents, the possibility of low temperature processing and good thermal stability. A linearly polarized long wavelength ultraviolet (LPUV) light (lambda = 350 nm) was used to initiate the crosslinking process and induce liquid crystal alignment on the discotic film surface. The induced LC directors were found parallel to the electric field direction of the LPUV light. A 1.2 degrees pre-tilt angle was achieved using a single exposure at a 30 degrees oblique angle.
Photo-induced liquid crystal alignment with discotic films
(2000-10-01)
Wu, LH
;
Lee, WC
;
Hsu, CS
;
Wu, ST
;
應用化學系
Photo-induced liquid crystal alignment using cross-linked discotic compounds was demonstrated. These discotic compounds exhibit good solubility in common organic solvents, 60 degreesC processing temperature and excellent thermal stability. A linearly polarized long wave ultraviolet light (lambda = 350 nm) was used to initiate the cross-linking process which, in turn, induces liquid crystal alignment on the discotic film surface. The polarity of discotic compounds plays an important role in affecting the pretilt angle of the liquid crystal alignment. A pretilt angle of 1 +/- 0.4 degrees was obtained using single exposure at oblique angle. Potential application of this new photo-alignment film for low temperature poly-silicon thin-film-transistor liquid crystal display is foreseeable.
Synthesis and characterization of ferroelectric side chain liquid crystal polymers bearing banana-shaped chiral mesogens
(2000-06-01)
Wu, LH
;
Chu, CS
;
Janarthanan, N
;
Hsu, CS
;
應用化學系
A series of ferroelectric liquid crystal polymers having banana-shaped side chain mesogens were synthesized through photo-polymerization of epoxide moiety. 2,5-disubstituted-thiophene sub-unit was used in synthesizing the banana-shaped monomers. These liquid crystal compounds were characterized by NMR, differential scanning calorimetry (DSC) and optical polarized microscopy (POM). Mesomorphism was investigated as a function of spacer units. All the synthesized low molar mass banana-shaped compounds exhibit smectic blue phase, but chiral smectic C phase could be observed only on compounds having longer spacer. The clearing temperature of low molar mass compounds fluctuated a little when spacer length varies. All polymers exhibit cholesteric mesophase and an observable glass transition. These liquid crystalline compounds reveal strong photoluminescence at visible region (lambda(max) = 475 nm for M9EPX) and have potential use in polarized organic light emitting diode materials.
Synthesis and characterization of naphthalene-substituted triphenylene discotic liquid crystals
(2001-01-01)
Wu, LH
;
Janarthanan, N
;
Hsu, CS
;
應用化學系
;
Department of Applied Chemistry
A series of naphthalene-substituted triphenylene liquid crystals (LCs), viz. triphenylene-2,3,6,7,10,11-hexayl hexakis(6-alkenyloxy-2-naphthoate)s (HTPnN compounds) were synthesized and characterized. Alkenyloxy groups containing three to eleven carbon atoms were used as peripheral spacers in these liquid crystals. The discotic liquid crystals synthesized trap one to three water molecules depending on the length of their peripheral spacers. Differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction measurements confirm both nematic (N-d) and rectangular disordered columnar (Col(rd)) phases for most of the synthesized discotic LCs. The clearing temperatures of these discotic LCs increase with increasing peripheral spacer length. A hybrid aligned HTP9N sample was investigated to evaluate its optical performance. Retardation values of this hybrid sample decrease with increasing wavelength and increase with increasing measuring angle.
Synthesis and characterization of thiophene-containing liquid crystals
(2000-11-01)
Wu, LH
;
Wang, YC
;
Hsu, CS
;
應用化學系
;
Department of Applied Chemistry
Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; ( ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.