Wen-Chin Lee

Synthesis of alkyl-branched main chain copolyimides and their effect on the pretilt angles of liquid crystal alignment

Three series of copolyimides containing long alkyl branches were synthesized using the twostep method via poly(amic acid) precursors and chemical imidization. Most of the copolyimides prepared are soluble in polar organic solvents. Good liquid crystal alignment was achieved by buffing the copolyimide films spin-coated onto indium tin oxide glass substrates. The measured liquid crystal pretilt angles range from 0.16° to 15.54°. The Titan simulation program was used to calculate the dipole of each dianhydride structure and correlate with the observed pretilt angles. Main chain copolyimides with a long alkyl side chain, small dipole, and linear, symmetric and rigid core structures are favourable for generating large pretilt angles.

Sensitivity of the Photo-Crosslinkable Polyimide for Liquid Crystal Alignment

Two methods were developed for enhancing the photosensitivity of the cinnamates-contained polyimide (PICA). One is to increase the content of cinnamate side chain in PICA main chain and another is to blend some photosensitizer into PICA films. Our experimental results show that increasing the rate of photo-dimerization decreases the alignment ability of the PICA films. Using the molecular simulation method, we successfully describe the photoalignment mechanism of the PICA films. It is also found that photo-induced LC alignment ability of the PICA film could be enhanced through choosing a proper coating solvent.

Photo-Induced Liquid Crystal Alignment with Discotic Films

Photo-induced liquid crystal alignment using cross-linked discotic compounds was demonstrated. These discotic compounds exhibit good solubility in common organic solvents, 60°C processing temperature and excellent thermal stability. A linearly polarized long wave ultraviolet light (λ=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° 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.

Liquid Crystal Alignment with a Photo-Crosslinkable and Solvent-Soluble Polyimide Film

A new photo-crosslinkable and solvent-soluble polyimide containing cinnamate side chains (PICA) was developed for aligning nematic liquid crystals (LCs). Good LC alignment was achieved by exposing a long-wave linearly polarized ultraviolet (LPUV) light to the PICA film. The LC alignment direction is found perpendicular to the polarization axis of the incident LPUV light. The uniform alignment of LC molecules induced by PICA films remains intact after being heated at 85°C for 450 hours. A small pretilt angle on the PICA film was generated by the double exposure method.

Photo-alignment of liquid crystals using a crosslinked discotic film

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 (λ = 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° pre-tilt angle was achieved using a single exposure at a 30° oblique angle.

Effects of doping type and concentration on precipitation of nanometer arsenic clusters in low-temperature-grown GaAs

In this study, the effects of doping type and concentration on arsenic precipitation in low-temperature-grown GaAs upon postgrowth annealing at 600, 700, and 800 °C were investigated. Three undoped/Si-doped/undoped (i-n-i) regions and three undoped/Be-doped/undoped (i-p-i) regions were grown by low-temperature molecular beam epitaxy. The results show that arsenic precipitation is dependent on doping type and doping concentration. Arsenic depletion was observed in all Be-doped layers for all annealing temperatures. However, a “dual” arsenic precipitation behavior was observed in Si-doped layers: As accumulates in [Si]=2×1018cm−3 doped layers, while it depletes in [Si]=2×1016 and 2×1017cm−3 doped layers. We attribute this “dual” As precipitation phenomenon in Si-doped layers to the different depletion depths.

The magnetic anisotropy properties of Ga0.93Mn0.07As by low-temperature annealing

The magnetic anisotropy properties of (Ga, Mn)As epilayers are dependent on strain, thickness, substrates orientation, measurement temperature and shape. Systematic study magnetic anisotropy is important owing to they will strongly influence its application of spintronic devices. The effect of low-temperature annealing on the lattice constant and magnetic properties of ternary (Ga, Mn)As grown on GaAs substrates has been intensely studied. Here, we report magnetic anisotropy properties of Ga0.93Mn0.07As by low-temperature annealing.The Ga0.93Mn0.07As sample was grown on (001) semi-insulating GaAs substrates by a Varian Modular GEN-II MBE system. The surface reconstruction of the samples during growth was monitored in situ with the reflection high-energy electron diffraction (RHEED). After growth, sample was cleaved into pieces for various characterizations. The Mn concentrations were determined by a calibration curve based on the electron microprobe analysis (EMPA). The crystalline structures were examined by double-crystal x-ray diffraction (DXRD). Magnetic properties were measured by a Quantum Design MPMS2 superconducting quantum interference device (SQUID) magnetometer. To measure the magnetic properties of Ga0.93Mn0.07As epilayers the samples are cooled down to 10 K under an external field of 1000 Oe applied along the measurement axis. Then, field is removed, and projection of the remnant magnetization along the measurement axis is recorded as a function of increasing temperature. The temperature dependence of magnetization (M-T) curves of as-grown and annealed samples, were measured in small magnetic field (H = 1 Oe) applied in-plane directions. The M-T curves show in Fig. 1. Post-annealing with 250 oC for 1.5 hrs, the Curie temperatures (TC) value increases from 80 Kto 160 K.The magnetic field dependence of magnetization (M-H) curves was measured with applied fields along [110], [-110], [100] and [010] axis, respectively. M-H curves of as-grown and annealed samples are shown. The easy axis can be rotated from [100] direction to [010] direction by low-temperature annealing. And the coercivity field is from 50 G decrease to 18 G. Detailed studies of the effect of low-temperature annealing on magnetic anisotropy properties were discussed.