Mechanism of unidirectional liquid-crystal alignment on polyimides with linearly polarized ultraviolet light exposure

Nishikawa, Michinori; Taheri, Bahman; West, John L.

doi:10.1063/1.121390

Cited by 116 publications

(60 citation statements)

References 16 publications

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“…In past years, this technique has been extensively used to align LC materials. [109][110][111][112] Recently, Chou et al demonstrated that this approach can also be applied to aligning small conjugated molecules, thereby fabricating OFETs with anisotropic electronic properties. [49] As shown in Figure 13a, on a photoaligned polyimide layer, the pentacene film is aligned perpendicularly to the polarization direction of the incident UV light, resulting in grains continuous along this direction.…”

Section: Alignment Layers Prepared By Photoirradiationmentioning

confidence: 98%

Controlled Deposition of Crystalline Organic Semiconductors for Field‐Effect‐Transistor Applications

et al. 2009

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The search for low‐cost, large‐area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors, which serve as one of the most important components for organic field‐effect transistors (OFETs). In the current review, we highlight deposition techniques that offer precise control over the location or in‐plane orientation of organic semiconductors. We focus on various vapor‐ and solution‐processing techniques for patterning organic single crystals in desired locations. Furthermore, the alignment of organic semiconductors via different methods relying on mechanical forces, alignment layers, epitaxial growth, and external magnetic and electric fields are surveyed. The advantages, limitations, and applications of these techniques in OFETs are also discussed.

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Section: Alignment Layers Prepared By Photoirradiationmentioning

confidence: 98%

Controlled Deposition of Crystalline Organic Semiconductors for Field‐Effect‐Transistor Applications

et al. 2009

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“…The irradiation of the film surface with linearly polarized UV light is a promising alternative. [6,9,10] The orientation of a polymer doped with an azo dye has been reported, however, it suffered from limited light stability. Later, stable photoinduced liquid-crystal (LC) alignment layers of more advanced materials, such as cinnamoyl side-chain polymers [11] or polyimides, [12] were introduced.…”

Section: Introductionmentioning

confidence: 99%

Polyimide Orientation Layers Prepared from Lyotropic Aromatic Poly(Amic Ethyl Ester)s

Neuber

Giesa

Schmidt³

2003

Adv Funct Materials

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The synthesis and characterization of liquid‐crystalline precursor polymer solutions[1] for polyimides permit for the first time the preparation of bulk‐ and surface‐oriented polyimide thin films from the nematic lyotropic state by shear. A special shearing technique was developed and optimized to orient viscous solutions into thin films with thicknesses below 100 nm. The films produced were thermally imidized and characterized by polarized light microscopy, as well as polarized FTIR and UV‐vis spectroscopy before and after imidization. The dichroic ratios (DRs) before imidization were determined as 5 by FTIR, and 4.5 by UV‐vis spectroscopies. After imidization the DRs increased to 14 and 7, respectively. The shear‐oriented layers possess a surface profile in the form of striations, which was characterized by mechanical surface scanning and atomic force microscopy (AFM). The profile height was determined in the nanometer range in contrast to the profile distance in the micrometer range, thus the latter is a magnitude larger than the film thickness. To quantify and compare the orientation potential of the obtained orientation layers, cells with a liquid‐crystalline host and a dichroic azo dye as guest were prepared. Interesting for this class of rod‐like polyimides is that layers, which were cast from low concentration isotropic solutions and rubbed, exhibited an almost doubled DR of 15 compared to analogously prepared alignment layers based on commercial flexible polyimide systems (DR = 8).

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“…This experimental design is similar to methods for measuring the azimuthal anchoring energy of liquid crystals on treated substrates [19]. The mechanism of liquid crystal alignment induced by a rubbed polymer layer itself is the subject of much study but is thought to include physical grooves, aligned polymer chains, and the van der Waals torque for surfaces in contact [20][21][22][23]. The last of these is equivalent to the short-ranged Casimir torque of our proposed study, though distinguishing the van der Waals effect from other alignment effects is difficult.…”

Section: Introductionmentioning

confidence: 99%

Rotation of a liquid crystal by the Casimir torque

Somers

Munday

2015

Phys. Rev. A

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We present a calculation of the Casimir torque acting on a liquid crystal near a birefringent crystal. In this system, a liquid crystal bulk is uniformly aligned at one surface and is twisted at the other surface by a birefringent crystal, e.g. barium titanate. The liquid crystal is separated from the solid crystal by an isotropic, transparent material such as SiO2. By varying the thickness of the deposited layer, we can observe the effect of retardation on the torque (which differentiates it from the close-range van der Waals torque). We find that a barium titanate slab would cause 5CB (4-cyano-4 -pentylbiphenyl) liquid crystal to rotate by 10 • through its bulk when separated by 35 nm of SiO2. The optical technique for measuring this twist is also outlined.

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Mechanism of unidirectional liquid-crystal alignment on polyimides with linearly polarized ultraviolet light exposure

Cited by 116 publications

References 16 publications

Controlled Deposition of Crystalline Organic Semiconductors for Field‐Effect‐Transistor Applications

Controlled Deposition of Crystalline Organic Semiconductors for Field‐Effect‐Transistor Applications

Polyimide Orientation Layers Prepared from Lyotropic Aromatic Poly(Amic Ethyl Ester)s

Rotation of a liquid crystal by the Casimir torque

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