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Patterning Smectic Liquid Crystals for OFETs at Low Temperature
摘要： The preparation of regular microstructures with liquid crystalline materials for organic field effect transistors (OFETs) is an attractive but challenging issue. However, it is usually limited by the difficulty of forming large-area single crystals aligned in a desirable direction. Herein, several terthiophene (TTP) smectic liquid crystals such as 8-TTP-8 and 12-TTP-11OH are patterned into highly crystalline microstripes by a sandwich system through a dewetting method. Morphology and orientation of the microstripes strongly depend on preparation temperature. Microstripes prepared below crystalline temperature are uniform, well-ordered, and show high field effect transistor (FET) mobility. Meanwhile, π–π stacking direction of the TTP backbone is perpendicular to the microstripe and the molecules stack in layer structure, standing up on the SiO2/Si substrate, which would provide an effective pathway for p-type charge transport. However, higher preparation temperatures at liquid crystalline or isotropic liquid range induce many defects in the crystal formation process and cause incline of the unit cell, thus leading to a sharp decrease in FET mobility. A possible mechanism of molecular stacking at different temperature range is proposed. This strategy promised to provide a new opportunity for the high cost-efficiency fabrication of OFETs.
关键词： OFETs,smectic liquid crystals,terthiophene,alignment,patterning
更新于2025-09-23 15:23:52
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Liquid Crystals - Self-Organized Soft Functional Materials for Advanced Applications || Bent-Core Liquid Crystals: Structures and Mesomorphic Properties
摘要： Bent-core (BC) molecules became an attractive liquid crystal class due to their potential use in smart displays and photonic devices. In contrast to calamitic mesogens, bent-shaped mesogens are self-organized superstructures with remarkable properties, given the presence of polar order in mesophases, although the molecules themselves are not chiral. A particular interest represents the biaxial nematic liquid crystal materials that are used in display technology and allow a faster switching response, compared to calamitic liquid crystals, with considerably reduced costs. This chapter briefly reviews the bent-core liquid crystals with three different core units in the structure: (1) 2,5-disubstituted oxadiazole, (2) 1,3-disubstituted benzene, and (3) 2,7-disubstituted naphthalene. To the central bent units (BUs) containing reactive functional groups of phenolic or aminic type, various mesogenic groups are symmetrically or asymmetrically connected, via esterification or condensation reactions. The obtained compounds showed biaxial nematic and/or smectic mesophases with high transition temperatures in the case of oxadiazole derivatives or cholesteric and banana-type mesophases with lower transition temperatures in the case of benzene and naphthalene derivatives.
关键词： naphthalene,nematic,azomesogens,resorcinol,cholesteric,bent-core molecules,oxadiazole,biaxial,smectic,liquid crystals
更新于2025-09-23 15:22:29
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Phase transitions in nanofilms of polar smectic liquid crystals with multilayer periodicity
摘要： Optical investigations and calculation of structures were performed on ?lms of liquid crystal with the sequence of phase transitions in the bulk sample SmC??SmC??SmA?I. We investigated freestanding nano?lms with number of smectic layers from 2 to 6 with thickness commensurate, smaller, or greater than the period of SmC?d4 phases of the bulk sample. The number of phase transitions increases as the ?lm thickness increases. Under temperature change the transitions occur with conservation as well as with change of the direction of ?lm polarization with respect to the molecular tilt plane. In thick ?lms at high temperature the ?lm can be switched by electric ?eld from the state with longitudinal (parallel to the molecular tilt plane) into a state with transverse (perpendicular to the molecular tilt plane) electric polarization. In the framework of Landau theory of phase transitions, structures and phase transitions in the bulk sample and in nano?lms were calculated; values of interlayer interactions were estimated. The molecular structure was assumed to be planar with different sequences of synclinic and anticlinic orderings in nearest layers. The results of calculations are in good agreement with experiment.
关键词： multilayer periodicity,Landau theory,phase transitions,nano?lms,polar smectic liquid crystals
更新于2025-09-23 15:21:01