CN115626909A - Ferroelectric smectic phase A liquid crystal material and method for realizing switching and storage of two-state polarization structure - Google Patents
Ferroelectric smectic phase A liquid crystal material and method for realizing switching and storage of two-state polarization structure Download PDFInfo
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Abstract
The invention discloses a ferroelectric smectic phase A liquid crystal material and a method for realizing the switching and storage of a two-state polarization structure. The invention is based on ferroelectric smectic phase A liquid crystal materials. Such materials have ferroelectric properties and are capable of exhibiting macroscopic polarity. By applying an electric field, the liquid crystal polarization structure is switched, and the switched polarization structure can still stably exist after the electric field is removed, so that the switching and the storage of the two-state polarization structure are realized. Has wide application prospect in the field of liquid crystal information storage devices.
Description
Technical Field
The invention belongs to the field of liquid crystal optical materials and application, and discloses a method for realizing polarization structure switching and storage of ferroelectric smectic phase A liquid crystal. Has wide application prospect in the fields of data storage and the like.
Background
Ferroelectric materials (Ferroelectrics) are known for their hysteresis relationship between their electric polarization P and electric field strength E, which is similar to the hysteresis relationship between magnetization M and magnetic field strength H in ferromagnetic materials (ferrographics). The ferroelectric phenomenon was first observed in rosette salt by the french scientist Joseph Valasek in 1920. The ferroelectric material has spontaneous polarization, and the polarization direction can be turned under the action of an external electric field, and is the main characteristic of the ferroelectric material. Conventional solid-based ferroelectrics, such as barium titanate, lead zirconate titanate, etc., have long been one of the indispensable basic stones for modern information technology, and have been widely used in data storage, sensors, mechanical actuators, and optoelectronic applications.
In addition to conventional solid materials, researchers have been concerned with the presence of ferroelectricity in soft fluids such as liquid crystals. In 1916, the famous physicist Born proposed the assumption that there is a ferroelectric nematic order in liquid crystal, and supposing that liquid crystal molecules in rod shape have a dipole moment (μ > 6D) large enough, at this time, dipole-dipole interaction between molecules is enough to overcome thermal fluctuation, so that the direction of electric dipole of liquid crystal molecules can be spontaneously aligned to generate macroscopic polarity. And the ferroelectric order can exist stably. Unfortunately, it has not been experimentally verified that such ferroelectric nematic phases can exist. It is generally recognized that rod-shaped liquid crystal molecules may have longitudinal or transverse dipoles, but longitudinal and transverse polarizations may be prevented, respectively, due to their head-to-tail equivalent directions and free rotation about the long axis of the molecules, so that liquid crystals generally exhibit non-polarity.
Until 1974, the first ferroelectric liquid crystal material, p-decyloxy benzylidene para-amino-2-methyl butyl cinnamate (DOBAMBC), was synthesized by Meyer in cooperation with Keller. This is a chiral smectic C phase liquid crystal having a layered structure, the molecules in each layer are tilted at an angle to the layer normal, there is an azimuthal twist between the layers, corresponding to the long axis of the molecule spiralling in the direction of the layer normal, so that the liquid crystal material exhibits chiral characteristics with a helical pitch. The discovery has milestone significance in the liquid crystal research field and even in ferroelectric families, and opens a new direction for researching novel liquid crystal materials and devices, but the chiral smectic C-phase liquid crystal is in a semi-solid state due to a layered structure with higher order degree, is basically free of fluidity, generally has more defect structures, and is limited in technical development in the fields of flexible photoelectric devices and displays. Over the next thirty years, many series of ferroelectric lamellar phase Liquid Crystal materials were developed, but the problem of ferroelectric stacking order of nematic phase in Liquid Crystal could not be realized, and experimental verification could not be obtained until a century later, in 2017, professor Kikuchi Hirotsuku of Kyun university, japan, synthesized a novel Liquid Crystal molecule DIO with a strong Polar nematic phase structure (A Fluid Liquid-crystalline Material with high highlymar Polar order, adv. Mater.2017,29,1702354). It has ultrahigh dielectric constant, several hundred times greater than that of traditional liquid crystal and very strong second harmonic response. The liquid crystal material has macroscopic polarization, and liquid crystal molecules can form a ferroelectric domain with a specific polarization direction under the action of an electric field. Such liquid crystals having Ferroelectric characteristics are called Ferroelectric nematic (Ferroelectric) liquid crystals. The ferroelectric nematic liquid crystal material is the first liquid-based ferroelectric material, and its appearance has widely expanded the category of ferroelectric substances. Unlike conventional ferroelectric materials, it has both ferroelectric properties and high fluidity as a liquid material. The method has great potential for realizing thin, flexible and reconfigurable ferroelectric devices. However, although ferroelectric nematic liquid crystal materials exhibit ultra-high electric field sensitivity over conventional ferroelectrics, their spontaneous polarization structure can be changed under the drive of a small applied electric field. However, due to the self-flowing viscosity, elastic effect and surface polarization anchoring effect of the ferroelectric nematic liquid crystal, the spontaneous polarization of the ferroelectric nematic liquid crystal material tends to return to the initial state after the external electric field is removed, and the ferroelectric memory capability is difficult to show, so that the application of the ferroelectric nematic liquid crystal material in the fields of storage devices and the like is limited.
In order to explore the relationship between the structure of the Ferroelectric nematic liquid crystal molecules and the ferroelectricity thereof, researchers designed and synthesized a large number of liquid crystal molecules, and observed a Ferroelectric Smectic a phase (Ferroelectric Smectic a) among the related polar derivatives of the Ferroelectric nematic liquid crystal material. The ferroelectric smectic A phase liquid crystal material has a structure similar to that of a conventional smectic A phase liquid crystal, and rod-like molecules are arranged in layers. Within the layer, the long axes of the molecules are parallel to each other and perpendicular to the layer plane, i.e. parallel to the layer normal direction. The molecules can rotate around their long axes, with the layer thickness corresponding to the length of the molecule. On the basis, local polarization in the ferroelectric smectic A phase liquid crystal layer is oriented and ordered along the molecular long axis direction, and spontaneous polarization is formed among molecules through dipole-dipole interaction, so that the ferroelectric smectic A phase liquid crystal layer has macroscopic polarity (see figure 3 for details). The ferroelectric smectic a phase liquid crystal material can form a uniform plane orientation texture with a single domain, and as a ferroelectric material having fluidity, the ferroelectric smectic a phase liquid crystal has a sensitive response to an electric field, and can induce a response even with a very small field strength. The spontaneous polarization direction of the liquid crystal can be driven to turn by applying an in-plane electric field to the ferroelectric smectic A phase liquid crystal. What is more surprising is that the ferroelectric smectic A liquid crystal has a tightly packed layer structure, and has high elasticity and flowing viscosity, and can reach a certain stable state with surface polarization anchoring. Even if the electric field is removed, the switched polarization structure does not return to the initial state (E = 0), so that the switching and memorizing of the liquid crystal polarization information can be realized. Meanwhile, compared with the traditional solid ferroelectric material, the liquid crystal material has good fluidity, and the appearance of ferroelectric smectic phase A liquid crystal brings dawn for developing a novel flexible storage device based on liquid substances.
Disclosure of Invention
The invention discloses a ferroelectric smectic phase A liquid crystal material, which can perform spontaneous polarization on the basis of the traditional smectic phase A layered structure, shows macroscopic polarity and has stronger second harmonic response characteristics. It behaves as a ferroelectric nematic phase at high temperatures and enters the ferroelectric smectic a phase at low temperatures. The ferroelectric smectic phase A can sensitively respond to a tiny electric field when in a ferroelectric smectic phase A state, and the spontaneous polarization structure can be switched by applying an in-plane electric field. The changed polarization structure can still stably exist after the electric field is removed. Compared with the traditional solid-based ferroelectric body, the ferroelectric smectic phase A liquid crystal material has good fluidity. Has wide application prospect in the field of flexible storage.
The aim of the invention is achieved by the following measures:
a ferroelectric smectic A phase liquid crystal material having macroscopic polarity based on a conventional smectic A phase layered structure. The ferroelectric smectic phase A material liquid crystal molecules are in a rod-shaped form, a dioxane structure with an alkyl chain at the head of the molecule plays a role in electron donating, and fluorine substituent groups in the same direction and a cyano group at the tail end play a role in electron withdrawing, so that the molecules have large dipole moment. The very strong dipole-dipole interaction aligns the electric dipole directors of the molecules in the same direction, exhibiting macroscopic polarity, i.e., ferroelectricity; and the polarization direction can be switched under the drive of an electric field, and the switched polarization structure can stably exist, thereby showing the switching and storing capability of the two-state polarization structure.
Further, the ferroelectric smectic A phase liquid crystal material has ferroelectric properties (see the attached figures 1 and 2 for details), and can show spontaneous polarization characteristics.
Further, the ferroelectric smectic phase A material is a DIO polar derivative, referred to as DIO-CN for short, and the structure is as follows:
further, the ferroelectric smectic A phase liquid crystal material can spontaneously form a uniform ferroelectric single domain under the rubbing condition that the liquid crystal box alignment layer is uniform and homodromous.
Further, the switching and the storage of the polarization structure of the ferroelectric smectic phase A liquid crystal material are realized based on the ferroelectric polarization principle.
Further, the switching of the polarization structure of the ferroelectric smectic A phase liquid crystal material is realized by applying in-plane electric field drive, and the spontaneous polarization direction of the material is changed by applying in-plane electric field opposite to the polarization direction.
Further, the ferroelectric smectic-A liquid crystal material has nonlinear optical characteristics, shows strong second harmonic response, and has corresponding intensity variation of the second harmonic signal during the switching of the polarization structure.
Furthermore, the polarization structure of the ferroelectric smectic phase A liquid crystal material is switched under the drive of an electric field, so that the ferroelectric smectic phase A liquid crystal material has an obvious electric field threshold value, and the threshold value is very small | E | -0.0096V/mum.
Further, the polarization switching process of the ferroelectric smectic a phase liquid crystal material is affected by the surface anchoring polarization energy, and the electric field threshold has different magnitude when the rubbing direction of the polyimide alignment layer of the liquid crystal cell is the same as or opposite to the electric field direction.
Further, the storage of the polarization structure of the ferroelectric smectic phase A liquid crystal material after switching can be realized by the parallel or antiparallel orientation layer friction direction and the polarization direction after molecular switching.
Furthermore, when the polarization structure of the ferroelectric smectic phase A liquid crystal material is switched, the intensity of the second harmonic wave of the ferroelectric smectic phase A liquid crystal material is changed along with the switching of the polarization structure.
A ferroelectric smectic A phase liquid crystal realizes the method that the two state polarizes the structure to switch over and keep, regard a piece of interdigital electrode glass and a piece of ordinary quartz glass as the base plate to make the liquid crystal cell, rub and orient after the base plate surface spin-coats polyimide, the base plate friction direction of two sides points to the identity direction; an in-plane electric field is provided by an interdigital electrode external function generator, and the macroscopic polarization direction of the ferroelectric smectic phase A liquid crystal material in the liquid crystal box is switched along with the direction of the electric field.
Compared with the prior art, the invention has the following advantages and beneficial effects:
the ferroelectric smectic A phase material DIO-CN has ferroelectric characteristics, and compared with a common ferroelectric liquid crystal material, the smectic A phase material DIO-CN has a compact layered structure, so that the ferroelectric smectic A phase material has higher liquid crystal elasticity and flow viscosity, and can reach an equilibrium state with surface anchoring polarization, thereby realizing the switching and retention of polarization information. Compared with the traditional solid-based ferroelectric material, the polarization switching of the ferroelectric smectic phase A material has ultrahigh electric field sensitivity, and the ferroelectric smectic phase A material has the characteristics of softness and easy processing as a fluid soft material and has wide application prospect in the field of flexible storage devices.
Drawings
FIG. 1 shows the ferroelectric hysteresis loop of a ferroelectric smectic A phase liquid crystal material DIO-CN at 90 ℃.
FIG. 2 is a polarization current response curve of a ferroelectric smectic A phase liquid crystal material DIO-CN at 90 ℃.
FIG. 3 is a schematic diagram comparing a ferroelectric smectic A phase liquid crystal structure with a conventional smectic A phase liquid crystal structure.
The common smectic phase A liquid crystal is equivalent in head and tail, and has no macroscopic polarity. The ferroelectric smectic A phase liquid crystal is 'head-to-tail non-equivalent' and has macroscopic polarity.
FIG. 4 is a view showing a liquid crystal cell structure in which an in-plane electric field is applied to a ferroelectric smectic A phase liquid crystal via interdigital electrodes, and when the direction of the electric field is opposite to the direction of spontaneous polarization, the polarization direction is changed.
FIG. 5 is a diagram of a second harmonic detection path
FIG. 6 shows the second harmonic intensity variation of DIO-CN at 90 deg.C (ferroelectric smectic A phase) driven by a periodic triangular wave electric field, where the straight line is the triangular wave electric field and the hollow dots are the second harmonic intensity.
Detailed Description
The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto, and may be carried out with reference to conventional techniques for process parameters not particularly noted.
Example 1
The method for forming the uniform plane orientation texture method of the ferroelectric smectic A phase liquid crystal in the liquid crystal box comprises the following steps:
two glass substrates (1X 1 cm) were prepared 2 ) One of the glass substrates is provided with an interdigital electrode formed by an ITO layer. After spin-coating polyimide films on both substrates, the substrates were uniformly rubbed with velvet cloth for orientation. The thickness of the liquid crystal cell can be determined using 5 μm spacer beads. Since the orientation of ferroelectric liquid crystal is affected by surface polarization energy, the rubbing direction can guide the direction of electric dipole director of liquid crystal molecules, and thus the rubbing directions of the two substrates need to be parallel and point to the same direction.
The ferroelectric smectic A phase liquid crystal is heated to melt and the liquid crystal wicks into the cell as shown in FIG. 4. The ferroelectric smectic phase A liquid crystal material DIO-CN has the following structure:
the temperature controller is used for heating and raising the temperature to enter a nematic phase, and then the temperature is slowly lowered from the nematic phase to a ferroelectric smectic phase A, so that the DIO-CN can form a uniform plane orientation texture, namely, the molecular electric dipole points are oriented in the same direction along the sagittal direction, and a ferroelectric single domain is formed.
Example 2
The ferroelectric smectic phase A liquid crystal two-state polarization structure switching and storing method is realized as follows:
in the homodromous parallel orientation liquid crystal box, a periodic triangular wave electric field is applied to the ferroelectric smectic phase A liquid crystal material through the interdigital electrode, the direction of the electric field is parallel to the friction direction, and the switching and the storage of a polarization structure are realized.
The first stage is as follows: the method comprises the following steps of applying a gradually increased positive electric field (E & gt 0, E = 0-0.07V/mum), wherein the polarization same as the direction of the positive electric field is not changed, the polarization opposite to the direction of the positive electric field starts to rotate towards the direction of the applied electric field after the electric field exceeds a threshold value, until the polarization direction is the same as the direction of the electric field, in the second stage, the positive electric field starts to gradually decrease to E =0, the polarization structure formed after switching is kept stable and does not change along with the decrease of the electric field, in the third stage, applying a gradually increased negative electric field (E & lt 0,E = -0.07-0V/mum), when the field strength exceeds the threshold value, if the polarization direction of the ferroelectric smectic A phase liquid crystal material is consistent with the direction of the negative electric field, the polarization structure does not change, and the polarization direction is opposite to the direction of the negative electric field, the liquid crystal molecules rotate towards the direction of the applied electric field until the polarization structure is parallel to the direction of the electric field, in the fourth stage, the negative electric field starts to gradually decrease to E =0, and the polarization structure formed after switching is kept stable and does not change along with the decrease of the electric field.
Example 3
The switching and storage of the double-state polarization structure of the second harmonic detection ferroelectric smectic phase A liquid crystal material are realized as follows:
the switching and the storage of the polarization structure are realized by applying a periodical triangular wave electric field in a plane to the ferroelectric smectic phase A liquid crystal material through the interdigital electrode, and the process can be observed by detecting the intensity change of a second harmonic signal.
The laser light source uses 1064nm pulse laser, which can generate 532nm second harmonic wave due to the nonlinear optical characteristics of ferroelectric smectic A phase liquid crystal, and the photomultiplier is used to detect the emergent second harmonic wave (see figure 5).
When the polarization direction is parallel to the polarization direction of the laser, the second harmonic signal detected by the photomultiplier is strongest. The second harmonic detected by the photomultiplier is the weakest when the polarization direction is perpendicular to the laser polarization direction (see fig. 6).
The above examples are preferred embodiments of the present invention, but the present invention is not limited to the above examples. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall fall within the scope of the invention.
It should be understood that although the terms first, second, etc. may be used herein to describe various information in one or more embodiments of the specification, these information should not be limited by these terms, which are used only for distinguishing between similar items and not necessarily for describing a sequential or chronological order of the features described in one or more embodiments of the specification. Furthermore, the terms "having," "including," and similar referents, are intended to cover a non-exclusive scope, such that a process, method, system, article, or apparatus that comprises a list of steps or modules is not necessarily limited to the particular details set forth, but may include other inherent information not expressly listed for such steps or modules.
Claims (10)
1. A ferroelectric smectic A phase liquid crystal material, characterized in that, the ferroelectric smectic A phase material has macroscopic polarity based on the traditional smectic A phase layered structure, the dioxane structure with alkyl chain at the head of the ferroelectric smectic A phase material acts as electron donating, the equidirectional fluorine substitution and the terminal cyano group act as electron withdrawing, so that the molecules have large dipole moment, strong dipole interaction, the molecules are arranged in the same direction, and the macroscopic polarity, namely ferroelectricity is displayed; and the polarization direction can be switched under the drive of an electric field, and the switched polarization structure can stably exist, thereby showing the switching and storing capability of the two-state polarization structure.
2. The ferroelectric smectic a liquid crystal material of claim 1, wherein the ferroelectric smectic a material is a DIO polar derivative, DIO-CN for short, and has the following structure:
3. the ferroelectric smectic phase a liquid crystal material as claimed in claim 1, wherein the ferroelectric smectic phase a liquid crystal material is capable of spontaneously forming uniform ferroelectric monodomains under uniform homeotropic rubbing conditions of the alignment layer.
4. A ferroelectric smectic a liquid crystal material as claimed in claim 1, wherein the switching and storing of the polarization structure of the ferroelectric smectic a liquid crystal material is based on ferroelectric polarization.
5. The ferroelectric smectic A liquid crystal material as claimed in claim 1, wherein the switching of the polarization structure of the ferroelectric smectic A liquid crystal material is driven by application of an in-plane electric field.
6. The ferroelectric smectic phase A liquid crystal as claimed in claim 1, wherein the ferroelectric smectic phase A liquid crystal material has a strong second harmonic response with a corresponding change in intensity of the second harmonic signal during switching of the polarization structure.
7. A ferroelectric smectic A liquid crystal material as claimed in claim 5, wherein the ferroelectric smectic A liquid crystal material exhibits a sharp electric field threshold with a small threshold of | E | to 0.0096V/μm due to switching of the polarization structure under the drive of an electric field.
8. A ferroelectric smectic A liquid crystal material as claimed in claim 5, wherein the polarization switching process of the ferroelectric smectic A liquid crystal material is effected by surface anchoring conditions, and the electric field threshold values have different magnitudes when the rubbing direction of the polyimide alignment layer of the liquid crystal cell is the same as or opposite to the electric field direction.
9. A ferroelectric smectic A liquid crystal material as claimed in claim 5, wherein the switched polarization structure of the ferroelectric smectic A liquid crystal material is preserved by aligning the rubbing direction of the alignment layer to be parallel or antiparallel to the polarization direction after molecular switching.
10. The method for realizing the switching and storage of the bimodal polarization structure of the ferroelectric smectic a phase liquid crystal according to claims 1 to 9, wherein an interdigital electrode glass and a common quartz glass are used as the substrates to make the liquid crystal cell, and the surfaces of the substrates are spin-coated with polyimide and then rubbing-aligned in the same direction; an in-plane electric field is provided by an interdigital electrode external function generator, and the macroscopic polarization direction of the ferroelectric smectic phase A liquid crystal material in the liquid crystal box is switched along with the direction of the electric field.
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| WO2023250192A3 (en) * | 2022-06-23 | 2024-02-08 | The Regents Of The University Of Colorado, A Body Corporate | Ferroelectric smectic a phase materials, devices including the materials, and methods of forming and using same |
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