JPH02160220A - Preparation of organic thin film single crystal - Google Patents

Abstract

PURPOSE:To obtain a org. thin film single crystal having superior quality and large area and to permit to control an orientation of an axis of polarity of the thin film single crystal by forming a crystal nucleus at >=40 deg.C using polar molecules having >=4 debye dipole moment, as solute. CONSTITUTION:A crystal nucleus is formed at >=40 deg.C when an org. thin film single crystal is prepd. from a soln. of an org. compd. inserted between substrates by a temp. lowering method or a slow evaporation method of a solvent, by regulating the org. compd. to a polar molecule having >=4 debye dipole moment. Thus, an org. thin film single crystal having superior quality a large breadth and a relatively large area and, an axis of polarity oriented in a face is obtd.

Description

[Detailed Description of the Invention] [Industrial Application Fields] The present invention is applicable to industrial fields that require high-quality organic thin film single crystals;
For example, the present invention relates to a method for producing organic thin film single crystals useful in the fields of optics, information processing, and communications.

[Conventional technology] Conventionally, the main method for producing thin film-like organic single crystals has been (
There are a) melt method and (b) solution method. In order to obtain a thin film-like single crystal, a method has been used in which a melt or solution is sandwiched between glass substrates and crystallized by lowering the temperature or by slow evaporation of the solvent.

(b) In the case of the melting method, there are restrictions such as using an organic compound that has a melting point and does not decompose or change in quality even in a molten state for a long time, but it is relatively easy to increase the area and is short. It has the characteristic that crystals can be produced in a short period of time. However, because the coefficient of thermal expansion of a substrate such as glass and that of an organic compound are significantly different, the large temperature difference between the crystallization temperature and room temperature can easily cause cracks in the grown thin film single crystal. There was a problem in that it was difficult to obtain a thin film single crystal.

(b) In the case of the solution method, there are fewer problems of thermal decomposition and thermal alteration as in the case of the melt method, and cracks due to the significantly different thermal expansion coefficients of the substrate and organic compound and the large temperature difference between the crystallization temperature and room temperature. There were no problems, and the thin film single crystals grown were of good quality, but due to the fact that the solute concentration was not high, the size was small, and the plane orientation (i.e. polar axis orientation) of the individual thin films grown was not constant. Generally, there was a problem that it was random.

Regarding the polar axis orientation control of thin film single crystal growth, there are two methods: 1. Select a substrate and perform epitaxial growth (liquid phase method); 2. Control orientation using an external field such as an ion beam (vapor phase method).
There have been attempts such as , , etc., but all of them are at the exploratory stage and the technology has not yet been established.

[Problems to be Solved by the Invention] The present invention aims to eliminate the drawbacks of the existing organic thin film single crystallization methods described above, and is capable of (1) achieving high quality and large area; (2) further improving the polar axis ( The present invention aims to provide a simple method for producing an organic thin film single crystal having an in-plane polar axis whose orientation can be controlled.

[Means for Solving the Problems] In order to achieve the above object, the present invention has the following configuration.

[In a method for producing an organic thin film single crystal from a solution of an organic compound sandwiched between substrates by temperature reduction or slow evaporation of a solvent, A. the organic compound is 4
It is characterized by being a polar molecule having a dipole moment greater than or equal to Debye, B, by performing crystal nucleation at a temperature of 40°C or higher, and by producing a thin film single crystal in which the polar axis is substantially within the plane of the thin film. A method for manufacturing an organic thin film single crystal with an in-plane polar axis. ” The feature of the present invention is that by forming a crystal nucleus at 40°C or higher using a polar molecule with a dipole moment of 4 Debye or more as a solute, ■ selectively forming a high-quality thin film single crystal with the polar axis in the plane. The following points have been made possible: (1) The area of the obtained thin film single crystal has been made larger.

The polar axis of a crystal here refers to the axis formed by the vector sum of the dipole moments of molecules, and exists only in polar crystals, that is, non-centrosymmetric crystals. Such non-centrosymmetric crystals are used, for example, in nonlinear optics.
It is useful as a second-order nonlinear optical material having harmonic generation, -order electro-optic effects, etc.

When crystal nuclei are formed at room temperature as has been conventionally done, single crystals having various polar axis orientations with respect to the thin film surface are precipitated. In addition, in a single crystal that grows by chance and has a polar axis within the plane of a thin film, the crystal growth rate in the direction of the polar axis is much higher than that in the direction perpendicular to the polar axis, so the thin film single crystal becomes narrow and needle-shaped. It's easy.

In contrast, in the present invention, a high-quality thin film single crystal having a relatively large area of polar axis in the plane can be selectively obtained. At this time,
Since the solute must be a polar molecule with a dipole moment of 4 Debye or more, dipole-dipole interactions between molecules and between substrate and molecules work during crystal nucleation at 40°C or higher, resulting in a constant dipole moment. It is assumed that oriented crystal nucleation occurs and plays a major role in the growth of thin film single crystals with in-plane polar axes. In addition, the reason why thin film single crystals with a relatively large area can be grown is that a higher solute concentration can be applied by increasing the temperature, and that the anisotropy of the growth rate is smaller. This is presumed to be due to increasing the crystal growth rate in the direction perpendicular to the polar axis direction relative to the crystal growth rate of .

However, when setting the temperature, care must be taken to prevent cracks in the thin film single crystal. In other words, it is necessary to set the temperature to a temperature lower than the melting point of the crystal and lower than the boiling point of the solvent, to ensure optimal crystal growth. The temperature is usually below 100°C when an inorganic material is used as the substrate, and below 200°C when an organic material is used.

The substrate used in the thin film single crystallization of the present invention may be of any material, such as metals, inorganic materials (semiconductors, etc.), organic materials, and composite materials thereof, as long as they are not affected by solutions. A glass substrate is useful in consideration of solvent properties.

The substrate material may be either crystalline or amorphous. If a crystalline substrate, especially a substrate having a lattice constant that matches well with a thin film single crystal, is selected, so-called heteroepitaxial growth may occur with respect to crystal growth.

Both slow evaporation and temperature drop methods are effective methods for forming and growing crystal nuclei, but rapid crystal nucleus formation and growth must be avoided. That is,
It is necessary to gradually form crystal nuclei at a temperature of 40° C. or higher. Furthermore, at the stage of crystal growth, if the crystal is grown rapidly, good results may not be obtained due to dislocation, formation of crystal nuclei having new random polar axis orientations, and the like.

Examples of compounds that form organic thin film single crystals, that is, polar molecules having a dipole moment of 4 debyes or more in the present invention, include π-electron conjugated compounds in which donor groups and acceptor groups are introduced at both ends. .

The dipole moment can be measured by the solution method described in Experimental Chemistry Course, page 3.38 (1957, Maruzen).

In the present invention, the donor group refers to Hammett's substituent constant σp
and satisfies σp<Q, and examples of halogen include amino, alkoxy, hydroxy, and alkylthionyl. In addition, the acceptor group is one that satisfies σp>Q with Hammett's substituent constant σp,
Examples of halogens include nitro, cyano, acetyl, and formyl.

Selecting a solute molecule whose dipole moment direction coincides with the long sleeve direction of the molecule is more effective in realizing the polar axis orientation control of the present invention. In this sense, the π-electron conjugated system into which the donor group and acceptor group are to be introduced is preferably a benzylidene aniline derivative, a stilbene derivative, etc., rather than a monocyclic system such as benzene.

Therefore, examples of preferred compounds include 4'nitrobenzylidene-3-acetamino-4-methoxyaniline,
4'-nitropenzylidene-4-methylaniline, 4'
- benzylidene aniline derivatives such as ditropendylidene-4-iodoaniline, 4-methoxy-3-methyl-
Examples include stilbene derivatives such as 4'-nitrostilbene and 4-bromo-4'-nitrostilbene.

The solvent to be used is: 1) a good solvent that does not dissolve the substrate, 2) has a moderately high boiling point, and 2) has good stability and has a polar axis in the plane. It is preferable.

As already mentioned, a good solvent is effective in improving the solution concentration and increasing the area of the thin film. A moderately high boiling point refers to a temperature higher than the temperature at which crystal nuclei are formed, preferably a temperature higher than the crystal nucleus formation temperature by 60° C. or more. In this sense, a moderately high boiling point refers to 100°C or higher.

That is, it is particularly preferable that the crystallization solvent is highly polar, hydrogen bonding, and has a relatively high boiling point. Solvents containing fragments of carbonyl, thiocarbonyl, ether, amino (-NH-), etc. in the molecule are effective; specific examples include N,N-dimethylformamide, dimethylsulfoxide, N,N-dimethylacetamide, Examples include N-methylpyrrolidone.

In the present invention, the thickness of the organic compound solution when it is sandwiched between the substrates (before temperature drop or slow evaporation) is generally determined by: temperature, etc.) and
It is determined by the wettability (affinity) between the solution and the substrate, the pressure acting between the substrates (determined by the weight and area of the substrate), and the surface smoothness of the substrate. Although it may exist, it is usually in the range of submicrons to several tens of microns, and preferably 0.01 μ to 100 μ. The simplest method is to change the factor ■,
Another method is to change the factor (2) using a spacer, but there are limits to its applicability. Therefore, it is preferable to change the factor (2) by a method such as surface treatment of the substrate, or to change the factor (2).

[Examples] The present invention will be described below using Examples, but the effectiveness of the present invention is not limited in any way by these Examples.

Example 1 A thin film single crystal of 4'-nitropenzylidene-3-acetamino-4-methoxyaniline (MNBA) having an in-plane polar axis was prepared by the method shown below.

Dissolve 50 mg of yellow powder crystals of MNBA in N,N-dimethylformamide (DMF) lcc, and
1 was sandwiched between 10 cm×locm soda glass substrates that had been washed with alkali and left to stand at 50° C. for 2 days, and thin film crystals were precipitated on the sides due to slow evaporation of the solvent. The maximum size of the thin film crystal is 1゜2 mrn x 15 mm N film thickness 80
00 angstroms. It was found by observation with a polarizing microscope and X-ray analysis that the thin film crystal is a single crystal, and all its plane orientations are the same, and that this crystal plane is an AC plane including a polar axis.

Furthermore, it was found that the polar axis was in-plane rotated by about 20 degrees from the growth axis direction of the thin film single crystal. Therefore, it was found that the width expansion in the thin film single crystal (see Comparative Example 1) was due to the relative promotion of growth in the direction perpendicular to the polar axis.

Example 2 4-methoxy-3-methyl-4-m=trostilbene (
A thin film single crystal of MMNS) having an in-plane polar axis was prepared by the following method.

150 mg of yellow powder crystals of MMNS was dissolved in 1 cc of N-methylpyrrolidone (N'MP), and 10
When μl was sandwiched between 10 cm×10 cm soda glass substrates that had been washed with alkali and left at 60° C. for 4 days, thin film crystals were precipitated on the sides due to slow evaporation of the solvent. The size of the thin film crystal is 2.4 mm x 15 mm at its largest, and the film thickness is 1.5 mm.
It was μm. It was found from observation with a polarizing microscope and X-ray analysis that the thin film crystal is a single crystal, and all its plane orientations are the same, and that this crystal plane is an AC plane including a polar axis.

Furthermore, it was found that the polar axis was in-plane rotated by about 30 degrees from the direction of elongation of the thin film single crystal. Therefore, it was found that the expansion of the width of the thin film single crystal (see Comparative Example 2) was the result of relatively promoting growth in the direction perpendicular to the polar axis.

Example 3 A thin film single crystal of MNBA having an in-plane polar axis was produced by the following method.

120 mg of yellow powder crystals of MNBA was dissolved in 1 cc of N,N-dimethylformamide (DMF) at about 85°C, and 10 μl of this was dissolved in 1 cc of alkali-washed solution.
Sandwiched between mX 10 cm soda glass substrates, 0.62
When the sample was slowly cooled from 80°C to 25°C at a cooling rate of 5°C/h (about 4 days), several thin film crystals were precipitated. During this time, it was observed that crystal nuclei were formed at a temperature of 60°C.

The maximum size of the thin film crystal is 2. 2mm×2
The film thickness was 4 mm and the film thickness was 1.1 μm. A thin film crystal is a single crystal, and all its planes have the same orientation, and observation with a polarizing microscope and X
This was revealed by line analysis.

The polar axis is in-plane rotated by about 20 degrees from the growth axis direction of the thin film single crystal, and the width of the thin film single crystal increases (
Comparative Example 3) was similar to Example 1 in that the growth in the direction perpendicular to the polar axis was also relatively promoted.

Comparative Example 1 The solution was kept at room temperature (approximately 25°C) and kept for 3 days.
When the same experiment as in Example 1 was carried out except that the time was changed for 1 day, thin film crystals were deposited on the 10-odd side due to slow evaporation of the solvent. As in Example 1, it was found that all of these were single crystals.

However, the thin film single crystal is elongated compared to that in Example 1, with a maximum size of 200 μm x 10 mm and a film thickness of 900 μm.
It was 0 angstrom.

In addition, when the thin film surface orientation was investigated by conoscope observation using a polarizing microscope and X-ray analysis, it was found that the surface orientation was not constant.
It was found that there was a mixture of types.

Comparative Example 2 An experiment similar to Example 2 was conducted, except that the solution was held at room temperature (approximately 25°C) and held for 7 days. Crystals precipitated. As in Example 2, it was found that all of these were single crystals.

The precipitated thin film single crystals had an elongated shape and a plate-like shape, and when the thin film surface orientation was examined by conoscope observation using a polarizing microscope and X-ray analysis, there were three types of surface orientation. There are two types of elongated thin films, one of which uses the AC plane that includes the polar axis as the thin film surface.
Maximum size is 400 μm x 15 mm, film thickness 1.2
It was μm.

Comparative Example 3 Using a DMF solution of MNBA (5 mg/cc), from room temperature (about 25°C) to 5°C (about 4 days), about 0.208
When the sample was slowly cooled at a temperature decreasing rate of 0.degree. C./h, thin film crystals were precipitated on the sides. All of these were single crystals. However, the thin film single crystal is elongated compared to that in Example 3, with a maximum size of 100 μm x 40 rn m and a film thickness of 70 μm.
00 angstroms. It was also found that there are five types of thin film plane orientations.

[Effects of the Invention] According to the present invention, a wide and relatively large area, a polar axis in the plane, and a high quality organic thin film single crystal can be easily obtained, and can be used for information processing such as optics, nonlinear optics, acousto-optics, etc. This can be widely used in the processing and communication fields.

Claims (1)

[Claims] (1) In a method for producing an organic thin film single crystal from a solution of an organic compound sandwiched between substrates by temperature reduction or slow evaporation of a solvent, A. the organic compound has a polarity having a dipole moment of 4 debyes or more; B. Forming crystal nuclei at a temperature of 40° C. or higher, producing a thin film single crystal in which the polar axis is substantially in the plane of the thin film. A method for producing organic thin film single crystals.