A raman method for characterizing domain structure of molecular ferroelectric materials

Abstract

This invention discloses a Raman method for characterizing the domain structure of molecular ferroelectric materials. The method includes the following steps: Step 1: Determine the crystal growth direction and crystal planes of the material; Step 2: Acquire variable-temperature Raman spectra; Step 3: Analyze the variable-temperature Raman spectra to determine the Raman vibrational modes closely related to polarization; Step 4: Calculate the angle between the characteristic Raman vibrational modes and the crystal growth direction; Step 5: Construct a polarization Raman testing system and acquire Raman spectra at different polarization angles; Step 6: Process the angle-dependent polarization Raman spectra; Step 7: Plot the "angle-peak intensity (peak width or peak position)" diagram of the characteristic Raman peaks; Step 8: Fit the data from Step 7 to determine the angle between the characteristic Raman vibrational modes and the crystal growth direction; Step 9: Perform polarization Raman imaging at the angle where the peak intensity (peak width or peak position) reaches its extreme value. This invention effectively fills the gap in non-contact, non-destructive characterization methods for the domain structure of molecular ferroelectric materials.

Description

Technical Field

[0001] This invention belongs to the field of domain structure characterization technology of molecular ferroelectric materials, and specifically relates to a characterization method based on polarized Raman scattering technology that enables non-contact and non-destructive visualization of the domain structure of molecular ferroelectric materials. Background Technology

[0002] Ferroelectric materials are widely used in non-volatile memories and microelectromechanical systems (MEMS) due to their unique polarization properties. In ferroelectrics, regions with identical spontaneous polarization are called domains, and studying ferroelectric polarization is equivalent to studying ferroelectric domains.

[0003] In recent years, with the surge in demand for flexible electronic devices, molecular ferroelectric materials, which are flexible, lightweight, environmentally friendly, and whose performance is comparable to that of inorganic ferroelectrics, have become increasingly popular.

[0004] Piezoelectric microscopy is currently the most widely used technique in the field of molecular ferroelectric domain structure research. However, its limitations, such as its contact and destructive testing capabilities and the difficulty of operation under external fields, have hindered its further application in domain structure research.

[0005] Raman spectroscopy is a molecular structure characterization technique based on the Raman effect. By analyzing the intensity, width, and position of Raman scattering peaks, various sample properties associated with the structure can be revealed. Confocal microRaman spectroscopy is a non-contact, non-destructive detection method with high spatial resolution. It requires no special sample preparation and allows for three-dimensional sample detection. Furthermore, it can acquire Raman spectra of samples under external fields (such as polarization, force fields, and temperature fields), greatly compensating for the shortcomings of existing domain structure characterization techniques. Polarized Raman spectroscopy is a technique that collects Raman spectra of materials under applied polarization conditions. It is achieved by selectively measuring Raman scattered light parallel or perpendicular to the polarization direction of the incident light. Polarized Raman spectroscopy has been widely used in the study of domain structures in inorganic ferroelectric materials, but research on domains in molecular ferroelectric materials remains lacking. Therefore, inventing a non-contact, non-destructive method for characterizing the domain structure of molecular ferroelectric materials is of great significance. Summary of the Invention

[0006] Purpose of the invention: This invention addresses the shortcomings of existing technologies in the field of domain structure characterization of molecular ferroelectric materials by providing a non-contact, non-destructive, high-resolution, and three-dimensional Raman method for characterization.

[0007] The technical solution of the present invention is as follows: A Raman method for characterizing the domain structure of molecular ferroelectric materials, comprising the following steps:

[0008] Step (1): Determine the crystal growth direction and crystal plane of the molecular ferroelectric material;

[0009] Step (2): Build a variable temperature Raman testing system, place the molecular ferroelectric material on the surface of the variable temperature Raman testing system, and collect the Raman spectra of the ferroelectric domains at different temperatures in situ.

[0010] Step (3): Analyze the variable-temperature Raman spectrum from step (2) to determine the characteristic Raman vibration modes closely related to ferroelectric spontaneous polarization;

[0011] Step (4): Calculate the angle between the characteristic Raman vibration mode determined in step (3) and the crystal growth direction;

[0012] Step (5): Build an angle-dependent polarization Raman testing system, place the molecular ferroelectric material on the surface of the angle-dependent polarization Raman testing system, and collect the polarization Raman spectra of the ferroelectric domains at different angles in situ.

[0013] Step (6): Perform data processing and analysis on the angle-dependent polarization Raman spectra of the ferroelectric domains obtained in step (5) to obtain the peak intensity, peak width, and peak position of the characteristic Raman peaks determined in step (3) under different polarization angles.

[0014] Step (7): Organize and plot the data obtained in step (6) to obtain the "angle-peak intensity (peak width or peak position)" relationship diagram;

[0015] Step (8): Fit the data from step (7), analyze the fitting results, and determine the angle between the above-mentioned characteristic Raman vibration mode and the crystal growth direction. Compare the experimental results with the theoretical calculation results in step 4. If the results are within the allowable error range, the experiment is correct.

[0016] The temperature range for the variable-temperature Raman spectroscopy acquisition in step (2) covers the phase transition temperature of molecular ferroelectric materials.

[0017] The criteria for judging the characteristic Raman vibration mode closely related to ferroelectric spontaneous polarization in step (3) are: the Raman peak corresponding to the Raman vibration mode changes significantly in peak position, peak intensity and peak width before and after the phase transition, and the Raman peak appears independently.

[0018] In step (5), the change of polarization angle is achieved in two ways: one is by fixing the sample and adjusting the polarization direction of the incident light and the scattered light; the other is by fixing the polarization direction of the incident light and the scattered light and rotating the sample.

[0019] In step (7), the relationship diagram uses the angle between the crystal growth direction and the polarization direction of the incident light as the x-axis and the Raman peak intensity (peak width or peak position) corresponding to the angle as the y-axis.

[0020] The method for visualizing ferroelectric domains based on the Raman method for characterizing the domain structure of molecular ferroelectric materials comprises the following steps:

[0021] Step (1): For the characterization of two non-180° ferroelectric domains, the molecular ferroelectric material is placed at the angle when the peak intensity (peak width or peak position) obtained in the Raman method is at its extreme value, and polarization Raman imaging is performed to visualize the ferroelectric domain structure.

[0022] Step (2): For the characterization of two non-180° ferroelectric domains, the peak intensity (peak width or peak position) data obtained from the Raman method should be divided to create an "angle-peak intensity ratio (peak width ratio or peak position ratio)" relationship graph.

[0023] Step (3): Fit the obtained data, analyze the data, determine the angle between the above-mentioned characteristic Raman vibration mode and the crystal growth direction, compare the experimental results with the theoretical calculation results, and if the error is within the acceptable range of the experiment, then the experiment is valid.

[0024] Step (4): Place the molecular ferroelectric material at the angle where the peak intensity ratio (peak width ratio or peak position ratio) obtained in step (2) reaches its extreme value, and perform polarization Raman imaging to visualize the ferroelectric domain structure.

[0025] In step (2), the curve is plotted with the angle between the crystal growth direction and the incident light polarization direction as the x-axis and the peak intensity ratio (peak width ratio or peak position ratio) corresponding to the angle as the y-axis.

[0026] Beneficial effects

[0027] Compared with existing molecular ferroelectric domain structure characterization techniques, this invention fills the gap in non-contact, non-destructive characterization methods. It not only has the advantages of high spatial resolution and three-dimensional detection, but can also be applied under superimposed external field conditions. Attached Figure Description

[0028] Figure 1 The temperature evolution Raman spectrum of the characteristic Raman peak in the example of this invention;

[0029] Figure 2 This is a schematic diagram of the angle-dependent polarization Raman experimental setup in an example of the present invention;

[0030] Figure labels: 1. Spectrometer, 2. Confocal microscope, 3. Analyzer, 4. Polarization direction of incident light, 5. Laser, 6. Dichroic mirror, 7. Sample stage, 8. Polarization direction of incident light;

[0031] Figure 3 This is a polar coordinate diagram of the "angle-peak intensity" of the characteristic Raman peak in Example 2 of the present invention;

[0032] Figure 4 This is a graph showing the "angle-peak intensity ratio" of the characteristic Raman peak in Example 3 of the present invention.

[0033] Figure 5This is a Raman image of a domain other than 180° in Example 3 of the present invention. Detailed Implementation

[0034] A Raman method for characterizing the domain structure of molecular ferroelectric materials includes the following steps:

[0035] Step 1: Determine the crystal growth direction of the molecular ferroelectric material.

[0036] Step 2: Set up a variable-temperature Raman testing system, place the molecular ferroelectric material on the surface of the variable-temperature Raman testing system, and collect the Raman spectra of the ferroelectric domains at different temperatures in situ.

[0037] Step 3: Analyze the temperature-varying Raman spectrum from Step 2. The Raman vibration modes corresponding to Raman peaks that show significant changes in peak intensity, peak width, and peak position before and after the phase transition and appear independently are identified as characteristic Raman vibration modes closely related to spontaneous polarization.

[0038] Step 4: Calculate the angle between the characteristic Raman vibration mode determined in Step 3 and the crystal growth direction.

[0039] Step 5: Set up an angle-dependent polarization Raman testing system. Place the molecular ferroelectric material on the surface of the angle-dependent polarization Raman testing system and collect the polarization Raman spectra of the ferroelectric domains at different angles in situ. The change of polarization angle can be controlled in two ways: one is to fix the sample and adjust the polarization direction of the incident light and the scattered light; the other is to fix the polarization direction of the incident light and the scattered light and rotate the sample.

[0040] Step 6: Perform data processing and analysis on the angle-dependent polarization Raman spectra of the ferroelectric domains obtained in Step 5 to obtain the peak intensity, peak width, and peak position data of the characteristic Raman peaks determined in Step 3 under different polarization angles.

[0041] Step 7: Organize the data obtained in Step 6, and plot the "angle-peak intensity (peak width or peak position)" relationship graph with the angle between the crystal growth direction and the polarization direction of the incident light as the x-axis and the Raman peak intensity (peak width or peak position) corresponding to the angle as the y-axis.

[0042] Step 8: Fit the data from Step 7, analyze the fitting results, and determine the angle between the aforementioned characteristic Raman vibration mode and the crystal growth direction. The experimental result is the same as the theoretical calculation result from Step 4.

[0043] Step 9: Place the molecular ferroelectric material at the angle where the peak intensity (peak width or peak position) obtained in Step 8 reaches its extreme value, and perform polarization Raman imaging to visualize the ferroelectric domain structure.

[0044] Step 10: For the characterization of non-180° ferroelectric domains, the peak intensity (peak width or peak position) data obtained in Step 6 should be divided, and the angle between the crystal growth direction and the polarization direction of the incident light should be taken as the x-axis, and the Raman peak intensity (peak width or peak position) corresponding to the angle should be taken as the y-axis to draw the "angle-peak intensity ratio (peak width ratio or peak position ratio)" relationship graph.

[0045] Step 11: Fit the data from Step 10, analyze the data, and determine the angle between the aforementioned characteristic Raman vibration mode and the crystal growth direction. The experimental result is the same as the theoretical calculation result from Step 4.

[0046] Step 12: Place the molecular ferroelectric material at the angle where the peak intensity ratio (peak width ratio or peak position ratio) obtained in Step 11 reaches its extreme value, and perform polarization Raman imaging to visualize the ferroelectric domain structure.

[0047] The following section uses the molecular ferroelectric material TMCMCdCl3 as an example, and, in conjunction with the accompanying drawings, illustrates the specific characterization process of this invention applied to non-180° ferroelectric domains:

[0048] Example 1

[0049] A Raman method for characterizing the domain structure of molecular ferroelectric materials, the preparation method of the TMCMCdCl3 thin film used in this embodiment specifically includes the following steps:

[0050] Step 1: Pretreatment of the indium tin oxide-glass substrate: The indium tin oxide-glass substrate (1.0×1.0cm) after being soaked, washed and dried with ethanol was placed in an ozone generator for 30 minutes to improve the hydrophilicity of the substrate.

[0051] Step 2: Take an appropriate amount of TMCMCdCl3 crystal powder and deionized water to prepare a precursor solution with a concentration of 20 mg / mL, and centrifuge and shake to make the solution uniform;

[0052] Step 3: Uniformly drop 20 μL of the precursor solution from Step 2 onto the indium tin oxide surface of the substrate treated in Step 1;

[0053] Step 4: Place the thin film sample from Step 3 on a constant temperature hot plate at 50°C and grow for 10 min to obtain the TMCMCdCl3 thin film.

[0054] Example 2

[0055] A Raman method for characterizing the domain structure of molecular ferroelectric materials. In this example, the confocal Raman spectrometer used is an Alpha 300 confocal Raman spectrometer from WITEC, employing a backscatter configuration. The characterization of non-180° domains in the TMCMCdCl3 prepared in Example 1 includes the following steps:

[0056] Step 1: Determine that the growth direction of the crystal on the thin film is along the c-axis of the crystal. PXRD test results show that its crystal plane is (110) plane.

[0057] Step 2: Set up a variable-temperature Raman spectroscopy acquisition system. The incident light source is a 532nm laser, the microscope magnification is 100X, the grating is 600gr / mm, and the spectral acquisition range is set to 70-3100cm². -1 The TMCdCl3 thin film was placed on an automatic temperature-controlled platform, and the platform temperature was maintained between 298K and 443K using temperature control software. The heating rate of the platform was set to 10K / min. The laser power was set to 5mW, the integration time for spectral acquisition was set to 20s, and the number of integrations was set to 1. Raman spectra were acquired at temperatures of 298K, 328K, 348K, 368K, 388K, 398K, 403K, 408K, 413K, 418K, 423K, 433K, and 443K, respectively, with the acquisition parameters remaining consistent throughout.

[0058] Step 3: Analyzing the temperature-dependent Raman spectra from Step 2, it was found that the Raman peak corresponding to the ν(C–Cl) stretching vibration of chloromethyl groups changed position from 808 cm⁻¹ before and after the ferroelectric-paraelectric phase transition (413 K). -1 It became 812cm -1 Both peak intensity and peak width changed significantly, and the Raman peak appeared independently, as shown in the experimental results. Figure 1 As shown in the figure. Therefore, this Raman peak was identified as a characteristic Raman peak closely related to polarization.

[0059] Step 4: Obtain the vector coordinates of the C–Cl bond in the chloromethyl group using the CIF file of the TMCMCdCl3 crystal, and calculate that the angle between the C–Cl bond and the crystal c-axis is 30°.

[0060] Step 5: Construct an angle-dependent polarization Raman testing system, such as... Figure 2 As shown, the incident light source was a 532nm laser with a fixed polarization direction. The microscope magnification was 100X, and the grating was 600gr / mm. With the incident light polarization direction fixed, the polarization direction of the collected scattered light was adjusted to ensure it remained parallel to the incident light polarization direction. The ferroelectric material was placed on the object plane of the parallel polarization testing system. The two non-180° domains of the TMCMCdCl3 thin film were labeled as domain A and domain B. The laser power was set to 10mW, the integration time for spectral acquisition was set to 20s, and the number of integrations was set to 1. The thin film sample was rotated within a 360° rotation period, and in-situ Raman spectra of domains A and B were acquired every 10°.

[0061] Step 6: Analyze and process the angle-dependent Raman spectra of the two non-180° domains obtained in Step 5. After uniformly subtracting the baseline, perform Lorentz function fitting on the Raman peak of the ν(C–Cl) stretching vibration of chloromethyl determined in Step 3 to obtain peak intensity data corresponding to different polarization angles.

[0062] Step 7: For the two non-180° domains, plot the "angle-peak intensity ratio" polar coordinate graph with the angle between the crystal c-axis and the polarization direction of the incident light as the x-axis and the normalized data of the Raman peak intensity corresponding to different angles as the y-axis.

[0063] Step 8: Fit the data using the function: y = abs(A*(cos(x+θ))^2), where y is the peak intensity of the Raman peak, x is the angle between the crystal's c-axis and the polarization direction of the incident light, θ is the angle between the crystal's c-axis and the polarization direction of the incident light when y is at its maximum (its value can be determined through linear curve fitting), and A is the fitting parameter used to measure the intensity (amplitude) of the fitted data (its value can also be determined through linear curve fitting). Figure 3 As shown, for both domain A and domain B, the angle between the ν(C–Cl) stretching vibration direction and the crystal c-axis is 30°. However, the angle dependence trends of the ν(C–Cl) stretching vibrations for the two domains do not coincide, indicating that domains A and B are indeed non-180° domains and can be distinguished by the angle dependence polarization Raman spectra of characteristic Raman peaks. The difference between the experimental result of 30° and the 37° calculated in step 4 is within the allowable range of experimental error.

[0064] This embodiment proposes a non-contact, non-destructive Raman characterization method for the non-180° domains of the multipolar molecular ferroelectric material TMCMCdCl3. Utilizing the strong correlation between the characteristic Raman vibrational mode—the ν(C–Cl) stretching vibration of chloromethyl groups and spontaneous polarization—two non-180° domains are effectively distinguished based on angle-dependent polarization Raman spectroscopy.

[0065] Example 3

[0066] A Raman method for characterizing the domain structure of molecular ferroelectric materials according to Example 2, the difference being that, to achieve visualization of two non-180° domains, it further includes the following steps:

[0067] Step 1: Divide the peak intensities of the two non-180° domains at different angles to obtain the peak intensity ratio of the two domains at different angles. Plot an "angle-peak intensity ratio" curve with the angle between the crystal's c-axis and the polarization direction of the incident light as the x-axis and the peak intensity ratio corresponding to different angles as the y-axis. For example... Figure 4 As shown, the peak intensity ratio of the Raman peaks corresponding to the ν(C–Cl) stretching vibrations of the two non-180° domains reaches a maximum at 45° and a minimum at 135°.

[0068] Step 2: Place the TMCMCdCl3 thin film on the object plane of a confocal micro-Raman system with parallel polarization at the angle corresponding to the maximum (minimum) value of the peak intensity ratio obtained in Step 8. Raman imaging is then performed. The incident light source is a 532nm laser with a fixed polarization direction. The microscope used is 100X, the grating is 600gr / mm, the laser power is 10mW, the step size is 250nm, the integration time for spectral acquisition is 1s, and the integration count is 1. The Raman images of the two non-180° domains are shown below. Figure 5 As shown, this demonstrates that the method of characterizing the non-180° domains of the molecular ferroelectric material TMCMCdCl3 using confocal micro Raman spectroscopy is effective.

[0069] This embodiment proposes a non-contact, non-destructive Raman characterization method for the non-180° domains of the multipolar molecular ferroelectric material TMCMCdCl3. Utilizing the strong correlation between the characteristic Raman vibrational mode—the ν(C–Cl) stretching vibration of chloromethyl groups and spontaneous polarization—and angle-dependent polarization Raman spectroscopy, the visualization of two non-180° domains is effectively achieved.

Claims

1. A Raman method for characterizing the domain structure of molecular ferroelectric materials, characterized in that, Includes the following steps: Step (1): Determine the crystal growth direction and crystal plane of the molecular ferroelectric material; Step (2): Build a variable temperature Raman testing system, place the molecular ferroelectric material on the surface of the variable temperature Raman testing system, and collect the Raman spectra of the ferroelectric domains at different temperatures in situ. Step (3): Analyze the variable-temperature Raman spectrum from step (2) to determine the characteristic Raman vibration modes closely related to ferroelectric spontaneous polarization; Step (4): Calculate the angle between the characteristic Raman vibration mode determined in step (3) and the crystal growth direction; Step (5): Build an angle-dependent polarization Raman testing system, place the molecular ferroelectric material on the surface of the angle-dependent polarization Raman testing system, and collect the polarization Raman spectra of the ferroelectric domains at different angles in situ. Step (6): Perform data processing and analysis on the angle-dependent polarization Raman spectra of the ferroelectric domains obtained in step (5) to obtain the peak intensity, peak width, and peak position of the characteristic Raman peaks determined in step (3) under different polarization angles; Step (7): Organize and plot the data obtained in step (6) to obtain the "angle-peak intensity" relationship diagram; Step (8) Fit the data from step (7), analyze the fitting results, and determine the angle between the above-mentioned characteristic Raman vibration mode and the crystal growth direction. Compare the determined angle with the theoretical calculation result in step 4. If the result is within the allowable error range, the experiment is correct.

2. The Raman method for characterizing the domain structure of molecular ferroelectric materials as described in claim 1, characterized in that, The temperature range of the variable-temperature Raman spectroscopy collected in step (2) covers the phase transition temperature of molecular ferroelectric materials.

3. The Raman method for characterizing the domain structure of molecular ferroelectric materials as described in claim 1, characterized in that, The criteria for judging the characteristic Raman vibration mode closely related to ferroelectric spontaneous polarization in step (3) are: the Raman peak corresponding to the Raman vibration mode changes significantly in peak position, peak intensity and peak width before and after the phase transition, and the Raman peak appears independently.

4. The Raman method for characterizing the domain structure of molecular ferroelectric materials as described in claim 1, characterized in that, In step (5), the change of polarization angle is achieved in two ways: one is by fixing the sample and adjusting the polarization direction of the incident light and the scattered light; the other is by fixing the polarization direction of the incident light and the scattered light and rotating the sample.

5. The Raman method for characterizing the domain structure of molecular ferroelectric materials as described in claim 2, characterized in that, In step (7), the relationship diagram uses the angle between the crystal growth direction and the polarization direction of the incident light as the x-axis and the Raman peak intensity corresponding to the angle as the y-axis.

6. A method for visualizing ferroelectric domains based on the Raman method for characterizing the domain structure of molecular ferroelectric materials according to any one of claims 1 to 5, characterized in that, The steps are as follows: Step (1): For the characterization of two non-180° ferroelectric domains, the molecular ferroelectric material is placed at the angle when the peak intensity is at its extreme value obtained in the Raman method, and polarization Raman imaging is performed to visualize the ferroelectric domain structure. Step (2): For the characterization of two non-180° ferroelectric domains, the peak intensity data obtained from the Raman method should be divided to create an "angle-peak intensity ratio" relationship graph. Step (3): Fit the obtained data, analyze the data, determine the angle between the above-mentioned characteristic Raman vibration mode and the crystal growth direction, compare the determined angle with the theoretical calculation result, and if the error is within the acceptable range of the experiment, then the experiment is valid; Step (4): Place the molecular ferroelectric material at the angle at which the peak intensity ratio obtained in step (2) reaches its extreme value, and perform polarization Raman imaging to visualize the ferroelectric domain structure.

7. The visualization method as described in claim 6, characterized in that, In step (2), the curve is plotted with the angle between the crystal growth direction and the incident light polarization direction as the x-axis and the peak intensity ratio corresponding to the angle as the y-axis.

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