Preparation method and application of chiral spiral superstructure film

Abstract

The invention discloses a preparation method and application of a chiral spiral superstructure film, and the preparation method comprises the following steps: pretreating a substrate, and carrying out surface hydrophilic treatment by using O2 plasma; preparing a (R-MBA) X (S-MBA) 1-XBiI4 precursor solution, wherein X is an enantiomer fraction; spin-coating the precursor solution on a substrate to form a thin film; the film is annealed on a heating plate until it completely crystallizes. The obtained chiral superstructure film is applied to the photoelectric fields including but not limited to circularly polarized light detection, circularly polarized light emitting diodes and nonlinear optical anti-counterfeiting. The invention provides a simple and effective method, the chiral metal halide semiconductor (MHS) spiral superstructure with amplification-chiral interaction can be self-assembled only by changing the enantiomer fraction X, and a chiral template or an external field is not needed. The invention opens up a new way for constructing mesoscopic chirality in chiral MHS and enhancing chiral optical activity.

Description

Technical Field

[0001] The present invention relates to the technical field of chiral material preparation and application, in particular to a preparation method and application of a chiral helical superstructure thin film. Background Art

[0002] Chirality widely exists in nature. If the mirror image of an object cannot coincide with the original object, it is called chiral. Chiral molecules and their mirror image structures are called enantiomers, which have different physical and chemical properties. The transfer of chiral information in nature between multiple length scales, the transmission of chiral information across multiple length scales, such as chiral communication from short-distance molecules / crystals to long-distance molecules / crystals, is crucial for amplifying the chiral optical properties of chiral semiconductors. Chiral organic-inorganic hybrid metal halide semiconductors (MHSs) have developed rapidly in recent years and have several advantages required for chiral optoelectronic integrated devices, including good solution processability, excellent optical properties, and tunable electronic conduction. Chiral MHSs have been used in many frontier applications, such as circularly polarized light-emitting diodes, nonlinear optics, circularly polarized light detectors, photovoltaics, and spintronics.

[0003] In the chiral MHS system, chiral organic cations cause the inorganic framework to form left-handed or right-handed helices and transfer this chiral preference to the entire crystal. For the stability of the chiral hybrid structure, chiral organic cations with a small volume are used, and the most common is chiral aromatic amine cations. Small chiral molecules with a maximum width of less than 1 nm cannot "feel" the obvious degree of light distortion, resulting in weak chiral optical activity. Usually, the asymmetry factor (g CD ) of the circular dichroism signal is less than 10 -4 . When these small chiral molecules are tightly helically stacked with the inorganic framework in the MHS crystal structure, the overlap between the electric and magnetic transition dipole moments increases, resulting in an enhanced g CD value. However, in the prior art, the g CD of chiral MHS is lower than 10 -2 , which cannot meet the sensitivity and accuracy requirements of actual polarization applications.

[0004] Learning from the experience of other chiral material systems, such as organic chiral supramolecules and inorganic nanocrystal chiral superstructures, enhancing the exciton coupling between molecules / crystals, and increasing the size of the long-range chiral superstructure unit (for example, the helical pitch is comparable to the emission / absorption light wavelength) have been proven to effectively improve the chiral optical activity by several orders of magnitude. Therefore, in order to further significantly improve the chiral optical activity of chiral MHS, efforts need to be made to design chiral helical superstructures. However, compared with organic small molecules and inorganic nanocrystal systems, the material and crystal structure composition in MHS is very complex, and it is a great challenge to significantly improve the chiral optical activity of MHS by designing and preparing chiral helical superstructures. Summary of the Invention

[0005] To overcome the above problems existing in the prior art, the present invention provides a method for preparing a chiral helical superstructure thin film and its application.

[0006] The technical solution adopted by the present invention to solve its technical problems is as follows: A method for preparing a chiral helical superstructure thin film, comprising the following steps: Step 1: After pre-treating the substrate, the surface of the substrate is treated with O 2 plasma. Step 2: Prepare a (R-MBA) X (S-MBA) 1-X BiI 4 precursor solution, where X is the enantiomeric fraction, R-MBA is R-(+)-α-methylbenzylamine, and S-MBA is S-(-)-α-methylbenzylamine; Step 3: Spin-coat the (R-MBA) X (S-MBA) 1-X BiI 4 precursor solution on the substrate to form a thin film; Step 4: Anneal the thin film obtained in Step 3 on a hot plate to make it fully crystallized.

[0007] In the above method for preparing a chiral helical superstructure thin film, the substrate pre-treatment in Step 1 is specifically to clean the substrate sequentially with detergent, ethanol, and deionized water in an ultrasonic cleaner.

[0008] In the above method for preparing a chiral helical superstructure thin film, the solvent of the (R-MBA) X (S-MBA) 1-X BiI 4 precursor solution in Step 2 is one of dimethylformamide, dimethyl sulfoxide, and ethanol.

[0009] In the above method for preparing a chiral helical superstructure thin film, Step 2 is specifically to dissolve R-(+)-α-methylbenzylammonium iodide, S-(-)-α-methylbenzylammonium iodide, and bismuth triiodide in a DMF solution.

[0010] In the above method for preparing a chiral helical superstructure thin film, Step 2 is specifically to mix and dissolve (R-MBA) BiI 4 , (S-MBA) BiI 4 single crystal powders in a DMF solution, where (R-MBA) BiI 4 , (S-MBA) BiI 4 single crystal powders are prepared by the cooling crystallization method.

[0011] The above preparation method of a chiral helical superstructure thin film, the (R-MBA) BiI 4 The preparation method of single crystal powder is as follows: In a glass bottle, bismuth trioxide and R-(+)-α-methylbenzylamine are dissolved in an aqueous hydroiodic acid solution; after sealing, the glass bottle is placed in an oven, and the oven is maintained at 120 °C for 6 hours to completely dissolve the solid. It is slowly cooled to room temperature at a cooling rate of 3 °C per hour, the crystals are filtered, washed with n-hexane, and then vacuum dried in an oven to obtain (R-MBA) BiI 4 single crystal powder.

[0012] An application of a chiral helical superstructure thin film, based on the chiral helical superstructure thin film prepared by the above preparation method, is applied to optoelectronic fields including but not limited to circularly polarized light detection, circularly polarized light-emitting diodes, and nonlinear optical anti-counterfeiting.

[0013] The beneficial effect of the present invention is that the present invention provides a simple and effective method, and a chiral MHS helical superstructure with enhanced chiral optical activity can be self-assembled only by using enantiomeric excess (ee) mixing. Compared with the g of enantiopure thin films CD , in the mixture of two chiral MHS enantiomers, an enantiomeric excess of 20% can induce the circular dichroism asymmetry factor (g CD ) to be amplified by more than 50 times. The strong interaction between heterochiral organic ligand cations in adjacent nanocrystals is probably the reason for inducing the helical self-assembly of chiral MHS nanocrystals. The present invention opens up a new way for constructing mesoscopic chirality and enhancing chiral optical activity in chiral MHS. Description of the Drawings

[0014] Figure 1 is the structure diagram of R-MBI and S-MBI single crystal powders of the present invention; Figure 2 is R of the present invention X S 1-X XRD pattern of -MBI thin film; Figure 3 is R of the present invention X S 1-X Polarizing optical microscope image of -MBI thin film. Two orthogonal polarizers are added in the test optical path. Among them, (a) is the polarizing optical microscope image of R-MBI thin film, and (b) is R 0.6 S 0.4 Polarizing optical microscope image of -MBI thin film, (c) is the polarizing optical microscope image of rac-MBI thin film, (d) is R 0.4 S 0.6 Polarizing optical microscope image of -MBI thin film, (e) is the polarizing optical microscope image of S-MBI thin film; Figure 4 is R of the present inventionX S 1-X - Atomic force microscopy images of MBI films, where (a) is the atomic force microscopy image of S-MBI film, (b) is the atomic force microscopy image of rac-MBI film, and (c) is R 0.6 S 0.4 - Atomic force microscopy image of MBI film, and (d) is R 0.4 S 0.6 - Atomic force microscopy image of MBI film; Figure 5 is R of the present invention X S 1-X - Chiroptical property diagrams of MBI films, where (a) is based on testing R-MBI, S-MBI, R from the front and back of the sample 0.4 S 0.6 - MBI and R 0.6 S 0.4 - CD results of MBI films, and the calculated true CD spectra; (b) is for several typical g CD peak positions of g CD values versus enantiomeric fraction X. For thin film samples with different enantiomeric fractions X, based on the true CD spectral data and absorption spectra, the calculated true g CD spectra, and several typical g CD peaks are selected. Detailed implementation manners

[0015] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific implementation manners.

[0016] This embodiment discloses a preparation method of a chiral superstructure film, specifically: Step 1, the substrate can be selected from quartz glass, glass, silicon wafers, etc. In this embodiment, a quartz glass substrate is used for linear optical detection, and a glass substrate is used for nonlinear optical detection. The used substrate is cleaned in an ultrasonic cleaner with detergent, ethanol, and deionized water in sequence for 30 minutes. Before film preparation, the substrate surface is treated with O 2 plasma for 10 minutes to increase its hydrophilicity.

[0017] Step 2, prepare (R-MBA) X (S-MBA) 1-X BiI 4 (abbreviated as R X S 1-X -MBI in this embodiment) solution, where X is the enantiomeric fraction and the solvent is dimethylformamide; Step 3, form a film by spin-coating 45 μL of the precursor solution on the substrate at a rotation speed of 2000 rpm for 20 seconds; Step 4: Anneal the fabricated film on a hot plate at 90 °C for 20 minutes to achieve complete crystallization.

[0018] In this embodiment, R X S 1-X -MBI precursor solution can be prepared by dissolving R-(+)-α-methylbenzylammonium iodide, S-(-)-α-methylbenzylammonium iodide, and bismuth triiodide in DMF solution, or by dissolving (R-MBA)BiI 4 (abbreviated as R-MBI in this embodiment), (S-MBA)BiI 4 (abbreviated as S-MBI in this embodiment) single crystal powder in DMF solution.

[0019] This embodiment provides a method for preparing R-MBI and S-MBI single crystal powder, which specifically includes: In a 50 mL glass bottle, dissolve Bi 2 O 3 (2.796 g, 6 mmol) and R-MBA (1.527 mL, 12 mmol) in 20 mL hydroiodic acid aqueous solution (without hypophosphorous acid). After sealing, place the glass bottle in an oven. First, keep the oven at 120 °C for 6 hours to completely dissolve Bi 2 O 3 , and then slowly cool it to room temperature at a cooling rate of about 3 degrees per hour. After these steps, red R-MBI crystals are formed. The crystals should be immediately filtered and washed with hexane, and thoroughly dried on a hot plate at 90 °C for 30 minutes to obtain R-MBI single crystal powder with a yield of 6 grams (60% based on the total Bi content). S-MBI follows the same crystal growth process, except that the chiral amine is replaced with S-(+)-α-methylbenzylamine (S-MBA).

[0020] The obtained R-MBI and S-MBI single crystal powder structures are as Figure 1 shown.

[0021] Among them, the materials for preparing R X S 1-X -MBI precursor solution, R-(+)-α-methylbenzylamine (R-MBA, 98% enantiomeric excess), S-(-)-α-methylbenzylamine (S-MBA, >99% gas chromatography purity), N,N-anhydrous dimethylformamide (DMF, 99.8%), bismuth trioxide (Bi 2 O 3, 99.99%), bismuth triiodide (BiI3, 99.99%) were all purchased from Aladdin (Shanghai, China). 45% aqueous hydroiodic acid solution was purchased from Sinopharm Chemical Reagent Co., Ltd. R-(+)-α-methylbenzylammonium iodide (R-MBAI, 99%), S-(-)-α-methylbenzylammonium iodide (S-MBAI, 99%) and α-methylbenzylammonium iodide (MBAI, 99%) were purchased from Xi'an Yuri Solar Co., Ltd.

[0022] In this example, R X S 1-X -MBI precursor solutions with various concentrations of X = 0, 0.4, 0.5, 0.6, 1 were prepared, and thin films were prepared using the precursor solutions with the above concentrations. The thin films were subjected to X-ray diffraction, and the results obtained are as Figure 2 shown. The X-ray diffraction (XRD) patterns of R X S 1-X -MBI (X = 0, 0.4, 0.5, 0.6, 1) thin films showed the same diffraction peaks. Among them, when X = 0, the thin film is denoted as S-MBI, when X = 1, the thin film is denoted as R-MBI, and when X = 0.4, the thin film is denoted as R 0.4 S 0.6 -MBI, when X = 0.5, the thin film is denoted as rac-MBI, and when X = 0.6, the thin film is denoted as R 0.6 S 0.4 -MBI. Two prominent XRD peaks can be attributed to the (011) and (012) planes, indicating that the thin films with enantiomeric excess form in the same crystal phase as the enantiopure R-MBI or S-MBI thin films. According to these main diffraction peaks, [BiI 4 - The octahedral chains are preferentially oriented parallel to the substrate plane.

[0023] For the morphological study of R X S 1-X -MBI thin films, as Figure 3 shown. From the polarized light microscopy (POM) images of the enantiopure thin films (i.e., R-MBI or S-MBI thin films), the surface is relatively flat and grainy domains are separated by boundaries. Adjacent domains exhibit different brightnesses, and as the sample rotates, each domain shows alternating light and dark, indicating the presence of mesoscopic local linear dichroism (LD) and linear birefringence (LB) within each domain. Compared with the enantiopure thin films, the enantiomeric mixture thin films contain propeller-shaped superstructural domains. For R 0.6 S 0.4 -MBI films, the radial dendritic texture retains a clockwise (i.e., left-handed) helical twist (as shown in (b) of Figure 3 ), and for R 0.4 S 0.6 ​- The MBI film retains a counterclockwise (i.e., right-handed) helical twist (as shown in (d) of Figure 3 ), while for the rac-MBI film, there is no twist (as shown in (c) of Figure 3 ). The macroscopic unidirectional twist related to the major enantiomeric fraction indicates the formation of a helical superstructure beyond crystal chirality.

[0024] Combined with atomic force microscopy ( Figure 4 ), the images detailedly capture the partially layered structure. In the enantiopure R-MBI and S-MBI thin films, the films are relatively flat, formed by the close packing of nanocrystals with a median diameter of about 50 nm. When the two enantiomers are mixed, 20% excess (X = 0.6) of R-MBI can induce the growth of "branches" with mainly clockwise helical twists, and at the same time, "leaves" with clockwise twists (span about ~5 μm) will also attach. In the rac-MBI thin film ( Figure 4 in (b)), the branches disappear, replaced by pairs of leaves with clockwise and counterclockwise twists respectively (span about ~2 μm). In addition, one of the leaves is composed of nanocrystals (also about 50 nm in diameter), arranged in a 3D S-shaped chain in the left-handed direction, and the same is true for the other leaf, but arranged in the right-handed direction ( Figure 4 in (b)). This example also systematically studies the circular dichroism (CD) response of the R X S 1-X -MBI thin film. Due to the general influence of macroscopic anisotropy (i.e., LD or / and LB) on chiral activity, the CD responses of each sample before and after sample flipping are measured (denoted as CDfront and CDback respectively). To exclude the influence of the LD-LB effect, the true CD of the R X S 1-X -MBI thin film is calculated as the average of CD front and CD back , and the true CD results are as shown in (a) of Figure 5 .

[0025] To quantitatively compare the CD response with other chiral systems, the true CD asymmetry factor (true gCD) of the R X S 1-X -MBI thin film is also calculated:

[0026] where Absorbance represents absorbance and CD is the measured true CD value. As shown in Figure 5As shown in (a), the true CD value of the R-MBI film at 538 nm is estimated to be ~20 mdeg, while the true CD value of the S-MBI film with the opposite sign is estimated to be ~16 mdeg. The true gCD of the R-MBI film is estimated to be ~5×10 -4 . R 0.4 S 0.6 -MBI's true CD is estimated to be ~970 mdeg at 542 nm, more than 50 times enhanced compared to the enantiopure S-MBI film. At the first peak at 550 nm, its true g CD is estimated to exceed -0.03, close to the highest true g CD values reported so far for low-dimensional chiral MHSs.

[0027] Figure 5 In (b), the variation curves of the true g X S 1-X values of each R CD -MBI film with different enantiomeric fractions are plotted. All the strongest true g CD peaks appear in the mixtures with X values of 0.4 or 0.6, showing a bimodal g CD across X = 0.5 (i.e., the racemic mixture). In the R X S 1-X -MBI films, these g CD -X dependences are closely related to the formation and ratio of the two chiral superstructures. Therefore, by adjusting the value of X, the magnitude, sign, and position of the CD of the film can be adjusted.

[0028] In this embodiment, a mesoscopic chiral superstructure is constructed by inducing a slight enantiomeric excess in the chiral MHSs film. This strong structure-property interaction in the chiral MHS superstructure can be further used in optoelectronic applications such as circularly polarized light detection, emission, and nonlinear optical anti-counterfeiting. In the field of chiral science, it is rare to induce the self-assembly of chiral nanoparticles with heterochiral interactions rather than homochiral interactions, and its underlying physical mechanism may help design other simple building blocks into layered functional structures.

[0029] The above embodiments are only exemplary embodiments of the present invention and are not used to limit the present invention. Those skilled in the art can make various modifications or equivalent replacements to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present invention.

Claims

1. A method for preparing a chiral helical superstructure film, characterized in that: The steps include: Step 1, after pre-treating the substrate, the surface of the substrate is treated with O2 plasma; Step 2, Preparation (R-MBA) X (S-MBA) 1-X BiI4 precursor solution, X is the enantiomeric fraction, R-MBA is R-(+)-α-methylbenzylamine, S-MBA is S-(-)-α-methylbenzylamine; Step 3, (R-MBA) X (S-MBA) 1-X The BiI4 precursor solution was spin-coated on the substrate to form a thin film; Step 4, annealing the thin film obtained in step 3 on a hot plate to make it completely crystallized.

2. The method for preparing a chiral helical superstructure film according to claim 1, characterized in that: The substrate pretreatment in step 1 is specifically cleaning with detergent, ethanol and deionized water in an ultrasonic cleaner in sequence.

3. The method for preparing a chiral helical superstructure film according to claim 1, characterized in that: Prepare (R-MBA) in step 2 X (S-MBA) 1-X The solvent of the BiI4 precursor solution is one of dimethylformamide, dimethyl sulfoxide and ethanol.

4. The method for preparing a chiral helical superstructure film according to claim 1, characterized in that: The step 2 specifically comprises dissolving R-(+)-α-methylbenzylammonium iodide, S-(-)-α-methylbenzylammonium iodide and bismuth triiodide in a dimethylformamide solution.

5. The method for preparing a chiral helical superstructure film according to claim 1, characterized in that: The step 2 specifically comprises mixing (R-MBA)BiI4 and (S-MBA)BiI4 single crystal powders in proportion and dissolving them in a DMF solution, wherein the (R-MBA)BiI4 and (S-MBA)BiI4 single crystal powders are prepared by a cooling crystallization method.

6. The method for preparing a chiral helical superstructure film according to claim 5, characterized in that: The method for preparing the (R-MBA)BiI4 single crystal powder is specifically as follows: in a glass bottle, bismuth trioxide and R-(+)-α-methylbenzylamine are dissolved in a hydroiodic acid aqueous solution; after sealing, the glass bottle is placed in an oven, and the oven is kept at 120°C for 6 hours to completely dissolve the solid, and the bottle is slowly cooled to room temperature at a cooling rate of 3°C per hour, the crystals are filtered and washed with n-hexane, and then vacuum dried in an oven to obtain the (R-MBA)BiI4 single crystal powder.

7. An application of a chiral helical superstructure film, characterized in that: The chiral helical superstructure film prepared by the preparation method according to any one of claims 1 to 6 is applied to optoelectronic fields including but not limited to circularly polarized light detection, circularly polarized light emitting diodes, and nonlinear optical anti-counterfeiting.

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