Preparation method and application of super-twisted spiral WS2 crystal for second-order nonlinear effect modulation

Abstract

The invention belongs to the technical field of photoelectric materials, and discloses a preparation method and application of a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation.The preparation method comprises the following steps that WS2 powder is placed in a first porcelain boat, a substrate is horizontally placed on a second porcelain boat, and deionized water is placed in a third porcelain boat; then, the first porcelain boat is arranged in a middle heating area of the tubular furnace, the second porcelain boat is arranged on the air outlet side, and the third porcelain boat is arranged on the air inlet side; meanwhile, a quartz glass cylinder is placed between the first porcelain boat and the third porcelain boat; introducing carrier gas, and heating to enable the heating temperature of the area where the first porcelain boat is located to be 1140-1160 DEG C; carrier gas is continuously introduced, meanwhile, the area where the first porcelain boat is located is kept at the constant temperature, and after the carrier gas carries the evaporated WS2 source to send the WS2 source to the SiO2 substrate slice and deposition is completed, the layered spiral WS2 two-dimensional nanometer material with the layer-by-layer twisting angle is obtained. The second-order nonlinear magnetic susceptibility of the obtained product is large and adjustable, and the method can be widely applied to the field of optics.

Description

Technical Field

[0001] The present invention belongs to the technical field of optoelectronic materials, and specifically relates to a preparation method and application of a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation. Background Art

[0002] The second-order nonlinear effect refers to a phenomenon that occurs when certain materials are exposed to strong light. This effect generally occurs in specific materials with broken central symmetry in the structure, and usually exhibits frequency doubling, optical parameter amplification, self-focusing and self-phase modulation. It is widely used in laser frequency conversion, material structure and characterization, information processing and other fields. Precise control of the second-order nonlinear effect can achieve faster response speed, higher operating efficiency and more options. At present, the control methods of the second-order nonlinear effect mainly include artificial microstructure design, external field adjustment, and phase matching adjustment. However, the key to artificial microstructure design lies in the microstructure of the material, which can only enhance the nonlinear effect within a specific wavelength range, limiting the application of a wide wavelength range. The application of common electric and magnetic fields by external fields requires complex experimental settings, and the application field is limited. Phase matching adjustment requires strict phase matching between the input wave and the output wave, and is sensitive to parameters such as the incident angle, wavelength, and temperature, which limits the frequency doubling efficiency.

[0003] It is considered a more advanced method to modulate the second-order nonlinear effect by regulating the nonlinear properties of two-dimensional materials. Two-dimensional materials (such as chalcogenide transition metal compounds MoS2, MoSe2, WS2, etc.) have clean surfaces and no dangling bonds. They have an atomically thin layer structure in the single-layer state, lack spatial inversion symmetry, show strong nonlinearity, and have a wide range of adjustable band gaps. These characteristics make them exhibit unique optoelectronic properties and show broad application prospects in the field of integrated nanodevices, so they have received widespread attention. In addition, the layers are bonded by van der Waals bonds, which can be easily mechanically peeled off and stacked into a moiré superlattice structure, thereby changing the symmetry of the material itself to achieve the regulation of the second-order nonlinear effect. However, the moiré superlattice interface formed by mechanically assisted transfer will be contaminated and uneven, and the second-order nonlinear characteristics are fixed after the preparation, and cannot be effectively regulated.

[0004] Theoretical calculations of two-dimensional layered materials show that the nonlinear properties of two-dimensional transition metal disulfides will be adjusted with different stacking methods. Therefore, the different morphologies of the synthesized moiré superlattice are crucial for studying the regulation of the second-order nonlinear effect, and the influence of the different torsion angles and the number of layers in the synthesis method on the structure is particularly important. The preparation method of super-twisted spiral WS2 crystals was introduced in a previous patent (CN118441261A), but its structural yield was low, which limited the effective regulation of its second-order nonlinear effect. Therefore, it is urgent to develop a super-twisted spiral WS2 crystal to achieve a wide wavelength range of the second-order nonlinear effect and modulation of the high second-order magnetic susceptibility. Summary of the invention

[0005] In order to solve the problems of moiré superlattice interface contamination and unevenness in the existing preparation technology, the present invention provides a method for preparing super-twisted spiral WS2 crystals for second-order nonlinear effect modulation, which combines screw dislocation growth, non-Euclidean substrate and gas wake effect to obtain super-twisted spiral WS2 crystals with clean surface and high yield, and then utilizes the reversible inversion symmetry formed by the periodic change of the interlayer torsion angle of the crystal to achieve the modulation of the second-order nonlinear effect in a wide wavelength range and high second-order magnetic susceptibility, which effectively solves the application problem that the existing technology cannot achieve adjustable second-order nonlinear effect in the same crystal by a simple method.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation, comprising the following steps:

[0007] Step 1: Place WS2 powder in a first porcelain boat (1), place a SiO2 / Si substrate horizontally on a second porcelain boat (2), and place deionized water in a third porcelain boat (3); then place the first porcelain boat (1) in the middle heating area of ​​a horizontal quartz tube (5) of a tube furnace (4), place the second porcelain boat (2) on the air outlet side of the horizontal quartz tube (5), and place the third porcelain boat (3) on the air inlet side of the horizontal quartz tube (5); at the same time, place a block (6) between the first porcelain boat (1) and the third porcelain boat (3), and make the tail flow area of ​​the block (6) cover the position of the second porcelain boat (2);

[0008] Step 2: introducing carrier gas, exhausting the air in the furnace and continuing to introduce carrier gas, and heating the horizontal quartz tube (5) through the tubular furnace (4) so ​​that the heating temperature of the area where the first porcelain boat (1) is located is 1140-1160° C.;

[0009] Step 3: Continue to introduce carrier gas, and keep the heating temperature of the area where the first porcelain boat (1) is located at a constant temperature of 1140-1160°C, waiting for the carrier gas to carry the evaporated WS2 source to the SiO2 substrate and complete the deposition. After the deposition is completed, wait for natural cooling, take out and obtain a layered spiral WS2 two-dimensional nanomaterial with a layer-by-layer twist angle.

[0010] The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation also includes the following steps: controlling the content of deionized water placed in the third porcelain boat (3) to control the twisting angle of the super-twisted spiral WS2 crystal, thereby controlling its second-order nonlinear effect.

[0011] The stopper is a horizontally placed quartz glass cylinder, and its cross-sectional area is 60-70% of the horizontal quartz tube (5).

[0012] The carrier gas is one of helium, neon and argon.

[0013] In step 2 and step 3, the flow rate of the carrier gas is kept constant within the range of 80-120 sccm, and the pressure inside the tube furnace is normal pressure.

[0014] In the step 1, the amount of deionized water used is 50 to 100 μL.

[0015] In step 3, the deposition time is 10-30 minutes.

[0016] In addition, the present invention also provides an application of a super-twisted spiral WS2 crystal in a second-order nonlinear effect, and the super-twisted spiral WS2 crystal is prepared according to the preparation method according to claim 1.

[0017] The application of placing a super-twisted spiral WS2 crystal in the second-order nonlinear effect controls the second-order nonlinear performance of the super-twisted spiral WS2 crystal by controlling the torsion angle of the super-twisted spiral WS2 crystal.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention provides a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation and a preparation method thereof, using WS2 powder as a source material, and adopting a carefully designed water-assisted chemical vapor deposition method to generate nucleation impurity points on a non-Euclidean substrate. By optimizing and controlling the raw material heating temperature, deposition temperature, carrier gas flow rate and deionized water content, the WS2 steam inside the tube furnace is in a low supersaturation state, and at the same time, a quartz glass cylinder is added as a block to form a wake effect, reduce the carrier gas flow rate and introduce gas disturbance, thereby promoting the formation of nucleation points and increasing the yield of super-twisted spiral WS2 crystals. Moreover, by regulating the deionized water content, the torsion angle of the super-twisted spiral WS2 crystal can be controlled.

[0020] The uniform torsion angle between layers of the super-twisted spiral WS2 crystal prepared by the present invention provides a new method for regulating the second-order nonlinear effect. The main mechanism is that the uniform torsion angle periodically modulates the inversion symmetry of the crystal, thereby realizing the regulation of the second-order nonlinear effect. With the help of an electrically controlled translation stage, it is possible to ensure that the 800nm ​​femtosecond laser is accurately excited on each layer. The second harmonic intensity generated by the super-twisted spiral WS2 crystal shows an oscillation dependence on the number of layers, and this is true within a wide wavelength range of 780nm-1320nm. In addition, the change in the interlayer stacking model, and then the atomic phase matching of the nonlinear dipoles inside the lattice is regulated, with different second-order nonlinear susceptibilities. After testing, the second-order nonlinear magnetic susceptibility is 0.68, 0.46, 0.23 and 0.21 nm / V at torsion angles of 0°, 13°, 16° and 21° respectively, which are all larger than the 2.04 pm / V value of the commercial frequency doubling crystal BBO. This improves the existing technology that cannot synthesize WS2 layered materials with adjustable second-order nonlinear effects of a single component through a one-step method. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the twisted spiral WS2 structure prepared in the present invention, wherein (a) is a schematic diagram of the growth process of the 0° twisted WS2 structure driven by the screw dislocation, and (b) is a three-dimensional oblique view of the corresponding structure;

[0022] Figure 2 A schematic diagram of constructing a non-Euclidean substrate in a one-step water-assisted chemical vapor deposition method of the present invention;

[0023] Figure 3 This is a principle analysis diagram of the super-twisted spiral WS2 crystal with controllable left-handed and right-handed angles in the present invention, where (a)-(c) are left-handed and (d)-(e) are right-handed.

[0024] Figure 4 This is a schematic diagram of the structure of the device used in Example 1 of the present invention;

[0025] Figure 5The optical images of the super-twisted WS2 spirals with different interlayer twist angles prepared in Example 1 of the present invention, wherein (a) is a left-handed super-twisted WS2 spiral with a twist angle of about 20° and a size of about 14 μm; (b) is a left-handed super-twisted WS2 spiral with a twist angle of about 24° and a size of about 12 μm; (c) is a left-handed super-twisted WS2 spiral with a twist angle of about 16° and a size of about 13 μm; (d) is a right-handed super-twisted WS2 spiral with a twist angle of about 13° and a size of about 15 μm; (e) is a right-handed super-twisted WS2 spiral with a twist angle of about 20° and a size of about 14 μm; (f) is a right-handed super-twisted WS2 spiral with a twist angle of about 17° and a size of about 12 μm;

[0026] Figure 6 The super-twisted WS2 helix with different interlayer twist angles prepared in Example 1 of the present invention Figure 5 The corresponding high-magnification AFM profile image;

[0027] Figure 7 The high-magnification AFM annotation image of the super-twisted WS2 helix of 13°, 16° and 20° selected in Example 1 of the present invention and the AFM height contour line image along the purple short line;

[0028] Figure 8 This is a schematic diagram of the optical path for performing a second harmonic test of the second-order nonlinear effect on the selected sample in Example 1 of the present invention;

[0029] Fig. 9 Figure 1 is a second harmonic signal intensity diagram related to the interlayer twist angle of the super-twisted WS2 spirals of 13°, 16° and 20° selected in Example 1 of the present invention, wherein (a), (d), (g) are optical images of the super-twisted WS2 spirals with twist angles of 13°, 16° and 20°, respectively, and the colored crosses indicate the area where the second harmonic signal is collected, (b), (e), (h) are layer-related second harmonic spectra of the super-twisted WS2 spirals with twist angles of 13°, 16° and 20°, respectively, and (c), (f), (i) are layer-dependent second harmonic intensity variation diagrams of the super-twisted WS2 spirals with twist angles of 13°, 16° and 20°, respectively.

[0030] Fig.10 Schematic diagram of the super-twisted WS2 helical structure with a twist angle of 20° in Example 1 of the present invention, wherein (a)-(d) are top views corresponding to 1 to 4 layers, (a1)-(d1) are side views corresponding to 1 to 4 layers, (e) is a side view of the structure of the helical WS2 twisted to the seventh layer, and dashed lines of different colors represent the restoration of spatial inversion symmetry;

[0031] Fig.11The second-order nonlinear magnetic susceptibilities obtained by testing the super-twisted WS2 spirals of 13°, 16° and 20° selected in Example 1 of the present invention are 0.46, 0.23 and 0.21 nm / V, respectively;

[0032] Fig.12 The scanning electron microscope (STEM) image of the super-twisted WS2 spiral of about 30° prepared in Example 2 of the present invention, wherein (a) is a high-angle annular dark field image (HAADF), in which the edges of different layers are highlighted in different colors, (b) is a position averaged convergent beam electron diffraction (PACBED) image collected from the purple dot area in (a), (c) is a comparison of the twist angle measured by the edge and diffraction with the number of layers, and (d) is a schematic diagram of the three-layer structure; DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0034] like Figure 1 As shown, the continuous layered growth of twisted spiral WS2 depends on the existence of dislocation lines, so the synthesis conditions of the material are particularly critical.

[0035] like Figure 2 As shown in the figure, during the material growth process, the presence or absence of water vapor will affect the unevenness of the substrate. When water vapor does not participate in the reaction, the substrate appears as a flat Euclidean substrate, and the period of one rotation of the surface is 2π; when water vapor participates, it will react with WS2 gas molecules under high temperature conditions, forming cone-like protrusions on the substrate, thus evolving into a non-Euclidean substrate, and the surface period of the protrusions will become 2π-θ°.

[0036] according to Figure 3It can be seen that the key to successfully synthesizing left-handed or right-handed super-twisted spiral WS2 crystals with different torsion angles lies in the control of the substrate. For Euclidean substrates (i.e., the surface is flat and without any undulations), the period of one rotation of the surface is 2π, which is exactly the same as the crystal period of the spiral WS2 material. When the WS2 material grows on this Euclidean substrate, the two periods match, so that there is almost no rotation angle between the layers, and finally a spiral WS2 two-dimensional layered material with basically parallel edges is formed. On non-Euclidean substrates (i.e., there are protrusions or irregularities on the surface), the period of the substrate surface is no longer 2π, resulting in two possible situations. If the dislocation center of the spiral WS2 is located at the center of the protrusion, the surface period at the protrusion will become 2π-θ°, while the crystal period of WS2 is still 2π. The two periods do not match, and WS2 will have an extra θ° for each rotation, thus forming a left-handed twisted spiral WS2 layered structure. On the contrary, if the dislocation center is located at the edge of the protrusion, since the period of the edge becomes 2π+θ°, the substrate will lack a crystal period of θ° for each rotation. This missing part needs to be completed in the next period, thus forming a right-handed twisted spiral WS2 layered structure.

[0037] Embodiment 1

[0038] Embodiment 1 of the present invention provides a method for modulating the second-order nonlinear effect in a super-twisted spiral WS2 crystal by regulating the interlayer torsion angle. Figure 4 The experimental setup shown is implemented, including the following steps:

[0039] Step 1: Place WS2 powder in the first porcelain boat 1, place the SiO2 / Si substrate horizontally on the second porcelain boat 2, and put deionized water in the third porcelain boat 3; then place the first porcelain boat 1 in the middle heating area of ​​the horizontal quartz tube 5 of the tube furnace 4, place the second porcelain boat 2 on the gas outlet side of the horizontal quartz tube 5, and place the third porcelain boat 3 on the gas inlet side of the horizontal quartz tube 5; at the same time, place a stopper 6 between the first porcelain boat 1 and the third porcelain boat 3, and make the tail area of ​​the stopper (6) cover the position of the second porcelain boat (2). The SiO2 / Si substrate is a composite layer including a silicon dioxide layer and a silicon layer, wherein the silicon dioxide layer on the surface of the substrate is used to grow a spiral WS2 layered structure, which is relatively thin, and the silicon layer is located at the bottom and is relatively thick.

[0040] Specifically, in this embodiment, the stopper (6) is a horizontally placed quartz glass cylinder, and its cross-sectional area is 60-70% of the horizontal quartz tube (5). Specifically, the stopper used in this embodiment is 2 cm long and 17 mm in diameter, and its tail flow area is approximately between 8.5 cm and 13.6 cm from the end of the stopper 6. Placing the SiO2 / Si substrate in the tail flow area of ​​the stopper 6 can form a tail flow effect, reduce the carrier gas flow rate and introduce gas disturbance, thereby promoting the formation of nucleation points and increasing the yield of super twisted spiral WS2 crystals.

[0041] Step 2: introducing carrier gas, exhausting the air in the furnace and then continuing to introduce carrier gas, and heating the horizontal quartz tube 5 through the tubular furnace 4 so that the heating temperature of the area where the first porcelain boat 1 is located is 1140-1160°C;

[0042] Step 3: Continue to introduce carrier gas, and keep the heating temperature of the area where the first porcelain boat 1 is located at a constant temperature of 1140-1160°C, waiting for the carrier gas to carry the evaporated WS2 source to the SiO2 substrate and complete the deposition. After the deposition is completed, wait for natural cooling, take out and obtain a layered spiral WS2 two-dimensional nanomaterial with a layer-by-layer twisting angle.

[0043] Specifically, in this embodiment, a SiO2 / Si wafer is used as a substrate, cut into 10 mm×15 mm size, ultrasonically cleaned in acetone and ethanol solution for 15 min, and blown dry with high-purity nitrogen to obtain a clean growth substrate. In addition, a quartz glass cylinder with a length of 20 mm and a diameter of 17 mm as a stopper 6 is also ultrasonically cleaned and dried in an oven at 60°C.

[0044] Specifically, in this embodiment, WS2 powder is used as raw material, a certain amount of WS2 powder is placed in porcelain boat 1, and the SiO2 / Si substrate is spread flat on porcelain boat 2. In addition, 50 μl of deionized water is dripped into porcelain boat 3; then porcelain boat 1 is placed in the center of the tube furnace, porcelain boat 2 is placed 8 cm away from the center of the tube furnace near the air outlet, porcelain boat 3 is placed at the edge of the air inlet, and block 6 is placed in the center of porcelain boat 1 and porcelain boat 3.

[0045] Specifically, in this embodiment, high-purity argon is used as the carrier gas, the flow rate is controlled to be 80 sccm, and the air in the furnace is evacuated. The carrier gas input is maintained, and the temperature of porcelain boat 1 is raised to 1150°C within 35 minutes (i.e., the temperature of the porcelain boat containing WS2 powder is 1150°C), at which temperature the WS2 powder can be effectively sublimated, and the temperature of porcelain boat 2 is 600-800°C (i.e., the porcelain boat containing SiO2 / Si substrate), and the temperature of porcelain boat 3 is 80-100°C (i.e., the porcelain boat containing deionized water).

[0046] Specifically, in this embodiment, the temperature is kept constant for 20 minutes, and the carrier gas carrying WS2 vapor is delivered to the SiO2 / Si substrate for deposition. After the reaction is completed, the tubular furnace is naturally cooled to room temperature, and the deposited SiO2 / Si substrate is taken out.

[0047] The synthesized super-twisted helical WS2 crystals on the SiO2 / Si substrate obtained in this example show two forms, left-handed and right-handed, under an optical microscope, with a size of about 10-20 μm. However, in this example, due to the lack of deionized water, the synthetic torsion angle range is mainly 10°-20°, such as Figure 5 and 6AFM (atomic force microscopy) profiles further revealed that these layered flakes exhibited a spiral morphology, with uniform twist angles between layers and a thickness of 0.7-0.8 nm for each layer, which is consistent with the thickness of a single layer of WS2, indicating the high crystalline quality of the synthesized material. Figure 7 shown.

[0048] In this example, the optical test of the prepared structure was carried out. Figure 8 The schematic diagram of the laser optical path for the second harmonic intensity test of the spiral structure is shown. The measurement of the second harmonic signal is carried out under a customized reflective confocal optical microscope. In the test, an 800nm ​​femtosecond laser is used to vertically incident on the WS2 thin film with a twisted spiral structure. An electric-controlled translation stage is used to accurately control the number of incident layers. A 100X objective lens (NA=0.9) is used to collect the reflected light signal, and the second harmonic signal is collected by a spectrometer through a short-pass filter. Crystals with twist angles of 13°, 16°, and 20° were tested, and the following results were obtained: Fig. 9 The results show the oscillatory dependence of the second harmonic intensity on the number of layers, confirming that the torsion angle will significantly change the crystal structure symmetry that affects the second harmonic intensity, causing the recovery and breaking of the spatial inversion symmetry, thereby achieving the modulation of the second-order nonlinear effect on the same crystal, such as Fig.10 As shown. It can also be observed that when the twist angle increases from 13° to 16° and then to 20°, the number of layers where the second harmonic intensity begins to decrease changes from 6 layers to 5 layers and then to 4 layers. This shows that the oscillation period is significantly shortened with the increase in the number of layers, which is consistent with the crystal structure characteristics of the three-fold rotational symmetry of the single-layer WS2 flake. In addition, as Fig.11 As shown in the figure, the second-order nonlinear susceptibility of the super-twisted spiral WS2 crystals with angles of 13°, 16° and 20° are calculated to be 0.68, 0.46, 0.23 and 0.21 nm / V respectively, indicating that the super-twisted spiral WS2 crystals with different torsion angles have different second-order nonlinear response capabilities under strong field conditions, especially in the field of second harmonic generation, which are larger than the 2.04 pm / V value of the commercial frequency doubling crystal BBO.

[0049] Embodiment 2

[0050] The second embodiment of the present invention provides a method for modulating the second-order nonlinear effect in a super-twisted spiral WS2 crystal by regulating the interlayer torsion angle. Similar to the first embodiment, the method is based on the following: Figure 4 The experimental setup shown is implemented, including the following steps:

[0051] 1) Take SiO2 / Si wafer as substrate, cut into 10mm×15mm size, ultrasonically clean in acetone and ethanol solution for 15min, blow dry with high-purity nitrogen to obtain a clean growth substrate. In addition, a quartz glass cylinder with a length of 20mm and a diameter of 17mm as a stopper is also ultrasonically cleaned and dried in an oven at 60℃.

[0052] 2) Using WS2 powder as raw material, place a certain amount of WS2 powder in porcelain boat 1, and spread the SiO2 / Si substrate flat on porcelain boat 2. In addition, drop 100 μl of deionized water into porcelain boat 3; then place porcelain boat 1 in the center of the tube furnace, place porcelain boat 2 near the air outlet, 8 cm away from the center of the tube furnace, place porcelain boat 3 at the edge of the air inlet, and place the quartz glass cylinder in the center of porcelain boats 1 and 3.

[0053] 3) Use high-purity argon as carrier gas, control the flow rate to 80 sccm, and exhaust the air in the furnace. Keep the carrier gas input, and raise the temperature of porcelain boat 1 to 1150°C within 35 minutes (i.e., the temperature of the porcelain boat containing WS2 powder is 1150°C). At this temperature, WS2 powder can be effectively sublimated. At this time, the temperature of porcelain boat 2 is 600-800°C (i.e., the porcelain boat containing SiO2 / Si substrate), and the temperature of porcelain boat 3 is 80-100°C (i.e., the porcelain boat containing deionized water).

[0054] 4) Maintain the temperature for 20 minutes, carry the WS2 vapor with the carrier gas to the SiO2 / Si substrate for deposition, and after the reaction is completed, cool the tube furnace naturally to room temperature and take out the deposited SiO2 / Si substrate.

[0055] Specifically, different from the first embodiment, in this embodiment, 100 μl of deionized water is placed in the porcelain boat 3. On the SiO2 / Si substrate obtained in this example, the synthesized continuous multilayer twisted spiral WS2 structure has a random twist angle interval of 20°-30°, which is caused by the increase of deionized water and the enlargement of the formed protrusions. Fig.12 As shown, scanning transmission electron microscopy (STEM) was performed on a super-twisted spiral WS2 crystal of about 30°, and its low-resolution high-angle annular dark field (HAADF) imaging and average electron diffraction pattern (PACBED) verified the accuracy of determining the twist angle along the edge, clearly demonstrating the consistent twist angle between adjacent layers.

[0056] Embodiment 3

[0057] Embodiment 3 of the present invention provides a method for modulating the second-order nonlinear effect in a super-twisted spiral WS2 crystal by controlling the interlayer torsion angle. Similar to Embodiment 1, it is based on Figure 4 The experimental setup shown is implemented, including the following steps:

[0058] 1) Take SiO2 / Si wafer as substrate, cut it into 10mm×15mm size, ultrasonically wash it in acetone and ethanol solution for 15min respectively, and blow dry it with high-purity nitrogen to obtain a clean growth substrate.

[0059] 2) Using WS2 powder as raw material, place a certain amount of WS2 powder in porcelain boat 1, and spread the SiO2 / Si substrate flat on porcelain boat 2. In addition, drop 50 μl of deionized water into porcelain boat 3; then place porcelain boat 1 in the center of the tube furnace, place porcelain boat 2 near the air outlet, 8 cm away from the center of the tube furnace, and place porcelain boat 3 at the edge of the air inlet.

[0060] 3) Use high-purity argon as carrier gas, control the flow rate to 80 sccm, and exhaust the air in the furnace. Keep the carrier gas input, and raise the temperature of porcelain boat 1 to 1150°C within 35 minutes (i.e., the temperature of the porcelain boat containing WS2 powder is 1150°C). At this temperature, WS2 powder can be effectively sublimated. At this time, the temperature of porcelain boat 2 is 600-800°C (i.e., the porcelain boat containing SiO2 / Si substrate), and the temperature of porcelain boat 3 is 80-100°C (i.e., the porcelain boat containing deionized water).

[0061] 4) Maintain the temperature for 20 minutes, carry the WS2 vapor with the carrier gas to the SiO2 / Si substrate for deposition, and after the reaction is completed, cool the tube furnace naturally to room temperature and take out the deposited SiO2 / Si substrate.

[0062] Specifically, different from the first embodiment, in this embodiment, a quartz glass cylinder is no longer placed in the center of the porcelain boat 1 and the porcelain boat 3 , but a quartz glass cylinder is placed as the stopper 6 .

[0063] The optical image of the synthesized super-twisted spiral WS2 crystal on the SiO2 / Si substrate obtained in this example shows that the left-handed / right-handed spiral WS2 structure with different twist angles is obtained, and the size is 10-20μm. Since the smooth quartz glass cylinder is no longer placed, the sample yield becomes lower. This is because the quartz glass cylinder will cause the formation of a wake zone, thereby reducing the gas velocity and introducing air flow disturbances to produce more nucleation points.

[0064] Embodiment 4

[0065] Embodiment 4 of the present invention provides a method for modulating the second-order nonlinear effect in a super-twisted spiral WS2 crystal by regulating the interlayer torsion angle. Similar to Embodiment 1, it is based on Figure 4 The experimental setup shown is implemented, including the following steps:

[0066] 1) Take SiO2 / Si wafer as substrate, cut into 10mm×15mm size, ultrasonically clean in acetone and ethanol solution for 15min, blow dry with high-purity nitrogen to obtain a clean growth substrate. In addition, a quartz glass cylinder with a length of 20mm and a diameter of 17mm as stopper 6 is also ultrasonically cleaned and dried in an oven at 60℃.

[0067] 2) Using WS2 powder as raw material, place a certain amount of WS2 powder in porcelain boat 1, and spread the SiO2 / Si substrate flat on porcelain boat 2. In addition, drop 50 μl of deionized water into porcelain boat 3; then place porcelain boat 1 in the center of the tube furnace, place porcelain boat 2 near the air outlet, 8 cm away from the center of the tube furnace, place porcelain boat 3 at the edge of the air inlet, and place the quartz glass cylinder in the center of porcelain boats 1 and 3.

[0068] 3) Use high-purity argon as carrier gas, control the flow rate to 120 sccm, and exhaust the air in the furnace. Keep the carrier gas input, and raise the temperature of porcelain boat 1 to 1150°C within 35 minutes (i.e., the temperature of the porcelain boat containing WS2 powder is 1150°C). At this temperature, WS2 powder can be effectively sublimated. At this time, the temperature of porcelain boat 2 is 600-800°C (i.e., the porcelain boat containing SiO2 / Si substrate), and the temperature of porcelain boat 3 is 80-100°C (i.e., the porcelain boat containing deionized water).

[0069] 4) Maintain the temperature for 20 minutes, carry the WS2 vapor with the carrier gas to the SiO2 / Si substrate for deposition, and after the reaction is completed, cool the tube furnace naturally to room temperature and take out the deposited SiO2 / Si substrate.

[0070] Specifically, different from the first embodiment, in this embodiment, the carrier gas flow rate is controlled to be 120 sccm, the air in the furnace is evacuated, and the flow rate is kept constant until the deposition is completed.

[0071] The optical image of the synthesized super-twisted spiral WS2 crystal on the SiO2 / Si substrate obtained in this example shows that left-handed / right-handed spiral WS2 structures with different twist angles are obtained, with a size between 8-15μm, which is relatively small, indicating that the carrier gas flow rate affects the size of the material.

[0072] Comparative Example 1

[0073] The other conditions were the same as those in Example 1, except that the temperature of the porcelain boat 1 containing the WS2 powder was set to 1200° C., and the temperature on the SiO2 / Si substrate was 900° C. The obtained precipitate was observed to be mostly dendritic, with only a few left-handed / right-handed helical WS2 structures.

[0074] Comparative Example 2

[0075] The other conditions are the same as those in Example 1, except that the temperature of the porcelain boat 1 containing the WS2 powder is set to 1050° C., and the temperature on the SiO2 / Si substrate is 700° C. The obtained precipitate is observed to find that the obtained product is less in size and smaller in size, and is basically in a non-crystalline state.

[0076] Principle and advantages: The present invention uses WS2 powder as the source material and adopts a carefully designed water-assisted chemical vapor deposition method to generate nucleation impurity points on a non-Euclidean substrate. By strictly controlling the raw material heating temperature, deposition temperature, carrier gas flow rate and deionized water content, the WS2 vapor inside the tube furnace is in a low supersaturated state. Under this condition, dislocation lines are easily generated during WS2 vapor deposition growth. When the dislocation line is located on a non-Euclidean substrate, due to the mismatch between the crystal period and the substrate period, different twisted spiral structures will be formed according to the position of the dislocation line: when the dislocation line is located at the center of the protrusion, the lattice period is greater than the substrate period, and a left-handed twisted spiral WS2 is generated; when the dislocation line is located at the edge of the protrusion, the lattice period is less than the substrate period, and a right-handed twisted spiral WS2 is generated. In addition, the deionized water content affects the twist angle: when the content is low, the twist angle is 10°-20°; when the content is high, the twist angle is 20°-30°. By properly placing a quartz glass cylinder as a stopper, reducing the carrier gas flow rate and introducing gas disturbance, the formation of nucleation points is promoted, thereby increasing the yield of super-twisted spiral WS2 crystals. This super-twisted spiral WS2 crystal belongs to the moiré superlattice system, and its uniform interlayer torsion angle provides a new method for regulating the second-order nonlinear effect. The main mechanism is that the uniform torsion angle periodically modulates the inversion symmetry of the crystal, thereby achieving the regulation of the second-order nonlinear effect.

[0077] The process of the present invention is simple, easy to control, and low in cost. The obtained sample has a layer-by-layer twisted structure with a twist angle range of 10°-30°. Under laser excitation, the sample shows the oscillation dependence of the second harmonic intensity on the number of layers, and the super-twisted spiral WS2 crystals with different twist angles have different second-order nonlinear magnetic susceptibilities, thereby realizing the adjustability of the second-order nonlinear effect with the twist angle and the number of layers. This highly crystalline super-twisted spiral WS2 crystal has broad application prospects in the field of micro-nano nonlinear integrated optoelectronic devices.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation, characterized in that: The following steps are involved: Step 1: Place WS2 powder in a first porcelain boat (1), place a SiO2 / Si substrate horizontally on a second porcelain boat (2), and place deionized water in a third porcelain boat (3); then place the first porcelain boat (1) in the middle heating area of ​​a horizontal quartz tube (5) of a tube furnace (4), place the second porcelain boat (2) on the air outlet side of the horizontal quartz tube (5), and place the third porcelain boat (3) on the air inlet side of the horizontal quartz tube (5); at the same time, place a block (6) between the first porcelain boat (1) and the third porcelain boat (3), and make the tail flow area of ​​the block (6) cover the position of the second porcelain boat (2); Step 2: introducing carrier gas, exhausting the air in the furnace and continuing to introduce carrier gas, and heating the horizontal quartz tube (5) through the tubular furnace (4) so ​​that the heating temperature of the area where the first porcelain boat (1) is located is 1140-1160° C.; Step 3: Continue to introduce carrier gas, and keep the heating temperature of the area where the first porcelain boat (1) is located at a constant temperature of 1140-1160°C, waiting for the carrier gas to carry the evaporated WS2 source to the SiO2 substrate and complete the deposition. After the deposition is completed, wait for natural cooling, take out and obtain a layered spiral WS2 two-dimensional nanomaterial with a layer-by-layer twist angle.

2. The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation according to claim 1, characterized in that: The following steps are also included: The content of deionized water placed in the third porcelain boat (3) is controlled to control the twist angle of the super twisted spiral WS2 crystal, thereby controlling its second-order nonlinear effect.

3. According to the method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation as described in claim 1, the stopper (6) is a horizontally placed quartz glass cylinder, and its cross-sectional area is 60-70% of the horizontal quartz tube (5).

4. The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation according to claim 1, characterized in that: The carrier gas is one of helium, neon and argon.

5. The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation according to claim 1, characterized in that: In step 2 and step 3, the flow rate of the carrier gas is kept constant within the range of 80-120 sccm, and the pressure inside the tube furnace (4) is normal pressure.

6. The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation according to claim 1, characterized in that: In the step 1, the amount of deionized water used is 50 to 100 μL.

7. The method for preparing a super-twisted spiral WS2 crystal for second-order nonlinear effect modulation according to claim 1, characterized in that: In step 3, the deposition time is 10-30 minutes.

8. An application of a super-twisted spiral WS2 crystal in second-order nonlinear effects, characterized in that: The super-twisted helical WS2 crystal Prepared according to the preparation method according to claim 1.

9. The application of a super twisted spiral WS2 crystal in second order nonlinear effect according to claim 8, characterized in that: By controlling the torsion angle of the super-twisted spiral WS2 crystal, its second-order nonlinear performance can be controlled.

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