Preparation method of MX2 / CrCl3 heterojunction

Abstract

The invention discloses a preparation method of an MX2 / CrCl3 heterojunction, which comprises the following steps of: cleaning a SiO2 / Si substrate to obtain a clean surface, adhering a block sample by using a 3M adhesive tape, obtaining a dense and uniformly distributed thick-layer material on the adhesive tape in a manner of repeatedly adhering and tearing two adhesive tapes, and then stripping a two-dimensional material from the surface of the thick-layer material on the adhesive tape by using PDMS (Polydimethylsiloxane) to obtain the MX2 / CrCl3 heterojunction. Single-layer MX2 and few-layer CrCl3 are identified by means of an optical microscope or a photoluminescence spectrum; the method comprises the following steps: sequentially stacking a few layers of CrCl3 and a single layer of MX2 on a cleaned SiO2 / Si substrate by virtue of a two-dimensional transfer platform so as to construct an MX2 / CrCl3 two-dimensional Van der Waals heterostructure; performing vacuum annealing treatment on the prepared MX2 / CrCl3 sample to enhance the interface coupling of the MX2 / CrCl3 sample so as to obtain an MX2 / CrCl3 two-dimensional Van der Waals heterojunction sample with relatively high quality; and the circularly polarized laser excites the MX2 / CrCl3 heterojunction to induce out-field-free room temperature valley polarization of MX2, and finally, non-resonant excited room temperature valley polarization under the action of no external field is realized.

Description

Technical Field

[0001] The present invention relates to two-dimensional materials, and more particularly, to a method for preparing an MX2 / CrCl3 heterojunction. Background Art

[0002] Two-dimensional transition metal dichalcogenides (TMDs) are a type of graphene-like two-dimensional layered material that has received extensive attention in recent years. Their chemical formula is generally expressed as MX2. Due to the broken inversion symmetry and spin-orbit coupling in monolayer MX2, the valley polarization of monolayer MX2 can be manipulated by circularly polarized light, making it an ideal carrier for studying valleytronics.

[0003] However, the valley polarization degree of monolayer MX2 has problems such as dependence on external fields (low temperature, external magnetic field, electric field, etc.), strict requirements for the photon energy of the excitation light (resonant excitation or near-resonant excitation is required), and short valley depolarization lifetime, which limit its application and development in valleytronics.

[0004] In addition, the reported methods for preparing two-dimensional van der Waals heterojunctions currently include chemical vapor deposition, wet transfer stacking, etc. However, these methods all have certain limitations, such as preparation efficiency issues, unstable quality of the prepared samples, and chemical reagent contamination. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing an MX2 / CrCl3 heterojunction to achieve the purpose of increasing the room-temperature valley degree of MX2.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: A method for preparing an MX2 / CrCl3 heterojunction, comprising the following steps: Step 1, clean the SiO2 / Si substrate to obtain a sample substrate with a clean surface; Step 2, repeatedly bond and tear the MX2 bulk material and the CrCl3 bulk material with tape, and the layered MX2 and layered CrCl3 are adhered to the tape; then use PDMS to lift the layered MX2 and layered CrCl3 on the tape to obtain a two-dimensional sample with MX2 and CrCl3 on the PDMS; Step 3, use an optical microscope to distinguish and select PDMS samples with monolayer WS2 and few-layer CrCl3 respectively; Step 4, utilize the thermal release property of PDMS, and with the aid of a two-dimensional transfer platform, stack few-layer CrCl3 and monolayer WS2 onto the cleaned SiO2 / Si substrate in sequence to complete the preparation of the MX2 / CrCl3 heterojunction; Step 5: Place the SiO2 / Si substrate with the MX2 / CrCl3 heterojunction into a tube furnace for vacuum annealing treatment; Step 6: Use circularly polarized laser to excite the MX2 / CrCl3 heterojunction to induce room-temperature valley polarization of MX2.

[0007] As a preferred embodiment, in Step 1, divide the SiO2 / Si wafer into rectangular SiO2 / Si substrates and place them in a beaker, then place the beaker in an ultrasonic cleaner for ultrasonic cleaning three times. Acetone, anhydrous ethanol, and deionized water are added to the beaker as cleaning agents during the three ultrasonic cleanings. After the first two ultrasonic cleanings, use a nitrogen gun to dry the surface of the SiO2 / Si substrate. After the third ultrasonic cleaning, take out the SiO2 / Si substrate and place it upside down on the surface of the filter paper, and finally put it into a drying oven for drying for later use.

[0008] As a preferred embodiment, in Step 2, for the MX2, M is Mo or W, and X is S or Se.

[0009] As a preferred embodiment, in Step 2, take a piece of 3M tape and pick up some visible samples from the bulk material; take a second piece of 3M tape, and with the sample area as the center, repeatedly bond and tear the two tapes to form a dense and evenly distributed sample area on the tape; cut the PDMS according to the size of the sample area; use pointed tweezers to remove the flexible protective layer of the PMDS, and then paste the freshly exposed surface of the PDMS to the sample area on the tape; after standing for 5 - 10 minutes, lift the PDMS from the tape to obtain a two-dimensional sample on the PDMS.

[0010] As a preferred embodiment, in Step 3, utilize the high luminescence efficiency of monolayer MX2 as a direct-bandgap semiconductor and distinguish it with the fluorescence mode or PL spectrum of an optical microscope.

[0011] As a preferred embodiment, in Step 3, utilize the characteristic that the thinner the two-dimensional CrCl3 layer, the more transparent it is, and then distinguish few-layer CrCl3.

[0012] As a preferred embodiment, in Step 4, fix the PDMS with CrCl3 to the robotic arm of the two-dimensional transfer platform through a glass slide; fix the SiO2 / Si substrate to the sample stage of the two-dimensional transfer platform; control the robotic arm of the two-dimensional transfer platform to descend so that the PDMS with CrCl3 and the SiO2 / Si substrate are in contact; heat the sample stage to 80 °C and stand for 10 - 20 minutes; lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, so that CrCl3 is transferred onto the SiO2 / Si substrate; use the same transfer process to stack monolayer MX2 on top of few-layer CrCl3.

[0013] As a preferred embodiment, the monolayer MX2 is vertically aligned with the few-layer CrCl3 sample on the SiO2 / Si substrate, and a part of the monolayer MX2 is transferred onto the SiO2 / Si substrate.

[0014] As a preferred embodiment, in step five, the SiO2 / Si substrate stacked with the MX2 / CrCl3 heterojunction sample is placed at the center of the temperature zone of the tube furnace, with a vacuum degree of 10 -4 Pa, an annealing temperature of 190 - 210 °C, and annealed for 0.5 - 1.5 hours.

[0015] As a preferred embodiment, in step five, the annealing temperature is 200 °C and annealed for 1 hour.

[0016] MX2 and chromium trichloride (CrCl3), as layered materials, the weak van der Waals force between their layers provides an ideal way for the simple construction of high-quality two-dimensional heterojunctions. In the present invention, a monolayer MX2 sample and a few-layer chromium trichloride (CrCl3) sample are obtained by mechanical exfoliation method, then a MX2 / CrCl3 van der Waals heterostructure is obtained by thermal release dry transfer, and the coupling of the heterojunction interface is enhanced by vacuum annealing, thereby preparing a high-quality MX2 / CrCl3 two-dimensional van der Waals heterojunction; and a circularly polarized excitation light is used to excite the MX2 / CrCl3 heterojunction sample, thereby obtaining room-temperature valley polarization of MX2. Description of the Drawings

[0017] Figure 1 It is a schematic diagram of the vacuum annealing device in Example 1 of the present invention; in the figure, 1 - tube furnace, 2 - quartz tube, 3 - heterojunction sample, 4 - vacuum pump, 5 - controller.

[0018] Figure 2 It is a schematic diagram of the WS2 / CrCl3 heterojunction sample prepared in Example 1 of the present invention; Figure 3 It is a bright-field mode image of the optical microscope of the WS2 / CrCl3 heterojunction sample prepared in Example 1 of the present invention; Figure 4 It is a fluorescence image of the WS2 / CrCl3 heterojunction sample prepared in Example 1 of the present invention; Figure 5 It is the PL spectrum of the monolayer WS2 region and the heterojunction region in the WS2 / CrCl3 sample prepared in Example 1 of the present invention; Figure 6 It is the Raman spectrum of the monolayer WS2 region, the few-layer CrCl3 region and the heterojunction region in the WS2 / CrCl3 sample prepared in Example 1 of the present invention; Figure 7 It is the helicity-resolved PL spectrum of the monolayer WS2 region in the WS2 / CrCl3 sample prepared in Example 1 of the present invention; Figure 8 It is the helicity-resolved PL spectrum of the heterojunction region in the WS2 / CrCl3 sample prepared in Example 1 of the present invention; Figure 9 It is the bright-field mode image of the optical microscope of the MoS2 / CrCl3 heterojunction sample prepared in Example 2 of the present invention. Detailed implementation manners

[0019] In a semiconductor heterojunction based on type-II band alignment, the spatial separation of electrons and holes can significantly extend the exciton lifetime. Combining with the valley-spin optical selectivity of monolayer MX2, the enhancement of its valley polarization lifetime and the improvement of the valley polarization degree can be achieved. In the screening of heterojunction materials, on the premise of meeting the type-II band alignment with MX2, we noticed the peculiar band-edge structure (approximate flat band) of two-dimensional chromium trihalide, which has a lower requirement for the momentum matching degree during charge transfer, thus generating a unique charge transfer behavior in the heterojunction. Considering the environmental stability of the material, two-dimensional chromium trichloride (CrCl3) was finally selected as the other material of the heterostructure. In view of this, the content of the present invention is to construct an MX2 / CrCl3 two-dimensional van der Waals heterojunction with monolayer MX2 and few-layer CrCl3, thereby realizing the improvement of the room-temperature valley polarization degree of MX2 without an external field, providing a new way to break through the bottleneck of the existing technology.

[0020] The overall concept of the present invention is to separately obtain high-quality monolayer MX2 and few-layer CrCl3 by mechanical exfoliation; then use the thermal release dry transfer technology to build the MX2 / CrCl3 van der Waals heterostructure; to further optimize the interface characteristics, the heterojunction sample is vacuum annealed, effectively enhancing the interface coupling effect of the heterojunction. Through this series of designed preparation processes, we successfully obtained high-quality MX2 / CrCl3 two-dimensional van der Waals heterojunction samples, and realized the enhancement of the room-temperature valley polarization degree of MX2 without an external field through the formation of the MX2 / CrCl3 heterojunction.

[0021] A method for preparing an MX2 / CrCl3 heterojunction provided by a typical implementation manner of the present invention includes the following steps 1 to 6.

[0022] Step 1, cleaning of the substrate.

[0023] Clean the SiO2 / Si substrate to obtain a sample substrate with a clean surface.

[0024] Specifically, the SiO2 / Si wafer was cut into multiple 1.2 cm × 0.8 cm rectangular SiO2 / Si substrates. Each cut SiO2 / Si substrate was placed face-up in a 5 ml beaker. Surface decontamination was performed using acetone (15 minutes of ultrasonic cleaning), anhydrous ethanol (15 minutes of ultrasonic cleaning), and deionized water (15 minutes of ultrasonic cleaning). After the first two ultrasonic cleanings, the SiO2 / Si substrate surface was dried using a nitrogen gun. After the third ultrasonic cleaning, the SiO2 / Si substrate was removed and placed upside down on filter paper. Finally, it was dried in a 60°C oven until ready for use.

[0025] Step 2: Prepare two-dimensional samples by mechanical exfoliation.

[0026] The MX2 bulk material and the CrCl3 bulk material were repeatedly bonded and torn apart using tape, and the layered MX2 and layered CrCl3 were adhered to the tape; then the layered MX2 and layered CrCl3 on the tape were peeled off using PDMS to obtain a two-dimensional sample with MX2 and CrCl3 on PDMS.

[0027] In the MX2, M is Mo or W, and X is S or Se. For example: WS2, MoS2 or WSe2, MoSe2.

[0028] For example, take a piece of 3M tape and stick a visible sample on the bulk material. Then, take a second piece of 3M tape and repeatedly stick and tear the two pieces together, centering on the sample area, to create a dense and evenly distributed sample area on the tape. Using two pieces of tape stacked together, rather than a single piece on its own, is important to avoid creases.

[0029] Cut the PDMS into the appropriate size according to the size of the sample area; use pointed tweezers to remove the flexible protective layer of PMDS, and then stick the fresh exposed surface of PDMS to the sample area on the tape; after standing for 5 to 10 minutes, peel the PDMS off the tape at a certain speed, so that we can obtain a two-dimensional sample on PDMS.

[0030] Step 3: Distinguish between single-layer MX2 and few-layer CrCl3.

[0031] Optical microscopy was used to distinguish and select PDMS samples with single-layer WS2 and few-layer CrCl3.

[0032] The few-layer CrCl3 has 20 to 140 layers.

[0033] Monolayer MX2 is a direct bandgap semiconductor, which has properties such as high luminescence efficiency, and can be distinguished with the help of the fluorescence mode of an optical microscope or the photoluminescence (PL) spectrum.

[0034] Utilize the characteristic of different light transmittance due to different numbers of layers of two-dimensional CrCl3 to distinguish few-layer CrCl3.

[0035] Step 4: Fabrication of the heterostructure.

[0036] Adopt the thermal release dry transfer technique. With the aid of a two-dimensional transfer platform, through precise alignment by the robotic arm and real-time observation by the microscope, few-layer CrCl3 and monolayer WS2 are successively stacked onto a cleaned SiO2 / Si substrate to complete the preparation of the MX2 / CrCl3 heterojunction.

[0037] Specifically, fix the PDMS with the CrCl3 sample on a transparent glass slide, and then fix the glass slide on the robotic arm of the two-dimensional transfer platform; fix the SiO2 / Si substrate on the sample stage of the two-dimensional transfer platform, with the robotic arm above the sample stage; by adjusting the microscope on the two-dimensional transfer platform, respectively find the CrCl3 sample to be transferred on the PDMS and the area to be transferred on the SiO2 / Si substrate, and align the two areas in the vertical direction; by controlling the robotic arm of the two-dimensional transfer platform to descend, so that the PDMS and the SiO2 / Si substrate are in contact; heat the sample stage to 80 °C and let it stand for 10 - 20 minutes; slowly lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, and thus the CrCl3 sample is transferred to the SiO2 / Si substrate.

[0038] For the step of stacking monolayer MX2 above the few-layer CrCl3 sample, it is the same as the CrCl3 sample transfer process described above and will not be elaborated here. The monolayer MX2 sample on the PDMS is aligned with the few-layer CrCl3 sample on the SiO2 / Si substrate in the vertical direction.

[0039] Step 5: Vacuum annealing treatment.

[0040] Put the SiO2 / Si substrate with the MX2 / CrCl3 heterojunction into a tube furnace for vacuum annealing treatment.

[0041] Place the SiO2 / Si substrate with the stacked MX2 / CrCl3 heterostructure sample in a corundum boat, and then place the boat at the center of the temperature zone of the tube furnace. Subsequently, install the vacuum annealing device; start the vacuum device and wait for the vacuum degree of the device to stabilize, and the pressure is usually around 10 -4 Pa; set the annealing program of the tube furnace, with the annealing temperature of 190 - 210 °C and annealing for 0.5 - 1.5 hours. Preferably, the annealing temperature is 200 °C and annealing for 1 hour. Then, start the tube furnace, wait for the heating program of the tube furnace to end and the furnace temperature to drop to room temperature, then terminate the operation of the vacuum system, and finally take out the sample.

[0042] Step 6: Circularly polarized laser excites the MX2 / CrCl3 heterojunction to induce room-temperature valley polarization in MX2.

[0043] Perform helicity-resolved PL spectroscopy measurements and use the spectral data to reflect the valley polarization degree of the test sample. Place a quarter-wave plate in the excitation optical path and adjust the angle between the linear polarization direction of the linearly polarized excitation light and the fast and slow axes of the quarter-wave plate to 45°, so as to generate chiral circularly polarized light; use circularly polarized light to excite the heterojunction region in the vacuum-annealed MX2 / CrCl3 sample, and utilize the valley circular dichroism selectivity of monolayer MX2 to excite one of the degenerate K valleys or K' valleys of MX2, thereby generating an initial imbalanced distribution of inter-valley carriers. Subsequently, due to the charge transfer between the MX2 layer and the CrCl3 layer in the MX2 / CrCl3 heterojunction, electrons and holes are respectively aggregated in the CrCl3 layer and MX2 to form interlayer excitons, and the interlayer excitons bind to the holes in the valence band of the excited valley of MX2, so that the valley polarization degree is maintained. Finally, room-temperature valley polarization under non-resonant excitation without an external field is achieved.

[0044] The following further illustrates the technical solutions claimed in the present invention through some embodiments. However, the embodiments are used to explain the implementation solutions of the present invention and do not exceed the scope of the present invention's theme. The protection scope of the present invention is not limited by the described embodiments. Unless otherwise specified, the materials and reagents used in the present invention can be obtained from commercial products in the art. Example 1

[0045] Divide the SiO2 / Si wafer into multiple rectangular SiO2 / Si substrates with a length of 1.2 cm and a width of 0.8 cm; place one of the cut SiO2 / Si substrates face up in a small beaker with a capacity of 5 ml; put the small beaker into an ultrasonic cleaner and ultrasonicate it three times, 15 minutes each time. During the three ultrasonication processes, add appropriate amounts of acetone / absolute ethanol / deionized water to the small beaker respectively, and pour out the waste liquid after each ultrasonication. After the first two ultrasonication processes, dry the surface of the SiO2 / Si substrate with a nitrogen gun. After the third ultrasonication is completed, take out the SiO2 / Si substrate and place it upside down on the surface of the filter paper, and finally put it into a drying oven at 60°C for drying for later use.

[0046] Take a piece of 3M tape and pick up some visible samples from the bulk material; take a second piece of 3M tape and repeatedly bond and tear the two tapes with the sample area as the center to form a dense and evenly distributed sample area on the tape; cut the PDMS into an appropriate size according to the size of the sample area; use a pair of pointed tweezers to remove the flexible protective layer of the PMDS, and then paste the freshly exposed surface of the PDMS onto the sample area on the tape; after standing for 5 - 10 minutes, lift the PDMS from the tape at a certain speed, and thus we obtain a two-dimensional sample on the PDMS.

[0047] Taking advantage of the high luminescence efficiency of monolayer WS2 as a direct-bandgap semiconductor, it can be resolved by means of the fluorescence mode of an optical microscope or PL spectroscopy; taking advantage of the characteristic that the thinner the two-dimensional CrCl3 layer is, the more transparent it is, few-layer CrCl3 can be resolved.

[0048] Fix the PDMS with the CrCl3 sample on a transparent glass slide, and then fix the glass slide on the robotic arm of the two-dimensional transfer platform; fix the SiO2 / Si substrate on the sample stage of the two-dimensional transfer platform, where the robotic arm is above the sample stage; by adjusting the microscope on the two-dimensional transfer platform, find the CrCl3 sample to be transferred on the PDMS and the transfer area on the SiO2 / Si substrate respectively, and align the two areas vertically; control the robotic arm of the two-dimensional transfer platform to descend, so that the PDMS and the SiO2 / Si substrate are attached; heat the sample stage to 80 °C and let it stand for 10 - 20 minutes; slowly lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, and thus the CrCl3 sample is transferred to the SiO2 / Si substrate. For the step of stacking monolayer WS2 on top of the few-layer CrCl3 sample, it is basically the same as the CrCl3 sample transfer process described above, with the difference being that before attaching the PDMS to the SiO2 / Si substrate in the previous step, the monolayer WS2 sample on the PDMS needs to be aligned vertically with the few-layer CrCl3 sample on the SiO2 / Si substrate.

[0049] Place the SiO2 / Si substrate with the stacked WS2 / CrCl3 heterojunction sample at the center of the temperature zone of the tube furnace, and then install the vacuum annealing device; start the vacuum device and wait for the vacuum degree of the device to stabilize at 10 -4 Pa; set the annealing program of the tube furnace to anneal at 200 °C for 1 hour, and then start the tube furnace; wait for the heating program of the tube furnace to end and the furnace temperature to drop to room temperature, then terminate the operation of the vacuum system, and finally take out the sample.

[0050] Perform helicity-resolved PL spectroscopy testing and use the spectral data to reflect the valley polarization state of the test sample. Place a quarter-wave plate in the excitation optical path and adjust the linear polarization direction of the 532 nm linearly polarized excitation light to form an angle of 45° with the fast axis and slow axis of the quarter-wave plate, so as to generate chiral circularly polarized light; use circularly polarized light to excite the heterojunction region in the vacuum-annealed WS2 / CrCl3 sample, and use the valley optical activity selectivity of monolayer WS2 to excite one of the degenerate K valleys or K′ valleys of WS2 energy, so as to generate an initial inter-valley carrier imbalance distribution. Subsequently, due to the charge transfer from the conduction band electrons of the WS2 layer to the conduction band of the CrCl3 layer in the WS2 / CrCl3 heterojunction, the valley polarization degree is maintained; The schematic diagram of the vacuum annealing device used in this example is as Figure 1As shown in the figure. The vacuum annealing device includes a tubular furnace 1, a quartz tube 2, a heterojunction sample 3, a vacuum pump 4, and a controller. The quartz tube 2 is arranged in the middle of the furnace chamber of the tubular furnace 1. One end of the vacuum pump 4 is connected to the quartz tube 2, and the controller 5 is connected to the vacuum pump 4. The vacuum pump 4 is a molecular pump and a mechanical pump.

[0051] The molecular pump and the mechanical pump are used to evacuate the quartz tube 2 with one end sealed in the tubular furnace 1 at the same time. During the operation of the system, the internal pressure of the quartz tube 2 is 10 -4 pa.

[0052] The schematic diagram of the WS2 / CrCl3 heterojunction prepared in this example is as Figure 2 , where the WS2 layer is located on the CrCl3 layer, and the WS2 / CrCl3 heterojunction is located on the SiO2 / Si substrate.

[0053] The bright-field mode image of the optical microscope of the WS2 / CrCl3 heterojunction sample prepared in this example is as Figure 3 . The position selected by the gray dashed box is the monolayer WS2, the position selected by the black dashed box is the few-layer CrCl3, and the overlapping part of the two dashed boxes is the WS2 / CrCl3 heterojunction region.

[0054] The fluorescence image of the WS2 / CrCl3 heterojunction sample prepared in this example is as Figure 4 , where the heterojunction region of the WS2 / CrCl3 sample is selected by the white dashed box. The darker the fluorescence image, the weaker the luminescence in this region. The luminescence of the heterojunction region is weaker than that of the monolayer WS2. This is because after photoexcitation, the conduction band electrons of WS2 transfer to CrCl3, thus quenching the luminescence of WS2.

[0055] The PL spectra of the monolayer WS2 region and the heterojunction region in the WS2 / CrCl3 sample prepared in this example are as Figure 5 , for the convenience of comparison in the spectrum, the PL intensity of the heterojunction region has been multiplied by 5. The PL emission peak of the monolayer WS2 is located at 618.1 nm. The PL intensity of the heterojunction region is reduced by 97.3% compared with the monolayer WS2 region, and the PL peak position is red-shifted by about 1.2 nm, proving that the interface coupling of this heterojunction is good.

[0056] The Raman spectra of the monolayer WS2 region, the few-layer CrCl3 region, and the heterojunction region in the WS2 / CrCl3 sample prepared in this example are as Figure 6 . In the Raman spectrum of the heterojunction, the Raman characteristic peaks of the monolayer WS2 and the few-layer CrCl3 can be clearly distinguished, proving that the coupling of the heterojunction is good.

[0057] The helicity-resolved PL spectrum of the monolayer WS2 region in the WS2 / CrCl3 sample prepared in this example is as Figure 7. In this spiral-resolved PL spectrum test, the excitation light was in a non-resonant excitation configuration of 532 nm, and the test was carried out at room temperature without an external field acting on the sample (such as a magnetic field, electric field, etc.). The two spectral lines SCP and OCP in the spectrum are signal components collected under the same and opposite chirality configurations as the excitation light, respectively. Among them, the signal at 618.7 nm is the A exciton state PL signal of the monolayer WS2. The coincidence of the SCP and OCP spectral lines here proves that under non-resonant excitation and without an external field, the monolayer WS2 cannot have room temperature valley polarization; at the same time, in the spectrum, the intensity contrast of the SCP and OCP spectral lines at 532 nm is about 20:1, which proves that the spiral-resolved configuration of the optical path we built is in line with expectations.

[0058] The spiral-resolved PL spectrum of the heterojunction region in the WS2 / CrCl3 sample prepared in this example is as follows Figure 8 .exist Figure 7 In the spiral-resolved PL spectrum test, except for placing a 550 nm long-wave filter at the signal collection point, other test conditions are the same as Figure 7 The test conditions are exactly the same. Figure 8 The non-overlap of the SCP and OCP lines near 621 nm in the spectrum proves the valley polarization state of the sample at this time, that is, constructing a WS2 / CrCl3 heterojunction can enable WS2 to still have room temperature valley polarization even under non-resonant excitation at 532 nm and without an external field.

[0059] In this example, a single-layer WS2 sample and a few-layer CrCl3 sample were obtained by mechanical exfoliation, and then a WS2 / CrCl3 van der Waals heterostructure was obtained by thermal release dry transfer. Vacuum annealing was then used to enhance the coupling of the heterojunction interface, thereby preparing a high-quality WS2 / CrCl3 two-dimensional van der Waals heterojunction. Finally, circularly polarized excitation light was used to excite the heterojunction region of the WS2 / CrCl3 sample, thereby obtaining the room-temperature valley polarization of WS2. Example 2

[0060] The SiO2 / Si wafer was divided into several rectangular SiO2 / Si substrates with a length of 1.2 cm and a width of 0.8 cm; a cut SiO2 / Si substrate was placed face up in a small beaker with a capacity of 5 ml; the small beaker was placed in an ultrasonic cleaner and ultrasonicated three times, each time for 15 minutes. During the three ultrasonications, appropriate amounts of acetone / anhydrous ethanol / deionized water were added to the small beaker respectively, and the waste liquid was poured out after each ultrasonication. After the first two ultrasonications, the surface of the SiO2 / Si substrate was blown dry with a nitrogen gun. After the third ultrasonication, the SiO2 / Si substrate was taken out and turned upside down on the surface of the filter paper, and finally placed in a drying oven at 60°C for drying.

[0061] Take a 3M tape and pick up some visible samples from the bulk material; take a second 3M tape, and repeatedly bond and tear the two tapes with the sample area as the center to form a dense and evenly distributed sample area on the tape; cut the PDMS into an appropriate size according to the size of the sample area; use a pair of pointed tweezers to remove the flexible protective layer of the PMDS, and then paste the freshly exposed surface of the PDMS onto the sample area on the tape; after standing for 5 - 10 minutes, lift the PDMS from the tape at a certain speed, and thus we obtain a two-dimensional sample on the PDMS.

[0062] Utilize the high luminous efficiency of monolayer MoS2 as a direct bandgap semiconductor, so that it can be resolved by means of the fluorescence mode of an optical microscope or PL spectrum; utilize the characteristic that the thinner the number of layers of two-dimensional CrCl3, the more transparent it is, and then distinguish few-layer CrCl3.

[0063] Fix the PDMS with the CrCl3 sample on a transparent glass slide, and then fix the glass slide on the robotic arm of the two-dimensional transfer platform; fix the SiO2 / Si substrate on the sample stage of the two-dimensional transfer platform, where the robotic arm is above the sample stage; by adjusting the microscope on the two-dimensional transfer platform, find the CrCl3 sample to be transferred on the PDMS and the area to be transferred on the SiO2 / Si substrate respectively, and align the two areas vertically; by controlling the robotic arm of the two-dimensional transfer platform to descend, the PDMS and the SiO2 / Si substrate are brought into contact; heat the sample stage to 80 °C and stand for 10 - 20 minutes; slowly lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, and thus the CrCl3 sample is transferred onto the SiO2 / Si substrate. For the step of stacking monolayer MoS2 above the few-layer CrCl3 sample, it is basically the same as the CrCl3 sample transfer process described above, with the difference being that in the previous step, the monolayer MoS2 sample on the PDMS needs to be aligned vertically with the few-layer CrCl3 sample on the SiO2 / Si substrate before the PDMS and the SiO2 / Si substrate are brought into contact.

[0064] Place the SiO2 / Si substrate with the stacked MoS2 / CrCl3 heterojunction sample at the center of the temperature zone of the tube furnace, and then install the vacuum annealing device; start the vacuum device and wait for the vacuum degree of the device to stabilize at 10 -4 Pa; set the annealing program of the tube furnace to anneal at 200 °C for 1 hour, and then start the tube furnace; wait for the heating program of the tube furnace to end and the furnace temperature to drop to room temperature, then terminate the operation of the vacuum system, and finally take out the sample.

[0065] Perform helicity-resolved PL spectroscopy measurements and use the spectral data to reflect the valley polarization state of the test sample. Place a quarter-wave plate in the excitation optical path and adjust the linear polarization direction of the linearly polarized excitation light to form a 45° angle with the fast axis and slow axis of the quarter-wave plate, thereby generating chiral circularly polarized light; use the circularly polarized light to excite the heterojunction region in the vacuum-annealed MoS2 / CrCl3 sample, and utilize the valley optical activity selectivity of monolayer MoS2 to excite one of the degenerate K valleys or K′ valleys in MoS2, thereby generating an initial imbalanced distribution of inter-valley carriers. Subsequently, due to the charge transfer between the MoS2 layer and the CrCl3 layer in the MoS2 / CrCl3 heterojunction, the valley polarization degree is maintained; The bright-field mode image of the prepared MoS2 / CrCl3 heterojunction sample in this example is as follows Figure 9 . The position outlined by the gray dashed box is monolayer MoS2, the position outlined by the black dashed box is few-layer CrCl3, and the overlapping part of the two dashed boxes is the MoS2 / CrCl3 heterojunction region. Example 3

[0066] Divide the SiO2 / Si wafer into multiple rectangular SiO2 / Si substrates with a length of 1.2 cm and a width of 0.8 cm; place one of the cut SiO2 / Si substrates face up in a small beaker with a capacity of 5 ml; put the small beaker into an ultrasonic cleaner and ultrasonicate it three times, 15 minutes each time. During the three ultrasonication processes, add appropriate amounts of acetone / absolute ethanol / deionized water to the small beaker respectively, and pour out the waste liquid after each ultrasonication. After the first two ultrasonication processes, dry the surface of the SiO2 / Si substrate with a nitrogen gun. After the third ultrasonication is completed, take out the SiO2 / Si substrate and place it upside down on the surface of the filter paper, and finally put it into a drying oven at 60 °C for drying for later use.

[0067] Take a piece of 3M tape and pick up some visible samples from the bulk material; take a second piece of 3M tape and repeatedly bond and tear the two tapes centered on the sample area to form a dense and evenly distributed sample area on the tape; cut the PDMS into an appropriate size according to the size of the sample area; use a pair of pointed tweezers to remove the flexible protective layer of the PMDS, and then paste the freshly exposed surface of the PDMS onto the sample area on the tape; after standing for 5 - 10 minutes, lift the PDMS from the tape at a certain speed, and thus we obtain a two-dimensional sample on the PDMS.

[0068] Utilize the high luminescence efficiency of monolayer WSe2 as a direct-bandgap semiconductor, so that it can be distinguished by the fluorescence mode or PL spectroscopy of an optical microscope; utilize the characteristic that the thinner the two-dimensional CrCl3 layer, the more transparent it is, and then distinguish few-layer CrCl3.

[0069] Fix the PDMS with the CrCl3 sample on a transparent glass slide, and then fix the glass slide to the robotic arm of the two-dimensional transfer platform; fix the SiO2 / Si substrate to the sample stage of the two-dimensional transfer platform, where the robotic arm is above the sample stage; by adjusting the microscope on the two-dimensional transfer platform, find the CrCl3 sample to be transferred on the PDMS and the area to be transferred on the SiO2 / Si substrate respectively, and align the two areas vertically; by controlling the robotic arm of the two-dimensional transfer platform to descend, the PDMS and the SiO2 / Si substrate are brought into contact; heat the sample stage to 80 °C and let it stand for 10 - 20 minutes; slowly lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, so that the CrCl3 sample is transferred to the SiO2 / Si substrate. For the step of stacking a single layer of WSe2 on top of the few-layer CrCl3 sample, it is basically the same as the CrCl3 sample transfer process described above, with the difference that: in the previous step, before bringing the PDMS into contact with the SiO2 / Si substrate, the single layer of WSe2 sample on the PDMS needs to be aligned vertically with the few-layer CrCl3 sample on the SiO2 / Si substrate.

[0070] Place the SiO2 / Si substrate with the stacked WSe2 / CrCl3 heterojunction sample at the center of the temperature zone of the tube furnace, and then install the vacuum annealing device; start the vacuum device and wait for the vacuum degree of the device to stabilize at 10 -4 Pa; set the annealing program of the tube furnace to anneal at 200 °C for 1 hour, and then start the tube furnace; wait for the heating program of the tube furnace to end and the furnace temperature to drop to room temperature, then terminate the operation of the vacuum system, and finally take out the sample.

[0071] Perform helical-resolved PL spectroscopy testing and use the spectral data to reflect the valley polarization state of the test sample. Place a quarter-wave plate in the excitation optical path and adjust the linear polarization direction of the linearly polarized excitation light to form an angle of 45° with the fast axis and slow axis of the quarter-wave plate, so as to generate chiral circularly polarized light; use the circularly polarized light to excite the heterojunction region in the vacuum-annealed WSe2 / CrCl3 sample, and use the valley optical activity selectivity of the single layer of WSe2 to excite one of the degenerate K valleys or K' valleys of WSe2, so as to generate an initial valley-carrier imbalance distribution. Subsequently, due to the charge transfer between the WSe2 layer and the CrCl3 layer in the WSe2 / CrCl3 heterojunction, the valley polarization degree is maintained; Example 4

[0072] The difference between this example and Example 1 is only that: set the annealing program of the tube furnace to anneal at 190 °C for 1.5 hours. Example 5

[0073] The difference between this example and Example 1 is only that: set the annealing program of the tube furnace to anneal at 210 °C for 0.5 hours.

[0074] The scope of protection claimed by the present invention is not limited to the above specific embodiments. For those skilled in the art, the present invention may have various modifications and changes, and any modifications, improvements, and equivalent replacements made within the concept and principle of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing an MX2 / CrCl3 heterojunction, characterized in that, It includes the following steps: Step 1: Clean the SiO2 / Si substrate to obtain a sample substrate with a clean surface; Step 2: Use tape to repeatedly bond and tear the MX2 bulk material and the CrCl3 bulk material respectively, and the layered MX2 and the layered CrCl3 are adhered to the tape; then use PDMS to lift the layered MX2 and the layered CrCl3 on the tape to obtain a two-dimensional sample with MX2 and CrCl3 on the PDMS; Step 3: Use an optical microscope to distinguish and select PDMS samples with monolayer WS2 and few-layer CrCl3 respectively; Step 4: Utilize the thermal release property of PDMS, with the help of a two-dimensional transfer platform, stack few-layer CrCl3 and monolayer WS2 onto the cleaned SiO2 / Si substrate in sequence to complete the preparation of the MX2 / CrCl3 heterojunction; Step 5: Put the SiO2 / Si substrate with the MX2 / CrCl3 heterojunction into a tube furnace for vacuum annealing treatment; Step 6: Use circularly polarized laser to excite the MX2 / CrCl3 heterojunction to induce room-temperature valley polarization of MX2.

2. The method according to claim 1, wherein: In Step 1, divide the SiO2 / Si wafer into rectangular SiO2 / Si substrates and put them into a beaker, then place them in an ultrasonic cleaner and ultrasonicate three times. Acetone, anhydrous ethanol, and deionized water are added to the beaker as cleaning agents during the three ultrasonications. After the first two ultrasonications, use a nitrogen gun to dry the surface of the SiO2 / Si substrate. After the third ultrasonication, take out the SiO2 / Si substrate and place it upside down on the surface of the filter paper, and finally put it into a drying oven for drying for later use.

3. The method according to claim 1 or 2, characterized in that: In Step 2, for the MX2, M is Mo or W, and X is S or Se.

4. The method according to claim 3, wherein: In Step 2, take a 3M tape and pick up some visible samples from the bulk material; take a second 3M tape, and repeatedly bond and tear the two tapes with the sample area as the center to form a dense and evenly distributed sample area on the tape; cut the PDMS according to the size of the sample area; use pointed tweezers to remove the flexible protective layer of the PMDS, and then paste the fresh exposed surface of the PDMS onto the sample area on the tape; after standing for 5 - 10 minutes, lift the PDMS from the tape to obtain a two-dimensional sample on the PDMS.

5. The method according to claim 1 or 4, characterized in that: In Step 3, utilize the high luminescence efficiency of monolayer MX2 as a direct bandgap semiconductor and distinguish it with the fluorescence mode or PL spectrum of an optical microscope.

6. The method according to claim 5, wherein: In Step 3, utilize the characteristic that the thinner the two-dimensional CrCl3 layer, the more transparent it is, and then distinguish few-layer CrCl3.

7. The method according to claim 1 or 6, characterized in that: In Step 4, fix the PDMS with CrCl3 to the robotic arm of the two-dimensional transfer platform through a glass slide; fix the SiO2 / Si substrate to the sample stage of the two-dimensional transfer platform; control the robotic arm of the two-dimensional transfer platform to descend so that the PDMS with CrCl3 and the SiO2 / Si substrate are in contact; heat the sample stage to 80°C and stand for 10 - 20 minutes; lift the robotic arm to separate the PDMS from the SiO2 / Si substrate, so that CrCl3 is transferred to the SiO2 / Si substrate; adopt the same transfer process to stack monolayer MX2 on top of the few-layer CrCl3.

8. The method according to claim 7, wherein: The single-layer MX2 is vertically aligned with the few-layer CrCl3 sample on the SiO2 / Si substrate, and part of the single-layer MX2 is transferred to the SiO2 / Si substrate.

9. The method according to claim 1 or 8, characterized in that: In step five, place the SiO2 / Si substrate stacked with MX2 / CrCl3 heterojunction samples at the center of the temperature zone of the tube furnace, with a vacuum of 10 -4 Pa, an annealing temperature of 190 - 210 °C, and anneal for 0.5 - 1.5 hours.

10. The method according to claim 9, wherein: In step five, the annealing temperature is 200 °C and the annealing time is 1 hour.

Images