Analysis method for anisotropic thermal transport characteristics of suspended WTe2 thin films

Abstract

The present invention relates to an analysis method for the anisotropic thermal transport characteristics of a suspended WTe₂ film, comprising the following steps: S1, providing a suspended WTe₂ film; S2, determining the lattice orientation of the suspended WTe₂ film; S3, performing photo-heating Raman and electro-heating Raman tests along different lattice directions, thereby calculating the in-plane thermal conductivity and the thermal transport anisotropy ratio of the suspended WTe₂ film along the zigzag direction and the armchair direction. By analyzing the anisotropic thermal transport characteristics of the suspended WTe₂ through the method of the present invention, it can provide reference and guidance for the structural design, performance optimization, and regulation and management of thermal dissipation of WTe₂ functional devices.

Description

Technical Field

[0001] The present invention relates to the technical field of applications of two-dimensional materials, and particularly to an analysis method for the anisotropic thermal transport characteristics of a suspended WTe2 thin film. Background Art

[0002] Tungsten ditelluride (WTe2) is an anisotropic two-dimensional material with a lattice structure of low symmetry. Compared with isotropic two-dimensional materials with a highly symmetric lattice structure, anisotropic two-dimensional materials have more diverse physical properties in aspects such as thermology, mechanics, and electricity in the plane. Figure 1 The following shows the low-symmetry lattice structure of tungsten ditelluride (WTe2): Among them, the W-Te-W atomic layer shows a square lattice through the vertical stacking along the z-axis by van der Waals forces. The W atoms form W-W chains along the x-axis, and the upper Te atomic layer is rotated 180° relative to the lower Te atomic layer, showing an obvious anisotropic structure. Among them, the x-axis is defined as the zigzag direction, and the y-axis is defined as the armchair direction.

[0003] Compared with isotropic materials, anisotropic two-dimensional materials can provide new degrees of freedom for the structural design, physical property regulation, and direction application exploration of electronic devices. Tungsten ditelluride (WTe2) is one of the low-symmetry two-dimensional materials that have received more attention in recent years. This material has advantages such as a large atomic mass, a low Debye temperature, an unsaturated giant magnetoresistance, and superconducting properties, and thus has broad application prospects for functional devices. For anisotropic two-dimensional materials, the low-symmetry lattice structure makes the phonon scattering of the material have directional differences, thus showing anisotropic thermal transport characteristics. Two-dimensional materials with thermal anisotropy can enable the heat generated during the operation of devices based on anisotropic two-dimensional materials to be preferentially dissipated along the direction with high thermal conductivity. With the development of devices, the size of transistors is gradually miniaturized, and the integration density is gradually increased. A large increase in power consumption per unit area will form local hot spots inside the device, and when the heat accumulation is serious, it may damage the circuit and thus affect the device life. Therefore, how to effectively manage the heat of the device to avoid heat accumulation is very important for the structural design and performance optimization of actual devices.

[0004] Based on this problem, it is necessary to focus on exploring and regulating the thermal properties of materials in the design of modern integrated circuits. Among the many excellent physical properties of anisotropic two-dimensional materials, understanding the anisotropic thermal transport characteristics will be beneficial to effectively manage the heat of the device and thus extend the service life of the device, and can provide valuable guidance for the structural design and performance optimization of WTe2 devices. Summary of the Invention

[0005] (I) Technical Problems to be Solved

[0006] In view of the problems of the prior art, the present invention provides an analysis method for the anisotropic thermal transport characteristics of a suspended WTe2 thin film. By analyzing the anisotropic thermal transport characteristics of the suspended WTe2, it can provide reference and guidance for the structural design, performance optimization, and regulation and management of heat dissipation of WTe2 functional devices.

[0007] (II) Technical Solution

[0008] An analysis method for the anisotropic thermal transport characteristics of a suspended WTe2 thin film, comprising:

[0009] S1. Provide a suspended WTe2 thin film; the suspended WTe2 thin film is a structure formed by covering a hole area on a silicon substrate with a WTe2 thin film;

[0010] S2. Determine the lattice orientation of the suspended WTe2 thin film;

[0011] S3. Perform Raman spectroscopy tests along the zigzag direction and the armchair direction of the suspended WTe2 thin film at different temperatures and different laser powers, analyze the variation law of the Raman characteristic peak frequency with temperature and laser power, and substitute it into the corresponding relational formulas I and II to calculate the first-order temperature coefficient χT and the first-order power coefficient χP of the suspended WTe2 thin film along the zigzag direction and the armchair direction respectively;

[0012] Among them, the relational formula of the Raman characteristic peak frequency with temperature is as follows:

[0013] ω = ω0 + χ T T Formula I

[0014] Among them, ω0 is the Raman characteristic peak frequency of the sample at a temperature of 0K, T is the current actual temperature, and χ T corresponds to the first-order temperature coefficient, that is, the relationship between the Raman characteristic peak frequency of the material and temperature;

[0015] The linear relational formula of the Raman characteristic peak frequency with power is as follows:

[0016] Δω = ω(P2) - ω(P1) = χ P( P2 - P1) = χ P ΔP Formula II

[0017] Among them, P is the laser power magnitude, Δω is the change in the Raman characteristic peak frequency brought about by the power increasing from P1 to P2, and χ P is the first-order power coefficient, that is, the relationship between the Raman characteristic frequency of the material and power;

[0018] S4. Measure the absorption rates of the test sample along the zigzag direction and the armchair direction at the excitation laser wavelength, and then combine with the sample thickness h to calculate the in-plane thermal conductivity and the thermal transport anisotropy ratio along the zigzag direction and the armchair direction of the suspended WTe2 film according to Formula III;

[0019] The in-plane thermal conductivity κ of the suspended WTe2 film sample is calculated according to the following formula:

[0020] κ = χ T (1 / 2πh)(δω / δP A ) -1 Formula III;

[0021] The thermal transport anisotropy ratio = κ zigzag / κa rmchair ;

[0022] where h is the thickness of the sample, δω is the change in the characteristic peak frequency after the sample absorbs the power of δP A , P A is the absorption power of the sample, P A = A × P, P is the magnitude of the laser power, and A is the absorption rate value of the sample at the test laser wavelength.

[0023] According to a preferred embodiment of the present invention, in S1, the preparation method of the suspended WTe2 film is as follows:

[0024] S11. Take a clean glass slide and wipe it clean with alcohol, cut a piece of PDMS and stick it on the glass slide for later use; cut a clean tape, and use tweezers to place a piece of WTe2 bulk material on one side surface of the tape;

[0025] S12. Stick the side of the tape with the WTe2 bulk material facing the PDMS on the glass slide onto the PDMS, and gently press to make the tape fit tightly with the PDMS; then lift one side of the tape and quickly peel the tape off the PDMS, so that a thin layer of the WTe2 bulk material on the tape covers the surface of the PDMS; place the glass slide and the PDMS as a whole under an optical microscope to observe, and select a sample area with uniform thickness for preparing the suspended WTe2 film;

[0026] S14. Provide a silicon substrate with multiple holes having a diameter of 12 μm. In an inverted manner, move the PDMS with a WTe2 thin layer and a glass slide with the WTe2 thin layer facing downwards close to and adhere to the silicon substrate, so that the selected sample area of the WTe2 thin layer corresponds to the hole area of the silicon substrate. During the adhesion, heat the silicon substrate, control the heating temperature at 105 - 115 °C and keep it warm for 8 - 12 min. Then, cool it down to transfer the selected sample area of the WTe2 thin layer on the PDMS to the holes of the silicon substrate, thereby obtaining a suspended WTe2 thin film.

[0027] According to a preferred embodiment of the present invention, in S2, the method for determining the lattice orientation of the suspended WTe2 thin film is as follows: Calibrate the lattice orientation through angle-resolved Raman spectroscopy; or first perform a preliminary identification of the lattice direction through an optical map, and then further determine the lattice orientation with the aid of angle-resolved Raman spectroscopy. Among them, it can be observed through an optical microscope that the edge along the zigzag direction is longer and straighter than the armchair direction. Therefore, a preliminary identification of the lattice direction can be made based on its optical map.

[0028] According to a preferred embodiment of the present invention, in S2, use a laser with an excitation wavelength, make the polarization direction of the incident laser parallel to the x-axis, and keep the direction of the analyzer the same as the polarization direction and unchanged. Perform angle-polarized Raman spectroscopy characteristic tests on the suspended WTe2 sample, and determine the lattice orientation of the suspended WTe2 thin film accordingly. The specific steps are as follows:

[0029] (1) Fix both the laser incident direction and the direction of the analyzer on the x-axis and place the sample on the rotating sample stage; among them, the initial placement direction of the sample is such that the initially identified zigzag direction in the optical map is parallel to the x-axis. At this time, the angle θ between the lattice direction of the sample and the laser incident direction is 0°.

[0030] (2) Drive the sample to rotate clockwise around the z-axis by different angles by the rotating sample stage to make the angle θ between the lattice direction of the sample and the laser incident direction vary between 0° and 360°. The test interval is 10 - 20° (preferably 15°), and obtain the angle-polarized Raman spectra of the sample at different angles.

[0031] (3) Extract the Raman characteristic peak intensity values corresponding to different angles θ, fit a polarization diagram, and further analyze the polarization diagram to obtain the lattice orientation of the sample.

[0032] According to a preferred embodiment of the present invention, during the analysis according to the polarization diagram, cooperate with the analysis of the relationship between the Raman intensity of the A1 mode and the angle θ according to the Placzek approximation method. In the parallel configuration, the unit vector of the incident light is e i = (cosθ, sinθ, 0), and the unit vector of the scattered light is e s=(cosθ, sinθ, 0), so the relationship between the Raman peak intensity of the A1 mode and the angle θ is described by the following formula:

[0033]

[0034] where a and b are the elements of the Raman tensor that determine the Raman peak intensity;

[0035] When a > b, the maximum value of the Raman peak intensity of the A1 mode appears at θ = 0° and θ = 180°, that is, the polarization direction of the incident light is parallel to the metal atom W-W chain of the WTe2 material and corresponds to the zigzag direction; when a < b, the maximum Raman peak intensity of the A1 mode is located at θ = 90° and θ = 270°, and at this time the polarization direction of the incident light is perpendicular to the metal atom W-W chain of the WTe2 material and corresponds to the armchair direction.

[0036] According to a preferred embodiment of the present invention, in S2, it further includes characterizing the thickness and surface uniformity of the prepared suspended WTe2 thin film by an atomic force microscope.

[0037] According to a preferred embodiment of the present invention, in S3, Raman spectroscopy tests are performed along the zigzag direction and the armchair direction of the suspended WTe2 thin film at different temperatures, specifically including:

[0038] (1) First, perform a variable-temperature test on the suspended WTe2 thin film, calibrate the relationship between the Raman characteristic peak frequency and temperature, and calibrate the Raman spectrum;

[0039] During the variable-temperature test, place the suspended WTe2 thin film sample in the sample chamber and seal it, then connect the sample chamber to the temperature controller and the liquid nitrogen tank; the controller controls the amount of liquid nitrogen input into the sample chamber by the liquid nitrogen tank for temperature control. After reaching the set temperature value and stabilizing for 3 - 5 minutes, perform Raman spectroscopy tests at the current temperature, so as to analyze the change of the Raman characteristic peak frequency of the suspended WTe2 thin film with temperature; during the test, the laser power is stabilized at ≤0.5 mW;

[0040] (2) Extract the 10 characteristic peak frequency of the A1 Raman mode of the suspended WTe2 thin film at different temperatures for plotting, and analyze to obtain 10 the curve of the characteristic peak frequency of the A1 Raman mode versus temperature change, and use formula I to fit this change curve. After fitting, obtain 10 the first-order temperature coefficient χ of the A1 mode along the zigzag direction and the armchair direction T .

[0041] Among them, the Raman spectra at different temperatures were kept in a low-temperature liquid nitrogen environment for testing to avoid damage to the sample at higher temperatures. During the Raman spectrum test, the laser power was kept within 0.5mW to prevent the laser from causing the sample temperature to rise and affect the experimental results and analysis. In the three main modes of the A1 Raman mode, 133cm -1 ( 4 A1), 163cm -1(8 A1) and 212cm -1 ( 10 In A1), 10 The A1 Raman characteristic peak is more sensitive to temperature, so in the following data extraction and analysis, select 10 A1 Raman characteristic peak The change of the Raman characteristic peak frequency of WTe2 material is analyzed to calculate its thermal conductivity.

[0042] According to a preferred embodiment of the present invention, in S3, Raman spectroscopy is tested along the zigzag direction and the armchair direction of the suspended WTe2 film at different laser powers, that is, the sample is heated by the laser power to characterize the in-situ Raman spectrum of the suspended WTe2 film along the zigzag direction and the armchair direction at different laser powers, and the relationship between the Raman characteristic peak frequency and the laser power is determined; specifically, the following steps are included:

[0043] Step 1: Focus the Raman laser spot on the center of the suspended WTe2 film, which corresponds to the center of the hole on the silicon substrate. The Raman spectrum is detected using a laser of the excitation wavelength and a 50x objective lens with an NA of 0.55. The laser spot is much smaller than the suspended area of the WTe2 film on the silicon substrate. The laser serves as both a heating source for the sample and a detection laser for the Raman spectrum.

[0044] Step 2: Increase the incident laser power from 106μW to 325μW and extract the sample under different laser powers. 10 A1 Raman characteristic peak frequency is plotted along the zigzag direction and the armchair direction as the laser power changes. The Raman characteristic peak frequency and laser power change curve is analyzed and fitted using formula II. 10 The first-order power coefficient χ of the A1 mode along the zigzag and armchair directions P .

[0045] In step 1, the laser spot diameter is only 616nm, which is much smaller than the suspended area of the WTe2 film on the silicon substrate (the diameter of the hole on the silicon substrate is 12μm). At this time, the heat transfer from the center of the suspended sample to the silicon substrate area at the edge of the hole can be ignored, which improves the detection accuracy. In step 2, the same laser power variation range causes 10The change of the A1 mode is more obvious. Therefore, in the subsequent extraction of the WTe2 thin film, 10 The frequency of the A1 Raman characteristic peak is plotted against the laser power along the zigzag direction and the armchair direction.

[0046] According to a preferred embodiment of the present invention, in S4, the method for testing the absorption rates corresponding to the excitation laser wavelength along the zigzag direction and the armchair direction of the test sample is as follows: Under the laser at the excitation wavelength (the same as the Raman detection laser wavelength mentioned above, usually 532 nm), fix the laser incident direction, rotate the suspended WTe2 thin film from 0° to 90°, and test the change trend of the absorption rate values in different directions; here, 0° corresponds to the zigzag direction of the sample, and 90° corresponds to the armchair direction of the sample.

[0047] According to a preferred embodiment of the present invention, when the excitation wavelength of the Raman spectroscopy test laser is 532 nm, the absorption rates of the suspended WTe2 thin film along the zigzag direction and the armchair direction are 55% ± 0.10% and 54% ± 0.10% respectively.

[0048] Substitute the first-order temperature coefficient χ T and the first-order power coefficient χ P、 The absorption rates corresponding to the excitation laser wavelength along the zigzag direction and the armchair direction of the sample, the sample thickness h, etc. are substituted into Formula III, and the in-plane thermal conductivities of the sample along the zigzag direction and the armchair direction can be obtained. The ratio of the in-plane thermal conductivity along the zigzag direction to the in-plane thermal conductivity along the armchair direction is calculated to obtain the thermal transport anisotropy ratio.

[0049] (III) Beneficial effects

[0050] By preparing the suspended WTe2 thin film and based on its low-symmetry lattice structure, the present invention calibrates the lattice orientation through angle-resolved Raman spectroscopy, and conducts temperature-variable and laser-power-variable photo-heating Raman tests and absorption rate tests along different lattice directions, and calculates and obtains the in-plane thermal conductivities and the thermal transport anisotropy ratio of the suspended WTe2 thin film along the zigzag direction and the armchair direction. Through analysis, it is obtained that the in-plane thermal conductivities of the suspended WTe2 thin film along the zigzag direction and the armchair direction are 4.45 W·m -1 .K -1 and 4.10 W·m -1 .K -1, which exhibits obvious thermal transport anisotropy, and the ratio of thermal transport anisotropy is about 1.09. Analyzing the anisotropic thermal transport characteristics of the suspended WTe2 thin film by the method of the present invention can provide reference and guidance for the structural design, performance optimization, and regulation and management of thermal dissipation of WTe2 functional devices. Description of the Drawings

[0051] Figure 1 is the low-symmetry lattice structure of WTe2, with blue being the metal atom W and gray being Te.

[0052] Figure 2 is the method for preparing the suspended WTe2 thin film sample by the mechanical exfoliation method.

[0053] Figure 3 is the suspended WTe2 thin film located in the hole area on the silicon substrate.

[0054] Figure 4 is the AFM image of the suspended WTe2 thin film in the hole area on the silicon substrate.

[0055] Figure 5 is the angular polarization Raman spectrum of the suspended WTe2 thin film at different angles. (a) is at different angles 4 A1, 8 A1 and 10 the trend chart of the change in the intensity of the A1 Raman characteristic peak; (b), (c), and (d) are the detailed enlarged views of the change in the intensity of the three Raman characteristic peaks with the angle, respectively.

[0056] Figure 6 is the color map of the angle-resolved Raman spectrum of the suspended WTe2 thin film at different angles.

[0057] Figure 7 is the polarization map of the change in the intensity of the Raman characteristic peak of the suspended WTe2 thin film at different angles; (a) is 3 the polarization map of the change in the intensity of the A2 Raman characteristic peak with the angle; (b), (c), and (d) are 4 A1, 8 A1, 10 the polarization map of the change in the intensity of the A1 Raman characteristic peak with the angle.

[0058] Figure 8 is the schematic diagram of the Raman temperature-variable test device.

[0059] Figure 9 At different temperatures, along the (a) zigzag direction and (b) armchair direction of the suspended WTe2 thin film 8 A1 and 10 the trend chart of the change in the frequency of the A1 Raman characteristic peak.

[0060] Figure 10 The linear fitting diagrams of the relationship between the frequency of the A1 Raman characteristic peak and the temperature change along the (a) zigzag direction and (b) armchair direction of the suspended WTe2 thin film 10 and the first-order temperature coefficient χ T are -0.0177 cm -1 .K -1 and 0.0126 cm - 1 .K -1 respectively.

[0061] Figure 11 Figure shows the schematic diagram of the optical heating Raman thermometry device.

[0062] Figure 12 The variation trend diagrams of the frequencies of the A1 and 8 A1 Raman characteristic peaks along the (a) zigzag direction and (b) armchair direction of the suspended WTe2 thin film under different laser powers 10 are shown.

[0063] Figure 13 The linear fitting diagrams of the relationship between the frequency of the A1 Raman characteristic peak and the laser power change along the (a) zigzag direction and (b) armchair direction of the suspended WTe2 thin film 10 and the first-order power coefficient χ P are -0.0069 cm -1 .μW -1 and -0.0053 cm -1 .μW -1 respectively.

[0064] Figure 14 Figure shows the change in the absorption rate of the suspended WTe2 thin film when the laser incident direction is fixed on the x-axis and the film rotates from 0° to 90°.

[0065] Figure 15 Figure shows the comparison of the in-plane thermal conductivities along the zigzag direction and armchair direction of the suspended WTe2 thin film; the abscissa in the figure is the lattice direction of the sample, and the ordinate is the value of the in-plane thermal conductivity. Detailed implementation manners

[0066] To better explain the present invention for easy understanding, the present invention will be described in detail below with reference to the accompanying drawings through specific implementation manners.

[0067] The present invention provides an analysis method for the anisotropic thermal transport characteristics of a suspended WTe2 thin film, which includes the following parts and will be described in detail below with reference to the accompanying drawings.

[0068] (1) Provide a suspended WTe2 thin film; the suspended WTe2 thin film is a structure formed by covering a hole area on a silicon substrate with a WTe2 thin film.

[0069] In the present invention, a mechanical exfoliation method is used to separate a bulk material to prepare a suspended WTe2 thin film. During the preparation process, the WTe2 bulk material is mechanically exfoliated by tape to obtain a WTe2 thin film on polydimethylsiloxane (PDMS), and then it is dry-transferred onto a silicon substrate with holes (multiple holes with a diameter of 12 μm are etched on the silicon substrate), so that the WTe2 thin film covers the holes, thereby forming a suspended WTe2 thin film region on the silicon wafer.

[0070] For the detailed process of preparing the suspended WTe2 thin film sample, see Figure 2 :

[0071] (1) Take a clean glass slide and wipe the debris on its surface with alcohol. Cut a piece of PDMS of any size and stick it in the middle of the glass slide for later use; (2) Cut a clean blue tape. Use tweezers to place a piece of WTe2 bulk material on one side of the tape, and repeatedly fold it until the area where the WTe2 bulk material is distributed on the tape is close to the size of the spare PDMS ( Figure 2 as shown in a)); (3) Gently stick the tape on the prepared PDMS / glass slide and gently press it to make the tape fit as closely as possible to the PDMS ( Figure 2 as shown in b)); (4) After close fitting, lift one side of the tape and quickly and completely peel it off from the PDMS / glass slide, so that the WTe2 material on the tape covers the surface of the PDMS ( Figure 2 as shown in c)); (5) Place the PDMS / glass slide under an optical microscope and select a sample area with uniform thickness and appropriate size for subsequent dry transfer method to prepare a suspended WTe2 thin film on the holes of the silicon substrate (multiple holes with a diameter of 12 μm are etched on the silicon substrate) ( Figure 2 as shown in d).

[0072] Among them, the process of dry transfer is as follows: Cut a piece of PDMS with a size equivalent to that of the transfer stage, attach it to the central area of the transfer stage, and attach the silicon substrate above the PDMS to fix the silicon substrate on the transfer stage with the help of PDMS; Under the microscope, use alcohol to wipe the holes and edges on the silicon substrate; Fix the PDMS with a WTe2 thin layer on its surface and the glass slide on the clip stage with the WTe2 thin layer facing down (i.e., in the order from top to bottom: glass slide / PDMS / WTe2), and find the selected sample area under the microscope; Adjust the position of the transfer stage to align the holes on the silicon substrate with the selected sample area; Manually lower the clip stage to make the PDMS approach the silicon substrate, and at the same time turn on the heating stage to heat the transfer stage carrying the silicon substrate to 60 °C (55 - 65 °C); Continue to manually lower the clip stage, observe the lowering progress with a microscope during the lowering process, stop manually lowering when the PDMS is close to the hole area on the silicon substrate, and heat the transfer stage at a step of 0.1 °C to make the PDMS under the silicon substrate expand thermally and lift the silicon substrate, so that it naturally fits with the PDMS above; When the WTe2 thin layer on the PDMS completely covers the holes on the silicon substrate, increase the heating rate, heat the transfer stage to about 110 °C (105 - 115 °C) to further increase the fitting tightness, and keep it warm for about 10 min (8 - 12 min); After the heat preservation ends, lower the temperature to make the transfer stage cool down slowly (the PDMS shrinks), until the glass slide / PDMS above is completely separated from the silicon substrate below. At this time, the selected sample area of the WTe2 thin layer on the PMDS on the clip stage is transferred to the hole area on the silicon substrate, and thus a suspended WTe2 thin film is prepared. The prepared suspended WTe2 thin film is subjected to subsequent morphological characterization and thermal transport performance testing.

[0073] According to the above method, the suspended WTe2 thin film located in the hole area on the silicon substrate as shown in Figure 3 is obtained. The suspended area is within the red circle in the figure; the hole diameter is 12 μm; the scale of the picture is 20 μm. From Figure 3 it can be seen that the coverage range of the WTe2 thin film is much larger than the hole size to obtain the suspended WTe2 thin film area. Since the dissociation energy in the zigzag direction is less than that in the armchair direction, during the mechanical exfoliation process, the WTe2 material is more likely to break along the zigzag direction, and thus the lattice direction can be preliminarily judged. Refer to Figure 3 the coordinate system in the lower left corner, and define the horizontal x-axis as the zigzag direction and the y-axis as the armchair direction.

[0074] (2) The thickness and surface uniformity of the suspended WTe2 thin film are characterized by atomic force microscopy, and the lattice orientation is further determined by angle-resolved Raman spectroscopy.

[0075] After the preparation of the suspended WTe2 thin film sample, the thickness and surface uniformity of the above-mentioned suspended WTe2 thin film were further characterized by atomic force microscopy (AFM) in tapping mode (mainly characterizing the morphology of the WTe2 material at the center of the hole region). From Figure 4 the height curve in, it can be seen that the thickness of the selected suspended WTe2 thin film here is about 50 nm, and the morphology map shows that the thickness of the WTe2 material is relatively uniform and can completely cover the holes on the silicon wafer. Figure 4 Figure Figure 4 is the AFM image of the suspended WTe2 thin film in the hole region on the silicon substrate; the small inset in the upper right corner is the height curve of the suspended WTe2 sample, with a thickness of about 50 nm; the scale bar of the image is 5 μm.

[0076] In addition, different WTe2 regions with different optical contrasts can be observed in the prepared sample under an optical microscope, corresponding to different sample thicknesses, and the sample region can be observed to be relatively flat. Thus, it can be seen that this exfoliation method can separate the WTe2 bulk material into a thin film with atomic-level thickness, relatively flat surface, few defects, and high mobility.

[0077] Next, it is also necessary to characterize its optical anisotropy and further calibrate its lattice orientation through angular resolved Raman spectroscopy tests to facilitate subsequent tests of the thermal transport properties along different lattice directions. Specifically, a rotatable sample stage is placed on the Raman spectrometer, and then the sample is placed at the center of the rotating stage. The angle θ between the lattice direction of the sample and the incident laser direction is changed by rotating the sample stage, and Raman spectroscopy tests are performed at different angles θ. The law of the change of the Raman characteristic peak intensity of the WTe2 material with the angle is analyzed through formulas to judge its lattice direction. During the test, a laser with an excitation wavelength of 532 nm is used, and the polarization direction of the incident laser and the direction of the analyzer are both parallel to the x-axis and remain unchanged, that is, the angular polarization Raman spectroscopy characteristics of the suspended WTe2 sample are always analyzed in the parallel configuration. The test process is as follows:

[0078] (1) Place it horizontally (parallel to the x-axis) at the center of the rotatable sample stage according to the initially identified zigzag direction in Figure 3 , and define the θ angle at this time as the initial 0°. The sample is rotated clockwise by adjusting the sample stage, so as to change the angle θ between the WTe2 lattice direction and the incident laser direction (the change range of θ is 0° - 360°, and the test interval angle is 15°) to obtain the Raman spectra of the suspended WTe2 thin film at different angles. Figure 5 Figure Figure 5 is the Raman spectra of some bands corresponding to different angles θ. The angles in the figure are the angles between the sample lattice direction and the laser incident direction (the change range is 0° - 360°), and the Raman peak intensity of the suspended WTe2 thin film changes with the angle. Among them, Figure 5(a) shows the variation trend of the Raman peak intensity of A1 at different angles. Figures (b)-(d) are the magnified details of the Raman spectra of the three main A1 modes at 133 cm 4 A1, 8 A1 and 10 the variation trend of the Raman peak intensity of A1; Figures (b)-(d) are the magnified details of the Raman spectra of the three main A1 modes at 133 cm -1 ( 4 A1), 163 cm -1 ( 8 A1) and 212 cm -1 ( 10 A1) at different angles θ.

[0079] (2) To observe more clearly the variation of the Raman peak intensity with the angle, the Raman peak intensity values of the suspended WTe2 thin film corresponding to different angles are extracted, and the relationship diagram between the Raman peak intensity and the angle is plotted. From this relationship diagram, a polarization diagram is fitted, and the lattice orientation of the sample is further analyzed through the polarization diagram.

[0080] As Figure 6 shown, it is the color map of the angle-resolved Raman spectrum of the suspended WTe2 thin film at different angles. The red part in the figure represents the region with the maximum peak intensity, and the blue part represents the region with the minimum peak intensity. It can be seen that the Raman peak intensity shows a periodic variation law with the change of the angle, and a polarization diagram is fitted. Through the polarization diagram, the periodic law of the peak intensity change with the angle is explored. Specifically, in the process of analyzing according to the polarization diagram, the relationship between the Raman intensity of the A1 and A2 modes and the angle θ can also be analyzed in combination with the Placzek approximation method, and the analysis results of the Placzek approximation method are mutually verified with the polarization diagram. As the angle θ changes from 0° to 360°, the Raman intensity changes of the A1 and A2 modes can be described by the following formula:

[0081]

[0082] is the Raman tensor, In the parallel configuration, the unit vector of the incident light is e i =(cosθ, sinθ, 0), and the unit vector of the scattered light is e s =(cosθ, sinθ, 0). Therefore, after substituting it into formula (1-1), the relationship between the Raman peak intensity of the A1 and A2 modes and the angle θ can be described as:

[0083]

[0084] where a, b, and d are the elements of the Raman tensor that determine the Raman peak intensity.

[0085] According to formulas (1-2) and (1-3), it can be seen that the intensity variation law of the A2-mode Raman characteristic peak of the suspended WTe2 thin film is quadruple symmetric with respect to the angle, while the intensity variation law of the A1-mode Raman characteristic peak is double symmetric with respect to the angle, which is consistent with Figure 7 the experimental results of the polarization diagrams drawn. At the same time, it can be seen from formula (1-2) that the variation law of the Raman peak intensity of the A1 mode is also related to the relationship between the Raman tensor elements a and b. When a > b, the maximum value of the Raman peak intensity of the A1 mode appears at θ = 0° and θ = 180°, that is, the polarization direction of the incident light is parallel to the metal atom W-W chain of the WTe2 material (corresponding to the zigzag direction), as 4 A1 and 8 the polarization directions of the A1 mode; when a < b, the maximum Raman peak intensity of the A1 mode is located at θ = 90° and θ = 270°, at this time the polarization direction of the incident light is perpendicular to the metal atom W-W chain of the WTe2 material (corresponding to the armchair direction), as 10 the polarization direction of the A1 mode is consistent with Figure 3 the preliminary judgment of the lattice orientation in

[0086] (3) Raman spectra were measured along the zigzag direction and the armchair direction of the suspended WTe2 thin film at different temperatures and different laser powers, the variation laws of the Raman characteristic peak frequencies with temperature and laser power were analyzed, and the first-order temperature coefficient χT and the first-order power coefficient χ P along the zigzag direction and the armchair direction of the suspended WTe2 thin film were calculated by substituting into the corresponding relational formulas I and II respectively.

[0087] First step, first perform a temperature-variable test on the suspended WTe2 thin film to calibrate the relationship between its Raman characteristic peak frequency and temperature, and calibrate the Raman spectrum. During the temperature-variable test, the sample was placed in the sample chamber of the temperature-variable test device as Figure 8 shown and sealed, and then the sample chamber was connected to the temperature controller and the liquid nitrogen tank. The temperature controller was adjusted to set the temperature of the sample chamber, that is, the amount of liquid nitrogen introduced into the sample chamber was adjusted for temperature control. After the temperature reading on the temperature controller reached the set value and stabilized for 3-5 minutes, the Raman spectrum was measured at the current temperature, so as to analyze the variation of the Raman characteristic peak frequency of the suspended WTe2 thin film with temperature. In order to avoid damaging the sample at higher temperatures, the Raman spectra at different temperatures were all measured in a low-temperature liquid nitrogen environment. In addition, during the Raman spectrum measurement process, the laser power was kept within 0.5 mW to avoid the laser causing the sample temperature to rise and affecting the analysis of the experimental results. As Figure 8 shown, for the suspended WTe2 thin film 8 A1 (located at 163 cm -1 ) and10 A1 (located at 212 cm -1 ) The variation of the peak position of the Raman mode with temperature (here the temperature range of the variable-temperature test is from 203 K to 273 K, and the temperature interval is 10 K). As can be seen from Figure 9 , as the temperature increases, 8 A1 and 10 the Raman peaks of the A1 mode both shift significantly towards the low-frequency direction, which is called red shift. This red shift phenomenon is caused by anharmonic lattice vibrations and thermal expansion brought about by temperature changes. It can also be observed from the figure that 8 A1 and 10 the red shift amounts of the A1 Raman characteristic peak frequencies with increasing temperature are significantly different. When the temperature changes from 203 K to 273 K, 8 the A1 Raman characteristic peak frequencies shift red by about 0.9 and 0.7 wavenumbers along the zigzag direction and the armchair direction respectively, while 10 the A1 Raman characteristic peak frequencies shift red by 1.2 and 0.8 wavenumbers along the zigzag direction and the armchair direction respectively; this research conclusion shows that 10 the A1 Raman characteristic peak is more sensitive to temperature. Therefore, in the subsequent data extraction and analysis, the 10 A1 Raman characteristic peak is selected to analyze the change of the Raman characteristic peak frequency of the WTe2 material with temperature to calculate its thermal conductivity.

[0088] Furthermore, the characteristic peak frequencies of the A1 Raman mode of the suspended WTe2 thin film at different temperatures are extracted for plotting, and the relationship between the Raman characteristic peak frequency and the temperature change is analyzed, and its Raman spectrum is calibrated. As 10 shown, as the temperature increases, Figure 10 the peak positions of the A1 Raman characteristic peak along the zigzag direction and the armchair direction both show a linear red shift trend, among which it shifts red by 1.2 wavenumbers along the zigzag direction and 0.8 wavenumbers along the armchair direction. Continuing to use the relationship formula (Formula I) between the Raman characteristic peak frequency and temperature to 10 fit the A1 Raman characteristic peak frequency and temperature change curve, and after fitting, the 10 first-order temperature coefficients χ of the A1 mode along the zigzag direction and the armchair direction 10 (representing the relationship between the Raman characteristic peak frequency of the suspended WTe2 thin film and temperature) are obtained, where the zigzag direction is -0.0177 ± 0.0008 cm T .K -1 , and the armchair direction is -0.0126 ± 0.0004 cm -1 .K -1 -1 ​, showing obvious anisotropy. Among them, the relationship formula between the Raman characteristic peak frequency and temperature is as follows:

[0089] ω = ω0 + χ T T (Formula I)

[0090] Among them, ω0 is the Raman characteristic peak frequency of the sample at a temperature of 0K, T is the current actual temperature, and χ T corresponds to the first-order temperature coefficient, that is, the relationship between the Raman characteristic peak frequency of the material and temperature.

[0091] For the WTe2 material with direction differences in the lattice structure, its first-order temperature coefficient χ T is different along the zigzag direction and the armchair direction, which is caused by the differences in the phonon group velocity and the mean free path in the zigzag and armchair directions.

[0092] In the second step, perform variable laser power tests of Raman thermometry, that is, heat the material through the laser power to characterize the in-situ Raman spectra of the suspended WTe2 thin film along the zigzag direction and the armchair direction under different laser powers, and determine the relationship between the Raman characteristic peak frequency and the laser power. During the test, the Raman laser spot is always focused on the central region of the suspended WTe2 thin film (i.e., the center position of the silicon substrate hole of the suspended WTe2 thin film), and the Raman spectrum is detected by a laser with an excitation wavelength of 532nm and a 50-fold (NA = 0.55) objective lens. The size of the laser spot (the diameter of the laser spot is 616nm) is much smaller than the suspended area of the WTe2 thin film on the silicon substrate (the diameter of the hole on the silicon substrate is 12μm), that is, the heat transfer from the center of the suspended sample to the substrate area at the edge of the hole can be ignored. As Figure 11 shown, it is a schematic diagram of the optical heating Raman thermometry device, where the laser serves as both the heating source of the sample and the detection laser of its Raman spectrum.

[0093] As Figure 12 shown, it is the Raman spectrum of the suspended WTe2 thin film when the incident laser power increases from 106 μW to 325 μW. As the laser power increases, the temperature of the area of the suspended WTe2 thin film irradiated by the laser rises, 8 A1 and 10 the frequencies of the A1 Raman characteristic peaks both show a red shift phenomenon of moving towards the low-frequency direction. Among them, when along the zigzag direction, 8 the frequency of the A1 Raman characteristic peak red-shifts by 0.8 wave numbers, while 10 the frequency of the A1 Raman characteristic peak red-shifts by 1.4 wave numbers. As described above, the 8 A1 and 10The change amounts of the A1 Raman characteristic peak frequencies are different, and 10 the change of the Raman characteristic peak of the A1 mode is more obvious.

[0094] Further, the 10 changes of the A1 Raman peak frequencies of the suspended WTe2 film along the zigzag direction and the armchair direction under different laser powers are extracted for plotting, and the relationship diagram between the Raman characteristic peak frequency and the laser power is analyzed. The relationship diagram shows that as the laser power increases, 10 the A1 Raman peak frequencies linearly redshift along both the zigzag direction and the armchair direction. Among them, it redshifts by 1.4 wave numbers along the zigzag direction and 1.1 wave numbers along the armchair direction. The relationship diagram between the Raman characteristic peak frequency and the laser power is fitted by the linear relationship formula (Formula II) of the Raman characteristic peak frequency varying with power, and the linear fitting diagram as shown in Figure 13 is obtained. Among them, the first-order power coefficients χ P (representing the relationship between the Raman characteristic peak frequency of the suspended WTe2 film and the laser power) along the zigzag direction and the armchair direction of the suspended WTe2 film are -0.0069±0.0002 cm -1 .μW -1 and -0.0053±0.0001 cm -1 .μW -1 respectively. That is, the sensitivity degree of the zigzag direction to the laser power is greater than that of the armchair direction, showing a more obvious redshift phenomenon.

[0095] The linear relationship formula of the above Raman characteristic peak frequency varying with power is as follows:

[0096] Δω = ω(P2) - ω(P1) = χ P (P2 - P1) = χ P ΔP (Formula II)

[0097] where P is the magnitude of the laser power, Δω is the change of the Raman characteristic peak frequency brought by the power increasing from P1 to P2, and χP is the first-order power coefficient, that is, the relationship between the Raman characteristic frequency of the material and the power.

[0098] (4) Measure the absorption rates corresponding to the excitation laser wavelength of the test sample along the zigzag direction and the armchair direction, and then combine with the sample thickness h to calculate the in-plane thermal conductivity and the ratio of thermal transport anisotropy of the suspended WTe2 film along the zigzag direction and the armchair direction according to Formula III;

[0099] The in-plane thermal conductivity κ of the suspended WTe2 film sample is calculated according to the following formula:

[0100] κ = χ T (1 / 2πh)(δω / δP A ) -1 Formula (III);

[0101] Thermal transport anisotropy ratio = κ zigzag / κa rmchair ;

[0102] where h is the thickness of the sample, δω is the change in the characteristic peak frequency of the sample after absorbing the power of δP A , P A is the absorption power of the sample, P A = AP, P is the magnitude of the laser power, and A is the absorption rate value of the sample at the test laser wavelength.

[0103] The method for measuring the absorption rates of the test sample along the zigzag direction and the armchair direction at the excitation laser wavelength is as follows: At a laser wavelength of 532 nm, fix the laser incident direction, rotate the suspended WTe2 film from 0° to 90°, and measure the change trend of the absorption rate values in different directions (as Figure 14 shown). According to the analysis of the above-mentioned diagonal polarization Raman spectrum, 0° here corresponds to the zigzag direction of the sample, and 90° corresponds to the armchair direction of the sample. Read the specific data to obtain the absorption rates of the suspended WTe2 film along the zigzag direction and the armchair direction as 55% and 54% respectively. In addition, considering the Poisson distribution of the continuous and stable number of photons during the test, as well as the influence of the noise and dark current read by the CCD on the experimental data, the error range of the experimental test is given as approximately 0.10%. Therefore, at a laser wavelength of 532 nm, the absorption rates of the suspended WTe2 film along the zigzag direction and the armchair direction are 55% ± 0.10% and 54% ± 0.10% respectively.

[0104] Substitute the thickness h and the first-order temperature coefficient χ T , the first-order power coefficient χ p , and the absorption rate values obtained from the above tests into Formula III to calculate the in-plane thermal conductivity values of the suspended WTe2 film along different lattice directions.

[0105] As Figure 15 shown, the in-plane thermal conductivities of the suspended WTe2 film along the zigzag direction and the armchair direction are 4.45 W·m -1 .K -1 and 4.10 W·m -1 .K -1 , the suspended WTe2 film sample shows obvious directional differences, and the thermal transport anisotropy ratio (k zigzag / karmchair ) is approximately 1.09. The anisotropy of this heat transport property can be attributed to the difference in the group velocity of the acoustic mode, that is, the group velocity along the zigzag direction is greater than that along the armchair direction.

[0106] The analysis of the anisotropic heat transport characteristics of the suspended WTe2 thin film by the method of the present invention can provide reference and guidance for the structural design, performance optimization, and regulation and management of heat dissipation of WTe2 functional devices.

[0107] 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 foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An analysis method for the anisotropic thermal transport characteristics of a suspended WTe2 thin film, characterized in that, It includes the following steps: S1. Provide a suspended WTe2 thin film; the suspended WTe2 thin film is a structure formed by covering the hole area of the silicon substrate with the WTe2 thin film; S2. Determine the lattice orientation of the suspended WTe2 thin film; S3. Perform Raman spectroscopy tests along the zigzag direction and armchair direction of the suspended WTe2 thin film at different temperatures and different laser powers respectively, analyze the variation law of the Raman characteristic peak frequency with temperature and laser power, and substitute into the corresponding relational formulas I and II to calculate the first-order temperature coefficient χT and the first-order power coefficient χP of the suspended WTe2 thin film along the zigzag direction and armchair direction respectively; Among them, the relational formula of the Raman characteristic peak frequency with temperature is as follows: ω = ω0 + χ T T formula I Among them, ω0 is the Raman characteristic peak frequency of the sample at a temperature of 0K, T is the current actual temperature, and χT corresponds to the first-order temperature coefficient, that is, the relationship between the Raman characteristic peak frequency of the material and temperature; The linear relational formula of the Raman characteristic peak frequency with power is as follows: Δω = ω(P2) - ω(P1) = χ P (P2 - P1) = χ P ΔP formula II Among them, P is the laser power magnitude, Δω is the change in the Raman characteristic peak frequency brought about by the power increasing from P1 to P2, and χP is the first-order power coefficient, that is, the relationship between the Raman characteristic frequency of the material and power; Among them, performing Raman spectroscopy tests along the zigzag direction and armchair direction of the suspended WTe2 thin film at different temperatures specifically includes: (1) First, perform a temperature-variable test on the suspended WTe2 thin film, calibrate the relationship between its Raman characteristic peak frequency and temperature, and calibrate the Raman spectrum; During the temperature-variable test, place the suspended WTe2 thin film sample in the sample chamber and seal it, and then connect the sample chamber to the temperature controller and the liquid nitrogen tank; the controller controls the amount of liquid nitrogen input into the sample chamber by the liquid nitrogen tank for temperature control. After reaching the set temperature value and stabilizing for 3 - 5 minutes, perform Raman spectroscopy tests at the current temperature to analyze the variation of the Raman characteristic peak frequency of the suspended WTe2 thin film with temperature; during the test, the laser power magnitude is stabilized at ≤0.5mW; (2) Extract the 10 characteristic peak frequencies of the A1 Raman mode of the suspended WTe2 thin film at different temperatures and plot them to analyze and obtain 10 the characteristic peak frequency - temperature change curve of the A1 Raman mode. Use formula I to fit this change curve, and after fitting, obtain 10 the first - order temperature coefficients χT of the A1 mode along the zigzag direction and the armchair direction; When performing Raman spectroscopy on the zigzag direction and armchair direction of the suspended WTe2 thin film at different laser powers, the Raman laser spot is always focused on the central region of the suspended WTe2 thin film. The diameter of the laser spot is 616 nm, the diameter of the suspended region of the WTe2 thin film is 12 μm, the incident laser power varies from 106 μW to 325 μW, and the 10 characteristic peak frequency of the A1 Raman mode of the suspended WTe2 thin film at different laser powers is extracted; The above-mentioned 10 The Raman shift of the A1 mode is 212 cm -1 ; S4. Test the absorbance corresponding to the excitation laser wavelength along the zigzag direction and armchair direction of the sample, and then combine with the sample thickness h to calculate the in-plane thermal conductivity and the thermal transport anisotropy ratio of the suspended WTe2 thin film along the zigzag direction and armchair direction according to formula III; The in-plane thermal conductivity κ of the suspended WTe2 thin film sample is calculated according to the following formula: κ = χ T (1 / 2πh)(δω / δP A ) -1 Formula III; Thermal transport anisotropy ratio = κ zigzag / κa rmchair ; where h is the thickness of the sample, δω is the change in the characteristic peak frequency of the sample after absorbing the power of δPA, and P A is the absorption power of the sample, and P A = A × P, where P is the laser power and A is the absorption rate value of the sample at the test laser wavelength.

2. The analysis method according to claim 1, wherein In S1, the preparation method of the suspended WTe2 thin film is: S11. Take a clean glass slide and wipe it clean with alcohol, cut a piece of PDMS and stick it on the glass slide for standby; cut a clean tape, and use tweezers to place a WTe2 bulk material on one surface of the tape; S12. Attach the side of the tape with WTe2 bulk material facing the PDMS on the glass slide to the PDMS, gently press to make the tape fit tightly with the PDMS; then lift one side of the tape and quickly peel it off from the PDMS, so that a thin layer of WTe2 bulk material on the tape covers the surface of the PDMS; place the glass slide and the PDMS as a whole under an optical microscope, and select a sample area with uniform thickness for preparing the suspended WTe2 thin film; S14. Provide a silicon substrate with multiple holes having a diameter of 12 μm. In an inverted manner, move the PDMS and the glass slide with the WTe2 thin layer covering the surface closer to and fit with the silicon substrate with the WTe2 thin layer facing downwards, so that the selected sample area of the WTe2 thin layer corresponds to the hole area of the silicon substrate. During the fitting process, heat the silicon substrate, control the heating temperature at 105 - 115 °C and keep it warm for 8 - 12 min. Then, cool it down to transfer the selected sample area of the WTe2 thin layer on the PDMS to the holes of the silicon substrate, thereby obtaining the suspended WTe2 thin film.

3. The analysis method according to claim 2, characterized in that In S2, the method for determining the lattice orientation of the suspended WTe2 thin film is: calibrate the lattice orientation through angle-resolved Raman spectroscopy; Or, first make a preliminary identification of the lattice direction through an optical map, and then further determine the lattice orientation with the help of angle-resolved Raman spectroscopy.

4. The analysis method according to claim 2, characterized in that, In S2, use a laser with an excitation wavelength, make the polarization direction of the incident laser parallel to the x-axis, keep the direction of the analyzer the same as the polarization direction and unchanged, perform angle-polarized Raman spectroscopy characterization on the suspended WTe2 sample, and determine the lattice orientation of the suspended WTe2 thin film accordingly. The specific steps are as follows: (1) Fix both the laser incident direction and the direction of the analyzer on the x-axis and place the sample on the rotating sample stage; among them, the initial placement direction of the sample is such that the preliminary identified zigzag direction in the optical map is parallel to the x-axis. At this time, the angle θ between the lattice direction of the sample and the laser incident direction is 0°; (2) Drive the sample to rotate clockwise around the z-axis by different angles by the rotating sample stage, so that the angle θ between the lattice direction of the sample and the laser incident direction changes between 0° and 360°, and the test interval is 10 - 20°, thereby obtaining the angle-polarized Raman spectra of the sample at different angles; (3) Extract the Raman characteristic peak intensity values corresponding to different angles θ, fit a polarization map, and further analyze the polarization map to obtain the lattice orientation of the sample.

5. The analysis method according to claim 4, characterized in that, In the process of analysis based on the polarization diagram, the relationship between the Raman intensity of the A1 mode and the included angle θ is analyzed in accordance with the Placzek approximation method. In the parallel configuration, the unit vector of the incident light is e i =(cosθ, sinθ, 0), and the unit vector of the scattered light is e s =(cosθ, sinθ, 0). Therefore, the relationship between the Raman peak intensity of the A1 mode and the angle θ is described by the following formula: Among them, a and b are the elements of the Raman tensor that determine the Raman peak intensity; When a > b, the maximum value of the Raman peak intensity of the A1 mode appears at θ = 0° and θ = 180°, that is, the polarization direction of the incident light is parallel to the metal atom W-W chain of the WTe2 material and corresponds to the zigzag direction; when a < b, the maximum Raman peak intensity of the A1 mode is located at θ = 90° and θ = 270°, and at this time the polarization direction of the incident light is perpendicular to the metal atom W-W chain of the WTe2 material and corresponds to the armchair direction.

6. The analysis method according to claim 2, characterized in that In S2, it also includes characterizing the thickness and surface uniformity of the prepared suspended WTe2 thin film by an atomic force microscope.

7. The analysis method according to claim 2, characterized in that, In S3, Raman spectroscopy tests were carried out at different laser powers along the zigzag direction and the armchair direction of the suspended WTe2 thin film, that is, the sample was heated by the laser power to characterize the in-situ Raman spectra of the suspended WTe2 thin film along the zigzag direction and the armchair direction at different laser powers, and to determine the relationship between the Raman characteristic peak frequency and the laser power; specifically including: Step 1: The Raman laser spot was always focused on the central region of the suspended WTe2 thin film, that is, the center of the hole corresponding to the silicon substrate. The Raman spectrum was detected by a laser with an excitation wavelength and an objective lens with a magnification of 50 and NA = 0.

55. The laser spot was much smaller than the suspended area of the WTe2 thin film on the silicon substrate. Among them, the laser served as both the heating source of the sample and the detection laser of the Raman spectrum; Step 2: Increase the incident laser power from 106 μW to 325 μW, and extract the 10 A1 Raman characteristic peak frequencies along the zigzag direction and the armchair direction of the sample under different laser powers are plotted, the Raman characteristic peak frequency versus laser power curve is analyzed, and the change curve is fitted using Equation II. After fitting, the 10 first-order power coefficients χ of the A1 mode along the zigzag direction and the armchair direction P .

8. The analysis method according to claim 2, characterized in that In S4, the method for testing the absorption rates of the sample along the zigzag direction and the armchair direction at the excitation laser wavelength was as follows: At the laser with the excitation wavelength, with the laser incident direction fixed, the suspended WTe2 thin film was rotated from 0° to 90°, and the change trend of the absorption rate values in different directions was tested; here, 0° corresponded to the zigzag direction of the sample, and 90° corresponded to the armchair direction of the sample.

9. The analysis method according to claim 1, wherein The excitation wavelength of the Raman spectroscopy test laser was 532 nm, and the absorption rates of the suspended WTe2 thin film along the zigzag direction and the armchair direction were 55% ± 0.10% and 54% ± 0.10% respectively.

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