Diamond anvil cell high pressure thermoelectric effect measuring device and measuring method

Abstract

The present application belongs to the technical field of material thermoelectric effect measurement, and particularly relates to a diamond anvil cell high-pressure thermoelectric effect measurement device and a measurement method. The measurement device comprises a diamond anvil cell and a multifunctional bottom electrode arranged on the lower anvil surface of the diamond anvil cell. The bottom electrode comprises a heating wire, a thermoelectric voltage measurement electrode, and first and second temperature measurement electrodes which are independently arranged and constitute a sample measurement area. During measurement, the sample to be measured is placed in the measurement area, a platinum strip is connected to the electrode and a measuring instrument to form a measurement system, after pressure calibration and temperature calibration, the heating wire is powered to establish a stable temperature difference and apply a vertical magnetic field, the temperature difference and voltage value are obtained through the electrodes and the measuring instrument, and then the Seebeck coefficient and the Nernst coefficient are calculated. The method can realize in-situ high-precision measurement of thermoelectric parameters of two-dimensional materials under high pressure, extremely low temperature and strong magnetic field environment, and provides key technical support for the research of thermoelectric materials in extreme environment and the development of devices.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material thermoelectric effect measurement, and particularly relates to a diamond anvil cell high-pressure thermoelectric effect measurement device and method for two-dimensional materials. BACKGROUND

[0002] Thermoelectric effect, as a kind of physical phenomenon directly realizing mutual conversion between heat energy and electric energy, has important application prospects in clean energy conversion and high-efficiency heat management system. Especially, the Seebeck effect (temperature difference leading to electric energy generation) and the Nernst effect (thermoelectric effect under the joint action of a magnetic field and a temperature gradient) have become the core physical mechanisms for developing high-efficiency thermoelectric conversion materials.

[0003] The thermoelectric conversion efficiency of a material is determined by a dimensionless figure of merit ZT=(S 2 σT) / κ, wherein S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. However, the ZT value of most current material systems is still at a low level, which leads to the difficulty of meeting the demand of industrial application in power generation efficiency and refrigeration performance, and becomes the main bottleneck for promoting the industrialization of thermoelectric technology.

[0004] The main challenge in improving the ZT value lies in breaking the inherent mutual restriction relationship among the Seebeck coefficient, the electrical conductivity, the thermal conductivity and other parameters. In recent years, two-dimensional layered materials are regarded as an important system for breaking the performance limitation of thermoelectric materials and realizing technical leap, because of the quantum confinement effect, the adjustable electronic structure, the rich interlayer coupling and the potential low-dimensional phonon transport characteristics. Theoretical research and preliminary experimental results show that by accurately regulating the interlayer spacing, the interlayer coupling strength and the interlayer interaction of two-dimensional materials, the energy band structure, the carrier concentration and mobility and the phonon scattering behavior can be effectively modulated, so as to greatly improve the thermoelectric conversion efficiency and the engineering application potential.

[0005] Applying external pressure is a clean and non-doped physical regulation method, which can effectively modulate the interlayer interaction of two-dimensional materials and avoid the carrier scattering caused by chemical disorder, and is suitable for developing high-performance thermoelectric materials for industrial applications. However, the existing thermoelectric measurement technology under pressure (such as a piston-cylinder device) is usually limited to a pressure below 3 GPa, and cannot reach the high-pressure region that may induce a stepwise improvement in thermoelectric performance or even the appearance of a new functional phase, which seriously limits the development and application of such materials in high-end thermoelectric energy systems.

[0006] Therefore, there is an urgent industrial demand for a new technology that can in-situ measure the high-precision thermoelectric parameters of two-dimensional materials under extremely high pressure conditions. SUMMARY

[0007] To solve the above technical problems, the present application first provides a diamond anvil cell high-pressure thermoelectric effect measuring device.

[0008] The technical scheme adopted by the present application is as follows:

[0009] The diamond anvil cell high-pressure thermoelectric effect measuring device for high-pressure thermoelectric effect measurement of two-dimensional materials comprises a diamond anvil cell, and a multifunctional bottom electrode is arranged on the lower anvil surface of the diamond anvil cell, wherein the multifunctional bottom electrode comprises a heating wire and a first temperature measuring electrode, a thermoelectric voltage measuring electrode and a second temperature measuring electrode which are arranged independently in sequence on one side of the heating wire; the thermoelectric voltage measuring electrode comprises four linear electrodes which are arranged in two rows in a symmetrical and spaced manner; the heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode and the second temperature measuring electrode are all Ti / Au composite plating films, and the heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode and the second temperature measuring electrode jointly constitute a sample measurement area.

[0010] Preferably, the heating wire, the first temperature measuring electrode and the second temperature measuring electrode are all in the form of a strip, and the heating wire is arranged in a serpentine shape to increase the heating area, and the first temperature measuring electrode and the second temperature measuring electrode are arranged in parallel with the thermoelectric voltage measuring electrode.

[0011] Preferably, in the thermoelectric voltage measuring electrode, the spacing between the two rows is less than the width of the sample to be measured, and the lengths of the first temperature measuring electrode and the second temperature measuring electrode are greater than the width of the sample to be measured.

[0012] Preferably, the widths of the first temperature measuring electrode, the second temperature measuring electrode and the heating wire are 1-2 μm, and the widths of the four linear electrodes are 2-3 μm.

[0013] Preferably, in the Ti / Au composite plating film, Ti is a bottom adhesion layer with a thickness of 5-10 nm, and Au is a surface conductive layer with a thickness of 30-50 nm.

[0014] Preferably, the preparation method of the measuring device comprises the following steps:

[0015] S1. The lower anvil surface of the diamond anvil cell is subjected to plasma cleaning, then the clean lower anvil surface is uniformly coated with photoresist and hardened, and then corresponding patterning treatment is performed according to the required arrangement position of the multifunctional bottom electrode;

[0016] S2. Ti film and Au film are sequentially evaporated on the lower anvil surface subjected to the patterning treatment;

[0017] S3. The excess photoresist is removed to obtain the prepared measuring device.

[0018] Preferably, in step S1, the photoresist uses the MMA / PMMA system, and the MMA is spin-coated at 4000 rpm for 30 s and then heated at 80 DEG C for 2 min, and then the PMMA is spin-coated at 4000 rpm for 60 s and then heated at 120 DEG C for 2 min, to complete the coating and hardening.

[0019] Preferably, in step S1, the process of the patterning treatment is that the lower anvil surface after hardening is subjected to electron beam exposure under a vacuum environment of 2x10 -5 mbar, then immersed in a developing solution for 20 s, and then immersed in isopropyl alcohol for 10 s for fixing; the developing solution is prepared from isopropyl alcohol and tetramethyl dioxane at a volume ratio of 2:1; in step S3, the photoresist is removed using acetone.

[0020] The application further provides a method for measuring high-pressure thermoelectric effect of a two-dimensional material, which is completed by using the measuring device as described above, and the specific steps are as follows:

[0021] Step one, place the two-dimensional material sample to be measured in the sample measuring area, and cover a layer of hexagonal boron nitride flake on the surface of the sample; one side of the sample is close to the heating wire but does not contact the heating wire; the sample forms good ohmic contact with the thermoelectric voltage measuring electrode and the first and second temperature measuring electrodes;

[0022] Step two, respectively connect the end portions of the heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode and the second temperature measuring electrode to the lower anvil surface edge by using platinum strips, and then respectively connect the platinum strips to the nanovolt meter, the current source and the temperature controller by using wires, to obtain a measuring system;

[0023] Step three, place the prepared lower anvil surface and the upper anvil surface of the diamond anvil in the center of the hydraulic machine, and then calibrate the pressure;

[0024] Step four, place the measuring system in a thermostat to cool down, to obtain the change rule of the resistance values of the first and second temperature measuring electrodes with temperature, for temperature calibration;

[0025] Step five, heat the heating wire to a set temperature, and after a stable temperature difference is generated at the two ends of the sample, obtain the temperature difference by the first and second temperature measuring electrodes, then apply a magnetic field perpendicular to the plane where the sample is located, and obtain the voltage between two linear electrodes in the same column of the thermoelectric voltage measuring electrode, in units of microvolts , and the voltage between two linear electrodes in different columns but symmetrically arranged in the thermoelectric voltage measuring electrode, in units of microvolts , and calculate the Seebeck coefficient of the sample according to the following formula and Nernst coefficient :

[0026] ;

[0027] wherein, T1 is the temperature of the end of the sample close to the heating wire measured by the first temperature measuring electrode, in Kelvin ; T2 is the temperature of the end of the sample far from the heating wire measured by the second temperature measuring electrode, in Kelvin ; L is the linear distance between two linear electrodes in the same column of thermoelectric voltage measuring electrodes, in microns ; D is the linear distance between the first temperature measuring electrode and the second temperature measuring electrode, in microns ;

[0028] ;

[0029] ;

[0030] wherein, B is the magnitude of the magnetic field perpendicular to the plane where the sample is located, in Tesla ; L' is the linear distance between two linear electrodes in the thermoelectric voltage measuring electrodes which are located in different columns but symmetrically arranged, in microns .

[0031] Preferably, in the step two, the nanovoltmeter is connected to the platinum strip connected to the thermoelectric voltage measuring electrodes, the current source is connected to the platinum strip connected to the heating wire, and two temperature controllers are provided, which are respectively connected to the platinum strips connected to the first temperature measuring electrode and the second temperature measuring electrode.

[0032] Preferably, in the step three, the method for pressure calibration is ruby fluorescence spectrum method.

[0033] Preferably, in the step four, the thermostat is a 12T Oxford cryostat, and the 12T Oxford cryostat is also used to generate the magnetic field perpendicular to the plane where the sample is located in the step five;

[0034] Preferably, in the step four, the specific method for temperature calibration is:

[0035] 1) cooling the measurement system to obtain a series of resistance-temperature raw numbers and fitting to obtain a resistance-temperature curve;

[0036] 2) generating 200 temperature points distributed as needed in the temperature range of 300K~3K according to the following formula:

[0037] ;

[0038] ;

[0039] In the formula, x takes natural numbers of 0, 1, 2…199;

[0040] 3) 200 temperature points corresponding to resistance values are obtained by interpolation method according to the resistance-temperature curve;

[0041] 4) for the low temperature interval of 20K~3K, the following power function formula is used to re-fit the resistance-temperature curve:

[0042] ;

[0043] In the formula, R is the resistance value, unit: ohm ; T is the temperature, unit: Kelvin ; y0, A are fitting constants, and n is the index;

[0044] 5) the resistance value corresponding to the temperature value of the 20K~3K region generated in step 2) is obtained by the formula in step 4), and the resistance value is used to replace the resistance value of the 20K~3K corresponding temperature region in step 3); After integration, the complete temperature-resistance calibration curve of 300K~3K is obtained, which is used for temperature measurement of the first temperature measuring electrode and the second temperature measuring electrode.

[0045] The beneficial effects of the present application are:

[0046] The measuring device is loaded in the diamond anvil, and through the integrated micro-heating wire, thermometer and double sets of measuring electrodes, a certain temperature gradient can be provided to the two-dimensional sample to be measured in a high-pressure environment to induce a thermoelectric response, and the temperature gradient and thermoelectric voltage can be accurately obtained, and the Seebeck coefficient and the Nernst coefficient can be calculated. Further, through the measuring device and the measuring method, multiple control parameters such as high pressure, extremely low temperature and strong magnetic field can be combined to study the change rule of the thermoelectric signal in extreme environment, clarify the correlation between various novel physical phenomena of two-dimensional materials, and have great value for developing the next generation of industrial waste heat power generation system, integrated solid-state cooler, and extreme environment (such as deep space, deep earth, deep sea exploration power) energy application. It also lays a solid experimental foundation for understanding the influence of quantum effect on thermoelectric behavior under high pressure and realizing on-demand design of thermoelectric materials. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 It is an optical microscope graph of the measuring device on the lower anvil surface of the diamond.

[0048] Figure 2 It is a structural schematic diagram of the measuring device, wherein a diagram is a device graph (on the lower anvil surface of the diamond) under the optical microscope, and b diagram is a structural schematic diagram of the multifunctional bottom electrode in the red dashed box in a diagram.

[0049] Figure 3 Schematic diagram of the positional relationship of each electrode in the multifunctional bottom electrode.

[0050] Figure 4 The resistance values of the first temperature measuring electrode and the second temperature measuring electrode obtained after calibration in Example 2 vary with temperature.

[0051] Figure 5 When the system temperature is 16 K, the temperature changes of the first temperature measuring electrode and the second temperature measuring electrode (a) and the temperature gradient changes with the current of the heating wire (b) when different currents are applied to the heating wire.

[0052] Figure 6 When the system temperature is stabilized at 4 K, 2 mA current is passed through the heating wire, ±12 T magnetic field scanning measurement is carried out under different pressures, and the calculated Seebeck (a) and Nernst thermoelectric signal (b) results are obtained.

[0053] Figure 7 MoTe2 sample cross-section height distribution map obtained using AFM. DETAILED DESCRIPTION

[0054] Unless otherwise specified, the terms used herein have meanings commonly understood by those skilled in the art.

[0055] The technical solutions of the present application will be described in more detail below in conjunction with specific embodiments:

[0056] Example 1

[0057] The preparation of the measuring device includes the following steps:

[0058] S1. Place the lower anvil surface of the diamond anvil cell (DAC) on the sample table of the plasma cleaner, make sure the anvil surface is facing up, close the cleaning chamber, and vacuum to the base pressure (usually 10 -2 ~10 -3 mbar). Introduce cleaning gas, set the gas flow, start the plasma generator, set the power to 50 W and the cleaning time to 10 min, after cleaning, turn off the plasma, stop the gas introduction, and slowly release the vacuum.

[0059] Remove the diamond lower anvil surface, avoid direct contact with the surface with your hands, use clean tweezers to place the diamond anvil surface firmly on the spin coater, evenly drop a few drops of MMA photoresist, spin at 4000 rpm for 30 s, then heat at 80°C for 2 min, then evenly drop PMMA photoresist, spin at 4000 rpm for 60 s, then heat at 120°C for 2 min.

[0060] The pre-designed pattern in the drawing software klayout is imported into the control terminal computer of the electron beam exposure system. After the photoresist is evenly homogenized on the anvil surface and placed under vacuum for 2 hours, it is placed in the vacuum chamber of the electron microscope. When the vacuum level reaches 2x10⁻⁶, the electron beam exposure system is prepared. -5 After mbar exposure, the anvil is located under the electron beam, the center is determined, and exposure begins.

[0061] After exposure, the sample is developed using a developer solution with a volume ratio of isopropanol:tetramethyldipentanone of 2:1 to remove the photoresist in the exposed areas. This process requires approximately 20 seconds of soaking, followed by fixation in isopropanol for about 10 seconds to stabilize the photoresist structure in the unexposed areas.

[0062] S2. Place the diamond anvil with the etched electrode in an electron beam evaporation and resistance evaporation composite coating instrument, and perform titanium (5 nm) / gold (30 nm) evaporation under vacuum to form a conductive layer.

[0063] S3. After the evaporation is completed, the sample is immersed in an acetone solution to remove the residual photoresist and the excess metal layer attached thereto. Finally, the required multifunctional bottom electrode is formed on the diamond anvil, and the measuring device is obtained.

[0064] Specifically, in this measuring device, the multifunctional bottom electrode includes a heating wire and a first temperature measuring electrode, a thermoelectric voltage measuring electrode, and a second temperature measuring electrode (Ti / Au composite coating) arranged independently and sequentially on one side of the heating wire. The heating wire, the first temperature measuring electrode, and the second temperature measuring electrode are all strip-shaped with a width of 1-2 μm. The heating wire is arranged in a serpentine pattern to increase the heating area, and the first and second temperature measuring electrodes are arranged linearly parallel to the thermoelectric voltage measuring electrode. The thermoelectric voltage measuring electrode includes four identical and parallel linear electrodes, each 2-3 μm wide, arranged symmetrically in two rows. The interval between the two rows of the thermoelectric voltage measuring electrode is less than the width of the sample to be measured, and the length of the first and second temperature measuring electrodes is greater than the width of the sample to be measured. The heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode, and the second temperature measuring electrode together constitute the sample measurement area.

[0065] The thermoelectric effect of two-dimensional materials was measured using the aforementioned measuring device, and the method was as follows:

[0066] Step one: Using a micromanipulator on the transfer platform, place the two-dimensional material sample to be tested in the sample measurement area. Then, using the same method, transfer a hexagonal boron nitride (hBN) sheet onto the surface of the sample to isolate it from air and prevent oxidation. See [link to relevant documentation]. Figure 1 One side of the sample is close to the heating wire but not in contact with it. The sample forms good ohmic contact with the thermoelectric voltage measuring electrode, the first temperature measuring electrode, and the second temperature measuring electrode.

[0067] The two-dimensional materials in this application include various single-crystal materials with different thermoelectric responses, such as molybdenum ditelluride (MoTe2) and rubidium vanadium triantimony pentoxide (RbV3Sb5).

[0068] In this step, care should be taken to ensure that the sample does not come into electrical contact with the heating wire during sample transfer. This is to ensure that the measured signal is entirely generated by the heat provided by the heating wire, thus guaranteeing the authenticity of the results. During the transfer process, the transfer process can be monitored in real time using an optical microscope or a micromanipulation system to ensure that the contact interface between the two-dimensional material and the electrode is flat and that the electrodes make ohmic contact. Ultimately, this achieves efficient integration of the two-dimensional material and the electrode circuit, laying the foundation for subsequent thermoelectric testing and device functionality.

[0069] Step 2: Connect the heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode, and the second temperature measuring electrode to the ends of the heating wire, extending them to the edge of the lower anvil. Then, use enameled copper wire to connect the platinum strip to the nanovoltmeter, the current source, and the temperature controller respectively to obtain the measurement system.

[0070] In the above connection, the nanovoltmeter is connected to the platinum bar that connects to the thermoelectric voltage measuring electrode, the current source is connected to the platinum bar that connects to the heating wire, and two temperature controllers are provided, which are respectively connected to the platinum bars that connect to the first temperature measuring electrode and the second temperature measuring electrode.

[0071] In this step, the measuring instruments used to accurately measure the thermoelectric signals of the Seebeck and Nernst coefficients of samples with thicknesses at the nanometer level under the high-pressure environment formed by the diamond anvil cell are, including but not limited to, a 2182A nanovolt voltmeter, a 6221 DC and AC current source, a Lake Shore 350 temperature controller, and a pressure calibration system.

[0072] Step 3: Place the prepared lower anvil and the upper anvil of the diamond anvil at the center of the hydraulic press, and then perform pressure calibration. The pressure calibration method is ruby ​​fluorescence spectroscopy, specifically:

[0073] A small amount of ruby ​​powder was placed on the lower anvil, ensuring that the ruby ​​powder and the two-dimensional sample to be tested were under the same pressure environment. The fluorescence spectrum of the ruby ​​powder was measured using an instrument, and the characteristic wavelengths were recorded. For example, a force of 200 kg is pre-applied to the DAC, and the pressure sensor value is reduced by tightening the screw, so that the pressure acts on the lower anvil surface, and the characteristic wavelength is recorded. Based on the characteristic wavelength of ruby ​​under standard atmospheric pressure. Calculate the standard pressure using the following formula. :

[0074] ;

[0075] This allows for accurate pressure measurement.

[0076] Step four: The measurement system is cooled in a 12T Oxford cryostat to obtain the resistance values ​​of the first and second temperature-sensing electrodes as a function of temperature, which is used for temperature calibration. Specifically:

[0077] 1) Cool down the measurement system to obtain a series of raw resistance-temperature data, and fit the resistance-temperature curve.

[0078] 2) Within the temperature range of 300K to 3K, generate 200 temperature points distributed as needed (x is a natural number from 0, 1, 2...199) according to the following formula, covering the entire temperature measurement range:

[0079] ;

[0080] ;

[0081] 3) For the resistance-temperature curve, the corresponding resistance values ​​are obtained on the curve using the 200 generated temperature points through interpolation, forming a temperature-resistance dataset;

[0082] 4) For the low-temperature range of 20K to 3K, the resistance-temperature curve is refitted using the following power function formula:

[0083] ;

[0084] In the formula, R is the resistance value, T is the temperature, y0 and A are fitting constants, and n is the exponent;

[0085] 5) Substitute the temperature values ​​of the 20K~3K region generated in step 2) into the formula in step 4) to obtain the corresponding resistance values. Replace the resistance values ​​of the corresponding temperature region of 20K~3K in step 3) with these resistance values. After integration, we obtain 200 optimized temperature-resistance datasets, forming a complete temperature-resistance calibration curve of 300K~3K.

[0086] The obtained temperature-resistance calibration curve is imported into the temperature controller. The resistance signals of the first and second temperature measuring electrodes collected in subsequent measurements are automatically converted into actual temperature values ​​according to the calibration curve, thus completing the real-time temperature display.

[0087] Step 5: Apply different currents to the heating wire at different stable system temperatures. (mA) to generate different thermal gradients, and obtain the temperature difference through the first and second temperature measuring electrodes. :

[0088] ;

[0089] In the formula, The temperature of the sample near the heating wire, measured by the first thermometric electrode, is expressed in Kelvin. ; The temperature measured by the second thermometric electrode at the end of the sample furthest from the heating wire, in Kelvin. ; The linear distance between linear electrodes in the same row of thermoelectric voltage measurement electrodes, in micrometers. ; The straight-line distance between the first and second temperature measuring electrodes is expressed in micrometers. .

[0090] Then, a magnetic field perpendicular to the plane of the sample was applied using a 12T Oxford cryostat, and the voltage between two linear electrodes in the same column of the electrode was measured by thermoelectric voltage measurement. The unit is microvolts. The voltage between two linear electrodes located in different columns but symmetrically arranged in the thermoelectric voltage measurement electrode system is measured. The unit is microvolts. Calculate the Seebeck coefficient of the sample according to the following formula. and Nernst coefficient :

[0091] ;

[0092] ;

[0093] In the formula, B is the magnitude of the magnetic field perpendicular to the plane containing the sample, measured in Tesla. ; The linear distance between two linear electrodes located in different columns but symmetrically arranged in the thermoelectric voltage measurement electrode system, in micrometers. .

[0094] In this application, the variation law of thermoelectric signal under extreme environment can be further studied by using multiple control parameters such as high pressure, extremely low temperature, and strong magnetic field.

[0095] Furthermore, after completing the test, the sample thickness is measured using an atomic force microscope (AFM) to obtain a cross-sectional height distribution map, thereby determining the sample thickness and providing a dimensional reference for analyzing material properties.

[0096] Example 2

[0097] Measurement of the high-pressure thermoelectric effect of a certain two-dimensional MoTe2 single crystal material:

[0098] 1. Preparation of MoTe2 two-dimensional single crystal materials:

[0099] A certain proportion of sealed single-crystal molybdenum tellurium (mixture) was placed in a KSL-1200X-J muffle furnace for annealing. Note that the sample should be placed in a position to ensure uniform heating and avoid local overheating or combustion. After annealing, it was slowly cooled to room temperature to obtain the two-dimensional single-crystal material MoTe2 in this example.

[0100] 2. Preparation of the measuring device:

[0101] Place the lower anvil of the diamond anvil cell (DAC) on the sample stage of the plasma cleaner, ensuring the anvil faces upwards. Close the cleaning chamber and evacuate to the baseline pressure (typically 10). -2 ~10 -3 (mbar). Introduce cleaning gas, set the gas flow rate, start the plasma generator, set the power to 50 W and the cleaning time to 10 min. After cleaning is complete, turn off the plasma, stop the gas supply, and slowly release the vacuum.

[0102] Remove the diamond anvil, avoiding direct contact with the surface with your hands. Use clean tweezers to firmly place the diamond anvil onto the spin coater. Spin coat a few drops of MMA photoresist evenly at 4000 rpm for 30 seconds, then heat at 80°C for 2 minutes. Next, spin coat PMMA photoresist evenly at 4000 rpm for 60 seconds, then heat at 120°C for 2 minutes.

[0103] The pre-designed pattern in the drawing software klayout is imported into the control terminal computer of the electron beam exposure system. After the anvil surface with homogenized photoresist is placed under vacuum for 2 hours, it is placed in the vacuum chamber of the electron microscope. When the vacuum degree reaches 2×10⁻⁶... -5 After mbar exposure, the anvil is located under the electron beam, the center is determined, and exposure begins.

[0104] After exposure, the sample is developed using a developer solution with a volume ratio of isopropanol:tetramethyldipentanone of 2:1 to remove the photoresist in the exposed areas. This process requires approximately 20 seconds of soaking, followed by fixation in isopropanol for about 10 seconds to stabilize the photoresist structure in the unexposed areas.

[0105] The diamond anvil with the etched electrode is placed in an electron beam evaporation and resistance evaporation composite coating instrument, and titanium (5 nm) / gold (30 nm) is deposited in a vacuum environment to form a conductive layer.

[0106] After vapor deposition, the diamond anvil is immersed in acetone solution and left to stand for 10 minutes. Then, it is ultrasonicated for 30 seconds to remove residual photoresist and excess metal layer, ultimately forming the desired multifunctional bottom electrode on the diamond anvil. Figure 1 As shown.

[0107] See Figures 2-3In this embodiment, the multifunctional bottom electrode on the diamond anvil cell mainly consists of a serpentine heating wire (approximately 600 μm in total length and 2 μm in line width), a strip-shaped first temperature measuring electrode (approximately 120 μm in length and 1.8 μm in line width), a second temperature measuring electrode (approximately 80 μm in length), and a thermoelectric voltage measuring electrode (2 μm in line width). The electrodes are arranged in the following order: heating wire, first temperature measuring electrode, thermoelectric voltage measuring electrode, and second temperature measuring electrode. All electrodes must be parallel and maintain an appropriate spacing. In this embodiment, the first temperature measuring electrode is approximately 4 μm away from the heating wire electrode, and the straight-line distance between the first and second temperature measuring electrodes is... Approximately 28 μm; the straight-line distance between linear electrodes in the same row of thermoelectric voltage measurement electrodes. The linear distance between two linear electrodes located in different columns but arranged symmetrically is approximately 14 μm. The distance is approximately 4 μm, excluding the electrode linewidth. The spacing between the electrodes is carefully chosen to ensure that each parallel electrode does not touch each other while being as close as possible to the heating wire to create a larger thermal gradient.

[0108] A MoTe2 single crystal sample was placed on a slightly viscous blue adhesive tape and mechanically peeled off to the desired thickness. Polydimethylsiloxane (PDMS) was then applied to the tape containing the sample, extracting a thinner portion of the sample. The PDMS was then observed under an optical microscope to identify samples with a smooth surface, suitable thickness, and size. A transfer platform micromanipulator was then used to precisely transfer the two-dimensional sample from the PDMS surface onto the sample measurement area of ​​the diamond anvil. It is crucial that the sample not come into contact with the heating wire. Figure 2 The dark blue MoTe2 sample (approximately 120 nm thick, 60 μm long and 50 μm wide) is shown in Figure a. A hexagonal boron nitride (hBN) sheet is then transferred onto the sample surface using the same method to isolate it from air and prevent oxidation. The principle for selecting the size and thickness of the hBN is that its size should completely cover the sample to achieve good air isolation, while the thickness is not critical and should be as uniform as possible. Finally, on the surface of the diamond anvil cell, from bottom to top, are the bottom electrode, the sample (MoTe2), and the hBN sheet, as shown. Figure 2 As shown in Figure b.

[0109] The electrodes of the lower diamond anvil are extended to the outside of the anvil using pre-cut platinum strips. Then, the platinum strips are connected and led to the outside of the upper diamond anvil using enameled copper wires of different colors. Finally, the upper diamond anvil is aligned with the lower diamond anvil and the upper anvil is closed to complete the fabrication of the diamond anvil cell (DAC) micro-nano thermoelectric device with two-dimensional material MoTe2.

[0110] 3. Measurement of the thermoelectric effect of MoTe2 under different pressures using the above measuring device:

[0111] First, a diamond anvil cell is placed in the center of a hydraulic press and a force of approximately 100 kg is applied. The pressure sensor reading is reduced by tightening a screw to apply pressure to the sample chamber. The prepared diamond anvil cell (DAC) micro-nano thermoelectric device is then placed in the pressure calibration system. Pressure calibration is performed using ruby ​​fluorescence spectroscopy. By monitoring the wavelength changes in the ruby ​​fluorescence spectrum, a redshift phenomenon is observed in the spectral wavelength as the pressure increases, thus achieving accurate pressure measurement.

[0112] ;

[0113] In the formula, and These represent the wavelengths of the spectrum at a specific pressurized pressure and a standard atmosphere, respectively, where p is the standard pressure. In this embodiment, the first pressure point tested was calibrated to 0.5 GPa.

[0114] The enameled copper wire of the prepared diamond anvil cell (DAC) thermoelectric device is soldered to the measuring rod of the Oxford cryostat, and the enameled copper wire is gently moved to ensure a stable connection with the measuring instrument.

[0115] The measuring rod was placed in a 12T Oxford cryostat for cooling. Simultaneously, the first and second temperature-sensing electrodes were connected to a Lake Shore 350 temperature control instrument, and their resistance changes with system temperature were recorded for later thermometer calibration. See Example 1 for details. The final calibration curve of the thermometer obtained in this embodiment is shown below. Figure 4 As shown, the 200 data points obtained by interpolation match the original data very well. The calibration curves of the first and second temperature measuring electrodes were then imported into the Lake Shore 350 temperature control instrument, which can then perform real-time conversion from resistance to temperature.

[0116] Different currents were applied to the heating wire at different stable system temperatures. Different thermal gradients are generated, and the temperature gradient is obtained by measuring the temperature difference between two temperature-sensing electrodes. The first temperature-sensing electrode is denoted as... The second temperature measuring electrode is denoted as The temperature gradient on the sample is then tested as follows:

[0117] ;

[0118] in The distance between the two electrodes used to measure the longitudinal thermoelectric voltage is d, where d is the distance between the two thermometers. In this example... It is 14 μm. The sample temperature is 28 μm. When a heating wire provides the heat source, the temperature on the sample is no longer the same as the system temperature; the temperature T at the center of the sample is taken. mid = ( T hot + T cold ) / 2 is taken as the actual temperature of the sample.

[0119] The specific temperature gradient can be adjusted based on the thermoelectric response of the material being measured and the adaptability of the system to different temperatures. In this embodiment, the temperature gradient provided by the micro heating wire to the sample at a system temperature of 16 K is as follows: Figure 5 As shown, since the substrate is diamond, the establishment of the temperature gradient is affected by the thermal conductivity of the substrate. As the system temperature increases, the temperature gradient generated by the same current will decrease. Therefore, the current of the heating wire will generally be greater at higher temperatures. The original data of the first thermometer, the second thermometer, the sample temperature, and the temperature gradient when different currents (1 mA to 8 mA) are applied to the heating wire are shown in Table 1.

[0120] Table 1. Raw data of temperature gradients under different currents

[0121] .

[0122] Following the thermometer calibration and temperature gradient detection under different system temperatures, a suitable temperature gradient was selected for the sample, and a corresponding current was applied to the heating wire. In this embodiment, since the temperature of the MoTe2 sample under test is generally at a low level, the system temperature was stabilized at 4 K. Subsequently, a 2 mA current was applied to the heating wire using a 6221 DC and AC microcurrent source (at which point the actual temperature on the sample was 7.4 K). Two 2182A nanovoltmeters were used to detect the voltage between two linear electrodes in the same column of the thermoelectric voltage measurement electrode. (Vertical electrode voltage) and the voltage between two linear electrodes located in different columns but arranged symmetrically. (Transverse electrode voltage), after the temperature gradient has stabilized, use a 12T Oxford cryostat to start at +12T. An experiment was conducted using a 12T magnetic field sweep, and the changes in voltage with magnetic field were recorded for two sets of data.

[0123] In this embodiment, the specific test pressures are 0.5 GPa, 1.3 GPa, 2.7 GPa, 4.5 GPa, 6.1 GPa, and 9.4 GPa. After each test, the device needs to be removed from the Oxford cryostat and placed on a hydraulic press for repressurization. The pressure calibration process needs to be repeated for each change in pressure value to obtain accurate pressure.

[0124] Following the above testing method, calculate the Seebeck coefficient S and the Nernst coefficient N according to the formulas:

[0125] ;

[0126] ;

[0127] In the formula, B is the magnitude of the magnetic field perpendicular to the plane containing the sample. The straight-line distance between two linear electrodes located in different columns but symmetrically arranged in a thermoelectric voltage measurement electrode, in this example... It is 4 μm.

[0128] Based on the series of results, we finally obtained... Figure 6 The series of thermoelectric signals shown indicate that the Seebeck coefficient S and Nernst coefficient N undergo a series of significant changes under different pressure values. The Seebeck signal shows a symmetrical trend with the positive and negative magnetic fields, and the sign has also flipped multiple times. In contrast, the Nernst signal shows an antisymmetric trend with the positive and negative magnetic fields, and its value decreases as the pressure increases.

[0129] After all pressure measurements are completed, the diamond anvil cell pressure pack is disassembled. The MoTe2 sample measured on the lower diamond anvil cell is placed under an atomic force microscope (AFM) to measure the sample thickness, obtaining a cross-sectional height distribution map to determine the measured MoTe2 sample thickness. Figure 7 As shown, the thickness of the MoTe2 sample in this embodiment is approximately 127 nm.

[0130] It is evident that the apparatus and method provided in this application have successfully acquired high-quality data and thermoelectric signals with certain physical significance.

[0131] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A diamond anvil cell high-pressure thermoelectric effect measuring device for measuring the high-pressure thermoelectric effect of two-dimensional materials, characterized in that, The device includes a diamond anvil cell, on the lower anvil surface of which a multifunctional bottom electrode is disposed. The multifunctional bottom electrode includes a heating wire and a first temperature measuring electrode, a thermoelectric voltage measuring electrode, and a second temperature measuring electrode arranged independently and sequentially on one side of the heating wire. The thermoelectric voltage measuring electrode includes four identical and parallel linear electrodes, which are symmetrically arranged in two rows with intervals. The heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode, and the second temperature measuring electrode are all Ti / Au composite coated. The heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode, and the second temperature measuring electrode together constitute the sample measurement area.

2. The diamond anvil cell high-voltage thermoelectric effect measuring device as described in claim 1, characterized in that, The heating wire, the first temperature measuring electrode, and the second temperature measuring electrode are all strip-shaped, with the heating wire arranged in a serpentine pattern to increase the heating area. The first temperature measuring electrode, the second temperature measuring electrode, and the thermoelectric voltage measuring electrode are arranged in parallel. The width of the first temperature measuring electrode, the second temperature measuring electrode, and the heating wire is 1~2 μm, and the width of the four linear electrodes is 2~3 μm.

3. The diamond anvil cell high-voltage thermoelectric effect measuring device as described in claim 2, characterized in that, In the thermoelectric voltage measuring electrodes, the interval between the two columns is smaller than the width of the sample to be measured, and the lengths of the first and second temperature measuring electrodes are greater than the width of the sample to be measured.

4. The diamond anvil cell high-voltage thermoelectric effect measuring device as described in claim 1, characterized in that, In the Ti / Au composite coating, Ti is the bottom adhesion layer with a thickness of 5~10 nm, and Au is the surface conductive layer with a thickness of 30~50 nm.

5. A diamond anvil cell high-pressure thermoelectric effect measuring device as described in any one of claims 1-4, characterized in that, The method for preparing the measuring device includes the following steps: S1. The lower anvil surface of the diamond anvil is plasma cleaned, and then photoresist is uniformly coated on the clean lower anvil surface and hardened. Then, the patterning is performed according to the required multifunctional bottom electrode arrangement position. S2. Sequentially deposit Ti thin films and Au thin films on the patterned lower anvil surface; S3. Remove excess photoresist to obtain the prepared measuring device.

6. The diamond anvil cell high-voltage thermoelectric effect measuring device as described in claim 5, characterized in that, In step S1, the photoresist uses an MMA / PMMA system. MMA is spin-coated at 4000 rpm for 30 seconds and then heated at 80°C for 2 minutes. Then PMMA is spin-coated at 4000 rpm for 60 seconds and then heated at 120°C for 2 minutes to complete the coating and hardening process.

7. The diamond anvil cell high-voltage thermoelectric effect measuring device as described in claim 5, characterized in that, In step S1, the graphic processing involves shaping the lower anvil surface after hardening into a 2x10 grid. -5 Electron beam exposure is performed under mbar vacuum, followed by immersion in developer for 20 seconds and then immersion in isopropanol for 10 seconds for fixing; the developer is prepared by mixing isopropanol and tetramethyldipentanone in a volume ratio of 2:1; in step S3, the photoresist is removed using acetone.

8. A method for measuring the high-pressure thermoelectric effect of two-dimensional materials, characterized in that, The measurement is performed using the measuring device described in any one of claims 1-4, and the specific steps are as follows: Step 1: Place the two-dimensional material sample to be tested in the sample measurement area and cover the surface of the sample with a layer of hexagonal boron nitride sheet. One side of the sample is close to the heating wire but avoids the heating wire. The sample forms good ohmic contact with the thermoelectric voltage measuring electrode, the first temperature measuring electrode, and the second temperature measuring electrode. Step 2: Connect the ends of the heating wire, the first temperature measuring electrode, the thermoelectric voltage measuring electrode, and the second temperature measuring electrode with platinum bars and extend them to the edge of the lower anvil. Then, use wires to connect the platinum bars to the nanovoltmeter, the current source, and the temperature controller respectively to obtain the measurement system. Step 3: Place the prepared lower anvil and the upper anvil of the diamond anvil at the center of the hydraulic press, and then perform pressure calibration; Step four: Place the measurement system in a thermostat to cool it down, and obtain the variation of the resistance values ​​of the first and second temperature measuring electrodes with temperature, which is used for temperature calibration. Step 5: The heating wire is energized and heated to the set temperature. After a stable temperature difference is generated between the two ends of the sample, the temperature difference is obtained through the first and second temperature measuring electrodes. Then, a magnetic field perpendicular to the plane of the sample is applied, and the voltage between two linear electrodes in the same column of the electrode is measured by thermoelectric voltage measurement. The unit is microvolts. The voltage between two linear electrodes located in different columns but symmetrically arranged in the thermoelectric voltage measurement electrode system is measured. The unit is microvolts. The Seebeck coefficient of the sample is calculated according to the following formula. and Nernst coefficient : In the formula, The temperature of the sample near the heating wire, measured by the first thermometric electrode, is expressed in Kelvin. ; The temperature measured by the second thermometric electrode at the end of the sample furthest from the heating wire, in Kelvin. ; The linear distance between linear electrodes in the same row of thermoelectric voltage measuring electrodes, in micrometers. ; The straight-line distance between the first and second temperature measuring electrodes is expressed in micrometers. ; In the formula, B is the magnitude of the magnetic field perpendicular to the plane containing the sample, measured in Tesla. ; The linear distance between two linear electrodes located in different columns but symmetrically arranged in the thermoelectric voltage measurement electrode system, in micrometers. .

9. The method for measuring the high-pressure thermoelectric effect of two-dimensional materials as described in claim 8, characterized in that, In step two, the nanovoltmeter is connected to the platinum strip connecting the thermoelectric voltage measuring electrode, the current source is connected to the platinum strip connecting the heating wire, and two temperature controllers are provided, which are respectively connected to the platinum strips connecting the first and second temperature measuring electrodes; in step three, the pressure calibration method is ruby ​​fluorescence spectroscopy; in step four, the thermostat is a 12T Oxford cryostat, which is also used to generate the magnetic field perpendicular to the plane of the sample as described in step five.

10. A method for measuring the high-pressure thermoelectric effect of a two-dimensional material as described in claim 8 or 9, characterized in that, In step four, the specific method for temperature calibration is as follows: 1) Cool down the measurement system to obtain a series of raw resistance-temperature values ​​and fit them to obtain a resistance-temperature curve; 2) Within the temperature range of 300K to 3K, generate 200 temperature points distributed as needed using the following formula: In the formula, Choose a natural number from 0, 1, 2...199; 3) Using interpolation, the resistance values ​​corresponding to the 200 generated temperature points are obtained based on the resistance-temperature curve; 4) For the low-temperature range of 20K to 3K, the resistance-temperature curve is refitted using the following power function formula: In the formula, R is the resistance value, and the unit is ohms. T represents temperature, measured in Kelvin. ; y0 and A are fitting constants, and n is the exponent; 5) Substitute the temperature value of the 20K~3K region generated in step 2) into the formula in step 4) to obtain the corresponding resistance value, and replace the resistance value of the 20K~3K region in step 3) with this resistance value; after integration, a complete temperature-resistance calibration curve of 300K~3K is obtained, which is used for temperature measurement of the first temperature measuring electrode and the second temperature measuring electrode.

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