Van der Waals heterojunction and charge unidirectional transfer detection method thereof

Abstract

The invention belongs to the field of two-dimensional materials, and particularly relates to a Van der Waals heterojunction and a detection method for unidirectional charge transfer of the Van der Waals heterojunction. Molybdenum telluride of a 1T'phase and tungsten diselenide of a 2H phase are stacked to form a Van der Waals heterostructure, the structure has more excellent physical and chemical properties, and possibility is provided for realizing more excellent photoelectric properties. By selecting a two-dimensional material with a proper band gap and utilizing the difference of energy band arrangement in a heterojunction, I-type energy band arrangement is formed, a one-way charge transfer channel is designed, and one-way charge transfer is achieved. A layer selective absorption pumping detection technology is adopted to research a one-way dynamic process of charge transfer: according to a band gap, photon energy 1.51 eV is selected to selectively excite a 1T '-MoTe2 atomic layer of a narrow band gap in a heterogeneous layer, and carriers are injected into the 1T'-MoTe2 atomic layer. The wavelength of detection light is selected to be 741 nm (matched with the band gap of WSe2), and the dynamic behavior of carriers in the WSe2 layer is detected.

Description

Technical Field

[0001] The present invention belongs to the field of two-dimensional materials, and in particular relates to a van der Waals heterojunction and a detection method for unidirectional charge transfer thereof. Background Art

[0002] Van der Waals heterojunctions are formed by stacking different two-dimensional materials through van der Waals forces and have unique electrical, optical and mechanical properties. In recent years, with the in-depth study of two-dimensional materials, van der Waals heterojunctions have shown great application potential in the fields of photoelectric detection and energy conversion. In terms of unidirectional charge transfer, the interlayer charge transfer phenomenon in van der Waals heterojunctions is the key to realizing its various novel physical properties. By regulating the interlayer charge transfer, precise control of the electrical, optical, magnetic and other properties of the heterojunction can be achieved. Therefore, the development of an effective method for detecting unidirectional charge transfer is of great significance for a deep understanding of the physical mechanism of van der Waals heterojunctions and the development of new devices.

[0003] 1) Van der Waals heterojunction

[0004] Van der Waals heterojunction is an interface structure formed by stacking different two-dimensional materials through van der Waals forces, which has unique physical and chemical properties. In the existing technology, van der Waals heterojunction has been widely used in the fields of photoelectric detection, gas separation, magnetic storage, etc.

[0005] 2) Charge unidirectional transfer detection technology

[0006] Unidirectional charge transfer refers to the phenomenon that under certain conditions, charge can only be transferred in one direction. In the prior art, unidirectional charge transfer technology has been applied to the fields of solar cells, energy collection systems, etc. At present, the methods or technologies for detecting unidirectional charge transfer include: electrochemical methods (cyclic voltammetry, AC impedance spectroscopy), spectroscopic methods (fluorescence spectroscopy, absorption spectroscopy) and imaging technology (scanning tunneling microscopy (STM), atomic force microscopy (AFM)), etc.

[0007] Most of the existing van der Waals heterojunctions have a 2H / 2H crystal structure, and very few have a 1T / 2H structure. In order to obtain heterojunctions with efficient charge transfer and energy transfer, it is urgent to develop new heterostructures with unique lattice structure combinations. In addition, the detection of the charge transfer path at the van der Waals heterojunction interface is still insufficient at this stage, and the unidirectional charge transfer process cannot be directly monitored, which limits its further application in high-performance photoelectric detection devices. Summary of the invention

[0008] The present invention aims to provide a novel (1T' / 2H type) van der Waals heterojunction and a detection method for unidirectional charge transfer thereof, so as to broaden the existing van der Waals heterojunction system and solve the problem of difficulty in detecting unidirectional charge transfer in the prior art.

[0009] In order to solve the above-mentioned technical problems, this application provides the following technical solutions:

[0010] The present invention provides a van der Waals heterojunction, wherein the van der Waals heterojunction is obtained by laminating a few layers of 1T′ phase molybdenum ditelluride and a single layer of 2H phase tungsten diselenide and then annealing at 125-135° C. for 4-6 hours in a protective atmosphere;

[0011] The 2H phase single-layer tungsten diselenide is a single-layer sample with an optical contrast ratio lower than 15% formed by mechanical exfoliation of a tungsten diselenide crystal.

[0012] Preferably, the method for preparing the 1T′ phase few-layer molybdenum ditelluride comprises the following steps:

[0013] S11: depositing a molybdenum film on the substrate by magnetron sputtering to obtain a molybdenum film substrate;

[0014] S12: Lay the molybdenum films in the two molybdenum film substrates together, place them in a container, heat them, and then cool them to room temperature (25±5°C) to obtain the 1T′ phase few-layer molybdenum ditelluride; the heating is carried out in an argon-hydrogen mixed gas, the heating temperature is 680-700°C, and the heating time is 14-16 minutes.

[0015] Furthermore, the substrate is selected from SiO2 / Si substrate; the thickness of the substrate is 300nm.

[0016] Furthermore, the thickness of the molybdenum film is 1 nm.

[0017] Furthermore, the container is selected from a corundum boat.

[0018] Furthermore, in the argon-hydrogen mixed gas, the volume ratio of hydrogen is 5%, and the volume ratio of argon is 95%.

[0019] Furthermore, in step S12, the time for cooling to room temperature is 85-95 minutes.

[0020] Preferably, the protective atmosphere is argon.

[0021] Preferably, the annealing is performed in a tube furnace.

[0022] The present invention also provides a method for detecting the unidirectional charge transfer of the above van der Waals heterojunction, which is performed using a femtosecond laser system composed of a titanium sapphire laser and an optical parametric oscillator, and comprises the following steps:

[0023] S21: selectively exciting the 1T′ phase few-layer MoTe in the van der Waals heterojunction using a pulsed laser with a photon energy of 1.51 eV and 820 nm;

[0024] S22: Probe the van der Waals heterojunction with a 741 nm detection pulse, and collect changes in reflection or absorption of the detection light by each layer in the van der Waals heterojunction after being excited to reflect changes in carrier concentration in real time.

[0025] The present invention proposes a novel (1T′ / 2H type) van der Waals heterojunction and a detection method for unidirectional charge transfer thereof, aiming to expand the van der Waals heterojunction system and promote the development of unidirectional charge transfer detection technology.

[0026] Novel van der Waals heterojunction structure:

[0027] The present invention uses 1T' phase molybdenum ditelluride (MoTe2) and 2H phase tungsten diselenide (WSe2) to stack to form a van der Waals heterostructure, such as Figure 1 As shown, the structure has better physical and chemical properties, providing new possibilities for achieving better optoelectronic performance.

[0028] Efficient one-way charge transfer channel:

[0029] By selecting a two-dimensional material with a suitable band gap and utilizing the difference in the energy band arrangement in the heterojunction, a type I (straddling) energy band arrangement ( Figure 2 ), design the charge transfer channel to realize the unidirectional transfer of charge.

[0030] Detection method of unidirectional charge transfer:

[0031] The pump-probe technique of layer-selective absorption was used to study the unidirectional dynamics of charge transfer: based on the band gap, a photon energy of 1.51 eV (lower than that of wide-bandgap semiconductor materials) was selected to selectively excite the narrow-bandgap 1T′-MoTe2 atomic layer in the heterolayer and inject carriers into it. The probe light wavelength was selected to be 741 nm (matching the bandgap of WSe2) to detect the carrier dynamics in the WSe2 layer.

[0032] The technical solution of the present invention has the following advantages over the prior art:

[0033] 1) The novel (1T′ / 2H type) van der Waals heterojunction of the present invention realizes efficient unidirectional charge transfer by optimizing the type and structure of semiconductor materials, thereby improving the efficiency of photoelectric detection and energy conversion.

[0034] 2) The charge unidirectional transfer detection method of the present invention adopts advanced detection technology, which can observe and record the charge transfer process in real time and accurately, and provides a powerful tool for studying and optimizing the performance of van der Waals heterojunctions.

[0035] 3) The technical solution of the present invention has broad application prospects and can be used to prepare high-performance photoelectric detectors, solar cells and other devices, contributing to the development of new energy and optoelectronic technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the crystal structure diagram of WSe2-MoTe2(1T′) heterostructure.

[0037] Figure 2 Schematic diagram of type I band alignment of WSe2 and 1T′-MoTe2.

[0038] Figure 3 Schematic diagram of unidirectional charge transfer in a 1T′ / 2H heterojunction under layer-selective pump-probe (a) and pump-probe setup (b). DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0040] Example 1

[0041] The crystal structures of the 1T′ phase few-layer molybdenum ditelluride (MoTe2) and the 2H phase single-layer tungsten diselenide (WSe2) were selected.

[0042] The thin film of MoTe2 with few layers of 1T′ phase was provided by the research group of Professor Jiao Liying of Tsinghua University. It was prepared by a one-step CVD (chemical vapor deposition) method. A 1nm thick Mo film was deposited on a 300nm thick SiO2 / Si substrate by magnetron sputtering. Under the conditions of 25℃ and 1MPa, two SiO2 / Si substrates with deposited Mo films were placed "face to face" in a corundum boat with a trapezoidal cross-section, and Te powder was loaded to the bottom of the corundum boat. In an atmosphere of Ar / H2 mixed gas (ratio of 95% / 5%) with a flow rate of 25sccm, the temperature of the tube furnace was heated from room temperature to 690℃ for 15 minutes; it was naturally cooled from 690℃ to room temperature 25℃ in 90 minutes to obtain a thin film of MoTe2 with few layers of 1T′ phase.

[0043] A single-layer WSe2 is prepared by mechanical exfoliation. The specific operation process of the mechanical exfoliation method is: use clean tweezers to place the bulk WSe2 crystal on a transparent tape or a blue tape. This tape is called a master tape. Then use a new tape (sub-tape) to stick it to the master tape multiple times to thin the bulk WSe2 material into a thin layer. Observe and calculate the optical contrast of the target WSe2 sample under the green channel under a microscope. If it is lower than 15%, it is a single-layer sample. The contrast of the single-layer sample prepared in this experiment is 11.6%, indicating that the mechanically exfoliated WSe2 is a single layer.

[0044] Then, using the weak viscoelasticity of polydimethylsiloxane (PDMS) prepared by our laboratory, the target single-layer WSe2 sample is directly peeled off onto the PDMS, and then the PDMS is flipped over. Based on the self-built or purchased transfer table, the target material on the PDMS is precisely aligned and fitted with the target substrate (1T′-MoTe2), and finally the PDMS is lifted up very slowly to complete the transfer of the target material, thereby preparing a 1T′ / 2H van der Waals heterojunction. The heterojunction is placed in a tube furnace and annealed at 130°C for 5 hours in an Ar gas atmosphere to obtain a van der Waals heterojunction with a high-quality interface.

[0045] Example 2

[0046] The crystal structures of the 1T′ phase few-layer molybdenum ditelluride (MoTe2) and the 2H phase single-layer tungsten diselenide (WSe2) were selected.

[0047] The few-layer 1T′ phase MoTe2 thin film was provided by the research group of Professor Jiao Liying of Tsinghua University. The specific preparation method is shown in Example 1.

[0048] A single-layer WSe2 was prepared by mechanical exfoliation. The specific operation process of the mechanical exfoliation method is as follows: use clean tweezers to place the bulk WSe2 crystal on a transparent tape or a blue tape. This tape is called a master tape. Then use a new tape (sub-tape) to stick it to the master tape multiple times to thin the bulk WSe2 material into a thin layer. The optical contrast of the target WSe2 sample under the green channel was observed and calculated under a microscope. It was less than 15%, indicating that the mechanically exfoliated WSe2 was a single layer.

[0049] Then, using the weak viscoelasticity of polydimethylsiloxane (PDMS) prepared by our laboratory, the target single-layer WSe2 sample is directly peeled off onto the PDMS, and then the PDMS is flipped over. Based on the self-built or purchased transfer table, the target material on the PDMS is precisely aligned and fitted with the target substrate (1T′-MoTe2), and finally the PDMS is lifted up very slowly to complete the transfer of the target material, thereby preparing a 1T′ / 2H van der Waals heterojunction. The heterojunction is placed in a tube furnace and annealed at 125°C for 4 hours in an Ar atmosphere to obtain a van der Waals heterojunction with a high-quality interface.

[0050] Example 3

[0051] The crystal structures of the 1T′ phase few-layer molybdenum ditelluride (MoTe2) and the 2H phase single-layer tungsten diselenide (WSe2) were selected.

[0052] The few-layer 1T′ phase MoTe2 thin film was provided by the research group of Professor Jiao Liying of Tsinghua University. The specific preparation method is shown in Example 1.

[0053] A single-layer WSe2 was prepared by mechanical exfoliation. The specific operation process of the mechanical exfoliation method is as follows: use clean tweezers to place the bulk WSe2 crystal on a transparent tape or a blue tape. This tape is called a master tape. Then use a new tape (sub-tape) to stick it to the master tape multiple times to thin the bulk WSe2 material into a thin layer. The optical contrast of the target WSe2 sample under the green channel was observed and calculated under a microscope. It was less than 15%, indicating that the mechanically exfoliated WSe2 was a single layer.

[0054] Then, using the weak viscoelasticity of polydimethylsiloxane (PDMS) prepared by our laboratory, the target single-layer WSe2 sample is directly peeled off onto the PDMS, and then the PDMS is flipped over. Based on the self-built or purchased transfer table, the target material on the PDMS is precisely aligned and fitted with the target substrate (1T′-MoTe2), and finally the PDMS is lifted very slowly to complete the transfer of the target material, thereby preparing a 1T′ / 2H van der Waals heterojunction. The heterojunction is placed in a tube furnace and annealed at 135°C for 6 hours in an Ar gas atmosphere to obtain a van der Waals heterojunction with a high-quality interface.

[0055] Example 4

[0056] According to theoretical calculations, it can be seen that MoTe2 and WSe2 can form a heterostructure with type I band alignment, so that the type I heterojunction can be designed to construct a unidirectional charge transfer channel.

[0057] The femtosecond laser system used in this embodiment is composed of a titanium sapphire laser (Spectra-Physics, MAI TAIHP) and an optical parametric oscillator (INSPIREAuto 50). The titanium sapphire laser outputs ultrashort pulses with a pulse width of 100fs, a frequency of 80MHZ, and a central wavelength of 820nm. After passing through a beam splitter, the pulses are divided into two beams. One beam enters the optical path through optical elements such as a reflector, and is collimated by a parallel aperture group and calibrated by a parallel lens group, and is directly used as a detection pulse laser; the other beam directly enters the parametric oscillator, and is tuned to output 741nm light as the detection light; the pump pulse and the detection pulse are focused at the same position of the sample through a lens. The linear displacement platform is used to control the time of the pump pulse to reach the sample, and the changes in the reflection or absorption of the detection light by the sample at different times after the sample is excited are collected to reflect the changes in the carrier concentration in real time.

[0058] A pulsed laser with a photon energy of 1.51eV (820nm) is used to selectively excite the 1T'-MoTe2 in the heterolayer of Example 1, and photogenerated carriers are injected therein. At this time, the pump light photon energy is lower than the band gap of WSe2, so it is not enough to excite the WSe2 atomic layer. The detection pulse is set to 741nm. The monolayer WSe2, heterojunction and 1T'-MoTe2 are detected respectively. Because the selected pump light does not excite WSe2, no differential reflection signal is observed in the monolayer WSe2. However, weak signals are observed in the WSe2-MoTe2 (1T') and 1T'-MoTe2 regions, and the peak and attenuation processes of the signals in the two regions are exactly the same. It should be noted that even though the peak energy flux of the pump light used here is 4 times the flux of the Pump light (3.02eV), the observed signal is only 1 / 10 of the original. This very small signal can be attributed to the lattice dynamics of 1T'-MoTe2. This part of the experimental results further proves that there is no charge transfer from 1T'-MoTe2 to WSe2 at the heterointerface, which is a favorable support for the WSe2-MoTe2 (1T') heterojunction to be type I band alignment.

[0059] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A van der Waals heterojunction, characterized in that: The van der Waals heterojunction is obtained by laminating a few layers of 1T′ phase molybdenum ditelluride and a single layer of 2H phase tungsten diselenide and annealing them at 125-135° C. for 4-6 hours in a protective atmosphere; The 2H phase single-layer tungsten diselenide is a single-layer sample with an optical contrast ratio lower than 15% formed by mechanical exfoliation of a tungsten diselenide crystal.

2. The van der Waals heterojunction according to claim 1, wherein: The preparation method of the 1T′ phase few-layer molybdenum ditelluride comprises the following steps: S11: depositing a molybdenum film on the substrate by magnetron sputtering to obtain a molybdenum film substrate; S12: Lay the molybdenum films in the two molybdenum film substrates together, place them in a container, heat them, and then cool them to room temperature to obtain the 1T′ phase few-layer molybdenum ditelluride; the heating is carried out in an argon-hydrogen mixed gas, the heating temperature is 680-700°C, and the heating time is 14-16 minutes.

3. The van der Waals heterojunction according to claim 2, characterized in that: The substrate is selected from SiO2 / Si substrate; the thickness of the substrate is 300nm.

4. The van der Waals heterojunction according to claim 2, wherein: The thickness of the molybdenum film is 1 nm.

5. The van der Waals heterojunction according to claim 2, characterized in that: The container is selected from a corundum boat.

6. The van der Waals heterojunction according to claim 2, characterized in that: In the argon-hydrogen mixed gas, the volume ratio of hydrogen is 5%, and the volume ratio of argon is 95%.

7. The van der Waals heterojunction according to claim 2, characterized in that: In step S12, the time for cooling to room temperature is 85-95 minutes.

8. The van der Waals heterojunction according to claim 1, characterized in that: The protective atmosphere is argon.

9. The van der Waals heterojunction according to claim 1, characterized in that: The annealing is carried out in a tube furnace.

10. A method for detecting unidirectional charge transfer of a van der Waals heterojunction according to any one of claims 1 to 9, characterized in that: The femtosecond laser system consisting of a titanium sapphire laser and an optical parametric oscillator is used, and the following steps are included: S21: selectively exciting the 1T′ phase few-layer MoTe in the van der Waals heterojunction using a pulsed laser with a photon energy of 1.51 eV and 820 nm; S22: Probe the van der Waals heterojunction with a 741 nm detection pulse, and collect changes in reflection or absorption of the detection light by each layer in the van der Waals heterojunction after being excited to reflect changes in carrier concentration in real time.

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